**Electroless Deposition of Nanolayered Metallic Coatings**

Jothi Sudagar, Rajendraprasad Tamilarasan, Udaykumar Sanjith, Raj Rajendran and Ravi Kumar

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68220

#### **Abstract**

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26 Nanoscaled Films and Layers

focus.

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Electroless metallic coating is referred as the deposition of a substrate material by the process of chemical or autocatalytic reduction of aqueous metal ions deposited to a substrate material without any external supply of power. Electroless nickel alloys are generally considered synonymous to the word "*electroless coating"* as ~90% of productions in industries are of this alloy coating. Rest of the electroless metallic coatings includes gold, copper, palladium, cobalt, silver, etc. These electroless metallic coatings (other than electroless nickel coatings) are also one of the vibrant areas in the field of materials properties and surface engineering research. From the year 2000 to till date, nearly 1000 SCI indexed research papers were published on this topic. However, no comprehensive studies about the recent progress on this topic were reported elsewhere so far. In this context, the present chapter aims to give a complete overview on various aspects of the rest of the electroless metallic nanocoatings/layer as a whole. More importance will be on the recent developments of the nanocharacteristics and future scopes.

**Keywords:** electroless deposition, metallic coating, electroless nanocoating, electroless gold, electroless copper, electroless palladium, electroless cobalt, electroless silver

## **1. Introduction**

The term "Electroless coating" is referred to as the reduction of aqueous metal ions plated to a substrate by autocatalytic or chemical means, in the absence of external current [1–3], and it disregards the technique used to perform coating in the absence of current such as immersion plating (deposition of copper on steel dipped in copper sulfate solution or nickel on steel dipped

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in chloride and boric acid bath) and the process of homogenous chemical reduction (silvering). The mechanism of autocatalytically deposited electroless metallic coatings differs in a way completely from coatings obtained by immersion plating or silvering, wherein in the latter case, nature of base material itself will behave as a reducing agent and does not necessitate any external reducing agent to initiate any metal ion to metal reduction. However, these processes have not gained wide acceptance, as it has poor adherence and non-productive behavior. Electroless process has received widespread acceptance in the market due to their exceptional anti-corrosion [4–6], wear resistance properties [3, 5–7] and also good for soldering and brazing applications [8]. Many metals like nickel, copper, gold, silver, palladium and cobalt are being deposited employing this technique [3, 9–12]. Rest of electroless coatings includes gold, copper, palladium, cobalt and silver [13–15]. These metallic coating/ layers (other than electroless nickel) are also one of the vibrant areas in the field of materials properties and surface engineering research. This review chapter provides an insight into the electroless metallic gold, copper, palladium, cobalt, silver, aluminum and the new processing technology in deposition mechanism and nanocharacteristics. Besides, more stress has been laid to understand the recent developments and its future scopes.

### **1.1. Historical overview**

The term "electroless plating" also defined as "autocatalytic" is nearly as old as electroplating. Von Liebig first described it in 1835 with the reduction of silver salts by reducing aldehydes. In 1988, metallic nickel deposition by electroless technique from an aqueous solution in the existence of hypophosphite (reducing agent) was first stated as a chemical tragedy by Wurtz [3, 16]. Roux, in the year 1911, positively reported that there was clear precipitation of metal in the form of powder. However, these workings were not practically used for any applications. Development in this field did not receive much of patronage until World War II. The method established by Brenner and Riddell for coating a layer of nickel-tungsten alloy in the inner parts of tubes by means of a citrate based bath containing an insoluble anode yielded the hypophosphite with uncommon reducing properties. The Patent Office of US declared that this patent, issued in 1950, is distinct and varied from the earlier patent owned by Roux, in which the process was unstructured and complete. In contrast, Brenner and Riddell's method was described as a controlled catalytic process as deposition process took place only on catalytically active surfaces dipped in the plating bath. Dr. Brenner later manifested that his patent was an unintended outcome comparable to the efforts of Wurtz and Roux. But he added that the patent was taken in order to safeguard US government rights. Actually, in 1963, a publicized US Army based technical report was printed that pronounced mostly about the work of Wurtz and Roux and gave much of the finding credit to Brenner and also covered by patent in 1956 [17]. This phenomenon ascribed to the action of chemical reduction of nickel ions was later identified.

After initiation of electroless nickel deposit, rest of electroless coatings was followed. In 1970, semiconductors and circuit boards were plated with thick, pure soft gold after the development of the first electroless gold coating bath at Bell Laboratories [14]. Narcus first reported the electroless copper deposit, and the first profitable application was realized by Cahill and Zeblisky et al. [18] in tartarate baths, where the reducing agent used was formaldehyde. Improved formulations of electroless copper [19] recorded a higher rate of plating with exceptionally stable conditions under diverse operating conditions. Baum et al. developed a process for selective deposition of copper by first selectively depositing palladium seeds in 1986. Palladium and alloys of palladium have been identified as a pecuniary alternative to applications involving gold plating. Electroless cobalt discovered along with nickel deposition by Brenner and Riddell [1–3]. This technique of electroless cobalt deposition has been exploited frequently for the preparation of magnetic films. In particular, thin films of cobalt have found application as recording media on account of their thinness, high coercivity, and high remanence. The commercial application of electroless silver was developed early in 1970 [20]. Electroless aluminum [21], electroless platinum [22], electroless ruthenium, and electroless rhodium were developed. In the early 1984, materials like plastics; ceramics, polymers and other non-conducting materials are coated by electroless technique before even being subjected to electroplating. This phenomenon led to the development of the rest of electroless metallic deposits. Meanwhile, the field of electroless coating chemistry has materialized as one of the promising and key areas of the surface engineering and metal finishing research, etc.

#### **1.2. Primary aspects of electroless metallic coating**

in chloride and boric acid bath) and the process of homogenous chemical reduction (silvering). The mechanism of autocatalytically deposited electroless metallic coatings differs in a way completely from coatings obtained by immersion plating or silvering, wherein in the latter case, nature of base material itself will behave as a reducing agent and does not necessitate any external reducing agent to initiate any metal ion to metal reduction. However, these processes have not gained wide acceptance, as it has poor adherence and non-productive behavior. Electroless process has received widespread acceptance in the market due to their exceptional anti-corrosion [4–6], wear resistance properties [3, 5–7] and also good for soldering and brazing applications [8]. Many metals like nickel, copper, gold, silver, palladium and cobalt are being deposited employing this technique [3, 9–12]. Rest of electroless coatings includes gold, copper, palladium, cobalt and silver [13–15]. These metallic coating/ layers (other than electroless nickel) are also one of the vibrant areas in the field of materials properties and surface engineering research. This review chapter provides an insight into the electroless metallic gold, copper, palladium, cobalt, silver, aluminum and the new processing technology in deposition mechanism and nanocharacteristics. Besides, more stress has been laid to understand the recent developments and its future scopes.

The term "electroless plating" also defined as "autocatalytic" is nearly as old as electroplating. Von Liebig first described it in 1835 with the reduction of silver salts by reducing aldehydes. In 1988, metallic nickel deposition by electroless technique from an aqueous solution in the existence of hypophosphite (reducing agent) was first stated as a chemical tragedy by Wurtz [3, 16]. Roux, in the year 1911, positively reported that there was clear precipitation of metal in the form of powder. However, these workings were not practically used for any applications. Development in this field did not receive much of patronage until World War II. The method established by Brenner and Riddell for coating a layer of nickel-tungsten alloy in the inner parts of tubes by means of a citrate based bath containing an insoluble anode yielded the hypophosphite with uncommon reducing properties. The Patent Office of US declared that this patent, issued in 1950, is distinct and varied from the earlier patent owned by Roux, in which the process was unstructured and complete. In contrast, Brenner and Riddell's method was described as a controlled catalytic process as deposition process took place only on catalytically active surfaces dipped in the plating bath. Dr. Brenner later manifested that his patent was an unintended outcome comparable to the efforts of Wurtz and Roux. But he added that the patent was taken in order to safeguard US government rights. Actually, in 1963, a publicized US Army based technical report was printed that pronounced mostly about the work of Wurtz and Roux and gave much of the finding credit to Brenner and also covered by patent in 1956 [17]. This phenomenon ascribed to the action of chemical reduction of nickel ions was later identified.

After initiation of electroless nickel deposit, rest of electroless coatings was followed. In 1970, semiconductors and circuit boards were plated with thick, pure soft gold after the development of the first electroless gold coating bath at Bell Laboratories [14]. Narcus first reported the electroless copper deposit, and the first profitable application was realized by Cahill and Zeblisky et al. [18] in tartarate baths, where the reducing agent used was formaldehyde. Improved formulations of electroless copper [19] recorded a higher rate of plating with exceptionally stable conditions under diverse operating conditions. Baum et al. developed a process for selective deposition of copper by first selectively depositing palladium seeds in

**1.1. Historical overview**

28 Nanoscaled Films and Layers

The basic elements of any electroless bath and their role are briefly reported in **Table 1**. A typical electroless gold, copper, palladium and cobalt plating baths available in literature are illustrated in **Table 2** [3, 23]


**Table 1.** Electroless bath components and their functions.



**Table 2.** Plating bath composition of typical electroless gold, copper, palladium, and cobalt with their major applications.

In the process of electroless deposition, the reduction of metal ions to metal takes place by the action of reducing agents, which are basically donors of electrons. The metal ions act as acceptors of electron and are subject to chemical reaction with the electron donors. This process is autocatalytic in which the acceleration of electroless chemical reaction subjects the reducing agent used to oxidize. The experimental apparatus shows the basic illustration of the setup usually used in experiments of electroless coatings (**Figure 1**). In addition to the basic setup, ultrasound improved the benefits in the electroless nickel, copper and cobalt plating [24].

**Figure 1.** Basic illustration of the apparatus used in electroless coating experiments.

## **2. Rest of the electroless metallic coatings**

### **2.1. Electroless gold**

In the process of electroless deposition, the reduction of metal ions to metal takes place by the action of reducing agents, which are basically donors of electrons. The metal ions act as acceptors of electron and are subject to chemical reaction with the electron donors. This process is autocatalytic in which the acceleration of electroless chemical reaction subjects the reducing agent used to oxidize. The experimental apparatus shows the basic illustration of the setup usually used in experiments of electroless coatings (**Figure 1**). In addition to the basic setup, ultrasound improved the benefits in the electroless nickel, copper and cobalt plating [24].

**Table 2.** Plating bath composition of typical electroless gold, copper, palladium, and cobalt with their major applications.

**Electroless bath Gold Copper Palladium Cobalt**

Rochelle salt, Ammonium hydroxide, pyridium-3-sulfonic acid, Potassium tartrate, Quadrol, EDTA.

Thiodiglycolic, Thiourea, Sodium cyanide, Vanadium pentoxide, Potassium ferrocyanide, MBT.

Hydrochloric, Sulfuric acids, Sodium and Potassium hydroxide.

Conductive layer prior to deposit on plastics or non-metallic, Printed circuit board and decorative purpose.

Ammonia, Methylamine, EDTA.

Thioorganic compound. Thiourea, Organic cyanide, Thiocyanates.

Ammonium hydroxide, Hydrochloric acid.

Printed circuit board, electronic switch contact, and it are alternative for electroless gold.

Sodium citrate, Succinic acid. Ammonium chloride, Citric

Urea, Thioorganic compounds.

Ammonium hydroxide, Sodium hydroxide.

Magnetic memory disc and storage devices in electronic industry.

acid.

Potassium citrate, sodium phosphate, sodium borate, Potassium tartrate.

Alkali hydrogen, fluoride,

Phosphoric acid, Sulfuric

Integrated circuit, chips, connector, and semiconductor devices in electronic Industry.

Acetyl acetone.

**Stabilizer(s)** Alkali metal cyanide,

**pH adjuster(s)** Potassium hydroxide,

acid.

**Complexing agent(s)**

30 Nanoscaled Films and Layers

**Major applications**

**Figure 1.** Basic illustration of the apparatus used in electroless coating experiments.

In most of the existing practices of electroless gold, coatings are produced initially by depositing a thin layer of immersion gold, followed by electroless gold plating [14, 15]. However, there are several shortcomings which depict electroless gold processes: (a) low deposition rates, (b) necessity to carefully regulate plating conditions, (c) substrates should be meticulously cleaned, (d) plating baths are likely to have relatively short lives, (e) stirring. Electroless gold deposition has been most successfully accomplished using a gold sulfite and ammonium salts of sulfite. Those salts of gold comprise of potassium gold chloride, gold cyanide and gold sulfite among which the most commonly used source is potassium gold cyanide. The reducing agents essential for electroless plating are as varied as the potential gold salts. A wide variety of reducing agents required for electroless are available as various potential gold salts such as sodium hypophosphite, dimethylamine borane, sodium borohydride, and hydrazine. It has been serving as the primary application for electronic industry to perform selective plating, thereby conserving plating costs and improve circuit design in integrated circuits [25]. There are some realistic cases of autocatalytic processes of coating gold with the ability of having achieved 99.99% purity. **Table 3** gives a typical bath composition for the electroless gold plating. Electroless plating of gold salts on germanium results in morphologically complex nanostructure metallic films [26]. Deposition simply takes place by means of galvanic displacement without the presence of pH adjusters, fluoride, complexing agents, or other external reducing agents. This facile method gives good control over deposition rate and surface morphology by proper variation of coating parameters such as temperature, immersion time and metal ion concentration. The preparation of Au nanowires of high aspect ratio by employing electroless reduction of gold in the hexagonally ordered, thiol-modified nanosized channels [27]. The outcomes evidently display that the development of the Au nanowires is templated by the channel structure of the base substrate. Ecofriendly electroless plating bath was developed by chloroauric acid (HAuCl<sup>4</sup> ) and hydrogen peroxide (H2 O2 ) for depositing a film of gold onto (3-aminopropyl) trimethoxysilane (APTMS). It could prove as a feasible replacement of using hazardous source of potassium gold cyanide [28].


**Table 3.** A typical electroless gold bath.

The various characteristics of electroless gold coatings attained from baths of borohydride have been abridged by Feldstein [29], and it is given in **Table 4**. This table shows mainly the physical properties of the gold plating (i.e., adhesion, density, porosity, and resistivity)

#### *2.1.1. Recent developments of electroless gold*

Electroless gold layer was developed on hydrogen-terminated Si substrate from aqueous hydrofluoric solutions [30]. The Au deposition is kinetically inadequate by diffusion at first, but then development of Au clusters is kinetically restricted by a surface reaction containing a fluoride species. This possibly necessitates for Au to be coated in a relatively mobile state initially which is only moderately discharged. Hou et al. reported about the preparation of gold films and affirmed the possibility for them to subsequently be used as base materials to aid the formation of self-assembled monolayers (SAMs) from alkanethiols [31]. The formation of SAMs on the films of electroless gold can be ascribed to two primary reasons. Firstly, any type of wet-chemical laboratory can be used to prepare electroless gold. In addition, electroless gold can be coated on intricate and complicated shapes of substrate, wherein no technique for the evaporation process could be recommended. The densely packed SAMs (prepared from hexadecanethiol), present on the surface of electroless gold, are only deposited as a thin film of evaporated gold. Electroless coating promoted the epitaxial growth of Au (111) on a seed layer of evaporated gold that makes it highly beneficial for microfabrication applications [32]. Wang et al. [33] have recently found that electroless Au microelectrodes could be fabricated on polycarbonate microfluidic chips with SAM, after cleaning the microelectrodes with plasma. The system comprised of a polycarbonate microfluidic chip with an electrochemical detector, a gravity pump, and an automatic sample loading and injection unit. Lei et al. [34] reported the analysis of prepared gold films with a surface plasmon resonance (SPR) device to detect the rapid and label-free detection of the white spot syndrome virus (WSSV). This method, key issues in the field of pisciculture and environmental toxicology, has been addressed and supplemented the expansion in the range of applications of the SPR technology. Schwarz et al. have recently [35] deposited a thin layer of soft gold onto polypyrrole and copper-coated paraaramid


**Table 4.** Properties of electroless gold deposits.

yarns. Copper ions go into the solution as less noble copper dissolves, and consequently, there is a reduction of gold ions in the solution thereby getting deposited on the yarn surface. Once a thick layer of gold has been deposited, further dissolution of copper cannot happen and the process of charge exchange gets stopped. As gold layer is now formed on the yarn surface, autocatalytic chemical reduction takes place, and deposition continues further. Result revealed that the electroless coated yarns exhibited improved mechanical properties plus excellent electrical conductivity and considerable resistance to washing. Also, electrochemical results displayed that the gold coated yarns can be used to measure biomedical signals as they are promising electrode materials. An effective and simple electroless deposition technique was proven to deposit mixed SnO2 -Pd-Au film for sensing hydrogen. Electrical conductivity of the film was improved by the co-deposited gold. The sensor was easy-to-use and can be fabricated easily. This sensor can be used in areas like hydrogen gas alarm in commercial or domestic security [36]. Conducting electroless gold pads on light-emitting diodes chips are fabricated which exhibit no color difference, reliable wire bonding ability, and high values of electrical conductivity. The hardness of pads formed by electroless plating is three times softer than those formed by evaporation and the force gauges [37]. The industrial application of this plating is feasible.

### *2.1.2. Nanocharacteristics of electroless gold*

The various characteristics of electroless gold coatings attained from baths of borohydride have been abridged by Feldstein [29], and it is given in **Table 4**. This table shows mainly the physical properties of the gold plating (i.e., adhesion, density, porosity, and resistivity)

Electroless gold layer was developed on hydrogen-terminated Si substrate from aqueous hydrofluoric solutions [30]. The Au deposition is kinetically inadequate by diffusion at first, but then development of Au clusters is kinetically restricted by a surface reaction containing a fluoride species. This possibly necessitates for Au to be coated in a relatively mobile state initially which is only moderately discharged. Hou et al. reported about the preparation of gold films and affirmed the possibility for them to subsequently be used as base materials to aid the formation of self-assembled monolayers (SAMs) from alkanethiols [31]. The formation of SAMs on the films of electroless gold can be ascribed to two primary reasons. Firstly, any type of wet-chemical laboratory can be used to prepare electroless gold. In addition, electroless gold can be coated on intricate and complicated shapes of substrate, wherein no technique for the evaporation process could be recommended. The densely packed SAMs (prepared from hexadecanethiol), present on the surface of electroless gold, are only deposited as a thin film of evaporated gold. Electroless coating promoted the epitaxial growth of Au (111) on a seed layer of evaporated gold that makes it highly beneficial for microfabrication applications [32]. Wang et al. [33] have recently found that electroless Au microelectrodes could be fabricated on polycarbonate microfluidic chips with SAM, after cleaning the microelectrodes with plasma. The system comprised of a polycarbonate microfluidic chip with an electrochemical detector, a gravity pump, and an automatic sample loading and injection unit. Lei et al. [34] reported the analysis of prepared gold films with a surface plasmon resonance (SPR) device to detect the rapid and label-free detection of the white spot syndrome virus (WSSV). This method, key issues in the field of pisciculture and environmental toxicology, has been addressed and supplemented the expansion in the range of applications of the SPR technology. Schwarz et al. have recently [35] deposited a thin layer of soft gold onto polypyrrole and copper-coated paraaramid

*2.1.1. Recent developments of electroless gold*

32 Nanoscaled Films and Layers

**Property Value**

Purity 99.90% Thermo compression bond ability Excellent

**Table 4.** Properties of electroless gold deposits.

Adhesion Excellent on metals

Density Bulk gold (19.3 g/cm3

Porosity ~Zero for deposits, ≥ 1 μm on uniform substrate

Resistivity Bulk gold (0.03 ohm/square at 1 µm)

Hardness Soft (Knoop 60–80)

)

Appearance Matt yellow

The procedure for fabricating high-yield integrated nano-gap electrodes concurrently on a single sample was developed, which contains iodine solution (usually known as tincture (a medical liquid)) and ascorbic acid [38]. Results revealed that methods like nanogap fabrication and the fabricated gold nano-gap electrodes are beneficial for realizing applications like mono-electron and molecular nanodevices. Ding et al. have [39] produced Au nanoparticles using conducting polymer nanoparticles through electroless collecting [AuCl<sup>4</sup> ]− from solutions. A considerable improvement of the recovery capability of Au was realized by the polypyrrole nanoparticle when compared to the film or polypyrrole powder, due to the high specific surface area. Researcher [40] investigated in wielding electroless plated Au as a provision for carbon nanotube (CNT) electrodes. Further, they stressed the need for developing electroless techniques to meet the requirements of creating a more heterogeneous, uniform layer of gold to reduce desorption of cysteamine monolayer. An innovative nanotemplating technique was established to produce spherical gold nanoparticles (NPs) or regular arrays of silane rings on silicon substrates through selective electroless plating on layers of particlelithographed silane [41]. This novel nanotemplating method can synthesize even and smooth metal NP arrays over huge areas to enhance the potential of improved spectral features in opto-plasmonic devices in spite of disregarding the requisites of large and expensive lithography and metal coating equipment. Research [42] defined a new electroless technique of depositing single-crystalline Au-NPs on and inside an organic single-matrix (SM) confined with both a stabilizer and a reducing agent. This process is appropriate for direct deposition of mechanically stable and optically transparent Au-NPs on and inside a SM. The development of palladium and platinum nanoparticles was made possible only with this method when the actual reaction was carried out on the surface of the SM in the presence of some gold nanocrystals.

#### **2.2. Electroless copper**

Electroless copper deposits have found their greatest applications for imparting a conductive layer for non-conductors before being coated by electrolytically. Electroless copper solutions resembling today's technology were first reported in 1957 by Cahill with the report of alkaline copper tartrate baths using formaldehyde as reducing agent. The pH range of 12.0–13.0 is generally optimal in formaldehyde-reduced baths. Formaldehyde-reduced electroless copper bath is given in **Table 5**. In printed-circuit boards industry, copper is deposited on the inner wall of the insulating hole that connects the two sides of the substrate by employing platingthrough-hole process, one of the techniques of electroless copper deposition. Electroless Cu plating is becoming attractive because of its favorable nature of electrical & thermal conductivity, ornamental surface and so on [15].

Among the various chelating agents in electroless Cu plating (ethylenediamine tetraacetic, triethanolamine, and ethylenediamine), studies recognized ethylenediamine as an outstanding grain-refining agent owing to its strong adsorption on the Cu surface [43]. Formaldehyde is the reducing agent used in the great majority of commercial electroless Cu baths and other reducing agents have also been used successfully. The copper deposit increases the surface roughness by using pyridine-2,6-dicarboxylic and 4-hydroxypyridine-2,6-dicarboxylic acids as Cu (II) ions ligands in formaldehyde having alkaline electroless Cu plating baths [44]. The surface roughness factor (i.e., ratio of real surface area (nano-scale roughness) to geometrical surface area) was found to be the highest for pyridine-2,6-dicarboxylic acid having solutions at pH 13.

#### *2.2.1. Recent developments of electroless copper*

One of the most important applications of electroless copper is the electronic industry. The electroless Cu/Ni/Au deposits had the attributes of excellent adhesion in addition to having a low sheet resistivity, which are prerequisites for low attenuation at microwave frequencies. The above coating has been recommended as a cost effective means for microwave components because the thickness of gold in the proposed system is much less than that of conventional chromium/gold. Researcher studied the effective acceleration of deposition of copper at a temperature range of 20–30°C by a factor of 2–4, even for lesser additions of


**Table 5.** A typical electroless copper bath.

ammonia (1–3 mM) [45]. Acidic electroless Cu on aluminum-seeded ABS plastics was an alternative option for plating plastics, [46] and this was critically criticized because of its very slow rate. Hanna et al. [47] studied the role of various organic additives (cytosine, pyridine, benzotriazole and 2-mercaptobenzothiozole) which stabilized electroless copper baths in addition to enhancing the rate of plating. The bath stability increased 20 times to that of the aeration lacking-bath, as an effect of mild air agitation. This is an unusual method of non-isothermal deposition to enhance the deposition rate without affecting the plating bath. The deposition rate increased with increasing temperature of the substrate; nevertheless, the influence of bath temperature cannot be ignored. Result showed that this non-isothermal technique allows for deposition rates ~12 µmh-1 at certain conditions, which is 3 times than conventional isothermal method [48]. Recently, found long-term stability of Cu/Pd nanoparticles by using poly-vinylpyrrodione (PVP). In his study, acceptable activity and superior stability have been exhibited by the newly developed Cu/Pd colloidal system thus, indicating its capacity as an auxiliary for the prevailing Pd/Sn-based activator in PCBs [49]. The reduction of Cu(II) autocatalytically by formaldehyde from solutions having saccharose as the ligand initiates at a level of pH beyond 12, increases with an additional increase in pH, reaches a maximum value at pH 12.75, and then decelerates at higher pH values [50]. Tamayo-Ariztondo et al. [51] prepared electroless Cu deposits preferably on the surfaces parallel to graphene layers, due to the exposure of π bonds in the outside surface, and deposition was subdued on surfaces perpendicular to the graphene layers. Result revealed that there is maximum coverage of electroless Cu along the plane of graphene, and it is reduced at the edges of the plane. Garcia et al. [52] narrated two methods based on the ligand-induced electroless plating (LIEP) process to obtain patterns of Cu onto flexible polymer substrates. The LIEP process permits plating of copper selectively with stable electrical properties onto flexible polymer substrates. Therefore, the LIEP process collaborated with any of those patterning methods could perhaps be a better auxiliary for the classical processes of cost-effective fabrication of large-area plastic electronic devices and to endure substantial mechanical deformation with only a negligible loss in performance. Researcher developed activation for the dielectric surface by the cobalt compounds. This palladium-free activation looks hopeful for the practical application due to its stability and can be potential compare to many of the patented Pd-free activation solutions [53].

#### *2.2.2. Nanocharacteristics of electroless copper*

**2.2. Electroless copper**

34 Nanoscaled Films and Layers

tivity, ornamental surface and so on [15].

*2.2.1. Recent developments of electroless copper*

Copper salt as Cu2+ 1.8 g/L Rochelle salt 25 g/L Formaldehyde as HCHO 10 g/L Sodium hydroxide 5 g/L 2-Mercaptobenzothiazole (MBT) <2 g/L pH 12 Temperature RT

**Electroless copper bath composition**

**Table 5.** A typical electroless copper bath.

Electroless copper deposits have found their greatest applications for imparting a conductive layer for non-conductors before being coated by electrolytically. Electroless copper solutions resembling today's technology were first reported in 1957 by Cahill with the report of alkaline copper tartrate baths using formaldehyde as reducing agent. The pH range of 12.0–13.0 is generally optimal in formaldehyde-reduced baths. Formaldehyde-reduced electroless copper bath is given in **Table 5**. In printed-circuit boards industry, copper is deposited on the inner wall of the insulating hole that connects the two sides of the substrate by employing platingthrough-hole process, one of the techniques of electroless copper deposition. Electroless Cu plating is becoming attractive because of its favorable nature of electrical & thermal conduc-

Among the various chelating agents in electroless Cu plating (ethylenediamine tetraacetic, triethanolamine, and ethylenediamine), studies recognized ethylenediamine as an outstanding grain-refining agent owing to its strong adsorption on the Cu surface [43]. Formaldehyde is the reducing agent used in the great majority of commercial electroless Cu baths and other reducing agents have also been used successfully. The copper deposit increases the surface roughness by using pyridine-2,6-dicarboxylic and 4-hydroxypyridine-2,6-dicarboxylic acids as Cu (II) ions ligands in formaldehyde having alkaline electroless Cu plating baths [44]. The surface roughness factor (i.e., ratio of real surface area (nano-scale roughness) to geometrical surface area) was found to be the highest for pyridine-2,6-dicarboxylic acid having solutions at pH 13.

One of the most important applications of electroless copper is the electronic industry. The electroless Cu/Ni/Au deposits had the attributes of excellent adhesion in addition to having a low sheet resistivity, which are prerequisites for low attenuation at microwave frequencies. The above coating has been recommended as a cost effective means for microwave components because the thickness of gold in the proposed system is much less than that of conventional chromium/gold. Researcher studied the effective acceleration of deposition of copper at a temperature range of 20–30°C by a factor of 2–4, even for lesser additions of

> A research thrust has been initiated [54] that sub-100 nm copper films of low resistivity be deposited by electroless means on SAM of 3-aminopropyltrimethoxysilane and activated by 5 nm gold nano-particles. The resistivity achieved in this process is fairly relatable to the effective resistivity expected by the International Technology Roadmap for Semiconductors (ITRS) for realization in 2010–2011 years for 45 nm ULSI metallization. Noteworthy developments in wetting of aluminum can be attained by applying an electroless Cu on Al<sup>2</sup> O3 and SiC (Al/Cu-Al<sup>2</sup> O3 and Al/Cu-SiC) ceramics [55]. The copper coating present in the interface hindered the reactivity of SiC toward Al thus causing a clean interface. Recently, [56] a new method has been proposed to coat the surface of fly ash particles with conducting metal Cu by electroless, where titania/ultraviolet radiation/metal catalyst-system has been used instead of conventional Pd/Sn-based activator. More work has to be done, whether the proposed one

will be suit for the rest. Organic protection coatings act as an interlayer for highly reactive metals (magnesium) to coat electroless Cu and Ni or its alloys, and the process is criticized for its long time process. Daoush et al. studied on enhancing the strength of interfacial bonding between Cu and CNT by acid treatment and electroless copper coating of multiwall CNT to produce CNT/Cu nanocomposite powder with different CNT volume fractions. The electrical conductivity decreased, and the hardness increased with increase in volume fraction of CNT. The yield strength of the sintered materials had enhanced by increasing the volume fraction of CNT except in case of 20 vol. % CNT/Cu composite where the material fractured even before yielding. In addition, the increase in volume fraction of CNT in copper matrix witnessed an increase in Young's modulus and a subsequent decrease in elongation [57]. Wear and mechanical behavior of reinforced carbon nanofibers improved by electroless Cu alloybased composites [58]. A new method proposed for plating a Cu layer onto an aramid film with a strong bonding by adhesion [59]. Deposition of electroless copper nanofilms on silicon and on polycrystalline germanium substrates seemed to cover an extensive area of the base material compared to other noble metals such as Au, Pt, Pd, and Ag [60]. Bruning et al. developed an in situ X-ray diffraction measurement for electroless Cu films. The strain evolution of films was determined based on three types of electrolyte. Within the experimental ambiguity, the correlation among stress and strain for the Cu films approves with the properties of bulk polycrystalline Cu [61]. The coating of Cu on particles of B4 C is required in order to synthesize metal-ceramic composites with improved sinterability and dispersability. A surface pretreatment similar to that of acid and alkali treated particles was carried out for B4 C particles. There was uniformity in the observed copper coating in alkali-treated particles at a level of pH-12, when matched to others. This is due to the effective elimination of impurities during the processes of production and processing of commercially existing B4 C [62]. The electroless Cu-P-SiC composite coating on carbon steel improved the anti-corrodibility behavior of deposits of electroless Cu [63].

Zangmeister and van Zee [64] observed the possibility of Cu to be deposited by the reducing action of formaldehyde on Cu2+ ions in 4-mercaptobenzoic acid SAMs, but inferred that the same cannot be performed on octadecanethiolate or 3-mercaptobenzoic acid SAMs. The formation of a surface-bound Cu complex when reduced by formaldehyde leads to the deposition of Cu metal. Garno et al. [65] observed another prospect for Cu to be deposited on COOH-terminated SAMs. Moreover, it was observed that smaller amounts of Cu could be deposited on CH3 -terminated SAMs, while narrowing likelihood of depositing Cu on OH-terminated SAMs. Recently, two main requirements for selective electroless deposition of Cu to take place were presented. Firstly, in cases where more than one type of functional group is present in the SAM, Cu2+ ions must interact with any one specific terminal group, if there is more than one group. Secondly, the temperature needs to be maintained adequately high to avert adsorption of non-specific Cu on non-interacting SAMs. It was significant, however, to maintain the reaction temperature in such a way, so that it was not too high to damage the SAM. The penetration of Cu into the Au/S interface through the monolayer was observed in addition to the deposition of Cu at the SAM/air interface. Moreover, the penetrating action through the SAM was noticed not to be ceasing even after plating was stopped [66]. This advises that electroless deposition of Cu taking place parallel with Cu evaporation may not be an appropriate technique to develop Cu/SAM/metal or Cu/SAM/semiconductor junctions in molecular architectures based applications. This is because of the gradual shifting of upper Cu metal contact with respect to time. Alternative conditions of electroless deposition and metal types (e.g., Au, Pt, and Pd,) should be encouraged and moreover be explored to find the scope for formation of more stable metal over-layers with slight or without any metal atom penetration. Electroless Cu plating developed on nanofibers with the attributes of [67] high modulus and high strength, and particularly high electric conductivity on the surface of poly (p-) phenylene benzobisoxazole. A simple electroless Cu-coated prepared on glass nanofiber with excellent conductivity. Actually, this method is simple, low-cost, and large production and can be stretched to make other metal coated glass fibers by distinctive conductivity [68].

### **2.3. Electroless palladium**

will be suit for the rest. Organic protection coatings act as an interlayer for highly reactive metals (magnesium) to coat electroless Cu and Ni or its alloys, and the process is criticized for its long time process. Daoush et al. studied on enhancing the strength of interfacial bonding between Cu and CNT by acid treatment and electroless copper coating of multiwall CNT to produce CNT/Cu nanocomposite powder with different CNT volume fractions. The electrical conductivity decreased, and the hardness increased with increase in volume fraction of CNT. The yield strength of the sintered materials had enhanced by increasing the volume fraction of CNT except in case of 20 vol. % CNT/Cu composite where the material fractured even before yielding. In addition, the increase in volume fraction of CNT in copper matrix witnessed an increase in Young's modulus and a subsequent decrease in elongation [57]. Wear and mechanical behavior of reinforced carbon nanofibers improved by electroless Cu alloybased composites [58]. A new method proposed for plating a Cu layer onto an aramid film with a strong bonding by adhesion [59]. Deposition of electroless copper nanofilms on silicon and on polycrystalline germanium substrates seemed to cover an extensive area of the base material compared to other noble metals such as Au, Pt, Pd, and Ag [60]. Bruning et al. developed an in situ X-ray diffraction measurement for electroless Cu films. The strain evolution of films was determined based on three types of electrolyte. Within the experimental ambiguity, the correlation among stress and strain for the Cu films approves with the properties of bulk

metal-ceramic composites with improved sinterability and dispersability. A surface pretreat-

was uniformity in the observed copper coating in alkali-treated particles at a level of pH-12, when matched to others. This is due to the effective elimination of impurities during the pro-

SiC composite coating on carbon steel improved the anti-corrodibility behavior of deposits of

Zangmeister and van Zee [64] observed the possibility of Cu to be deposited by the reducing action of formaldehyde on Cu2+ ions in 4-mercaptobenzoic acid SAMs, but inferred that the same cannot be performed on octadecanethiolate or 3-mercaptobenzoic acid SAMs. The formation of a surface-bound Cu complex when reduced by formaldehyde leads to the deposition of Cu metal. Garno et al. [65] observed another prospect for Cu to be deposited on COOH-terminated SAMs. Moreover, it was observed that smaller amounts of Cu could

OH-terminated SAMs. Recently, two main requirements for selective electroless deposition of Cu to take place were presented. Firstly, in cases where more than one type of functional group is present in the SAM, Cu2+ ions must interact with any one specific terminal group, if there is more than one group. Secondly, the temperature needs to be maintained adequately high to avert adsorption of non-specific Cu on non-interacting SAMs. It was significant, however, to maintain the reaction temperature in such a way, so that it was not too high to damage the SAM. The penetration of Cu into the Au/S interface through the monolayer was observed in addition to the deposition of Cu at the SAM/air interface. Moreover, the penetrating action through the SAM was noticed not to be ceasing even after plating was stopped [66]. This advises that electroless deposition of Cu taking place parallel with Cu evaporation may not


ment similar to that of acid and alkali treated particles was carried out for B4

C is required in order to synthesize

C [62]. The electroless Cu-P-

C particles. There

polycrystalline Cu [61]. The coating of Cu on particles of B4

electroless Cu [63].

36 Nanoscaled Films and Layers

be deposited on CH3

cesses of production and processing of commercially existing B4

Palladium and palladium alloys can be deposited by the electroless mechanism using hypophosphite or hydrazine reducing agent. Palladium deposits find application in electrical contacts and connectors and serve as a diffusion barrier between metals such as copper and gold. It has been established as an economic substitute to gold plating. It has also been used as a best replacement for rhodium for wear application [69]. **Table 6** gives the constituents and composition details for an electroless palladium (hypophosphite-reduced) bath. The deposit can be hardened or be bonded to electroless nickel or be acquired with desired coating characteristics by means of varying specific bath components or their composition. For example, deposits with greater bond strength than the actual tensile strength of the palladium plate itself also can be attained. The plating can be direct in metals like stainless steel and nickel, whereas copper, brass, and other copper alloys would require an electroless nickel preplate.

The catalytic properties in Palladium have marked the metal notable in serving several applications of chemical and automotive industries. The usage of the metal is, however, been limited owing to high cost and difficulty to put it to realistic processing for other commercial applications. It is, yet, studied and proposed that thin films of palladium plated by electroless means onto ceramic sponge or other support materials may well be used effectually as reaction or auto emission catalysts. The metal and its alloys have been established as a profitable


**Table 6.** A typical electroless palladium bath.

substitute to gold plating. In spite of the differences in some of the properties of Pd and Au such as melting point and deposit hardness (**Table 7**), one character the metals share in common is the superior oxidation resistance.

#### *2.3.1. Recent developments and nanocharacteristics of electroless palladium*

It is often proposed that Pd/Sn-based activator is used to activate the substrate surface to deposit electroless Cu. In some cases, electroless Pd acts as an activator [70]. Recently, electroless Pd was coated on Iridium (Ir) and tungsten (W) substrates. The thickness of Pd was 20 and 30 Å on Ir and W substrates, respectively. A very strong adhesion of the electroless Cu to Ir and W was observed, when Pd was used as a catalytic layer [71]. In practical application point of view, modification of Pd by electroless on ZrO<sup>2</sup> -TiO2 selective layer produced membranes, which detached hydrogen and nitrogen gases in the Knudsen diffusion domain (H2 :N2 ~3.75). In addition, using γ-aminopropyltriethoxysilane (γ-APTES) will ultimately lead to the proficient electroless deposition of amorphous and porous layers of Pd [72]. A method of vacuum electroless plating for synthesizing of thin dense Pd membranes on porous alumina tubes was developed. The diverse application of these membranes was ascribed to their high permeation performance, good thermal durability, and favorable chemical stability. Furthermore, these membranes had withstood tests for thermal durability over 470 h under H2 or Ar atmosphere for both cycles of temperature cycles and gas-exchange. Additionally, these membranes offered a strong resistance to fluctuations in the chemical stability under various H2 mixtures over a varied pressure and temperature range for 2000 h. No significant


**Table 7.** A comparison between electroless palladium and gold.

changes in hydrogen permeation performance were observed, and sustained hydrogen permeation for the acute temperature fluctuations was exhibited. Another application oriented in electroless Pd membranes onto cordierite mini-channel network was developed recently and testified hydrogen separation [73]. Initially, the cordierite channels were coated in its interiors with alumina layers as supporting layers, consisting of a layer of micropowder alumina that covered a structure of bare cordierites structure of high porosity, and followed by a layer of nanopowder alumina resulting in an even surface free of defects for electroless plating of defect-free palladium films. The perm-selective palladium films fabricated in this new support structure also permits for extraction of hydrogen from hydrocarbon fuels in this design of integrated membrane reformers [74]. In some cases, the function of electroless Pd film on stainless steel is to increase the electrode potential and boost the rate of passivation of steel to endure strong corrosive environments [69]. Strukova et al. developed Pd-Au, Pd-Ag, Pd-Ni, Pd-Pb, and ternary Pd-Au-Ni alloy system onto different metallic substrates. The coatings of Pd-Au, Pd-Ag, and Pd-Ni have a solid solution structure, whereas Pd-Pb is intermetallic compound whose films after deposition comprised of nanocrystalline grains with sizes in the range of 11–35 nm [75]. Self-supported thin Pd alloys [76] membranes without defects were prepared for hydrogen permeation performance. The subsequent self-supported Pd membranes, less than 10 µm in thickness, demonstrate exceptional performance of hydrogen permeation and an extensive selectivity. In addition, a new type of non-alloy Ru/Pd composite membrane fabricated for hydrogen separation [77].

#### **2.4. Electroless cobalt**

substitute to gold plating. In spite of the differences in some of the properties of Pd and Au such as melting point and deposit hardness (**Table 7**), one character the metals share in com-

It is often proposed that Pd/Sn-based activator is used to activate the substrate surface to deposit electroless Cu. In some cases, electroless Pd acts as an activator [70]. Recently, electroless Pd was coated on Iridium (Ir) and tungsten (W) substrates. The thickness of Pd was 20 and 30 Å on Ir and W substrates, respectively. A very strong adhesion of the electroless Cu to Ir and W was observed, when Pd was used as a catalytic layer [71]. In practical applica-

membranes, which detached hydrogen and nitrogen gases in the Knudsen diffusion domain

 or Ar atmosphere for both cycles of temperature cycles and gas-exchange. Additionally, these membranes offered a strong resistance to fluctuations in the chemical stability under

**Property Electroless palladium Electroless gold**

Magnetic Paramagnetic Diamagnetic Density 12.0 g/cc 19.3 g/cc Molecular weight 106 197 Melting point 1555°C 1063°C Boiling point 3140°C 2660°C Hardness 50–200 HV 20–150 HV Elongation 24% 44% Tensile strength 24 Ksi 18 Ksi

Co-efficient of expansion 6.5 µ-in./in./°F 7.9 µ-in./in./°F Maximum thickness Unlimited Unlimited

Color Silver/White Yellow Crystal FCC FCC

Resistivity 10.5 × 108

Reacts with HNO3

**Table 7.** A comparison between electroless palladium and gold.

mixtures over a varied pressure and temperature range for 2000 h. No significant

ohm-m 2.2 × 108

; HF H2

~3.75). In addition, using γ-aminopropyltriethoxysilane (γ-APTES) will ultimately lead to the proficient electroless deposition of amorphous and porous layers of Pd [72]. A method of vacuum electroless plating for synthesizing of thin dense Pd membranes on porous alumina tubes was developed. The diverse application of these membranes was ascribed to their high permeation performance, good thermal durability, and favorable chemical stability. Furthermore, these membranes had withstood tests for thermal durability over 470 h under


ohm-m

SO4

; KCN; aqua regia

selective layer produced

*2.3.1. Recent developments and nanocharacteristics of electroless palladium*

tion point of view, modification of Pd by electroless on ZrO<sup>2</sup>

mon is the superior oxidation resistance.

38 Nanoscaled Films and Layers

(H2 :N2

H2

various H2

Cobalt deposits are mainly produced from alkaline-hypophosphite baths. These coatings are produced from sodium hypophosphite-based solutions at a slightly alkaline pH range at elevated temperatures. The electroless cobalt deposition by hypophosphite is always supplemented by the co-deposition of phosphorus. The fabrication of magnetic storage devices with high area recording density generally requires magnetic material that is soft in nature. Its application includes large variety of magnetic properties and has found their major applications for switching and memory storage devices [78]. **Table 8** gives the information on constituents and composition of a typical electroless cobalt bath. Thin electroless Co deposits have their applications in the electronics industry (magnetic memory discs and storage devices) exclusively for


**Table 8.** A typical electroless cobalt bath.

their magnetic properties. In ternary Ni-Co-P alloy coatings, the Co content displayed a linear dependency with both pH of bath and temperature and was also influenced by the composition of Co sulfate in the electrolyte. It showed a spherical nodular structure with finer, compact grains and a homogeneous crystal structure. In addition, the maximum hardness was 804 HV50 for the deposits having 19% cobalt [79]. The maleic and succinic acid maintained the stability, long plating of the bath, and morphology of the as-plated deposits [80]. The deposition rate was more in the bath with succinic acid addition. These results are, though, inadequate for reporting evidences of electroless Co-based alloys with magnetic properties due to lack of systematic study on the conditions of preparation and intrinsic magnetic properties of the material.

#### *2.4.1. Recent developments and nanocharacteristics of electroless cobalt*

In Ni-Co-B ternary alloy coatings, the saturation magnetic moment was found to be increasing with rise in content of cobalt in the deposit and with prolonged annealing of the deposit [81]. SiC (Co-P-SiC) entrapment was more favored within the cobalt matrix composite coatings than Ni (Ni-P-SiC) matrix. The magnetic properties of the electroless Co-Fe-P films formed out of a stable sulfate bath [82]. In high pH bath with heat treatment films of Co-Fe-P, showed good soft magnetic properties. The magnetic properties of Co-W-P films have been studied by many authors [83–84]. Magagnin et al. [85] have proposed that electroless cobalt-phosphorus acts as a metallization barrier for copper in lead-free soldering. However, electroless Co-P/ Au finish with about 4 wt. % P content sturdily restricts inter diffusion and formation of intermetallic compounds when paralleled to Ni-P/Au finish with Sn-Pb and Sn-Ag-Cu solder alloys. Furthermore, the shear test results suggested that Co-P/Au deposit having higher joint strength than Ni-P/Au deposit. Jiang et al. [86] have prepared SiC-W/Co nanocomposite particles by electroless cobalt on SiC whiskers. The bonding between the substrate and the cobalt coating is so weak due to which the thermal stability of SiC-W/Co composite is low. Hence, a research thrust has been initiated toward encouraging further investigation on the thermal stability of metal coating plated on whiskers. The electroless Co composites have not seemed to have received widespread attention and application; however, future prospects will be very attractive. The Co-Zn-P thin film coated nano-diamond materials were the basis for providing a feasible solution over the expensive cobalt material in the synthesis of this type of magnetic thin film materials. These magnetic film materials have been witnessing myriad prospective applications in the field of Magnetic Abrasive Lapping Materials in the near future [87]. CNT were decorated with FeCO using one-step polymer-stabilization activation step and low-cost electroless deposition. The approach recommends a viable method that can be followed for preparing nanoparticles of FeCo in application of cancer thermotherapy [88].

#### **2.5. Electroless silver**

Silver can be deposited from dimethylamine borane-based baths. The major application is to coat in the interior of waveguides. Many authors have reported and illustrated evidences of depositing electroless silver on different substrates. Abbott et al reported studies carried out about Ag coating on copper substrates using an ionic liquid [89]. A super hydrophobic surface formed by developing of a monolayer of polyfluoroalkyl thiol layer on copper or zinc substrate followed by electroless deposition of silver was reported by Larmour et al [90]. Various methods for depositing silver on silicon substrates of definite patterns and plain Si substrate have also been recently explored [91].

#### *2.5.1. Recent developments and nanocharacteristics of electroless silver*

their magnetic properties. In ternary Ni-Co-P alloy coatings, the Co content displayed a linear dependency with both pH of bath and temperature and was also influenced by the composition of Co sulfate in the electrolyte. It showed a spherical nodular structure with finer, compact grains and a homogeneous crystal structure. In addition, the maximum hardness was 804 HV50 for the deposits having 19% cobalt [79]. The maleic and succinic acid maintained the stability, long plating of the bath, and morphology of the as-plated deposits [80]. The deposition rate was more in the bath with succinic acid addition. These results are, though, inadequate for reporting evidences of electroless Co-based alloys with magnetic properties due to lack of systematic study on the conditions of preparation and intrinsic magnetic properties of the material.

In Ni-Co-B ternary alloy coatings, the saturation magnetic moment was found to be increasing with rise in content of cobalt in the deposit and with prolonged annealing of the deposit [81]. SiC (Co-P-SiC) entrapment was more favored within the cobalt matrix composite coatings than Ni (Ni-P-SiC) matrix. The magnetic properties of the electroless Co-Fe-P films formed out of a stable sulfate bath [82]. In high pH bath with heat treatment films of Co-Fe-P, showed good soft magnetic properties. The magnetic properties of Co-W-P films have been studied by many authors [83–84]. Magagnin et al. [85] have proposed that electroless cobalt-phosphorus acts as a metallization barrier for copper in lead-free soldering. However, electroless Co-P/ Au finish with about 4 wt. % P content sturdily restricts inter diffusion and formation of intermetallic compounds when paralleled to Ni-P/Au finish with Sn-Pb and Sn-Ag-Cu solder alloys. Furthermore, the shear test results suggested that Co-P/Au deposit having higher joint strength than Ni-P/Au deposit. Jiang et al. [86] have prepared SiC-W/Co nanocomposite particles by electroless cobalt on SiC whiskers. The bonding between the substrate and the cobalt coating is so weak due to which the thermal stability of SiC-W/Co composite is low. Hence, a research thrust has been initiated toward encouraging further investigation on the thermal stability of metal coating plated on whiskers. The electroless Co composites have not seemed to have received widespread attention and application; however, future prospects will be very attractive. The Co-Zn-P thin film coated nano-diamond materials were the basis for providing a feasible solution over the expensive cobalt material in the synthesis of this type of magnetic thin film materials. These magnetic film materials have been witnessing myriad prospective applications in the field of Magnetic Abrasive Lapping Materials in the near future [87]. CNT were decorated with FeCO using one-step polymer-stabilization activation step and low-cost electroless deposition. The approach recommends a viable method that can be followed for

*2.4.1. Recent developments and nanocharacteristics of electroless cobalt*

preparing nanoparticles of FeCo in application of cancer thermotherapy [88].

Silver can be deposited from dimethylamine borane-based baths. The major application is to coat in the interior of waveguides. Many authors have reported and illustrated evidences of depositing electroless silver on different substrates. Abbott et al reported studies carried out about Ag coating on copper substrates using an ionic liquid [89]. A super hydrophobic surface formed by developing of a monolayer of polyfluoroalkyl thiol layer on copper or zinc substrate followed by electroless deposition of silver was reported by Larmour et al [90]. Various

**2.5. Electroless silver**

40 Nanoscaled Films and Layers

Electroless Ag nanoparticles were effectively deposited on ZnO nanorod surfaces for the purpose of decreasing the infrared emissivity values, due to its high reflectance and will lead to innovative options for producing materials of low infrared emissivity by doping metal to semiconductor materials. To impart electrical conductivity to non-conducting glass particles, possibility to deposit electroless Ag over the glass was reported to be feasible and efficient [92]. Electroless Ag was deposited on calcite and was first reported by Srikanth and Jeevanandam [93]. In their report, lower concentration of silver ions (e.g., 0.01 M AgNO<sup>3</sup> ) and shorter deposition times (e.g., 30 min) led to the formation of silver nanoparticles on calcite. Sun et al. [94] coated a uniform silver film about 50-nm thick on a graphite nanosheet surface by an enhanced electroless plating using 3-aminepropyltrimethoxysilane. This silver-coated graphite nanosheet exhibited high conductivity that was equivalent to that of the silver powder. There is still a need for new and simple methods for electroless deposition of silver metal on different substrates. A protocol was developed for a solid templating mask, which is utilized for the electroless modification of sulfate-terminated polystyrene spheres with caps comprised of silver nanoparticles. Miyoshi et al. developed an electroconductive Ag nanoarray on a Si wafer. The fabrication of nano-interconnections in electric circuits, nanowire grid polarizers, molecular sensors, and other functional devices has been attained with promising attributes of pattern and material variety on the scale of ≤50 nm for the above technique. Ag nanoparticles on hydrogenated SiNx :H layers for photovoltaic applications. A novel activation procedure was developed, via electroless Ag deposition and comparable to the wet Sn-Ag activation. The investigated approach may find applications in the fabrication of metal microstructures and nanostructures on various substrates and is projected to have numerous applications in catalysis, plasmonic devices, sensors, and many other fields. Radke et al. successfully fabricated 3D metallic bichiral crystals via direct laser writing and electroless silver plating and this method exposes a route toward very complex 3D plasmonic structures in the optical range, for example, toroidal structures with completely unusual and novel types of optical resonances. Electroless Ag coating on tetraethoxy silane-bridged fiber glass has lowest electrical resistance of 1.56 × 103 Ω/cm<sup>2</sup> and good mechanical stability. Kim et al. developed uniform compact silver layer by ecofriendly electroless method on a Fe/TiO2 /Ag core-shell structure [95–98].

#### **2.6. Miscellaneous electroless coatings**

#### *2.6.1. Electroless aluminum*

Electroless aluminum is capable of becoming one of the beneficial methods to develop thin films of Al and aluminum wiring at a very low cost. It is very difficult to perform electro deposition of aluminum in an aqueous solution because aluminum is not a very noble metal. In spite of being able to deposit Al from a room-temperature ionic liquid [99], there have been no established techniques well in virtue. A method used for the electroless plating of Al based on using AlCl<sup>3</sup> -1-ethyl-3-methylimidazolium chloride (AlCl<sup>3</sup> -EMIC) ionic liquid as the electrolyte and lithium hydride (LiH) as the solid reducing agent. It is criticized for its bath composition (contained LiH) as it was challenging to control the bath condition and stability. The main reasons for the limited use of LiH-based baths are that LiH solubility in the plating bath is very low and usually supplemented with excessive temperature during the deposition reaction. Recently, the same group has further investigated electroless aluminum plating based on using AlCl<sup>3</sup> -EMIC ionic liquid with di-isobutyl aluminum hydride (DIBAH) as a liquid reducing agent [100]. The DIBAH-based baths were easier to control and regulate for stability than that containing LiH. However, the reports on this topic such as film composition, plating condition, and reaction mechanism are still scarce. If the plating technique is established, it would widen the likelihood of obtaining thin and thick film coatings on the substrates of insulating material and intricate structures without electricity.

### *2.6.2. Electroless platinum and its alloy*

Electroless bath and method of coating platinum and platinum alloys contain up to about 20% rhodium, up to about 10% iridium, and up to about 10% ruthenium on an active surface, wherein the bath is an alkaline solution containing about 2 to about 20 g/L of platinum, an alkali metal hydroxide to give a minimum bath pH of about 8, up to about 1 g/L of hydrazine. Electroless platinum deposits in the absence of the stabilizer have catalytic properties, whereas platinum and platinum alloy deposits in the presence of the stabilizer are bright. Electroless coating of platinum [101] group metals has reasonable descriptions on techniques for preparing solutions and setting up conditions of plating. However, all the information on individual processes was obtained only from the patent literature, and the basic interpretation of those processes is not well known. Besides, in a few exceptional cases, there are still lacunae about the detailed information on process characteristics and deposit properties.

#### *2.6.3. Electroless ruthenium*

Electroless ruthenium developed in a patent by using Ru-nitosylammine complexes with hyrazine. Hydroxylamine added to the bath acts as a stabilizer as it is generally done to the similar baths of electroless platinum. The baths contain both [Ru(No)(OH)(NH3 ) 4 ]2+ and [Ru(No) (NH3 ) 3 ]3+. The active ruthenium species are either added as their chloride salts or produced in-situ from other ruthenium salts, such as RuCl3 or K2 [Ru(No)Cl3 ] with NaNo2 and NH4 OH. The inventor found that the low operating temperature brands this process of ruthenium deposition highly suitable for materials that are mercurial at high temperatures [102].

### *2.6.4. Electroless rhodium*

Electroless rhodium deposits were developed by Strejcek [103] by using hydrazine as the reducing agent. Rhodium bath solution: 0.1 g RhCl3 .3H2 O + 100 ml water and a large excess of NaNo2 (10 g). After heating in the range of 95–98°C for about 30 min, the color of the solution changes from red to pale yellow. After cooling, 5 ml of conc. (NH<sup>3</sup> )n Rh (No2 ) 4 a copper wire (with Al foil contacting) and a nickel sheet was immersed in this solution. With continuous agitation and heating, a 2% solution of N2 H4 .H2 O was added drop wise. At 60°C, a bright deposit of rhodium was deposited.

## **3. Conclusions and future perspectives**

electrolyte and lithium hydride (LiH) as the solid reducing agent. It is criticized for its bath composition (contained LiH) as it was challenging to control the bath condition and stability. The main reasons for the limited use of LiH-based baths are that LiH solubility in the plating bath is very low and usually supplemented with excessive temperature during the deposition reaction. Recently, the same group has further investigated electroless aluminum plating

liquid reducing agent [100]. The DIBAH-based baths were easier to control and regulate for stability than that containing LiH. However, the reports on this topic such as film composition, plating condition, and reaction mechanism are still scarce. If the plating technique is established, it would widen the likelihood of obtaining thin and thick film coatings on the

Electroless bath and method of coating platinum and platinum alloys contain up to about 20% rhodium, up to about 10% iridium, and up to about 10% ruthenium on an active surface, wherein the bath is an alkaline solution containing about 2 to about 20 g/L of platinum, an alkali metal hydroxide to give a minimum bath pH of about 8, up to about 1 g/L of hydrazine. Electroless platinum deposits in the absence of the stabilizer have catalytic properties, whereas platinum and platinum alloy deposits in the presence of the stabilizer are bright. Electroless coating of platinum [101] group metals has reasonable descriptions on techniques for preparing solutions and setting up conditions of plating. However, all the information on individual processes was obtained only from the patent literature, and the basic interpretation of those processes is not well known. Besides, in a few exceptional cases, there are still lacunae about the detailed information on process characteristics and deposit

Electroless ruthenium developed in a patent by using Ru-nitosylammine complexes with hyrazine. Hydroxylamine added to the bath acts as a stabilizer as it is generally done to the simi-

The inventor found that the low operating temperature brands this process of ruthenium

Electroless rhodium deposits were developed by Strejcek [103] by using hydrazine as the

NaNo2 (10 g). After heating in the range of 95–98°C for about 30 min, the color of the solution

(with Al foil contacting) and a nickel sheet was immersed in this solution. With continuous

H4 .H2

deposition highly suitable for materials that are mercurial at high temperatures [102].

]3+. The active ruthenium species are either added as their chloride salts or produced

or K2

.3H2

[Ru(No)Cl3

) 4

] with NaNo2 and NH4

O + 100 ml water and a large excess of

) 4

)n Rh (No2

O was added drop wise. At 60°C, a bright

]2+ and [Ru(No)

a copper wire

OH.

lar baths of electroless platinum. The baths contain both [Ru(No)(OH)(NH3

in-situ from other ruthenium salts, such as RuCl3

reducing agent. Rhodium bath solution: 0.1 g RhCl3

agitation and heating, a 2% solution of N2

deposit of rhodium was deposited.

changes from red to pale yellow. After cooling, 5 ml of conc. (NH<sup>3</sup>

substrates of insulating material and intricate structures without electricity.


based on using AlCl<sup>3</sup>

42 Nanoscaled Films and Layers

properties.

(NH3 ) 3

*2.6.3. Electroless ruthenium*

*2.6.4. Electroless rhodium*

*2.6.2. Electroless platinum and its alloy*

The literature briefs and demonstrates the numerous attempts prepared to identify the interdependence of the parameters which influence the performance of electroless nanolayered metallic coatings (concentration, the problems, easier for the impending user to prepare a bath). There is scope for further research and empirical analysis to be done toward formulating a rest electroless plating bath that would be reliable for extensive application than the baths existing at present.

Simple work is needed to replace of using hazardous source of potassium gold cyanide in electroless Au layer coatings. Long-term stability of Cu/Pd nanoparticles requires lot of research, and more work has to be done, whether the proposed one will be suit for the rest. The thermal stability of SiC-W/Co composite layer remains less because of the weak bonding between the substrate and deposit. This Co composite seems to be inadequate, future prospects will be very attractive. This silver-coated graphite nanosheets exhibited excellent conductivity, equivalent to silver powder. Finding of new and simple approaches is needed for the deposition of silver metal on numerous substrates. If the electroless Al plating technique is established, it will be able to achieve the thin and thick film coating on the substrates of shielding material and complicated structures. The reports on this topic are still scarce. It was difficult to make critical judgment on the practical usefulness of electroless platinum and its platinum alloy. Nevertheless, future work is to improve the existing process as well as to develop new process useful for today's application.

It is expected that this review, together with the ideas proposed by the authors, will be helpful toward the development of newer practical applications. These studies have highlighted commercial viability for rest of electroless processes.

## **Acknowledgements**

We would like to thank Indian Institute of Technology Madras and B.S. Abdur Rahman University, Chennai, India, for their kind permission and support. We confirm that there are no conflicts of interest.

## **Author details**

Jothi Sudagar1, 2\*, Rajendraprasad Tamilarasan3 , Udaykumar Sanjith<sup>3</sup> , Raj Rajendran3 and Ravi Kumar1

\*Address all correspondence to: sendme2sudagar@gmail.com

1 Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India

2 Department of Physics, Vellore Institute of Technology-Amaravati, Andhrapradesh, India

3 Department of Mechanical Engineering, School of Mechanical Sciences, B.S. Abdur Rahman University, Chennai, Tamil Nadu, India

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## **Laser Prepared Thin Films for Optoelectronic Applications**

Marcela Socol, Gabriel Socol, Nicoleta Preda, Anca Stanculescu and Florin Stanculescu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67659

#### **Abstract**

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50 Nanoscaled Films and Layers

2246–2254.

Patent, 3,486,928.

TiO2

Laser techniques such as pulsed laser deposition, combinatorial pulsed laser deposition, and matrix-assisted pulsed laser evaporation were used to deposit thin films for optoelectronic applications. High-quality transparent conductor oxide films ITO, AZO, and IZO were deposited on polyethylene terephthalate by PLD, an important experimental parameter being the target-substrate distance. The TCO films present a high transparency (>95%) and a reduced electrical resistivity (5 × 10−4 Ωcm) characteristics very useful for their integration in the flexible electronics. In<sup>x</sup> Zn1−xO films with a compositional library were obtained by CPLD. These films are featured by a high optical transmission (>95%), the lowest resistivity (8.6 × 10−4 Ωcm) being observed for an indium content of about 44–49 at.%. Organic heterostructures based on arylenevinylene oligomers (P78 and P13) or arylene polymers (AMC16 and AMC22) were obtained by MAPLE. In the case of ITO/P78/Alq3/Al heterostructures, a higher current value is obtained when the film thickness increases. Also, a photovoltaic effect was observed for heterostructures based on AMC16 or AMC22 deposited on ITO covered by a thin layer of PEDOT:PSS. Due to their optical and electrical properties, such organic heterostructures can be interesting for the organic photovoltaic cells (OPV) applications.

**Keywords:** PLD, CPLD, TCO, MAPLE, organic thin films

## **1. Introduction**

During the time, the deposition methods have been developed or/and adapted to process materials with special properties as thin films. It is well known that between the deposition methods and the quality of the obtained layers exist a strong correlation.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

From the actual deposition techniques, those based on laser prove their great potential in the field of the thin films processing for different applications. Moreover, the methods based on pulsed laser have been widely implied in the preparation of transparent conductor oxide (TCO), organic films, nitrides, biomaterials, etc. [1–4].

TCO are materials integrated in applications such as organic photovoltaic cells (OPV), organic light-emitting devices (OLED), organic field-effect transistors (OFET), or smart windows [5–8]. Generally, in order to be used in optoelectronic applications, these materials must satisfy special requirements such as high optical transmittance in visible parts of the spectrum, significant reflectance in IR, and a reduced electrical resistivity [9, 10]. The most used TCO is indium tin oxide (ITO) based on indium, a rare and expensive material, deposited on glass or flexible substrates [1, 5]. Beside ITO, other TCO material used is ZnO, especially ZnO doped with elements from the III group as aluminum, indium, or gallium for improving its conductivity [11–13]. It was reported that using an aluminum-doped zinc oxide (AZO) electrode, which has absorption under 380 nm, can avoid the degradation of the cell due to the UV irradiation in the OPV applications [14]. Indium zinc oxide (IZO) films are characterized by a high electrical mobility and increased carrier density [15, 16].

Organics are a class of materials containing a large variety of compounds as small molecules, oligomers, and polymers that, lately, have been intensively studied in order to replace the inorganic materials in different domains. The most important applications of the organic semiconductors are OPV, OLED, and OFET [17–19]. Thus, Heliatek reported that a higher efficiency (13.2%) can be obtained in the field of the organic materials using a combination of the three oligomers [20]. Organic light-emitting devices are already implemented in applications as displays (in TV sets or in mobile phone) and lighting sources. OFET are used in the sensor applications [21]. A great advantage of these organic materials comes from the facts that they are ecofriendly and at low cost fabricated by a large variety of deposition techniques.

In this chapter, some of our contributions in the field of the TCO layers and organic thin films deposited by laser techniques (pulsed laser deposition (PLD), combinatorial pulsed laser deposition (CPLD), and matrix-assisted pulsed laser evaporation (MAPLE)) are presented. All these materials have been studied to be further integrated into OPV applications: TCO (ITO, AZO, and IZO) as transparent electrodes (anode) and organic semiconductors as active layers. The TCO materials obtained by PLD and CPLD present suitable optical and electrical properties as high transparency and reduced electrical resistivity. The MAPLE prepared organic films are characterized by a large absorption domain and adequate electrical properties.

## **2. Insights into the laser-based techniques for deposition of thin films—basic principles and experimental set-ups**

Various deposition methods based on chemical or physical processes are used to obtain different materials in thin films form. The technique is chosen to obtain coatings with the expected properties for targeted applications, by tuning the parameters implied in the deposition process. In the field of OPV applications, the requested TCO can be obtained using methods such as RF magnetron sputtering, oxygen ion beam-assisted deposition, spray chemical vapor deposition (CVD), PLD, and spray pyrolysis [22–26].

For the preparation of the organic active layer, methods such as vacuum thermal evaporation, spin-coating, Langmuir-Blodgett, inkjet printing, and MAPLE can be used [27–31].

In the following are briefly summarized the laser methods used in our work for the preparation of TCO and organic layers, to understand their basic principles, and the way in which the appropriate technique can be chosen in order to obtain layers with adequate properties.

## **2.1. Pulsed laser deposition (PLD)**

From the actual deposition techniques, those based on laser prove their great potential in the field of the thin films processing for different applications. Moreover, the methods based on pulsed laser have been widely implied in the preparation of transparent conductor oxide

TCO are materials integrated in applications such as organic photovoltaic cells (OPV), organic light-emitting devices (OLED), organic field-effect transistors (OFET), or smart windows [5–8]. Generally, in order to be used in optoelectronic applications, these materials must satisfy special requirements such as high optical transmittance in visible parts of the spectrum, significant reflectance in IR, and a reduced electrical resistivity [9, 10]. The most used TCO is indium tin oxide (ITO) based on indium, a rare and expensive material, deposited on glass or flexible substrates [1, 5]. Beside ITO, other TCO material used is ZnO, especially ZnO doped with elements from the III group as aluminum, indium, or gallium for improving its conductivity [11–13]. It was reported that using an aluminum-doped zinc oxide (AZO) electrode, which has absorption under 380 nm, can avoid the degradation of the cell due to the UV irradiation in the OPV applications [14]. Indium zinc oxide (IZO) films are characterized by a high electrical mobility and increased carrier density [15, 16]. Organics are a class of materials containing a large variety of compounds as small molecules, oligomers, and polymers that, lately, have been intensively studied in order to replace the inorganic materials in different domains. The most important applications of the organic semiconductors are OPV, OLED, and OFET [17–19]. Thus, Heliatek reported that a higher efficiency (13.2%) can be obtained in the field of the organic materials using a combination of the three oligomers [20]. Organic light-emitting devices are already implemented in applications as displays (in TV sets or in mobile phone) and lighting sources. OFET are used in the sensor applications [21]. A great advantage of these organic materials comes from the facts that they

are ecofriendly and at low cost fabricated by a large variety of deposition techniques.

**2. Insights into the laser-based techniques for deposition of thin** 

Various deposition methods based on chemical or physical processes are used to obtain different materials in thin films form. The technique is chosen to obtain coatings with the expected

**films—basic principles and experimental set-ups**

In this chapter, some of our contributions in the field of the TCO layers and organic thin films deposited by laser techniques (pulsed laser deposition (PLD), combinatorial pulsed laser deposition (CPLD), and matrix-assisted pulsed laser evaporation (MAPLE)) are presented. All these materials have been studied to be further integrated into OPV applications: TCO (ITO, AZO, and IZO) as transparent electrodes (anode) and organic semiconductors as active layers. The TCO materials obtained by PLD and CPLD present suitable optical and electrical properties as high transparency and reduced electrical resistivity. The MAPLE prepared organic films are characterized by a large absorption domain and adequate electrical

(TCO), organic films, nitrides, biomaterials, etc. [1–4].

52 Nanoscaled Films and Layers

properties.

PLD is a deposition technique widely used in the preparations of the thin films based on material or on combination of materials. High-quality coatings with special properties can be performed by PLD. Materials with complicated composition can be transferred by PLD on substrate without changing their stoichiometry [3]. During the material transfer process, a high-intensity laser source falls on a solid target containing materials used for deposition inside a vacuum chamber or filled with inert gas as nitrogen (N2 ) or reactive gas as oxygen (O2 ). Over a particular value of the incident laser intensity, the target elements are heated above their evaporation temperature (evaporation threshold). The materials are ejected from the target forming the plasma plume and moved toward the substrate. The plasma species that have sufficient energy condenses on the substrate producing the nucleation and the thin film grow up [32]. The target is rotated in order to prevent the local deterioration, which can affect the uniformity and quality of the obtained coating. A typical PLD experimental set-up is presented in **Figure 1**. Into a PLD process, the most important parameters are laser fluence, deposition rate, substrate temperature, target-substrate distance, and number of laser pulses [33].

**Figure 1.** Schematic representation of PLD set-up.

## **2.2. Combinatorial pulsed laser deposition (CPLD)**

In the beginning, the CPLD was introduced, in chemistry, in the fields of drugs from the necessity to develop new active molecules [34, 35], but this technique can be applied for a variety of materials: metals, semiconductors, polymers [36–38], etc. This deposition method is a proper tool to obtain doped materials like TCO layers.

The great advantage of the CPLD over PLD is the possibility to perform in a single experiment samples with different composition. By comparison, in order to find the sample with the best properties using PLD technique, it is necessary to carry out a lot of samples with different compositions; a time-consuming process using CPLD is obtained, a so-called composition library. Practically, along the deposition substrate in each point, the concentration of the thin films is different. In **Figure 2**, a CPLD deposition set-up is illustrated. In comparison with the PLD, in the CPLD deposition process is involved targets with different composition, situated at certain distance to each other [39]. An optical beam-splitter is used to split the laser beam into two beams. The targets are simultaneously ablated, generating intermixed films [40].

**Figure 2.** Schematic representation of CPLD set-up.

## **2.3. Matrix-assisted pulsed laser evaporation (MAPLE)**

**2.2. Combinatorial pulsed laser deposition (CPLD)**

a proper tool to obtain doped materials like TCO layers.

intermixed films [40].

54 Nanoscaled Films and Layers

**Figure 2.** Schematic representation of CPLD set-up.

In the beginning, the CPLD was introduced, in chemistry, in the fields of drugs from the necessity to develop new active molecules [34, 35], but this technique can be applied for a variety of materials: metals, semiconductors, polymers [36–38], etc. This deposition method is

The great advantage of the CPLD over PLD is the possibility to perform in a single experiment samples with different composition. By comparison, in order to find the sample with the best properties using PLD technique, it is necessary to carry out a lot of samples with different compositions; a time-consuming process using CPLD is obtained, a so-called composition library. Practically, along the deposition substrate in each point, the concentration of the thin films is different. In **Figure 2**, a CPLD deposition set-up is illustrated. In comparison with the PLD, in the CPLD deposition process is involved targets with different composition, situated at certain distance to each other [39]. An optical beam-splitter is used to split the laser beam into two beams. The targets are simultaneously ablated, generating This technique is also derived from PLD method; Piqué et al. [41] mentioned that this technique was first introduced by Epstein in 1997 [42]. MAPLE has the advantage that it can process soft materials (organics) that could not be transferred by other techniques because there is the risk that takes place—a decomposition of the materials. In MAPLE, the target is formed from the materials (one or more) that must be deposited, and an adequate solvent is used as matrix [43, 44]. The solvent is chosen to obtain a homogeneous mixture (concentration usually below 3%) and to be compatible with the used laser wavelength. The formed mixture (organic material and solvent) is subsequently frozen in liquid nitrogen to form a solid target. During the deposition, the laser energy is absorbed by the solvent and transformed into thermal energy, enabling the evaporation of the solvent, and this being pumped away by the vacuum system while the material of interest reaches the substrate [45–47].

In MAPLE, smaller fluence are used (under 0.5 J/cm2 ) in order to prevent the deterioration of the materials [44]. Another great advantage compared with methods used for the deposition of the organic materials is the possibility to obtain stacked layers without deterioration of the preliminary deposited layer [31]. **Figure 3** presents a schematic representation of the experimental set-up used in MAPLE deposition.

In **Table 1**, are presented comparatively the pulsed laser techniques described before which can be used to prepare thin films from different materials with thickness from nano to micrometers.

**Figure 3.** Schematic representation of MAPLE set-up used for the deposition of organic films.


**Table 1.** Advantages of pulsed laser techniques for deposition of thin films.

## **3. Transparent conductive oxide (TCO) thin films deposited by PLD or CPLD—influence of the deposition conditions on their structural, morphological, optical, and electrical properties**

The great interest in the field of TCO is proved by the high number of the research articles, reviews, books, or chapter books existent in the literature regarding this topic [48–56]. Various attempts were made to find the TCO with high optical and electrical properties and the best method to obtain these properties. The first TCO preparation is attributed to Badeker and dates from 1907, when CdO was obtained by the thermal oxidation of a Cd film deposited by sputtering [57]. ITO was frequently used in different applications, being one of the most-studied materials. In time, materials such as In<sup>2</sup> O3 , CdO, ZnO, and SnO2 were also addressed as TCO. n type semiconductors can be doped in order to improve their electrical conductivity, obtaining materials such as In<sup>2</sup> O3 -ZnO, In<sup>2</sup> O3 -SnO2 , Ga<sup>2</sup> O3 -In<sup>2</sup> O3 , In<sup>2</sup> O3 -MgInO4 , ZnSnO3 -ZnIn<sup>2</sup> O5 , ZnIn<sup>2</sup> O5 -GaInO3, or MgIn2 O4 -Zn<sup>2</sup> In2 O5 [58]. Several attempts were made to prepare p type semiconductors: CuGaO2 , SrCu2 O2 , CuAl<sup>2</sup> O2 , and CuCrO<sup>2</sup> [59–62].

There are many studies regarding the TCO prepared by PLD, the best result being reported for ITO thin films (7.2 × 10−5 Ωcm electrical resistivity and ~90% transparency [63]). Also, by PLD, ITO layers with a smooth surface (root mean square (RMS) ~ 4.5 Å) were obtained [64]. The performances of the ITO deposited by PLD are superior to that presented by commercially available ITO deposited by sputtering.

The TCO film represents a key element in all applications, including OPV, due to the fact that through this electrode passes the light depending on its optical transmittance [65].

Our results regarding the TCO films obtained by PLD and CPLD and their complex characterization by techniques such as X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-VIS) and atomic force microscopy (AFM) are presented in the next section.

### **3.1. Indium tin oxide (ITO)**

**3. Transparent conductive oxide (TCO) thin films deposited by PLD or CPLD—influence of the deposition conditions on their structural,** 

**Method PLD CPLD MAPLE**

Deposition rate (nm/pulse) 0.01–0.5 0.01–0.5 0.1–0.5 Typical film thickness (nm) 1–5000 10–1000 10–5000

Combination of any inorganic materials

Laser ablation of solid targets with different composition

) 1–10 1–10 0.05–0.5

material with complex and very complex stoichiometry

Class of materials Suitable for any inorganic

Principle of the laser transfer Laser ablation of solid

targets

The great interest in the field of TCO is proved by the high number of the research articles, reviews, books, or chapter books existent in the literature regarding this topic [48–56]. Various attempts were made to find the TCO with high optical and electrical properties and the best method to obtain these properties. The first TCO preparation is attributed to Badeker and dates from 1907, when CdO was obtained by the thermal oxidation of a Cd film deposited by sputtering [57]. ITO was frequently used in different applications, being one of

addressed as TCO. n type semiconductors can be doped in order to improve their electrical

There are many studies regarding the TCO prepared by PLD, the best result being reported for ITO thin films (7.2 × 10−5 Ωcm electrical resistivity and ~90% transparency [63]). Also, by PLD, ITO layers with a smooth surface (root mean square (RMS) ~ 4.5 Å) were obtained [64]. The performances of the ITO deposited by PLD are superior to that presented by commercially

The TCO film represents a key element in all applications, including OPV, due to the fact that

Our results regarding the TCO films obtained by PLD and CPLD and their complex characterization by techniques such as X-ray diffraction (XRD), ultraviolet-visible spectroscopy

through this electrode passes the light depending on its optical transmittance [65].

(UV-VIS) and atomic force microscopy (AFM) are presented in the next section.


, CuAl<sup>2</sup>

O3

O4 -Zn<sup>2</sup> In2 O5

, SrCu2 O2 O3

O3 -SnO2

O2

, CdO, ZnO, and SnO2

[58]. Several attempts were made to

[59–62].

Suitable for deposition of organic compounds and laser transfer of inorganic or organic nanostructures (e.g., nanoparticles, nanotubes, nano-sheets,

Laser evaporation of frozen composite targets under ablation threshold

etc.)

, Ga<sup>2</sup> O3 -In<sup>2</sup> O3 , In<sup>2</sup> O3

, and CuCrO<sup>2</sup>

were also


,

**morphological, optical, and electrical properties**

**Table 1.** Advantages of pulsed laser techniques for deposition of thin films.

the most-studied materials. In time, materials such as In<sup>2</sup>


conductivity, obtaining materials such as In<sup>2</sup>

prepare p type semiconductors: CuGaO2

available ITO deposited by sputtering.

ZnSnO3


Typical laser fluences (J/cm2

56 Nanoscaled Films and Layers

This material was the subject of many research studies and is the most used electrode in OPV applications because it seems to present the best electrical conductivity and optical properties. It is used as hole injector material, having a work function between 4.5 and 5 eV [33].

Because of the technological progress in the field of flexible electronics, light-weight and cheap TCO materials that are compatible with plastic substrates are necessary.

A PLD experimental set-up like that presented in **Figure 1** was used to prepare ITO thin films on polyethylene terephthalate (PET) substrate. An excimer laser with KrF\* (model COMPex-Pro 205, Lambda Physics-Coherent), λ = 248 nm, and τFWHM = 25 nm was used to irradiate the ITO target (SCI engineered materials) in an ultrahigh vacuum chamber [1]. The depositions were made at room temperature, at 2 J/cm2 laser fluence, and with 10 Hz repetition rate. The oxygen pressure was between 1 and 1.5 Pa. We have selected different deposition parameters: target-substrate distance (4, 6, or 8 cm) and the number of laser pulses (6000, 9000, or 12,000). The sample was labeled as ITO1 (4 cm distance and 6000 pulses), ITO2 (6 cm distance and 9000 pulses), and ITO3 (8 cm distance and 12,000 pulses).

Analyzing the ITO samples from the structural point of view (**Figure 4a**), it was found that the films present a lower crystallinity. The ITO1 film deposited at the lower target-substrate distance appears as a small and large peak at 35.2° corresponding to the (400) diffraction plane. An increase in the diffraction peak from 35.2° is remarked for the ITO2. A supplementary peak at ~30.2° (a reduced one) that corresponds to (222) diffraction plane of ITO [66] is also disclosed. The film ITO3 deposited at higher target-substrate distance is amorphous. This means that there is an optimum for the deposition condition for assuring an increased crystallinity of the ITO layer.

The UV-VIS spectra for ITO samples (**Figure 4b**) record subtracting the contribution of the flexible substrate and show the differences in the transmission degree. The sample ITO1 presents the lowest transparency (70–75%) that is being attributed to the presence of some defects (cracks) generated during a deposition process at this distance and the defects that scatter the light.

**Figure 4.** XRD patterns (a) and transmission spectra (b) of the ITO1, ITO2, and ITO3.

In PLD, the deposition appears as species with an increased kinetic energy that can affect the deposition substrate and the way in which this takes place is the nucleation process [1]. Defects can appear due to the energy transferred through the collision of the energetic items such as atoms, ions, and molecules, with the substrate atoms or with the atoms previously deposited. The adatoms must have enough time and mobility to form films with a good adherence at reduced substrate temperature [67].

The transmittance of the film increases with the increase of the target-substrate distance, reaching up to ~90% for the film deposited at a higher distance (ITO3).

**Figure 5** shows the topographic images obtained by AFM. The appearance of the cracks in the ITO1 film can be observed. Also for the second sample, it was obtained morphologically with cracks. These cracks are characteristic to ITO deposited on PET substrates in some experimental conditions, which were being reported by the other authors also [68]. The smallest RMS value (2.4 nm) was obtained for the ITO3 film, compared to ITO1 (RMS = 15.5 nm) and to ITO2 (RMS = 31.5 nm), the ITO3 being the film showing the highest transmittance. This aspect of the sample is different from the other two, no cracks being observed on it.

In terms of the electrical resistivity, the sample ITO2 was measured with a good resistivity (5.9 × 10−4 Ωcm). Taking into account that this is the sample with the increased crystallinity, it can be concluded that there is a correlation between its crystallinity and its electrical properties. The sample ITO3 that is amorphous is featured by an increased resistivity (9.7 × 10−4 Ωcm). Higher value for the films is obtained by PLD at room temperature and had been reported by other authors [69]. The electrical properties of the TCO layers can be improved by heating the substrate is mentioned in the literature [70].

## **3.2. Aluminum-doped zinc oxide (AZO)**

Another studied TCO with n type conduction is AZO, which is a nontoxic candidate having properties close to ITO. AZO samples were prepared by PLD on PET substrate in the same condition as ITO using a target with 2% Al content (SCI engineered materials). The samples prepared in different geometrical configuration were also labeled as AZO1 (4 cm), AZO2 (6 cm), and AZO3 (8 cm).

**Figure 5.** AFM images of the ITO1 (a), ITO2 (b), and ITO3 (c).

As for ITO, defects like cracks were remarked on the AZO layers deposited at 4 cm targetsubstrate distance (AZO1) and at 6 cm (AZO2). Films with crystalline quality (**Figure 6a**) were obtained at 6 and 8 cm. All AZO samples show a diffraction peak situated at ~34°, corresponding to (002) plane of ZnO [71]. The broadening of this peak from 34° for AZO1 is attributed to the presence of the lattice strain in this film, resulting in a peeling effect of the flayer.

In PLD, the deposition appears as species with an increased kinetic energy that can affect the deposition substrate and the way in which this takes place is the nucleation process [1]. Defects can appear due to the energy transferred through the collision of the energetic items such as atoms, ions, and molecules, with the substrate atoms or with the atoms previously deposited. The adatoms must have enough time and mobility to form films with a good

The transmittance of the film increases with the increase of the target-substrate distance,

**Figure 5** shows the topographic images obtained by AFM. The appearance of the cracks in the ITO1 film can be observed. Also for the second sample, it was obtained morphologically with cracks. These cracks are characteristic to ITO deposited on PET substrates in some experimental conditions, which were being reported by the other authors also [68]. The smallest RMS value (2.4 nm) was obtained for the ITO3 film, compared to ITO1 (RMS = 15.5 nm) and to ITO2 (RMS = 31.5 nm), the ITO3 being the film showing the highest transmittance. This aspect of the sample is different from the other two, no cracks being

In terms of the electrical resistivity, the sample ITO2 was measured with a good resistivity (5.9 × 10−4 Ωcm). Taking into account that this is the sample with the increased crystallinity, it can be concluded that there is a correlation between its crystallinity and its electrical properties. The sample ITO3 that is amorphous is featured by an increased resistivity (9.7 × 10−4 Ωcm). Higher value for the films is obtained by PLD at room temperature and had been reported by other authors [69]. The electrical properties of the TCO layers can be improved by heating the

Another studied TCO with n type conduction is AZO, which is a nontoxic candidate having properties close to ITO. AZO samples were prepared by PLD on PET substrate in the same condition as ITO using a target with 2% Al content (SCI engineered materials). The samples prepared in different geometrical configuration were also labeled as AZO1 (4 cm), AZO2

reaching up to ~90% for the film deposited at a higher distance (ITO3).

adherence at reduced substrate temperature [67].

substrate is mentioned in the literature [70].

**Figure 5.** AFM images of the ITO1 (a), ITO2 (b), and ITO3 (c).

**3.2. Aluminum-doped zinc oxide (AZO)**

(6 cm), and AZO3 (8 cm).

observed on it.

58 Nanoscaled Films and Layers

The UV-VIS spectra in **Figure 6b** can be observed has a higher transmittance (over 90%) obtained for AZO3 coating compared with AZO1 and AZO2. A high optical quality is achieved for the film obtained at 8 cm target-substrate distance because, after target ejection, probably the energetic species have enough time to be thermalized in collision with the oxygen molecule when the distance is increased between target and PET substrate.

The AZO films present a morphology with long aggregates (**Figure 7**). The AZO3 has the bigger aggregates (with size between 0.4 and 1 μm) that are oriented in the same direction. This is in concordance with the higher transmittance observed for this layer due to the reduction of the light scattering on the grain boundaries. The AZO2 film presents the morphology similarly to AZO3 but with smaller aggregates, the film having lower RMS value (5.2 nm) compared with AZO1 (RMS = 12.4 nm) and AZO3 (RMS = 8.5 nm).

**Figure 6.** XRD patterns (a) and transmission spectra (b) of the AZO1, AZO2, and AZO3.

**Figure 7.** AFM images of the AZO1 (a), AZO2 (b), and AZO3 (c).

The best electrical resistivity (6.8 × 10−4 Ωcm) was found for the AZO2 film, with the AZO3 sample having increased in value (2.1 × 10−3 Ωcm). Due to the plasma expansion between target and substrate at a distance of 8 cm, it could appear that some oxidation effects affect the ZnO stoichiometry. Collisions and recombination can appear due to the oxygen background. Nevertheless, the resistivity values are better than other reported values (1.1 × 10−3 Ωcm) for AZO deposited on PET by RF magnetron sputtering at 100°C [72].

## **3.3. Indium zinc oxide (IZO)**

Competitive optical and electrical properties were found for IZO electrode in comparison with that reported for ITO. IZO has a work function of ~5.2 eV being framed also as an n type semiconductor. The same deposition parameters used before were preserved to deposit IZO by PLD on PET from a solid target formed with In in atomic concentration, In/(In + Zn), of 70% by mixing In2 O3 and ZnO powders (both from Aldrich). Also, in this case, the samples were labeled in accordance with the target-substrate distance: IZO1 (4 cm), IZO2 (6 cm), and IZO3 (8 cm).

As is reported in the literature [25], generally, IZO layers deposited at room temperature are amorphous (**Figure 8a**). A broad peak appears between 31 and 34° only for the IZO1 film. This peak contains two contributions one attributed to (111) diffraction plane of In2 O3 , which is usually situated ~31° and another to (101) diffraction plane of ZnO, which appears at 34° [25]. For the IZO1 and IZO2 layers, no diffraction peaks were observed.

If for the ITO and AZO, we have observed the appearance of a significant number of defects on the samples deposited at 4 and 6 cm, in the case of IZO, defects were evidenced just for the samples performed at the lower target-substrate distance (IZO1). For IZO layers, the transmittance is reduced compared with ITO and AZO (**Figure 8b**). No noticeable differences appear in the transmission spectra of the IZO films deposited at different target-substrate distances, the transmittance having between 75 and 88%.

**Figure 8.** XRD patterns (a) and transmission spectra (b) of the IZO1, IZXO2, and IZO3.

The RMS values of the IZO films were evaluated by AFM (**Figure 9**). Thus, for the IZO3 sample, low RMS value (2.1 nm) was interpolated from AFM measurements. The other two films (IZO1 and IZO2) present higher RMS values (47.4 and 21.2 nm). The great value obtained for the IZO1 can be attributed to the presence of the cracks in this film. The best value for IZO3 is comparable with that obtained for ITO1. In order to be adequate for device applications, in addition to possess good optical and electrical properties, the films must be characterized by a smooth surface [33].

The best electrical resistivity (6.8 × 10−4 Ωcm) was found for the AZO2 film, with the AZO3 sample having increased in value (2.1 × 10−3 Ωcm). Due to the plasma expansion between target and substrate at a distance of 8 cm, it could appear that some oxidation effects affect the ZnO stoichiometry. Collisions and recombination can appear due to the oxygen background. Nevertheless, the resistivity values are better than other reported values (1.1 × 10−3 Ωcm) for

Competitive optical and electrical properties were found for IZO electrode in comparison with that reported for ITO. IZO has a work function of ~5.2 eV being framed also as an n type semiconductor. The same deposition parameters used before were preserved to deposit IZO by PLD on PET from a solid target formed with In in atomic concentration, In/(In + Zn), of

were labeled in accordance with the target-substrate distance: IZO1 (4 cm), IZO2 (6 cm), and

As is reported in the literature [25], generally, IZO layers deposited at room temperature are amorphous (**Figure 8a**). A broad peak appears between 31 and 34° only for the IZO1 film. This

usually situated ~31° and another to (101) diffraction plane of ZnO, which appears at 34° [25].

If for the ITO and AZO, we have observed the appearance of a significant number of defects on the samples deposited at 4 and 6 cm, in the case of IZO, defects were evidenced just for the samples performed at the lower target-substrate distance (IZO1). For IZO layers, the transmittance is reduced compared with ITO and AZO (**Figure 8b**). No noticeable differences appear in the transmission spectra of the IZO films deposited at different target-substrate distances,

peak contains two contributions one attributed to (111) diffraction plane of In2

For the IZO1 and IZO2 layers, no diffraction peaks were observed.

**Figure 8.** XRD patterns (a) and transmission spectra (b) of the IZO1, IZXO2, and IZO3.

and ZnO powders (both from Aldrich). Also, in this case, the samples

O3

, which is

AZO deposited on PET by RF magnetron sputtering at 100°C [72].

**3.3. Indium zinc oxide (IZO)**

O3

the transmittance having between 75 and 88%.

70% by mixing In2

60 Nanoscaled Films and Layers

IZO3 (8 cm).

The resistivity values obtained for IZO samples were much better, compared with ITO and AZO layers, being ranged in 5.4–6.7 × 10−4 Ωcm domain. These values are in agreement with other results obtained for coatings made on glass substrate [25].

Considering all the investigated properties for ITO, AZO, and IZO deposited by PLD on the flexible substrate, it can be concluded that an important parameter in PLD process is the target-substrate distance. At a higher distance (8 cm) smooth films are obtained characterized by increased transmittances compared with the films obtained at 4 or 6 cm distance. Films with high transparency (over 95%) can be obtained by this laser technique.

As was already pointed, another technique to obtain TCO films is CPLD; this method allows to obtain doped films with a compositional gradient along the deposition substrate.

Using the same laser beam and a combinatorial geometry as that presented in **Figure 2**, IZO films at 3 J/cm2 laser fluence were deposited. Two targets with atomic In concentration, In/(In + Zn), of 28 and 56 at.% or 42 and 70 at.% were made by mixing In2 O3 and ZnO powders [73]. The combinatorial samples were obtained in 1 Pa oxygen atmosphere and at room temperature. The laser repetition rate was 10 HZ, and the number of pulses was 3000. Substrates for deposition were 26 × 76 mm microscope glass slides. The distance between the targets and substrate was 5 cm. In order to make a relevant comparison, samples from each target, but just by PLD, were also performed. The samples obtained by CPLD were IZOCMB1 (with 28% and 56%) and IZOCMB2 (with 42% and 70%). The sample produced by PLD are labeled as IZO1A (28%), IZO1B (56%), IZO2A(42%), and IZO2B(70%).

The UV-VIS spectra of the films deposited by PLD are presented in **Figure 10a**. The UV-VIS spectra of the combinatorial samples, IZOCMB1 presented in **Figure 10b** and IZOCMB2 presented in **Figure 10c**, were collected in three points corresponding to L, C,

**Figure 9.** AFM images of the films deposited on PET substrate IZO1 (a), IZO2 (b), and IZO3 (c).

and R position (see **Figure 2**). The spectra are presented subtracting the glass contribution. Independently of the method used for deposition, PLD or CPLD, the samples show a high transmittance (up to 95%) in 500–1000 nm domain, which is a good premise for this TCO material.

For the films obtained by CPLD, a nonlinear variation in the composition of the films has been revealed by Energy dispersive X-ray spectroscopy (EDS) measurements (**Figure 11**). Data from 2.5 mm consecutive section along L-R line were mediated in order to establish the In, In/(In + Zn) content. Moreover, values from four areas were used to obtain the average In content from the target 1(TG1) and target 2 (TG2). We have obtained 28 and 56 at.% In, In/ (In + Zn) atomic concentration for the first CPLD target and 42 and 70 at.% In, In/(In + Zn) for the second CPLD target. The films IZOCMB1 and IZOCMB2 have atomic concentration in In between L and R position, of 27–33 and 36–52 at.%, respectively. The lower In content is attributed to the ZnO preferential nucleation on glass substrate.

The AFM images obtained on the combinatorial films (in different areas between L and R points) are presented in **Figure 12**. The films are smooth, and the RMS roughness values are ranged between 7.1 and 26.2 nm for the IZOCMB1 film and 1.0 and 7.3 nm for the IZOCMB2 film.

**Figure 10.** Transmission spectra of the IZO1A(curve1), IZO1B(curve2), IZO2A(curve3), IZO2B(curve4) (a) of the IZOCMB1 (b) and IZOCMB2 (c) in different areas corresponding to positions L, C, and R.

**Figure 11.** Elemental composition profiles of the IZO films deposited on glass substrate by CPLD using two targets with various In atomic concentration: (a) −28% (TG1) and 56% (TG2) and (b) −42% (TG1) and 70% (TG2).

The variation of RMS as a function by distance is given in **Figure 13**. The RMS mean value from three neighbor areas was used to plot each point in **Figure 13**. A decrease in the RMS value was observed with the increase in the In content between L and R points. This observation is in agreement with other results presented in the literature [74].

The modification of the electrical resistivity was also investigated along L-R direction of the films. The IZOCMB1 film presents a lower resistivity (2.3 × 10−3 Ωcm) for 28.8–29.5 at.% In content (**Figure 14a**), and IZOCMB2 film has a lower resistivity (8.6 × 10−4 Ωcm) in 44–49 at.% of In content region (**Figure 14b**). The literature reported similar data for the minimum resistivity obtained for samples with similar In atomic concentration deposited by magnetron sputtering [75, 76].

In conclusion, Inx Zn1−xO (27≤ × ≤52) systems were performed by CPLD. The best resistivity value obtained was 8.6 × 10−4 Ωcm that correspond to 44–46 at.% In content domain. All investigated films present a high optical transparency (~95%). This technique is useful for preparing TCO films with different composition and adequate optical and electrical properties.

**Figure 12.** AFM images of IZOCMB1 (a) and IZOCMB2 (b).

and R position (see **Figure 2**). The spectra are presented subtracting the glass contribution. Independently of the method used for deposition, PLD or CPLD, the samples show a high transmittance (up to 95%) in 500–1000 nm domain, which is a good premise for this TCO

For the films obtained by CPLD, a nonlinear variation in the composition of the films has been revealed by Energy dispersive X-ray spectroscopy (EDS) measurements (**Figure 11**). Data from 2.5 mm consecutive section along L-R line were mediated in order to establish the In, In/(In + Zn) content. Moreover, values from four areas were used to obtain the average In content from the target 1(TG1) and target 2 (TG2). We have obtained 28 and 56 at.% In, In/ (In + Zn) atomic concentration for the first CPLD target and 42 and 70 at.% In, In/(In + Zn) for the second CPLD target. The films IZOCMB1 and IZOCMB2 have atomic concentration in In between L and R position, of 27–33 and 36–52 at.%, respectively. The lower In content is

The AFM images obtained on the combinatorial films (in different areas between L and R points) are presented in **Figure 12**. The films are smooth, and the RMS roughness values are ranged between 7.1 and 26.2 nm for the IZOCMB1 film and 1.0 and 7.3 nm for the

**Figure 10.** Transmission spectra of the IZO1A(curve1), IZO1B(curve2), IZO2A(curve3), IZO2B(curve4) (a) of the

**Figure 11.** Elemental composition profiles of the IZO films deposited on glass substrate by CPLD using two targets with

various In atomic concentration: (a) −28% (TG1) and 56% (TG2) and (b) −42% (TG1) and 70% (TG2).

IZOCMB1 (b) and IZOCMB2 (c) in different areas corresponding to positions L, C, and R.

attributed to the ZnO preferential nucleation on glass substrate.

material.

62 Nanoscaled Films and Layers

IZOCMB2 film.

**Figure 13.** RMS roughness profiles of the IZOCMB1 (a) and IZOCMB2 (b) between L and R positions.

**Figure 14.** Electrical resistivity profiles of the IZOCMB1 (a) and IZOCMB2 (b).

## **4. Organic thin films deposited by MAPLE—properties and applications**

At the same time with the development of the organic materials domain, a large number of deposition methods had been adapted to obtain them as thin films. The most used technique to deposit organic layers was vacuum evaporation, but for materials as oligomers or polymers, more suitable are techniques involving solution because the risk to destroy the molecular chains during the deposition is reduced. The spin-coating method was frequently used as deposition technique for the polymers thin films preparation.

In the last years, the MAPLE method was introduced in order to process such organic materials, taking into account that MAPLE allows their transfer with the preservation of their chemical composition.

Subsequently are presented the organic thin layers prepared by MAPLE. These were investigated from morphological (AFM) and optical (UV-VIS, photoluminescence spectroscopy— PL, Fourier transform infrared spectroscopy (FTIR), and from electrical (current-voltage characteristics) point of view.

## **4.1. Oligomers based on arylenevinylene compounds**

Arylenevinylene oligomers, 1,4-*bis* [4-(N,N-diphenylamino)phenylvinyl] benzene (P78) and 3,3-*bis* (N-hexylcarbazole)vinylbenzene (P13), with electron-donating groups (triphenylamine and N-alkylcarbazole), are used in combination with *tris*(8-hydroxyquinolinato)aluminum salt (Alq3 ) to prepare by MAPLE organic heterostructures with one or two layers. The molecular formula of the oligomers used in this study was presented in **Figure 15**. The concentration of the organic material in the dimethyl sulfoxide (DMSO), used as a solvent, was 2.5% (w/v). The depositions were made at 5 Hz laser frequency and at lower fluence (250 mJ/cm2 ). In the heterostructures with two layers, the first deposited layer was the oligomer (P 13 or P78) and the second layer was Alq3 [77]. The number of laser pulses were 80,000 for 1P13, 1P78, and 1Alq3 samples and 160,000 for 2P13, 2P78, and 2Alq3 samples.

The FTIR spectra (**Figure 16**) of the MAPLE-deposited layers have been analyzed and was concluded that no materials decomposition appear during the laser transfer. The peaks characteristic to P13 and P78 compounds were identified. The peak situated at 960 cm−1 is characteristic to HC=HC trans-out-of-plane bending vibration [78], whereas the δ (C-N) stretching vibration appears at 1154 cm−1 in P13 layer and at 1328 cm−1 in P78. The band from 1589 cm−1 in P78 is assigned to the vibration of the C-C phenyl group. The peaks from 1598 cm−1, 1491 cm−1, and 1477 cm−1 are due to the ν (C-C) vibration in monosubstituted benzene [78]. The Alq<sup>3</sup> layer presents vibration characteristic to the following chromophoric groups: between 600 and 900 cm−1 =C-H stretching, at 1475 cm−1 aromatic C=C stretching, at 1390 cm−1 C=N bond, at 1600 cm−1 the quinolinic ring [79].

**Figure 15.** Chemical structures of the arylenevinylene oligomers: P13 (a) and P78 (b).

**4. Organic thin films deposited by MAPLE—properties and applications**

At the same time with the development of the organic materials domain, a large number of deposition methods had been adapted to obtain them as thin films. The most used technique to deposit organic layers was vacuum evaporation, but for materials as oligomers or polymers, more suitable are techniques involving solution because the risk to destroy the molecular chains during the deposition is reduced. The spin-coating method was frequently used as

In the last years, the MAPLE method was introduced in order to process such organic materials, taking into account that MAPLE allows their transfer with the preservation of their chemi-

Subsequently are presented the organic thin layers prepared by MAPLE. These were investigated from morphological (AFM) and optical (UV-VIS, photoluminescence spectroscopy— PL, Fourier transform infrared spectroscopy (FTIR), and from electrical (current-voltage

Arylenevinylene oligomers, 1,4-*bis* [4-(N,N-diphenylamino)phenylvinyl] benzene (P78) and 3,3-*bis* (N-hexylcarbazole)vinylbenzene (P13), with electron-donating groups (triphenylamine and N-alkylcarbazole), are used in combination with *tris*(8-hydroxyquinolinato)aluminum

lar formula of the oligomers used in this study was presented in **Figure 15**. The concentration of the organic material in the dimethyl sulfoxide (DMSO), used as a solvent, was 2.5% (w/v).

heterostructures with two layers, the first deposited layer was the oligomer (P 13 or P78) and the second layer was Alq3 [77]. The number of laser pulses were 80,000 for 1P13, 1P78, and

The depositions were made at 5 Hz laser frequency and at lower fluence (250 mJ/cm2

) to prepare by MAPLE organic heterostructures with one or two layers. The molecu-

samples.

). In the

deposition technique for the polymers thin films preparation.

**Figure 14.** Electrical resistivity profiles of the IZOCMB1 (a) and IZOCMB2 (b).

**4.1. Oligomers based on arylenevinylene compounds**

1Alq3 samples and 160,000 for 2P13, 2P78, and 2Alq3

cal composition.

64 Nanoscaled Films and Layers

salt (Alq3

characteristics) point of view.

**Figure 16.** FTIR spectra of the organic thin films deposited on Si substrate by MAPLE: P13 (curve 1), P78 (curve 2), and Alq3 (curve 3).

The UV-VIS spectra of the P13 and P78 layers deposited by MAPLE on ITO and with an additional Alq<sup>3</sup> layer are presented in **Figure 17(a)** and **(b)**. The structures realized with two organic materials are featured by a great transparency, 60% for λ > 550 nm. Absorption maxima attributed to the electronic π-π\* transitions of the conjugated backbone [80] were evidenced. The absorption maxima characteristic to these materials and for the Alq3, situated at low wavelength are hidden by the ITO substrate [81].

The Alq<sup>3</sup> is frequently used in OLED applications due to its emission properties. The photoluminescence spectra of oligomers and Alq3 , obtained at 350 nm excitation wavelength are given in **Figure 17(c)** and **(d)**. The emission band with the maximum situated at 523 nm is attributed to the presence of the Alq3 meridional stereoisomer [80]. P13 oligomer showed two emission maxima (490 and 525 nm), while P78 discloses a raw maxima at 500 nm [80], the prepared films preserving the emission properties of the start materials.

**Figure 17.** Transmission spectra (a, b) and photoluminescence spectra (c, d) of the organic thin films based on arylenevinylene oligomer (P13 (a, c) and P78 (b, d)) deposited on glass/ITO substrate by MAPLE. For transmission spectra: glass/ITO (curve 1), glass/ITO/arylenevinylene oligomer (curve 2), and glass/ITO/arylenevinylene oligomer/ Alq3 (curve 3). For photoluminescence spectra: glass/ITO/arylenevinylene oligomer (curve 1), glass/ITO/Alq3 (curve 2), and glass/ITO/arylenevinylene oligomer/Alq3 (curve 3).

Different topographies of the films deposited by MAPLE are presented in **Figure 18**. The globular morphology is characteristic to MAPLE process [2, 11]. The RMS values obtained by interpolation of a single layer deposited on ITO evidenced that the P78 film (24.6 nm) presents a higher roughness compared with P13 (8.2 nm) (**Table 2**).

The UV-VIS spectra of the P13 and P78 layers deposited by MAPLE on ITO and with an

organic materials are featured by a great transparency, 60% for λ > 550 nm. Absorption maxima attributed to the electronic π-π\* transitions of the conjugated backbone [80] were evidenced. The absorption maxima characteristic to these materials and for the Alq3, situated at

in **Figure 17(c)** and **(d)**. The emission band with the maximum situated at 523 nm is attributed

maxima (490 and 525 nm), while P78 discloses a raw maxima at 500 nm [80], the prepared

**Figure 17.** Transmission spectra (a, b) and photoluminescence spectra (c, d) of the organic thin films based on arylenevinylene oligomer (P13 (a, c) and P78 (b, d)) deposited on glass/ITO substrate by MAPLE. For transmission spectra: glass/ITO (curve 1), glass/ITO/arylenevinylene oligomer (curve 2), and glass/ITO/arylenevinylene oligomer/ Alq3 (curve 3). For photoluminescence spectra: glass/ITO/arylenevinylene oligomer (curve 1), glass/ITO/Alq3 (curve 2),

layer are presented in **Figure 17(a)** and **(b)**. The structures realized with two

, obtained at 350 nm excitation wavelength are given

meridional stereoisomer [80]. P13 oligomer showed two emission

is frequently used in OLED applications due to its emission properties. The photolu-

additional Alq<sup>3</sup>

66 Nanoscaled Films and Layers

The Alq<sup>3</sup>

low wavelength are hidden by the ITO substrate [81].

films preserving the emission properties of the start materials.

minescence spectra of oligomers and Alq3

and glass/ITO/arylenevinylene oligomer/Alq3 (curve 3).

to the presence of the Alq3

The samples made with a doubled number of pulses present RMS values comparable with that obtained for the samples realized with 80,000 laser pulses. A decrease in the RMS values was observed for the samples based on P78 and Alq3 double layers, while an increase was seen for the sample containing P13 and Alq3 (**Table 2**). The higher roughness of P78 seems to favor a better fit of Alq3 molecules.

The I-V characteristics recorded in 0–10 V domain in the dark are presented in **Figure 19**. No rectifying properties were observed for the investigated heterostructures. A good current value (8 × 10−6 A at 1 V) was evidenced in the sample based on P78 and Alq3 with thicker layers. The lowest current value was presented by the sample with Alq<sup>3</sup> layer realized at 160,000 laser pulses.

**Figure 18.** AFM images of the organic thin films deposited on glass/ITO substrate by MAPLE: glass/ITO/1P13 (a), glass/ ITO/1P78 (b), glass/ITO/1Alq3 (c), glass/ITO/1P13/1Alq3 (d), and glass/ITO/1P78/1Alq3 (e).


**Table 2.** The RMS values obtained from the AFM on the organic thin films.

**Figure 19.** Current—voltage characteristics, in logarithmic representation, of the heterostructures based on arylenevinylene oligomer deposited on glass/ITO substrate by MAPLE using different no. of pulses (80,000 or 160,000): glass/ ITO/Alq3/Al (curve 1—80,000 pulses, curve 2—160,000 pulses), glass/ITO/P13/Alq3/Al (curve 3—80,000 pulses, curve 4—160,000 pulses, curve 5—160,000 pulses, rev. bias), glass/ITO/P78/Alq3/Al (curve 6—80,000 pulses, curve 7—160,000 pulses).

Regarding the I-V characteristics of the heterostructures based on oligomers, it was remarked that their behavior is different. If, in the heterostructure with P78, a high current value was obtained for thicker layer (ITO/2P78/2Alq3 /Al), for the heterostructure with P13, the higher current value was recorded for thinner layers (ITO/1P13/1Alq3 /Al). No relevant changes are expected in the charge flow because there are no significant differences in the energetic barriers at IOT/P78 and ITO/P13 interfaces (**Figure 20**).

As was supposed by Gao [82], at the Al/Alq3 interface, it can appear as a dipole layer, marked by a potential shift of −0.9 V, which determines a lowering of the Al cathode Fermi level to −5.2 eV, close to the Alq<sup>3</sup> HOMO level. The compounds have energy level positions that determine ohmic behavior or the appearance of the space charge limited currents (SCLC).

Analyzing at 1 V applied voltage, it is observed that in the heterostructure with Alq3 single layer formed with two layers, the current increases from 1.5 × 10−12 A for the ITO/2Alq3 / Al structure at 7 × 10−9 A and at 8 × 10−6 A for the ITO/2P13/2Alq3 /Al structure and for the ITO/2P78/2Alq3 /Al, respectively.

Thin films from oligomers and Alq3 were deposited by MAPLE, and the films preserved the optical properties of the raw powders materials. In the heterostructure containing P78 and Alq3 as stacked layers, the current value can be increased by using a thicker P78 layer.

### **4.2. Polymers based on arylene compounds**

Regarding the I-V characteristics of the heterostructures based on oligomers, it was remarked that their behavior is different. If, in the heterostructure with P78, a high current value was

**Figure 19.** Current—voltage characteristics, in logarithmic representation, of the heterostructures based on arylenevinylene oligomer deposited on glass/ITO substrate by MAPLE using different no. of pulses (80,000 or 160,000): glass/ ITO/Alq3/Al (curve 1—80,000 pulses, curve 2—160,000 pulses), glass/ITO/P13/Alq3/Al (curve 3—80,000 pulses, curve 4—160,000 pulses, curve 5—160,000 pulses, rev. bias), glass/ITO/P78/Alq3/Al (curve 6—80,000 pulses, curve 7—160,000

expected in the charge flow because there are no significant differences in the energetic barri-

by a potential shift of −0.9 V, which determines a lowering of the Al cathode Fermi level to −5.2 eV,

Analyzing at 1 V applied voltage, it is observed that in the heterostructure with Alq3 single layer formed with two layers, the current increases from 1.5 × 10−12 A for the ITO/2Alq3

the optical properties of the raw powders materials. In the heterostructure containing

ohmic behavior or the appearance of the space charge limited currents (SCLC).

Al structure at 7 × 10−9 A and at 8 × 10−6 A for the ITO/2P13/2Alq3

HOMO level. The compounds have energy level positions that determine

as stacked layers, the current value can be increased by using a thicker

/Al), for the heterostructure with P13, the higher

interface, it can appear as a dipole layer, marked

were deposited by MAPLE, and the films preserved

/Al). No relevant changes are

/Al structure and for the

/

obtained for thicker layer (ITO/2P78/2Alq3

As was supposed by Gao [82], at the Al/Alq3

close to the Alq<sup>3</sup>

pulses).

68 Nanoscaled Films and Layers

ITO/2P78/2Alq3

P78 and Alq3

P78 layer.

ers at IOT/P78 and ITO/P13 interfaces (**Figure 20**).

/Al, respectively.

Thin films from oligomers and Alq3

current value was recorded for thinner layers (ITO/1P13/1Alq3

From the first generation of the organic photovoltaic cells realized with a single organic layer between anode and cathode [83], numerous attempts were made to improve the final cell parameters, either by developing new materials (as polymers with special properties) or using different cell architectures.

MAPLE method was used to obtain thin films from arylene-based polymers. The used polymers were poly[N-(2-ethylhexyl)2.7-carbazolyl vinylene]/AMC16 and poly[N-(2-ethylhexyl)2.7 carbazolyl 1.4-phenyleneethynylene]/AMC22 with the chemical structure presented in **Figure 21**. The same laser source mentioned above was used for the MAPLE deposition. The chloroform was used to obtain a target with 3 g/l concentration. We have deposited thin films on ITO, ITO covered with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and Si substrates in the following experimental conditions: 250 mJ/cm2 laser fluence and 30,000 laser pulses [84].

**Figure 20.** Band diagrams of the heterostructures based on arylenevinylene oligomer deposited on glass/ITO substrate by MAPLE: ITO/P13/Alq3/Al (a) and ITO/P78/Alq3/Al (b).

**Figure 21.** Chemical structures of the arylene polymers: AMC16 (a) and AMC22 (b).

The UV-VIS spectra (**Figure 22a**) and (**b**) of the organic film deposited by MAPLE were given in comparison with drop-cast realized films on glass substrate. A high transmittance is observed both for MAPLE films and for the drop-cast coatings (**Figure 22a**). The absence of the absorption maxima from 375 nm in the layers prepared by MAPLE (observed for the films deposited by drop-casting) is attributed to some modifications in the electronic structures of the polymeric films, determined by some differences in the arrangement of the molecules on the substrate surface. If for the preparing drop-cast film, the solvent is evaporated at room temperature, in MAPLE the solvent is thermally evaporated during the film deposition [85, 86]. Some cluster can appear, and this can affect the polymer backbone configuration [87, 88]. The polymers deposited by MAPLE on ITO and ITO/PEDOT:PSS (**Figure 22b**) exhibit a lower transmittance compared with the same layers prepared on glass.

The AFM images of the AMC 16 and AMC 22 films prepared by MAPLE on ITO and on ITO/PEDOT:PSS are given in **Figure 23**. The AMC22 polymer (with a reduced conjugated chain) forms layers with an increased roughness (RMS = 42 nm), when it is deposited on ITO/ PEDOT:PSS substrate compared with the same layer deposited on ITO (RMS = 36 nm). In the case of AMC 16 with longer conjugated length, an increased roughness was obtained for the sample made on simple ITO substrate (RMS = 49.5 nm) compared with the sample realized on ITO/PEDOT:PSS substrate (RMS = 37 nm).

The I-V characteristics plotted in the dark and under illumination put into evidence the appearance of the photovoltaic effect in the polymeric films realized on ITO covered by an additional PEDOT:PSS layer (**Figure 24**). A higher current density (2.5 × 10−9 A/cm2 ) was obtained in the dark at 0.5 V for the structure prepared with AMC 16 compared with that made with AMC 22 (~3 × 10−10 A/cm2 ). The best photovoltaic parameters (open-circuit voltage-VOC, short-circuit current-ISC, and fill factor-FF) were shown by the cells based on AMC 16 (VOC = 0.303, ISC = 12.7 × 10−9 A, and FF = 29%) compared with the cell based on AMC22 (VOC = 0.073, ISC = 8.2 × 10−11 A, and

**Figure 22.** Transmission of the organic thin films based on arylene polymers (AMC16 and AMC22) deposited on glass (a), glass/ITO (b), and glass/ITO/PEDOT-PSS (b) substrates by drop-cast (only on glass) and MAPLE. On glass—AMC16 (curve 1—drop cast, curve 3) and AMC22 (curve 2—drop cast, curve 4), on glass/ITO—AMC16 (curve 1) and AMC22 (curve 3), on glass/ITO/PEDOT-PSS—AMC16 (curve 2) and AMC22 (curve 4).

FF = 25.9%). This sustains that a better collection of the charge appears in the cell structure realized with the polymer having a longer conjugation length. On the other hand, this is the sample with a lower roughness (RMS = 37 nm) of the active layer favoring the charge carrier transport.

The UV-VIS spectra (**Figure 22a**) and (**b**) of the organic film deposited by MAPLE were given in comparison with drop-cast realized films on glass substrate. A high transmittance is observed both for MAPLE films and for the drop-cast coatings (**Figure 22a**). The absence of the absorption maxima from 375 nm in the layers prepared by MAPLE (observed for the films deposited by drop-casting) is attributed to some modifications in the electronic structures of the polymeric films, determined by some differences in the arrangement of the molecules on the substrate surface. If for the preparing drop-cast film, the solvent is evaporated at room temperature, in MAPLE the solvent is thermally evaporated during the film deposition [85, 86]. Some cluster can appear, and this can affect the polymer backbone configuration [87, 88]. The polymers deposited by MAPLE on ITO and ITO/PEDOT:PSS (**Figure 22b**) exhibit a lower

The AFM images of the AMC 16 and AMC 22 films prepared by MAPLE on ITO and on ITO/PEDOT:PSS are given in **Figure 23**. The AMC22 polymer (with a reduced conjugated chain) forms layers with an increased roughness (RMS = 42 nm), when it is deposited on ITO/ PEDOT:PSS substrate compared with the same layer deposited on ITO (RMS = 36 nm). In the case of AMC 16 with longer conjugated length, an increased roughness was obtained for the sample made on simple ITO substrate (RMS = 49.5 nm) compared with the sample realized on

The I-V characteristics plotted in the dark and under illumination put into evidence the appearance of the photovoltaic effect in the polymeric films realized on ITO covered by an additional

dark at 0.5 V for the structure prepared with AMC 16 compared with that made with AMC

current-ISC, and fill factor-FF) were shown by the cells based on AMC 16 (VOC = 0.303, ISC = 12.7 × 10−9 A, and FF = 29%) compared with the cell based on AMC22 (VOC = 0.073, ISC = 8.2 × 10−11 A, and

**Figure 22.** Transmission of the organic thin films based on arylene polymers (AMC16 and AMC22) deposited on glass (a), glass/ITO (b), and glass/ITO/PEDOT-PSS (b) substrates by drop-cast (only on glass) and MAPLE. On glass—AMC16 (curve 1—drop cast, curve 3) and AMC22 (curve 2—drop cast, curve 4), on glass/ITO—AMC16 (curve 1) and AMC22

(curve 3), on glass/ITO/PEDOT-PSS—AMC16 (curve 2) and AMC22 (curve 4).

). The best photovoltaic parameters (open-circuit voltage-VOC, short-circuit

) was obtained in the

transmittance compared with the same layers prepared on glass.

PEDOT:PSS layer (**Figure 24**). A higher current density (2.5 × 10−9 A/cm2

ITO/PEDOT:PSS substrate (RMS = 37 nm).

22 (~3 × 10−10 A/cm2

70 Nanoscaled Films and Layers

**Figure 23.** AFM images of the organic thin films deposited on glass/ITO (a, b) and glass/ITO/PEDOT-PSS (c, d) substrate by MAPLE: AMC16 (a, c) and AMC22 (b, d).

**Figure 24.** Current-voltage characteristics of the heterostructures based on arylene polymers deposited on glass/ITO/ PEDOT-PSS substrate by MAPLE in dark (curves 1) and light (curves 2) conditions: glass/ITO/PEDOT-PSS/AMC16/Al (a) and glass/ITO/PEDOT-PSS/AMC22/Al (b).

Taking into account the presented results, it can be concluded that using MAPLE, polymeric thin films are deposited, characterized by good absorption in the blue-green region of the solar spectrum. The structures realized on ITO/PEDOT:PSS with AMC 16 and AMC 22 present photovoltaic effect, meaning that these materials can be taken into consideration for further applications in the OPV domain.

## **5. Conclusion**

Summarizing, various thin films were deposited by PLD, CPLD, and MAPLE, the laser deposition technique being chosen in accordance with the material type (TCO or soft organic materials).

TCO films as ITO, AZO, and IZO are prepared by PLD on plastic substrate that presents a high transparency (~95%) and a reduced electrical resistivity (5 × 10−4 Ωcm), and the characteristics are very useful for integrating them in flexible electronics. An important parameter in the PLD experiments is the target-substrate distance. At a higher distance (8 cm), films free of cracks with a high transmittance and a reduced roughness were obtained. The exhibited electrical and optical properties are very good; all depositions were performed at room temperature without heating of the substrates.

Inx Zn1−xO films were obtained by CPLD, using two targets with atomic In concentration In/ (In + Zn), of 28 at.% and 56 at.% or 42 at.% and 70 at.%. The layers were analyzed from optical, electrical, and morphological point of view. We evidenced a high optical transmission, >95%. The lowest resistivity (8.6 × 10−4 Ωcm) was observed for an In content of about 44–49 at%. This technique is useful in the deposition of materials with different composition, each sample yielding practically a library of data, avoiding in this way the unnecessary loss of time. It is observed that the roughness of the samples decreases with the increase of the In content.

Layers based on arylenevinylene oligomers (P13, P78) and Alq3 were transferred by MAPLE without any material deterioration. Organic heterostructures with one or two organic layers were deposited. The optical properties of the start compounds were preserved. The prepared organic films present a good transmittance in the visible domain and the emission bands characteristic to the oligomers and to the Alq3. The globular morphology characteristic to MAPLE process was remarked. The I-V characteristics were symmetric, and the injector contact behavior was evidenced for the prepared heterostructures. For ITO/P78/Alq3 /Al heterostructures, the I-V plots evidenced dependence between the current value and the thickness of the organic layers.

Thin films from new arylene-based polymers were also processed by MAPLE. A good absorption was evidenced in the blue-green domain of the solar spectrum for the samples prepared with each polymer. The appearance of the photovoltaic effect was remarked for the heterostructures based on AMC16 film and AMC22 film deposited on ITO covered by a thin layer of PEDOT:PSS, which confirm that this buffer layer favors the charge carrier collection. The heterostructure based on the polymer with longer conjugation length present the higher dark current density. Due to their optical and electrical properties, such organic heterostructures can be interesting for OPV applications.

## **Acknowledgements**

Taking into account the presented results, it can be concluded that using MAPLE, polymeric thin films are deposited, characterized by good absorption in the blue-green region of the solar spectrum. The structures realized on ITO/PEDOT:PSS with AMC 16 and AMC 22 present photovoltaic effect, meaning that these materials can be taken into consideration for fur-

Summarizing, various thin films were deposited by PLD, CPLD, and MAPLE, the laser deposition technique being chosen in accordance with the material type (TCO or soft organic

TCO films as ITO, AZO, and IZO are prepared by PLD on plastic substrate that presents a high transparency (~95%) and a reduced electrical resistivity (5 × 10−4 Ωcm), and the characteristics are very useful for integrating them in flexible electronics. An important parameter in the PLD experiments is the target-substrate distance. At a higher distance (8 cm), films free of cracks with a high transmittance and a reduced roughness were obtained. The exhibited electrical and optical properties are very good; all depositions were performed at room temperature

Zn1−xO films were obtained by CPLD, using two targets with atomic In concentration In/ (In + Zn), of 28 at.% and 56 at.% or 42 at.% and 70 at.%. The layers were analyzed from optical, electrical, and morphological point of view. We evidenced a high optical transmission, >95%. The lowest resistivity (8.6 × 10−4 Ωcm) was observed for an In content of about 44–49 at%. This technique is useful in the deposition of materials with different composition, each sample yielding practically a library of data, avoiding in this way the unnecessary loss of time. It is observed that the roughness of the samples decreases with the increase of the

Layers based on arylenevinylene oligomers (P13, P78) and Alq3 were transferred by MAPLE without any material deterioration. Organic heterostructures with one or two organic layers were deposited. The optical properties of the start compounds were preserved. The prepared organic films present a good transmittance in the visible domain and the emission bands characteristic to the oligomers and to the Alq3. The globular morphology characteristic to MAPLE process was remarked. The I-V characteristics were symmetric, and the injector contact

tures, the I-V plots evidenced dependence between the current value and the thickness of the

Thin films from new arylene-based polymers were also processed by MAPLE. A good absorption was evidenced in the blue-green domain of the solar spectrum for the samples prepared with each polymer. The appearance of the photovoltaic effect was remarked for the heterostructures based on AMC16 film and AMC22 film deposited on ITO covered by a thin layer of PEDOT:PSS, which confirm that this buffer layer favors the charge carrier collection. The

/Al heterostruc-

behavior was evidenced for the prepared heterostructures. For ITO/P78/Alq3

ther applications in the OPV domain.

without heating of the substrates.

**5. Conclusion**

72 Nanoscaled Films and Layers

materials).

Inx

In content.

organic layers.

The authors thank to the group from ICMPP Iasi (led by Dr. M. Grigoras) for the synthesis of the oligomers and of the polymers. The work has been funded by the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, projects TE 188/2014, PN-II-RU-TE-2014-4-1590 and the National Authority for Research and Innovation in the frame of Core Program - contract 4N/2016 and contract PN16-480102.

## **Author details**

Marcela Socol1 \*, Gabriel Socol2 , Nicoleta Preda1 , Anca Stanculescu1 and Florin Stanculescu3

\*Address all correspondence to: marcela.socol@infim.ro


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