**3.4 Metal tellurides**

The minimum studied materials among the chalcogen members *via* SILAR technique are metal tellurides because of the unavailability of the appropriate anionic precursors. Na2TeO3 or ethanolic Te or TeO2 with NaBH4 is the mostly used anionic precursor performed as the source of Te<sup>2</sup> to form tellurides. For example, 0.1 M CuSO4. 5H2O (pH: 5) and 0.05 M Na2TeO3 (pH: 9) react at an ambient temperature to synthesize Cu2Te film. Till now, CdTe, Cu2Te, La2Te3, Cu7Te4, and Bi2Te3 thin films were fabricated and investigated *via* SILAR technique having potential uses in case of radiation detectors, photovoltaics, and thermo-electric devices [63–65]. More scientific research is expected to understand and control the characteristics of such fabricated films to build outstanding optoelectronic devices.

### **4. Recent advances by SILAR**

The optoelectronic properties of SILAR grown thin films have been demonstrated in many more applications, for example, supercapacitors, photovoltaics, photoelectrochemical water splitting, gas sensors, and many more. The technique seems to be simpler and represents an efficient way to fabricate devices. Three potential applications such as supercapacitors, photovoltaics, and photoelectrochemical water splitting will be discussed in the following section.

#### **4.1 Supercapacitors**

The rapid progress in state-of-the-art tools has guided to a profound reliance on energy storage devices. Satellites, electric vehicles, laptops, cellphones, and sensors need some species of energy storage to function properly. The lead-acid battery was the first device, discovered around the 1800s, and most common storage energy till today. Supercapacitors, another promising energy storage device, well known as electrochemical capacitor or ultracapacitor creates a gap bridging role between conventional capacitors and batteries [74]. They can offer 1 2 orders of higher magnitude of power density than rechargeable batteries as well as supply much more energy than traditional dielectric capacitors.

A supercapacitor works following two-charge storage mechanisms: (i) surface ion adsorption such as electric double-layer capacitance (EDLC) and (ii) redox reactions such as pseudo capacitance. Supercapacitors reveal an extraordinary set of features in comparison with batteries, for instance, high-power density, low maintenance cost, reliable cycling life, fast rates of charge or discharge, and safe operation as well as offer versatile powering solutions to many appeals ranging from portable consumer electronic appliance and electric automobiles to large-scale smart utility grids. Nevertheless, carbon-based EDLC supercapacitors show very low energy densities, which are limited through the finite electrical charge separation at the interface of electrolyte and electrode materials, as well as the approachability of surface area. Consequently, efforts to surge the energy densities of supercapacitors have involved the application of better pseudo-capacitance electrode supplies, equipped by conducting polymers and nanostructured metal oxides, bearing the low cost of high-power density as well as chemical stability, which have the significance of phase changes and faradaic reactions in it [74].

Several types of metal oxides, sulfides, and tellurides have been used in supercapacitor device fabrication so far, by utilizing the ever-fast-growing technique SILAR as summarized in **Tables 3** and **4**. Initially the single metal oxides or sulfides such as CuO, NiO, NiMoO4, WO3, Bi2O3, Mn3O4, or MnS have been prepared by following SILAR technique and then tested for the supercapacitor behaviors to acquire the results of specific capacitance with their retention stability using cyclic voltammetry (CV) with the assistance of 3-electrode measurement system. Higher capacitance was attained at the lower scan rate and/or lower current density during such measurements and usually a relatively small quantity of electrochemical active material was developed atop of the working electrode. Moreover, the performance found using the 3-electrode system is higher than 2-electrode test cells, and the latter can be either a symmetric (S) or asymmetric (A) cell. Generally, in a symmetric cell both positive and negative electrodes are alike, whereas they are different active materials in an asymmetric cell.

*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*


**Table 3.**

*Properties of various electrode materials deposited by SILAR for electrochemical capacitors by 3-electrode system.*

Not only binary, but also ternary or even doped metal oxides, sulfides or tellurides were synthesized *via* SILAR for supercapacitor device application. For example, ZnCo2O4 and ZnFe2O4 were synthesized via SILAR technique from binary cationic solutions in the presence of Zn and Co (or Fe) precursors and demonstrate high energy density of 9.67 and 28 Wh kg�<sup>1</sup> as well as power density of 1451 and 7970 W kg�<sup>1</sup> , respectively. La2S3 and La2Te3 with mesoporous pine-leaf structure prepared with SILAR showed 35 and 60 WhKg�<sup>1</sup> energy density and power density of 1260 and 7220 WKg�<sup>1</sup> , respectively. A flexible La2Te3jLiClO4-PVAjLa2Te3 supercapacitor cell was further fabricated and is represented as in **Figure 3**.

Hybrid supercapacitors, EDLC and pseudo-capacitance, build of charge storage mechanisms reduce the superior features of the device. On the other hand, among the other EDLC electrode materials multiwalled carbon nanotubes (MWCNTs) fascinated major interest due to their favorable features such as high surface area and mesoporous network, good mechanical strength and flexibility, excellent electrical conductivity, and chemical stability. The facile synthesis of composites of metal oxides with carbon materials was facilitated by SILAR as well. For instance, the fabrication of NiO/ MWCNTs nanohybrid thin films *via* SILAR and the specific capacitance was as high as


#### **Table 4.**

*Supercapacitors performance of SILAR grown films measured by 2-electrode system.*

#### **Figure 3.**

*Schematic diagrams of La2Te3*j*LiClO4-PVA*j*La2Te3 supercapacitor device [65].*

1727 Fg�<sup>1</sup> and current density 5 mAcm�<sup>2</sup> with 91% retention ability after 2000 cycles as demonstrated in the **Figure 4** [84]. Moreover, an analogous synthesis style was employed to NiCoO*x*/Carbon-black hybrid thin films accomplishing coatings with a high specific capacitance of 1811 Fg�<sup>1</sup> at 0.5 mAcm�<sup>2</sup> [85].

Therefore, SILAR is a unique as well as multipurpose technique to fabricate thin films for supercapacitor device application with superior power and energy densities in comparison with other available and more conventional deposition techniques, justifying the quality of the SILAR growth thin films.

*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*

#### **Figure 4.**

*Assembly of highly flexible symmetric NiO/MWCNTs-NiO/MWCNTs nanohybrid device: (a) image of flexible NiO/MWCNTs thin film deposited on the stainless steel, (b) NiO/MWCNTs nanohybrid thin film electrodes with closed ends (2 3 cm<sup>2</sup> area), (c) coating of the electrode by PVA/LiClO4 gel electrolyte, (d) flexible supercapacitor built under the 1 ton pressure through sandwiching the two-gel electrolyte coated electrodes, (e) two flexible supercapacitors in series can successfully light a LED [84].*

#### **4.2. Solar cells**

Photovoltaic (PV) is a simple device, which promotes the direct conversion of light radiation into electrical energy by following the photovoltaic effect [91]. The discovery of such a device for the conversion of sunlight radiation directly into electricity was first carried out during the late 1800s. C. Fritts first demonstrated the solid-state PV by fabricating a thin layer of Au on Se semiconductor material [92]. At American Telephone and Telegraph Bell Laboratory the modern PV was discovered by Ohl in 1946 [93] but demonstrated by Chapin, Fuller and Pearson in 1954 [94]. The cell was fabricated by single-crystal Si wafer having an efficiency of 5%.

At present for feasible use, extensive research is going for efficiency enhancement of solar cells, as the efficiency of solar cells is one of the very vibrant parameters to promote this technology. Over the years, the efficiency of single crystal-Si solar cells has shown a sound development. In 1950s, it was only 15% and nowadays it is improved to around 26.7% [95]. The commercial efficiency of Si solar cell is approaching in between 12% and 15%, while the theoretical Shockley-Queisser (SQ) limit energy conversion efficiency is of around 28% [96]. The PV cell and module market was mostly occupied on first-generation Si-based cells until 2004, for example, sc- and poly-Si cells, which covered about 85% of the overall international PV modules market. In the meantime, thin film cells or second-generation PV have exhibited great advantages, for instance, the ease of large area fabrication and usage of minimum materials, though their market share was much smaller in comparison with the Si-cells [97]. After 2005, the developments were spurred by the sharp increase in the country's implementation of solar energy due to the rapid advancement of the PV production industry in China. The price of PVs is generally supplemented by the strict requirement for fabricating high-purity materials such as GaAs and Si, or the rareearth elements such as CIGS. The element, In, is rare and can be certainly exhausted, which might affect the prospect of such PVs. Later, CdTe thin-film PVs have increased longing in the market of South-Eastern countries. But Cd has a serious environmental distress, which is due to its high toxicity [98, 99]. For example, chronic Cd exposure breeds an extensive acute and chronic effects in humans [100, 101]. Moreover, Cd is a rare earth element and it will also generate a higher cost within the demand in future. Further, a third generation of devices has newly developed in addition to the thin-film solar cells, based on fresh organic materials such as dyesensitized solar cells (DSSC) [102], quantum dot solar cells (QDSC), perovskites, bulk heterojunctions, having innovative device architectures with the usage of multiple exciton generation, upconverting layers, and others. Though, organic materials-built PVs have small life spans as the nature of the materials, for example, thermal stability [103] or concerns of electrolyte-based variability [104]. Inherently, a mostly striking new field of PV devices using metal oxide (MO) semiconductors has performed [105]. Atop of the MO thin films, favorable next-generation PV cells such as exciting thin absorber cells [106], DSSC [107], and QDSC [46, 108] are built as they are promising applicants for being stable, eco-friendly, and ultra-low-cost PV materials.

SILAR accounts itself directly into the third generation, by affording ultra-thin, compositionally fabricated by layers of several semiconductors that could be subjugated in a diversity of device architectures. Besides Si, most of the absorbers in PVs are conventionally II–VI, III–V semiconductors, or organic polymers, or small molecules, or perovskites. Still, the number of metal oxides is not adequate, which can be effectively used as absorber layers. Consequently, research into SILAR-grown light absorber layers for PV device applications has been aimed mainly on selective transition metal oxides, sulfides, and selenides. These materials have drawn incredible interest in technological and scientific research due to their unique optical, electrical, and mechanical properties in the past few decades [109–111]. Nevertheless, there are some examples of SILAR grown metal oxides, sulfides, and selenides applied in different types of solar cells such as thin film, DSSC, perovskite, and QDSC as summarized in **Table 5**. The layers were used not only to achieve high efficiency, but also served diverse roles inside the PVs such as light absorber, selective charge transport (electrons or holes), and passivation.

In many cases, the core light-absorbing layers, within the solar cells, fabricated *via* SILAR have been investigated. For example, in ITO/CdS/PbS/C heterojunction solar cell, n-type layer, CdS thin films were deposited by CBD on transparent conductive oxide (ITO) substrates, whereas PbS film by SILAR using different deposition cycles, 15, 20, 30, 40 and 60 to obtain different thicknesses, showed that 40 cycles PbS film has a greater photovoltaic conversion efficiency [115]. In another study, *p*-type CuO was utilized as photo-absorber in the p-CuO/n-Si heterojunction cell [123], where a vibrant role of the SILAR deposition was observed in the overall device performance, depending mainly on the concentration of the copper precursor solution. In an all-oxide solar cell, NiO/Cu2O or CuO/ZnO/SnO2, both Cu2O and CuO fabricated were examined as light absorber fabricated by SILAR and the hole transporting layer (*p*-type NiO), buffer layer (ZnO) as well as *n*-type SnO2 were deposited by sol-gel method [124]. The cell having Cu2O showed better performance than CuO, which is due to the reduced conductivity, mobility, and carrier concentration of CuO. However, the study showed an overall

### *Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*


#### **Table 5.**

*SILAR growth PV cells demonstrating with the cell properties.*

efficiency over 1%. In a different study, heterojunction solar cells have been fabricated between layers of p-type CuS and n-type Ag2S deposited *via* SILAR method and Sn2+ and Al3+ heterovalent dopants are introduced in Ag2S so that Fermi energy of the semiconductor can be modified to alter the band diagram of pn junctions. The Sn2+ doped Ag2S resulted in better solar cell parameters with an efficiency of 2.85% as compared to that based on Al3+-doped Ag2S, which consists of many defect states due to mismatch in ionic radii of the cation and the dopant ions [44].

Further, metal oxides were worked in charge transportation in between different layers in solar cells, and both electron transport layer (ETL) and hole transport layer (HTL) can enhance the performance of PVs. Since the early 1990s, TiO2 is one of the key materials used as ETL owing to its wide popularity in DSSC [120]. In a recent study, TiO2 nanocrystalline film was directly deposited using SILAR at 90°C for

perovskite solar cell applications and used as an ETL [114]. Due to the fast charge transport, kinetics and slow charge recombination process of the TiO2 ETL synthesized from the solutions of TiCl4 and hot K2S2O8, with subsequent annealing at 450°C, advances the efficiency of the cell to around 10%. Further, a couple of studies showed the deposition of TiO2 layers from solutions of TiCl3 and NaOH [27, 125] followed by annealing at 400°C, as ETLs in DSSC with the modest efficiency of just over 1%. Other SILAR-fabricated layers used as ETLs in PVs consist of ZnO and ZrO2 as interfacial layer attached to porous TiO2, both demonstrated performance in DSSC [102, 126].

In this study, Cu2O thin films were introduced as a HTL in a planar perovskite solar cell and successfully enhanced the efficiency of the cell to around 8.23%, as shown in **Figure 5**(a-c). The Cu2O films were deposited *via* SILAR by followed the complexation reaction of copper and ammonia with H2O2 [112]. The methylammonium lead triiodide (MAPbI3) perovskite layer is sandwiched between a *p*-type Cu2O HTL layer and another *n*-type PCBM (phenyl-C61-butyric acid methyl ester) ETL layer, respectively. The Cu2O films demonstrated suitable band structure after annealing at 170°C and boosted device performances better than conventional sol-gel-deposited NiO and Cu-doped NiO hole transport layers, confirming the quality of the SILAR-Cu2O.

In a report of 2009, Lee et al. showed a novel technique for preparing selenide (Se<sup>2</sup>) by the SILAR process in pursuit of efficient QD-sensitized solar cells atop of mesoporous TiO2 photoanodes. After several optimization of the QD-sensitized TiO2 films *via* regenerative photoelectrochemical cells in presence of a cobalt redox couple [Co(o-phen)3 2+/3+], with a final layer of CdTe, the overall efficiencies of the was recorded around 4.2% at 100 Wm<sup>2</sup> [127]. To find the answer to a question, "How does a SILAR CdSe film grow?" Becker et al. tuned the deposition steps to suppress interfacial charge recombination in FTO/TiO2/CdSe/Na2S: S/CoS2 cell by showing an efficiency of 3.53% [117]. Recently, in another report, SILAR- and CBD-grown CdSe-sensitized TiO2 solar cells were examined concentrating on the influences of two commonly used QD deposition techniques [119], and atop of pre-accumulated CdS seed layers, a successful CdSe deposition was performed. The PEC of both the cells has been recorded as 4.85%, for CBD grown CdS/CdSe cell, whereas for SILAR grown cell the value was 3.89%. One research group enhanced the PCE of CdS/CdSe/S2-S/RGO/Cu2S cell to 5.4% by employing Mn2+ doping of CdS *via* SILAR method [118], whereas another group reported on a PbS: Hg QD-sensitized solar cell by Hg2+ doping into PbS employing similar deposition technique and showed an unprecedentedly high JSC of 30 mA/cm2 with the PEC of 5.6% [120]. More studies are ongoing with great efforts to find new alternative, clean, and environment-friendly energy resources due to the increasing demands.

#### **Figure 5.**

*(a) Cell structure, (b) schematic energy level diagram; the dashed line represents the Fermi energy after contact (c) current-voltage characteristics of under dark and a white light illumination condition of Cu2O/MAPbI3/PCBM heterojunction [112].*
