**Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications**

Marcela Socol, Nicoleta Preda, Anca Stanculescu, Florin Stanculescu and Gabriel Socol Florin Stanculescu and Gabriel Socol Additional information is available at the end of the chapter

Marcela Socol, Nicoleta Preda, Anca Stanculescu,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67702

#### **Abstract**

Multilayer or blend heterostructures based on porphyrins and phthalocyanines were obtained on different substrates using VTE and MAPLE methods. Stacked structures based on ZnPc and C60 with NTCDA were prepared by VTE on ITO/glass, their cur‐ rent value being increased by the deposition of the materials in an inverted configu‐ ration or by using ITO/PEDOT:PSS as a substrate. Multilayer structures comprising ZnPc and NTCDA were fabricated by MAPLE on an AZO/glass. Treating the AZO in oxygen plasma, a higher current value was obtained for the deposited heterostructures. The oxygen plasma treatment can increase the work function of the AZO resulting in a decrease of the energetic barrier from AZO/organic interface and finally improving the charge transport. Stacked layers or blend heterostructures having ZnPc, MgPc and TPyP were deposited by MAPLE on ITO/PET. In the case of those containing MgPc and TPyP, an increase in the current value (in dark) was obtained for the blend compared to the stacked layer configuration. For those with ZnPc and TPyP, under illumination, a pho‐ tovoltaic effect was observed for the blend structure. All heterostructures are featured by a large absorption in the visible domain of the solar spectrum and suitable electrical properties for their use in OPV applications.

**Keywords:** ZnPc, TPyP, MgPc, VTE, MAPLE

### **1. Introduction**

During the last years, the organic materials have attracted the attention of researchers because they can be used in different types of applications: organic photovoltaic (OPV) cells, organic‐ based light‐emitting devices (OLEDs) and organic field effect transistors (OFETs) [1–6]. Heliatek reports a conversion efficiency of about 13.2% for an OPV fabricated by vacuum

© 2016 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. © 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.

evaporation and having three absorbers [7]. OLEDs are already integrated in commercially available devices such as mobile phone displays, TV sets, etc.

The field of organic materials for applications in photovoltaic cells has begun in 1906 and 1913 with the observation of the anthracene photoconductivity [8, 9]. Kearns introduced, in 1958, the first organic photovoltaic cell with a film based on magnesium phthalocyanine (MgPc) [10]. In 1986, Tang makes a step forward for the OPV, fabricating a photovoltaic cell with two organic layers in configuration donor/acceptor (D/A) using copper phthalocyanine (CuPc) as a donor and perylenediimide (PDI) as an acceptor [11]. Since then the photovoltaic effect (PV) was reported in different organic compounds such as porphyrins, phthalocyanines or their derivatives [12]. The organic materials are part from the third generation of photovoltaic, after those based on inorganic materials (the first and second generation).

Comparatively with the inorganic compounds, the organic materials present the following advantages: they can be deposited at low temperature (decreasing in this way the processions costs), are compatible with plastic substrates (a good premise for the flexible electronics) and their properties can be tuned by various processing techniques which allow their deposition even on a large area. In photovoltaic cells, organic compounds present absorption coefficients greater than 105 cm−1 allowing an increased absorption of the incident light even under 100 nm [13]. The light collection efficiency is dependent on the organic active layer thickness and the absorption properties of the used materials [14].

Porphyrins and phthalocyanines are the most used organic compounds as active layers in photovoltaic cells due to their several absorption maxima in the visible part of the solar spectrum (less than 700 nm [14, 15]). Furthermore, in the porphyrins and derivatives, the range of absorption spectrum in the near infrared part can be increased due to the extended conjugation [14].

The impact of the porphyrins and phthalocyanines on the OPV domain can be evaluated, as shown in **Figure 1**, which contains the histograms with the publications number (from ISI web of science) from the last 5 years (2012 to 2016) having as subject porphyrins or phthalocyanines

**Figure 1.** Number of publications per year in the last five years having as topic porphyrins or phthalocyanines and solar cell.

and solar cells. Moreover, it has to be mentioned that there is a journal entirely dedicated to these organic compounds.

evaporation and having three absorbers [7]. OLEDs are already integrated in commercially

The field of organic materials for applications in photovoltaic cells has begun in 1906 and 1913 with the observation of the anthracene photoconductivity [8, 9]. Kearns introduced, in 1958, the first organic photovoltaic cell with a film based on magnesium phthalocyanine (MgPc) [10]. In 1986, Tang makes a step forward for the OPV, fabricating a photovoltaic cell with two organic layers in configuration donor/acceptor (D/A) using copper phthalocyanine (CuPc) as a donor and perylenediimide (PDI) as an acceptor [11]. Since then the photovoltaic effect (PV) was reported in different organic compounds such as porphyrins, phthalocyanines or their derivatives [12]. The organic materials are part from the third generation of photovoltaic, after

Comparatively with the inorganic compounds, the organic materials present the following advantages: they can be deposited at low temperature (decreasing in this way the processions costs), are compatible with plastic substrates (a good premise for the flexible electronics) and their properties can be tuned by various processing techniques which allow their deposition even on a large area. In photovoltaic cells, organic compounds present absorption coefficients

[13]. The light collection efficiency is dependent on the organic active layer thickness and the

Porphyrins and phthalocyanines are the most used organic compounds as active layers in photovoltaic cells due to their several absorption maxima in the visible part of the solar spectrum (less than 700 nm [14, 15]). Furthermore, in the porphyrins and derivatives, the range of absorption spectrum in the near infrared part can be increased due to the extended

The impact of the porphyrins and phthalocyanines on the OPV domain can be evaluated, as shown in **Figure 1**, which contains the histograms with the publications number (from ISI web of science) from the last 5 years (2012 to 2016) having as subject porphyrins or phthalocyanines

**Figure 1.** Number of publications per year in the last five years having as topic porphyrins or phthalocyanines and solar cell.

cm−1 allowing an increased absorption of the incident light even under 100 nm

available devices such as mobile phone displays, TV sets, etc.

86 Phthalocyanines and Some Current Applications

those based on inorganic materials (the first and second generation).

absorption properties of the used materials [14].

greater than 105

conjugation [14].

Photovoltaic cells based on porphyrins with high performances were achieved. Thus, in 2011 Yella et al. reported 12.3% efficiency for a structure with a zinc porphyrin (YD2‐oC8) co‐sen‐ sitised with Y123 deposited on a TiO2 [16]. Also, in 2014, a conversion efficiency of about 13% was obtained for a porphyrin dye, coded SM315 [17]. In 2015, a teoretical study made for a new porphyrin‐based molecular complex shows that an open circuit voltage of about ~1.8 V can be obtained using this kind of materials [18].

Additionally, the bioinspired structures of porphyrins can be attractive in different forms (nanoparticles, nanosheets, nanorods and nanorings, nanowires, nanotubes, aggregates) as summarised by Monti et al. [19] in applications as catalysts (for O2 reduction or H<sup>2</sup> O oxidation [20]), sensors [21], in photodynamic therapy as photosensitizers [22], for drug delivery [23] and for the treatment of tumours [24].

One of the most important advantages of the phthalocyanines over other organic materials is their increased value of the exciton diffusion length, which is usually in the range of 10 nm [25]. Thus, for CuPc a diffusion length of about ~68 nm was reported [26]. Increased cell per‐ formances (efficiency) were also recorded for the OPV based on phthalocyanines: 3.6% for a double layer cell with CuPc and C60 [27], 4.2% for a structure with 1,4,8,11,15,18,22,25‐octahex‐ ylphthalocyanine (C6PcH2 ) and [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) prepared by spin‐coating [28], 5% for a cell also with CuPc:C60 [29] and the highest reported efficiency of about 5.7 % was achieved also for a structure based on CuPc and C60 [30]. And in the field of the perovskite cells was reported an increased efficiency (11.75%) for a structure containing a ZnPc thin film as a donor material [31].

Complementary, the phthalocyanines and their derivatives have wide range of applications such as OLEDs, gas sensors and optical communications [32]. These compounds are promising candidates in the non‐linear optical devices due to their large third‐order non‐linearity [32, 33]. They are also used in therapy of cancer, infectious or neurodegenerative diseases [34] and in the xerographic photoreceptors of laser printers due to the strong Q‐band absorption [35].

In this chapter, we summarised some of our results regarding the preparation and characteri‐ sation of porphyrins and metallic phthalocyanine layers for applications in OPV. These mate‐ rials were obtained as thin films (in multilayer structures or blends) on solid (glass coated with indium tin oxide‐ITO or aluminium‐doped zinc oxide‐AZO) or on a flexible substrate (polyethylene terephthalate‐PET coated with ITO).
