**5. Organic solar cells based on thin polymer films**

The technique of formation of thin films of polyanilines and fullerene-containing polymers by vacuum deposition from a Knudsen effusion cell was used [36]. The length of the cylindrical

**Figure 6.** (a) An energy level diagram of the PANI/FCM system; (b) process of photon absorption and charge separation in this structure; (c) multilayer film structure of OSC.

cell was 25 mm, the internal diameter was 4 mm and the working temperature varied within the range 500–650 K. Thermal heating of fullerene-containing monomers (FCMs) during deposition led to their polymerization. Some thin films were formed by the spin coating technique from a solution of fullerene-containing monomers. All the obtained films were uniform in thickness, and their conductivity was about 0.1–1.0 mS/cm.

The analysis of the dependences obtained in this study allows the assumption that the main mechanism of charge carrier transfer through the interface between the metal substrate and the polymer film is the Schottky thermionic emission, which determines carrier transport in the temperature range from 300 to 450 K. This confirms the conclusion that the transfer of charge carriers through the metal-polymer interface occurs as a result of the above-barrier transport. In this case, the barrier height is determined by the difference between the work function of the metal and the electron affinity of the polymer. For example, the calculation according to the results of the electrophysical measurements for film samples of copolymers **15** gives the barrier height of 0.77 eV (**Table 1**). Taking into account that the work function of aluminum is 4.26 eV and the electron affinity of the polymer lies in the range from 3.5 to 3.6 eV, we obtain the barrier height ranging from 0.76 to 0.66 eV, that is, we have the value comparable to that calculated within the framework of the Schottky model. Since the field addition in Eq. (1) does not exceed 0.1 eV, it is ignored. Thus, the above calculations are further evidence in favor of the model of above-barrier transport at the metal-polymer interface. The obtained values of HOMO and LUMO indicate that the polyanilines studied in our work can be used for the development of new organic solar cells [36, 37]. The short-circuit current of the photo-converter is closely related to the difference in the energy between the HOMO of the PANI (donor) and the LUMO of the acceptor. The most appropriate acceptor can be represented by a methanofullerene [38]. This difference also determines the open-circuit voltage. Moreover, the band gap of the donor determines the minimum energy or the maximum wavelength of the absorbed photons. For the effective absorption in the visible part of the

**, V ЕHOMO, eV\* ЕLUMO, eV\*\* Eg**

 0.54 −1.07 −5.31 −3.73 1.61 1.55 0.71 0.49 −1.11 −5.29 −3.69 1.60 1.52 0.69 0.44 −1.13 −5.24 −3.67 1.57 1.68 0.77 0.29 −1.25 −5.09 −3.55 1.54 1.53 0.70

**, eV ϕB, eV**

**CVA EP**

solar spectrum, the band gap should be in the range from 1.4 to 1.5 eV.

**5. Organic solar cells based on thin polymer films**

ple of a solar energy photoconverter.

№ **Eox 1**

\**E*HOMO = −(*Eox*

\*\**E*LUMO = −(*Ered*

*1* + 4.8) (eV)

*<sup>1</sup>* + 4.8) (eV)

CVA—cyclic voltammetry; EP—electrophysical measurements.

**Table 1.** Electrochemical characteristics of the synthesized polyaniline derivatives.

**, V E red**

94 Emerging Solar Energy Materials

**1**

Thus, the poly-2-(1-methyl-2-buten-1-yl)aniline/methanofullerene heterojunction, which is composed of newly synthesized compounds, is optimal for manufacturing a laboratory sam-

The technique of formation of thin films of polyanilines and fullerene-containing polymers by vacuum deposition from a Knudsen effusion cell was used [36]. The length of the cylindrical To increase the conductivity of polyaniline layers, the temperature conditions of deposition from the Knudsen cell were selected. The temperature range of 500–550 K proved to be the most optimal. In addition, protonation of the freshly prepared films in vapors of hydrochloric acid solution was carried out. For PANI films a conductivity value of 1.0 mS/cm was achieved as a result.

The surface condition and thickness of the deposited films were monitored on the basis of analysis of AFM images obtained by a NanoScan 3D. The thickness of photoactive layers varied and took on values within the range 100–200 nm. It should be noted that a too large thickness of the films leads to exciton recombination and reduces the efficiency of charge separation. On the contrary, the incident photon absorption and quantity of formed excitons decrease in overly thin films.

The organic solar cell test samples based on the donor-acceptor polymer systems described earlier were formed on a glass substrate with conductive and transparent ITO layers. Resistance of ITO layers was about of 10 Ω/**□**. For experimental structures of the OSC in this research the following organic substances were used: PANI, conventional fullerene and a novel synthesized monomer—monosubstituted methanofullerene derivative [38] (**Figure 6a** and **b**). The aluminum films fabricated by thermo-diffusion deposition in vacuum were applied as the upper electrode. **Figure 6c** presents the structure of the OSC in which thin films of PANI and fullerene-containing polymers were used as photoactive layers.

The current–voltage characteristics (CV characteristics) of all the prepared OSC samples were measured and the numerical values of such parameters such as open-circuit voltage, short-circuit current, filling factor and PCE were calculated on their basis. Measuring the CV characteristics of a photovoltaic cell is usually done by exposing it to steady-state illumination and a known temperature. The sun or a sunlight simulator can act as a light source. Estimations of the coefficient of efficiency were based on standard sun intensity *P*<sup>0</sup> = 1000 W/m<sup>2</sup> (AM 1.5 G conditions).

The values of these parameters for the various OSC experimental structures studied in this work appeared to be Jsc = 0.6–1.8 mA/cm<sup>2</sup> (short-circuit current), Voc = 0.6–0.8 V (open-circuit voltage) and FF = 0.6–0.8 (filling factor). The highest values of PCE for the investigated organic solar cells were about 2%. These values were obtained for the structures based on methanofullerene derivatives.

Thus, it was demonstrated that a combination of PANI with fullerene-containing polymers is very important for formation of OSC on the basis of binary donor-acceptor systems. The solar cells investigated here differ from earlier ones [39] that they can be fabricated on the flexible substrates.
