Organic solar cells (OSCs) continue to attract attention because of their simple fabrication, flexibility, large-scale production, wide material sources and low cost. In recent years, there has been significant progress toward developing OSCs that are highly efficient, flexible, lightweight, and stable under environmental conditions. Transport layers (TLs) play a key role in the operation of organic solar cells and their optimization is essential for maximizing their performance. The major role of TLs is to provide the efficient transport of majority carriers from the active layer to the corresponding electrode. For this reason, the mobility of majority carriers in TLs should be as high as possible. Increasing the mobility of majority carriers in TLs is most often achieved by doping [1].

In this presentation the impact of TLs doping on power conversion efficiency (PCE) of OSCs is analyzed. The reported analysis is conducted by OghmaNano software. As a first step the experimental current density-voltage (J-V) characteristics of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell, taken from literature [2] is reproduced by OghmaNano numerical simulator. The obtained set of simulation parameters are used in the further analysis. We limit ourselves to the analysis of the role of hole transport layer (HTL) doping, the PEDOT:PSS in this case, because in regular OSC configuration electron transport layer (ETL) is too thin to exhibit transport properties. The acceptor concentration in HTL N_a is varied in the range 10¹⁵-10²³cm^-3, which is somewhat wider than the values often used in experiments. Usually, in experiments N_a is given in weight fraction which changes from 10^-5 to 10^-1 that can be converted into 10¹⁷-10²¹cm^-3. The PCE slightly increases till N_a reaches approximately 10¹⁷cm^-3 which is the value comparable with effective densities of states (N_c, N_v) in the HTL and the active layer (AL). After that PCE abruptly decreases for N_a between 10¹⁷ and 10¹⁸cm^-3, which is followed by slight decrease with further increase of N_a. On the other hand, the change of hole mobility in HTL μ_p^HTL in the range 10^-5- 1cm²/Vs leads to PCE enhancement until μ_p^HTL=10^-2cm²/Vs (the value comparable with the hole mobility in the AL), after which the PCE starts to fall. The range of values in which μ_p^HTL is varied is also taken from the literature for PEDOT:PSS. According to previous, we conclude that doping simultaneously exerts two opposite effects on the PCE in OSCs. While an increase in N_a lowers the PCE, consequent enhancement of μ_p^HTL generally improves the PCE until the mobility of holes in the AL is reached. For this reason, it is expected that at lower N_a when is also low, the influence of μ_p^HTL predominates and PCE increase with doping, while when higher N_a and correspondingly higher μ_p^HTL values ​​are applied, doping lead to a decrease in PCE. Therefore, there will be some optimal N_a value at which the PCE will have a maximum. This is fully consistent with the experimental results that can be found in [1].