A simulation study on p-doping level of polymer host material in P3HT: PCBM bulk heterojunction solar cells

In this study, we investigate the influence of doping on the charge transfer and device characteristics parameters in the bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). Organic semiconductors are also known to be not pure and they have defects and impurities, some of them are being charged and act as p-type or n-type dopants. Calculations of the solar cell characteristics parameters versus the p-doping level have been done at three different n-dopings (N_d) that consist of 5×10¹⁷ cm^-3, 10¹⁸ cm^-3, and 5×10¹⁸ cm^-3. We perform the analysis of the doping concentration through the drift-diffusion model, and calculate the current and voltage doping dependency. We find that at three different n-dopant levels, optimum p-type doping is about N_p=6×10¹⁸ cm^-3. Simulation results have shown that by increasing doping level, V_oc monotonically increases by doping. Cell efficiency reaches its maximum at somewhat higher doping as FF has its peak at N_p=3×10¹⁸ cm^-3. Moreover, this paper demonstrates that the optimum value for the p-doping is about N_p=6×10¹⁸ cm^-3 and optimum value for n-dopant is N_d=10¹⁸ cm^-3, respectively. The simulated results confirm that doping considerably affects the performance of organic solar cells.