Effect of doping on the short-circuit current and open-circuit voltage of polymer solar cells

Abstract

The change in doping density in P3HT:PCBM based polymer solar cells (PSCs) with different processing solvents and with/without post-fabrication thermal treatment is investigated with capacitance-voltage measurement and optical microscopic imaging. The results suggest that both slow drying and thermal treatment facilitate the phase-separation and crystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. Direct links between the doping density and the performance of the PSCs, specifically the short-circuit current (Jsc) and open-circuit voltage (Voc), are observed. The results show that doping density is one of the decisive factors affecting the photocurrent of the PSCs. Lower doping density leads to a wider depletion region, which is beneficial for carrier collection. The agreement between the calculation and the experiment suggests that the Voc increases monotonically with increasing doping densities in the PSCs. These rules consistently explain our results on the change of Jsc and Voc after thermal annealing in the PSCs with different processing solvents.