Band unpinning and photovoltaic model for P3HT:PCBM organic bulk heterojunctions under illumination

Capacitance analysis of P3HT:PCBM bulk heterojunction solar cells, in dark and under illumination, shows a linear Mott–Schottky characteristic at moderate reverse bias, indicating p-doping of the organic blend. The flatband potential under illumination is displaced negatively about 0.6 V with respect to dark conditions. A basic photovoltaic model is developed to explain this, in terms of electron transfer via surface states at the metal/organic interface. Surface states with a slow exchange kinetics, become charged under illumination, unpinning the band and decreasing the depletion layer at the electron extraction contact. This becomes a major factor limiting the performance of bulk heterojunction solar cells.