Influence of charge carriers mobility and lifetime on the performance of bulk heterojunction organic solar cells

In this work, we investigated the influence of the mobility (mu) and the lifetime (tau) of charge carriers on the short-circuit current, the fill factor, and the efficiency of a bulk heterojunction solar cell based on a P3HT:PCBM biphasic film. Measurements of the photocurrent as a function of the applied voltage J(ph)(V), photo-CELIV, and the transient photovoltage (TPV) were carried out at different temperatures (100-340 K). With the help of an analytical model for the photocurrent, we shed light on which recombination mechanism is dominant along the J(ph)(V) curve and quantitatively evaluated the impact of the mu tau product on the cell parameters. We conclude that for voltages close to the open-circuit voltage, nongeminate recombination is the main loss factor in a bulk heterojunction solar cell, while as the photocurrent approaches its saturated value, the loss factor is essentially defined by the dissociation probability (P) of the charge transfer states (CTC states). Thermomechanical analysis (DMA) provided additional information about structural changes of the P3HT:PCBM active layer, which can be correlated with changes in the loss factor and in the cell parameters. (C) 2015 Elsevier B.V. All rights reserved.