Effects of bulk and interfacial charge accumulation on fill factor in organic solar cells

Abstract

The effects of charge accumulation on fill factor (FF) are studied by using light intensity-dependent current-voltage and capacitance-voltage measurements based on poly-{bi(dodecyl) thiophene-thieno[ 3,4-c]pyrrole-4,6-dione}:1(3-methyloxycarbonyl)propyl-1-phenyl [6,6] (PBTTPD:PCBM) solar cells. We find that the FF of un-annealed device shows a non-monotonic dependence of light intensity: first an increase and then a decrease with increasing light intensity. This suggests that both bulk charge accumulation and recombination determine the FF before thermal annealing. Furthermore, the morphological enhancement of PBTTPD: PCBM film leads to a monotonic decrease of FF as increasing light intensity. This indicates that only surface charge accumulation plays an important role in the determination of the FF after thermal annealing. (C) 2013 AIP Publishing LLC.