Interpreting the Density of States Extracted from Organic Solar Cells Using Transient Photocurrent Measurements

The energetic distribution of trapped carrier states (DoS) in organic photovoltaic (OPV) devices is a key device parameter which controls carrier mobility and the recombination rate; as such, it can ultimately limit device efficiency. Recent studies have attempted to measure the DoS from working OPV devices using transient photocurrent methods adapted from the time-of-flight (ToF) method originally developed to measure mobility in thick unipolar devices. While a method to extract the DoS from OPV devices using a simple optoelectronic means would be valuable, analysis is complicated by the presence of both electrons and holes in the bipolar organic solar cells. The presence of both carrier species leads to distortion of the extracted DoS due to (a) recombination losses removing carriers from the photocurrent transient thus changing its shape and (b) both LUMO and HOMO DoS features being observed simultaneously in any measurement. In this paper we use a detailed device model to determine the conditions under which the DoS can safely be extracted from the transient photocurrent from bipolar devices. We show that under conditions of reverse bias it is possible to extract the undistorted DoS from a working OPV device. We apply our method to estimate the DoS in a bulk heterojunction solar cell made of a novel low band gap, diketopyrrolopyrrole-based polymer blended with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) and solar cells made of poly(3 hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) annealed over a range of temperatures.