Charge transfer states as traps in organic solar cells (Presentation Recording)

Abstract

We investigate the NIR time-resolved photoluminescence of a series of P3HT:PC61BM solar cells with varying blend ratios after preferential excitation of the PC61BM and P3HT components respectively. Besides the rapid and diffusion-limited quenching of singlet excitons we resolve a weak emission feature in the near-infrared that our measurements confirm comes from interfacial charge-transfer (CT) states. This CT state emission becomes stronger for samples with an excess of PC61BM, and also after selective excitation of the PC61BM component. In this way, we show that these NIR time-resolved photoluminescence measurements provide an accurate method of observing subtle changes in the formation and dynamics of CT states at organic heterojunctions due to its high selectivity, and suggest that PC61BM excitons are more likely to lead to geminately recombining CT states than are the excitons created on P3HT. We also measure the temperature dependence of the transient NIR photoluminescence and find that while the intensity of the NIR emission is temperature dependent, its lifetime is not. This interesting observation suggests that the CT states we observe are formed through a precursor state which can either form separated charges or CT states, and that the relative yield of these two pools is temperature dependent. Furthermore, it indicates that charges within these relaxed CT states are trapped at the donor-acceptor interface and cannot contribute to free-charge generation via thermal activation anymore.