Abstract
Polymer solar cells (PSCs) suffer energy loss due to a number of reasons including high thermalisation, large exciton binding energy and electron-hole recombination within the active layer of the device. Although much effort has gone into addressing the above, interface recombination between a FTO cathode and ZnO electron transport layer (ETL) has remained relatively unnoticed. In this paper, an efficient inverted PSC is demonstrated by introducing a solution-processed SnO2 film between the ZnO ETL and FTO cathode. The use of the SnO2 layer, improves significantly all the device performance parameters of PSCs based on P3HT:PC61BM and PCDTBT:PC71BM system, achieving higher power conversion efficiency (PCE) up to 4.25% and 7.16%, compared to that without the SnO2 film (PCE 3.10% and 5.52%). The improved performance is attributed to the enhanced optical transmission, the reduced energy barrier and suppression of carrier recombination at the interface between the ZnO layer and cathode. Furthermore, the SnO2 film is shown to facilitate electron injection as well as effective hole blocking from the active layer. This study provides an efficient approach to optimize the device performance of PSCs by interfacial modification at bottom conducive electrode.