Understanding the low voltage losses in high-performance non-fullerene acceptor-based organic solar cells

Despite the rapid increase in power conversion efficiency (PCE) of non-fullerene acceptor (NFA) based solar cells in recent years, organic photovoltaic (OPV) devices exhibit considerably larger voltage losses compared to their highly-efficient inorganic counterparts ( e.g. Si or GaAs). Further material optimizations and strategies to reduce the voltage losses in OPV systems are required to close the gap to inorganic PV technologies and allow for efficiencies surpassing 20%. The main focus of this study is on understanding the voltage losses in high performance NFA-based solar cells, as furthering the knowledge in this area is critical in promoting their recent success. In this article, the open circuit voltage losses observed in high-performance D18:Y6 organic solar cells with a PCE of 16% are investigated in detail. The voltage losses of D18:Y6 devices are compared to fullerene-devices consisting of D18 and PC71BM in order to highlight the differences between non-fullerene and fullerene acceptors. A low open-circuit voltage loss of 0.51 V has been found for Y6-based devices suggesting a 0.29 V lower voltage loss compared to PC71BM-based devices (0.8 V). The observed differences can be explained by the high-lying charge transfer state energy in Y6-based solar cells and the strong emissivity of the pristine acceptor. Both properties seem to be prerequisites for efficient OPV systems with low voltage losses. Based on the experimental results, we suggest two design strategies to further improve the performance of OPV systems. A detailed voltage loss analysis of organic solar cells based on the record breaking material combination D18:Y6 and their fullerene-based counterparts (D18:PC70BM) cast light on the superior performance of state-of-the-art non-fullerene acceptors.