Organic solar cells have witnessed a rapid improvement in device performance over the past few years, now achieving an exceptional power conversion efficiency of over 18% under standard solar irradiation (over 30% under indoor illumination) in a single junction device, rapidly closing the performance gap with potentially competing technologies such as crystalline silicon and halide perovskite solar cells. This upsurge in performance is primarily driven by the emergence of non-fullerene organic small molecular and polymeric acceptors, surpassing conventional fullerene acceptors due to stronger optical absorption, optimal energy levels and potentially lower cost in synthesis and purification. This is further coupled with major advances in device design (e.g. ternary bulk-heterojunction blends, novel device interlayers), placing organic solar cells in an unprecedentedly promising position for potential large scale commercialisation in multiple application areas.

A further key factor to realise the full commercialisation potential of organic solar cells is stability, that is, a PV device must have a sufficiently long lifespan that exceeds the required operational period for a particular application. For example, it is desirable for a PV device lifespan of >20 years for building integrated applications, whereas a shorter lifespan of ~5-10 years may be sufficient for powering indoor autonomous sensors. The modest device stability has been a widely-recognised and long-standing challenge for conventional fullerene-based organic solar cells, with multiple degradation mechanisms already identified that result in rapid losses of device performance under illumination, ambient air and thermal stress conditions. Nevertheless, the recent transition of organic solar cells from fullerene acceptors to non-fullerene acceptors, as well as the major advances in their molecular and device design, has brought exciting opportunities to fully overcome this challenge.

In this talk I will give a summary of the recent research progress of my group in understanding the degradation mechanisms of non-fullerene organic solar cells. I will highlight the distinct roles of the donor and acceptor materials as well as the device organic/inorganic interfaces in the degradation, and propose potential strategies to overcome these degradation mechanisms.