Reducing burn-in voltage loss in polymer solar cells by increasing the polymer crystallinity

Organic photovoltaics (OPV) are a promising technology with the potential for large scale production at very low cost. An industrial application of this technology could signicantly contribute to the global energy production, but was impeded by low efficiencies and lifetimes of organic photovoltaics. With power conversion efficiencies recently surpassing 10%, it is now imperative to investigate and improve the lifetime of polymer– fullerene solar cells. The lifetime of OPV can depend on many variables and results between different laboratories are sometimes difficult to compare. We show how to distinguish multiple simultaneously occurring degradation mechanisms and how to identify their inuence on different device parameters. We nd that a high degree of crystallinity signicantly reduces the initial light induced burn-in degradation which provides a general design rule for organic solar cells. In order to commercialize polymer solar cells, the fast initial performance losses present in many high efficiency materials will have to be managed. This burn-in degradation is caused by light-induced traps and its characteristics depend onwhich polymer is used.We show that the light-induced traps are in the bulk of the active layer and we find a direct correlation between their presence and the open-circuit voltage loss in devices made with amorphous polymers. Solar cells made with crystalline polymers do not show characteristic open circuit voltage losses, even though light-induced traps are also present in these devices. This indicates that crystalline materials are more resistant against the influence of traps on device performance. Recent work on crystalline materials has shown there is an energetic driving force for charge carriers to leave amorphous, mixed regions of bulk heterojunctions, and charges are dominantly transported in pure, ordered phases. This energetic landscape allows efficient charge generation as well as extraction and also may benefit the stability against light-induced traps.