Traps and transport resistance: the next frontier for stable state-of-the-art non-fullerene acceptor solar cells

TL;DR

This study investigates degradation mechanisms in high-performance organic solar cells, revealing that trap formation and transport resistance are key factors limiting device stability and lifetime.

Contribution

It identifies thermally induced trap state formation as the primary cause of thermal degradation, highlighting the importance of suppressing trap formation to enhance stability.

Findings

Trap states increase non-radiative recombination.

Transport resistance due to traps reduces fill factor.

Device lifetime can be improved by reducing trap formation.

Abstract

Stability is one of the most important challenges facing organic solar cells (OSC) on their path to commercialization. In the high-performance material system PM6:Y6 studied here, investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits open-circuit voltage and lowers charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.