Investigation of different degradation pathways for organic photovoltaics at different temperatures

Abstract

The lifetime of organic photovoltaics (OPVs) is a significant challenge for the technology to become commercially viable. Mechanisms of thermally induced degradation are extremely complex due to the nature of the materials and structures in OPVs, each with varying responses to thermal cycling. To date, approaches have focused on small-scale devices, with a limited number of studies focusing on cells made by scalable methods, leaving the stability of materials for large scale devices a substantial unsolved challenge. Ageing OPVs at elevated temperatures is normally performed to reduce experimental times but care must be taken to not introduce new degradation pathways which are not relevant for actual operational conditions. In this paper, we have investigated the thermal degradation of active layer blend of PPDT2FBT:PC₆₁BM that has been deposited via slot-die coating. By combining surface and thin-film characterisation techniques with dynamic mechanical thermal analysis (DMTA), we have investigated the thermal behaviour of the blend, and how it changes upon annealing at two elevated temperatures; 85 °C, and 120 °C. Our results show that at 120 °C large crystals made of PC₆₁BM are formed already after 8 h while at 85 °C large crystals did not form even after 6 months annealing. The fullerene crystallisation at 120 °C plays a significant role in fast thermal degradation of the solar cells, whereas at 85 °C the change in thermal behaviour is dominated by small morphological changes that affect changes in OPV performance. These results clearly shows that care must be taken when performing accelerated ageing of OPV materials.