Temperature-dependent thermal behavior of BTP-4F-12-based organic solar cells

Heat is one key factor contributing to performance decreases, which would lead to inevitable morphological changes in the active layers. Common research with ex-situ characterizations ignored the degradation process kinetics, which hinders a comprehensive insight into the underlying thermal degradation mechanisms in organic solar cells (OSCs). In this study, the device thermal stability of BTP-4F-12-based solar cells is investigated with operando tracking of grazing-incidence wide/small-angle X-ray scattering (GIWAXS/GISAXS), providing a deep understanding of temperature-dependent degradation processes. The OSCs show a harsh open-circuit voltage (V_OC) loss with increasing temperature, which recovers mostly after getting cooled to low temperature. This behavior is attributed to the charge carrier recombination, π-π stacking distances, and aggregated domains at various temperatures. The irreversible loss of FF and short-circuit current density (J_SC) during aging is due to changes in crystallinity and dense π-π stacking. Furthermore, no obvious correlation is found for the sharp decreased FF for the final aged solar cells, suggesting that such a degradation originates not from high temperature but more likely from the heating/cooling process. PBDBTCl-DTBT:BTP-4F-12 solar cells suffer from a more severe thermal degradation compared with PBDB-TF-T1:BTP-4F-12, where the bad miscibility of donor and acceptor is not beneficial to an optimized stable active layer and the intrinsic thermal properties of the polymer donor also affect significantly the stability of the blend films and solar cells. This study reveals a temperature-dependent thermal degradation of OSCs, which broadens our knowledge from common ex-situ characterizations and deepens our understanding of thermal degradation mechanism.