Thermal stabilization of the bulk-heterojunction morphology in polymer:Fullerene solar cells using a bisazide cross-linker

After enhancing the power conversion efficiencies of organic solar cells beyond 10%, their long term stability has become the most urgent challenge in order to eventually integrate organic solar cells into end‐user products. Solar devices may have to endure harsh conditions already during their fabrication, typically requiring lamination temperatures up to 120 °C, critical for the initial performance of organic solar cells. In this work, polymer:fullerene bulk‐heterojunctions are fabricated with significantly enhanced thermal stability at 120 °C and beyond, by locking the bulk‐heterojunction morphology through incorporating the novel cross‐linkable bisazide 1,2‐bis((4‐(azidomethyl)phenethyl)thio)ethane (TBA‐X). Bulk‐heterojunctions comprising various light‐harvesting polymers and the industrially relevant fullerene acceptor PC61BM are investigated. Upon thermal annealing, the reference blends without the cross‐linking TBA‐X exhibit only moderate thermal stability and a relative loss of more than 70% of their initial performance, mainly originating from aggregation of the fullerene. In contrast, polymer:fullerene blends comprising TBA‐X retain up to 90% of their initial performance despite the harsh thermal annealing at 120 °C for up to 200 h.