2, Fraunhofer Institute for Applied Polymer Research, Potsdam, , Germany
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 the fabrication of tiles or façade elements, typically requiring lamination temperatures up to 120°C, critical for the initial performance of organic solar modules.
In this work, we demonstrate polymer:fullerene bulk-heterojunctions with significantly enhanced thermal stability at 120°C and beyond, by incorporating a novel crosslinkable bisazide that can lock the bulk-heterojunction morphology. The bisazide molecule is easy to synthesize and offers large-scale accessibility.
The solar cells clearly outperform the thermal stability of reference devices without the crosslinking bisazides.
We investigated bulk-heterojunctions comprising a variety of light-harvesting copolymers, combined with the industrially relevant fullerene acceptor PC61BM. Upon thermal annealing, the reference blends without the crosslinking bisazides exhibit only moderate thermal stability and loose more than 70% of their initial performance, mainly originating from crystallization and aggregation of the fullerene. In contrast, polymer:fullerene blends comprising 7wt.% bisazide crosslinkers show effectively no degradation but retain their initial performance: Even after 200 hours of continuous thermal annealing at 120°C, the respective solar cells still exhibit over 90% of their initial performance.