Microstructure instabilities in solution-processed organic bulk-heterojunction solar cells

The performance of organic solar cells (OSCs) is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. In this work we examine the reliability and stability of bulk-heterojunction (BHJ) microstructures of a highly-efficiency OSC based on PCE11 as the donor and PCBM as the acceptor. The so called burn-in degradation is identified as a spinodal de-mixing due to the low miscibility of donor and acceptor, which is turned out to be a major challenge for the development of stable and efficient OSCs. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. The construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells [1].