Factors contributing to degradation of organic photovoltaic cells

The performance of organic photovoltaic (OPV) cells has considerably improved over the past decade, and now exceeds an energy conversion efficiency of 10%, the benchmark for practical use. One factor contributing to this increase is the development of pi-conjugated linear acceptor molecules as alternatives to fullerene derivatives. Although conversion efficiency is an important issue for practical applications, device stability is also necessary for commercialization of OPV technologies. To date, long-term stability has been examined in OPV devices based on fullerenes. However, the factors involved in performance degradation remain poorly understood. It is difficult to address this problem because of the multilayer nature of the devices and many factors involved in degradation. In recent OPV devices based on pi-conjugated molecules as acceptors, there have been few reports on stability under continuous light irradiation. Ensuring the stability of the pi-conjugated molecules under illumination and electrical operation is critical for achieving practical use of OPV devices. In this paper, we comprehensively analyzed OPV devices based on EH-IDTBR as an acceptor material. We identified a decrease in OPV performance of approximately 50% under 100-h light irradiation, which we attribute to increased resistance of the organic semiconductor layer. The increase in resistance was caused by a decrease in the number of carriers, suggesting that structural changes in the EH-IDTBR molecule are an important factor affecting degradation.