Role of donor-acceptor domain formation and interface states in initial degradation of P3HT:PCBM-based solar cells

This work is devoted to identifying the degradation mechanism in various structures of a poly(3-hexylthiophene) (P3HT): 6,6-phenyl C(61)-butyric acid methyl ester (PCBM)-based solar cell. We have tried to identify the dominant initial degradation mechanism on the basis of experimental studies carried out on different structures of an organic solar cell. It is known that many of these problems can be solved by means of the following: using proper electrodes and a suitable annealing temperature and duration, improving the morphology of the active film and maintaining a donor-acceptor phase-segregated ordered network as far as possible. The present studies have been carried out both in the dark and under illumination, and it is suggested that initial degradation plays a key role in device performance. The dominant degradation mechanism is the growth of the donor-acceptor complex with time, which not only reduces the effective surface area but also hampers the charge separation. The little change in V(OC) and the significant change in J(SC) suggest that once the LiF/Al electrode is improved, one must modify the structure either by introducing a thin layer of high-molecular-weight P3HT between PEDOT: PSS (poly(3,4-ethylenedioxythiopene) poly(styrenesulfonate)) and photoactive P3HT: PCBM layers or by introducing an optimized content of P3HT nanofibrils/nanoparticles into the P3HT: PCBM blend. The best structure was found to be ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al and it can be improved by the above two methods.