Organic photovoltaics (OPVs) are promising candidates for providing a low cost, widespread energy source by converting sunlight into electricity. Solution-processable active layers have predominantly consisted of a conjugated polymer donor blended with a fullerene derivative as the acceptor. Although fullerene derivatives have been the dominant electron acceptors in organic photovoltaics, they have limitations, including poor absorption in the visible to near-IR and limited tuning of the energy levels. There is strong interest in discovering new electron acceptors that may overcome these limitations. We found that zinc(II) complexes of azadipyrromethenes (ADP) are promising acceptors, with strong absorption in the visible to near-IR, high electron affinity, low reorganization for electron transfer, tunable energy levels, and large non-planar shape. Power conversion efficiency of 1-4% were obtained when blended with poly(3-hexylthiophene) (P3HT) as the donor. The best performance was obtained for Zn(WS3)2, where phenylethynyl substituents are added at the pyrollic positions. The presence of arylethynyl substituents in the pyrollic positions suppresses crystallization and is key in obtaining a favorable morphology in blends and a high efficiency. To further tune the properties of Zn(WS3)2, fluorination at various positions were explored. The effect of fluorination of electrochemical and photovoltaic properties varied depending on the position. The best results were obtained with -CF3 groups on the phenylehynyl substituents. Photocurrent versus light intensity measurements revealed that device performance in these systems is limited by bimolecular recombination, and that fluorination of Zn(WS3)2 reduces bimolecular recombination. Further experiments are underway to understand these results so that we can further reduce bimolecular recombination and enhance device performance using these non-conventional conjugated molecular electron acceptors.

Azadipyrromethene-based complexes as electron acceptor for bulk heterojunction organic solar cells

254th National Meeting (2017)

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