Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.171
Publication date: 18th July 2019
Non-fullerene acceptors have emerged as promising fullerene replacements in organic photovoltaics and recently power conversion efficiencies have surpassed 16%. Despite the rapid progress, fundamental understanding of the photo-physical processes is still lacking and identifying the loss mechanisms in devices is important for future material design and device optimization. Here, we investigate the impact of polymer main chain and side chain substitution in BDT-thiophene copolymers on the performance of three bulk heterojunction solar cells that use ITIC as nonfullerene acceptor: PBDT[2HT]:ITIC (2.5%), PBDT(T)[2H]T:ITIC (4.2%) and PBDT(T)[2F]T:ITIC (9.8%). We observe more geminate recombination in the alkoxy-substituted PBDT[2HT]:ITIC blend in addition to nongeminate recombination of free charges, limiting both the short circuit current and fill factor. The alkylthiophene-substituted PBDT(T)[2HT]:ITIC blend exhibits less geminate recombination but significant nongeminate recombination, limiting the fill factor to about 40%, while backbone fluorination in PBDT(T)[2F]T:ITIC leads to fast and efficient charge separation and significantly reduced non-geminate recombination, resulting in fill factors in excess of 60%. Time-delayed collection field measurements showed that charge generation in PBDT(T)[2F]T:ITIC is field-independent, while a weak field dependence is observed for the PBDT[2H]T:ITIC system. Our findings provide important structure-property relations for the design of novel polymer:NFA systems.
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