Alkylthienyl side groups in conjugated polymers enable localization of π electron density and facilitate efficient hole transfer

Recent advances in molecular design have seen peripheral alkoxy groups replaced with alkylthienyl substituents in conjugated polymers for high-performing organic photovoltaics. However, little is known about the mechanistic origins behind this improvement in performance. In this work, transient absorption spectroscopy is used in conjunction with resonance Raman spectroscopy to shed light on this question. Alkoxy-substituted polymer PBDB is compared with alkylthienyl-substituted PBDB-T in blends with two different acceptors. Larger charge photogeneration yields are observed for PBDB-T:ITIC-Th compared to the PBDB:ITIC-Th blend due to more efficient hole transfer. However, an active triplet formation mechanism via charge recombination leaves the two blends with similar polaron populations on longer timescales. Importantly, resonance Raman spectroscopy demonstrates that the alkylthienyl groups in PBDB-T enable a stronger coupling of the S1 state to the benzodithiophene unit compared to the benzodithiophene-dione unit. The observation that this localization of electron density on the benzodithiophene unit only occurs for PBDB-T, and most prominently in the presence of the ITIC-Th, suggests a strong interaction between the PBDB-T and ITIC-Th. This interaction may facilitate the more efficient hole transfer and greater charge photogeneration yields observed for PBDB-T:ITIC-Th. These results demonstrate the mechanistic origin of a valuable structure-function relationship.