Ultrafast charge generation pathways in photovoltaic blends based on novel star-shaped conjugated molecules

The quest for new materials is one of the main factors propelling recent advances in organic photovoltaics. Star-shaped small molecules (SSMs) have been proven promising candidates as perspective donor material due to the increase in numbers of excitation pathways caused by the degeneracy of the lowest unoccupied molecular orbital (LUMO) level. In order to unravel the pathways of the initial photon-to-charge conversion, the photovoltaic blends based on three different SSMs with a generic structure of N(phenylene-nthiophene-dicyanovinyl-alkyl)₃ (n = 1-3), and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor are investigated by ultrafast photoinduced absorption spectroscopy assisted by density functional theory calculations. It is shown that both electron transfer from SSMs to PC₇₁BM and hole transfer from PC₇₁BM to SSMs are equally significant for generation of long-lived charges. In contrast, intramolecular (intra-SSM) charge separation results in geminate recombination and therefore constitutes a loss channel. Overall, up to 60% of long-lived separated charges are generated at the optimal PC₇₁BM concentrations. The obtained results suggest that further improvement of the SSM-based solar cells is feasible via optimization of blend morphology and by suppressing the intra-SSM recombination channel. Novel donor materials for organic solar cells, triphenyl-based star-shaped small molecules, are studied using ultrafast spectroscopy and density functional theory calculations. Both electron- and hole-transfer pathways are found to be equally important for generation of the separated charges in blends with [6,6]-phenyl-C₇₁-butyric acid methyl ester. The results provide routes for further optimization of small-molecule organic solar cells.