The development of photovoltaic polymers based on C3h -symmetric benzotrithiophene (C3h -BTT), an analogue of the well-known benzodithiophene (BDT) donor unit, has been severely limited due to difficult processability. Here the authors report the preparation of solution-processable C3h -BTT-based hyperbranched conjugated polymer nanoparticles (BTT-HCPNs) with tunable particle sizes via Stille miniemulsion polymerization. Compared with the corresponding star-shaped small molecule (C3h -BTT core with three diketopyrrolopyrrole arms, BTT-3DPP) with a wide bandgap (1.83 eV), both BTT-HCPNs show strong aggregation even in dilute solutions, leading to much-extended absorption (up to ≈1000 nm) and lower bandgaps (1.38 eV). The larger BTT-HCPN particle exhibits stronger aggregation and more extended absorption than the smaller one. As a result, solar cells fabricated from BTT-HCPNs/PC71 BM solutions show a power conversion efficiency up to 1.51% after DIO additive treatment, much higher than that of BTT-3DPP (0.31%).
Keywords: Stille polymerization in miniemulsions; benzotrithiophene; hyperbranched conjugated polymer nanoparticles; polymer solar cells; solution-processability.
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