Solution-Processable Hyperbranched Conjugated Polymer Nanoparticles Based on C3h-Symmetric Benzotrithiophene for Polymer Solar Cells

The development of photovoltaic polymers based on C(3)h-symmetric benzotrithiophene (C(3)h-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 C(3)h-BTT-based hyperbranched conjugated polymer nanoparticles (BTT-HCPNs) with tunable particle sizes via Stille miniemulsion polymerization. Compared with the corresponding star-shaped small molecule (C(3)h-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 approximate 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/PC71BM solutions show a power conversion efficiency up to 1.51% after DIO additive treatment, much higher than that of BTT-3DPP (0.31%).