Solution‐Processable Hyperbranched Conjugated Polymer Nanoparticles Based on C 3 h ‐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 ≈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/PC 71 BM solutions show a power conversion efficiency up to 1.51% after DIO additive treatment, much higher than that of BTT‐3DPP (0.31%).