BODIPY-Based Conjugated Polymers for Use as Dopant-Free Hole Transporting Materials for Durable Perovskite Solar Cells: Selective Tuning of HOMO/LUMO Levels.

Recently, perovskite solar cells (PSCs) have emerged as an excellent photovoltaic device owing to the outstanding power conversion efficiency (PCE). Nevertheless, device instability remains a critical issue in this field. To overcome device instability without deteriorating PCE, dopant-free hole transporting materials (HTMs) are needed to separate the air-sensitive perovskite layer from extrinsic factors, which induce its degradation. Herein, we developed novel conjugate polymers of benzo[1,2- b:4,5- b']dithiophene (BDT) and 4,4-difluoro-4-bora-3 a,4 a-diaza- s-indacene (BODIPY) for use as HTMs without dopants. The pBDT-BODIPY polymer allows individual "dialing" of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) levels with small modifications to the molecular structure, enabling study of the impact of the frontier molecular orbital on PSC performance. Different alkyl chains on BDT can minutely adjust the HOMO level, and meso-substituents on BODIPYs can selectively set the LUMO level of the resulting polymers. Application of BODIPY-containing polymer into the perovskite solar cell as an HTM leads to a high PCE value (16.02%) and exceptional solar cell stability shown by the fact that over 80% of its original PCE value was maintained after 10 days under ambient air conditions.