Symmetry-Breaking Strategy Yields Dopant-Free Small Molecule Hole Transport Materials for Inorganic Perovskite Solar Cells with 20.58% Efficiency and Outstanding Stability.

Inorganic perovskites are known for their excellent photothermal stability; however, the photothermal stability of all-inorganic n-i-p perovskite solar cells (PSCs) is compromised due to ion diffusion and free radical-induced degradation caused by the use of doped spiro-OMeTAD hole transport materials (HTMs). In this study, two isomeric donor-acceptor-donor (D-A-D) type small molecules, namely HBT and HiBT, were developed and used as dopant-free HTMs, using 2,1,3-benzothiadiazole or benzo[d][1,2,3]thiadiazole as acceptor moieties. The HiBT molecule, with its symmetry-breaking features, exhibits a large dipole moment, enhanced coordination-active sites, and a well-aligned energy level structure, all of which contribute to passivating perovskite surface defects and improving free charge separation. As a result, inorganic CsPbI PSCs with HiBT HTM achieved an impressive power conversion efficiency (PCE) of 20.58%, the highest reported for dopant-free HTM-based inorganic PSCs. Moreover, the enhanced hydrophobic properties of HiBT molecules, coupled with their ability to passivate perovskite surface defects, contribute to significantly improved device stability. The unencapsulated devices based on HiBT HTM retained over 83% and 80% of their initial efficiency after being stored at 85 °C for 50 days and undergoing maximum power point (MPP) tracking at 85 °C for 1100 h, respectively. These results highlight that the symmetry-breaking strategy is an exceptionally effective approach for designing efficient, dopant-free small molecule HTMs, significantly contributing to both the high efficiency and enhanced stability of all-inorganic PSCs.