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 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 CsPbI3 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.