Helical Polycyclic Heteroaromatic as Hole Transport Material for Perovskite Solar Cell: Remarkable Impact of Alkyl Substitution Position

Polycyclic aromatic hydrocarbons play a critical role in the development of organic semiconductors. This study unravels the impact of alkyl substitution position on the molecular energy level, glass transition temperature, diffusion of external species, and hole transport of a pyrrole-rich, helical polycyclic heteroaromatic, that is TBPC. Compared to terminal substitution, internal hexyl substitution results in steric repulsion with TBPC, weakening & pi;& pi; stacking and therefore improving thin film morphology. Internal substitution also reduces energy disorder, lowers reorganization energy, and increases intermolecular transfer integrals, leading to enhanced hole mobility. Notably, the organic semiconductor with internal hexyl substitution (TBPC-611) exhibits a higher molecular packing density, resulting in a markedly higher glass transition temperature and slower diffusion of external species. Using TBPC-611 as the hole transport material, this work successfully fabricates perovskite solar cells with an average power conversion efficiency above 24%, showing good photostability and thermostability. These findings contribute new insights to the development of high-performance organic semiconductors.