Strategic Design, Synthesis, and Computational Characterization of Hole Transport Materials for Lead-Free Perovskite Solar Cells

Lead-free perovskites based on nontoxic titanium(IV) are promising candidates for photovoltaic applications due to their improved intrinsic/environmental stability compared to the lead analogues in metal halide perovskite solar cells (PSCs). However, their yet lower power conversion efficiencies (PCEs) predominantly owing to a lack of compatible charge transport layers limit their commercial viability. Here, we synthesized and characterized two series of hole-transporting materials (HTMs) based on fluorene and benzothiadiazole cores functionalized with halogen-substituted indoloquinoxaline arms. Employing experimental and first-principles density functional theory calculations, the structure-property relationships and electrochemical, optical, and charge transport characteristics of these HTMs were examined. The synthesized HTMs showed low-lying highest occupied molecular orbital (HOMO) energy levels at -5.73 to -6.04 eV having ideal band alignment with the cesium titanium(IV) bromide (Cs2TiBr6) perovskite material. The HTMs exhibited minimal absorption in the visible region (lambda max abs <= 422 nm) with negligible overlap with the photoactive perovskite absorber Cs2TiBr6. Computational analysis further revealed the HTMs' ability to possess high charge separation and transfer potential, characterized by high charge hopping rates, robust mobility, and lower exciton binding energy compared to benchmark Spiro-OMeTAD. Photovoltaic device simulations using SCAPS-1D software projected promising performance for PSCs incorporating these HTMs, with open-circuit voltage (V OC) ranging between 1.29 and 1.32 V and predicted PCE surpassing 18%. The study introduces a new class of HTM candidates with low-lying HOMOs and tailored electronic properties, presenting a compelling alternative to Spiro-OMeTAD for lead-free PSCs.