Strategic\nDesign, Synthesis, and Computational Characterization\nof Hole Transport Materials for Lead-Free Perovskite Solar Cells

Lead-free perovskites\nbased on nontoxic titanium(IV) are promising\ncandidates for photovoltaic applications due to their improved intrinsic/environmental\nstability compared to the lead analogues in metal halide perovskite\nsolar cells (PSCs). However, their yet lower power conversion efficiencies\n(PCEs) predominantly owing to a lack of compatible charge transport\nlayers limit their commercial viability. Here, we synthesized and\ncharacterized two series of hole-transporting materials (HTMs) based\non fluorene and benzothiadiazole cores functionalized with halogen-substituted\nindoloquinoxaline arms. Employing experimental and first-principles\ndensity functional theory calculations, the structure–property\nrelationships and electrochemical, optical, and charge transport characteristics\nof these HTMs were examined. The synthesized HTMs showed low-lying\nhighest occupied molecular orbital (HOMO) energy levels at −5.73\nto −6.04 eV having ideal band alignment with the cesium titanium(IV)\nbromide (Cs<sub>2</sub>TiBr<sub>6</sub>) perovskite material. The\nHTMs exhibited minimal absorption in the visible region (λ<sub>max</sub><sup>abs</sup> ≤ 422\nnm) with negligible overlap with the photoactive perovskite absorber\nCs<sub>2</sub>TiBr<sub>6</sub>. Computational analysis further revealed\nthe HTMs’ ability to possess high charge separation and transfer\npotential, characterized by high charge hopping rates, robust mobility,\nand lower exciton binding energy compared to benchmark Spiro-OMeTAD.\nPhotovoltaic device simulations using SCAPS-1D software projected\npromising performance for PSCs incorporating these HTMs, with open-circuit\nvoltage (<i>V</i><sub>OC</sub>) ranging between 1.29 and\n1.32 V and predicted PCE surpassing 18%. The study introduces a new\nclass of HTM candidates with low-lying HOMOs and tailored electronic\nproperties, presenting a compelling alternative to Spiro-OMeTAD for\nlead-free PSCs.