Design, Characterization, and Performance Evaluation of Highly Efficient Lead‐Free Gradient‐Structured Perovskite Solar Cells With Carbon Nanotube–Based Hole Transport Material

ABSTRACT This study addresses the challenges of toxicity, high cost, and stability in perovskite solar cells (PSCs) while advancing the development of efficient lead‐free alternatives. Using density functional theory (DFT) calculations and SCAPS‐1D simulations, we designed and optimized CsSnCl 3 PSCs, identifying it as a promising absorber material due to its direct bandgap of 1.52 eV, ideal for photovoltaic (PV) applications. Among various electron transport layers (ETLs), tungsten disulfide (WS 2 ) exhibited superior energy level matching, whereas carbon nanotube (CNT) was selected as the hole transport layer (HTL) for its excellent charge extraction and recombination suppression. Optimization of the CsSnCl 3 absorber thickness (600 nm) and trap density ( N t = 1 × 10 14 cm −3 ) improved film quality and carrier lifetime, leading to enhanced device performance. Gradient doping ( G ), combined with charge transport layer (CTL) and interface engineering, played a key role in efficiency enhancement. A doping gradient of G = 20 optimized energy band alignment, strengthened the built‐in electric field, and reduced recombination losses, facilitating efficient charge carrier separation and transport. Even with just three gradient‐doped sub‐layers, significant performance improvements were observed. By systematically refining key device parameters, we achieved a record‐high power conversion efficiency (PCE) of 25.77%, with V oc = 1.154 V, J sc = 25.60 mA/cm 2 , and FF = 87.21%, surpassing previously reported values. Additionally, carbon (C) was identified as a cost‐effective and efficient back electrode material. Temperature studies further demonstrated that CsSnCl 3 ‐based PSCs perform optimally in low‐temperature environments, reinforcing their suitability for diverse operating conditions. These findings establish CsSnCl 3 PSCs as a promising lead‐free alternative for next‐generation photovoltaics. The combination of gradient doping, optimized CTLs, and interface engineering paves the way for high‐efficiency, stable, and environmentally sustainable solar cells (SCs).