Tailoring the Microstructure and Optoelectronic Properties of CsPbI₃ via Synergistic Potassium Doping and Surface Passivation for High-Efficiency Perovskite Solar Cells

Inorganic CsPbI 3 perovskite solar cells (PSCs) have attracted significant attention due to their superior thermal stability compared to hybrid perovskites, but their practical application is limited by phase instability and suboptimal optoelectronic properties. Cs 1– x K x PbI 3 thin films were synthesized with varying K contents ( x = 0%, 1%, 5%, 10%), and their structural, morphological, and electronic characteristics were systematically investigated. Here, we demonstrate a dual strategy of potassium doping and cetyltrimethylammonium bromide (CTAB) passivation to simultaneously enhance the structural stability and photovoltaic performance of CsPbI 3 films. Systematic characterization reveals that 5% K + doping optimizes film morphology, yielding larger (162 nm), more isotropic grains with reduced surface roughness (48.3 nm) and suppressed nonperovskite phase formation. The doped films exhibit enhanced light absorption (α = 1.97 × 10 5 cm –1 ), reduced electronic disorder (Urbach energy = 116 meV), and improved charge transport, while CTAB treatment further passivates surface defects. Consequently, champion devices achieve a power conversion efficiency (PCE) of 15.00%, a 39% improvement over undoped devices (10.81%), with simultaneous gains in open-circuit voltage (1.07 V), short-circuit current (19.39 mA cm –2 ), and fill factor (72.29%). The performance enhancement is attributed to lower series resistance (34.18 Ω cm 2 ) and improved energy level alignment. Beyond 5% doping, excessive K + incorporation degrades film quality, highlighting an optimal threshold for defect passivation vs lattice distortion. This work provides a comprehensive materials engineering approach to stabilize inorganic perovskites while advancing their photovoltaic performance.