Lead-Free BaCoX 3 -Based Perovskite Solar Cells: A Simulation Framework for Highly Efficient Photovoltaic Response

Lead-free perovskite solar cells have encountered difficulties in closing the efficiency gap with lead-based counterparts, mostly due to inherent material limitations. This work presents a notable paradigm shift: the use of half-metallic ferromagnetic perovskites, specifically the BaCoX3 (X = Cl, Br, I) series, as photovoltaic absorbers, leveraging their spin polarization, optimal bandgap tunability, defects tolerance, and exceptional carrier transport characteristics. Through comprehensive numerical simulations, we meticulously optimize the architectures of charge transport layers (TiO2 as the electron transport layer and Spiro-OMeTAD as the hole transport layer), including thickness parameters, doping densities, and defect tolerances, to clarify the fundamental device physics of these unconventional absorbers. Our optimized BaCoI3-based device attains a high power-conversion efficiency of 25.5% (short-circuit current = 31.5 mA cm−2, open-circuit voltage = 0.92 V, fill factor = 84%), significantly exceeding reported lead-free barium-based perovskites and challenging leading lead-based counterparts. BaCoI3 also has remarkable defect tolerance, maintains operating efficiency at trap densities of $10^{17}$ cm−3, and has a temperature coefficient of −2.1 mV K−1, confirming its practical usefulness in real-world environments. Our optimization methodology delineates the device performance at a realistic bulk trap density of $10^{14}$ cm−3, a number often seen in experimentally manufactured thin films, resulting in the reported 25.5% efficiency. In contrast, the optimal low-trap regime ( $10^{11}$ cm−3) yields efficiencies of up to 37.3% for BaCoI3; however, attaining such ultralow defect densities presents major obstacles to existing solution-processing methods. Consequently, the practically attainable $10^{14}$ cm−3 defect-density scenario serves as a dependable and reproducible performance benchmark. This work shows that half-metallic ferromagnetism, often considered incompatible with solar performance, potentially achieves both high efficiency and complete lead elimination. The BaCoI3 design provides an effective foundation for sustainable, high-performance photovoltaics and establishes a novel domain for multifunctional spintronic-photovoltaic devices.