Transport Limitations and Defect-Mediated Recombination in Lead- Free Double Perovskite Cs₂AgSbI₆: Implications for Solar-Cell Design

Even though the defect-mediated recombination severely limits the photovoltaic performance of lead-free double perovskites, they have become a promising alternative to Pb-based absorbers. The transport mechanisms and the optoelectronic properties of Cs₂AgSbI₆ are systematically investigated in this work by combining impedance analysis, electrical transport, photoluminescence and optical spectroscopy. A highly disordered electronic structure dominated by nonradiative recombination is shown through a non-ideal charge transport, a strong photoluminescence quenching and a large Urbach energy (~ 90 meV). Based on SCAPS-1D, numerical simulations are carried out using a representative FTO/TiO₂/Cs₂AgSbI₆/Spiro-OMeTAD architecture in order to translate the experimentally observed limitations into a device-level framework. The Shockley–Read–Hall (SRH) recombination is consistently identified by the simulations as the dominant loss pathway, which results in limited photovoltaic parameters and reduced quasi-Fermi level splitting. Defect density is emphasized as the primary bottleneck in Cs₂AgSbI₆-based devices through the strong agreement between numerical trends and experimental material signatures. These results not only offer clear guidelines for defect mitigation strategies towards improved lead-free perovskite solar cells but also provide device-relevant losses, recombination and a coherent physical picture linking disorder.