Cu/Ag-Sb-I Rudorffite Thin Films for Photovoltaic Applications

In the search for lead-free perovskites, silver pnictohalides recently gained attention as novel perovskite-inspired materials for photovoltaics due to their high stability, low toxicity, and promising early efficiencies, especially for indoor applications. Recent research on such "rudorffites" mainly addresses silver bismuth iodides (Ag-Bi-I), while their antimony analogues are hardly investigated due to intrinsic challenges in the synthesis of Sb-based thin films. Here, we establish a synthetic route to prepare Ag-Sb-I thin films by employing thiourea as a Lewis-base additive. Thin film morphologies were further optimized by alloying them with Cu, resulting in solar cells with an improved power conversion efficiency of 0.7% by reducing undesired side phases. Density functional theory calculations and optical characterization methods support the incorporation of Cu into a Cu1-x Agx SbI4 phase, keeping the overall stoichiometry and band gap virtually unchanged upon alloying. Our results further reveal the detrimental role of Ag point defects representing trap states in the band gap, being responsible for low open-circuit voltages and subgap absorption and emission features. Moreover, additional minor amounts of Bi are shown to boost the efficiency and stabilize the performance over a wider compositional range. Despite the remaining challenges regarding device performance, we demonstrate a strong increase in external quantum efficiency when reducing the light intensity, highlighting the potential of Ag-Sb-I rudorffites for indoor photovoltaics.