Cesium Titanium(IV) Bromide Thin Films Based Stable Lead-free Perovskite Solar Cells

Summary Significant effort is being devoted to the search for all-inorganic Pb-free halide perovskites (HPs) for photovoltaic applications. However, candidate HPs that combine all the desirable attributes — ease of synthesis, favorable bandgaps, outstanding optoelectronic properties, high stability, no toxicity — are extremely rare. Here, we demonstrate experimentally the promise of cesium titanium(IV) bromide (Cs 2 TiBr 6 ), a part of the Ti-based vacancy-ordered double-perovskite halides family, in perovskite solar cells (PSCs). We show, for the first time, that high-quality Cs 2 TiBr 6 thin films can be prepared through a facile low-temperature vapor-based method. These films exhibit a favorable bandgap of ∼1.8 eV, long and balanced carrier-diffusion lengths exceeding 100 nm, suitable energy levels, and superior intrinsic and environmental stability. The first demonstration of Cs 2 TiBr 6 thin films-based PSCs shows stable efficiency of up to 3.3%. Insights into the Cs 2 TiBr 6 film-formation mechanisms and the PSC device operation are provided, pointing to directions for improving Ti-based PSCs.