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

Abstract

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 (Cs2TiBr6), 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 Cs2TiBr6 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 Cs2TiBr6 thin films-based PSCs shows stable efficiency of up to 3.3%. Insights into the Cs2TiBr6 film-formation mechanisms and the PSC device operation are provided, pointing to directions for improving Ti-based PSCs. Halide perovskites (HPs) have attracted a great deal of attention in the field of photovoltaics (PVs) in recent years. The efficiency of perovskite solar cells (PSCs) has seen an unprecedentedly rapid rise within a short period of time. However, the toxicity of Pb and the volatility of the organic ions in the state-of-the-art HP light absorbers have been recognized as the two major obstacles in the path of PSC development and commercialization. Thus, the search is on for all-inorganic, Pb-free HPs for use in PSCs. In this study, we demonstrate experimentally the promise of a new candidate HP: cesium titanium(IV) bromide, a member of a larger family of Ti-based HPs that are all-inorganic, non-toxic, and contain only earth-abundant elements. The exploration of PSCs based on this new family of HPs is likely to have a lasting impact on the global landscape of environmentally friendly PVs, and these materials may also find use in other promising (opto)electronic applications. Cesium titanium(IV) halide perovskites (HPs) are promising all-inorganic, Pb-free materials for perovskite solar cells (PSCs). Here we show that high-quality, uniform thin films of Cs2TiBr6 HP can be prepared through a facile low-temperature vapor-based method. These thin films exhibit a favorable bandgap of ∼1.8 eV, long and balanced carrier-diffusion lengths >100 nm, suitable energy levels, and superior intrinsic and environmental stability. The first planar-heterojunction PSCs based on Cs2TiBr6 thin films show a stable efficiency of up to 3.3%.