Vertically-aligned quasi-2D cesium lead halide perovskite solar cells

Quasi-2D cesium lead halide perovskites exhibit better stability than 3D CsPbI3 due to the steric hindrance, decreased surface energy, and improved hydrophobicity brought by long-chain organic spacers, such as phenylethylammonium (PEA) cations. However, crystal-orientation-induced poor charge transport and phase impurities greatly limit their performance. Here, we report effective approaches to tuning the crystal orientation of quasi-2D crystalline films in inorganic cesium lead halide perovskites. By reducing the N,N-dimethylformamide/dimethyl sulfoxide ratio and introducing MACl in the precursor solutions, we are able to achieve vertically-aligned PEA(2)Cs(3)Pb(4)I(13) perovskite films with enhanced film coverage, decreased trap density, and improved phase purity. The optimized solar cell devices show over 70% improvement in photocurrent which results in a 30% enhancement in efficiency. The unencapsulated cesium-based 2D device preserved 70% of its initial efficiency after being exposed to humid air for 30 days.