Pure two-dimensional (2D) perovskite (n = 1)-based perovskite solar cells (PSCs) have been proven to have excellent stability against humidity, but the photovoltaic performance is very poor due to the parallel orientation to the substrate and mismatched energy alignment in the PSC device. We report herein a novel bulky organic cation of 3-aminopropionitrile (3-APN) for constructing a pure 2D hybrid lead-iodide perovskite. The crystal structure of (3-APN)2PbI4 features a stable layered and undistorted PbI6 octahedral geometry (∠Pb-I-Pb = 180°) with a small I···I distance (4.66 Å), and the crystals grow in a dominant out-of-plane direction to the substrate. In addition, the existence of an intramolecular H bond between cyano groups and ammonium heads result in an appropriate valence band level of (3-APN)2PbI4 for a well-matched energy level alignment in the device, benefitting the interfacial charge transfer and hence a better photovoltaic performance. As a result, the PSC with the pure 2D (3-APN)2PbI4 perovskite-based PSC achieves a power conversion efficiency of 3.39%, which is the highest value thus far for the pure 2D lead-iodide perovskite family, to the best of our knowledge. More importantly, this pure 2D (3-APN)2PbI4 perovskite-based PSC demonstrates excellent stability against humidity. This work demonstrates that there is great potential to realize efficient and stable pure 2D perovskite-based PSCs through the wise design of organic cations.
Keywords: 3-aminopropionitrile; crystal orientation; energy level alignment; perovskite solar cells; pure 2D.