2, Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, , Switzerland
Organic-inorganic layered perovskite type compounds with a basic formula of (RNH3)2(CH3NH3)n-1PbnHal3n+1 (R is usually an alkyl chain, and X – halogen) are widely studied due to their intriguing electronic, optical properties, and structural characteristics. These compounds are made of slabs of Pb-X corner sharing octahedra with small methylammonium (CH3NH3+) cations filling inter-slab cavities. Long alkyl chain ammonium cations are situated in-between the layers, they play a role of a potential wall. Tuning of the band gap energy of these compounds is possible through the modification of the carbon chain length in organic amines as well as by changing the thickness of the inorganic layer.
In our study, we eliminated primary amines and replaced typically used methylammonium by cesium expecting higher thermal and chemical stability. As an interlayer component a guanidinium was chosen due to its thermodynamic stability, high basicity (pKa = 13.6), and strong hydrogen-bonds capabilities. The guanidinium cation is not able to maintain a three dimensional network in APbI3 system due to its crystal ionic radii, therefore, it could situate itself in the interlayer space.
All obtained compounds: Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4 (II), and Cs2[C(NH2)3]Pb2Br7 (III), are two–dimensional, air stable, and possess a reasonable temperature stability (up to 300 °C). Compounds I-II are luminescent at moderate cooling, and the compound III is emissive already at room temperature. Photoresponsivities in the range of 1-10 mA×W-1 were measured with compounds I and III. According to DFT calculations I, II and III are semiconducting compounds with resulting (calculated) band gaps of approximately 2.7 eV for II, 2.2 eV for III, and 2.0 eV for I. It is common to the three that the highest occupied bands and lowest empty ones have dominant halide-p and Pb-p contributions, respectively.