Preparation and characterization of layered two-dimensional germanium halide perovskite

Although it has been only ten years since perovskite materials was considered to be a potential candidate for solar cells and aroused wide and intense attention and investigation then, with the incipient power conversion efficiency (PCE) of 3.9% in 2009, perovskite solar cells have soared recently, reaching a power conversion efficiency over 23%. And due to the excellent photoelectronic properties, such as tunable bandgaps, high panchromatic absorption and charge carrier diffusion length, organometal halide perovskites have made prominent progress. In addition to solar cells, they could be applied in a variety of regions, ranging from lasing, light emitting devices, sensing, photodetectors as well as X-ray and particle-detection. As the representing perovskite material, methylammonium lead halide perovskite compound has achieved high power conversion efficiency, enabling exploration in new lead- free region. People have been trying to replace lead which is toxic with environment-friendly elements like tin (Sn) and germanium (Ge). In our work represented in the thesis, we mainly concentrated on the layered twodimensional (CH3(CH2)3NH3)2(CH3NH3)n-1GenX3n+1 (X=Br, I) perovskites. For the first time, pure layered (CH3(CH2)3NH3)2GeBr4 (n=1) was fabricated successfully, which could be certified by the comparison of powder X-ray diffraction pattern between the experimental and calculated result. Furthermore, we constructed the unit cell of (CH3(CH2)3NH3)2GeBr4 and the three-dimensional CH3NH3GeBr3, along with their crystallographic parameters. From the crystal data, we know that the layered (CH3(CH2)3NH3)2GeBr4 adopts a monoclinic distortion structure crystallizing in the polar R21/c space group, showing lower symmetry than that of CH3NH3GeBr3, who crystallizes in the R3m space group of a trigonal crystal system. For the synthesis of n=2 compound, the X-ray diffraction result showed that the crystals were composed of a majority of (CH3(CH2)3NH3)2(CH3NH3)Ge2X7 and a small fraction of n=3 analogue. But for the n=3 experiment, intense diffraction peaks of CH3NH3GeBr3 were detected, proving the overwhelming proportion of n=3 compound. If comparing the diffraction peaks of the n=1, n=2 and n=3 analogues, we can find a tendency of left shift to lower angles as the two-dimensional perovskite layers grow thicker by introducing new inorganic layers into the crystal structure which is resulted from the expansion of unit cells by adding one single perovskite layer at one time as the value of n increases. The reflectance and time-resolved photoluminescence spectroscopy analysis demonstrates a tendency that the optical bandgaps (Eg) in the (CH3(CH2)3NH3)2(CH3NH3)n-1GenBr3n+1 series decrease with increased thickness of the inorganic layers, which could be attributed to the quantum confinement effects. From the estimated bandgaps (probably larger than 2eV) and the fitted decay time, we can find that even though the (CH3(CH2)3NH3)2(CH3NH3)n-1GenBr3n+1 series are not suitable for solar cells, they are still promising and attracting materials in the solar absorber field. In addition to bromide perovskite, we also synthesized layered two-dimensional (CH3(CH2)3NH3)2GeI4 perovskite respecting to n=1. The compound was synthesized by the same cooling crystallization method with different experimental parameters. Unlike the (CH3(CH2)3NH3)2(CH3NH3)n-1GenBr3n+1 series, who were precipitated in yellow with a lamellar morphology, (CH3(CH2)3NH3)2GeI4 crystallized in a needle-like morphology, displaying bright red color.