Abstract. Using first principles calculations, we investigate the geometric and electronic structures of organic-inorganic hybrid perovskite, FAPbX 3 (FA = CH(NH 2 ) 2 + ; X = Cl, Br, I). Since the organic molecule in the centre of the 3D hybrid perovskite is the key for its characteristics, here we compare FAPbX 3 with MAPbX 3 (MA = CH 3 NH 3 + ). The band gap of the former is smaller than the latter. Particularly, the calculated band gap of FAPbI 3 , 1.40 eV, is close to the experimental data, 1.41 eV. Furthermore, we analyze their orbitals, density of states and the spatial distribution of the charges, revealing that FAPbX 3 can produce and transfer more excitons than MAPbX 3 does.Keywords: Solar cell, formamidinium, methylammonium, geometric structure, band gap.
IntroductionPerovskite solar cells (PSCs) have been the most promising devices towards the renewable energy generation recently, whose highest efficiency, 21.1%, was achieved in 2016 [1]. In addition to their efficient light absorption and high carrier mobility [2-4], they are semiconductors with adjustable band gaps, which have the benefits to absorb different wavelengths of light [5] and to fit into the solar devices well [6]. However, the performance of perovskite depends on its structural order which is temperaturedependent even in typical solar cell operating conditions. For example, methylammonium (MA, CH 3 NH 3 + ) lead iodide (PbI 3 ) undergoes a phase transition between tetragonal and cubic symmetry within 54 and 57 • C [7,8]. Increasing the temperature increases the kinetic energy of the organic molecule in the perovskite [9,10] and creates volatile molecular defects towards the structure degradation [11]. These factors affect the PSC durability. Improving the perovskite stability thus is the key issue for solar cells, and formamidinium (FA, CH(NH 2 ) 2 + ) cations were recently suggested, by plane-wave first-principles calculations, to replace MA inside the inorganic metrix [12,13], due to that the former can interact with the inorganic cage stronger than the latter does, to reduce the release of volatile species [13] or alter the covalent/ionic character of Pb-I bonds [12]. On the other hand, mixing FA with MA is experimentally shown to be a route to stabilize the perovskite [8,14] and improve the power conversion efficiency [14]. However, first-principles calculations with the linear combination of atomic orbital (LCAO) basis-set, which are economical and feasible for charge transport studies, on this issue are still limited to date. In this work, we investigate the geometric and electronic properties of organic-inorganic hybrid perovskite, FAPbX 3 (X = Cl, Br, I) from first principles. Particularly, their geometries, band structures, orbitals, density of states, and the charge densities are analysed, revealing the electronic and optical properties, as well as the stability, of such materials. In addition, we compare the band gap of FAPbI 3 with the measured data. Finally, we compare the electronic features of FAPbX 3 with MAPbX 3 .
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