First-Principles Study on Electronic Structures of FAPbX3 (X = Cl, Br, I) Hybrid Perovskites

Abstract

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 . Comp...

Importance of Electronic Correlations and Unusual Excitonic Effects in Formamidinium Lead Halide Perovskites

“…Therefore, this MAPbI 3 band structure is used, as it is based on a bodycentered tetragonal Brillouin zone [shown in Fig. 4(b)], whereas previous simulations of FAPbI 3 are calculated using a simple cubic Brillouin zone [40]. This can even be used for the lower temperatures due to the lack of phase change seen in the dielectric function.…”

Structural, electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation

“…The band gaps calculated from DFT-PBE and DFT-vdW are decreased by about 1.0 eV aer inclusion of the SOC effects. The HSE06 calculation including SOC effects has a band gap that is about 0.82 eV lower, decreasing from 1.9937 eV to 20 6.42, 24 -1.368, 19 1.40, 20 1.75, 21 1.45, 24 1.43, 37 1.43, 38 0.224, 19 0.66, 21 0.82, 24 1.47, 38 Experimental 6.3503, 11 6.3620, 37 6.3558, 39 -1.53, 11 1.489, 9 1.45, 39 - 1.1732 eV. The good consistency of the band-gap energy with experiments in the DFT calculations is caused by the strong relativistic effect of the Pb atoms offsetting the underestimation of the band gap by typical DFT calculations.…”

“…Pan et al investigate the geometric and electronic structures of hybrid organic-inorganic perovskites FAPbX 3 (X ¼ Cl, Br, I). 20 Quarti et al studied the exibility structural and dynamics electronic properties of MAPbI 3 and FAPbI 3 using ab initio molecular dynamics simulations. 21 Kanno et al theoretically studied the rotational potential energy surface of FAPbI 3 .…”

Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites

“…This transition must conserve energy, momentum and spin, which is a general requirement of photoemission spectroscopy 36,37 . The second issue follows from this requirement, where for a FA-based perovskite at room temperature (cubic phase), the band gap was calculated to be at the high-momentum R point in the Brillouin zone 38 .To maintain momentum conservation, this would require an additional photon energy of about 11 eV (assuming a cubic lattice with a ~ 6.3 Å) on top of the energy needed to overcome the work function and binding energy. Since we only have probe energies of 4.65 and 6.2 eV, we cannot directly probe such a high momentum point in the Brillouin zone, and this would prevent us from observing free carriers in the conduction or valence bands.…”