For all six compositions, neutral (001) surfaces with two possible surface termination of slabs, i.e., MAX- and BX 2- surface terminations are considered. For surface calculations, the slab models are constructed from the corresponding optimized bulk perovskite structures. For each bulk structure, lattice parameter and position of atoms are optimized until the force on each atom is below 0.02 eV/Å. COMPUTATIONAL METHODSĮxperimental lattice parameters are chosen as starting lattice parameters, 26,37 when it is available. Understanding of the surface structures will form the preliminary basis to comprehend the more complex interfacial atomic structure and properties. Further, we have highlighted the importance of the crystal symmetry and size of the halide atom and its bond distance with B atom as factors responsible for reduction of band gap at the surface compared to the bulk counterpart. The orientation of polar ion ( C H 3 N H 3 +), the metal-halide interatomic distance, B-X-B angle as well as band gap are compared between the surface with their corresponding bulk structures, to gain a thorough understanding of the change in properties from bulk to surface. 18,28 In this study, we restrict our computations to more commonly observed (001) surface. It should be pointed out that both theory and experiment have confirmed that (110) and (001) charge neutral surfaces are stabilized in the tetragonal phase of the perovskite. In particular, we will present electronic structure calculations of the surface atomic structure and electronic properties of tetragonal MABX 3, with B=Pb or Sn, and X=I, Br, or Cl. In this paper, we aim to develop a more comprehensive understanding of the surface structure and electronic properties of MABX 3 for different B site and halide composition. 11 Further, in the hybrid halide perovskite, most of the defects are formed in the valence band or the conduction band making them less likely to form deep trap defects within the band gap. The unique features of halide perovskite includes its band gap (≈2 eV), greater optical absorption spectra (up to a wavelength of ∼800 nm) compared to the conventional thin-film solar cell absorber (about ∼600 nm wavelength), 1,8–10 high carrier mobility and long (>1 μm) electron-hole diffusion length. 7 Beside the PCE, MABX 3 family of perovskites also exhibit remarkable photovoltaic features compared to conventional solar absorbers like CdTe, and amorphous Si. 1,2 The power conversion efficiency (PCE) of these halide perovskite has risen from 3.8% to 24.2%, 2–6 since the first use of halide perovskite as a light absorber in solar cells. Organic-inorganic hybrid halide MABX 3 perovskites (MA/methylammonium = CH 3NH 3 B = Sn or Pb X = I, Br, or Cl) have recently emerged as one of the most promising materials for photovoltaic technology. Further, for all perovskite compositions investigated, no midgap surface electronic states were observed. Both structural features and the band gap at the surface are found to be dependent on the crystal symmetry and chemistry of the perovskite. Such changes in structural features at the perovskite surface lead to change in the band gap between surface and bulk perovskite. Our results reveal that the structural properties like B-X distances, B-X-B angles, and orientation of MA are different between bulk to perovskite surface. ![]() Next, we compared calculated electronic properties of the surface with the corresponding bulk values. Here we have used electronic structure calculations to determine the structural and electronic properties at the surface of MABX 3 (MA = CH 3NH 3 B = Sn or Pb X= I, Br, or Cl) perovskites. The photovoltaic performance of hybrid halide perovskite is linked to its atomic and electronic structure at the surface. ![]() ![]() Organic-inorganic hybrid halide perovskites are emerging as one of the potential materials in the photovoltaic community due to its attractive power conversion efficiency and cost-effective device fabrication.
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