One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

X − ⋅(H 2 O) n=1–4  [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies.

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.

Articles you may be interested inFlexible, ab initio potential, and dipole moment surfaces for water. I. Tests and applications for clusters up to the 22-mer Erratum: "Structures, energies, and vibrational spectra of water undecamer and dodecamer: An ab initio study" [Structures, energies, and vibrational spectra of water undecamer and dodecamer: An ab initio study

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F − (H 2 O) n  (n=1–6) clusters have been studied using ab initio calculations. This is an extensive work to search for various low-lying energy conformers, for example, including 13 conformers for n=6. Our predicted enthalpies and free energies are in good agreement with experimental values. For n=4 and 6, both internal and surface structures are almost isoenergetic at 0 K, while internal structures are favored with increasing temperature due to the entropic effect. For n=5, the internal structure is favored at both 0 and 298 K under 1 atm. These are contrasted to the favored surface structures in other small aqua–halide complexes. The ionization potential, charge-transferto-solvent (CTTS) energy, and O–H stretching vibrational spectra are reported to facilitate future experimental work. Many-body interaction potential analyses are presented to help improve the potential functions used in molecular simulations. The higher order many-body interaction energies are found to be important to compare the energetics of the various conformers and compare the stability of the internal over the surface state.

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low-lying energy structure. However, high levels of ab initio calculation predict that a number of low-lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two-body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between −5.0 and −4.7 kcal (mol dimer)−1, while the three-body corrections be taken into account to a large extent.

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX2 (X = F, Cl, Br) has been demonstrated to improve the photovoltaic performance of Sn halide perovskite solar cells, the mechanistic roles of SnX2 in the performance enhancement have not yet been studied appropriately. Here we perform a comparative study of CsSnI3 films and devices and examine how SnX2 additives affect their stability, and the results are corroborated by first-principles-based theoretical calculations. Unlike the conventional belief that the additives annihilate defects, we find that the additives effectively passivate the surface and stabilize the perovskite phase, promoting the stability of CsSnI3. Our mechanism suggests that SnBr2, which shows ca. 100 h of prolonged stability along with a high power conversion efficiency of 4.3%, is the best additive for enhancing the stability of CsSnI3.