Cesium lead iodide (CsPbI3) has recently emerged as a promising solar photovoltaic absorber. However, the cubic perovskite (α-phase) remains stable only at high temperature and reverts to a photoinactive nonperovskite (δ-phase) CsPbI3 at room temperature. In this work, the formation energies and transition energy levels of intrinsic point defects in γ- (more stable than α-phase) and δ-phases have been studied systematically by first-principles calculations. It is found that CsPbI3 exhibits a unipolar self-doping behavior (p-type conductivity), which is in contrast to CH3NH3PbI3. Most of the intrinsic defects induce deeper transition energy levels in δ-phase than in γ-phase. This is due to the small Pb–I–Pb bond angles in δ-phase that results in the weak antibonding character of valence band maximum (VBM). However, the strong antibonding character of VBM plays a critical role in keeping defect tolerance in semiconductors. Therefore, these results indicate the importance of the large metal–halide–metal bond angle for the performance of perovskite solar cells.

A study of the growth, structure determination, and electronic properties of spiro-OMeTAD single crystals.

Band structure of transition metal oxides plays a critical role in many applications such as photo-catalysis, photovoltaics, and electroluminescent devices. In this work we report findings that the band structure of MoO3 can be significantly altered by a distortion in the octahedral coordination structure. We discovered that, in addition to epitaxial type of structural strain, chemical force such as hydrogen inclusion can also cause extended lattice distortion. The lattice distortion in hydrogenated MoO3 led to a significant reduction of the energy gap, overshadowing the Moss-Burstein effect of band filling. Charge doping simulations revealed that filling of conduction band drives the lattice distortion. This suggests that any charge transfer or n-type electron doping could lead to lattice distortion and consequentially a reduction in energy gap.

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We present results of density functional theory (DFT) calculations of the adsorption of hydrogen molecules on Ti-decorated graphene. Our results indicate that the binding energies of molecular hydrogen on Ti-decorated graphene can be dramatically enhanced to 0.23-0.60 eV. The hybridization of the Ti 3d orbitals with the H(2) σ and σ* orbitals plays a central role in the enhanced binding. There is also a contribution from the attractive interaction between the surface dipole and the dipole of polarized H(2). It can be expected that Ti-decorated graphene could be considered as a potential high-capacity hydrogen storage medium.

The cuprate material Bi2Sr2CaCu2O8 (BSCCO-2212) is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunnelling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby.PACS numbers: 74.25. Bt,74.25.Jb,74.40.+k STM measurements of impurities and other inhomogeneities in the cuprates have opened a new window on high temperature superconductivity and raised a host of new questions about the way these impurities interact with their environment [1,2,3]. It has been traditionally assumed that an impurity acts as a localized screened Coulomb potential, and that its principal effect is the modification of quasiparticle wavefunctions nearby. This effect is large and observable as a consequence of the dwave symmetry of the superconducting state [4]. Within such models, the superconducting order parameter is suppressed around the impurity, but this phenomenon is not usually essential for qualitative predictions. Recently, a phenomenological analysis [5] of STM experiments imaging interstitial oxygen atoms [6] suggested that such impurities might have a much more striking effect, to wit, the local modulation of the electronic pair interaction, leading to large amplitude modulations of the superconducting order parameter. If this is true, an understanding of the local changes in electronic structure and couplings to collective excitations of the material might tell us which aspects are of crucial importance to the pairing interaction itself.Naturally occurring impurities which dope the CuO 2 planes in as-grown Bi 2 Sr 2 CaCu 2 O 8 crystals [7] include both the roughly 3% excess Bi substituting on the Sr sites, as well as oxygen interstitials, whose concentration depends on the annealing sequence and determines the net doping of the sample. Both sets of dopant atoms have recently been imaged by STM [6,9]. In the case of the O interstitials, which appear as bright spots on the Bi 2 Sr 2 CaCu 2 O 8 surface at a bias of -960mV, a remarkable set of correlations was established by McElroy et al.[6] between the positions of the impurities and the magnitude of the local superconducting gap (as defined by the position of the coherence peak in the conductance signal), as well as the magnitude of the LDOS at different energies. Local doping disorder was therefore claimed to be responsible for the nanoscale inhomogeneities observed earlier in the same material [10,11,12,13], but the positive dopant-...

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Based on first-principles density functional theory calculations, we systemically study the properties of two-dimensional buckled single-layer bismuth (b-bismuthene).