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Recent theoretical and experimental reports have shown that the perovskite CH3NH3PbI3 exhibits unique ambipolar self-doping properties. Here, we show by density-functional theory calculation that its sister perovskite, CH3NH3PbBr3, exhibits a unipolar self-doping behavior--CH3NH3PbBr3 presents only good p-type conductivity under thermal equilibrium growth conditions. We further show that despite a large bandgap of 2.2 eV, all dominant defects in CH3NH3PbBr3 create shallow levels, which partially explains the ultra-high open-circuit voltages achieved by CH3NH3PbBr3-based thin-film solar cells. Our results suggest that the perovskite CH3NH3PbBr3 can be both an excellent solar cell absorber and a promising low-cost hole-transport material for lead halide perovskite solar cells. [ABSTRACT FROM AUTHOR]

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to earner collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells. [ABSTRACT FROM AUTHOR]

Through the use of aberration corrected scanning transmission electron microscopy, the atomic configuration of CdTe intragrain Shockley partial dislocation pairs has been determined: Single Cd and Te columns are present at opposite ends of both intrinsic and extrinsic stacking faults. These columns have threefold and fivefold coordination, indicating the presence of dangling bonds. Counterintuitively, density-functional theory calculations show that these dislocation cores do not act as recombination centers; instead, they lead to local band bending that separates electrons and holes and reduces undesirable carrier recombination. [ABSTRACT FROM AUTHOR]

Using first-principles band structure calculations, we investigate the crystal-field and spin-orbit splittings at the valence-band edge of ZnO and their dependence on the strain. Different from other conventional semiconductors, the variation of the valence-band splitting of ZnO shows a strong nonlinear dependence on the strain and the slope of the crystal-field splitting as a function of strain can even change sign. Our analysis shows that this unusual behavior in ZnO is due to the strong coupling between Zn 3d states and oxygen 2p states. A mapping of the valence-band ordering in ZnO under different strain levels is provided that will be useful in designing ZnO-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

In the present Letter, we report on a combined ab initio density functional theory calculation, multislice simulation, and electron holography study, performed on a &Sgr;9 grain boundary (GB) in a CuGaSe2 bicrystal, which exhibits a lower symmetry compared with highly symmetric &Sgr;3 GBs. We find an electrostatic potential well at the &Sgr;9 GB of 0.8 V in depth and 1.3 nm in width, which in comparison with results from &Sgr;3 and random GBs exhibits the trend of increasing potential-well depths with lower symmetry. The presence of this potential well at the &Sgr;9 GB can be explained conclusively by a reduced density of atoms at the GB. Considering experimental limitations in resolution, we demonstrate quantitative agreement of experiment and theory. [ABSTRACT FROM AUTHOR]

The kesterite Cu2ZnSnS4 (CZTS) is attracting considerable interest because first-principles calculations predict that its electronic properties must be similar to their associated chalcopyrite Cu(In,Ga)Se2 (CIGS) compounds [Chen et al., Phys. Rev. B 81, 245204 (2010)]. Here, the authors report on first experimental evidence of the close resemblance in the luminescence of Cu-poor kesterites and Cu-poor chalcopyrites used in photovoltaic applications. Microluminescence measurements suggest that even the very distinct electronic structure of grain boundaries in CIGS is present, to some extent, in CZTS. The similarities between CIGS and CZTS become more pronounced as the efficiency of the CZTS solar cells gradually increases. The implications of these results for the future development of CZTS solar cells are discussed. [ABSTRACT FROM AUTHOR]

The intrinsic and extrinsic doping properties of BiVO4, i.e., the formation energies and transition energy levels of defects and impurities, have been studied systematically by first-principles density-functional theory. We find that for doping caused by intrinsic defects, O vacancies are shallow donors and Bi vacancies are shallow acceptors. However, these defects compensate each other and can only lead to moderate n-type and p-type conductivities at Bi-rich and O-rich growth conditions, respectively. To obtain BiVO4 with high n-type and p-type conductivities, which are required for forming Ohmic contacts, extrinsic doping using foreign impurities is necessary. Our results reveal that Sr, Ca, Na, and K atoms on Bi sites are very shallow acceptors and have rather low formation energies. The calculated Fermi-level pinning positions predict that doping of these impurities under oxygen-rich growth conditions should result in outstanding p-type conductivity. Substitutional Mo and W atoms on V sites are very shallow donors and have very low formation energies. Fermi-level pinning position calculations expect the doping of Mo and W under oxygen-poor growth conditions to produce excellent n-type conductivity. Also discussed is the dependence of formation energies and transition energies of defects on the atomic size and atomic chemical potential trends. [ABSTRACT FROM AUTHOR]

Using band symmetry analysis and density functional theory calculations, we reveal the origin of why oxygen vacancy (VO) energy levels are shallow in some ABO3 perovskites, such as SrTiO3, but are deep in some others, such as LaAlO3. We show that this diverse behavior can be explained by the symmetry of the perovskite structure and the location (A or B site) of the metal atoms with low d orbital energies, such as Ti and La atoms. When the conduction band minimum (CBM) is an antibonding Γ12 state, which is usually associated with the metal atom with low d orbital energies at the A site (e.g., LaAlO3), then the VO energy levels are deep inside the gap. Otherwise, if the CBM is the nonbonding Γ25′ state, which is usually associated with metal atoms with low d orbital energies at the B site (e.g., SrTiO3), then the VO energy levels are shallow and often above the CBM. The VO energy level is also deep for some uncommon ABO3 perovskite materials that possess a low s orbital, or large-size cations, and an antibonding Γ1 state CBM, such as ZnTiO3. Our results, therefore, provide guidelines for designing ABO3 perovskite materials with desired functional behaviors. [ABSTRACT FROM AUTHOR]