236 research outputs found
Intrinsic Defects and Electronic Conductivity of TaON: First-Principles Insights
As a compound in between the tantalum oxide and nitride, the tantalum
oxynitride TaON is expected to combine their advantages and act as an efficient
visible-light-driven photocatalyst. In this letter, using hybrid functional
calculations we show that TaON has different defect properties from the binary
tantalum oxide and nitride: (i) instead of O or N vacancies or Ta
interstitials, the antisite is the dominant defect, which determines its
intrinsic n-type conductivity and the p-type doping difficulty; (ii) the
antisite has a shallower donor level than O or N vacancies, with a delocalized
distribution composed mainly of the Ta orbitals, which gives rise to
better electronic conductivity in the oxynitride than in the oxide and nitride.
The phase stability analysis reveals that the easy oxidation of TaON is
inevitable under O rich conditions, and a relatively O poor condition is
required to synthesize stoichiometric TaON samples
Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution
We introduce an approach to calculate the thermodynamic oxidation and
reduction potentials of semiconductors in aqueous solution. By combining a
newly-developed ab initio calculation for compound formation energy and band
alignment with electrochemistry experimental data, this approach can be used to
predict the stability of almost any compound semiconductor in aqueous solution.
30 photocatalytic semiconductors have been studied, and a graph (a simplified
Pourbaix diagram) showing their valence/conduction band levels and
oxidation/reduction potentials is produced. Based on this graph, we have
studied the stabilities and trends against the oxidative and reductive
photocorrosion for compound semiconductors. We found that, only metal oxides
can be thermodynamically stable when used as the n-type photoanodes. All the
non-oxides are unstable due to easy oxidation by the photogenerated holes, but
they can be resistant to the reduction by electrons, thus stable as the p-type
photocathodes
Self-regulation mechanism for charged point defects in hybrid halide perovskites
Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3)
exhibit unusually low free carrier concentrations despite being processed at
low-temperatures from solution. We demonstrate, through quantum mechanical
calculations, that the origin of this phenomenon is a prevalence of ionic over
electronic disorder in stoichiometric materials. Schottky defect formation
provides a mechanism to self-regulate the concentration of charge carriers
through ionic compensation of charged point defects. The equilibrium charged
vacancy concentration is predicted to exceed 0.4% at room temperature. This
behaviour, which goes against established defect conventions for inorganic
semiconductors, has implications for photovoltaic performance
First-principles study on the effective masses of zinc-blend-derived Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se)
The electron and hole effective masses of kesterite (KS) and stannite (ST)
structured Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se) semiconductors are
systematically studied using first-principles calculations. We find that the
electron effective masses are almost isotropic, while strong anisotropy is
observed for the hole effective mass. The electron effective masses are
typically much smaller than the hole effective masses for all studied
compounds. The ordering of the topmost three valence bands and the
corresponding hole effective masses of the KS and ST structures are different
due to the different sign of the crystal-field splitting. The electron and hole
effective masses of Se-based compounds are significantly smaller compared to
the corresponding S-based compounds. They also decrease as the atomic number of
the group IV elements (Si, Ge, Sn) increases, but the decrease is less notable
than that caused by the substitution of S by Se.Comment: 14 pages, 6 figures, 2 table
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