X. G. Gong

Defect physics of the kesterite thin-film solar cell absorber Cu2ZnSnS4

Cu2ZnSnS4 is one of the most promising quaternary absorber materials for thin-film solar cells. Examination of the thermodynamic stability of this quaternary compound reveals that the stable chemical potential region for the formation of stoichiometric compound is small. Under these conditions, the dominant defect will be p-type CuZn antisite, which has an acceptor level deeper than the Cu vacancy. The dominant self-compensated defect pair in this quaternary compound is [CuZn−+ZnCu+]0, which leads to the formation of various polytype structures of Cu2ZnSnS4. We propose that to maximize the solar cell performance, growth of Cu2ZnSnS4 under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided by kinetic barriers.

Abundance of CuZn + SnZn and 2CuZn + SnZn defect clusters in kesterite solar cells

Kesterite solar cells show the highest efficiency when the absorber layers (Cu2ZnSnS4 [CZTS], Cu2ZnSnSe4 [CZTSe] and their alloys) are non-stoichiometric with Cu/(Zn+Sn)≈0.8 and Zn/Sn≈1.2. The fundamental cause is so far not understood. Using a first-principles theory, we show that passivated defect clusters such as CuZn+SnZn and 2CuZn+SnZn have high concentrations even in stoichiometric samples with Cu/(Zn+Sn) and Zn/Sn ratios near 1. The partially passivated CuZn+SnZn cluster produces a deep donor level in the band gap of CZTS, and the fully passivated 2CuZn+SnZn cluster causes a significant band gap decrease. Both effects are detrimental to photovoltaic performance, so Zn-rich and Cu, Sn-poor conditions are required to prevent their formation and increase the efficiency. The donor level is relatively shallower in CZTSe than in CZTS, which gives an explanation to the higher efficiency obtained in Cu2ZnSn(S,Se)4 (CZTSSe) cells with high Se content.

Origin of the superior conductivity of perovskite Ba(Sr)SnO3

ASnO3 (A = Ba, Sr) are unique perovskite oxides in that they have superior electron conductivity despite their wide optical band gaps. Using first-principles band structure calculations, we show that the small electron effective masses, thus, good electron conductivity of ASnO3 can be attributed to the large size of Sn in this system that gives the conduction band edge with antibonding Sn and Os characters. Moreover, we show that ASnO3 can be easily doped by La with shallow LaA(+/0) donor level. Our results, therefore, explain why the perovskite BaSnO3, SrSnO3, and their alloys are promising candidates for transparent conducting oxides.

First-principles study on the effective masses of zinc-blend-derived Cu2Zn−IV−VI4 (IV = Sn, Ge, Si and VI = S, Se)

The electron and hole effective masses of kesterite (KS) and stannite (ST) structured Cu2Zn−IV−VI4 (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 anisotropies are observed for the hole effective masses. 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.

Ab initio calculation of hydrostatic absolute deformation potential of semiconductors

The hydrostatic absolute deformation potential (ADP) of the valence-band maximum state is one of the most important properties of semiconductors. Yet, it has been calculated in the past only using assumptions that have not been rigorously approved. In this letter, we present an approach to calculate the hydrostatic ADP of Si, GaAs, and ZnSe using an ab initio all-electron method and lattice harmonic expansions. We show that the calculated ADP is independent of the selection of the reference energy levels. The calculated ADPs are all positive for the three systems. However, as the p-d coupling increases in the II-VI compounds, the ADP decreases.The hydrostatic absolute deformation potential (ADP) of the valence-band maximum state is one of the most important properties of semiconductors. Yet, it has been calculated in the past only using assumptions that have not been rigorously approved. In this letter, we present an approach to calculate the hydrostatic ADP of Si, GaAs, and ZnSe using an ab initio all-electron method and lattice harmonic expansions. We show that the calculated ADP is independent of the selection of the reference energy levels. The calculated ADPs are all positive for the three systems. However, as the p-d coupling increases in the II-VI compounds, the ADP decreases.

Band structure engineering of multinary chalcogenide topological insulators

Topological insulators (TIs) have been found in strained binary HgTe and ternary I-III-VI

Crystal structure and defect reactions in the kesterite solar cell absorber Cu2ZnSnS4 (CZTS): Theoretical insights

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Effects of Y doping on the structural stability and defect properties of cubic HfO2

chalcopyrite compounds such as CuTlSe

Unified model of ferroelectricity induced by spin order

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Energetics and electronic structure of aluminum point defects in HfO2: A first-principles study

which have inverted band structures. However, the nontrivial band gaps of these existing binary and ternary TIs are limited to small values, usually around 10 meV or less. In this work, we reveal that a large nontrivial band gap requires the material to have a large negative crystal field splitting