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The electronic structures of chalcopyrite CuIn1-xGaxSe2 have recently been reported to have strongly anisotropic and non-parabolic valence bands (VBs) even close to the Γ-point VB maximum. Also, the lowest conduction band (CB) is non-parabolic for energies 50-100 meV above the CB minimum. The details in the band-edge dispersion govern the material's electrical properties. In this study, we, therefore, analyze the electronic structure of the three uppermost VBs and the lowest CB in CuIn1-xGaxSe2 (x = 0, 0.5, and 1). The parameterized band dispersions are explored, and the density-of-states (DOS) as well as the constant energy surfaces are calculated and analyzed. The carrier concentration and the Fermi energy EF in the intrinsic alloys as functions of the temperature is determined from the DOS. The carrier concentration in p-type materials is modeled by assuming the presence of Cu vacancies as the acceptor type defect. We demonstrate that the non-parabolicity of the energy bands strongly affects the total DOS. Therefore, it is important to take into account full band dispersion of the VBs and CB when analyzing the free carrier concentration, like for instance, in studies of electronic transport and/or measurements that involve strong excitation conditions. [ABSTRACT FROM AUTHOR]

To accelerate environmental friendly thin-film photovoltaic technologies, earth-abundant, non-toxic, and low-cost materials are demanded. We study the compounds of Cu2Sn1 −xGe xS3 and Cu2Sn1− xSi xS3 ( x = 0, 0.5, and 1) employing first-principles method within the density functional theory. The compounds have comparable band dispersions. The band-gap energies Eg can be tailored by cation alloying the Sn atoms with Ge or Si. The gap energies of Cu2Sn1− xGe xS3 and Cu2Sn1− xSi xS3, with x = 0, 0.5, and 1, vary almost linearly from 0.83 to 1.43 eV and 2.60 eV, respectively. However, the gap energy of Cu2SiS3 does not follow the linear relation for x > 0.8. The effective electron masses at the Γ-point of the lowest conduction band are almost isotropic for all materials, which are between 0.15 m0 and 0.25 m0. On the other hand, the effective hole masses of the topmost valence band show very strong anisotropy for all compounds. In the (010) direction, the hole masses are estimated to be between 1.01 m0 and 1.85 m0, while between 0.11 m0 and 0.41 m0 in the (001) direction. Calculations reveal that all compounds have high absorption coefficients that are comparable with that of Cu2ZnSnS4. The absorptions in the energy region from Eg + 0.5 eV to Eg + 1.0 eV are even higher for Ge- and Si-alloying of Cu2SnS3, compared with Cu2ZnSnS4. The high-frequency dielectric constants of the compounds are between 6.8 and 8.9. Cu2Sn1− xGe xS3 and Cu2Sn1− xSi xS3 can be considered as potential candidates for absorber materials in thin-film solar cells. [ABSTRACT FROM AUTHOR]

Abstract: Parameterization of the electronic band structure of CuIn1−x Ga x Se2 (x =0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [E j (k); j = v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the Γ-point valence-band maximum E v1(0). Also the lowest conduction band E c1(k) is anisotropic and non-parabolic for energies ~ 0.05eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn1−x Ga x Se2 need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses m j (k), and also an average energy-dependent approximation of the masses m j (E). [Copyright &y& Elsevier]