Battal G. Yalcin

Thermoelectric properties of stannite-phase CuZn2AS4 (CZAS; A=Al, Ga and In) nanocrystals for solar energy conversion applications

Abstract The current study aimed to comprehensively investigate structural, electronic, optical and transport properties of quaternary semiconductor CuZn2AS4 (CZAS; A=Al, Ga and In) nanocrystals (NCs). Based on energy considerations, the stannite structure (I-42m; No. 121) is found to be more stable than the kesterite (I-4; No.82) and wurtzite (P63mc; No.186) type structures. By means of hybrid functional calculations, these nanocrystals have direct band gap of 0.81–1.71 eV with a high absorption coefficient of >104 cm−1, which are well-suited for use in solar energy-conversion applications. Some of the latest advances in applications of these nanocrystals in thermoelectric applications are also highlighted in the current study. It is observed that transport coefficients of these materials are found to be nearly direction independent and isotropic. All three samples are p-type conductors at room temperature. Especially, the Seebeck coefficient of CuZn2AlS4 is even larger than that of CuZn2GaS4 and CuZn2InS4 under the studied carrier concentration and temperature region. The maximum figure of merit (ZT) reaches 0.982 (0.977), 0.984 (0.974) and 0.53 (0.955) for p-type (n-type) CuZn2AlS4, CuZn2GaS4, and CuZn2InS4, respectively, at 300 K. The high Seebeck coefficients, high figure of merit and low thermal conductivities make these systems good candidates for high-efficiency thermoelectric conversion applications.

Structural, electronic, optical, vibrational and transport properties of CuBX2 (X = S, Se, Te) chalcopyrites

The structural, electronic and optical properties of CuBX2 (X = S, Se, Te) chalcopyrite semiconductors have been studied using the full-potential (linearized) augmented plane-wave (FP(L)APW) method based on the density functional theory (DFT) within the Yukawa screened-PBE0 (YS-PBE0) hybrid functional as implemented in the WIEN2k package. We have found that our calculated structural and electronic parameters such as lattice parameter, tetragonal ratio, anion displacement and energy band gap are in very good agreement with previous experimental results. We have also presented the real and imaginary parts of the dielectric function, refractive index and absorption coefficients to describe optical properties of the investigated chalcopyrite semiconductors. Furthermore, the phonon dispersion curves and corresponding density of states have been studied by using a linear response approach based on the density functional perturbation theory implemented in the Quantum ESPRESSO code. Finally, transport properties such as the Seebeck coefficient, thermal and electrical conductivity and the figure of merit for these materials have been calculated using the semi-classical Boltzmann theory as implemented in the BoltzTraP code.

Structural, mechanical, electronic and optical properties of BBi, BP and their ternary alloys BBi1−x P x

The ground state properties of BBi, BP and their ternary alloys BBi1−x P x are reported using first-principles calculations based on density functional theory (DFT). The modified Becke–Johnson (mBJ) potential together with the generalized gradient approximation (GGA) for the correlation potential has been used here as it is a superior method for estimating band inversion strength and band order. The zincblende phase is found to be more stable than the other phases for all studied materials. The calculated lattice constants exhibit a small deviation from the linear Vegards law with a downward bowing value of 0.11 A. The calculated ground state parameters for the studied binary compounds agree with available theoretical and experimental results. The bandgap value of the studied materials calculated with the mBJ potential is considerably enhanced with respect to values from the GGA functional. Optical properties have been calculated and analysed with photon incident energy up to 21.0 eV. The spin–orbit interaction (SOI) has also been considered for structural and electronic calculations and the results are compared with those of non-SOI calculations. The real and imaginary parts of the dielectric function have also been calculated and discussed.

Optoelectronic and thermoelectric response of Ca5Al2Sb6 to shift of band gap from direct to indirect

The structural, optoelectronic and thermoelectric properties of Ca5Al2Sb6 under applied external pressures have been studied using the full potential linear augmented plane wave method. WIEN2k code is used with considering the generalized gradient approximation (GGA), modified Becke–Johnson (MBJ) and modified Becke–Johnson + spin orbit (mBJ + SO) functionals based on density functional theory (DFT). From electronic results, the size of the band gap decreases with increasing pressure and the nature of the band gap shift from direct to the indirect. In high pressure (>35.7 GPa by mBJ + SO), the band gap is also completely disappeared and the nature of compound is changed to the metallic. The calculated anisotropic optical properties such as the static dielectric function, increase with decreasing the size of band gap and increasing of pressure. As a novel result, the thermoelectric performance of n-type and p-type doping of Ca5Al2Sb6 is related to the value of pressure. According to the thermoelectric results, the n-type one has the highest ZT in comparison with the p-type Ca5Al2Sb6 material.

Structural and electronic properties of InNxP1-x alloy in full range (0 ≤ x ≤1)

Abstract We have performed first-principles method to investigate structural and electronic properties of InNxP1−x ternary semiconductor alloy in full range (0 ≤ x ≤ 1) using density functional theory. We have used modified Becke–Johnson potential to obtain accurate band gap results. From the electronic band structure calculation we have found that InNxP1−x become metal between 47 and 80% of nitrogen concentration. Additional to our band gap calculations, we have also used the band anticrossing model. The band anticrossing model supplies a simple, analytical expression to calculate the physical properties, such as the electronic and optical properties, of III-NxV1−x alloys. The knowledge of the electron density of states is required to understand and clarify some properties of materials such as the band structures, bonding character and dielectric function. In order to have a deeper understanding of these properties of the studied materials, the total and partial density of states has been calculated. Finally, we have calculated the total bowing parameter b of studied alloys, together with three contributions bVD, bCE, and bSR due to volume deformation, different atomic electron negativities and structural relaxation, respectively.