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The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation. [ABSTRACT FROM AUTHOR]

Optical absorption measurements at different temperatures on CuIn5Se8 single crystals are reported. Single crystals are grown by chemical vapor transport (CVT) and the importance of growing and measuring the properties of CuIn5Se8 single crystals lies in the possibility of obtaining more reliable data on the physical properties of the material of interest.

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons. [ABSTRACT FROM AUTHOR]

The polarized Raman spectra of the Cu2ZnSnSe4 and Cu2ZnGeSe4 single crystals were measured for various in-plane rotation angles on the basal (1 1 2) crystal facet. The position of up to 15 (for Cu2ZnSnSe4) and 16 (for Cu2ZnGeSe4) Raman peaks was determined in the spectral region 50-300 cm-1. From the analysis of the experimental dependence of the intensity of the Raman peaks with the rotation angle, a symmetry assignment for most of the detected modes and an estimation of numerical values of Raman tensor elements were derived. The kesterite type structure of Cu2ZnSnSe4 and Cu2ZnGeSe4 single crystals was established by the quantity of the observed nonpolar A and polar B (TO+LO) symmetry modes. [ABSTRACT FROM AUTHOR]

The dielectric functions of Cu2ZnGeS4 bulk crystals grown by the Bridgman method were measured over the energy range 1.4 to 4.7 eV at room temperature using variable angle spectroscopic ellipsometry. The observed structures in the dielectric functions were adjusted using the Adachi's model and attributed to interband transitions E0, E1A, and E1B at Γ:(000), N(A):2π/a(0.5 0.5 0.5), and T(Z):2π/a(0 0 0.5) points of the first Brillouin zone, respectively. The model parameters (threshold energy, strength, and broadening) have been determined using the simulated annealing algorithm. The decrease in the first gap, E0, has been attributed to a higher Ge-S hybridization. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. [ABSTRACT FROM AUTHOR]

Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu(In1-xGax)5Se8 crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies E0 and E1A are found to shift linearly to higher energies as the gallium content increases. [ABSTRACT FROM AUTHOR]

Analysis of the optical properties of bulk Cu(In1-xGax)3Se5 mixed crystals synthesized from the elements as a function of the Ga content is presented. Measurements of the complex dielectric function [variant_greek_epsilon](ω)=[variant_greek_epsilon]1(ω)+i[variant_greek_epsilon]2(ω) were performed at room temperature in the photon energy range of 0.8–4.7 eV using a variable angle of incidence ellipsometer. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. The structure observed in the dielectric functions attributed to the interband transitions E0, E1A, and E1B has been modeled using a modification of the Adachi’s model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap and higher critical points, have been determined using the simulated annealing algorithm. The values of E0 and E1A are found to increase linearly with the increasing Ga content. [ABSTRACT FROM AUTHOR]

The complex dielectric functions, [variant_greek_epsilon](ω)=[variant_greek_epsilon]1(ω)+i[variant_greek_epsilon]2(ω), of CuIn3Se5 and CuIn5Se8 crystals with different Cu contents have been determined in the 0.8–4.7 eV photon energy range by using spectroscopic ellipsometry. The spectral dependence of the real, [variant_greek_epsilon]1(ω), and imaginary, [variant_greek_epsilon]2(ω), parts of [variant_greek_epsilon](ω), as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity, has been modeled by using a modification of Adachi’s model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap, E0, and to higher critical points, have been determined by using the simulated annealing algorithm. [ABSTRACT FROM AUTHOR]

Spectral dependence of the real [variant_greek_epsilon]1(ω) and imaginary [variant_greek_epsilon]2(ω) parts of the complex dielectric function, complex refractive index, absorption coefficient, and normal-incidence reflectivity of CuGa5Se8 crystals with slightly different Cu contents are modeled using Adachi’s model for interband transitions. The results are in good agreement with the experimental data over the entire range of photon energies. The model parameters are determined using the simulated annealing algorithm. [ABSTRACT FROM AUTHOR]

Single crystals of CuIn5Se8 have been grown by chemical vapor transport. The crystals show a deviation from stoichiometry. The temperature dependence of their optical absorption spectra was investigated in the temperature range of 10–300 K. The variation of the energy gap with temperature was studied by means of a three-parameter thermodynamic model, the Einstein model, and the Pässler model. The values of the band gap at T=0 K, a dimensionless constant related to the electron-phonon coupling, an effective and a cutoff phonon energy have been estimated. It was also found that the major contribution of phonons to the shift of Eg vs T in CuIn5Se8 is mainly from optical phonons. The presence of Urbach’s tail just below the band edge in the absorption spectra of CuIn3Se5 has been observed. It was shown that the static structural disorders contribute mainly to the absorption below the direct band gap. [ABSTRACT FROM AUTHOR]

Single crystals of CuIn5Se8 have been grown by chemical vapor transport. The crystals show a deviation from stoichiometry. The temperature dependence of their optical absorption spectra was investigated in the temperature range of 10–300 K. The variation of the energy gap with temperature was studied by means of a three-parameter thermodynamic model, the Einstein model, and the Pässler model. The values of the band gap at T=0 K, a dimensionless constant related to the electron-phonon coupling, an effective and a cutoff phonon energy have been estimated. It was also found that the major contribution of phonons to the shift of Eg vs T in CuIn5Se8 is mainly from optical phonons. The presence of Urbach’s tail just below the band edge in the absorption spectra of CuIn3Se5 has been observed. It was shown that the static structural disorders contribute mainly to the absorption below the direct band gap. [ABSTRACT FROM AUTHOR]