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Cu2Sn1-xGexS3 (CTGS) which is composed of earth-abundant and non-toxic elements is a promising material for the absorber layer of thin-film solar cells. In this study, the optical properties of CTGS bulk single crystals with varying germanium content were investigated by temperature and excitation power dependent photoluminescence (PL) measurements. At low-temperature, excitons and broad defect-related bands were observed in the PL spectra from all samples. These PL bands indicate a blue-shift with an increase in the germanium content, which suggested the formation of solid-solutions in CTGS. The broad band was dominated by donor-acceptor pair (DAP) recombination luminescence. The DAP bands were due to the transition of carriers between shallow acceptors and relatively deep donors for any alloy composition. Band-to-band (BB) recombination luminescence was also observed from all samples at room temperature. The band gap energies were varied from 0.933 to 1.544 eV with an increase in the germanium content, which was determined by spectral fitting of the BB bands. In addition, a small optical bowing parameter b, of ca. 0.1 eV was determined, which indicates that the band gap energy of CTGS can be controlled almost linearly by varying the alloy composition. Therefore, the optimum band gap energy for single-junction solar cells can be achieved by control of the CTGS alloy composition. [ABSTRACT FROM AUTHOR]

Temperature‐dependent optical absorption spectra from transmittance and reflectance measurements of Cu2SnS3 (CTS) thin films which are promising material for solar cells were investigated. Thin film CTS samples with Cu‐poor, near‐stoichiometric, and Cu‐rich compositions were prepared on glass substrates by thermal co‐evaporation and sulfurization. At low temperature, three band‐to‐band (BB) transitions that are allowed between triple upper valence bands and a single lower conduction band were observed for all samples. The square of the product of the absorption coefficient and the photon energy was plotted to estimate the band gap energy for three bands of the Cu‐poor sample. On the other hand, excitonic (EX) transitions that corresponded to three bands were observed for the Cu‐rich and near‐stoichiometric samples. Temperature‐dependent optical absorption spectra with the triple BB and EX transitions were resolved using a simplified fitting equation with Lorentzian functions as the EX transitions. The band gap, EX transition, and exciton binding energies for the lowest energy band of the Cu‐rich sample were determined to be 0.945 eV, 0.936 eV, and 8.9 meV at 6 K, respectively. In the low‐temperature region, anomalous blue‐shifts of the estimated band gap energy with increasing temperature were obtained for three bands of all samples. [ABSTRACT FROM AUTHOR]

The optical properties of Cu2GeS3 bulk crystals grown by chemical vapor transport were characterized using photoluminescence (PL) spectroscopy. PL spectra from the Cu2GeS3 bulk crystals were analyzed as functions of excitation power and temperature. The crystal structure of the PL measurement samples was determined as monoclinic Cu2GeS3 by Raman spectroscopy. The PL spectra contained two main bands of exciton (1.59 eV) and donor-acceptor pair (DAP, 1.45-1.55 eV) recombination luminescence. In addition, phonon-assisted transitions of bound excitons were observed on the DAP recombination luminescence. The exciton band was composed of free- and bound-exciton luminescence. The thermal activation energy of the bound-exciton luminescence was estimated to be about 9 meV. The activation energy of bound-exciton luminescence was approximately identical to the peak energy difference of the free- and bound-exciton luminescence. [ABSTRACT FROM AUTHOR]

Cu2SnS3 (CTS) is a promising compound for use as an absorber layer in thin-film solar cells because it is made up of low-cost and abundant elements. However, the band gap energy of 0.94 eV for monoclinic Cu2SnS3 is less than the optimal value for absorber layers in single-junction solar cells. On the other hand, the band gap energy of Cu2GeS3 (CGS) is approximately 1.5-1.6 eV. Therefore, in this study, we carried out the synthesis of Cu2Sn1− xGe xS3, which is a solid solution of Cu2SnS3 and Cu2GeS3. The structure of the prepared samples was investigated using X-ray diffraction and Raman analyses. Diffuse reflectance spectra were obtained using a UV-Vis-NIR spectrometer and used to estimate the band gap energy. In X-ray diffraction patterns from the synthesized compounds, the diffraction peaks were found to shift to higher 2 θ values with increasing x, indicating a decrease in the lattice constants. In addition, the estimated band gap energy increased from 0.86 to 1.53 eV with increasing x. Cu2Sn1 − xGe xS3 is, therefore, a potential candidate for use as the absorber layer in thin film solar cells because its band gap energy can be adjusted simply by varying its composition. [ABSTRACT FROM AUTHOR]

The ternary compound semiconductor Cu2SnS3 (CTS) is formed from non-toxic materials that are abundant in Earth's crust and other low-cost elements. CTS has also been reported to a band gap energy range of 0.93-1.77 eV and an absorption coefficient of 1.0×104 cm-1. Consequently, CTS is a potential material for the p-type absorber layer of thin film solar cells. In this study, we examined the dependence of the optical, electrical and photovoltaic properties of CTS thin films on the Cu/Sn composition ratio. CTS thin films were fabricated by sulfurizing evaporated Cu-Sn precursors with different Cu/Sn composition ratios at 560 °C for 2 h in a N2 atmosphere and sulfur vapor. The Cu/Sn composition ratios of the sulfurized films were determined to be 1.6-2.5 by X-ray fluorescence. The solar cell comprising a CTS thin film with a Cu/Sn composition ratio of 1.77 exhibited the best performance among the cells examined, an open-circuit voltage of 242 mV, a short-circuit current density of 28.9 mA/cm2, a fill factor of 41.7% and a conversion efficiency of 2.92% were obtained. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Cu2SnS3 (CTS) is a promising compound for use as an absorber layer in thin-film solar cells because it is made up of low-cost and abundant elements. However, the band gap energy of 0.94 eV for monoclinic Cu2SnS3 is less than the optimal value for absorber layers in single-junction solar cells. On the other hand, the band gap energy of Cu2GeS3 (CGS) is approximately 1.5-1.6 eV. Therefore, in this study, we carried out the synthesis of Cu2Sn1− xGe xS3, which is a solid solution of Cu2SnS3 and Cu2GeS3. The structure of the prepared samples was investigated using X-ray diffraction and Raman analyses. Diffuse reflectance spectra were obtained using a UV-Vis-NIR spectrometer and used to estimate the band gap energy. In X-ray diffraction patterns from the synthesized compounds, the diffraction peaks were found to shift to higher 2 θ values with increasing x, indicating a decrease in the lattice constants. In addition, the estimated band gap energy increased from 0.86 to 1.53 eV with increasing x. Cu2Sn1 − xGe xS3 is, therefore, a potential candidate for use as the absorber layer in thin film solar cells because its band gap energy can be adjusted simply by varying its composition. [ABSTRACT FROM AUTHOR]