Naoya Aihara

Cu2SnS3 Thin-Film Solar Cells from Electroplated Precursors

Cu2SnS3 (CTS) contains non-rare metals and it has suitable optical characteristics for the absorber layer of thin-film solar cells. In this study, CTS thin films were fabricated by sulfurizing Cu?Sn precursors deposited by co-electrodeposition. Solar cells with a structure glass/Mo/CTS/CdS/ZnO:Al/Al were fabricated from the films. The best cell had an efficiency of 2.84%. A relatively high conversion efficiency was obtained from films with Cu/Sn?2.

Sulfurization temperature dependences of photovoltaic properties in Cu2SnS3-based thin-film solar cells

We report on the sulfurization of metal-alloyed precursors in Cu2SnS3 (CTS)-based thin-film solar cells. CTS thin films were prepared through the sulfurization of Cu?Sn alloy precursors at sulfurization temperatures of 500?580 ?C for 2 h in a N2 atmosphere with sulfur vapor. The Cu/Sn composition ratios of the sulfurized films were determined by X-ray fluorescence analysis to be in the range of 1.77?1.89. The photovoltaic properties of CTS-based solar cells improved with increasing sulfurization temperature owing to the higher external quantum efficiency at long wavelengths. The solar cell comprising a CTS thin film with a sulfurization temperature of 580 ?C exhibited the optimum performance among the cells examined: an open-circuit voltage of 244 mV, a short-circuit current density of 29 mA/cm2, a fill factor of 0.385, and a conversion efficiency of 2.7% were obtained.

Fabrication of Cu2GeS3-based thin film solar cells by sulfurization of Cu/Ge stacked precursors

In- and Se-free Cu2GeS3 thin films were prepared by thermal evaporation followed by sulfurization, and photovoltaic cells with a glass/Mo/Cu2GeS3/CdS/ZnO:Al/Al structure were fabricated. The composition ratios of the obtained films were Cu/Ge = 1.96 and S/metal = 0.92 on glass, and Cu/Ge = 2.08 and S/metal = 0.94 on a Mo-coated glass substrate. By X-ray diffraction measurement, the sulfurized films were identified to be Cu2GeS3. By optical measurement, the band gap energy was estimated to be 1.5–1.6 eV. In the visible region, a Cu2GeS3 film has an optical absorption coefficient that is on the order of 104 cm−1. A solar cell fabricated using the Cu2GeS3 thin film exhibited an open-circuit voltage of 380 mV and a conversion efficiency of 1.70%.

Donor-acceptor pair recombination luminescence from monoclinic Cu2SnS3 thin film

The defect levels in Cu2SnS3 (CTS) were investigated using photoluminescence (PL) spectroscopy. A CTS thin film was prepared on a soda-lime glass/molybdenum substrate by thermal co-evaporation and sulfurization. The crystal structure was determined to be monoclinic, and the compositional ratios of Cu/Sn and S/Metal were determined to be 1.8 and 1.2, respectively. The photon energy of the PL spectra observed from the CTS thin film was lower than that previously reported. All fitted PL peaks were associated with defect related luminescence. The PL peaks observed at 0.843 and 0.867 eV were assigned to donor-acceptor pair recombination luminescence, the thermal activation energies of which were determined to be 22.9 and 24.8 meV, respectively.

Exciton luminescence from Cu2SnS3 bulk crystals

The optical properties of Cu2SnS3 (CTS) bulk crystals grown by chemical vapor transport were studied by photoluminescence (PL) spectroscopy. The PL spectra from the CTS bulk crystals were analyzed as a function of excitation power and temperature. The main phase of the as-grown samples was determined to be monoclinic CTS by Raman spectroscopy. The observed PL spectra from the CTS bulk crystals were composed of peaks corresponding to free-exciton, two bound-excitons, and donor-acceptor pair recombination luminescence. The peak energies for the free-exciton and two bound-exciton emissions were 0.9317, 0.9291, and 0.9260 eV, respectively, at temperature of 4.2 K. The bound-exciton luminescence was not observed above 30 K. The thermal activation energies for the free-exciton and two bound-exciton emissions were 6.5, 4.8, and 5.2 meV, respectively. The fundamental band gap in the CTS bulk crystals was expected to be ca. 0.94 eV.

Photoluminescence characterization of Cu2Sn1-xGexS3 bulk single crystals

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.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, ...