ZnO and Mg(OH)2 thin films were grown by chemical bath deposition (CBD) and applied to the fabrication of Cu(InGa)(SSe)2 (CIGSSe) solar cells. From the X-ray diffraction (XRD) measurements, the CBD-ZnO film was preferentially oriented toward the (002) plane and the CBD-Mg(OH)2 film was oriented toward the (001) plane. The optical and electrical properties of CBD-ZnO and CBD-Mg(OH)2 thin films on a fused silica substrate were investigated and it was found that the films showed a high resistivity and a high transmittance in the visible light wavelength regions. A cell efficiency of 14.3% (Voc: 557 mV, Jsc: 35.5 mA/cm2, F.F.: 0.721) was achieved using a CBD-ZnO buffer layer.
Using an optimized system comprising concentrator photovoltaic (CPV) modules, electrolyzers, and DC/DC converters for dynamic control, we achieved a one-day solar-to-hydrogen (STH) efficiency of 18.78% — the highest value for a sub-kilowatt-scale photovoltaic (PV) system under outdoor operation. We clarified that incorporating high-efficiency CPV modules and DC/DC converters in PV-driven solar hydrogen production systems is essential for obtaining the highest STH efficiency. Furthermore, we noted that the overpotential of electrochemical (EC) cells is a bottleneck for the improvement in STH efficiency; therefore, future work should focus on reducing such an overpotential.
Cu(InGa)Se2 (CIGS) thin films were grown by three-stage growth process using a molecular beam deposition system on a Mo/soda lime glass (SLG) substrate. The raman scattering spectroscopy of as-grown samples with a high Ga composition showed the presence of Cu2-x
Se compounds, which are thought to be responsible for the degradation of the open circuit voltage of solar cells. Rapid thermal annealing (RTA) using an infrared lamp in forming gas (N2 95% + H2 5%) ambient under atmospheric pressure was examined in order to remove the Cu2-x
Se compounds, and it was found that hydrogen is necessary to remove the Cu2-x
Se compounds. The influence of RTA conditions on device performance was investigated. A Cu(In0.4Ga0.6)Se2 (energy band gap: E
g=1.37 eV) solar cell with an efficiency of 11.7%, particularly open circuit voltage (V
oc) increased from 640 to 750 mV, was obtained by RTA at 400°C.
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