Developments in CIGS Energy Conversion Devices—From Solar Cells to Hydrogen Evolution

Cu(In,Ga)Se₂ (CIGS) and its derivative materials are versatile, as well as resource and cost-efficient and thus, attractive for photovoltaic energy conversion device applications. To date, we have developed lightweight and flexible Cu(In,Ga)Se₂ (CIGS) minimodules using a monolithically interconnected structure with independently certified photovoltaic efficiency values greater than 18% (number of cells: 17, designated area: 68 cm², without Ag or S alloying). This accomplishment was the culmination of techniques for controlling alkali-metal doping, the activation of metastable acceptors, and a minimodule fabrication process developed on flexible substrates. Modification of surface and interface, and bulk crystal quality by alloying with Ag or S, or other elements is expected to be a promising approach for further enhancements of the photovoltaic efficiency value. In addition, it has been suggested that mechanically scribed cell edges (equivalent to the side wall of P3 in the module fabrication process) can be an important factor in the degradation of device performance; therefore, there is room for improvement with proper passivation/termination treatments in the cell separation process. In fact, significant increases in the shunt resistance, fill factor (FF), and thus, photovoltaic efficiency values, and suppression of photovoltaic performance deterioration under low irradiance conditions were observed for CIGS solar cells fabricated using a photolithography-based cell separation process in lieu of a conventional mechanical scribing process.<br/>Wide-bandgap CIGS-based photoabsorber materials are promising candidates for photocathode applications for the splitting of water into hydrogen as well as top-cell applications in tandem solar cells. We have demonstrated an over 8% half-cell solar-to-hydrogen (HC-STH) efficiency using ternary CuGaSe₂ (CGS)-based thin-films. This HC-STH value was demonstrated for a photocathode that underwent surface modification of CGS layer with RbF (q.e. not a postdeposition treatment). It has also been suggested that such surface modification of CGS thin-films with a Cu-deficient phase layer (CDL) may be effective in enhancing both the HC-STH and the open circuit voltage (<i>V</i>_OC) values of corresponding CGS solar cell devices. A CGS/CdS solar cell fabricated with a CDL demonstrated an independently certified photovoltaic efficiency value of 11.05% with a <i>V</i>_OC of 0.960 V, a short circuit current density of 15.9 mA/cm², and a FF of 72.4% (designated area: 0.497 cm², without RbF). These results suggest that improving wide-bandgap CIGS-based device performance requires different approaches from those used for conventional narrow-bandgap CIGS devices.