Interfacial Alkali Diffusion Control in Chalcopyrite Thin-Film Solar Cells

Alkali elements, specifically sodium (Na), are key materials to enhance the energy conversion efficiencies of chalcopyrite and related thin-film photovoltaic solar cells. Recently, the effect of potassium (K) has also attracted attention because elemental K has unique effects different from Na as well as a similar beneficial effect in improving device performance. In this study, the control of selective alkali K and Na diffusion into chalcopyrite thin-films from soda-lime glass substrates, which serve as the monolithic alkali source material and contain both K and Na, is demonstrated using ternary CuGaSe₂. Elemental K is found to be incorporated in the several ten nanometer thick Cu-deficient region, which is formed on the CuGaSe₂ film surface, while Na is ejected, although both K and Na diffuse from the substrate to the CuGaSe₂ film surface during growth. The alkali [K]/[Na] concentration ratio in the surface region of CuGaSe₂ films strongly depends on the film structure and can be controlled by growth parameters under the same substrate temperature conditions. The results we present here offer new concepts necessary to explore and develop emerging new chalcopyrite and related materials and optimize their applications

Si-Doping Effects in Cu(In,Ga)Se₂ Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells

We found that elemental Si-doped Cu­(In,Ga)­Se₂ (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance

Group III Elemental Composition Dependence of RbF Postdeposition Treatment Effects on Cu(In,Ga)Se₂ Thin Films and Solar Cells

The effects of RbF postdeposition treatment (RbF-PDT) on Cu­(In,Ga)­Se₂, CuInSe₂, and CuGaSe₂ thin films and solar cell devices are comparatively studied. Similar to the effect of the KF postdeposition treatment (KF-PDT), Cu­(In,Ga)­Se₂ and CuInSe₂ film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe₂. The device properties of the In-containing and In-free Cu­(In,Ga)­Se₂ solar cells also show contrasting results, namely, Cu­(In,Ga)­Se₂ or CuInSe₂ devices show an increase in the open circuit voltage (<i>V</i>_oc) and fill factor (FF) values and almost constant or a slight decrease in the short-circuit current density (<i>J</i>_sc) values with RbF-PDT, whereas CuGaSe₂ devices show no significant improvements in the <i>V</i>_oc and FF values but a substantial increase in the <i>J</i>_sc values. These results suggest that the alkali effects on the Cu­(In,Ga)­Se₂ film and device properties strongly depend on the group III elemental composition in the Cu­(In,Ga)­Se₂ films as well as alkali-metal species