Interfacial Alkali Diffusion Control in Chalcopyrite Thin-Film Solar Cells
2014
Ishizuka, Shogo | Yamada, Akimasa | Fons, Paul J. | Shibata, Hajime | Niki, Shigeru
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.
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