Polycrystalline CuGaSe₂ thin film growth and photovoltaic devices fabricated on alkali-free and alkali-containing substrates

Polycrystalline thin films of ternary CuGaSe₂ (CGS), which are one of the potential materials for wide-gap top cells in tandem structure photovoltaic devices, are grown on alkali-free and alkali-containing substrates. The effects of alkali-metals on CGS film morphology, grain size, and photovoltaic device properties are studied. High temperature (>600 °C) growth of CGS films is also investigated with a focus upon variations in alkali-metal distribution profiles in CGS films and the formation of a surface Cu-deficient layer (CDL), which plays an important role in p-n junction formation. The use of a high growth temperature is effective in realizing large grain sizes due to enhanced elemental migration during growth, whereas the formation of a surface CDL is diminished and the alkali-distribution profiles in CGS films are modified resulting in a decrease in carrier density. It is suggested that large CGS grain sizes contribute to obtaining high photocurrent values, but coping with modifications in surface/interface and carrier density with alkali-metals is important to enhance overall photovoltaic performance.