CuGaSe2 Thin Film Solar Cells: Challenges for Developing Highly Efficient Wide-Gap Chalcopyrite Photovoltaics

CuGaSe2, with a band-gap energy of 1.7 eV, is expected to be a practical material useful for wide-gap top cells in tandem structure solar cells. In contrast to the success of narrow-gap Cu(In,Ga)Se-2 devices which are already in commercial production, there is room for further improvement before considering practical applications for CuGaSe2 devices. In this review, the developments in CuGaSe2 devices reported to date are surveyed from the perspective of wide-gap chalcopyrite photovoltaics with a focus upon ternary CuGaSe2 films and solar cells. Differences observed between CuGaSe2 and CuInSe2 or Cu(In,Ga)Se-2 films and device properties and correlations with the alkali-effects are also discussed.