CuGaSe 2 Thin Film Solar Cells: Challenges for Developing Highly Efficient Wide‐Gap Chalcopyrite Photovoltaics

CuGaSe 2 , 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 CuGaSe 2 devices. In this review, the developments in CuGaSe 2 devices reported to date are surveyed from the perspective of wide‐gap chalcopyrite photovoltaics with a focus upon ternary CuGaSe 2 films and solar cells. Differences observed between CuGaSe 2 and CuInSe 2 or Cu(In,Ga)Se 2 films and device properties and correlations with the alkali‐effects are also discussed.