Structural and electrical properties of polycrystalline CuGaSe2 thin films have been studied by changing the Ga/Cu ratio in the films. CuGaSe2 thin films with various Ga/Cu ratio were grown over Mo-coated soda-lime glass substrates. With the increase of Ga content in CuGaSe2, morphology of the films was found to deteriorate which is associated with the smaller grain size and the appearance of impurity phases presumably due to the phase transition from the chalcopyrite structure to the defect-related phase on the surface of the films. Properties of the Ga poor films were affected by the Cu rich secondary phases. Electrical properties of the films were strongly influenced by the structural properties and degraded with increasing the Ga/Cu ratio in the film. Device performances, fabricated with the corresponding CuGaSe2 films, were found to be correlated with the Ga/Cu ratio in the films and consistent with the observed structural and electrical properties. 1. Introduction Chalcopyrite Cu(In,Ga)Se2, abbreviated as CIGS, is one of the most promising materials to realize high-efficiency, low-cost thin film solar cell. Efficiency of 19.9% has already been achieved for the CIGS-based solar cell [1]. As the ideal CIGS bandgap for highest conversion efficiency is speculated theoretically to be around 1.4？eV [2], CuGaSe2 ( ) with a bandgap of 1.68？eV [3] can be considered as a leading material to enable the highest possible efficiency. Moreover, the large band gap makes the CuGaSe2, an ideal absorber material for the top cell in a photovoltaic tandem device together with CuInSe2 as the bottom cell absorber [4]. However, so far, CuGaSe2 solar cells with a CdS buffer have achieved efficiency of around 9.3% for thin film [5] and 9.7% for single crystal solar cells [6]. Therefore, a better understanding of the material properties of CuGaSe2 is needed to realize efficiency beyond the current level. The electrical, optical, and microstructural properties of CIGS films are dominated by the various intrinsic defects originated from the off stoichiometry of the film composition [7–9]. Moreover, deviation from the ideal stoichiometry during growth of this material is reported to contain some secondary phases preferably segregated on the surface of the film. Particularly, formation of the Cu(In,Ga)3Se5, Cu(In,Ga)2Se3.5, and so forth phases on the surface of the slightly Cu-poor film (Ga/Cu-rich) and Cu-Se related secondary phase in the Cu-rich film is a commonly observed phenomenon in CIGS material grown by various methods [10, 11] and reported to have significant impact on
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