Abstract
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.
Highlights
Chalcopyrite Cu(In,Ga)Se2, abbreviated as CIGS, is one of the most promising materials to realize high-efficiency, lowcost thin film solar cell
As the ideal CIGS bandgap for highest conversion efficiency is speculated theoretically to be around 1.4 eV [2], CuGaSe2 (x = Ga/ In + Ga = 1.0) with a bandgap of 1.68 eV [3] can be considered as a leading material to enable the highest possible efficiency
Polycrystalline CuGaSe2 thin films with the typical thickness of 2 μm were grown over Mo-coated soda-lime glass (SLG) substrates through a three-stage coevaporation process using molecular beam epitaxy system [14]
Summary
Chalcopyrite Cu(In,Ga)Se2, abbreviated as CIGS, is one of the most promising materials to realize high-efficiency, lowcost thin film solar cell. 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 the material properties as well as fabricated device performances [12, 13]. Advances in OptoElectronics cell performances in connection with the above mentioned secondary phases are scarce specially due to the relatively high defect formation energy of this defect in standard grown CuGaSe2 films. We have systematically varied the Ga/Cu ratio in the CuGaSe2 absorber layer to study the effect of Ga content on the electrical and structural properties of the film. Correlation between the performances of the fabricated solar cells and various Ga/Cu ratios in the absorber layer has been investigated
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