$CuInSe_2$ films with direct band gap and high absorption coefficient is one of the promising absorbing materials for heterojunction solar cell. $CuInSe_2$ solar cells have been successfully fabricated by several methods. The two distinguished methods with solar high-efficiency are coevaporation of Cu, In, Se elements and selenization of predeposited Cu and In metals. Recently, it has been reported that $Cu(In_{1-x}Ga_x)Se_2$ solar cells prepared by elemental coevaporation method exhibited the efficiency of 17.7%. But, the elemental coevaporation method requires expensive high-temperature Knudsen cells, because of low vapor pressures of Cu, Ga and In. In this study, $In_2Se_3$ and $Cu_2Se$ compounds are selected as evaporation sources because they have relatively high vapor pressure at low temperature. In the case, $CuInSe_2$ films can be prepared in a simple evaporation chamber without Knudsen cells. $CuInSe_2$ films were prepared on CdS substrate to fabricate Au/$CuInSe_2$/CdS/ITO/ glass superstrate-type solar cells.
CdS films were grown by chemical bath deposition (CBD) process using cadmium salts and thiouria solution on ITO/glass substrate at the condition of pH = 12 and solution temperature of 90℃ and annealed at 400℃ for 30 min in $H_2$. $CuInSe_2$ films were prepared by the sequential evaporation of $In_2Se_3$ and $Cu_2Se$ on glass and CdS/ITO/glass substrates. The substrate temperature and $Cu_2Se/In_2Se_3$ ratio were varied during $CuInSe_2$ film preparation.
The $In_2Se_3$ were first evaporated on CdS/ITO/glass substrate at substrate temperature ranging from 250℃ to 450℃. The structure and morphology of the $CuInSe_2$ may depend on the those of In-selenide films. The evaporation of $In_2Se_3$ at a substrate temperature of 250℃ formed a smooth amorphous structure. The phase of the In-selenide films deposited at 250℃ and 350℃ was InSe after 600℃. Both annealed films showed a smooth and densely packed structure.
$CuInSe_2$ films were directly formed by the evaporation $Cu_2Se$ on In-selenide films at 600℃. The $CuInSe_2$ films with the $In_2Se_3/Cu_2Se$ molar ratio of 2 showed a smooth and dense morphology with small grain sizes.
The microstructure of $CuInSe_2$ films depends on the [In]/[Cu] ratio. The $In_2Se_3/Cu_2Se$ molar ratio during $CuInSe_2$ films formation varied 1.43, 1.25, 0.83, 0.5. As the $In_2Se_3/Cu_2Se$ molar ratio increased, $CuInSe_2$ films had a smaller grain size. When the $In_2Se_3/Cu_2Se$ molar ratio was below 1.0, $CuInSe_2$ films had a large grain size and $Cu_2Se$ second phase appeared. The $CuInSe_2$ films with $In_2Se_3/Cu_2Se$ molar ratio 1.25 had a smooth and densely packed structure.
However, high-temperature formation of $CuInSe_2$ films at 600℃ resulted in the interdiffusion of Cu and Cd at $CuInSe_2/CdS$ interface. As a consequence, the CdS buffer layer was destroyed. So, the formation temperature of $CuInSe_2$ films should be lowered. The morphology of $CuInSe_2$ films formed on CdS/ITO/glass at 450℃ was smooth and densely-packed.
Finally, $Au/CuInSe_2/CdS/ITO$/glass solar cells ($In_2Se_3/Cu_2Se$ molar ratio = 1.25, evaporation temperature of $Cu_2Se=600℃, evaporation temperature of $In_2Se_3=250℃) were prepared. The cells showed a diode I-V characteristics, but was not well reproducible. The result suggests that the CdS buffer layer should be better defined and not be destroyed during $CuInSe_2$ formation to fabricate reproducible junction properties.
