P-type polycrystalline copper indium diselenide, $CuInSe_2$, thin films are used as absorbers in CdS/$CuInSe_2$ solar cells. It is necessary to enhance the photovoltaic properties of $CuInSe_2$-based solar cells that $CuInSe_2$ films have large grain size and (112) texture in order to decrease interface states in grain boundary and mismatch states of junction. On that purpose, we fabricated $CuInSe_2$ thin films by the sequential evaporation of $Cu_2Se$ and $In_2Se_3$ on $In_2Se_3$ films evaporated on a glass or a Mo/glass substrate. The substrate temperature and the additional evaporation of $In_2Se_3$ films were varied to investigate their effect on the properties of $CuInSe_2$ thin films.
The evaporation of $In_2Se_3$ at the substrate temperature of 400℃ formed extremly smooth $In_2Se_3$ thin films with a densely packed grain structure. The smoothness and densely packed grain structures were retained in finished $CuInSe_2$ films, which had maximum (112) texture and minimum second phases that were found to be $Cu_2Se$.
The sequential evaporation of $Cu_2Se$ and $In_2Se_3$ on $In_2Se_3$ films at the substrate temperature of 700℃ formed very smooth $CuInSe_2$ thin films, which had the maximum grain size of 3 ㎛ and (112) texture. As the substrate temperature increased, the electrical resistivities of $CuInSe_2$ films decreased. That was probably because the hole concentration of $CuInSe_2$ films was enhanced by an increasing amount of $Cu_2Se$ phases with the high hole concentration. $CuInSe_2$ films should have the deviation from its stoichiometric composition because of the dissipation of indium and selenium at high substrate temperature. It was thought that the deviation formed intrinsic defects working as acceptors and so enhanced the hole concentration.
As the additional evaporation of $In_2Se_3$ films increased, the amount of $Cu_2Se$ phases and the (112) texture decreased and the microstructure of $CuInSe_2$ films became rougher. The electrical resistivities of $CuInSe_2$ films increased as the additional evaporation of $In_2Se_3$ films increased. That was because the additional evaporation of $In_2Se_3$ compensated the deviation of its stoichiometric composition and consumed $Cu_2Se$ phases and so the hole concentration of $CuInSe_2$ films decreased.
The optical band gap of $CuInSe_2$ thin films were measured to 1.04eV and did not change so much by the substrate temperature of evaporation of $Cu_2Se$ or the additional evaporation of $In_2Se_3$.
$CuInSe_2$ thin films prepared on a Mo/glass substrate had a similar microstructures to those of films prepared on a glass substrate.
