Structural properties and growth mechanism of CuGaSe2 thin films for solar cells grown by two-source CVD

The high direct bandgap of E-g = 1.68 eV makes CuGaSe2 a promising candidate for the top cell of a multijunction device with a CuInSe2 (E-g = 1.02 eV) bottom cell. Photovoltaic devices based on CuGaSe2 single crystals grown in a closed system with iodine as transport agent have already reached an efficiency of 9.7 %. We used open-tube CVD for the deposition of polycrystalline CuGaSe2 thin films. Cu2Se and Ga2Se3 powders were used as source materials and were volatilized by I-2/H-2 and HCl/H-2, respectively. Deposition of Ga- and Cu-rich thin films on plain and Mo-coated glass substrates was achieved by varying the Cu(g)/Ga(g) ratio in the gas phase at T-sub = 500 degreesC. Ga-rich films were deposited at [Cu]/[Ga] ratios up to 1/4. They were single-phase as confirmed by XRD measurements. Their grain size increased from 0.5 to 2 mum with increasing Cu content in the gas phase. Cu-rich films showed Cu2-xSe secondary phases which could be identified by Raman-Spectroscopy measurements. These secondary phases could be removed by wet-chemical KCN etching. XRD measurements revealed an increase in the lattice constants a and c with increasing Cu content in the films. This was attributed to a reduced density of Cu vacancies (V-cu) with increasing Cu content in the films. Layer deposition for different time intervals and subsequent characterization by SEM and XRD allowed postulation of a model for the growth mechanism of CVD-grown CuGaSe2 thin films.