Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells

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Abstract
The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-based polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of CdS with different bath temperatures (60 and 80 °C) are compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber layer. This precipitation cannot be avoided and seems to be a principal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning the interaction with the absorber layer surface. For bath temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe2 interface region consists of a continuous transition zone with low defect density, whereas for the 60 °C recipe the interface is sharper, but the CdS layer contains a high density of stacking faults. The structure of the CdS layer depends also on the bath temperature and the growth orientation of the CuGaSe2 grains. CdS(80 °C) crystallizes predominantly in the zincblende structure and contains less linear and planar defects than CdS(60 °C) which tends to incorporate hexagonal regions in the cubic matrix. Strains due to lattice mismatch as well as mixture between wurtzite and zincblende structures were revealed in high resolution transmission electron microscopy (HRTEM) images of the CdS(60 °C) layer. For CdS(80 °C) the strain is relaxed by twinning and small-angle grain boundaries which were imaged by HRTEM. A suitable CdS buffer layer for Cu-rich absorber layers could not be obtained by CBD because of either the low crystal quality [CdS(60 °C)] or the formation of Cu–S inclusions [CdS(80 °C)]. The enhanced interaction with the Ga-rich absorber layer and improved quality of CdS(80 °C) results in an improved device performance of Ga-rich CuGaSe2-based solar cells.