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

The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-b ased polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of C dS with different bath temperatures (60 and 80 degrees C) are compared. Coh erent Cu-Se precipitates are observed in both cases in the grains of the ab sorber layer. This precipitation cannot be avoided and seems to be a princi pal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning t he interaction with the absorber layer surface. For bath temperatures of 80 degrees C the interaction is much stronger and Cu-S inclusions are found i n the buffer layer. These may be responsible for shunts across the pn junct ion. Owing to the reduced interaction of the CdS deposited at 60 degrees C there are no Cu-S inclusions. For the 80 degrees C recipe the CdS/CuGaSe2 i nterface region consists of a continuous transition zone with low defect de nsity, whereas for the 60 degrees C recipe the interface is sharper, but th e CdS layer contains a high density of stacking faults. The structure of th e CdS layer depends also on the bath temperature and the growth orientation of the CuGaSe2 grains. CdS(80 degrees C) crystallizes predominantly in the zincblende structure and contains less linear and planar defects than CdS( 60 degrees C) which tends to incorporate hexagonal regions in the cubic mat rix. Strains due to lattice mismatch as well as mixture between wurtzite an d zincblende structures were revealed in high resolution transmission elect ron microscopy (HRTEM) images of the CdS(60 degrees C) layer. For CdS(80 de grees 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 absorb er layers could not be obtained by CBD because of either the low crystal qu ality [CdS(60 degrees C)] or the formation of Cu-S inclusions [CdS(80 degre es C)]. The enhanced interaction with the Ga-rich absorber layer and improv ed quality of CdS(80 degrees C) results in an improved device performance o f Ga-rich CuGaSe2-based solar cells. (C) 1999 American Institute of Physics . [S0021-8979(99)03101-1].