Microstructure of polycrystalline CuInSe₂/Cd(Zn)S heterojunction solar cells

Polycrystalline CuInSe₂/Cd(Zn)S heterojunction solar cells deposited on Corning 7059 or soda-lime glass are characterized structurally and chemically by scanning electron microscopy and transmission electron microscopy in conjunction with energy-dispersive analysis of X-rays. Scanning electron micrographs reveal rough and uneven surfaces and cross-sectional morphologies of the Cd(Zn)S and CuInSe₂ layers. The crystallography and defect structure of the individual Cd(Zn)S, CuInSe₂ and molybdenum layers are examined by conventional and high resolution transmission electron microscopy. The crystal structures for Cd(Zn)S, CuInSe₂ and molybdenum are wurtzite, chalcopyrite and b.c.c. respectively. The Cd(Zn)S layer exhibits stacking faults on hexagonal basal planes. Planar defects such as twins and stacking faults on {112}_chalcopyrite planes are identified in the CuInSe₂ layer. The most significant features obtained from these cross-sections are (i) the lateral non-uniformity of the Cd(Zn)S and CuInSe₂ layers, (ii) the intimate bonding between these two layers, and an epitaxial relationship between grains of Cd(Zn)S and CuInSe₂ at the interface ({0001}_Cd(Zn)S ∥ {112}_CuInSe2), and (iii) the presence of voids and fractures in the CuInSe₂ layer. A correlation between the formation of fractures and voids and the defect structure in CuInSe₂ layer, and the mechanical stresses induced by differential thermal contraction of the substrate/film assembly is discussed.