Correlations of Single-Crystal CuInSe2 Surface Processing with Defect Levels and Cell Performance

The effects of various surface treatments used in the fabrication of CuInSe₂-based solar cells are investigated. These treatments include polishing, etching, and annealing. In particular, high-resolution photoluminescence (PL) and deep-level transient spectroscopy (DLTS) are used to identify the dominant defect states in cleaved and processed cell structures (including heterostructure formation with (Cd,Zn)S and Schottky barriers with Al on p-CuInSe₂ single crystals). These results are correlated with the junction electrical characteristics. Atomic-level images using spectroscopic scanning tunneling microscopy (SSTM) confirm the physical nature of the defect levels. Radiative recombination levels originating from the processing are identified near the surface region of the CuInSe₂ crystals. The energy and depth locations of these states that evolve from the formation of a defect/nonstoichiometric interfacial layer can limit the performance of the (Cd,Zn)S/single-crystal CuInSe₂ cells. The four major trapping levels have been confirmed by DLTS measurements. Two of these are shallow levels in the energy regions 100-114 and 150-185 meV, and two are deep levels in the range 340-385 and 475-496 meV, respectively. In the case of Al/p-CuInSe₂ Schottky barrier structures, only the shallow levels are detected. These results contribute to the understanding of the nature of the electrical junctions and the physical interfaces that control the electronic properties of the CuInSe₂ single crystals and the performance of the associated devices.