Improving the characteristics of CdS and CIAS films and the performances of CIAS solar cells by electrodeposition Cu–Se/CIAS binary structure precursors on FTO substrate

Abstract

Cu-poor electrodeposited CuIn1−xAlxSe2 (CIAS) precursor films were prepared to investigate the alteration in surface morphology of post-annealed CIAS films through post-annealing temperature adjustment. Scanning electron microscopy (SEM) and atomic force microscope (AFM) analyses demonstrated that surface morphology and root–mean–square (RMS) roughness of post-annealed CIAS films exhibited uneven and rough triangular structures. The crystal size of post-annealed CIAS films can be increased by increasing post-annealing temperature. The precursor film structure was modified by substituting Cu–Se/CIAS binary structure with CIAS single structure to proceed with the investigation. The apparent variation in surface morphology of post-annealed CIAS films changed from rough triangular structures to smooth round structures, and the RMS roughness of post-annealed CIAS films was reduced to <100 nm. The reduction was attributed to the formation of Cu–Se liquid phases during the post-annealing process, which enhanced elemental migration, recombination, and promotion of large grains and smooth surface formation. X-ray diffraction patterns showed three preferred growth orientations along the (112), (204/220), and (116/312) planes with chalcopyrite structures for all species. In addition, the characteristics of surface morphology, RMS roughness, and current measurement of subsequently deposited cadmium sulfide (CdS) film were studied and examined via SEM and AFM analyses. The surface morphology of CdS films deposited on binary structure post-annealed CIAS films exhibited smoothness, compactness, small RMS roughness, and large crystals with round and film-like structure. The AFM current images indicated that the distribution of leakage current paths was greatly diminished by changing the precursor film structure from CIAS single structure to Cu–Se/CIAS binary structure. The dark current–voltage characteristics of the CdS/CIAS heterojunctions showed that the reverse dark current density was decreased by approximately one order of magnitude from 4.02 × 10−4 (single structure) to 4.26 × 10−5 A/cm2 (binary structure). Furthermore, the conversion efficiency of CIAS solar cells was enhanced from 0.52 (single structure) to 1.44 % (binary structure) with increase in Voc and Jsc.