Effects of photo-assisted electrodeposited on CuInSe2 thin films
© Chang et al.; licensee Springer. 2014
Received: 23 June 2014
Accepted: 5 November 2014
Published: 9 December 2014
Photo-assisted one-step electrodeposition has been applied to help in forming smooth and dense CuInSe2 films. The difference in surface morphology and crystalline quality between CuInSe2 films with various photo-assistance has been investigated. In the photo-assisted electrodeposition process, the many kinds of lamps providing maximum light intensity at about 380 to 620 nm were used as light source to be irradiated onto the surface of Mo-coated soda-lime glass substrates. The results suggested effects of photo-assistance including activating surface diffusion and growing high-crystalline quality films with reduced defects during electrodeposition.
Chalcopyrite (CH) semiconductors have been applied as absorber layers for polycrystalline thin-film solar cells and extensively studied due to their importance in optoelectronic applications. CuInSe2 (CIS) thin films are characterized as by a suitable band gap, a high absorption coefficient that exceeds 105 cm-1, and good stability . Among ternary compounds, CIS-based thin films are the most promising for use as optical absorbers for high-efficiency solar cells as they match the solar spectrum. Various methods have been used for the growth of CIS films, such as metalorganic vapor-phase epitaxy , molecular beam epitaxy , flash evaporation , coevaporation [5, 6], and electrodeposition . Several methods based on vacuum techniques have been developed to prepare CIS layers. A photo-assisted molecular beam epitaxy (MBE) process using an Hg lamp has been investigated by Tseng et al. . They found that the photo-assisted MBE process dramatically reduced the epitaxial temperature to 300°C, where photons may supply an additional energy to the film surface and activate the surface diffusion and the dissociation of Se2 and Se4 molecules . Nakada et al.  also proposed a laser-assisted deposition (LAD) process using the MBE system. In the process, a pulsed excimer laser (λ =248 nm) and pulsed YAG laser (λ =1,064, 532, 355, and 266 nm) were irradiated onto the substrate surface during Cu (In,Ga)Se2 (CIGS) deposition by the three-stage process. They found the cell performance improved for all CIGS solar cells with different Ga contents, and the grain size of CIGS thin films became large and a (1 1 2)-preferred orientation was enhanced by the LAD process. The photo-assisted electrodeposition has been used to help in forming smooth and dense CIS films . Accordingly, seeking possibilities for better electrodeposited film qualities, we hereby applied the photo-assistance to electrodeposition and aimed to find out the differences between electrodeposited CIS films with various wavelength photo-assistance. The crystallographic properties of CIS thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy.
Growth experiments were performed in a room-temperature solution, under slow stirring of the bath. Approximately 1.7-μm-thick CIS layers were obtained after 20 min of deposition. On completion of growth, CIS films were removed from the bath, rinsed with deionized water, and dried in an argon stream.
The surface morphology and chemical composition of the films were characterized by SEM (Philips XL-40FEG (Philips, Amsterdam, The Netherlands)) and energy-dispersive spectroscopy (EDS). AFM was used in contact mode. The Raman spectra were obtained in the backscattering configuration at room temperature with unpolarized light using a spectrometer (DILOR XY 800 (J Y, Inc., Edison, NJ, USA)) and an Argon-ion laser with a 514.5-nm wavelength as the light source. The phase composition and the crystallographic structure were analyzed by XRD using a multipurpose thin-film X-ray diffractometer (Bruker D8 SSS (Bruker AXS, Inc., Madison, USA)).
Results and discussion
EDS analysis of CIS layer deposited at various wavelengths of photo-assistance
Various wavelengths of photo-assistance
Cu at %
In at %
Se at %
380 to 476 nm
476 to 495 nm
495 to 570 nm
570 to 590 nm
590 to 620 nm
The (1 1 2) peak intensity shows more crystalline CIS with increasing intensity of photo-assistance. When the intensity of photo-assistance was increased, the CIS (1 1 2) peak intensity also increased. The (1 1 2) peak intensity shows more crystalline CIS at the wavelength of photo-assistance at about 380 to 476 nm. This is presumably because of an enhanced surface migration of reduced atoms by intensity of photo-assistance. If the atoms hold a high kinetic energy on the substrate surface, they can move to appropriate positions and form a more stable lattice plane that is a closed-packed (1 1 2) plane of chalcopyrite structure .
In summary, we have developed a method of various wavelengths photo-assistance electrodepositing for improving morphological and crystalline qualities of CIS thin films. At the photo-assistance of wavelengths of 380 to 476 nm, smooth and dense surface morphology can be achieved due to activation of surface processes with a lower potential (-0.7 V vs. Ag/AgCl). From the XRD pattern, an enhancement in (1 1 2)-preferred orientation is observed, suggesting a formation of a closed-packed (1 1 2) plane of chalcopyrite structure. Moreover, the enhancement in crystalline quality by photo-assistance of higher electron volt remains conspicuous after the following annealing step. From Raman spectroscopy, we notice a reduction in secondary phases such as Cu-Se and Se after applying photo-assistance on CIS films deposited at -0.7 V (vs. Ag/AgCl), and the increase in intensity of peaks in Raman spectra can also be referred to a better crystalline quality. In conclusion, the higher electron volt photo-assisted electrodeposition is effective to improve not only the surface morphology but crystalline quality of electrodeposited CIS thin films.
This work was supported by the National Science Council of Taiwan (Grant No. NSC 103-2623-E-006-010). The authors thank National Cheng Kung University, Tainan, Taiwan, for technical support, and the University also assisted in meeting the publication costs of this article.
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