Abstract
CIGS polycrystalline thin films were successfully fabricated by one-step cathodic electrodeposition on Mo-coated glass. In this study, we applied a galvanometry mode with three-electrode potentiostatic systems to produce a constant concentration electroplating solution, which were composed of CuCl2, InCl3, GaCl3, and SeO2. Then these as-electrodeposited films were annealed in argon atmosphere and characterized by X-ray diffraction. The results revealed that annealing treatment significantly improved the crystallinity of electrodeposited films and formed CIGS chalcopyrite structure, but at low applied deposition voltage (−950 mV versus SCE) there appeared second phase. The cross-section morphology revealed that applied voltage at −1350 mV versus SCE has uniform deposition, and higher applied voltage made grain more unobvious. The deposition rate and current density are proportional to deposition potential, and hydrogen was generated apparently when applying potential beyond −1750 mV versus SCE. It was found that the CIGS compound did not match exact stoichiometry of . This result suggests the possibility of controlling the property of thin films by varying the applied potential during electrodeposition.
1. Introduction
CuInSe2 (CIS) and Cu(In, Ga)Se2 (CIGS) to be candidate materials as absorber material for the thin film photovoltaic device are popular because of high optical absorption and direct band gap. We present that the optimum band gap for solar cell is 1.45 eV. But of CuInSe2 film is near 1.02 eV at room temperature, not exactly located within the best operation area. CIGS cells offer a tunable direct band gap by adjusting the ratio of In to Ga to maximize the absorption of the solar spectrum. To add Ga with appropriate concentration to CIS material could form CuInxGa1−xSe2 and adjust the band gap ranges from 1.02 eV (pure CuInSe2) to 1.68 eV (pure CuGaSe2) [1]. This property could be utilized to fabricate multijunction devices for increasing efficiency. Taking CIGS film as an absorber can apparently cause efficiencies of up to 19.5% on a laboratory scale. CIGS cells also have the highest absorption coefficient of any thin film ( cm−1), which allows for greater than 99% of the incoming photons to be absorbed within the first micron of the material. Cost would be down because of low material requirement. It is estimated that the theoretical efficiency could reach 25% to 30%. After mass production the cost would probably be about US$0.03/W. In sum, a CIGS modules device will have great competition potential for solar energy in the future.
There are different techniques to prepare the CIGS absorber layer including both physical and chemical methods. The main concern for CIGS solar cell is to develop a low-cost and large-scale production technology. It is desirable technically to form the concerned thin film by a simple, safe, and inexpensive procedure. Many researches have reported the fabrication of CIGS by different approaches [2–16]. Xu et al. reported that applying a novel single source three-stage evaporation process to fabricate CIGS thin film solar cells with a power conversion efficiency of 10.6%. This technique enables the deposition of CIGS thin films using a conventional one-source-at-a-time evaporator and device quality films are formed in situ without selenization [3]. Chen et al. reported one-step fabrication of the chalcopyrite CIGS absorber layer without excess Se supply during/after deposition or postselenization treatments with an efficiency of 10.14% by using pulse DC sputtering route [9]. Later, Chen et al. discussed the characteristics of one-step sputtered CIGS films including compositional, structural, optical, morphological, and electrical properties. For the device fabrication, the authors claimed that Mo back contacts dominated the growth behavior of CIGS films deposited at high temperature [10]. Duchatelet et al. presented new advances on an atmospheric-based deposition process for Cu(In, Ga)Se2 with an efficiency of 12.4%. The electrodeposition consisted of a Cu-In-Ga mixed oxide/hydroxide layer from an aqueous solution at room temperature, followed by a thermochemical reduction and selenization. This process enables the one-step codeposition of the three elements, from a simple aqueous electrolyte containing nitrate ions as oxygen precursor, with fast growing rates and precise control of composition [11]. Wang et al. reported a novel approach for the fabrication of chalcopyrite CuInxGa1−xSe2 thin film solar cells by inkjet printing with a power conversion efficiency of 5.04%. The authors claimed that inkjet printing in atmospheric environment offers an opportunity for the direct patterning of absorber materials at large scale. Moreover, inkjet printing increases the raw material utilization ratio compared to more wasteful vacuum-based deposition techniques [12]. The techniques for the deposition of the CIGS layer can be either vacuum or solution-based methods [13–15]. Although the cell efficiencies obtained from CIGS layer fabricated by the vacuum methods were higher, the cheap and convenient solution-based approaches still attract a lot of research interest as a potential alternative [16].
Among various solution-based methods, electrodeposition is a potentially suitable preparation method for obtaining the low-cost CIGS layer. One-step electrodeposition, especially, has the most various advantages for high throughput and large-area fabrication, because of its low processing cost, the fact that there is no vacuum system needed, its high deposition speed, the fact that there is no use of toxic gases, its low operation temperature, and its simple operation steps. The whole process involves electrodeposition of precursor films of Cu-In-Ga-Se alloy and subsequent recrystallization by thermal annealing at high temperature. The most important part is to optimize deposition parameters and conditions to offer the prospect of achieving stoichiometry. The main emphasis and purpose of this paper focused on details of one-step electrodeposition process. It took depositions’ potentials to be process parameters. The goal of this work was to study the effect of the different deposition conditions and treatments used on the structural and morphological properties of CIGS in order to find out the trend of obtaining high quality precursor layers. Grain growth during annealing was observed. The mechanism of film growth and influence of hydrogen generated at cathode are also discussed.
2. Experimental Details
2.1. Thin Film Preparation
One-step electrodeposition was carried out potentiostatically in an aqueous solution containing CuCl2, InCl3, GaCl3, SeO2, and LiCl; the concentration of each electrolyte is shown in Table 1. No stirring was employed during deposition in first cases; then follow-up cases are under stirring with the velocity using a magnetic bar to improve the uniformity of the deposited sample during deposition (to protect the sample from a bubble generated under the electrolyte solution). The solution temperature was under room temperature without heating and recirculation. Diluted HCl was added drop by drop to adjust the pH of the solution to be between 1.9 and 2.0. It used a classical three-electrode potential static device with platinum (Pt) coated titanium gauze as an auxiliary electrode. Saturated calomel electrode (SCE) is to be a reference for measurement of the local potential of the cathode. The working electrode was a 1-μm thick Mo film on a glass substrate (with active deposition area of 1.0 cm2) deposited by DC sputtering on soda-lime glass. Ultrasonic cleaner cleaned the specimens with deionized water and acetone. Then they were dried in flowing air. The electrodeposition was controlled using an EG&G Mod. 263 Potentiostat/Galvanostat. To minimize the experimental error, electrodeposition was conducted under the same conditions (concentration of the electrolyte, pH of solution, supporting electrolyte, sample size, and magnetic stirring), except for applied bias. The applied potential was varied from −1950 mV to −950 mV versus SCE [17]. During electrodepositing the current variation was recorded. After electrodeposition precursor thin films were annealed in a tube furnace for 1 hour in Ar atmosphere at 400°C to form CIGS without oxidation and to improve their crystalline properties.
2.2. Thin Film Characterization
The specimens are characterized X-ray diffraction (XRD) by X-ray Diffractometer Rigaku using Cu Kα radiation ( Å) to confirm the formation and crystallization of the quaternary compound. The data obtained by XRD was compared with JCPDS file cards in order to determine the phases of the films. The field emission scanning electron microscopy (FESEM) images were used to demonstrate the surface morphology. EDX was done to define compound stoichiometry. The roughness of sample surfaces was detected by atomic force microscopy (AFM).
3. Results and Discussion
The anions of Cu2+, In3+, Ga2+, and are believed to exist in the solution. There are reduction reactions which are shown as follows [18, 19]. The optimum reduction potentials of each element listed after chemical formula are standard reduction potentials:
Polarization curve was conducted as in Figure 1. The deviation of these reaction potentials from the theoretically expected values was caused by various factors, such as the concentration of the electrolyte, pH of the solution, used substrate size, supporting electrolyte, and the absence or presence of magnetic stirring [20–23]. Based on theory and [17], the most applicable potential to codeposit these four components should be located between −950 mV versus SCE and −1950 mV versus SCE.
After a round of experiments without stirring, it was found that at higher bias (over −1750 mV versus SCE) a great deal of hydrogen was produced in 10 seconds. Figure 2 shows the i-t curves under different potentials. The current density of low bias (below −1350 mV versus SCE) would tend to stabilize and made i-t curves horizontal, but for high ones the values of current density were raised with time. Observing the surface after experiments, some areas had no film deposition because of seethe or bubbles adherence. And when presented in aqueous deposition baths, In and Ga deposition efficiencies are limited by H+ reduction, which cause composition inhomogeneity and pinholes in the film and hence limit the cell efficiencies [18]. To eliminate the effect of hydrogen generation, thereafter electrodeposition was performed by stirring with a magnetic bar.
Figure 3 shows the representative XRD pattern for stoichiometry CIGS samples, annealed and as-annealed, prepared at −1350 mV versus SCE. In the as-deposited sample, pattern just showed a peak at and the preferred formation of compound is not formed. Furthermore, the uniformity of the thin film is not good without magnetic stirring being used in the deposition procedure. After annealing the sample (400°C, 1 hours, Ar ambient), the quaternary compound, CIGS, is dominantly formed to be chalcopyrite structure in the deposited film. Sharp peaks at , 44.7°, and 53.0° corresponding to the diffraction of the (112), (220), and (312) planes were formed. The value for (112) plane changed from 28.7° to 26.9°. The phenomenon could be due to the facts that the residual stress would exist in the thin film during electrodeposition processes. The residual stress might lead to lattice anisotropic deformation. The diffraction peak shifted from high to low degree indicated that the lattice spaces increased after annealing. Furthermore, it suggested that the residual stress may be responsible for tensile stress. Figure 4 presents XRD patterns of the samples and thus heat treated after electrodeposition by different deposition potentials. Formation of the main phase of chalcopyrite structure is observed in all samples except −1950 mV one, which had only one preferred orientation at . However, in cases of potentials −950 mV and −1150 mV, there appeared another peak at . But the peak disappeared when applied potential was beyond −1350 mV. It suggested that would have been a secondary phase at lower electrodeposition bias. The peak of secondary phase corresponded to (203) plane of Cu-In alloyed film that annealed to be crystallized to provide the C11In9 phase (stable up to 320°C) [18]. The small positive shift in value of (112) peak indicated the incorporation of Ga and In sites and hence a decrease in lattice parameter [24].
Figure 5 shows the EDX compositional analysis data, which are normalized to Cu = 1, of the precursor films prepared by electrodeposition at different potentials and corresponded morphologies of top view by SEM. It notes that samples at which −950 mV and −1150 mV were applied are excluded because they did not detect Ga element in thin film. Surface morphology revealed a dendrite surface affected by the electrolyte concentration and duration of deposition which is due to limited ion concentration [18]. When electrodeposition process begins, a compact thin film comprising metal elements is formed on the substrate. Based on our group’s previous experimental results, it showed that the electrochemical kinetic behavior of the CIGS thin film is strongly influenced by the structure of the electrical double layer existing between the substrate and the electrolyte with different concentrations of Ga3+. With an increase in the electrodeposition time, the kinetic behavior of this electrodeposition system was gradually dominated by the diffusion process rather than the charge-transfer process [25]. In this study, we also observed similar kinetic behavior. With time increasing, the diffusion-controlled reaction gradually dominated the mechanism of the electrodeposition of CIGS layers. The limitation of the electrolyte concentration reaches the substrate, which ions consumed and deposited CIGS. Then, the dendrite structure appeared on the surface [26]. From the SEM images, it could be observed that when more potential was applied, the dendrite size decreased and surface porosity increased. This indicated that the number of nucleation sites in the case of lower deposition potentials is less than that of the higher one. It means that the coalescence of the elements occurs in fewer nucleated sites. Compositional analysis showed that stoichiometry of specimen prepared at –1550 mV was the closest to ratio with . It seems the applied potential and compound stoichiometry are independent, and composition of In, Ga, and Se had same trend with potential when normalized to Cu = 1. To investigate the elemental distribution of CIGS film prepared at −1350 mV via electrodeposition, it was examined by SEM equipped with EDX. The compositional mapping of Cu, In, Ga, and Se exhibited uniform distribution of each element as shown in Figure 6, indicating that no second phases existed on the surface of film.
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Figure 7 shows the morphology of the cross-section. As mentioned in the last paragraph, the deposition is limited by the electrolyte concentration that reaches the substrate. At lower applied potential, the metal ions in the bath were consumed slower and diffused more sufficiently than with higher applied potential conduction. Deposit film prepared at −1350 mV was uniform and orderly. The bottom part of −1550 mV cross-section is dense and compact, but with deposition time increasing, the follow-up deposit would become less smooth because of the effect of point discharge and diffusion. Even at −1750 mV and −1950 mV samples, some areas of Mo substrate were rarely deposited and some areas had masses of deposition. Table 2 shows the roughness compared with different deposition time measured by AFM. The roughness root mean square (RMS) and roughness average (Ra) values varied significantly between 20 seconds and 40 seconds; after 40 seconds the roughness varied slightly. It suggested that a large number of atoms did not generate and deposit on substrate surface at initial stage (from 0 to 20 seconds). When certain area of substrate surface had covered the early deposits, it would get advantages to enhance mass transfer process progress with short diffusion distance and high electrical potential gradients. Then the surface would become coarse in this stage (after 40 seconds). Figure 8 shows images and plots of the surface average height. The AFM was used to study the nucleation and development of roughness of the growth of CIGS film. The curve indicated that the roughness increased with deposition time. It was the evidence to explain the surface of thin film deposition influenced by point discharge and diffusion. At first, metal ions at cathode vicinity were depleted in the beginning, but ions cannot supply deposition on time completely. Some areas that had deposits would get advantages, such as short distance for diffusion and high electrical potential gradients due to high charge density. These effects enhanced mass transfer process progress at these knobby parts; then the surface would not worsen uniformly with time. To avoid this behavior, deposition potential, operation time, and stirring should be considered as control parameters. The theoretical thickness of the electrodeposited CIGS layer is determined by the following relationship [27]: , where is the current density (mA/cm2), is the molecular weight of CIGS (for CuIn0.5Ga0.5Se2 is 315.18 g), is the time for which the current is passed, is the density of CIGS, is the Faraday constant (96500 C), and is the number of electrons transferred. Based on this equation, one could evaluate the deposited thickness of the CIGS material which was controlled.
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4. Conclusions
One-step cathodic electrodeposition of CIGS thin films has been prepared, and the XRD patterns show that annealing treatment improved the crystallization of electrodeposited films and formed CIGS chalcopyrite structure, and at low applied deposition voltage (below −1150 mV versus SCE) there appeared the second phase of Cu-In alloys. The morphology of cross-section displayed that applied voltage at −1350 mV versus SCE has uniform deposition. Ga element could not be detected by EDX when deposition potential is below −1150 mV versus SCE. It was found that the compound did not match exact stoichiometry of . Varying the applied potentials during electrodeposition could control the properties and compounds of CIGS thin film, but the effect was restricted. It should therefore be operated in coordination with electrolyte concentration adjustment in solution to achieve a more ideal composition.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract no. NSC 99-2113-M-259-003 and Nanotechnology Research Center, National Dong Hwa University, for FE-SEM morphology measurements.