Prepare dispersed CIS nano-scale particles and spray coating CIS absorber layers using nano-scale precursors
© Liou et al.; licensee Springer. 2014
Received: 31 October 2013
Accepted: 27 November 2013
Published: 1 January 2014
In this study, the Mo-electrode thin films were deposited by a two-stepped process, and the high-purity copper indium selenide-based powder (CuInSe2, CIS) was fabricated by hydrothermal process by Nanowin Technology Co. Ltd. From the X-ray pattern of the CIS precursor, the mainly crystalline phase was CIS, and the almost undetectable CuSe phase was observed. Because the CIS powder was aggregated into micro-scale particles and the average particle sizes were approximately 3 to 8 μm, the CIS power was ground into nano-scale particles, then the 6 wt.% CIS particles were dispersed into isopropyl alcohol to get the solution for spray coating method. Then, 0.1 ml CIS solution was sprayed on the 20 mm × 10 mm Mo/glass substrates, and the heat treatment for the nano-scale CIS solution under various parameters was carried out in a selenization furnace. The annealing temperature was set at 550°C, and the annealing time was changed from 5 to 30 min, without extra Se content was added in the furnace. The influences of annealing time on the densification, crystallization, resistivity (ρ), hall mobility (μ), and carrier concentration of the CIS absorber layers were well investigated in this study.
KeywordsNano-scale particle Spray coating method CIS absorber layer Annealing
In the past, the major developments for the solar cells were on the single-crystalline and multi-crystalline Si-based materials. However, those solar cells will spend too many materials, and they have the shortcoming of the high-temperature-dependence properties, i.e., their efficiencies are critically decreased as the temperature is increased from 40°C to 80°C. Single-crystalline Si-based solar cells, however, have been known to have two major disadvantages of low photoelectric conversion rate and expensive cost of single-crystalline silicon wafer . To overcome those problems, some researchers have examined the II-IV compound semiconductor solar cell [2, 3]. Among those, the CuInSe (CIS) and CuIn1−xGa x Se2 (CIGS) systems are known to have some advantages such as non-toxicity, long-time stability, and high conversion efficiency . For that, the CIS and CIGS thin films are being studied as promising absorber material for high-efficiency, low-cost, thin-film solar cells. The inherent advantages of the direct band gap material CIS and CIGS thin-film solar cells are based on its high absorption and therewith low layer thickness required for light absorption. The resultant potential for cost reduction, light weight, and flexible applications makes the CIS and CIGS absorber layer an all-round candidate for cheap large-area module technology as well as special architectural and space applications .
To further increase the applicability and profitability, a further improvement in the fabrication process of the CIS and CIGS thin films is necessary. In the past, CIS and CIGS absorber layers could be prepared by various methods, sputtering and co-evaporation are two of the most popular methods to deposit CIS and CIGS absorber layers. Wuerz et al. used the co-evaporation process to fabricate the highly efficient CIS absorber layers on different substrates  and Hsu et al. used the sputtering and selenization processes to deposit the CIGS absorber layers . Traditional vacuum methods are too complicated and difficult because those methods require a large number of expensive equipments, when the number of process parameters increases. Also, there are many non-vacuum methods were investigated, including spray pyrolysis , electrodeposit , and non-vacuum particle-based techniques . It can be easily assumed that the process cost could be lowered by non-vacuum thick-film process such as screen printing, though nano-sized powders of the CIS and CIGS precursors are needed for the paste. For synthesis of the nano-sized CIS and CIGS powders, the solvothermal method has been mainly adopted, for it can easily control particle characteristics and produces much amount of powder . However, single-phase powders of CIS and CIGS have never been synthesized by the solvothermal method [11–13].
The spray pyrolysis method (SPM) is a very important non-vacuum deposition method to fabricate thin films because it is a relatively simple and inexpensive non-vacuum deposition method for large-area coating . In this study, the micro-sized CIS powder was synthesized by the hydrothermal process by Nanowin Technology Co. Ltd. Because the formed CIS powder was aggregated in the micro-scale, for that we ground the CIS powder by the ball milling method. Particle-size change during process has been observed by Field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) patterns to examine the effect of adding dispersant or not and grinding time on particle size. A SPM method was used to develop the CIS absorber layers with high densification structure. However, only few efforts had been made to systematically investigate the effects of thermal-treated parameters in a selenization furnace on the physical and electrical properties of the CIS absorber layers. We would investigate the effects of annealing parameters on the physical and electrical properties of the CIS absorber layers. The feasibility of the crystalline phase CIS by controlling RTA-treated temperature and time has been checked.
After finding the optimum grinding time and KD1 content, the 6 wt.% CIS particle was dispersed into isopropyl alcohol (IPA) to get the solution for SPM to prepare the CIS absorber layers. The organic/CIS composite films were formed by spray coating method (SCM) on Mo/glass, and then the organic/CIS composite films were annealed for 5 min by the rapid temperature annealing (RTA) process in selenization furnace (the chamber size is 5 cm × 5 cm × 4 cm) under different annealing parameters to remove the used organic and crystallize the CIS absorber layers. Then, 550°C was used as the annealing temperature, without extra Se content was put in the furnace during the annealing process, and the annealing time was changed from 5 to 30 min. After annealing process, the crystalline structure was examined using the XRD pattern and the surface morphology and cross section observations of the CIS absorber layers were examined by FESEM, respectively. The electrical resistivity and the Hall-effect coefficients were measured using a Bio-Rad Hall set-up.
Results and discussion
Both the carrier concentration and the carrier mobility contribute to the conductivity. The resistivity of the all CIS absorber layers were in the region of 3.17 to 6.42 × 10−4 Ω-cm and the minimum resistivity of 2.17 × 10−4 Ω-cm appeared at the 20 min-annealed CIS films.
After finding the optimum grinding time, the CIGS powder had the average particle sizes approximately 20 to 50 nm. As the grinding time was 1, 2, 3, and 4 h, the FWHM values of the (112) peak were 0.37°, 0.37°, 0.38°, 0.38°, and 0.38° for CIS without KD1 addition and the FWHM values of the (112) peak were 0.38°, 0.43°, 0.47°, and 0.52° for CIS with KD1 addition, respectively. As annealing temperature was 550°C and annealing time of CIS absorber layers was 5, 10, 20, and 30 min, the FWHM values of the (112) peak was 0.496, 0.472, 0.424, and 0.371, respectively. In this study, the thicknesses of the annealed CIS absorption layers were around 1,905 ± 53 nm. The carrier concentration had a maximum of 1.01 × 1022 cm–3 at 30 min and the mobility had a minimum of 1.01 cm2/V-s at 30 min. The resistivity of all the CIS absorber layers was in the region of 3.17 to 6.42 × 10−4 Ω-cm and the minimum resistivity of 2.17 × 10−4 Ω-cm appeared at the 20-min-annealed CIS films.
The authors acknowledge financial supports of NSC 102-2622-E-390 -002-CC3 and NSC 102-2221-E-390-027.
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