Preperation Of CIGS Absorbers By Magnetron Sputtering Of Quaternary Target And The Fabrication Of CIGS Photovoltaic Devices

Cu(In_1-xGa_x)Se₂?CIGS? solar cell is one of the most promising thin-film solar cells for practical application due to its outstanding advantages, such as high efficiency, good stability and good low-light performances. It has been successfully commercialized in recent years. In 2015, the Zentrum für Sonnenenergie- und Wasserstoff-Forschung?ZSW??Center for Solar Energy and Hydrogen Research? has demonstrated the CIGS solar cells with new record efficiencies up to 21.7%. In a remarkable feat, scientists at EMPA, the Swiss Federal Laboratories for Materials Science and Technology, have developed thin film solar cells on flexible polymer foils with a new record efficiency of 20.4% for converting sunlight into electricity. But these high-efficiency CIGS solar cells were fabricated by co-evaporation process which need the extremely harsh conditions, and difficult to achieve continuous production. Although the solution method can greatly reduce the production cost of CIGS solar cells, the maximum efficiency of solution process only reached about 70% of a co-evaporation method. Magnetron sputtering process has a variety of advantages for fabricating CIGS solar cells, such as the good reproducibility, high purity as-prepared film and continuous production possibility, so it has attracted the extensive attentions of researchers. Postselenization of sputtered alloy precursor layers using H2 Se gas or Se vapor is one of the mostly adopted methods to fabricate the CIGS absorber layers. Fabrication CIGS absorbers from a single quaternary target composed of Cu, In, Ga, and Se is another sputtering method. However, the stoichiometry deviation and abundant secondary phase make it difficult to fabricate high quality CIGS absorbers by direct sputtering the quaternary CIGS target. On the other hand, high toxic H₂Se was involved to selenize the CIGS absorber. In order to resolve such problems, the CIGS absorber was prepared by sputtering of CIGS quaternary target. The detailed composition, structure, morphologies and device characteristics of CIGS absorbers and corresponding devices were also investigated. Main research works are listed as follows.1. We studied RF sputtering deposition CIGS absorber layers using the single-source CIGS ceramic target with stoichiometric ratio?Cu: In: Ga: Se = 1: 0.7: 0.3: 2?.We found that the composition of sputtering target and the resulting CIGS thin film existed a huge difference, and has great relevance with the work distance between the target and the substrate. The Cu/?In+Ga? ratio of as-prepared CIGS films decreased from 1.29 to 0.96 when the working distance become increased. But the selenized CIGS films with the Cu/?In+Ga?>1 ratio, showed a copper-rich composition, which is mainly due to the copper-rich CIGS crystal growth mechanism. We determined the preferential growth direction of the crystallographic planes by X-ray diffraction, and determined the crystal structure by the Raman spectroscopy. The selenized CIGS films showed the absence of impurity phases. The photoelectric conversion efficiency only received 0.65% using such process by fabricating the CIGS solar cells. The work in this chapter showed it is feasible to fabricate the CIGS solar cells by RF sputtering deposition process using the single-source CIGS ceramic target with stoichiometric ratio, but further optimization was needed.2. We studied the fabrication conditions and characteristics of CIGS absorber layers using RF sputtering deposition with the copper-poor CIGS ceramic target?Cu:In:Ga:Se = 1:0.7:0.3:2?. We found the copper-poor CIGS ceramic target could fabricate the copper-poor as-prepared CIGS films, and the selenized CIGS films still kept the copper-poor composition. The CIGS solar cell efficiencies had the great relevance with the selenization temperature. When the annealed temperature is 550 ?, it achieved the maximum efficiency; when the temperature is lower or higher than 550 ?, the solar cell efficiency is lower than this value.The device performance parameters were reported based on the active area of 0.21 cm², excluding the area of the Al grid electrode??5% of total device area?. The as-fabricated device with a selenization temperature of 550 ? showed a PCE of 5.02%?Voc = 0.413 V, Jsc = 15.64 mA/cm², FF = 63.84%?. Such production process significantly improved the efficiency of CIGS solar cells compared with the RF sputtering deposition CIGS absorber layers using the stoichiometric ratio CIGS ceramic target.3. We studied RF and DC sputtering deposition high-quality CIGS absorber layers using the stoichiometric CIGS ceramic target?Cu: In: Ga: Se = 1: 0.7: 0.3: 2? and indium metal target. By optimizing the DC sputtering process of indium target, we fabricated a series of copper-poor CIGS thin films, and we effectively regulated the composition of CIGS thin films. We characterized the as-prepared CIGS films by morphology and component analysis, and characterized the selenized CIGS films by morphology, composition and structure. The as-fabricated device with the optimized prepared process showed a PCE of 10.29%?Voc = 0.532 V, Jsc = 32.02 mA/cm², FF = 60.50%? based on the active area of 0.36 cm².This paper systematically studied sputtering deposition CIGS absorber layers using the CIGS ceramic target with different stoichiometric ratio, and systematically studied the relationship of the composition of target and the composition, morphology, and structure of selenized CIGS films.We found it was hard to fabricate high-efficiency CIGS solar cells by sputtering CIGS ceramic target with stoichiometric ratio, and the photoelectric conversion efficiency only received 0.65%. When we used the copper-poor CIGS ceramic target as sputtering source, the device performance could improved to 5.02%. By optimizing the fabrication process of RF and DC sputtering deposition CIGS absorber layers using the stoichiometric CIGS ceramic target and indium metal target, the as-fabricated CIGS device with the highest efficiency showed a PCE of 10.29%.