Characteristics of Cu(In,Ga)Se2 Thin Film Solar Cells Fabricated by RF Sputtering of Single CIGS Targets

Abstract

Solar cells have attracted great attention with the increase of problems such as an energy crisis and an environmental pollution. Particularly, thin film solar cells have been extensively investigated due to their small consumption of raw materials and possibility of lightweight. Among them, Cu(In,Ga)Se2 (CIGS) solar cell has attracted worldwide attention due to its merits of high conversion efficiency and stability. However, a very complicated fabrication process, so-called three-stage co-evaporation process, is required for the achievement of the highest conversion efficiency of ~20%. In addition, this method has a difficulty to produce large-area film with uniform composition. In this thesis, the properties of CIGS films and CIGS solar cells prepared by RF sputtering of single CIGS targets were investigated. In addition, antireflection (AR) coating and transparent conducting Al-doped ZnO (AZO) films was studied, which are necessary to enhance efficiency. The major results are as in the following. First, multi-layer antireflection (AR) coatings such as double- and triple-layer with ZnS and MgF2 were investigated. For this study, ZnS and ZnS-MgF2 composite films were prepared on soda-lime glass substrates and their optical constants were determined by envelope method. And MgF2 film was deposited on GaAs substrate by rf magnetron sputtering and their optical constants were determined by spectroscopic ellipsometry. In particular, ZnS-MgF2 composite films were fabricated by co-sputtering of ZnS and MgF2 target to obtain intermediate refractive index material for a middle layer in the triple-layer AR coating and these films exhibited the desired intermediate refractive index. All films were highly transparent in the range from visible and near-infrared (IR). In the case of ZnS-MgF2 composite film, optical absorption edge was continuously shifted to short wavelength and refractive index decreased with increasing concentration of Mg and F. Based on the extracted optical constants, single-, double- and triple-layer AR coatings on GaAs substrates were designed by using a quarter-wave thickness at a normal incidence and fabricated by rf magnetron sputtering. The experimental results of AR coating were compared to simulated results. Low reflectance could be obtained from single-layer AR coating only at a specific wavelength and could be obtained from multi-layer AR coating at wide wavelength regime. Additionally, incident angle dependence of the reflectance of the multi-layer AR coatings was also investigated and showed different behavior according to a number of layers. Second, the characteristics of the transparent conducting Al-doped ZnO (AZO) films were investigated. Al-doped ZnO films were deposited on soda-lime glass substrates by using dc magnetron sputtering as a function of argon gas pressure, O2/Ar gas ratio and substrate temperature, and their electrical and optical properties were investigated. As a result, the resistivity of the AZO films decreased with decreasing argon gas pressure or O2/Ar gas ratio. As the substrate temperature was increased, resistivity decreased continuously and the lowest value of resistivity was obtained at the temperature of 250 °C in pure Ar and 250-300 °C in O2/Ar ratio of 1.23%, respectively. The higher substrate temperature increased the resistivity a little. The transmittance was found to be very sensitive to O2/Ar gas ratio and substrate temperature. Addition of a very small amount of oxygen to argon (1.23% of O2/Ar ratio) or slight increase of the substrate temperature from room temperature to 150 °C enhanced the transmittance in visible region remarkably. Conclusively, the AZO films with low resistivity of order of 10- 4 Ωcm and high transparency in visible region could be prepared at the substrate temperatures of above 150 °C (the lowest resistivity of 3.19 × 10- 4 Ωcm at 250 °C) by dc magnetron sputtering and these films are applicable to various fields which require transparent conducting oxide films as well as solar cell field. Third, the properties of CIGS absorber layers prepared by RF sputtering without selenization were investigated. For this study, various single CIGS sputtering targets with the nominal compositions of Cux(In0.7Ga0.3)Sey (x=0.95~1, y=2~2.5) were fabricated. Employing these targets, CIGS films were deposited on Mo coated soda lime glass (SLG) substrates by using RF magnetron sputtering. When the CIGS precursor film deposited at room temperature using a stoichiometric CIGS target was annealed in an Ar atmosphere, grain growth of the CIGS film did not occur regardless of the post-annealing temperature. Grain size of the CIGS films was increased with increasing a deposition temperature. However, rough and porous surface morphology with the facetted grains was obtained and therefore very low efficiencies were achieved. For enhancing the efficiency and obtaining the smooth surface morphology, various CIGS targets with the nominal compositions of Cu0.95(In0.7Ga0.3)Se2.5 and Cu0.95(In0.7Ga0.3)Se2.1 were used to fabricate CIGS absorber layer. Grain size of these films was smaller than that of the CIGS film fabricated using a stoichiometric CIGS target. This result is thought to be caused by Cu-poor composition of these CIGS targets. Although the CIGS films with a smoother surface morphology can be obtained by using Cu-poor CIGS targets, surface morphology becames rough with increasing deposition temperature. Conversion efficiency of about 2% was obtaind from the CIGS solar cell fabricated using the Cu0.95(In0.7Ga0.3)Se2.5 target. However, the CIGS solar cells fabricated using the Cu0.95(In0.7Ga0.3)Se2.1 target, containing the smaller quantity of Se, showed the lower conversion efficiency. Meanwhile, for supplement of selenium, CIGS films fabaricated by using the single target of Cu(In0.7Ga0.3)Se2.5 powder were annealed in Ar atmosphere with selenium. These films showed the densely packed surface morphology. Conversion efficiency of the solar cells fabricated using these CIGS films was enhanced. Consequentially, selenization process is beneficial to improvement of structural and electrical properties. Hence, in the next study, the effect of selenization of CIGS precursor film was systematically investigated. Finally, the characteristics of CIGS absorber layers after selenization of CIGS precursor film deposited at room temperature were systematically investigated. In this study, the CIGS targets with various compositions of Cux(In0.7Ga0.3)Sey (where x=0.9, 1

y=2, 2.2, 2.5) were fabricated. Utilizing these targets, the CIGS precursor films were produced by RF magnetron sputtering at room temperature. The CIGS precursor films were then selenized in a tube furnace under Se atmosphere using Se powder. From the stoichiometric CIGS target, Cu-rich CIGS films were obtained and very low conversion efficiencies were achieved due to inappropriate composition and porous and rough surface morphology with large grains. In contrast, the CIGS films fabricated by using the Cu-poor CIGS target showed the device-quality stoichiometry ratio and densely packed surface morphology with small grains, as well as the chalcopyrite structure without the second phases. Increase of the Se content in the single CIGS target led to decrease of the Cu content and increase of the (In+Ga) content in the CIGS film, which created the ordered vacancy compound (OVC) phases and increased the band gap (Eg) of the CIGS film resulting in increase of the open-circuit voltage (Voc). The difference between Eg/q - 0.5V and Voc was remarkably decreased with the formation of the OVC phases. The highest efficiency of 6.88% could be achieved from the CIGS solar cell fabricated by using the Cu-poor and Se-excess target with the nominal composition of Cu0.9(In0.7Ga0.3)Se2.2. Consequently, the composition of the single CIGS target was found to play a key role in the structural and electrical properties of the CIGS film and CIGS thin film solar cell efficiency. Characteristics of CIGS films selenized using mixing powders with various weight ratios of alumina to selenium were invesitigaed. For this experiment, the CIGS precursor films were deposited using the single CIGS targets with the nominal compositions of Cu0.9(In,Ga)Se2 and Cu0.9(In,Ga)Se2.5. As the results, regardless of target composition, grain size was increased with increasing the Se ratio of mixing powder. The MoSe2 layer was formed between the CIGS and the Mo layer and its thickness was also increased with increasing the Se ratio of mixing powder. The MoSe2-layer thickness of the CIGS film fabricated using the Cu0.9(In0.7Ga0.3)Se2.5 target is thicker than that of the CIGS film fabricated using the Cu0.9(In0.7Ga0.3)Se2 target because of larger amount of Se in the CIGS film fabricated using the Cu0.9(In0.7Ga0.3)Se2.5 target. From the auger electron spectroscopy (AES) depth profiles after selenization of the CIGS films, it was found that each element of the CIGS film was evenly distributed in the CIGS films without grading of elements. This means that the CIGS phases including the OVC phase in the CIGS film fabricated using Cu0.9(In0.7Ga0.3)Se2.5 target are evenly distributed. The open-circuit voltages of the CIGS solar cells fabricated using the Cu0.9(In0.7Ga0.3)Se2.5 target were generally higher than those of solar cells fabricated using the Cu0.9(In0.7Ga0.3)Se2 target. However, the open-circuit voltage of the CIGS solar cells fabricated using the Cu0.9(In0.7Ga0.3)Se2.5 target was decreased with decreasing Se ratio of mixing powder used for selenization. Also, characteristics of CIGS films selenized using nanocrystalline precursor film were invesitigaed. CIGS films were deposited at various temperatures using a single quaternary Cu0.9(In0.7Ga0.3)Se2.5 target. Based on the results of XRD patterns and TEM images, as-deposited CIGS films fabricated at the temperatures of 120 °C and 130 °C were considered as the nano-crystalline CIGS precursor films and selenized at 475 °C for 30min. The CIGS films showed a (112) preferred orientation. All of the selenized CIGS films had the similar composition regardless of deposition temperature although Cu content in the film was slightly different. From the Raman spectra of the CIGS films, the existence of OVC phase which can lead to decrease of open-circuit voltage loss was identified. The CIGS film with smooth and dense surface morphology as well as the large grain size was obtained by increase of Se ratio in a tube furnace and using the nano-crystalline CIGS precursor film. And the thickness of the MoSe2 layer was decreased after selenization of the nano-crystalline CIGS precursor film compared to that of the amorphous CIGS precursor film. The highest efficiency was achieved from the CIGS solar cell fabricated using the nanocrystalline CIGS precursor film deposited at 130 °C and its value is 8.09 % with the open-circuit voltage of 0.571V, short-circuit current density of 26.58 mA/cm2 and the fill factor of 53.28%. This result is thought to be achieved by the beneficial factors such as the existence of an OVC phase, the thickness reduction of a MoSe2 layer, smooth surface morphology and the decrease of defect density achieved by dense CIGS film with large grain.