Abstract
Recently, solar energy has drawn much attention due to the development of renewable energy. Various solar cells have been widely studied for the application of solar energy to replace the conventional power for terrestrial application, including single/multi junction GaAs, crystalline Si cells (single/ multi crystalline), thin film technologies (CIGS, CdTe, amorphous-Si), and other emerging technologies. In this dissertation, the main research focus is on CIGS thin film technology. We have developed a simplified fabrication process and investigated the characteristics of CIGS solar cells prepared by the one-step sputtering process. In the first topic of this dissertation, we demonstrated the feasibility of one-step sputtering process for the fabrication of CIGS absorbers. By using pulse DC sputtering from a single quaternary CIGS target, the chalcopyrite structure is spontaneously developed on the substrate at 500 oC even without extra Se supply. The obtained CIGS absorber layer possesses unique columnar grains with (112) preferred orientation, which is quite different from those prepared by co-evaporation process. In addition, the characterization of one-step sputtered CIGS films and devices were also addressed. The best efficiency of 8.22 % was achieved at the area of 0.4 cm2 with open circuit voltage (Voc) of 505 mV, short circuit current density (Jsc) of 24.76 mA/cm2 and the fill factor (FF) of 0.66 by using a nearly stoichiometric CIGS target. The second topic of this dissertation focuses on the target composition effect on the CIGS films and devices. By tuning the composition of CIGS targets, we modified the film composition from Cu-rich to slight Cu-poor. In addition, the Cu2-xSe second phase was suppressed by using a modified CIGS target. Even no KCN treatment is required to obtain device quality CIGS films. The best device efficiency of 10.14 % was achieved at the area of 0.4 cm2 with open circuit voltage (Voc) of 505 mV, short circuit current density (Jsc) of 32.3 mA/cm2 and the fill factor (FF) of 0.63 by using a modified CIGS target. Furthermore, a non-destructive compositional mapping is demonstrated for the investigation of compositional distribution on the top surface and at deep level of CIGS films by using surface sensitive Auger electron spectroscopy (AES) and bulk sensitive field emission micro-analyzer (FE-EPMA). We found the inhomogeneous distribution of Se in the Cu-rich CIGS films may be related to the low photocurrent response in the long wavelength region. . The third topic in this dissertation focuses on the improvement of device performance. Na incorporation and nanostructure were introduced to modify the electrical and optical behaviors of CIGS solar cells. Sputtered NaF was first reported in this dissertation which could be integrated with our one-step sputtering process of CIGS absorbers. Better device efficiency of 10.41 % was observed while the NaF was added. Furthermore, we also developed a template-free technique to create nanotip arrays on the surface CIGS absorbers to enhance light harvesting, which could improve the collection of photocurrent.