| Summary: | 碩士 === 國立清華大學 === 先進光源科技學位學程 === 102 === In this study, we investigated the depth-dependent compositions and band structure of Cu(In,Ga)(Se,S)2-based solar cell device. In order to measure the elemental composition distribution and the band structure of the multiple-layered films, we polished the CIGSSe-based solar cell with a gradient along the normal direction of the sample to observe the variations of elemental distributions, chemical bonds and electronic band structure by means of X-ray photoemission spectroscopy (XPS). The structural characteristic at the interfaces of layers were also investigated by using X-ray absorption spectroscopy (XAS).
According to the observation of the band offset at the interface between CdS and CIGSSe layers, we can deduce that the conduction band corresponds to the cliff type (Ec= -0.47 eV). Therefore, this type of band structure is possibly increased the recombination probability at the interface and lead to a limitation in the open circuit voltage. The fitting results of B 1s photoelectron spectra reveals that in the bottom of ZnO layer, the concentration of the dopant boron becomes increased with respect to the boron oxide. Because the carrier concentration is increased in the region of ZnO layer near CdS, it would be beneficial to improve the carrier transport. Copper depletion was observed in the whole region of CIGSSe layer, which plays a role of acceptor due to copper vacancies and facilitates the formation of p-type semiconductor. The concentration ratio of In/Ga is decreased from the top to the bottom of the CIGSSe layer. The optimal band-gap distribution could be achieved by controlling the In/Ga ratios. We also found that the Mo(S,Se)2 layer was formed at the interface of CIGSSe and Mo layers, providing an ohmic contact and increasing the open circuit voltage to improve the device performance of the CIGSSe solar cell.
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