SnO₂ Films Prepared By PECVD And Their Applications On CdS/CdTe Thin Film Solar Cells

Cadmium telluride-based thin-film solar cells have become the research focus of solar cells in recent years. A reduction in the thickness of the CdS layer could increase the transmittance of visible light, so the CdTe thin-film solar cells with higher short-circuit current densities (Jsc) and efficiency could be obtained. However, as the CdS window layer turns thinner, it's necessary to use a high-resistance transparent (HRT) blocking layer between the transparent conducting oxide (TCO) and the CdS layer. Such blocking layer must satisfy the requirements as following: it has high bulk resistance, its crystalline lattice is matched well with that of TCO, and its surface is much smoother than the one of TCO thin film. The researches about HRT blocking layers have been advanced abroad, but the research reports on HRT in our country have rarely been found. There are two respects of the researches about HRT films. One is that if SnO2:F is used as TCO, the un-doped SnO2 thin films prepared by LPCVD is adopted as HRT buffer layer. Another respect is: if CdSnO4 is used as TCO, ZnSnO4 thin films prepared by sputtering are chosen as HRT buffer layer. Nonetheless, these fabrication technologies have some disadvantages. Hence, we bring forward the new technological course, which is to deposit the un-doped SnO2 thin film by the PECVD method. The influence of the deposition condition and post treatment on the structure and character of SnO2 thin film is investigated in the thesis. First of all, the crystalline properties of the as-deposited and annealed films were studied by X-ray diffraction, X-ray Photo Electronic Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The results show that the as-prepared tin oxide thin films were transferred from amorphous state to polycrystalline state through annealing. The average surface roughness of the annealed thin films decreases and the crystal grains grow more completely. The concentration of carriers and Hall coefficients increase with annealing temperature, and the dark conductivity increases by 10 times. It was found that there are some Cl atoms in the films by virtue of using the SnCl4 as Sn resource. However, the Cl atomic content in the films decreases as the annealing temperature increases.Secondly, the applications of the un-doped SnO2 thin films prepared by PECVD after annealing as the blocking layer between the TCO and CdS window layer of CdTe thin film solar cells are studied. The effects of HRT blocking layers on the performance of the cells are analyzed, and the parameters of CdTe solar cells of two kinds with and without HRT blocking layer are compared. The results show that HRT blocking layer is useful to collect and transmit carrier. The obvious improvements in performance of the CdTe solar cells are observed. The open-circuit voltage increases by 4.59% while the short-circuit current Isc decreases by 2.19%, which leads to FF and efficiencyηincrease by 6.05% and 10.5%, respectively.Finally, the experimental results and the feasibility of adopting the thin films prepared by PECVD as buffer layers are discussed.