Device Stability Of CdTe Thin-film Solar Cells With Different Back Contacts

Cadmium telluride (CdTe) is a Ⅱ-Ⅵ direct bandgap semiconductor with a band gap of 1.45 eV, in response to the best solar spectrum. Besides, CdTe thin film has a high absorption coefficient, low manufacturing cost and can form heterojunction solar cell with CdS thin film which has great competitiveness in the photovoltaic market and attracts widespread attention. However, CdTe solar cell performance is degraded under illumination, bias, humidity, thermal stress or gas atmosphere quite often. The degradation may come from several causes:poor back contact stability, poor ohmic contact in CdTe, poor CdS/CdTe junction, etc. It is essential to study the stability of CdTe solar cell, the only compound thin film solar cell can compete with crystalline silicon solar cell.In Chapter Ⅰ, I have reviewed the history and background of solar cells, and introduced the structure, principle and current-voltage characteristics of solar cells. Finally, I summarized the material properties and preparation of CdTe thin film solar cells.In Chapter Ⅱ, I have discussed the stability of CdTe solar cells with different back contacts. CdTe thin film solar cells with three types of back contact structures (Au, Cu/Au, MoOx/Mo) were subjected to heat stressing at 40 ℃ to 100 ℃ under air environment and at open-circuit bias. The device degradation was investigated using current-voltage (Ⅰ-Ⅴ), capacitance-voltage (C-V) and X-ray diffraction (XRD) measurements. The XRD studies revealed that the solar cell with Cu/Au back contact exhibited high stability because of the formation of Cu2-xTe, prohibiting the oxidation of tellurium rich layer, while the cell with Au back contact was found to form TeO2, which increased the height of schottky barrier at the back contact. The solar cell with the MoOx/Mo back contact was quite stable at the beginning of the stressing test, however, it degraded under high temperature. This was induced by the reduction of the oxygen denisity in MoOx through the oxidation of the Te rich layer. The study also applied "double diode model" to Au, MoOx/Mo cells to analyze the oxidation of back contact during the aging process by fitting light 1-V-T curve.In Chapter 111, I have studied the C-V variation of CdTe solar cells with different back contacts under dark and temperature elevating condition. I obtained much information from carrier concentration of PN junction, built-in voltage, the depletion region width changing, and further verified that the diffusion of Cu in CdTe cell with Cu/Au back contact and the reaction of Te elemental with MoOx thin film in MoOx/Mo cell at high temperature.