Interface passivation by Al2O3 and SnO2 at the CdTe solar cell front contact

Al2O3 and SnO2 thin layers employed as the passivation layer for MgxZn1-xO(MZO)/CdTe thin-film solar cells were grown by RF magnetron sputtering. Time-resolved photoluminescence (TRPL)spectroscopy spectra show that the minority carrier lifetime of the MZO/Al2O3/CdTe structure is much longer than that of the MZO/CdTe structure, indicating that Al2O3 has a strong passivation effect on CdTe. However, the CdTe solar cells with Al2O3 as the passivation layer demonstrate serious “S-kink” in the light current density-voltage curves. The band alignment of solar cells with different passivation layer were analyzed by X-ray photoelectron spectroscopy (XPS). The large conduction band offset between Al2O3 and CdTe was found to be responsible for the “S-kink” for the solar cells with Al2O3. By controlling the thickness of the SnO2 layer, quantum size effect can be used to adjust the conduction band offset between SnO2 and CdTe, thus reducing the “S-kink” phenomenon, improving the filling factor and short-circuit current density. A solar cell with an efficiency of 9.7% has been fabricated. This study demonstrates that large conduction band offset is the main reason for the “S-kink” in the J-V curves, and that reducing the conduction band offset can effectively decrease the “S-kink” phenomenon and increase the solar cell efficiency.