The dependence of apparent band gap （EgaDD） on buffer layer thickness has been measured. The result shows that the EgaDD of the cell with a buffer layer is widened by introduction of the buffer layer

which contributes to decrease the reverse saturation current Io and improve the VocThe C graded buffer layer can not only increase the inner electric field and decrease the recombination current in I layer to a certain degree but also result in the fast degradation of the performance of cells. The thickness of buffer layer must properly be chosen in order to guarantee both the improvement of efficiency and the stability of the cells.