Performance dependence on grading width of a-SiGe:H component solar cells

For the bottom cell in a tandem structure, a-SiGe:H solar cells are often applied, because the red response of these cells is higher. In order to optimize the performance of these cells, the germanium concentration in the intrinsic layer is profiled. In this paper, the authors aim to understand, using computer simulations, the variation of external parameters of solar cells with the band-gap grading width near the interfaces. Experimental results show that grading is essential, but that the grading width near the p-i interface needs to be as small as possible, whereas that near the i-n interface needs to be as large as possible. Varying the grading width mainly affects the fill factor of the cells. Computer simulations indicate that the profile of the slope of the valence band tail in the graded regions is important, because it influences the density of state distribution in the cell