Graded Layer Modification for High Efficiency Hydrogenated Amorphous Silicon-Germanium Solar Cells

Hydrogenated amorphous silicon-germanium (a-SiGe:H) solar cells are fabricated with different thicknesses of the i/n graded layer and profiling shapes for appropriate band gap profiling. Comparison of the solar parameters between the U-shape profiling and the exponential shape (E-shape) profiling has been carried out at the same total thickness. In the U-shape profiling, as the thickness of the i/n graded layer increase, the fill factor (FF) and open circuit voltage (V-oc) of p-i-n single-junction a-SiGe:H solar cells increase, but the short circuit current (J(sc)) of cells decreases. In the E-shape profiling, the J(sc) of the a-SiGe:H cell is enhanced without significant losses in V-oc. For further analysis, a modified E-shape profiling is incorporated in a-Si:H/a-SiGe:H double-junction cells, which has resulted in the improvement of V-oc and FF of double-junction cells to 1.67 V and 0.753, respectively, without significant reduction in J(sc, SiGe)(QE), 12.58 mA/cm(2). (C) 2012 The Japan Society of Applied Physics