Examination of the mid-gap unit cell absorber and P/I interface properties needed for 15% a-SiC:H/a-Si:H/a-SiGe:H multi-junction cells: numerical modeling study

The authors have performed numerical modeling of the high-band gap, mid-band gap, and low-band gap unit cells of a-SiC:H/a-Si:H/a-SiGe:H triple junction solar cell structures. This report focuses on the mid-band gap unit cell of these structures and examines the effect of the i-layer quality and the p-i interface on this unit cell´s performance. The purpose of this study is to determine reasonable performance goals for the a-Si:H mid gap unit cell of a 15% efficient multijunction structure as well as to prioritize the effect of certain material parameters. The authors find that when optimizing the cell by reducing recombination, the defect density in the absorber layer, not the p/i interface layer, is the key to improving cell efficiency. Further, of the three components of the efficiency (J_SC , V_OC, and the fill factor), improvements in the efficiency, due to reduced recombination, are more readily gained by increasing the fill factor than by trying to raise the open circuit voltage