Role of bandgap grading for the performance of a-SiGe:H based solar cells

The influence of bandgap grading and bandgap discontinuities on the performance of a-SiGe:H based multi-junction solar cells is investigated. Different types of bandgap grading of the i-layer, like linear profiling in asymmetrical v- and u-form, are studied with respect to their influence on the solar cell parameters under white, red and blue light illumination. Additionally, the cell series with different bandgap designs are light soaked under filtered red light for 300 h and 3000 h. Optimum performance is found for an asymmetrical v-shape with a band gap minimum close to the p/i interface. Using the concept of bandgap grading, a-Si:H/a-SiGe:H tandem and a-Si:H/a-Si: H/a-SiGe:H triple solar cells have been fabricated with efficiencies above 10%.