GaAs0.75P0.25 Directly Grown on Si Towards Cost-Effective High-Efficiency Multi-Junction Silicon Photovoltaics |
Jonas Grutke, Tadeo Schweigstill, Andreas W. Bett, Frank Dimroth, Oliver Höhn, David Lackner, Jens Ohlmann Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, --, Germany |
The combination of III-V-based solar cells with a conventional silicon solar cell is a promising approach for the development of highly efficient and stable tandem photovoltaic devices. We present results from our ongoing efforts towards high-quality GaAs0.75P0.25/Si dual-junction solar cells prepared by direct MOVPE growth. One focus is placed on the reduction of defects in the GaP nucleation layer on silicon. Refining the nucleation process yields low stacking fault densities of <2x105 cm-2. The main influences on the stacking fault density are the Ga precursor flow during quasi monolayer deposition and the process interruption between precursor pulses. In the subsequent GaAsxP1-x buffer structure which is used to overcome the lattice mismatch between Si and GaAs0.75P0.25, the behavior of threading dislocations is investigated. Independent of the grading rate of the first buffer layers, the threading dislocation density peaks around GaAs0.2P0.8 and continuously decreases afterwards. A series of GaAs0.75P0.25 single-junction solar cells on Silicon as well as GaAs0.75P0.25/Si dual-junction solar cells were grown on a set of chosen buffer structures with a threading dislocation density of 7x106 cm-2. Cells are processed with a novel rapid low-cost mask and plate approach to significantly reduce processing efforts. A detailed discussion of the cell performance will be presented at the conference. |