III-V Multi-junction Solar cell on Si
With 25.6% record efficiency , market-dominant crystalline silicon (c-Si) photovoltaic technologies are already very close to their theoretical maximum of about 29% . On the other hand III-V-based multijunction solar cells have been able to achieve efficiencies close to 40% without concentration and close to 45% with concentration  but their high manufacturing costs make their high-scale development unlikely without the use of concentration systems. New strategies are thus needed to fabricate low cost high efficiency (>30%) photovoltaic solar cells. Monolithic growth of III-V materials on a silicon substrate in a two-junction tandem architecture, the Si substrate acting as a bottom cell, presents an elegant and industry relevant pathway to achieve low cost high efficiency (≈35%) solar cells. The main challenge of this technology lies in the difference of lattice parameters between the III-V epilayers and the Si substrate, causing the nucleation and propagation of threading dislocations (TDs) in the top cell. These TDs act as recombination centers and, in the case of a high defect density, can seriously impair the performances of the cell.
The goal of the III-V/Si photovoltaic project is to demonstrate the potential of the technology, using the unique MBE facilities of UCL Photonics Group to grow III-V solar cells on Si with a low threading dislocation density (TDD). Promising results have already demonstrated for III-V on Si lasers using dislocation filter layers . First a model has been developed to assess the impact of TDs on the performances of GaAsP/Si tandem solar cells . In particular we show that efficiencies over 30% are achievable with TDDs under 106cm-2 and over 35% with TDDs under 105cm-2. Following these theoretical results, 1.7eV AlGaAs solar cells have been grown on Si with low TDD and relatively high Voc. Further work is being carried out to improve on these encouraging initial results and develop 1.7eV III-V top cells with a high enough material quality for integration in a future record-breaking III-V/Si tandem dual junction solar cell.
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