Design, fabrication, characterization, and analysis of wide band gap gallium phosphide solar cells and gallium phosphide on silicon

Diaz, Martin
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University of Delaware
The cost of electricity generated by solar cells is an important factor limiting the competitiveness of photovoltaics in the energy economy. A reduction of this cost by increasing the conversion efficiency can be achieved with the application of multi-junction structures composed of III-V materials grown on silicon (Si), because Si is a well developed technology and less expensive than III-V or Ge substrates. It has been found that gallium phosphide (GaP) is a good III-V material candidate for high quality epitaxial growth on Si due to its lattice matching with Si. The GaP material can serve as the top solar cell above a Si solar cell in a multi-junction system or a platform for subsequent III-V solar cell growth. In this work, liquid phase epitaxy (LPE) was used to grow GaP on Si (111) substrates. The epitaxial layers have been characterized by SEM and EDS. Using a systematic approach to improve the quality of the GaP epitaxial layers, semi-uniform layers with a 2-4% Si concentration have been achieved. These epitaxial layers should provide a good platform for further growth of III-V solar cells. Also, in this work, a GaP solar cell device with a VOC of 1.56 V, a JSCof 1.2 mA, and an efficiency of 1.04% under AM 1.5G without any anti-reflection coating has been designed and fabricated. A GaAsP/Si 2-junction three terminal solar cell device with an efficiency of 37% has also been designed in this work. Future work includes the incorporation of the III-V epitaxial growth on Si using LPE with these device designs to fabricate high-efficiency multi-junction device structures.