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