Characterization of InxGa1-xN/GaN MQWs Heterostructures for Solar Cell Applications

29th European Photovoltaic Solar Energy Conference and Exhibition; 221-224
We report a comprehensive comparison of structural and electrical investigations of high quality and high indium content InGaN epilayers in forms of bulk and multiple quantum wells (MQWs) to be used as active regions in solar cells. Solar cell designs have active regions that include 200 nm In0.26Ga0.74N and InxGa1-xN/GaN (x~ 0.10, x~0.20 and x~0.30) MQWs repeated for 25 and 35 cycles which were grown on sapphire (0001) by metal-organic chemical vapor deposition (MOCVD). The surface characterization of the epilayers was investigated by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The detailed characterization of crystalline quality and the morphological evolution of high In content InGaN MQWs were studied by High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) in addition to compositional data provided by Energy dispersive X-ray spectroscopy (EDX) and High resolution X-ray diffraction (HRXRD). It was found that changing the type of high In content InGaN active layer from bulk to very thin QW epilayers enhances greatly the crystalline quality by decreasing the defect density and the phase separation within the InGaN epilayers to a large extent. For solar cells based on InxGa1-xN/GaN MQWs (x~0.10, x~0.20 and x~0.30), decreasing both the growth thickness of InGaN QWs and the number of wells improved the structural and surface quality. The designs with fully strained 25 cycles InxGa1-xN/GaN MQWs (varied from x~0.10 to 0.30) epilayers shows very low defect density and excellent structural quality showing that they are highly promising for future applications such as solar cells. Besides, very high quality crystalline structure of the solar cell having 25 cycles of In0.1Ga0.9N (3 nm)/GaN (10 nm) MQWs, which was fabricated into a solar cell, showed a photovoltaic response. In addition, owing to excellent mechanical robustness, high resistance to energy radiation, alignment of energy bands favorable with silicon, strong optical absorption, and good thermal stability, InGaN based solar cells can be explored to be integrated with standard silicon PV technology. The innovative hybrid integration of InGaN solar cells with silicon solar cells and their incorporation in very high efficiency modules of reduced area would lead to a major break-through in the photovoltaic domain.