Karl Franzens University Graz | Graz University of Technology | |
Advanced transmission electron microscopy for the development of high‐efficiency solar cells Highly efficient multi-junction solar cells based on III-V compound semiconductor materials are applied for power generation in terrestrial concentration photovoltaics and in space applications. Solar-electric conversion efficiencies well above 40% have been obtained for concentrator solar cells grown by metal-organic vapor epitaxy on Ge cells. The high-resolution imaging and spectroscopic methods of transmission electron microscopy (TEM) can be applied successfully to supporting the development of novel solar cell concepts. Two application areas will be illustrated: concepts for buffer layer systems that control defects and layer strains in III-V dual-junction cells grown by metal organic vapor phase epitaxy on Ge and Si solar cells, and analyses of transparent and electrically conductive bond interfaces that are formed by direct wafer bonding of dual-junction solar cells grown on GaAs to separately fabricated Si solar cells. By applying aberration-corrected scanning TEM, combined with elemental analyses by electron-energy loss spectroscopy, the distributions of elements at the Ga/Si interfaces in wafer-bonded solar cells can be measured with high detection sensitivity, thus allowing to interpret the electrical properties of the direct wafer bond. Optimized solar cell concepts resulted in active cell regions with significantly reduced defect densities. The success of such concept developments led to a GaInP/GaAs//Ge cell with a record efficiency of 41.1 % and to a GaInP/GaAs//Si solar cell with a maximum efficiency of 30%. Current research aiming at higher solar cell efficiencies will be briefly addressed. |