Fabrication and characterization of thin-film GaAs solar cells peeled-off from Si substrates

GaAs based III-V group solar cells are the most efficient and radiation resistant cell technologies in the photovoltaic technologies at present. The high cost of single crystal III-V wafers limits the use of GaAs in photovoltaic applications. Therefore, heteroepitaxial growth of group III-V materials and devices on cheaper Si substrate has been attracting huge interests recently. Also, epitaxial lift-off (ELO) technique can be used to reduce the substrate cost more possible by re-using the substrate to grow solar cell multiple times in sequential way. In this work, GaAs solar cell structures were grown in reverse order on 4o miscut (100) Si substrates by molecular beam epitaxy (MBE) system. Thin-film cell structures were designed as p-type and n-type with a different base layer thicknesses. Effect of thermal cycle annealing process has been investigated to enhance crystal quality. Strain layer super lattice (SLS) was used in the cell structures as dislocation filter. Two different processing methods were applied to obtain flexible thin-film GaAs solar cell. The processing stages of these two methods have been studied in detail and the effects of processing methods on thin-film quality were compared and analyzed. Optical and structural characterization of both grown samples and fabricated thin-film cells have been studied in detail by using various characterization techniques such as scanning electron microscope, transmission electron microscopy, high resolution X-ray diffraction and photoluminescence spectroscopy. Besides, quantum efficiency and I-V curve of flexible and lightweight GaAs thin-film solar cells peeled-off from Si substrate were fabricated and tested for the first time in this thesis study.