Fabrication and characterization of GaAsxP1-x single junction solar cell on Si for III-V/Si tandem solar cell

Sammanfattning: Silicon based solar cells have been used as photovoltaic devices for decades due to reasonable cost and environment- friendly nature of silicon. But the conversion efficiency of silicon solar cell is limited; for instance, the maximum conversion efficiency of a crystalline silicon solar cell available in the market developed by Kaneka Corporation is 26 % [1]. In comparison, III-V compound semiconductor multi-junction solar cells are the most efficient solar cells with efficiency of 47.1% [2]. However, due to high-cost substrate materials, III-V solar cells are not the best option for large scale production in real life. Therefore, integration of III-V compound semiconductors on silicon substrate has been studied to obtain III-V/Si multi junction solar cells with high conversion efficiency with reasonable price. To this end, we studied epitaxial growth of on GaAs deposited on Si.This thesis presents the characterization results of the above epitaxial layer and fabrication of a single junction solar cell on GaAs coated Si substrate and its performance.In the first part of the project, epitaxial layer grown by Hydride Vapor Phase Epitaxy (HVPE) on different kinds of substrates at different growth conditions are characterized to identify the optimized growth conditions and a suitable substrate. Samples are characterized by High Resolution X-ray Diffraction (HRXRD) and photoluminescence (PL) to determine the composition of and its crystalline quality and by optical microscope to assess the surface morphology. Scanning Electron Microscope (SEM) is used to study the depth of the dry etched structures.The second part of the project deals with the fabrication process consisting of 21 steps to obtain a single junction solar cell structure on GaAs/Si. This process flow will be explained in some detail along with a brief description of several tools in cleanroom that have been used for this purpose.Finally, in the third part, devices are characterized to investigate their performance. Transmission Line Method (TLM) is used to obtain important parameters such as specific contact resistance. Current- voltage (I-V) relation of solar cell is investigated to acquire its efficiency. The lowest specific contact resistance measured in this project is for p-contact (for 4041DV- cell 8) and the highest efficiency measured is 1.64% (for 4041DV- cell 6).In conclusion, although the results obtained are far from the state-of-the art results, this work has laid the foundation for future work that can lead to a breakthrough in fabricating multi-junction tandem solar cell on silicon.