Development and optimization of high-efficiency thin-film silicon based solar cells using VHF PECVD techniques

The objective of this dissertation was an intensive experimental study of development and optimization of high-efficiency thin-film silicon based solar cells made using very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD) techniques. New growth regimes have been developed and established in our Thin Film Silicon Photovoltaic Laboratory at the University of Toledo for preparation of high quality hydrogenated amorphous silicon (a-Si:H), hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated nanocrystalline silicon (nc-Si:H) intrinsic (i-) layers at high deposition rates using advanced VHF PECVD techniques. Si:H and SiGe:H deposition phase diagrams have also been developed using real-time spectroscopic ellipsometry. Using phase diagram concepts for guidance, high efficiency a-Si:H, a-SiGe:H and nc-Si:H based single-junction solar cells of a n-i-p structure, a-Si:H/nc-Si:H tandem-junction solar cells of a n-i-p/n-i-p structure and a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cells of a n-i-p/n-i-p/n-i-p structure have been fabricated and optimized with intrinsic layers made using the VHF PECVD process.General strategies for guidance toward optimum thin-film silicon solar cell fabrication have been generated in this research on a basis of a previous intensive research on thin-film silicon based solar cell fabrication and characterization: The best a-Si:H and a-SiGe:H solar cells could be achieved with high quality a-Si and a-SiGe i-layers made in proto-crystalline regime close to the transition to the amorphous and nanocrystalline mixed (a+nc)-Si:H phase, while the best nc-Si:H solar cells could be obtained by having their high quality i-layers made in single-phase nanocrystalline silicon near the mixed (a+nc)-Si:H phase transition regime. With the guidance of these strategies, initial efficiencies of 9.99% for 7.3 A/s a-Si n-i-p single-junction solar cells, 11.3% for 9 A/s a-SiGe single-junction solar cells, 9.81% for 8 A/s nc-Si single-junction solar cells, 12.2% for a-Si/nc-Si tandem cells, and 12.4% for a-Si/a-SiGe/nc-Si triple cells have all been achieved with their corresponding intrinsic-layers made by VHF PECVD at high rates. Stabilized efficiencies of 9.46% for 2.2 A/s nc-Si single-junction solar cells, 10.5% for 3 A/s a-Si/nc-Si tandem-junction solar cells, 11.0% for 3 A/s a-Si/a-SiGe/nc-Si triple-junction solar cells have been obtained.