Hydrogenation of Dislocation-Limited Heteroepitaxial Silicon Solar Cells

Post-deposition hydrogenation by remote plasma significantly improves performance of heteroepitaxial silicon (Si) solar cells. Heteroepitaxial deposition of thin crystal Si on sapphire for photovoltaics (PV) is an excellent model system for developing the PV technology platform of film c-Si on inexpensive Al₂O₃-coated (100) biaxially-textured metal foils. Without hydrogenation PV conversion efficiencies are less than 1% in our model system, due to carrier recombination at electrically-active dislocations and other growth defects. Hydrogenation dramatically improves performance, with low-temperature hydrogenation at 350oC being more effective than hydrogenation at 610°C. Spectral quantum efficiency, secondary ion mass spectrometry (SIMS), and vibrational Si-H_x Raman spectroscopy measurements elucidate the effects of hydrogenation on the materials and devices. Quantum efficiency increases at wavelengths >400 nm, indicating hydrogenation is mostly affecting the bulk of the cells. SIMS detects nearly 100 times more hydrogen atoms in our cells than available dangling bonds along all dislocations. Yet, Raman spectroscopy indicates that only low temperature hydrogenation creates Si-H_x bonds; trapped hydrogen does not stably passivate dangling-bond recombination sites at high temperatures.