A study of crystallinity in amorphous Si thin films for silicon heterojunction solar cells

In this work we analyzed the crystallinity of hydrogenated amorphous Si thin films deposited on n-type Si substrates using the effective medium approximation (EMA) method of a spectroscopic ellipsometer (SE) and evaluated their passivation quality by measuring effective carrier lifetime (τ_eff) and implied V_oc using quasi-steady-state photo conductance decay (QSSPC) simultaneously. The crystalline volume fraction of doped a-Si:H layers using RF-PECVD was controlled from ∼0% (nearly full amorphous phase) to above 90% (nearly polycrystalline phase) through varying deposition conditions. The passivation property depended on the crystallinity more strongly for p-a-Si:H than n-a-Si:H of which crystallinity was more sensitive to deposition rate relatively. The implied V_oc above 650 mV was achieved with crystallinity less than about 5% for p-a-Si:H and 20% for n-a-Si:H. The HRTEM images confirmed the reliability of SE analysis with EMA modeling and showed the maximum part of crystalline phase exists at the interface of a-Si:H and c-Si in the form of epitaxial growth configuration. By the optimization of each a-Si:H deposition conditions 17.17% the cell efficiency was accomplished on non-textured substrate.