Hydrogenation Mechanisms of Poly-Si/SiOx Passivating Contacts by Different Capping Layers

Herein, posttreatment techniques of phosphorus-doped poly-Si/SiO_x passivating contacts, including forming gas annealing (FGA), atomic layer deposition (ALD) of hydrogenated aluminum oxide (AlO_x:H), and plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated silicon nitride (SiN_x:H), are investigated and compared in terms of their application to silicon solar cells. A simple FGA posttreatment produces a significant increase in the implied open circuit voltage (iV_oc) and the effective minority-carrier lifetime (τ_eff) of high-resistivity crystalline Si (c-Si) samples, whereas low-resistivity samples show a minimal change. Treatment by means of AlO_x:H and/or SiN_x:H followed by postdeposition FGA results in a universal increase in τ_eff and iV_oc for all substrate resistivities (as high as 12.5 ms and 728 mV for 100 Ω cm and 5.4 ms and 727 mV for 2 Ω cm n-type c-Si substrates). In addition, both the FGA and AlO_x:H + FGA techniques can inject sufficient hydrogen into the samples to passivate defects at the SiO_x/c-Si and poly-Si/SiO_x interfaces. However, this hydrogen concentration is insufficient to neutralize both the nonradiative defects inside the poly-Si films and dangling bonds associated with the amorphous Si phase present in them. The hydrogen injected by the SiN_x:H + FGA technique can passivate both the interfaces and the defects and dangling bonds within the poly-Si film. These results are confirmed by low-temperature photoluminescence spectroscopy, Fourier transform infrared spectroscopy, and dynamic secondary-ion mass spectrometry measurements.