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

Herein, posttreatment techniques of phosphorus-doped poly-Si/SiOx passivating contacts, including forming gas annealing (FGA), atomic layer deposition (ALD) of hydrogenated aluminum oxide (AlOx:H), and plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated silicon nitride (SiNx: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 (tau(eff)) of high-resistivity crystalline Si (c-Si) samples, whereas low-resistivity samples show a minimal change. Treatment by means of AlOx:H and/or SiNx:H followed by postdeposition FGA results in a universal increase in tau(eff) and iV(oc) for all substrate resistivities (as high as 12.5 ms and 728 mV for 100 omega cm and 5.4 ms and 727 mV for 2 omega cm n-type c-Si substrates). In addition, both the FGA and AlOx:H + FGA techniques can inject sufficient hydrogen into the samples to passivate defects at the SiOx/c-Si and poly-Si/SiOx 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 SiNx: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.