Investigating Hydrogen Behavior at a-Si:H/c-Si Interfaces through Electrical Measurement and Chemical Analysis

Metastable Si-H bonds in amorphous silicon (a-Si:H) are widely accepted as a risk factor to reliability in silicon heterojunction solar cells. The metastability of the Si-H bond is linked to light-induced degradation, manifesting as losses in open-circuit voltage and fill factor. Despite extensive studies on metastability, the dynamics of defect movement and charge redistribution are still not fully understood, making it challenging to quantitatively link hydrogen movement to the charge states of defects. To better understand the mechanism involved, here metal-insulator-semiconductor test structures incorporating intrinsic a-Si:H/p-type a-Si:H (n-i-p) or intrinsic a-Si:H/n-type a-Si:H (n-i-n) stacks on n-type silicon substrates were investigated through illuminated bias-temperature stress. We employed cryogenic Capacitance-Voltage (C-V) measurements to identify the motion of mobile ions and used Glow Discharge Optical Emission Spectrometry (GD-OES) for the depth profiling of the hydrogen concentration. Results of C-V indicate that positive charges primarily build up in i-p structures under reverse bias conditions, likely due to hole trapping and subsequent breaking of weak Si-H bonds. Additionally, GD-OES confirms that significant hydrogen diffusion occurs exclusively in i-p structures. Results suggest that hydrogen movement is crucial to changes in mobile charge distribution that may affect passivation, particularly on the p-type side. Further computational analysis will be conducted to explore the dynamics involved to predict the movement of hydrogen under long-term operation.