Hydrogen Concentration in Photovoltaic a-Si:H Annealed at Different Temperatures Measured by Neutron Reflectometry

TL;DR

This study investigates how hydrogen in amorphous silicon (a-Si:H) desorbs at high temperatures using neutron reflectometry, revealing a critical temperature around 425°C and a linear relationship between hydrogen content and device performance.

Contribution

It combines neutron and X-ray reflectometry with photoconductance measurements to precisely analyze hydrogen stability and defect passivation in a-Si:H/Si bilayers at elevated temperatures.

Findings

Hydrogen desorbs at approximately 425°C.

Hydrogen content correlates linearly with minority carrier lifetime.

Hydrogen passivates one defect per atom.

Abstract

Amorphous hydrogenated silicon (a-Si:H) is an important material for surface defect passivation of photovoltaic silicon (Si) wafers in order to reduce their recombination losses. The material is however unstable with regards to hydrogen (H) desorption at elevated temperatures, which can be an issue during processing and device manufacturing. In this work we determine the temperature stability of a-Si:H by structural characterization of a-Si:H/Si bilayers with neutron- (NR) and X-ray reflectometry (XRR) combined with photoconductance measurements yielding the minority carrer lifetime. The neutrons are sensitive to light elements such as H, while the X-rays which are insensitive to the H-concentration, provide an independent constraint on the layer structure. It is shown that H-desorption takes place at a temperature of approximately T = $425\,^{\circ}\mathrm{C}$ and that hydrogen content and minority carrier lifetimes have a strongly correlated linear relationship, which can be interpreted as one hydrogen atom passivating one defect.