What can we learn from the saturation of light-induced defects in amorphous hydrogenated silicon?

Abstract

At a sufficiently high photon flux the density of light-induced defects in amorphous hydrogenated silicon (a-Si:H) can be made to saturate within a few hours. The saturated defect density $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$ is independent of light intensity, of temperature and of further illumination over a wide range of conditions. Therefore, $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$ can serve as a robust criterion for the rapid evaluation of the stability of a-Si:H. In this paper we show that $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$ is correlated with the rate of defect buildup during light-soaking, so that the entire defect history and the useful life of a particular sample may be inferred from $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$. So far, we have measured values of $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$ between 4 × 10¹⁶ and 2 × 10¹⁷ cm⁻³ in electronic-grade a-Si:H. $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$, and thus the rate of defect buildup, rises with the value of the band gap and with the hydrogen content; it drops with increasing deposition temperature. $\text{N}{}_{\mathrm{<mtext>sat</mtext>}}$ is not correlated with either the initial defect density or the Urbach energy. We demonstrate an application of the correlation between the saturated defect density and the optical gap to predict the long-term performance of solar cells.