Studying Light-Induced Degradation by Lifetime Decay Analysis: Excellent Fit to Solution of Simple Second-Order Rate Equation

Abstract

Abstract-Twenty different boron-doped Czochralski silicon materials have been analyzed for light induced degradation. The carrier lifetime degradation was monitored by an automated quasi steady state photoconductance setup with an externally controlled bias lamp for in-situ illumination between measurements. Logarithmic plots of the time resolved lifetime decays clearly displayed the previously reported rapid and slow decays, but a satisfactory fit to a single exponential function could not be achieved. We found, however, that both decay curves, for all the investigated samples, can be fitted very well to the solution of a simple second order rate equation. This indicates that the defect generation process can be described by second order reaction kinetics. The new information is used to discuss the role of holes in the defect reaction and the rate determining steps of the rapid and slow defect reactions.

Accelerated formation of the boron–oxygen complex in p‐type Czochralski silicon

“…In contrast, the accelerated defect formation rate with increasing illumination intensity may imply a strong dependence upon the total hole concentration during the annealing process, which in these experiments is estimated to vary between 1.0 × 10 16 cm –3 for an illumination intensity of 2.1 × 10 17 photons/cm 2 /s to 5.2 × 10 16 cm –3 for an illumination intensity of 1.68 × 10 19 photons/cm 2 /s. This would agree with the previous results presented in which holes are required for the degradation reaction , as well as with the observation of Rougieux et al of an excess carrier dependent defect formation rate in compensated n‐type silicon. It would also agree with speculation by Macdonald et al that the rate of defect formation may possess a dependence on excess hole density.…”

“…A possible explanation is that in this work the illumination intensities used here are high enough to result in a significant increase in the majority carrier concentration which would not have occurred in the previous work. The saturated defect concentration appeared to be independent of the hole concentration during the process, supporting the conclusions of several in the literature that this parameter depends upon p 0 . In contrast, the accelerated defect formation rate with increasing illumination intensity may imply a strong dependence upon the total hole concentration during the annealing process, which in these experiments is estimated to vary between 1.0 × 10 16 cm –3 for an illumination intensity of 2.1 × 10 17 photons/cm 2 /s to 5.2 × 10 16 cm –3 for an illumination intensity of 1.68 × 10 19 photons/cm 2 /s.…”

“…If a dependence of the defect formation upon the hole concentration is accepted it would be expected to be either linear or quadratic based upon results in the Refs. . In which case it may therefore be supposed that instead of the reported quadratic dependence of degradation rate upon p 0 the actual dependence should be replaced with either pp 0 or p 2 to reflect the enhancement of the defect formation rate under medium to high injection levels.…”

“…In the same study the saturated concentration of the defect showed a quadratic dependence on the interstitial oxygen concentration as well as a linear dependence on N A . However, recent observations on compensated p‐type material doped with boron and phosphorus have demonstrated that the rate of formation and the saturated defect concentration depend upon the equilibrium hole concentration ( p 0 ), rather than the boron concentration, with the rate being proportional to p 0 2 . This relationship was shown experimentally by varying the bulk doping in both standard Cz silicon as well as in compensated silicon.…”

Eliminating Light-Induced Degradation in Commercial p-Type Czochralski Silicon Solar Cells

“…On the one hand, a quadratic dependence of the defect formation rate on the equilibrium hole concentration (p 0 ) was observed [24,27,28,94,128]. On the other hand, a dependence of the defect formation rate on the illumination intensity was also observed for low illumination intensities, up to approximately 0.01 suns, at which the reaction rate saturated [8,129].…”