Modulation of Carrier‐Induced Defect Kinetics in Multi‐Crystalline Silicon PERC Cells Through Dark Annealing

Abstract

In this letter, we report on significant changes caused after dark annealing to the kinetics of the carrier‐induced defect, present in p‐type multi‐crystalline silicon PERC cells. The characteristic shapes of the degradation and regeneration curves under light soaking at 75 °C are dramatically altered, depending on the temperature of an initial dark anneal on the non‐degraded cell. Dark annealing for a fixed time (2.5 h) at temperatures of 200 °C or below, is found to accelerate both the subsequent degradation and regeneration rate and the degradation severity, while at higher temperatures it appears that a possible second defect with a significantly longer degradation and regeneration rate is activated. Through a further increase of the dark annealing temperature, the magnitude of this slow degradation is suppressed. This data provides essential information into the role that thermal history plays in the behavior of the still unidentified defect or defects, which is crucial for future studies of the degradation and methods to mitigate it.

Possible Candidates for Impurities in mc‐Si Wafers Responsible for Light‐Induced Lifetime Degradation and Regeneration

“…Since they were not able to explain this effect by known light‐induced degradation processes such as the boron‐oxygen defect activation or the iron‐boron pair dissociation, they attributed the efficiency loss to a new degradation mechanism. The increasing number of studies on this phenomenon in recent years underlines the practical and fundamental relevance of the effect, which is sometimes denoted “LeTID” (Light and elevated Temperature Induced Degradation) . In order to elucidate the fundamental degradation mechanism, we recently performed a comprehensive lifetime study where we showed that (i) after completed degradation, a full regeneration of the lifetime is observed on a timescale of several hundred hours, (ii) the degradation/regeneration cycle is trigged by a rapid thermal annealing (RTA) treatment and no degradation is observed with no or a low‐temperature RTA treatment, and (iii) the thinner the mc‐Si wafer the faster the regeneration appeared to take place.…”

Optimization of Belt Furnace Anneal to Reduce Light and Elevated Temperature Induced Degradation of Effective Carrier Lifetime of P‐Type Multicrystalline Silicon Wafers

“…As noted by Bredemeier in ref., LeTID cannot be described by a single exponential decay function. It is proposed that LeTID has at least two defects involved, as also mentioned by Chan et al The two defects are a type‐1 defect, which has a fast degradation, and a type‐2 defect which has less degradation and saturates after a few hours of illumination at elevated temperature.…”

“…A 200 °C dark anneal for 8 min is reported in the literature to be able to restore the lifetime of a degraded sample to its initial value, but the effect was found to be reversible with samples degrading again upon illumination . However, as demonstrated by Chan et al a dark anneal alters the defect kinetics and hence it may be possible to permanently suppress the LeTID by changing the state of degradation causing defects to the non‐degrading state using a dark anneal at different temperature. Hence, in this study we subject the multicrystalline samples at different stages (pre‐ and post‐LeTID) to a dark anneal at moderate temperature (300–550 °C) for 20 min.…”

“…Chan et al reported that the thermal history of the sample has an impact on LeTID. Similarly, the different durations spent at the peak firing temperature can not be ruled out from having an influence on the LeTID behavior.…”

Impact of black silicon on light‐ and elevated temperature‐induced degradation in industrial passivated emitter and rear cells

“…7,8 LeTID may result in over 10% relative efficiency loss in cells with dielectrically-passivated rear side, 7,9,10 which is a significant issue for the PV industry that is shifting towards passivated emitter and rear cells (PERC). 2 Hence, several PV research groups, academic institutions, and companies have recently investigated methods to mitigate the detrimental phenomenon, eg, via dark annealing, 11 reduced peak temperature or ramp rates of fast firing, [12][13][14][15] application of high-intensity illumination at elevated temperature, 16 or phosphorus diffusion gettering of LeTID-causing impurities. 17,18 In this work, we apply a b-Si nanostructure produced by deep reactive ion etching (DRIE) to PERC cells fabricated on typical LeTIDsensitive mc-Si material.…”