Mitigating Light and Elevated Temperature Induced Degradation in Multicrystalline Silicon Wafers and PERC Solar Cells Using Phosphorus Diffusion Gettering

Abstract

Reducing light and elevated temperature induced degradation (LeTID) is important for the industrial success of PERC (passivated emitter and rear cell) solar cells fabricated on high-performance multicrystalline silicon (multi-Si) wafers. Although there has been a lot of progress in understanding the degradation kinetics, the defect(s) responsible for LeTID in multi-Si wafers at elevated temperatures (%80 C, 1 Sun) has yet to be identified. In this study, the authors look at the possibility of using phosphorus diffusion gettering (PDG) for reducing LeTID in multi-Si wafers and solar cells. By measuring light induced defect concentrations in multi-Si wafers before and after LeTID, the authors observe that PDG can substantially reduce the average defect concentration. Trace element analysis using inductively coupled plasma mass spectrometry reveals that multi-Si wafers from the edge of the ingot contain a high concentration of Cu, Ni and Ti in grains that degrade more than neighboring grains. To explore PDG for reducing LeTID in multi-Si PERC solar cells, the authors fabricate cells with two different emitter profiles. Etching back a heavily diffused emitter to obtain a high sheet resistance is observed to improve the LeTID performance of the solar cells, an effect that is very likely related to a reduced impurity concentration within the wafer.

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

“…Zuschlag et al showed that P‐diffused SiN x ‐passivated mc‐Si lifetime samples experienced only slight degradation, whereas a stronger and faster LeTID was observed in ungettered sister samples . Chakraborty et al presented similar results and reported that phosphorus diffusion gettering reduced the concentration of light‐induced defects, which were suspected to be Cu, Ni, and Ti, by nearly one order of magnitude . Interestingly, our previous study demonstrated that heavy phosphorus‐doping in the b‐Si spikes after an industry‐typical POCl 3 diffusion process enhances iron segregation to the emitter, which substantially improves minority carrier lifetime in iron‐contaminated substrates .…”

“…Recent studies suggest that P or Al‐gettering can suppress LeTID to some extent, especially in material with low lifetime . Zuschlag et al showed that P‐diffused SiN x ‐passivated mc‐Si lifetime samples experienced only slight degradation, whereas a stronger and faster LeTID was observed in ungettered sister samples .…”

“…17 Chakraborty et al presented similar results and reported that phosphorus diffusion gettering reduced the concentration of light-induced defects, which were suspected to be Cu, Ni, and Ti, by nearly one order of magnitude. 18 Interestingly, our previous study demonstrated that heavy phosphorus-doping in the b-Si spikes after an industry-typical POCl 3 diffusion process enhances iron segregation to the emitter, which substantially improves minority carrier lifetime in iron-contaminated substrates. 6 Consistent with earlier reports, [19][20][21] sheet resistance is indeed substantially lower in the b-Si than in the acidic-textured cells (~50 Ω/□ vs~80 Ω/□).…”

“…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. Industrial b-Si solar cells are processed together with acidic-textured references following the standard PERC process at the production line of SolarWorld Industries GmbH.…”

“…LeTID may result in over 10% relative efficiency loss in cells with dielectrically‐passivated rear side, which is a significant issue for the PV industry that is shifting towards passivated emitter and rear cells (PERC) . Hence, several PV research groups, academic institutions, and companies have recently investigated methods to mitigate the detrimental phenomenon, eg, via dark annealing, reduced peak temperature or ramp rates of fast firing, application of high‐intensity illumination at elevated temperature, or phosphorus diffusion gettering of LeTID‐causing impurities …”