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The exposure to light can cause a loss in solar cell performance which is obviously undesirable. It has been an active research topic for decades with a special focus on the boron-oxygen (BO) defect in monocrystalline silicon. However, it is not such a serious concern in multicrystalline silicon (mc-Si) due to inherently lower oxygen content and the fact that the BO defect intensity scales with the square of the oxygen concentration. Recently, a novel degradation mechanism in mc-Si has been observed [1], which has significantly longer timescales than BO degradation and more pronounced at higher temperatures and was consequently referred to as light and elevated temperature induced degradation (LeTID).

Defect Properties

When mc-Si solar wafers and cells are treated at elevated temperatures, a significant reduction in effective minority carrier lifetime and efficiency is observed. Interestingly, after prolonged exposure, they recover/regenerate to their initial values. Unfortunately, it can take years to decades for this complete cycle [2], resulting in significant yield loss in installed PV systems making this a serious concern for the PV industry. Although LeTID was first observed in mc-Si, a similar defect has been confirmed in Czochralski silicon (Cz-Si) [3], float zone (FZ-Si) [4]and n-type silicon [5]. In addition, annealing in the dark has shown to induce an identical defect, indicating that it is more correct to refer to carrier induced degradation (CID) instead of light-induced degradation as the presence of illumination is not a prerequisite for the formation of the defect.

Figure 1: Evolution of effective minority carrier lifetime as a function of treatment time. This sample underwent accelerated degradation with high-intensity laser illumination (top) and this sample was degraded at 175 °C in the dark (bottom).

Crucial parameters and postulated causes

The characteristics of the defect has shown to depend strongly on the solar cell architecture, the degradation being significantly enhanced in Passivated emitter rear contact (PERC) cells compared to the cells with an aluminium back surface field (Al-BSF) [6]. It has also shown to depend on the position of the wafer in the parent ingot, on the gettering processes used, and on the on the presence of grain boundaries. More importantly, the degradation has shown to be significantly influenced by thermal treatments. Dark annealing hugely impacts the degradation reaction kinetics [7], [8] . High-temperature firing is an important step in solar cell processing and can alter the rate of degradation. An escalated degradation was observed when samples are fired at high temperatures with minimal or no degradation in non-fired samples [9]. Surface passivation layers have also been reported to influence the rate of degradation, with highest observed degradation on samples passivated with hydrogen-rich passivation layers like silicon nitrides (SiN_x:H) fired at >800°C. The wide range of factors affecting LeTID make it a challenging mechanism to study. However, several possible causes have been postulated. These include metallic impurities like copper, nickel and cobalt [10]. The actual source of degradation is still unknown, however, recently, there is an increased discussion on the involvement of hydrogen in causing LeTID [7], [8], [11]–[14].

Mitigation strategies

While the cause of degradation is debated, numerous mitigation techniques have been proposed. Most common amongst is reducing the firing temperature [9], [12]. Modulation in the firing profile, with variation in cooling rates has also been suggested [15]. Other solutions include accelerating the degradation and accelerated degradation with a second firing step at a lower temperature [9], [16]. Moreover, changing the wafer properties [12] and thickness [10] has also shown to reduce the degradation. Within PV industry, various companies have identified modifications in their process flows to minimise the level of LeTID to an acceptable level, typically in the range up to 3% relative, but this was done in a fully empirical way without fully understanding the defect. Since the discovery of the defect, it has been intensively studied and significant progress has been made in understanding the defect and corresponding methods have been developed that almost completely mitigate LeTID. It is expected that more sophisticated newly developed/or adapted LeTID-free processing tools will be released in the near future.

Figure 2: Photoluminescence images of high-performance mc-Si wafers. Images were taken after high-temperature firing, at the fully degraded state, and after regeneration.

[1] – K. Rampseck, S. Zimmermann, H. Nagel, A. Metz, and G. Yvonne, “Light Induced Degradation of Rear Passivated Mc-Si Solar Cells,” 27th Eur. Photovolt. Sol. Energy Conf. Exhib., vol. 1, pp. 861–865, 2012.

[2] – F. Kersten, et al., “System Performance loss due to LeTID,” Energy Procedia, 2017.

[3] – D. Chen et al., “Evidence of an identical firing-activated carrier-induced defect in monocrystalline and multicrystalline silicon,” Sol. Energy Mater. Sol. Cells, vol. 172, pp. 293–300, 2017.

[4] – T. Niewelt, J. Schön, F. Schindler, and M. C. Schubert, “Understanding the light ‐ induced degradation at elevated temperatures : Similarities between multicrystalline and floatzone p ‐ type silicon,” Proceeding of the 33rd EUPVSEC, 10.4229/EUPVSEC20172017-2BP.1.4 (2018).

[5] – D. Chen et al., “Hydrogen induced degradation: A possible mechanism for light- and elevated temperature- induced degradation in n-type silicon,” Sol. Energy Mater. Sol. Cells, vol. 185, no. March, pp. 174–182, 2018.

[6] – M. Padmanabhan et al., “Light-induced degradation and regeneration of multicrystalline silicon Al-BSF and PERC solar cells,” Phys. status solidi – Rapid Res. Lett., vol. 10, no. 12, pp. 874–881, 2016.

[7] – C. Chan et al., “Modulation of Carrier-Induced Defect Kinetics in Multi-Crystalline Silicon PERC Cells Through Dark Annealing,” Sol. RRL, vol. 1, no. 2, p. 1600028, 2017.

[8] – T. H. Fung et al., “A four-state kinetic model for the carrier-induced degradation in multicrystalline silicon: Introducing the reservoir state,” Sol. Energy Mater. Sol. Cells, vol. 184, no. February, pp. 48–56, 2018.

[9] – C. E. Chan et al., “Rapid Stabilization of High-Performance Multicrystalline P-type Silicon PERC Cells,” IEEE J. Photovoltaics, vol. 6, no. 6, pp. 1473–1479, 2016.

[10] – D. Bredemeier, D. C. Walter, and J. Schmidt, “Possible Candidates for Impurities in mc-Si Wafers Responsible for Light-Induced Lifetime Degradation and Regeneration,” Sol. RRL, vol. 2, no. 1, p. 1700159, 2018.

[11] – M. A. Jensen, A. E. Morishige, J. Hofstetter, D. B. Needleman, and T. Buonassisi, “Evolution of LeTID Defects in p-Type Multicrystalline Silicon During Degradation and Regeneration,” IEEE J. Photovoltaics, vol. 7, no. 4, pp. 980–987, 2017.

[12] – K. Nakayashiki et al., “Engineering Solutions and Root-Cause Analysis for Light-Induced Degradation in p-Type Multicrystalline Silicon PERC Modules,” IEEE J. Photovoltaics, vol. 6, no. 4, pp. 860–868, 2016.