Light And Elevated Temperature Induced Degradation Research Articles

Silicon Nitride Deposition: Impact on Lifetime and Light-Induced Degradation at Elevated Temperature in Multicrystalline Silicon

Light and elevated temperature induced degradation (LeTID) in multicrystalline (mc) silicon (Si) is known to be sensitive to the silicon nitride (SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H) deposition and the firing, but the reason remains unknown. Three plasma-enhanced chemical vapor deposition (PECVD) tools are used to deposit five different layers, causing significant differences in the initial lifetimes before and after firing. The impact of the different depositions can be effectively compared by replacing the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H with well-passivating room-temperature iodine ethanol passivation (IE). The IE passivation reveals that the firing of SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H and subsequent bulk hydrogenation leads to a lifetime increase only in the gettered samples. Although the PECVD temperature load affects the bulk lifetime, it does not cause LeTID without SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H deposition. LeTID is only detected in samples fired with SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H, but once the LeTID precursors have been activated, the PECVD layer can be etched and the wafer thinned down without affecting LeTID formation. No link is found between the LeTID defect density and the bulk-hydrogenated lifetime. Instead the LeTID defect density appears to decrease with decreasing PECVD deposition time and temperature.

Controlling Light- and Elevated-Temperature-Induced Degradation With Thin Film Barrier Layers

In this article, we investigate the extent of lifetime degradation attributed to light- and elevated-temperature-induced degradation (LeTID) in p- type multicrystalline silicon wafers passivated with different configurations of hydrogenated silicon nitride (SiNx:H) and aluminum oxide (AlOx:H). We also demonstrate a significant difference between AlOx:H layers grown by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) with respect to the extent of LeTID. When ALD AlOx:H is placed underneath a PECVD SiNx:H layer, as used in a passivated emitter and rear solar cell, a lower extent of LeTID is observed compared with the case when a single PECVD SiNx:H layer is used. On the other hand, the LeTID extent is significantly increased when an ALD AlOx:H is grown on top of the PECVD SiNx:H film. Remarkably, when a PECVD AlOx:H is used underneath the PECVD SiNx:H film, an increase in the LeTID extent is observed. Building on our current understanding of LeTID, we explain these results with the role of ALD AlOx:H in impeding the hydrogen diffusion from the dielectric stack into the c-Si bulk, while PECVD AlOx:H seems to act as an additional hydrogen source. These observations support the hypothesis that hydrogen is playing a key role in LeTID and provide solar cell manufacturers with a new method to reduce LeTID in their solar cells.

Excessive light-induced degradation in boron-doped Cz silicon PERC triggered by dark annealing

This work investigates the impact of annealing at elevated temperatures on the light-induced degradation (LID) of passivated emitter and rear cells (PERC) processed on boron-doped Czochralski-grown silicon substrates. The boron-oxygen (BO) defect has been stabilised prior to annealing and subsequent LID treatment. Excessive LID of up to 19.1 %rel. is observed upon illumination after extended dark annealing at 150 °C for 552 h, which is well above the BO defect-related LID of 5.6 %rel. measured upon illumination after cell processing if BO is not stabilised. Light and elevated Temperature Induced Degradation (LeTID), iron-boron pairs and surface recombination are excluded as root causes for the observed increased LID, which shows a similar behaviour as the BO defect but which cannot be explained by the well-established three-state model of the BO defect with the assumption of an empty regenerated state prior to BO stabilisation. Two speculative hypothesis for an explanation are (i) that further BO defect precursors are formed, which could be described via a reservoir or (ii) that a high percentage of the in-principle available BO defects are already in the stabilised state even without dedicated BO defect stabilisation. This increased LID does not occur when at least a small level of excess carrier concentration is induced during extended annealing and, hence, is expected not to occur during field operation. However, the observed behaviour is highly relevant for accelerated aging testing such as, e.g., damp heat testing during IEC and UL certification.

Annealing prior to contact firing: A potential new approach to suppress LeTID

In this work, we introduce a new approach to suppress light and elevated temperature-induced degradation (LeTID) by applying a pre-fire annealing step using rapid thermal processing (RTP) and discuss the impact of this process on the evolution of bulk and surface lifetime components. We demonstrate that pre-fire annealing at low temperatures and/or shorter holding times allows a significant amount of hydrogen to migrate into the bulk to passivate bulk defects as including grain boundaries and dislocation clusters, without causing surface deterioration. As such, the addition of the pre-fire annealing step results in larger improvements in bulk and surface lifetime than that of the control samples. These conditions also significantly suppress LeTID. Increasing pre-fire annealing temperature and duration is shown to completely mitigate LeTID. However, this process may cause surface deterioration, possibly due to the excessive effusion of hydrogen out of the dielectric layer. Injection-dependent lifetime analysis shows that at the most degraded state, the bulk lifetime of the pre-fire annealed samples (650 °C–1 min and 3 min) remains relatively higher (∼110 μs–∼120 μs) than that of the control sample (∼40 μs). Applying pre-fire annealing process at 700 °C on Cz-Si samples and testing the boron-oxygen (B-O) generation behavior suggest that these processes cause a reduction in the hydrogen concentration in the bulk, resulting in slower B-O regeneration rate and reduction of regeneration extent. This result also implies that the suppression of LeTID in mc-Si by applying a pre-fire thermal treatment is likely due to a reduction of hydrogen in the bulk, and this highlights that the proposed method of pre-fire annealing may be unsuitable for material such as p-type Cz silicon subjected to B-O related degradation.

Evaluating the Impact of SiNx Thickness on Lifetime Degradation in Silicon

There has been continuous effort to understand the cause of light- and elevated-temperature-induced degradation (LeTID) in silicon solar cells; however, the actual origin of the defect is still under investigation. Multiple reports in the literature suggest the involvement of hydrogen in activating the recombination-active defect that is responsible for this degradation. In this paper, we investigate the influence of the amount of in-diffused hydrogen in the bulk on the degradation in silicon lifetime test structures. We examine this by varying the thickness of hydrogenated silicon nitride (SiNx:H) before high-temperature firing. Fourier transform infrared spectroscopy is performed to confirm that the hydrogen content in SiNx:H film scales with its thickness. We observe that an increase in the thickness of hydrogen-rich SiNx:H leads to an almost proportional increase in the extent of defect concentration in multicrystalline silicon wafers. We attribute this increase to the higher amount of hydrogen released from thicker SiNx:H layers into the bulk during firing. This paper provides further evidence for the involvement of hydrogen in the formation of the LeTID defect in silicon.

Role of post-metallization anneal sequence and forming gas anneal to mitigate light and elevated temperature induced degradation of multicrystalline silicon solar cells

The mystery surrounding the role of hydrogen in light and elevated temperature induced degradation (LeTID) of multicrystalline silicon (mc-Si) solar cells is attracting a lot of interest in the photovoltaic community. It is still unclear whether hydrogen plays a role in the degradation or the regeneration phase, with convincing evidence reported for both. In this paper, we base our work on the hypothesis that hydrogen plays a role in both phases depending on its charge state. Hydrogen in at least one charge state can passivate the LeTID defect. Thus, it is possible to mitigate LeTID in mc-Si by controlling the charge state of hydrogen. We observe in our experiments that the sequence of different thermal anneals impacts the degree of degradation. We also explore the possibility of mitigating LeTID by annealing the samples in a forming gas atmosphere. P-type mc-Si samples that received a forming gas anneal (FGA) show an improvement of the effective carrier lifetime when subjected to 1-Sun light soaking at 80 °C. Mc-Si PERC solar cells fabricated by incorporating a FGA anneal step show about 1.5% (relative) maximum drop in efficiency during 300 h of light soaking, as compared to about 3.2% (relative) maximum drop for cells fabricated by annealing in atmospheric air (without FGA), and 5.3% (relative) maximum drop for non-annealed cells. It thus seems that, after optimization, FGA is an easy to implement solution to minimise LeTID in industrial mc-Si solar cells.

The Impact of Thermal Treatment on Light-Induced Degradation of Multicrystalline Silicon PERC Solar Cell

Multicrystalline silicon (mc-Si) PERC (passivated emitter and rear cell) solar cells suffer from severe light-induced degradation (LID), which mainly consists of two mechanisms, namely, BO-LID (boron–oxygen complex-related LID) and LeTID (light and elevated temperature induced degradation). The impact of thermal treatment on the LID of a mc-Si PERC solar cell is investigated in this work. The LID of mc-Si PERC solar cells could be alleviated by lowering the peak temperature of thermal treatment (namely sintering), perhaps because fewer impurities present in mc-Si tended to dissolve into interstitial atoms, which have the tendency to form LeTID-related recombination active complexes. The LID could also be effectively restrained by partially replacing the boron dopant with gallium, which is ascribed to the decreased amount of boron–oxygen (B–O) complexes. This work provides a facile way to solve the severe LID problem in mc-Si PERC solar cells in mass production.

Assessing the Impact of Thermal Profiles on the Elimination of Light- and Elevated-Temperature-Induced Degradation

Light- and elevated-temperature-induced degradation (LeTID) in p-type multicrystalline silicon has a severe impact on the effective minority carrier lifetime of silicon and remains a crucial challenge for solar cell manufacturers. The precise cause of the degradation is yet to be confirmed; however, several approaches have been presented to reduce the extent of degradation. This paper presents insights on the impact of thermal budgets and cooling rates during post-firing illuminated anneals and their role in changing the lifetime and mitigating LeTID for thermal processes between 350 and 500 °C. We demonstrate that the thermal budget of these processes plays a crucial role in LeTID suppression and that the cooling rate only plays a role during short treatment durations (≤1 min). For the parameter space studied, we show that annealing for an appropriate time and temperature can both enhance the minority carrier lifetime and completely suppress the LeTID, with the injection-dependent Shockley–Read–Hall lifetime analysis indicating that the recombination activity of the LeTID defects in the bulk has been eliminated. Finally, this paper demonstrates a process that results in a stable lifetime after 800 h of conventional light-soaking at 75 °C.

Low-temperature dark anneal as pre-treatment for LeTID in multicrystalline silicon

Light and elevated temperature induced degradation (LeTID) is currently a severe issue in crystalline silicon photovoltaics, which has led to numerous efforts to both understand the mechanism and to mitigate it. Here we show that a low-temperature dark anneal performed as the last step in typical solar cell processing influences greatly LeTID characteristics, both the strength of the degradation and the degradation kinetics. While a relatively short anneal in the temperature range of 200–240 °C can be detrimental to LeTID by doubling the degradation intensity, an optimized anneal at 300 °C shows the opposite trend providing an efficient means to eliminate LeTID. Furthermore, we show that the simulated recombination activity of metal precipitation and dissolution during the dark anneal correlates with the experiments, suggesting a possible explanation for the LeTID mechanism.

Bulk and Surface‐Related Degradation in Lifetime Samples Made of Czochralski Silicon Passivated by Plasma‐Enhanced Chemical Vapor Deposited Layer Stacks

Significant bulk‐related degradation (BRD) is followed by surface‐related degradation (SRD) of effective excess charge carrier lifetime in lifetime samples made of Czochralski silicon during illuminated treatment at 80–150 °C. Samples are passivated with either AlOx:H/SiOxNy:H/SiNx:H or SiOxNy:H/SiNx:H stacks stemming entirely from plasma‐enhanced chemical vapor deposition. Samples show strong variations in BRD depending on passivation stacks and treatment conditions, and a potential link to light and elevated temperature‐induced degradation (LeTID) is discussed. All samples are fired in a belt furnace, and variations of firing temperature and belt speed are shown to influence SRD slightly. SRD is furthermore accelerated with increasing treatment temperature and an apparent activation energy Eapp = 1.07 ± 0.02 eV is determined in SiOxNy:H/SiNx:H passivated samples. Interpretation of Eapp is, however, difficult as both changes in interfacial defect and fixed charge density occur in parallel during SRD.

Impact of Dark Annealing on the Kinetics of Light- and Elevated-Temperature-Induced Degradation

Light- and elevated-temperature-induced degradation (LeTID) has been shown to have a significant detrimental impact on p-type multicrystalline silicon solar cells and, in particular, on passivated emitter and rear cells. Previous studies have shown that defect kinetics can be modulated for samples that are dark annealed prior to light soaking at elevated temperature. In this work, we show that while short annealing durations help accelerate both degradation and recovery rates to different extents, extended annealing instead instigates a retarding effect. Our results confirm that thermally induced degradation and regeneration mechanisms can be observed during dark annealing. The results also suggest that the response to this yet undetermined defect mechanism not only depends on the initial dark annealing temperature, but it is also highly dependent on the stage of the dark annealing degradation and regeneration cycle reached before beginning light soaking. Finally, we propose a refined model of three generalized modes to describe the changes in LeTID kinetics after dark annealing.

Light and elevated temperature induced degradation (LeTID) in perovskite solar cells and development of stable semi-transparent cells

The stability of perovskite solar cells (PSCs) is one of the major challenges to their commercialization. In this work, the stability of state-of-the-art mesoporous PSCs employing multi-cation mixed-halide perovskite and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as the hole-transport-layer (HTL) is studied under illumination at an elevated temperature in N2 environment. The Rb in the quadruple-cation perovskite is found to segregate when the film undergoes aging under simultaneous light and heat exposure. More importantly, the PTAA layer prevents diffusion of gold into the perovskite layer when aged at 85 °C in the dark, but not under light. As a result, the PSCs thermally aged under light degrade much more severely than cells thermally aged in the dark. Therefore, the term Light and elevated Temperature Induced Degradation (LeTID) is introduced for PSCs. The effect is also evident for PCSs employing a copper(II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPC) HTL. This effect seriously impacts the operational stability of PSCs and it might not be detected with the current stress tests defined in the IEC-61646 standard. PSCs with a transparent contact and robust perovskite composition are developed with improved stability against LeTID. The cells retain more than 90% of the initial efficiency after > 160 h operating under illumination at 85 °C in N2 environment.

Evaluating root cause: The distinct roles of hydrogen and firing in activating light- and elevated temperature-induced degradation

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Correlation of the LeTID amplitude to the Aluminium bulk concentration and Oxygen precipitation in PERC solar cells

We performed an extensive study of light induced degradation experiments in PERC solar cells made of Boron doped Cz Silicon material from several experimental crystals. Strong LeTID (Light and Elevated Temperature Induced Degradation) amplitudes were found in cells originating from the top and tail part of the crystals. A statistical analysis reveals a strong correlation to the bulk concentration of Aluminium in cells originating from wafers close to the tail. Close to the top, the LeTID effect is strongly enhanced whenever Oxygen precipitation occurs in the cell process.

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

Light and elevated temperature induced degradation (LeTID) of the effective charge carrier lifetime significantly lowers the efficiency of multicrystalline silicon (mc‐Si) solar cells and is a major challenge currently faced by the silicon photovoltaic industry. Optimization of the temperature profile used in the rapid thermal anneal (RTA) step of the metallization line has been found to significantly reduce LeTID of mc‐Si solar cells. Hence, the authors experimentally study the impacts of varying the RTA process parameters on the LeTID behavior of mc‐Si lifetime samples. It is shown that a low peak temperature, slow ramp‐up rate (slow belt speed) reduce LeTID in mc‐Si lifetime samples. Also, subsequent dark anneal at a moderate temperature (≈300–550 °C) further reduces LeTID. Samples already subjected to LeTID conditions also benefit from the post‐degradation dark anneal. The recovered effective carrier lifetime of the degraded samples dark annealed at 550 °C is even higher than the post‐firing effective carrier lifetime value. The defect state achieved post‐dark anneal at 550 °C is stable for the studied period of ≈60 h, unlike the well‐known case of low‐temperature dark anneals (&lt;200 °C) where samples degrade again when subjected to LeTID conditions. The authors believe that the optimized conditions identified in this work can be applied to mass‐produced mc‐Si solar cells.

Hydrogen induced degradation: A possible mechanism for light- and elevated temperature- induced degradation in n-type silicon

In this work, we demonstrate a form of minority carrier degradation on n-type Cz silicon that affects both the bulk and surface related lifetimes. We identify three key behaviors of the degradation mechanism; 1) a firing dependence for the extent of degradation, 2) the appearance of bulk degradation when wafers are fired in the presence of a diffused emitter and 3) a firing related apparent surface degradation when wafers are fired in the absence of an emitter. We further report a defect capture cross-section ratio of σn/σp = 0.028 ± 0.003 for the defect in n-type. Utilizing our understanding of light and elevated temperature induced degradation (LeTID) in p-type silicon, we demonstrate that the degradation behaviors in both n-type and p-type silicon are closely correlated. In light of numerous reports on the involvement of hydrogen, the potential role of a hydrogen-induced degradation mechanism is discussed in both p- and n-type silicon, particularly in relation to the diffusion of hydrogen and influence of hydrogen-dopant interactions.

On improved passivation stability on highly-doped crystalline silicon and the long-term stability of regenerated Cz-Si

Different surface passivation approaches are applied on Cz-Si and FZ-Si samples and long-term stability is investigated during treatments at 60–80 °C and up to 1 sun equivalent illumination intensity. It is shown that SiNx:H and AlOx:H/SiNx:H surface passivation show a much more stable passivation quality when deposited on P-diffused and B-diffused surfaces, respectively. Long-term measurements lead to the conclusion that Cz-Si samples fired at measured peak temperatures up to 750 °C are very stable after regeneration of bulk defects. Samples fired at 850 °C show much stronger bulk-related degradation potentially linked to light and elevated temperature induced degradation (LeTID). Furthermore, Cz-Si samples fired at 850 °C express an instable behavior after a regeneration treatment.

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

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.

Solubility and Diffusivity: Important Metrics in the Search for the Root Cause of Light- and Elevated Temperature-Induced Degradation

Light- and elevated temperature-induced degradation (LeTID) is a detrimental effect observed under operating conditions in p-type multicrystalline silicon (mc-Si) solar cells. In this contribution, we employ synchrotron-based techniques to study the dissolution of precipitates due to different firing processes at grain boundaries in LeTID-affected mc-Si. The synchrotron measurements show clear dissolution of collocated metal precipitates during firing. We compare our observations with degradation behavior in the same wafers. The experimental results are complemented with process simulations to provide insight into the change in bulk point defect concentration due to firing. Several studies have proposed that LeTID is caused by metal-rich precipitate dissolution during contact firing, and we find that the solubility and diffusivity are promising screening metrics to identify metals that are compatible with this hypothesis. While slower and less soluble elements (e.g., Fe and Cr) are not compatible according to our simulations, the point defect concentrations of faster and more soluble elements (e.g., Cu and Ni) increase after a high-temperature firing process, primarily due to emitter segregation rather than precipitate dissolution. These results are a useful complement to lifetime spectroscopy techniques, and can be used to evaluate additional candidates in the search for the root cause of LeTID.

System performance loss due to LeTID

If not adequately suppressed, Light and elevated Temperature Induced Degradation (LeTID) has been shown to cause severe degradation of multicrystalline (mc-Si) silicon solar cells and modules with passivated emitter and rear cell (PERC). Within this work, the system performance of LeTID-sensitive mc-Si modules is investigated when operated in temperate and mediterranean climates, and a correlation to predict the observed field performance based on accelerated laboratory testing is presented. Severe degradation induced by LeTID of up to 7% in maximum output power is detected after one thousand hours of laboratory testing, which is shown to correspond to ~ 3 years of field installation time in Cyprus. In contrast, LeTID-sensitive modules installed in Germany show a degradation of 2.5% within the same time period. Due to lower irradiance values and module temperature in Germany the LeTID rate is lower than in Cyprus. A significant dependence of system performance loss and energy yield due to LeTID on installation site is shown. Hence, the loss in system performance due to LeTID affects one-to-one the levelized cost of electricity (LCOE) compared to reference without LeTID. Furthermore, it is shown that LeTID in the field can be suppressed by applying Hanwha Q CELLS’ Q.ANTUM technology, independently of installation site.