Light And Elevated Temperature Induced Degradation Research Articles

Insights into the Hydrogen‐Related Mechanism behind Defect Formation during Light‐ and Elevated‐Temperature‐Induced Degradation

By investigating the formation of light‐ and elevated‐temperature‐induced degradation (LeTID) defects under dark annealing conditions alongside the formation of the boron–hydrogen complex, it is found that both formations are limited by presumably the same reaction. Starting with this observation, two possible mechanisms of LeTID defect formation, a hydrogen‐related association and dissociation process, are discussed. Including the current knowledge on hydrogen in silicon as well as on the reverse reaction to the defect formation, the dissociation mechanism seems to be more likely: the LeTID precursor is composed of hydrogen being bound to another complex. During dark annealing, the hydrogen dissociates and binds to its sink, which herein is boron. The remaining complex is recombination active and leads thus to the observed degradation.

The Role of Metal-Catalyzed Chemical Etching Black Silicon in the Reduction of Light- and Elevated Temperature-Induced Degradation in P-Type Multicrystalline Wafers

Light- and elevated temperature-induced degradation (LeTID) of silicon wafers reduces the energy yield of photovoltaic modules in the field. A previous study demonstrated that this degradation is greatly reduced in samples with a surface textured with black silicon formed via reactive ion etching. However, the reason for this reduction is currently not understood. This article extends that result to a range of nano-textures formed using commercial metal-catalyzed chemical etching. Lifetime test structures are fabricated on multicrystalline wafers with six different chemical nano-textures and the results are compared to planar reference samples. The article demonstrates a reduction in the extent of light-induced degradation compared to planar reference samples for all six textures. The extent of LeTID is observed to reduce with decreasing front surface reflectance and increasing front surface area. The article finds that wafer thickness, phosphorus gettering, and fast-firing cannot explain the result.

Gallium‐Doped Silicon for High‐Efficiency Commercial Passivated Emitter and Rear Solar Cells

Czochralski‐grown gallium‐doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron‐doped silicon. Second, the stability of gallium‐doped PERC cells are monitored under illumination (>3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature‐induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID‐like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron–oxygen degradation does not influence any cells that are studied. Secondary‐ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light‐induced degradation, Ga‐doped silicon is still more stable than unstabilized B‐doped silicon under illumination.

High-intensity illumination treatments against LeTID – Intensity and temperature dependence of stability and inline feasibility

We study the mitigation of light- and elevated temperature-induced degradation (LeTID) with high-intensity illumination treatments, placing special emphasis on inline feasibility. After the treatments, we investigate the stability upon degradation conditions close to the recently suggested standard, which allows estimating the LeTID behavior during the operating lifetime of solar modules in the field. Subsequently, we map the stability at different treatment intensities and temperatures achievable with an air-cooled tool. We show that, when applying short treatment times, the stability improves with increasing treatment intensity and deteriorates steeply with rising temperature above an optimum region around 250 °C. However, these intensity- and temperature-dependent differences largely vanish when increasing the treatment time sufficiently. We also investigate the significance of darkness/illumination during the cooling ramp from the treatment temperature in view of the LeTID stability. We discuss our results based on suggested defect models of LeTID, and provide hypotheses of the origin of the instabilities observed at the high treatment temperatures. After identifying optimal treatments, we demonstrate that the energy yield loss due to LeTID reduces by over 60% after an inline-feasible process consisting of only 30 s of high-intensity illumination, combined with cooling of the samples from the process temperature under a lower-intensity illumination.

LID and LETID evolution of PV modules during outdoor operation and indoor tests

Light Induced Degradation (LID) and Light and Elevated Temperature Induced Degradation (LETID) manifest with carrier injection due to light or forward bias and can lead to performance losses during the first months or years of operation in the field. We are investigating the effects of common LETID indoor test conditions and the module temperature under outdoor exposure on the evolution of BO LID and LETID over time. The investigations are based on experimental data from twelve structurally identical mono-crystalline and two multi-crystalline PERC PV modules, which underwent a detailed experiment including five different indoor test sequences and an outdoor test. Changes in the module performance are discussed based on the current knowledge on state transitions of the BO defect and LETID. Temporary recovery of the LETID defect was used to distinguish LETID from other degradation mechanisms. Our results confirm the importance of BO stabilization prior to LETID tests as it is included in the current IEC TS draft for LETID detection. We also show that too strong acceleration of the processes can lead to misinterpretation of LETID test results. Under dark storage conditions, destabilization of BO defects was found to already evolve at temperatures as low as 75 °C and a likely alteration of subsequent LETID was observed. The performance changes under outdoor exposure can be explained with the same mechanisms as investigated under indoor experiments and reveal reversible seasonal recovery effects. Furthermore, the influence of different module operating temperatures on the evolution of both, BO LID and LETID is presented and evaluated.

Impact of Substrate Thickness on the Degradation in Multicrystalline Silicon

Light and elevated-temperature-induced degradation (LeTID) is a well-known phenomenon that reduces the bulk lifetime in silicon wafers. The cause of this degradation mechanism is still under investigation. However, a wide range of empirical trends that correlate LeTID with multiple physical and processing parameters have been reported, including the observation that wafers thinner than 120 μm do not show significant LeTID. In this work, we extend that study by varying the thickness of the wafers, the temperature of the firing step, and testing LeTID at the accelerated stability testing conditions. We demonstrate that the extent of degradation reduces with the thickness of the wafer, in agreement with the earlier work. However, silicon wafers with a thickness below 120 μm still suffer from LeTID when fired at sufficiently high temperatures, demonstrating that thinner wafers are not inherently immune to LeTID. By performing accelerated testing using a high-intensity laser and fitting the degradation and regeneration data, we observe that thinner wafers do not necessarily exhibit a faster recovery, as suggested earlier. However, their reduced degradation extent could be a consequence of relatively higher out-diffusion of hydrogen per unit volume in thinner wafers during firing. We further report that the method used for thinning the wafers results in a variation in the surface morphology of the samples, and that may partly be responsible for the observed correlation between the thickness of the wafers and LeTID extent. Finally, we discuss how these new findings can be explained by the involvement of hydrogen and other impurities in LeTID.

Defect engineering in cast mono‐like silicon: A review

AbstractCast mono‐like silicon is a promising material for next‐generation silicon solar cells with higher efficiency and lower cost compared with currently commercialized silicon solar cells. So far, cast mono‐like silicon technique still faces several problems, including multicrystallization, dislocation clusters, sub‐grain boundaries, and impurity contamination, which hinder its mass‐scale applications in photovoltaic industry. In this review, we will introduce these common problems in turn and discuss a multitude of the reported studies on eliminating these problems. Furthermore, light and elevated temperature‐induced degradation (LeTID) was recently reported to be a severe problem of cast mono‐like silicon solar cells, which can cause power loss over 10% relative. This review will introduce the behaviors and the proposed mechanisms of LeTID as well as the methods to suppress LeTID in Section 4. In the subsequent section, the efficiency potential and distribution of cast mono‐like silicon solar cells are discussed. At last, a comprehensive summary will be given for this review.

(Invited) ALD Enabling High-Efficiency Solar Cells

Atomic layer deposition (ALD) can synthesize materials with atomic-scale precision. The ability to tune the material composition, film thickness with excellent conformality, low-temperature processing, and in-situ real-time monitoring makes this technique very appealing for a wide range of applications. In this contribution, we will show that ALD is already widely used in the manufacturing of industrial silicon solar cells, where ALD aluminum oxide is used for the passivation of the rear of PERC (Passivated Emitter and Rear Contact) solar cells. ALD aluminum oxide layers can also be used to reduce the contact resistance and firing temperature for phosphorous diffused surfaces. We will show that ALD layers can also avoid potential induced degradation (PID, a possible failure mode for large photovoltaic systems) at the solar cell level. ALD capping layers can also be used to reduce light and elevated temperature induced degradation (LeTID), which is a recently identified failure mode for high-efficiency solar cells, by acting as a blocking layer for hydrogen. Finally, we will show that ALD layers are enabling world-record CZTS (copper-zinc-tin-sulphide) solar cells.

Study on the Relationship between BO–LID and LeTID in Czochralski-Grown Monocrystalline Silicon

Most research about Light and elevated Temperature Induced Degradation (LeTID) is focused on multicrystalline silicon (mc-Si). In this work, the degradation kinetics of Czochralski-grown monocrystalline silicon (Cz-Si) induced by light at an elevated temperature were studied in detail. The lifetime evolutions over time during (1) light soaking (LS), (2) dark annealing–light soaking (DA–LS), and (3) DA–LS cycling experiments were analyzed. Ratios of the capture coefficients for the electrons and holes (k-values) were used to characterize the possible defects responsible for degradation. We found that the behavior of degradation and recovery under light soaking with or without a dark annealing treatment was mostly like boron–oxygen (BO)-related degradation but gave k-values from 19 to 25. In the DA–LS cycling experiment, the max degradation amplitudes hardly changed from the second cycle, and the k-values decreased with an increase in the cycling number. We then analyzed the possible reactions in Cz-Si and discuss the relationship between BO defects and LeTID.

Temporary Recovery of the Defect Responsible for Light- and Elevated Temperature-Induced Degradation: Insights Into the Physical Mechanisms Behind LeTID

The effect of light- and elevated temperature-induced degradation (LeTID) can be nonpermanently reversed by charge carrier injection below the degradation temperature (commonly used degradation temperatures are above ~70 °C). In this study, we show that the rate of temporary recovery depends strongly on the excess carrier density. We observe that the order of the reaction changes from pseudo-zero to first with increasing injection. The rate decreases slightly with increasing temperature. Since the samples can go through multiple degradation/recovery cycles without distinct changes in the degradation kinetics, the experimentally accessible recovered and degraded states are interpreted as manifestations of the equilibrium concentrations of the defect responsible for LeTID at different temperatures. Based on our observations, we argue that the process underlying LeTID degradation is the dissociation of a precursor rather than an association of two or more components. In light of the relation between LeTID susceptibility and bulk hydrogen concentration, we hypothesize that the LeTID precursor dissociates into the LeTID defect and monatomic hydrogen. Numerical simulations of the coupled rate equations including hydrogen interactions well reproduce the experimental observations; according to these results, the presence of a sink for the atomic hydrogen such as dopant atoms is paramount for the LeTID degradation.

Light‐ and elevated temperature‐induced degradation impact on bifacial modules using accelerated aging tests, electroluminescence, and photovoltaic plant modeling

AbstractThe authors report on the light‐ and elevated temperature‐induced degradation (LeTID) effect observed on bifacial photovoltaic modules and its potential impact on photovoltaic plants performance. Indoor LeTID quantification using indoor carrier‐induced degradation (CID) is carried out using current injection. Power measurements yielded higher LeTID sensitivity for the rear side of bifacial modules compared to the front side, hence leading to a variation of the bifaciality factor by several percentage points. The difficulty in evaluating the maximal power degradation caused by LeTID is also highlighted as a reduced number of samples are used most of the time and as cells within a single module do not have always the same performance evolution trends. Using indoor CID results with the help of empirical fitting and Arrhenius relation, the yield impact of LeTID on a bifacial power plant is simulated under three different climates. Modeling results help to identify the main parameters related to LeTID modules sensitivity that impact photovoltaic (PV) plants yield: maximal power degradation, stabilized power value after regeneration, activation energy value, and LeTID kinetics. In some cases, the yield variation caused by LeTID sensitive modules could be mitigated by carefully selecting the modules as a function of climatic conditions.

Stabilization of light-induced effects in Si modules for IEC 61215 design qualification

Standardized testing of commercial photovoltaic modules is widely used around the world and reduces risks of module failures. Such testing also reduces financial risks for module manufacturers and customers. This work examines the expected impact of certain defects in silicon (Si) modules during standardized accelerated testing. Specifically, the behavior of boron-oxygen (BO) light-induced degradation (LID) and light- and elevated temperature-induced degradation (LeTID) are simulated during some of the stress tests in IEC61215. LID and LeTID reaction rates at qualification temperatures are estimated from earlier published data. It is demonstrated the BO-related LID may cause some false positives and false negatives when IEC61215 tests are performed as prescribed in the 2016 published version. Possible stabilization steps to avoid these false results are suggested. State changes for the defects causing LeTID occur much more slowly than those causing BO LID, and therefore LeTID is predicted to have a lesser impact on the IEC61215 stress tests results.

Evolution of LeTID defects in industrial multi-crystalline silicon wafers under laser illumination - Dependency of wafer position in brick and temperature

In the past few years, a hot topic in both research and industrialization of p-type multi-crystalline silicon (mc-Si) solar cells is to investigate the mechanism, measurement and mitigation of the light- and elevated temperature-induced degradation (LeTID), which has been found to be a bulk-sensitive degradation behavior and is dependent on the degradation condition. In this paper, we study the influence of silicon bulk property on LeTID from five representative positions along a relative low resistivity (0.82–1.33 Ω cm) p-type mc-Si brick. The evolution of degradation and regeneration under different laser illumination conditions are investigated. Identical defect capture cross section ratio k values of ~35 at mid-gap along the brick height are found. For the first time, the activation energy for degradation (Ea,deg) and regeneration (Ea,reg) along the brick are studied, which shows Ea,deg = Ea,reg and a tendency of larger values towards the brick bottom. These results indicate that the LeTID in the whole brick might be induced by a single defect and thus mitigated in a single manner. Besides, a laser illumination of 45 kW/m2 at 142 °C for 100 s is able to induce over 90% degradation for all wafers, which could be used as a universal and fast LeTID test condition.

The Role of Dark Annealing in Light and Elevated Temperature Induced Degradation in p-Type Mono-Like Silicon

We have studied lifetime instabilities in p-type boron-doped mono-like silicon during light soaking (LS) and dark annealing (DA) at different temperatures, and their behavior upon LS/DA cycling at various degradation and regeneration stages. Despite having similar capture cross section ratios, it is found that the defects responsible for the degradation under illumination and in the dark could stem from two separate reactions, with hydrogen being the common precursor. A model for light and elevated temperature induced degradation (LeTID) is presented based on our experimental findings. It is proposed that hydrogen atoms originally bound in the silicon nitride layer are released into the silicon bulk above a certain firing temperature, which then interact with some other species in the silicon bulk under illumination, causing the LeTID degradation. During the cooling ramp of the firing process or extended DA, hydrogen in the silicon bulk starts to effuse into the ambient, reducing the amount of hydrogen remaining in the silicon bulk, and correspondingly affecting their LeTID behavior. The proposed model provides new insights to help understand complex LeTID behaviors reported in the literature, including its dependence on the firing profile, sample thickness, dopant type, and DA pretreatment.

Light and Elevated Temperature Induced Degradation (LeTID) in a Utility-Scale Photovoltaic System

We present a detailed case study of degradation in monocrystalline silicon photovoltaic modules operating in a utility-scale power plant over the course of approximately three years. We present the results of degradation analysis on arrays within the site, and find that five of the six arrays degraded faster than the best performing array, even though the arrays consist of modules of the same manufacturer and model. We also describe the results of extensive laboratory characterization of modules returned from the field, including module- and cell-level current-voltage characterization, luminescence imaging, and accelerated testing. The laboratory test results and the field performance are consistent with light and elevated temperature induced degradation (LeTID). Notably, we observe differences in back contact technology between affected and unaffected modules. This article also demonstrates a method to identify possible LeTID degradation in the field and confirm the result with laboratory testing of a small number of modules.

Surface related degradation phenomena in P-type multi-crystalline silicon at elevated temperature and illumination

As a non-negligible part of light and elevated temperature induced degradation (LeTID), the studies of surface related degradation are rare, and the complex mechanism hinders the research in this field. In this paper, the degradation behaviors of passivation quality on the surface of P-type multi-crystalline silicon were mainly analyzed during LeTID. Results shown that effective minority carrier lifetime of passivated mc-Si wafers decreased in the range of 20–50% after it got to the full degraded during light soaking at 80 °C. The fixed charges at the interface between passivation layer and silicon bulk increased slightly after full degradation, which had little effect on field-effect passivation. Long-term illumination resulted in a certain increase in interface states density (Dit) in the bottom half of bandgap. According to the result that the increment of Dit of ungettered samples was higher than that of phosphorus diffusion gettered samples, it implied that the increase of interface defects might be caused by the diffuse of impurities in the bulk to the surface during the process of LeTID.

Light and elevated temperature induced degradation in B–Ga co-doped cast mono Si PERC solar cells

The utilization of boron-doped Si solar cells based on the structure of a passivated emitter and rear cell (PERC) in the solar industry has increased recently. However, this type of high efficiency solar cell is exposed to a so-called light and elevated temperature induced degradation (LeTID). A suppressing of mc-Si LeTID has been studied through a regeneration treatment at high temperature under illumination or in the dark, but most was about the lifetime samples. In this work, to evaluate the applicability of regeneration and annealing at industrial relevant conditions, industrially made B–Ga co-doped cast-mono Si PERC solar cells were treated in a light soaking tool in which the intensity of light and substrate temperature could be adjusted separately and a rapid thermal process (RTP) system. This treatment was evaluated during a subsequent intentional degradation under conditions of 75 °C with an LED white light source at an intensity of 1 kW/m2. The results showed that properly regenerated samples by high intensity illumination at elevated temperatures suffered from the least degradation, while untreated solar cells had most severe degradation. The RTP method could improve the performance of the solar cells but the RTP-treated samples were less stable than the regenerated samples. It demonstrates the application of a fast (around 20 min) regeneration method could be coupled in mass production. Further, RTP treatment combining with an accelerated regeneration step may be a potential method to provide both the improved performance and high anti-LeTID properties in Si PERC solar cells.

Eliminating light- and elevated temperature-induced degradation in P-type PERC solar cells by a two-step thermal process

Light- and elevated temperature-induced degradation (LeTID) can have a severe impact on the carrier lifetime of silicon substrates used in solar cell production and thus remains a crucial challenge for manufacturers. In this work, we introduce a two-step annealing process to mitigate LeTID in multi-crystalline silicon (mc-Si) passivated emitter and rear cell (PERC) solar cells. We demonstrate that the first annealing step (450 °C) with a slow belt speed (0.5 m/min) plays a primary role in mitigating LeTID in the cells, but also results in an increase in contact resistance. The application of a second annealing step at a similar temperature (400–500 °C) with a faster belt speed (1.4 m/min) recovers the contact resistance whilst maintaining the stability of the cell. Applying this approach to the p-type mc-Si PERC solar cells resulted in a reduction of efficiency loss during light soaking from ~6%rel (control) to ~1%rel (treated sample). This finding is significant for p-type mc-Si solar cell manufacturers, as the process can be applied to finished cells using a standard belt firing furnace to stabilise cell efficiency for long term operation in the field.

Hydrogen-induced degradation: Explaining the mechanism behind light- and elevated temperature-induced degradation in n- and p-type silicon

Light- and elevated temperature-induced degradation (LeTID) has been extensively studied on p-type silicon materials with increasing evidence suggesting the involvement of hydrogen. Recent findings of the identical phenomenon in n-type silicon wafers have further opened up new areas of understanding into the inherent behavior and root cause of the defect. In this work, we compare LeTID observed in both p- and n-type silicon wafers under both dark and illuminated annealing conditions, highlighting previously unobserved similarities in defect formation and recovery kinetics. We report thermal activation energies of the LeTID-related degradation and recovery in n-type silicon to be 0.76 ± 0.02 eV and 0.97 ± 0.01 eV without illumination, respectively, and 0.70 ± 0.05 eV and 0.83 ± 0.15 eV under illumination (0.02 kWm−2), respectively. Furthermore, we present additional experimentation demonstrating the thermal and illumination dependency of surface-related degradation (SRD) in n-type silicon. We report an extracted activation energy of this SRD of 0.38 ± 0.10 eV. Through modelling of the hydrogen charge state fractions, we speculate that the behavior of LeTID both in the dark and under illumination may be explained by the migration of and interactions between charged hydrogen species and dopant atoms within the diffused layers and the silicon bulk.

Influencing Light and Elevated Temperature Induced Degradation and Surface-Related Degradation Kinetics in Float-Zone Silicon by Varying the Initial Sample State

Light and elevated temperature induced degradation (LeTID) kinetics in float-zone silicon are investigated by varying the initial sample state, composed of different base material, base doping, SiN x :H films, and subsequent firing, and/or annealing steps. The approach of deliberately changing the initial sample state is shown to allow for specific studies of influences of LeTID kinetics. Bulk- and surface-related degradations are examined separately and the influence on the kinetics of bulk- and surface-related degradation is illustrated by a four-state and three-state model, respectively. In case of bulk-related degradation, an increase in defect density because of the firing step is shown, whereas the annealing step has an inverse effect. Both temperature steps—individually and combined—influence the transition rates of bulk-related degradation and regeneration by presumably changing the distribution of a defect precursor. For surface-related degradation, the firing step reduces the transition rate from the initial to the degraded state. In addition, the influence of a comparably humid atmosphere and the absence of UV light are found to be negligible.