Dark Annealing Research Articles

Ag‐Dependent Behavior Threshold and Metastability in Wide‐Gap (Ag,Cu)(In,Ga)Se2Solar Cells

Wide‐gap, high‐Ga (Ag,Cu)(In,Ga)Se2thin‐film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 °C, and light soaking. A 1:4 ratio of Ag to Cu enhances initial device performance significantly, with excess Ag enhancing carrier collection at the expense of open‐circuit voltage and fill factor for close‐stoichiometric devices. For off‐stoichiometric devices, increased open‐circuit voltages are offset by losses in carrier collection. Efficiency degradation after treatments is typically increased with additional Ag alloying. A second observation is a behavior threshold identified slightly below an Ag to Cu ratio of 1:1. For compositions below the threshold, the doping response to light soaking and dark annealing is similar to that exhibited by low‐Ga Cu(In,Ga)Se2. Above the threshold, light soaking reduces net doping and that dark annealing can even increase net doping. Furthermore, devices above the threshold exhibit a far greater doping responsivity than those below and display a strong dependence of initial performance and stability on group‐I/group‐III stoichiometry. A third observation is that all devices lose ≈1–2% (absolute) in efficiency after a 3 h light soak, indicating that this loss originates from the high‐Ga content (1:3 In:Ga), rather than the Ag alloying.

Light‐Induced Frenkel Defect Pair Formation Can Lead to Phase‐Segregation of Otherwise Miscible Halide Perovskite Alloys

AbstractAlloys of ABX3 halide perovskites (HP) exhibit unique phase behavior compared to traditional III‐V and II‐VI semiconductor alloys used in solar cells. While the latter typically have good mutual miscibility when their mixed components are size matched, and phase‐segregate when size mismatched, HP alloys show good miscibility in the dark but can phase‐segregate under light. Quantum mechanical calculations described herein reveal light‐induced defect formation and migration hold the key. Specifically, the interaction between a halogen vacancy VX with halogen interstitial Xi forming together a Frenkel‐pair defect emerges as the enabler for phase‐segregation in HP alloys. At a threshold bromine composition in the Br‐I alloys, the photogenerated holes in the valence band localize, creating thereby a doubly‐charged iodine Frenkel‐pair (VI + Ii)2+. Faster migration of iodine over bromine interstitial into the vacant iodine VI site leads to the formation of iodine‐rich and iodine‐depleted regions, establishing phase‐segregation. Removal of the mobile defects–the agent of segregation–by dark thermal annealing, supplies the opposing force, leading to reversal of phase‐segregation. This atomistic understanding can enable some control of the phase‐segregation by selecting substituting elements on the B site–such as replacing some Pb by Sn–that are unable to form stable Frenkel defects.

A comparative study on alternative industrial manufacturing routes for bifacial n‐PERT silicon solar cells

AbstractBifacial n‐PERT solar cells, taking advantage of the superior material quality of n‐type Si wafers and the high bifaciality potential of the design, can be fabricated by the existing industrial manufacturing methods. Here, we compare five alternative process flows to manufacture the bifacial n‐PERT solar cells. The complexities of their fabrication routes vary mainly according to the applied doping methods (either diffusion or ion implantation) and the rear side morphology (either alkaline textured or hardly etched pyramidal surface). We also analyze the performance of the devices fabricated by each process flow. Our results indicate that the solar cells with a B‐diffused emitter and P‐implanted BSF yield higher efficiencies than the others. In addition, the n‐PERT solar cells with hardly etched pyramidal rear surfaces have slightly higher efficiency than those with textured rear surfaces. Moreover, we studied the responses of the n‐PERT solar cells against the degradation and regeneration processes applied to boron‐doped p‐type Si solar cells under dark annealing, light soaking, and illuminated annealing. They are not significantly influenced by light‐induced degradation (LID); however, the fill factor (FF) value of the cell with hardly etched pyramidal rear surfaces is sensitive to the illuminated annealing process.

Annealing effects on Cu(In,Ga)Se2 solar cells irradiated by high-fluence proton beam

Radiation tolerance of Cu(In,Ga)Se2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 1016 MeVneq cm−2, however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Hydrogen Reactions in c‐Si:B During Illumination at Room Temperature

Herein, effects related to reactions involving hydrogen during carrier injection at room temperature in boron‐doped Czochralski‐grown silicon wafers are investigated. It is shown that these conditions lead to boron–oxygen defect regeneration. Under these conditions, bulk material quality degradation induced by a dark annealing can be temporarily recovered in the same way as light and elevated temperature‐induced degradation. Dissociation of boron–hydrogen pairs by carrier injection at room temperature is observed in parallel. These observations are discussed within the framework of known hydrogen reactions. With this study, significant aspects of hydrogen‐related meta‐stabilities at temperatures that are most relevant for solar modules in moderate climates are covered.

Investigation of Light-Induced Degradation in B-Doped Mono-Like Silicon PERC Cells by a Cycling Test With Light Soaking and Dark Annealing

We investigated light-induced degradation (LID) of passivated emitter and rear contact cells fabricated from B-doped cast-grown mono-like Si by conducting a cycling test comprising light soaking (illumination intensity of 1 sun at 95 °C) and dark annealing (at 175 °C for 30 min). Degradation and regeneration were observed in both the first light soaking (LS1) and subsequent light soaking after dark annealing (LS2 and LS3), but the extent and timescale were significantly different. The normalized defect density (NDD) during light soaking was analyzed by fitting based on first-order consecutive reaction models. Systematic residuals between the NDD and the three-state consecutive reaction model were observed in the early and regeneration regions during LS1. We discussed the scenarios that could explain the cause of these residuals, and the values of the reaction rate constants and capture cross-section ratio suggested that two types of LIDs occurred simultaneously in LS1. We concluded that one of the two types of LIDs was light- and elevated-temperature-induced degradation, but we could not identify the other LID with a larger extent and timescale.

On the Kinetics of Light‐Induced Enhancement Effect in Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells are one of the promising technologies for next‐generation high‐efficiency crystalline silicon solar cells in photovoltaic industry. Recently, an interesting light‐induced performance enhancement (LIE) effect has been observed in SHJ solar cells. Herein, the kinetics and stability of the LIE effect during repeated cycles of light soaking and subsequent dark annealing at 150 °C are investigated. It is found that VOC of SHJ solar cells increases with light soaking time following stretched exponential, but can be reverted by subsequent dark annealing. This means that LIE effect in SHJ solar cells is not stable. The underlying mechanism of this phenomenon should be attributed to the hydrogen motion in a‐Si:H layers. It is also demonstrated that increasing illumination intensity leads to faster (shorter time constant) and stronger (higher saturation level of ΔVOC) LIE effect.

Comparison of LID and Electrical Injection Regeneration of PERC and Al-BSF Solar Cells from a Cz-Si Ingot

In order to study the effect of device structures and silicon wafer positions on light-induced degradation (LID) and regeneration, five groups of industrial PERC and Al-BSF solar cells were fabricated by using silicon wafers from different positions of a B-doped Czochralski silicon (Cz-Si) ingot. Then, the cells were subjected to a dark annealing (200 °C, 30 min), the first LID (45 °C, 1 sun, 12 h), an electrical injection regeneration (175 °C, 18 A, 30 min) and the second LID (45 °C, 1 sun, 12 h) in order, and the variations of performance of the cells with processing time were measured. It was found that after the electrical injection regeneration, the efficiency losses of PERC cells decreased from 1.28–1.76%absolute in the first LID to 0.09–0.16%absolute in the second LID, while those of Al-BSF cells decreased from 0.3–0.66%absolute in the first LID to 0 in the second LID. The efficiency losses of PERC cells during the first LID were caused by the co-action of B-O-defect-induced LID (BO-LID) and dissociation of Fe-B pairs, and the latter contributed 5.81–9.56% of the efficiency loss, while those of Al-BSF cells during the first LID were almost contributed by BO-LID solely. For both kinds of cells, the cells made from the silicon wafers from middle of the ingot had the best performance throughout the experiment. In addition, the LID and regeneration treatments only affected the spectral response of the cells in the wavelength larger than 700 nm.

Improved Surface Passivation of Rear SiNx/AlOx Stacks in p‐Type Boron‐Doped Czochralski Silicon Wafers During Dark Annealing at Elevated Temperatures

Herein, the surface passivation quality of p‐type boron‐doped Czochralski silicon (Cz‐Si) wafers with passivated emitter and rear cell (PERC) structure during 200 °C dark annealing is investigated. During such a treatment, a reduction in the saturation current density (J 0) along with the degradation and regeneration of minority carrier lifetime can only be observed in PERC‐structured wafers that fast fired at high temperatures instead of in unfired wafers, which is possibly correlated with hydrogen injected into the silicon bulk during fast firing. Besides, no notable variations in J 0 can be observed in fast‐fired SiN x /n + /p‐Si/n + /SiN x symmetrical structure wafers during the same treatment, although their minority carrier lifetime undergoes degradation and regeneration cycles, demonstrating that the improved surface passivation comes from the rear surface of PERC structure. The capacitance–voltage measurement on the fabricated rear surface samples with metal‐insulator‐semiconductor (MIS) structure further demonstrates that the improved surface passivation is not related to field‐effect passivation. By comparing the surface state density of rear surface during dark annealing, it can be concluded that the improved rear surface passivation originates from the hydrogenation of surface defects such as dangling bonds, possibly by the outdiffused hydrogen from silicon bulk during such a treatment.

Electronic Properties of the Boron–Oxygen Defect Precursor of the Light-Induced Degradation in Silicon

In this work, we study the electronic properties of the boron–oxygen precursor defect responsible for light-induced degradation in crystalline silicon via deep-level transient spectroscopy. Even though this degradation has been known for many decades, and has resulted in severe silicon solar cell degradation, its root cause is poorly understood. The detection of a deep defect correlated to BO degradation has only been reported recently using deep-level transient spectroscopy. The resulting trap has been labeled as H390. In this work, we confirm the existence of this trap and report on the observation of a second defect (H400), which appears to be similarly associated with the BO light-induced degradation. To the best of our knowledge, this level has not been previously reported and could result from a second latent form of the BO light-induced degradation defect, a topic that has been largely debated in the literature. The concentration of both levels increases following dark annealing and decreases following minority carrier injection. The two levels disappear during BO activation and appear during BO deactivation, confirming a link between the observed levels and the latent state of the BO defect. Furthermore, consistent with previous studies, the two traps are metastable and change configurations after hole capture. Finally, we determine the minority carrier capture cross-section of the active BO defect by comparing the latent defect concentration with the minority carrier lifetime.

Formation Dynamics of BH and GaH‐Pairs in Crystalline Silicon During Dark Annealing

In crystalline silicon, atomic hydrogen released from hydrogen dimers forms acceptor–hydrogen pairs during annealing in the dark at elevated temperatures. In this study, the formation of boron–hydrogen (BH) and gallium–hydrogen (GaH) pairs in 1 Ω cm silicon is investigated at temperatures ranging from 140 to 220 °C. Acceptor–hydrogen concentrations in the low range are quantified by means of highly sensitive resistance measurements. GaH pairs are generally found to form faster than BH pairs. Arrhenius analysis shows a difference in activation energy (BH: 1.20 eV, GaH: 1.04 eV) while the trial frequency is the same ().

On the kinetics of the exchange of hydrogen between hydrogen–boron pairs and hydrogen dimers in crystalline silicon

Silicon samples after fast-firing with a hydrogen-rich silicon nitride layer on their surfaces can contain high concentrations of hydrogen (up to 6 × 1015 cm−3 in this study). Directly after fast-firing, this hydrogen is mostly present in a neutral dimeric state. Subsequent dark annealing applied in a temperature range between 140 and 175 °C leads to a conversion of dimers into HB pairs, which means that a significant number of boron acceptors are electrically inactive. The concentration of inactive boron, the hydrogen–boron (HB) pair concentration [HB], can thus be determined by measuring the change in specific resistivity before and after annealing. In our study, after the initial anneal for HB pair formation, the same samples are subsequently annealed at stepwise increasing higher temperatures, which leads to a partial backward conversion of HB pairs into neutral hydrogen dimers. This is indicated by a gradual reduction of the resistivity upon increasing the annealing temperature. By measuring the transient curves [HB](t) during each temperature step on samples with different boron content, we extract the parameters for the exchange between the HB pairs and the hydrogen dimers within the framework of our presented physical model. Apart from the backward formation of hydrogen dimers, from HB pairs upon increasing temperature, prolonged annealing at the same temperature leads to a transition of HB pairs into a more stable form of hydrogen dimers, compared to the initial form after fast-firing. This transition is also included in our proposed defect model.

Influence of the Bulk Resistivity on Silicon Heterojunction Solar Cells and Module Reliability

Recent developments in industry on surface passivation open the possibility of using less doped substrates in silicon solar cells. We investigate how the bulk resistivity affects the performance of silicon cells and the reliability of modules. Herein, n‐ and p‐type silicon heterojunction cells with bulk resistivities between 3 and 15 000 Ωcm are studied. We measure the current–voltage characteristics of n‐type cells across the resistivity range, and we find comparable responses to illumination intensities between 0.1 and 1 suns. The cells with bulk resistivities over 1000 Ωcm show breakdown voltages larger than −1000 V, almost two orders of magnitude higher than in typical commercial cells. Although modules have bypass‐diodes to prevent cells from going into breakdown, higher breakdown voltages can improve the reliability of modules in case of bypass‐diode failure and reduce the module cost by easing the number of bypass‐diodes required. Finally, the cells have been submitted to light soaking. The float‐zone p‐type cells with bulk resistivities over 10 000 Ωcm are less sensitive to light‐induced degradation than cells with bulk resistivities below 10 Ωcm. The former show to recover few hours after light soaking, while the latter recover only after dark annealing.

Investigation of minority carrier traps in p-type mc-Si: Effect of firing and laser annealing

Recently, it has been shown that the investigation of minority carrier traps (traps) is a useful method to study defects in silicon wafers. In this paper, we report the presence of traps in p-type multicrystalline silicon with a photoconductance decay time constant of 1.9 ± 0.4 s (at 30 °C). It is shown that the density of traps is significantly reduced after firing. However, this reduction in trap density is metastable, and it recovers by short dark annealing at 100 °C or after several days of storage at room temperature. In contrast, laser annealing is shown to eliminate the traps in fired wafers, while no change in the trap density is observed for wafers that have not been fired. Further dark annealing of those wafers does not recover the traps, suggesting that this trap annihilation is not reversible.

Study on Boron–Hydrogen Pairs in Bare and Passivated Float‐Zone Silicon Wafers

This study deals with the dynamics of the formation and dissociation of boron–hydrogen (BH) pairs in crystalline silicon during a rapid high‐temperature treatment and subsequent dark annealing between 200 and 300 °C. Highly accurate resistivity measurements are used to detect BH pairs in chemically polished B‐doped float‐zone silicon. It is found that an unexpecteded high amount of hydrogen is present in the as‐purchased wafers. Hydrogen is initially mostly paired to boron but can be dissolved by a short high‐temperature firing step. If a firing step (530 °C) is applied to bare, unpassivated Si wafers, most of the initial BH pairs are dissolved, and hydrogen dimers () form. With increasing peak temperature, an increasing amount of hydrogen leaves the system, while the proportion of BH increases. Additional hydrogen can be introduced by firing a wafer passivated with plasma‐enhanced chemical vapor deposition (PECVD) H. A three‐state model shows a good agreement with the measured data for both bare and coated samples as well as for different annealing temperatures. With increasing dark annealing temperatures, the BH dynamics accelerates, whereas the maximum BH concentration reached decreases. For temperatures above 280 °C, significant changes in the reaction dynamics are observed.

Kinetics of light and elevated temperature-induced degradation in cast mono p-type silicon

Cast mono Silicon (Si) technology has the advantages of low cost, i.e., multicrystalline silicon (mc-Si), and high efficiency, i.e., Czoch-ralski-grown silicon (Cz-Si). Cast mono Si has a similar performance to current Cz-Si counterparts with a solar cell structure, i.e., a passivated emitter and rear cell (PERC). However, the severity of light and elevated temperature-induced degradation (LeTID) on cast mono Si solar cells may be similar to mc-Si, not Cz-Si. To clarify the LeTID kinetics in the p-type cast mono silicon, we observed the effect that LeTID had on the carrier lifetime. First, a series of carrier lifetime measurements were performed to investigate LeTID kinetics in the accelerated regeneration process via high intensity illumination at elevated temperatures and modulating kinetics via dark annealing before illumination. Next, we documented the evolution of a carrier lifetime to investigate the stability of regenerated state at 75 °C and light intensity of ~ 1 kW/m2. The results show that the effect of dark annealing on LeTID kinetic modulation could be explained by the B-H defect precursor concentration and defect reservoir theory, which originate from the results for mc-Si. An accelerated regeneration method that combines dark annealing and illumination, with high intensity at an elevated temperature, can yield a higher value and improved LeTID stability in terms of the lifetime, as compared with illumination without pre-annealing. This implies that this mixed process may be a potential method to provide both improved performance and high anti-LeTID properties in Si PERC solar cells.

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.

New insights on LeTID/BO-LID in p-type mono-crystalline silicon

In this work, light-induced degradation (LID) experiments were performed on p-type boron-doped Czochralski silicon (Cz-Si) wafers under different conditions, aiming to separately characterize boron-oxygen related LID (BO-LID) and “light and elevated temperature-induced degradation” (LeTID). We found that the regeneration rate of BO-LID, as well as the extent and kinetics of LeTID, were strongly influenced by peak firing temperature. These dependences can be explained by the variation of hydrogen concentration, which was modulated by peak firing temperature. Furthermore, we found that extending the duration of dark annealing pre-treatment decelerates the subsequent regeneration of BO defects. This result revealed that, under dark annealing, the evolution of LeTID was accompanied by a notable decrease in the mobile hydrogen concentration. Supported by this phenomenon, a hydrogen-related model based on reversible reaction is proposed to describe the LeTID behavior in crystalline silicon.

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.