While progress has been made in understanding the behaviour of the recently identified carrier-induced degradation mechanism in p-type multicrystalline silicon solar cells, little is currently known ...about the root cause of the defect or its possible existence in other materials. In this work, we present evidence suggesting that the defect also exists in Czochralski grown monocrystalline silicon wafers. For both mono- and multicrystalline silicon we demonstrate: 1) the presence of a degradation and recovery of bulk minority carrier lifetime induced by either illuminated or dark annealing; 2) a modulation in the magnitude of degradation by varying the firing conditions; and 3) capture cross-section ratios of 39.4 ± 4.9 and 33.4 ± 1.5 in monocrystalline and multicrystalline silicon, respectively. The results indicate that the recently identified degradation mechanism does not only occur in multicrystalline silicon from illuminated annealing at elevated temperatures, but it is also induced by dark annealing at elevated temperatures, and that the degradation can occur in Czochralski grown silicon.
•Identical carrier-induced degradation in mono- and multi-crystalline silicon materials.•Degradation and recovery cycle induced via a purely thermal route.•Defect activation and degradation mechanics have a dependency on firing conditions.•Shockley-Read-Hall statistics show similarities to defects formed under illumination.
Light-induced or, more broadly, carrier-induced degradation (CID) in high-performance multicrystalline silicon (TIP mc-Si) solar cells remains a serious issue for many manufacturers, and the root ...cause of the degradation is still unknown. In this paper, the impact of firing temperature on the stability of lifetime test structures is investigated, and it is found that substantial CID can be triggered if peak temperatures exceed approximately 700 °C. We then investigate two pathways to stabilize the performance of industrially produced TIP mc-Si passivated emitter rear contact cells which have been fired at CID-activating temperatures (~740 °C-800 °C) currently required for silver contact formation. The first is a fast-firing approach, whereby it is demonstrated that an additional firing step at a reduced temperature after cell metallization can suppress the extent of V oc degradation by up to 80%. The second approach is the accelerated degradation and subsequent recovery of carrier lifetime through the use of high-intensity illumination during annealing at elevated temperatures. A 30 s process is found to suppress the maximum extent of degradation in V oc by up to 60% and up to 80% for longer processes. Ultimately, the results suggest that a combined approach of fast-firing and a high-intensity-illuminated anneal could achieve the best results in terms of V oc , stability.
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.
•N-type silicon is susceptible to LeTID previously observed only on p-type silicon.•A surplus of hydrogen in the silicon bulk is believed to be responsible for LeTID.•Diffused emitters play a role in instigating bulk or surface degradation mechanisms.•Hydrogen redistribution and diffusion towards surfaces is related to LeTID recovery.
In this work, we present new insight into the multi-crystalline silicon carrier-induced defect (CID) by performing multiple degradation and regeneration cycles and further investigation on the ...partial recovery of mc-CID through extended dark annealing (DA). The maximum normalised defect density was found to decay exponentially with the number of cycles, suggesting that the defect precursors were slowly depleted by DA. A four-state kinetic model is proposed by introducing a reservoir state. Simulation results generated by mathematical modelling based on the proposed state diagrams exhibited good agreement with the experimental results. Extended DA on a partially recovered sample combined with simulation results suggests that the capability of defect formation through DA and the existence of the reservoir state proposed herein were the root causes for the partial recovery reported in the literature. Finally, the change in bound hydrogen state is speculated to cause the modulation of mc-CID formation. A qualitative reservoir model based on the interaction between hydrogen molecules (H2), boron-hydrogen pairs (B-H) and free hydrogen (H+, H°) is proposed and further discussed.
•A four-state kinetic model for the degradation in mc-Si is introduced.•The possible root cause for partial recovery by dark annealing in mc-Si is proposed.•The maximum defect density exponentially decreases with cycling.•A reservoir model based on the interaction between H2, B-H and H(+, 0) is discussed.
We apply ray tracing to compute the light-generated current I L within each solar cell of a bifacial tracking module, and circuit modeling to quantify how the spatial variability in I L (i.e., ...current mismatch) reduces the module's output power P MP . We find that 10 million rays are required to accurately map I L . for a central module in a photovoltaic (PV) system at a given insolation condition. The relative reduction in P MP is found to be 1) greatest in the middle of the day for sunny conditions, 2) independent of time for very cloudy conditions, 3) higher for edge modules than central modules, 4) higher for one-high portrait configurations than for two-high, and 5) higher when the ground albedo is higher. We trace 2 billion rays on 2000 parallel cores to solve a module's annual energy yield for a system located at Golden CO with a sandy soil. The yield reduction in a one-high configuration due to nonuniform illumination is 0.23% for a central module and 0.35% for an edge module. Thus, in this example, mismatch loss due to nonuniform illumination within an individual tracking module is relatively low, despite the rear of the module being shaded by a torque tube.
The optimal tilt β opt of the modules in a single-axis tracker is often determined by assuming all sunlight is direct and ground reflectance is zero. Prior works, however, have demonstrated that β ...opt is smaller when diffuse light is significant. In this article, we determine how β opt decreases as the conditions change from clear sky to overcast, accounting for many complicated effects like row-to-row shading of diffuse light, ground reflection, structural shading, cell-to-cell mismatch, and angular and spectral dependencies. We find that when compared to monofacial systems, bifacial systems tend to have a higher β opt when it is cloudier and a lower β opt when it is sunnier. We also quantify the increase in annual yield that arises when accounting for indirect light at three example sites with different climates, finding it to be ∼30% lower for a bifacial system than a monofacial system; the gain was 0.8%-1.5% for bifacial systems and 1.1%-2.1% for monofacial systems, where the highest gain was attained in the cloudiest climate.
A slow forming carrier-induced degradation effect has previously been reported for p-type multi-crystalline silicon (mc-Si) solar cells and has been observed to be most severe in passivated emitter ...and rear contact (PERC) designs. The as yet undetermined defect (or defects) responsible for this degradation is typically described as being light-induced. However, in this work we confirm that a directly equivalent degradation also occurs when subjecting a cell to current injection, thus the effect can therefore be more accurately described as being carrier-induced. The defect can take years to form under normal operating conditions, but acceleration of this formation and an apparent subsequent passivation through the use of high intensity illumination at elevated temperatures has recently been demonstrated. In this work we further investigate this approach, analyzing the effects of temperature and time on degradation, regeneration, and resulting stability, as well as the variations in treatment response for mc-Si materials from two different manufacturers. Susceptibility to carrier-induced degradation after 225h of light soaking at 70°C is shown to be reduced by 80% using a rapid, 10s treatment with an illumination intensity of 44.8kw/m2 at 200°C. Stability is shown to further improve by extending treatment time but is reduced with increasing treatment temperature.
•Slow forming LID present in mc-Si cells is confirmed to also be activated by current.•Effect found to varying extents in mc-Si PERC cells from different manufacturers.•Treatments using high irradiance and elevated temperature processes are investigated.•Degradation reduced by up to 80 percent using a rapid 10 s treatment.•Extending treatment time further suppressed degradation.
Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and ...representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.
A three-state model is used to explore the influence of the accelerated formation of recombination-active defect complexes on the mitigation of carrier-induced degradation in p-type silicon ...containing boron and oxygen. Defect formation is observed to be a critical factor for the speed at which carrier-induced degradation can be mitigated. Defect formation also plays a critical role in determining the effectiveness of mitigation at elevated temperatures. It is observed that under conventional conditions, at a processing temperature of 200 °C, approximately 170 s are required to form and passivate 99% of possible defects. The experimentally demonstrated improved effectiveness of carrier-induced defect passivation with a process time of 10 s at temperatures over 300 °C is consistent with a substantial acceleration of defect formation.
This paper discusses the influence of different solar cell loss mechanisms at low light intensities and presents a simple method for the analysis of solar cell performance under various illumination ...intensities below 1 sun. Suns-PL and Suns - V oc are used to measure the intensity-dependent pseudo I-V curves of symmetric test structures and of finished silicon solar cells in an intensity range between 1 sun and 10-3 suns. The solar cell parameters from the pseudo I-V curves are compared with the parameters evaluated by intensity-dependent measurements of the whole I-V curve. The pseudo efficiency and pseudo fill factor are found to be in good agreement with the real values at low intensities as the influence of the series resistance vanishes. Based on this finding, we compare the passivation quality of silicon dioxide and silicon nitride in combination with emitter windows on test structures. Above 0.1 suns, both passivation layers show similar performance. Below 0.1 suns, the pseudo fill factors and pseudo efficiencies of the silicon nitride passivated sample are strongly reduced compared with the sample with silicon dioxide. The open-circuit voltage starts differing below 0.01 suns.
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