• Energy Research

AbstractHere we report on modeling kinetics of the boron-oxygen defect system in crystalline silicon solar cells. The model, as supported by experimental data, highlights the importance of defect formation for mitigating carrier-induced degradation. The inability to rapidly and effectively passivate boron-oxygen defects is primarily due to the unavailability of the defects for passivation, rather than any “weakness” of the passivation reaction. The theoretical long-term stability of modules in the field is investigated as a worst-case scenario using typical meteorological year data and the System Advisor Model (SAM). With effective mounting of the modules, the modelling indicates that even in desert locations, destabilisation of the passivation is no concern within 40 years. We also incorporate the quadratic dependence of the defect formation rate on the total hole concentration, and highlight the influence of changing doping densities or changing illumination intensity on the CID mitigation process.

Abstract In this paper, we study a light-induced degradation (LID) mechanism observed in commercial n-type silicon heterojunction (SHJ) solar cells at elevated temperatures using dark- and illuminated annealing for a broad range of illumination intensities (1–40 kWm−2) at temperatures from 25 to 180 °C. Three key results are identified. Firstly, an increase in solar conversion efficiency (η) of up to 0.3% absolute is observed after 3 min of dark annealing at 160 °C, attributed to improved surface passivation and a reduction in series resistance. Secondly, a temperature-dependent light-induced degradation behaviour is observed at temperatures as low as 85 °C under 1-sun equivalent illumination, with increasing degradation extent and rate for increasing temperatures. At 160 °C, an average η loss of 0.5 ± 0.3% absolute is observed after only 5 min and exceeding 0.8% in some cells. Thirdly, a subsequent light intensity-dependent recovery occurs with continued illumination exposure. Under 1-sun illumination at 160 °C, a reduction in net η loss up to 0.05 ± 0.1% absolute is observed after 2 h. Increasing the illumination intensity to 40 kWm−2 accelerates the recovery and can result in a net η improvement of 0.2% absolute at 150 °C within 100 s. The results suggest that attempts to improve the efficiency of SHJ solar cells using illuminated annealing could be detrimental to cell performance if not carefully optimised. Further investigation is required to identify the exact nature of the underlying defect mechanism(s) and develop appropriate mitigation strategies on commercially suitable timescales.

Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in the field due to boron–oxygen (BO) defects. Commercial hydrogenation processes can now passivate such defects; however, this passivation can be destabilized under certain conditions. Module operating temperatures are rarely considered in defect studies, and yet are critical to understanding the degradation and passivation destabilization that may occur in the field. Here we show that the module operating temperatures are highly dependent on location and mounting, and the impact this has on BO defects in the field. The System Advisor Model is fed with typical meteorological year data from four locations around the world (Hamburg, Sydney, Tucson, and Wuhan) to predict module operating temperatures. We investigate three PV system mounting types: building integrated (BIPV), rack-mounted rooftop, and rack mounted on flat ground for a centralized system. BO defect reactions are then simulated, using a three-state model based on experimental values published in the literature and the predicted module operating temperatures. The simulation shows that the BIPV module in Tucson reaches 94 °C and stays above 50 °C for over 1600 h per year. These conditions could destabilize over one-third of passivated BO defects, resulting in a 0.4% absolute efficiency loss for the modules in this work. This absolute efficiency loss could be double for higher efficiency solar cell structures, and modules. On the other hand, passivation of BO defects can occur in the field if hydrogen is present and the module is under the right environmental conditions. It is therefore important to consider the specific installation location and type (or predicted operating temperatures) to determine the best way to treat BO defects. Modules that experience such extreme sustained conditions should be manufactured to ensure incorporation of hydrogen to enable passivation of BO defects in the field, thereby enabling a “self-repairing module.”

Abstract—Recently, an n-type multi-crystalline silicon (mc-Si) was observed to be susceptible to degradation under illumination at elevated temperatureswith similarities to carrier-induced degradation in p-type mc-Si. In this study, we demonstrate degradation and regeneration of the effective lifetime of non-diffused n-type mc-Si wafers using illuminated and dark annealing conditions at moderate temperatures. Under illuminated annealing conditions, the degradation and regeneration rates of the n-type mc-Si are observed to be slower than those of the p-type mc-Si; however, the opposite trend was observed under dark annealing conditions. The carrier-induced degradation kinetics of the n-type wafers can be described by degradation and regeneration that occur simultaneously, and the activation energies have been identified to be 1.23 ± 0.16 eV for the degradation process and 1.34 ± 0.08 eV for the regeneration. Surprisingly, no degradation was observed in n-type mc-Si under dark annealing above 160 °C. Rather, at these conditions, a two-stage improvement in the lifetime was observed. Although degradation occurs after a subsequent laser treatment, the stable lifetime at the end of the degradation is still slightly higher than its initial value.

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.

Abstract 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.

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.

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.