• Energy Research

AbstractAt present, the commercially dominant and rapidly expanding PV‐device technology is based on the passivated emitter and rear cell (PERC) design developed at UNSW. However, this technology has been found to suffer from a carrier‐induced degradation commonly referred to as ‘light‐ and elevated temperature‐induced degradation’ (LeTID) and can result in up to 16% relative performance losses. LeTID was recently shown to occur in almost every type of silicon wafer, independent of the doping material. Even though the degradation mechanism is known to recover under normal operation conditions, it is a lengthy process that drastically affects the energy yield, stability and, ultimately, the levelized cost of electricity (LCOE) of installed systems. Despite the joint effort of many research groups, the root cause of the degradation is still unknown. Here, we provide an overview of the existing literature and describe key LeTID characteristics and how these have led to the development of various theories of the underlying mechanism. Further, given the continuously appearing and strong evidence of hydrogen involvement in LeTID, many mitigation methods concerning hydrogenation have been suggested. We discuss such reported methods, bearing in mind crucial consumer necessities in terms of sustained cell performance and minimised LCOE.

AbstractThe understanding and development of advanced hydrogenation processes for silicon solar cells are presented. Hydrogen passivation is incorporated into virtually all silicon solar cells, yet the properties of hydrogen in silicon are still poorly understood. This is largely due to the complex behaviour of hydrogen in silicon and its ability to exist in many different forms in the lattice. For commercial solar cells, hydrogen is introduced into the device through the deposition of hydrogen‐containing dielectric layers and the subsequent metallisation firing process. This process can readily passivate structural defects such as grain boundaries but is ineffective at passivating numerous defects in silicon solar cells such as the boron‐oxygen complex, responsible for light‐induced degradation in p‐type Czochralski silicon. This difficulty is due to the need to first form the boron‐oxygen defect and also due to atomic hydrogen naturally occupying low‐mobility and low‐reactivity charge states. However, these challenges can be overcome using advanced hydrogenation processes incorporating excess carrier generation from illumination or current injection that increase the concentration of the highly mobile and reactive neutral charge state. As a result, after fast firing, additional low‐temperature advanced hydrogenation processes incorporating illumination can be implemented to enable the passivation of difficult defects like the boron‐oxygen complex. With the implementation of such processes for industrial silicon solar cells, efficiency improvements of 1.1% absolute can be obtained.

Laser doping is a typical industrial method to introduce a local highly doped region in silicon solar cells to form a selective emitter. Such a process inherently introduces defects that can be a concern to the overall performance of the solar cell. Here, we investigate the effectiveness of laser-induced defect (LasID) passivation on lifetime test structures through different annealing processes, including high-temperature belt-furnace firing, low-temperature belt-furnace annealing, and an advanced hydrogenation process (AHP) for n+ laser-doped selective emitters. We demonstrate clear advantages of post treatment using a rapid 10 s AHP at 300 °C when the lifetime structures are prefired. For the examined laser speeds of 0.5–6 m/s (sheet resistances of 4--70 Ω/□), AHP is the most effective treatment method. For example, for a typical laser doping speed of 4 m/s, starting from the same effective carrier lifetime of 36.9±2.4 μs after laser-doping step for all the passivation treatments, the AHP not only surpasses the conventional approaches by showing the highest recovery of the effective carrier lifetime (∼79% compared with ∼63% and ∼41% for the firing and belt-furnace annealing treatments, respectively) and dark saturation current density reduction in the regions affected by LasIDs but also simultaneously suppresses light-induced degradation (maximum of 4% effective lifetime degradation with respect to the passivated state, as opposed to 14% and 16% degradation for the firing and belt-furnace annealing treatments, respectively) common in Cz grown boron-doped p-type monocrystalline silicon.

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.

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.