• Energy Research

In this article, we investigate the extent of lifetime degradation attributed to light- and elevated-temperature-induced degradation (LeTID) in p- type multicrystalline silicon wafers passivated with different configurations of hydrogenated silicon nitride (SiNx:H) and aluminum oxide (AlOx:H). We also demonstrate a significant difference between AlOx:H layers grown by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) with respect to the extent of LeTID. When ALD AlOx:H is placed underneath a PECVD SiNx:H layer, as used in a passivated emitter and rear solar cell, a lower extent of LeTID is observed compared with the case when a single PECVD SiNx:H layer is used. On the other hand, the LeTID extent is significantly increased when an ALD AlOx:H is grown on top of the PECVD SiNx:H film. Remarkably, when a PECVD AlOx:H is used underneath the PECVD SiNx:H film, an increase in the LeTID extent is observed. Building on our current understanding of LeTID, we explain these results with the role of ALD AlOx:H in impeding the hydrogen diffusion from the dielectric stack into the c-Si bulk, while PECVD AlOx:H seems to act as an additional hydrogen source. These observations support the hypothesis that hydrogen is playing a key role in LeTID and provide solar cell manufacturers with a new method to reduce LeTID in their solar cells.

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.

AbstractIn this work, we integrate defect engineering methods of gettering and hydrogenation into silicon heterojunction solar cells fabricated using low‐lifetime commercial‐grade p‐type Czochralski‐grown monocrystalline and high‐performance multicrystalline wafers. We independently assess the impact of gettering on the removal of bulk impurities such as iron as well as the impact of hydrogenation on the passivation of grain boundaries and B‐O defects. Furthermore, we report for the first time the susceptibility of heterojunction devices to light‐ and elevated temperature–induced degradation and investigate the onset of such degradation during device fabrication. Lastly, we demonstrate solar cells with independently verified 1‐sun open‐circuit voltages of 707 and 702 mV on monocrystalline and multicrystalline silicon wafers, respectively, with a starting bulk minority‐carrier lifetime below 40 microseconds. These remarkably high open‐circuit voltages reveal the potential of inexpensive low‐lifetime p‐type silicon wafers for making devices with efficiencies without needing to shift towards n‐type substrates.