Inference from scientific literatureLiao opened his presentation by discussing phenomena observed in the scientific literature regarding the effects of UV at both the cell and module levels. Referring to Paper 1 titled, “The effect of light soaking on crystalline silicon surface passivation by atomic layer deposited Al2O3”, NUS, 2013, the effect of prolonged light soaking and subsequent dark treatment on single layer Al2O3 thin film layer was investigated. Irrespective of the n-type or p-type FZ-Si surface, enhanced passivation after light soaking was observed. However, this passivation was not stable under UV light exposure which reduced the passivation level to its initial level, noted Liao.The paper titled, “Effect of UV illumination on the passivation quality of AlOx/c-Si Interface”, ISFH, 2016, investigated the effect of UV illumination on single-layer silicon nitride (SiNy) as well as on AlOx/SiNy stacks. Following a firing step, the AlOx layer showed an improvement in surface recombination velocities (SRV) during the UV illuminations due to the increased negative fix charge. However, AlOx/SiNy stacks showed very stable performance, no matter whether on a p-type surface or with the glass or EVA to simulate at the module level. On the contrary, SiNy film experienced degradation after UV illumination, says Liao.Furthermore, in paper 3, the effect of UV illumination was experimented on front-side emitters with AlOx/p+ type Si as well as on the rear-side SiNy/n+ type Si interfaces which showed stable performance. However, the cell structure, featuring front-side SiNy/n+ type Si interfaces, exhibited performance degradation.Liao further emphasized that the 2022 SLAC NAL paper, Comparison of UV-Induced Degradation of High-Efficiency Silicon PV Modules with Different Cell Architectures, revealed that the n-PERT cell structure – equivalent to TOPCon's front-side architecture – exhibited varying degradation levels across 4 different module samples under prolonged UV illumination. The higher degradation levels observed in samples C and D, compared to the stable performance of samples E and F, suggest that solutions to mitigate UV degradation exist even at the device level. This result concludes that the degradation differences might be due to variations in the passivation layer deposition steps or the doped profile of the front emitter, explained Liao.However, a subsequent scientific paper showed that the thickness of remote plasma-enhanced chemical vapor deposited (PECVD) SiNx films, used for passivating the p-type crystalline silicon interface, also plays a critical role. The increase in thickness from 20 nm to 140 nm reduced the UV degradation significantly.Liao presented a comparison table of photon energy (or optical bandgap energy, in eV) for UV illumination and various passivation materials, including Si-H bonds, AlOx, Si-rich SiNx, and N-rich SiNx. Compared to the UV illumination bandgap energy range of 3.1 eV to 4.428 eV, AlOx has a bandgap between 6 eV and 8.8 eV, while the other materials' bandgap energies overlap with the UV illumination range. These results explain why AlOx reacts minimally to UV radiation compared to the other interfaces with the materials mentioned above.Leadmicro summarized its conclusions, based on inferences from the referenced papers, as follows: changes in the composition, density, and thickness of the ARC significantly affect the UV dose at the Si interface. Improving AlOx density helps address the UVID issue, and, contrary to expectations, ALD Al2O3 does not cause UVID. In fact, ALD Al2O3 can help reduce the impact of UVID.
