The role of hydrogen in firing and light-soaking effects on doped polysilicon passivating contacts for silicon solar cells

Kang, Di

The role of hydrogen in firing and light-soaking effects on doped polysilicon passivating contacts for silicon solar cells

Date

2022

Authors

Kang, Di

Abstract

High-temperature firing treatment used in commercial screen-printed solar cells can directly deteriorate the surface passivation, and indirectly impact the long-term stability of the cell performance by triggering degradation in both the surface and the silicon bulk. This thesis studies the firing and long-term performance of polycrystalline silicon on silicon oxide (poly-Si/SiOx) passivating contacts, and light and elevated temperature induced degradation (LeTID) in various types of silicon wafers. This thesis first examines the firing stability of n-type phosphorus-doped and p-type boron-doped poly-Si/SiOx structures. N-type poly-Si exhibits a substantial increase in the recombination current density parameter J0 after firing at 800oC, with the extent of degradation sensitive to numerous factors, such as the firing temperature, the phosphorus diffusion conditions, and the dielectric coating layers. In comparison, p-type poly-Si shows higher firing stability than n-type poly-Si. With the assistance of various characterization tools, such as Fourier transform infrared (FTIR) and secondary ion mass spectrometry (SIMS), it is found that hydrogen diffusion during firing determines the final passivation quality. An optimum amount of hydrogen surrounding the interfacial SiOx is beneficial to achieve high-level surface passivation after firing, while an excess or insufficient amount of hydrogen is detrimental. The improved firing stability in p-type poly-Si could be owing to the lower effective hydrogen diffusivity, preventing the accumulation of excess hydrogen, which contributes to the degradation observed in n-type poly-Si. This thesis then studies the long-term stability of the poly-Si/SiOx passivating contacts and silicon wafers. Firing can activate a degradation and a subsequent recovery in the surface passivation of n-type poly-Si upon light soaking at temperatures between 75oC and 200oC, with the extent of the degradation depending on the samples, such as the SiNx coating layers and the poly-Si films, and the test conditions, such as the light-soaking temperature and the light intensity. Hydrogen could help with the regeneration, evident from the observation that samples with silicon nitride (SiNx) films removed after firing suffer a significantly larger degradation without showing recovery. Moreover, firing can lead to degradation in the silicon bulk upon light soaking at elevated temperature, which is known as LeTID. This thesis compares LeTID on p-type boron-doped and n-type phosphorus-doped mono-like silicon and float zone silicon. While all the studied materials exhibit degradation, n-type materials generally suffer a lower degradation extent than their p-type counterparts. Moreover, LeTID is affected by the SiNx film properties, with the degradation extent increasing with the Si-N bond density measured by FTIR and the refractive index of the SiNx films given by ellipsometry, which indicates that hydrogen is also responsible for LeTID. The degree of degradation is found to be reduced by phosphorus diffusion gettering and decreasing SiNx deposition temperature, revealing a potential solution to mitigate LeTID.