Textured silicon surface passivation by high‐rate expanding thermal plasma deposited SiN and thermal SiO₂/SiN stacks for crystalline silicon solar cells

Abstract

Expanding thermal plasma (ETP) deposited silicon nitride (SiN) with optical properties suited for the use as antireflection coating (ARC) on silicon solar cells has been used as passivation layer on textured monocrystalline silicon wafers. The surface passivation behavior of these high-rate (>5 nm/s) deposited SiN films has been investigated for single layer passivation schemes and for thermal SiO 2 /SiN stack systems before and after a thermal treatment that is normally used for contact-firing. It is shown that as-deposited ETP SiN used as a single passivation layer almost matches the performance of a thermal oxide. Furthermore, the SiN passivation behavior improves after a contact-firing step, while the thermal oxide passivation degrades which makes ETP SiN a better alternative for single passivation layer schemes in combination with a contact-firing step. Moreover, using the ETP SiN as a part of a thermal SiO 2 /SiN stack proves to be the best alternative by realizing very low dark saturation current densities of <20 fA/cm 2 on textured solar-grade FZ silicon wafers and this is further improved to <10 fA/cm 2 after the anneal step. Optical and electrical film characterizations have also been carried out on these SiN layers in order to study the behavior of the SiN before and after the thermal treatment.

Protection of Si-SiO<inf>2</inf> interfaces from damp heat by overlying SiN<inf>x</inf> and Si<inf>3</inf>N<inf>4</inf> coatings

“…This suggests the deposition or annealing conditions of the SiNx were not optimised, since it is possible to attain better passivation with SiNx/Si02 than with Si02 alone [16].…”

Recombination at textured silicon surfaces passivated with silicon dioxide

“…Finally, we note that the lowest J 0E measured on phosphorus-diffused rantex is 8 fA/ cm 2 . 53 The phosphorus diffusion was 140 ⍀ / sq and passivated with an SiO 2 / SiN x stack, where the SiO 2 was 40 nm and grown by a dry thermal oxidation at 1050°C, and the SiN x was 40-70 nm and deposited by chemical vapor using an expanding thermal plasma. The J 0E was ϳ50 fA/ cm 2 after thermal oxidation ͑no hydrogenation͒, ϳ20 fA/ cm 2 after SiN x deposition and 8 fA/ cm 2 after firing for SiN x films of high refractive index.…”

Manufacturing metrology for c-Si module reliability and durability Part II: Cell manufacturing

“…The deposition of SiN x on Si wafers can induce stress, thereby affecting the mechanical properties of wafer and potentially impacting the reliability of the finished cell. By measuring wafer bow before and after deposition, multiple authors have found that the stress induced is dependent on the stoichiometry of the SiN x film, with the resulting stress being tensile [210,211] in some cases and compressive in others [212]. Wan et al show an increase in the calculated stress from x = 0.8 to x = 1.2 (where x is the [N]/[Si] ratio in the film after growth) with a peak occurring at ≈1.2, followed by a reduction in stress as x increases beyond this point [212].…”

“…Induced tensile [210,211] or compressive [212] stress caused by SiN x deposition Fourier transform infrared (FTIR) Wafer curvature tests [212] IR polariscopy [40] Raman/µ-Raman spectroscopy [213] Nanoindentation [214] Blistering…”