Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J 0 ) and interface state densities (D it ) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The D it values are (3 ± 0.2) Â 10 11 and (8 ± 2) Â 10 11 eV/cm 2 when fired at 620 C and 900 C, respectively. The activation energy in an Arrhenius fit of D it versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si H bonds at the poly-Si/SiO x interface is not the root cause of depassivation. Postfiring annealing at 425 C can improve the passivation again. Samples with SiN x capping layers show an increase in J 0 up to about 100 fA/cm 2 by firing, which can be attributed to blistering and is not reversed by annealing at 425 C. On the other hand, blistering does not occur in poly-Si samples capped with AlO x layers or AlO x /SiN y stacks, and J 0 values of 2-5 fA/cm 2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J 0 and D it : thermal stress at the SiO z interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level.
The response of commercial n‐type silicon heterojunction (SHJ) solar cells to illuminated annealing at temperatures between 75 and 180 °C is reported on. Although a slight increase in efficiency of 0.1% absolute occurs at 75 °C under 1 sun illumination after 20 h, annealing at higher temperatures (85–180 °C) results in significant degradation in cell performance, and only occurs in the presence of illumination. At 160 °C, a loss in η up to 1% absolute is observed under 1 sun light soaking (LS) in as little as 2 min. Further illuminated annealing leads to a subsequent partial or full recovery of cell performance. At 160 °C, after an initial VOC degradation exceeding 10 mV within 3 min, a complete recovery is obtained after 60 min of illuminated annealing. The results indicate the potential presence of a light‐induced degradation mechanism in n‐type SHJ cells, suggesting care must be taken when using LS to improve efficiency. In addition, significant variability in the maximum extent of degradation indicates a high degree of cell‐to‐cell variation in expression of this degradation mechanism. The exact nature of the underlying defect mechanism(s) governing degradation and recovery dynamics remains uncertain and requires further studies.
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