Impact of surface doping profile and passivation layers on surface-related degradation in silicon PERC solar cells

Abstract

Illuminated solar cells are susceptible to various degradation mechanisms that can act to reduce the total energy yield when deployed. One potentially severe form is an increase in carrier recombination in the surface regions. This effect has been reported at both the undoped rear surface and phosphorous diffused emitter of PERC solar cells. This work investigates the influence of a range of surface conditions on the surface-related degradation (SRD) behaviour in PERC solar cells. It is shown that SRD is strongly affected by the doping profile of phosphorous emitters, the use of thin thermal oxides with SiN x :H dielectric passivation layers, the substrate material, and the configuration of the rear surface passivation. It finds that more lightly doped emitters result in more front side SRD, with its extent increasing with the introduction of the SiO 2 /SiN x :H surface passivation layers. Czochralski silicon (Cz-Si) wafers were observed to be significantly more susceptible to surface degradation than multi-crystalline silicon (mc-Si) wafers, which we attribute to less trapping of hydrogen in the bulk of those substrates. On the rear side of PERC cells, surface degradation was only observed in structures that incorporated the combination of SiO 2 /SiN x :H rear layers. No SRD was observed in the existing Al 2 O 3 /SiN x :H technology used in the industrial PERC cells studied. However, the results presented have implications for future commercial solar cell technologies, which are transitioning towards lightly doped emitters and commonly incorporate thermal oxides for surface passivation. • The introduction of more lightly doped emitters leads to severe SRD, with its extent increasing with the introduction of theSiO 2 . • Cz-Si wafers were observed to be substantially more susceptible to surface degradation than mc-Si wafers. • Rear side SRD was only observed in PERC precursors that incorporated the combination of SiO 2 /SiN x :H rear layers.