Characterization of absorption losses in rear side n-type polycrystalline silicon passivating contacts

Abstract

Passivating contacts consisting of polycrystalline silicon (poly-Si) and thin silicon-oxide (SiOx) layers facilitate a significant reduction of recombination losses in silicon solar cells. Nevertheless, these gains come with short circuit current density (Jsc) losses due to parasitic absorption by the poly-Si. Even if the passivating contacts are employed at the rear side only, absorption, particularly due to free carriers (FCA) in the heavily doped poly-Si, may still lead to significant Jsc losses. In this work, these losses are characterized as a function of the poly-Si thickness (tpoly) by the analysis of front reflectance spectra in the infrared (IR). For this study, two sets of samples with different n-type full-area poly-Si passivating contacts at the rear are compared to references with a phosphorus(P)-diffused back surface field (BSF) instead. For the two sets, Jsc losses with respect to the references (ΔJsc) are 0.10 mA/cm2 and 0.42 mA/cm2 per 100 nm thick poly-Si, respectively. The difference between the two values is studied by Hall measurements and interpreted to be due to the over three times as high free carrier concentration (ND,act) in the poly-Si layers of the second set of samples as the first set. On the other hand, lifetime measurements showed an excellent passivation yielding an implied open circuit voltage (iVoc) up to 736 mV only for the samples with the more heavily doped poly-Si, whereas iVoc of 683 mV was measured for the first set, which indicates a trade-off between absorption losses and passivation quality.