Ex-situ phosphorus-doped polycrystalline silicon passivating contacts for high-efficiency solar cells by physical vapour deposition

We present ex-situ phosphorus-doped polycrystalline silicon (poly-Si) passivating contacts fabricated by the physical vapour deposition method, specifically sputtering. This technique is currently of great interest to the photovoltaics (PV) community due to its low deposition temperature, small footprint, high throughput, and use of low-hazard, solid targets and gases. We optimise the performance of the passivating contacts by adjusting various parameters from the sputtering of amorphous Si films to the formation of the poly-Si layers by a high-temperature thermal diffusion (with phosphorus oxychloride – POCl3, or phosphorus spin-on glass as the dopant sources). For the case of POCl3 diffusion, a high iVoc of ∼720 mV together with a low J0 of 2.6 fA/cm2 after a hydrogenation treatment by atomic layer deposition (ALD) AlOx and forming gas annealing (FGA), and a low contact resistivity ρc of less than 10 mΩ cm2 were achieved. Meanwhile, with spin-on doping, a low average iVoc of ∼600 mV was observed. This variation in performance could be explained by the difference in the crystallinity and the diffusion mechanism of the sputtered Si films after different subsequent thermal diffusion processes. Quokka3 simulations show that cell efficiencies as high as 25% are possible using the optimised passivating contacts. The results demonstrate the potential of these PVD poly-Si films as a promising candidate for future industrial solar cells.