Progress in the surface passivation of silicon solar cells

Abstract

In order to increase the efficiency of silicon-wafer-based solar cells in production well above 20%, it is indispensable to improve the currently applied level of surface passivation at the front as well as at the rear of the cells. This paper focuses on two main challenges: (i) the low-temperature passivation of lowly doped p-type silicon surfaces at the cell rear and (ii) the passivation of highly boron-doped p + emitter surfaces as used at the front of solar cells on high-lifetime n-type silicon wafers. In the past, low surface recombination velocities (< 20 cm/s) have been achieved on low-resistivity (~1 Ωcm) p-type silicon using plasma-enhanced chemical-vapour-deposited (PECVD) silicon nitride (SiNx) as well as amorphous silicon (a-Si). However, the high density of fixed positive charges within the PECVD-SiNx layer induces an inversion layer at the rear of p-type Si cells, producing a detrimental parasitic shunting, which reduces the short-circuit current density by up to 3 mA/cm 2 . The passivation quality of a-Si on the other hand is very temperature sensitive. More recently it has been shown that atomic-layer-deposited (ALD) alumin- ium oxide (Al2O3) provides an outstanding level of surface passivation, which can be attributed to its extremely high negative fixed charge density in combination with the very gentle deposition technique ALD, leading to low inter- face state densities. The application of these ALD-Al2O3 layers to the rear of p-type solar cells shows that this new passivation scheme is indeed suitable for high efficiencies and that due to the large negative fixed charge density no parasitic shunting occurs. We also demonstrate that ALD-Al2O3 seems to be the ideal passivation layer for boron- doped p + emitter surfaces. In a direct comparison with other passivation schemes, it is found that Al2O3 even outper- forms optimized thermally grown SiO2 and opens the possibility of achieving very large open-circuit voltages up to Voc = 740 mV.