Atomic-layer deposited Nb2O5 as transparent passivating electron contact for c-Si solar cells

Passivating contacts based on metal oxides have proven to enable high energy conversion efficiencies for crystalline silicon (c-Si) solar cells at low processing complexity. In this work, the potential of atomic-layer deposited (ALD) Nb2O5 as novel electron-selective passivating contact is explored in terms of recombination parameter J0 and contact resistivity ρc. It is shown that after forming gas annealing, ALD Nb2O5 can provide adequate surface passivation with J0 values down to 25-30 fA/cm2. On HF-treated c-Si surfaces a minimum film thickness of ~ 3 nm is required to achieve this high level of passivation, whereas on surfaces with a wet-chemical SiO2 interlayer the high passivation level is persistent down to film thicknesses of only 1 nm. Ohmic n-type contacts have been achieved using Al as contacting metal, where annealing the samples after Al contacting proved crucial for obtaining good contact properties. Low contact resistivity values of 70 and 124 mΩ cm2 for 1 and 2 nm Nb2O5 films, respectively, have been achieved on c-Si substrates that received an HF treatment prior to Nb2O5 deposition. Transmission electron microscopy imaging shows that on such surfaces the annealing treatment leads to the formation of a (1.7±0.2) nm interfacial oxide in between the c-Si substrate and the Nb2O5 film. The presented results demonstrate the potential of ALD Nb2O5 as electron-selective passivating contact and directions for future research are outlined.