Atomic layer deposited Zn _x Ni _1−x O: A thermally stable hole selective contact for silicon solar cells

Atomic layer deposited zinc nickel oxide (ZNO, Zn _x Ni _1−x O) films with Zn concentrations of 0.09, 0.46, and 0.62 are investigated for application as a hole-selective contact for crystalline silicon solar cells. The ZNO films were found to be p-type by evaluating their contact performance on p-Si. A direct contact between ZNO and p-Si showed perfect ohmic behaviour. Spectroscopic ellipsometry measurements revealed a high optical transparency of the ZNO films with a bandgap of >3 eV. X-ray photoelectron spectroscopy confirmed a much lower valence band offset between ZNO and p-Si than the conduction band offset, which is favourable for selective hole extraction on p-Si. In addition, subsequent annealing at 200 °C significantly decreased the contact resistivity, and annealing temperatures up to 500 °C did not degrade its contact performance. A minimal contact resistivity of 21.5 mΩ·cm ² was obtained on p-type c-Si for a 3.4 nm ZNO film with a Zn concentration of 0.62 after annealing at 200 °C. These results demonstrate the advantages of Zn _x Ni _1−x O as a hole-selective contact for crystalline p-Si solar cells.