Effective Hydrogenation of Poly-Si Passivating Contacts by Atomic-Layer-Deposited Nickel Oxide

In recent years, passivating contacts based on SiO2/poly-Si have proven to be an enabling technology for Si solar cells. Effective hydrogenation of the interfacial SiO2 is vital for realizing efficient contacts. Hydrogen-rich dielectrics, such as SiNx and Al2O3, are commonly employed for hydrogenation, whereas also recently, n-type conductive oxides, such as In2O3:Sn and ZnO, have been demonstrated to yield excellent hydrogenation. This study presents the use of a p-type metal oxide, specifically NiO, as a suitable hydrogenation source. The p-type character of NiO makes it an interesting candidate for hydrogenation because of its poten tial use in selective contacting structures. Herein, we show that NiO, synthesized by atomic layer deposition (ALD), can be used to hydrogenate poly-Si/SiO2 contacts effectively. Furthermore, we benchmark its hydrogenation performance to the established ALD ZnO/Al2O3 stack and provide insights into the hydrogenation process. On planar surfaces, NiO yields almost as excellent results as ZnO/Al2O3 stacks, whereas it lags behind on more challenging textured surfaces. Interestingly, even though elastic recoil detection analysis reveals that ALD NiO is rich in hydrogen, secondary ion mass spectrometry measurements show that, when NiO is com pared to the ZnO/Al2O3 stack, less hydrogen is present at the Si/SiO2 interface after annealing. This is explained from effusion measurements, which show substantial effusion of hydrogen from NiO around 300 °C. Hence, Al2O3 capping is further employed to prevent hydrogen loss and on textured wafers, the NiO/Al2O3 stacks on poly-Si achieve an implied open-circuit voltage of 728 mV, confirming the excellent hydrogenation from ALD metal oxides.