Surface passivation and carrier selectivity of the thermal-atomic-layer-deposited TiO₂ on crystalline silicon

Here, we demonstrate the use of an ultrathin TiO₂ film as a passivating carrier-selective contact for silicon photovoltaics. The effective lifetime, surface recombination velocity, and diode quality dependence on TiO2 deposition temperature with and without a thin tunneling oxide interlayer (SiO₂ or Al₂O₃) on p-type crystalline silicon (c-Si) are reported. 5-, 10-, and 20-nm-thick TiO₂ films were deposited by thermal atomic layer deposition (ALD) in the temperature range of 80–300 °C using titanium tetrachloride (TiCl₄) and water. TiO₂ thin-film passivation layers alone result in a lower effective carrier lifetime compared with that with an interlayer. However, SiO₂ and Al₂O₃ interlayers enhance the TiO₂ passivation of c-Si surfaces. Further annealing at 200 °C in N₂ gas enhances the surface passivation quality of TiO2 tremendously. From these findings, design principles for TiO₂–Si heterojunction with optimized photovoltage, interface quality, and electron extraction to maximize the photovoltage of TiO₂–Si heterojunction photovoltaic cells are formulated. Diode behaviour was analysed with the help of experimental, analytical, and simulation methods. It is predicted that TiO₂ with a high carrier concentration is a preferable candidate for high-performance solar cells. The possible reasons for performance degradation in those devices with and without interlayers are also discussed.