Titanium oxide: A re-emerging optical and passivating material for silicon solar cells

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by a thin layer of thermal atomic layer deposited (ALD) titanium oxide (TiO₂). Surface recombination velocities of 0.8 cm/s, 2.5 cm/s and 9.8 cm/s have been obtained on n-type 10 cm, 1 Ω cm and p-type 1 Ω cm undiffused wafers, respectively. Recombination current densities of 19 fA/cm² and 780 fA/cm² have been measured on 120 Ω/□ boron diffused p⁺ and 100 Ω/□ phosphorus diffused n⁺ regions. In addition to providing a superior passivation on p⁺ over n⁺ (c-Si) surfaces, the ALD TiO₂ layers produces a strong injection-dependent effective lifetime on the n-type substrates, both of which are consistent with the possible presence of negative charge in the passivating layer. The recombination at the interface between TiO₂ and planar <100>, planar <111> and alkaline textured (c-Si) surfaces with upright pyramids is compared. We find that (i) a planar <111> surface exhibits a 1.39 times higher recombination than a planar <100> surface, and (ii) after area correction, the ratio of recombination on the textured and planar <111> surfaces is 1.37. A thin film of TiO₂ deposited by ALD has been applied to the front surface of a rear locally diffused p⁺nn⁺ front junction solar cell, performing the dual role of surface passivation and single-layer antireflection coating on the textured p⁺ diffusion. The best solar cell achieved V_oc=655 mV, FF=79.9% and efficiency=20.45%. The results presented in this work demonstrate that TiO₂ re-emerging as a suitable optical and passivating material to produce high performance solar cells.