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 Omega cm and p-type 1 Omega cm undiffused wafers, respectively. Recombination current densities of 19 fA/cm(2) and 780 fA/cm(2) have been measured on 120 Omega/square boron diffused p(+) and 100 Omega/square 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 137. 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. (C) 2016 Elsevier B.V. All rights reserved.