Atomic Layer Deposition TiO2 Films and TiO2/SiNx Stacks Applied for Silicon Solar Cells

Zu-Po Yang,Hsyi-En Cheng,I-Hsuan Chang,Ing-Song Yu

doi:10.3390/app6080233

Abstract

Titanium oxide (TiO2) films and TiO2/SiNx stacks have potential in surface passivation, anti-reflection coatings and carrier-selective contact layers for crystalline Si solar cells. A Si wafer, deposited with 8-nm-thick TiO2 film by atomic layer deposition, has a surface recombination velocity as low as 14.93 cm/s at the injection level of 1.0 × 1015 cm−3. However, the performance of silicon surface passivation of the deposited TiO2 film declines as its thickness increases, probably because of the stress effects, phase transformation, atomic hydrogen and thermal stability of amorphous TiO2 films. For the characterization of 66-nm-thick TiO2 film, the results of transmission electron microscopy show that the anatase TiO2 crystallinity forms close to the surface of the Si. Secondary ion mass spectrometry shows the atomic hydrogen at the interface of TiO2 and Si which serves for chemical passivation. The crystal size of anatase TiO2 and the homogeneity of TiO2 film can be deduced by the measurements of Raman spectroscopy and spectroscopic ellipsometry, respectively. For the passivating contacts of solar cells, in addition, a stack composed of 8-nm-thick TiO2 film and a plasma-enhanced chemical-vapor-deposited 72-nm-thick SiNx layer has been investigated. From the results of the measurement of the reflectivity and effective carrier lifetime, TiO2/SiNx stacks on Si wafers perform with low reflectivity and some degree of surface passivation for the Si wafer.

Highlights

Dielectric thin-film materials capped on top of solar cells serve important roles as an anti-reflection coating layer for photon harvesting, and as a surface passivation layer to reduce the surface recombination loss for crystalline silicon (c-Si) solar cells
We investigated the growth and phase transformation of TiO2 films deposited by atomic layer deposition (ALD) at the temperature of 200 ◦ C by using transmission electron microscopy (TEM)
Si substrate was placed in the reaction chamber and the chamber temperature was raised to 200 ◦ C for the ALD deposition [28]

Summary

Introduction

Dielectric thin-film materials capped on top of solar cells serve important roles as an anti-reflection coating layer for photon harvesting, and as a surface passivation layer to reduce the surface recombination loss for crystalline silicon (c-Si) solar cells. As the thickness of c-Si solar cells decreases due to lowering the manufacturing cost of solar cells, Si surface passivation becomes a more and more important issue for high efficiency solar cells [1]. Beside the use of dielectric thin films, Si surface passivation depends on the deposition techniques, treatments of surface cleaning, and types of Si substrates. Al2 O3 thin films deposited by ALD have shown great performance in surface passivation for industrial c-Si solar cells [5,6,7]. ALD bilayer coatings can provide a function for surface

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Conclusion