A Comprehensive Fundamental Understanding of Atomic Layer Deposited Titanium Oxide Films for c-Si Solar Cell Applications

Abstract

IEEE Titanium oxide (TiO<formula><tex>$_{x}$</tex></formula>) film shows excellent surface passivation of crystalline silicon (c-Si) as well as carrier selectivity. The coexistence of both these properties along with extraordinary optical properties of TiO<formula><tex>$_{x}$</tex></formula> opens new opportunities to design novel high-performance c-Si solar cells. However, many inconsistent findings have also been reported in the literature that limit the application of TiO<formula><tex>$_{x}$</tex></formula>. A greater fundamental insight into TiO<formula><tex>$_{x}$</tex></formula> films therefore needs to be developed in order to produce very high-efficiency c-Si solar cells. In this article, a comprehensive study of atomic layer deposited (ALD) TiO<formula><tex>$_{x}$</tex></formula> films is presented, including the processing as well as material, electrical and optical properties. The effects of deposition temperature from 100 to 400&#x00A0;&#x00B0;C and postdeposition annealing treatment on the properties of TiO<formula><tex>$_{x}$</tex></formula> films are examined in terms of the ALD growth process and the composition, stoichiometry, crystallographic structure, bonding environment, elemental, chemical, electrical, and optical properties of the films. The screening of the temperature suggests that it is one of the key parameters that can be used to control the TiO<formula><tex>$_{x}$</tex></formula> film properties for different applications, by giving excellent c-Si surface passivation with a surface recombination velocity as low as 3.7&#x00A0;cm&#x002F;s. Further analysis of the underlying passivation mechanism suggests that TiO<formula><tex>$_{x}$</tex></formula> films in amorphous phase combined with an interface oxide (SiO<formula><tex>$_{y}$</tex></formula>&#x002B;TiO<formula><tex>$_{x}$</tex></formula>) and a small amount of Cl are favorable for achieving good c-Si passivation. These findings significantly improve the fundamental understanding of TiO<formula><tex>$_{x}$</tex></formula> films from their growth to application, and could enable the control of their properties for future device development.