Charge Carrier Dynamics in TiO2 Thin Films for Photonic Applications

Abstract

With the advent of the modern world, the growing demand of power requires sustainable ways to generate energy. The depletion of fossil fuels is not only contributing to global warming but has also driven the need for exploiting more renewable energy resources. Solar energy emerges as a promising alternative to the fossil fuels which can pave the way for a cleaner and more sustainable future.

For this motive, many photocatalysts have been designed to harvest the solar energy. Among them, titanium dioxide (TiO<SUB>2</SUB>) is a versatile semiconductor material that holds an immense potential for various photocatalytic and photovoltaic applications. Long lifetime of the charge carriers is a paramount requirement for the optimal performance of many photonic devices. Herein, TiO<SUB>2</SUB> thin films prepared by atomic layer deposition (ALD) are well investigated using femtosecond transient absorption spectroscopy and the results are used to identify optimal parameters to improve their optoelectronic properties. Transient absorption spectroscopy enables monitoring the dynamics of photogenerated charge carriers from subpicosecond timescale up to nanoseconds.

Photoexcitation induces change in the charge carrier distribution which changes the refractive index within the TiO<SUB>2</SUB> thin films. Hence, both transient transmittance (TT) and transient reflectance (TR) modes have been employed to gain insights on the changes in carrier distribution over time. The results of this Thesis indicate that the lifetime of carriers can be enhanced when the ALD growth temperature is first optimized to 150 <SUP>◦</SUP>C and the samples are further heat-treated at ≥ 300 <SUP>◦</SUP>C which converts them to anatase phase. These optimized TiO<SUB>2</SUB> films were deposited as corrosion protection layers over Si and due to their favorable conduction band position, were employed to optimize interfacial charge transfer in TiO<SUB>2</SUB>–Si heterojunctions using TR spectroscopy. This optimization will facilitate efficient carrier transport from Si through the TiO<SUB>2</SUB> layer to the metal contacts and catalytic centers in photovoltaic and photoelectrochemical devices, respectively, enhancing their overall performance.