Investigation of Heterostructure Photoanodes for Solar Energy Conversion

Abstract

Given the continuous escalation in the rate of energy consumption, fossil fuels, which presently meet ~86% of the global energy demand, are anticipated to run out by the end of 21^st century. Moreover, increasing concern of global warming from greenhouse gases emitted by fossil fuels, drives us to explore viable alternatives, such as renewable energy sources. Solar energy, the cleanest form of renewable energy, strikes the earth annually with a staggering 3× 10²⁴ joules which is ~10,000 times more than our global energy consumption. Abundance of solar energy, can be exploited by converting and storing them into forms like electricity and H₂ by means of photovoltaics and photoelectrochemical H₂ generation, respectively. Ever since the discovery of the photoelectric effect by Edmond Becquerel, there has been extensive research on converting light into electric power or chemical fuels. Here, our research is focused mainly on two types of PEC cells, firstly DSSCs, which is the regenerative cell that converts sunlight into electric power leaving no net chemical change behind. Secondly, a photoelectrochemical cell where there are two redox systems: one reacting with the holes at the surface of the n-type semiconductor photoanode producing oxygen, and the other, reacting with the electrons entering the counter-electrode yielding hydrogen.

The quest for efficient and stable PEC cells has led to extensive research on photoanode materials, which play a key role in the charge dynamics of the overall photoelectrochemical device. Nanoparticle based photoanodes, have received significant attention mainly due to their high surface area and facile fabrication methods. Anatase TiO₂ has been widely used as a photoanode to fabricate photoelectrochemical cells, because of the ultra-fast electron injection rates from the excited sensitizer into the TiO₂ nanoparticles. But high electron recombination rates due to low electron mobility in TiO₂ limits its use. SnO₂ on the other hand is a promising photoanode material because of its higher electronic mobility and large band gap. Mobility reported in both single crystal SnO₂ as well as nanostructures are orders of magnitude higher than TiO₂. In addition, SnO₂ has a low sensitivity to UV degradation due to its larger band gap, and hence has better long term stability. Our goal is to fabricate SnO₂-TiO₂ heterostructure photoanodes by a straight forward chemical post treatment approach, to combine the advantages of higher conduction band edge of TiO2, and the high stability and exceptional electronic mobility of SnO₂. Moreover, SnO₂-TiO₂ heterojunction has a type-II band alignment, which facilitates charge separation and transport.

In the first part of this thesis, we report the fabrication and characterization of DSSCs based on SnO₂-TiO₂ photoanodes. Firstly, FTO coated conducting glass substrates were treated with TiO_x or TiCl₄ precursor solutions to create a blocking layer before tape casting the SnO2 mesoporous anode. In addition, SnO₂ photoanodes were treated with the same precursor solutions to deposit a TiO₂ passivating layer covering the SnO₂ particles.