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Investigation of heterostructure photoanodes for solar energy conversion.

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Basu, Kaustubh (2018). Investigation of heterostructure photoanodes for solar energy conversion. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 215 p.

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Résumé

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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 21st 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 TiOx 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. We found that the modification enhances the short circuit current, open-circuit voltage and fill factor, leading to nearly 2-fold increase in power conversion efficiency, from 1.48% without any treatment, to 2.85% achieved with TiCl4 treatment. The superior photovoltaic performance of the DSSCs assembled with modified photoanode is attributed to enhanced electron lifetime and suppression of electron recombination to the electrolyte, as confirmed by EIS carried out under dark condition. These results indicate that modification of the FTO and SnO2 anode by TiO2 can play a major role in maximizing the photo conversion efficiency. Although, nanoparticle-based photoanodes exhibit high surface to volume ratio, the main drawback in using nanoparticles, is that mesoporous networks experience high density of grain boundaries, which facilitate charge recombination. Carbon-based nanostructures such as graphene and carbon nanotubes possess exceptional mechanical and electronic properties, can be used as charge directing structures in mesoporous nanoparticle networks. In the next part of this thesis, we report the effect of incorporation of graphene microplatelets into SnO2-TiO2 mesoporous anode to boost the performance and long-term stability of DSSCs. DSSCs were fabricated by incorporating different concentrations of graphene microplatelets (up to 0.50 wt.%) into the SnO2-TiO2 mesoporous network utilizing a fast and large area-scalable technique. At optimized concentration of graphene (0.03 wt.%), highest PCE of 3.37% was achieved, which is ~16% higher than for SnO2-TiO2 photoanodes based DSSC. This enhancement of PCE can be attributed to higher electron lifetime and reduced charge recombination in SnO2-TiO2/graphene photoanodes as confirmed by transient photovoltage decay and impedance spectroscopy. Furthermore, the performance of the solar cells was recorded for 200 hours of continuous illumination under simulated sunlight (AM 1.5G) for long-term stability measurements. Our results demonstrated that the addition of graphene microplatelets results in superior stability of DSSCs, where the drop in PCE was a mere 8%, while a sharp plummet of 30% in PCE was observed in case of SnO2-TiO2 photoanodes. These findings are encouraging, and establish SnO2-TiO2/graphene architecture as a promising photoanode towards efficient and stable DSSC. In the final part of our thesis, we introduce our novel SnO2-TiO2/graphene photoanodes for fabrication of PEC cells for H2 generation. H2, being a zero emission fuel, is expected to be a major player in the future energy scenario especially in the automobile fuel sector. However, presently large scale industrial production of H2 relies on utilization of fossil fuels, resulting in greenhouse gas emissions. PEC water splitting, exploiting solar energy as a clean energy source and implementing colloidal QDs as sensitizers is a promising approach for H2 generation due to the QD’s size-tunable optical properties. However, the challenge of long term stability of the QDs is still unresolved. Here, we introduce a highly stable QD-based PEC device for H2 generation using a photoanode based on a SnO2–TiO2 heterostructure, sensitized by CdSe/CdS core/thick-shell QDs. This hybrid photoanode architecture leads to an appreciable saturated photocurrent density of ~4.7 mA/cm2, retaining an unprecedented ~96% of its initial current density after two hours, and sustaining ~93% after five hours of continuous irradiation under AM 1.5G (100 mW/cm2) simulated solar spectrum. Transient PL measurements demonstrate that the heterostructured SnO2-TiO2 photoanode exhibits faster electron transfer compared with the bare TiO2 photoanode. The lower electron transfer rate in the TiO2 photoanode can be attributed to slow electron kinetics in the ultraviolet regime, revealed by ultrafast transient absorption spectroscopy. Graphene microplatelets were further introduced into the heterostructured photoanode, which boosted the photocurrent density to ~5.6 mA/cm2. Our results clearly demonstrate that SnO2–TiO2 heterostructured photoanode holds significant potential for developing highly stable PEC cells.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Vetrone, Fiorenzo
Co-directeurs de mémoire/thèse: Rosei, Federico
Informations complémentaires: Résumé avec symboles
Mots-clés libres: énergie et matériaux
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 25 juill. 2019 18:04
Dernière modification: 25 juill. 2019 18:30
URI: http://espace.inrs.ca/id/eprint/8450

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