Jérémie Brillet

APPLICATION OF HIGHLY ORDERED TIO2 NANOTUBE ARRAYS IN FLEXIBLE DYE- SENSITIZED SOLAR CELLS

TiO(2) nanotube arrays prepared by electrochemical anodization of Ti foils show impressive light to electricity conversion efficiency in the dye-sensitized solar cells (DSCs). The length of the TiO(2) nanotube arrays (5-14 microm) was controlled by varying the anodization time from 2 to 20 h. The influence of nanotube lengths on the photovoltaic performance of DSCs was investigated by impedance. A flexible DSC using TiO(2) nanotube arrays on a Ti foil as a working electrode and polyethylene naphthalate (ITO/PEN) as counterelectrode in combination with solvent-free ionic liquid electrolyte achieved 3.6% photovoltaic conversion efficiency under simulated AM 1.5 sunlight.

Decoupling Feature Size and Functionality in Solution-Processed, Porous Hematite Electrodes for Solar Water Splitting

We introduce a simple solution-based strategy to decouple morphological and functional effects of annealing nanostructured, porous electrodes by encapsulation with a SiO(2) confinement scaffold before high temperature treatment. We demonstrate the effectiveness of this approach using porous hematite (α-Fe(2)O(3)) photoanodes applied for the storage of solar energy via water splitting and show that the feature size and electrode functionality due to dopant activation can be independently controlled. This allows a significant increase in water oxidation photocurrent from 1.57 mA cm(-2) (in the control case) to 2.34 mA cm(-2) under standard illumination conditions in 1 M NaOH electrolyte-the highest reported for a solution-processed hematite photoanode. This increase is attributed to the improved quantum efficiency, especially with longer wavelength photons, due to a smaller particle size, which is afforded by our encapsulation strategy.

Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers

The onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers. It is proposed that the corundum-type overlayers released lattice strain of the ultrathin hematite layer and decreased the density of surface states. Particularly, a Ga2O3 overlayer exhibited an enhanced photocurrent attributed to stoichiometric water splitting near the onset potential. The photocurrent was sustained over a day, attesting to its outstanding performance and durability for water splitting.

Examining architectures of photoanode–photovoltaic tandem cells for solar water splitting

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode parts.

High-efficiency dye-sensitized solar cell with three-dimensional photoanode.

Herein, we present a straightforward bottom-up synthesis of a high electron mobility and highly light scattering macroporous photoanode for dye-sensitized solar cells. The dense three-dimensional Al/ZnO, SnO(2), or TiO(2) host integrates a conformal passivation thin film to reduce recombination and a large surface-area mesoporous anatase guest for high dye loading. This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs. An increase in photovoltage of up to 110 mV over state-of-the-art DSC is demonstrated.

Solar hydrogen production with semiconductor metal oxides: new directions in experiment and theory

An overview of a collaborative experimental and theoretical effort toward efficient hydrogen production via photoelectrochemical splitting of water into di-hydrogen and di-oxygen is presented here. We present state-of-the-art experimental studies using hematite and TiO(2) functionalized with gold nanoparticles as photoanode materials, and theoretical studies on electro and photo-catalysis of water on a range of metal oxide semiconductor materials, including recently developed implementation of self-interaction corrected energy functionals.

High-efficiency solid-state dye-sensitized solar cells: fast charge extraction through self-assembled 3D fibrous network of crystalline TiO2 nanowires.

Herein, we present a novel morphology for solid-state dye-sensitized solar cells based on the simple and straightforward self-assembly of nanorods into a 3D fibrous network of fused single-crystalline anatase nanowires. This architecture offers a high roughness factor, significant light scattering, and up to several orders of magnitude faster electron transport to reach a near-record-breaking conversion efficiency of 4.9%.

A Ga2O3 underlayer as an isomorphic template for ultrathin hematite films toward efficient photoelectrochemical water splitting

Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.

Solid-state dye-sensitized solar cells using TiO2nanotube arrays on FTO glass

Highly ordered, vertically oriented TiO2nanotube arrays were prepared by potentiostatic anodization of titanium on FTO-coated glass substrate and for the first time successfully applied in the fabrication of solid-state dye sensitized solar cells (SSDSCs), giving a power conversion efficiency of 1.67% measured under an irradiation of air mass 1.5 global (AM 1.5 G) full sunlight. Furthermore, 3.8% efficiency was reached with a 2.8 µm thin TiO2nanotube array film based on a metal free organic dye using ionic liquid electrolyte.

Controlling photo-activity of solution-processed hematite electrodes for solar water splitting

Hematite is a promising material for solar energy conversion via photo-electrochemical water splitting. However, the precise control of substitutional doping and nanometer feature size is important for high photon harvesting efficiency. Doped and nanostructured hematite electrodes can be prepared by a simple solution-based colloidal approach however, a high temperature (800°C) annealing is required to activate the dopant atoms. This high temperature annealing step also increases the particle size above the dimension necessary for high photon harvesting efficiencies. Here we investigate a strategy to control the two kinetic processes occurring during sintering (particle size increase and dopant diffusion/activation) by incorporating Ti dopant directly into the colloid solution and reducing the annealing time. We find that this strategy leads to porous, high-surface area hematite electrodes giving a solar photocurrent density of 1.1 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode (RHE) under standard testing conditions where only 0.56 mA cm-2 was observed at 1.23 V vs. RHE with our previous work. In addition, scanning electron micrographs examining the morphology of the electrodes suggests that our kinetic strategy is indeed effective and that further optimization may result in higher photocurrents.