Exploiting defects in TiO2 inverse opal for enhanced photoelectrochemical water splitting

In this work, we report on defects generation in TiO2 inverse opal (IO) nanostructures by electrochemical reduction in order to increase photocatalytic activity and improve photoelectrochemical (PEC) water splitting performance. Macroporous structures, such as inverse opals, have attracted a lot of attention for energy-related applications because of their large surface area, interconnected pores, and ability to enhance light-matter interaction. Photocurrent density of electrochemically reduced TiO2-IO increased by almost 4 times, compared to pristine TiO2-IO photoelectrodes. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses confirm the presence of oxygen vacancies in electrochemically reduced TiO2-IO photoelectrodes. Oxygen vacancies extend the absorption of TiO2 from the UV to visible region. The incident photon-to-current efficiency (IPCE) increased by almost 3 times in the absorption (UV) region of TiO2 and slightly in the visible region. Impedance studies show improved electrical conductivity, longer photogenerated electron lifetime, and a negative shift of the flatband potential, which are attributed to oxygen vacancies acting as electron donors. The Fermi level shifts to be closer to the conduction band edge of TiO2-IO.Australian Research Council (ARC)

Improved photoelectrochemical performance of GaN nanopillar photoanodes

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.ARC grant DP140103278 (2014-2016) - H.H. Tan, Nitride-based Compound Semiconductors for Solar Water Splittin

Homogeneous photosensitization of complex TiO 2 nanostructures for efficient solar energy conversion

TiO 2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO 2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO 2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO 2 substrates. PEC tests made to CdSe-sensitized TiO 2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ∼15.7 mA/cm 2 at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction

Photoelectrochemical studies of InGaN/GaN MQW photoanodes

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm-2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.ARC grant DP140103278 (2014-2016) - H.H. Tan, Nitride-based Compound Semiconductors for Solar Water Splittin

Perovskite Photovoltaic Integrated CdS/TiO2 Photoanode for Unbiased Photoelectrochemical Hydrogen Generation

Photoelectrolysis of water using solar energy into storable and environment-friendly chemical fuel in the form of hydrogen provides a potential solution to address the environmental concerns and fulfill future energy requirements in a sustainable manner. Achieving efficient and spontaneous hydrogen evolution in water using solar light as the only energy input is a highly desirable but a difficult target. In this work, we report perovskite solar cell integrated CdS-based photoanode for unbiased photoelectrochemical hydrogen evolution. An integrated tandem device consisting of mesoporous CdS/TiO2 photoanode paired with a triple-cation perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) solar cell is developed via a facile fabrication route. The proposed photovoltaic integrated photoanode presents an efficient tandem configuration with high optical transparency to long-wavelength photons and strong photoelectrochemical conversions from short-wavelength photons. On the basis of this integrated tandem device, an unbiased photocurrent density of 7.8 mA/cm2 is demonstrated under AM1.5G illumination.ARC grant DP140103278 (2014-2016) - H.H. Tan, Nitride-based Compound Semiconductors for Solar Water Splittin

Ultrathin Ta2O5 electron-selective contacts for high efficiency InP solar cells

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA/cm2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc

Tantalum oxide electron-selective heterocontacts for silicon photovoltaics and photoelectrochemical water reduction

Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen. In this work, we demonstrate a nanoscale tantalum oxide (TaOx, ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaOx is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaOx interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications

Novel inverse opal based nanostructures using atomic layer deposition for photoelectrochemical cell applications

Three-dimensional (3D) inverse opal (IO) photonic crystals are potential candidate materials for nanostructured photoanode to realize highly efficient photoelectrochemical (PEC) solar energy harvesting. IO nanostructure possesses several beneficial characteristics for light harvesting such as direct electron transport paths for longer electron diffusion lengths, highly percolated pore structure for close interfacial contacts with the electrolyte, light trapping to improve photon absorptions and high surface area for greater photosensitizer loading. This thesis focuses on the design and fabrication of IO based nanostructures using atomic layer deposition (ALD) and investigation of PEC hydrogen generation using the IO nanostructures as photoanode of a PEC cell. Manly, detailed studies are performed to evaluate the optical and PEC performances of the IO based nanostructures and establish the key parameters which enable efficient PEC solar energy harvesting. TiO2 IO nanostructures are developed by controlled infiltrations of self-assembled polystyrene opal templates using ALD method. ALD infiltration kinetics is investigated using stop-flow process in comparison with conventional continuous flow (pulse-purge) process. The deposition in opal templates is found to be limited by the Knudsen flow of precursor gases into the nanostructure templates. Stop-flow process is shown to enable the controlled filling of high aspect ratio opal templates with filling fractions close to theoretical maximum and deposition cycle time one order magnitude lower than continuous-flow process. As fabricated TiO2 IO samples using stop-flow ALD are photosensitized with CdS quantum dots and applied as a photoanode for visible light driven PEC hydrogen generation. It is observed that TiO2 IO photoanodes possess fast photoresponse and efficient charge transfer properties. Photocurrent density and IPCE maxima are found to increase with decreasing pore size due to the increase in quantum dots loading. Bilayer photoanodes based on optically and electrically coupled ZnO nanowire array with TiO2 IO are developed using ALD to improve photon absorptions and light harvesting efficiency through photonic crystal light manipulations. Results have revealed that the bilayer nanoarchitectures contribute to the improved light harvesting through Bragg diffractions and slow light. Nevertheless, surface area shortage is observed to be the main limitation for PEC performance of these photoanodes. In order to address the surface area requirement, we have designed 3D ordered nanobushes by hierarchically integrating dense networks of ZnO nanowires with TiO2 IO. Tuning the diameter of the IO shells is shown to promote high diffuse scattering. A comparative PEC performance investigation has demonstrated the promise of the nanobushes photoanode for highly efficient PEC solar energy harvesting as supported by greatly improved specific surface area and strong light scattering.DOCTOR OF PHILOSOPHY (MSE

Nanostructured Photoelectrodes via Template-Assisted Fabrication

Nanostructured materials are crucial to the light harvesting and power conversion efficiency of solar energy conversion systems. The template-assisted fabrication method offers a versatile route to produce nanostructured materials of controlled morphology and optoelectronic properties. Toward this, self-assembled opal colloidal crystals and nanoporous anodic aluminum oxide have been used as templates to produce nanostructures of a wide range of materials with different dimensionalities and enhanced photocatalytic properties. This chapter introduces various approaches developed for producing the templates and methods of infiltrating them with the desired photoactive materials. This chapter also critically assesses the advantages and limitations of the proposed methods, and summarizes the beneficial outcomes realized using nanostructures developed through the template-assisted route for photocatalysisWe acknowledge The Australian Research Council (ARC) and The Australian National Fabrication Facility (ANFF) for providing support for this work