Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - EpiAnodes (Heteroepitaxial α-Fe2O3 photoanodes for solar water splitting)

Teaser

Within the past 40 years, tremendous progress has been made in both the efficiency and cost reduction of photovoltaic (PV) cells that convert sunlight to electricity. However, one of the main limitations of using solar power as an energy source is that the electricity must be...

Summary

Within the past 40 years, tremendous progress has been made in both the efficiency and cost reduction of photovoltaic (PV) cells that convert sunlight to electricity. However, one of the main limitations of using solar power as an energy source is that the electricity must be used immediately or stored in a secondary device . Photoelectrochemical (PEC) cells combined in tandem with PV cells offer a solution to this problem by using solar radiation (light) to electrolyze water and generate hydrogen which can then be converted to electricity using fuel cells or be used to synthesize and store hydrocarbon fuels by hydrogenation of CO2 .

Iron oxide (also known as α-Fe2O3, hematite, or rust) has become the leading candidate for use as a photoanode material in PEC cells due to its stability in aqueous solutions, 2.1 eV band gap, vast abundance, and low cost. However, hematite has extremely poor charge carrier mobility, and a very short lifetime of photogenerated carriers. The combination of these two characteristics results in a collection length of only 2-20 nm for the photogenerated minority charge carriers (holes). This is significantly shorter than the optical absorption depth of light in hematite. As a result, significant bulk recombination occurs for thicker films on planar substrates, preventing the holes from reaching the semiconductor/liquid interface and thereby reducing efficiency. The host’s (Prof. Avner Rothschild) research group at the Technion Institute of Technology invented a new approach towards overcoming the trade-off between light absorption and charge carrier collection length. Rather than decoupling the optical and electrical path lengths, a significant amount of light is instead trapped in ultrathin films. Nevertheless, state-of-the-art hematite photoanodes still operate at approximately 30-40% of the theoretical efficiency.

This project focused on a multi-faceted research plan that addressed the salient challenges facing efficient water splitting in hematite photoanodes, seeking to improve the photoconversion efficiency of ultrathin film α-Fe2O3 photoanodes and gain fundamental understanding of the effects of thin film processing on the structure and properties of α-Fe2O3 ultrathin films. Four specific objectives were listed and met by a combination of thin film engineering coupled with materials, electrical, and photoelectrochemical characterization. These objectives included the development of heteroepitaxial hematite thin film photoanodes, determination of optimal doping profiles, development of selective hole and electron blocking layers, and investigation of orientation dependent properties of hematite photoanodes.

Work performed

This work covered a span of two years and resulted in six published peer-reviewed publications along with two manuscripts currently under revision and multiple more in preparation. The main results are described below in detail based on the objectives designated in the initial grant proposal.

Objective 1: Deposition of high quality heteroepitaxial α-Fe2O3(0001) films on platinized sapphire substrates

Heteroepitaxial multilayer Pt(111)/Fe2O3(0001) films were deposited on sapphire c-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively. The films were highly crystalline (Figure 1 adapted from ref 1), displaying an in-plane mosaic spread of less than 1° and a homogenous surface morphology with roughness of ∼3 Å. Ellipsometry and UV-vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al2O3(0001)/Pt(111)/Fe2O3(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes. This work was published in reference 1 listed below. Additionally, the heteroepitaxial photoanodes produced in this work are being used to explore various effects in hematite films

Objective 2: Determination of optimal doping profile for heteroepitaxial α-Fe2O3 films

Heterogeneous doping profiles were generated in 30 nm thick hematite stacks on F:SnO2-coated glass substrates with 25 nm thick SnO2 underlayers in order to investigate the effect of different doping profiles on photoelectrochemical performance and compare with homogeneously doped counterpart photoelectrodes. Heterogeneously doped photoelectrodes displayed both high plateau photocurrent and low onset potential, with the highest performance achieved for the specimen with Ti-doped, undoped, and Zn-doped layers at the bottom, center, and top parts of the stack, respectively. This demonstrates the potential of heterogeneous doping to improve the performance of hematite photoelectrodes for solar water splitting.

Objective 3. Development and integration of heteroepitaxial hole and electron blocking layers to suppress hole injection into the substrate and electron injection into the electrolyte.
Both deposition of underlayers and overlayers were investigated. Promising improvements in performance were observed for heteroepitaxial overlayers. This work is yet to be published. For overlayers, it was found that ion permeable overlayers deposited by wet chemistry methods were superior to dense heteroepitaxial overlayers. Transparent FeNiOx overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (α-Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the co-catalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the FeNiOx overlayers. This manuscript is currently in preparation.

Objective 4. Investigation of α-Fe2O3 heteroepitaxy and orientational relationships on various reflective substrates.
The orientation dependence on the photoelectrochemical properties of Sn-doped hematite photoanodes was studied by means of heteroepitaxial film growth. Nb-doped SnO2 (NTO) was first grown heteroepitaxially on c, a, r, and m plane single crystal sapphire substrates in three different orientations. Hematite was then grown in the (001), (110), and (100) orientations on the NTO films. Cathodic shifts in the onset potential for water photo-oxidation of up to 170 mV were observed for (110) and (100) oriented hematite photoanodes as compared to (001) oriented films. (Figure 2)

Final results

The work performed in this project has yielded both significant insight into the fundamental physics and chemistry guiding the performance of hematite photoanodes as well as provided a promising path towards the development of state-of-the-art high performance hematite photoanodes based on thin film non-porous architectures. Both of these advances have brought a boost to the chances of success and commercialization of ultrathin water splitting devices. The growing need for energy conversion and storage is critical as the price of PV continues to fall. Further development of the technologies developed in this work towards scale up and large scale deployment may provide a path towards more efficient and affordable water splitting devices.

Website & more info

More info: https://emd.net.technion.ac.il/.