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Energy Generation Materials

Novel materials for energy generation

a) Solar Cells

We are interested in innovating and developing oxides in solar cells either as hole or electron transport layers, or for new absorbers or both.  Oxides are excellent materials as transport layers as they can be doped to enable good band matching across interfaces. They are also highly stable and can be easily interfaced with organics or hybrid materials. AP-SALD is an excellent, in-situ method to uniformally coat of thin oxide layers onto nanostructures or uneven surfaces.

An example of recent work is our proposal and demonstration of BiOI as a promising new absorber material (Hoye RLZ, Lee LC, Kurchin RC, Huq TN, Zhang KHL, Sponseller M, Nienhaus L, Brandt RE, Polizzotti, JA, Kursumovic A, Bawendi MG, Bulovic V, Stevanovic V, Buonassisi T, MacManus-Driscoll JL, Strongly enhanced photovoltaic performance and defect physics of air-stable bismuth oxyiodide (BiOI), Advanced Materials Sep 2017; 36: 1702176).

By careful, low temperature fabrication and device engineering (including chemical vapour transport-grown BiOI, AP-SALD growth of n-type ZnO sol-gel growth of p-type NiO), high performance BiOI PV cells were made (Fig. 1). We demonstrated very high quantum efficiencies and stability of the cells. A press release on the work is here.

We are actively further developing the BiOI cells and also exploring and developing devices from other new Pb-based perovskite-inspired absorbers materials.

Figure 1. Highly stable, all-oxide solar cells with new BiOI absorber material. a) Cross-section micrograph of layered cells including electron transport layer of ZnO and hole transport layer of NiO; b) External quantum efficiencies of cells showing 80% efficiency; c fully fabricated cells. Images from: DOI: 10.1002/adma.201702176

Figure 1. Highly stable, all-oxide solar cells with new BiOI absorber material. a) Cross-section micrograph of layered cells including electron transport layer of ZnO and hole transport layer of NiO; b) External quantum efficiencies of cells showing 80% efficiency; c fully fabricated cells. Images from: DOI: 10.1002/adma.201702176

b) Ionitronics and Ionics

Ionic materials are critical components for fuel cells, gas separation membranes and solid state batteries, and memristors.

In the ionitronics area, discoveries of non-negligible nanoionic memristive signals has sparked off many research activities, towards a universal device involving terabit memories, logic operators, neuristors and nanobatteries. A key question is how to create and tune nanionic channels. To this end, we have created new kinds of ordered nanostructured thin films by a one-step process. These films have highly controlled nanionic pathways in Sm-doped CeO2 (SDC) nanocolumns and electronic pathways  along vertical interfaces in SrTiO3. Using these structures, we have demonstrated electroforming-free nanoscaffold memristors with extreme uniformity, tunability and high density. In particular, we have shown that while keeping the high resistance state (HRS) fixed, we can carefully tune the low resistance state (LRS) by tuning the ionic conductivity of the SDC, using different Sm doping levels (Fig. 2).

Figure 2. TEM cross section of nanocomposite memristor films and On-Off Ratios of films with the LRS stated controlled by the Sm-doped CeO2 (SDC) nanopillars in the films. Images from  DOI:10.1038/ncomms12373.

Figure 2. TEM cross section of nanocomposite memristor films and On-Off Ratios of films with the LRS stated controlled by the Sm-doped CeO2 (SDC) nanopillars in the films. Images from  DOI:10.1038/ncomms12373.

In the ionics area, we have shown how nanocomposite films, it is possible to increase ionic conductivity by 2-3 orders of magnitude by inducing much greater crystalline perfection and by straining ionic conducting nanopillars (Fig. 3). This gives great promise for micro-solid oxide fuel cells where thin film ionic conductors with much higher ionic conductivity is required to enable them to operate at lower temperatures (no more than 300°C is ideal). A press release on this work is here: http://www.enterprise.cam.ac.uk/news/novel-fuel-cell-electrolyte-developed-to-offer-cleaner-more-efficient-energy/

Figure 3. Enhanced ionic conduction in ionic nanopillars (YSZ, SDC and SrZrO3) related to crystalline perfection and strain. Images from APL Materials 5, 042304 (2017) and Advanced Functional Materials 25, 4328 (2015).

Figure 3. Enhanced ionic conduction in ionic nanopillars (YSZ, SDC and SrZrO3) related to crystalline perfection and strain. Images from APL Materials 5, 042304 (2017) and Advanced Functional Materials 25, 4328 (2015).

c) Photocatalytic materials

Converting solar energy into chemical energy by photoelectrochemical (PEC) water splitting is a promising approach for sustainable energy production. Owing to the versatility in their chemical and physical properties, transition metal perovskite oxides have emerged as a new category of highly efficient photocatalysts for PEC water splitting.

To overcome the limitations of single-component perovskite oxide photocatalysts, solid solutions consisting of two different perovskite materials have been actively investigated. To understand the underlying mechanism for the enhanced PEC water splitting in mixed perovskites, we explored ideal epitaxial thin films of the BiFeO3-SrTiO3 (BFO-STO) solid solution. We determined that the conduction band minimum position is raised and an exponential tail of trap states from the hybridized Ti 3d and Fe 3d orbitals emerges near the conduction band edge when STO is added to BFO. The presence of these trap states strongly suppresses the fast electron-hole recombination, increases the carrier lifetime, and improves the photocurrent density in the visible-light region.

photocatalytic

Fig.4. (a) Schematic diagram of band positions relative to the vacuum level and NHE for the BFO50-STO50 film. A, B, and C represent the photo-excited carrier relaxation and recombination processes. (b) The dependence of carrier decay lifetime τ2 as a function of STO composition. (c) Polarization curves of BFO, BFO75-STO25, BFO50-STO50, BFO25-STO75, and STO films. Images from Advanced Energy Materials 8, 1801972 (2018).

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