Andrew Watt
Associate Professor of Materials
+44 1865 613456
Andrew leads a group of 14 scientists on energy materials research from the synthesis of new nanomaterials through to the fabrication, characterization and application of devices. The group has considerable experience in the synthesis of nanomaterials, thin film device fabrication, optoelectronic materials characterisation and advanced transmission electron microscopy techniques. Recent highlights include, the demonstration of vacuum thermal evaporation of conducting polymers for solar cells, high resolution TEM imaging of polymer lamellae in bulk heterojunction photovoltaic devices, and the initial demonstration of a SnS nanocrystal heterojunction thin film photovoltaic devices.
Selected Publications
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Active Metamaterials with Negative Static Electric Susceptibility.
March 2020|Journal article|Advanced materials (Deerfield Beach, Fla.)Although well-established textbook arguments suggest that static electric susceptibility χ(0) must be positive in "all bodies," it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ(0) < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ(0) < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ(0) < 0 and a design concept to achieve "true" χ(0) < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability -1 < χ(0) < 0, resulting in experimentally-determined magnitudes that are over one thousand times greater than those predicted previously. Since χ(0) < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation. -
Solvodynamic Printing As A High Resolution Printing Method.
July 2019|Journal article|Sci RepPrinting techniques are becoming increasingly prevalent in modern manufacturing. However, its biggest drawback is the limit in printing resolution. In this paper, we present solvodynamic printing as a novel printing system which aims to improve print resolution by incorporating an additional immiscible carrier solvent into the ink delivery system. The resolution is improved due to the solvent-solvent interactions between the ink and the carrier solvent which alter the contact angle of the ink on the substrate and limit the printed feature size. We demonstrate the proof of concept of solvodynamic printing by printing silver nanoparticle inks on a polyethylene naphthalate substrate. Silver nanoparticle tracks with widths of 35.2 ± 7.0 μm were achieved using a 300 μm nozzle. This is equivalent to 11.7 ± 2.3% of the nozzle diameter. The result shows great potential in solvodynamic printing as not many modern printing techniques can achieve such nozzle to feature size ratios. -
Carbon Nanotubes for Quantum Dot Photovoltaics with Enhanced Light Management and Charge Transport
December 2018|Journal article|ACS PHOTONICScolloidal quantum dots, photovoltaics, single-walled carbon nanotube, hole transport layer, charge carrier transfer, optical spacer effect -
Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells.
July 2018|Journal article|LangmuirThe performance of devices containing colloidal quantum dot (CQD) films is strongly dependent on the surface chemistry of the CQDs they contain. Multistep surface treatments, which combine two or more strategies, are important for creating films with high carrier mobility that are well passivated against trap states and oxidation. Here, we examine the effect of a number of these surface treatments on PbS CQD films, including cation exchange to form PbS/CdS core/shell CQDs, and solid-state ligand-exchange treatments with Cl, Br, I, and 1,2-ethanedithiol (EDT) ligands. Using laboratory-based and synchrotron-radiation-excited X-ray photoelectron spectroscopy (XPS), we examine the compositions of the surface layer before and after treatment, and correlate this with the performance data and stability in air. We find that halide ion treatments may etch the CQD surfaces, with detrimental effects on the air stability and solar cell device performance caused by a reduction in the proportion of passivated surface sites. We show that films made up of PbS/CdS CQDs are particularly prone to this, suggesting Cd is more easily etched from the surface than Pb. However, by choosing a less aggressive ligand treatment, a good coverage of passivators on the surface can be achieved. We show that halide anions bind preferentially to surface Pb (rather than Cd). By isolating the part of XPS signal originating from the topmost surface layer of the CQD, we show that air stability is correlated with the total number of passivating agents (halide + EDT + Cd) at the surface. -
Two-Photon Absorption and Two-Photon-Induced Gain in Perovskite Quantum Dots.
June 2018|Journal article|J Phys Chem LettPerovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 106 GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm2. We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process. -
Modelling of a vacuum metallization patterning method for organic electronics
February 2018|Journal article|SURFACE & COATINGS TECHNOLOGYFlexography, Metallization, Roll-to-roll, Organic transistor, Vacuum, Patterning -
Flexography Printing for Organic Thin Film Transistors
January 2018|Journal article|MATERIALS TODAY-PROCEEDINGSflexography, diacrylate, organic electronics, vacuum, patterning -
The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling.
May 2017|Journal article|NanoscaleAchieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport. -
Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.
August 2016|Journal article|ACS applied materials & interfacesThe band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation. -
High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.
June 2016|Journal article|ACS applied materials & interfacesHybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.