Materials challenges for sustainable energy technologies

Traditional thermoelectrics based on Bi₂Te₃ are well established commercially but restricted to low temperature applications; concerns about stability, cost and environmental issues have limited their wider exploitation. The last 20 years have witnessed significant improvements in the performance of a wide variety of thermoelectric materials, with maximum values of the thermoelectric figure of merit (ZT) exceeding 2.5 in some cases. Whilst peak ZT is important there is growing recognition of stability issues and the need for a wide ‘thermal window’ over which the thermoelectric module can be employed. In order to improve the material performance (by maximising ZT) efforts have focused on reducing thermal conductivity and electrical resistivity. One strategy is to employ microstructural engineering at the nanoscale to increase phonon scattering in order to reduce thermal conductivity. By taking examples from systems including oxides, selenides and tellurides and materials exhibiting self-assembly nanostructures, the nature and benefits of interface structures will be examined. Details of atom level structures revealed by use of high resolution TEM and information from DFT modelling can reveal important mechanisms. Finally, the potential benefits of oxide/metal-carbon interactions will be outlined.

Professor Bob Freer, University of Manchester, UK

Professor Bob Freer, University of Manchester, UK

Bob Freer is Professor of Ceramics at the University of Manchester. He has over 25 years’ experience with electronic ceramics, particularly microwave and thermoelectric materials. He has published over 230 refereed papers, holds one patent and given many invited conference papers. Professor Freer is a joint coordinator of the EPSRC supported Thermoelectric Network and chaired the EU programme COST 525: Advanced Electronic Ceramics, and the Foresight Task Force on Advanced Ceramics. He served as President of the International Ceramic Federation, and held roles in the European Ceramic Society. He is an Editor of the Journal of the European Ceramic Society. Recent work includes combined experimental-modelling investigations of oxide thermoelectrics, and EPSRC-JST Partnership in oxide thermoelectrics with Japan.

Heat is a ubiquitous source of energy. About half of the energy that the sun delivers to the Earth is in the form of infrared radiation. Moreover, two/thirds of the energy produced for human consumption is lost in the form of heat. In this scenario, solid state heat-to-electricity converters, ie thermoelectrics, have strong potential to harvest part of this untapped diluted energy source. Widespread use of thermoelectric generators has been thus far elusive due to the relatively high cost of the technology, and the fact that most thermoelectric materials operating at low temperatures are based on non-abundant or toxic materials. Solution processed carbon based materials are currently being investigated as a very promising alternative for low-cost, low temperature thermoelectrics. In this talk the researcher will describe the progress and challenges for three carbon based systems: highly doped conjugated polymers, carbon nanotubes, and composites thereof. The researcher will focus on our current understanding of what governs the main material thermoelectric properties, namely electrical conductivity, Seebeck coefficient and thermal conductivity.

Dr Mariano Campoy-Quiles, Institutes of Material Sciences of Barcelona (ICMAB-CSIC), Spain

Dr Mariano Campoy-Quiles, Institutes of Material Sciences of Barcelona (ICMAB-CSIC), Spain

Dr Mariano Campoy Quiles is a physicist and his core expertise lies in combining novel material processing methods with advanced spectroscopic techniques focusing on renewable energy applications such as organic photovoltaics and thermoelectric generators. He was awarded a PhD in Experimental Physics from Imperial College London (2005), a fellowship from the Japan Society for the Promotion of Science (2007), a Ramon y Cajal research fellowship (2009), a permanent position as tenured scientist of CSIC (2012), the Most Outstanding Young Researcher in Experimental Physics Award (from the Spanish Royal Society of Physics and Fundación BBVA) (2012), an individual European Research Council Consolidator grant (2014), and promoted to Research Scientist of CSIC (2017). His achievements are solidly based on the consistent excellence of his published research including three papers in Nature Materials, and a stream of significant papers in excellent journals (72 publications, 3900 citations, and an h-index of 32).

An important component of the development in thermoelectric materials, for waste heat recovery systems, has been the investigation of doping, nano-engineering and dimensionality reduction. Part of the reason is that these approaches improve the efficiency via the lowering of the thermal conductivity. The challenge is however, to lower the thermal conductivity while maintaining a high electrical conductivity and Seebeck coefficient. Atomistic simulation has the advantage of being able to examine the effect of interfaces and doping on each of the key properties separately, and thereby help devise a strategy for finding the structure and composition for optimum thermoelectric performance. The scope of atomistic simulation will be illustrated, by first reviewing the approach we use for generating interfaces and calculating properties then giving several recent examples. These will include the group’s work on SrTiO₃ where the group has explored the effect of nanostructuring on suppressing thermal conductivity by means of the introduction of grain boundaries. The researchers demonstrate that structural features impact strongly on the mean free path of the phonons and are responsible for the enhanced phonon scattering. The electronic and thermal properties can also be adjusted through doping, from cation substitution to graphene incorporation. In each case the major influence is normally at the interfaces.

Professor Steve Parker, University of Bath, UK

Professor Steve Parker, University of Bath, UK

Steve Parker is a Professor of Chemistry at the University of Bath and has been studying the properties of materials and minerals using computational techniques for nearly 40 years leading to over 300 publications. His interest is in developing and applying atomistic simulation techniques to model the structure, thermodynamics and transport in materials and at their interfaces with particular emphasis on materials problems for environmental benefit. These range from identifying the efficiency of different materials for pollutant remediation and cellulose dissolution to the search and characterisation of materials in energy generation and storage, including nuclear and thermoelectric materials. The thermoelectric materials present a particularly interesting challenge to computer simulation due to the complexity of the structures, stoichiometry and microstructure and the requirement to optimise electronic and thermal conductivity.

Professor Durrant will address some of the key challenges and opportunities for organic and perovskite based solar cells. He will start by discussing the motivations for such printed photovoltaic technologies, and the market opportunities for these technologies. He will discuss some of the recent advances in the efficiencies and stabilities of these devices, including in particular the development of non-fullerene acceptors for organic solar cells, and stability limitations for both organic and perovskite solar cells.

Professor James Durrant FRS, Imperial College London, Swansea University

Professor James Durrant FRS, Imperial College London, Swansea University

James Durrant is Professor of Photochemistry in the Department of Chemistry, Imperial College London and Sêr Cymru Solar Professor, College of Engineering University of Swansea. His research addresses the photochemistry of new materials for solar energy conversion – targeting both solar cells (photovoltaics) and solar to fuel (ie: artificial photosynthesis). It is based around employing transient optical and optoelectronic techniques to address materials function, and thereby elucidate design principles which can help guide technological development. His research is currently addressing the development and functional characterisation of organic and perovskite solar cells, and photoelectrodes and photocatalysts for solar driven fuel synthesis. In addition to his core research activities, Professor Durrant leads Imperial’s Centre for Plastic Electronics and the Welsh Government funded Sêr Cymru Solar initiative. He also founded the UK’s Solar Fuels Network, and was founding Deputy Director of Imperial’s Energy Futures Laboratory. His awards include both the Environment (2009) and Tilden (2012) Prizes of the RSC. He was elected a Fellow of the Learned Society of Wales in 2016.

The combination of electrical conductivity and optical transparency in a single material gives transparent conducting oxides (TCOs) an important role in modern optoelectronic applications such as in solar cells, flat panel displays, and smart coatings. The most commercially successful TCO so far is tin doped indium oxide (Indium Tin Oxide – ITO), which has become the industrial standard TCO for many optoelectronics applications: the ITO market share was 93% in 2013. Its widespread use stems from the fact that lower resistivities have been achieved in ITO than in any other TCO; resistivities in ITO have reached as low as 7.2 × 10^-5Ω cm, while retaining >90% visible transparency. In recent years, the demand for ITO has increased considerably, mainly due to the continuing replacement of cathode ray tube technology with flat screen displays. However, indium is quite a rare metal, having an abundance in the Earth’s crust of only 160 ppb by weight, compared with abundances for Zn and Sn of 79000 ppb and 2200 ppb respectively, and is often found in unstable geopolitical areas. The overwhelming demand for ITO has led to large fluctuations in the cost of indium over the past decade. There has thus been a drive in recent years to develop reduced-indium and indium-free materials which can replace ITO as the dominant industrial TCO. In this talk Dr Scanlon will outline a new doping mechanism, and a new TCO which should usurp ITO as the industry standard.

Dr David Scanlon, University College London, UK

Dr David Scanlon, University College London, UK

David O Scanlon is a Reader in Computational, Inorganic and Materials Chemistry at University College London (UCL). He was awarded his PhD in Chemistry from Trinity College Dublin in 2011, and moved later that year to take up a Ramsay Fellowship in the Department of Chemistry at UCL. He leads a group focused on computationally driven materials design at UCL, especially within the remit of solid state materials for renewable energy applications.

One way of improving the efficiency of dye-sensitised solar cells is to use two photoelectrodes in a tandem device, one harvesting the high energy photons, and the other harvesting the low energy photons [1]. This enables the photovoltage to be increased, whilst maximizing light harvesting across the solar spectrum. Despite their promise, a tandem cell with a higher efficiency than the state-of-the-art ‘Grätzel’ cell has not yet been achieved. This is because the performances of photocathodes are significantly lower than TiO₂-based anodes, and the p-type concept has been largely unexplored since the first device was prepared in 1999 [2]. The small potential difference between the valence band of the NiO, p-type semiconductor, and the redox potential of the electrolyte and the faster charge-recombination reactions compared to the TiO₂ system limits the efficiency. In recent years the researchers have made progress by developing new photosensitisers [3]. In parallel the researchers have investigated the charge-transfer processes to determine the mechanism and limitations to efficiency [4]. This has increased our understanding of the redox processes at the dye/electrolyte and NiO/electrolyte interfaces [5]. The fundamental limitation of these devices arises from the NiO material itself and the researchers have re-focussed our efforts on finding a replacement transparent p-type semiconductor. The group’s strategy and recent results will be presented. Recent work to expand the applications to photoelectrochemical water splitting for energy storage will be described, briefly [6].

1. EA Gibson, AL Smeigh, L Le Pleux, L Hammarström, F Odobel, G Boschloo, A Hagfeldt., Angew. Chem Int Ed 2009, 48, 4402 –4405.

2. J He, H Lindström, A Hagfeldt, S Lindquist, J Phys Chem B, 1999, 103, 8940–8943.

3. CJ Wood, GH Summers, EA Gibson. Chem Commun 2015, 51, 3915 – 3918.

4. J-F Lefebvre, X-Z Sun, JA Calladine, MW George, EA Gibson. 2014, 50, 5258 – 5260. G Boschloo,  EA Gibson, A Hagfeldt J Phys Chem Lett, 2011, 2, 3016–302. EA Gibson, L Le Pleux, J Fortage, Y Pellegrin, E Blart, F Odobel, A Hagfeldt, G Boschloo, Langmuir 2012, 28, 6485–6493.

5. FA Black, CJ Wood, S Ngwerume, GH Summers, IP Clark, M Towrie, Jason E Camp, EA Gibson, Faraday Discussions, 2017, 198, 449 – 461. L D'Amario, R Jiang, U Cappel, EA Gibson, G Boschloo, H Rensmo, L Sun, L Hammarström, H Tian, ACS Appl Mater  Interfaces. 2017, 9, 33470–33477.

6. EA Gibson, Chem Soc Rev, 2017, 46, 6194 – 6209. N Põldme, L O’Reilly, I Fletcher, I Sazanovich, M Towrie, C Long, JG Vos, MT Pryce, EA Gibson Chem Sci In press.

Dr Libby Gibson, Newcastle University, UK

Dr Libby Gibson, Newcastle University, UK

Libby Gibson is a Lecturer in Physical Chemistry at Newcastle University. Prior to her current role, she held a University of Nottingham Anne McLaren Research Fellowship and a Royal Society Dorothy Hodgkin Research Fellowship. She obtained her PhD in 2007 from the University of York, supervised by Robin Perutz FRS and Anne-Kathrin Duhme-Klair. Research in her group focuses on solar cell and solar fuel devices that function at a molecular level and challenge the conventional solid-state photovoltaic technologies. Her current ERC-funded project focuses on developing transparent p-type semiconductors for tandem solar cells and artificial photosynthesis.

During his lecture Professor Palomares will present his group latest results on the characterisation of different type of solar cells from DSSC and OPV to MAPI using advanced photo-induced time resolved techniques [1-4]. Using PICE (Photo-induced charge extraction), PIT-PV (Photo-induced Transient PhotoVoltage) and other techniques, the researchers have been able to distinguish between capacitive electronic charge, and a larger amount of charge due to the intrinsic properties of the perovskite material. Moreover, the results allow us to compare different materials, used as hole transport materials (HTM), and the relationship between their HOMO and LUMO energy levels, the solar cell efficiency and the charge losses due to interfacial charge recombination processes occurring at the device under illumination. These techniques and the measurements carried out are key to understand the device function and improve further the efficiency and stability on perovskite MAPI based solar cells.

1.  1) JM Marin-Beloqui; L Lanzetta; E Palomares; Decreasing Charge Losses in Perovskite Solar Cells Through mp-TiO₂/MAPI Interface Engineering. Chem Mater 2016, 28, 207-213.

3.  3) A Matas Adams; JM Marin-Beloqui; G Stoica; E Palomares; The influence of the mesoporous TiO₂ scaffold on the performance of methyl ammonium lead iodide (MAPI) perovskite solar cells: charge injection, charge recombination and solar cell efficiency relationship. J Mater Chem A 2015, 3, 22154-22161.

4.  4)  L Cabau; I Garcia-Benito; A Molina-Ontoria; NF Montcada; N Martin; A Vidal-Ferran; E Palomares; Diarylamino-substituted tetraarylethene (TAE) as an efficient and robust hole transport material for 11% methyl ammonium lead iodide perovskite solar cells. Chem Commun 2015, 51, 13980-13982.

Professor Emilio Palomares, ICIQ Tarragona, Spain

Professor Emilio Palomares, ICIQ Tarragona, Spain

Emilio Palomares is ICREA Research Professor at the Institute of Chemical Research of Catalonia (ICIQ). He joined ICIQ in 2006 as a tenure-track and was appointed ICREA Researcher in 2008. In 2009 he was selected as ERCstg by the European Research Council. Emilio’s research interests span a range of targets with emphasis on materials for energy production and bio-applications, in the context of electron transfer processes, photovoltaic applications and nanoscience. In the ten years since he joined ICIQ in 2006, Professor Palomares has published over 150 papers in peer reviewed journals about molecular photovoltaic devices and novel materials for bio-application in human health. Emilio has given over 50 lectures in scientific meetings and research institutions, and supervised ten PhD theses. He has been invited as guest editor for several special issues (ChemSusChem) in well-known international journals. He serves as a member of the Advisory Editorial Board of Energy and Environmental Science and is Fellow of the Royal Society of Chemistry. Professor Palomares has received several research awards including the Marie Curie European Fellowship in 2002, The Young Researcher Fellowship at the International Conference on Photochemical Conversion and Storage of Solar Energy in 2002, The Ramon y Cajal Fellowship in 2003, The Roscoe Medal in 2004 at the Younger European Chemist’s Conference (Highlights of European Chemical Research and R&D), The Invited Sigma-Aldrich Lecture for Young Chemists, 2004, The Young Chemist Research Award by The Spanish Royal Society of Chemistry in 2006 and The INNOVA Award by the SusChem Spanish Technology Platform for Sustainable Chemistry in 2010.