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.

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.

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 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.

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.

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.

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.