Optimisation of charge carrier mobility in nanoporous metal oxide films

High surface area nanoporous films formed by sintering metal oxide nanoparticles are highly stable, non-toxic and inexpensive to produce on an industrial scale. They find a wide range of applications in gas sensing and catalysis where high surface area is essential to maximise the interaction of molecules with the film. They also find applications as charge transport layers in third generation solar cells, e.g. dye- or perovskite-sensitised cells, where efficient photoinjection of electrons and holes is ensured by coating nanoporous films with a light absorbing material. For solar cells, as well as for other important applications of nanoporous films such as electrodes in fuel cells and photoelectrochemical cells, good charge carrier mobility is also an essential requirement. Unfortunately, despite their numerous advantages, the electronic mobility of nanoporous oxide films is in general very poor. For example, the mobilities of nanoporous TiO2, ZnO and SnO2 films have been shown to be between two and four orders of magnitude smaller than those of corresponding single crystals. This low mobility is a key factor limiting the efficiency of (photo-)electrochemical and photovoltaic applications and is usually attributed to increased charge carrier trapping at surfaces and at interfaces between nanoparticles.

Since charge trapping is associated with ions near surfaces we hypothesise that it should be possible to eliminate these traps by suitable chemical modification of the surfaces of nanoparticles prior to sintering into a film. This approach would retain the advantages of nanoporous films in terms of high surface area, non-toxicity and processability while improving mobility. Such modifications have been attempted previously, but due to the lack of understanding on the origin of charge trapping or the effects of surface modification, success has been limited. Here, we propose to combine the predictive power of first principles theoretical modelling with structural, spectroscopic and photophysical materials characterisation, in order to quantify the factors responsible for charge trapping at surface and interfaces in nanoporous oxide films at an atomistic level. Once validated and refined on unmodified films, theoretical methods will be used to assess modification strategies to reduce charge-trapping. In particular, we will consider the incorporation/substitution of anions and cations near the surface of oxide nanoparticles to eliminate the problematic trapping sites. The ability to theoretically screen various possible modification routes (i.e. different cations and anions) is a key advantage of our proposed approach. Application, testing and optimisation of such strategies may offer a new paradigm for knowledge-led design of solar oxide materials.

We aim to demonstrate the effectiveness of our approach by increasing the mobility of nanostructured TiO2 and ZrO2 to deliver an improvement in the efficiency of perovskite-sensitised solar cells, which are emerging as an attractive third generation photovoltaic technology. The size of the third generation photovoltaic market is predicted to grow to $38bn by 2022, making this an area with significant potential for economic impact. Improving the mobility of nanoporous oxides could bring the efficiency of these devices from their current level (about 20%) to closer to the theoretical maximum of about 30%. An increase in overall efficiency from 20% to only 23% percent would increase the total power output by 15%, which when coupled with lower manufacturing costs would make the technology very attractive. We will work with leading manufacturers of nano-TiO2 (Cristal) and perovskite-sensitised solar cells (Dyesol Limited) to test the performance of our modified films. More generally, the ability to tailor the electronic properties of interfaces in nanoporous films by controlled modification should find applications in other technologies including sensing, catalysis and electronics.

ECONOMY: The development of industrially relevant processes to improve the mobility of nanoporous films has significant potential for economic impact. For example, perovskite-sensitised solar cells (PSSC) are emerging as an attractive third generation photovoltaic technology due to their already demonstrated high efficiency (>20%), low cost and processability. The global market for third generation photovoltaics was valued at $16bn in 2014 and is predicted to grow to $38bn by 2022. Dyesol Limited, one of the world's leading alternative photovoltaics manufacturers and a partner on this project, is investing AUS $120M into R&D for PSSCs (including a new laboratory planned in the UK). We estimate improvements in the mobility of the nanoporous oxide films could deliver improvements in efficiency of >3%, which when coupled with the lower manufacturing costs would make the technology very attractive. We will work with Dyesol Limited to incorporate modified nanoporous films into perovskite-sensitised solar cells to quantify the improvement in efficiency relative to unmodified films. Cristal Global, an international leader in the manufacture of TiO2 nanoparticles, has a significant presence in the UK (manufacturing plant in Stalinborough, UK with a dedicated R&D facility). They are seeing growing demand for well-defined nanoparticles for nanoporous films in PSSCs and for wider applications including sensors, DeNOx photocatalysis, electronics and refining catalysis. We will work closely with Cristal Global to identify and pursue opportunities throughout the project. The development of new material optimisation processes for metal oxides would open up new product opportunities and increase the UK's competitiveness in this sector.

SOCIETY: Reducing carbon dioxide emissions, improving energy security, and reducing energy costs are priorities high on the agenda of many governments internationally. The development of technologies that can tap into the vast amount of energy delivered by our sun can make a significant contribution towards addressing these issues. Improving the efficiency of third generation solar cell technologies and other emerging energy technologies requiring high mobility nanoporous films (e.g. photoelectrochemical cells for solar fuel synthesis) will make them much more affordable, encouraging large-scale adoption. Therefore, while the project is fundamental in nature it underpins the optimisation of materials for solar applications and ultimately may help deliver cleaner and more efficient solar technologies for the wider benefit of society.

KNOWLEDGE: In addressing its objectives, this project will develop new methodologies with significant potential for impact outside the immediate area of the research. For example, theoretical methods for accurate predictions of the charge trapping properties of TiO2 and ZrO2 will be developed. These methods (which will be made publically available) may find diverse applications in the varied fields where these materials are studied - for example in catalysis, electronics and medicine.

PEOPLE: The project will nurture the development of two postdoctoral research assistants (PDRAs) who will be an asset to the UK whether they continue in academia, contribute to emerging industries, or take up skilled positions in the wider economy. PDRA training will address the need to develop skills across disciplines and they will be given opportunities to develop transferable organisation and management skills (e.g. through conferences and outreach activities). The PDRAs will also be actively involved in an outreach activity targeted primarily at students (14-18) involving a virtual reality 'flight' through a nanoparticle that will be developed in collaboration with the Digital Creativity Hub in the Department of Theatre, Film and Television at York. The aim is to excite, inform and inspire students about cutting edge materials research and its role in technology.