3D-Localisation - Three Dimensionally Defined Non-Fullerene Acceptors

Sunlight presents an essentially infinite source of energy. Converting it into electricity, heat, or chemical energy is among the most appealing and effective approaches to tackling the energy crisis and reducing the impact of human activity induced climate change. Organic solar cells are one emerging technology that can aid in the transition to a renewable economy. They are lightweight, flexible devices which utilise readily available organic molecules and can be processed by energy-efficient, non-thermal methods unlike traditional silicon devices. The development of these devices has relied upon fullerenes as electron acceptor materials.

Fullerenes are molecular forms of carbon with a spherical, soccer ball-like geometry which gives rise to delocalisation of electrons across the entire surface of the molecule. This structure attributes fullerene with a variety of unique properties, they can reversibly accept up to six electrons and can transport charges efficiently in three dimensions. However, it is now well-established that using fullerenes places strict limitations on organic solar cell performance. Fullerenes absorb sunlight only poorly and they participate in processes which are destructive to the device while under operation. Compounding this, they are expensive to produce and purchase, and are extremely challenging to chemically modify with any degree of control. This means that their optical and electronic properties cannot be easily tuned for solar cells or any other specific application. Ultimately, the use of fullerenes is non-sustainable therefore new non-fullerene acceptors are urgently required if these green energy technologies are to realise their full potential.

This project takes a holistic view of the beneficial and detrimental properties of fullerenes and will use this approach to produce a completely new class of non-fullerene acceptors. These will serve to impact hugely on the delivery of renewable energy sources. There are two key facets to this approach:

1) The use of three-dimensional molecular structures as a central scaffold. These will facilitate electronic delocalisation in three dimensions.
2) By attaching selected heterocyclic side groups to these scaffolds, solar absorbance will be maximised, and the electrochemical and morphological properties of these new molecules will be controlled in a facile manner.

This represents a step-change in the development of useful non-fullerene acceptors. A new generation of molecular materials for use in energy conversion technologies will be produced, and design rules for attaining truly fullerene-like behaviour in general, and for any application, will be established. In contrast with much of the existing work on organic electronic materials, which focusses upon molecules and polymers composed of planar heterocyclic fragments, exploring chemical space in three dimensions is key to the work proposed here. This adds significantly to the novelty of our approach.

The necessity to develop alternative energy sources for fossil fuel alternatives is stark. The Paris accord signed in 2016 committed 195 countries to reducing the global temperature increase to below +1.5 degrees Celsius above pre-industrial levels by 2100. Despite this, climate change scepticism and political pressure has led the US to abandon these agreements, exploration for oil reserves in pristine areas of the Arctic and Antarctic is underway, the global population and energy demand continue to grow, and sea levels continue to rise as the polar icecaps melt.

Semiconducting organic molecules remain among the foremost candidates to function in renewable thin film and building integrated solar cells, low power lighting and even communication - all of which serve to reduce our reliance on the combustion of fossil fuels and minimise our carbon footprint, beneficially impacting upon climate change. The UK can, and must, continue to support the advance of these new classes of functional materials and the emerging technologies which they can facilitate. This will maintain the UK's presence as both a leading competitive research force and a progressive environmentally responsible society.

Organic electronics and nanotechnology are important sectors of UK PLC. The UK has pioneered nanotechnology and remains one of the primary drivers in the field of organic and printed electronics. In this context, the earliest commercial drives for organic light emitting device (OLED) technologies came from Cambridge Display Technology (CDT) - the first spin-out from the University of Cambridge to go public - which was acquired by Sumitomo in 2007 for $285M. OLEDs are now rapidly maturing into an established technology with the latest smartphones employing OLED screens. Despite their commercialisation, academic interest in OLED materials continues unabated which highlights just how important fundamental research is to the continued development of this field. In 2014 Allied Market Research indicated that the market for organic electronics, dominated by displays and solar cells, would reach $79.6B by 2020 while drives to reduce fossil fuel use and increase renewable energy supplies mean that the solar energy industry will increase to $422 billion in 2022 from $86 billion in 2015. Companies currently utilising fullerenes in organic solar cell devices will save money and energy by using our new materials. The modular nature of our molecules will permit a level of device optimisation which is simply not possible with fullerenes. This will lead to more efficient, longer lasting, and cheaper devices.

Fullerenes present opportunities in numerous other applications including spintronics, therapeutics and thermoelectrics. However, recurring themes in why their use halts at the laboratory or prototype stage include high cost, challenging purification and inflexibility. Our design concept is adaptable to any specific application which seeks fullerene-like behaviour alongside tuneable properties. This concept will therefore impact on all fields which have seen potential in the useful properties of fullerene. This will serve to drive long-considered, and even well-developed, but currently impractical new technologies towards commercialisation and thereby wealth generation.

More than ever, people are engaged with environmental policies and the move to a green economy, while also utilising consumer electronics and utilities which are of immediate relevance to our proposed work. Coupled with the UK's historic, Nobel prize winning, connection to fullerene this project also presents an excellent opportunity to engage the general public with UK physical sciences research and inspire the next generation of scientists.