Organic semiconductors form the basis of optoelectronic devices such as organic light emitting diodes, organic solar cells and organic field-effect transistors. These devices are very thin, lightweight and flexible, and can be fabricated over large areas from solution by simple printing techniques under ambient conditions. One of the most developed areas in organic electronics is that of organic light emitting diode (OLED) based displays for mobile phones, tablets and televisions; these make a major contribution to a multi-billion dollar global industry. We have recently discovered a method to produce an OLED that emits circularly-polarized light. This very simple method involves blending a conventional light emitting semiconducting polymer with a helically shaped molecule known as a helicene. The helically shaped molecule somehow causes the spaghetti-like polymer chains to change conformation, resulting in the polymer emitting circularly-polarized (CP) light. Such a CP emitting OLED has application in conventional OLED displays, 3-D OLED displays, and as backlights in liquid-crystal displays. For example, to improve contrast, most displays contain a top circularly-polarized filter to remove back-reflected light. A CP emitting OLED would allow all the emitted light to pass through this filter, potentially doubling the energy efficiency of the display. This would reduce global energy consumption, increase battery life of portable products, and increase display lifetime. Other potential applications of CP emitting OLEDs include protein detection in biomedicine, optical spintronics, optical quantum computing, and conventional and quantum optical telecommunication. Despite the success of our simple and novel method, we do not understand the principle of how mixing the polymer with the helicene results in a change in the polymers structure allowing it to emit CP light. The purpose of this grant is to actually understand this process, finding out exactly how the polymers structure is affected by blending with the helicene. We will investigate how this phase forms when the mixture is heated and cooled; how this changes with polymer chain length and polymer/helicene ratio; how we can generate this phase by solution processing using different solvents and different annealing treatments; whether a helical liquid crystal phase can form; and how these factors affect thin film morphology and microstructure. We will also investigate the CP light emission process, looking at factors such as whether CP light absorbed by the helicene can result in CP emission from the polymer. The particular semiconducting polymer concerned (F8BT) is known to transport both electrons and holes; we will investigate charge transport in the novel helical phase. We will take all the processing knowledge and create an optimized CP-OLED, with a high CP light output, brightness and efficiency. We will also investigate other similar semiconducting polymers which emit CP light when blended with helicenes, looking at their structural, spectroscopic and electronic properties. This project involves an ideal synergy between teams in the Departments of Physics and Chemistry at Imperial College London and in the Department of Materials Science and Engineering at the University of Sheffield, the former covering spectroscopic, electronic and device measurements and the latter covering structural and morphological measurements and modeling. It will also involve a partnership with Cambridge Display Technology (UK), supplying high performance polymers and expertise, and enabling rapid transfer of the technology towards the marketplace.

This project is clearly focused on understanding one of the most fundamental aspects of organic light emitting devices, which as yet is little recognised as being so important. With CDT we have shown that up to 37% of light coming from an OLED device arises from triplet fusion, in itself an absolutely fundamental excitonic process which if not at all well understood. By elucidating the triplet fusion mechanism and demonstrating that 0.50 singlet yield is possible with correct triplet energy levels we aim to build devices that could attain internal quantum efficiencies of 0.62 from fluorescent emitters. This would provide a real step change in OLED performances both for displays and lighting. Together with CDT we shall work to develop a new device architecture to fully exploit this possibility. This will be a highly adventurous piece of applied science. We will also collaborate with Kyushu University on small molecule systems in evaporated devices. A range of state of the art spectroscopic techniques will be used, most developed in and unique to Durham, to fully elucidate the photophysical process underpinning triplet triplet annihilation and triplet fusion, some of the most fundamental excitonic processes in organic materials but still poorly understood. We will determine how singlet yield from triplet fusion is controlled by energy levels of the molecule. Further, we shall also determine the efficiency of upper excited state triplet inter system crossing to the singlet manifold, a further process which we believe also contributes to the overall singlet yield both in an optical experiment and in an OLED device. Together with CDT will will develop a novel device based on a polymer dyad where triplet fusion is maximised and aim to produce fluorescent OLEDs with efficiencies approaching those of phosphorescent devices. In this way we can produce blue fluorescent devices of high efficiency without the short lifetime associated with phosphorescent systems. This would be a major step change for OLED device. This project has the full support of Cambridge Display Technology who will provide £45,000 to off set the EPSRC cost of the project and another £30,000 in new materials developed specifically for this project and further resource for modelling and project management.

There has been remarkable progress over the last decade in making displays, lighting panels, solar cells and lasers out of flexible, plastic materials. This has a wide range of potential applications, such as roll up TV displays or having power generation, sensors and data communications systems woven into your clothing. The technology of organic LEDs has now matured to the degree that OLED displays are mass produced in consumer products such as mobile phones. The next generations of plastic electronics products will include OLED lighting, solar cells and lasers. It is now clear however that to deliver the technology for these demanding applications it is necessary to develop a deeper understanding of the basic materials physics. In all of these devices the physics of the excited states of molecules plays a crucial role in performance. In OLEDs the efficiency at high brightness is limited by the absorption due to charge carriers and various interactions that quench the light emission from excited states. In lasers there is a delicate interplay of the excited state physics and laser losses, and so far little is known about how the chemical and structural properties of the materials may be used to control this. This proposal seeks to develop this understanding by bringing together the expertise of two groups: one who are experts in measuring the optoelectronic performance of these polymers and in their application for photonics, and the other who are experts in the quantum theory of organic materials. Through a combination of theory and experiment we will aim to understand the complex excited state interactions of organic semiconductors, and uncover new design strategies to control these processes. This would help us to optimise the performance (e.g. efficiency and brightness) of current devices; and enable new generations of photonic devices based on these materials. We will make optical measurements of the fundamental excited-state processes in the materials and their behaviour under device conditions. Using state-of-the-art techniques in quantum mechanics we can also simulate the microscopic physics which gives rise to these effects. Measuring these interactions in working devices is particularly demanding and to achieve this we will also draw on specific complementary expertise from our project partners at Cambridge Display Technologies and the University of Alicante. We will then apply our new knowledge of excited states to the operation of a range of organic devices including OLEDs, lasers, solar cells and optical amplifiers. We will quantify the significance of the different excited state interactions and develop design strategies that can minimise parasitic processes and optimise operation.

Strongly-coupled microcavities are a fascinating system for the exploration of the fundamental physics of the interactions between light and matter. Under such circumstances, the emissive states in such microcavities are termed 'polaritons', and can be described as an admixture between an exciton and a confined cavity photon. The optical properties of polaritons can be very different from their constituent parts (excitons and cavity photons), and thus there is a significant opportunity to explore new fundamental processes, and develop new types of devices that may find applications as low-threshold lasers, optical-amplifiers and high-speed optical switches. At present, the majority of work done on the strong-coupling regime in microcavities has centred on structures that contain inorganic semiconductors (either III-V, II-VI or GaN based materials). We have however pioneered the study of strong-coupled microcavities containing organic (carbon-based) semiconductors, which are anticipated permit new effects to be engineered. Despite the importance of organic-semiconductors in a range of optoelectronic devices (LEDs, photovoltaics, FETs, lasers etc) relatively little is understood regarding the microscopic processes that occur in strongly-coupled organic microcavities.Development of a basic understanding of non-linear processes and properties of organic-semiconductors in strongly-coupled microcavities will thus be a key area that we will address in this project. Key components of the research include studies the interactions between organic-polaritons and vibrational modes of the molecular semiconductor and the generation of organic exciton-polaritons at high density following electrical injection of carriers. We will also explore the fabrication and optical properties of 'hybrid-semiconductor' microcavities and devices (containing organic and inorganic semiconductors), and will study optically-driven energy-transfer between the different types of excitation using both linear and ultra-fast measurements. We are confident that our work will provide new fundamental insights into the optical properties of organic-polaritons (including relaxation and condensation), the transfer of excitations between different semiconductor materials via a cavity photon over large distances (> 100 nm) and the generation of new electrically-driven polariton devices. We believe that we are in an excellent position to undertake such an ambitious programme of research due to our world-leading expertise in strongly coupled organic semiconductor microcavities (Sheffield), and two-colour ultra-fast spectroscopy of microcavities (Southampton).

Force sensing plays a key technological role in providing tactile feedback in automated systems thus maximising efficiency in industrial applications (i.e., pick-and-place robots, robotic welding) and enabling novel devices and applications (i.e., video games controllers and smart homes). Now that human-oriented technologies (i.e., electronic and robotic skins, prosthetics, surgical robotic arms and rehabilitative devices, force-sensitive buttons on smartphones) are becoming a ubiquitous part of daily life, the requirement for improved force sensors is self-evident. Sensors that mimic human tactile receptors have been developed. However, the existing devices do not satisfy technological needs of flexibility, stretch-ability, high force and spatial resolution, self-powering and adaptability to measure both static and dynamic forces. Therefore, new forms of sensor are essential and this research programme aims to tackle this technological need by proposing a new transformative device featuring a force sensitive flexible and stretchable material with embedded well-aligned and ordered nanowires (a smart nanocomposite material). The smart nanocomposite is made using a unique and innovative approach that involves filling a polymer with well-ordered and aligned high aspect ratio nanowires (well-defined geometrical shape with length much greater than width). This approach differentiates substantially from the usual conservative methods where low aspect ratio nanoparticles (imperfect spherical shapes) are randomly dispersed and distributed into polymers. In this way, the transformative strategy of organising the nanowires in well-ordered patterns will overcome the disadvantages and limitations of present sensors such as low area/force/position resolution, limited functionality (measuring either static or dynamic forces) and low adaptability to different applications (flexible but not stretchable). The intrinsic discrete particle aspect and piezoelectric nature of the nanowires enables sensor operation in a combined resistive and piezoelectric functionality and thus enables both static and dynamics force measurements with the same device. The device will be driven with low DC bias voltage (low power consumption and zero-power when operating in "piezoelectric mode"), and will provide modularity, flexibility and stretch-ability for optimal surface conformability (i.e., adaptability to a wide range of systems and geometries). The sensor prototypes will be tested against commercially available sensors and their force resolution, flexibility, stretch-ability and reliability will be compared under different bending conditions. In summary, the research programme has three main objectives: to create a combined resistive and piezoelectric smart nanocomposite; use the smart nanocomposite to develop a flexible and stretchable force sensor for both static and dynamic measurements; and to test and compare the developed devices against commercially available sensors. The research will benefit those fields in which force sensing is needed (static, dynamic, impact force measurements). The first targeted application will involve integration of the devices into robotic arms to provide tactile feedback. However, the proposed approach of developing a smart nanocomposite that will enhance the performance of a sensing device has the potential to revolutionise the force sensing market, greatly improve current applications (i.e. robotics) and target novel applications including force sensing on humans (grippers, hands/feet sensors), smart clothes for healthcare and fashion, sports equipment and gadgets (currently limited solely to position or acceleration sensing).