Origin of the Strong Induced Chiroptical Effect in Semiconducting Polymer/Helicene Blends

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.

The science proposed herein would have significant international impact on current academic and industrial knowledge in a diverse range of disciplines including chemistry, physics, materials science, biomedicine and engineering. Moreover, advances in the development of induced chiroptical properties in a photoluminescent/electroluminescent semiconducting polymer via helicene doping is likely to promote new research directions; for example, chiral photonics involving structures on 10-100 nm length scales; hybrid chiral organic-metamaterial devices; inducing chirality in biologically active polymers; varying mechanical properties by inducing chirality in non-conjugated polymers.

The translational aspect of this proposal would be of high interest to a number of key industrial companies in the UK involved in plastic electronics research, including Cambridge Display Technology (CDT), FlexEnable, Merck, Molecular Vision, Ambicare Health, Power OLEDs, Lomox, FlexInk and Ossila. Technological advances resulting from this work would have clear ramifications in terms of wealth creation within the UK, with, for example, the current prospects for solution-processed organic light emitting diodes (OLEDs) in rigid and flexible displays, either as the directly emitting pixel element in an OLED display or as a liquid crystal display (LCD) backlight. Indeed, active matrix OLED (AMOLED) and LCD are the two dominant technologies across the large-area TV, medium-sized tablet/PC, and small mobile display areas, the total display market being worth several billion dollars per annum. The technological exploitation of this proposal has the potential to impact this industry in the next few years both within the UK and globally, thereby benefiting those involved in the production, distribution and sale of displays.

Taking displays as an example of the exploitation of this work, a novel solution-processable, high performance circularly polarized (CP) light emitting OLED would significantly improve both display efficiency and display lifetime. This would reduce global energy consumption, decrease display carbon footprint, and extend portable product battery lifetime, therefore being of benefit to society as a whole. Additionally, thin, compact and patternable CP light emitting devices could have a strong impact in current and future technologies such as biomedical optoelectronic sensors, conventional and quantum optical telecommunication, optical spintronics, and integrated optical quantum computing.

The project partner CDT will directly benefit from this proposal, allowing them to have access to processing and optimization knowledge to fabricate high performance CP emitting polymer OLEDs.

The further training of the researchers involved in this proposal, including their involvement and exposure to an interdisciplinary project involving physicists, chemists and material scientists and industry, would also have clear economic and societal impact. Through communication and outreach activities, scientific ideas behind the project will be popularized and this will contribute to increasing public awareness and understanding of science and technology.