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

Lead Research Organisation: University of Sheffield
Department Name: Materials Science and Engineering

Abstract

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.

Publications

10 25 50

publication icon
Cheng H (2018) Trigonal columnar self-assembly of bent phasmid mesogens. in Chemical communications (Cambridge, England)

publication icon
Fall W (2019) An Ising transition of chessboard tilings in a honeycomb liquid crystal in Molecular Systems Design & Engineering

publication icon
Li YX (2019) New Type of Columnar Liquid Crystal Superlattice in Double-Taper Ionic Minidendrons. in Chemistry (Weinheim an der Bergstrasse, Germany)

 
Title A cover picture for Chemistry A European Journal 
Description A new approach aimed at widening the range and complexity of potential mesophases by introducing double-taper shaped molecules is reported. In such molecules based on simple model minidendrons, a new type of hexagonal columnar liquid-crystal superlattice consisting of two kinds of columns, ionic and non-ionic, has been obtained. This points the way to creating complex self-assembly of mixed columns, mixed spheres, or mixed columns and spheres, and novel materials with double functionality. 
Type Of Art Artwork 
Year Produced 2019 
Impact Increased readership of the particular paper of ours. 
URL https://onlinelibrary.wiley.com/doi/full/10.1002/chem.201904203
 
Title Cover image for W.S.Fall et al, Soft Matter, 2019, 15, 22. 
Description Artistic impression of the molecular ejection transitions in columnar LCs. 
Type Of Art Image 
Year Produced 2019 
Impact Promoting the findings in the publication to a wider audience. 
URL https://pubs.rsc.org/en/content/articlepdf/2019/sm/c8sm01851k
 
Title Cover image for paper A. Lehmann et al. ChemComm 2018, 54, 12306. 
Description A cover image showing the molecular models of different phases formed by Y-shaped molecules and their transitions, with the POM pictures of the phases as the background. 
Type Of Art Image 
Year Produced 2018 
Impact Promoting the findings in the publication to a wider audience. 
URL https://pubs.rsc.org/en/content/articlepdf/2018/cc/c8cc06281a
 
Description The key development so far is that we have established the processing conditions that would maximize the chiral-optical properties of the polymer films doped with chiral dopant, including the control of dopant concentration, film thickness and most importantly, thermal annealing temperature and time. Unprecedentedly high CD (Circular Dichroism) properties in such thin film samples, that they absorb left-handed and right-handed circular polarized light very differently, have been realized. We are also coming to a better understanding the structural origin of such strong induced chiral-optical properties, and how it can be controlled in the thermal annealing process.

We have further identified different crystalline phases F8BT polymers could form and how their formation can be controlled by their preparation and thermal history. The different behaviour, and different phases shown, in bulk and in thin films have been recognized. The key to the best chiroptical behaviour relies on the propagation of chiral order, which can only be achieved by annealing above the glass transition of the polymer, from a homogeneously mixed polymer/helicene system. Crystallization reduces the chiral order, so prolonged thermal annealing which leads to crystallization should be avoided.
Exploitation Route The extremely high CD properties of the film can be made into filters of left- or right-handed Circular polarized light for applications. We are currently working with our collaborators to fabricate organic OLEDs on the basis of such films, with the potential of emitting pure circularly polarized light. This should have vast translational potential in photonic technologies, including optical communication, quantum-based optical computing, as highly efficient liquid crystal display (LCD) backlights, and in active matrix OLED (AMOLED) displays. For example, current AMOLED displays employ a circular polarising filter (consisting of a linear polariser and quarter wave plate/film) to improve contrast by reducing external reflected light. However, nearly 50% of the transmitted light from the OLED emission is cut out by the filter, dramatically reducing overall display efficiency. An electroluminescent material that directly emits circularly polarised light will result in lower losses in brightness at the filter, and improve both efficiency and lifetime.
Sectors Chemicals,Digital/Communication/Information Technologies (including Software),Electronics

 
Description Spontaneous Induction and Amplification of Macroscopic Homochirality in Isotropic Liquid and Liquid Crystals
Amount £412,149 (GBP)
Funding ID EP/T003294/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2020 
End 02/2023
 
Description Imperial College London, Department of Physics and Department of Chemistry 
Organisation Cambridge Display Technology
Country United Kingdom 
Sector Private 
PI Contribution We are working on investigating the various crystalline/liquid crystalline polymorphs present in the semiconducting polymers, and how they are affected by doping and processing conditions. These involves detailed XRD studies, on bulk, thin film and fibre samples, a lot of times involve the use of synchrotron radiation sources. Molecular modelling, simulation of diffraction patterns are also included. In addition, synchrotron radiation CD spectroscopy which enables direct in-situ observation of development of chirality in doped thin films, will be carried out as well. In the future, samples prepared in device configuration will be studied in full detail as well.
Collaborator Contribution Our collaborators at Department of Physics, ICL will investigate the sample preparation conditions and their effect on the device properties such as CP electro-luminescences. Constant discussions between us helps us to find the best route to prepare low-cost high quality CP emitting devices. Partners from Department of Chemistry ICL provide us with helicene and other chiral dopants for the project. CDT supplies us with semiconducting polymers of different molar mass and dispersity.
Impact Helicene compounds have been synthesized and provided by our collaborator. Initial combined CD and XRD studies have clarified the origin of the induced chirality in semi-conducting polymers, that it is the formation of a particular crystalline structure, presumely chiral but in pure compound it is a racemic mixture, and the inclusion of chiral dopants serving as nucleation agents and so either left- or right-handed version of the crystal dominates.
Start Year 2017
 
Description Imperial College London, Department of Physics and Department of Chemistry 
Organisation Imperial College London
Department Department of Chemistry
Country United Kingdom 
Sector Academic/University 
PI Contribution We are working on investigating the various crystalline/liquid crystalline polymorphs present in the semiconducting polymers, and how they are affected by doping and processing conditions. These involves detailed XRD studies, on bulk, thin film and fibre samples, a lot of times involve the use of synchrotron radiation sources. Molecular modelling, simulation of diffraction patterns are also included. In addition, synchrotron radiation CD spectroscopy which enables direct in-situ observation of development of chirality in doped thin films, will be carried out as well. In the future, samples prepared in device configuration will be studied in full detail as well.
Collaborator Contribution Our collaborators at Department of Physics, ICL will investigate the sample preparation conditions and their effect on the device properties such as CP electro-luminescences. Constant discussions between us helps us to find the best route to prepare low-cost high quality CP emitting devices. Partners from Department of Chemistry ICL provide us with helicene and other chiral dopants for the project. CDT supplies us with semiconducting polymers of different molar mass and dispersity.
Impact Helicene compounds have been synthesized and provided by our collaborator. Initial combined CD and XRD studies have clarified the origin of the induced chirality in semi-conducting polymers, that it is the formation of a particular crystalline structure, presumely chiral but in pure compound it is a racemic mixture, and the inclusion of chiral dopants serving as nucleation agents and so either left- or right-handed version of the crystal dominates.
Start Year 2017
 
Description Imperial College London, Department of Physics and Department of Chemistry 
Organisation Imperial College London
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution We are working on investigating the various crystalline/liquid crystalline polymorphs present in the semiconducting polymers, and how they are affected by doping and processing conditions. These involves detailed XRD studies, on bulk, thin film and fibre samples, a lot of times involve the use of synchrotron radiation sources. Molecular modelling, simulation of diffraction patterns are also included. In addition, synchrotron radiation CD spectroscopy which enables direct in-situ observation of development of chirality in doped thin films, will be carried out as well. In the future, samples prepared in device configuration will be studied in full detail as well.
Collaborator Contribution Our collaborators at Department of Physics, ICL will investigate the sample preparation conditions and their effect on the device properties such as CP electro-luminescences. Constant discussions between us helps us to find the best route to prepare low-cost high quality CP emitting devices. Partners from Department of Chemistry ICL provide us with helicene and other chiral dopants for the project. CDT supplies us with semiconducting polymers of different molar mass and dispersity.
Impact Helicene compounds have been synthesized and provided by our collaborator. Initial combined CD and XRD studies have clarified the origin of the induced chirality in semi-conducting polymers, that it is the formation of a particular crystalline structure, presumely chiral but in pure compound it is a racemic mixture, and the inclusion of chiral dopants serving as nucleation agents and so either left- or right-handed version of the crystal dominates.
Start Year 2017