LIQUID CRYSTALLINE HYBRID DIELECTRICS FOR MONODOMAIN ORGANIC SEMICONDUCTORS

Lead Research Organisation: University of Hull
Department Name: Physical Sciences

Abstract

The twentieth century saw an explosion in semiconductor electronics from the first transistor, which was used in hearing aids, to the ultrafast computers of today. A similar surge is anticipated for Plastic Electronics based on a new type of semiconducting material which is soft and flexible rather than hard and brittle. Plastic Electronics is considered a disruptive technology, not displacing conventional electronics, but creating new markets because it enables the printing of electronic materials at low temperatures so that plastic, fabric, paper and other flexible materials can be used as substrates. Printing minimises the waste of materials and low cost roll-to-roll manufacturing can be used because the substrates are flexible. New applications include intelligent or interactive packaging, RFID tags, e-readers, flexible power sources and lighting panels. The organic field effect transistor (OFET) is the fundamental building block of plastic electronics and is used to amplify and switch electronic signals. The organic semiconducting channel connects the source and drain electrodes and is separated from the gate electrode by an insulating dielectric. A positive/negative gate voltage induces negative/positive charges at the insulator/semiconductor interface and so controls the conductivity of the semiconductor and consequently the current flowing between the source and drain. The future success of the industry depends on the availability of high performance solution processable materials and low voltage device operation. The semiconductors must have high electron and hole mobility (velocity/electric field) achieved by the hopping of carriers between closely spaced molecular sites. A new class of lamellar polymers, mostly developed in the UK, provides the required state-of the art performance because of their macromolecular self-organisation. However a major problem is that the materials are only well-ordered in microscopic domains; trapping in grain boundaries and poor interconnectivity between domains substantially reduce performance and reliability. The low voltage operation of OFETs requires that the gate insulators have a high dielectric constant.
We propose novel insulating dielectrics for OFETs to simultaneously align the plastic semiconductors and ensure low voltage operation. They will be solution processable at low temperatures for compatibility with printing and other large area manufacturing techniques. We will synthesise and characterise the new materials and test their performance using state of the art semiconductors. We will engage with industrial end-users to ensure that our technology is exploited so contributing to the high-tech economy in an area where the UK is already pre-eminent. We anticipate that our novel insulators will provide monodomain order over large areas to the overlying semiconductor and so will enhance OFET performance and stability. Hence we aim to hasten the commercialisation of Plastic Electronics.

Planned Impact

Plastic Electronics is forecast to become a major disruptive technology, not replacing conventional electronics but opening new markets. It is considered disruptive because it enables the printing of electronic materials at low temperatures so that plastic, fabric, paper and other flexible materials can be used as substrates. Printing minimises the waste of materials and low cost roll-to-roll manufacturing can be used because the substrates are flexible. New applications include intelligent or interactive packaging, RFID tags, e-readers, flexible power sources, novelty items and lighting panels. The global market for Plastic Electronics is under $5 billion dollars now but is predicted to grow to over $330 billion by 2027 (IDTechEx). The future success of the industry depends on the availability of high performance, solution processable, insulating and semiconducting materials, which ensure high performance organic semiconductor devices with low voltage device operation. Very little work has been done on developing the insulating materials although high quality, semiconducting, organics are widely known. We aim to hasten the advent of Plastic Electronics by the development of a novel solution processable insulating film for use in low-voltage integrated circuits. The material has novel features to enhance device performance and reliability.
The entire supply chain of Plastic Electronics, from chemical companies through component fabrication and integration to high-volume product manufacturers, would benefit economically from our research. Chemical manufacturers and print companies would benefit by the synthesis and ink-formulation of large quantities of our novel materials. Our insulating materials are compatible with both polymer and small molecule semiconductors and so will be applicable no matter which material type dominates commercially. The UK is predominant in the area of organic semiconductors with a large patent base and SME activity and so will benefit particularly form the improved manufacturability of Plastic Electronics devices. The technologies involved are at a very early stage so that the UK may emerge as a manufacturing base. The general public will benefit from the availability of low cost and high specification products and the creation of high tech jobs. In the shorter term the demonstration of innovative research and the availability of researchers with interdisciplinary training will help ensure that the UK remains an attractive location to attract inward investment.
The new applications generated by Plastic Electronics would contribute to improved quality of life and health for the general public. For example, a new generation of flexible e-readers would be both ergonomic and energy efficient; medical sensors could be embedded in patches or dressings etc. There is also a potential cultural impact as Plastic Electronics has the potential to drive panelled displays with unusual shapes or aesthetics in architecture, fashion or art.

Publications

10 25 50
 
Description Plastic Electronics is based on a new type of semiconducting material which is soft and flexible rather than hard and brittle. Plastic Electronics is considered a disruptive technology, not displacing conventional electronics, but creating new markets because it enables the printing of electronic materials at low temperatures so that plastic, fabric, paper and other flexible materials can be used as substrates. New applications include intelligent or interactive packaging, RFID tags, e-readers, flexible power sources and lighting panels. The organic field effect transistor (OFET) is the fundamental building block of plastic electronics and is used to amplify and switch electronic signals. A new class of lamellar polymers, mostly developed in the UK, provides the required state-of the art performance because of their macromolecular self-organisation. However a major problem is that the materials are only well-ordered in microscopic domains; trapping in grain boundaries and poor interconnectivity between domains substantially reduce performance and reliability. The low voltage operation of OFETs requires that the gate insulators have a high dielectric constant.
We have synthesized and studied novel insulating dielectrics for OFETs based on nanorods of titanium oxide which are capped by ligands. The nanorods form lyotropic liquid crystal phases and thin films of the nanorods were aligned in one direction by solution shearing. Doped titanium oxide and hafnium oxide nanorods are also being developed as dielectric thin films with low leakage currents. The materials have been characterised using a wide variety of techniques including X-ray diffraction, transmission electron microscopy, electrical measurements, photoelectron spectroscopy, fourier transform infra-red, etc.
We found that the resistance of thin films of titanium oxide nanorods can be switched and reset between two stable states on application of negative and positive voltage pulses respectively. Hence they can be used as memristors or memory switching devices. Such devices could potentially replace transistors in future computers because of their higher circuit density. They can also emulate biological synapses, and so can be used to model learning and forgetting in the human brain. The materials have two advantages for low-cost and high performance manufacturing: solution processability and the ability to fabricate multi-layer devices since insoluble films can be made by crosslinking the ligands by irradiation with ultraviolet light. We are studying the mechanism of resistive switching and have identified two different modes of switching dependent on how the thin films are treated. Capacitance switching is also observed.
Finally the nanorod materials are being investigated as polarization sensitive ultraviolet detectors.
Exploitation Route The memory switching application of our materials was not in our original application but is extremely interesting because of its applicability to future computing and the modelling of human memory. We have not yet tested the dielectric materials in transistors because of the high leakage currents.
Sectors Chemicals

Digital/Communication/Information Technologies (including Software)

Electronics

Energy

Manufacturing

including Industrial Biotechology

 
Description Higher Education Innovation funding
Amount £206,148 (GBP)
Organisation Higher Education Funding Council for England 
Sector Public
Country United Kingdom
Start 04/2012 
End 06/2014
 
Description Research Project grants
Amount £217,972 (GBP)
Funding ID F/00 181/S 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 08/2010 
End 11/2012
 
Description Brazilian Exchange 
Organisation Pontifical Catholic University of Rio Grande do Sul
Country Brazil 
Sector Academic/University 
PI Contribution Sharing of know-how of liquid crystal research
Collaborator Contribution Sharing of know-how of synthetic chemistry
Impact Joint publications and increased understanding between Brazilian and UK science methodologies
Start Year 2012
 
Description Liquid crystalline hybrid dielectrics for monodomain organic semiconductors 
Organisation Imperial College London
Country United Kingdom 
Sector Academic/University 
PI Contribution Joint grant with Imperial College
Collaborator Contribution Joint grant
Impact Resesrch is ongoing with joint papers in preparation
Start Year 2012
 
Company Name Polar OLED 
Description Polar OLED manufactures materials for use with printed OLED displays, wearables and lighting. 
Year Established 2008 
Impact N/A
Website http://www.polaroled.com
 
Description Alumni talk 2013 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Increased alumni awareness of current research in facullty

N/A
Year(s) Of Engagement Activity 2013
 
Description ChelthenhamFestival 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Talk promoted understanding of applications of liquid crystals research and its current research direction at the University of Hull.

Media interest in Hull connection with liquid crystals
Year(s) Of Engagement Activity 2013
 
Description Christmas Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Talk sparked interest in liquid crystal science in Hull, its heritage and current research directions.

Local media coverage of Liquid Crystal activities in HUll University
Year(s) Of Engagement Activity 2013
 
Description Faculty Science festival 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Workshop related to current research topics with activities for children and adults

Increased profile of cutting edge research at the University of Hull
Year(s) Of Engagement Activity 2012,2013,2014
 
Description Heritage Open Day 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Improved awarenesss of heritage and current research in liquid crystals at the University of Hull

N/A
Year(s) Of Engagement Activity 2014
 
Description RSCpublic lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Lively discussion followed talk

Increased awareness of cutting edge research into liquid crystals at Hull University
Year(s) Of Engagement Activity 2014
 
Description University's week2014 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Talk and workshop sparked interest in non-traditional applications of liquid crystals.

N/A
Year(s) Of Engagement Activity 2014