The Campbell group has an active research programme in organic semiconductors and their devices. Materials include both conjugated polymers and molecular semiconductors; devices include both organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). Key areas of interest are:
- Organic Chiral Optoelectronics
- Large-Area Contact Printed Plastic Electronics
- Organic Semiconductor Device Physics
- Biomedical Organic Electronic Sensors
- Novel Organic Semiconductor Materials, Devices and Fabrication Methods
ORGANIC CHIRAL OPTOELECTRONICS
Working closely with the Matthew Fuchter group in the Department of Chemistry, we have an ongoing research program on organic chiral optoelectronic materials and devices. To date our focus has been on using molecular semiconductors known as helicenes, which have an intrinsically chiral helically shaped structure. We have used helicenes directly in devices, or as a molecular dopant to induce chirality in a normally achiral light emitting polymer. So far this has been targeted towards developing circularly polarised (CP) light emitting polymer OLEDs, CP light emitting phosphoresent OLEDs (PHOLEDs), and CP light detecting phototransistors. Such devices have application in energy efficient displays, quantum optical computing and telecommunication, and biomedical imaging and sensing.
LARGE-AREA CONTACT PRINTED PLASTIC ELECTRONICS
We have been developing the use of gravure contact printing as a fabrication method for organic large area electronics (OLAE). Gravure is an extremely high volume, roll-to-roll (R2R) compatible printing technique conventionally used to print packaging, magazines and postage stamps. We have been using it to print semiconducting and conducting polymers, molecular semiconductors, dielectrics, inorganic electron injection layers and metal nanoparticles. This is on large area, flexible plastic substrates, using gravure with conventional photolithographic techniques in a batch process flow, and combining this with other techniques such as nanoimprint lithography (NIL), zone-casting and molecular evaporation. Target devices are OLEDs for flexible, lightweight displays and lighting, and OFETs and their circuits for flexible display TFT backplanes and drivers, RFID tags, and large area sensor arrays. Recent work includes: investigating the science behind printing materials for organic electronics; gravure printed ultra-thin dielectrics for low-voltage transistors; printed polymer and molecular semiconductor inverters; self-aligned nanoscale channel-length complimentary inverters and MHz transistors; printed organic complimentary logic circuits on plastic, comparing ink-jet to gravure and demonstrating NAND gates.
ORGANIC SEMICONDUCTOR DEVICE PHYSICS
My work in the area of the organic semiconductor devices physics focuses on charge transport, charge injection and charge trapping in polymer and molecular semiconductors and their devices.
Examples of this work include: showing that transport in poly(phenylene-vinylene) diodes involved space charge limited conduction (SCLC) with a trap distribution, the latter made up of the deep density-of-states (DOS) tail sites; mapping out this DOS using an impurity trap state in polymer Schottky diodes using deep level transient spectroscopy; using dark injection, time-of-flight and current-voltage techniques to demonstrate trap-free SCLC in polyfluorene copolymer diodes; identifying the strong electron transport in the bis-thiophene copolymer of polyfluorene; identifying fast and slow trapping at the injecting interface as the cause of the reduced injection efficiency in polymer diodes with interlayers.
More recent work has focused on charge transport in polyfluorene copolymer diodes and transistors, showing that these amorphous materials have a charge carrier density independent mobility. This is in direct contradiction to many theoretical models, and has important implications for charge transport in all polymer semiconductors.
BIOMEDICAL ORGANIC ELECTRONIC SENSORS
Initial work focused on investigating conducting polymers as coatings for neural stimulation electrodes. Working within the EC FP7 OrgBio project, we are currently printing arrays of electrolyte gated organic field-effect transistors (EGOFETs) on plastic for use as biochemical detectors, and plan to develop these as neural sensors. Another area of interest is the use of circularly polarised light in biomedical imaging and biomolecule detection.
Novel Organic Semiconductor Materials, Devices and Fabrication Methods
We have investigated new polymers and molecular semiconductors, transport and injection layer materials, electrodes, substrates and other device components; new device structures including organic-inorganic hybrid devices; and new organic semiconductor deposition and device fabrication methods.
Recently we have been developing ambient, solution processable metal oxides and hybyrid materials as hole or electron injection layer materials for polymer semiconductor diodes and transistors. With Martyn McLachlan in the Department of Materials Science, we have fabricated hybrid LEDs with an inverted polymer bilayer structure using zinc oxide planar and nanorod electron injecting electrodes. We have also been developing the use of a confined crystallisation technique to fabricate well-controlled and positioned molecular semiconductor single crystals on planar substrates for devices such as high mobility transistors and phototransistors.
CURRENT AND PREVIOUS RESEARCH COLLABORATORS
- Internal:- Joachim Steinke (Chemistry), Matthew Fuchter (Chemistry), Martin Heeney (Chemistry), Ian McCulloch (Chemistry), Donal Bradley (Physics), Ji-Seon Kim (Physics), Martyn McLachlan (Materials).
- Industry:- Cambridge Display Technology (UK), Merck (UK), OSRAM Optosemicon (USA).
- National Laboratory:- Rutherford Appleton Laboratory (UK), Fraunhofer IPMS (DE).
- Academia:- University of Florida (UK), Cornell University and CHESS (USA), University of St Andrews (UK), University of Sheffield (UK), University of Bath (UK).
- EC FP6 CONTACT project:- Swatch R&D, Merck, IMEC, University of Ilmenau and Schlaefli Maschinen.
- EC FP7 POLARIC project:- BASF, Swatch R&D, AMO, Obducat Technologies, 3D-Micromac, Micro Resist Technology, VTT, IMEC, CSEM, Joanneum Research, Fraunhofer EMFT and the University of Cardiff.
- EC FP7 OrgBio project:- TU Munich, EMSE, CNRS, Dublin City University, Linkoping University, University of Rijeci, UNIBA, University del Pais Vasco, Plasma Solution, Helmholtz ZMD, St Microelectronics and Ibidi.
PostDoctoral Research Assistants
Dr Ben Muir
Dr Xuhua Wang
Dr Nildo C. da Costa
Dr Ying Yang
Dr Julian Farmer
Dr Monika Voigt
Dr Katherine Whitehead
PHD STUDENTS (GRADUATED)
Jorge C. D. Faria