Organic light emitting diodes are an emerging display technology that promise cheap, flexible, vibrant and diverse displays. Information display panels and television screens can be implemented in ways never before imagined. Traditional displays, utilising CRT, LCD and plasma technology all have significant limitations that prohibit their use in one way or another. Organic light emitting diodes (OLEDs) are comprised of a thin layer of light emitting organic material sandwiched between two electrical contacts. Because the material itself is emitting coloured light (and not filtering out white light to give the appearance of colour, as is the case with LCDs) the displays appear vibrant, rich, detailed and have excellent viewing angle. We study OLED materials and fabricate OLED devices. Our research involves understanding how the properties of the materials relate to their structure, and in turn how they affect the device performance. Materials are supplied through collaborations with leading academic and industrial teams. This includes our long-standing collaboration with Prof. Paul Burn on light-emitting dendrimers, leading to the world's most efficient solution processable devices.



OLEDs

Human eye response of three colour components (red, green and blue) to photons of different wavelengths.

Thin films of polymers, dendrimers or solution processed small molecules emit light when an electrical current is passed through them. This results in flat solid state devices which are very thin and are available in a wide range of colours. OLEDs are thus suitable for making ultra-thin full colour displays. Human eyes have three different components to their colour response which correspond to red, green and blue. This is why our modern colour displays have red, green and blue pixels. The graph on the left shows the responses of these three colour components to photons of different wavelengths.









Commission internationale de l'éclairage diagram. This shows how the mixing of red, green and blue colours will appear to human eyes, and is applied when making OLED devices to determine their apparent colour. Picure by Sakurambo

By multiplying these spectra by the electroluminescence spectrum of the OLED device under investigation we can find out quantitatively how red, green and blue it will be. By plotting the faction of red and green response against one another we can draw a plot of all colours that are possible. This is known as the 1931 Commission internationale de l'eclairage (CIE) diagram and a representation of it is shown on the right. The actual diagram includes colours that can't be displayed by your monitor because the pixels making it up are not pure red, green and blue. By selecting the properties of the electrical contacts, blending materials and by molecular engineering OLEDs can be optimised to deliver high luminous and quantum efficiencies. By using these methods our group has produced solution processable dendrimer devices with external quantum efficiencies up to 16%[1], a number close to the theoretical maximum.



Fluorescent and phosphorescent materials emit light when excited by ultraviolet radiation. This is because the high energy photon promotes an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). When the electron falls back into the HOMO it can emitting light, this is called photoluminescence. This process can also be driven by removing electrons from the HOMO to create holes and then injecting electrons directly into the LUMO that can fall into them producing light. This is called electroluminescence. In order for light to be emitted from an OLED an electron and hole must recombine radiatively. To do this efficiently an equal number of electrons and holes must be injected into the device, these charge carriers must travel to reach one another and recombine, additionally when they recombine there must be a good chance a photon is emitted.



We study the motion of charge carriers in our materials using Time of Flight and the light emission and absorption properties using various photophysical measurement techniques, including: photoluminescence quantum yield, time correlated single photon counting, gated CCD spectrometry, a streak camera, femtosecond optical gating and transient absorption. By understanding these processes new materials can be designed in order to produce highly efficient solution processable devices.