Dresden 2009 – wissenschaftliches Programm
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SYDI: Symposium SKM Dissertation Prize 2009
SYDI 1: SKM Dissertation Prize 2009
SYDI 1.2: Vortrag
Montag, 23. März 2009, 11:00–11:30, BAR SCHÖ
OLEDs Setting Out in New Directions – From Displays to Sensors — •Malte C. Gather — Department Chemie, Universität zu Köln, Germany
When organic light-emitting diodes (OLEDs) were developed some 20 years ago, the focus was on their implementation as light sources in flat-panel displays. Despite the great technical challenges faced at that time – especially with regard to their insufficient lifetime – displays consisting of hundreds of thousands of red, green and blue emitting OLEDs are now being commercialized. In close conjunction with the technologically driven development, the academic work on OLEDs and on aromatic hydrocarbons – the material class, on which these devices are based – grew into a very respectable field of research. And while much of the industrial work still focuses on the display market, the academic work greatly benefits from the tremendous progress achieved over the last few years, for example by borrowing the technology for new device configurations, which of course all pose interesting new scientific questions. My talk will reflect this exchange between (1) the very much display-oriented work on OLEDs, (2) the thorough characterisation, modelling and optimization of these devices, and (3) the transfer of the technology to novel applications, for example in metrology.
Defining lateral structure in organic semiconductors at micron resolution has turned out difficult but is essential for example to fabricate the red, green and blue emitting pixels of a full-colour display. An interesting approach to achieve this is to use so called “smart photoresists” which are electroluminescent and/or semi-conducting organic materials that can be structured by conventional photolithography in very much the same way as a photoresist. This is of special interest for high-resolution displays or microdisplays where the required resolution is not accessible with competing technologies such as ink-jet printing. I will describe the development of the smart photoresist process and present results on the first prototype of a full-color OLED display based thereon.[1]
Apart from the increasingly apparent commercial value of this new technique, the smart photoresist method is also a very useful tool for basic science: One can use the technique to define stacks consisting of an unlimited number of layers of different materials.[2, 3] For example one can confine the emissive layer to a thin sheet with a thickness of a few nanometres and thus study thin-film interference effects in active devices. In combination with electromodulation spectroscopy, such devices can also be used to investigate the electric field distribution within OLEDs.[4]
Since the smart photoresist process enables defining virtually any micro or nano-structure in an organic semiconductor, completely new device concepts become feasible. One example are light sources for “lab-on-a-chip” systems: By sandwiching an electroluminescent material with a relatively high refractive index between to layers of hole and electron conducting materials of lower refractive index, a waveguide structure is formed. If electrodes are applied to the two outer layers this waveguide can be electrically pumped, with the generated radiation being guided in the core layer of the waveguide.[5] Because organic semiconductors can be more easily processed on glass or flexible substrates than their inorganic counterparts, this self-emissive waveguide structure might prove very useful as a light source on future lab-on-a-chip systems or highly integrated sensors.
[1] M. C. Gather, A. Köhnen, A. Falcou, H. Becker, K. Meerholz, Adv Funct Mater 2007, 17, 191.
[2] N. Rehmann, C. Ulbricht, A. Köhnen, P. Zacharias, M. C. Gather, D. Hertel, U. S. Schubert, K. Meerholz, Adv Mater 2008, 20, 129.
[3] P. Zacharias, M. C. Gather, M. Rojahn, O. Nuyken, K. Meerholz, Angewandte Chemie 2007, 46, 4388.
[4] M. C. Gather, R. Jin, J. C. deMello, D. D. C. Bradley, K. Meerholz, Appl. Phys. B accepted.
[5] M. C. Gather, F. Ventsch, K. Meerholz, Adv Mater 2008, 20, 1966.