We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below , the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above , however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of . As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices.
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15 July 2009
Research Article|
July 23 2009
Improving the lifetime of white polymeric organic light-emitting diodes
Malte C. Gather;
Malte C. Gather
a)
1Department of Chemistry
, Universität zu Köln
, Luxemburger Str. 116, 50939 Köln, Germany
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Sebastian Köber;
Sebastian Köber
1Department of Chemistry
, Universität zu Köln
, Luxemburger Str. 116, 50939 Köln, Germany
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Susanne Heun;
Susanne Heun
2
Merck KGaA
, LC/BU OLED/PC, Industriepark Höchst, Building G864, 65926 Frankfurt a. M., Germany
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Klaus Meerholz
Klaus Meerholz
b)
1Department of Chemistry
, Universität zu Köln
, Luxemburger Str. 116, 50939 Köln, Germany
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a)
Present address: Wellman Center, Harvard Medical School, 40 Blossom Street, Boston, MA 02114.
b)
Electronic mail: [email protected].
J. Appl. Phys. 106, 024506 (2009)
Article history
Received:
March 13 2009
Accepted:
June 19 2009
Citation
Malte C. Gather, Sebastian Köber, Susanne Heun, Klaus Meerholz; Improving the lifetime of white polymeric organic light-emitting diodes. J. Appl. Phys. 15 July 2009; 106 (2): 024506. https://doi.org/10.1063/1.3176502
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