Biwu Ma, Bumjoon Kim, Daniel Poulsen, Sebastian Zonte

The characteristics of organic material in general, such as processability and low cost, have made organic light emitting diodes (OLEDs) an emerging new technology with the potential to enable robust, lighter, thinner and more efficient display devices. These characteristics also make OLEDs an attractive potential alternative to the current standards in multipurpose lighting, incandescent and fluorescent bulbs. The most efficient OLEDs reported to date have been multilayer structures with heavy metal phosphorescent emitters, in which the hole-transporting layer (HTL), phosphorescent emitting layer (EL), and electron-transporting layer (ETL) are sandwiched between two electrodes. The advantages of a multilayer structure include facilitated charge carrier injection through reducing the respective injection barriers, enhanced recombination of electrons and holes in the EL, and decreased exciton quenching by the electrodes. The use of phosphorescent materials enables harvesting both singlet and triplet excitons leading to a theoretical internal quantum efficiency of 100%. Such efficient multilayer devices have been realized for small molecules through layer-by-layer vapor deposition, a very successful technique that is however limited to thermally stable low-molecular-weight materials and is relatively expensive and time-consuming. On the other hand, fabrication of devices from solution using macromolecules would be an attractive alternative for mass production of devices with large areas. At Berkeley, our research focuses on development of new materials and techniques for solution processing organic electroluminescent devices.

Solution Processing Polymer LEDs via Cross-Linking

To achieve multilayer structure for high efficient devices via solution processing, we have been working on the development of materials with “resist” properties, that is, soluble precursors, which can be cured photochemically or thermally to yield insoluble films. Several types of materials that achieve this goal have been developed including small molecules and simple styrenic polymers containing both OLED functionalities and thermal- or photo- crosslinkable units. One of these successful examples is a copolymer that contains a benzocyclobutene (BCB) thermally reactive functional group as well as a N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) hole-transporting moiety. (Figure 1) This copolymer can be completely cross-linked to form smooth insoluble films through reaction of the benzocyclobutene (BCB) units at 200 °C. Using this novel thermally cross-linkable polymer, a highly efficient solution-processed multilayer green emitting phosphorescent device with external quantum efficiency of 10.4% at 350 cd/m² has been demonstrated.

• Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R. Physical Review B 1999, 60, 14422-14428.
• Ma, B.; Lauterwasser, F.; Deng, L.; Zonte, C. S.; Kim, B. J.; Fréchet, J. M. J.; Borek, C.; Thompson, M. E. Chem. Mater. 2007, 19, 4827-4832.
• Ma, B.; Kim, B. J.; Deng, L.; Poulsen, D. A.; Thompson, M. E.; Fréchet, J. M. J. Macromolecules 2007, 40(23); 8156-8161.