OLED and PLED Devices Employing Electrogenerated, Intramolecular Charge-Transfer Fluorescence

• Published in: Chemphyschem 2003-04, Vol.4 (4), p.349-358
• Main Authors: Goes, Marijn, Verhoeven, Jan W., Hofstraat, Hans, Brunner, Klemens
• Format: Article
• Language: English
• Subjects: Applied sciences; charge carrier; charge transfer; donor-acceptor systems; doping; Electronics; Exact sciences and technology; injection; luminescence; Optoelectronic devices; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200390061

The light generating mechanism of a series of light emitting diodes with electron donor–bridge–acceptor systems (D–b–A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly‐N‐vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge‐transfer (CT) fluorescence of the donor–bridge–acceptor systems occurred readily in OLED devices with a D–b–A system as the emissive layer. In diodes with PS as the host matrix, hole‐injection and electron‐injection occurred directly in the D–b–A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole‐injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D–b–A molecule, whereas electron‐injection at the cathode side occurred directly into the D–b–A molecules. Charge‐hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence. Getting the blues: Blue light is readily produced by hole and electron injection in a thin layer containing electron donor–bridge–acceptor molecules between a transparent anode and a metal cathode, which leads to population of the strongly (electro)luminescent, intramolecular charge‐transfer state (see graphic).