Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

• Published in: Applied physics. B, Lasers and optics Lasers and optics, 2009-04, Vol.95 (1), p.113-124
• Main Authors: Gather, M. C., Jin, R., de Mello, J., Bradley, D. D. C., Meerholz, K.
• Format: Article
• Language: English
• Subjects: Anodes; Applied sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Devices; Electric potential; Electronics; Emissivity; Engineering; Exact sciences and technology; Lasers; Multilayers; Optical Devices; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of specific thin films; Optics; Optoelectronic devices; Organic light emitting diodes; Photonics; Physical Chemistry; Physics; Physics and Astronomy; Polymers; organic compounds; Quantum Optics; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Spectroscopy; Voltage
• ISSN: 0946-2171; 1432-0649
• DOI: 10.1007/s00340-009-3364-8

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.