Direct observation of the potential distribution within organic light emitting diodes under operation

• Published in: Physica status solidi. PSS-RRL. Rapid research letters 2015-08, Vol.9 (8), p.475-479
• Main Authors: Weigel, Christian S., Kowalsky, Wolfgang, Saive, Rebecca
• Format: Article
• Language: English
• Subjects: Biophysics; Cathodes; Charge; charge transport; Degradation; Devices; Electric potential; Ion beams; Light emitting diodes; Microscopy; Organic light emitting diodes; potential distribution; scanning Kelvin probe microscopy
• ISSN: 1862-6254; 1862-6270
• DOI: 10.1002/pssr.201510223

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq3)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq3 layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) Direct visualization of the potential distribution within an Alq3/NPB organic light emitting diode under operation is realized by focused ion beam (FIB) preparation of device cross sections and subsequent scanning Kelvin probe microscopy. Results are consistent with previously performed capacitance–voltage measurements and show charge accumulation at the interface between the different organic materials. Furthermore, the effect of degraded or mismatched cathode contacts was visualized.