Oral: Nanoscale electrical inhomogeneity observed in organic light-emitting diodes

The understanding of organic light-emitting diodes (OLEDs) is built on the assumption that these thin film devices can be described by one-dimensional (1D) models that exploit their planar symmetry. In contrast, 3D Monte Carlo models predict that injection, transport, and recombination in OLEDs are, in fact, locally inhomogeneous and filamentary on the nanoscale due to weak electronic coupling and strong energetic disorder in organic semiconductor films.¹ Using high-resolution optical microscopy, we report sub-micron electroluminescence (EL) inhomogeneities in doped electrosphosphorescent and bilayer fluorescent OLEDs. The local EL intensity varies by 20% relative to the mean and has a characteristic length scale on the order of microns in diffraction-limited images. The EL pattern is stable over time, though it includes EL “hot spots” that flicker on a timescale of seconds. Importantly, the pattern of inhomogeneity in a given region changes when viewing the highest versus lowest energy components of the emission spectrum, suggesting that the effect is related to energetic disorder and local current flow in the organic stack. These results provide experimental evidence supporting nanoscale electrical inhomogeneities in thin film organic electronic devices and may have important implications for OLED performance and operating lifetime.

[1] J.J. van der Holst, M.A. Uijttewaal, B. Ramachandhran, R. Coehoorn, P.A. Bobbert, G.A. de Wijs, and R.A. de Groot, Physical Review B 79, (2009).