Examining organic light-emitting diode defects through scanning probe microscopy: Technique development and inherent spatial variations

Inherent spatial variations in the characteristics of organic light-emitting diodes (OLEDs) have been revealed through the use of new scanning probe microscopy and impedance spectroscopy techniques. These variations are attributed to fluctuations in the charge trap density at the anode/organic interface that occur over 20-200 mum2 regions. Although the origins of these fluctuations are unknown, we hypothesis that ordering in the first 15 nm of the organics may contribute to these variations. These fluctuations could also be attributed to regional variations in the ITO surface roughness as determined by atomic force microscopy (AFM), or to regional carbon contamination, as demonstrated through secondary ion mass spectrometry (SIMS).; To qualitatively determine the electronic trap density, impedance spectroscopy is employed. Though the examination of frequency dependent complex conduction in OLEDs, it is demonstrated that the parasitic role of traps can be limited through the passivation of ITO prior to device fabrication. The subsequent decrease in trap capacitance results in the observance of negative capacitance (NC) acting in series with these traps. The source of NC is the accumulation of space charge in the device, and the interplay between NC and the magnitude of the parasitic trap capacitance provides a measure of the trap density in this buried interface.; The examination of spatial variations in light emission, current density, and impedance were facilitated by the development of new atomic force microscope (AFM) techniques. The first technique, Atomic Force Electroluminescence Microscopy (AFEM), couples a photomultiplier tube with the conductive AFM to enable simultaneous spatial mapping of light emission, current, and quantum efficiency. Through the development of large arrays of nanoscaled OLEDs, intensity variations at both the micro- and nano-scales are revealed.; The second technique, Bridge-Enhanced Nanoscale Impedance Microscopy (BE-NIM), couples the AFM with a lock-in-amplifier to monitor variations in the sample impedance while using an electronic bridge to improve resolution. By performing AFEM and BE-NIM on devices exhibiting inter-device electronic variations and through the understanding of the interplay between NC and parasitic trap capacitance, it is confirmed that intensity and charge transport variations result from local fluctuations in the trap density.