Masters Defense: Effects of Gate Thickness, Gate Morphology and Emissive Layer Material On Organic Light Emitting Varactor Capacitor Performance, July 26, 2018, 3pm

Eric Henderson, Masters Candidate
Public Presentation in Olin Physical Laboratory, Room 101
Thursday, July 26, 2018, at 3:00 PM
Dave Carroll, PhD, Advisor

The defense will follow.


Light is essential to our modern world. To keep up with the demands of power efficiency and broadened applicability, new lighting technologies must be invented. Organic light emitting varactors (OLEVs) represent a promising future in this regard, and these are explored in this study.  OLEVs have a semiconducting gating layer, as well as the typical light emitting and buffer layers of an OLED. Under AC driving, this gate allows control of current flow during the lighting part of the power cycle thereby allowing the maximum theoretical efficiency of the OLEV to exceed that of the DC driven OLED. However, there are numerous factors that remain to be understood about the morphology of the gate-emitter complex, before such maximum efficiencies can be approached.

In this work, a simple capacitive model for the OLEV is introduced and a direct correlation between gate layer thickness, resonant frequency and brightness is demonstrated. Trends in brightness with the gate layer properties are directly in line with expectations from this simple model. Surprisingly, luminance with field strength measurements showed little variation with thin film morphology. This suggests that field concentration across the films (due to hillock formation during gate film growth) was effectively planarized, thereby yielding a length scale for acceptable roughness in device fabrication. Morphological effects where, however, observed in the critical failure fields of thicker films.

While this work focused on magnetron sputtered ZnO layers with organic emitters, the gate-emitter paring of ZnO with a hybrid inorganic-organic perovskite, MAPbBr3, was attempted. In this system charge accumulation at the gate – emitter interface should be screened leading to lower barrier heights during the power cycle. This allowed for a direct study of the effect of these barrier heights in systems with high carrier mobility.  Indeed, the ZnO/Perovskite OLEV did not produce light as might be expected with extreme leakage currents. In contrast, the OLED counterparts do light up suggesting that this leakage current is the primary source of failure.