CHARGE TRAPPING IN POLYMER DIELECTRICS AND POTENTIALS AT ORGANIC DONOR-ACCEPTOR JUNCTIONS—THE ROLE OF INTERFACE AND BULK CONTRIBUTIONS

Organic electronics have attracted increasing interest during the past decade due to their potential applications in transparent, large-area, printable, and stretchable devices. Solution based material deposition considerably reduces processing costs, and allows the use of non-standard substrates in device design. Many organic electronic device parameters are controlled by interfacial as well as bulk properties. Organic donor-acceptor junctions are relevant to organic photovoltaics (OPVs) as well as organic light emitting diodes (OLEDs). In an OPV, interfacial potentials between the hole transporting (donor) organic semiconductor (OSC) and electron transporting OSC (acceptor) lead to separation and recombination of electrons and holes. The mechanisms behind interfacial potential formation in organic donor-acceptor junctions are not fully understood and are an active area of study. In this thesis, the interfacial potential was measured, and interface and bulk contributions were separated by fabricating lateral organic donor-acceptor junctions both with and without a gap between the donor and acceptor materials. Contact between the donor and acceptor materials increases the interfacial potential beyond that calculated from bulk values. This can be explained through differences in electron affinity of the donor and acceptor, and also by differences in the delocalization of molecular orbitals (MOs) of the two OSC films. Greater delocalization of MOs allows for electron donation to adjacent molecules, a surprising result in organic electronics. In addition, the effect of the substrate on the potential was examined. The field is persistently negative on the acceptor side when the junction is made on a SiO2 substrate. When Al2O3, a substrate with higher dielectric constant, is used, the field decreases in one case, and reverses in the other. For organic field effect transistors (OFETs), the instability of switching voltages is an interface-dominated issue which causes the device left on to turn off over time, referred to as bias stress. Bias stress, caused by charges trapped at the dielectric/OSC interface, can be quantified by a shift in the threshold voltage (Vth) of the device. This thesis discusses localizing trapped charges in an OFET dielectric to control bias stress and operating voltages. By changing numbers and positions of trapped charges in the dielectric, the voltage at which the OFET turns on can be defined, and by pre-populating interfacial traps before running the device, bias stress may be reduced. In this thesis, charging of bilayer and trilayer dielectrics made from in-house synthesized ‘chargeable’ substituted polymers was studied. There was greater stabilization of trapped charges at the dielectric/OSC interface in chargeable polymers adjacent to the OSC, indicating charging occurs through an interface-driven mechanism. However, when they were encapsulated such that the chargeable polymer was situated between two layers of unsubstituted polymer, there was less response to charging than in the fully unsubstituted control. This reduction in bias stress susceptibility could stem from the bulk dielectric polarization of the chargeable layer, which counteracts the charge trapping mechanism at the dielectric/OSC interface