Engineering interfacial gates for enhanced functionality in organic optoelectronic devices

Organic semiconductors are a novel class of electronic materials currently receiving attention for next generation information display, highly efficient solid-state lighting, and low-cost solar energy conversion applications. These materials are of interest for their high performance and also for their compatibility with high throughput, roll-to-roll processing techniques that could ultimately enable low-cost device and system fabrication. In all of these applications, a critical component to enhanced device design and overall high performance is the ability to engineer the confinement and motion of molecular excited states created under electrical (as in a light-emitting device, LED) or optical excitation (as in a solar cell). Unlike the more familiar case of charge carriers (electrons or holes), it has to date proved difficult to affect the motion of charge-neutral molecular excited states through the application of an external voltage. The proposed work will develop novel, specially designed interfaces capable of acting as gates, permitting the funneling of molecular excited states to specific locations in a device. In an LED, this could permit an enhanced efficiency of luminescence, while in a solar cell this could permit a more efficient conversion of molecular excited states into useable electric current using simple device architectures. Overall, the proposed work will lead to the design of advanced optoelectronic devices for light-emission and photoconversion, among others.

Technical:

The design of many organic optoelectronic devices including photovoltaic cells (OPVs) and light-emitting devices (OLEDs) is heavily impacted by the excitonic character of these materials. Excitation of an organic semiconductor leads to the formation of tightly bound excitons that resist dissociation by both thermal and electric means. In OPVs, the exciton must be dissociated into its component charge carriers in order to generate a photocurrent. Dissociation is typically realized at an electron donor-acceptor interface, necessitating efficient exciton diffusion from the point of photogeneration to the point of dissociation. A similar challenge in exciton management exists in OLEDs, where it is critical to confine excitons to the device emissive-layer, and avoid spatial overlap between the exciton and polaron densities to reduce non-radiative quenching processes. In state-of-the-art devices, problems related to exciton management are often dealt with by affecting large-scale changes in device architecture, which while effective, avoid addressing the actual processes responsible for transport. For instance, in OPVs, exciton harvesting is maximized through the use of blended donor-acceptor structures known as a bulk heterojunctions. In OLEDs, confinement is often realized through the use of wide-energy gap blocking layers. This proposal forges an alternate and more direct approach to the challenge of exciton management. Work is centered on the use of architectures that directly impact the rates of excitonic energy transfer responsible for diffusion. Preliminary results suggest that it is possible to construct one-way excitonic gates, which overcome the limitations of normal diffusion and permit direct exciton transport. The novelty in this approach is in the use of device design concepts that directly address deficiencies in exciton migration. The ability to direct exciton transport has broad implications for the design of OPVs and OLEDs in terms of addressing the fundamental excitonic challenges inherent in device operation.