Engineering Exciton Transport in Organic Semiconductor Thin Films for Photoconversion and Electroluminescence

Abstract

Organic semiconductors continue to grow in commercialization, driven by widely tunable material optical properties, compatibility with high-throughput processing, and demonstrations of flexible and foldable devices. Their optoelectronic functionality is dictated largely by the behavior of molecular excited states termed excitons. In this work, we demonstrate several approaches to manipulate exciton transport, confinement, and recombination, with applications to photoconversion and electroluminescence.In organic photovoltaic cells (OPVs), optically generated excitons must be dissociated into their component charge carriers to realize a photocurrent. Dissociation is generally carried out at a heterojunction between electron donating and accepting materials. As such, it is crucial that excitons can diffuse to this interface before natural decay. A long diffusion length (LD) can be achieved by populating long-lived, spin triplet excitons, which are challenging to generate directly through optical absorption. One approach relies on the incorporation of a sensitizing guest capable of funneling host singlet excitons into triplets. Specifically, we measure the intrinsic LD of non-luminescent excitons and elucidate the factors limiting the performance of OPVs based on a host-guest system. A second approach to populating triplets involves the use of a singlet fission material capable of splitting a singlet into a pair of triplets. Here, our interest is in understanding the role of thin film grain structure and processing condition on triplet LD. Indeed, we achieve a nearly 40% increase in the triplet LD of pentacene, and find that low-energy triplets are potentially less susceptible than singlets to quenching at grain boundaries. In organic light-emitting devices (OLEDs), bimolecular exciton-polaron and exciton-exciton quenching processes are known to reduce device efficiency and lifetime. Here, we demonstrate a novel architecture designed to spatially separate the species involved in such processes, where triplets are generated in a phosphorescent injector, migrate through an adjacent spacer, and undergo triplet-triplet annihilation upconversion for subsequent fluorescence. High-purity deep blue electroluminescence is realized with CIE chromaticity co-ordinates of (0.15, 0.13) as well as remarkably increased device lifetime. This dissertation demonstrates the advances that can be made in device design and performance with systematic engineering of exciton transport.