Characterizing and engineering nanoscale energy transport in organic semiconductors
Main list: Physics and Astronomy Colloquia
Organic semiconductors are conjugated molecular materials whose facile thin film processing and tunable optoelectronic properties have made them of interest for applications in light-emission, detection, and solar photoconversion. In addition, devices made from organic semiconductors can be integrated with a broad range of substrates allowing them to be mechanically flexible, enabling novel form and functionality. In contrast to conventional inorganic semiconductors, the excited state in these materials is a tightly-bound electron-hole pair termed an exciton. In organic semiconductor devices, exciton migration and recombination strongly dictate device design and performance. In an organic photovoltaic cell (OPV), excitons must be efficiently transported to a dissociating heterojunction in order to realize efficient photocurrent generation. In the simplest OPVs, performance is limited by an unfavorable trade-off between a short exciton diffusion length (LD) and the optical absorption length. Consequently, state-of-the-art devices rely on a morphology-optimized mixture of the active materials to reduce the distance an exciton must migrate. Frequently, fluorescence quenching methods are used to experimentally probe LD for potential active materials. Unfortunately, many promising active materials are non-luminescent, or rely on the formation of non-radiative spin-triplet excited states. These systems are not amenable to fluorescence-based methods and hence, values of LD are less frequently reported. While photocurrent spectroscopy can be used to probe optically dark states, such methods often require assumptions to be made about unknown charge carrier recombination losses.
In the first part of this talk, a device-based measurement technique will be described that is capable of yielding the intrinsic material LD despite the presence of unknown recombination losses. The method is first applied to extract LD for luminescent materials to demonstrate agreement with conventional fluorescence-based measurements. We also examine a series of dark small molecule and polymer semiconductors, as well as active materials that exhibit singlet fission to form dark triplet excitons. The broad usefulness of the method is further demonstrated by also extracting LD for a thin film of CdSe quantum dots. The second part of the talk will examine methods to directly engineer LD, including variations in molecular structure, thin film crystallinity, and the population of long lived spin-triplet states.
Prof. Russell Holmes
University of Minnesota
Dept of Physics, University Of St. Andrews North Haugh
Lecture theatre C
March 8, 2019
From: 10h00 To: 11h00