School Colloquia Series - Dane McCamey - Spin in Organic Optoelectronic Materials and Devices
Speaker: Dane McCamey
Abstract: Excitons are central to the operation of a wide variety of optoelectronic materials and devices. Whilst the energy and lifetime of excitons is fairly easy to access with optical methods, the role of their spin is more challenging to access. In this talk I will discuss two situations in which the spin of excitons plays a role in determining optoelectronic properties, and describe spin based spectroscopic approaches for investigating these effects.
In the first half of the talk I will focus on the conductivity of organic light emitting diodes, and particularly the impact of exciton spin on conductivity of these devices. I will describe electrically detected approaches to measuring pulsed electron spin resonance, which provide the sensitive needed to identify the influence of spin on these thin film devices.1,2
The second topic I will cover is Singlet exciton fission - a process wherein an optically prepared singlet state splits into two triplet excitons. If harnessed efficiently, this process can be exploited to enhance the photocurrent of solar cells, raising the limiting power conversion efficiency from 33.7% to 45.9% under 1 Sun. Fission has been characterised with widely used optical spectroscopies, such as pump-probe transient absorption, which drive electronic transitions and give details about the rate and yield of fission. However experimental insight into the nature of the triplet pair state generated upon fission, (TT), is lacking.
I will describe a series of continuous wave and pulsed electron spin resonance experiments on engineered molecules designed to optimise singlet fission. These bipentacene molecules show signatures of strongly coupled quintet states at short times, which evolve toward isolated triplets at longer times. By varying the length of the coupling between the two pentacenes, we modify the spin-spin coupling between the resultant excitons, demonstrating how molecular structure influences the pathways and dynamics of the fission process3, and providing a pathway toward engineering optimised singlet fission material.
1. D. R. McCamey et al., Nature Materials 7, 723 (2008)
2. W. J. Baker et al., Physical Review B 92, 041201(R) (2015)
3. M. J. Y. Tayebjee et al., Nature Physics (2017)