Ultrafast measurement of photophysical processes in organic semiconductors

Abstract

The aim of the work presented in this thesis was to investigate both the photophysical properties of organic semiconductors used in photovoltaic and LED devices, and the processes which occur in OPV devices on a sub-nanosecond timescale. The main experimental technique used was transient absorption (TA). TA was used alongside other techniques such as time resolved fluorescence, time resolved electric field induced second harmonic (TREFISH), and spectroelectrochemistry, which allowed for more accurate assignment of spectral features.

The anion absorption features for PC₆₀BM and PC₇₀BM were measured, and were observed to be very similar to the anion absorption of corresponding bare fullerenes, but slightly blue-shifted. An absorption peak observed in the near-infrared offers the best option for tracking electron dynamics.

The excited state absorption spectra of a range of fluorene based co-polymers were measured. For most materials, behaviour consistent with structural relaxation was observed. Additionally, the change in excited state absorption of a copolymer with a ratio of 95 fluorene units to 5 phenylenediamine units allowed for an intrachain diffusion coefficient to be estimated for polyfluorene.

Exciton diffusion was also measured in P3HT, PTB7 and PC₇₀BM. For these three materials, it was observed that processes dependent on exciton diffusion did not proceed with a single, time independent rate constant. Instead, finite reaction radii, dispersive energy gradients and film morphology all introduced time dependence to dynamics.

For the photovoltaic blend P3HT-PC₆₀BM, polarisation sensitive TA was used to monitor charge separation following both P3HT and CT excitation. Results suggested charge separation was efficient regardless of excitation energy. For PTB7-PC₇₀BM, blends, spectroelectrochemistry was used to identify photo-induced absorption features relating to bound and free charge, and track the charge separation process. The results obtained suggest the high electron mobility provided by carbon fullerenes may be the key to efficient charge separation.

Date of Award	23 Jun 2016
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ifor Samuel (Supervisor)