Femtosecond spectroscopy

We aim to understand fundamental electronic processes in functional organic and hybrid semiconductors as well as operational mechanisms of OLEDs, solar cells and other optoelectronic devices to help to improve their performance. We study energy and charge transfer, charge pair dissociation, extraction and recombination, forward and reverse intersystem crossing, exciton-photon interactions and other relevant processes. The main experimental methods that we use are time-resolved photoluminescence and broadband pump-probe spectroscopies.

An arrray of equipment used to interpret tuneable femtosecond light pulses

In these experiments we use widely tuneable femtosecond light pulses generated using state-of-the-art optical parametric amplifiers ORPHEUS and ORPHEUS-N‑3H pumped by regenerative amplifier PHAROS from Light Conversion and sub-nanosecond pulses from diode-pumped lasers.  For broadband transient absorption we use white light continuum probes and InGaAs photodetector arrays which cover the spectral range from 500 nm to 1600 nm. Optical and electronic delays of synchronized laser pulses allow us to perform measurements on a wide time scale from 50 fs to milliseconds. We use fluorescence up-conversion spectroscopy and streak cameras to measure photoluminescence dynamics over a wide time scale from 100 fs to milliseconds and broad spectral range from ultraviolet to 1600 nm in near infrared. We also use luminescence microscopy in combination with time-correlated single photon counting to image exciton diffusion in functional materials.

The highlights of recent work include reports on the role of exciton diffusion and charge delocalisation in developing efficient solar cells and quantitative characterisation of fluorescence quenching in OLEDs at high current densities.  We also published a comprehensive review on light harvesting and charge separation in organic solar cells.

Examples of Current Publications

Hole delocalization as a driving force for charge pair dissociation in organic photovoltaics. B. Matherson; A. Ruseckas; S. J. Pearson and I. D. W. Samuel. Materials Horizons, 6, 1050-1056 (2019)

Engineered exciton diffusion length enhances device efficiency in small molecule photovoltaics, by Sajjad M. T. et al, Journal of Materials Chemistry A. 6, p. 9445-9450 (2018).

Exciton-polaron interactions in polyfluorene films with β phase, by Montilla F., Ruseckas A. & Samuel I. D. W., Journal of Physical Chemistry C. 122, p. 9766-9772 (2018)

Hot-Hole Cooling Controls the Initial Ultrafast Relaxation in Methylammonium Lead Iodide Perovskite. G. J. Hedley, C. Quarti, J. Harwell, O. Prezhdo,  D. Beljonne and I. D. W Samuel Scientific Reports; 8, 8115 (2018)

Time-resolved studies of energy transfer in thin films of green and red fluorescent proteins. J. Zajac, M. Schubert, T. J. Roland, C. Keum, I. D. W. Samuel and M. C. Gather. Advanced Functional Materials. 28, 24, 1706300 (2018)

Light harvesting for organic photovoltaics, by Hedley G. J., Ruseckas A. & Samuel I. D. W., Chemical Reviews 117, p. 796-837 (2017).