Ultrafast spectroscopy to explore photophysics in materials for optoelectronics

The Clarke group use ultrafast spectroscopy to study photophysical properties of conjugated organic materials for optoelectronic applications.

The Clarke group focuses on the laser spectroscopy of conjugated organic materials such as conducting polymers. In particular, we use vibrational and transient absorption spectroscopy to explore these materials for applications such as organic photovoltaics, seeking to establish structure-function relationships to enhance device efficiencies. 

We use transient absorption spectroscopy across multiple timescales, from femtoseconds to milliseconds, to investigate charge photogeneration and recombination processes in conjugated materials. We are particularly interested in how the charge carriers – which generate the electricity in an organic photovoltaic device – interact with other excited states such as charge transfer states and triplets. 

Examples of projects where we have utilised ultrafast TAS in particular to gain insight into material photophysics include exploring the influences of symmetry breaking on triplet formation,1 circumventing the energy gap law to enhance excited state lifetimes in red-emitters,2 probing triplet-charge annihilation as a loss mechanism in organic photovoltaics,3 elucidating intrinsic charge carrier photogeneration in pristine conjugated polymers,4 examining the effects of minimal driving force for charge separation,5 and uncovering ultrafast spin-mixing mechanisms.6 

Going beyond TAS, time-resolved vibrational spectroscopy combines ultrafast dynamics information with high structural sensitivity: the “holy grail” of spectroscopy. Vibrational spectroscopy is highly sensitive and small changes in geometry are detectable, and thus is capable of substantially more detailed structural information compared to TAS. Ultrafast Raman spectroscopy has, for example, been proven to distinguish between bulk and interfacial excited states. We have recently been extending our research into this field, and have already discovered intriguing results relating to anion localisation in state-of-the-art non-fullerene acceptors.

• Medina Rivero, S.; Alonso-Navarro, M. J.; Tonnelé, C.; Marín-Beloqui, J. M.; Suárez-Blas, F.; Clarke, T. M.; Kang, S.; Oh, J.; Ramos, M. M.; Kim, D. et al. V-Shaped Tröger Oligothiophenes Boost Triplet Formation by CT Mediation and Symmetry Breaking. JACS 2023, 145 (50), 27295-27306.
• Marin-Beloqui, J. M.; Congrave, D. G.; Toolan, D. T. W.; Montanaro, S.; Guo, J.; Wright, I. A.; Clarke, T. M.; Bronstein, H.; Dimitrov, S. D. Generating Long-Lived Triplet Excited States in Narrow Bandgap Conjugated Polymers. JACS 2023, 145 (6), 3507-3514.
• Marin-Beloqui, J. M.; Toolan, D. T. W.; Panjwani, N. A.; Limbu, S.; Kim, J.-S.; Clarke, T. M. Triplet-Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics. Adv. Energy Mater. 2021, 11 (24), 2100539.
• Shaikh, J.; Congrave, D. G.; Forster, A.; Minotto, A.; Cacialli, F.; Hele, T. J. H.; Penfold, T. J.; Bronstein, H.; Clarke, T. M. Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer. Chem. Sci. 2021, 12 (23), 8165-8177.
• Vezie, M. S.; Azzouzi, M.; Telford, A. M.; Hopper, T. R.; Sieval, A. B.; Hummelen, J. C.; Fallon, K.; Bronstein, H.; Kirchartz, T.; Bakulin, A. A. et al. Impact of Marginal Exciton–Charge-Transfer State Offset on Charge Generation and Recombination in Polymer:Fullerene Solar Cells. ACS Energy Letters 2019, 4 (9), 2096-2103.
• Salvadori, E.; Luke, N.; Shaikh, J.; Leventis, A.; Bronstein, H.; Kay, C. W. M.; Clarke, T. M. Ultra-fast spin-mixing in a diketopyrrolopyrrole monomer/fullerene blend charge transfer state. J. Mater. Chem. A 2017, 5 (46), 24335-24343.