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Non-radiative processes in organic semiconductors"

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Non-radiative deactivation of neutral excited states in organic semiconductors can determine the overall efficiency of devices such as solar cells or lasers. For energy harvesting in solar cells, for example, ultrafast non-radiative multiexciton generation through ‘singlet fission’ could prove fundamental for future increases in efficiency. On the other hand, in organic semiconductor lasers, all non-radiative processes should be avoided and high photoluminescence (PL) quantum efficiencies are required.

As an example of my work, I will present a spectroscopic study of two non-radiative processes in conjugated polymers. The first demonstrates how internal conversion is coupled with molecular conformational reorganization. In general, conformational change occurs on a timescale defined by the energy of the main vibrational mode and the rate of energy dissipation. Typically, for a conformational change such as a twist around the backbone of a conjugated molecule, this occurs on the tens of picoseconds timescale. However, I will demonstrate that in certain circumstances the molecule, in this case an oligofluorene, can change conformation over two orders of magnitude faster (that is sub-100 fs) in a manner analogous to inertial solvent reorganization demonstrated in the 1990s. Theoretical simulations demonstrate that non-adiabatic transitions during internal conversion can efficiently convert electronic potential energy into torsional kinetic energy, providing the ‘kick’ that prompts sub-100 fs torsional reorganization.

The second example is of singlet fission in a conjugated polymer. Singlet fission is a spin-allowed process in organic semiconductors whereby a singlet exciton separates into two spatially distinct triplet excitons. Singlet fission has been recently extensive studied in small molecule systems such as pentacene owing to its potential for solar cells. Unfortunately in these systems, singlet fission efficiency is highly dependent on intermolecular packing, making implementation in solar cells problematic. To overcome this problem, we demonstrate intrachain singlet fission in the solution-processable and stable conjugated polymer poly(thienylene-vinylene). I will discuss the mechanism of singlet fission in this polymer and the potential for its use as a sensitizer.

This talk is part of the Special Departmental Seminars series.

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