Events

Singlet Exciton Fission in Polyacenes: Photophysics and Photovoltaic Applications

December 13, 2011 at 3pm/34-401A

Mark Wilson
Optoelectronics Group, University of Cambridge

Abstract:
The development of novel technologies for harvesting solar energy is a major contemporary research effort in the physical sciences. However, the efficiency of any single-bandgap photovoltaic device under solar irradiation has a fundamental limit because sub-bandgap photons are not absorbed and the excess energy of super-bandgap photons is wasted as heat. An attractive method to circumvent this limit is to sensitize a ‘red-absorbing’ solar cell with a ‘blue-absorbing’ material which generates multiple electron-hole pairs. This is possible in some organic semiconductors via ‘singlet fission’, where a spin-singlet bound electron-hole pair (exciton) ‘splits’ to form two triplet excitons, each with roughly half of the singlet energy.

Although singlet fission has been historically observed in molecular crystals, we recently used transient absorption spectroscopy to demonstrate that it occurs rapidly (~70 fs) and efficiently (>85%) in easily-fabricated evaporated films of pentacene and that fission-generated triplet excitons undergo long-range diffusion (>40nm) and are dissociated at a pentacene/C60 heterointerface. These results are consistent with reported photon-to-electron quantum efficiencies that exceed 100% and have led us to fabricate a proof-of-concept photovoltaic device where an evaporated pentacene film absorbs visible light and generates pairs of triplets via fission, while a second layer of inorganic colloidal quantum dots generates charge from transferred triplets as well as directly-absorbed infrared photons. Further transient absorption measurements address the mechanism of exciton fission, as questions remain as to whether singlet fission is also rapid and efficient larger-bandgap acenes and whether fission is mediated by a ‘dark’ (one-photon inaccessible) multiexcitonic state.

Bio:
Hailing from Port Colborne, Canada, Mark received a B.A. (History) (2008), and a B.Sc. (2006) and M.Sc. (2008) in Engineering Physics from Queen’s University, Kingston. Working with Prof. James Fraser, his thesis concerned the ultrafast dynamics of photoluminescence from individual, air-suspended, single-walled carbon nanotubes. He is presently working towards a Ph.D. in Physics under the supervision of Sir Richard Friend at the University of Cambridge’s Cavendish Laboratory.