School of Natural and Environmental Sciences

Event items

Harnessing singlet exciton fission to break the Shockley–Queisser limit

NECEM Seminar - Dr Akshay Rao, University of Cambridge

Date/Time: Wednesday 16 January 2019, 13:00 - 14:00

Venue: Barbara Strang Teaching Centre, Room 1.46

Abstract

Photovoltaics made of silicon are the most widely deployed in the world today. Despite rapid reductions in cost over the past decade, the efficiencies of the best silicon cells have not improved by more than 2% in the last 20 years.

This is largely because silicon PV have been well optimized and is close to the Shockley-Queisser limit on efficiency [1], that applies to all single-junction solar cells, and is 29% for an ideal silicon cell (bandgap 1.1 eV).

The main loss is such devices is thermalization, i.e. high-energy photons in the solar spectrum produce one electron-hole pair just as the absorption of lower-energy photons does, but the energy of photons in excess of the bandgap is lost as heat as the carriers relax to the band edges.

Singlet exciton fission is a carrier multiplication process in organic semiconductors (OSCs) [2]. Within OCSs the absorption of a photon leads to the formation of a bound electron-hole pair, an exciton. The photogenerated exciton is in a spin-0 singlet configuration.

However, these systems also posses a lower-energy spin-1 triplet exciton state and under the right conditions the initially photogenerated singlet exciton can convert to a pair of triplet excitons, a process termed singlet fission.

In this talk I will outline the basic physics of singlet fission. I will present results from ultrafast spectroscopy studies that elucidate that quantum mechanical dynamics [3-4] of this process and discuss the transfer of the triplet excitons formed vis fission to inorganic nanocrystals [5] and how novel organic-inorganic nanostructures could be used to create a new generation of photovoltaics that can overcome thermalisation losses and could break through the Shockley-Queisser limit [6].

References

[1] Shockley, W. & Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. Journal of Applied Physics 32, (1961).
[2] Wilson et al., Singlet Exciton Fission in Polycrystalline Pentacene: From Photophysics toward Devices. Accounts of Chemical Research, 46, 1330, (2013), 10.1021/ar300345h.
[3] Musser et al., Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission, Nature Physics, (2015), 10.1038/nphys3241
[4] Bakulin et al., “Real-Time Observation of Multiexcitonic States and Ultrafast Singlet Fission Using Coherent 2D Electronic Spectroscopy”, Nature Chemistry, doi:10.1038/nchem.2371.
[5] Tabachnyk et al., Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals. Nature Materials, 13, 1033-1038, (2014), 10.1038/nmat4093
[6] Rao, Nature Reviews Materials, (2017) doi:10.1038/natrevmats.2017.63

Biography

Akshay Rao is group leader, EPSRC Early Career Fellow and Winton Advanced Research Fellow at the Cavendish Laboratory, University of Cambridge. His research interests include the electronic and optical properties of molecular semiconductors, quantum dots and 2D semiconductors.

He has worked extensively on organic and nanostructured inorganic semiconductors, using ultrafast spectroscopy to elucidate the how charge delocalisation, ballistic motion and vibronic coupling underlie the physics of these systems.

Akshay received his undergraduate degree from St Stephen’s College, University of Delhi in 2006 and his MSc from the University of Sheffield in 2007. He received his PhD from the University of Cambridge in 2011, working in the group of Prof. Sir Richard Friend.

From 2011 to 2014 he held a Research Fellowship in Cambridge. As of October 2014, he leads an independent research group (www.rao.oe.phy.cam.ac.uk) at Cambridge. His work has been recognized with the award of the 2017 Henry Moseley Medal of the Institute of Physics, for outstanding early career contributions to experimental physics.