Photovoltaics (PV) 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 twenty 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, Figure1.  

In this talk I will outline the basic physics of singlet fission and how it could be used to create a new generation of photovoltaics that can overcome thermalisation losses and could break through the Shockley-Queisser limit. The key quantum mechanical dynamics [3,4] underpinning singlet fission will be discussued and how then enable fast and energy efficent carrier multiplciation. New resonant energy transfer schemes [5], combining organic and inorganic semiconductors will be introudenced which could allow singelt fission to be harnessed within device architectures compatible with current inorganic PV. These could allow the efficiency of silicon PV to be raised from 29% to 38%, without the need to multijunction cells.

[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)

[3] Musser et al., Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission, Nature Physics, (2015)

[4] Bakulin, et al., Nature Chemistry, (2015)

[5] Tabachnyk et al., Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals. Nature Materials, 13, 1033-1038, (2014)