Singlet Exciton Fission: A Path to Break the Shockley-Queisser Limit on the Efficiency of Photovoltaics

DIPC Seminars

Speaker

Akshay Rao (Cambridge Univ., UK)

When

2016/09/13
14:00

Place

Donostia International Physics Center

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**Singlet Exciton Fission: A Path to Break the Shockley-Queisser Limit on the Efficiency of Photovoltaics** ** ** 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, Figure 1. 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. Key challenges, from understating the quantum mechanical dynamics **[3-4]** of singlet fission to fabricating novel organic-inorganic nanostructures that can harness this phenomena **[5]** , will be discussed. [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 _ _