Excitonic solar cells: New Approaches to Photovoltaic Solar Energy Conversion Alison Walker Department of Physics University of Bath, UK Modelling Electroactive Conjugated Materials at the Multiscale

Lecture scheme Excitonic solar cellsLecture 1: Excitonic solar cells Modelling excitonic solar cellsLecture 2: Modelling excitonic solar cells An excellent textbook on all types of solar cells is P Würfel Physics of Solar Cells Wiley-VCH 2 nd Edition 2009 Can be obtained in paperback For animations of organic device applications see Linked from the Modecom website

How an Si solar cell works

Created by blending together two semiconducting polymers Thin, lightweight and flexible Can be integrated into other materials Very cheap to manufacture and run (potential for less than 1 $/W) Short energy payback time (less than one year) Polymer blend solar cells

Organic Photovoltaic & Display Devices

Photovoltaic Device Exciton holes electrons LUMO HOMO Interface MRS bulletin 1 These are often made from blends of an electron and a hole conductor

Display Device holes Exciton electrons LUMO HOMO Prototype of Flexible OLED Display driven by Organic TFT

Performance measures Power conversion efficiency  depends on Short circuit current density J SC Open circuit voltage V OC Fill factor FF FF = max(JV) J SC V OC J SC J V OC V max(JV) dark illuminated

Excitonic solar cells all organic: polymer and/or molecular hybrid organic/inorganic dye-sensitized cell

cathode anode F Organic solar cell operation

Electrode Electron conductor Hole conductor Electrode e-e- h+h+ Exciton hopping between chromophores Exciton Migration in photovoltaics

Charge separation

Reproduced from McNeill, Westenhoff, Groves, J. Phys. Chem. C 111, (2007) Create a range of morphologies with different feature sizes using an Ising model Periodic boundary conditions in y and z Disordered morphology

(a) Interfacial area 3  10 6 nm 2 (b) Interfacial area 1  10 6 nm 2 (c) Interfacial area 0.2  10 6 nm 2 Snaith 3, Peumans 4

Reproduced from Chen, Lin, Ko; Appl. Phys. Lett (2008) Theoretically very efficient, but very difficult to make Rods

Continuous charge transport pathways, no disconnected or ‘cul-de-sac’ features Free from islands A practical way of achieving a similar efficiency to the rods? Gyroids

Dye-sensitised solar cells

Sony Flower power: Lanterns powered by dye-sensitized cells G24i cells incorporated in sails: Nantucket race week 2008

Light harvesting

Energetics of injection from sensitizer dye

Equilibrium in the Dark Electron Fermi level

Photostationary State under Illumination (open circuit) energy electron quasi Fermi level redox Fermi level injection back reactions

Competition between electron collection and loss by reaction with tri-iodide Electron transport to contact Electrons lost by transfer to I 3 - ions electron transport by field-free random walk

generation transport back reaction with I 3 - continuity equation The continuity equation for free electrons in the cell (illumination from anode side) screening by the electrolyte eliminates internal field so no drift term Electron transport and ‘recombination’ Ignore trappping/detrapping for stationary conditions  n = 1/k cb [I 3 - ]

O O O cb vb substrate electrolyte surface states TiO 2 Shunting via the conducting glass substrate Negligible at short circuit Increases exponentially with forward bias

Energy conduction band full traps band gap Multiple trapping/release of electrons slows diffusion Trap occupancy depends on light intensity empty traps

The Electron Diffusion Length D n is the electron diffusion coefficient  n is the electron lifetime A Key Cell Parameter

Summary overall Excitonic solar cells are based on the creation of excitons in an organic absorber and their subsequent dissociation at an interface Excitonic cells can be all organic or hybrid organic-inorganic and can include a dye sensitizer The way excitonic cells work is quite different from the 1 st generation Si solar cells It is important to understand the details of the operation of excitonic cells before these cells can be exploited

Stavros Athanasopoulos Diego Martinez Pete Watkins Jonny Williams Thodoris Papadopoulos Robin Kimber Eric Maluta Acknowledgements

Funding European Commission FP6 UK Engineering and Physical Sciences Research Council Royal Society Cambridge Display Technology Sharp Laboratories of Europe

References 1.Reviews in MRS bulletin Jan (2005) 2.A B Walker et al J Phys Cond Matt (2002) 3.A C Grimsdale et al Adv Funct Mat 12, 729 (2002) 4.D Beljonne et al Proc Nat Acad Sci 99, (2002) 5.G Lieser et al Macromol 33, 4490 (2000) 6.E Hennebicq et al J Am Chem Soc 127, 4744 (2005) 7.L M Herz et al Phys Rev B 70, (2004) 8.J-L Brédas et al Chem Rev 104, 4971 (2004) 9.J Kirkpatrick, J Nelson J Chem Phys 123, (2005)