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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 • Lecture 1: Excitonic solar cells • Lecture 2: Modelling excitonic solar cells An excellent textbook on all types of solar cells is P Würfel Physics of Solar Cells Wiley-VCH 2nd Edition 2009 Can be obtained in paperback For animations of organic device applications see http://www.bath.ac.uk/news/multimedia/?20070417 Linked from the Modecom website http://www.modecom-euproject.org/publicns.htm How an Si solar cell works www.soton.ac.uk/~solar/intro/tech6.htm Polymer blend solar cells •Created by blending together two semiconducting polymers •Thin, lightweight and flexible •Can be integrated into other materials http://www.sciencedaily.com/releases/2008/02/080206154631.htm •Very cheap to manufacture and run (potential for less than 1 $/W) •Short energy payback time (less than one year) Organic Photovoltaic & Display Devices Photovoltaic Device Exciton LUMO electrons holes Interface HOMO These are often made from blends of an electron and a hole conductor MRS bulletin1 Display Device Prototype of Flexible OLED Display driven by Organic TFT electrons LUMO HOMO holes Exciton Performance measures Power conversion efficiency depends on • Short circuit current density JSC • Open circuit voltage VOC • Fill factor FF J dark FF = max(JV) JSC VOC FFJ scVoc max(JV) Pin max( JV ) Pin JSC VOC illuminated V Excitonic solar cells • all organic: polymer and/or molecular • hybrid organic/inorganic • dye-sensitized cell Organic solar cell operation anode cathode F Hole conductor Electron conductor Exciton Migration in photovoltaics Electrode Exciton hopping between chromophores eh+ Electrode Charge separation Disordered morphology Create a range of morphologies with different feature sizes using an Ising model Periodic boundary conditions in y and z Reproduced from McNeill, Westenhoff, Groves, J. Phys. Chem. C 111, 19153-19160 (2007) (a) Interfacial area 3106 nm2 (b) Interfacial area 1106 nm2 (c) Interfacial area 0.2106 nm2 Snaith3, Peumans4 Rods •Theoretically very efficient, but very difficult to make Reproduced from Chen, Lin, Ko; Appl. Phys. Lett. 92 023307 (2008) Gyroids •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? Dye-sensitised solar cells Sony Flower power: Lanterns powered by dye-sensitized cells G24i cells incorporated in sails: Nantucket race week 2008 Light harvesting adsorbed dye layer TiO2 nanoparticle Energetics of injection from sensitizer dye energy lumo cb redox homo vb TiO2 electrolyte Pt Equilibrium in the Dark electron energy dye Electron Fermi level SnO2 (F) TiO2 redox system Pt Photostationary State under Illumination (open circuit) energy injection dye electron quasi Fermi level back reactions qUphoto redox Fermi level SnO2(F) TiO2 redox system Pt Competition between electron collection and loss by reaction with tri-iodide Electrons lost by transfer to I3- ions Electron transport to contact electron transport by field-free random walk Electron transport and ‘recombination’ screening by the electrolyte eliminates internal field so no drift term Ignore trappping/detrapping for stationary conditions n n n n0 x Ie Dn 2 t x n 2 generation transport back reaction with I3- n = 1/kcb [I3-] The continuity equation for free electrons in the cell (illumination from anode side) Shunting via the conducting glass substrate TiO2 cb O surface states O vb O electrolyte substrate Negligible at short circuit Increases exponentially with forward bias Multiple trapping/release of electrons slows diffusion conduction band Energy empty traps band gap full traps Trap occupancy depends on light intensity A Key Cell Parameter Ln Dn n The Electron Diffusion Length Dn is the electron diffusion coefficient n is the electron lifetime 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 1st generation Si solar cells • It is important to understand the details of the operation of excitonic cells before these cells can be exploited Acknowledgements Stavros Athanasopoulos Diego Martinez Pete Watkins Jonny Williams Thodoris Papadopoulos Robin Kimber Eric Maluta Funding • European Commission FP6 • UK Engineering and Physical Sciences Research Council • Royal Society • Cambridge Display Technology • Sharp Laboratories of Europe References 1. 2. 3. 4. 5. 6. 7. 8. 9. Reviews in MRS bulletin Jan 2005 30 10-52 (2005) A B Walker et al J Phys Cond Matt 14 9825 (2002) A C Grimsdale et al Adv Funct Mat 12, 729 (2002) D Beljonne et al Proc Nat Acad Sci 99, 10982 (2002) G Lieser et al Macromol 33, 4490 (2000) E Hennebicq et al J Am Chem Soc 127, 4744 (2005) L M Herz et al Phys Rev B 70, 165207 (2004) J-L Brédas et al Chem Rev 104, 4971 (2004) J Kirkpatrick, J Nelson J Chem Phys 123, 084703 (2005)