Excitonic solar cells: New Approaches to Photovoltaic Solar Energy Conversion

1. 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

2. 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

3. How an Si solar cell works www.soton.ac.uk/~solar/intro/tech6.htm

6. Polymer blend solar cells • 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) http://www.sciencedaily.com/releases/2008/02/080206154631.htm

7. Organic Photovoltaic & Display Devices

8. Photovoltaic Device Exciton LUMO electrons holes HOMO Interface These are often made from blends of an electron and a hole conductor MRS bulletin1

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

10. J dark VOC max(JV) V illuminated JSC Performance measures Power conversion efficiency  depends on • Short circuit current density JSC • Open circuit voltage VOC • Fill factor FF FF = max(JV)JSC VOC

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

12. Organic solar cell operation anode cathode F

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

14. Charge separation

15. 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)

16. (a) Interfacial area 3106 nm2 (b) Interfacial area 1106 nm2 (c) Interfacial area 0.2106 nm2 Snaith3, Peumans4

17. Rods • Theoretically very efficient, but very difficult to make Reproduced from Chen, Lin, Ko; Appl. Phys. Lett. 92 023307 (2008)

18. Novel bicontinuous morphologies 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?

19. Comparison with other morphologies

20. Dye-sensitised solar cells

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

22. Light harvesting

24. Energetics of injection from sensitizer dye

25. Equilibrium in the Dark Electron Fermi level

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

27. 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

28. Electron transport and ‘recombination’ screening by the electrolyte eliminates internal field so no drift term Ignore trappping/detrapping for stationary conditions transport back reaction with I3- generation tn= 1/kcb [I3-] Thecontinuity equationfor free electrons in the cell (illumination from anode side)

29. Shunting via the conducting glass substrate TiO2 cb O surface states O vb Negligible at short circuit Increases exponentially with forward bias O electrolyte substrate

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

31. A Key Cell Parameter The Electron Diffusion Length Dnis the electron diffusion coefficient tnis the electron lifetime

32. 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

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

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

35. References • Reviews in MRS bulletinJan 200530 10-52 (2005) • A B Walker et al J Phys Cond Matt14 9825 (2002) • A C Grimsdale et al Adv Funct Mat12, 729 (2002) • D Beljonne et al Proc Nat Acad Sci99, 10982 (2002) • G Lieser et al Macromol33, 4490 (2000) • E Hennebicq et al J Am Chem Soc127, 4744 (2005) • L M Herz et al Phys Rev B70, 165207 (2004) • J-L Brédas et al Chem Rev104, 4971 (2004) • J Kirkpatrick, J Nelson J Chem Phys123, 084703 (2005)