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Excitonic solar cells: New Approaches to Photovoltaic Solar Energy Conversion. Alison Walker Department of Physics University of Bath, UK. Mod elling E lectroactive Co njugated Materials at the M ultiscale. Lecture scheme. Lecture 1: Excitonic solar cells
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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 • 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
Photovoltaic Device Exciton LUMO electrons holes HOMO Interface These are often made from blends of an electron and a hole conductor MRS bulletin1
electrons LUMO HOMO Exciton Display Device Prototype of Flexible OLED Display driven by Organic TFT holes
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
Excitonic solar cells • all organic: polymer and/or molecular • hybrid organic/inorganic • dye-sensitized cell
Organic solar cell operation anode cathode F
Exciton Migration in photovoltaics Electrode Exciton hopping between chromophores Electron conductor e- h+ Hole conductor Electrode
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)
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?
Sony Flower power: Lanterns powered by dye-sensitized cells G24i cells incorporated in sails: Nantucket race week 2008
Equilibrium in the Dark Electron Fermi level
Photostationary State under Illumination (open circuit) energy injection electron quasi Fermi level back reactions redox Fermi level
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 transport back reaction with I3- generation tn= 1/kcb [I3-] Thecontinuity equationfor free electrons in the cell (illumination from anode side)
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
Multiple trapping/release of electrons slows diffusion conduction band Energy empty traps full traps band gap Trap occupancy depends on light intensity
A Key Cell Parameter The Electron Diffusion Length Dnis the electron diffusion coefficient tnis 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 • 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)