Transcript Monte Carlo Modelling of Exciton Diffusion in Polyfluorenes
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
• Lecture 2:
Modelling excitonic solar cells An excellent textbook on all types of solar cells is P W ürfel
Physics of Solar Cells
Wiley-VCH 2 nd 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
http://www.sciencedaily.com/releases/2008/02/080206154631.htm
•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)
Organic Photovoltaic & Display Devices
LUMO
Photovoltaic Device
Exciton electrons holes Interface HOMO These are often made from blends of an electron and a hole conductor MRS bulletin 1
Display Device
Prototype of Flexible OLED Display driven by Organic TFT LUMO holes Exciton electrons HOMO
Performance measures
Power conversion efficiency depends on • Short circuit current density
J SC
• Open circuit voltage
V OC
• Fill factor
FF J
dark FF = max(JV)
J SC V OC V OC
FFJ sc V oc P in
max(
JV
)
P in J SC
max(JV) illuminated
V
Excitonic solar cells
• all organic: polymer and/or molecular • hybrid organic/inorganic • dye-sensitized cell
Organic solar cell operation
cathode anode
F
Exciton Migration in photovoltaics Electrode Exciton hopping between chromophores e h + 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)
Snaith 3 , Peumans 4 (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
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 TiO 2 nanoparticle
energy cb Energetics of injection from sensitizer dye lumo homo redox vb TiO 2 electrolyte Pt
Equilibrium in the Dark electron energy dye SnO 2 (F) TiO 2 redox system Pt Electron Fermi level
Photostationary State under Illumination (open circuit) energy injection dye back reactions electron quasi Fermi level qU photo redox Fermi level SnO 2 (F) TiO 2 redox system Pt
Competition between electron collection and loss by reaction with tri-iodide Electrons lost by transfer to I 3 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
t
Ie
x
generation
D n
n
2
x
2 transport 0
n
back reaction with I 3 n = 1/ k cb [I 3 ] The continuity equation for free electrons in the cell (illumination from anode side)
Shunting via the conducting glass substrate TiO 2 cb surface states O O vb O electrolyte Negligible at short circuit Increases exponentially with forward bias substrate
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
L n
D
n n
The Electron Diffusion Length
D n
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 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
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
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References
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 Br édas et al Chem Rev 104 , 4971 (2004) J Kirkpatrick, J Nelson J Chem Phys 123 , 084703 (2005)