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

1.

2.

3.

4.

5.

6.

7.

8.

9.

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)