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
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

FFJscVoc
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
3106 nm2
(b) Interfacial area
1106 nm2
(c) Interfacial area
0.2106 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
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