Excitonic Solar Cells: 

The Challenges of Efficiency and Durability
 

Garry Rumbles 

Chemical and Biosciences Center 

NREL 

Golden, Colorado
 

NREL/PR-270-43347 

Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, California 
Or. 

“Convince an old skeptic like me that OPV has a future.” 

• Do not use anecdotal evidence. 

– e.g. Organic Light-Emitting Diodes, Liquid Crystal Displays, Conventional 

Photographs, Photocopiers, Car paint. 

• No comparison with other thin-film technologies. 

• From the viewpoint of basic science . 
q

• How efficient is it? 

• How long does it last? 

• How big is it? 

• How many can we make? 

• How much do they cost? 

• Can they be made sustainable? 

• Do they contain toxic components? 

• How much energy required to build? 

• How do they work? 
•	 Examples of Excitonic Solar Cells (XSC) 

–	 Dye-sensitized (Grätzel) solar cell (DSSC). 

–	 Two-layer, planar (Tang) solar cell. 

–	 Bulk heterojunction (Sariciftci and Heeger) solar cell. 

–	 Hybrids. 

•	 Why ‘Excitonic’? Coupled/correlated electron-hole pairs. 

–	 Absorbed photon produces a neutral excitation, not free carriers. 

Therefore, a dissociation interface is required. 
Dye-sensitised TiO2 solar cell Blended (bulk heterojunction) OPV 

O’Regan and Grätzel, Nature, 353, 737 (1991) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. H

Two-layer OPV cell Science 270, 1789 (1995)

C.W.Tang, APL, 48(2), 183 (1986) 

• Large titania surface area to • Solution processible. 

increase absorption of adsorbed • Planar and simple. • Short exciton diffusion length

dye. 

• Vacuum deposited molecular films. • Ultrafast and efficient exciton • Ultrafast and efficient electron dissociation. injection from dye to titania. • Absorption by both layers.

• Redox couple to transport holes.

N

N

Ru

N

N

N

C

S

N

C

S

HOOC

COOTBA

COOH

COOTBA

N N N

N

N

N

N

N

Cu

N

O

N

N

O

N

n

O

O

O

O

N719

PV

CuPc

MEH-P

PCBM
j() 

•	 Solution processible. 

– Both the PEDOT:PSS and Active layer can be deposited from solution. 

(e.g. Spin, Spray, Ultrasonic, Ink-jet, Doctor blade) 

•	 Short exciton diffusion lengths. 

–	 Requires two components of the Active layer, the Donor and Acceptor, 

to phase separate and excitons not have to travel more than 10 nm. 

•	 Ultrafast and efficient exciton dissociation. 

–	 Interface between Donor and Acceptor must efficiently dissociate 

exciton and inhibit recombination. 
ypy(yp)

S

C6H13

n S

C6H13

S

C6H13

P3HT

O

O

P

PEDOT

PSS
• P3HT absorbs photon. 

• Exciton diffuses to interface. 

• Exciton dissociates. 

• Carriers diffuse, driven by chemical potential, to electrodes. 

– Electrons through PCBM network to cathode. 

– Holes through P3HT phase to anode. 
y 

•	 P3HT absorbs further to the red, with optical bandgap at < 2 eV. 

–	 But Short Exciton lifetime, τ, ~ 300 - 500 ps. 

•	 Exciton migrates to interface 

–	 But Diffusion length < 10 nm. Hence blend. 

•	 Exciton dissociates to electron and hole at interface with PCBM. 

–	 Ultrafast, 100% yield, minimal recombination. 

•	 Carrier recombination is minimal. 

–	 Why? 

•	 Holes transport through P3HT. 

–	 Hole mobility, μh, > 10-3 cm2/Vs 

•	 Electrons transport (hopping) through PCBM. 

–	 Electron mobility, μe, > 10-3 cm2/Vs 

•	 Summary: 

–	 P3HT is a good absorber, it is ordered and therefore exhibits good hole 

(and possibly exciton) mobility. 

–	 PCBM forms an ideal donor-acceptor interface with P3HT. 
Now Realistic How? 

Voc 

(V) 0.62 0.65 

Jsc 

(mA/cm2) 10 14 

Increase absorption 

(OD 2), reduce 

recombination 

FF 

(%) 67 75 

Reduce series resistance 

from 

50 to 15 Ω

Efficiency 

(%) (4.2)* 6.8 

p

* Yang Yang UCLA



Look for ‘report from Degradation Summit’. Defining standards for OPV
p py 
y

• Oxygen, Water and UV light. 

– Reduction 

– Sensitization 

• Contacts 

– Acidity of PSS 

– Delamination 

– Reducing metals 

• Oxidising species 

– Polaron+

• Reducing species 

– Polaron-, PCBM-

• Adventious dopants!

• Thermal 

– Internal conversion

Extrinsic effects Intrinsic effects
Now Realistic Plan* How? 

Voc 

(V) 0.62 0.65 1.1 Lower LUMO (and HOMO) 

Jsc 

(mA/cm2) 10 14 25 Shift absorption to red 

FF 

(%) 67 75 80 Improve transparent electrode 

Efficiency 

(%) (4.2) 6.8 22 Larger carrier mobilities 

p

* Sean Shaheen. Department of Physics, DU and NREL.
Voc
py 

Voc

5.5 eV

3.2 eV

Polymer1
py 

Voc

5.5 eV

4.5 eV

Polymer2
yg 

• Exciton binding energy 

• Still exciton dissociation? 

• Still minimal recombination (Marcus theory)? 

• Same morphology! 

– Mobilities same?

• Any successes?
gg py

S

S

S

S C16H33

C16H33

n

T0T0TT16

N

S

N

S S

n

ZZ50 (PCPDTBT)

S

S

S

S

n

C12H25

C12H25 T12T12TT0

g 
p
y p

• How efficient is it? 

• How long does it last? 

• How big is it? 

• How many can we make? 

• How much do they cost? 

• Can they be made sustainable? 

• Do they contain toxic components? 

• How much energy required to build? 

• How do they work? 
pyp
• Ultrasonic spray deposition • Several ~ 3% devices • Thick active layers - spun at 

200 rpm 

• Reducing electrode roughness 

is key 

• PEDOT can be eliminated 

40

200

Current Density (mA/cm

2

)

-1.0-0.50.00.51.0

Voltage(V)

ITOReference

η=3.5%

J

sc

=11.09

Vo

c

=570.6

FF=55%

60

40

200

Current Density (mA/cm

2

)

-1.0-0.50.00.51.0

Voltage(V)

SWCNT

η=3.5%

J

sc

=10.65

Voc=571.8

FF=57.6%

15

1050

-5

Current Density (mA/cm

2

)

-1.0-0.50.00.51.0

Voltage(V)

NOPEDOT

η=1.05%

J

sc

=5.4

V

oc

=491.1

FF=39.2%

40

30

20

10

IPCE

400500600700800

Wavelength(nm)

CNT

ITOref
x 

pp

Kim et al., Adv. Mat. 18, 572 (2006)
p
y 

• Increase efficiencies 

incrementally for P3HT:PCBM up 

to a limit of ~7%. 

• Provides a foundation on to 

which new concepts can be 

tested. 

• Learn how bulk heterojunction 

works. Understand role of 

interface states. 

• Test new electrodes. 

• Test new acceptors. (Remember 

unique property of C60). 

• Chemistry is the tool to pave the 

way forward. 

• New red absorbers with more 

tolerance to attack by oxygen/

water. 

• Gain a fundamental 

understanding of how devices 

work. 

• Foundation for third-generation 

concepts. 
g

• Nikos Kopidakis 

• Sean Shaheen 

• David Ginley 

• Jao van de Lagemaat 

• Teresa Barnes 

• Jeff Blackburn 

• Rob Tenent 

• Brian Gregg 

• Dana Olson 

• Andrew Ferguson 

• Matt Reese 

• Tom Reilly 

• Anthony Morfa 

• Matt White 

• Xerxes Steirer 

• Alex Nardes 

• Ben Rupert 

• Erkan Kose 

• Will Rance 

Funding: US DOE. Office of EERE. Office of Science.