1

Approaching or exceeding the

limit in organic photovoltaics

Xiaoyang Zhu

$$ NSF, DOE

2

How to escape the Coulomb trap?

• Energetic alignment

• Hot CT excitons

• Gradients

• ……. 1.0

0.5

0.0

6004002000-200

1.0

0.5

0.0

1.0

0.5

0.0

1.0

0.5

0.0

150100500

hn1 =

4.17 eV

4.38 eV

4.59 eV

4.77 eV

Bin

din

g e

nerg

y

1s

1s

2s

3s

IPS

Pump-probe delay (fs)

MRS Bulletin (2010)

Acct. Chem. Res. (2009)

3

Model: phthalocyanine/fullerene

Loren Kaake

(UCSB)

A. Jailaubekov

(Cymer)

Nature Mater.

TR-SHG

4

Direct & indirect formation of interfacial

CT excitons

• For initiation exciton in CuPc,

interfacial CT exciton forms in 80-

170 fs.

• Direct exciton in the optical gap

(hv ~ 1.5 eV) leads to the

instantaneous formation of

interfacial CT excitons.

5

Hot CT exciton cooling sets the time

limit in charge separation at interfaces

e Wai-lun Chan

(U. Kansas)

John Tritsch

Energ

y

Distance to

interface

TR-2PPE

6

How to escape the Coulomb trap?

• Hot CT excitons

• Gradient, gradient, gradient

• ……

http://echwaluphotography.wordpress.com/

“Mind the gap!”

7

Exceeding the Shockley-Queisser Limit

The SQ limit:

1 photon in,

1 e-h pair out.

J. Appl. Phys. 32

(1961) 510.

Multiple excitons: 1

photon in, 2 or more

e-h pairs out

Science 334 (2011) 1541

Nature Chem. 4 (2012)

Hot carrier cell: Share

the hot electron or hot

hole energy

Science 328 (2010) 1543

Nano Lett. 12 (2012) 1588

h

8

Implementation of singlet fission in solar cells

hv

So

S1

T1

hv2So

S1 +

Chromophore 2 Chromophore 1

Leo et al. JAP 106, 064511 (2009).

Baldo et al. Nano Lett. 11, 1495 (2011).

Greenham, Friend, et al. Nano Lett. 12, 1053 (2012);

Nature Comm. 3, 1019 (2012).

Hanna & Nozik, JAP 100, 074510 (2006)

Paci, et al. JACS 128, 16546 (2006).

So + S1

1TT( ) T1 +T1

9

Singlet Fission & multi-electron transfer

hv

Wai-lun Chan

(U. Kansas)

John Tritsch

Science 334 (2011) 1541; Nature Chem. 4 (2012); JACS (2012) pending

So + S1

1TT( ) T1 +T1

??

10

Seeing the multiexciton state in pentacene

1.83 eV T1

0.86 eV S0

T1 2x ET1 - ES1

= -0.11 eV

S1

Time-resolved two-photon photoemission Science 334 (2011) 1541

S1 ÛME[ ] ME* T1 +T1

hv1 = 2.15 eV; hv2 = 4.65 eV;

100 fs

lifetime

11

The coherent mechanism is also responsible

for endothermic singlet fission in tetracene

hv = 2.32

2.3 eV S1 T1 1.25 eV

How is the energy barrier

overcome in singlet fission?

D 2T1 - S1( ) » 0.2 eV

Nature Chem. 4 (2012) S1 ÛME[ ] lives for 7 ps.

12

There is no energy relaxation from ME to 2T1 in

tetracene

S1

2xT1

ME

Nature Chem. 4 (2012)

13

The higher energy ME state is accessed by coherent

electronic coupling on an ultrafast time scale.

S1

2T1 ME

Tetracene

Pentacene

S1 2T1

ME

Interaction with the

bath: dephasing &

entropic driving force

14

Harvesting singlet fission for solar energy

conversion: one vs. two electron transfer

JACS pending

15

Singlet fission occurs faster than electron

transfer at the pentacene/C60 interface

Singlet Multiexciton

+ triplets

Science 334 (2011) 1541

16

Singlet fission is not competitive with electron

transfer at the tetracene/C60 interface

SF ET

2 × T1

7 ps C60

0.5 ps

26 ps

[S1 ME]

JACS pending

17

Simultaneous ET to LUMO & LUMO+1 of C60

CT2 (LUMO+1), 60%

CT1 (LUMO), 40%

JACS pending

18

Ultrafast harvesting from the multiexciton state in the

quantum superposition may increase fission yield

p12

Singlet exciton

cooling, decay,

1 e transfer

...

Multiple exciton

localization,

cooling,

2 e transfer,

…

Nbi

Nex= P1®2

g 2

g1

Shabaev, Efros, Nozik

Nano Lett. 6, 2856 (2006)

g 1 g 2

2xET

S1 ÛME[ ] ME* T1 +T1

2xET 2xET 1xET

g 2g 1

19

Design principles for harvesting singlet fission in OPVs

• Singlet fission faster than electron

transfer, as is the case for

pentacene but not tetracene.

• For slow SF, limiting interfacial

charge transfer, but recombination

and T - T annihilation is a concern.

• Energy selective electron transfer

from the ME state: need designer

electron acceptors!

• Enhancing the carrier multiplication

yield by electron transfer from the

ME state.

20

• Escaping the Coulomb trap with hot CT excitons –

mind the gap!

Phys. Rev. Lett. 101 (2008) 196403; Acct. Chem. Res. 42 (2009) 1779;

Nature Mater. pending.

• Exceeding the limit in OPVs with singlet fission –

harvesting triplets or the quantum coherence

Science 334 (2011) 1541; Nature Chem. 4 (2012); JACS pending

Conclusions