1

Exciton dissociation in solar cells:

Xiaoyang Zhu

Department of ChemistryUniversity of Minnesota, Minneapolis

t (fs)

h!

3h!

e-

Pc

Bi

E, k

2

Acknowledgement

Organic semiconductors: Mutthias Muntwiler, Qingxin Yang

Inorganic quantum dots: William Tisdale, Kenrick Williams Prof. Eray Aydil, Prof. David Norris

Funding: NSF (DMR, MRSEC, NIRT) DOE (Solar Initiative) MN-IREE (Initiative for Renewable Energy & Environment)

3

Outline

• Introduction - Excitons & photovoltaics

• Charge transfer (CT) excitons on organic

semiconductor surfaces:

• Direct time domain observation of 2D polarons

• Extracting hot or multiple carriers from inorganic

quantum dots:

4

The conventional solar cell: p-n junction

UMN solar car

p

n Conduction band

Valence band

Fermi

e

h

• Light absorption & charge separation occur in same location

• Charge separation by build-in electric field

5

Excitonic solar cells

electrolyte

I-/I3-

transparent

anodecathode

Leschkies et al., Nano Lett (2007)

M Gratzel, Nature (2001),

DA

Tang, APL (1986)

Donor

Acceptor

e

h

6

The Coulomb barrier problem

+

-

Exciton: an electron-hole pair

-+

Charge transfer excitonat a D/A interface

Large binding energy

(>> kT) due to the low

dielectric constant!

D A

e2

-4!""or

Molecular or Frenkel excitonin an organic semiconductoror inorganic quantum dot

V =

7

What drives exciton dissociation?

Frenkel or

BEex

BECT

• How does a Frenkel exciton dissociate?

#LUMO > BEex

• How does a CT exciton

separate?

#µ, but …

8

Probing electron dynamics in the time domain:

femtosecond two-photon photoemission

Lindstrom, Zhu Chem. Rev. (06)

n = 1 image state, 2ML nonane/Cu(111)

e!

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

Bin

din

g E

nerg

y (

eV

)

40003000200010000

Pump-Probe Delay (fs)

9

How do charge carriers separate in an

organic heterojunction solar cell?

DA

10

Previous suggestion of “long distance”

geminate e-h pair in exciton dissociation

Morteani et al. PRL 92, 247402 (2004) Adv. Mater. 15, 1708 (2003)

11

Previous evidence for the need of excess

energetic driving force in exciton dissociation

Ohkita et al. JACS 130, 3030 (2008)

Nanocrystalline polythiophene/PCBM

Amorphous polythiophene/PCBM

Coulombically bound radical pair (BRP)

12

Charge transfer exciton: a toy model

D A

e

+ro

r

1.0eV

0.8

0.6

0.4

0.2

0.0

EB (

eV

)

100806040200

z (Å)

1s

1p 2s 1d

500-50 x (Å)

1s

1p

2s

13

Experimental realization of the toy model

for CT excitons

!

"Pc

= 5.3,"o

= 1.0 # " = 3.2

14

What is the binding energy of a CT exciton (geminate pair)?

How to overcome this binding energy in charge separation?

Image potential state as the electron

acceptor

(Geminate pair)

15

The model system: crystalline pentacene

thin films grown on Bi

Sadowski et al, Appl Phys Lett 2005

Thayer et al, Phys Rev Lett 2005

16

2PPE spectra: IPS & CT exciton

300

200

100

01.0 0.5

EB (eV)

Pc coverage (ML) 4.0 3.0

2.0

1.3 1.0

CT1s

IPS

1h!

(Bi)-1.2

3.0

3.45

0 eV

4.16 eV

h!1 = 4.16 eV

h!2 = 1.39 eV

Bi

17

Spatial distribution of IPS & CT exciton "

Nonane overlayer quenches bothIPS and the CT exciton state

|"|2 is on the surface

130 fs

70 fs

IPS shows dispersion &CT exciton state is localized.

meff = 1.5 me

18

Origin of charge transfer excitons

¯ =?¡ 1

?+ 1

(atomic units)!

[jackson, classical electrodynamics]

E

xy

z

E

x

! =" #1

" +1$ =

2

" +1

ro

r

19

Simulation results: the delocalized image band

?= 5

z0 = ¡ 7ºA

Lxy = 800ºA

1s

1p

2s

2p

20

Simulation results: the localized CT excitons

1s 1d1p 1f

2p 2f

3s 3p 3d 3f

2s 2d

...

...

angular momentum lz

nodes

E = -0.846 eV -0.657 -0.552 -0.493

-0.593 -0.528 -0.484 -0.453

-0.503 -0.473 -0.449 -0.433

100 Å

21

Simulation: CT exciton binding energies

energy scale ECTE

– internal Coulomb energy ofthe exciton

– 0 = bottom of IPS band(continuum) !

1s 0.450 eV

1p 0.265 eV

2s 0.197 eV

1d 0.156 eV

2p 0.132 eV

3s 0.107 eV

?= 5:3

zh = ¡ 2:7ºAhzi 1s = 3:6ºA

hzi 1s ¡ zh = 6:3ºA

20

10

0

z (Å

)

1s|_|_

1.0

0.8

0.6

0.4

0.2

E CTE

,1s

(eV

)

-6 -4 -2 0zh (Å)

3

4

5

6_

0.4

0.2

0.0

E CTE

(eV

)

6040200x (Å)

1s

1p2s

1d2p3s

-10

0

10

y (Å

)

1s

-10

0

10y

(Å)

1p

-10

0

10

y (Å

)

-10 0 10x (Å)

2s

a b

c

22

Seeing hot excitons at higher h!

UV pump from HOMO

20

10

0

z (Å

)

1s|_|_

1.0

0.8

0.6

0.4

0.2

E CT

E,1

s (

eV

)

-6 -4 -2 0zh (Å)

3

4

5

6_

0.4

0.2

0.0

E CTE

(eV

)

6040200x (Å)

1s

1p2s

1d2p3s

-10

0

10

y (Å

)

1s

-10

0

10

y (Å

)

1p

-10

0

10

y (Å

)

-10 0 10x (Å)

2s

a b

c

400

300

200

100

0

kcounts/s

3.63.43.23.02.82.62.4

eV

1s

2s

IPS3s

UV only or long pump-probe delay

HOMO 1p forbidden

1 ML pentacene

Pump: 4.38 eV

Probe: 1.46 eV

23

3.6

3.4

3.2

3.0

2.8

E -

EF

erm

i (eV

)

4002000-200

Pump-probe delay (fs)

CT excitons - the complete picture

Pump: 4.38 eV; Probe: 1.46 eV

1 ML pentacene

1s

2s

3s+

IPS6x10

5

5

4

3

2

1

0

Photo

ele

ctr

on inte

nsity (

c/s

)

3.83.63.43.23.02.82.62.4

E - EFermi (eV)

0 fs

160 fs

300 fs

! t =

CT1s

CT2s

IPS

CT3s + ...

24

All CT excitons show no dispersion:

photo-ionization destroys the quasi-particle!

3.6 3.6

3.4 3.4

3.2 3.2

3.0 3.0

2.8 2.8

2.6 2.6

2.4 2.4

E -

EF

erm

i (e

V)

140135130

Detection angle

Dispersion measurementcarried out at h!1 = 4.38eV & h!2 = 1.46 eV; timedelay = 80 fs.

1 ML pentacene

Ekin=

hk( )2

2mIPS

3s

2s

1d?

1s

m= 1.5 me

25

>20 nm pentacene/Si(111)

Pump: 4.59 eV; Probe: 1.53 eV

6x105

5

4

3

2

1

0

Photo

ele

ctr

on inte

nsity (

c/s

)

4.24.03.83.63.43.23.02.8

E - EFermi (eV)

! t / fs

300

0

400

CT1s

CT2s

CT3s + ...

IPS

4.0

3.8

3.6

3.4

3.2

E-E

F (

eV

)

6004002000-200

Pump-probe delay (fs)

3

2

1

0x10

5

CT excitons on thick pentacene films

26

Why do CT excitons on pentacene decay so fast?

Short lifetimes: resonance with LUMO+x bands

3.6

3.4

3.2

3.0

2.8

E-E

F (

eV

)

5004003002001000-100-200

Pump-probe delay (fs)

4.0

3.8

3.6

3.4

3.2E

-EF (

eV

)

5004003002001000-100-200

Pump-probe delay (fs)

1 ML > 10 ML

110 fs

60 fs

40 fs

90 fs

120 fs

27

Surface CT excitons are general: tetracene

4.2

4.0

3.8

3.6

3.4

E-E

F (

eV

)

8006004002000-200

Time delay (fs)

h!1=4.77 eV

4.2

4.0

3.8

3.6

3.4

E-E

F (

eV

)

8006004002000-200

Time delay (fs)

h!1=4.59 eV

4.2

4.0

3.8

3.6

3.4

E-E

F (

eV

)

8006004002000-200

Time delay (fs)

h!1=4.28 eV

!20 nm tetracene/Si(111)!Tuning h! to reach eachresonance

CT1s

IPS (n=1)

CT1sIPS (standing up)

IPS (lying down)

CT1s

28

The difference between surface CT

excitons and the toy D/A model

2s

1.0eV

0.8

0.6

0.4

0.2

0.0

EB (

eV

)

100A806040200

z (Å)

1s

2s

1p

1d

IPS band bottom

2s

1.0eV

0.8

0.6

0.4

0.2

0.0

EB (

eV

)

100806040200

z (Å)

1s

1p 2s 1d

CT exciton at surfaceCT exciton at D/A interface

29

The “hot” mechanism for charge carrier

separation in organic photovoltaic

• Less CT binding energy• Longer lifetime• More delocalization • Better electronic coupling (energy denominator!)

n > 1

Conclusion:Charge separation at aD/A interface in OPVmust involve “hot” CTexcitons!

30

Other interesting systems……

Donor

Acceptor

Donor

Acceptor

31

Summary 1

• Charge transfer excitons are seen on thecrystalline pentacene and tetracene surfaces.

• Hot CT excitons are necessary in organicheterojunction photovoltaic.

32

Challenge to theory/simulation:breaking up the quasiparticle

D A

+ $e

kD

kA

"( , )k $

k = kA - kD

What is the role of delocalization

on interfacial charge separation?

33

Polarons & localization

Electronic-nuclear coupling Defects

Energetics of self-trapping:

!

"EST = Epol + Eloc + Ebond

Polarization gain(negative)

Localization penalty(positive)

!

" 0.5 # BW

BW

!

"p # "x $ h

E

k

LUMO band

Appl. Phys. Lett. 05

34

4.2

4.0

3.8

3.6

3.4

8006004002000-200

4.77 eV

Tetracene/Silicon(111), 150 K, thick film (> 20 nm)

n = 1

n = 2

1s1

1s2

Energy relaxation negligible:No lattice polaron formation.

Self trapping not observed.

35

2D polarons that are absent in 3D

Model: NaCl/Cu(111)

Not an organic, but same physics…

Cl-

Cl-

Na+

Na+

Cl-

Cl-

Na+

Na+

ee

36

Polaron formation in time-energy domain

3ML NaCl/Cu(111)

Interface polaron

image polaron

Pump-probe delay (fs)

NaCl conduction band (CB)#trap = 62 ± 8 fs, #decay = 120 ± 20 fs$E = 60±10 meV

NaCl interface band (IB)#trap = 135 ± 47 fs, #decay = 230 ± 20 fs$E = -85 ± 10 meV

Muntwiler & Zhu, Phys. Rev. Lett. 07.

37

Where is the |%|2 ? Effect of an overlayer

3ML NaCl/Cu(111) 1 ML nonane/3ML NaCl/Cu(111)

“bulk” polaron

NaCl/Cu Interface polaron

image polaron

38

Polaron formation in the momentum domain

Parallel dispersions 3 ML NaCl/Cu(111)

Conduction band

Interface band

39

Why does an electron polaron form in a

2D NaCl film but not in 3D NaCl?

Cl-

Cl-

Na

Na+

Cl-

Cl-

Na+

Na+

e

!

"EST = Epol + Eloc + Ebond

Bulk crystal

-1.0 eV

-0.5 eV

Conduction bandwidth = 5.0 eV

2.5 eV

!

"EST#1.0 eV

Thin film (1 unit cell thick)

-1.0 eV 1.7 eV

-1.0 eV

!

"EST# $0.3eV

More deformable and narrower bandwidth in a thin film.

Delocalization localization!

Does it apply to thin films in general? Quantum wells?

40

Conclusion 2

•TR-2PPE of NaCl/Cu(111) providestime-domain evidence of a 2D polaron.

•Polarons or self-trapped excitons mustbe important to exciton dissociation atorganic D/A interfaces (not directlyobserved in the time domain yet…..)

41

Challenge to theory/simulation:delocalization + localization

D A

+ $e

kD

kA

"( , )k $

k = kA - kD

Now add localization from electron-nuclear coupling & trapping!

My head is spinning…