The role of molecular structure andconformation in polymer opto-electronicsCharge separation: Molecular structure
Enrico Da Como
Conjugated polymers
polythiophene
Conjugated polymers
polythiophene
Bottom – up design for electronics
Optical, electrical & ordering properties arise at the molecular scale
Structure-function relationships
Polymer solar cells: transport, recombination & efficiency
-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75-12
-8
-4
0
4
8
Cu
rre
nt d
en
sity (
mA
/cm
2)
Voltage (V)
Solar cells
1. Absorption of light and photogeneration of excitons
Mott-Wannier
~ 0.1 eV
Large radius
Charge transfer excitons
~ 0.1 – 1.0 eV
Localised between molecules
Frenkel excitons
~ 0.5 – 1.0 eV
Localised on molecule
Solar cells
E
e-
e-
e-
h+
h+
h+
h+
h+ DriftDiffusion
e-
e-
eV
Built-in Potential
pn junction, heterojunction
2. Exciton dissociation & 3. Transport of charge
Solar cells
2. Exciton dissociation & 3. Transport of charge
Donor-Acceptor system
S
-
+
G. Yu, … A. J. Heeger, Science 207, 1789 (1995)
Y. Yang & Solarmer Nature Photonics 3, 649 (2009)
Polymer/fullerene photovoltaics
> 8% efficiency on lab cells
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Polymer/fullerene photovoltaics
Polymer/fullerene photovoltaics
Polymer/fullerene photovoltaics
Polymer/fullerene photovoltaics
The race for 10 %
Konarka's Power Plastic
Achieves World Record 8.3%
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Physics on different length scales
Efficiency
Layers & interfaces
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Mesoscopic scalebulk heterojunction
Physics on different length scales
Charge transport
Morphology & molecular ordering
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Molecular scaleDonor-Acceptor
Mesoscopic scalebulk heterojunction
Physics on different length scales
Exciton generation & dissociation
molecular ordering & mobility
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Molecular scaleDonor-Acceptor
Charge transport
Charge separation
Physics on different length scales
Mesoscopic scalebulk heterojunction
Exciton generation & dissociation
molecular ordering & mobility
Photovoltaic action: competing mechanisms?E
ne
rgy
Charge separation
absorption
Polymer Fullerene
HOMO
HOMO
LUMO
LUMO
En
erg
y
Charge separation
absorption
Large donor-acceptor interface
Polymer Fullerene
morphology & mobility
HOMO
LUMO
HOMO
LUMO
Photovoltaic action: competing mechanisms?
Efficiency = JscVocFF
Pin
Polmyer solar cell: device parameters
-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75-12
-8
-4
0
4
8
Curr
en
t density (
mA
/cm
2)
Voltage (V)
= 2.9 %
P3HT:PCBM
Short circuit current Jsclight absorption
transport
Fill factor FFcharge collection
Open circuit voltage VocHOMO-LUMO offset
Charge transfer @ polymer:fullerene interface
structure
conformation
ordering
Donor-acceptor distance
Excitons in polymers
Frenkel exciton (~ 0.5 eV – 1 eV)Intra (inter) chain excitationLifetime ~ns, diffusion length ~ 10 – 20 nm
Polymer structure, conformation & excitons
Excitons in polymers
Frenkel exciton (~ 0.5 eV – 1 eV)Inter or intrachain excitationLifetime ~ns, diffusion length ~ 10 – 20 nm
Chemical structure, excitons, long range ordering
Excitons in polymersE
ne
rgy
HOMO
LUMO
En
erg
y
S0
S1
S2
T1
absorption
Excitons in Polymer:fullerene systems
Charge transfer excitonCoulomb bound electron-hole pair @ the donor-acceptor interface
Excitons in Polymer:fullerene systems
Polymer Fullerene
HOMO
HOMO
LUMO
LUMO
En
erg
y
S0
S1
S2
T1CTE?
Excitons in Polymer:fullerene systems
Polymer Fullerene
HOMO
HOMO
LUMO
LUMO
En
erg
y
S0
S1
S2
T1CTE?
Where is the CTE energetically?What role does it play in charge transfer/recombination?CTE vs molecular structure, conformation and ordering?
Excitons in Polymer:fullerene systems
Why do CTEs dissociate?Field dependence
Only 60 % of CTEs dissociate in polymer fullerene solar cells at room temperature
V. Mihailetchi, L. Koster, J. Hummelen, P. Blom, Phys. Rev. Lett. 93, 216601 (2004)
Excitons in Polymer:fullerene systems
Are CTEs a necessary step for charge separation?
Voc limited by CTE
Polymer Fullerene
HOMO
LUMO
LUMO
Excitons in Polymer:fullerene systems
Are CTEs a necessary step for charge separation?
Polymer Fullerene
HOMO
LUMO
LUMO
Veldman et al., JACS 2008
Change molecular ordering, interface states
Excitons in Polymer:fullerene systems
Mixed amorphous & crystalline polymer regions enhance charge separation
Higher charge separation efficiency with engineered heterojunctions
Bulk properties influence CTE dissociation
Charge transfer @ polymer:fullerene interface
Acceptor concentration
En
erg
y(e
V)
-6.1
-5.4
-3.2
-4.2
HOMO
LUMO
HOMO
LUMO
MDMO-PPV/PCBM
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.0
0.4
0.8
1.2 PCBM
PL
(a.u
.)
0.0
0.4
0.8
1.2 MDMO-PPV pristine
PL
(a.u
.)
Probing recombination with PL spectroscopy
Energy (eV)
Adv. Funct. Mater. 19, 3662 (2009)
En
erg
y(e
V)
-6.1
-5.4
-3.2
-4.2
HOMO
LUMO
HOMO
LUMO
MDMO-PPV/PCBM 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.0
0.4
0.8
1.2 MDMO-PPV/PCBM blendP
L (
a.u
.)
0.0
0.4
0.8
1.2 PCBM pristine
PL
(a.u
.)
0.0
0.4
0.8
1.2 MDMO-PPV pristine
PL
(a.u
.)
CTE
Energy (eV)
Probing recombination with PL spectroscopy
Adv. Funct. Mater. 19, 3662 (2009)
0.8 1.2 1.6 2.00
1x105
2x105
3x105
4x105
PL
(a
.u.)
Energy (eV)0.8 1.2 1.6 2.0
Energy (eV)
80 wt % PCBM60 wt % PCBM
0.8 1.2 1.6 2.0
Energy (eV)
20 wt % PCBM
Vary the donor-acceptor interface
Adv. Funct. Mater. 19, 3662 (2009)
0.8 1.2 1.6 2.00
1x105
2x105
3x105
4x105
PL
(a
.u.)
Energy (eV)0.8 1.2 1.6 2.0
Energy (eV)
80 wt % PCBM60 wt % PCBM
0.8 1.2 1.6 2.0
Energy (eV)
20 wt % PCBM
Vary the donor-acceptor interface
Adv. Funct. Mater. 19, 3662 (2009)
CTE dissociation depends on acceptor concentration
Increased probability of exciton dissociation
Arkhipov et al., Appl. Phys. Lett. 2003 82, 4605.
Charge transfer @ polymer:fullerene interface
Donor/Acceptor structure
The role of the fullerene acceptor
En
erg
y (
eV
)
HOMO
LUMO
HOMO
LUMO
Donor/acceptor
PCBM
bis-PCBM
DPM
MDMO-PPV
VOC
Appl. Phys. Lett . 97 023301 (2010)
CTE recombination
Appl. Phys. Lett . 97 023301 (2010)
Anti-Correlation of PLCTE intensity and JSC
0 20 40 60 80 100 120 140 160 180 2000.0
0.2
0.4
0.6
0.8
1.0
1.2
PL
CT
E (
arb
. u
.)
JSC
µA/cm2
Appl. Phys. Lett . 97 023301 (2010)
Anticorrelation Jsc and CTE
Morphology and transport
bis-PCBM PCBM
me=2 10-4 cm2/Vs me=8 10-3 cm2/Vsme= 1 10-3cm2/Vs
Appl. Phys. Lett . 97 023301 (2010)
Long range ordering? Transport?
Changing morphology with chain regioregularityRegiorandom P3HT Regioregular P3HT
Amorphous vs. Polycrystalline
Adv. Funct. Mater. 19, 3662 (2009)
1.0 1.5 2.0
ra-P3HT
PCBM
Energy (eV)
X10
RE-P3HT
RE-P3HT/PCBM
Changing morphology with chain regioregularity
100 nm
= 2.1%
PL I
nte
nsity
PL I
nte
nsity
= 0.9%
1.0 1.5 2.0
ra-P3HT
ra-P3HT/PCBM
Energy (eV)
EnergyEnergy
100 nm
Regiorandom P3HT Regioregular P3HT
What is the role of donor-acceptor distance?
Model system: „low band gap“ polymers
PCPDT-BT
M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003)
N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007)
Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec,
Macromolecules 40, 1981 (2007).
„Low bandgap“ co-polymers for better light absorption
dithiophene
benzodiathiazole
LUMO
HOMO
Increasing solar cell efficiency
PCPDT-BT
M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003)
N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007)
Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec,
Macromolecules 40, 1981 (2007).
„Low bandgap“ co-polymers for better light absorption
dithiophene
benzodiathiazole
Low-bandgap copolymers
500 1000 1500 2000
Abs
orpt
ion
(arb
. uni
ts)
Wavelength (nm)
800 nm
PCPDT-2TBT
PCPDT-BDT
PCPDT-2TTP
PCPDT-BT
800 nm
660 nm
800 nm
Low-bandgap copolymers
Tautz et al submitted
Stronger vs weaker acceptor
Shifting the donor-acceptor centre of mass
IR Absorption
HOMO -1
LUMO +1
Measuring IR absorption of chemically
induced polarons
HOMO
LUMO
e-P1
Where are the polarons?
-0.1
0.0
0.1
500 1000 1500 2000 2500 3000 3500
-0.1
0.0
0.1
-0.1
0.0
0.1
P1
Ch
emic
ally
in
du
ced
O
D (
arb
. u
.)
GB
P1
GB
Probe
Wavelength [nm]
Probe
P2
P1
GB
Ex
Ex P2
GB
P1
P2
-5
0
5
0
5
-10
0
10
-0.1
0.0
0.1
-5
0
5
P2
Probe
P2
IR Absorption
HOMO -1
LUMO +1
Measuring IR absorption of chemically
induced polarons
HOMO
LUMO
e-P1
Where are the polarons?
-0.1
0.0
0.1
500 1000 1500 2000 2500 3000 3500
-0.1
0.0
0.1
-0.1
0.0
0.1
P1
Ch
emic
ally
in
du
ced
O
D (
arb
. u
.)
GB
P1
GB
Wavelength [nm]
P2
P1
GB
P2
GB
P1
P2
-0.1
0.0
0.1
P2
IR Absorption
HOMO -1
LUMO +1
Measuring IR absorption of chemically
induced polarons
HOMO
LUMO
e-P1
Where are the polarons?
-0.1
0.0
0.1
500 1000 1500 2000 2500 3000 3500
-0.1
0.0
0.1
-0.1
0.0
0.1
P1
Ch
emic
ally
in
du
ced
O
D (
arb
. u
.)
GB
P1
GB
Probe
Probe
Wavelength [nm]
Probe
Ex P2
P1
GB
Ex
Ex P2
GB
P1
P2
-5
0
5
0
5
-10
0
10
Op
tica
lly
in
du
ced
(1
0-4)
-0.1
0.0
0.1
-5
0
5
P2
Probe
Polaron formation in realtime
10
20
30
10
20
10
20
Time delay (fs)
PCPDT-BT
Po
laro
n p
air
yie
ld (
%)
P3HT
PCPDT-BDT
PCPDT-2TBT
PCPDT-2TTP
IRF
-1000 -750 -500 -250 0 250 500 7500
10
20
10
20
D A
D A
D A
D A
UU U U-
= 15.9%
= 21.4%
= 13.9%
= 7.9%
= 23.6%
Polaron formation in realtime
10
20
30
10
20
10
20
Time delay (fs)
PCPDT-BT
Po
laro
n p
air
yie
ld (
%)
P3HT
PCPDT-BDT
PCPDT-2TBT
PCPDT-2TTP
IRF
-1000 -750 -500 -250 0 250 500 7500
10
20
10
20
D A
D A
D A
D A
UU U U-
= 15.9%
= 21.4%
= 13.9%
= 7.9%
= 23.6%
Polaron formation in realtime
10
20
30
10
20
10
20
Time delay (fs)
PCPDT-BT
Po
laro
n p
air
yie
ld (
%)
P3HT
PCPDT-BDT
PCPDT-2TBT
PCPDT-2TTP
IRF
-1000 -750 -500 -250 0 250 500 7500
10
20
10
20
D A
D A
D A
D A
UU U U-
= 15.9%
= 21.4%
= 13.9%
= 7.9%
= 23.6%
Acceptor strength only slightly influencing efficiency
Important role of spatial separation
Charge transfer @ polymer:fullerene interface
structure
conformation
ordering
Donor-acceptor distance
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Mesoscopic scalebulk heterojunction
Physics on different length scales
Charge transport
Morphology & molecular ordering
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Molecular scaleDonor-Acceptor
Mesoscopic scalebulk heterojunction
Physics on different length scales
Exciton generation & dissociation
molecular ordering & mobility
How to improve efficiency at every length scale?
Conformation & structure
Long range ordering
Charge transfer
Adv. Funct. Mater. 19, 3662 (2009)
1.0 1.5 2.0
ra-P3HT
PCBM
Energy (eV)
X10
RE-P3HT
RE-P3HT/PCBM
Changing morphology with chain regioregularity
100 nm
= 2.1%
PL I
nte
nsity
PL I
nte
nsity
= 0.9%
1.0 1.5 2.0
ra-P3HT
ra-P3HT/PCBM
Energy (eV)
EnergyEnergy
100 nm
Regiorandom P3HT Regioregular P3HT
100 nm
The effect of long range ordering
AnnealedNot Annealed
= 2.1% = 4.0%
100 nm
AnnealedNot Annealed
= 2.1% = 4.0%
1.0 1.5 2.0
PL
in
ten
sity
X10
RE-P3HT/PCBM
RE-P3HT/PCBM
(annealed)
Energy (eV)Adv. Funct. Mater. 19, 3662 (2009)
The effect of long range ordering
100 nm
AnnealedNot Annealed
= 2.1% = 4.0%
Adv. Funct. Mater. 19, 3662 (2009) J. App. Phys.100, 043702 (2006)
Ambipolar transportUnipolar (hole) transport
The effect of long range ordering
How to induce long range ordering?
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Conformation & structure
Long range orderingDoping
Charge transfer
Increase mobility without changing morphology?
Increasing mobility by molecular doping
P doping by electron transfer in the
ground state
F4TCNQ
Yim et al., Adv Mater, 2008, 20Zhang et al., Phys Rev B, 2010, 81
Zhang et al., Adv Func Mater, 2009, 19
Increasing mobility by molecular doping
P doping by electron transfer in the
ground state
PCPDTBT:PCBM
F4TCNQ
SPP1355
Fill tail states with excess
charge carriers
Increase Mobility
+
Energ
y (
eV
)
Disordered film
Increasing mobility by molecular doping
g(E)
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Charge transport
Charge separation
Improvement in charge separation, mobility, efficiency
Photocurrent & Efficiency?
0.9 1.0 1.1 1.2 1.3 1.4
PCBM
PCPDTBT
1.0 1.2 1.4 1.6 1.8
PL inte
nsity (
arb
.units)
Energy (eV)
PCPDTBT/PCBM
Energy (eV)
PL inte
nsity (
arb
. units)
Doping & Charge separation
+
-
0.9 1.0 1.1 1.2 1.3 1.4
PCBM
PCPDTBT
1.0 1.2 1.4 1.6 1.8
PL inte
nsity (
arb
.units)
Energy (eV)
PCPDTBT/PCBM
Energy (eV)
PL inte
nsity (
arb
. units)
+
-
0%
1%
3%
4%
Doping & CTE recombination
0 200 400 600 800
Time (ps)
0%
2%
4%
5%
No
rm.
PL
inte
nsity
Doping & CTE recombination
0 100 200 300
0%
2%
4%
5%
PL inte
nsity (
arb
. u
nits)
Time (ps)
Lower density of CTE or very fast dissociation with doping?
Doping & CTE recombination
Doping & Polaron formation
Janssen et al. Adv. Mater
(2010)-T/
T x
10
4
0 50 100 150 200 250 3000
2
Time delay (ps)
-
T/T
(x10
-3)
0%
EProbe
Doping & Polaron formation
Janssen et al. Adv. Mater
(2010)-T/
T x
10
4
0
2
0 50 100 150 200 250 3000
22%
-
T/T
(x10
-3)
0%
Time delay (ps)
EProbe
Doping & Polaron formation
Janssen et al. Adv. Mater
(2010)-T/
T x
10
4
0
2
0
2
0 50 100 150 200 250 3000
24%
2%
-
T/T
(x10
-3)
0%
Time delay (ps)
EProbe
Doping & Polaron formation
Janssen et al. Adv. Mater
(2010)-T/
T x
10
4
0
2
0
2
0
2
0 50 100 150 200 250 3000
25%
4%
2%
-
T/T
(x10
-3)
0%
Time delay (ps)
EProbe
PCPDTBT PCBM
En
erg
y
tCT-r
tCT-ftFR-r
tP-f
tP-r
0 100 200 300
0%
2%
4%
5%
PL
in
ten
sity (
arb
. u
nits)
Time (ps)
Time Delay (ps)
0
2
0.0
1.6
0
2
0.0
1.6
0
2
0.0
1.6
0
2
0.0
1.6
0 50 100 150 200 250 300 Po
laro
n d
en
sity (
x1
01
7/c
m3)
0%
-T
/T (
x1
0-4)
2%
4%
5%
Rate equation model
PCPDTBT PCBM
En
erg
y
tCT-r
tCT-ftFR-r
tP-f
tP-r
0 100 200 300
0%
2%
4%
5%
PL
in
ten
sity (
arb
. u
nits)
Time (ps)
Time Delay (ps)
0
2
0.0
1.6
0
2
0.0
1.6
0
2
0.0
1.6
0
2
0.0
1.6
0 50 100 150 200 250 300 Po
laro
n d
en
sity (
x1
01
7/c
m3)
0%
-T
/T (
x1
0-4)
2%
4%
5%
Doping
[%] tFR-r tCT-f tP-f tCT-r tP-r[ps] [ps] [ps] [ps] [ps]
0 125 0.2 0.2 300 1400
2 125 0.5 0.2 300 1000
4 125 0.95 0.2 300 400
5 0.15 0.95 0.2 250 300
Rate equation model
Decrease in CTE emission and larger density of polarons with doping
Conclusion: doping helps!
Phys. Rev. Lett. 107, 127402 (2011)
Substrate
Anode
Transport layer
Active layer: Polymer/fullerene
Metal contact
Charge transport
Charge separation
Improvement in charge separation, mobility, efficiency
Efficiency?
09.03.2015 Präsentationstitel 80
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