design and synthesis of conjugated polymers towards ......uv-vis stability in solution Ł absorption...
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Design and Synthesis of Design and Synthesis of Conjugated Polymers towards Conjugated Polymers towards Organic Solar Cell ApplicationsOrganic Solar Cell Applications
Toshihiro Okamoto, Ying Jiang, Zhenan Bao
� Solar energy � plentiful and free� Organic vs inorganic
� Low cost� High-speed production (solution, roll-to-roll
processing)� Mountable on flexible substrates� Chemical flexibility for modifications via
synthetic methods� Downside: low efficiency, instability
Organic Organic PhotovoltaicsPhotovoltaics (OPV)(OPV)
Photovoltaic energy conversionPhotovoltaic energy conversion
hν
-
+
Donor Acceptor
E
Anode Cathode
LUMOLUMO
HOMOHOMO
ABS COLLDISSTOTALη = × η×ηη
1. Light absorption and exciton formation.
2. Charge dissociationat heterojunction.
3. Charge transport and collection at electrodes.
Double-layer heterojunction
Bulk heterojunction
Organic Heterojunction Solar CellsOrganic Heterojunction Solar Cells
Ordered bulk heterojunction
5.5Bulk heterojunction
15 � 20Target for OPV
> 30Inorganic solar cell
Efficiency (%)Type of Device
The idealmaterial
for asolar cell
Highabsorptioncoefficient
Solubility
Low bandgap(<1.7 eV)
High mobility
Exhibits percolating morphology
Stability
PCE (%)
FF
VOC (V)
ISC(mA/cm2)
3.0[1]
0.56
0.80
5.4
~10-7[3]
2.2[4]
0.42
0.59
8.9
~10-6 � 10-4
1.6
Fluorenecopolymers
4.6[2]
Best performance
reported 0.65
0.65
12.5
~0.01-0.1Mobility (cm2/V s)
~1.9 � 2.2[5]2.0 � 2.2HOMO-LUMO gap (eV)
Structure
P3HTsPPVsMaterial
Representative/Promising classes of materials toRepresentative/Promising classes of materials to--datedate
1. Chem. Commun., 2003, 2116�2118; 2. Solar Energy Materials & Solar Cells 91 (2007) 1187�1193; 3. J. Phys. Chem. B 2004, 108, 5235-5242; 4. Adv Mater 2006, 18, 2169-2173; 5. Adv. Funct. Mater. 2007, 17, 1071-1078
Desirable bandgap: < 1.7 eV, mobility > 0.01cm2/Vs
Our approach: Our approach: aceneacene--containing conjugated polymerscontaining conjugated polymers
Low Low bandgapbandgap conjugated conjugated pentacenepentacene derivativesderivatives
� Pentacene� Optical band gap 2.1 eV in solution, 1.77 eV in thin film
� Further reduction in band gap by extending conjugation
−1.626 eV
−7.612 eV
−1.772 eV
−7.644 eV
0.146 eV0.146 eV
0.032 eV0.032 eVHOMO
LUMO
ADTADT--fluorenefluorene copolymercopolymer
2.4
2.9
3.8
PDI
-5.23
-5.22
-5.24
HOMO (eV)
2.02
2.05
2.03
Optical Band Gap
(eV)
24×103
23×103
131×103
Mw
10×1032-Ethylhexyl
8×103Dodecyl
46×103Octyl
MnR
Copolymer (R = octyl) in toluene
Copolymer film (R = octyl)
Eg = 2.2 eV;More stable than pentacene
UVUV--VisVis stability in solutionstability in solution� Absorption measured in ODCB solution
� Decrease in absorption with t1/2 ~ 600 min
� Pentacene t1/2 (575 nm) ~ 250 min in dark1
300 400 500 600 700 8000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Abs
orba
nce
Wavelength (nm)
0 min 5 min 15 min 30 min 60 min 240 min 420 min 1500 min (25 hr)
YJ2-36a
1 Chem Mater, Vol 16, No 24, 4980
Absorbance at 587nm vs Time
00.050.1
0.150.2
0.250.3
0.35
0 500 1000 1500 2000
Time (min)
Abs
orba
nce
Copolymer 1
Thin film fieldThin film field--effect mobilityeffect mobility
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0-100-80-60-40-200
VDS (V)
I DS ( µ
A)
Vg=20
Vg=10
Vg=00
Vg=-10
Vg=-20
Vg=-30
Vg=-40
Vg=-50
Vg=-60
Vg=-70
Vg=-80
Vg=-90
Vg=-100
1E-09
1E-08
1E-07
1E-06
1E-05
-100 -80 -60 -40 -20 0 20
VG (V)
- ID
S ( µ
Am
ps )
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
(- I D
S )1/
2 (A
)1/2
µ=0.0090 cm2/Vs
On/Off = ~4000
Vth = -5.5 V
Preliminary solar cell resultsPreliminary solar cell results
-0.5 0.0 0.5 1.0-5
0
5VOC = 495 mV
Jsc = 4.05 mA/cm2
FF = 0.38Eff. = 0.76%
Cur
rent
Den
sity
(mA
/cm
2 )
Voltage (V)
Toluene and ODCB Light Toluene and ODCB Dark ODCB Light ODCB Dark
High seriesresistancepossibly dueto amorphousstructure.
1:1 TIPSEDTAF:PCBM C60
Higher VOC'spossible.
TIPSEPTIPSEP--fluorenefluorene copolymercopolymer
2.06
2.77
3.21
PDI
17×103
49×103
117×103
Mw
1.80
1.79
1.78
Optical Band Gap
(eV)
5.17
5.19
5.20
HOMO(eV)
8.2×1032-Ethylhexyl
36×103Octyl
18×103Dodecyl
R Mn
Eg = 1.86 eV
TIPSEPTIPSEP--dTfluorenedTfluorene copolymercopolymerMonomer synthesis:addition of thiophene linkers
Copolymerization
1.67
1.74
Eg(eV)
457745On Glass
443715In ODCB
λmax (nm)
λonset (nm)
UV-Vis absorption spectra of TIPSEP-dTfluorene copolymer
7: R = C12H25
TIPSEPTIPSEP--diethynylbenzenediethynylbenzene copolymercopolymer
Modest solubility in common organic solvents (< 5 mg/ml)
Dark Green Film(Polymer 9)
2.41
2.21
PDI
1.76
1.68
Eg (eV)
58×103
25×103
Mw
24×1032-Ethylhexyl
11×103Dodecyl
MnR Films ODCB
Ref.) S. L. Buchwald and G. C. Fu et al. Org. Lett. 2000, 2, 1729
Summary and future workSummary and future work
� Molecular design towards stable, low bandgap materials for OPV
� Synthesized copolymers with promising properties
� Continue to build and characterize a library of pentacene-derivative-containing copolymers
� Fabricate devices to assess materials
AcknowledgementsAcknowledgements
SynthesisMichelle Senatore (UG)
Fei Qu (UG)
Transistor DeviceDr. Hector Becerril
The Air Force Office of Scientific Research, AFRL
Solar Cell DeviceDr. Alex MyerJack Parmer
Professor Mike McGehee