30
Vikram Kuppa School of Energy, Environmental, Biological and Medical Engineering College of Engineering and Applied Science University of Cincinnati [email protected]
Vikram Kuppa
School of Energy, Environmental, Biological and Medical Engineering
College of Engineering and Applied Science
University of Cincinnati
Fei Yu
Yan Jin
d ld iAndrew Mulderig
Greg BeaucageGreg Beaucage
― Renewable― Potential for High coverage― Low emission
1 US DOE Energy Information Administration (2012), Annual Energy Review 2011U. Mich., Center for Sustainable Systems. 2012. “U.S. Renewable Energy Factsheet”. Pub No. CSS03‐12
Organic Photovoltaics (OPVs)
Solar power offers unique advantages
― no mech. parts
― flexible & customizable
― relatively expensive
― storage
IIIa generation ‐ Si & Ge cells are efficient but expensive
OPVs are of IIIb type: low to moderate efficiency yp y
― processed at lower T
― versatile manufacturing
― low efficiency
― inadequate spectral coverageversatile manufacturing
― distinctive mechanical & optical properties
― tunability
― inadequate spectral coverage
― poor mobility of charges
y
― cheap
Functioning of OPVs
hν
−+)
(− +)
( )(− (− +)
Incident radiation produces e‐h pairs (excitons)
Exciton motion length & time scales ~ 100 ps, 5‐20 nm
Morphology of active +
material is KEY−
OPV Materials
― Blend of polymer(s) and/or additive – bulk heterojunction (BHJ)
― Traditional BHJs have about 50% of polymer, and 50% PCBM
(fullerene derivative)(fullerene derivative)
― PCBM only for charge conduction and exciton dissociation
Critical Issues― Critical Issues
― Increase fraction of conjugated polymer
― Helps capture more sunlight
― Improves efficiency
― Improve charge transport
Importance of interfaces in OPV devices
D-A interfaceLUMODonor
LUMO 0 3eVEDonor
HOMODonorh+
EAcceptor
LUMOAcceptor
e-
0.3eV
HOMOAcceptor
D-A interface facilitates exciton dissociationElectron transfer from donor(semiconducting polymer) to acceptorExciton dissociation is energetically favorableExciton diffusion length(~10 nm)
polymer) to acceptor
7
g ( )D-A interfacial area is determined by morphology
Typical OPVs
P3HT PCBM
+
+
P3HT F8TBT
+
P3HTF8BT
+
McNeill & Greenham, Adv. Mater. 2009 21, 3840Kim et al., Chem. Mater 2004 16(23), 4813
Polymer Blend OPVs
― Mix of semiconducting polymers
― Both components active & capture sunlight
― Morphology control is again key
― Critical Issues
― Poor charge mobilities persistg p
― Greater recombination losses
Crystallization of polymers and blend miscibility ?― Crystallization of polymers and blend miscibility ?
― Free charge formation and transport
V l― Voltage
Solution ?
Pristine graphene
― Excellent conductivity and high aspect ratio
― Percolation paths at very low concentration
― OPVs with chemically modified graphenes were reported*
F ti li d h t h l t i b h i― Functionalized sheets show poor electronic behavior
TEM image of pristine graphene flake
t=0.35 nm
Scale bar=50nmlateral~200-500nm
*Liu, Z. et al., Adv. Mater., 2008 20(20), 3924 Yu, D. et al., ACS Nano, 2010 4(10), 5633; Yu, D. et al., J. Phys. Chem. Lett., 2011 2(10), 1113
Graphene‐based OPVs
+
−
P3HT(~90.99%)PCBM(~9%)PCBM(~9%)Graphene(~0.01%)
― Three‐fold enhancement in efficiency― Increase in current – better mobility
Novel device physics
Yu, Bahner & Kuppa, Key Engr. Mater. 2012 21, 3840Yu & Kuppa, Mater. Lett. 2013 99, 72
― Novel device physics
Current focus ‐ ternary blends
+
−
P3HT (59.9%)F8BT (39.9%)Graphene (~0.2%)
Device Fabrication
Anode― Patterned ITO as bottom electrode
PEDOT PSS b i ti
Anode
― PEDOT:PSS by spin coating
― Active layer with graphene by spinActive layer with graphene by spin coating
LiF and Aluminum― LiF and Aluminum
Cathode― Fabricated and annealed in N2
Solar Cell Parameters
14Deibel and Dyakonov, Rep. Prog. Phys., 2010 73(9), 096401
Cell Performance
Open circuit voltage ‐ Voc
1.5
1.4
V) 1.3
Voc
(V
1.2
0.00 0.02 0.04 0.06 0.08 0.101.1
graphene concentration (mg/ml)
Cell PerformanceShort circuit current ‐ Jsc
0.6
0.5
0.4
mA
/cm
2 )
0.3Jsc
(m
0 1
0.2
0.10 0.05 0.1 0.15 0.2
graphene concentration (mg/ml)
Cell PerformanceFill factor ‐ FF
0.21
0.20
F
0.19
FF
0 180.00 0.02 0.04 0.06 0.08 0.10
0.18
graphene concentration (mg/ml)
Cell Performance
0.16
0.14
(%)
0.1
0.12
ffic
ienc
y (
0.08
ef
0 04
0.06
0.040 0.05 0.1 0.15 0.2
graphene concentration (mg/ml)
Cell PerformanceExternal quantum efficiency ‐ EQE
PF10PF10 G0 0254 PF10 G0.025 PF10 G0.05 PF10 G0.1PF10 G0.2
3
%)
2
EQE
(%
1
300 350 400 450 500 550 600 650 7000
Wavelength (nm)
Device physics ‐ recombination
-0.2
0.0
alpha=0.67
alpha=0.73
-0 6
-0.4
mA
/cm
2 ))
alpha=0.74
alpha=0.66alpha=0.62
Jsc ~ Iα
-0.8
0.6
G 0ent d
ensi
ty(m
p
alpha=0.90
-1.2
-1.0 G 0 G 0.025 G 0.05 G 0.1G 0 2
Log
(Cur
re
alpha=0.93
alpha=0.91
alpha=0.84
1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6-1.6
-1.4 G 0.2 Linear Fit
alpha=0.99
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6Log (Intensity(mW/cm2))
(a)α = 1 for geminateα = 0.5 for bimolecular
graphene dependence of α
1.00 part 1 alphapart 2 alpha
Jsc ~ Iα0.90
0.95part 2 alpha
0.80
0.85
Alp
ha
0.70
0.75
A
0.00 0.05 0.10 0.15 0.200.60
0.65
Graphene concentration (mg/ml)(b)
Role of Graphene
I i iE i i IntrinsicCharge transport
Extrinsic
Morphology of blend
Mobility
Morphology of blend
Structure of P3HT & F8BT
RecombinationCrystallization & Aggregation
UV‐VIS of thin films
Peaks at 550‐600nm w/ Increasing [Gr]
P3HT/F8BT 6/4 P3HT/F8BT/G 6/4/0.05 P3HT/F8BT/G 6/4/0.1 P3HT/F8BT/G 6/4/0.2 Increasing [Gr]
P3HT crystallites
Abso
rptio
n
Nucleating agent ?
Nor
mal
ized
350 400 450 500 550 600 650 700Wavelength (nm)
Concentration dependence
0.15‐0.18
0.15
0.18
0 5 0 8
0.12‐0.15
0.09‐0.12
0.12
iency (%
) 0.06‐0.09
0.03‐0.06
10
0.06
0.09
effici
8
100.03
0.10.05 [P3HT+F8BT] (mg/ml)
60.0250[graphene] (mg/ml)
Thickness dependence
0 18
0 12
0.15
0.18
(%) 0.15-0.18
0.12-0.15
0 06
0.09
0.12
effic
ienc
y 0.09-0.120.06-0.090.03-0.06
0.10.2
0.03
0.06e 0.03 0.06
00.025
0.050.1
film thickess( )
graphene concentration (mg/ml)(nm)
New paradigms in OPV BHJs− Graphene enhances charge transport ‐ high Jsc , FF and η
− Cells with majority active layer are now viablej y y− Better harnessing of solar energy− Improved mobility
− Morphology of blend is altered – enhanced crystallization
− Intrinsic and extrinsic effects are observed
− Complex influence of thickness & concentrationComplex influence of thickness & concentration
− Synergistic role of high‐aspect ratio graphene additives
Jin, Yu and Kuppa, (manuscript in preparation)
Choice of solvent: polymer chain packing
Choice of donor and acceptor materials: band gap and miscibility
Donor-acceptor ratio: domain size
Annealing conditions: reorganize polymer chains, crystallizationOther post-production treatments: DC voltage during annealing for ordered structure *
Morphology Performance
28Pictures source: Dennler, Scharber and Brabec, Adv. Mater. 2009, 21(13): p. 1323-1338. * Padinger, Rittberger and Sariciftci, Adv. Funct. Mater., 2003. 13(1): p. 85-88.
BHJ featuresBHJ featuresPolymer:Fullerene BHJ devicePolymer:Fullerene BHJ device
High interfacial area for exciton dissociationBicontinuous network for charge transportg p50:50 w/w P3HT:PCBM for optimum performanceIncrease P3HT ratio to capture more solar energyp gy
P3HT PCBM
F t W kFuture Work
Better dispersed and oriented graphene via morphological controlmorphological control
Increase FF by reducing interfacial roughnessIncrease FF by reducing interfacial roughness
St bilit d d i l tiStability and device encapsulationFY and VKK thank UC and the URC for funding and support
