enma490 final presentation sp06
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Utilizing CarbonNanotubes to ImproveEfficiency of OrganicSolar Cells
ENMA 490 Spring 2006
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Motivation
Problem: Lack of power in remotelocations
Possible solution: Organic solar cells areless expensive and more portable thanconventional solar cells
Main issue: Inadequate efficiency
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What We DidFocus: Increase the efficiency through theaddition of carbon nanotubes
Research Goal: Model a basic device andpropose an ideal structure for moreefficient power generation
Experimental Goal: Build selected devicesto test parameters
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Project OrganizationResearch TeamErik Lowery
Nathan Fierro Adam HaughtonRichard Elkins
Experimental TeamErin Flanagan
Scott WilsonMatt StairMichael Kasser
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How Organic Solar Cells Work
High Work Function Electrode
Acceptor Material
Low Work FunctionElectrode
Donor Material
1. Photon absorption, excitons arecreated
2. Excitons diffusion to aninterface
3. Charge separation due toelectric fields at the interface.
4. Separated charges travel to theelectrodes.
E
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Critical Design Issues
Exciton creation via photon absorptionMaterial absorption characteristics
Exciton diffusion to junctionInterfaces within exciton diffusion length(nanoscale structure)
Charge separationDonor/Acceptor band alignmentTransport of charge to electrodes
High charge mobility
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The Active Layer
Composed of an electron donor andelectron acceptor3 types of junctions
BilayerDiffuse BilayerBulk heterojunction
Usually the excitons from the electrondonor are responsible for the photocurrent
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Electron AcceptorMEH-PPV-CN
Electron acceptorCN group
Increased bandalignmentHigher electron affinity
Electrical PropertiesPoor charge mobility
Optical PropertiesPeak emission at 558nmPeak absorption at 405nm (~3eV)
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 1 2 3 4 5 6
-250
-50
150
350
550
750
950
1150
1350
1550
1750
Energy, eV
I r r a d
i a n c e
( W / m ^ 2 )
A b s o r p t
i o n
( a r b . U
n i t s ) MEH-PPV-CN
Solar Spectrum
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Electron DonorCarbon Nanotubes
Orders of magnitudebetter conductance
than polymersOur nanotubesspecifications (Zyvex)
FunctionalizedDiameter: 5-15 nmLength: 0.5-5 micronsMWNT (60% metallic40% semiconducting)
AFM Amplitude Scan
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Electron Donor (cont.)Carbon Nanotubes
Optical PropertiesDiameterSW vs. MWChirality (Semi-conducting vs metallic)
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Modeling
Model GeometryPhotogeneration of ExcitonsExciton Transport to JunctionElectron Hole Separation
Charge Transport to Electrode
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Model Geometry
ITO
CNT
MEH-PPV-CN
Al
Incoming Light
Define A to be the area perpendicular to the incominglight.
X=0
X=L
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Photogeneration of Excitons
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Photogeneration of ExcitonsStart with Beer-Lambert absorption equation:
Arrive at expression for # Photons absorbed per unit area, per unittimeUse either blackbody approximation or numerical data for thesolar spectrum (S
inc)
2
1 0
)(
0
)(
)(
)()(
)(),(
)(),(
d d ehc
S x I
d ehc
S x I
eS xS
x
Inc
x
Inc
x
Inc
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Exciton Transport to JunctionDiffusion Model
Initial and Boundary Conditions
)(),(),(),(
2
2
x I At xu Rdx
t xud D
dt
t xdu
0)0,(
0),(0),0(
xu
t Lu
t u Excitons destroyed at CNT/Electrode InterfaceExcitons destroyed at CNT/Polymer Junction
Initially, assume ground state, no excitonsanywhere.
Diffusion Term Decay Term, simpletime-dependentmodel
Source Term,accounts for excitongeneration
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Charge Transport to Electrode
Holes move along CNTsHole Mobility ~ 3000 cm2/Vs
Electrons move along MEH-PPV-CNElectron Mobility ~ 3.3x10-7 cm2/Vs
Current density is directly related tomobility; Increased mobility leads to highercurrent densities.
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Modeling Summary
CNT/MEH-PPV junctions within diffusionlength of exciton generation pointsThickness Optimization Problem:
Maximizing thickness gives more excitonsMinimizing thickness leads to higher current
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Ideal Structure
AlMEH-PPV-CN
NanotubesITO
Nanoscalemixing
Nanoscale mixing allows excitons to charge separate beforethey recombine
Structure allows for the bulk heterojunction and minimizesthe travel distance to the electrodes
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Experimental Design
Experimental design parameters
CNT concentrationMethod of mixingSpin ParametersSolvents
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Device Process Flow
ITO
2.5 mm.7 mm
.4 mm
.2 mm
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Device Process Flow
PEDOT ~100nm Al contacts ~600
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Active Layer
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Device Process Flow
LiF ~ 20
Al contacts
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Final Product
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Nanotube
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Experimental Results
y = 0.0038x - 1E-06
-0.005
-0.004
-0.003
-0.002
-0.001
0
0.001
0.002
0.003
0.004
0.005
-1.5 -1 -0.5 0 0.5 1 1.
V
A
Device 4 dark
Linear (Device 4 dark)
Pure CNT acted like a resistor, R >350 .
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Experimental Design Issues We
AddressedNanotube Processing
Method of dispersion
Type of solventConcentration CNT
amount of CNT in solvent
CNT to PolymerDiffused junction vs. bulk heterojunction
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Results Summary Absorption spectra measured AFM to check spatial distribution ofnanotubesNo successful devices madePossible causes:
CNT shortingFunctionalized CNTs might be a problem
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Conclusions
Experimental:Device process recipe needs to be refined
Solve experimental design problems to work onsuccessful deviceModeling:
Diffusion model considerations point towards
improving efficiency by creating nanoscale structureNeed to consider charge transport in more detail
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Acknowledgements
We would like to thank the followingpeople/organizations:
Dr. Gary RubloffDr. Danilo RomeroLaboratory for Physical Sciences
Zyvex
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