enma490 final presentation sp06

8/13/2019 ENMA490 Final Presentation Sp06

1/31

Utilizing CarbonNanotubes to ImproveEfficiency of OrganicSolar Cells

ENMA 490 Spring 2006


2/31

Motivation

Problem: Lack of power in remotelocations

Possible solution: Organic solar cells areless expensive and more portable thanconventional solar cells

Main issue: Inadequate efficiency


3/31

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


4/31

Project OrganizationResearch TeamErik Lowery

Nathan Fierro Adam HaughtonRichard Elkins

Experimental TeamErin Flanagan

Scott WilsonMatt StairMichael Kasser


5/31

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


6/31

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


7/31

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


8/31

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


9/31

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


10/31

Electron Donor (cont.)Carbon Nanotubes

Optical PropertiesDiameterSW vs. MWChirality (Semi-conducting vs metallic)


11/31

Modeling

Model GeometryPhotogeneration of ExcitonsExciton Transport to JunctionElectron Hole Separation

Charge Transport to Electrode


12/31

Model Geometry

ITO

CNT

MEH-PPV-CN

Al

Incoming Light

Define A to be the area perpendicular to the incominglight.

X=0

X=L


13/31

Photogeneration of Excitons


14/31

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


15/31

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


16/31

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.


17/31

Modeling Summary

CNT/MEH-PPV junctions within diffusionlength of exciton generation pointsThickness Optimization Problem:

Maximizing thickness gives more excitonsMinimizing thickness leads to higher current


18/31

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


19/31

Experimental Design

Experimental design parameters

CNT concentrationMethod of mixingSpin ParametersSolvents


20/31

Device Process Flow

ITO

2.5 mm.7 mm

.4 mm

.2 mm


21/31

Device Process Flow

PEDOT ~100nm Al contacts ~600


22/31

Active Layer


23/31

Device Process Flow

LiF ~ 20

Al contacts


24/31

Final Product


25/31

Nanotube


26/31


27/31

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 .


28/31

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


29/31

Results Summary Absorption spectra measured AFM to check spatial distribution ofnanotubesNo successful devices madePossible causes:

CNT shortingFunctionalized CNTs might be a problem


30/31

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


31/31

Acknowledgements

We would like to thank the followingpeople/organizations:

Dr. Gary RubloffDr. Danilo RomeroLaboratory for Physical Sciences

Zyvex

enma490 final presentation sp06

Documents