Utilizing Carbon Nanotubes to Improve Efficiency of Organic Solar Cells

ENMA 490 Spring 2006

Motivation

Problem: Lack of power in remote locations

Possible solution: Organic solar cells are less expensive and more portable than conventional solar cells

Main issue: Inadequate efficiency

What We Did

Focus: Increase the efficiency through the addition of carbon nanotubes

Research Goal: Model a basic device and propose an ideal structure for more efficient power generation

Experimental Goal: Build selected devices to test parameters

Project Organization

Research TeamErik Lowery Nathan FierroAdam HaughtonRichard Elkins

Experimental TeamErin FlanaganScott WilsonMatt StairMichael Kasser

How Organic Solar Cells Work

High Work Function Electrode

Acceptor Material

Low Work Function Electrode

Donor Material

1. Photon absorption, excitons are created

2. Excitons diffusion to an interface

3. Charge separation due to electric fields at the interface.

4. Separated charges travel to the electrodes.

E

Critical Design Issues

Exciton creation via photon absorptionMaterial absorption characteristics

Exciton diffusion to junction Interfaces within exciton diffusion length (nanoscale

structure) Charge separation

Donor/Acceptor band alignment Transport of charge to electrodes

High charge mobility

The Active Layer

Composed of an electron donor and electron acceptor

3 types of junctionsBilayerDiffuse BilayerBulk heterojunction

Usually the excitons from the electron donor are responsible for the photocurrent

Electron Acceptor MEH-PPV-CN

Electron acceptor CN group Increased band alignment Higher electron affinity

Electrical Properties Poor charge mobility

Optical Properties Peak emission at 558 nm Peak absorption at 405

nm (~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

Irra

dian

ce (

W/m

^2)

Abs

orpt

ion

(arb

. U

nits

) MEH-PPV-CN

Solar Spectrum

Electron Donor Carbon Nanotubes

Orders of magnitude better conductance than polymers

Our nanotubes specifications (Zyvex)

Functionalized Diameter: 5-15 nm Length: 0.5-5 microns MWNT (60% metallic

40% semiconducting)

AFM Amplitude Scan

Electron Donor (cont.) Carbon Nanotubes

Optical Properties Diameter SW vs. MW Chirality (Semi-

conducting vs metallic)

Modeling

Model Geometry Photogeneration of Excitons Exciton Transport to Junction Electron Hole Separation Charge Transport to Electrode

Model Geometry

ITO

CNT

MEH-PPV-CN

Al

Incoming Light

Define A to be the area perpendicular to the incoming light.

X=0X=L

Photogeneration of Excitons

Photogeneration of Excitons

Start with Beer-Lambert absorption equation:

Arrive at expression for # Photons absorbed per unit area, per unit time Use either blackbody approximation or numerical data for the solar

spectrum (Sinc)

2

1 0

)(

0

)(

)(

)()(

)(),(

)(),(

ddehc

SxI

dehc

SxI

eSxS

xInc

xInc

xInc

Exciton Transport to Junction Diffusion Model

Initial and Boundary Conditions

)(),(),(),(

2

2

xIAtxuRdx

txudD

dt

txdu

0)0,(

0),(

0),0(

xu

tLu

tu Excitons destroyed at CNT/Electrode Interface

Excitons destroyed at CNT/Polymer Junction

Initially, assume ground state, no excitons anywhere.

Diffusion Term Decay Term, simple time-dependent model

Source Term, accounts for exciton generation

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 to mobility; Increased mobility leads to higher current densities.

Modeling Summary

CNT/MEH-PPV junctions within diffusion length of exciton generation points

Thickness Optimization Problem:Maximizing thickness gives more excitonsMinimizing thickness leads to higher current

Ideal Structure

Al

MEH-PPV-CN

NanotubesITO

Nanoscale mixing

Nanoscale mixing allows excitons to charge separate before they recombine

Structure allows for the bulk heterojunction and minimizes the travel distance to the electrodes

Experimental Design

Experimental design parametersCNT concentrationMethod of mixingSpin ParametersSolvents

Device Process Flow

ITO

2.5 mm.7 mm

.4 mm

.2 mm

Device Process Flow

PEDOT ~100nm

Al contacts ~600 Å

Active Layer

Device Process Flow

LiF ~ 20 Å

Al contacts

Final Product

Nanotube

Experimental Results

Pure polymer devices acted like diodes. Light emission was observed at higher currents (8 mA)

Experimental Results

Pure CNT acted like a resistor, R >350Ω.

Experimental Design Issues We Addressed Nanotube Processing

Method of dispersionType of solvent

Concentration CNT amount of CNT in solvent CNT to Polymer

Diffused junction vs. bulk heterojunction

Results Summary Absorption spectra measured AFM to check spatial distribution of

nanotubes No successful devices made Possible causes:

CNT shortingFunctionalized CNTs might be a problem

Conclusions

Experimental: Device process recipe needs to be refined Solve experimental design problems to work on

successful device

Modeling: Diffusion model considerations point towards

improving efficiency by creating nanoscale structure Need to consider charge transport in more detail

Acknowledgements

We would like to thank the following people/organizations:Dr. Gary RubloffDr. Danilo RomeroLaboratory for Physical SciencesZyvex