1. Utilizing Carbon Nanotubes to Improve Efficiency of Organic Solar Cells
ENMA 490 Spring 2006

2. 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
The need for power in remote locations has given rise to the need for a more portable solar cell
Organic solar cells have shown great potential for the solution
The problem lies in the inadequate efficiency currently found in organic solar cells

3. 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

4. Project Organization Research Team Erik Lowery Nathan Fierro
Adam Haughton
Richard Elkins
Experimental Team
Erin Flanagan
Scott Wilson
Matt Stair
Michael Kasser
Abstract/ project overview

5. How Organic Solar Cells Work
Photon absorption, excitons are created
Excitons diffusion to an interface
Charge separation due to electric fields at the interface.
Separated charges travel to the electrodes.
High Work Function Electrode
Donor Material E
Acceptor Material
Basic Structure
Low Work Function Electrode

6. Critical Design Issues
Exciton creation via photon absorption
Material 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
Steps here mimic the ones from the previous slide, but want to emphasize the material properties that are important at each step.
Photon absorption – want good absorption within the solar spectrum
Exciton Diffusion – nanoscale structure that has interfaces within the diffusion length of the excitons
Charge Separation – Energy difference between the donor/acceptor forces charges to separate
Transport of charges – good mobility

7. The Active Layer Composed of an electron donor and electron acceptor
3 types of junctions
Bilayer
Diffuse Bilayer
Bulk heterojunction
Usually the excitons from the electron donor are responsible for the photocurrent
And now Scott is going to go into more details on the materials we used and how we approached modeling the problem.

8. Electron Acceptor MEH-PPV-CN Electron acceptor Electrical Properties
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)
Solar Spectrum
MEH-PPV-CN
Irradiance (W/m^2)
Absorption (arb. Units)
FIX GRAPH HERE
Energy, eV

9. Electron Donor Carbon Nanotubes
Orders of magnitude better conductance than polymers
Our nanotubes specifications (Zyvex)
Functionalized
Diameter: 5-15 nm
Length: microns
MWNT (60% metallic 40% semiconducting)
Length hard to estimate
AFM Amplitude Scan

10. Electron Donor (cont.) Carbon Nanotubes Optical Properties Diameter
SW vs. MW
Chirality (Semi-conducting vs metallic)
Chose which graphs and key points you want to emphasize here… SWNT v MWNT

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

12. Model Geometry Incoming Light ITO CNT MEH-PPV-CN Al
X=0
X=L
ITO
CNT
MEH-PPV-CN Al
Define A to be the area perpendicular to the incoming light.

13. Photogeneration of Excitons

14. 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)

15. Exciton Transport to Junction
Diffusion Model
Initial and Boundary Conditions
Diffusion Term
Decay Term, simple time-dependent model
Source Term, accounts for exciton generation
Term by term breakdown, be prepared to answer questions about each term…
Excitons destroyed at CNT/Electrode Interface
Excitons destroyed at CNT/Polymer Junction
Initially, assume ground state, no excitons anywhere.

16. Charge Transport to Electrode
Holes move along CNTs
Hole Mobility ~ 3000 cm2/Vs
Electrons move along MEH-PPV-CN
Electron Mobility ~ 3.3x10-7 cm2/Vs
Current density is directly related to mobility; Increased mobility leads to higher current densities.
Read up on this charge transport model

17. Modeling Summary
CNT/MEH-PPV junctions within diffusion length of exciton generation points
Thickness Optimization Problem:
Maximizing thickness gives more excitons
Minimizing thickness leads to higher current

18. Ideal Structure
ITO
Nanotubes
Nanoscale mixing
MEH-PPV-CN Al
In our goals we say we modeled this, are we putting that into the report?
Nanoscale mixing allows excitons to charge separate before they recombine
Structure allows for the bulk heterojunction and minimizes the travel distance to the electrodes

19. Experimental Design Experimental design parameters CNT concentration
Method of mixing
Spin Parameters
Solvents
Details need to filled in here by Erin or Matt

20. Device Process Flow
ITO
.4 mm
.7 mm
.2 mm
2.5 mm

21. Device Process Flow
PEDOT ~100nm
Al contacts ~600 Å

22. Active Layer

23. Device Process Flow
LiF ~ 20 Å
Al contacts

24. Final Product

25. Nanotube

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

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

28. Experimental Design Issues We Addressed
Nanotube Processing
Method of dispersion
Type of solvent
Concentration CNT
amount of CNT in solvent
CNT to Polymer
Diffused junction vs. bulk heterojunction

29. Results Summary Absorption spectra measured
AFM to check spatial distribution of nanotubes
No successful devices made
Possible causes:
CNT shorting
Functionalized CNTs might be a problem

30. Conclusions Experimental: Modeling:
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

31. Acknowledgements
We would like to thank the following people/organizations:
Dr. Gary Rubloff
Dr. Danilo Romero
Laboratory for Physical Sciences
Zyvex