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 Team Erik Lowery Nathan Fierro Adam Haughton Richard Elkins Experimental Team Erin Flanagan Scott Wilson Matt Stair Michael 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 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

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

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) Energy, eV Irradiance (W/m^2) Absorption (arb. Units) 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: 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=0 X=L

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 (S inc )

Exciton Transport to Junction Diffusion Model Initial and Boundary Conditions 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 CNTs  Hole Mobility ~ 3000 cm 2 /Vs Electrons move along MEH-PPV-CN  Electron Mobility ~ 3.3x10 -7 cm 2 /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 excitons  Minimizing thickness leads to higher current

Ideal Structure Al MEH-PPV-CN Nanotubes ITO 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 parameters  CNT concentration  Method of mixing  Spin Parameters  Solvents

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 dispersion  Type 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 shorting  Functionalized 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 Rubloff  Dr. Danilo Romero  Laboratory for Physical Sciences  Zyvex