Introduction

The choice of carbon nanotubes is based upon their infinitesimal diameter and resulting large surface area to volume ratio, resilient structural integrity and high-speed carrier transport capabilities facilitated by ballistic conductivity. Quantum dots will also be integrated on account of their high quantum efficiency, high surface to volume ratio, as well as their optical non-linearity[2],[3].

Carbon Nanotubes:

• are single layer carbon atom (graphene) sheets of hexagonal structure rolled up into seamless cylinders

• are of two types – Single-Walled Nanotubes (SWNTs) or Mulit-Walled Nanotubes (MWNTs)

• SWNTs consist of only one cylinder with diameter between 0.33 and 5nm and length between 2 and 10nm this is the type of nanotube that has been used for this project • have great strength (50 times stronger than steel while only a quarter as dense) providing for a more resilient device

• are one-dimensional structures thereby enabling ballistic transport

Quantum Dots:

• are nanostructured semiconductors whose exitons are quantum confined in all planes of 3 dimensional space hence they can be considered a 0-dimensional structure • range from 2-10nm (10 to 50 atoms) in diameter

• have a very high surface to volume ratio due to their small size, thus more photons are absorbed per square centimeter by a quantum dot as opposed to bulk material (photon absorption is a surface process) resulting in a high quantum efficiency

Background

Due to increasing demands for renewable, environmentally-friendly means of energy generation and production, there is an impetus to overcome the many challenges faced in the design of cheaper and more efficient systems that will harness energy from natural sources. Solar energy is one such natural energy resource. One of the many problems faced is the production of economical solar cells. Current solar cell technology can be improved in terms of processing costs, scalability, flexibility of the solar cells and weight of devices[1]. Two materials of interest in this field of research include carbon nanotubes and quantum dots.

Analysis of Carbon Nanotubes and Quantum Dots in a Photovoltaic DeviceFrancis Smith, Mohammad Faisal HalimFaculty mentor: Prof. Roger Dorsinville

Non-Linear Optics Laboratory Department of Electrical Engineering, The City College of New York, New York, NY 10031

Abstract

We synthesized and characterized photovoltaic cells (solar cells) composed of carbon nanotubes and quantum dots encapsulated in a polymer matrix. The quantum dots act as photo-absorbers while the carbon nanotubes act as electrical conduits between these active centers and the electrodes.

Results References

[1]: B.J. Landi et al, Solar Energy Materials & Solar Cells 87 (2005) 733–746[2]: Kanwal, Alokik. A Review of Carbon Nanotube Field Effect Transistors (Version 2.0). (2003).[3]: “Quantum Dots Explained.” Evident Technologies. 2008. http://www.evidenttech.com/quantum-dots-explained.html. 06 July 2009.[4]: Avila et al, Molecular Mechanics Applied to Single-Walled Carbon Nanotubes. Mat. Res. [online]. 2008, vol.11, n.3 [cited 2009-07-15], pp. 325-333[5]: Pileni, Marie-Paule, Nature Materials 2, 145–150 (2003)[6]: Chaiwat Engtrakul, et al, Self-Assembly of Linear Arrays of Semiconductor Nanoparticles on Carbon Single-Walled Nanotubes,J. Phys. Chem. B, 2006, 110 (50), 25153-25157

The synthesized quantum dots have exciton peaks at 415nm, 380nm, and 350nm so they absorb very efficiently at these wavelengths (see Figure 7).

The absorption curve of single-walled carbon nanotubes (SWCNTs) are much broader than that of the quantum dots but there is an absorption peak at 440nm (see Figure 8).

Figure 2: Quantum Dot Structure, Ref 5

Figure 1: Carbon Nanotube Structure, Ref 4

Methods and MaterialsQuantum Dot Preparation:

• 5ml of CdNTA and 0.6ml of decylamine were placed in one beaker and 7.5mlof Na2SeSO3 was placed in another beaker.

• The temperatures of the two beakers were equalized in a water bath at 55oCfor 2-3 minutes.

• The contents of the two beakers were then combined and stirred manually.

• 5ml of toluene were then added to the mixture, stirred manually and allowed tosit for 5 minutes.

• 5ml of toluene was again added, stirred and set for 5 minutes. Toluene addition,in this manner, was continued until a total of 20 ml was added to the quantum dot (QD) solution.

• After the final addition of 5ml toluene, the QD solution was left in the bath for 5 minutes and then removed to cool at room temperature for ½ an hour.

• The CdSe QD solution was then separated from the water suspension by decantation, combined with a polymer solution (0.05g of polystyrene (PS) per 1ml of toluene), and sonicated for 48 hours before being stored in multiple, small, sealed cuvettes in a refrigerator.

Single Walled Carbon Nanotube (SWCNT) Purification:

• 100mg of unpurified SWCNT were heated at 470oC for 1 hour.

• The sample was cooled down in air, washed with HCL, and sonicated for 30 minutes.

• The sample was then centrifuged at 6000 RPM for 5 minutes and then at 13000 RPM for 5 minutes.

• The HCL was then pipetted out and the SWCNT were washed three times.

• The washing process included sonication with distilled water for 30 minutes, centrifuging at 6000 rpm for 5 minutes then at 1300rpm for 5 minutes, followed by the removal of distilled water via pipette.

• Finally, the thoroughly washed SWCNT were dried at 100oC.

Material Combinations:

The SWCNT and QDs were characterized in the following combinations: SWCNT, SWCNT + PS, QD, QD + PS, SWCNT + QD, and SWCNT + QD + PS.

Film Making:

The same film making procedure was used on all organic solutions:

25.The spin-coater was spun (with a clean quartz disk) at 100 rpm.

27.The sample was added drop wise to the center of the disk with a pipette until the entiresurface was covered.

29.The speed was then slowly increased to 1000 rpm, maintained at that level for 20 secondsand then slowly reduced to 200 rpm for 20 minutes in order to dry the sample.

31.The spin speed was then quickly increased to 2000 rpm. (This instantaneous speedchange was necessary to flush out any solution that may have leaked under the surfaceand collected at the bottom of the disk.)

33.New layers were added using the same procedure.

35.When a film of desired thickness was obtained, the bottom of disk was cleaned with a wipesoaked in acetone.

Characterization:

Each material was characterized for its absorption spectrum (using an absorption spectrometer) and two-photon absorption and non-linear refractive index characteristics (using open and closed aperture z-scans, respectively) both in solution and as film.

Figure 7: Absorption Pattern of CdSe Quantum Dot Solution Figure 8: Absorption Pattern of SWCNT Solution

Z-Scans were performed on the CdSe quantum dots to investigate the two photon absorption characteristics (see Figure 9). Non-linear refractive index changes (self-lensing effects) were measured in the polymers that were used as the matrices suspending the nanomaterials. Figure 10 shows the non-linear refractive index changes in CS2, which was used to calibrate the setup used. Data from the samples are comparted with data from the CS2 readings in order to calculate the non-linear refractive index changes in the samples.

Figure 9: Z-Scan of CdSe Quantum Dot Solution at 13uJ and 532nm

DiscussionFigures 7 and 9 show that quantum dots are not only good absorbers at 415nm, 380nm, and 350nm but given a high enough intensity are also absorbent at 532nm essentially giving them good spectrum of absorbance. This coupled with the fact that carbon nanotubes can be functionalized to conduct electrons from quantum dots [6] indicate that very efficient photovoltaic cells can be created which are flexible (active materials are embedded in a polymer matrix) and at the same time are cheaper to produce than single-crystalline or poly-crystalline silicon. Furthermore, other quantum dots may also be embedded in the material to increase the spectral range of these solar cells.

Future Work

Film Making:

Functionalized carbon nanotubes with quantum dots in a polymer solution will be spin-coated into thin films from a solution in toluene or chloroform. The films will be grown to varying thicknesses and various polymers will be utilized (including P3OT, PMMA, and PS).

Electrode Attachment:

Upon perfecting the spin coating technique required for each material combination, the film will be spin coated onto quartz plates, half the surface of which has been coated with ITO (ITO forming 1 electrode). The other electrode of the photovoltaic cell will be attached on top of the film after which radiometric studies will be performed on the solar cells to characterize them for energy conversion efficiency.

Figure 4: Purified SWCNT

Figure 5: Film Making

Acknowledgements

I would like to thank my mentors Professor Roger Dorsinville, Professor Ardie Walser for allowing me the opportunity to work in their laboratories. I would like to thank Professor Lusik Hovhannisyan for working with me. I would also like to thank Professor Mohammed Ali Ummy for contributing his time to the project. Finally, I would like to thank my supervisor and co-worker Mohammed Faissal Halim for his patience and support.

Figure 6: Z-Scan Setup

YAG Laser

Laser Attenuation Optics

Scanning Platform

Complete Z-Scan Setup

Figure 3: CdSe QD Solution

Figure 10: Z-Scan of CS2 Solutionat 8uJ and 1064nm

3rd OrderOptical non-linearsusceptibility ofSample Light Intensity

Incident on Sample

Light IntensityIncident on CS2

3rd OrderOptical non-linearsusceptibility ofCS2

Path Lengthin CS2

Path Lengthin Sample

Peak-ValleyDifferencein CS2

Peak-ValleyDifferencein Sample

=

x

x

x

x

x

The following equation is used to calculate the 3rd order non-linear optical susceptibility of any given sample: