GREEN NANO TECHNOLOGY

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					GREEN NANO TECHNOLOGY

Materials Engineering 297 Christy Peters 10 May 2006

Abstract Many avenues are being explored to find alternative energy sources and to increase the efficiency of the resources we are already using. The field of efficiently utilizing solar radiation is one of the most promising today because of the large amount of sunlight available on this planet. Conventional solar power technology has two main drawbacks. The cost of manufacturing the components used in solar cells is high, and the efficiency of these devices is low. The use of nano technology is being investigated as a way to overcome these drawbacks. Nano particles have unique qualities which enable them to be tuned to absorb a wider range of wavelengths available in sunlight, thereby increasing efficiency. Their use lowers the cost of manufacture. This paper will explore the possibilities that nano particles offer to the field of solar energy. Introduction Traditional solar technology has relied on silicon as the semiconductor in solar cells. Its job is to absorb photons of light, thereby exciting electrons from the valence band to the conduction band and creating positively charged holes. Keeping these charge carriers separated and getting the electrons to flow and provide power is the basic idea behind getting usable energy from the sun. One limitation to this type of a set up is the band gap of silicon. It cannot be changed easily in the bulk material. Only light that possesses energy equal to this band gap is utilized. Silicon absorbs light in the 500 – 1000 nm range. See Figure 1[1]. Lower energy light passes through the cell unused. Higher energy light does excite electrons to the conduction band, but any energy beyond the band gap energy is lost as heat. If these excited electrons aren’t captured and redirected, they will spontaneously recombine with the created holes, and the energy will be lost as heat or light.

High quality silicon needed for these cells is expensive, as is the batch rather than continuous processing used in manufacturing these cells. Thin films made from less expensive cadmium telluride or cadmium sulfide make the cells more efficient, but vacuum deposition required in the manufacturing still keeps costs high.

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Quantum Dots or Nanocrystals One area of research focuses on quantum dots or nanocrystals. They are made of materials from periodic groups II – VI, III – V, or IV – VI. They range in size from 2 – 10 nm in diameter. Electrons in bulk semiconductor material have energy levels that are so close together that they behave as if the levels were the same. This characteristic sets the band gap energy at a fixed amount. Quantum dots behave differently. An exciton is defined as an excited electron–hole pair. The average distance between an excited electron and a hole is called the Exciton Bohr Radius.

Figure 2. The Exciton Bohr Radius in relation to a quantum dot. [1]

In bulk material, this radius is much smaller than the semiconductor crystal. But nanocrystal diameters are smaller than this Bohr radius. See Figure 2. Because of this, the “continuous band” of electron energy levels no longer can be viewed as continuous. The energy levels become discrete, and quantum confinement is seen to operate. The difference of a few atoms between two quantum dots alters the band gap boundaries. Small nanocrystals absorb shorter wavelengths or bluer light, whereas larger nanocrystals absorb longer wavelengths or redder light. Changing the shape of the dot also changes the band gap energy level as shown in Figure 3.

Figure 3. The relationship of size of quantum dot to the light absorbed.[1]

Multiple excitons can be generated in quantum dot solar cells. If the irradiated energy is 2 – 4 times the band gap energy, two or more excitons are generated in PbSe 3

and PbS quantum dots. These particular quantum dots also absorb infrared radiation otherwise wasted by conventional cells. Tunable band gaps which increase the range of sunlight captured, and less expensive materials and processing are areas of active research. As exciting as these advances are, they represent only part of the challenge of efficient solar energy conversion. Once these electrons are excited, they must be prevented from recombining with holes, and they must be transported to electrodes. Gratzel Cells In the early 1990’s, Professor Gratzel developed a solar cell utilizing a dye that absorbs light. This arrangement is shown in Figure 4.

Figure 4. Basic Gratzel cell composition.[2]

Nanoparticles of titanium dioxide, widely available and less expensive than silicon, are used in these cells. TiO2 has a wide band gap and is able to conduct electrons efficiently once they are in the conduction band. Its band gap is too wide to absorb much sunlight, but when the TiO2 particles are coated with a metalorganic ruthenium-based dye, the dye absorbs light, becomes oxidized (loses electrons), and injects these electrons into the TiO2. These diffuse to the electrode while the holes pass to the LiI electrolyte. The electrons pass through an external load, doing work, then flow to the counter electrode. Here the electrons are carried by iodine ions to regenerate the dye through reduction (gain of electrons). Figure 5 shows this regeneration cycle.

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Figure 5. Mechanism of light absorption, electron injection and subsequent transport.[2]

Gratzel cells demonstrate 10% power conversion efficiencies which are comparable to those achieved by current silicon devices.[7] However, the dyes are sensitive to heat and light. Research is being done to replace the dye molecules with semiconducting InP, CdSe, CdS, and PbS nanocrystals. These absorb light over a broader range than the organic dyes. Nanocrystals prepared with protective shells are more stable than organic dyes and will not photo-bleach or degrade as easily as the dyes do from heat and UV light. Conjugated Polymers Conjugated polymers, polymers with “alternating single and double carbon – carbon bonds along the polymer backbones”,[3] are being utilized in photovoltaic cells. The bonding between the carbons is sp-2 hybridized, leaving one unhybridized p-z orbital which comes out of the plane of the polymer. A delocalized electron cloud results from the pi–orbitals as shown in Figure 6. Single and double carbon–carbon bonds have different lengths, and consequently have different energies. Because the bonds alternate along the backbone, they create an energy gap and act as semiconductors.

Figure 6. A carbon-carbon bond showing the delocalized electron cloud from the unhybridized p-z orbital.[3]

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Hybridization of 3s and 3p orbitals gives four identical hybrid orbitals. Neighboring atoms overlap to form bonding and anti-bonding orbitals. The bonding orbitals overlap, becoming the valence band. The anti-bonding orbitals overlap, becoming the conduction band.[3] These polymers are less expensive to make than inorganic photovoltaics and are being explored for use in solar cells. Here in Figure 7 is one such arrangement using PPV (poly (2-methoxy-5-(3’,7’-dimethyl octoxy)-p-phenylene vinylene) as the electron donor and PCBM (1-(3(methoxy carbonyl) propyl-1-phenyl [6,6] methono-fullerene) as the electron acceptor.

Figure 7. A polymer solar cell on the left, and the photovoltaic process of the cell on the right.[4]

An improved fullerene (C70) that shows better absorption in the visible spectrum has been made. Its structure along with its isomers is shown in Figure 8.

Figure 8. The improved fullerene (C70) and its isomers.[5]

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The graph in Figure 9 shows the broader absorption range achieved with the C70 fullerene. Its line on the graph is the solid one.

Figure 9. Graph showing the improved range of absorption of the C70 fullerene.[5]

Use of polymers as electron donors to replace dyes in the Grazel cells is also being investigated. MEV-PPV (poly (2-methoxy-5(2-ethyl) hexoxy-phenylene-vinylene) is layered with titanium dioxide (TiO2) film. The TiO2 layer has particles 80 nm wide with 20 nm pore diameters between them.[6] The polymer flows into these pores, enhancing the interface between the electron donor (the polymer) and the electron acceptor (TiO2). This shortens the exciton diffusion length and overcomes the degradation of the ruthenium-based dyes. The mechanism of electron transfer from the polymer to the TiO2 nanoparticle occurs because of a lowering of electron energy upon transfer as illustrated in Figure 10.

Figure 10. Diagram showing the alignment of energy levels at the TiO2/MEH-PPV interface. An electron in the polymer (MEH-PPV) lowers its energy by transferring to a TiO2 nanoparticle.[6]

Polymers and Inorganic Nanocrystals Conjugated polymers have low electron mobilities and contain electron traps like oxygen.[8] They also degrade from UV light. Another avenue of research explores the blending of polymers with inorganic nanocrystals. This combination provides an interface for charge transfer. It gives the broader absorption range and higher carrier mobilities of inorganic semiconductors and the lower expense of solution processing and

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low-temperature synthesis of organic semiconductors.[8] Higher carrier mobility means faster charge transport to electrodes and less loss due to recombination. Initial low efficiencies of these blends is due to the electrons’ becoming trapped in dead-ends in the polymer matrix as they hop between nanocrystals. The electrons eventually recombine with holes in the polymers, never arriving at the electrodes. Nanocrystals grown into rods transport electrons along their long axis. When they are densely packed into aggregates, power conversion efficiencies rise from 0.25% to 2% under 514 nm illumination.[7] Another study shows that the band gap of these nanorods can be tuned by altering the radius of the rod. In this study, cadmium selenide (CdSe) nanorods are used with the conjugated polymer poly (3-hexylthiophene) (P3HT). See Figure 11A for the structure of P3HT. CdSe is electron-accepting and P3HT is hole-accepting. See Figure 11B for the energy level diagram. They have complementary absorption spectra and together can absorb light from 300 to 720 nm. The solar cell shown in Figure 11C was created “by spin-casting a solution of 90% by weight (90 wt %) CdSe nanorods in P3HT onto an indium tin oxide glass substrate coated with poly (ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS, a conducting polymer) with aluminum as the top contact.” [8]

Figure 11.(A) The structure of P3HT. (B) Energy level diagram showing the charge transfer of electrons to CdSe and holes to P3HT. (C) Structure of the solar cell device using a thin film of CdSe/P3HT blend between an aluminum electrode and a transparent conducting electrode made of PEDOT:PSS (polystyrene sulfonic acid, a conducting polymer).This is deposited on an indium tin oxide substrate.[8]

Under simulated sunlight, the power conversion efficiencies reached 1.7% for CdSe nanorods which were 7 nm by 60 nm. These larger nanorods presented a challenge in processing, because they are less soluble in the polymer matrix than rods with smaller dimensions. To overcome this solubility problem, the nanocrystals are coated with a surfactant that controls their growth, both in size and shape, and aids in their dispersion in organic solvents. The organic solvents mix well with the conjugated polymers and are needed to process these blends. However, the coatings needed in processing are 8

insulating and inhibit electronic interaction at the interface of the crystals and the polymers. Washing with pyridine strips off this coating, thereby restoring the charge transfer capabilities of the nanorods. When branched nanocrystals are compared to nanorods, the external quantum efficiency (EQE) or “the percentage of electrons collected per incident photon”[8] nearly doubles as shown on the graph in Figure 12A. This is due to a reduction in chargehopping steps for the electrons and results in a power-conversion efficiency of 1.8% in sunlight.[9] CdSe tetrapods, nanocrystals with four rod-like arms, (see Figure 12B) having 200 nm-long arms provide transport paths for charges without a single hopping step. Due to their configurations, they can do this without special attention given to their orientation.

A

B

Figure 12.(A) Graph showing the higher external quantum efficiencies of branched nanocrystals when compared with nanorods.(B) CdSe tetrapods as viewed by transmission electron microscopy.[9]

Nanocrystal-nanocrystal combinations One drawback of organic-based devices is their sensitivity to the environment, in particular to photo-oxidation. They degrade from UV light. Recent research is focusing on the use of two types of inorganic semi conducting nanocrystals with energy levels that are staggered. The materials used are rod shaped CdSe and CdTe (cadmium telluride) nanocrystals. Figure 13 shows an energy diagram of valence and conducting band levels for CdTe and CdSe.

Valence Bands

Conduction Bands

Figure 13. An energy diagram showing the staggered energy levels of CdTe and CdSe.[11]

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Thin films of CdTe are spin cast on indium tin oxide (ITO) glass and annealed for 15 minutes at 200°C to remove any remaining solvent. Then, thin films of CdSe are deposited on the CdTe layer. Aluminum is thermally deposited as a reflective top coat.[11] The bilayer has a higher external quantum efficiency (EQE) than CdSe or CdTe alone. The EQE is also higher than the one observed for a mixture of the two materials. See Figure 14.
Bilayer Mixed CdTe CdSe

Figure 14. Graph comparing external quantum efficiencies of bilayer and blend (CdTe and CdSe) devices as compared with single-material devices of CdTe and CdSe.[11]

These films are electrically insulating in the dark, but show conductivity when exposed to sunlight. The excitons (hole-electron pairs) separate as follows: the holes find lower energy states in the CdTe and diffuse into the ITO. The electrons find lower energy states in the CdSe and diffuse toward the aluminum as shown in Figure 15.

Figure 15. Schematic showing the layers in a nanocrystal-nanocrystal device and the charge transfer between layers.

Both of these starting materials are undoped, and diffusion is the driving force for carrier extraction rather than the field created from a depletion zone as in p-n junctions. There is charge separation as a result of this diffusion, and the chance of recombination of the holes and electrons is reduced.

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Annealing and sintering the nanocrystals minimizes traps, improves carrier transport and raises external quantum efficiencies to almost 70% as shown in Figure 16.

Figure 16. Graph showing the external quantum efficiency improvement and the broadening of the spectrum absorbed when the bilayer is annealed and sintered.[11]

These cells have a power conversion efficiency of 2.1%. Just by using a calcium top contact capped with aluminum increases the power conversion efficiency to 2.9%. Unlike the organic solar cells, these cells show no sensitivity to photo-oxidation. In fact, they show a 13.6% increase in efficiency as a result of atmospheric aging.[11] Summary and Conclusion In an attempt to find a cheaper, more durable and more efficient way of harnessing energy from the sun, researchers have turned to nanocrystals for some answers. Traditional silicon solar cells are expensive to make, and bulk silicon has a band gap that is not easily changed. This limits the range of sunlight that these devices can utilize. Band gaps in quantum dots can be altered by changing the size of the dots. Gratzel cells have the same power conversion efficiencies as silicon cells, but they utilize dyes which photo-degrade. Using nanocrystals and conducting polymers to replace the dyes offers some hope of providing more durable devices. All-organic devices made from conducting polymers are less expensive than traditional solar cells but present challenges of low electron mobility, electron traps and degradation from UV light. Polymer and inorganic nanocrystal devices overcome the low electron mobilities and electron traps, but still photo-degrade. All inorganic nanocrystal-nanocrystal devices have high electron mobilities, few electron traps, and are not sensitive to photo-oxidation. These look to be the most stable devices developed to date. While much work remains to be done to develop efficient and inexpensive solar cells, the progress achieved thus far is very promising.

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References: 1. 2. 3. 4. http://www.evidenttech.com/applications/solar-cell-white-paper/qdot-promise.php http://www.bath.ac.uk/chemistry/electrochemistry/gratzel.html http://www.physics.ucsc.edu/~sacarter/polymer.shtml http://www.research.philips.com/technologies/misc/matanalysis/downloads/solar. pdf 5. Hummelen, J.C. (Kees). “Improved Fullerene Materials for Plastic Photovoltaics.”http://www.esqsec.unibe.ch/%5Cpub%5Cpub_232.pdf 6. Arango, A.C., Carter, S.A., and Brock, P.J. “Charge Transfer in Photovoltaics Consisting of Interpenetrating Networks of Conjugate Polymers and TiO2 Nanoparticles.” Applied Physics Lectures Volume 74. Number 12. 22 March 1999 p 1698-1699. 7. Huynh, Wendy U., Peng, Xiaogang, and Alivisatos, A. Paul. “CdSe Nanocrystal Rods / Poly (3-hexylthiophene) Composite Photovoltaic Devices.” Advanced Materials 1999, 11, No.11 p 923-927. 8. Huynh, Wendy U., Dittmer, Janke J., Alivisatos, A. Paul. “Hybrid NanorodPolymer Solar Cells.” Science Volume 295 29 March 2002 p 2425-2427. 9. Milliron, Delia J., Gur, Ilan, and Alivisatos, A. Paul. “Hybrid OrganicNanocrystal Solar Cells.” MRS Bulletin Volume 30 January 2005 p 41-44. 10. Ziebarth, Dr. Jonathan of Nanosys Inc. Personal Interview 27 March 2006. 11. Gur, Ilan, Fromer, Neil A., Geier, Michael L., and Alivisatos, A. Paul. “Air-Stable All-Inorganic Nanocrystal Solar Cells Processed from Solution.” Science Volume 310 21 October 2005 p 462-465. 12. http://en.wikipedia.org/wiki/Solar_cell

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