Introduction
In 1st term paper, I mentioned electrical conductivity of conducting polymer, types of conducting polymer and application areas of conducting polymer. There are many application areas of conducting polymer. currently most focus is given to organic light emitting diodes(OLED) and organic polymer solar cell.
so in this final term paper, I'd like to talk about polymer(plastic) solar cell based on conducting polymers.
Polymer solar cells are a type of organic solar cell (also called plastic solar cell), or organic photovoltaic cell that produce electricity from sunlight using polymers. It is a relatively novel technology, they are being researched by universities, national laboratories and several companies around the world.
Currently, many solar cells in the world are made from a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips (wafer silicon). The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies.
Compared to silicon-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), disposable, inexpensive to fabricate (sometimes using printed electronics), flexible, customizable on the molecular level, and have lower potential for negative environmental impact.
Operational Principle of Polymer Solar Cell
Organic photovoltaics are comprised of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy bandgap between these orbitals determines which wavelength of light can be absorbed.
Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4eV. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbed the photon is the donor, and the material acquiring the electron is called the acceptor. The polymer chain is the donor and the fullerene is the acceptor. After dissociation has taken place, the electron and hole may still be joined as a geminate pair and an electric field is then required to separate them.
After exciton dissociation, the electron and hole must be collected at contacts. However, charge carrier mobility now begins to play a major role: if mobility is not sufficiently high, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other. In that case, space-charge limited photocurrent (SCLP) hampers device performance.
As an example of the processes involved in device operation, organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron transfer from the polymer chain to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle, or polaron (P+), occurs on the polymer chain and the fullerene becomes an ion-radical C60- Polarons are highly mobile along the length of the polymer chain and can diffuse away.
Polymer solar cell Structure and Fabrication Method
Almost all polymer solar cells have a planar-layered structure, where the organic light-absorbing layer is sandwiched between two different electrodes. One of the electrodes must be (semi-) transparent, often Indium–tin-oxide (ITO), but a thin metal layer can also be used. The other electrode is very often aluminium (calcium, magnesium, gold and others are also used). Basically, the underlying principle of a light-harvesting organic PV cell (sometimes referred to as photodetecting diodes) is the reverse of the principle in light emitting diodes (LEDs) (see Fig. 1) and the development of the two are somewhat related.
Fig. 1. A PV device (Left) is the reverse of a LED (Right). In both cases an organic material is sandwiched between two electrodes. Typical electrode materials are shown in the figure. In PVs electrons are collected at the metal electrode and holes are collected at the transparent electrode. The reverse happens in a LED: electrons are introduced at the metal electrode (cathode), which recombine with holes introduced at the transparent electrode (anode).
Production of the active layer of a polymer solar cell is simple: first the donor and acceptor materials are dissolved in a solvent and are then excited through dropping, spinning or pressing these onto a suitable substrate. After the solvent has evaporated, a homogeneous film of a thickness of approx. 100 nanometer (nm) is formed – this is about one two-hundredth of the thickness of a human hair. The simple printing of polymers onto a film promises low manufacturing costs for “solar cells off the role”. Application of existing printing technologies could facilitate large-scale technical implementation.
Polymer Solar cell Materials
Organic photovoltaics refers to solar cells on the basis of organic semiconductor materials (mostly dyes) that can generate electric current from light. The Graetzel cell, an electrochemical dye-sensitised solar cell named after its inventor Michael Grätzel of the Swiss Technical University Lausanne, for example, uses the chlorophyll molecule of a leaf with which plants convert sunlight into chemical energy. A variant of organic photovoltaics are electrically conductive polymers (hydrocarbon polymers) used by Alan Heeger. The principle according to which both organic as well as polymer solar cells function is based on a transfer of electrons that is initiated by sunlight, the so-called Donor-Acceptor System.
The photoactive layers in organic and polymer solar cells usually consist of an electron donor and an electron acceptor material. For organic solar cells, dyes from the group of the so-called phthalocyanines are used as donors and molecules from the hydrocarbon atoms (with fullerene) are used as acceptors. The layers are generally created through the separation of materials from the gaseous phase in vacuum.
In polymer solar cells compound polymers are used as donors and, in some cases, also as acceptors. Often fullerenes are utilised as electron acceptors. The structure of a polymer solar cell is similar to that of an organic solar cell, although its photoactive layer consists of a donor-acceptor combination.
Due to high exciton binding energy, polymer solar cells consist of a blend of two materials, one acting as an electron donor, and the other as an acceptor, to promote exciton dissociation.2 The acceptor functions as electron transporter, while the donor transports holes. Figure 2 shows the chemical structures and energy levels of a typical electron-donating polymer, poly-3-hexylthiophene (P3HT), and electron-accepting fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).
Figure 2. Chemical structures and energy levels of F8TBT, PCBM, and P3HT. The energy diagrams show the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the materials, and the arrows indicate the direction of electron (e−) and hole (h+) transfer. R = C6H13.
Scheme 2. shows the chemical structure of fullerene based polymer as an acceptor.
Scheme 2. Chemical Structure of fullerene based polymer as an acceptor
Current research & commercial status
For the reasons described above, polymer solar cells are not generally produced commercially today. One exception is the company Konarka Technologies, which in 2008 announced the company has opened the largest roll-to-roll flexible thin film solar manufacturing facility in the world, preparing for the commercialization and mass production of its patent-protected thin film solar material, Power Plastic. Located in New Bedford, Massachusetts, the 250,000 square foot building was previously the location for Polaroid Corporation’s most advanced printing technologies.
Please, see the below linked youtube vedio. it explains the current research status and the future of polymer solar cell very well.
http://www.youtube.com/watch?v=ilI5rl2fQ_Q
The initial cells from the factory are 3-5% efficient, and only last a couple years, but the company has stated that it would eventually be able to improve both the efficiency and durability. The company expects to initially sell the cells in for number of niche applications: For example, in laptop-recharging briefcases, put into tents, umbrellas, and awnings, and as window tinting (since the cells can be made transparent).
Nature Photonics This week ( 05/01/2009 ), in the online edition of Nature Photonics, researchers reported on polymer solar cells that convert about 6.1 percent of the energy in sunlight into electricity—inching a bit closer to the 10 percent that they say will be needed to gain a significant foothold in the market. (Conventional silicon cells are about 15 percent efficient.) The new efficiency numbers "show that we're in the game," says Alan Heeger, a professor of physics at the University of California, Santa Barbara, who led the research. Heeger shared the Nobel Prize in Chemistry in 2000 for his role in the development of the first conducting polymers, and he's cofounder and chief scientist at Konarka, a plastic solar cell company headquartered in Lowell, MA.
The California researchers' results compare very favorably with previous published descriptions of polymer solar cells, whose efficiency has hovered around 5 percent. Konarka says that the company's cells, which use different materials than the cells made in Heeger's university lab, have recently been rated at about 6.4 percent.
Organic materials are still limited to visible light," says Yang, but much of the sun's energy is in the neighboring part of the spectrumthe infraredso polymer scientists are working on solar-cell materials that can also absorb this band. The University of Chicago's Yu, who is collaborating with Solarmer Energy, says that the company has used his polymers, which absorb shorter-wavelength light, to make cells that should achieve more than 7 percent efficiency, but he cannot disclose the details because the results have not yet been published.
Reference
1. Wikipedia < Polymer Solar Cell >
2. Dr.Sotzing Class Materials. UCONN IMS Polymer Program.
3. 6 November 2007, SPIE Newsroom. < Polymers show versatility in organic solar cells >
4. Nature Materials 5, 675 - 676 (2006) < Solar cells: Pictures from the blended zone >
5. Chem. Rev. 2007, 107, 1324-1338 < Conjugated Polymer-Based Organic Solar Cells >
6. Solar Energy Materials & Solar Cells 83 (2004) 125–146 < A brief history of the development of organic and polymeric photovoltaics >
7. Photonics Polymer Lab. MSE, KJIST, South Korea, Lecture Note.
8. http://www.youtube.com/watch?v=ilI5rl2fQ_Q&feature=PlayList&p=48102B42BCF02D29&index=0, Konarka Inc.
9. Adv. Mater. 2009, 21, 1–16. < Polymer-Fullerene Bulk-Heterojunction Solar Cells >
10. Nature Photonics 3, 297 - 302 (2009) < Bulk heterojunction solar cells with internal quantum efficiency approaching 100% >
