Polymer Solar Cell
Currently, commercial solar cells are made from a refined, highly purified silicon crystal, similar to the material 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), potentially disposable and inexpensive to fabricate (sometimes using printed electronics), flexible, customizable on the molecular level, and have lower potential for negative environmental impact. An example device is shown in Fig. 1. The disadvantages of polymer solar cells are also serious: they offer about 1/3 of the efficiency of hard materials, and they are relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.
Fig. 2. Polymer chain with diffusing polaron surrounded by fullerene molecules
Device physics
The following discussion is based primarily on Mayer et al.’s review, cited below. 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 electrons that result from carbon p orbital hybridization. These 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 -* 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 1.4 eV. 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. In Fig. 2, 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. Both the polaron and ion-radical possess spin S= , so the charge photoinduction and separation processes can be controlled by the Electron Paramagnetic Resonance method.
Architectures
This section is derived largely from Mayer’s review, referenced below. The simplest architecture that may be used for an organic PV device is a planar heterojunction, shown in Fig. 1. A film of active polymer (donor) and a film of electron acceptor are sandwiched between contacts in a planar configuration. Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor. However, planar heterojunctions are inherently inefficient; because charge carriers have diffusion lengths of just 3-10 nm in typical organic semiconductors, planar cells must be thin to enable successful diffusion to contacts, but the thinner the cell, the less light it can absorb.
Bulk heterojunctions (BHJs) address this shortcoming. In a BHJ, the electron donor and acceptor materials are blended together and cast as a mixture that then phase-separates. Regions of each material in the device are separated by only several nanometers, a distance optimized for carrier diffusion. Although devices based on BHJs are a significant improvement over planar designs, BHJs require sensitive control over materials morphology on the nanoscale. A great number of variables, including choice of materials, solvents, and the donor-acceptor weight ratio can dramatically affect the BHJ structure that results. These factors can make rationally optimizing BHJs difficult.
The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). This paradigm eliminates much of the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO2. Holes still must diffuse along the length of the pore through the polymer to a contact, so OHJs do have thickness limitations. Mitigating the hole mobility bottleneck will thus be key to further enhancing OHJ device performance, but controlling morphology inside the confines of the pores is challenging.
Engineers at the University of California, San Diego (UCSD) have employed “nanowires” to boost the efficiency of organic solar cells .
Conclusion
At the moment, an open question is to what degree polymer solar cells can commercially compete with silicon solar cells and the other thin-film cells. The silicon solar cell industry has the important industrial advantage of being able to leverage the infrastructure developed for the computer industry. Besides, the present efficiency of polymer solar cells lies near 5 percent, much below the value for silicon cells. Polymer solar cells also suffer from environmental degradation. Good protective coatings are still to be developed.
Still, organic PV devices show great promise for decreasing the cost of solar energy to the point where it may become widespread in the decades ahead. While great progress has been made in the last ten years with respect to understanding the chemistry, physics, and materials science of organic photovoltaics, work remains to be done to further improve their performance. Specifically, novel nanostructures must be optimized to promote charge carrier diffusion; transport must be enhanced through control of order and morphology; and interface engineering must be applied to the problem of charge transfer across interfaces. Novel molecular chemistries and materials offer hope for revolutionary, rather than evolutionary, breakthroughs in device efficiencies in the future.
Current 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 opened a factory with the capacity to produce a gigawatt’s worth of polymer-fullerene solar cells each year. 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).
Other second generation solar cells
Dye-sensitized solar cell
Hybrid solar cell
Nanocrystal solar cell
Photoelectrochemical cell
See also
Layer (electronics)
Substrate
References
N.S. Sariciftci, L. Smilowitz, A.J. Heeger,F. Wudl, Photoinduced Electron Transfer from Conducting Polymers onto Buckminsterfullerene, Science 258, (1992) 1474
N.S. Sariciftci, A.J. Heeger, Photophysics, charge separation and device applications of conjugated polymer/fullerene composites, in Handbook of Organic Conductive Molecules and Polymers, edited by H.S.Nalwa, 1, Wiley, Chichester, New York, 1997, Ch. 8, p.p. 413-455
lastic Solar Cells Christoph J. Brabec, N. Serdar Sariciftci, Jan Kees Hummelen, Advanced Functional Materials, Vol. 11 No: 1, pp.15-26 (2001)
Organic Photovoltaics, Christoph Brabec, Vladimir Dyakonov, Jrgen Parisi and Niyazi Serdar Sariciftci (eds.), Springer Verlag (2003) ISBN: 3-540-00405
Organic Photovoltaics: Mechanisms, Materials, and Devices (Optical Engineering), (Sam-Shajing Sun and Niyazi Serdar Sariciftci (eds.), CRC Press (Taylor&Francis Group) ISBN: 0-8247-5963-X, Boca Raton, 2005
A. Mayer, S. Scully, B. Hardin, M. Rowell, M. McGehee, Polymer-based solar cells, Materials Today 10, (2007) 28. doi:10.1016/S1369-7021(07)70276-6
H. Hoppe and N. S. Sariciftci, Polymer Solar Cells, p. 1-86, in Photoresponsive Polymers II, Eds.: S. R. Marder and K.-S. Lee, Advances in Polymer Science, Springer, ISBN: 978-3-540-69452-6, Berlin-Heidelberg (2008)
Footnotes
^ Joachim Luther, Michael Nast, M. Norbert Fisch, Dirk Christoffers, Fritz Pfisterer, Dieter Meissner, Joachim Nitsch “Solar Technology” 2002, Wiley-VCH, 2008, Weinheim. doi:10.1002/14356007.a24_369
^ http://ucsdnews.ucsd.edu/newsrel/science/05-08Nanowires.asp
^ a b Mass Production of Plastic Solar Cells, Technology Review Magazine, Kevin Bullis, October 17, 2008.
External links
NREL reports
LIOS – Linzer Institut fr Organische Solarzellen, Johannes Kepler Universitt Linz, sterreich
Quantsol 1998
Categories: Photovoltaic technologies | Thin-film cells | Organic photovoltaics | Fullerenes | Infrared photovoltaics
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