Toward Textile-based Solar Cells
A fibre-based organic photovoltaic may form the building block of cost-effective, energy-harvesting textiles
A 100km 2 area covered with 10% efficient solar cells can produce enough electricity to satisfy the national requirement.  Unfortunately, the total area of cells produced and installed to date is 1,000 times smaller than needed. Despite the high annual growth rate of the photovoltaic (PV) industry, current manufacturing methods face a scalability barrier that makes fulfilling demand unlikely in the next 20 years.
Manufacturing of organic pigment-based solar cells could be expanded, because the dyes are made at the commodity scale. In addition, device-quality organic thin films can be deposited onto virtually any kind of substrate at room temperature  without the need to make crystalline bonds between the two.  Unfortunately, the relatively low efficiency—about 5%—of organic solar cells, the need for expensive ingredients like indium tin oxide (ITO), and the substantial installation costs prevent widespread deployment.
To address these challenges, we developed an ITO-free, fiber-based organic cell that could become the building block in rapid, cost-effective manufacturing of energy-harvesting textiles. Researchers have studied optoelectronic devices on fabrics for large-scale electronics, [4–6] sensing, thermoelectric generation,  and lighting.  Others have tried fiber-shaped solar cells using polycrystalline silicon,  dye-sensitized titanium dioxide, [11,12] and polymers.  We developed  and fabricated a fiber-based organic PV (OPV) cell in a precise and quantifiable manner, with a configuration that facilitates scale-up. This method enables us to directly compare our device to its planar analogues.
A typical OPV cell consists of active organic materials sandwiched between two electrodes. We created the proof-of-principle fiber cell by depositing the electrodes and organic layers conformally onto a fiber using thermal evaporation. (We used vacuum thermal evaporation, an industrial method for metalizing food packaging, because it enables precise thickness control and scalability.) Light is absorbed through the outer electrode, which is made of an ultrathin metal film.
Compared to the conventional ITO electrode, ours is less transmissive and more reflective at oblique incidence angles, which decreases photocurrent and cell efficiency. Nevertheless, the fiber geometry has advantages in realistic usage conditions. Typical solar cell efficiency depends strongly on the angle of illumination. To maintain efficiency, these cells must use solar tracking. By contrast, the symmetry of the fiber device keeps power generation constant with incidence angle. This property enables the OPV fiber to outperform its planar analogue by 30% or more when hit with diffuse light, or with sunlight coming from a range of positions over the course of the day. Furthermore, the solar cell itself is just a thin ( ~200nm) coating on a fiber, which may allow the energy-harvesting fabric to be much lighter than thin-film, fabric-laminated designs.
The materials used in the initial study limited the power conversion efficiency of the organic material combination to 1% in the planar configuration, and only 0.5% for a single fiber. However, theoretical efficiency values for OPV cells reach more than 15%, 15 and values of ~5% have been published 16 for other (purified) organic compounds. We plan to use variants of these compounds for future deposition on fibers. We and others have also shown that antireflection coatings, 17,18 which can function simultaneously as protective barrier films, may further improve efficiency.
In addition, scalability of fiber-based OPVs may be within reach. Using our deposition system with single-point evaporation sources, we have shown that cells spanning 3cm of the fiber (near the system limit for forming uniform layer thickness) performed nearly identically. This result motivates our efforts to fabricate longer fibers and weave them together.
Finally, the United States imports the equivalent of 250km 2 of textiles annually. 19 The coloring dyes used in textiles are also manufactured at the commodity scale. These dyes are often quite similar, if not identical in their chemical structure, to those used in small molecular organic solar cells. Thus, cost-effective manufacturing and deployment of fiber-based solar cells is feasible, and I believe that the versatility of the fiber form factor will continue to inspire research on this topic.
1. D. Ginley, A. Martin, R. Green, Solar energy conversion toward 1 terawatt, MRS Bull. 33, pp. 355, 2008.
2. Y. Zhao, K. An, S. Chen, B. O’Connor, K. Pipe, M. Shtein, Localized current injection and submicron organic light-emitting device on a pyramidal atomic force microscopy tip, Nano Lett. 7, pp. 3645, 2007.
3. S. Forrest, Ultrathin organic films by organic molecular beam deposition and related techniques, Chem. Rev. 97, pp. 1793, 1997.
4. M. Hamedi, R. Forchheimer, O. Inganas, Towards woven logic from organic electronic fibres, Nat. Mater. 6, pp. 357, 2007.
5. J. Lee, V. Subramanian, Weave patterned organic transistors on fiber for e-textiles, IEEE Trans. Electron Dev. 52, pp. 269, 2005.
6. R. Service, Electronic textiles charge ahead, Science 301, pp. 909, 2003.
7. M. Bayindir, F. Sorin, A. Abouraddy, J. Viens, S. Hart, J. Joannopoulos, Y. Fink, Metal-insulator-semiconductor optoelectronic fibres, Nature 431, pp. 826, 2004.
8. A. Yadav, K. Pipe, M. Shtein, Fiber-based flexible thermoelectric power generator, J. Power Sources 175, pp. 909, 2008.
9. B. O’Connor, K. An, Y. Zhao, K. Pipe, M. Shtein, Fiber shaped organic light emitting device, Adv. Mater. 19, pp. 3897, 2007.
10. H. Kuraseko, T. Nakamura, S. Toda, H. Koaizawa, H. Jia, M. Kondo, Development of flexible fiber-type poly-Si solar cell, IEEE 4th World Conf. Photovolt. Energy Convers. 2, pp. 1380, 2006.
11. X. Fan, Z. Chu, L. Chen, C. Zhang, F. Wang, Y. Tang, J. Sun, D. Zou, Fibrous flexible solid-type dye-sensitized solar cells without transparent conducting oxide, Appl. Phys. Lett. 92, pp. 113510, 2008.
12. J. Ramier, C. Plummer, Y. Leterrier, J. Manson, B. Eckert, R. Gaudiana, Mechanical integrity of dye-sensitized photovoltaic fibers, Renewable Energy 33, pp. 314, 2008.
13. J. Liu, M. Namboothiry, D. Carroll, Fiber-based architectures for organic photovoltaics, Appl. Phys. Lett. 90, pp. 063501, 2007.
14. B. O’Connor, K. Pipe, M. Shtein, Fiber based organic photovoltaic devices, Appl. Phys. Lett. 92, pp. 193306, 2006.
15. G. Dennler, M. Scharber, T. Ameri, P. Denk, K. Forberich, C. Waldauf, C. Brabec, Design rules for donors in bulk-heterojunction tandem solar cells: towards 15 percent energy-conversion efficiency, Adv. Mater. 20, pp. 579, 2008.
16. W. Ma, C. Yang, X. Gong, K. Lee, A. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15, pp. 1617, 2005.
17. B. O’Connor, K. An, K. Pipe, Y. Zhao, M. Shtein, Enhanced optical field intensity distribution in organic photovoltaic devices using external coatings, Appl. Phys. Lett. 89, pp. 233502, 2006.
18. M. Agrawal, P. Peumans, Broadband optical absorption enhancement through coherent light trapping in thin-film photovoltaic cells, Opt. Express 16, pp. 5385, 2008.
19. US textile imports from January to March 2008. Accessed 21 August 2008. http://www.textileinfo.com/en/news/2008_02/0714_05.html
Materials Science and Engineering
University of Michigan
Ann Arbor, MI
Max Shtein has been a professor of materials science and engineering at the University of Michigan since 2004. He received his PhD at Princeton University the same year, specializing in vapor phase deposition and vapor jet printing of organic optoelectronic devices.
Source: University of Michigan , http://spie.org/x27087.xml?pf=true&highlight=x2358