Publish your research in the Journal of Nanophotonics

Solar & Alternative Energy

Toward textile-based solar cells

Max Shtein

A fiber-based organic photovoltaic may form the building block of cost- effective, energy-harvesting textiles.

A 100km² area covered with 10% efficient solar cells can produce enough electricity to satisfy the national requirement.¹ 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 temperature² without the need to make crystalline bonds between the two.³ 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.

Figure 1. The large yellow square represents an area of land that would need to be covered with 10% efficient solar cells to satisfy the national energy demand. In comparison, the arrow points to a small red square whose relative area represents the worldwide area of solar cells currently manufactured and installed. The area of the fabrics image represents the total square meters of textiles imported to the United States. The bottom right image shows the fiber-based organic solar cell, photographed near a penny for size comparison.

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,⁷ thermoelectric generation,⁸ and lighting.⁹ Others have tried fiber-shaped solar cells using polycrystalline silicon,¹⁰ dye-sensitized titanium dioxide,^11,12 and polymers.¹³ We developed¹⁴ 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 published¹⁶ 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 250km²of 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.

Max Shtein

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.

References:

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

DOI: 10.1117/2.1200808.1240

A 100km2 area covered with 10% efficient solar cells can produce enough electricity to satisfy the national requirement.

Hang on... assuming the grid to support such a vast network of technology could be flawless (and leaving aside the arguments against covering an appropriately sunny region with photovoltaic arrays of any type 62 miles on a side), these critters have a DC output, meaning that an even larger array has to be constructed to handle the conversion losses to feed the national AC grid (and give or take conversion from there to jump the voltage). Add to that the additional capacity needed during shunts (shunting would be a necessity during malfunctions, which could be due to wind, gee, ya think? Or lightning strikes, decline in efficiency of individual cells, plane crashes, debris from space, manufacturing flaws, installation mistakes, TERRORIST ATTACK, etc), plus growth in demand...

Electricity costs would go up on the generation side, which means such a massive array won't be built until the consumption side stops being regulated, or at least becomes more sanely regulated (for example, electricity should cost more at peak consumption times -- else it isn't going to be generated in a more expensive manner, ever; as an analogy, gasoline costs more when demand rises or supply thins).

From the files:

'Denim' solar panels to clothe future buildings
by Jenny Hogan
15:30 15 February 03
Unlike conventional solar cells, the new, cheap material has no rigid silicon base. Instead, it is made of thousands of inexpensive silicon beads sandwiched between two thin layers of aluminium foil and sealed on both sides with plastic. Each bead functions as a tiny solar cell, absorbing sunlight and converting it into electricity. The aluminium sheets give the material physical strength and act as electrical contacts... The bumpy surface presented by the spheres offers a large area for absorbing light, giving the material an overall efficiency of 11 per cent. This is comparable to the performance of conventional photovoltaic cells, and much better than proposed flexible designs based on conducting polymers.
Solar cells aiming for full spectrum efficiency
by Jenny Hogan
10:15 08 December 02
Today's best cells have layers of two different semiconductors stacked together to absorb light at different energies but they still only manage to use 30 per cent of the Sun's energy. Theorists have calculated which two bandgaps would give a maximum efficiency of 50 per cent, but until now they have not had the semiconductors to do the job. Now Wladek Walukiewicz and his team at the Lawrence Berkeley National Laboratory in California have found a material that fits the bill - a semiconductor called indium gallium nitride (InGaN)... There is one catch. Scientists had previously overlooked InGaN because its bandgap range was thought to be much smaller - data books quote the lower limit to be twice as high as Walukiewicz claims. The difference may be due to the purity of the semiconductor. The samples Walukiewicz tested were made using a painstaking, and prohibitively expensive, method to grow very pure crystals of InGaN one atomic layer at a time. The team now hopes to collaborate with the National Renewable Energy Laboratory in Colorado to try to build cheap InGaN solar cells.
SunPower Announces
World's Most Efficient,
Low-Cost Silicon Solar Cell
by Bobby Ram & Agnes Toan
May 12, 2003
Based on a unique rear-contact design -- which maximizes the working cell area, hides unsightly wires and makes automated production easier -- the A-300 achieves over 20 percent efficiency compared with currently available cells in the 12% - 15% range. The National Renewable Energy Laboratory (NREL) has verified 20.4 percent conversion efficiency for the A-300... [T]he A-300 silicon solar cell delivers 3-kW in less than 17 square meters, allowing SunPower's customers to trump the world's best area-efficient construction designs... The A-300 cells eliminate front-side metallization by utilizing a patented rear-contact design to provide more cosmetic uniformity and maximize energy generation.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.

Related Links

Solar & Alternative Energy

Toward textile-based solar cells