Toward textile-based solar cells
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.
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 250km2 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.
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.
