Dye-Sensitized Solar Cells
DOE supports research and development projects aimed at increasing the efficiency and lifetime of dye-sensitized solar cells (DSSCs). Below are a list of current projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.
DSSCs were developed in the early 1990s by Michael Grätzel of the Federal Polytechnic University in Lausanne, Switzerland. Although not yet in commercial production, dye-sensitized cells have achieved laboratory efficiencies of 12.3%. These solar cells rely on stable and easy-to-produce materials, making them an attractive alternative to polymer, bulk-heterojunction organic photovoltaics (OPV).
DSSCs have three separate components: the absorber dye, a metal-oxide electron-transfer network, and an electrolytic hole-transfer layer. The first component—the dye—is an organic or organometallic molecule with strong absorption in the visible spectrum that effectively absorbs a photon and generates an exciton. The second component—the metal-oxide electron-transfer layer—is a highly porous nanoscaffold that serves as a "home" for dye molecules and also acts as an electron highway between the excited dye molecule and the anode. The last component of the cell—the electrolyte or hole-transfer layer—replenishes these now electron-deficient dye molecules by ferrying away holes from the dye to the cathode.
Dye-sensitized cells have low efficiencies and limited durability compared to leading PV technologies. To increase durability, work is being done to convert the liquid hole-transport layer into a solid-state layer. Improved encapsulation will similarly help to mitigate reliability concerns.
Traditional dye-sensitized cells used dyes that incorporated ruthenium (Ru), an expensive metal. Significant research has been performed on zinc- or iron-based organometallic dyes and organic dyes, and the conversion efficiency using these dyes has recently surpassed that of ruthenium dyes. However, further improvements are still necessary to make dye-sensitized cells affordable and ready for market.
The benefits of dye-sensitized solar cells include:
- Low cost and ease of manufacturing: They hold the promise of being a very low-cost technology with great ease of manufacture.
- Abundant materials: They rely on stable and abundant resource materials, especially if ruthenium-free dyes are used.
Dye-sensitized cells are not currently produced in large-scale quantities.
Typically, the semiconductor framework anode is deposited first. This is a highly porous nanoscaffold typically made from titanium dioxide (TiO2), although occasionally tin dioxide (SnO2) or other metal oxides are used, as well. Typical deposition methods for nanoparticle networks include spin-coating or doctor-blade application of a colloidal nanoparticle solution, followed by sintering.
Dye is introduced into this network by soaking the metal-oxide anode in a dye solution. Dyes are often modeled after chlorophyll and are commonly porphyrins coordinated around a metal center. Originally, ruthenium was used, although recent efforts have successfully replaced ruthenium with zinc-based or metal-free dyes. During soaking, carboxylate, sulfonate, or phosphonate groups on the dye chelate to the metal oxide, providing strong adhesion.
The last production step is to create a hole transport layer. A liquid electrolyte redox couple, such as iodide/iodine (I-/I3-), accomplishes this with a complicated series of redox reactions that ferry positive charge to the cathode. This process is effective, but the liquid nature of the redox couple makes these solar cells difficult to install in the field. Liquids have a tendency to expand and contract as the temperature changes, which can cause cracks in the cell over time. Solid-state remedies are being developed—such as polymer conductors, polymer-electrolyte gel frameworks, and small-molecule hole conductors. Electrolytes are injected into the metal-oxide framework so as to make contact with both the dye molecules and the cathode.