In today's society, it is becoming ever important to find alternative sources of energy that are both cheap and efficient. Solar cells have become one of the most widely-researched methods of obtaining energy in "greener" ways than burning fossil fuels, etc. One of the new variants on the solar cell that is currently being researched is the dye-sensitized solar cell (DSC), which was invented by Michael Gratzel and Brian O'Regan in 1991. Where conventional systems take advantage of the semiconductor to absorb light and transport charge carriers, DSCs separate these two functions. A sensitizer, which is anchored to the surface of a wide band semiconductor, absorbs sunlight. When light is incident on the dye, electrons are injected from the dye into the conduction band of the solid, accounting for the charge separation. The electrons are then transported in the conduction band of the semiconductor to the charge collector. Using sensitizers with a broad absorption band along with nanocrystalline oxide films (most commonly titanium oxide) allows for the efficient capture of a large fraction of sunlight over a large spectral range (from the UV to the near IR range).
With conversion efficiencies of over 10% obtained, DSCs provide a technically and economically viable alternative to present day pn junction photovoltaic devices.  DSCs are made from low-cost materials and do not require any elaborate or complicated machinery to operate. Additionally, they can be engineered into flexible sheets and are quite durable, being able to withstand minor events such as hail. Conventionally used liquid-based dye-sensitized solar cells (DSCs) can have efficiencies as high as 11.1%, they often suffer from potential leakage and corrosion problems, sparking research in solid-state DSCs (ss-DSCs). [2,3] Although this conversion efficiency is less than the best of thin film cells, in theory its price-to-performance ratio (kWh/(m2-annum-dollar)) is high enough to make it an attractive alternative to fossil fuel electric generation. 
The dye used in dye-sensitized solar cells is extremely efficient at converting absorbed photons into free electrons in the titanium oxide layer. However, the current is limited to how many photons the dye can actually absorb—the photons that do get absorbed are the ones that ultimately produce the current. The rate at which the photons are absorbed depends on the overlap between the absorption spectrum of the titanium oxide layer (or other nanocrystalline oxide film used) and the solar flux spectrum. The maximum possible photocurrent is dependent on the overlap between these two spectra. Typically used dye molecules generally have poorer absorption in the red part of the spectrum compared to silicon, meaning that fewer of the photons in sunlight are available for current generation in comparison to silicon. Current dye-sensitized solar cells offer about 18 mA/cm2 of current.  The peak power production for current DSCs is ~11% when combined with a fill factor of 45%. 
Given the efficiency and low cost of materials needed to fabricate dye-sensitized solar cells, DSCs are an attractive replacement for existing technologies in "low-density" applications such as rooftop solar collectors, though the technology still has a way to go before it can be an attractive alternative for large-scale deployments as well. However, even a small increase in conversion efficiency for these new age solar cells could make them suitable for large-scale roles as well, as the efficiency of the cell would be worth the cost of utilizing more DSCs.
Another advantage DSCs have over traditional solar cells is the fact that direct injection of a photon into the nanocrystalline metal oxide layer evades the possibility of an electron recombining with a hole. This circumvents the problem of no current being generated when recombination occurs. Whereas a hole is generated when an electron is excited across the bandgap in traditional cells, no hole is generated in a DSC when an electron is injected. Instead, only an extra electron is added. While it is theoretically and energetically possible for an electron to recombine with the dye, the rate of this happening is negligible compared to the rate at which electrons are supplied by the electrolyte.  Due to these favorable kinetics, DSCs will also work in low light conditions (i.e. cloudy skies and indirect sunlight). So little light is needed, that it has been suggested that DSCs be used indoors - light could be absorbed from the various lights that are usually used to illuminate indoor rooms. 
Another advantage that DSCs offer, due in part to their mechanical robustness, is the fact that they have higher efficiencies at higher temperatures than traditional solar cells typically do. DSCs are able to radiate heat away much more efficiently than traditional silicon cells and operate at lower internal temperatures, since they are usually built with only a thin layer of conductive plastic on the front layer, versus the more insulating glass box that is typically used for silicon solar cells.
Despite these myriad advantages, DSCs do have a disadvantage. The major disadvantage is that the liquid electrolyte used in DSCs is temperature-sensitive. At low temperatures, the electrolyte can freeze, thus rendering the solar cell completely unusable. At high temperatures, the liquid electrolyte expands, making sealing the solar panels a major problem. The use of a liquid electrolyte causes some serious additional problems such as potential potential instability, limitation of maximum operation temperature, danger of evaporation, and extra cost for forming an electrical series connection. 
Dye-sensitized solar cells have become an incredible alternative to solid-state p-n- junction devices. With conversion efficiencies of over 11% having already been obtained, DSCs have a bright future in becoming a major contributor to renewable electricity generation, and with ongoing research, the efficiencies can only improve. Future research will focus on improving the short circuit current density by extending the light response of the sensitizers in the near-infrared spectral region, and substantial gains are expected from introducing ordered oxide mesostructures and controlling the interfacial charge recombination by manipulating the cell on the molecular level. With the work being done on DSCs, along with the already impressive numbers that these cells have put up, DSCs are a viable source of renewable energy for the future.
© 2010 Brian Luk. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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