|Fig. 1: Electron transfer scheme of typical dye-sensitized solar cell. (Source: Wikimedia Commons)|
Dye-sensitized solar cells (DSSCs) may be applied as photovoltaic windows in energy-sustainable buildings, an important consideration given 20-40% of the total energy drain in developed comes from building use.  Unlike other solar cells, DSSCs are naturally transparent. They readily achieve efficiencies over 10% while remaining very cost-effective.  As illustrated in Fig. 1, the defining architecture of a typical DSSC consists of photosensitive dye adsorbed onto the surface of TiO2 nanostructures, the former of which mediates light absorption and electron injection into the latter.  DSSCs have historically used metal-based dyes that are both expensive and not environmentally sound, pushing efforts to develop efficient organic alternatives.  Until recently, however, the efficiency of organic dyes has fallen short of metal-based ones. 
In 2012, Cheng et al. created a DSSC from a novel cyanine-based dye and achieved 5.6% average efficiency under 100 mW per square centimeter; this efficiency is defined as the ratio of useful electricity to incident power.  Then, by creating a DSSC with this dye and 2 previously studied ones, an efficiency of 8.2% was achieved. (While the DSSC certainly absorbs some light and is thus not perfectly transparent, it is still possible to see through the solar cell, unlike, say, a silicon solar cell with a macroscopically thick silicon substrate.) This was an instance of improving solar cell performance by exploiting co-sensitization, where different dyes with complementary UV-vis absorption spectra are adsorbed to obtain broad spectral absorption. Then, in 2015, Kakiage et al. broke the world record in DSSC efficiency with organic dyes for the first time, which had traditionally been held by metal-based dyes, to demonstrate the efficacy of this general technique. 
While initial studies where dyes for co-sensitization are chosen based on complementary absorption spectra have been promising, other factors that determine the ultimate efficiency of co-sensitized DSSCs are not well understood.  On a single molecule level, computation will be necessary to efficiently explore new molecular architectures (namely what chemical motifs should be chosen for the electronic donor, electron acceptor, and adsorbing group to titania) and how they should be arranged. These studies will require first-principles calculations, which have not been widely applied in this field. Moving beyond the molecular level, future studies will also need to take into account how dye molecules interact with both identical and other molecules. For instance, a major gap in understanding of dye-to-dye interaction is how the molecular structures of the dye pairs affect their aggregation on the surface of titania nanostructures, and how aggregation then affects ultimate efficiency through quenching or charge transfer, which may be understood through computational methods.  Thus, future computational studies have potential to drastically improve the efficiency of co-sensitization of organic dyes for solar cells.
© Derek Wang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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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