(258k) Harnessing Disorder in a Novel Nanomaterial for Light Harvesting Applications

Explorations of energy sources for the ever-increasing global population and its expected exponential increase in demand and consumption with the progression of this century have led to an array of creative designs. Of these, solar energy technology holds great promise given the now well-known calculation that within approximately one and a half hours, the energy incident upon the earthâ??s surface has the capacity to power the world for one year. To harness this immense energy source, light harvesting technologies have expanded beyond the traditional silicon-based photovoltaic designs, mainly due to their thermodynamic restriction of an approximate 33% maximum efficiency, known as the Shockley-Queisser limit. Out of necessity, new designs have arisen including multi-junction, perovskite, plasmonic, and dye-sensitized solar cells. The last of these are promising because of their generally low cost materials and their flexibility, allowing them to be implemented in a variety of locations. Currently, however, they are experiencing an efficiency limit of ~10% due ultimately to charge transport issues, such as injected electron recombination with the electrolyte, instead of with the semiconductor material. Research with dye-sensitized solar cells is presently focused on improving the absorption capabilities of dyes and effective carrier transportation in order to circumvent this restriction.

A variety of light sensitive structures have been examined in our lab, including elongated ZnO rods in N719 dye, an array of nanoparticle systems, and TiO₂ inverse opals. Building on this foundation of fabrication experience, we now seek to explore a quantum dot sensitized solar cell. This novel material consists of â??ultrasmallâ?? quantum dots, i.e. sub 2 nm diameters, which exhibit a phenomenon known as fluxionality, stemming from their high degree of disorder. Nearly 90% of the atoms in a QD of this size are on the surface and as a result these surface atoms switch bonds with their neighbors, altering their band gaps at femtosecond intervals. With the design proposed here, this fluxionality property will be used to absorb across the visible spectrum and into UV range as well by using the proximity of the UV-absorbing material to space the band gaps of the quantum dot farther apart. This in turn increases the carrier lifetime and allows for the capture of excited electrons.

Herein we report the behavior of this new quantum dot sensitized cell and compare its efficiency to that of N719 and other dyes commonly used in dye sensitized solar cells. We also note the key differences between the absorption capabilities of this novel structure and other quantum dot structures such as core-shell designs.