nanoscience. This efficiency objective provides a strong motivation for a program of basic
research that aims to understand and control all the factors that determine cell performance in
nanostructured systems. Building this knowledge base will provide the platform from which to
launch an effort to achieve efficiencies beyond the Shockley-Queisser limit by incorporation of
approaches such as multijunction cells and photon up-conversion.
RESEARCH DIRECTIONS
Multiple Charge Carrier Generation
Calculated thermodynamic efficiency limits in single-junction solar cells (~32%) assume that
absorption of an individual photon results in the formation of a single electron-hole pair and that
all photon energy in excess of the energy gap is lost as heat. This limit, however, can be
surpassed via multiple exciton (electron-hole pair) generation (MEG) by single-photon
absorption as was predicted (Nozik 2001; Nozik 2002) and observed optically in PbSe and PbS
quantum dots (Schaller and Klimov 2004; Ellingson et al. 2005). The ability to generate multiple
charge carriers upon absorption of one photon could lead to greatly enhanced photocurrent and,
ultimately, to very high efficiency solar cells.
Exploit the Unique Properties of Nanostructured Systems to Develop New Cells
with Solar Efficiencies of 20%
Current mesoporous nanocrystalline films used in dye-sensitized solar cells consist of a random
nanoparticle network and a disordered pore structure. Such films are characterized by slow
electron transport. Moreover, because of the wide particle distribution and disordered nature of
the pores, not all of the internal surface area of a film is accessible to the sensitizer. Also, it is
difficult to fill the pores completely with viscous, quasi-solid, or solid ionically or electronic
conductors, which serve to transfer photogenerated holes away from the sensitizers following
charge separation. Development of ordered nanostructured, inorganic electrodes could lead to
more effective incorporation of ionically or electronically conducting materials (ionic gels,
polymers, etc.) within the pore structure and potentially to faster charge transport. Also, more
uniformly sized particles coupled with periodic order could facilitate films favoring preferred
crystal faces for optimizing charge separation. Developing new stable, near-infrared absorbing
molecular and quantum confined sensitizers with increased red absorbance would allow for
thinner TiO 2 layers, which would result in lower charge recombination and higher overall
efficiency. Confining photons to a high-refractive-index sensitized nanostructured oxide film is
another approach to enhance the red response of the cells. For instance, a two-layer structure
consisting of submicron spheres and a nanoparticulate TiO 2 layer has been used to enhance light
collection owing to multiple scattering. Incorporation of more advanced light management
strategies, such as photonic band gaps, also offers promise for enhancing the red response of the
cell.
Also, relatively unexplored are self-assembling molecular, supermolecular, and inorganic
interface layers having, for example, a broad spectral response and/or the electronic capability of
directing the resulting energy vectorially as excitons or charges toward the nanostructure