Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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nanoscience. This efficiency objective provides a strong motivation <strong>for</strong> a program <strong>of</strong> basic<br />
research that aims to understand and control all the factors that determine cell per<strong>for</strong>mance in<br />
nanostructured systems. Building this knowledge base will provide the plat<strong>for</strong>m from which to<br />
launch an ef<strong>for</strong>t to achieve efficiencies beyond the Shockley-Queisser limit by incorporation <strong>of</strong><br />
approaches such as multijunction cells and photon up-conversion.<br />
RESEARCH DIRECTIONS<br />
Multiple Charge Carrier Generation<br />
Calculated thermodynamic efficiency limits in single-junction solar cells (~32%) assume that<br />
absorption <strong>of</strong> an individual photon results in the <strong>for</strong>mation <strong>of</strong> a single electron-hole pair and that<br />
all photon energy in excess <strong>of</strong> the energy gap is lost as heat. This limit, however, can be<br />
surpassed via multiple exciton (electron-hole pair) generation (MEG) by single-photon<br />
absorption as was predicted (Nozik 2001; Nozik 2002) and observed optically in PbSe and PbS<br />
quantum dots (Schaller and Klimov 2004; Ellingson et al. 2005). The ability to generate multiple<br />
charge carriers upon absorption <strong>of</strong> one photon could lead to greatly enhanced photocurrent and,<br />
ultimately, to very high efficiency solar cells.<br />
Exploit the Unique Properties <strong>of</strong> Nanostructured Systems to Develop New Cells<br />
with <strong>Solar</strong> Efficiencies <strong>of</strong> 20%<br />
Current mesoporous nanocrystalline films used in dye-sensitized solar cells consist <strong>of</strong> a random<br />
nanoparticle network and a disordered pore structure. Such films are characterized by slow<br />
electron transport. Moreover, because <strong>of</strong> the wide particle distribution and disordered nature <strong>of</strong><br />
the pores, not all <strong>of</strong> the internal surface area <strong>of</strong> a film is accessible to the sensitizer. Also, it is<br />
difficult to fill the pores completely with viscous, quasi-solid, or solid ionically or electronic<br />
conductors, which serve to transfer photogenerated holes away from the sensitizers following<br />
charge separation. Development <strong>of</strong> ordered nanostructured, inorganic electrodes could lead to<br />
more effective incorporation <strong>of</strong> ionically or electronically conducting materials (ionic gels,<br />
polymers, etc.) within the pore structure and potentially to faster charge transport. Also, more<br />
uni<strong>for</strong>mly sized particles coupled with periodic order could facilitate films favoring preferred<br />
crystal faces <strong>for</strong> optimizing charge separation. Developing new stable, near-infrared absorbing<br />
molecular and quantum confined sensitizers with increased red absorbance would allow <strong>for</strong><br />
thinner TiO2 layers, which would result in lower charge recombination and higher overall<br />
efficiency. Confining photons to a high-refractive-index sensitized nanostructured oxide film is<br />
another approach to enhance the red response <strong>of</strong> the cells. For instance, a two-layer structure<br />
consisting <strong>of</strong> submicron spheres and a nanoparticulate TiO2 layer has been used to enhance light<br />
collection owing to multiple scattering. Incorporation <strong>of</strong> more advanced light management<br />
strategies, such as photonic band gaps, also <strong>of</strong>fers promise <strong>for</strong> enhancing the red response <strong>of</strong> the<br />
cell.<br />
Also, relatively unexplored are self-assembling molecular, supermolecular, and inorganic<br />
interface layers having, <strong>for</strong> example, a broad spectral response and/or the electronic capability <strong>of</strong><br />
directing the resulting energy vectorially as excitons or charges toward the nanostructure<br />
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