Basic Research Needs for Solar Energy Utilization - Office of ...

• Light harvesting,
• Advanced light management,
• Control of charge separation and recombination,
• Control of charge carrier transport to the contacts,
• Design of multijunction systems, and
• Discovery of molecular routes to multiple carrier pair generation and photon
up-conversion.
The fabrication of new nanostructured systems is opening up new possibilities for a range of
devices, including batteries, sensors, and optoelectronics. Template synthesis methods using
surfactants or block copolymers, for example, allow very precise control of the shape, size, and
distribution of regular pores in oxide nanostructures. In addition, new methods for preparing
highly organized nanostructures by chemical or electrochemical methods are showing
considerable promise. Application of these exciting new developments to nanostructured photoelectrochemical
solar cells will allow optimization of the different elements of functionality that
are essential for high performance.
Light harvesting can be achieved by using strongly absorbing molecular dyes or semiconductor
nanoparticles, nanorods, nanocylinders, or nanowires. Tuning of the absorption spectrum and
energy levels of both types of sensitizer represents an important challenge. For efficient light
harvesting, the absorption spectrum of the sensitizer needs to extend to the optimum band gap
value of around 1.4 eV. At the same time, the energy levels of the sensitizer [highest occupied
molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels for dyes and
valence/conduction band energies for semiconductor nanoparticles} must be fine-tuned to
optimize the injection and regeneration steps. In the case of dyes, tuning can be achieved by
modifying molecular structure, whereas size selection and surface modification can be used to
tune semiconductor nanoparticles and nanowires. In both cases, it will be possible to enhance
light-harvesting performance by using appropriate light management techniques.
Enhancing the performance of nanostructured solar cells requires understanding and controlling
electron injection and subsequent recombination (either with the oxidized dye or with the ”hole”
in the contacting medium). The dynamics of these processes are sensitive to the interfacial
structure and the molecular structure of the sensitizer. Exciting opportunities exist for the design
of sensitizer molecules that incorporate the ability to remove the hole from the interface toward
the bulk of the contacting phase before recombination can occur. In addition, core shell oxide
structures can be used to control the rate of the electron injection and recombination processes at
the interface.
Transport of electrons and holes in nanostructured solar cells plays an important part in
determining cell efficiency. In the case of electrolyte-based cells, the competition between carrier
collection and recombination places constraints on the thickness of the device that are much less
stringent than those in other systems. However, when the electrolyte is replaced by an alternative
hole-conducting medium, such as a molecular solid or polymer, recombination limits the
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