Light as Energy

Figure. Designing a system with a Type I band alignment (left) promotes radiative recombination (i.e. when the excited electron loses energy it gives off a photon) which can be useful in LEDs. A Type II band alignment (right) can provide long lived excited electrons which can be used for photocatalysis or solar cells.

All light is made of photons, small "packets" of energy, the energy of which depends on the color, or wavelength, of the light. When a material absorbs a photon of light, the energy of the photon is transferred to an electron in the material, we call this an excited electron. This process is happening all of the time in every colored object that we see - the dye molecules in a red shirt are constantly absorbing blue, green, and yellow photons and reflecting red photons back to our eyes. In most objects, soon after the photon's energy is transferred to an electron, the electron loses that energy as heat, but, if the system is designed correctly, we can use that excited electron to do work. Depending on the system, these excited electrons can be driven through a wire as in a solar cell, or they can be used directly to perform a reaction as in solar hydrogen production.

Harvesting Light with Nanostrucures

Figure. Schematic of the different charge transfer pathways in a Quantum Dot Solar Cell.

During the last decade, nanomaterials have emerged as the new building blocks to construct these types of light energy harvesting assemblies. Organic and inorganic hybrid structures that exhibit improved selectivity and efficiency towards catalytic processes have been designed. Size dependent properties such as size quantization effects in semiconductor nanoparticles and quantized charging effects in metal nanoparticles provides the basis for developing new and effective systems. These nanostructures provide innovative strategies for designing next generation energy conversion devices. Recent efforts to synthesize nanostructures with well defined geometrical shapes (e.g., solid and hollow spheres, prisms, rods, wires) and their assembly as 2- and 3- dimensional assemblies has further expanded the possibility of developing new strategies for light energy conversion.

There are three major ways that one can utilize nanostructures for the design of solar energy conversion devices. The first is to mimic the photosynthesis with donor-acceptor molecular assemblies and clusters. The second is the semiconductor assisted photocatalysis to produce fuels such as hydrogen. The third, and most promising category, is the nanostructure semiconductor based solar cells.

Check out these links for more info!

• Solar Energy - Beyond the Hype - Webinar by Dr. Prashant Kamat on the clean energy challenge and solar energy.
• A Delicious Solar Cell - A solar cell made from Doughnuts and Tea by former group member Blake Farrow (Youtube video)