Solar cell research
Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 °C).
[1][2] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, with a particle size of a few micrometers, and may therefore offer new opportunities for development of solar cell technologies.
These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption).
In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.
This technology combines the advantages of crystalline silicon as a solar cell material (abundance, non-toxicity, high efficiency, long-term stability) with the cost savings of using a thin-film approach.
However, organic solar cells generally suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable.
Additionally, the conjugated double bond systems in the polymers which carry the charge, react more readily with light and oxygen.
Although the research is still in its infancy, quantum dot modified photovoltaics may be able to achieve up to 42% energy conversion efficiency due to multiple exciton generation (MEG).
With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low-bandgap solar cells.
The ultimate goal for both wafer-based and alternative photovoltaic concepts is to produce solar electricity at a cost comparable to currently market-dominant coal, natural gas, and nuclear power in order to make it the leading primary energy source.
To achieve this it may be necessary to reduce the cost of installed solar systems from currently about US$1.80 (for bulk Si technologies) to about US$0.50 per Watt peak power.
[13] Researchers at Idaho National Laboratory, along with partners at Lightwave Power Inc.[14] in Cambridge, MA and Patrick Pinhero of the University of Missouri, have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources, which garnered two 2007 Nano50 awards.
While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to computers and mobile phones with higher efficiency than traditional solar cells.
Used to replace conventional window glass, the installation surface area could be large, leading to potential uses that take advantage of the combined functions of power generation, lighting and temperature control.
This transparent, UV-absorbing system was achieved by using an organic-inorganic heterostructure made of the p-type semiconducting polymer PEDOT:PSS film deposited on a Nb-doped strontium titanate substrate.
[20] Others have reported extending the absorption range of single-junction photovoltaic cells by doping a wide band gap transparent semiconductor such as GaN with a transition metal such as manganese.
Three-dimensional solar cells that capture nearly all of the light that strikes them and could boost the efficiency of photovoltaic systems while reducing their size, weight and mechanical complexity are under development.
The new 3D solar cells, created at the Georgia Tech Research Institute, capture photons from sunlight using an array of miniature “tower” structures that resemble high-rise buildings in a city street grid.
They rely on luminescence, typically fluorescence, in media such as liquids, glasses, or plastics treated with a suitable coating or dopant.
The structures are configured to direct the output from a large input area onto a small converter, where the concentrated energy generates photoelectricity.
For example, at Massachusetts Institute of Technology researchers have developed approaches for conversion of windows into sunlight concentrators for generation of electricity.
[30] Metamaterials are heterogeneous materials employing the juxtaposition of many microscopic elements, giving rise to properties not seen in ordinary solids.
[35] Luque and Marti first derived a theoretical limit for an IB device with one midgap energy level using detailed balance.
