After Gutenberg

Titanium dioxide solar cells are cheaper and cleaner to manufacture than traditional silicon solar cells.

R & D Magazine (TinyURL) relates the possibility of inexpensively converting solar to chemical and electrical energy with a bio-mimicry approach to solar power, i.e., a ‘leaf-inspired’ PV (Photo Voltaic) cell design.

The U.S. alone uses about 30 trillion kilowatt-hours of energy each year, and with energy usage in other regions of the world rapidly growing, it is clear that developing alternative sources is imperative. Each square meter at mid-latitude locations in the U.S. receives 4 to 5 kilowatt-hours of solar energy per day; so tapping into that energy source seems an obvious path to alleviate energy shortages. But traditional methods of capturing solar energy are expensive to generate and deploy.

After 15 years of research there seems to progress in developing an efficient dye-sensitized solar cell. A venture first announced in 2005, DyeSol Titanium solar cells now are being manufactured.

A photonic crystal added underneath a layer of dye-sensitized titania nanocrystals enhances the efficiency by forcing reflection back into the light-absorbing region.

This is an example of solar cells that are other than only crystalline silicon. Several developers are banking on a silicon shortage and next-generation technology that “will be a cost-competitive — and energy efficient — alternative to silicon solar modules.” A recent Renewable Access Energy article quotes Graeme Sweeney, Shell, Executive Vice-President of Renewables, Hydrogen and CO2, as observing:

“Based on our R&D experience in Munich, where the laboratory line delivered record 13.5% efficiency, we believe this facility can achieve industry-leading performance amongst thin-film technologies.”

Shell Erneuerbare Energien GmbH and Saint-Gobain Glass Deutschland GmbH, recently announced plans to join forces and create AVANCIS — an entity that will develop, produce and market next generation solar technology based on Shell’s advanced Copper-Indium-Selenium (CIS) thin-film deposited on glass.

This is one of several, previously noted, nanotechnology developments that eventually may result in widespread use of solar power because the third generation technologies are significantly more efficient. Today, the University of Idaho released a story about Chemist Pam Shapiro and work on CIS (Copper, Indium and Selenium) quantum dots.

The difference between the Idaho research and other previous efforts is that the quantum dots are embedded between layers of a solar cell, absorbing energy otherwise wasted due to overheating. Quantum dots or carbon nanotubes capture more solar energy by absorbing different wavelengths, whereas nanocrystalline solar cells have “a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections.”

There’s plenty of solar energy around, the problem is converting it to a usable form. Because of the established infrastructure developed for distributing and using electrical energy, one of the most desirable approaches is to convert solar energy to electrical energy. Photovoltaic (PV) cells do just that, by transferring energy from an incident photon to an electron. That absorption occurs when the energy of the incident photon matches the energy needed to push an electron from one energy level to a higher available energy level. The trick is harvesting the energetic electrons before other naturally occurring processes return them to their lower energy state.

Traditional PV cells, for their part, are constructed from semiconductor crystals. The band gap of the semiconductor is tuned to match the energy available in sunlight so that a decent percentage of the incident light is converted into electrical energy. But then the real battle begins. If left to itself, the energetic photon will drop back down into the vacancy it left in the lower energy band. So PV cells are engineered with an electric field across the absorption region. When electrons are promoted into the higher energy band of the semiconductor, the applied electric field rapidly draws them away, eventually to an electrode that harvests them. But any disturbance in the process as the electron travels to the electrode will tend to bring the electron back to its lower energy state, so great care must be taken to ensure the purity and order of the semiconductor.

Idaho is not the only surprising location for innovation in solar energy technology. In 2004 Professor Vivian Alberts and a team at the University of Johannesburg announced that they had “developed and patented a novel manufacturing technique that finally makes it possible to construct CIGS (Copper, Indium, Gallium and Selenium) solar panels at a very low cost.”

The new CIGS panels developed by the University of Johannesburg team use a thin, flexible, “photo responsive” metal alloy coating only 5 micron thick. (Compared to a human hair at 20 micrometers, and silicon photo voltaic cells at 350 micrometers.) The developers claim that the panels have a useful life of about 20 years and an ROI within the first 1-2 years of operation because they are about 25% of the price of standard PV panels.

While reducing the cost of solar power by printing thin film solar cells is laudable and developers of third generation, full-spectrum* solar cells are claiming remarkable efficiencies, the greater goal, of course, is artificial photosynthesis:

In 1991, Prof. Michael Grätzel and his colleague Brian O’Regan, at the Ecole Polytechnique Federale de Lausanne in Switzerland developed a device architecture that introduced a new conceptual approach to PV energy generation. It would be more accurate to state they refashioned an old idea—a three-billion- year-old idea. They reasoned that the light-harvesting approach used by plants for billions of years must have features that could be adapted for use in PV cells.

One of the primary features is a separation between the light absorption and electron transfer mechanisms. An energetic electron generated in chlorophyll is rapidly transferred from molecule to molecule until it reaches a chlorophyll reaction center—a chlorophyll molecule modified with a metallic atom to modify its electron energy level structure. The reaction center then transfers the electron to an energy storage molecule. But this leaves the chlorophyll “short” an electron. It grabs the electron it needs from a surrounding water molecule. Grätzel followed that same model.

Images are from The Wasielewski Group @ Northwestern University

Detailed molecular structures of the two light-harvesting proteins, LH1 and LH2, and the reaction center (RC) from a specific species of purple photosynthetic bacteria. The view is looking down onto the plane of the membrane in which these proteins reside. Green plant photosynthesis uses a larger number of proteins, as well as greater numbers of energy and electron transfer cofactors.

While significant progress has been made, there remain challenges facing researchers before we see fourth generation solar cells. Primarily, the challenges are in mimicking the reaction center; an artificial leaf must be able to replicate the enzyme complex that splits water.

Thanks and a snap of the snout to The Big Gav.

* Note: The intermediate band allows absorption of photons at three different energy levels, corresponding to the three different band gaps. In particular, low-energy photons are captured that would pass through a conventional solar cell.