THE NEWEST IN SOLAR CELLS

Solar energy needs to be much, much less expensive than it is right now. Scientists are working (make that racing) to find the best way to increase the efficiency of materials that, when struck by the energy of the sun, generate an electric current they can capture and use.

The science is pretty complicated but the basic idea is pretty easy. When the sun hits certain materials, electrons are knocked loose and flow. If the material has a wire running through it, the electrons will flow to and along the wire and can be sent into an electrical system as electricity (to light lights, be stored in a battery, sent to the grid, etc.).

The materials commonly seen in panels on rooftops lose so much of the sun’s energy to heat that they are pretty inefficient at sending along the electrons. Materials that don’t heat up aren't useful because they also don’t have electrons that break loose and flow.

Scientists first used, and still mainly use, silicon to make solar photovoltaic (photo = light, voltaic = having electrical voltage) panels because it was a pretty good conductor of electrons (a semiconductor) without too bad of a heating problem. Then they began altering the silicon (monocrystalline, polycrystalline).

Thinking they could get more electrons and less heat with other materials, electrical engineers began trying combinations to catch a wider spectrum of light. What has emerged is “thin film.” Thin film is made several ways from several different materials (cadmium telluride and copper-indium-gallium-diselenide are the most prevalent) but the basic idea is the same: Catch a wider spectrum of light with a cheaper, easier to use material.

The thin film materials are layered. Each layer or combination of layers captures a different segment of sunlight’s spectrum. Catching more of sunlight's spectrum becomes especially important in varying kinds of light and angles of light. The total electron flow in thin film materials is less than that of silicon photovoltaic panels but thin film materials are less expensive so the overall cost effectiveness is better.

All of these silicon photovoltaic and thin film materials are now being used to effectively generate electricity from sunlight and none is nearly cheap enough to make in huge enough volumes to fully replace the current greenhouse gas-emitting energies now used to generate electricity (in the absence of a cost on emissions, but that's another matter).

Scientists like Nobel laureate Alan J. Heeger, whose lab’s breakthrough is reported below, and Caltech’s solar energy eminence grise Nathan Lewis believe the solution is in using advanced chemistry to grow nanomaterials that have a vertical dimension of electron conduction as well as the horizontal flow of electrons that happens in the other materials. This approach allows for a very high density of electron flow. So far it has been done mainly with platinum, a very, very expensive metal. The scientists are working (make that racing) to grow highly conductive nanomaterials using very common metals like iron combined with oxygen.

Nanostructures are “grown” in the lab because they are too small to structurally manipulate with precision. The study reported below from Professor Heeger’s lab discovered that adding alkanedithiols will grow the chemical combinations into more conductive structures. The scientists can only speculate as to why.

The next generation of solar materials will come from work along these lines. Nobody can say exactly when. Scientific details are available at the links to the Journal article below or at Dr. Lewis' highly accessible website.

Quoted in Science Daily: "These data provide a better understanding of correlation between the nano-scale morphology of the bulk heterojunction film and the solar cell performance…"

Typical thin film structure. Layers capture more of the sun's light spectrum. Electrons still flow horizontally. (click to enlarge)

Toward The Next Generation Of High-efficiency Plastic Solar Cells
March 19, 2008 (American Chemical Society via Science Daily)

WHO
Alan J. Heeger (head researcher/winner, Nobel Prize in Chemistry), Jae Kwan Lee, Wan Li Ma, Christoph J. Brabec, Jonathan Yuen, Ji Sun Moon, Jin Young Kim, Kwanghee Lee, Guillermo C. Bazan

Nanostructures channel the electron flow differently. (slide from Dr. Lewis - click to enlarge)

WHAT
Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells essentially reports success at growing nanomaterials with higher conductive properties using a chemical catalyst-like substance.

WHEN
- Heeger won the Nobel Prize in Chemistry in 2000.
- The study was submitted in November 2007 and, following the usual peer review and editing processes, was published March 19.

Structures grown in Dr. Heeger's lab. This is the complicated science part. (click to enlarge)

WHERE
Published in the Journal of the American Chemical Society

WHY
- Heeger’s Nobel Prize was for work in conducting polymers.
- Using bulk heterojunction materials (semiconducting polymers and nonoscale fullerenes) the scientists have found a way to increase efficiency by incorporating alkanedithiols.
- Heeger’s lab bumped efficiency from 3.4% to 5.1%.

There's a lot of action in all the competing thin film configurations but its nothing compared to the action there will be when the materials scientists start mastering nanoSolar. (click to enlarge)

QUOTES
Article abstract: Two criteria for processing additives introduced to control the morphology of bulk heterojunction (BHJ) materials for use in solar cells have been identified: (i) selective (differential) solubility of the fullerene component and (ii) higher boiling point than the host solvent. Using these criteria, we have investigated the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphology by selective solubility of the fullerene component is demonstrated using these high boiling point processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the reference device) to 5.1%.