The idea of everyone having their own solar cell array painted onto a flat surface of their house came one more step closer to reality last week with the announcement by Rice University of a new organic solar cell that is both flexible and stretchy.
According to their website, chemical engineering professor Rafael Verduzco and his team have developed a photovoltaic material based on the organic polythiophene that allows bending while still providing an output current. Because they can bow they can be applied to almost any surface.
As you may know, existing silicon cells are very glass-like and fragile because they are made from a poly crystalline material and will crack at the slightest uneven pressure. This I know for a fact because I have about five lawn path illumination lights that don’t work anymore due to fractured solar cells. When silicon cleaves it sometimes shorts out the cell from parasitic current paths along the crack, yielding an output voltage of exactly zero.
The idea of getting free electricity from sunlight goes back well over 150 years. First demonstrated by French physicist Edmond Becquerel (father of Antoine Henri Becquerel of radioactivity fame) who at age 19, in 1839, built the world’s first photovoltaic cell using silver chloride in his father’s laboratory. Forty years later, Willoughby Smith first described the “Effect of Light on Selenium during the passage of an Electric Current” in a February 1873 issue of Nature.
It wasn’t until 1946 when Russell Ohl, working at Bell laboratories, patented the modern junction semiconductor solar cell, this one being composed of silicon. The practicality of using solar cells to produce electrical power gained much prominence with their incorporation onto the many satellites launched in the late 1950s. Nowadays they are commonplace at roadside call boxes and flashing traffic signals.
If silicon solar cells work so well why concentrate on organic cells? It’s the price. Silicon cells are still too costly. Organic solar cells on the other hand can be mass produced like paint and cost one-tenth of their inorganic counterparts.
Look at the numbers: The price of a silicon solar cell has dropped from $20 per watt in the mid-1970s to about a dollar per watt today, faithfully keeping track of the observation known as Swanson’s law, (similar to Moore’s Law) that states that solar cell prices fall 20 percent for every doubling of industry capacity to produce them.
Because silicon cells require crystals to be grown from a hot furnace, the process is expensive. The search for an organic cell that can be easily sprayed on has taken root even though the efficiencies of paintable devices are still much lower than crystalline types. For the record, typical silicon solar cells you see on people’s roofs have a power efficiency of about 20 percent when tilted properly and exposed directly to the Sun. Organic cells are still below the 3 percent level but they are slowly improving.
Somewhere along the line a claim was made by Konarka, a Massachusetts company, that they achieved an efficiency of 8.3 percent for an organic cell but did not publish how they did it. This announcement strangely occurred several months before they received $20 million in government grants as part of the 2009 stimulus under the Obama administration. A check of their website to see if they have made any recent improvements shows that Konarka is now bankrupt.
But no matter, organic solar cells will be the way of the future because they are so easy to make. A typical cell can be processed from liquid solutions and offers the possibility of a simple roll-to-roll printing process. The films are usually quite thin, about 100 nanometers, of some polymer compound such as polyphenylene vinylene or copper phthalocyanine (a blue organic pigment).
Much like the silicon cell with a pn junction, the active region of an organic device consists of two vastly different materials, one an electron donor and one an acceptor. When a photon of light strikes the junction an electron-hole pair is formed that separates to their respective electrodes, forming an electric field and hence voltage that can be used to supply electrical current.
Organic cells are somewhat sensitive to mechanical distortion but not as much as a crystalline silicon cell. According to Professor Verduzco, “If you stretch or bend things (too much), you get cracks in the active layer and the device fails.” To limit this problem and allow the cell to be easily constructed, he explained his technique: “Our idea was to stick with the materials that have been carefully developed over 20 years and that we know work, and find a way to improve their mechanical properties.”
With this in mind, the Rice researchers mixed in some sulfur-based thiolene reagents whose molecules blend with the light sensitive polymers and crosslink them with each other, providing inherent flexibility.
At about 20 percent thiolene, they found that cells retained their efficiency and gained flexibility. According to Verduzco: “The chemistry is mild, fast, and efficient.”
Will we see photovoltaic Rustoleum someday?