Sun light is perhaps the only source of energy that can keep up with the world’s ever increasing appetite for energy. More solar energy strikes the Earth in one hour than all the global fossil fuels combined can provide in a year. However, utilizing this solar energy is not trivial. Humans have used solar energy every time they’ve ignited fires for heat by focusing the sun’s rays on a pile of dry twigs. But this process is far from efficient. Plants are experts at converting energy from the sun into chemical energy, in the form of starch and other sugars, through the process of photosynthesis. But even plants are not very efficient at capturing the sun’s energy.

For human use, sun light must be converted directly into electricity for it to be practical as a source of versatile energy. Given the increasing concern about our reliance on fossil fuels, researchers are actively investigating ways to harness the power of the sun. A device called a solar cell might be the answer.

Converting Light into Electricity

Solar cells are devices designed to turn sunlight into electricity. When light hits the right kind of material, energy carried by the light can transfer to tiny subatomic particles, called electrons, in the material. Electrons are negatively charged particles which are in constant motion, circling their respective nuclei in prescribed orbits. If the energy from the light is great enough, the electrons become “excited” and start roaming round the material more freely. If one collects these excited electrons from the material, it is possible to generate an electric current, which is simply a net flow of electrons. The simplest solar cell consists of a light-absorbing material sandwiched between two electrodes, which are regions that collect the whirling electrons. These electrodes are then hooked up to an external circuit to power anything from a handheld calculator to a satellite in space.

Currently, the most common material used to make solar cells is silicon semiconductor — the same material that is used to make computer chips. Silicon is very good at absorbing light. It absorbs about two-thirds of the solar spectrum, which consists of all visible light, some UV, and some infrared. Solar cells made of silicon are efficient, converting 20-40% of the energy they absorb into electricity. One drawback of using silicon for solar cells is that silicon itself is very expensive. Solar power today costs about 4 to 8 times more than electricity generated from natural gas: you pay about 4 cents per kilowatt-hour for natural gas electricity in your home and solar power would cost you 15-30 cents per kilowatt hour. High cost is the major factor limiting the adoption of solar as a mainstream source of energy today.

Plastic Solar Cells

Given the high price of silicon solar cells, many research groups and companies are looking into alternative materials for solar cells. Some groups are abandoning traditional semiconductors altogether and are switching to plastics. In addition to being cheaper, plastics are also more flexible than silicon. They can be printed on to surfaces and even painted over large areas on almost any surface, from the walls of a building to the top of your car.

Making solar cells out of plastics is not easy though. Conventional plastics are electrical insulators — electrons can’t travel through them at all. Because electrons need to be collected from the light-absorbing material in order to generate an electric current, using a material that impedes electron movement makes this process nearly impossible. But in 1977, a research group led by Dr. Alan Heeger discovered a plastic material with high conductivity. Within this new class of material, generally referred to as conductive polymers, electrons can buzz around freely as if they were in a silicon semiconductor. This great discovery won Heeger and his colleagues the 2000 Nobel Prize in Chemistry. This class of conducting polymers makes plastic solar cells possible (and it is also the material that enables the functioning of Organic Light Emitting Diodes (OLEDS) used in flat panel displays for TVs!).

The Power of Two

Plastic solar cells may be the wave of the future, but the limitation right now is their low efficiency, typically around 2-4% (compared to the 20-40% efficiency with which silicon can convert absorbed energy into electricity). Plastic solar cells are not very efficient mainly because of the difficulty in collecting the excited electrons compounded by the relatively narrow range of light that conductive polymers can absorb in the first place. A record efficiency of 6.5% was achieved by Heeger and his colleagues and reported last month in the journal Science. They discovered a new way to stack multiple polymer layers to produce a “tandem” plastic solar cell. Tandem cells are common in conventional solar panels to increase the power output. They can absorb a broader spectrum of sunlight by using multiple layers of cells that are optimized to capture different colors of light. Until now however, this tandem structure could not be used with plastic solar cells because spraying multiple layers of conductive polymer on top of each other caused them to mix, diminishing the overall performance of the solar cell.

Heeger and colleagues were able to overcome this mixing problem by introducing a boundary between the two cells. They reported the development of a solar cell in which the bottom layer is filled with a polymer that absorbs both infrared and ultraviolet light, while the top layer is filled with a polymer that absorbs mostly blue and green light. A thin film of titanium oxide separates these two layers. Physically, this layer of titanium oxide seals the bottom cell and provides support for the top cell. Electrically, titanium oxide, being conductive like a metal, collects and transmits the energy captured by the cells to an external electrical circuit. Further improvements in efficiency are expected as researchers gain experience with the new materials and processing methods.

Challenges Ahead

Solar power holds big promise as a long-term sustainable source of energy. Assuming solar cells can be made with 10% efficiency, we will need about 62,000 square miles of solar cells (about the size of North Dakota) to meet the energy needs of the U.S Yet, solar power constitutes only 0.04% of total world power generation today. Cost is the main reason that limits its popularity; the adoption of cheaper materials will hopefully drive down the price tag. The dependence on weather and location is another issue: obviously solar cells work best when and where sunlight is abundant – that’s why California is enthusiastically promoting solar energy. The storage of energy and transmission of energy (for example, from sunny states to cloudy/rainy states) generated by solar cells are challenges still to be overcome. Much research and development lies ahead, but with the rate of improvement in solar cell technology and its intrinsic friendliness to the environment – solar power does not generate CO2! – it might not be long before solar power truly “outshines” other energy sources.

–Sindy K.Y. Tang, Harvard’s School of Engineering and Applied Sciences

For More Information:

Original article about the development of tandem plastic solar cells by Dr. Alan Heeger and colleagues in the journal Science:
< http://www.sciencemag.org/cgi/content/abstract/317/5835/222 >

A presentation about the energy problem by Dr. Steve Chu, Nobel Laureate in Physics (1997), director of Lawrence Berkeley National Laboratory:
< http://www.lbl.gov/Publications/Director/assets/docs/Berkeley_Repertory_Theater_outreach.pdf >

Comparison of the cost of power generated by different methods:
< http://peswiki.com/energy/Directory:Cents_Per_Kilowatt-Hour >

The California Solar Initiative:
< http://www.gosolarcalifornia.ca.gov/csi/index.html >