Most people in this country are probably aware of the ongoing energy crisis in the world. We are continuing to use energy at an ever-increasing rate, yet simultaneously our energy supplies are endangered by regulations, decreasing natural resources, and a huge oil spill directly off our coastline. So-called “renewable” energy sources such as wind and solar power are promising, but trying to capture that natural energy is proving to be difficult. Solar power is perhaps the most promising, but engineers are continuing to concentrate solely on silicon as the material of choice. New research from Cornell University shows that silicon may not be the best, after all.
Organic electronics is a term that describes the use of organic molecules (molecules containing carbon) as the active circuitry components in an electronic device. The notion of using organic molecules to carry electric current takes a little getting used to. For most of us, our experience with organic compounds and electricity takes the form of coating our household wiring with an organic plastic, to protect against accidental electrocution. Just a thin layer of the polymer is able to transform a live copper wire into something that we will pick up and handle without a second thought. So if organic plastics are such good insulators, how can they also be the material of choice for carrying an electric current? How can they be good conductors?
The key lies in the molecular structure of certain organic molecules. A normal organic molecule, such as the structures we use in insulation, consists of carbon atoms connected by simple bonds that scientists called “sigma” bonds. These bonds are easy to understand and the two electrons contained within each bond are relatively constrained; they don’t have a lot of freedom of movement. So, if we introduce an electric charge at one end of the molecule, the charge stays in place. There is no mechanism available for the electric charge to travel through the molecule and out the other side.
However, certain organic molecules have a molecular structure that consists of alternating single and double bonds. This type of arrangement relies on not only sigma bonds, but also an additional type of bond known as “pi” bonds. The electrons that make up the pi bonds are mobile, and can jump from atom to atom as long as the alternating single-double bonds continue, a phenomenon known as conjugation. Some conjugated polymers have hundreds of pi bonds, all lined up, and this allows a charge introduced at one end of the molecule to travel the length of the material.
Scientists are now looking to use some of these conjugated materials in solar cells. A solar cell (also called a photovolatic cell) is a material which transforms the energy of an incoming light beam into useful electricity. One groups results were recently reported in the science journal Nature: Chemistry. The approach is exciting because the molecules they were studying – phthalocyanines – are extremely common industrial chemicals used for textile dyes. The raw material cost is therefore very cheap, which has always been a thorn in the side of silicon-based solar cells. By exposing these dye molecules to an acid catalyst along with another common organic molecule (catechol), the dyes began to form an ordered three-dimensional lattice. The ordering is key to this approach as in order for an electric charge to hop from one molecule to another (through space), the pi bonds of molecule #1 must be precisely aligned with the pi bonds of molecule #2. If this condition is met, a pathway is produced that will allow the charge to be transported.
The process of the molecular ordering is reversible, which allows for errors in packing introduced in the course of the reaction to be reversed. Because of favorable interactions between side chains, often the desired highly-ordered state in a layered array is also a low-energy conformation. This means that the molecules keep attempting the packing until they settle into the desired arrangement. The final lattice structure performs very well as the active component in a solar cell, without the need for silicon.
Beyond cost savings and the ease of the self-assembly process, a final advantage of this approach is that the phthalocyanines absorb a large amount of incoming sunlight. This ensures that no energy is lost. Silicon-based solar cells, however, are very inefficient at capturing the incoming sunlight, as most of the wavelengths of light are either scattered or unabsorbed; this wastes most of the energy supply for the solar cell and helps to explain why silicon solar cells are so inefficient. Organic solar cells have the promise to perform much better, and this self-ordering dye lattice will most likely be a top contender in the months and years to come.