Imagine having cheap, printable solar cells at your fingertips, woven into your clothing, streaming power into your mobile electronics. Organic electronics, a field which includes organic solar cells and organic transistor circuits, has many advantages, the biggest of which is the ability to be printed easily, and the ability to flex without breaking.
Greg Scholes and Elisabetta Collini of University of Toronto's Department of Chemistry have discovered the mechanism behind how light triggers the transport of electricity in one promising organic solar material. Their new discovery could be applied to producing cheaper and more efficient cells.
The team looked at conjugated polymers, one of the most efficient organic materials available for solar power production. These long molecules can be also used in transistors and LEDs. They behave roughly like semiconductors, while retaining important organic characteristics. When exposed to light, they produce energy, which is transferred down their chain, eventually reaching other molecules and finally leaving the cell.
However, the way this transfer occurs was poorly understood, until now, making it difficult to optimize. Explains Professor Collini, "One of the biggest obstacles to organic solar cells is that it is difficult to control what happens after light is absorbed: whether the desired property is transmitting energy, storing information or emitting light. Our experiment suggests it is possible to achieve control using quantum effects, even under relatively normal conditions."
Her colleague, Professor Scholes describes this quantum effect responsible for the transmission, noted, "We found that the ultrafast movement of energy through and between molecules happens by a quantum-mechanical mechanism rather than through random hopping, even at room temperature. This is extraordinary and will greatly influence future work in the field because everyone thought that these kinds of quantum effects could only operate in complex systems at very low temperatures."
As well as being a breakthrough for organic electronics, the discovery could also yield advances in quantum computing. One of the key obstacles currently in quantum computing is the ultra-low temperatures needed to maintain useful quantum effects. The discovery of such effects occurring at room temperature could lead to important breakthroughs in building quantum computers.
On a technical level, the team used bursts from an ultrashort laser to trigger atoms of the polymer into a quantum-mechanical state. In this state the atoms were simultaneously in the ground state and at the energy level of the absorb photon. This phenomenon is a type of superposition known as quantum coherence. They used more lasers and a sophisticated measuring technique to then determine if the quantum state was being transferred between atoms on the chain.
Describes Professor Scholes, "It turns out that it only moves along polymer chains. The chemical framework that makes up the chain is a crucial ingredient for enabling quantum coherent energy transfer. In the absence of the chemical framework, energy is funneled by chance, rather than design. This means that a chemical property – structure -- can be used to steer the ultrafast migration of energy using quantum coherence. The unique properties of conjugated polymers continue to surprise us."
The findings are reported in the Jan. 16 edition of the journal Science. The work was sponsored by the Natural Sciences and Engineering Research Council of Canada.