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Posted: Jan 03, 2014
Modifying the molecular structure of a polymer can increase solar cell efficiency
(Nanowerk News) Researchers from North Carolina State University and the Chinese Academy of Sciences have found an easy way to modify the molecular structure of a polymer commonly used in solar cells. Their modification can increase solar cell efficiency by more than 30 percent.
Polymer-based solar cells have two domains, consisting of an electron acceptor and an electron donor material. Excitons are the energy particles created by solar cells when light is absorbed. In order to be harnessed effectively as an energy source, excitons must be able to travel quickly to the interface of the donor and acceptor domains and retain as much of the lightís energy as possible.
Side chain modification improves solar cell efficiency from 5.6% to 7.5%.
One way to increase solar cell efficiency is to adjust the difference between the highest occupied molecular orbit (HOMO) of the acceptor and lowest unoccupied molecular orbit (LUMO) levels of the polymer so that the exciton can be harvested with minimal loss. One of the most common ways to accomplish this is by adding a fluorine atom to the polymerís molecular backbone, a difficult, multi-step process that can increase the solar cellís performance, but has considerable material fabrication costs.
PBT-OP was not only easier to make than other commonly used polymers, but a simple manipulation of its chemical structure gave it a lower HOMO level than had been seen in other polymers with the same molecular backbone. PBT-OP showed an open circuit voltage (the voltage available from a solar cell) value of 0.78 volts, a 36 percent increase over the ~ 0.6 volt average from similar polymers.
According to NC State physicist and co-author Harald Ade, the teamís approach has several advantages. ďThe possible drawback in changing the molecular structure of these materials is that you may enhance one aspect of the solar cell but inadvertently create unintended consequences in devices that defeat the initial intent,Ē he says. ďIn this case, we have found a chemically easy way to change the electronic structure and enhance device efficiency by capturing a lager fraction of the lightís energy, without changing the materialís ability to absorb, create and transport energy.Ē