There is now an easy way to modify the molecular structure of a polymer commonly used in solar cells that could end up 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 ends up absorbed. In order to harness it 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 light’s energy as possible, said researchers from North Carolina State University and the Chinese Academy of Sciences.
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 the exciton can end up harvested with minimal loss. One of the most common ways to accomplish this is by adding a fluorine atom to the polymer’s molecular backbone, a difficult, multi-step process that can increase the solar cell’s performance, but has considerable material fabrication costs.
A team of chemists led by Jianhui Hou from the Chinese Academy of Sciences created a polymer known as PBT-OP from two commercially available monomers and one easily synthesized monomer. Wei Ma, a post-doctoral physics researcher from NC State and corresponding author on a paper describing the research, conducted the X-ray analysis of the polymer’s structure and the donor:acceptor morphology.
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 was 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.
The team’s approach has several advantages.
“The 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,” said NC State physicist and co-author Harald Ade. “In 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 light’s energy, without changing the material’s ability to absorb, create and transport energy.”