During photosynthesis, plants only convert about 10% of the light they receive from the sun into usable hydrogen to fuel the reaction. Last summer, a group of researchers were able to break the world record for laboratory efficiency by reaching 44.7% with a new cell, with 50% as the ultimate goal. This record could by smashed by a new hybrid material using both organic and inorganic materials that could propel solar cells to an efficiency exceeding 95%. The research was led by Maxim Tabachnyk of the University of Cambridge and the paper was published in Nature Materials.
When photons are absorbed by solar cells, they generate particles known as excitons, which facilitate interaction between light and matter. Excitons have two varieties, spin-singlet and spin-triplet. Spin-singlet excitons are optically bright and easy to harvest using solar cells. Spin-triplets are optically ‘dark’ and hard to capture. That said, spin-triplets can produce two electrons for every photon, maximizing efficiency.
Silicon, an inorganic material, is what is most commonly used within solar cells. Though it might not be the most efficient material, scientists have a great deal of experience in manufacturing circuits with it, making the development process a bit easier. Silicon cells can only absorb spin-singlet excitons, getting one electron per photon that is absorbed.
Pentacene, an organic molecule composed of five fused benzene rings that is found in leaves, is readily able to absorb photons that lead to efficient spin-triplet excitons. However, it isn’t very good at trapping the electrons so they can be used. The solution, Tabachnyk’s team surmised, was to combine the two materials and get the best of both worlds.
“The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” Tabachnyk said in a press release. “If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells.”
The hybrid material allows the organic pentacene to absorb the dark triplets and transfer them immediately to an inorganic semiconductor. The team used laser spectroscopy that produces a pulse of light every few femtoseconds (10-15 seconds) and found that the material was over 95% efficient on delivering the triplets to the semiconductor, where the electrons could be harvested.
The team is going forward with the research and working on a way to apply this knowledge to create hybrid systems in a cost-effective manner. They are currently working on developing a low-cost organic coating that will absorb the dark triplets and pass them along to a silicon solar cell that will harvest the energy, increasing the efficiency of the system.
“Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic semiconductors, could enable us to further push the efficiency of inorganic solar cells, like those made of silicon,” noted Akshay Rao, the project’s principal investigator.