When multiple excitons form in organic molecules, singlet fission is involved. In this electronic doubling process, an excited singlet state splits (fissions) to form two excited triplet states. As Campos and Sfeir explain, singlet fission has been observed previously in other organic molecules. But the yields have been low and conditions constrained. For example, singlet fission typically requires crystalline films and proper alignment between nearest molecular neighbors. In such samples, fission leads to a pair of excitons that reside on adjacent organic molecules. By contrast, in the new Columbia-Brookhaven study, charge multiplication happens within a single solution-phase polymer chain, which might simplify the manufacture of devices that exploit this phenomenon in the future. “It is wonderful that a conjugated polymer has now been found that performs singlet fission efficiently,” says Josef Michl, a University of Colorado, Boulder, chemistry professor and specialist in photochemistry and photophysics. Describing the team’s use of the donor-acceptor concept for this purpose as “clever,” Michl remarks that the long-term impact of the discovery depends on finding ways to extend the extremely short lifetimes of the excited triplet states. Indeed, finding a strategy for making materials that undergo singlet fission in high yield is just the first step, Campos acknowledges. Now the big challenges are fine-tuning the properties of these materials to extract the charges and integrating these compounds into a functioning solar cell. /articles/93/i9/TwoOne-Deal-Solar-Cells.html 20150302 Coupling electron donors and acceptors in single organic molecules might one day double device efficiency 93 9 /magazine/93/09309.html /departments/.html Two-for-One Deal In Solar Cells Science & Technology Mitch Jacoby Coupling electron donors (benzodithiophenes) with electron acceptors (oxidized thiophene units, or TDOs, shown in red) yields organic polymers prone to forming multiple excitons (top row).
by Mitch Jacoby |
March 02, 2015