More economically and environmentally attractive than typical solar panels, organic solar cells are also less efficient in converting sunlight to electricity. A process that could boost the efficiency of these cells by rapidly converting more of sunlight’s energy to electrical charges instead of losing it to heat has been documented by U.S. Department of Energy’s Lawrence Berkeley National Laboratory scientists.

The researchers identified the source of an ultrafast and efficient process that spawns several carriers of electrical charge from a single particle of light in organic crystals that are integral to organic solar cell technology. The singlet fission mechanism is likened to the splitting of atomic nuclei in nuclear fission to create two lighter atoms from a heavier one. This reaction can occur in just tens of femtoseconds (quadrillionths of a second), before other competing effects can undermine energy gains.

An optically excited spin-singlet state (red), which features electron-hole pairs, splits into a pair of spin-triplet states (blue). The individual triplets have equal and opposite center-of-mass momenta – they behave like waves moving in opposite directions along a crystal. The gray and white spheres represent carbon and hydrogen atoms, respectively. Credit: Florian Brown-Altvater/Berkeley LabIn the splitting process, a composite particle composed of an electron and its partner hole – a vacant electron position in a material’s atomic structure that behaves like a particle in carrying a positive charge – rapidly converts into two electron-hole pairs. This doubles the charge-carrying potential in the material while avoiding the loss of energy as heat.

Past efforts to investigate singlet fission focused on just a few molecules within the material – in this case, the crystallized form of pentacene, which is composed of hydrogen and carbon. These approaches may have oversimplified the effects driving the process, so the researchers began with a large-scale view of the overall structure of the crystallized pentacene, and particularly its symmetry.

Calculations conclude that in order to efficiently double these electron-hole pairs, the sampled material should display a specific kind of symmetry, or repeated combinations of molecules, within its crystal structure. The efficiency of the singlet fission process appears to rely heavily on the number of molecules packed within each repeating pattern in the crystal, and on a particular type of symmetry marked by a 180-degree rotation and mirroring of patterns. This relationship between symmetry and efficiency, the researchers found, allows them to make powerful predictions on the efficiency of the overall fission.

Those predictions can only be possible, though, if the electron-hole pairs in the sample behave as wavelike objects moving throughout the whole crystal like waves in an ocean. This method also provided new insight about the splitting process, and how the newly created pairs must behave like waves propagating in opposite directions.

The research is published in the journal Physical Review Letters.