All 19 expected cyclopropa-fullerene isomers of the organic photovoltaic electron acceptor material bis-phenyl-C₆₂-butyric acid methyl ester ("bisPCBM") have been isolated from the as-produced isomer mixture by peak-recycling high pressure liquid chromatography to a purity exceeding 99.5%, and their frontier orbitals characterised by UV-Vis absorption spectroscopy and cyclic voltammetry.

Using purified isomers in bisPCBM-based OPVs is expected to yield an increase in efficiency through the removal of energetic disorder and the removal of morphological disorder contributing to higher device currents; and through the using on isomers with higher LUMOs than the average for the isomer mixture contributing to an increase in device voltage. On the latter point above, an important factor in OPV efficiency is that the open-circuit voltage (“V_OC”) of OPVs scales linearly with the energy difference between the highest occupied molecular orbitals (“HOMOs”) of the donors and the lowest unoccupied molecular orbitals (“LUMOs”) of the acceptors. Hence, for a given donor, V_OC is proportional to the LUMO level.

The LUMOs of the purified isomers were found to vary from -3.90 to -3.73 eV. As such, all isomers have LUMOs are above that of PCBM (-3.92 eV). Furthermore, the LUMOs of 13 of the 18 isomers are above that of the isomer mixture (-3.82 eV). OPV devices based on each of those 13 individual isomers will each have a higher V_OC than that of present devices based on the mixture. One isomer, with a LUMO ca. 100 meV above that of the mixture (and ca. 200 meV above that of PCBM), is particularly promising.

Preliminary results on two unoptimized P3HT/BisPCBM OPV devices based on the most abundant isomer with (a) a below average LUMO, and (b) an above average LUMO, gave V_OCs of 0.77 V and 0.83 V, respectively; which compares favourably with V_OCs of 0.63 V and 0.73 V of fully optimized P3HT/PCBM and P3HT/bisPCBM devices, respectively.