The as-produced isomer mixture of the organic photovoltaic device acceptor material bis-[60]PCBM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry.

A total of 19 isomers (from a potential total of 21isomers) were purified from the mixture to a standard exceeding 99.5% with respect to other isomers. Three different columns, silica, 5PYE and 5PBB, were used and tried on each isomer to achieve the highest purification efficiency. The UV-Vis spectra and cyclic voltammograms demonstrate that each of the isomers exhibit different electronic properties. Their HOMO, LUMO and HOMO-LUMO gaps range from, -5.673 to -5.402 eV, from -3.901 to -3.729 eV, and from 1.664 to 1.883 eV, respectively.

Based on comparisons with known UV-vis absorption spectra of analogous materials we conclude that there is correlation between HPLC retention time and the relative positions of the addends; in that generally the closer the addends are to each other the longer the retention time of the isomer, and vice versa. These comparisons have enables us to tentatively, assign each of the isomers to one of the 8 bond types identified by Hirsch and co-workers1,2 (Trans1, Trans2, Trans3, Trans4, equatorial, Cis3, Cis2 and Cis1), such that the identity of the isomers is reduced to being one of only 2 or 3 isomers.

The LUMO levels of the majority of the isomers are above that of the isomer mixture, and hence, OPV devices based on each of those isomers are predicted to have a V_OC higher than that of present devices based on the mixture (with the most promising isomer being 94 meV above that of the mixture and 188 meV above that of [60]PCBM).

Furthermore, with the removal of energetic and morphological disorder associated with the normal mixture of bis-[60]PCBM isomers, it is also predicted that OPV devices based on all of the isomers will have better charge carrier transport properties, and thereby potentially higher currents, than those based on the mixture.

Preliminary analysis of an unoptimized P3HT/bisPCBM device based on the isomer with the highest LUMO indicates an open circuit voltage that is 210 meV above that obtained from the PCBM-based analogous, which is very significant given that the target voltage for OPVs is about 1 V. In addition, time-of-flight conductivity measurements indicate that electron carrier mobility for this isomer is similar to that of PCBM. Optimisation of a is underway and in addition to the abovementioned purification and frontier orbital the findings, the results of the performance of optimised devices will be presented at the meeting.