Impact of Fullerene Structure on Nanoscale Morphology and Miscibility and Correlation of Performance on Small Molecules: Fullerene Solar Cell

This manuscript reports the impact of fullerene structure on the morphology and miscibility of small molecules via a fullerene bulk heterojunction solar cell. The small angle neutron scattering and neutron reflectometry measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering miscibility, domain sizes, interfacial area between the small molecule and fullerene, and depth profiles in the mixtures. These results indicate that the bis-adduct fullerenes exhibit lower miscibility in small molecules. Correlation of miscibility and morphology to photovoltaic properties indicates that small molecule/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in small molecule organic photovoltaics. A higher open circuit voltage was obtained for bis-adduct fullerene devices which, however, does not translate to an increased power conversion efficiency. This decrease in performance is associated with the lower miscibility of bis-fullerene, which decreases the probability of the dissociation of excitons and enhances charge recombination rate in the miscible region. A quantitative analysis shows that an increase in the average separation of fullerenes in the miscible region is detrimental to electron transport in the miscible region, especially for a distance greater than ∼11 Å.