  We report both transport measurements and spectroscopic data of
polymer/fullerene blend photovoltaics using a small library of fullerene esters
to correlate device properties with a range of functionality and structural
diversity of the ester substituent. We observe that minor structural changes
can lead to significant differences in device efficiency and lifetime.
Surprisingly, we have found that isomeric R-groups in the fullerene ester-based
devices we have studied have very different efficiencies. The characteristic
lifetimes derived from both transport and spectroscopic measurements are
generally comparable, however some more rapid effects in specific fullerene
esters are not observed spectroscopically. Our results support using a library
approach for optimizing device performance in these systems.
