On the unipolarity of charge transport in methanofullerene diodes

Fullerenes are electron transporting organic semiconductors with a wide range of applications. In particular, methanofullerenes have been the preferred choice for solution-processed solar cells and photodiodes. The wide applicability of fullerenes as both 'n-type' transport materials and electron acceptors is clear. However, what is still a matter of debate is whether the fullerenes can also support efficient transport of holes, particularly in diode geometries. In this letter, we utilize a number of recently developed experimental methods for selective electron and hole mobility measurements. We show for the two most widely used solution processable fullerenes, PC70- and-PC60BM, that whilst both exhibit electron mobilities as high as 10(-3) cm(2)/Vs, their hole mobilities are < 10(-9) cm(2)/Vs. Thus charge transport in these fullerenes can be considered predominantly unipolar in diode configurations.