Substrate-correlated interface polarity reversal at a carbon nanotube/fullerene junction

Understanding of the electronic structure at organic donor/acceptor interfaces is vital to further develop organic photovoltaic technologies. We have observed a reversed interfacial potential difference at a carbon nanotube/fullerene interface when it is formed on aluminum oxide (Al2O3) compared to when it is formed on silicon dioxide (SiO2). We do not observe this in a polymer/fullerene junction. We hypothesize the cause is a combination of poorer screening of a more reactive substrate (Al2O3) by the nanotubes (which are a thinner, more conductive film than the polymer film) and greater static dipolar disorder at the Al2O3 interface. The static dipolar disorder leads to stable trapping of otherwise mobile charges, which may originate from the carbon nanotubes, the externally applied potential, or low lying static charges from the insulating substrate.