In recent years it has been shown that the efficiency of organic photovoltaic (OPV) cells can be improved by the addition of electron acceptors into the polymer matrix. Carbon nanotubes (CNTs) have the potential to provide a more continuous path for electron transport, thus further increasing the efficiency of OPVs. A drawback to conjugated fullerene/polymer composite OPV is the energy loss of the electron during its transport from the conducting band of the polymer to the conducting band of the fullerene. While it may be possible to integrate raw CNTs into OPV cells, additional gains in efficiency may be possible by altering the bandgap of the CNTs. In this paper, it is demonstrated by cyclovoltammetry that the bulk bandgap of CNTs as measured by cyclic voltammetry may be modified and controlled by the use of sidewall functionalization with diazonium salts.