To efficiently harvest the terrestrial solar emission in photovoltaic devices, the absorption spectrum of the photoactive material has to match the solar spectrum as well as possible. Materials which absorb at photon energies where the maximum of the solar emission occurs (around 1.8 eV are very important for photon harvesting. The synthesis of Poly-( N -dodecyl-2,5-bis(2'-thienyl)pyrrole-(2,1,3-benzothiadiazole)) (PTPTB), a low bandgap polymer, and its use in 'bulk heterojunction' photovoltaic devices was recently reported. For better understanding of the properties of this material, spectroelectrochemical, optical spectroscopic and cyclovoltammetric measurements as well as electrochemical voltage spectroscopy (EVS) were carried out to determine both the electrochemical and the optical bandgap. The comparison of the results allows the prediction of charge carrier formation via light exposure.