For efficiently harvesting the terrestrial solar spectrum in conjugated polymer based solar cells, low bandgap polymers with a bandgap < 1.8 eV are needed. The photophysics of such low band gap conjugated polymers as well as their excited state interactions with electron acceptors such as fullerenes are of importance when using them in photovoltaic devices. In this work we present a device structural study on the soluble low bandgap polymer PTPTB, consisting of alternating electron-rich dodecyl-2,5-bis (2'-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units. The bandgap of this polymer, determined by electrochemistry and by optical absorption, is 1.6 eV The performance of the photovoltaic devices is discussed in terms of spectrally resolved photocurrent measurements, AM1.5 measurements and temperature dependent I-V spectroscopy. Strategies to utilize this polymer for bulk heterojunction tandem solar cells with either a wide band gap polymer or in conjunction with strongly absorbing small molecular dyes are discussed.