Organic photovoltaic (OPV) devices are one of the most promising applications of organic semiconductors due to their compatibility with flexible plastic substrates resulting in light weight, inexpensive and decorative products. For a long time poly(3-hexylthiophene) (P3HT) has been the polymer of choice in organic photovoltaic devices in combination with [6,6]-phenyl-C61-butyric acid methylester (PCBM) as acceptor. However, recent research has focused on polymers with improved absorbance and processability that can assure higher efficiencies and longer lifetimes. Low band gap polymers (LBGs) absorb light above 600 nm and are believed to increase the efficiency of OPVs due to a better overlap with the solar spectrum. This has been fully demonstrated with a power conversion efficiency (PCE) above 9% [1]. Side chains are necessary on both donor and acceptor units to assure processability and the nature of these chains impacts the morphology of the photoactive layer (PAL) and its evolution. This work report synthesis and characterization of a series of LBG polymers based on the donor unit 4,4′-bis(2-ethylhexyl)-5,5’-dithieno[3,2-b:2′,3′-d]silole (DTS) and the soluble acceptor chromophore 3,6-dithiophen-2-yl-2, 5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) [2], with different alkyl side chain on the acceptor unit. The DTS-DPP series was chosen because it is representative of a large number of donor-acceptor LBG polymers and provided an easy accessible and useful template to discover the importance of the type of side-chain used on the polymer optoelectronic and thermal properties. These properties have been fully investigated by UV-vis spectroscopy, photoluminescence, cyclic voltammetry, TGA and DSC. Furthermore the effect of these side chains on the morphology of polymer and polymer:PCBM blend thin films has been studied by Atomic Force Microscopy (AFM). First studies on polymer:PCBM devices have been conducted, in order to investigate any effect on their photovoltaic properties. To have a better understanding of the influence of the different side chains on the electrical performances, studies on the stability of polymer thin films were performed. With this aim, the evolution of thin polymer films exposed to UV-Vis irradiation has been monitored using UV-vis spectroscopy and AFM. Reference [1] www.solarmer.com [2] Huo L. et al., Macromolecules, 2009, 42(17), 6564–6571 Acknowledgments The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2011 under grant agreement ESTABLIS n° 290022).