A donor-acceptor type of narrow-bangap polymer, poly[(4,4-didodecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl- alt-(2,1,3-benzoxadiazole)-4,7-diyl], was synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. Processing a BHJ solar cell with the narrow-bandgap polymer and a fullerene derivative (PC 71BM) results in a smooth surface morphology, and better performance is observed when 1-chloronaphthalene solvent additive is used. The smooth surface enables the introduction of a titanium suboxide (TiOx) optical spacer between the BHJ layer and the electrode, which increases light absorption by redistributing the light intensity. TiOx incorporation leads to increases in the device performance and lifetime. Further optimization of the BHJ solar cell using the narrow-bandgap polymer and PC71BM was achieved by end-capping the chain ends with thiophene fragments. This structural modification yields materials which are more thermally robust and that can be used in devices with thicker films; important considerations for enabling mass production of plastic solar cells. In addition, a new donor-acceptor type small molecule using a dithienothiophene (DTT) unit and a diketopyrrolopyrrole (DPP) derivative has been synthesized for application in organic solar cells. The topographic surfaces of the solar cell devices blending with PC71 BM showed large cluster formation which caused poor device performance. The use of additive prevented cluster formation leading to improved device characteristics.

It was found that allowing solutions of a low bandgap polymer to stand in a solvent of marginal quality leads to interchain aggregation. These supramolecular structures lead to thin films with higher charge carrier mobilities and internal order, as determined by the fabrication of thin film transistors and grazing-incidence wide angle X-ray scattering (GIWAXS) measurements, respectively. Aging of solutions is therefore a straightforward method to modify the optoelectronic properties of solution-processable organic semiconductors. Furthermore, by dipping into hexane, it is possible to efficiently eliminate the low molecular weight component and improve molecular ordering. These changes improve the performance of field-effect transistors. The correlation between the nanoscalar structural features and the electrical properties enables us to determine both the appropriate dipping time and how the low molecular weight component influences electronic properties.