(553e) Device Performance Is Independent of Crystallite Texture in Conjugated Block Copolymer Photovoltaics

Conjugated block copolymers have the potential to tune donor/acceptor interfaces and the mesoscale structure within the active layer of organic photovoltaic devices.  The ability to control and modify the micro-phase separation of the copolymer can offer a useful platform in understand the relationship between chemical structure, nanoscale morphology, and photovoltaic device performance.  We have demonstrated that utilizing poly(3-hexylthiophene)−block−poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (P3HT-b-PFTBT) as the active layer of solar cells leads to power conversion efficiencies near 3% and remarkable 1.2 V open-circuit voltages without the use of a fullerene acceptor.  We also find that we can control the P3HT crystallite orientation from “face-on” (π-stacking predominantly out-of-plane) to “edge-on” (π-stacking mainly in-plane) by altering the solvent casting conditions.  Nevertheless, our device performance is independent of the crystallite orientation, which is contrary to our current understanding – face-on crystal orientations are expected to maximize charge transport in solar cells.  We hypothesize that the ubiquitous broad distribution of crystallite textures always provides pathways for charge extraction, regardless of the average orientation of P3HT crystals.  This phenomenon is explored in model materials, where we have made perturbations to the P3HT-b-PFTBT structure by adding side chains to the TBT unit.  The combination of energy-filtered TEM, grazing-incidence wide angle X-ray scattering and soft X-ray scattering provides a comprehensive picture of the role of the molecular structure on the morphology.