Manipulating Organic Photovoltaic Thin Film Morphology through Interface Modification and Additives

The conversion of sunlight to energy using organic photovoltaic devices (OPVs) is viewed as a promising route to sustainable energy generation. The drive to improve the power conversion efficiency (PCE) of OPVs has rightfully attracted a great deal of attention; however, overcoming the shortfall in PCE remains a significant challenge. While various factors such as inadequate hole transport mobility of donor-type conjugated polymers, limited solubility and miscibility with fullerene acceptors, and inefficient light harvesting are obstacles, controlling the morphology of the bulk heterojunction (BHJ) active layer remains a particularly thorny multi-dimensional problem. In this presentation, efforts to address this through the use of well-defined, end-functional poly(3-hexylthiophene)s (P3HTs) that allow interfacial layers that straddle the anode-BHJ interface will be described. Neutron reflectivity studies, which enable the laterally averaged depth profile of P3HT donor and PCBM acceptors in BHJ-like thin films to be measured, show that the interfacial “P3HT brush” layer impacts morphology, leading to a more uniform composition profile through the film. Coupled with various annealing strategies and the use of additives, these studies point to possibilities for tuning BHJ morphology and optimizing performance.