ORGANIC PHOTOVOLTAICS: INTEGRATING NON-FULLERENE ACCEPTORS INTO SOLUTION-PROCESSED DEVICES
In recent decades the demand for low-cost and sustainable energy sources has fueled the growth of photovoltaic research. Among the various photovoltaic technologies, organic semiconductors offer a light-weight and inherently-flexible solution that can be fabricated in a roll-to-roll process, significantly reducing fabrication and installation costs. Additionally, due to their transparent nature a number of previously unutilized architectural surfaces such as windows, building facades, and vehicle panels can be employed for energy production. Among solution-processed organic photovoltaics, fullerenes have dominated as the highest performing electron acceptor materials. However, their low absorption in the solar spectrum and high-energy synthesis is undesirable. Non-fullerene acceptors have the potential for low-cost synthesis while providing complementary absorption to the donor material, enhancing photocurrent. This dissertation presents the design, characterization, and integration into solar cells of novel non-fullerene acceptors. A family of related push-pull chromophores with phthalimide or naphthalimide end-groups, as well as perylene diimide-based acceptors were characterized using a combination of ultraviolet-visible absorption spectroscopy and ultraviolet photoemission spectroscopy. The acceptor molecules were integrated into thin-film transistors to measure field-effect mobilities. Solution-processed bulk heterojunction photovoltaic devices were fabricated and characterized using atomic force microscopy and by measuring current density-voltage curves and external quantum efficiencies while under illumination. The best performing devices achieved power conversion efficiencies of 5.5 %, where overall performance was limited by domain sizes in the blend films. Difficulties in forming nanoscale domain sizes in blend films are presented and discussed. A low-cost nanoembossing technique utilizing anodized aluminum oxide templates is presented to address the associated challenges with bulk heterojunctions. Initial results show that small-molecule acceptor films can be nanostructured prior to donor material deposition. This presents a viable method for fabricating large-area modules with predetermined nanoscale domain sizes, that is compatible with roll-to-roll processing.