Watching the Ink Dry: In-situ Measurements to Monitor Film Formation and Function from Organic Semiconducting Inks

Solution processing of electrically active layers is a promising route to the sustainable manufacturing of functional components on diverse substrates such as flexible foils and textiles. Typically, solution processing does not result in the thermodynamic equilibrium form; instead metastable, kinetically trapped structures dominate. This allows great flexibility in the ability to tailor both film structure and performance by processing details. The sensitivity of structure and performance to processing is manifest in the fabrication of bulk heterojunction (BHJ) active layers for organic photovoltaics. Organic photovoltaic devices are a promising route to lower costs via roll-to-roll manufacturing. In a BHJ, nano-scale phase separation into nominally bicontinuous donor and acceptor rich regions enables both exciton dissociation and charge extraction. The performance of BHJ based devices is a strong function of the active layer structure and the optimized device structure is, in general, not the equilibrium structure. Recently it has become common to optimize BHJ film formation by introducing small amounts of processing additives. However, the mechanisms by which these additives effect the film formation are not known. We present results from the use of labscale photon based techniques, such as spectroscopic ellipsometry and photoluminescence and synchrotron based grazing incidence x-ray scattering (both wide angle and small angle) on the time evolution of films cast by blade coating, a material conservative model for scalable manufacturing processes. The in-situ measurements provide detailed insights into film thickness, composition, and microstructure. We will discuss highlights from studies of the additive effect in film formation of both polymer and small molecule based BHJs. Multiple mechanisms are revealed with common themes related to control of aggregation through solvent quality and control of growth kinetics via plasticization. We observe that, when elevated solution temperatures are required, significant differences between spin coating and isothermal deposition such as blade coating can arise. We demonstrate that isothermal blade coating is an excellent prototyping tool for slot-die coating by establishing comparable morphologies for small piece blade coating and continuous web slot-die coating.
Dr. Christopher L. Soles leads the Functional Polymers Group in the Materials Measurement Laboratory at the National Institute of Standards and Technology (NIST). His research group focuses on developing measurement methods that help facilitate the commercialization of polymeric materials in technologies related to semiconductor fabrication, printed and flexible electronics, and membranes for ion transport and water filtration. In 1993 he received Bachelors of Science degrees from the University of Michigan in Mechanical Engineering as well as Materials Science and Engineering. In 1998 he completed is Doctorate in Materials Science and Engineering at the University of Michigan under the guidance of Professor Albert Yee, studying the effects of water transport in aerospace epoxy resins. After his Ph D he received a NIST-NRC Postdoctoral Fellowship to work with Dr. Wen-li Wu of the NIST Polymers Division and in 2002 made the transition to a permanent research staff scientist. His areas of technical expertise include thin films and polymers under confinement, polymer dynamics, lithographic pattering, organic semiconductors for printed and flexible electronics, porous materials, and polymer membranes for water filtration, ion transport, and electrochemical energy storage and delivery. He has published over 150 peer-reviewed publications, an h-index of 33, one US Patent, and received several prestigious awards including the Presidential Early Career Award for Science and Engineering (2006) and the United States Department of Commerce Bronze (2006, 2008) and Silver (2006) Medals, and the Arthur S Flemming Award from The George Washington University for outstanding research contributions to the Federal Government (2010). In 2011 he was selected as a Fellow of the American Physical Society by the Division of Polymer Physics and he currently is the Vice Chair of the Polymeric Material: Science & Engineering (PMSE) Division of the American Chemical Society.