Solution processing of electrically active layers is a promising route to 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 film structure (and performance) by processing details. The sensitivity of structure and performance to processing is clearly manifest in the fabrication of bulk heterojunction (BHJ) active layers for organic photovoltaics and aligned semiconducting layers for organic transistors.  In a BHJ, nanoscale phase separation into donor and acceptor rich regions enables both exciton dissociation and charge extraction. The performance of BHJ based devices is thus a strong function of the active layer 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. We will discuss highlights from studies of the additive effect in 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. Due to the pi-stacking motif of most semiconducting polymers, the local mobility is intrinsically anisotropic; therefore, fabrication of aligned films enables optimization of transistor device performance. Because of its directional character, blade coating can result in aligned films. We will present results from blade coating a number of high performing polymer materials. Depending on polymer and deposition regime, highly aligned films can be produced. The aligned films typically exhibit significantly greater mobility than isotropic films. Detailed morphological studies suggest that the improvement in transport is related to alignment of the inter-crystallite regions (tie-chains). In-situ studies indicate that the coating induced alignment nucleates near the air interface and is facilitated by the presence of a lyotropic liquid crystal phase.<br/>

<i>In situ</i> studies of nanoscale structure development in printed organic electronics

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.

In situ studies of nanoscale structure development in printed organic electronics