4D Microspectroscopy Explores Orientation and Aggregations in π-Conjugated Polymer Films Prepared by Brush Printing

The orientation of polymers improves their mechanical, electrical, and optical properties, but aggregates alter these properties. Unfortunately, there is no definitive way to control aggregates and quantify orientations by distinguishing between polymer chains and aggregates. Herein, we show 4D microspectroscopy to examine brush-printed oriented films of π-conjugated polymers. Three-dimensional (x-y-z) and 1D (photon energy) components based on polarized-fluorescence spectra and film thickness at identical positions were measured. Stochastic analysis of 4D data for a brush-printed OLED film (30 × 30 μm², 900 pixels) distinguished orientations of polymer chains and their aggregates with a 1 μm x-y resolution and a z-range of 20-1800 nm. Polymer chains in thin regions (t < 50 nm) were oriented parallel to the brush-printing direction, whereas aggregates were oriented perpendicularly in thicker regions (t > 1000 nm). This difference was attributed to shear stress, uneven thickness, and capillary forces. The generality of the 4D method was also examined using conventional drop casting.