Properties of Charged Conjugated Polymers Deposited by Electrospraying on Substrates with an Insulating Oxide Layer

In the course of electrospraying in a vacuum, polymers acquire electric charges, which are distributed along their backbone. Here, we demonstrate that these charges have a strong impact on the molecular ordering and spectroscopic properties of conjugated polymers when deposited on substrates with an insulating oxide layer. To this end, we electrosprayed varying amounts of poly(3-hexylthiophene) (P3HT) onto silicon substrates with oxide layers of different thickness. The resulting morphologies were explored by atomic force microscopy. The corresponding spectroscopic properties, including their changes in time upon illumination, were derived from photoluminescence (PL) measurements. On insulating substrates, charges were retained on the polymer chains, causing long-range electrostatic repulsion and the formation of largely separated droplet-like objects containing multiple chains. These objects exhibited significant PL intensities for photons with a wavelength (λ) less than ca. 600 nm, suggesting the lack of π–π stacking interactions between P3HT chains. Conversely, on substrates with a thin insulating oxide layer, charges were dissipated comparatively easily in the substrate, allowing attractive intermolecular interactions to prevail. As a consequence, P3HT aggregates were generated, which emitted photons with λ ≥ 600 nm, spectroscopically similar to uncharged P3HT in spin-coated films. Furthermore, on substrates with a thick oxide layer, we demonstrate that polymer chains may lose their charges upon illumination, yielding a PL spectrum similar to that of aggregated P3HT observed in spin-coated films.