Effect of Additives on the Surface Morphology, Energetics, and Contact Resistance of PEDOT:PSS.

For a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film employed in a device stack, charge must pass through both the bulk of the film and interfaces between adjacent layers. Thus, charge transport is governed by both bulk and contact resistances. However, for ultrathin films (e.g., flexible devices, thin-film transistors, printed electronics, solar cells), interfacial properties can dominate over the bulk properties, making contact resistance a significant determinant of device performance. For most device applications, the bulk conductivity of PEDOT:PSS is typically improved by blending additives into the solid film. Doping PEDOT:PSS with secondary dopants (e.g., polar small molecules), in particular, increases the bulk conductivity by inducing a more favorable solid morphology. However, the effects of these morphological changes on the contact resistance (which play a bigger role at smaller length scales) are relatively unstudied. In this work, we use transfer length method (TLM) measurements to decouple the bulk resistance from the contact resistance of PEDOT:PSS films incorporating several common additives. These additives include secondary dopants, a silane crosslinker (typically used to stabilize the PEDOT:PSS film), and multi-walled carbon nanotubes (conductive fillers). Using conductive atomic force microscopy, Kelvin probe force microscopy, Raman spectroscopy, and photoelectron spectroscopy, we connect changes in the contact resistance to changes in the surface morphology and energetics as governed by the blended additives. We find that the contact resistance at the PEDOT:PSS/silver interface can be reduced by (1) increasing the ratio of PEDOT to PSS chains, (2) decreasing the work function, (3) decreasing the benzoid-to-quinoid ratio at the surface of the solid film, (4) increasing the film uniformity and contact area, and (5) increasing the phase-segregated morphology of the solid film.

Solvent-Free Transfer of Freestanding Large-Area Conjugated Polymer Films for Optoelectronic Applications.

Conventional processes for depositing thin films of conjugated polymers are restricted to those based on vapor, liquid, and solution-phase precursors. Each of these methods bear some limitations. For example, low-bandgap polymers with alternating donor-acceptor structures cannot be deposited from the vapor phase, and solution-phase deposition is always subject to issues related to the incompatibility of the substrate with the solvent. Here, a technique to enable deposition of large-area, ultra-thin films (≈20 nm or more), which are transferred from the surface of water, is demonstrated. From the water, these pre-solidified films can then be transferred to a desired substrate, circumventing limitations such as solvent orthogonality. The quality of these films is characterized by a variety of imaging and electrochemical measurements. Mechanical toughness is identified as a limiting property of polymer compatibility, along with some strategies to address this limitation. As a demonstration, the films are used as the hole-transport layer in perovskite solar cells, in which their performance is shown to be comparable to controls formed by spin-coating.

Comparison of the Mechanical Properties of a Conjugated Polymer Deposited Using Spin Coating, Interfacial Spreading, Solution Shearing, and Spray Coating.

The mechanical properties of π-conjugated (semiconducting) polymers are a key determinant of the stability and manufacturability of devices envisioned for applications in energy and healthcare. These properties─including modulus, extensibility, toughness, and strength─are influenced by the morphology of the solid film, which depends on the method of processing. To date, the majority of work done on the mechanical properties of semiconducting polymers has been performed on films deposited by spin coating, a process not amenable to the manufacturing of large-area films. Here, we compare the mechanical properties of thin films of regioregular poly(3-heptylthiophene) (P3HpT) produced by three scalable deposition processes─interfacial spreading, solution shearing, and spray coating─and spin coating (as a reference). Our results lead to four principal conclusions. (1) Spray-coated films have poor mechanical robustness due to defects and inhomogeneous thickness. (2) Sheared films show the highest modulus, strength, and toughness, likely resulting from a decrease in free volume. (3) Interfacially spread films show a lower modulus but greater fracture strain than spin-coated films. (4) The trends observed in the tensile behavior of films cast using different deposition processes held true for both P3HpT and poly(3-butylthiophene) (P3BT), an analogue with a higher glass transition temperature. Grazing incidence X-ray diffraction and ultraviolet-visible spectroscopy reveal many notable differences in the solid structures of P3HpT films generated by all four processes. While these morphological differences provide possible explanations for differences in the electronic properties (hole mobility), we find that the mechanical properties of the film are dominated by the free volume and surface topography. In field-effect transistors, spread films had mobilities more than 1 magnitude greater than any other films, likely due to a relatively high proportion of edge-on texturing and long coherence length in the crystalline domains. Overall, spread films offer the best combination of deformability and charge-transport properties.