Amorphous Poly-3-Hexythiophene Thin Films via Electrospray Deposition under High Vacuum

Conjugated polymers (CPs) are heavily explored for their application in flexible organic electronic devices such as- organic photovoltaics, organic light emitting diodes, organic transistors and organic thermoelectrics. This interest in CPs mainly stems from their high flexibility and ease of processibility on the large-scale by using cost-efficient solution-based techniques. Moreover, unlike their inorganic counterparts, the materials class of organic semiconductors offers the striking advantage that their optoelectronic properties can be easily tuned by their chemical structure including the chain length. However, CPs are thermally fragile thus intrinsically inhibiting vacuum deposition by thermal evaporation. Therefore, CPs are typically processed by solution-based techniques only. There, however, the extent of polymer pre-aggregation and film growth is substantially influenced by the choice of the solvent and other difficult to control experimental factors. The resulting microstructure in CP films can, in addition, not be improved by processing at elevated temperatures, which are largely inaccessible for common solvents. In contrast to conjugated organic molecules (COMs) that can be processed in-vacuo to form crystalline films of high coherence lengths, prototypical CPs such as regioregular poly(3-hexylthiophene) (P3HT) are semi-crystalline when grown from solution with the ratio of crystalline to amorphous film portion being hard to control.<br/>For CPs, molecular doping enables tuning the thin-film conductivity over orders of magnitude. However, the film crystallinity has huge impact on the charge carrier mobility in CP heterostructures as well as pristine and doped CP films, where it has been argued that amorphous and crystalline film portions react differently to dopant exposure. Due to the intrinsic semi-crystalline nature of CPs such as P3HT in thin films, information experimentally gained comprises superimposed signatures from the crystalline and amorphous regions of the CP. Therefore, preparing amorphous CP films represents the most straightforward experimental approach to disentangle this information. In contrast to <i>improving</i> the structural properties of semi-crystalline CP films, however, deliberately <i>reducing</i> their crystallinity and achieving <i>amorphous</i> films has not been achieved so far. Here, we demonstrate that room-temperature high vacuum electrospray deposition (HV-ESD) of P3HT enables reproducibly establishing such films and provide evidence for their amorphicity by synchrotron grazing-incidence X-ray diffraction (GIXRD). In addition to details of the experimental procedure of HV-ESD and the properties of pristine P3HT films established therewith, we will discuss their doping phenomenology with the strong electron acceptor 2,3,5,6-Tetrafluoro-tetracyanoquinodimethane (F4TCNQ).