Electron-to Hole Transport Change Induced by Solvent Vapor Annealing of Naphthalene Diimide Doped with Poly(3-Hexylthiophene)

Herein we report on fabrication and properties of organic field-effect transistors (OFETs) based on the spray-coated films of N,N′-dioctyl naphthalene diimide (NDIC8) doped with 2.4 wt% of poly (3-hexylthiophene) (P3HT). OFETs with the untreated NDIC8:P3HT films revealed electron conductivity [μe* = 5 × 10–4 cm2×(Vs)−1]. After the annealing in chloroform vapor the NDIC8:P3HT films revealed the hole transport only [μh* = 0.9 × 10–4 cm2×(Vs)−1]. Due to the chemical nature and energy levels, the hole transport was not expected for NDIC8-based system. Polarized optical- and scanning electron microscopies indicated that the solvent vapor annealing of the NDIC8:P3HT films caused a transition of their fine-grained morphology to the network of branched, dendritic crystallites. Grazing incidence wide-angle X-ray scattering studies indicated that the above transition was accompanied by a change in the crystal structure of NDIC8. The isotropic crystal structure of NDIC8 in the untreated film was identical to the known crystal structure of the bulk NDIC8. After the solvent annealing the crystal structure of NDIC8 changed to a not-yet-reported polymorph, that, unlike in the untreated film, was partially oriented with respect to the OFET substrate

Phase Diagrams of poly(3-hexylthiophene):N,N’-alkyl Substituted Naphthalene Diimides Blends

Fabrication of electronic devices based on organic semiconductors at the fundamental level can be considered a problem of engineering of polycrystalline materials. Tuning the charge transport properties in such materials requires controlling of size, perfection, distribution, and orientation of crystalline domains in the semiconducting films1. Those parameters can be controlled through film formation and post-processing conditions, such as thermal annealing. While the casting parameters (solvent, temperature etc.) exert an influence on the primary crystallization and formation of polycrystalline system, the annealing induces the further growth of crystalline domains, increases perfection of crystal packing but may also trigger phase transitions of the ordered phases crystallized from solution2. Insufficient insight into the processes occurring upon the annealing may lead to erroneous conclusions related to properties - especially when samples reveal a complex phase behavior like the one observed in the case of aromatic, conjugated molecules or polymers. These materials may show multiple packings in crystalline phases as well as multiple thermally-induced transitions between crystalline and liquid crystalline phases before reaching the isotropic state3. Different packing of molecules and hence different geometrical overlap of aromatic moieties may affect charge transport properties4. Thus in our research we concentrated on relationships between packing of molecules in the ordered domains and electronic properties of materials.The study was focused on the blends of poly(3-hexylthiophene) (P3HT) with butyl- or hexyl- or octyl-substituted naphthalene diimides (NDI). In our experimental approach we have applied differential scanning calorimetry (DSC) to determine phase transition points and X-ray diffraction (XRD) to gain insights into the structure of molecular assemblies in the system. These studies were supplemented with images from scanning electron microscopy (SEM) providing information about the morphology of the blends. In order to correlate the structure and morphology with charge transport properties, the blends were tested in organic field effect transistors (OFETs).Results of our experiments indicated that packing of molecules in crystalline phases and mesophases as well as phase transition temperatures were related to molecular architecture of NDI and compositions of the blends. XRD and DSC data were used to determine phase diagrams for the blends of P3HT with NDI. It was found that decreasing content of NDI in the blends caused a notable decrease of the isotropization temperature of the NDI component. The melting point of P3HT also significantly drops as its content in the blend decreases. The profile of the isotropization points of the P3HT:NDI blends plotted as a function of composition reveals a clear minimum resembling the eutectic point. In some of the blends, we observed additional transitions that may suggest formation of new crystalline phases occurring neither in pure P3HT nor pure NDI. The studies of the blends by means of scanning electron microscopy indicated that compositions of the blends also exerted an influence on the crystalline morphology. The studies electronic properties in the model field-effect transistors (OFETs) revealed that the blends with the uniform, fine-grained polycrystalline morphologies enable balanced ambipolar charge carrier transport. This, once again, indicates that the fine tuning of the blends composition is crucial for engineering of organic electronic devices.References[1] Brady M. A. et al., Soft Matter, 7, 11065 (2011)[2] Chlebosz D. et al., Dyes and Pigments, 140, 491 (2017)[3] Yuan Y. et al., Polymer, 105, 88 (2016)[4] Kakinuma T. el al., Journal of Materials Chemistry C, 1, 5395 (2013