Phase Transitions and Formation of a Monolayer-Type Structure in Thin Oligothiophene Films: Exploration with a Combined In Situ X-ray Diffraction and Electrical Measurements.

A combination of in situ electrical and grazing-incidence X-ray diffraction (GIXD) is a powerful tool for studies of correlations between the microstructure and charge transport in thin organic films. The information provided by such experimental approach can help optimizing the performance of the films as active layers of organic electronic devices. In this work, such combination of techniques was used to investigate the phase transitions in vacuum-deposited thin films of a common organic semiconductor dihexyl-quarterthiophene (DH4T). A transition from the initial highly crystalline phase to a mesophase was detected upon heating, while only a partial backward transition was observed upon cooling to room temperature. In situ electrical conductivity measurements revealed the impact of both transitions on charge transport. This is partly accounted for by the fact that the initial crystalline phase is characterized by inclination of molecules in the plane perpendicular to the π-π stacking direction, whereas the mesophase is built of molecules tilted in the direction of π-π stacking. Importantly, in addition to the two phases of DH4T characteristic of the bulk, a third interfacial substrate-stabilized monolayer-type phase was observed. The existence of such interfacial structure can have important implications for the charge mobility, being especially favorable for lateral two-dimensional charge transport in the organic field-effect transistors geometry.

Large-Size Single-Crystal Oligothiophene-Based Monolayers for Field-Effect Transistors.

High structural quality of crystalline organic semiconductors is the basis of their superior electrical performance. Recent progress in quasi two-dimensional (2D) organic semiconductor films challenges bulk single crystals because both demonstrate competing charge-carrier mobilities. As the thinnest molecular semiconductors, monolayers offer numerous advantages such as unmatched flexibility and light transparency as well  they are an excellent platform for sensing. Oligothiophene-based materials are among the most promising ones for light-emitting applications because of the combination of efficient luminescence and decent charge-carrier mobility. Here, we demonstrate single-crystal monolayers of unprecedented structural order grown from four alkyl-substituted thiophene and thiophene-phenylene oligomers. The monolayer crystals with lateral dimensions up to 3 mm were grown from the solution on substrates with various surface energies and roughness by drop or spin-casting with subsequent slow solvent evaporation. Our data indicate that 2D crystallization resulting in single-crystal monolayers occurs at the receding gas-solution-substrate contact line. The structural properties of the monolayers were studied by grazing-incidence X-ray diffraction/reflectivity, atomic force and differential interference contrast microscopies, and imaging spectroscopic ellipsometry. These highly ordered monolayers demonstrated an excellent performance in organic field-effect transistors approaching the best values reported for the thiophene or thiophene-phenylene oligomers. Our findings pave the way for efficient monolayer organic electronics highlighting the high potential of simple solution-processing techniques for the growth of large-size single-crystal monolayers with excellent structural order and electrical performance competing against bulk single crystals.

Local structure of semicrystalline P3HT films probed by nanofocused coherent X-rays.

The hidden structural properties of semicrystalline polymer films are revealed by nanofocused X-ray scattering studies. X-ray cross-correlation analysis (XCCA) is employed to diffraction patterns from blends of poly(3-hexylthiophene) (P3HT) with gold nanoparticles (AuNPs). Spatially resolved maps of orientational distribution of crystalline domains allow us to distinguish sample regions of predominant face-on morphology, with a continuous transition to edge-on morphology. The average size of crystalline domains was determined to be of the order of 10 nm. As compared to pristine P3HT film, the P3HT/AuNPs blend is characterized by substantial ordering of crystalline domains, which can be induced by Au nanoparticles. The inhomogeneous structure of the polymer film is clearly visualized on the spatially resolved nanoscale 2D maps obtained using XCCA. Our results suggest that the observed changes of the polymer matrix within crystalline regions can be attributed to nanoconfinement in the presence of gold nanoparticles.

Local Orientational Structure of a P3HT π-π Conjugated Network Investigated by X-ray Nanodiffraction.

We employed nanobeam X-ray diffraction using an X-ray spot size of 150 nm to investigate the local structure of P3HT thin films. We derived nanoscale real space maps of the X-ray diffraction properties at the π-π (020) diffraction peak. The X-ray data reveal a complex nanoscale structure of the polymer network with strong local variation where some areas of the film display a rather high degree of angular order. We quantify both the magnitude and direction of the angular order. Our results provide new insights into the local structural properties and connectivity of P3HT films.

Direct correlation between electric and structural properties during solidification of poly(3-hexylthiophene) drop-cast films.

Structural and electrical properties of semicrystalline P3HT cast films onto Si/SiO(2) surface are studied during the solidification under applied electric field in lateral OFET geometry. During evaporation of the solvent, the formation of P3HT crystallites is monitored simultaneously by time-resolved X-ray diffraction and by source-drain current measurements. The electrical current is reaching its maximum in two pronounced regimes already before complete solidification of the polymer as detected by X-ray diffraction intensities. The monitored complex time dependence of current and X-ray intensities reveals a highest conducting level for the gel-like state.