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

Mikayelyan et al. Nanoscale Research Letters https://doi.org/10.1186/s11671-019-3009-8 (2019) 14:185 NANO EXPRESS Open Access 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 Eduard Mikayelyan1, Linda Grodd1, Viachaslau Ksianzou2, Daniel Wesner3, Alexander I. Rodygin4,5, Holger Schönherr3, Yuriy N. Luponosov6, Sergei A. Ponomarenko6,7, Dimitri A. Ivanov4,5,8,9*, Ullrich Pietsch1 and Souren Grigorian1,10* Abstract 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. Keywords: Thin films, Interfacial monolayers, Oligomers, Quarterthiophenes, In situ GIXD, Phase transitions, Mesophase Introduction Organic semiconductors constitute an important class of materials due to their exceptional combination of mechanical flexibility and low cost allowing production of large-area electronic devices. They are used as functional layers in various organic electronic circuits such as organic field-effect transistors (OFETs), organic light emitting diodes * Correspondence: ; 4 Faculty of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University, GSP-1, Leninskie gory1, Moscow, Russian Federation119991 1 Department of Physics, University of Siegen, Walter-Flex-Strasse 3, 57072 Siegen, Germany Full list of author information is available at the end of the article (OLEDs), organic photovoltaics (OPVs), and others [1, 2]. Understanding the relation between the active layer structure and device properties is crucial for the optimization of the performance of the devices based thereupon. One of the common techniques for the structural analysis of organic semiconductor materials is X-ray diffraction. In particular, grazing-incidence X-ray diffraction (GIXD) using intense synchrotron X-ray beams is a powerful tool providing sensitivity to organization of the interfacial regions of the organic films close to the substrate and probing the thicknesses on the order of a few monolayers that are mainly responsible for the charge transport. © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Mikayelyan et al. Nanoscale Research Letters (2019) 14:185 For optimization of the performance of such devices as organic field-effect transistors (OFETs), it is important to consider that the structure of thin films can significantly differ from that of bulk single crystals [3]. Commonly, organic semiconductor films cast on substrate form crystallites, which may be randomly oriented with respect to the substrate surface. If the crystallite orientation is random in 3D, Bragg diffraction peaks corresponding to same d-value form a ring-like pattern. If the random orientation is restricted to the plane parallel to the substrate, well-defined Bragg spots appear allowing to analyze the film texture. Consequently, in the case of sufficiently intense diffraction peaks, 2D-GIXD is a suitable technique for in situ investigations of thin-film structures during various processes such as solidification and post-annealing [4–6] Generally, the rod-like oligothiophene molecules deposited on substrates exhibit nearly upright orientation, with the long molecular axis being almost perpendicular to the substrate surface [7]. Accordingly, the π-π stacking direction is largely oriented parallel to the substrate surface, which is advantageous for the OFET geometry. For utilizing in solution-processable electronics, the solubility improvement by aliphatic end group substitution is common [8, 9]. It is known that the increasing number of thiophene units increases the charge carrier mobility at the cost of decreasing the solubility. For this reason, the optimum length of the thiophene core is considered to be quarterthiophene (4T) [10]. Oligothiophenes are the most studied organic semiconductor materials [11]. These rod-like molecules provide a relatively high mobility in thin films caused by preferential π-π stacking [12] and are promising for applications in organic electronics [13–15]. Dihexyl-quaterthiophene (DH4T) is one of the well-known oligothiophenes [16–20]. Based on differential scanning calorimetry (DSC), two endotherms were reported, one at 81 °C and the other at 181 °C, where the first one is conventionally attributed to a transition to the mesophase and the second one to the isotropization [10, 21, 22]. Previously, the monoclinic structure of single D4HT crystals was analyzed by electron diffraction [23]. Furthermore, annealing of DH4T fibers revealed two crystallographic phases corresponding to the initial phase and the mesophase [10]. In the case of thin films [21], the structure of the mesophase was associated with a tilted pseudohexatic smectic structure, whereas in the study of fibers, it was identified as a crystalline phase II [10]. Apart from the rich polymorphism in the bulk, the considered organic molecules are often prone to formation of the so-called surface-induced polymorphs, or surface-mediated polymorphs [24, 25]. In this case, nucleation occurs in the proximity of a surface and results in a structure different from any of the bulk polymorphs. Such surface-induced structures can be Page (...truncated)