Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films

Abstract

The research of materials for organic fi eldeffect transistors (OFETs) has been extended to the exploration for new materials suitable for the fabrication of polycrystalline thin fi lms by solution processes such as spincoating and ink-jet techniques instead of costly vacuum evaporation techniques. In fact, solution-processed polycrystalline thin fi lms from precursor materials, [ 1 , 2 ] such as 6,13-bis(triisopropylsilylethynyl)-pentacene, [ 3 ] and alkylated oligothiophene, [4][5][6] hexabenzocoronene, [ 7 ] and benzothienobenzothiophene (BTBT) [ 8 ] derivatives, attained high fi eld-effect transistor (FET) mobility over 1 cm 2 V − 1 s − 1 comparable to that of vacuum evaporated fi lms of pentacene. [ 9 ] In industrial applications of FETs, uniformity of the fi lms is another important requirement because FETs are often used in a transistor array, for example, for active matrices in display devices. Therefore, the time-consuming optimization of fi lmcoating parameters such as solvents, concentrations and evaporation rates of the solvent, and the surface wetting condition of the substrates is essential. Towards this, polymer materials have some advantages in general thanks to their fi lm-forming ability, as is demonstrated in polythiophenes (P3HT), [ 10,11 ] polyfl uorenes (F8), [ 12 ] and poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT). [ 13 ] In this paper, we demonstrate the availability of liquid crystalline materials for fabricating polycrystalline thin fi lms for OFET applications in terms of both surface morphology and area uniformity, and discuss its origin.We selected a liquid crystalline terthiophene ( 1 , ω , ω '-dioctylterthiophene (8-TTP-8)) as a model material for liquid crystals and a BTBT ( 2 , 2,7-didecylbenzothienobenzothiophene (C 10 -BTBT)) derivative as another example, the chemical structures of which are shown in Figure 1 . In order to investigate the effect of spin-coating temperature on the resulting fi lms in detail, we paid much attention to the actual temperature at spin-coating, and kept the solution temperature on the substrate before spin-coating and the fi lm temperature after spincoating the same. The procedural details are described in the Supporting Information. Figure 2 shows the optical microscope textures of the fi lms spin-coated with 1 wt% p -xylene solution of 8-TTP-8 at various temperatures. Judging from the textures, the morphology of the resulting polycrystalline fi lms of 8-TTP-8, which exhibited smectic C, F, and G (SmC, SmF, and SmG) phases in a temperature range from 89 to 58 ° C, [ 14 ] strongly depended on the spin-coating temperature: the polycrystalline fi lm spin-coated at 38 ° C had a crystal phase that consisted of many microcrystalline fl akes as shown in Figure 2 a; the fi lm spin-coated at 102 ° C had an isotropic phase, was quite inhomogeneous, and looked like crystal droplets as shown in Figure 2 c. On the other hand, the polycrystalline fi lm spin-coated at 88 ° C had a SmC phase and was quite uniform on the millimeter scale as sho...

Single crystal growth in spin-coated films of polymorphic phthalocyanine derivative under solvent vapor

“…Liquid-crystalline organic semiconductors are attracting much attention owing to their features, such as organization of highly ordered crystalline structures and high carrier mobility in devices. [9][10][11] In addition, C6PcH 2 shows polymorphic characteristics as reported recently. 12 Because crystal structures in domains and grain boundaries between the domains are critical for carrier mobility, controlling transitions of crystal structure accompanied by a transformation of grain boundaries in the films is thought to be necessary for improving carrier mobility in films showing multiple crystal structures.…”

Dynamics of Monolayer–Island Transitions in 2,7‐Dioctyl‐benzothienobenzthiophene Thin Films

“…Our observations can therefore explain the interesting finding for C8-BTBT-C8 employed as active layer in thin-film transistors, for which enhanced performance is observed with films prepared at liquid-crystalline temperatures. [15] Figure 4. AFM height images of the system after cooling from the liquid-crystalline phase at A) fast (10 K min À1 ) and C) slow (0.8 K min À1 ) cooling rates.…”

A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology

“…By dissolving the material in an appropriate solvent and spin coating the solutions at a temperature where the pure material is in the smectic state, they achieved excellent films with very high uniformity and long-range order [33], cf. Fig.…”