A Zone‐Casting Technique for Device Fabrication of Field‐Effect Transistors Based on Discotic Hexa‐peri‐hexabenzocoronene

Abstract

Organic semiconductors have attracted considerable interest over the last decade due to an immense improvement in the performance of electronic devices based on these materials. This attention has mainly been focused on conjugated polymers and oligomers, as well as small molecules which can be utilized as active layers in devices such as field-effect transistors (FETs), [1,2] photovoltaic cells, [3] and light-emitting diodes.[4] An interesting group of materials with potential application as organic semiconductors in electronic devices are columnar discotics. [5,6] These mesogens consist of an aromatic core, which can be chemically modified by peripheral substitution (e.g., with alkyl chains), and self-assemble into one-dimensional (1D) columnar superstructures that then arrange in a two-dimensional (2D) lattice. The overlapping of the p orbitals of adjacent molecules within the columns ensures 1D intracolumnar charge-carrier transport. Another essential requirement for undisturbed 1D charge migration along the columns is a high degree of long-range order in the active layer which is deposited between the electrodes. [7] Local defects at domain boundaries in unoriented layers can trap charge carriers and significantly decrease the device performance. Thus, the development of appropriate processing techniques became an essential challenge for the fabrication of unperturbed long-range-oriented organic semiconductors. This close relationship between supramolecular structure and electronic properties has been investigated impressively for planar metallophthalocyanine (Pc) and metalloporphyrin derivatives, in which the charge-carrier mobility of the holes depends strongly on the processing technique.[8±11] For vacuum-deposited thin layers of phthalocyanine, the mobility varied from 10 ±4 cm 2 V ±1 s ±1 for nickel Pc [8] to 0.02 cm 2 V ±1 s ±1 for copper Pc. [9,10] Other processing techniques, such as solution deposition onto substrates with a friction-oriented poly(tetrafluoroethylene) (PTFE) layer [12] and the Langmuir±Blodgett (LB) method, require chemical substitution of Pc, which results in decreased mobility in comparison to samples prepared by vacuum deposition.[13]The history of discotic liquid-crystalline hexa-peri-hexabenzocoronene (HBC) derivatives as semiconductors is significantly shorter than that of phthalocyanines. Nevertheless, HBC derivatives have been successfully exploited in photovoltaic devices and field-effect transistors.[14] FETs were prepared by solution casting on substrates with the pre-oriented PTFE layer, resulting in uniaxial columnar order with an edge-on arrangement of the molecules.[15] The high supramolecular orientation was confirmed by field-effect anisotropy: the charge-carrier mobilities along the columns were significantly higher than in the perpendicular direction. [16] The high anisotropy of the charge-carrier mobility was demonstrated by flash-photolysis time-resolved microwave conductivity measurements.[17] The LB technique [18] and zone crystallization [19] have also been re...

Liquid crystalline organic semiconductors for organic transistor applications

“…There have been many reports on the FET performance of OFETs fabricated from crystalline thin films of liquid crystals including di-thienyl-naphthalene, 23 hexabenzocoronene, 24 bis-(5'-hexylthiophen-2'-yl)-2,6-Anthracene 25 and dialkyl-BTBT derivatives, 4 as shown in Figure 3. However, the liquid crystallinity in these materials was overlooked, and its benefits have not been utilized in device fabrication.…”

Theoretical study of stability and charge-transport properties of coronene molecule and some of its halogenated derivatives: A path to ambipolar organic-based materials?

“…14,17 Hexabenzocoronene is known to form discotic liquid-crystalline materials with molecules uniaxially oriented along the transport direction with field-effect mobilities reaching 0.01 cm 2 V −1 s −1 . 18,19 However, the substitution pattern followed here may also bring additional non-covalent through-space interactions, 20,21 complementarily to π · · ·π and C−H· · ·π interactions largely found in acene-based materials, which also deserve to be closely investigated by theoretical methods. Relying again on existing pentacene data, it is known how per-halogenation 22 tune from p-type towards n-type semiconductors concomitantly with opening of the herringbone angle 23 typical of acenelike structures.…”

One-dimensional self-confinement promotes polymorph selection in large-area organic semiconductor thin films

“…These methods generally yield little preferential crystallographic orientation in the plane of the generated thin films without additional patterning processes. By contrast, methods such as zone casting, dip coating, flow coating, knife coating and doctor blading have been shown to produce thin films with preferential in-plane crystallographic orientation over large areas [25][26][27] . The solutionshearing method was recently proposed as a large-area coating method capable of producing preferential in-plane crystallographic alignment 5,28 .…”