Dongwei Zhang

Molecular phase engineering of organic semiconductors based on a [1]benzothieno[3,2-b][1]benzothiophene core

Two environmentally and thermally stable [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-hexylphenyl)[1]benzothieno[3,2-b][1]benzothiophene (C6-Ph-BTBT), are prepared by Suzuki coupling. Organic thin-film transistors with a top-contact and bottom-gate based on BTBT, Ph-BTBT and C6-Ph-BTBT are fabricated by vacuum-deposition on octyltrichlorosilane treated Si/SiO2 substrates. Experimental results show that the thin-film based on BTBT sublimes instantly after the deposition of electrodes, and no semiconductor signal is detected. Ph-BTBT shows a mobility of 0.034 cm2 V−1 s−1. Furthermore, C6-Ph-BTBT exhibits three liquid crystal phases (Sm A, Sm E and Sm K or H) and achieves the highest hole mobility of 4.6 cm2 V−1 s−1 with an on/off ratio of 2.2 × 107 for polycrystalline organic thin-film transistors in ambient air. The present study exemplifies that the introduction of mesoscopic order in molecular design provides a general approach for the high electronic performance of organic semiconductors.

Highly responsive phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene single crystal

Herein, thin film and single crystal phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene (BOPAnt) are systematically studied. High quality BOPAnt single crystals are grown via the PVT (physical vapor transport) method to fabricate bottom gate top contact phototransistors. The thin film phototransistor shows a photoresponsivity of 9.75 A W−1 under 1 mW cm−2 blue LED illumination, whereas under the same conditions the single crystal based phototransistor displays a photoresponsivity of 414 A W−1. Furthermore, using low power monochromatic light with the wavelength of 350 nm, the photoresponsivity of 3100 A W−1 under 0.11 mW cm−2 illumination and EQE of 9.5 × 105% are obtained for the BOPAnt single crystal phototransistor with the channel length of 65 μm. The much higher photoresponsivity of the single crystal based phototransistor is due to its much higher exciton diffusion length compared to that of the thin film device, which is also evident by the large Von shift towards the positive voltage direction. The photoswitching behaviors of the single crystal based phototransistor are also studied. It is observed that after a short warming up period, the single crystal based phototransistor shows stable switching behavior.

Thermal and Optical Modulation of the Carrier Mobility in OTFTs Based on an Azo-anthracene Liquid Crystal Organic Semiconductor

One of the most striking features of organic semiconductors compared with their corresponding inorganic counterparts is their molecular diversity. The major challenge in organic semiconductor material technology is creating molecular structural motifs to develop multifunctional materials in order to achieve the desired functionalities yet to optimize the specific device performance. Azo-compounds, because of their special photoresponsive property, have attracted extensive interest in photonic and optoelectronic applications; if incorporated wisely in the organic semiconductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photo modulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystal semiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthesized for application in organic thin-film transistors (OTFTs). The UV-vis spectra of APDPD exhibit reversible photoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on its liquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films are characterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry (DSC), UV-vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study reveals that our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.

In-plane isotropic charge transport characteristics of single-crystal FETs with high mobility based on 2,6-bis(4-methoxyphenyl)anthracene: experimental cum theoretical assessment

In-plane isotropic charge transport in single crystals is desirable in large-area single-crystal thin-film transistor (FET) arrays because it is independent of crystal direction. However, most organic semiconductors show anisotropic charge transport, while only a few show isotropic or moderately isotropic charge transport characteristics. We report a highly isotropic charge transport semiconductor material, 2,6-bis(4-methoxyphenyl)anthracene (BOPAnt), and demonstrate BOPAnt-based single-crystal FETs with a high mobility of 13–16 cm2 V−1 s−1 and an anisotropic mobility ratio (μmax/μmin) of approximately 1.23, the lowest value yet reported. Using the single-crystal structure of BOPAnt, the rectangular diffraction patterns of the molecular lattice parameters in single-crystal thin films were analysed by transmission electron microscopy and polarized optical microscopy. The highly isotropic properties are attributed to the molecular structure enhancement by incorporation of methoxyphenyl units; our analysis revealed an orthorhombic lattice arrangement in the solid state, with the molecules packed in an unusual fashion in this particular stacking mode. In addition, hole mobility calculations combining the transfer integrals and the reorganization energy are used to explain the charge-transport properties.

Blending crystalline/liquid crystalline small molecule semiconductors: A strategy towards high performance organic thin film transistors

Solution processed small molecule polycrystalline thin films often suffer from the problems of inhomogeneity and discontinuity. Here, we describe a strategy to solve these problems through deposition of the active layer from a blended solution of crystalline (2-phenyl[1]benzothieno[3,2-b][1]benzothiophene, Ph-BTBT) and liquid crystalline (2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, C12-Ph-BTBT) small molecule semiconductors with the hot spin-coating method. Organic thin film transistors with average hole mobility approaching 1 cm2/V s, much higher than that of single component devices, have been demonstrated, mainly due to the improved uniformity, continuity, crystallinity, and stronger intermolecular π-π stacking in blend thin films. Our results indicate that the crystalline/liquid crystalline semiconductor blend method is an effective way to enhance the performance of organic transistors.

Effects of a highly lipophilic substituent on the environmental stability of naphthalene tetracarboxylic diimide-based n-channel thin-film transistors

N,N′-Bis(4-trifluoromethylthiobenzyl)naphthalene-1,4,5,8-tetracarboxylic acid diimide (NTCDI-BSCF3) is synthesized. It shows a similar molecular packing structure and intermolecular transfer integral to N,N′-bis(4-trifluoromethoxybenzyl)naphthalene-1,4,5,8-tetracarboxylic acid diimide (NTCDI-BOCF3), but demonstrates different behaviors in terms of electron mobility and air stability. NTCDI-BSCF3 based organic thin-film transistors (OTFTs) exhibit much better environmental stability when compared with NTCDI-BOCF3 due to their high hydrophobicity which prevents the diffusion of moisture and oxygen into the devices. In addition, the electron mobility of NTCDI-BSCF3 shows good thermal stability in relation to the deposition temperature, and achieves a value as high as 0.17 cm2 (V s)−1 in air, although it is lower than that of NTCDI-BOCF3. The lower mobility may be attributed to the unexpected crystal growth mode after the deposition of the second monolayer and an insufficient quality of the thin films of NTCDI-BSCF3, especially their inadequate crystallinity. This contrasts with the Stranski–Krastanov (SK) (layer-plus-island) growth mode with the expected crystal growth direction and good crystallinity of NTCDI-BOCF3. Nevertheless, it can be concluded that the introduction of the trifluoromethanesulfenyl (SCF3) group at the N-group of naphthalene tetracarboxylic diimide (NTCDI) is an effective approach for enhancing the environmental stability of NTCDI based n-channel OTFTs.

Phenyl substitution in tetracene: a promising strategy to boost charge mobility in thin film transistors

Tetracene, one of the polyacene derivatives, shows eminent optical and electronic properties with relatively high stability. To take advantage of the intrinsic properties of the tetracene molecule and explore new semiconductors, herein, we report the design and synthesis of two novel p-channel tetracene derivatives, 2-(4-dodecyl-phenyl)-tetracene (C12-Ph-TET) and 2-phenyl-tetracene (Ph-TET). Top contact OTFTs were fabricated using these two materials as semiconductor layers, with charge mobilities up to 1.80 cm2 V−1 s−1 and 1.08 cm2 V−1 s−1, respectively. Our molecular modeling results indicate that the introduction of phenyl into tetracene can improve the efficient charge transport in electronic devices as a result of the increased electronic coupling between the two neighboring planes of the molecules. AFM images of the thermally evaporated thin films of these two materials show large grains, which correspond to the high mobilities of these devices. Consequently, the mobility of our OTFTs based on C12-Ph-TET is the highest for OTFTs based on tetracene derivatives reported to date. The single crystal analyses show the existence of π–π stacking interactions within the molecules with the introduction of mono-phenyl substituents, which is the main cause of the increased mobility. The impressive properties of these two materials indicate that the introduction of alkyl-phenyl and phenyl group could be an excellent method to improve the properties of the organic semiconductor materials.