Aiyuan Li

Influence of heteroatoms on the charge mobility of anthracene derivatives

The introduction of polarizable heteroatom, such as O, and S, attached peripheral side chains of conjugated moieties such as polyacenes has not been systematically investigated. To study such effects, and to explore semiconductors with both high charge mobility and luminescence properties, we present a comparative systematic study of heteroatom effects on the conduction of organic semiconductors in a representative series of new organic semiconductors based on the blue phenyl-anthracene molecule core. Elucidated by the single-crystal X-ray analysis, thin film XRD and AFM measurements, a correlation between the molecular structure variation, film ordering, and charge mobility has been established. Quantum chemistry calculations combined with the Marcus–Hush electron transfer theory interpret the transport parameters. The anisotropic transport properties of these compounds were suggested by the DFT predictions and the high hole mobility in BEPAnt and BOPAnt is contributed mainly by the parallel packing of these compounds with the highest ∥μh; these results are in good agreement with the experimental observations. Heteroatoms are demonstrated to influence the charge mobility dramatically. Our systematic investigation will provide valuable guidance for a judicious material design of semiconductors for OTFT applications.

Unlocking the potential of diketopyrrolopyrrole-based solar cells by a pre-solvent annealing method in all-solution processing

In this work, we report a solution processible diketopyrrolopyrrole anthracene based diblock copolymer synthesized via a rapid direct C–H arylation coupling method. After optimizing the combination of solvent and thermol annealing process, we investigated the characteristics of this di-block low band gap copolymer used as a donor part in bulk hetero-junction solar cells. The influence of the polymer:fullerene ratio (PDPP-ANT:PC61BM) and annealing conditions on the photo active film nanomorphology and device performance has been studied in detail. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal that the resulting film morphology strongly depends on the fullerene ratio before heat treatment. Power conversion efficiencies increase, more than four-fold, to 1.7% for regular architectures with high Voc (0.73 V), Jsc (4.75 mA cm−2) and FF (49.0%), compared with the unannealed device 0.37% (with Voc (0.54 V), Jsc (1.83 mA cm−2), FF (37.8%)). Furthermore, the two broad absorption bands of the PDPP-ANT materials show their potential to be applied in ternary polymer solar cells and tandem polymer solar cells.

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.

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.

Traps induced memory effect in rubrene single crystal phototransistor

In this report, phototransistors based on rubrene single crystals have been fabricated using octyltrichlorosilane (OTS) treated SiO2 as a substrate and memory effect has been observed and studied. Memory writing realized by the combined stimulation of applied gate voltage and light illumination was observed to be strongly dependent on the applied gate voltage. When the applied gate voltage, Vg = 100 V, only 500 ms writing time was found to be sufficient to achieve the memory window of 25 V. Long retention time of over 10 000 s as well as stable writing-reading-erasing-circle were also observed in the rubrene single crystal based phototransistor device. Even at high temperature of 100 °C, the device demonstrated stable memory effect. Thus we attributed the excellent memory effect to the deep electron traps. To clarify further the origin of persistent photocurrent, strongly hydrophobic fluorinated polyimide (FPI) was used as a dielectric layer to exclude the hydroxyl groups at the semiconductor/insulator interface. Compared to the OTS treated SiO2 based device, the photocurrent was saturated in a shorter time in FPI based counterpart, regardless of the light intensity and persistent photoconductivity was also observed. X-ray photoelectron spectroscopy analysis of rubrene single crystal revealed the existence of characteristic oxidation states, both in the high resolution C 1s and O 1s spectra, which behave as electron traps. In conclusion, we attribute the photo memory effect in rubrene single crystal based device to the oxygen related defects as well as to the hydroxyl groups at the dielectric/single crystal interface.In this report, phototransistors based on rubrene single crystals have been fabricated using octyltrichlorosilane (OTS) treated SiO2 as a substrate and memory effect has been observed and studied. Memory writing realized by the combined stimulation of applied gate voltage and light illumination was observed to be strongly dependent on the applied gate voltage. When the applied gate voltage, Vg = 100 V, only 500 ms writing time was found to be sufficient to achieve the memory window of 25 V. Long retention time of over 10 000 s as well as stable writing-reading-erasing-circle were also observed in the rubrene single crystal based phototransistor device. Even at high temperature of 100 °C, the device demonstrated stable memory effect. Thus we attributed the excellent memory effect to the deep electron traps. To clarify further the origin of persistent photocurrent, strongly hydrophobic fluorinated polyimide (FPI) was used as a dielectric layer to exclude the hydroxyl groups at the semiconductor/insulator in...

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.

A chrysene-based liquid crystalline semiconductor for organic thin-film transistors

We report the synthesis and characterization of a non-liquid crystalline material, 2-phenylchrysene (Ph-CHR), and a liquid crystalline material, 2-(4-dodecyl phenyl)chrysene (C12-Ph-CHR), and discuss their organic thin-film transistor (OTFT) device performances. The concept of designing chrysene derivatives is derived from the similar electronic structure of chrysene and [1]benzothieno[3,2-b][1]benzothiophene (BTBT), which is a very famous core for organic semiconductors. OTFTs, based on Ph-CHR and C12-Ph-CHR, are fabricated by vacuum-deposition on octyltrichlorosilane (OTS(8)) treated Si/SiO2 substrates. Our experimental results show that OTFTs based on Ph-CHR possess higher mobility than OTFTs based on BTBT derivative 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT). The liquid crystalline material C12-Ph-CHR achieves the highest carrier mobility of up to 3.07 cm2 V−1 s−1 with an on/off ratio of 3.3 × 106 for polycrystalline organic thin-film transistors in ambient air. The results indicate that the chrysene derivative C12-Ph-CHR is a promising candidate for applications in electronic devices.

Self-supported hysteresis-free flexible organic thermal transistor based on commercial graphite paper

Due to their high thermal conductivity, stability, light weight, and low cost, graphite products are widely used as thermally conductive materials in current electronic devices and are promising materials for future flexible electronics. However, the intrinsic high rough surface of graphite severely impedes the fabrication of thermal transistors based on graphite products. On the other hand, most of the flexible thermal transistors reported to date are based on polymer substrates, whose thermal conductivities are extremely low for thermal sensing. To address these issues herein, a flexible commercial graphite paper with high thermal conductivity was used as both the substrate and the back gate of thermal transistors. Fluorinated polyimide was also synthesized as a high performance dielectric material and was skillfully blade-coated on a flexible graphite paper to reduce the surface roughness. As a result, the as-fabricated flexible device exhibits extremely low hysteresis, wide operating temperature range (20–100 °C), high stability, and temperature sensing performance. Moreover, the as-fabricated pentacene device reached the mobility of 0.146 cm2 V−1 s−1, which is highly competitive among the reported flexible organic thermal transistors. Such thermal transistors are promising for integration in current electronic devices and promote the diversity of the flexible transistor substrates.Due to their high thermal conductivity, stability, light weight, and low cost, graphite products are widely used as thermally conductive materials in current electronic devices and are promising materials for future flexible electronics. However, the intrinsic high rough surface of graphite severely impedes the fabrication of thermal transistors based on graphite products. On the other hand, most of the flexible thermal transistors reported to date are based on polymer substrates, whose thermal conductivities are extremely low for thermal sensing. To address these issues herein, a flexible commercial graphite paper with high thermal conductivity was used as both the substrate and the back gate of thermal transistors. Fluorinated polyimide was also synthesized as a high performance dielectric material and was skillfully blade-coated on a flexible graphite paper to reduce the surface roughness. As a result, the as-fabricated flexible device exhibits extremely low hysteresis, wide operating temperature range...