Yugeng Wen

Recent Progress in n‐Channel Organic Thin‐Film Transistors

Particular attention has been focused on n-channel organic thin-film transistors (OTFTs) during the last few years, and the potentially cost-effective circuitry-based applications in flexible electronics, such as flexible radiofrequency identity tags, smart labels, and simple displays, will benefit from this fast development. This article reviews recent progress in performance and molecular design of n-channel semiconductors in the past five years, and limitations and practicable solutions for n-channel OTFTs are dealt with from the viewpoint of OTFT constitution and geometry, molecular design, and thin-film growth conditions. Strategy methodology is especially highlighted with an aim to investigate basic issues in this field.

Oxygen-Aided Synthesis of Polycrystalline Graphene on Silicon Dioxide Substrates

We report the metal-catalyst-free synthesis of high-quality polycrystalline graphene on dielectric substrates [silicon dioxide (SiO(2)) or quartz] using an oxygen-aided chemical vapor deposition (CVD) process. The growth was carried out using a CVD system at atmospheric pressure. After high-temperature activation of the growth substrates in air, high-quality polycrystalline graphene is subsequently grown on SiO(2) by utilizing the oxygen-based nucleation sites. The growth mechanism is analogous to that of growth for single-walled carbon nanotubes. Graphene-modified SiO(2) substrates can be directly used in transparent conducting films and field-effect devices. The carrier mobilities are about 531 cm(2) V(-1) s(-1) in air and 472 cm(2) V(-1) s(-1) in N(2), which are close to that of metal-catalyzed polycrystalline graphene. The method avoids the need for either a metal catalyst or a complicated and skilled postgrowth transfer process and is compatible with current silicon processing techniques.

Bulky 4-tritylphenylethynyl substituted boradiazaindacene: pure red emission, relatively large Stokes shift and inhibition of self-quenching

Bulky 4-tritylphenylethynyl substituted boradiazaindacene with pure red emission, relatively large Stokes shift, high fluorescence quantum yield, and low self-quenching was efficiently synthesized and qualified as a potential EL dopant.

Two‐Stage Metal‐Catalyst‐Free Growth of High‐Quality Polycrystalline Graphene Films on Silicon Nitride Substrates

By using two-stage, metal-catalyst-free chemical vapor deposition (CVD), it is demonstrated that high-quality polycrystalline graphene films can directly grow on silicon nitride substrates. The carrier mobility can reach about 1500 cm(2) V(-1) s(-1) , which is about three times the value of those grown on SiO(2) /Si substrates, and also is better than some examples of metal-catalyzed graphene, reflecting the good quality of the graphene lattice.

Large-area, flexible imaging arrays constructed by light-charge organic memories

Existing organic imaging circuits, which offer attractive benefits of light weight, low cost and flexibility, are exclusively based on phototransistor or photodiode arrays. One shortcoming of these photo-sensors is that the light signal should keep invariant throughout the whole pixel-addressing and reading process. As a feasible solution, we synthesized a new charge storage molecule and embedded it into a device, which we call light-charge organic memory (LCOM). In LCOM, the functionalities of photo-sensor and non-volatile memory are integrated. Thanks to the deliberate engineering of electronic structure and self-organization process at the interface, 92% of the stored charges, which are linearly controlled by the quantity of light, retain after 20000 s. The stored charges can also be non-destructively read and erased by a simple voltage program. These results pave the way to large-area, flexible imaging circuits and demonstrate a bright future of small molecular materials in non-volatile memory.

Field dependent and high light sensitive organic phototransistors based on linear asymmetric organic semiconductor

The authors reported organic phototransistors (PTs) with octadecyltrichlorosilane treated SiO2/Si substrate based on anthra[2,3-b]benzo[d]thiophene. The organic PTs show a high mobility of 0.4 cm2 V−1 s−1, a maximum photoresponsivity of about 1000 A/W, and photocurrent/dark-current ratio of around 800 under white light irradiation with 30 μW/cm2. The data are comparable with those of PTs based on amorphous silicon. Meanwhile, it is interesting to see that the devices show a high air-stable property and high photosensitivity via electric-field enhanced process. All these phenomena were attributed to the intrinsic optic-electronic property of the organic semiconductor and optic-electric field effect.

Morphology Optimization for the Fabrication of High Mobility Thin‐Film Transistors

low-cost, [ 4–6 ] large-area electrical devices [ 7 , 8 ] and low-voltage driving circuits of soft displays. [ 9–11 ] Currently, the low mobility of OTFTs is the main challenge for the industrial application of organic devices. [ 12–14 ] The fi ndings from a large number of researchers so far suggest that the charge carrier transport of OTFTs is extremely dependent on the semiconductor morphology, especially the fi rst several molecular layers near the dielectric surface. [ 15 , 16 ] Therefore, convenient techniques that could optimize the morphology of semiconductors next to the semiconductor/dielectric interface are crucial for the fabrication of high-mobility OTFTs, which can rival conventional inorganic TFTs. [ 17–19 ] From the literature [ 20 ] it is known that single-crystal pentacene transistors with high mobility values can be obtained using pentacenequinone, which represents the most abundant impurity in the crystal as the dielectric layer. However, this method is of limited interest for the fabrication of OTFTs. To date, the modifi cation of the dielectric surface has gained considerable industrial and academic interests because of their potential to enhance the performances of OTFTs. [ 21,22 ] Lately, Bao and coworkers have gained high-mobility OTFTs by modifying the dielectric surfaces with high quality self-assembled monolayers (SAMs). [ 23 , 24 ] The high thin-fi lm mobility of pentacene, up to 3 cm 2 /Vs, [ 24 ] is due to the morphology optimization induced by the ultra fl at SAMs layer that has been modifi ed on the SiO 2 dielectric. Analogously, Schwartz and coworkers have also gained high-mobility OTFTs by modifying the SiO 2 dielectric with organiphosphonate SAMs recently (the average mobility of a pentacene FET is about 2.5 cm 2 /Vs). [ 25 ]

Solvent‐Assisted Re‐annealing of Polymer Films for Solution‐Processable Organic Field‐Effect Transistors

Solution-processable organic semiconductors are promising candidates as active layers in large-area, low-cost electronic applications since these materials can be deposited by various solution-processing techniques, such as spin-coating, jetprinting, and screen printing etc. Recently, significant progress has been achieved in solution-processable organic field-effect transistors (OFETs). As a demonstration, the field-effect mobility of solution-processable OFETs is comparable to that of amorphous-silicon based devices. On the other hand, polymers have attracted increasing attention because of their significant advantages in terms of solution rheology and mechanical properties. It has been demonstrated by different groups that many excellent polymers including regioregular poly(3-hexylthiophene) (P3HT), poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), poly(3,30-didodecylquaterthiophene) (PQT-12), and cyclopentadithiophenebenzothiadiazole copolymer (CDT-BTZ) possess high field-effect mobility. Despite these achievements, the mobility of polymer FETs is still lower than those of smallmolecule semiconductor-based OFETs. In addition, the device stabilities remain to be improved to meet the requirement of actual application. Therefore, further improvement of device characteristics by molecular design as well as device engineering is desirable to boost further development of polymer FETs. For a typical OFET, ordered molecular packing of organic semiconductors usually results in high extracted mobilities. However, the realization of good molecular ordering of polymers is a big challenge since the molecular packing relies on the self-organization from solution. Consequently, great efforts have been devoted to the exploration of a novel technique to improve the molecular ordering of polymer layers and the device performances of the corresponding OFETs. For polymer semiconductors, the solvent is an important factor that dominates the performance of polymer devices. For example, both the boiling point and polarity of the solvent, which influence the molecular self-organization time and solution wettability of a dielectric layer, can affect the molecular ordering and film continuity, respectively, and dramatically influence the device performance. More interestingly, it has been demonstrated that solvent-vapor annealing is an effective way to improve the crystallization of solution-processable triethylsilylethynyl anthradithiophene (TIPS). By using the vapor-annealing treatment, the device performance of TIPS OFETs increases from 0.002 to 0.2 cmV 1 s . It proved that the existence of solvent can facilitate the ordered packing of organicmolecules and represents a novel approach to improve the device performance of solution-processed OFETs. Different from solvent vapor, the solvent liquid can serve as the carrier for functional materials including encapsulation materials and polymer dielectrics to form dissolved solutions. The functional solution can be utilized to construct multilayer devices like top-gate OFETs by solution-processing techniques. Unfortunately, it is widely believed that the direct introduction of common solvents to the organic layer usually destroys the film and decreases the device performance. Therefore, orthogonal solvents are generally applied during the deposition process of encapsulation layers or top dielectric layers. The selection of appropriate solvents or the design of particular functional materials, therefore, is required. In this Communication, we discovered that the introduction of a small amount of common solvent into the polymer active layer followed by a re-annealing process is an effective way to improve the device performance of polymer OFETs. The fabrication of different polymer OFETs proved that a 2–4 fold improvement of device performance can be achieved after the solvent-assisted re-annealing treatment. More interestingly, polymer OFETs with an encapsulation layer and top-gate OFETs are successfully constructed with the same mechanism. The solvent-assisted re-annealing treatment thus offers multifunctional applications in polymer OFETs. Scheme 1 shows the chemical structure of the polymers used in this study including P1, P2, and P3HT. P1 and P2 are copolymers that incorporate rigid dithieno[3,2-b:20,30-d]thiophene moieties and single thiophene units, which were synthesized according to the our previous report. All these polymers

Interfacial heterogeneity of surface energy in organic field-effect transistors.

Organic fi eld-effect transistors (OFETs) have attracted considerable interest because of their potential applications in many fi elds. [1–3] A large amount of research has shown that the charge carrier transport in OFETs is extremely dependent on the properties of the interface between the dielectric and semiconductor layers. [ 4 , 5 ] Thus, a clear understanding of the nature of the interfacial effects between the semiconductor and dielectric in OFETs is crucial for fabricating high performance OFETs. To date, although the interfacial effects between the dielectric and semiconductor have been widely studied, [6–10] unfortunately, the conclusions in the published results are often inconsistent or even contradict each other, and the experimental differences in the literature aggravates the confusion concerning the nature of interfacial effects. In previous literature, the interfacial effects have usually been investigated and described with a traditional ‘statistic-average’ method, which implicitly assumes either that the whole interface has a homogeneity of interfacial states, or that any heterogeneity in interfacial states has no impact on OFET performance. [ 6–10 ] However, it has been shown recently that the performance of an OFET is highly sensitive to the presence of heterogeneities in interfacial states, while little dependence on variations in the ‘statistical-average’ properties has been observed. [11–15] Thus, the neglect of interfacial heterogeneity is a possible source of the inconsistencies and contradictions between previously published results, and a detailed understanding of the interfacial effects induced by the heterogeneous nature of the interface between the dielectric and semiconductor is of great importance in terms of both fundamental science as well as practical applications. The surface energy of the dielectric is one of the interfacial factors that has been considered to have a large impact on the performance of OFETs and has been extensively studied. [ 16–18 ]

Top-Gate Organic Thin-Film Transistors Constructed by a General Lamination Approach

www.MaterialsViews.com C O M M U Top-Gate Organic Thin-Film Transistors Constructed by a General Lamination Approach N IC A T By Lei Zhang , Chong-an Di , * Yan Zhao , Yunlong Guo , Xiangnan Sun , Yugeng Wen , Weiyi Zhou , Xiaowei Zhan , * Gui Yu , and Yunqi Liu * IO N Solution-processed organic thin-fi lm transistors (OTFTs) have received increasing interest because they are thought to be promising candidates for low-cost electronics applications. [ 1 , 2 ] Organic materials offer the benefi t that they can be printed on fl exible polymeric substrates at low temperature by means of solutionbased techniques, which results in a dramatic reduction of manufacturing costs. However, very few organic semiconducting materials possess high environmental stability, hindering their practical applications. Bottom-gate OTFTs are a conventional device structure for material-testing purpose. Unfortunately, the oxygen and moisture can easily penetrate into the active layer and degrade the device performances for bottom-gate OTFTs because the organic semiconducting materials are exposed to air. [ 3–5 ] On the contrary, in top-gate OTFTs the active layer is self-encapsulated by the gate dielectric layer and the gate electrode and, therefore, the device stability is signifi cantly improved. [ 6 , 7 ] It has been recently demonstrated by different groups that top-gate OTFTs exhibit higher stability than bottom-gate devices. [ 8 ] As a result, fabrication of high-performance top-gate OTFTs is highly desirable and have received considerable attention. The dielectric/organic semiconductor interface, the most important one in OTFTs, plays a key role in the device performance, because the conductive channel is located on the few molecular layers near the gate dielectric layer. [ 9–11 ] Consequently, deposition of the dielectric layer on the organic semiconductor is a key step and a challenging task in the fabrication of top-gate OTFTs, especially solution processed devices. So far, the use of orthogonal solvents is the most widely used approach for the fabrication of solution-processed top-gate OTFTs. [ 12–14 ] Nonetheless, orthogonal solvents have to be selected carefully to minimize any adverse effects during the deposition of the organic semiconducting layer and the dielectric layer in two successive steps. Despite the signifi cant progress made over the past few years with the use of this approach, some problems remain. Firstly, although it is possible to ensure that the solvent used for the deposition of the insulating layer does not dissolve the