Top-contact Organic Field-effect Transistors Research Articles

Low Band Gap Furan‐Flanked Diketopyrrolopyrrole‐Naphthobisthiadiazole Based Conjugated Polymer/Stretchable Blend for Organic Field Effect Transistors

AbstractN‐type organic semiconducting materials that are compatible in stretchable organic field effect transistors (OFETs) still lag in performance behind that of p‐type materials. Herein, a n‐type conjugated polymer (DPPF‐NTz) is reported that comprises a furan flanked diketopyrrolopyrrole (DPPF) as a monomer and napthobisthiadiazole (NTz) as a comonomer units, respectively, in a conjugated polymer backbone. The low band gap of 1.34 eV and suitable frontier energy levels allow its utilization in OFETs as an n‐type semiconducting material. Optimized bottom‐gate top contact OFETs based on chloroform and chloroform: o‐dichlorobenzene processed DPPF‐NTz showed a maximum electron mobility (µe) of 0.00042 cm2 V⁻¹ s⁻¹ and 0.00078 cm2 V⁻¹ s⁻¹, respectively, in devices annealed at 150 °C. Interestingly, upon mixing the DPPF‐NTz with a stretchable polymer, polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS), yielded a stretchable semiconducting polymer composite, which displayed an enhanced µe of 0.0024 cm2 V⁻¹ s⁻¹ in devices annealed at 250 °C over pristine DPPF‐NTz. The improved µe and mechanical stretchability of the DPPF‐NTz: SEBS polymer blend over pristine DPPF‐NTz polymer is examined by nano‐mechanical atomic force microscopy. The research investigation finding provides a critical insight into the structural and nano‐mechanical properties of n‐type stretchable polymer semiconductors, which are essential for the development of next‐generation wearable OFETs.

Fabrication and Characterization of P3HT/MoS₂ Thin-Film Based Ammonia Sensor Operated at Room Temperature

The composites of polymers and two-dimensional (2D) materials show promising applications in the area of gas sensing. In this work, we report an efficient ammonia gas (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) sensor based on poly(3-hexylthiophene)/ molybdenum disulfide (P3HT/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanocomposite. The sensing device has been fabricated in bottom-gate top contact organic field-effect transistor (OFET) assembly using P3HT/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as active channel material. The changes in the electrical response of OFET has been measured and analyzed for various ammonia gas concentration at room temperature. The sensing device in the form of an OFET structure is preferred due to multi-parameter characteristics to explore gas sensing applications. The active sensing layer has been fabricated via a self-assembled, cost-effective floating film transfer (FTM) technique. An optimized uniform sensing film of thickness 25±3 nm is used to analyze the change in the electrical characteristic of the device in terms of I/O characteristics, mobility, threshold voltage, trapped charge density, etc., for various ammonia concentrations. The OFET with nanofiber morphology of mobility 0.147 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s shows the threshold voltage of −3.78 V (in the air) and changes to −10.71 V after 100 ppm ammonia gas exposure. The device has shown a limit of detection (LOD) of 904 ppb and a sensing response of 63.45% at 100 ppm ammonia concentration. The extraction of multi-parameter sensing characteristics of the device has been conducted in a closed chamber with an ambient environment. The fabricated sensor is therefore having potential applications in high-performance ammonia sensing applications.

Improved Self-Assembly of P3HT with Pyrene-Functionalized Methacrylates.

A block copolymer with discotic liquid crystalline behavior was synthesized using Grignard metathesis polymerization (GRIM) and initiators for continuous activator regeneration atom transfer radical polymerization (ICAR-ATRP). A novel discotic liquid crystalline mesogen, 6-(pyren-1-yloxy)hexyl methacrylate (PyMA), comprises a block that is attached to regioregular poly(3-hexylthiophene) (rr-P3HT) generated by GRIM and subjected to end-group modification. Due to the continuous regeneration of Cu+ in the reaction mixture in ICAR-ATRP compared to conventional methods, the synthesis was successfully performed with less catalyst. The purity and yield of the final product are increased by eliminating rigorous post-synthesis purification. Stacked pyrene units have contributed to the enhanced long-range π–π interactions and aligning of the P3HT block as observed in thin-film X-ray diffraction (XRD). Furthermore, field-effect mobilities in the order of 10–2 cm2 V–1 s–1 in bottom-gate, top-contact organic field-effect transistors (OFETs) suggest an enhancement in charge transport due to the discotic electron-rich pyrene units that help mitigate the insulating effect of the methacrylate backbone. The formation of uniform microdomains of P3HT-b-poly(PyMA) observed with tapping mode atomic force microscopy (TMAFM) on the channel regions of OFETs indicates the unique packing of the block copolymer in comparison to pristine P3HT. Thermotropic properties of the novel discotic mesogen in the presence and absence of P3HT were observed with both the poly(3-hexylthiophene)-b-poly(6-(pyren-1-yloxy)hexyl methacrylate) (P3HT-b-poly(PyMA)) block copolymer and poly(6-(pyren-1-yloxy)hexyl methacrylate) (poly(PyMA)) homopolymer using polarized optical microscopy (POM) and differential scanning calorimetry (DSC).

Heptacene: Synthesis and Its Hole-Transfer Property in Stable Thin Films.

Heptacene (1) has been produced via a monoketone precursor, 2, which was prepared from 1,2,4,5-tetrabromobenzene in nine steps in a total yield of 10 %. Compound 2 was converted to 1 quantitatively by heating at 202 °C. Heptacene exhibited high thermal stability in the solid state without any observable change over two months. To investigate the potential value of 1 as a material for p-type organic field-effect transistors (OFETs), top-contact OFET devices were fabricated by vacuum deposition of 1 onto a hexamethyldisilazane (HMDS)/SiO2 /Si substrate. The best hole mobility performance was 2.2 cm2 V-1 s-1 . This is the first report of stable heptacene being used in an effective device and examined for its charge carrier properties.

Two-step direct heteroarylation reaction towards 2,5-bis(alkoxy)benzene polymer for organic field-effect transistors processed with tetrahydrofuran

A 2,5-bis(alkoxy)benzene polymer POB-TNT has been newly designed and synthesized via very convenient two-step direct heteroarylation reaction. Its physicochemical properties have been investigated. POB-TNT demonstrates typical [Formula: see text]-type carrier transport characteristic in unencapsulated bottom-gate and top-contact organic field-effect transistors (OFETs) processed with relatively green tetrahydrofuran solvent. The results highlight a readily accessible and low-cost-oriented polymer semiconductor for non-halogenated solvent-processed OFET application.

Picene and PTCDI based solution processable ambipolar OFETs

Facile and efficient solution-processed bottom gate top contact organic field-effect transistor was fabricated by employing the active layer of picene (donor, D) and N,N′-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (acceptor, A). Balanced hole (0.12 cm2/Vs) and electron (0.10 cm2/Vs) mobility with Ion/off of 104 ratio were obtained for 1:1 ratio of D/A blend. On increasing the ratio of either D or A, the charge carrier mobility and Ion/off ratio improved than that of the pristine molecules. Maximum hole (µmax,h) and electron mobilities (µmax,e) were achieved up to 0.44 cm2/Vs for 3:1 and 0.25 cm2/Vs for 1:3, (D/A) respectively. This improvement is due to the donor phase function as the trap center for minority holes and decreased trap density of the dielectric layer, and vice versa. High ionization potential (− 5.71 eV) of 3:1 and lower electron affinity of (− 3.09 eV) of 1:3 supports the fine tuning of frontier molecular orbitals in the blend. The additional peak formed for the blends at high negative potential of − 1.3 V in cyclic voltammetry supports the molecular level electronic interactions of D and A. Thermal studies supported the high thermal stability of D/A blends and SEM analysis of thin films indicated their efficient molecular packing. Quasi-π–π stacking owing to the large π conjugated plane and the crystallinity of the films are well proved by GIXRD. DFT calculations also supported the electronic distribution of the molecules. The electron density of states (DOS) of pristine D and A molecules specifies the non-negligible interaction coupling among the molecules. This D/A pair has unlimited prospective for plentiful electronic applications in non-volatile memory devices, inverters and logic circuits.

High performance n-type organic field-effect transistors based on halogenated derivatives of naphthalene tetracarboxylic diimides

We synthesized three kinds of naphthalene tetracarboxylic diimides (NDIs) containing halogenated phenyl groups by one-step reaction, and they were used as organic semiconductor layers in top-contact Organic field-effect transistors (OFETs). Besides, we adopted SiO2/Si substrates that treated by N-octadecylphosphonic acid (ODPA). In addition, a layer of 4, 4′, 4″-triphenylamine (n-3-methylphenyl-n-phenylamine) triphenylamine (m-MTDATA) was deposited by vacuum between the organic semiconductor layer and the gold electrode as the modification material. It was found through experiments that all the organic materials synthesized by us showed n-type organic semiconductor characteristics. Happily, the device made of NDI-FAN attained the highest electron mobility of 2.2 × 10−1 cm2v−1s−1 at room temperature and in air. The crystallinity and morphology of the films were characterized by x-ray diffraction (XRD) and atomic force microscope (AFM). The results expose that introducing electron withdrawing substituents into naphthalene tetracarboxylic diimides besides inserting ODPA and m-MTDATA are effective ways to achieve high electron mobility.

Aggregation Behavior and High Charge-Carrier OFET-Mobility of Functionalized Phenanthro[9,10-d]imidazoles

We designed and synthesized phenanthro[9,10-d]imidazoles (PIs) in a D–A fashion to achieve a balance of charge carriers. Solution processable bottom gate–top contact organic field-effect transistor...

Comparison of semiconductor growth and charge transport on hydrophobic polymer dielectrics of organic field-effect transistors: Cytop vs. polystyrene

Abstract Organic field-effect transistors (OFETs) have been fabricated by deposition of an organic semiconductor on inorganic dielectric layers like SiO2. However, inorganic oxide dielectrics have hydrophilic surface properties, which interrupts the growth of organic semiconductors with highly aligned and π-extended domains, and provide many absorbing sites for moisture that work as trap sites for charge carriers. To overcome this problem, a proper surface-treatment method is required to improve the morphology of organic semiconductors. In this study, we treat SiO2 dielectrics with polystyrene (PS) and Cytop (a fluorinated cross-linkable polymer) to tune the surface properties when fabricating pentacene-based bottom-gate top-contact OFETs, and analyzed the influence of surface functionalization on the growth of the pentacene crystals under different deposition conditions and electrical device characteristics. By comparing the morphological features of pentacene with the device characteristics, the effect of hydrophobicity on the morphology of the pentacene polymorph, strongly related to the charge-transport mechanism, could be described.

Augmenting n-Type Performance of Ambipolar Top-Contact Organic Thin-Film Transistors by Self-Generated Interlayers

We report n-type performance enhancement of an ambipolar donor–acceptor (D–A) polymer by suppressing its p-type behavior using a simple and versatile methodology of interlayer processing in top-contact organic field-effect transistors. In this approach, a judiciously selected interlayer material, polyethylene glycol (PEG), is codeposited as an additive with the semiconducting polymer DPP-T-TT active layer. During top-contact aluminum deposition, the PEG molecules migrate from within the bulk film to the organic/Al interface because of additive–metal interactions. The formation of the PEG interlayer was confirmed by X-ray photoelectron spectroscopy and its effect on the polymeric-interfacial microstructure was studied using angle-dependent grazing-incidence wide-angle X-ray scattering. We find that the PEG interlayer has two significant contributions to the n-type characteristics of the device: it suppresses hole injection while improving electron injection by tuning the effective work-function, and it enh...

An origin of the irreproducibility of hole injection barrier from Au top-contact electrodes and its influence on device performance in top-contact organic field-effect transistors

Abstract To understand the origin of the irreproducibility of the hole injection barrier at the interface between Au top-contact electrodes and organic semiconductors, we performed a systematic study by focusing on the deposition rate of Au. As a benchmarking material system, we fabricated field-effect transistors with pentacene active layers and Au top-contact electrodes evaporated with the deposition rate from 0.5 to 10 A/s. From the contact potential differences (ΔCPDs) between Au and pentacene measured with Kelvin-probe force microscopy, cross-sectional structures with scanning transmission electron microscopy, crystallographic structures with X-ray diffraction, and Au depth profiles with secondary ion mass spectrometry, we concluded that the variation of ΔCPD is due to embedded Au nanoclusters in pentacene with the average diameter of 4–6 nm. These Au nanoclusters, of which depth significantly varies by the Au deposition rate, modulate the ΔCPD and interfere with the reproducibility of hole injection barrier. As a result, gate threshold voltage and field-effect mobility also become irreproducible.

Nature of vacuum-deposited electrode induced thermal irradiation damage on organic transistors

At present, the direct experimental observation and confirmation of thermal damage on semiconductors remain unexplored. This report presents clear evidence of the thermal irradiation damage on the electrode contact region of an organic semiconductor in the electrode deposition process by applying an ultralong uniform rubrene single crystal to simultaneously fabricate vacuum-deposited electrode and stamped electrode top-contact organic field-effect transistors (OFETs). The surface potential measurements combined with the transfer characteristics show that the thermal irradiation damage causes the HOMO level of rubrene shift downwards by 0.17 eV, resulting in the enhanced contact resistance and hence the lowered field-effect mobility. This interesting experimental discovery clarifies the nature of vacuum-deposited electrode induced thermal irradiation damage on OFETs, will enhance the understanding and development of contact engineering to further improve the performance of organic electronics.

Direct Photolithography on Molecular Crystals for High Performance Organic Optoelectronic Devices.

Organic crystals are generated via the bottom-up self-assembly of molecular building blocks which are held together through weak noncovalent interactions. Although they revealed extraordinary charge transport characteristics, their labile nature represents a major drawback toward their integration in optoelectronic devices when the use of sophisticated patterning techniques is required. Here we have devised a radically new method to enable the use of photolithography directly on molecular crystals, with a spatial resolution below 300 nm, thereby allowing the precise wiring up of multiple crystals on demand. Two archetypal organic crystals, i.e., p-type 2,7-diphenyl[1]benzothieno[3,2- b][1]benzothiophene (Dph-BTBT) nanoflakes and n-type N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires, have been exploited as active materials to realize high-performance top-contact organic field-effect transistors (OFETs), inverter and p-n heterojunction photovoltaic devices supported on plastic substrate. The compatibility of our direct photolithography technique with organic molecular crystals is key for exploiting the full potential of organic electronics for sophisticated large-area devices and logic circuitries, thus paving the way toward novel applications in plastic (opto)electronics.

O-Carborane, Ferrocene, and Phthalocyanine Triad for High-Mobility Organic Field-Effect Transistors.

An unsymmetrical zinc phthalocyanine with ferrocenylcarborane linked to the phthalocyanine ring through a phenylethynyl spacer was designed for organic field-effect transistor (OFET). The unsymmetrical phthalocyanine derivatives were characterized using a wide range of spectroscopic and electrochemical methods. In particular, the ferrocenylcarborane structure was unambiguously revealed based on the single-crystal X-ray diffraction analysis. In-depth investigations of the electrochemical properties demonstrated that the ferrocenylcarborane insertion extended the electrochemical character of ferrocenylcarborane-substituted phthalocyanine (7). Moreover, in the anodic potential scans, the oxidative electropolymerization of etynylphthalocyanine (6) and 7 was recorded. To clarify the effect of the insertion of ferrocenylcarborane (2) on the field-effect mobility, solution-processed films of 2, 6, and 7 were used as an active layer to fabricate the bottom-gate top-contact OFET devices. An analysis of the output and transfer characteristics of the fabricated devices indicated that the phthalocyanine derivative functionalized with ferrocenylcarborane moiety has great potential in the production of high-mobility OFET.

Incorporation of 2,6-Connected Azulene Units into the Backbone of Conjugated Polymers: Towards High-Performance Organic Optoelectronic Materials.

Azulene is a promising candidate for constructing optoelectronic materials. An effective strategy is presented to obtain high-performance conjugated polymers by incorporating 2,6-connected azulene units into the polymeric backbone, and two conjugated copolymers P(TBAzDI-TPD) and P(TBAzDI-TFB) were designed and synthesized based on this strategy. They are the first two examples for 2,6-connected azulene-based conjugated polymers and exhibit unipolar n-type transistor performance with an electron mobility of up to 0.42 cm2 V-1 s-1 , which is among the highest values for n-type polymeric semiconductors in bottom-gate top-contact organic field-effect transistors. Preliminary all-polymer solar cell devices with P(TBAzDI-TPD) as the electron acceptor and PTB7-Th as the electron donor display a power conversion efficiency of 1.82 %.

Influence of phase-separated morphology on small molecule/polymer blend organic field-effect transistors fabricated using electrostatic spray deposition

Bottom-gate top-contact organic field-effect transistors (OFETs) based on 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and poly(methyl methacrylate) (PMMA) blends were fabricated using electrostatic spray deposition (ESD). Solvent-vapor annealing (SVA) was conducted as a post-deposition treatment to induce vertical phase separation of the blend films. The phase-separated morphology was investigated by selective etching of TIPS pentacene with cyclohexane. A relatively rough surface with some pinholes was observed in the etched film without SVA, suggesting that vertical phase separation was not sufficient during the ESD process. The phase-separated morphology was modified remarkably by SVA, and a SVA-treated blend device with a well-defined phase-separated structure exhibited enhanced performance and operational stability compared to a device based on the as-deposited blend.

1,2,5,6-Naphthalenediimide-based conjugated copolymers linked by ethynyl units

Two copolymers of P1 and P2 comprising benzothiadiazole, 1,4-bis(dodecyloxy)-benzene units were synthesized by Sonogashira coupling polymerization based on ethynyl-linked 1,2,5,6-naphthalenediimide. Their thermal, optical, electrochemical as well as charge transport properties were studied. Bottom-gate top-contact organic field-effect transistors (OFETs) measurements of P1 and P2 thin films showed different charge transport behaviors. P1 displayed pure electron transport behaviors in OFETs with electron mobility up to 10−3 cm2·V−1·s−1, while P2 exhibited hole transport features. The molecular structure analysis revealed that the structure of P1 has the acceptor-linker-acceptor′ (A-L-A′) characteristic, and P2 possesses the donor-linker-acceptor (D-L-A) structure feature. The results demonstrate that different molecular structures lead them to have distinct charge transport behaviors. In particular, the first pure electron transport copolymer in OFETs based on 1,2,5,6-naphthalenediimide is achieved.

Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field-Effect Transistors.

The charge-carrier mobility of organic semiconducting polymers is known to be enhanced when the energetic disorder of the polymer is minimized. Fused, planar aromatic ring structures contribute to reducing the polymer conformational disorder, as demonstrated by polymers containing the indacenodithiophene (IDT) repeat unit, which have both a low Urbach energy and a high mobility in thin-film-transistor (TFT) devices. Expanding on this design motif, copolymers containing the dithiopheneindenofluorene repeat unit are synthesized, which extends the fused aromatic structure with two additional phenyl rings, further rigidifying the polymer backbone. A range of copolymers are prepared and their electrical properties and thin-film morphology evaluated, with the co-benzothiadiazole polymer having a twofold increase in hole mobility when compared to the IDT analog, reaching values of almost 3 cm2 V-1 s-1 in bottom-gate top-contact organic field-effect transistors.

High performance n-channel organic field-effect transistors based on halogenated derivatives of 1,2,4,5-benzenetetracarboxylic diimides

Three kind of 1,2,4,5-benzenetetracarboxylic diimides (BTDs) with halogenated phenyl groups were synthesized through one-step reaction with high yields. Top-contact organic field-effect transistors (OFETs) were fabricated via vacuum deposition of BTDs as the semiconducting channel materials on N-octadecylphosphonic acid (ODPA) treated SiO2/Si substrates. The electronic characterization was measured in ambient condition. All these derivatives exhibit excellent n-channel OFET transport with the highest mobility up to 9.2×10−2cm2v−1s−1 for BTD-ClAN deposited at room temperature. The crystallinity and morphology of film were improved by modified the substrate and demonstrated by X-ray diffraction (XRD) and atomic force microscopy (AFM). Our results indicates that introducing electron-withdrawing substituents into 1,2,4,5-benzenetetracarboxylic diimides is an effective way to achieve high OFETs performance.

Vertical phase separation of 6,13-bis(triisopropylsilylethynyl) pentacene/poly(methyl methacrylate) blends prepared by electrostatic spray deposition for organic field-effect transistors

Blend films composed of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and poly(methyl methacrylate) (PMMA) were prepared by electrostatic spray deposition (ESD). ESD is considered as an intermediate process between dry and wet processes since the solvent present in small droplets can almost be evaporated before arriving at the substrate. Post-drying treatments with the time-consuming evaporation of residual solvents can be omitted. However, it is still not clear that a vertically phase-separated structure can be formed in the ESD process since the vertical phase separation of the blend films is associated with the solvent evaporation. In this study, we fabricated bottom-gate, top-contact organic field-effect transistors based on the blend films prepared by ESD and the devices exhibited transistor behavior with small hysteresis. This result demonstrates that the vertical phase separation of a blend film (upper TIPS pentacene active layer/bottom PMMA gate insulator) can occur in the facile one-step ESD process.