Solution-processed Organic Field-effect Transistors Research Articles

Boosting the performance of polymer field-effect transistors through fluorine atom substitutions and comparing the effect of various acceptor units and channel mobilities

Three conjugated copolymers, PDPP-2FTVT, PIID-2FTVT, and PNDI-2FTVT based on the 2,2′-(1E)-1,2-ethenediylbis[3-fluorothiophene] (2FTVT) core with varying acceptor units, diketopyrrolopyrrole (DPP), isoindigo (IID), and naphthalenediimide (NDI) were developed for organic field-effect transistors (OFETs). Combined electron-withdrawing fluorine atoms and vinyl linkage in the 2FTVT unit not only reduce the frontier molecular orbital energies but also extended the absorption profile and planarize the polymer backbone, manifesting synergetic effect on the strengthened molecular interactions and charge-carrier properties. The resulted copolymer, PDPP-2FTVT show highly reliable thin film transistor properties with highest hole mobility of 1.93 cm2 V−1 s−1 compared to PIID-2FTV and PNDI-2FTVT. PIID-2FTV copolymer show ambipolar characteristic with hole and electron mobilities of 0.71 and 0.03 cm2 V−1 s−1 respectively while PNDI-2FTVT exhibited the highest electron mobility of 0.20 cm2 V−1 s−1 among three copolymers. Thin film micro-structure characterization reveal that the three copolymers formed lamellar, edge-on molecular packing (for PDPP-2FTVT and PIID-2FTV), increased crystallinity with smaller π-π stacking distances, and strengthen the planarity after thermal annealing. These results contribute important progresses in solution-processed OFET and could provide a greater possibility of 2FTVT based copolymers for commercial application.

Acetylene-extended triarylamines for solution-processable p-channel OFETs

New acetylene-extended molecules were developed as a semiconducting layer for application in solution-processable organic field-effect transistors through coupling reactions using substituted triphenylamines as wings and diacetylene as spacer units. The influence of donor ability and spacer strength on the electrical properties was explored by developing different acetylene-extended triarylamines with D–π and D–π–π–D architectures and their structure–properties analysis. Their optical studies revealed narrow Stoke's shift and optical bandgap values, especially for highly electron-rich systems. Encouraging results supported by their electrochemical studies that revealed high HOMO energy levels ranging from –5.1 to –5.3 eV suggested a low energy barrier and good hole-transporting ability, hence, their applicability as p-channel OFETs. Furthermore, the extended conjugation in D–π–π–D based molecules relative to their corresponding D–π system caused enhanced molecular orbital overlap along the triphenylamine and facilitated good charge transport characteristics. Upon fabrication as the semiconducting channel in top-gate/bottom-contact OFET devices, the acetylene-extended D–π–π–D system showed good p-channel properties with hole mobilities up to 1.2 cm2/Vs and the highest on/off ratio of 107.

Ambient stable solution-processed organic field effect transistors from electron deficient planar aromatics: effect of end-groups on ambient stability.

Ambient stable solution processed n-channel organic field effect transistors (OFETs) are essential for next-generation low-cost organic electronic devices. Several molecular features, such as suitable orbital energy levels, easy synthetic steps, etc., must be considered while designing efficient active layer materials. Here, we report a case of improved ambient stability of solution-processed n-type OFETs upon suitable end-groups substitution of the active layer materials. A pair of core-substituted napthalenediimide (NDIFCN2 and EHNDICN2) derivatives with alkyl and perfluorinated end groups are considered. The transistor devices made out of these two derivatives exhibited largely different ambient stability behavior. The superior device stability (more than 25 days under ambient conditions) of one of the derivatives (NDIFCN2) was ascribed to the presence of fluorinated end groups that function as hydrophobic guard units inhibiting moisture infiltration into the active layer, thereby achieving ambient stability under humid conditions (>65% relative atmospheric humidity). Molecular level optical and electrochemical properties, thermal stability, and the solution-processed (spin coat and drop cast active layers) device characteristics are described in detail. Our findings highlight the requirement of hydrophobic end groups or sidechains for ambient stability of active layer materials, along with deep LUMO levels for ambient stability.

Towards sustainable, solution-processed organic field-effect transistors using cashew gum as the gate dielectric

To realize low-cost, environmentally friendly electronic devices and circuits, there is currently a strong trend to explore plant-based dielectric materials because they can be responsibly sourced from agricultural or forest vegetation, are generally water-soluble, and possess good electrical insulator properties. In this contribution, organic field-effect transistors (OFETs) using a biopolymer dielectric obtained from exudates of Anacardium occidentale Linn. trees, namely, cashew gum (CG), are reported. To characterise the physical and dielectric properties of the gum, thin films and metal-insulator-metal (MIM) capacitors were prepared and characterized. To evaluate the material’s performance in OFETs, bottom-gate top-contact (BGTC) p-channel poly [3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo (3,4-c)pyrrole-1,4-dione) thieno (3,2-b) thiophene]:polymethyl methacrylate (DPPTTT:PMMA) transistors were engineered and studied. The fabricated MIM capacitors display a comparatively high areal capacitance of 260 nF/cm2 at 1 kHz for 130 nm thick films. As a result, the solution-processed DPPTTT:PMMA OFETs favourably operate at 3 V with the average saturation field-effect mobility equal to 0.20 cm2/Vs., threshold voltage around −1.4 V, subthreshold swing in the region of 250 mV/dec, and ON/OFF current ratio well above 103. As such, cashew gum emerges as a promising dielectric for sustainable manufacturing of solution-processed organic FETs.

Electron-transporting boron-doped polycyclic aromatic hydrocarbons: Facile synthesis and heteroatom doping positions-modulated optoelectronic properties

While heteroatom doping serves as a powerful strategy for devising novel polycyclic aromatic hydrocarbons (PAHs), the further fine-tuning of optoelectronic properties via the precisely altering of doping patterns remains a challenge. Herein, by changing the doping positions of heteroatoms in a diindenopyrene skeleton, we report two isomeric boron, sulfur-embedded PAHs, named Anti-B2S2 and Syn-B2S2, as electron transporting semiconductors. Detailed structure-property relationship studies revealed that the varied heteroatom positions not only change their physicochemical properties, but also largely affect their solid-state packing modes and Lewis base-triggered photophysical responses. With their low-lying frontier molecular orbital levels, n-type characteristics with electron mobilities up to 1.5 × 10−3 cm2 V−1 s−1 were achieved in solution-processed organic field-effect transistors. Our work revealed the critical role of controlling heteroatom doping patterns for designing advanced PAHs.

An integrate-and-fire neuron circuit made from printed organic field-effect transistors

Neuromorphic circuits that mimic the operation of nerve cells are promising approaches to interface with or emulate the operation of biological neural network. These types of electronic circuits have generally been realised by fabrication using well-established silicon technology but herein we report the fabrication of a simple neuromorphic circuit, specifically, an integrate-and-fire (I&F) circuit, using solution-processed OFETs. It consists of p-type organic field-effect transistors (OFETs) realised with parylene-dielectric, printed silver electrodes and a solution processed semiconductor on a flexible plastic substrate. As the behaviour of the fabricated I&F circuit is predictable by simulation, the reported process allows for customised circuit design and provides the basis to realise a variety of flexible neuromorphic circuit designs. • Neuromorphic circuits can mimic the electrical spiking of nerve cells. • A simple neuromorphic circuit made using printed organic field-effect transistors. • Performance of a printed integrate and fire circuit can be modelled by simulation.

Influence of the cooling rate in annealing post-treatment on small molecule with siloxane side chains for solution-processed organic field-effect transistors

In this study, a donor-acceptor-type small molecule possessing a narrow bandgap with siloxane-terminated side chains, namely, S-LBG, was synthesized and characterized to be used as a semiconducting layer in organic field-effect transistors (OFETs). The diketopyrrolopyrrole (DPP)-based small molecule contains silaindacenodithiophene as an electron-donor core and 2-(1,1-dicyanomethylene)rhodanine as an electron-accepting end group. S-LBG films show highly crystalline structures after thermal annealing treatment owing to the strong intermolecular interactions between its siloxane side chains; furthermore, a smooth-surface film with a large polycrystalline grain size can be fabricated by controlling the cooling rate after annealing. Solution-processed OFETs using cooled S-LBG films at a rate of 1 °C/min and exhibits a high hole mobility of 0.102 cm2 V−1 s−1 with a π–π stacking distance of 0.659 nm.

Engineering Semiconductor Layer Using Halonaphthalene Additives for Organic Opto-Inverter Circuits

Solution-processable organic field-effect transistors (OFETs) are at the forefront of the future of electronic circuits. They have the potential for low-cost, large-scale fabrication on a variety of substrates. However, their application in electronic circuits is challenged by the unforeseen presence of defects leading to lower mobility. In this contribution, the application of a photoactive polymer-based phototransistor in a digital electronic circuit is demonstrated. Initially, a solvent additive engineering strategy is adopted to tune the thin film morphology and reduce morphology-related defects, resulting in improved device performance. The incorporation of 1-bromonaphthalene improves mobility from 0.09 to 0.50 cm2 V–1 s–1 with enhancement in both photoabsorption and photostability. The results are well supported by electrical characterization and photophysical and morphological studies. Thereafter, a unique architecture of an opto-inverter is presented using the OFET. In this work, an optoelectronic logic NOT gate is also fabricated by utilizing a simple resistive load circuit. This circuit is also demonstrated to perform the combined functionality of a logic gate and a transducer. Furthermore, this technique can be easily implemented for minimizing the circuit components and complexity by replacing a photodetector and a NOT gate with a single opto-inverter in applications requiring both.

Effects of solution-shearing process parameters on charge carrier mobility in green solvent-processed organic field-effect transistors

In this work, we explored the effect of solution-shearing processing parameters, coating speed and substrate temperature, on the electrical properties of solution-processed organic field-effect transistors (OFETs) based on 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) when green solvents are used. Organic semiconductor thin films were solution-sheared with various coating speeds and substrate temperatures and film morphologies/microstructures were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. Our data demonstrate that crystal growth of Ph-BTBT-C10 molecule is highly affected by the two solution shearing processing parameters and, especially, the OFET devices fabricated with cyclohexanone as green solvent exhibited the charge carrier mobility as high as 5.1 cm2/Vs with optimized processing condition. This work could further provide a guideline for process optimization for fabricating high-performance green solvent-processed OFETs.

High Mobility and ON/OFF Ratio of Solution-Processable p-Channel OFETs from Arylacetylene End-Capped Alkoxyphenanthrenes.

New arylacetylene end-capped alkoxyphenanthrenes were synthesized and demonstrated as the best active layer for solution-processable p-channel organic field-effect transistors. The alkoxy chain embedded compounds exhibited enhanced solubility and induced non-covalent interactions resulting in effective molecular packing. The 'Lewis soft' heteroatoms direct the most stable conformation with dihedral angles possible for molecular interactions, and energy levels. DFT studies supported the fine-tuning of FMOs, with high HOMO levels ∼-5.2 eV ensuring a low barrier for charge injection. OFET devices exhibited a maximum charge carrier mobility up to 1.30 cm2 /Vs with the highest ON/OFF ratio of 107 . The strong π-π interactions and the crystallinity of the films are well supported by GIXRD and SEM analysis.

Chlorine‐Substituted N‐Heteroacene Analogues Acting as Organic Semiconductors for Solution‐Processed n‐type Organic Field‐Effect Transistors

High performance solution processable n-type organic semiconductor is an essential element to realize low-cost, all organic and flexible composite logic circuits. In the design of n-type semiconducting materials, tuning the LUMO level of compounds is a key point. As a strong electron withdrawing unit, the introduction of chlorine atom into the chemical structure can increase the electron affinity of the material and reduce the LUMO energy level. Here, a series chlorine substituted N-heteroacene analogues of 6,7,8,9-tetrachloro-4,11-bis(4-((2-ethylhexyl)oxy)phenyl)-[1,2,5]thiadiazolo[3,4-b]phenazine (O4Cl), 6,7,8,9-tetrachloro-4,11-bis(4-((2-ethylhexyl)thio)phenyl)-[1,2,5]thiadiazolo[3,4-b]phenazine (S4Cl), 1,2,3,4,8,9,10,11-octachloro-6,13-bis(4-((2-ethylhexyl)oxy)phenyl)quinoxalino[2,3-b]phenazine (8Cl) and 12Cl have been synthesized and characterized. Solution-processed organic field-effect transistors (OFETs) based on these four compounds exhibit good electron mobilities of 0.04 cm2 V-1 s-1 , 0.01 cm2 V-1 s-1 , 2×10-3 cm2 V-1 s-1 and 3×10-3 cm2 V-1 s-1 , respectively, under ambient conditions. The results suggest that these chlorine substituted π-conjugated N-heteroacene analogues are promising n-type semiconductors in OFET applications.

Small molecules based on strongly electron-deficient aza-isatinylidene malononitrile for solution-processed n-type field-effect transistors

In this paper, three donor–acceptor (D/A) small molecules (M1, M2, and M3) based on aza-isatinylidene malononitrile (AIMN) and diketopyrrolopyrrole (DPP) containing different N-alkyl chains in AIMN unit were synthesized and characterized for application in solution-processable organic field-effect transistors (OFETs). The D/A small molecules with strongly electron-withdrawing AIMN and DPP units exhibited broad absorption spectra, low bandgaps, and deep lowest unoccupied molecular orbital energy levels (< – 4.0 eV). Consequently, OFET devices displayed unipolar electron transport characteristics with the highest electron mobilities of 0.26, 0.22, and 0.0079 cm2V–1s–1 for M1, M2, and M3, respectively. The small molecules with linear side chains showed much higher field-effect performances than that of branched side chains- based small molecule, due to the strong π–π stacking and appropriate morphology. This work indicated that the strongly electron-deficient AIMN should be a potential unit for constructing the n-type solution-processable small molecular semiconductors.

Sensitivity of gas sensors enhanced by functionalization of hexabenzoperylene in solution-processed monolayer organic field effect transistors.

Solution-processed monolayer films consisting of unfunctionalized and functionalized hexabenzoperylenes in a single homogeneous phase have enabled highly sensitive detection of NH3 and NO2 on the basis of organic field effect transistors.

Functionalized D/A–A–D quinolines for application in solution-processable p-channel organic field-effect transistors

New functionalized quinolines with a D/A–A–D architecture and pi-end-groups at terminals were designed and synthesized. OFETs fabricated from compounds with a D–A–D architecture exhibited p-channel transistor characteristics with high hole mobilities.

Charge transfer complexes of a benzothienobenzothiophene derivative and their implementation as active layer in solution-processed thin film organic field-effect transistors

Single crystals of novel (C8O-BTBT-OC8)(FxTCNQ) charge complexes (CT) are prepared and fully characterised. Solution processed films of the (C8O-BTBT-OC8)(F4TCNQ) CT are implemented in organic field-effect transistors giving an n-type behaviour.

Brewers’ spent grain (BSG)-based green dielectric materials for low-voltage operating solution-processed organic field-effect transistors

A brewers’ spent grain (BSG) based dielectric material was used in organic field-effect transistors (OFETs). The fabricated devices showed a maximum hole mobility of 2.30 cm2 V−1 s−1.

Small molecule semiconductors based on hemi-isoindigo and diketopyrrolopyrrole for solution-processed organic field-effect transistors

Two small semiconductor molecules based on diketopyrrolopyrrole (DPP) and hemi-isoindigo units with and without fluorination were designed and synthesized for investigating the solution-processed organic field-effect transistors (OFETs). Bottom-gate/top-contact (BG/TC) OFETs were fabricated to explore carrier transport characteristics. The combination of density functional theory (DFT) calculation, grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM) allowed us to understand the relationship between structures and properties. The results revealed the small molecule with fluorination (DPP-T-FMI) exhibited better field-effect performance owing to the improved crystal packing and morphology of thin-films.

Significant Performance Improvement in n-Channel Organic Field-Effect Transistors with C60 :C70 Co-Crystals Induced by Poly(2-ethyl-2-oxazoline) Nanodots.

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C60 -Ih )[5,6]fullerene (C60 ) and (C70 -D5h(6) )[5,6]fullerene (C70 ) blend films, with an increase from 0.1 to 2.1 cm2 V-1 s-1 . The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C60 :C70 co-crystal structure within the blend films.

Synergistic Effects of Self-Assembled Monolayers in Solution-Processed 6,13-Bis(triisopropylsilylethynyl)Pentacene Transistors.

Organic semiconductors are highly interface-sensitive, and therefore chemical functionalization using self-assembled monolayers (SAMs) is often adopted to tailor their properties. This study clarifies the synergistic effects of electrode and dielectric SAMs on the behavior of solution-processed organic field-effect transistors (OFETs). Utilization of a self-consistent device model enables a physically robust treatment of the measured electrical characteristics of the OFETs, thus providing highly reliable materials, interface, morphology, and transport parameters. These parameters are further extended and correlated to build a comprehensive picture on trap energy and injection-transport relationship, finally revealing a set of fundamental insights into chemically modified OFETs.

From the Mechanism to the Device in Polymer-Assisted Rubrene Crystallization

The rational design of organic crystalline materials is exceedingly challenging owing to the insufficient understanding of crystallization mechanisms. Here, we used a polymer (polystyrene) as a crystallization medium for an organic semiconductor (rubrene). This enabled a slow crystallization process whose mechanism was elucidated via direct transmission electron microscopy imaging and further employed to control crystallization and fabricate devices based on rubrene crystals. The elucidated mechanism involved (1) the initial formation of amorphous aggregates; (2) nucleation within the aggregates; (3) crystal growth; (4) break up into single nanocrystals; and (5) oriented attachment of nanocrystals to form platelet-like crystals. This mechanistic insight indicated that uniform nonclassical nucleation can be realized using a precise temperature control, leading to high-quality rubrene crystals. By employing this methodology, we fabricated solution-processed organic field-effect transistors (OFETs) and organic phototransistors (OPTs) that exhibited high mobility, reproducibility, and environmental stability. The devices showed an average mobility of 1 ± 0.8 cm2 V–1 s–1, a threshold voltage of −10 ± 6 V, and an on/off ratio of up to 106. Under white light irradiation, rubrene OPTs exhibited strong photoresponse with a photo/dark current ratio of P ≈ 105. Our work demonstrated that mechanistic information can be employed to fabricate high-quality OFETs, having implications for the rational design of crystalline organic electronic materials.