Organic neutral π-monoradicals are promising semiconductors with balanced ambipolar carrier-transport abilities, which arise from virtually identical spatial distribution of their singly occupied and unoccupied molecular orbitals, SOMO(α) and SOMO(β), respectively. Herein, we disclose a boron-stabilized triphenylmethyl radical that shows outstanding thermal stability and resistance toward atmospheric conditions due to the substantial spin delocalization. The radical is used to fabricate organic Mott-insulator transistors that operate at room temperature, wherein the radical exhibits well-balanced ambipolar carrier transport properties.

PbS quantum dots (QDs) are remarkable semiconducting materials, which are compatible with low-cost solution-processed electronic device fabrication. Understanding the doping of these materials is one of the great research interests, as it is a necessary step to improve the device performance as well as to enhance the applicability of this system for diverse optoelectronic applications. Here, we report the efficient doping of the PbS QD films with the use of solution-processable organic molecules. By engineering the energy levels of the donor molecules and the PbS QDs through the use of different cross-linking ligands, we are able to control the characteristics of PbS field-effect transistors (FETs) from ambipolar to strongly n-type. Because the doping promotes trap filling, the charge carrier mobility is improved up to 0.64 cm2 V–1 s–1, which is the highest mobility reported for low-temperature processed PbS FETs employing SiO2 as the gate dielectric. The doping also reduces the contact resistance of the devices, which can also explain the origin of the increased mobility.Keywords: benzyl viologen; doping; field-effect transistors; ligands; quantum dots;

We perform a quantitative analysis of the trap density of states (trap DOS) in PbS quantum dot field-effect transistors (QD-FETs), which utilize several polymer gate insulators with a wide range of dielectric constants. With increasing gate dielectric constant, we observe increasing trap DOS close to the lowest unoccupied molecular orbital (LUMO) of the QDs. In addition, this increase is also consistently followed by broadening of the trap DOS. We rationalize that the increase and broadening of the spectral trap distribution originate from dipolar disorder as well as polaronic interactions, which are appearing at strong dielectric polarization. Interestingly, the increased polaron-induced traps do not show any negative effect on the charge carrier mobility in our QD devices at the highest applied gate voltage, giving the possibility to fabricate efficient low-voltage QD devices without suppressing carrier transport.Keywords: field-effect transistors; high-k; PbS quantum dots; polaron; trap states;

Recent progress in the development of organic semiconductor materials has improved the performance of both p- and n-type transistors. Currently, it is anticipated that the next step in the evolution of electronics will be to establish a reliable fabrication technique for integrated electronic devices such as plastic sensor films and radio-frequency identification (RFID) tags. Herein, a new fabrication process to grow line-shaped organic single-crystalline films with widths on the order of one mm is reported. To realize large-scale complementary logic circuits, it is necessary to precisely control the growth conditions of p-type and n-type semiconductors when painting on different areas on the same substrate. This method makes it possible to fabricate highly oriented organic thin films elongated over a few millimeters for both p- and n-type semiconductors next to one another. The p-type and n-type semiconductor crystals grown by this technique exhibit excellent average mobilities of 4.9 cm2 V–1 s–1 and 0.16 cm2 V–1 s–1 respectively. A prototypical RFID tag based on single crystals fabricated with the presented technique is also demonstrated. This tag was able to transfer 4-bit digital signals including the information from a temperature sensor through near-field wireless communication at the commercially usable frequency of 13.56 MHz.

Organic semiconductors are already widely used in electronics. Nevertheless, their fundamental properties are still being debated. In particular, charge transport, which determines the performance of organic light-emitting diodes, solar cells and organic field-effect transistors, has been described by a wide range of complementary but incompatible theories. These theories involve either localized charge carriers hopping from molecule to molecule, leading to incoherent charge transport, or delocalized charge carriers moving around freely in the semiconductor, leading to coherent transport. In this communication, we reveal the first experimental evidence that charge coherence can be tuned from partially to fully coherent in one and the same material—pentacene—showing a continuous transition from one transport mechanism to the other. Microscopically, the transport mechanism depends on the overlap of adjacent molecular orbitals, which in turn is sensitive to molecular thermal fluctuations. We control these fluctuations through moderate variations of pressure and temperature, leading to a mobility increase of 75%. We quantify the influence of these thermal fluctuations by estimating the critical value below which fully coherent charge transport emerges. The ability to control thermal fluctuations and therefore to effectively tune the charge coherence is an important key to improving charge transport in soft molecular materials.

•Organic temperature detectors that consist of flexible temperature sensors and read-out circuits are presented.•Analog signals from the temperature sensors are output as digital data.•Organic temperature sensors of the resistance-change type are proposed.•Organic analog-to-digital converters using solution-processed organic semiconductor films are presented.We present an organic temperature detector that consists of an organic temperature sensor and an organic complementary read-out circuit. The temperature sensor is a Wheatstone bridge composed of temperature-sensitive polymer films and metal films. The read-out circuit receives an analog temperature signal from the temperature sensor and outputs a 1-bit digital signal that reflects whether the temperature has exceeded a threshold temperature or not. For more detailed read-out, detection of the temperature range the temperature sensor is within also demonstrated using an organic 2-bit analog-to-digital converter as a read-out circuit.

We developed an effective and steady solution-processing technique for a small molecule–type semiconductor, C10–DNBDT–NW, by adding an amorphous PMMA polymer to produce stable growth of a two-dimensional large-area single-crystalline thin film by effective phase separation at a crucially faster processing speed compared to the case without the addition of a polymer. By using this solution-processing technique, it is noteworthy that the single-crystalline films of C10–DNBDT–NW/PMMA exhibit the highest and average mobilities of 17 and 10.6 cm2/Vs, respectively. Furthermore, we also show the limitations of two-dimensional continuous growth of a single-crystalline film in terms of the solution technique.

The structural properties of ionic liquid/rubrene single-crystal interfaces were investigated using frequency modulation atomic force microscopy. The spontaneous dissolution of rubrene molecules into the ionic liquid was triggered by surface defects such as rubrene oxide defects, and the dissolution rate strongly depended on the initial conditions of the rubrene surface. Dissolution of the second rubrene layer was slower due to the lower defect density, leading to the formation of a clean interface irrespective of the initial conditions. Molecular-resolution images were easily obtained at the interface, and their corrugation patterns changed with the applied force. Force curve measurements revealed that a few solvation layers of ionic liquid molecules formed at the interface, and the force needed to penetrate the solvation layers was an order of magnitude smaller than typical ionic liquid/inorganic solid interfaces. These specific properties are discussed with respect to electric double-layer transistors based on the ionic liquid/rubrene single-crystal interface.

•Organic single-crystal transistors are fabricated by an all-solution process.•The mobility reaches 6.9 cm2/V s at an operating voltage below 5 V with little hysteresis.•An organic single-crystal enables a high mobility and a small trap density at the surface of the organic layer.High-mobility organic single-crystal field-effect transistors of 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]-dithiophene (C10-DNBDT) operating at low driving voltage are fabricated by an all-solution process. A field-effect mobility as high as 6.9 cm2/V s is achieved at a driving voltage below 5 V, a voltage as low as in battery-operated devices, for example. A low density of trap states is realized at the surface of the solution-processed organic single-crystal films, so that the typical subthreshold swing is less than 0.4 V/decade even on a reasonably thick amorphous polymer gate dielectrics with reliable insulation. The high carrier mobility and low interface trap density at the surface of the C10-DNBDT crystals are both responsible for the development of the high-performance all-solution processed transistors.

•Electroless gold plating is used to form contacts on organic semiconductors.•Top-contact devices are fabricated with short channel lengths of 5 μm.•The contact resistance with the plated Au is as low as 1.4 kΩ cm on PEN substrates.The establishment of a reliable vacuum-free method for the formation of electrical contacts on high-performance organic semiconductors has become an urgent task due to rapid progress made in the development of solution-processable high-mobility organic field-effect transistors (OFETs). We have recently proposed that electroless plating, a standard technology to mass produce wirings in currently commercialized electronic devices, is suited for high-performance solution-crystallized OFETs. A low contact resistance at the source and drain electrodes is necessary with organic semiconductors for high-speed device operation; therefore, we have evaluated the contact resistance using the transfer line method. A top-contact geometry with sufficient contact area is employed to achieve stable carrier injection, which has enabled contact resistances as low as 1.4 kΩ cm on a polyethylene naphthalate substrate at a gate voltage of −10 V. This marks outstanding performance among the solution-processed metal electrodes reported for OFETs, particularly on plastic substrates. The result indicates that high-quality boundaries with minimized trap densities are realized due to the mild conditions of the electroless plating process at room temperature.

We report a facile synthetic protocol for preparation of dinaphtho[2,3-b:2′,3′-d]furan (DNF–V) derivatives. DNF–V derivatives showed high emissive behaviour in solid. A solution-crystallized transistor based on alkylated DNF–V derivatives showed an excellent carrier mobility of up to 1.3 cm2 V−1 s−1, thereby proving to be a new solution-processable active organic semiconductor with high emission and high mobility.

•Single-crystal OFETs are fabricated from solution onto patterned substrates.•Ultrathin gate insulators are employed in order to reduce operation voltage.•High mobility, 5.2 cm2/V s, is achieved at the low operation voltage of −2 V.Single-crystalline organic transistors of 3,11-didecyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW) and 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) were fabricated by solution processes on top of the patterned hybrid ultrathin gate dielectrics consisting of 3.6 nm-thick aluminum oxide and self-assembled monolayers (SAMs). Due to the excellent crystallinity of the channel films, bottom-gate and top-contact field-effect transistors exhibited the average field-effect mobility of 3.7 cm2/V s and 4.3 cm2/V s for C10-DNBDT-NW and C10-DNTT, respectively. These are the first successful devices of solution-processed single-crystalline transistors on ultrathin gate dielectrics with the mobility above 1 cm2/V s, opening the way to develop low-power-consumption and high-performance printed circuits.Graphical abstract

We report dinaphtho[1,2-b:2′,1′-d]chalcogenophenes as a new class of highly potential p-type semiconductors. As the result of comprehensive investigations including computational studies based on single-crystal structures successfully determined by X-ray analysis, the impacts of the chalcogen atoms are substantially manifested in their molecular orbitals, crystal packing structures, charge-transporting properties, and the device performances at the end. Among them, dinaphtho[1,2-b:2′,1′-d]thiophene and dinaphtho[1,2-b:2′,1′-d]selenophene achieved a hole mobility of up to 1.6 and 2.0 cm2 V–1 s–1, respectively, accompanied by an Ion/Ioff ratio as high as 104–105 in their single-crystal organic field-effect transistors. Such high performances are attributed to the large orbital coefficient on the chalcogen atoms and the ideal packing structures induced by chalcogen-bridged W-shaped molecules.Keywords: chalcogenophene; crystal engineering; organic field-effect transistors; organic single crystal;

Frequency-modulation atomic force microscopy (FM-AFM) was employed to reveal the structural properties of a rubrene single crystal immersed in an ionic liquid. We found large vacancies formed by the anisotropic dissolution of rubrene molecules. Molecular resolution imaging revealed that structures of FM-AFM images deviated from the bulk-terminated structure.

•Flexible solution-processed OFET arrays with vertical short channels were fabricated.•The OFETs are air-stable with output current-density over 0.3 A/cm2 under low voltage.•The new semiconducting polymer and structural design to achieve the high performance.Flexible air-stable short-channel polymer organic field-effect transistor (OFET) arrays with high saturated output current density are demonstrated by utilizing a novel solution-processed naphthobisthiadiazole (NTz) based donor–acceptor semiconducting polymer (PNTz4T) and designing a three-dimensional vertical channel structure with an extremely large ratio of channel width to channel length. The saturated mean field-effect mobility of 0.16 cm2/V s of the short-channel polymer devices remains over one month resulting in air-stable OFET arrays with high on/off ratio over 106 and powerful current–density exceeding 0.3 A/cm2 under low operation voltage, both of which meet the requirements for such applications as driving organic light-emitting diodes in active-matrix displays.

•Contact resistance (Rc) of high-performance organic field-effect transistors was systematically investigated.•Influences of electron acceptor layer, active-layer thickness, and alkyl side chain on Rc were studied.•The lowest Rc of 110 W·cm was attained and high mobility of 6 cm2/Vs was kept for polycrystalline OFETs at short channels.Realization of high-frequency low-cost organic electronics requires high-mobility organic field-effect transistors (OFETs) with short channels, where influence of contact resistance becomes more serious than either lower mobility or longer channel devices. To reduce the contact resistance, we systematically and quantitatively investigate the influence of the lowest unoccupied molecular orbital (LUMO) level of an electron acceptor layer, the active layer thickness, and the side chain of active layer itself on contact resistance of top-contact high-mobility OFETs through a series of comparative analysis. We find that the acceptor of 1,3,4,5,7,8-hexafluoro tetracyano naphtha quinodimethane (F6TNAP) with a deeper LUMO level is efficient for carrier injection and that the bulk resistance plays an important role in such devices. By optimizing the parameters, we get the lowest contact resistance of only 110 Ω cm, and thus recorded effective mobility of 8.0 cm2/V s is attained for polycrystalline thin film transistors and still kept as high as 6 cm2/V s at shorter channel lengths.Graphical abstract

We report a facile synthetic protocol for preparation of dinaphtho[2,3-b:2′,3′-d]furan (DNF–V) derivatives. DNF–V derivatives showed high emissive behaviour in solid. A solution-crystallized transistor based on alkylated DNF–V derivatives showed an excellent carrier mobility of up to 1.3 cm2 V−1 s−1, thereby proving to be a new solution-processable active organic semiconductor with high emission and high mobility.

Frequency-modulation atomic force microscopy (FM-AFM) was employed to reveal the structural properties of a rubrene single crystal immersed in an ionic liquid. We found large vacancies formed by the anisotropic dissolution of rubrene molecules. Molecular resolution imaging revealed that structures of FM-AFM images deviated from the bulk-terminated structure.

The structural properties of ionic liquid/rubrene single-crystal interfaces were investigated using frequency modulation atomic force microscopy. The spontaneous dissolution of rubrene molecules into the ionic liquid was triggered by surface defects such as rubrene oxide defects, and the dissolution rate strongly depended on the initial conditions of the rubrene surface. Dissolution of the second rubrene layer was slower due to the lower defect density, leading to the formation of a clean interface irrespective of the initial conditions. Molecular-resolution images were easily obtained at the interface, and their corrugation patterns changed with the applied force. Force curve measurements revealed that a few solvation layers of ionic liquid molecules formed at the interface, and the force needed to penetrate the solvation layers was an order of magnitude smaller than typical ionic liquid/inorganic solid interfaces. These specific properties are discussed with respect to electric double-layer transistors based on the ionic liquid/rubrene single-crystal interface.