Multicolor nanofiber based organic light-emitting transistors

Self-assembled, molecular crystalline nanofibers form microscale light-emitters for future nanophotonic applications. Such organic nanofibers exhibit many interesting optoelectronic properties, including polarized photo- and electroluminescence, waveguiding, and emission color tunability. Surface-grown nanofibers from two different molecules are implemented in an organic field-effect transistor platform by a double roll-printing scheme. Roll-printing multiple types of nanofibers onto the same device provides a fast and simple alternative to multilayer devices. The combination of nanofibers made from para-hexaphenylene and 5,5′-di-4-biphenyl-2,2′-bithiophene results in a nanofiber based organic light-emitting transistor (OLET) which emits both blue and green light. A comparison of measured electrical transport and electroluminescence (EL) properties results in a correlation between the threshold voltage for transport and the onset voltage for EL emission.     

Ambipolar organic transistors based on isoindigo derivatives

Structural and transistor properties of isoindigo derivatives are investigated. The unsubstituted isoindigo affords two polymorphs in addition to the reported brickwork structure; one has a stacking structure analogous to indigo, and another consists of nonplanar molecules. The unsubstituted isoindigo exhibits ambipolar transistor properties with the hole and electron mobilities more than 0.01 cm²/Vs, and 6.6′-diphenylisoindigo shows ambipolar transistor properties with the hole/electron mobilities of 0.037/0.027 cm²/Vs. Isoindigo derivatives with electron withdrawing groups show only electron transport, indicating that the lower limit of the HOMO level showing the hole transport is −5.7 eV.     

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm² V⁻¹ s⁻¹ are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.     

Field-effect transistors based on organic single crystals grown by an improved vapor phase method

High-quality organic single crystals are produced directly onto the substrates using an improved vapor phase method. Unlike the conventional vapor phase methods, the present method is characterized by forming a large-sized crystal to which semiconductor devices can readily be made. The relevant method requires small space of only a 10-cm cube in which a couple of plates are put in close proximity. The crystal growth is carried out nearly at the thermodynamic equilibrium within the narrow space surrounded with the two plates. Thin single crystals of several hundreds of micrometers in size are grown on one of those plates. For the organic materials to be crystallized, we have chosen 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) and 5,5-diphenyl-2,2′:5′,2″:5″,2:5,2-quinquethiophene (P5T) from among thiophene/phenylene co-oligomers. The resulting crystals are well-defined polygons, each side reflecting the specific crystallographic orientation. In particular, those grown on self-assembled monolayers are exceedingly flat and free from cracks. We have directly fabricated top-contact field-effect transistors on these crystals. The devices exhibit the excellent performance and keep it both in air and in vacuum for a maximum of a hundred days.     

Highly sensitive H2S sensors based on ultrathin organic single-crystal microplate transistors

Based on ultrathin dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2) single-crystal microplates, the highly sensitive organic field-effect H₂S sensors are realized at room temperature. The response is as high as 1.2 × 10⁶% in 50 ppm H₂S. This value is extremely high for H₂S sensors, and is three orders of magnitude higher than that of the most reported semiconductor gas sensors. The response/recovery time is respectively as low as 2 min and 1 min in 50 ppm H₂S. The detect limitation is as low as 0.5 ppm. The ultrathin single-crystal microplates provide direct and efficient ways for the analytes' activities within the conducting channel, and therefore mainly account for the improved sensing performance. The excellent sensing performance of ultrathin Ph5T2 single-crystal microplate transistors reveals the capacity of developing highly sensitive room-temperature sensors.     

Controllable molecular doping in organic single crystals toward high-efficiency light-emitting devices

Organic single-crystalline semiconductors have drawn significant attention in the area of organic electronic and optoelectronic devices due to their superiorities of highly ordered structure, high carrier mobility and low impurity content. Molecular doping technique has made great progress in improving device performance via optimizing the optical and electrical properties of organic semiconductors. In particular, this technique has been attempted by taking fluorescent dye-molecules as the emissive dopants to tune emission color and improve device performance of organic single crystals. Up to now, there are few reports about the use of molecular doping in organic single crystals to optimize their intrinsic electrical properties. Here, we have introduced the controllable molecular doping as a feasible approach toward manipulating charge carrier transport properties of organic single crystals. Upon optimization of doping concentration, balanced carrier transport can be realized in 5,5′-bis(4-trifluoromethyl phenyl) [2,2’] bithiophene (P2TCF₃)-doped 1,4-bis(4-methylstyryl) benzene (BSB–Me) crystals. Organic light-emitting devices (OLEDs) based on these doped crystals achieve a maximum luminance of 423 cd/m² and current efficiency of 0.48 cd/A. It demonstrates that high-efficiency crystal-based OLEDs are of great significance for the development of organic electronics, especially for display and lighting applications.     

Influence of source/drain electrodes on external quantum efficiency of ambipolar organic light-emitting transistors

The influence of source/drain (S/D) electrodes on the external quantum efficiency (EQE) of ambipolar organic light-emitting transistors (OLETs) based on fluorene-type polymer films is investigated. The electrical properties and the maximum EQE value of the device with indium tin oxide (ITO) S/D electrodes are almost the same as those of the device with Ag S/D electrodes. A relatively high EQE of 1% is achieved regardless of the emission site for the OLET with ITO. In contrast, the EQE of the OLET with Ag is low when the emission occurs close to the S/D electrodes. The maximum EQE of the device with Ag is obtained when the emission is observed in the middle of the channel. It is found that the exciton quenching by Ag electrodes significantly influences the low EQE of the OLET with Ag electrodes. The achievement of high EQE regardless of the emission site is attributable to both better carrier injection and lower exciton quenching at the interface of S/D electrodes for the OLET with ITO.     

Flexible organic field-effect transistors with TIPS-Pentacene crystals exhibiting high electrical stability upon bending

Flexible organic field-effect transistors with high electrical stability upon bending are demonstrated on indium tin oxide coated polyethylene terephthalate substrates with TIPS-Pentacene semiconductor crystals formed by drop casting on a hybrid gate dielectric consisting hafnium dioxide grown by atomic layer deposition and spin coated poly(4-vinylphenol). Fabricated devices exhibited excellent p-channel characteristics with field-effect mobility up to 0.12 cm²/Vs with high current on/off ratio >10⁴ and low threshold voltage of −0.2 V. Device performance was slightly affected by mechanical strain applied by bending for 5 min with radius varying from 12.5 mm to as low as 5.0 mm; and a high stability in performance was demonstrated upon applying constant tensile strain for more than 48 h at bending radius of 5.0 mm. It was found that strain induced changes in the device performance primarily occur due to increase in dielectric surface roughness; and the semiconductor-dielectric interface uniformity is influenced more with magnitude of strain rather than its duration.     

High performance organic field-effect transistors using ambient deposition of tetracene single crystals

Organic molecular crystals (OMCs) are of significant interest due to their potential use in transistors, photovoltaic devices, light emitting diodes, and other applications. However, conventional vacuum-based methods of growing crystalline OMC films are costly and provide limited control over crystal growth. In this study, we present a new method for preparing high performance single-crystal tetracene field-effect transistors under near-ambient conditions using organic vapor-liquid-solid (OVLS) deposition. We find that the mobility of OVLS-grown tetracene is comparable to high quality crystalline films prepared by physical vapor deposition. These results establish OVLS deposition as a relatively low cost, low substrate temperature, and ambient pressure method for growing high quality OMC films for device applications.     

In-plane light emission of organic light-emitting transistors with bilayer structure using ambipolar semiconducting polymers

The concept of using an ambipolar bilayer semiconducting heterostructure in organic light-emitting transistors (OLETs) is introduced to provide a new approach to achieve surface emission. The properties of top-gate-type bilayer OLETs with ambipolar materials based on two types of fluorene-type polymers used as an emissive layer and an electron blocking layer are investigated. Line-shaped yellow–green emission occurs near a hole-injection electrode. When hole transport is dominant in the upper layer which acts as an electron blocking layer, and electrons are injected into the lower layer, an in-plane light-emitting pattern is observed. The measured in-plane emission zone confirms that both hole and electron transport are determined to occur mainly along the different organic layers between the source and drain electrodes, and an in-plane recombination zone of electrons and holes exists near the bilayer organic interface. This work is anticipated to be useful for the development of in-plane light-emitting transistors.     

Solution-processed organic crystals written directly with a rollerball pen for field-effect transistors

To deposit organic semiconducting crystals from solution, we propose the use of a rollerball pen as a simple and promising tool. These organic crystal grains of dioctylbenzothienobenzothiophene measured several hundred micrometers. The fabricated OFETs exhibited good device performance with a field-effect mobility (μ_FET) of 0.7 cm²/Vs and an on-off ratio of more than 10⁷. Simulation results reveal that the flow behavior of solution from the pen refill tube to the substrate intrinsically enhances the formation of large organic crystals.     

Low-voltage p-channel, n-channel and ambipolar organic thin-film transistors based on an ultrathin inorganic/polymer hybrid gate dielectric layer

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated low-voltage operating (below 3V) p-channel, n-channel and ambipolar OTFTs based on pentacene or/and C₆₀ as the active layers, respectively, with an ultrathin AlO_X/poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) hybrid layer as the gate dielectric. Benefited from the enhanced crystallinity of C₆₀ layer and greatly reduced density of electron trapping states at the interface of channel/dielectric due to the insertion of ultrathin pentacene layer between C₆₀ and P(MMA–GMA), high electron mobility can be achieved in present pentacene/C₆₀ heterostructure based ambipolar OTFTs. The effect of the thickness of pentacene layer and the deposition sequence of pentacene and C₆₀ on the device performance of OTFTs was studied. The highest electron mobility of 3.50 cm²/V s and hole mobility of 0.25 cm²/V s were achieved in the ambipolar OTFT with a pentacene (3.0 nm)/C₆₀ (30 nm) heterostructure.     

Design,fabrication and application of organic power converters: Driving light-emitting electrochemical cells from the AC mains

The design, fabrication and operation of a range of functional power converter circuits, based on diode-configured organic field-effect transistors as the rectifying unit and capable of transforming a high AC input voltage to a selectable DC voltage, are presented. The converter functionality is demonstrated by selecting and tuning its constituents so that it can effectively drive a low-voltage organic electronic device, a light-emitting electrochemical cell (LEC), when connected to high-voltage AC mains. It is established that the preferred converter circuit for this task comprises an organic full-wave rectifier and a regulation resistor but is void of a smoothing capacitor, and that such a circuit connected to the AC mains (230 V, 50 Hz) successfully can drive an LEC to bright luminance (360 cd m⁻²) and high efficiency (6.4 cd A⁻¹).     

Top-gate type,ambipolar, phosphorescent light-emitting transistors utilizing liquid-crystalline semiconducting polymers by the thermal diffusion method

In order to obtain phosphorescent emission from organic field-effect transistors, thermal diffusion of a phosphorescent dye at the interface between the active layer and gate insulator was controlled by utilizing the fact that liquid-crystalline semiconducting polymers self-organize due to reorientation of the molecules and increase in the size of the crystalline regions during thermal annealing. For top-gate type devices using poly(alkylfluorene) doped with a red emissive phosphorescent material, ambipolar characteristics and red emission from the phosphorescent material were clearly observed. We demonstrate the possibility of producing phosphorescent organic light-emitting transistors using a liquid-crystalline semiconducting polymer doped with phosphorescent emissive dopants by the thermal diffusion method.     

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

On the transient response of organic electrochemical transistors

We present a universal model for the transient drain current response in organic electrochemical transistors (OECTs). Using equivalent circuits and charge injection physics, we are able to predict the drain current in OECT devices upon application of a gate voltage input. The model is applicable to both plain and membrane-functionalized devices, and allows us to extract useful physical quantities such as resistances and capacitances, which are related to functional properties of the system. We are also able to use the model to reconstruct the magnitude and shape in time of an applied voltage source based on the observed drain current response. This was experimentally demonstrated for drain current measurements under an applied action potential.     

Megahertz operation of flexible low-voltage organic thin-film transistors

Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1 μm have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT). The transistors have an effective field-effect mobility of 1.2 cm²/V s, an on/off ratio of 10⁷, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 ns per stage at a supply voltage of 3 V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V.     

Below-one-volt organic thin-film transistors with large on/off current ratios

In many of the applications envisioned for organic thin-film transistors (TFTs), the electrical power will be supplied by small batteries or energy harvesters, which implies that it will be beneficial if the TFTs can be operated with voltages of 1 V or even below 1 V. At the same time, the TFTs should have large on/off current ratios, especially for applications in digital circuits and active matrices. Here we demonstrate p-channel and n-channel organic TFTs fabricated on a flexible plastic substrate that have a turn-on voltage of exactly 0 V, a subthreshold slope of 100 mV/decade, and an on/off current ratio of 2 × 10⁵ when operated with gate-source voltages between 0 and 0.7 V. Complementary inverters fabricated using these TFTs have a small-signal gain of 90 and a minimum noise margin of 79% at a supply voltage of 0.7 V. Complementary ring oscillators can be operated with supply voltages as small as 0.4 V.     

High-speed organic single-crystal transistors gated with short-channel air gaps: Efficient hole and electron injection in organic semiconductor crystals

Short-channel, high-mobility organic filed-effect transistors (OFETs) are developed based on single crystals gated with short-channel air gaps. The high hole mobility of 10 cm²/Vs for rubrene, and high electron mobility of 4 cm²/Vs for PDIF-CN₂ crystals are demonstrated even with a short channel length of 6 μm. Such performance is due to low contact resistance in these devices estimated to be as low as ～0.5 kΩ cm at gate voltage of ?4 V for rubrene. With the benefit of the short channel length of 4.5 μm in a new device architecture with less parasitic capacitance, the cutoff frequency of the rubrene air–gap device was estimated to be as high as 25 MHz for drain voltage of ?15 V, which is the fastest reported for p-type OFETs, operating in ambient conditions.     

Highly photosensitive thienoacene single crystal microplate transistors via optimized dielectric

High photosensitivity and high photocurrent gain have been obtained based on dielectric optimized dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2) single crystal microplate transistors. In our experiments, the PMMA dielectric device shows the best operational stability without hysteresis effect. Based on such an optimized device, the photoelectric properties of the Ph5T2 single crystal microplates have been studied for the first time. The Ph5T2 phototransistor has the high photosensitivity at 21 mA W⁻¹ and the extremely high photocurrent gain (I_light/I_dark) at 6.8 × 10⁵. The photocurrent gain is higher than that of the most reported organic phototransistors (OPTs), and is in a class with the highest photocurrent gain for the reported values so far. This confirms that Ph5T2 is a photosensitive material and shows it promising potential in photoswitches and phototransistors.