Structural phase transition in pentacene caused by molecular doping and its effect on charge carrier mobility

The structural properties and charge carrier mobility of pentacene doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes.     

Interface dipole: Effects on threshold voltage and mobility for both amorphous and poly-crystalline organic field effect transistors

We report a detailed comparison on the role of a self-assembled monolayer (SAM) of dipolar molecules on the threshold voltage and charge carrier mobility of organic field-effect transistor (OFET) made of both amorphous and polycrystalline organic semiconductors. We show that the same relationship between the threshold voltage and the dipole-induced charges in the SAM holds when both types of devices are fabricated on strictly identical base substrates. Charge carrier mobilities, almost constant for amorphous OFET, are not affected by the dipole in the SAMs, while for polycrystalline OFET (pentacene) the large variation of charge carrier mobilities is related to change in the organic film structure (mostly grain size).     

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C₆₀ as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4 cm²/Vs. Devices with pentacene showed a mobility of 0.16 cm²/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ～0.3 cm²/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.     

Bottom-contact small-molecule n-type organic field effect transistors achieved via simultaneous modification of electrode and dielectric surfaces

Low-voltage, n-type organic field effect transistors (OFETs) with simultaneously modified bottom-contact (BC) electrodes and dielectric were compared to their top-contact (TC) counterparts. The devices modified with 6-phenoxyhexylphosphonic acid (Ph6PA) self-assembled monolayer (SAM) showed similar performance, morphology, and contact resistance. Electron mobility of C₆₀ devices were 0.212 and 0.320 cm² V⁻¹ s⁻¹ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) devices were 0.04 and 0.06 cm² V⁻¹ s⁻¹ for TC and BC devices, respectively. Low contact resistance between 11 and 45 kΩ cm was found regardless of device architecture or n-type semiconductor used. This work shows it is possible to fabricate solution processable low-voltage bottom-contact devices with performance that is similar or better than their top-contact counterparts without the addition of complex and time-consuming processing steps.     

High performance organic thin film transistors using chemically modified bottom contacts and dielectric surfaces

A compatible process of orthogonal self-assembled monolayers (SAMs) is applied to intentionally modify the bottom contacts and gate dielectric surfaces of organic thin film transistors (OTFTs). This efficient interface modification is first achieved by 4-fluorothiophenol (4-FTP) SAM to chemically treat the silver source–drain (S/D) contacts while the silicon oxide (SiO₂) dielectric interface is further primed by either hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS-C8). Results show that the field effect mobilities of the bottom-gate bottom-contact PTDPPTFT4 transistors were significantly improved to 0.91 cm² V⁻¹ s⁻¹.     

Dialkylated dibenzotetrathienoacene derivative as semiconductor for organic field effect transistors

A novel semiconductor material based on dialkylated thienoacene is designed and synthesized. The dihexyl-substituted dibenzotetrathienoacene derivative C6-DBTTA exhibits high stability which is evidenced by thermogravimetric analysis (TGA), UV–vis spectroscopy and electrochemistry. X-ray diffraction measurements of the vacuum-evaporated thin films show strong diffraction and indicate that the molecules are stacked nearly perpendicular to the substrate. AFM images reveal that the morphology of thin films depended on the deposition temperature. Thin film FETs devices based on C6-DBTTA were constructed and showed high mobility up to 0.48 cm² V⁻¹ s⁻¹ and an on/off ratio over 10⁷. These results suggest that this new dihexylated thienoacene is an important organic semiconductor for field effect transistors.     

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.     

High mobility multibit nonvolatile memory elements based organic field effect transistors with large hysteresis

High mobility multibit nonvolatile memory elements based on organic field effect transistors with a thin layer of polyquinoline (PQ) were reported. The devices show a high mobility of 1.5 cm² V⁻¹ s⁻¹ in the saturation region which is among the best reported for nonvolatile organic memory transistors. The multibit nonvolatile memory elements can be operated at voltage less than 100 V with good stability under continuous operation condition and show long retention time. The different initial scanning positive gate voltages to −100 V result in several ON states, while the scanning gate voltage from −100 V to positive voltage leads to same OFF state. The charge trapping model of electrons into the PQ layer was used to explain the origin of the memory properties.     

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.     

Electric field induced ferroelectric-surface modification for high mobility organic field effect transistors

Inherent spontaneous polarization in ferroelectric-dielectric polymer PVDF-TrFE (Poly[(vinylidenefluoride-co-trifluoroethylene]) and an external electric field induced surface modification procedure are utilized to enhance organic field effect transistor (OFET) characteristics. The increase in the carrier mobility of the electric-field (EF) treated device correlates with the EF magnitude and evolution of dielectric microstructure and exhibits an enhancement beyond 300%. The enhanced interfacial transport property appears to have its origin in the dipolar orientation and nanostructure evolution at the interface.     

Channel length influenced contribution of hole and electron trapping effect to threshold voltage stability in organic field effect transistors

Effect of channel length on hysteresis and threshold voltage shift in copper phthalocyanine (CuPc) based organic field effect transistors was studied. Contrary to expectation, longer channel length devices exhibited minimum threshold voltage shift. Influence of channel length on the contribution of hole and electron trapping to threshold voltage stability was determined. Shortest channel length devices exhibited highest electron trapping effect while longest channel devices exhibited minimum hole as well as electron trapping. Lower hole trap effect for longer channel length devices was suggested to be due to reduced longitudinal field between source and drain electrodes while minimum electron trapping was attributed to suppression of drain current by increased hole trap centres.     

Facile polymer gate dielectric surface-modification for organic thin-film transistors using self-assembled surfactant layer

We demonstrate facile polymer gate dielectric surface-modification method for organic thin-film transistors (OTFTs). We simply introduce self-assembled surfactant layer onto the top surface of poly(4-vinylphenol) (PVP) dielectric by spin coating PVP solution mixed with sodium dodecyl sulfate and tridecafluorohexane-1-sulfonic acid potassium salt as additive agents. The surfactant-modified PVP layer acquires various merits compared to pristine PVP layer in terms of surface smoothness and hydrophobicity, as confirmed by contact angle measurement, atomic force microscopy analyses, grazing incident X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy. The resulting OTFTs with the conventional semiconducting poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active layer and surfactant-modified PVP as the dielectric layer reveal overall ascendency over the OTFT with pristine PVP, especially in terms of operating hysteresis and reliability. The effects of hydrophobicity of surfactants on the surface properties of PVP as well as the OTFT performances are fully discussed in conjunction with various characterization tools.     

Sensitivity enhancement of a flexible MEMS strain sensor by a field effect transistor in an all organic approach

We report a low-cost piezoresistive nanocomposite based organic micro electro mechanical system (MEMS) strain sensor that has been combined to an organic field effect transistor (OFET) with the objective of amplifying the sensitivity of the sensor. When the MEMS cantilever is strained by a mechanical deflection, the resulting variation of resistivity influences the gate voltage (V_GS) of the OFET and, hence, changes the drain current (I_DS) of the transistor. The present combination allows an enhancement of sensitivity to strain by a factor 3.7, compared to the direct detection of resistance changes of the nanocomposite. As a consequence, a low limit of detection of 24 ppm has been estimated in terms of strain transduction efficiency. Furthermore, the organic microsystem exhibits a short response time and operates reversibly with an excellent robustness.     

Impact of injection limitations on the contact resistance and the carrier mobility of organic field effect transistors

The contact resistance as well as the mobility have developed to key performance indicators for benchmarking organic field-effect transistors. Typically, conventional methods for silicon transistors are employed for their extraction thereby ignoring the peculiarities of organic transistors. This work outlines the required conditions for using conventional extraction techniques for the contact resistance and the mobility based on TCAD simulations and experimental data. Our experimental data contain both staggered and coplanar structures fabricated by exploiting different optimization techniques like SAM treated electrodes, different shearing speeds, PS blending and silicon oxide functionalization. In addition, the work clarifies how injection limited current–voltage characteristics can affect high-performance organic field-effect transistors. Finally, we introduce a semi-physical model for the contact resistance to accurately interpret extracted benchmark parameters by means of the transfer length method (TLM). Guidelines to use conventional extraction techniques with special emphasis on TLM are also provided.     

All solution-processed organic single-crystal transistors with high mobility and low-voltage operation

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.     

Electroabsorption studies of organic p-i-n solar cells: Increase of the built-in voltage by higher doping concentration in the hole transport layer

The built-in voltage in solar cells has a significant influence on the extraction of photogenerated charge carriers. For small molecule organic solar cells based on the p-i-n structure, we investigate the dependence of the built-in voltage on the work function of both the hole transport layer and the electrode material. The model system investigated here consists of a planar heterojunction with N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as donor and Buckminster Fullerene (C₆₀) as acceptor material. A higher concentration of the dopant C₆₀F₃₆ in the hole transport layer induces a shift of the work function towards the transport level. The resulting increase of the built-in voltage is studied using electroabsorption spectroscopy, measuring the change in absorption (Stark effect) caused by an externally applied electric field. An evaluation of these electroabsorption spectra as a function of the applied DC voltage enables the direct measurement of the built-in voltage. It is also shown that an increased built-in voltage does lead to a larger short-circuit current as well as a larger fill factor.     

Performance enhancement of solution-processed InZnO thin-film transistors by Al doping and surface passivation

Solution-processed oxide semiconductors have been considered as a potential alternative to vacuum-based ones in printable electronics. However, despite spin-coated InZnO (IZO) thin-film transistors (TFTs) have shown a relatively high mobility, the lack of carrier suppressor and the high sensitivity to oxygen and water molecules in ambient air make them potentially suffer issues of poor stability. In this work, Al is used as the third cation doping element to study the effects on the electrical, optoelectronic, and physical properties of IZO TFTs. A hydrophobic self-assembled monolayer called octadecyltrimethoxysilane is introduced as the surface passivation layer, aiming to reduce the effects from air and understand the importance of top surface conditions in solution-processed, ultra-thin oxide TFTs. Owing to the reduced trap states within the film and at the top surface enabled by the doping and passivation, the optimized TFTs show an increased current on/off ratio, a reduced drain current hysteresis, and a significantly enhanced bias stress stability, compared with the untreated ones. By combining with high-capacitance AlO_x, TFTs with a low operating voltage of 1.5 V, a current on/off ratio of > 10⁴ and a mobility of 4.6 cm²/(V·s) are demonstrated, suggesting the promising features for future low-cost, low-power electronics.     

Effects of iodine doping on small molecule organic semiconductors for high charge carrier mobility and photoconductivity

Here we report the effects of iodine doping on small molecule organic semiconductors. Thin films of semiconducting p-DTS(FBTTh₂)₂ doped with 1–5 wt% iodine were fabricated and their photo-physical, crystallographic, morphological, and electrical properties were systematically analyzed. The doping significantly increased the energetic distance between the highest occupied molecular orbital (HOMO) and Fermi level of p-DTS(FBTTh₂)₂, typical for p-type doping. In addition, depletion mode transistor measurements showed an increase in the hole concentration with increasing dopant concentration. From grazing incidence X-ray diffraction (GIXD) analyses of iodine-doped p-DTS(FBTTh₂)₂ films, we observed significant changes in the crystal orientation at the optimal doping ratio of 1 wt%. Atomic force microscopy (AFM) analyses showed morphological changes with respect to dopant concentrations, which were in good agreement with the GIXD results. As a result, accumulation mode transistor measurements demonstrated an increase in the hole mobility by 54% at the optimized doping concentration compared to an undoped device. Furthermore, photoconductive device operation revealed that iodine-doping can induce dramatically enhanced photo-responsivity as high as 2.08 A/W. We demonstrate that iodine doping can be a simple and effective method for enhancing the performance of small molecule-based electronic devices, by optimizing the energy level configuration as well as enhancing intermolecular interactions.     

Fast detection of blood gases by solution gated organic field effect transistors

We characterize the electrochemical stability of the organic semiconductor Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) in aqueous solutions. Electrochemical stability of DNTT in solution is validated by cyclic voltammetry and demonstrated by solution gating of DNTT organic field effect transistors (OFETs). Then, we investigate the response time of DNTT OFETs to ammonia, a common blood gas. For bare OFETs, the response time to ammonia is 1–2s only. The exact response time depends on the DNTT film morphology; the fastest response is obtained for pronounced 3D (Volmer-Weber) growth. By comparing OFETs with and without a semipermeable parylene-C encapsulation layer, the influence of the capping on the response time is investigated. An encapsulation layer of 86 nm prolongs the response time to 100s, indicating that parylene-C acts as an efficient diffusion barrier for ammonia.