Orders‐of‐Magnitude Reduction of the Contact Resistance in Short‐Channel Hot Embossed Organic Thin Film Transistors by Oxidative Treatment of Au‐Electrodes

In this study we report on the optimization of the contact resistance by surface treatment in short‐channel bottom‐contact OTFTs based on pentacene as semiconductor and SiO₂ as gate dielectric. The devices have been fabricated by means of nanoimprint lithography with channel lengths in the range of 0.3 μm < L < 3.0 μm. In order to reduce the contact resistance the Au source‐ and drain‐contacts were subjected to a special UV/ozone treatment, which induced the formation of a thin AuO_x layer. It turned out, that the treatment is very effective (i) in decreasing the hole‐injection barrier between Au and pentacene and (ii) in improving the morphology of pentacene on top of the Au contacts and thus reducing the access resistance of carriers to the channel. Contact resistance values as low as 80 Ω cm were achieved for gate voltages well above the threshold. In devices with untreated contacts, the charge carrier mobility shows a power‐law dependence on the channel length, which is closely related to the contact resistance and to the grain‐size of the pentacene crystallites. Devices with UV/ozone treated contacts of very low resistance, however, exhibit a charge carrier mobility in the range of 0.3 cm² V^–1 s^–1 < μ < 0.4 cm² V^–1 s^–1 independent of the channel length.     

High-mobility pentacene phototransistor with nanostructured SiO2 gate dielectric synthesized by sol-gel method

We have fabricated a pentacene based phototransistor by employing a modified nanostructured SiO₂ gate dielectric. The photosensing properties of the pentacene thin film transistor fabricated on n-Si substrate with nanostructured SiO₂ as gate dielectric have been investigated. The photocurrent of the transistor increases with an increase in illumination intensity. This suggests that the pentacene thin film transistor behaves as a phototransistor with p-channel characteristics. The photosensitivity and responsivity values of the transistor are 630.4 and 0.10 A/W, respectively at the off state under AM 1.5 light illumination. The field effect mobility of the pentacene phototransistor was also found to be 2.96 cm²/Vs. The nanostructured surface of the gate possibly is the cause of the high-mobility value of the phototransistor due to light scattering from the increased surface area.     

Experimental and numerical analysis of channel-length-dependent electrical properties in bottom-gate,bottom-contact organic thin-film transistors with Schottky contact

Channel length dependence of field-effect mobility and source/drain parasitic resistance in pentacene thin-film transistors with a bottom-gate, bottom-contact configuration was investigated. Schottky barrier effect such as nonlinear behaviors in transistor output characteristics appeared and became more prominent for shorter channel length less than 10 μm, raising some concerns for a simple utilization of conventional parameter extraction methods. Therefore the gate-voltage-dependent hole mobility and the source/drain parasitic resistance in the pentacene transistors were evaluated with the aid of device simulation accounting for Schottky contact with a thermionic field emission model. The hole mobility in the channel region shows smaller values with shorter channel length even after removing the influence of Schottky barrier, suggesting that some disordered semiconductor layers with low carrier mobility exist near the contact electrode. This experimental data analysis with the simulation enables us to discuss and understand in detail the operation mechanism of bottom-gate, bottom-contact transistors by considering properly each process of charge carrier injection, carrier flow near the contact region, and actual channel transport.     

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.     

The Effect of Gate‐Dielectric Surface Energy on Pentacene Morphology and Organic Field‐Effect Transistor Characteristics

The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field‐effect transistors (FETs) are reported, using surface‐energy‐controllable poly(imide‐siloxane)s as gate‐dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three‐dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low‐surface‐energy gate dielectric is larger by a factor of about five, compared to their high‐surface‐energy counterparts. In pentacene growth on the low‐surface‐energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low‐surface‐energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high‐surface‐energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.     

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ? 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.     

MoS_2激子效应的压电光声研究

本文首先利用光声压电法,研究了MoS_2单晶在540-740nm波段的室温本征吸收边的光声吸收光谱,得到了在2.0eV附近的A、B两个强激子的吸收峰及杂质峰A,与以前报道的用透射光谱法测得的吸收谱及光电流谱的结果一致.     

Temperature gradient controlled crystal growth from TIPS pentacene-poly(α-methyl styrene) blends for improving performance of organic thin film transistors

6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) from simple drop casting typically forms crystals with random orientation and poor areal coverage, which leads to device-to-device performance variation of organic thin film transistors (OTFTs). Previously, a temperature gradient technique was developed to address these problems. However, this approach simultaneously introduced thermal cracks due to the thermally induced stress during crystallization. These thermal cracks accounted for a reduction of charge transport, thereby impacting the device performance of TIPS pentacene based OTFTs. In this work, an insulating polymer, poly(α-methyl styrene) (PαMS) was blended with TIPS pentacene to relieve the thermal stress and effectively prevent the generation of thermal cracks. The results demonstrate that the incorporation of PαMS polymer combined with the temperature gradient technique improves both the hole mobility and performance consistency of TIPS pentacene based OTFTs.     

Pentacene bottom-contact thin film transistors with solution-processed BZT gate dielectrics

The solution-processed high-k barium zirconate titanate (BZT) as gate dielectrics for bottom-gate pentacene-based organic thin film transistor (OTFT) applications is presented. To reduce the transistor threshold voltage, higher work function metals (Au) is used as the gate electrodes. The threshold voltage is efficiently decreased from −3.6 to −2.15 V as compared to that of Al. In addition, the UV/ozone was employed to treat the Au (source/drain) surface to improve the poor crystalline of pentacene grown on Au. Moreover, the surface morphologies and orientations of pentacene films were analyzed through atomic force microscopy (AFM) and X-ray diffraction. As the results, the stack of pentacene molecules from disorder state changed to vertical growth on the Au surface. Finally, the electrical properties of pentacene-based thin film transistors exhibit high field-effect mobility of 4.5 cm²/V·s, low subthreshold swing of 260 mV/decade, high on/off ratio of 1.4 × 10⁵ and low operation voltage of −5 V. These results are better than the reported data using bottom contact pentacene OTFTs.     

An Orientation‐Controlled Pentacene Film Aligned by Photoaligned Polyimide for Organic Thin‐Film Transistor Applications

We have demonstrated that the photoalignment method could be used to control the structural anisotropy of pentacene films, an active semiconducting layer, in thin‐film transistors (TFTs) with conspicuously anisotropic electrical characteristics. The photoaligned pentacene films were characterized with respect to structure and morphology using X‐ray diffraction and atomic force microscopy. Compared to the pentacene films that are not controlled, a maximal 25‐times increase in field‐effect mobility (up to 0.75 cm V^–1 s^–1) has been achieved in the photoaligned pentacene‐based TFTs by aligning pentacene orientation parallel to the direction of current flow with the help of a photoaligned polyimide layer. Mobility anisotropic ratio in the range of 2.7–8.3 for the current flow parallel and perpendicular to the alignment of the photoaligned pentacene films have been observed for photoaligned pentacene‐based TFTs.     

Enhanced Performance Consistency in Nanoparticle/TIPS Pentacene‐Based Organic Thin Film Transistors

In this study, inorganic silica nanoparticles are used to manipulate the morphology of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin films and the performance of solution‐processed organic thin‐film transistors (OTFTs). This approach is taken to control crystal anisotropy, which is the origin of poor consistency in TIPS pentacene based OTFT devices. Thin film active layers are produced by drop‐casting mixtures of SiO₂ nanoparticles and TIPS pentacene. The resultant drop‐cast films yield improved morphological uniformity at ～10% SiO₂ loading, which also leads to a 3‐fold increase in average mobility and nearly 4 times reduction in the ratio of measured mobility standard deviation (μ_Stdev) to average mobility (μ_Avg). Grazing‐incidence X‐ray diffraction, scanning and transmission electron microscopy as well as polarized optical microscopy are used to investigate the nanoparticle‐mediated TIPS pentacene crystallization. The experimental results suggest that the SiO₂ nanoparticles mostly aggregate at TIPS pentacene grain boundaries, and 10% nanoparticle concentration effectively reduces the undesirable crystal misorientation without considerably compromising TIPS pentacene crystallinity.     

Morphology transformations of pentacene on an UV-patternable polymer insulator and their effect on performance of flexible organic thin-film transistors

This work presents a low temperature with high resolution capability UV-patternable polymer, i.e. mr-UVCur06, for use as a gate insulator in OTFTs, by investigating the morphology transformation of pentacene deposited on the mr-UVCur06. The device structure is polyethylene terephthalate (PET)/indium-tin oxide (ITO)/mr-UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patterned by UV light in a low-temperature process. UV/ozone post-treatment of the patternable mr-UVCur06 can illuminate organic contaminants from its surface and increases surface energy. Experimental results indicate that a high surface energy existing at the mr-UVCur06 surface has produced in a larger ratio of I_{thin film phase}/I_{triclinic bulk phase} in pentacene. Then, the distance of pentacene molecular crystal structure, which is arranged in the thin film phase, is shorter than that in triclinic bulk phase. The performance of pentacene-based OTFTs can be enhanced with few contaminants and a high surface energy on the UV-patternable gate insulator. By performing UV/ozone post-treatment on the mr-UVCur06 insulator surface for 60 s, the OTFTs demonstrate a mobility, on/off drain current ratio, and V_T of 0.34 cm²/V s, 5.5 × 10⁴, and 2.5 V, respectively. Furthermore, the low-temperature photopatternable polymer dielectric paves the way for a relatively easy and low-cost fabrication of an OTFT array without expensive and complicated photolithography and dry etching.     

Raman spectroscopy applied to reveal polycrystalline grain structures and carrier transport properties of organic semiconductor films: Application to pentacene-based organic transistors

We have fabricated high performance polycrystalline pentacene-based thin-film transistors using several dielectrics with different surface properties, including inorganic oxide and polymeric materials. These materials provide excellent samples for the analysis of charge transport properties, particularly the impact of the grain boundary and the molecular structural quality within the grain on the efficient charge transport. The carrier transport in polycrystalline organic films with grain structures is often interpreted using the grain boundary model. Assuming a large amount of charges are trapped at the boundaries, the model neglects the microstructural quality inside a grain. According to joint experimental and theoretical Raman spectra and normal modes analysis on these pentacene films, we present a new observation that the microscopic hopping transport parameters, i.e., intermolecular interactions and reorganization energy, in polycrystalline films govern the carrier transport. An obvious, positive correlation is found between the mobility and the molecular vibrational characteristics, especially the intermolecular vibrational coupling energy, under all varieties of grain size morphology. MicroRaman mapping methodology reveals that the grain size of pentacene films should not be taken for granted in structural quality and efficient charge transport. The microstructural qualities inside the grain play an important role in efficient charge transport.     

Insight into the charge transport and degradation mechanisms in organic transistors operating at elevated temperatures in air

Operational stability of organic devices at above-room-temperatures in ambient environment is of imminent practical importance. In this report, we have investigated the charge transport and degradation mechanisms in pentacene based organic field effect transistors (OFETs) operating in the temperatures ranging from 25 °C to 150 °C under ambient conditions. The thin film characterizations techniques (X-ray photoelectron spectroscopy, X-ray diffraction and atomic force microscopy) were used to establish the structural and chemical stability of pentacene thin films at temperatures up to 150 °C in ambient conditions. The electrical behavior of OFETs varies differently in different temperature bracket. Mobility, at temperatures below 110 °C, is found to be thermally activated in presence of traps and temperature independent in absence of traps. At temperatures above 110 °C mobility degrades due to polymorphism in pentacene or interfacial properties. The degradation of mobility is compensated with the decrease in threshold voltage at high temperatures and OFETs are operational at temperatures as high as 190 °C. 70 °C has been identified as the optimum temperature of operation for our OFETs where both device behavior and material properties are stable enough to ensure sustainable performance.     

Mechanical and electrical stability of PEDOT:PTS and Au source/drain electrodes for bottom contact OTFTs on plastic films under bending conditions

Two types of source/drain (S/D) electrodes, formed of poly(3,4-ethylenedioxythiophene):p-toluenesulfonate (PEDOT:PTS) conducting polymer and Au, were fabricated and mounted on flexible pentacene transistors. Several properties of the S/D electrodes were investigated: the adhesion between electrode and pentacene, the failure induced by mechanical bending, the electrical properties including contact resistance, and the field-effect mobility. The measurement of adhesive force using a 90° peel tester revealed that the adhesive force of the pentacene–PEDOT:PTS interface was 200 N/m, while that of Au–pentacene was too small to be measured. The poor adhesion between the Au and pentacene led to the local delamination of the Au film, starting at a strain of approximately 1% and with a rapid increase of delamination density with increasing strain. The strain-induced mechanical damage has a direct effect on contact resistance, which in turn degrades mobility. In contrast, no delamination is observed with the high adhesive force at the pentacene–PEDOT:PTS interface, which thus led to a slight increase of contact resistance and mobility with strain. Consequently, the pentacene–PEDOT:PTS contacts provide mechanical and electrical stability in flexible organic thin film transistors (OTFTs) under bending tests.     

Following Chemical Charge Trapping in Pentacene Thin Films by Selective Impurity Doping and Wavelength‐Resolved Electric Force Microscopy

Charge trapping is one of several factors that limit the performance of organic electronic materials, yet even in pentacene, a prototypical small‐molecule semiconductor, the precise chemical nature of charge trapping remains poorly understood. Here the effects of three chemical trap‐precursor candidates are examined by layering thin‐film pentacene transistors with different pentacene defect species. The resulting charge trapping is studied in each device via scanning‐probe electric force microscopy coupled with variable‐wavelength sample illumination. Firstly, it is found that layering with pentacen‐6(13H)‐one (PHO) readily produces uniform charge trapping everywhere in the transistor channel, as expected for an active blanket‐deposited trap‐precursor. However, layering with 6,13‐dihydropentacene (DHP) produces fewer, more‐isolated traps, closely resembling the surface potential distribution in pristine pentacene thin films. Secondly, the rates of trap‐clearing versus illuminating wavelength (trap‐clearing spectra) are measured, revealing enhanced trap‐clearing rates at wavelengths assigned to the absorption of either pentacene or the charged trap species. The trap‐clearing spectrum for the PHO‐layered sample closely resembles the spectrum obtained from pentacene aged in a working transistor, while the trap‐clearing spectrum for the DHP‐layered sample resembles the spectrum observed in pristine pentacene. We conclude that PHO competently creates traps in pentacene that match the expected trap‐clearing spectrum for degraded pentacene, while DHP does not, and that the chemical trap species in aged pentacene is very likely PHO⁺.     

Enhancement of Interconnectivity in the Channels of Pentacene Thin‐Film Transistors and Its Effect on Field‐Effect Mobility

With the aim of improving the field‐effect mobility of transistors by promoting the interconnectivity of the grains in pentacene thin films, deposition conditions of the pentacene molecules using one‐step (total thickness of layer 50 nm: 0.1 Å s^–1) and two‐step (first layer 10 nm: 0.1 Å s^–1, second layer 40 nm: 4.0 Å s^–1) depositions are controlled. Significantly, it is found that the continuities of the pentacene thin films vary with the deposition conditions of the pentacene molecules. Specifically, a smaller number of voids is observed at the interface for the two‐step deposition, which results in field‐effect mobilities as high as 1.2 cm² V^–1 s^–1; these are higher by more than a factor of two than those of the pentacene films deposited in one step. This remarkable increase in field‐effect mobility is due in particular to the interconnectivity of the pentacene grains near the insulator substrate.     

Understanding Lattice Strain‐Controlled Charge Transport in Organic Semiconductors: A Computational Study

The softness and anisotropy of organic semiconductors offer unique properties. Recently, solution‐sheared thin‐films of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐P) with nonequilibrium single‐crystal domains have shown much higher charge mobilities than unstrained ones (Nature 2011 , 480, 504). However, to achieve efficient and targeted modulation of charge transport in organic semiconductors, a detailed microscopic understanding of the structure–property relationship is needed. In this work, motivated by the experimental studies, the relationship between lattice strain, molecular packing, and charge carrier mobility of TIPS‐P crystals is elucidated. By employing a multiscale theoretical approach combining nonequilibrium molecular dynamics, first‐principles calculations, and kinetic Monte Carlo simulations using charge‐transfer rates based on the tunneling enabled hopping model, charge‐transport properties of TIPS‐P under various lattice strains are investigated. Shear‐strained TIPS‐P indeed exhibits one‐dimensional charge transport, which agrees with the experiments. Furthermore, either shear or tensile strain lead to mobility enhancement, but with strong charge‐transport anisotropy. In addition, a combination of shear and tensile strains could not only enhance mobility, but also decrease anisotropy. By combining the shear and tensile strains, almost isotropic charge transport could be realized in TIPS‐P crystal with the hole mobility improved by at least one order of magnitude. This approach enables a deep understanding of the effect of lattice strain on charge carrier transport properties in organic semiconductors.