Thickness‐Dependent Structural Evolutions and Growth Models in Relation to Carrier Transport Properties in Polycrystalline Pentacene Thin Films

Thickness‐dependent crystal structure, surface morphology, surface energy, and molecular structure and microstructure of a series of polycrystalline pentacene films with different film thickness ranging from several monolayers to the several hundred nanometers have been investigated using X‐ray diffraction (XRD), atomic force microscopy (AFM), contact angle meter, and Raman spectroscopy. XRD studies indicate that thin film polymorphs transformation behaviours are from the orthorhombic phase to the thin‐film phase and then to the triclinic bulk phase as measured by the increased tilt angle (θ_tilt) of the pentacene molecule from the c‐axis toward the a‐axis. We propose a growth model that rationalizes the θ_tilt increased along with increasing film thickness in terms of grain size and surface energy varying with film growth using AFM combined with contact angle measurements. The vibrational characterizations of pentacene molecules in different thickness films were investigated by Raman spectroscopy compared to density functional theory calculations of an isolated molecule. In combination with XRD and AFM the method enables us to distinguish the molecular microstructures in different thin film polymorphs. We proposed a methodology to probe the microscopic parameters determining the carrier transport properties based on Davydov splitting and the characteristics of aromatic C–C stretching modes in Raman spectra. When compared to the triclinic bulk phase at a high thickness, we suggest that the first few monolayer structures located at the dielectric surface could have inferior carrier transport properties due to weak intermolecular interactions, large molecular relaxation energy, and more grain boundaries.     

Controlled Topology of Block Copolymer Gate Insulators by Selective Etching of Cylindrical Microdomains in Pentacene Organic Thin Film Transistors

We investigate the effect of surface topology of a block copolymer/neutral surface/SiO₂ trilayered gate insulator on the properties of pentacene organic thin film transistor (OTFT) by the controlled etching of self assembled poly(styrene‐b‐methyl methacrylate) (PS‐b‐PMMA) block copolymer. The rms roughness of the uppermost block copolymer film directly in contact with pentacenes was systematically controlled from 0.27 nm to approximately 12.5 nm by the selective etching of cylindrical PMMA microdomains hexagonally packed and aligned perpendicular to SiO₂ layer with 20 and 38 nm of diameter and periodicity, respectively. Both mobility and On/Off ratio were significantly reduced by more than 3 orders of magnitudes with the film roughness in OTFTs having 60 nm thick pentacene active layer. The poor device performance observed with the etched thin film of block copolymer dielectric is attributed to a defective pentacene active layer and the mixed crystalline structure consisting of thin film and bulk phase arising from the massive nucleation of pentacene preferentially at the edge of each cylindrical etched hole.     

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.     

Effect of active layer thickness on environmental stability of printed thin-film transistor

We have studied the effect of active layer thickness on the performance and environmental stability of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) thin-film transistor. The organic thin-film transistors (OTFTs) were fabricated by inkjet printing using a solution based TIPS pentacene. To get thick organic semiconductor, the surface of gate insulator was treated with n-octyltrichlorosilane (OTS-C₈) before jetting. The on-currents of the OTFT with ～1 μm active layer decreases a little in air, but the OTFT with 0.05 μm TIPS pentacene shows a significant degradation in drain currents.     

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.     

Organic thin film transistors with double insulator layers

We have investigated a double-layer structured gate dielectric for the organic thin films transistor (OTFT) with the purpose of improving the performance of the SiO₂ gate insulator. A 50 nm PMMA layer was coated on top of the SiO₂ gate insulator as organic insulator layer. The results demonstrated that using inorganic/organic compound insulator as the gate dielectric layers is an effective method to fabricate OTFTs with improved electric characteristics and decreased leakage current. Electrical parameters such as carrier mobility and on/off ratio by field effect measurement have been calculated. OTFT based on highly doped Si substrate with a field-effect mobility of 0.004 cm²/V s and on/off ratio of 10⁴ have been obtained.     

Hybridization of a low-temperature processable polyimide gate insulator for high performance pentacene thin-film transistors

We have synthesized a novel fully soluble and low-temperature processable polyimide gate insulator (KSPI) through the one-step condensation polymerization of the monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride and 4,4-diaminodiphenylmethane. Fully imidized KSPI was found to be completely soluble in organic solvents such as N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), γ-butyrolactone, dimethylsulfoxide (DMSO), and 2-butoxyethanol. Thin films of KSPI can be fabricated at only 150 °C and a pentacene OTFT with KSPI as a gate dielectric was found to exhibit a field effect mobility of 0.22 cm²/V s. To obtain a high performance organic thin-film transistor (OTFT), the KSPI surface was modified in our new technique by hybridization with a non-polar side chain containing a polyimide insulator (PI). The carrier mobility of a pentacene OTFT with a hybridized polyimide gate insulator (BPI-3) was found to be 0.92 cm²/V s. Our new low-temperature processable polyimides show promise as gate dielectrics for OTFTs.     

Fabrication of the flexible pentacene thin-film transistors on 304 and 430 stainless steel (SS) substrate

Flexible organic thin-film transistors (OTFT) were fabricated on 304 and 430 stainless steel (SS) substrate with aluminum oxide as a gate insulator and pentacene as an organic semiconductor. Chemical mechanical polishing (CMP) process was used to study the effect of the SS roughens on the dielectric properties of the gate insulator and OTFT characteristics. The surface roughness was decreased from 33.8 nm for 304 SS and 19.5 nm for 430 SS down to ～2.5 nm. The leakage current of the metal–insulator–metal (MIM) structure (Au/Al₂O₃/SS) was reduced with polishing. Mobility and on/off ratio of pentacene TFT with bare SS showed a wide range of values between 0.005 and 0.36 cm²/Vs and between 10³ and 10⁵ depending on the location in the substrate. Pentacene TFTs on polished SS showed an improved performance with a mobility of 0.24–0.42 cm²/Vs regardless of the location in the substrate and on/off ratio of ～10⁵. With self assembled monolayer formation of octadecyltrichlorosilane (OTS) on insulator surface, mobility and on/off ratio of pentacene TFT on polished SS was improved up to 0.85cm²/Vs and ～10⁶. I–V characteristics of pentacene TFT with OTS treated Al₂O₃/304 SS was also obtained in the bent state with a bending diameter (D) of 24, 45 or 70 mm and it was confirmed that the device performed well both in the linear regime and the saturation regime.     

Organic–inorganic hybrid materials as solution processible gate insulator for organic thin film transistors

We synthesized organic–inorganic hybrid materials (hybrimers) using a simple non-hydrolytic sol–gel reaction and applied the materials as gate insulators in organic thin film transistors (OTFTs). The hybrimer thin films had smooth and hydrophobic surfaces, and were stable with solvents. In addition, the hybrimer thin films had good electrical properties such as low leakage current and high dielectric strength. The performance of the OTFT with hybrimer gate insulator fabricated by drop casting of regioregular poly(3-hexylthiophene) (P3HT) was similar to that of OTFT with hexamethyldisilazane (HMDS) treated thermally grown SiO₂. The hysteresis of RR-P3HT based OTFT with hybrimer gate insulator was negligible.     

High air stability of threshold voltage on gate bias stress in pentacene TFTs with a hydroxyl-free and amorphous fluoropolymer as gate insulators

We investigated the air stabilities of threshold voltages (V_th) on gate bias stress in pentacene thin-film transistors (TFTs) with a hydroxyl-free and amorphous fluoropolymer as gate insulators. The 40-nm-thick thin films of spin-coated fluoropolymer had excellent electrical insulating properties, and the pentacene TFTs exhibited negligible current hysteresis, low leakage current, a field-effect mobility of 0.45 cm²/Vs and an on/off current ratio of 3 × 10⁷ when it was operated at −20 V in ambient air. After a gate bias stress of 10⁴ s, a small V_th shift below 1.1 V was obtained despite non-passivation of the pentacene layer. We have discussed that the excellent air stability of V_th was attributed to the insulator surface without hydroxyl groups.     

Tunable Crystal Nanostructures of Pentacene Thin Films on Gate Dielectrics Possessing Surface‐Order Control

To enhance the electrical performance of pentacene‐based field‐effect transistors (FETs) by tuning the surface‐induced ordering of pentacene crystals, we controlled the physical interactions at the semiconductor/gate dielectric (SiO₂) interface by inserting a hydrophobic self‐assembled monolayer (SAM, CH₃‐terminal) of organoalkyl‐silanes with an alkyl chain length of C8, C12, C16, or C18, as a complementary interlayer. We found that, depending on the physical structure of the dielectric surfaces, which was found to depend on the alkyl chain length of the SAM (ordered for C18 and disordered for C8), the pentacene nano‐layers in contact with the SAM could adopt two competing crystalline phases—a “thin‐film phase” and “bulk phase” – which affected the π‐conjugated nanostructures in the ultrathin and subsequently thick films. The field‐effect mobilities of the FET devices varied by more than a factor of 3 depending on the alkyl chain length of the SAM, reaching values as high as 0.6 cm² V^?1 s^?1 for the disordered SAM‐treated SiO₂ gate‐dielectric. This remarkable change in device performance can be explained by the production of well π‐conjugated and large crystal grains in the pentacene nanolayers formed on a disordered SAM surface. The enhanced electrical properties observed for systems with disordered SAMs can be attributed to the surfaces of these SAMs having fewer nucleation sites and a higher lateral diffusion rate of the first seeding pentacene molecules on the dielectric surfaces, due to the disordered and more mobile surface state of the short alkyl SAM.     

Performance comparison of pentacene organic field-effect transistors with SiO2 modified with octyltrichlorosilane or octadecyltrichlorosilane

Performance of pentacene organic field-effect transistors (OFETs) is significantly improved by treatment of SiO₂ with octyltrichlorosilane (OTS-8) compared to octadecyltrichlorosilane (OTS-18). The average hole mobility in these OFETs is increased from 0.4 to 0.8 cm²/Vs when treating the dielectric with OTS-8 versus OTS-18 treated devices. The atomic force microscope (AFM) images show that the OTS-8 treated surface produces much larger grains of pentacene (∼500 nm) compared to OTS-18 (∼100 nm). X-ray diffraction (XRD) results confirmed that the pentacene on OTS-8 is more crystalline compared to the pentacene on OTS-18, resulting in higher hole mobility.     

Effect of post-fabrication thermal annealing on Fermi-level pinning phenomenon in metal-pentacene junctions

In this work, we investigated the material properties of pentacene films with XRD and AFM analyses as it were annealed from 25 to 150 °C in N₂ ambient. Electrical characterization of the films was also performed by transfer length method (TLM) at each temperature. These results were then correlated with the Fermi level pinning phenomenon in Al- and Ti-pentacene junctions. For both junctions, it was found that as the surface quality of the pentacene films changed with increase in annealing temperature, the hole-barrier heights (h-BH) that were modulated by Fermi level pinning were effectively reduced.     

Influence of La substitution on the electrical properties of metal-ferroelectric (BiFeO3)-insulator (CeO2)-semiconductor nonvolatile memory structures

Metal-multiferroic (La-substituted BiFeO₃)-insulator (CeO₂)-semiconductor (MFIS) capacitors has been fabricated. The crystalline phase and amount of La³⁺ substitution at Bi-site were investigated by XRD and XPS in the postannealing temperature range from 500 to 700 °C, respectively. The microstructure and interfacial layer between CeO₂ and Si substrate were characterized by HRTEM. The memory windows as functions of insulator film thickness and DC power for La were measured. The maximum memory window is about 1.9 V under ±6 V applied voltage. The ferroelectric polarization increases with increasing substitution amount. The morphologies of La-substituted BiFeO₃ films were also studied by AFM.     

Facile synthetic route to implement a fully bendable organic metal–insulator–semiconductor device on polyimide sheet

Triblock copolymer surfactant, HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H (i.e. P123)-based nanocrystalline (nc)-TiO₂ thin film had been synthesized on organic flexible polyimide (PI) sheet for their application in organic metal–insulator–semiconductor (MIS) device. The nc-TiO₂ film over PI was successfully deposited for the first time by a systematic solution proceeds dip-coating method and by the assistance of triblock copolymer surfactant. The effect of annealing temperature (270 °C, 5 h) on the texture, morphology and time-induced hydrophilicity was studied by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and contact angle system, respectively, to examine the chemical composition of the film and the contact angle. The surface morphology of the semiconducting layer of organic pentacene was also investigated by using AFM and XRD, and confirmed that continuous crystalline film growth had occurred on the nc-TiO₂ surface over flexible PI sheet. The semiconductor–dielectric interface of pentacene and nc-TiO₂ films was characterized by current–voltage and capacitance–voltage measurements. This interface measurement in cross-link MIS structured device yielded a low leakage current density of 8.7 × 10^?12 A cm^?2 at 0 to ?5 V, maximum capacitance of 102.3 pF at 1 MHz and estimated dielectric constant value of 28.8. Furthermore, assessment of quality study of nc-TiO₂ film in real-life flexibility tests for different types of bending settings with high durability (c.a. 30 days) demonstrated a better comprehension of dielectric properties over flexible PI sheet. We expected them to have a keen interest in the scientific study, which could be an alternate opportunity to the excellent dielectric–semiconductor interface at economic and low temperature processing for large-area flexible field-effect transistors and sensors.     

Pentacene organic field-effect transistors with polymeric dielectric interfaces: Performance and stability

Low-voltage pentacene organic field-effect transistors (OFETs) with different gate dielectric interfaces are studied and their performance in terms of electrical properties and operational stability is compared. Overall high electrical performance is demonstrated at low voltage by using a 100 nm-thick high-κ gate dielectric layer of aluminum oxide (Al₂O₃) fabricated by atomic layer deposition (ALD) and modified with hydroxyl-free low-κ polymers like polystyrene (PS), divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) (Cyclotene™, Dow Chemicals), and as well as with the widely used octadecyl-trichlorosilane (OTS). Devices with PS and BCB dielectric surfaces exhibit almost similar electrical performance with high field-effect mobilities, low subthreshold voltages, and high on/off current ratios. The higher mobility in pentacene transistors with PS can be correlated to the better structural ordering of pentacene films, as demonstrated by atomic force microscopy (AFM) images and X-ray diffraction (XRD). The devices with PS show good electrical stability under bias stress conditions (V_GS = V_DS = −10 V for 1 h), resulting in a negligible drop (2%) in saturation current (I_DS) in comparison to that in devices with OTS (12%), and to a very high decay (30%) for the devices with BCB.     

XRD and EXAFS studies of crystallisation in films

This paper reports a detailed structural study on the nucleation of t-HfO₂ nanocrystals in thin films of 70SiO₂–30HfO₂ prepared by sol–gel route on v-SiO₂ substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 °C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L₃-edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO₂ nanocrystals in the tetragonal phase after heat treatment at 1000 °C for 30 min, and a partial phase transformation to the monoclinic phase (m-HfO₂) starts after heat treatment at 1200 °C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 °C for 24 h are present in mixed phases.     

Pentacene organic thin-film transistors with solution-based gelatin dielectric

Gelatin is a natural protein in the field of food, pharmaceutical and tissue engineering, which works very well as the gate dielectric for pentacene organic thin-film transistors (OTFTs). An aqueous solution process has been applied to form a gelatin thin film on poly(ethylene terephthalate) (PET) or glass by spin-coating and subsequent casting. The device performance of pentacene OTFTs depend on the bloom number (molecular weight) of gelatin. The pentacene OTFT with 300 bloom gelatin as the gate dielectric in air ambient exhibits the best performance with an average field-effect mobility (μ_FE) value of ca. 16 cm² V^?1 s^?1 in the saturation regime and a low threshold voltage of ?1 V. The high performance of the pentacene OTFT in air ambient is attributed to the water resided in gelatin. The crystal quality of pentacene is not the key factor for the high performance.     

Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition

An electrostatic spray deposition (ESD) method was applied to prepare both crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and insulating films of poly(methyl methacrylate) (PMMA) for fabricating top-gate single-crystal organic field-effect transistors (OFETs). The electrical characteristics of the top-gate device were compared to those of the bottom-gate one (SiO₂ bottom-gate insulator) with the same active layer, and the lower charge-trap density at the interface between the top-gate insulator and single-crystalline active layer was demonstrated. The drain current compression in the output characteristics of the top-gate device, however, occurred due to the large parasitic resistance between the source/drain electrodes and accumulation channel. Reducing the thickness of the single-crystalline active layer resulted in a high charge-carrier mobility of 0.29 cm²/V s (channel length of 5 μm).     

并五苯有机薄膜晶体管电学性能研究

制作了以并五苯为半导体有源层材料的有机薄膜晶体管。用热氧化的方法制备了一层230nm的二氧化硅栅绝缘层并用原子力显微镜(AFM)分析了表面形貌。研究了器件的电学性能,得到的并五苯有机薄膜晶体管器件载流子迁移率为8.9×10-3cm2/V.s,器件的阈值电压和开关电流比分别为-8.2V和1.0×104。