Microstructure transformations induced by modified-layers on pentacene polymorphic films and their effect on performance of organic thin-film transistor

Phenyltrimethoxysilane was used to modify SiO₂ insulator and significantly enhanced the pentacene based organic thin-film transistors (OTFTs). The crystal structure, surface morphology, molecular structure and microstructure of pentacene polymorphic films with and without the modifications were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and contact angle meter. XRD studies reveal a decreased tilt angle (θ_T) of pentacene molecules from c-axis toward a-axis, indicating that polymorphs transformation from the “triclinic bulk” phase to the “thin film” phase and orthorhombic phase occurs. AFM images show that the surface roughness of gate insulators has no influence on performance of the pentacene based OTFT. These results provide strong evidence that the performance improvement of OTFT after PhTMS modification of SiO₂ insulator surface is related to the microstructure transformation of the semiconductor. It suggests that the modified-layer may alter the molecular geometry and further induce structural phase transitions in the pentacene films for the performance improvement.     

An Organic Thin‐Film Transistor with a Photoinitiator‐Free Photosensitive Polyimide as Gate Insulator

This investigation deals with the synthesis and detailed study of a photoinitiator‐free photosensitive polyimide gate insulator for organic thin‐film transistors (OTFTs), one of the most important components of active‐matrix displays on plastic substrates. The photosensitive polyimide precursor poly(amic acid) is prepared from the aromatic dianhydride 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) and the novel aromatic diamine 7‐(3,5‐diaminobenzoyloxy)coumarine (DACM). The photosensitivity of the poly(amic acid) film is investigated using a high‐pressure mercury lamp at 280–310 nm. The pattern resolution of the photocured film was about 50 μm. The surface morphology of the films before and after the photopatterning process is also investigated. In addition, we have fabricated pentacene OTFTs with the photoinitiator‐free photosensitive polyimide as gate insulator. The OTFT characteristics are discussed in more detail with respect to the electrical properties of the photosensitive polyimide thin film.     

Flexible AM OLED panel driven by bottom-contact OTFTs

Active matrix organic-light-emitting-diode (AM OLED) panels, driven by organic thin-film transistors (OTFT), have been successfully fabricated on a flexible plastic substrate. The pixel circuit consists of two bottom-contact pentacene OTFTs working as switching and driving transistors. The panel has 16 /spl times/ 16 pixels, each of which have an OLED using a phosphorescent material with an emission efficiency of 30 cd/A. A tantalum oxide (Ta/sub 2/O/sub 5/) film with a dielectric constant of 24, prepared by the anodization of Tantalum (Ta), was used as the gate insulator of the OTFTs. The passivation layer on the OTFTs was formed by a layer of silicon dioxide (SiO/sub 2/) and two layers of polyvinyl alcohol. Using OTFTs with a Ta/sub 2/O/sub 5/ gate insulator, the authors have realized a flexible active matrix OLED panel driven with a low voltage of -12 V.     

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.     

Environmental and operational stability of solution-processed 6,13-bis(triisopropyl-silylethynyl) pentacene thin film transistors

We report operational and environmental stability of solution-processed organic thin film transistors (OTFTs) using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene). Typical drop-cast TIPS-pentacene OTFTs show strong molecular ordering and relatively stable characteristics with air and illumination compared to vapor-deposited pentacene OTFTs. For short channel length devices, TIPS-pentacene OTFTs undergo significant degradation with bias-stress, possibly due to operation at large drive currents and large local power dissipation as well as built-in charges in the channel area.     

The Role of OTS Density on Pentacene and C60 Nucleation,Thin Film Growth,and Transistor Performance

In organic thin film transistors (OTFTs), charge transport occurs in the first few monolayers of the semiconductor near the semiconductor/dielectric interface. Previous work has investigated the roles of dielectric surface energy, roughness, and chemical functionality on performance. However, large discrepancies in performance, even with apparently identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. Here, a crystalline, dense octadecylsilane (OTS) surface modification layer is found that promotes two‐dimensional semiconductor growth. Higher mobility is consistently achieved for films deposited on crystalline OTS compared to on disordered OTS, with mobilities as high as 5.3 and 2.3 cm² V^?1 s^?1 for C₆₀ and pentacene, respectively. This is a significant step toward morphological control of organic semiconductors which is directly linked to their thin film charge carrier transport.     

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.     

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⁺.     

A novel method for soluble hydrophilic anthracene derivative self-assembled on APTES in air

A hydrophilic organic semiconductor, anthracene derivative (ATC) was self-assembled on hydrophilic SAM 3-aminopropyltriethoxysilane (APTES) by solution process in air. The APTES self-assembled on the insulator surface was found to help to align the packing of ATC thus improve the crystallinity of ATC thin film. The first attempt to fabricate organic thin film transistors (OTFTs) using self-assembled ATC via this approach was made with and the performance of OTFTs was also measured in ambient condition.     

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.     

Selective Nucleation of Organic Single Crystals from Vapor Phase on Nanoscopically Rough Surfaces

Organic field‐effect transistors (OFETs) are attractive for microelectronic applications such as sensor arrays or flexible displays, due to their adequate performance and relatively low production costs. Organic single‐crystal transistors have emerged as benchmark devices for studying the intrinsic charge‐transport properties in organic semiconductor materials. Conventional approaches for growing organic single crystals result in uncontrollable dimensions and the formation of extremely fragile crystals. In addition, the hand‐selection and placement of individual crystals on a device structure represents a severe limitation for producing arrays of single‐crystal transistors with high density and reasonable throughput. As a result, the application of organic single‐crystal transistors has been restricted to fundamental charge transport studies, with their commercial application not yet realizable. We recently reported a materials‐general method of fabricating large‐area arrays of patterned organic single crystals. Microcontact‐printed octadecyltriethoxysilane (OTS) film domains on smooth, inert substrates were found to act as preferential nucleation sites for single crystals for a broad range of organic semiconductor materials, such as pentacene, tetracene, rubrene and C₆₀. In order to understand the underlying mechanism of preferential nucleation, the stamped OTS domains and the contact plane between the OTS domains and the organic crystals were inspected by atomic force microscopy (AFM) and optical microscopy. Our analysis suggests that crystals nucleate at the base of tall OTS pillars that form the significantly rough surface in the stamped domains. The selective nucleation inside the rough surface regions is discussed by means of a rate‐equation model of the growth process.     

Simultaneous Modification of Bottom‐Contact Electrode and Dielectric Surfaces for Organic Thin‐Film Transistors Through Single‐Component Spin‐Cast Monolayers

An efficient process is developed by spin‐coating a single‐component, self‐assembled monolayer (SAM) to simultaneously modify the bottom‐contact electrode and dielectric surfaces of organic thin‐film transistors (OTFTs). This effi cient interface modifi cation is achieved using n‐alkyl phosphonic acid based SAMs to prime silver bottom‐contacts and hafnium oxide (HfO₂) dielectrics in low‐voltage OTFTs. Surface characterization using near edge X‐ray absorption fi ne structure (NEXAFS) spectroscopy, X‐ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry suggest this process yields structurally well‐defi ned phosphonate SAMs on both metal and oxide surfaces. Rational selection of the alkyl length of the SAM leads to greatly enhanced performance for both n‐channel (C₆₀) and p‐channel (pentacene) based OTFTs. Specifi cally, SAMs of n‐octylphos‐phonic acid (OPA) provide both low‐contact resistance at the bottom‐contact electrodes and excellent interfacial properties for compact semiconductor grain growth with high carrier mobilities. OTFTs based on OPA modifi ed silver electrode/HfO₂ dielectric bottom‐contact structures can be operated using < 3V with low contact resistance (down to 700 Ohm‐cm), low subthreshold swing (as low as 75 mV dec^?1), high on/off current ratios of 107, and charge carrier mobilities as high as 4.6 and 0.8 cm² V^?1 s^?1, for C60 and pentacene, respectively. These results demonstrate that this is a simple and efficient process for improving the performance of bottom‐contact OTFTs.     

Organic Thin‐Film Transistors: Simultaneous Modification of Bottom‐Contact Electrode and Dielectric Surfaces for Organic Thin‐Film Transistors Through Single‐Component Spin‐Cast Monolayers (Adv. Funct. Mater. 8/2011)

An efficient process is developed by spin‐coating a single‐component, self‐assembled monolayer (SAM) to simultaneously modify the bottom‐contact electrode and dielectric surfaces of organic thin‐film transistors (OTFTs). This effi cient interface modifi cation is achieved using n‐alkyl phosphonic acid based SAMs to prime silver bottom‐contacts and hafnium oxide (HfO₂) dielectrics in low‐voltage OTFTs. Surface characterization using near edge X‐ray absorption fi ne structure (NEXAFS) spectroscopy, X‐ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry suggest this process yields structurally well‐defi ned phosphonate SAMs on both metal and oxide surfaces. Rational selection of the alkyl length of the SAM leads to greatly enhanced performance for both n‐channel (C₆₀) and p‐channel (pentacene) based OTFTs. Specifi cally, SAMs of n‐octylphos‐phonic acid (OPA) provide both low‐contact resistance at the bottom‐contact electrodes and excellent interfacial properties for compact semiconductor grain growth with high carrier mobilities. OTFTs based on OPA modifi ed silver electrode/HfO₂ dielectric bottom‐contact structures can be operated using < 3V with low contact resistance (down to 700 Ohm‐cm), low subthreshold swing (as low as 75 mV dec^?1), high on/off current ratios of 107, and charge carrier mobilities as high as 4.6 and 0.8 cm² V^?1 s^?1, for C60 and pentacene, respectively. These results demonstrate that this is a simple and efficient process for improving the performance of bottom‐contact 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.     

High‐Resolution Characterization of Pentacene/Polyaniline Interfaces in Thin‐Film Transistors

We present the first detailed report that directly correlates the reduced contact resistance in organic thin‐film transistors (TFTs) with fundamental structural and morphological characterization at the organic semiconductor/conducting polymer interface. Specifically, the pentacene grains are similar in size and continuous across the channel/electrode interface in bottom‐contact TFTs with polyaniline (PANI) electrodes. On a molecular level, the fused rings of pentacene are oriented perpendicular to the surface both in the channel and on PANI. Accordingly, the contact resistance is small in such devices. In TFTs with gold electrodes, however, the pentacene grains are different in size and are discontinuous across the channel/electrode interface. Further, the fused rings of pentacene are oriented perpendicular to the channel surface and parallel to the gold surface. Such differences across the channel/electrode interface lead to structural and electronic disorder, which manifests itself as a significantly larger contact resistance in devices with gold electrodes.     

A label-free,organic transistor-based biosensor by introducing electric bias during DNA immobilization

We report an improved label-free DNA sensor based on pentacene organic thin-film transistors (OTFTs). OTFT with top-contact structure was first fabricated by using pentacene as the active layer. Different electric biases were introduced between the source and gate contacts of OTFT during the single-stranded DNA (ssDNA) immobilization process in order to improve the immobilization efficiency of DNA molecules on the pentacene substrate. Atomic force microscopy (AFM) images show the application of positive bias can significantly improve the amount of the immobilized ssDNA. The potentiostatic effect of bias can induce more ssDNA molecules onto the pentacene surface, which leads to the improvement of sensor sensitivity reflected by the electric signals of OTFTs. Furthermore, an optimized immobilization time of 30 min was obtained at a constant bias that exhibits the highest immobilization efficiency. With this design, the optimized process conditions (+50 V bias and an immobilization of 30 min) for the ssDNA immobilization on the pentacene film were also obtained. As a result, the introduction of bias during immobilizing ssDNA molecules has been proposed to improve the immobilized efficiency, which provides an effective path to enhance the sensitivity of OTFT-based DNA sensors.     

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.     

Improvement of mechanical and electrical stabilities of flexible organic thin film transistor by using adhesive organic interlayer

A newly synthesized organic material, oligo(3-methylsulfanylthiophene), was applied as an adhesive interlayer between the gold source/drain electrodes and the pentacene semiconductor layer in flexible, bottom-gated, organic thin film transistors (OTFTs). The cyclic bending tests showed that the electrical properties of the devices with the thermally evaporated interlayer were more stable than those of the device with no interlayer. The interlayer also reduced the contact resistance between Au and the pentacene layer. These results indicate that the interlayer is very useful in enhancing the mechanical and electrical stabilities of the OTFTs under repetitive mechanical bending as well as the electrical performance.     

易于集成的有机薄膜场效应晶体管的制备

用有机半导体并五苯作为有源层,聚四氟乙烯作为绝缘层,采用全蒸镀方式在真空室一次性制备了正装结构的有机薄膜场效应晶体管(OTFT)。薄的有机绝缘层使得器件工作在低电压下,有机薄膜场效应晶体管易于与显示像素(有机发光二极管(OLED))集成在同一个透明的刚性或者柔性衬底上。研究了有机薄膜场效应晶体管的源漏接触电阻和沟道电阻对器件性能的影响,结果表明接触电阻是影响器件性能的主要因素。在透明的玻璃衬底上实现了有机薄膜场效应晶体管对同一衬底上100μm×200μm红色有机发光二极管的驱动。     

Self‐Assembled,Millimeter‐Sized TIPS‐Pentacene Spherulites Grown on Partially Crosslinked Polymer Gate Dielectric

Here, a highly crystalline and self‐assembled 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pentacene) thin films formed by simple spin‐coating for the fabrication of high‐performance solution‐processed organic field‐effect transistors (OFETs) are reported. Rather than using semiconducting organic small‐molecule–insulating polymer blends for an active layer of an organic transistor, TIPS‐Pentacene organic semiconductor is separately self‐assembled on partially crosslinked poly‐4‐vinylphenol:poly(melamine‐co‐formaldehyde) (PVP:PMF) gate dielectric, which results in a vertically segregated semiconductor‐dielectric film with millimeter‐sized spherulite‐crystalline morphology of TIPS‐Pentacene. The structural and electrical properties of TIPS‐Pentacene/PVP:PMF films have been studied using a combination of polarized optical microscopy, atomic force microscopy, 2D‐grazing incidence wide‐angle X‐ray scattering, and secondary ion mass spectrometry. It is finally demonstrated a high‐performance OFETs with a maximum hole mobility of 3.40 cm² V^?1 s^?1 which is, to the best of our knowledge, one of the highest mobility values for TIPS‐Pentacene OFETs fabricated using a conventional solution process. It is expected that this new deposition method would be applicable to other small molecular semiconductor–curable polymer gate dielectric systems for high‐performance organic electronic applications.