Improved pentacene growth continuity for enhancing the performance of pentacene-based organic thin-film transistors

This study proposes an alternative planar bottom-contact (pBC) structure to enhance the electrical performance of pentacene-based organic thin-film transistors (OTFTs). This pBC structure uses a bilayer dielectric to control planarization with a precise etch depth and introduces a bilayer photoresist lift-off method to ensure that planarization produces an optimum flatness. Because of the improved growth continuity of pentacene near the edge of the source/drain electrodes, the contact resistance between the source/drain and the pentacene was reduced significantly, thereby enhancing the electrical performance of OTFTs. The mechanism for the enhanced performance was also verified by a physics-based numerical simulation.     

Teflon/SiO2 bilayer passivation for improving the electrical reliability of pentacene-based organic thin-film transistors

We demonstrate a bilayer passivation method using a Teflon and SiO₂ combination layer to improve the electrical reliability of pentacene-based organic thin-film transistors (OTFTs). The Teflon was deposited as a buffer layer using a thermal evaporator that exhibited good compatibility with the underlying pentacene channel layer, and can effectively protect the OTFTs from plasma damage during the SiO₂ deposition process, resulting in a negligible initial performance drop in OTFTs. Furthermore, because of the excellent moisture barrier ability of SiO₂, the OTFTs exhibited good time-dependent electrical performance, even after 168 h of aging in ambient air with 60–80% relative humidity.     

Environmental effects on the electrical behavior of pentacene thin-film transistors with a poly(methyl methacrylate) gate insulator

The electrical properties of top-contact pentacene thin-film transistors (TFTs) with a poly(methyl methacrylate) (PMMA) gate dielectric were analyzed in air and vacuum environments. Compared to the vacuum case, the pentacene TFT in air exhibited lower drain currents and more pronounced shifts in the threshold voltage upon reversal of the gate voltage sweep direction, together with a decrease in the field-effect mobility. These characteristic variations were explained in terms of two distinctive actions of polar H₂O molecules in pentacene TFT. H₂O molecules were suggested to diffuse under the source and drain contacts and interrupt the charge injection into the pentacene film, whereas those that permeate at the pentacene/PMMA interface retard hole depletion in and around the TFT channel. The diffusion process was much slower than the permeation process. The degraded TFT characteristics in air could be recovered mostly by storing the device under vacuum, which suggests that the air instability of TFTs is due mainly to the physical adsorption of H₂O molecules within the pentacene film.     

Extended lifetime of pentacene thin-film transistor with polyvinyl alcohol (PVA)/layered silicate nanocomposite passivation layer

We prepared a novel organic passivation material to protect organic thin-film transistors (OTFTs) from H₂O and O₂ using a polyvinyl alcohol (PVA)/hydrophilic layered silicate (HLS) nanocomposite system. Using up to a 3 wt% layered silicate to PVA weight concentration, a highly homogeneous nanocomposite solution was prepared. In addition, a PVA/HLS nanocomposite solution formed a smooth film layer by the spin-coating method with RMS surface roughness of about 1 nm. An OTFT device with PVA passivation showed a decrease of 23% in field effect mobility after passivation. However, a pentacene TFT device with PVA/HLS nanocomposite passivation showed no significant initial performance drop after passivation, and mobility was even slightly improved. Pentacene TFT-passivated PVA/HLS showed 2860 h of TFT lifetime (the time required to reduce the mobility by one-half of the initial mobility after passivation), which is almost twice the lifetime of pentacene TFT with PVA passivation (1320 h). We propose that a layered silicate containing PVA nanocomposite film can be used as an effective organic passivation layer in an OTFT.     

Two-step surface modification for bottom-contact structured pentacene thin-film transistors

We investigated surface treatment effects of hexamethyldisilazane (HMDS), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and l-cysteine on gold source/drain electrodes in bottom-contact structured pentacene thin-film transistors (TFTs). The treatment methods include spin coating and immersing. We have also researched on two-step treatment based on the combination of each treatment methods. The highest device performance was achieved by treating gold S/D electrodes with l-cysteine first and PEDOT:PSS afterwards, showing field effect mobility up to 0.35 cm²/V·s. l-cysteine can reduce the contact resistance between metal and semiconductor layer, and PEDOT:PSS acted as a hole transporting layer while HMDS decreased the surface energy, which enlarged the grain size of pentacene on it.     

Patchable thin-film strain gauges based on pentacene transistors

We present a patchable thin-film strain gauge for which output current responds sensitively to external strain. For this work, integrated organic thin-film transistors using pentacene as an active component were fabricated on a freestanding polyurethaneacrylate film with high flexibility and adhesive properties providing patchability. The device can be easily mounted onto non-flat surfaces, and the output characteristics show a strong correlation with the structural strain of freestanding polymeric film, which allows the external strain applied to the device to be gauged. In addition, a surface shape can be detected after mounting the device onto a non-flat surface, and the thickness of a complex structure can be inversely calculated using a calibration curve. It is anticipated that these results will be applied to the development of various patchable sensors and thickness measurement systems, which can lead to further applications.     

Organic thin-film field-effect transistors with MoO3/Al electrode and OTS/SiO2 bilayer gate insulator

An organic thin-film transistor (OTFTs) having OTS/SiO₂ bilayer gate insulator and MoO₃/Al electrode configuration between gate insulator and source–drain (S–D) electrodes has been investigated. Thermally grown SiO₂ layer is used as the OTFT gate dielectric and copper phthalocyanine (CuPc) for an active layer. We have found that using silane coupling agents, octadecyltrichlorosilane (OTS) on SiO₂, surface energy of SiO₂ gate dielectric is reduced; consequently, the device performance has been improved significantly. This OTS/SiO₂ bilayer gate insulator configuration increases the field-effect mobility, reduces the threshold voltage and improves the on/off ratios simultaneously. The device with MoO₃/Al electrode has similar source–drain current (I_DS) compared to the device with Au electrode at same gate voltage. Our results indicate that using double-layer of insulator and modified electrode is an effective way to improve OTFT performance.     

High performance tetrathienoacene-DDP based polymer thin-film transistors using a photo-patternable epoxy gate insulating layer

High performance organic thin-film transistors (OTFTs) are fabricated on an epoxy based photo-patternable organic gate insulating layer (p-OGI) using a top contact thin-film transistor configuration. This negative tone p-OGI material is composed of an epoxy type polymer resin, a polymeric epoxy cross-linker, and a sulfonium photoacid generator (PAG). Features from p-OGI can be precisely patterned down to ∼3 μm via i-line photolithography. In order to evaluate the potential of this epoxy type resin as a gate insulator, we evaluated the dielectric properties of the p-OGI and its gate insulating performance upon fabricating solution processed OTFTs using an organic semiconductor (OSC), namely tetrathienoacene-DPP copolymer (PTDPPTFT4). Results show that the PTDPPTFT4 based OTFTs with this p-OGI exhibit field-effect mobilities up to 1 cm² V⁻¹ s⁻¹, indicating the potential of high performance solution processed OTFT based on an epoxy based p-OGI/OSC system.     

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.     

Performance improvement mechanisms of pentacene-based organic thin-film transistors using TPD buffer layer

To improve the performances of pentacene-based organic thin-film transistors (OTFTs), a TPD buffer layer was inserted between the Au metal electrode and the pentacene channel layer. As shown by the ultraviolet photoelectron spectroscopy measurement, the Au work function was increased from 4.61 eV for Au in direct contact with pentacene to 4.74 eV and 4.78 eV for the sample inserted with 2-nm-thick and 3-nm-thick TPD buffer layers, respectively, between the Au metal electrode and the pentacene channel layer. Moreover, the contact resistance was reduced from 1 MΩ to 0.1 MΩ by inserting a 2-nm-thick TPD buffer layer. Compared with the transconductance of 2.67 × 10^?7 S, the field-effect mobility of 0.46 cm²/V s, and the substhreshold swing of 1.78 V/decade for the conventional pentacene-based OTFTs without TPD buffer layer, the transconductance, the field-effect mobility, and the subthreshold swing were improved to 9.77 × 10^?7 S, 1.68 cm²/V s, and 1.46 V/decade, respectively, for the pentacene-based OTFTs inserted with a 2-nm-thick TPD buffer layer. By considering the trade-off between the increase of Au work function and the tunneling effect, the optimal thickness of the TPD buffer layer in the pentacene-based OTFTs was 2 nm.     

A label-free biosensor based on organic transistors by using the interaction of mercapto DNA and gold electrodes

Organic thin-film transistors (OTFTs) with Au electrodes were successfully used as transducers for label-free deoxyribonucleic acid (DNA) sensors. Single-strand DNA (ssDNA), perfectly-matched double-strand DNA (dsDNA) and mis-matched DNA were immobilized on the surface of the source/drain electrodes of three OTFT devices respectively. The ssDNA molecules with mercapto group (–SH) can be well immobilized on the surface of Au electrode by chemical bond between –SH and Au atom. According to the significant difference in channel current, which was attributed to the changed contact resistances by introducing different DNA molecules on Au electrode, ssDNA, matched-dsDNA and mismatched-dsDNA were differentiated successfully in the experiments. The results may provide a promising approach for detecting DNA specificity and hybridization with label-free.     

Interfacial modifying layer-driven high-performance organic thin-film transistors and their nitrogen dioxide gas sensors

Organic thin-film transistors (OTFTs) based on bottom-gate bottom-contact configuration were fabricated by inserting two kinds of modifying layers at the interface of source/drain electrode and organic semiconductor, while nitrogen dioxide (NO₂) sensing capability was also evaluated based on the obtained OTFTs. Compared to OTFT without interfacial layer, the field-effect mobility (μ) was enhanced from 0.018 cm²/Vs to 0.15 cm²/Vs by incorporating with MoO_x interfacial layer. Moreover, when exposed to 30 ppm NO₂, the saturation current and μ of OTFT with MoO_x interfacial layer increase 22.7% and 26.7%, respectively, while in original OTFT, the values are only 3.0% and 3.7%, respectively. The mechanism of performance improvement of OTFT sensor was systematically studied by focusing on the interface of source/drain electrode and organic semiconductor. The reduced contact resistance leads to higher μ, meanwhile, pentacene morphology modulation on MoO_x contributes to better diffusion of NO₂ molecules. As a result, higher μ and more diffused gas molecules enhance the gas sensing property of the transistor.     

Ionic self-assembled monolayer for low contact resistance in inkjet-printed coplanar structure organic thin-film transistors

To reduce the contact resistance in inkjet-printed organic thin-film transistors (OTFTs), the use of a newly synthesized ionic self-assembled monolayer (SAM) consisting of an anchoring group, a linker group, and an ionic functional group, is investigated. According to the gated transmission line method (TLM) measurements of a series of OTFT devices, where one type has no charge injection layer, another type having a pentafluorobenzenethiol (PFBT) injection layer, and a third type containing a (6-mercaptohexyl)trimethylammonium bromide (MTAB) ionic SAM, the latter exhibits the lowest contact resistance value of ∼3.1 K Ω cm. The OTFTs without charge injection layer and with the PFBT SAM have relatively higher contact resistance values of ∼6.4 K Ω cm and ∼5.0 K Ω cm, respectively. The reduced contact resistance in the OTFTs with ionic SAMs is attributed to the large charge carrier density induced by the ionic SAM, which allows sufficient tunneling-assisted injection of the carriers from the metal electrode to the polymer semiconductor. These results suggest that the use of appropriate ionic SAM injection layer is an effective way to reduce the contact resistance, hence improving the charge transport characteristics of inkjet-printed OTFTs.     

Polyimide/polyvinyl alcohol bilayer gate insulator for low-voltage organic thin-film transistors

In this paper, we report the fabrication of a polyimide/polyvinyl alcohol (PVA) bilayer gate insulator for low-voltage organic thin-film transistors (TFTs). The introduction of a PVA layer to form a bilayer structure improves the dielectric and insulating properties of the gate insulator. Organic TFTs with 150 nm-thick polyimide and PVA gate insulators were inactive at low operation voltages below 5 V. Conversely, organic TFTs with 150 nm-thick polyimide/PVA bilayer gate insulators exhibited excellent device performances. Our results suggest that the introduction of a PVA layer with a high dielectric constant could be a simple and efficient way to improve the device performance of low-voltage organic TFTs.     

Crosslinked polymer-mixture gate insulator for high-performance organic thin-film transistors

In this paper, we report on the fabrication of a crosslinked polymer-mixture gate insulator for high-performance organic thin-film transistors (TFTs). We used cyanoethylated pullulan (CEP) as a crosslinkable high-k polymer matrix and poly(ethylene-alt-maleic anhydride) (PEMA) as a polymeric crosslinking agent. Because PEMA has a high number of functional groups reactive to the hydroxyl groups of CEP, the use of PEMA is effective for minimizing the amount of remaining hydroxyl groups strongly related to the large current hysteresis and high off current of the organic TFTs. To investigate the potential of the CEP-PEMA mixture as a gate insulator, we fabricated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) TFTs. The C₈-BTBT TFT with the 60 nm-thick CEP-PEMA gate insulator showed excellent TFT performance with a field-effect mobility of 1.4 cm²/V s and an on/off ratio of 2.4 × 10⁶.     

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.     

Silver nanowire-polymer composite electrode for high performance solution-processed thin-film transistors

Silver nanowires (AgNWs)/poly-(3,4-ethylenedioxythiophene/polystyrene sulphonate) (PEDOT:PSS) composite films as conductive electrode for OTFTs were prepared, and their optical and electrical properties were investigated. The conductive composite films used in this study afforded low sheet resistance of <140 Ω/sq and transmittance as high as 70% in the visible region. For the composite film with 0.1 wt.% of AgNWs, contact resistance as low as 2.7 × 10⁴ Ω cm was obtained, as examined by Transfer length model (TLM) analysis, and work function of the corresponding film was 5.0 eV. Furthermore, the composite films were employed as source and drain electrodes for top-gate/bottom-contact organic thin-film transistors (OTFTs) based on solution-processed 5,11-bistriethylsilylethynyl anthradithiophene (TES-ADT) as organic semiconductor, and the resulting device showed high electrical performance with carrier mobility as high as 0.21 cm²/V s.     

Effects of film microstructure on the bias stability of pentacene field-effect transistors

In this report, the effects of film microstructure on the bias stability of pentacene field-effect transistors (FETs) were investigated. To control the microstructure of pentacene film, substrate temperature was changed from 25 to 90 °C during pentacene deposition. As the substrate temperature increased, pentacene grain size increased (or grain boundary (GB) decreased) because of the elevated surface diffusion of pentacene molecules. Accordingly, field-effect mobility increased up to 1.52 cm²/V. In contrast, bias stability showed totally different characteristics: samples prepared at high substrate temperatures exhibited the lowest degree of bias stability. This GB independent charge trapping phenomenon was solved by examining molecular scale ordering within the intragrain regions. The pentacene film grown at 90 °C showed the largest percentage of pentacene molecules with bulk crystalline structures. This inhomogeneity in the pentacene microstructure induces crystal mismatch within intragrain region, thereby providing deep trap sites for gate-bias stress driven instability. Our study shows that GB is not the main sites for bias stress related charge trapping, rather the molecular orientation within intragrain region is responsible for the charge trapping events. In this regard, the control of molecular scale ordering is important to obtain OFETs with a high bias stability.     

A tunable organic inverter based on groove patterned pentacene thin film transistors using soft-contact lamination

Organic zero drive load inverters based on pentacene thin film transistors with periodic groove patterned dielectrics are fabricated using nanoimprinting and soft-contact lamination methods. Depletion mode transistor behavior is achieved when the current direction is parallel to groove direction and enhancement mode transistor behavior is achieved when the directions are crossed. An organic inverter is created after connecting two soft-contact laminated transistors. The electrical performance of the drive transistor can be varied and the organic inverter is tunable. This is done by utilizing a PDMS stamp with the source-drain electrode and changing the angle between the current direction and groove direction. The gain and symmetry of the VTC is improved by using an enhancement mode transistor where the source-drain electrode formed by thermal evaporation instead of being a soft contact-laminated device.