Rapid UV-A photo detection using a BTBT organic thin-film transistor enhanced by a 1,5-dichloro-9,10-dintiro-anthracene acceptor

A UV-A sensitive phototransistor was demonstrated using an organic semiconductor 2,7-dipentyl[1]benzothieno[3,2- b ][1] benzothiophene (C5-BTBT) and a strong electron acceptor 1,5-dichloro-9,10-dintiro-anthracene (2Cl-2NO₂-Anth) which is not photo-isomerizable. At 0 gate bias the photoresponsivity of the device begins to saturate at an incident power P_inc = 1 mW/cm², where a photocurrent-to-dark current ratio (P) of P > 10⁵ is observed with a photoresponsivity of 9 A/W. The photoresponsivity was increased with the decrease of P_inc, reaching 40 A/W at a P_inc = 0.0427 mW/cm². A persistent photocurrent with a P > 10⁵ was observed for more than 2 h, demonstrating the potential use for rewritable photo memory. By applying a gate voltage program consisting of a reset pulse of −90 V every 2nd data point, the device can perform as a UV sensor or switch at the timescale of 600 ms.     

A diketopyrrolopyrrole-based regular terpolymer bearing two different π-extended donor units and its application in solar cells

In this study, to adjust the desired molecular energy levels and bandgap energies of polymers for photovoltaic applications, a regular terpolymer structure was designed. A new regular terpolymer, poly(DPP4T-alt-TBP), containing diketopyrrolopyrrole (DPP), BT, and BP units in the repeating group was successfully synthesized. The DPP-BT monomeric unit in polymer backbone enhanced chain packing and induced a high-lying highest occupied molecular orbital (HOMO) level and the DPP-BP segment induced a deeper HOMO level. The lowest unoccupied molecular orbital (LUMO) level of the terpolymer was also controlled in a similar manner. The HOMO level of the terpolymer was similar to that of poly(DPP-alt-BP), and the energies of the LUMOs were governed by the DPP-BT unit. The polymer chain arrangement of the terpolymer on the substrate was observed to be a mix of face-on and edge-on orientations, which is a different chain arrangement mode to those shown in both poly(DPP-alt-BP) and poly(DPP-alt-BT). A TFT fabricated with poly(DPP4T-alt-TBP) had a charge carrier mobility of 0.59 cm² V⁻¹ s⁻¹ and a moderately high current on/off ratio. Furthermore, a polymer solar cell containing the terpolymer and PC₇₁BM had a power conversion efficiency of 4.54%, which is significantly higher than those of the PCEs of poly(DPP-alt-BP) and poly(DPP-alt-BT)-based solar cells with identical device configurations.     

The color change in polychromatic organic light-emitting field-effect transistors: Optical filtering versus reemission

In this contribution the color conversion process of a polychromatic organic light-emitting field-effect transistor (OLET) is revisited on the basis of an analytic device model. The device of interest consists of a color conversion layer out of rubrene on top of a monochromatic light-emitting transistor based on poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT). The model describes the relation of color coordinate and emission intensity – set by the applied drain and gate biases – linking the optoelectronic response of the employed monochromatic OLET to the optical processes occurring in the color conversion layer. The model shows that the color shift is rather due to partial absorption of the F8BT emission by rubrene than, as was claimed earlier, due to a color conversion process by absorption and reemission in the conversion layer. In addition to the earlier publication, it will be demonstrated that such a device allows for an independent electrical tunability of emission intensity and color coordinate within the color span of the F8BT and the rubrene spectrum being a unique feature of such a polychromatic light-emitting field-effect transistor.     

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.     

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.     

Electrical responses of short-channel organic transistor prepared by solution-processed organic crystal wire mask

We report a formation of a solution-grown single crystal wire mask for the fabrication of short-channel organic field-effect transistor with enhanced dynamic response time. The various channel length, ranging from submicrometer to a few micrometers, were obtained by controlling the concentration of solution and processing conditions. We fabricated p- and n-channel bottom-contact organic field-effect transistors using pentacene and PTCDI-C₁₃, respectively, and static and dynamic electrical characteristics of the devices were investigated. The highest and average field-effect hole mobility values were found to be 0.892 cm²/V s and 0.192 cm²/V s, respectively. The load type inverter based on the short-channel transistor connected with a 2 MΩ resistor showed a clear switching response when square wave input signals up to 1 kHz were applied at V_DD = −60 V.     

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.     

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.     

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-speed organic transistors with three-dimensional organic channels and organic rectifiers based on them operating above 20 MHz

Three-dimensional organic transistors (3D-OFETs) comprising vertical short channels are developed to raise the operational speed of organic transistors. The devices with a short-channel length of 0.8 μm and reduced parasitic capacitance operate at up to 20 MHz with an applied drain voltage of −15 V. Organic rectifiers based on the diode-connected 3D-OFETs are also demonstrated to operate at above 20 MHz, even with an applied effective voltage of about 4 V, which is higher than the speed of radio frequency identification tags of 13.56 MHz required in near field communication. These techniques boost the performance of organic transistors and can help to realize the breakthrough for practical applications of organic logic circuits used as key components in various flexible or plastic devices.     

High-performance solution-processed organic transistors with electroless-plated electrodes

Electroless-plated gold and platinum films are used as source and drain electrodes in high-performance solution-processed organic field-effect transistors (OFETs), representing a promising large-area, near-room-temperature and vacuum-free technique to form low-resistance metal-to-semiconductor interfaces in ambient atmosphere. Developing non-displacement conditions using a Pt-colloidal catalyst for soft electroless plating, the electrodes are deposited on crystallized thin films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) without significant damage to the semiconductor material. The top-contact OFETs show remarkable performance, with a mobility of 6.0 cm² V^?1 s^?1. The method represents a practical fabrication technique to mass-produce circuitry arrays of nearly best-performing OFETs for the printed electronics industry.     

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.     

High-speed organic single-crystal transistors gated with short-channel air gaps: Efficient hole and electron injection in organic semiconductor crystals

Short-channel, high-mobility organic filed-effect transistors (OFETs) are developed based on single crystals gated with short-channel air gaps. The high hole mobility of 10 cm²/Vs for rubrene, and high electron mobility of 4 cm²/Vs for PDIF-CN₂ crystals are demonstrated even with a short channel length of 6 μm. Such performance is due to low contact resistance in these devices estimated to be as low as ～0.5 kΩ cm at gate voltage of ?4 V for rubrene. With the benefit of the short channel length of 4.5 μm in a new device architecture with less parasitic capacitance, the cutoff frequency of the rubrene air–gap device was estimated to be as high as 25 MHz for drain voltage of ?15 V, which is the fastest reported for p-type OFETs, operating in ambient conditions.     

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.     

High-performance pentacene thin-film transistor with ZrLaO gate dielectric passivated by fluorine incorporation

Pentacene thin-film transistor with high-κ ZrLaO gate dielectric has been fabricated for the first time. After treating the dielectric in a fluorine plasma, the carrier mobility of the transistor can be greatly improved to 0.717 cm²/V s, which is more than 40 times that of one without plasma treatment. The major reasons should be larger pentacene grains and fewer traps in the device with gate dielectric passivated by the fluorine plasma. AFM confirms that relatively large and high pentacene islands form on the plasma-treated dielectrics in the initial growth stage, and the growth pattern obviously follows the Vollmer–Weber growth model. Furthermore, the surfaces of the dielectrics with different plasma treatment times are investigated by AFM, XPS and contact-angle measurement to reveal the mechanism/effects of the fluorine incorporation. Lastly, after exposure to atmosphere without encapsulation for 6 months, all the devices still display good transistor characteristics.