Tuning Interfacial Properties by Spontaneously Generated Organic Interlayers in Top‐Contact‐Structured Organic Transistors

Controlling the interfacial properties between the electrode and active layer in organic field‐effect transistors (OFETs) can significantly affect their contact properties, resulting in improvements in device performance. However, it is difficult to apply to top‐contact‐structured OFETs (one of the most useful device structures) because of serious damage to the organic active layer by exposing solvent. Here, a spontaneously controlled approach is explored for optimizing the interface between the top‐contacted source/drain electrode and the polymer active layer to improve the contact resistance (R_C). To achieve this goal, a small amount of interface‐functionalizing species is blended with the p‐type polymer semiconductor and functionalized at the interface region at once through a thermal process. The R_C values dramatically decrease after introduction of the interfacial functionalization to 15.9 kΩ cm, compared to the 113.4 kΩ cm for the pristine case. In addition, the average field‐effect mobilities of the OFET devices increase more than three times, to a maximum value of 0.25 cm² V^?1 s^?1 compared to the pristine case (0.041 cm² V^?1 s^?1), and the threshold voltages also converge to zero. This study overcomes all the shortcomings observed in the existing results related to controlling the interface of top‐contact OFETs by solving the discomfort of the interface optimization process.     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

Electroless‐Plated Gold Contacts for High‐Performance,Low Contact Resistance Organic Thin Film Transistors

Deposition of metallic electrodes on a semiconductor medium is an indispensable factor in governing carrier injection, and a metal/semiconductor contact that can be formed via solution process is highly desired in printed electronics. However, fine‐patterning the solution processes of metallic electrodes without damaging the excellent electronic properties of organic semiconductors (OSCs) is still a challenge. In this work, electroless plating, a metal coating technique that involves auto‐catalytic reaction in an aqueous solution, is used to fabricate top‐contact organic thin‐film transistors (OTFTs). An electroless‐plated gold pattern with a spatial resolution of 10 micrometers is transferred and laminated on a monolayer of OSCs to serve as a hole‐injection electrode. The fabricated OTFTs exhibit reasonably high field‐effect mobility of up to 13 cm² V^?1 s^?1 and decent contact resistance as low as 120 Ω · cm, which implies that an ideal metal/semiconductor contact can be realized. This electroless plating technique can provide possibilities for practical mass production of organic integrated circuits because it is in principle cost‐effective, capable of covering large areas, high‐vacuum free, and environmentally friendly.     

Enhanced Charge Injection Through Nanostructured Electrodes for Organic Field Effect Transistors

Nanosphere lithography is used to process nanopore‐structured electrodes, which are applied into the fabrication of bottom‐gate, bottom‐contact configuration organic field effect transistors (OFETs) to serve as source/drain elecrodes. The introduction of this nanopore‐structure electrode facilitates the forming of nanopore‐structure pentacene layers with small grain boundaries at the electrode interface, and then reduces the contact resistance, contact‐induces the growth of pentacene and accordingly improves the mobility of charge carriers in the OFETs about 20 times as compared with results in literature through enhancing the charge carrier injection. It is believed that this structure of electrode is a valuable approach for improving organic filed effect transistors.     

Hydrocarbons‐Driven Crystallization of Polymer Semiconductors for Low‐Temperature Fabrication of High‐Performance Organic Field‐Effect Transistors

While many high‐performance polymer semiconductors are reported for organic field‐effect transistors (OFETs), most require a high‐temperature postdeposition annealing of channel semiconductors to achieve high performance. This negates the fundamental attribute of OFETs being a low‐cost alternative to conventional high‐cost silicon technologies. A facile solution process is developed through which high‐performance OFETs can be fabricated without thermal annealing. The process involves incorporation of an incompatible hydrocarbon binder or wax into the channel semiconductor composition to drive rapid phase separation and instantaneous crystallization of polymer semiconductor at room temperature. The resulting composite channel semiconductor film manifests a nano/microporous surface morphology with a continuous semiconductor nanowire network. OFET mobility of up to about 5 cm² V^?1 s^?1 and on/off ratio ≥ 10⁶ are attained. These are hitherto benchmark performance characteristics for room‐temperature, solution‐processed polymer OFETs, which are functionally useful for many impactful applications.     

Molecular Packing‐Induced Transition between Ambipolar and Unipolar Behavior in Dithiophene‐4,9‐dione‐Containing Organic Semiconductors

By changing the packing motif of the conjugated cores and the thin‐film microstructures, unipolar organic semiconductors may be converted into ambipolar materials. A combined experimental and theoretical investigation is conducted on the thin‐film organic field‐effect transistors (OFETs) of three organic semiconductors that have the same conjugated core structure of s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione but with different n‐alkyl groups. The optical and electrochemical measurements suggest that the three organic semiconductors have very similar energy levels; however, their OFETs exhibit dramatically different transport characteristics. Transistors based on compound 1a or 1c show ambipolar transport properties, while those based on compound 1b show p‐type unipolar behavior. Specifically, compound 1c is characterized as a good ambipolar semiconductor with the highest electron mobility of 0.22 cm² V^?1 s^?1 and the highest hole mobility of 0.03 cm² V^?1 s^?1. Complementary metal oxide semiconductor (CMOS) inverters incorporated with compound 1c show sharp inversions with high gains above 50. Theoretical investigations reveal that the drastic difference in the transport properties of the three materials is due to the difference in their molecular packing and film microstructures.     

Stable Delocalized Singlet Biradical Hydrocarbon for Organic Field‐Effect Transistors

Delocalized singlet biradical hydrocarbons hold promise as new semiconducting materials for high‐performance organic devices. However, to date biradical organic molecules have attracted little attention as a material for organic electronic devices. Here, this work shows that films of a crystallized diphenyl derivative of s‐indacenodiphenalene (Ph₂‐IDPL) exhibit high ambipolar mobilities in organic field‐effect transistors (OFETs). Furthermore, OFETs fabricated using Ph₂‐IDPL single crystals show high hole mobility (μ_h = 7.2 × 10^?1 cm² V^?1 s^?1) comparable to that of amorphous Si. Additionally, high on/off ratios are achieved for Ph₂‐IDPL by inserting self‐assembled mono­layer of alkanethiol between the semiconducting layer and the Au electrodes. These findings open a door to the application of ambipolar OFETs to organic electronics such as complementary metal oxide semiconductor logic circuits.     

Insulator Polarization Mechanisms in Polyelectrolyte‐Gated Organic Field‐Effect Transistors

Electrolyte‐gated organic field‐effect transistors (OFETs) hold promise for robust printed electronics operating at low voltages. The polarization mechanism of thin solid electrolyte films, the gate insulator in such OFETs, is still unclear and appears to limit the transient current characteristics of the transistors. Here, the polarization response of a thin proton membrane, a poly(styrenesulfonic acid) film, is controlled by varying the relative humidity. The formation of the conducting transistor channel follows the polarization of the polyelectrolyte, such that the drain transient current characteristics versus the time are rationalized by three different polarization mechanisms: the dipolar relaxation at high frequencies, the ionic relaxation (migration) at intermediate frequencies, and the electric double‐layer formation at the polyelectrolyte interfaces at low frequencies. The electric double layers of polyelectrolyte capacitors are formed in ～1 µs at humid conditions and an effective capacitance per area of 10 µF cm^?2 is obtained at 1 MHz, thus suggesting that this class of OFETs might operate at up to 1 MHz at 1 V.     

Organic Light‐Emitting Transistors Containing a Laterally Arranged Heterojunction

An approach to produce organic light‐emitting transistors (OLETs) containing a laterally arranged heterojunction structure, which minimizes exciton quenching at the metal electrodes, is described. This device configuration provides an organic light‐emitting diode (OLED) structure where the anode (source) electrode, hole‐transport material (field‐effect material), light‐emitting material, and cathode (drain) electrode are laterally arranged, thus offering a chance to control the electroluminescent intensity by changing the gate bias. Pentacene and tris(8‐quinolinolato)aluminum (Alq₃) are employed as the field‐effect and light‐emitting materials, respectively. The laterally arranged heterojunction structures are achieved by successively inclined deposition of the field‐effect and light‐emitting materials. After deposition of pentacene, a narrow gap of about 10–20 nm between the drain electrode and pentacene was obtained, thereby creating an opportunity to fabricate a laterally arranged heterojunction. In the OLETs, unsymmetrical source and drain electrodes, that is, Au and LiF/Al ones, are used to ensure efficient injection of holes and electrons. Visible‐light emission from OLETs is observed under ambient atmosphere. This result is ascribed to efficient carrier injection and transport, formation of a heterojunction, as well as good luminescence from the organic emissive layer. The device structure serves as an excellent model system for OLETs and demonstrates a general concept of adjusting the charge‐carrier injection and transport, as well as the electroluminescent properties, by forming laterally arranged heterojunctions.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

Effect of Gate Electrode Work‐Function on Source Charge Injection in Electrolyte‐Gated Organic Field‐Effect Transistors

Systematic investigation of the contact resistance in electrolyte‐gated organic field‐effect transistors (OFETs) demonstrates a dependence of source charge injection versus gate electrode work function. This analysis reveals contact‐limitations at the source metal‐semiconductor interface and shows that the contact resistance increases as low work function metals are used as the gate electrode. These findings are attributed to the establishment of a built‐in potential that is high enough to prevent the Fermi‐level pinning at the metal‐organic interface. This results in an unfavorable energetic alignment of the source electrode with the valence band of the organic semiconductor. Since the operating voltage in the electrolyte‐gated devices is on the same order as the variation of the work functions, it is possible to tune the contact resistance over more than one order of magnitude by varying the gate metal.     

Vertical Organic Field‐Effect Transistors

Field‐effect transistors are the fundamental building blocks for electronic circuits and processors. Compared with inorganic transistors, organic field‐effect transistors (OFETs), featuring low cost, low weight, and easy fabrication, are attractive for large‐area flexible electronic devices. At present, OFETs with planar structures are widely investigated device structures in organic electronics and optoelectronics; however, they face enormous challenges in realizing large current density, fast operation speed, and outstanding mechanical flexibility for advancing their potential commercialized applications. In this context, vertical organic field‐effect transistors (VOFETs), composed of vertically stacked source/drain electrodes, could provide an effective approach for solving these questions due to their inherent small channel length and unique working principles. Since the first report of VOFETs in 2004, impressive progress has been witnessed in this field with the improvement of device performance. The aim of this review is to give a systematical summary of VOFETs with a special focus on device structure optimization for improved performance and potential applications demonstrated by VOFETs. An overview of the development of VOFETs along with current challenges and perspectives is also discussed. It is hoped that this review is timely and valuable for the next step in the rapid development of VOFETs and their related research fields.     

Mercury‐Mediated Organic Semiconductor Surface Doping Monitored by Electrolyte‐Gated Field‐Effect Transistors

Surface doping allows tuning the electronic structure of semiconductors at near‐surface regime and is normally accomplished through the deposition of an ultrathin layer on top or below the host material. Surface doping is particularly appealing in organic field‐effect transistors (OFETs) where charge transport takes place at the first monolayers close to the dielectric surface. However, due to fabrication restrictions that OFET architecture imparts, this is extremely challenging. Here, it is demonstrated that mercury cations, Hg²⁺, can be exploited to control doping levels at the top surface of a thin film of a p‐type organic semiconductor blended with polystyrene. Electrolyte‐ or water‐gated field‐effect transistors, which have its conductive channel at the top surface of the organic thin film, turn out to be a powerful tool for monitoring the process. A positive shift of the threshold voltage is observed in the devices upon Hg²⁺ exposure. Remarkably, this interaction has been proved to be specific to Hg²⁺ with respect to other divalent cations and sensitive down to nanomolar concentrations. Hence, this work also opens new perspectives for employing organic electronic transducers in portable sensors for the detection of an extremely harmful water pollutant without the need of using specific receptors.     

The Influence of Film Morphology in High‐Mobility Small‐Molecule:Polymer Blend Organic Transistors

Organic field‐effect transistors (OFETs) based upon blends of small molecular semiconductors and polymers show promise for high performance organic electronics applications. Here the charge transport characteristics of high mobility p‐channel organic transistors based on 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene:poly(triarylamine) blend films are investigated. By simple alteration of the film processing conditions two distinct film microstructures can be obtained: one characterized by small spherulitic grains (SG) and one by large grains (LG). Charge transport measurements reveal thermally activated hole transport in both SG and LG film microstructures with two distinct temperature regimes. For temperatures >115 K, gate voltage dependent activation energies (E_A) in the range of 25–60 meV are derived. At temperatures <115 K, the activation energies are smaller and typically in the range 5–30 meV. For both film microstructures hole transport appears to be dominated by trapping at the grain boundaries. Estimates of the trap densities suggests that LG films with fewer grain boundaries are characterized by a reduced number of traps that are less energetically disordered but deeper in energy than for small SG films. The effects of source and drain electrode treatment with self‐assembled monolayers (SAMs) on current injection is also investigated. Fluorinated thiol SAMs were found to alter the work function of gold electrodes by up to ～1 eV leading to a lower contact resistance. However, charge transport analysis suggests that electrode work function is not the only parameter to consider for efficient charge injection.     

High‐Speed Organic Single‐Crystal Transistor Responding to Very High Frequency Band

Although high carrier mobility organic field‐effect transistors (OFETs) are required for high‐speed device applications, improving the carrier mobility alone does not lead to high‐speed operation. Because the cut‐off frequency is determined predominantly by the total resistance and parasitic capacitance of a transistor, it is necessary to miniaturize OFETs while reducing these factors. Depositing a dopant layer only at the metal/semiconductor interface is an effective technique to reduce the contact resistance. However, fine‐patterning techniques for a dopant layer are still challenging especially for a top‐contact solution‐processed OFET geometry because organic semiconductors are vulnerable to chemical damage by solvents. In this work, high‐resolution, damage‐free patterning of a dopant layer is developed to fabricate short‐channel OFETs with a dopant interlayer inserted at the contacts. The fabricated OFETs exhibit high mobility exceeding 10 cm² V^?1 s^?1 together with a reasonably low contact resistance, allowing for high frequency operation at 38 MHz. In addition, a diode‐connected OFET shows a rectifying capability of up to 78 MHz at an applied voltage of 5 V. This shows that an OFET can respond to the very high frequency band, which is beneficial for long‐distance wireless communication.     

Controlling Electron and Hole Charge Injection in Ambipolar Organic Field‐Effect Transistors by Self‐Assembled Monolayers

Controlling contact resistance in organic field‐effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ambipolar and/or light‐emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) ambipolar OFETs using thiol‐based self‐assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM‐induced work‐function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM‐induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top‐gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.     

Highly Stable Contact Doping in Organic Field Effect Transistors by Dopant‐Blockade Method

In organic device applications, a high contact resistance between metal electrodes and organic semiconductors prevents an efficient charge injection and extraction, which fundamentally limits the device performance. Recently, various contact doping methods have been reported as an effective way to resolve the contact resistance problem. However, the contact doping has not been explored extensively in organic field effect transistors (OFETs) due to dopant diffusion problem, which significantly degrades the device stability by damaging the ON/OFF switching performance. Here, the stability of a contact doping method is improved by incorporating “dopant‐blockade molecules” in the poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) film in order to suppress the diffusion of the dopant molecules. By carefully selecting the dopant‐blockade molecules for effectively blocking the dopant diffusion paths, the ON/OFF ratio of PBTTT OFETs can be maintained over 2 months. This work will maximize the potential of OFETs by employing the contact doping method as a promising route toward resolving the contact resistance problem.     

Low-resistance contacts of electroless-plated metals with high-mobility organic semiconductors: Novel organic field-effect transistors with solution-processed electrodes

The establishment of a reliable vacuum-free method for the formation of electrical contacts on high-performance organic semiconductors has become an urgent task due to rapid progress made in the development of solution-processable high-mobility organic field-effect transistors (OFETs). We have recently proposed that electroless plating, a standard technology to mass produce wirings in currently commercialized electronic devices, is suited for high-performance solution-crystallized OFETs. A low contact resistance at the source and drain electrodes is necessary with organic semiconductors for high-speed device operation; therefore, we have evaluated the contact resistance using the transfer line method. A top-contact geometry with sufficient contact area is employed to achieve stable carrier injection, which has enabled contact resistances as low as 1.4 kΩ cm on a polyethylene naphthalate substrate at a gate voltage of −10 V. This marks outstanding performance among the solution-processed metal electrodes reported for OFETs, particularly on plastic substrates. The result indicates that high-quality boundaries with minimized trap densities are realized due to the mild conditions of the electroless plating process at room temperature.     

Enhanced Performance of Fullerene n‐Channel Field‐Effect Transistors with Titanium Sub‐Oxide Injection Layer

Enhanced performance of n‐channel organic field‐effect transistors (OFETs) is demonstrated by introducing a titanium sub‐oxide (TiO_x) injection layer. The n‐channel OFETs utilize [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PC₆₁BM) or [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM) as the semiconductor in the channel. With the TiO_x injection layer, the electron mobilities of PC₆₁BM and PC₇₁BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R_c) shows significantly decreased R_c values for FETs with the TiO_x layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO_x layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO_x. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO_x layer acts as a passivation layer that prevents penetration of O₂ and H₂O; devices with the TiO_x injection layer exhibit a significant improvement in lifetime when exposed to air.     

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm²V⁻¹s⁻¹, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.