Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6 V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0 × 10⁻³ and 2.8 × 10⁻³ cm² V⁻¹ s⁻¹) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.     

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm² V⁻¹ s⁻¹ are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.     

利用MoO3为电极修饰层的F16CuPc/ CuPc异质结的双极性有机场效应晶体管

在本文中,我们利用钛青铜(CuPc)和氟化钛青铜(F16CuPc)作为空穴传输层和电子传输层的制备了具有异质结结构的有机场效应晶体管(OFETs)。与单层的F16CuPc晶体管相比,异质结结构的晶体管的电子迁移率从3.1×10-3cm2/Vs提高至8.7×10-3cm2/vs,然而,空穴的传输行为却没有被观测到。为了提高空穴的注入能力,我们利用MoO3对源-漏电极进行了修饰,有效地改善了空穴注入。并进一步证实了MoO3的引入使得器件的接触电阻变小,平衡了电子和空穴的注入,从而最终实现了器件的双极性传输。     

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.     

Visible‐Near Infrared Absorbing Polymers Containing Thienoisoindigo and Electron‐Rich Units for Organic Transistors with Tunable Polarity

Systematic creation of polymeric semiconductors from novel building blocks is critical for improving charge transport properties in organic field‐effect transistors (OFETs). A series of ultralow‐bandgap polymers containing thienoisoindigo (TIIG) as a thiophene analogue of isoindigo (IIG) is synthesized. The UV‐Vis absorptions of the TIIG‐based polymers ( PTIIG‐T , PTIIG‐Se , and PTIIG‐DT ) exhibit broad bands covering the visible to near‐infrared range of up to 1600 nm. All the polymers exhibit unipolar p‐channel operations with regard to gold contacts. PTIIG‐DT with centrosymmetric donor exhibits a maximum mobility of 0.20 cm² V^?1 s^?1 under gold contacts, which is higher than those of the other polymers containing axisymmetric donors. Intriguingly, OFETs fabricated with aluminum electrodes show ambipolar charge transport with hole and electron mobilities of up to 0.28 ( PTIIG‐DT ) and 0.03 ( PTIIG‐T ) cm² V^?1 s^?1, respectively. This is a record value for the hitherto reported TIIG‐based OFETs. The finding demonstrates that TIIG‐based polymers can potentially function as either unipolar or ambipolar semiconductors without reliance on the degree of electron affinity of the co‐monomers.     

Ambipolar transport in transparent and flexible all-organic heterojunction field effect transistors at ambient conditions

We report on the realization of fully flexible and transparent n-type and ambipolar all-organic OFETs. A double layer, pentacene-C60 heterojunction, was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography MicroContact Printing (μCP). Interestingly, as demonstrated by atomic force microscopy and X-ray diffraction investigations, growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to obtain n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C60. As a result, it was possible to realize n-type and ambipolar all-organic OFETs by optimizing the thicknesses of the pentacene buffer layer. All devices, measured in air, worked in accumulation mode with mobilities up to 1 × 10⁻² cm²/V s and 3.5 × 10⁻⁴ cm²/V s for p-type and n-type regimes, respectively. This is particularly interesting because it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. These results confirm the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials.     

The origins in the transformation of ambipolar to n-type pentacene-based organic field-effect transistors

With aluminum (Al) source–drain electrodes, the transfer characteristics of pentacene-based organic field-effect transistors (OFETs) change from ambipolar to n-type after 24 h of storage in a nitrogen-filled glove box Chang et al. (2011) [16]. The time-dependent decrease of hole current is associated with the interfacial reaction at the Al source–drain electrodes and pentacene, which was studied by in-situ ultraviolet photoemission spectroscopy and X-ray photoelectron spectroscopy in this work. Experimental results indicate that the interface of the Al and pentacene is partially oxidized, but the similar oxidation was not observed at the interface of the pentacene and silver. The time-dependent oxidization of Al and pentacene creates an interfacial barrier to suppress the hole injection from Al electrodes (extraction of electrons from pentacene). However, it shows minor effect in the injection of electrons from Al electrode. Since the rate of oxidation is related to the contact area of the pentacene and Al, co-evaporating a thin Al:pentacene interlayer between the pentacene and Al electrodes expands the contact surface and accelerates the reaction, which is suitable for the fabrication of n-type only pentacene-based OFETs. This study highlights the impact of the interfacial reaction in Al/pentacene interface for the transformation of ambipolar to n-type OFETs.     

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.     

Reconfigurable Multifunctional Ambipolar Polymer-Blend Transistors with Improved Switching-Off Capability

Ambipolar field-effect transistors allowing both holes and electrons transport can work in different states, which are attractive for simplifying the manufacture of circuits and endowing the circuits with reconfigurable multi-functionalities. However, conventional ambipolar transistors intrinsically suffer from poor switching-off capability because the gate electrode is not able to simultaneously deplete holes and electrons across the entire transport channel, which hurdles the practical applications. This study shows that the switching-off capability of polymer ambipolar transistor is significantly improved by up to three orders by introducing non-uniformly distributed compensation potentials along the channel to synchronically tune the charge transport at different channel locations. The non-uniform compensation potential is experimentally generated by the non-uniformly distributed electret charges, which are pre-injected into the insulators from source and drain electrodes. By this method, both n-type and p-type operations with high mobility (2.2 and 0.8 cm²s⁻¹V⁻¹ respectively) and high on/off ratio (10⁵) are obtained in the same device, and the different states are reversibly switchable. Moreover, this method endows the device with diverse device characteristics and reconfigurable multi-functionalities, which promotes the application of ambipolar transistors in complementary metal-oxide semiconductors-like circuits and some emerging electronics, including reconfigurable devices, multi-level memories, and artificial synapses.     

Low-voltage organic transistors and inverters using HfOx dielectrics

Based on the p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13), low-voltage organic field-effect transistors (OFETs) and inverters using hafnium (Hf)-based dielectrics were produced and characterized. All the pristine and cyclic olefin copolymer (COC)-passivated HfO_x gate dielectrics were deposited by the solution-processed sol–gel chemistry, and organic thin films were deposited on the dielectrics by the neutral cluster beam deposition method. In comparison to the pristine HfO_x-based OFETs, the COC-passivated transistors showed better device performance: higher hole and electron mobilities, reduced hysteresis, decreased trap densities, and particularly improved operational stability of n-type transistors. The inverters composed of the optimized p- and n-type OFETs with the asymmetric Au and LiF/Al electrodes using COC-passivated HfO_x dielectrics exhibited high gains and good noise margins under ambient conditions.     

Gradual Controlling the Work Function of Metal Electrodes by Solution‐Processed Mixed Interlayers for Ambipolar Polymer Field‐Effect Transistors and Circuits

In this paper, a technique using mixed transition‐metal oxides as contact interlayers to modulate both the electron‐ and hole‐injections in ambipolar organic field‐effect transistors (OFETs) is presented. The cesium carbonate (Cs₂CO₃) and vanadium pentoixide (V₂O₅) are found to greatly and independently improve the charge injection properties for electrons and holes in the ambipolar OFETs using organic semiconductor of diketopyrrolopyrrolethieno[3,2‐b]thiophene copolymer (DPPT‐TT) and contact electrodes of molybdenum (Mo). When Cs₂CO₃ and V₂O₅ are blended at various mixing ratios, they are observed to very finely and constantly regulate the Mo's work function from ?4.2 eV to ?4.8 eV, leading to high electron‐ and hole‐mobilities as high as 2.6 and 2.98 cm² V^?1 s^?1, respectively. The most remarkable finding is that the device characteristics and device performance can be gradually controlled by adjusting the composition of mixed‐oxide interlayers, which is highly desired for such applications as complementary circuitry that requires well matched n‐channel and p‐channel device operations. Therefore, such simple interface engineering in conjunction with utilization of ambipolar semiconductors can truly enable the promising low‐cost and soft organic electronics for extensive applications.     

Low temperature,solution-processed ambipolar field-effect transistors based on polymer/self-assembled monolayer modified InOx hybrid structures for balanced hole and electron mobilities exceeding 1 cm2 V−1 s−1

Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures were investigated. An amorphous indium oxide (InO_x) film, as the n-type semiconducting layer, was prepared with an environmentally friendly method and annealed at a low temperature; and a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer, FBT-Th₄(1,4), was spin-coated on the InO_x film as the p-type semiconducting layer. To improve the p-type mobility, a self-assembled monolayer (SAM) of octadecyl-phosphonic acid was introduced to modify the surface of InO_x. The ambipolar FETs showed high and well-balanced hole and electron mobilities of 1.1 and 1.5 cm² V⁻¹ s⁻¹, respectively. Furthermore we found that ambipolar FETs could be integrated into functional complementary metal oxide semiconductor (CMOS)-like inverters.     

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

A graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (10² S cm^?1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.     

Solution‐Processed Ambipolar Field‐Effect Transistor Based on Diketopyrrolopyrrole Functionalized with Benzothiadiazole

Ambipolar charge transport in a solution‐processed small molecule 4,7‐bis{2‐[2,5‐bis(2‐ethylhexyl)‐3‐(5‐hexyl‐2,2′:5′,2″‐terthiophene‐5″‐yl)‐pyrrolo[3,4‐c]pyrrolo‐1,4‐dione‐6‐yl]‐thiophene‐5‐yl}‐2,1,3‐benzothiadiazole (BTDPP2) transistor has been investigated and shows a balanced field‐effect mobility of electrons and holes of up to ～10^?2 cm² V^?1 s^?1. Using low‐work‐function top electrodes such as Ba, the electron injection barrier is largely reduced. The observed ambipolar transport can be enhanced over one order of magnitude compared to devices using Al or Au electrodes. The field‐effect mobility increases upon thermal annealing at 150 °C due to the formation of large crystalline domains, as shown by atomic force microscopy and X‐ray diffraction. Organic inverter circuits based on BTDPP2 ambipolar transistors display a gain of over 25.     

3,7-Bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT) based polymer with balanced ambipolar charge transport performance

A novel acceptor building block, 3,7-bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT), is developed to construct a donor-acceptor polymer PIBDTBT-40. This polymer has favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for balanced ambipolar charge transport. Organic thin film transistors (OTFTs) based on this polymer shows well-balanced ambipolar characteristics with electron mobility of 0.14 cm² V⁻¹ s⁻¹ and hole mobility of 0.10 cm² V⁻¹ s⁻¹ in bottom-gate bottom-contact devices. This polymer is a promising semiconductor for solution processable organic electronics such as CMOS-like logic circuits.     

Self-assembled monolayer modification of silver source-drain electrodes for high-performance pentacene organic field-effect transistors

We demonstrated excellent performance improvement of bottom-contact pentacene-based organic thin film transistors (OTFTs) fabricated at room-temperature with silver electrodes modified by self-assembled monolayers (SAMs) of binary mixtures of n-alkanethiol (n-decanethiol, HDT) and the fluorinated analog (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, FDT). The OTFTs with modified silver (Ag) electrodes exhibit carrier mobility of 0.21 cm²/V s, which is faster than most of bottom-contact pentacene-based OTFTs fabricated at room-temperature with gold (Au) electrodes. The threshold voltage is reduced from −30 V of the devices with Au electrodes to −5.4 V of the devices with modified Ag electrodes. The hole injection barrier is also reduced with modified Ag as indicated by ultraviolet photoemission spectroscopy. The enhancement of the saturation current and the mobility of the devices are due to both the reduction of hole injection barriers and the continuous grain size of pentacene on top of electrodes and dielectrics.     

Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al₂O₃ bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01 cm² V^?1 s^?1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.     

Synergistic Use of Pyridine and Selenophene in a Diketopyrrolopyrrole‐Based Conjugated Polymer Enhances the Electron Mobility in Organic Transistors

To achieve semiconducting materials with high electron mobility in organic field‐effect transistors (OFETs), low‐lying energy levels (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) and favorable molecular packing and ordering are two crucial factors. Here, it is reported that the incorporation of pyridine and selenophene into the backbone of a diketopyrrolopyrrole (DPP)‐based copolymer produces a high‐electron‐mobility semiconductor, PDPPy‐Se. Compared with analogous polymers based on other DPP derivatives and selenophene, PDPPy‐Se features a lower LUMO that can decrease the electron transfer barrier for more effective electron injection, and simultaneously a lower HOMO that, however, can increase the hole transfer barrier to suppress the hole injection. Combined with thermal annealing at 240 °C for thin film morphology optimization to achieve large‐scale crystallite domains with tight molecular packing for effective charge transport along the conducting channel, OFET devices fabricated with PDPPy‐Se exhibit an n‐type‐dominant performance with an electron mobility (μ_e) as high as 2.22 cm² V^?1 s^?1 and a hole/electron mobility ratio (μ_h/μ_e) of 0.26. Overall, this study demonstrates a simple yet effective approach to boost the electron mobility in organic transistors by synergistic use of pyridine and selenophene in the backbone of a DPP‐based copolymer.     

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.     

Switch the n-type to ambipolar transfer characteristics by illumination in n-type pentacene-based organic field-effect transistors

This work investigates the suppression of n-channel and the switch of transfer characteristics (from n-type to ambipolar) by illumination in n-type pentacene-based organic field-effect transistors (OFETs). The illumination outcomes differently on the output characteristics of OFETs, which markedly decreases the magnitude of drain current (n-channel) and shifts the turn-on voltage to a higher positive bias in the n-type regime, but induces the formation of p-channel in the p-type regime. We attribute that the trapped negative charges in the device as induced by illumination electrostatically shield the effective electrical field applied to the gate with source/drain electrodes and modulate the device performance. The result of quasi-static capacitance–voltage measurement agrees well with the modulations of the transfer characteristics for n-type OFETs by illumination. In addition, the de-trapping of charges recovers the n-type only output characteristics of pentacene-based OFETs. This study highlights the unique photo responses of n-type pentacene-based OFETs to the development of phototransistors of distinct output characteristics operated in n- and p-type regime.