Conventional Organic Thin-film Transistors Research Articles

전기화학적으로 산화된 투과성 베이스 전극을 갖는 수직구조 유기 트랜지스터

Vertical organic thin film transistors (VOTFTs) are emerging electronic devices that can overcome the limitations of conventional organic thin film transistors (OTFTs) based on lateral architecture. Here, organic permeable base transistors (OPBTs) are formed with an oxide layer on the base electrode through an electrochemical process called anodization, and this oxide layer acts as a dielectric layer. Through this electrochemical method, the manufacturing process is simplified and the charge carrier transport of the VOTFT is improved. In this study, the thickness of AlOx formed by anodizing the Al electrode was calculated through the capacitance-voltage measurement of the metal-insulator-metal device, and OPBT operations were observed according to the anodizing potential and interfacial layer, respectively. In addition, we observed the specific behaviors of vertical OPBT depending on the applied potential correlations between middle base electrode and bottom collector electrode.

(Invited) Modeling Oect Devices for Circuit Simulations

Organic electrochemical transistors (OECTs) can be distinguished from conventional organic thin film transistors (OTFTs) by the use of an electrolyte in their gate stack. This results in a unique set of device properties, most notably, mixed ionic and electronic transport. Modeling of these devices has mainly been realized using MOSFET-like equations (often called the ‘Bernards model’) and extensions, which use a circuit diagram to include the different parasitics and capacitances involved [1]. While these models provide qualitative agreements with OECT devices, their quantitative agreement may not be sufficient for realizing circuit simulations. Physically, the discrepancy between the model and the devices are likely to stem from the assumption that the electrolyte behaves as a parallel plate capacitor, as in a conventional MOSFET. Recently, another model based on thermodynamics has been proposed that treats the OECT as a thermodynamic binary system with entropic mixing as the driving force for device operation [2]. Here we explore these different models and then show how they can be used to understand circuit behavior of OECTs.The authors thank the ANR and the Bundesministerium für Bildung und Forschung (BMBF) for funding of the BAYOEN project under contract ANR-21-FAI1-0006-01 and 01IS21089.[1] J Rivnay et al Organic electrochemical transistors. Nature Reviews Materials, 3(2):1–14, 2018[2] M. Cucchi et al ‘Thermodynamics of Organic Electrochemical Transistors’, accessed on researchsquare.com, https://doi.org/10.21203/rs.3.rs-1143463/v1 Figure 1

Solution-processed pseudo-vertical organic transistors based on TIPS-pentacene

Powerful and versatile transistors are indispensable to realize the vision of future flexible electronics, e.g., for wearable active-matrix displays, smart sensors, or radio-frequency identification tags. Organic thin-film transistors are considered to be a perfect match for flexible electronics. However, conventional organic thin-film transistors still do not match the demanding performance targets of many applications. Furthermore, often complex and expensive fabrication steps are employed for the fabrication of hero-devices, which is not compatible with the paradigm of low-cost production of flexible electronics.In this contribution, we present the first solution-processed vertical organic field-effect transistor (VOFET) with good on-state performance comparable to vacuum-processed VOFETs. This approach unites the advantages of a low-temperature, low-cost solution processing with an ultra-short channel transistor concept possibly enabling large-area, low-cost flexible electronics. We examine the influence of different crystal morphologies (spherulitic and ribbons) on the transistor performance by using spin- and shear-coating as solution-based deposition methods. The solution-processed VOFETs reach channel width-normalized transconductances of up to 0.26 μS/μm with a charge carrier mobility of 4.8 cm2/V. S-parameter measurements finally verify that transition frequencies up to 6 MHz are reachable with shear-coated TIPS-pentacene. However, compared to small molecule-based VOFETs, the solution-processed VOFETs show an unfavorably high off-state current and hysteresis, which are explained by background doping and charge carrier trapping. Hence, in order to advance with, further optimization of the semiconductor material and the insulator–semiconductor interface is thus required to qualify solution-processed VOFETs for commercial applications.

Ultralow-Noise Organic Transistors Based on Polymeric Gate Dielectrics with Self-Assembled Modifiers.

In this study, ultralow 1/f noise organic thin-film transistors (OTFTs) based on parylene gate dielectrics modified with triptycene (Trip) modifiers were fabricated. The fabricated OTFTs showed the lowest 1/f noise level among those of previously reported OTFTs. It is well known that 1/f noise causes degradation of signal integrity in analog and digital circuits. However, conventional OTFTs still possess high 1/f noise levels, and the factors that strongly affect 1/f noise are still ambiguous. In this work, the effect of gate dielectric surface on 1/f noise was investigated. First, by comparing OTFTs composed of various channel lengths, we revealed that contact resistance did not affect 1/f noise. Second, we compared parylene OTFTs with and without a self-assembled Trip modifier layer in terms of 1/f noise and trap density of states (Trap DOS). The experiments revealed that a specific Trip modifier layer suppresses the shallow Trap DOS in the OTFTs, leading to a low 1/f noise. Moreover, the 1/f noise level and Trap DOS of various kinds of OTFTs were comprehensively compared, which highlighted that the 1/f noise of OTFTs strongly depends on the gate dielectric surface. Finally, detailed analysis of the gate dielectric interface led us to conclude that the disorder of gate dielectrics and the crystalline quality of semiconductor films are related to shallow Trap DOS, which correlates with 1/f noise.

Organic field-effect transistor based on paramagnetic Cu(II) neutral complexes coordinated by Schiff base-type TTF ligands

We have succeeded in preparation of novel paramagnetic Cu(II) complexes composed of tetrathiafulvalene (TTF)-based ligands as electrical conducting units, [CuII(Bz-OMe-tBu-sae-TTF)2], [CuII(EDT-bissae-TTF)] and [CuII(Bz-bis(OMe-tBu-sae)-TTF)]. Electrochemical measurements of these neutral complexes revealed that they exhibited two pairs of redox waves attributed to the oxidation of TTF moieties. Among the newly synthesized TTF complexes, only [CuII(EDT-bissae-TTF)] gave a radical salt by electrochemical crystallization, but its resistive behavior was insulating. Field-effect transistors (FET) were fabricated with spin-coated films of [CuII(Bz-OMe-tBu-sae-TTF)2] and [CuII(EDT-bissae-TTF)], and their FET performances were very low compared with the conventional organic thin film transistors (OTFT) based on TTF derivatives. Atomic force microscopy (AFM) images of the spin-coated films indicated the non-uniform film surfaces with many small grains leading to the large grain boundaries, which probably causes the low carrier mobility of the FET devices.

Improve the n-type performance in organic heterojunction transistors by controlling thickness of copper phthalocyanine

N -type C 60 thin film transistors with heterojunction were fabricated by introducing copper phthalocyanine (CuPc) modifying layer between polymer dielectrics and C 60 semiconductor. Comparing with conventional organic thin film transistors (OTFTs) based on single C 60 layer, such devices can not only boost the electron mobility up to 3 times, but also well control threshold voltage V T and off current I off by optimizing CuPc thickness. The performance improvement can be attributed to the increase of free electron carriers by the dipole effect at the C 60 /CuPc interface, and better C 60 film quality grown upon CuPc.

Fabrication of an organic thin-film transistor by direct deposit of a pentacene layer onto a silicon substrate

Contrary to the conventional bottom-contact and top-contact structure, a new organic thin-film transistor structure has been developed in this work based on good crystallinity of the pentacene layer and on large grain sizes. Both these characteristics are obtained when the material is deposited directly onto a silicon substrate. In addition to depositing the pentacene layer at the bottom of the transistor, we adopted an HfO 2 and Si 3N 4 stack as the gate dielectric using sputtering to lower the operation voltage and to reduce the gate leakage current. This proposed organic thin-film transistor shows an acceptable carrier mobility of 0.21 cm 2 V −1 s −1 and a threshold voltage of −7 V. Because the gate dielectric is placed on top of the pentacene active layer, the new structure has a five-fold life-time improvement when compared with a conventional organic thin-film transistor without a passivation layer.

Improving the performance of the organic thin-film transistors with thin insulating lithium fluoride buffer layer

The pentacene-based organic thin-film transistors (OTFTs) with a thin insulating lithium fluoride (LiF) buffer layer between the pentacene and source/drain electrodes were fabricated. Compared with conventional OTFTs, the introduction of the buffer layer (1 nm) leads to field-effect mobility increases from 0.16 to 0.5 cm 2/Vs, and threshold voltage downshifts from −19 to −8 V for the linear region. The on/off current ratio is improved to a level of 10 5 for the off-state current decreasing. These improvements are attributed to (i) tunneling injection through the LiF layer and (ii) interface dipole energy barrier decreasing and contact resistance reduction between pentacene and Au. The results demonstrate that it is an effective method to improve the device characteristics by using a buffer layer.

Characteristics of Vertical Type Organic Transistor Using N-Type Material and its Application for OLED

We have fabricated static induction organic transistor of vertical type in order to improve the problems of conventional organic thin film transistor (OTFT). The vertical type organic transistors were fabricated by using n-type organic semiconductor materials such as F16CuPc, NTCDA and PTCDI C-8. The effects of carrier mobility of organic active materials, the thickness of thin film and gate structure on current–voltage (I–V) and transistor characteristics were investigated. The vertical type organic transistor using PTCDI C-8 exhibited low operation voltage and high on–off ratio. In addition, we have examined the feasibility of application in organic light emitting transistor using a light emitting polymer.

Organic thin-film transistors with improved characteristics using lutetium bisphthalocyanine as a buffer layer

Organic thin-film transistors (OTFTs) with a buffer layer sandwiched between source/drain electrodes and organic semiconductor are demonstrated. An intrinsic molecular semiconductor, Lutetium bisphthalocyanine (LuPc2), is used as the buffer layer due to its high carrier density (1016cm3). Compared with conventional OTFTs, the introduction of the buffer layer leads to on-state current increases from 700nA to 2.5μA, field-effect mobility increases from 0.7×10−2 to 1.58×10−2cm2∕Vs, and threshold voltage downshifts from −21 to −11V for the linear region. The on/off current ratio is improved to a level of 104. Mechanisms of performance improvement are attributed to include the difference of the Fermi level and interface dipolar between LuPc2 and Au. Our results demonstrate that it is an effective method to improve linear region characteristics by using a molecular semiconductor as the buffer layer.