Free Access
Issue
Eur. Phys. J. Appl. Phys.
Volume 91, Number 2, August 2020
Article Number 20201
Number of page(s) 8
Section Physics of Organic Materials and Devices
DOI https://doi.org/10.1051/epjap/2020200120
Published online 25 August 2020

© EDP Sciences, 2020

1 Introduction

The replacement of traditional semiconductors by organic molecular materials has enabled totally new concepts and products, as these materials are inherently flexible and can be processed from non-conventional methods such as low-cost additive printing [13]. Over the past few decades, the extremely active research on ‘organic electronics’ has seen a successful commercialization of organic light-emitting diodes (OLEDs) [4], while many other candidates are under continuous development to become the next key player.

Organic field-effect transistors (OFETs) are one of such emerging device building blocks [5,6], showing the performance on a par with or exceeding that of amorphous silicon thin-film transistors [7], with significantly added values from the use of flexible and printable materials [8]. Now, OFETs are broadly investigated as a key ingredient in display backplanes [9], active-matrix imagers [10], biosensors [11], and neuromorphic electronics [12]. Furthermore, the materials’ multi-functionality, interface sensitivity, and synthetic tunability are most likely to extend the technological scope in the near future [1316].

Although a relatively low charge-carrier mobility and environmental stability of organic semiconductors were major issues in the early days of OFETs, there have been tremendous progresses in these properties thanks to the dedicated materials developments and device engineering [7,17]. However, a high operating voltage of OFETs (typically over 10 V) is still a widespread concern and it therefore represents a particularly formidable challenge toward mobile consumer electronics and long-term embedded applications where the efficient power management is a critical problem.

This article is a focused review on important advances in nanodielectric materials and structures aimed at low-voltage OFETs. First, the key motivations behind the nanodielectric approaches are conceptualized, based on the calculations and analyses that reflect the unique properties of organic materials. This is followed by an extensive categorization of reported devices, for which state-of-the-art OFETs incorporating various hybridized nanodielectric structures are introduced. The final part of this contribution makes critical comments on future developments, helping to carry out a wide range of related researches on materials, devices, and integrated systems.

2 Key motivations

Here, the predictable operation voltage window of an OFET is systematically analyzed. We take the functional expression for the drain current (ID) in the saturation regime of OFETs as a tool, which is written as (1)

where W is the channel width, L is the channel length, μ is the semiconductor mobility, Cdiel is the dielectric capacitance per unit area, VG is the gate voltage, and VT is the threshold voltage. The parallel-plate capacitor model dictates (2)

where ɛ0 is the vacuum permittivity, kdiel is the dielectric constant of the utilized insulator, and tdiel is the dielectric film thickness. Combining (1) and (2), the explicit expression for the dielectric properties is given as (3)

For illustration purposes, we fixed several parameters as WL= 10, μ = 1 cm2 V−1s−1, and kdiel = 3. The value of kdiel was taken as an average of polymeric insulators widely used in OFETs [18]. After this, we can simulate the value of ID as a function of two remaining parameter groups which are tdiel and (VGVT). Figure 1 shows the calculation results with tdiel from 5 to 500 nm and (VGVT) from 1 to 50 V. This analysis clarifies what are the reasonable parametric combinations to reach a sufficient on-state current level, which we indicate here as a 1–10 μA band that is typically required for display applications [19]. Here, we can obviously notice that, with a supply voltage of the order of 10 V and higher, a thick dielectric is no problem to reach that current level. However, if driven by a reasonably low voltage below 5 V, a transistor can produce a desirable level of ID only when a downscaling of the dielectric is properly conducted. With (VGVT) = 1 V, tdiel should enterinto the sub-10 nm regime, addressing the necessity of a nanometer-scale insulting structure.

Therefore, it is clearly understandable that a moderate charge-carrier mobility in an organic semiconductor, which is at least several orders lower than that of crystalline silicon, forces more strict dimensional constraints on the dielectric medium to supply a comparable output current by an OFET. In other words, this simple analysis partly explains why such high voltages have been frequently utilized in research papers on OFETs (particularly those with no dielectric downscaling), and it also demonstrates the high relevance and potentials of the nanodielectrics approaches to organic semiconductor technologies. Below, we highlight more specific motivations for the dielectric research in the context of OFETs.

  • An ultra-thin dielectric material is preferred, but the leakage current density (Jleak) should be mitigated at the same time [20]. Generally, this constitutes an challenging trade-off to consider when designing an OFET device. The techniques to preparing a robust nanoscale dielectric broadly concern the control of surface roughness, the crystallinity and film morphology, and the hybridization of materials resulting in a composite-type film.

  • There are a huge variety of methods to fabricate a dielectric layer, but the solution-based coating should be the top priority, not to sacrifice the low-cost processability of organic materials on thermally sensitive substrates [21]. In this case, care has to be taken to secure a uniform and reproducible layer thickness (which is generally more difficult to achieve as compared to vacuum-based deposition).

  • Although the analysis in Figure 1 assumed a constant and relatively low kdiel (reasonable for a polymer dielectric), the use of a high-k dielectric material is another possible route towards low-voltage OFETs [22]. On the other hand, this generally requires the use of an inorganic insulating material [23], which may cause a chemical incompatibility with an organic semiconductor or a limited solution processability.

thumbnail Fig. 1

The predictive calculation of ID in OFETs under a number of different combinations of (VGVT) and tdiel. The semi-transparent band between 1 and 10 μA indicates the relevant current level to be produced in technological applications.

3 Recent advances

OFETs incorporating nanodielectrics architecture is being actively researched as a driver for low-voltage organic electronic systems. In this section, three groups of the dielectric materials (or compositions) are introduced, with their basic properties and representative device demonstrations.

3.1 Cross-linked polymers

Polymer-based insulating films have a clear advantage of simple processing and mechanical flexibility, therefore have been a natural material of choice since the early stage of OFET research [24]. However, the insulating performance of a polymer dielectric can be low especially at a small thickness, due to the increase in gate electric field and/or the structural and morphological imperfections leading to a substantial leakage current and a possible breakdown. Therefore, the inclusion of a cross-linking agent in a polymeric solution and its post-deposition activation has been a promising method to enhance the mechanical strength of a film at the molecular level, thus enabling a thin yet robust insulating medium.

In 2007, Noh et al. reported on the downscaling of self-aligned, all-printed OFETs utilizing such a cross-linked polymer dielectric (Figs. 2a and b) [25]. In this paper, a prototypical material poly(methyl methacrylate) (PMMA) was blended with a small amount of 1,6-bis(trichlorosilyl)hexane to prepare the cross-linked PMMA (c-PMMA). As shown in Figure 2a, the chlorosilane compound formed a siloxane network tightly cross-linking with the chains of PMMA matrix. The insulating properties of the c-PMMA were tested in a metal-insulator-metal (MIM) capacitor structure. Figure 2b shows that the strength of the film was highly enhanced, as evidenced by the practically no thickness dependence of the leakage currents (at the same electric field, rather than voltage), enabling the use of an ultra-thin solution-cast dielectric. The measured Jleak of the cross-linked films was of the order of 100 nA/cm2 at 1 MV/cm. The authors fabricated self-aligned OFETs coupling a 30-nm-thick c-PMMA film with poly(dioctylfluorene-cobithiophene) (F8T2) or poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) as an organic semiconductor, which operated below 10 V and with a high ac transit frequency (fT) up to 1.6 MHz [29].

The similar polymer cross-linking techniques were adopted in different materials and transistor structures [30]. In 2016, Jung et al. reported on low-voltage flexible n-type OFETs using a cross-linked polymer dielectric (Fig. 2c) [26]. We note that most high-performance organic semiconductors have a p-type (or hole-transporting) property [15], and developing comparable n-type (or electron-transporting) materials and devices is an important topic to realizing complementary organic circuit technologies [3133]. The authors employed a c-PMMA (tdiel = 100 nm) on a polyethylene terephthalate (PET) plastic substrate and fabricated OFETs based on triisopropylsilylethynyl-tetracyanotriphenodioxazine (TIPS-TPDO-tetraCN) n-type molecular semiconductor. As shown in Figure 2c, the devices performed well within a 5-V range, and the flexibility test revealed that the devices could be bended at a radius (r) down to 7 mm.

There have been also significant research efforts on diversifying the materials toolbox, by synthesizing new polymers and cross-linking agents to improve processing and film performances. In 2014, Li et al. proposed the thermal azide-alkyne cycloaddition (TAAC) reaction as a low-temperature cross-linking method in polymer dielectrics (Fig. 2d) [27]. These authors mixed an azide-containing styrenic polymer and an alkyne-containing counterpart (1:1 weight ratio), which was successfully cross-linked at 100 °C. The new TAAC-synthesized dielectric film (tdiel = 150 nm) showed a kdiel of 2.6 and a Jleak of the order of 10 nA/cm2 at 1 MV/cm. Also, the film was stable against further chemical exposure, allowing for the solution-coating of a TIPS-pentacene:polystyrene (PS) semiconductor [37] film on top to make an OFET. Figure 2d shows the electrical properties of this device, where an excellent on-off current ratio at a low operation voltage of − 5V was achieved.

In 2017, Wang et al. published a series of novel cross-linkable high-k coplymer dielectrics for high-energy-density capacitors and OFETs (Fig. 2e) [28]. In this study, reversible-addition fragmentation chain transfer (RAFT) polymerization was utilized to produce poly(2-(methylsulfonyl)ethyl methacrylate-co-glycidyl methacrylate) (poly(MSEMA-co-GMA)) cross-linkable at 80 °C with a kdiel in the range of 9 to 12. The authors compared three different feed ratios for polymerization from 3:1 to 10:1 (MSEMA:GMA, molar ratio), and the material at 10:1, named P(MSEMA-GMA)-10, exhibited the best dielectric properties, including a low Jleak of 10 nA/cm2 at 1 MV/cm. The OFETs with the P(MSEMA-GMA)-10 dielectric and dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) semiconductor [38,39] afforded a low-voltage operation at − 4 V, whose electrical behaviors are presented in 2e.

thumbnail Fig. 2

(a) Reaction scheme illustrating the cross-linking of PMMA insulator. (b) Current densities as a function of electric field measured on an MIM capacitor made of c-PMMA with different thicknesses. Reproduced with permission from [25]. Copyright Nature Publishing Group, 2007. (c) Transfer characteristics of the n-type TIPS-TPDO-tetraCN transistors with the c-PMMA dielectric showing a low gate-leakage current (IG) and a mechanical bending stability. Reproduced with permission from [26]. Copyright American Chemical Society, 2016. (d) Transfercharacteristics of the TIPS-pentacene: PS OFET incorporating the TAAC-synthesized polymer dielectric. Reproducedwith permission from [27]. Copyright Royal Society of Chemistry, 2014. (e) Output characteristics of the DNTT-based OFET comprising the poly(MSEMA-co-GMA) dielectric. Reproduced with permission from [28]. Copyright Royal Society of Chemistry, 2017.

3.2 Self-assembled monolayers

A self-assembled monolayer (SAM), referring to a closely packed nanoscale molecular film adsorbed on a specific binding surface [4042], is another promising strategy for low-voltage organic electronics. Especially, SAMs can ideally have a thickness equivalent to their molecular length, and therefore offering the possibility for ultimate down-scaling of tdiel and a high Cdiel. Also, many SAM materials have a chemical structure composed of an anchoring end group and a long (non-conjugated) alkyl chain, the latter of which can provide a good electrical insulating property.

In 2005, Yoon et al. reported on molecular-scale nanodielectrics for low-voltage OFETs (Figs. 3a and b) [34]. The authors proposed the idea of σ-π molecular multilayers, which are formed by a sequential layer-by-layer coating of different chemical SAMs. As shown in Figure 3a, the three molecules named as Alk, Stb, and Cap included different chemical moieties, which were intended to improve the structural packing and dielectric properties. Three well-defined nanodielectrics (I, II, and III) were prepared combining these base molecules with the tdiel of only 2.3 to 5.5 nm. Figure 3b shows the electrical leakage current characteristics measured on the metal-insulator-semiconductor (MIS) diodes with the different SAM nanodielectrics. This result evidences the systematic reduction of leakage by increasing the number of layers, with an estimated Jleak as low as 10 nA/cm2 at 1 MV/cm for the structure III. These dielectrics were combined with a range of organic semiconductors to demonstrate low-voltage OFETs, which exhibited a sub-1-V operation.

As the molecular-scale SAM nanodielectrics witness promising results, significant efforts have been put into understanding their basic structure-property relationships. In 2015, Kraft et al. reported on a systematic analysis of OFETs combining different semiconductors and SAM dielectric materials (Fig. 3c) [35]. Especially, an alkyl-phosphonic acid SAM (HC14-PA) and a fluoroalkyl-phosphonic acid SAM (FC18-PA) have been directly compared to address the impact of molecular fluorination. Figure 3c reveals that the OFETs with the FC18 -PA nanodielectric had a substantially more positive VT than those with HC14-PA (each group contains OFETs with three different p-type semiconducting molecules), while all the devices enabled an outstanding low-voltage switching. The observed VT shift is associated with the highly electronegative F-containing surface that pre-accumulates holes in the semiconductor layer [4346]. This study is an excellent demonstration that the chemical structure of the SAMs can substantially tune the overall electrical behaviors of OFETs.

Recently, the development of practical circuits employing SAM-dielectric-based low-voltage OFETs has received a growing attention. In 2019, Kraft et al. reported on high-performance low-voltage OFETs and complementary ring oscillators using self-assembled nanodielectrics (Figs. 3d and e) [36]. They fabricated p-type DNTT transistors and n-type Polyera ActivInk N1100 transistors incorporating the alkyl-phosphonic acid (tdiel = 5.4 nm) and fluoroalkyl-phosphonic acid SAM (tdiel = 5.7 nm), respectively. As shown in Figure 3d, the well-managed low-temperature chemical processing of the functional materials allowed for the demonstration of OFET circuits on a real 5-Euro banknote, which may show the potential for anti-counterfeit technologies, for instance. Complementary metal-oxide-semiconductor (CMOS) like integration was possible through the simultaneous use of the DNTT and N1100 OFETs, for fabricating low-voltage 11-stage complementary ring oscillators. Figure 3e shows that, at L = 1 and a supply voltage of 4 V, the signal propagation delay as low as 10 μs per stage was achieved on a banknote, which was only slightly higher than the reference circuit made on a polyethylene naphthalate (PEN) plastic substrate.

thumbnail Fig. 3

(a) Schematic illustration of the OFETs incorporating self-assembled nanodielectrics with different molecular structures. (b) Leakage current density versus applied voltage in MIS capacitors. Reproduced with permission from [34]. Copyright National Academy of Sciences, 2005. (c) Transfer characteristics of the p-type OFETs based on HC14 -PA and FC18-PA SAM dielectrics. Reproduced with permission from [35]. Copyright Wiley-VCH, 2014. (d) Photograph of the low-voltage OFET circuits on a banknote. (e) Signal delay per stage performances measured on the 11-stage complementary ring oscillators. Reproduced with permission from [36]. Copyright Wiley-VCH, 2018.

3.3 Sol-gel metal oxides

Sol-gel synthesis is a traditional method to prepare various inorganic solids (typically oxides) starting from a solution-phase chemical precursor [47,48]. Recently, sol-gel-derived functional oxide thin films have gained a tremendous attention for their integration in practical electronic devices [4953]. Especially, the combination of an intrinsically high kdiel, structural robustness, and solution-processability of sol-gel metal-oxide dielectrics is a highly promising strategy for realizing low-voltage OFETs on flexible substrates.

In 2015, Sung et al. reported on the use of sol-gel synthesized TiO2 thin films as an OFET gate dielectric (Figs. 4a and b) [54]. Previously, TiO2 thin films were mostly employed as a good electrical conductor (e.g. an electron transport layer in solar cells) [55,56]. The authors, however, found that an optimized thermal annealing can produce an excellent dielectric film. As shown in Figure 4a, the solution containing titanium(IV) isopropoxide was directly spin-cast on the gate substrate, which underwent hydrolysis and polycondensation reactions to become a dense, solidified TiO2 thin film. One important finding was that nano-crystalline domains develop at a high temperature annealing over 350 °C, while a moderate temperatureof 250 °C resulted in an amorphous-phase TiO2 material (transmission-electron microscopy (TEM) image in Fig. 4b). Since the electrical conductivity is associated with the material’s crystallinity, the low-temperature TiO2 film (tdiel = 23 nm) featured a reduced leakage conduction (Jleak of the order 100 nA/cm2 at 1 MV/cm), with an estimated high kdiel of 27. The pentacene OFETs including this sol-gel synthesized amorphous TiO2 dielectric were successfully operated at a low voltage of −1 V.

Researches on the circuit integration of OFETs based on sol-gel metal-oxide dielectrics have also been broadly conducted. In 2015, Shin et al. reported on the complementary organic inverters that incorporate a sol-gel-prepared cross-linked oxide dielectric (Figs. 4c and d) [57]. Employing 1,6-bis(trimethoxysily)hexane (BTMH) as a cross-linker (Fig. 4c), the cross-linked zirconia blend (CZB) dielectric was fabricated by spin-coating a blend solution containing zirconium chloride (ZrCl4) precursor, which was then activated at 150 °C. The BTMH was shown to improve the insulating property through the formation of a tightly intermixed solid network with ZrOx, reducing Jleak to 100 nA/cm2 at 1 MV/cm. The p-type pentacene and n-type F16CuPc OFETs sharing the same 30-nm CZB dielectric were connected to form a complementary digital inverter, which exhibited an average voltage gain of 13.3 at a supply voltage of 4 V (Fig. 4d). This result indicates that the hybridization of a dielectric medium, through the formation of a multi-component film and/or a cross-linked network can be a useful technique to further manipulating sol-gel oxide materials.

In 2016, Sun et al. employed the sol-gel Al2 O3 dielectric film to demonstrate low-voltage organic inverters (Fig. 4e and f) [58]. This film was synthesized by spin-coating of an aluminum acetate precursor solution, with a post-deposition thermal annealing at 170 °C. The kdiel of a 50-nm-thick Al2 O3 film was around 7, which also showed a low Jleak around 10 nA/cm2 at 1 MV/cm. The atomic-force microscopy (AFM) image and X-ray diffraction (XRD) pattern in Figure 4e illustrate that the semiconducting pentacene film grown on this Al2 O3 film has a good crystallinity thanks to the low surface roughness of the dielectric. The excellent dielectric property and the compatibility with the high-quality organic channels allowed for the fabrication of a high-performance complementary inverter based on pentacene and N,N’-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) semiconductors. This inverter had a high voltage gain over 40 at a low supply voltage below 3 V (Fig. 4f).

thumbnail Fig. 4

(a) Illustration of the solution-based sol-gel synthesis of TiO2 dielectric film on the OFET substrate. (b) Cross-sectional TEM image of the TiO2 film annealed at 250 °C (scale bar: 10 nm). Reproduced with permission from [54]. Copyright American Chemical Society, 2015. (c) Chemical structure of the BTMH cross-linker. (d) Structure of the complementary organic inverter based on the hybrid CZB dielectric. Reproduced with permission from [57]. Copyright Institute of Physics, 2015. (e) XRD pattern of the pentacene thin film grown on the surface of sol-gel Al2 O3 (inset: AFM image). (f) Voltage-transfer characteristics of the organic complementary inverter at different supply voltages (VDD) (inset: circuit diagram). Reproduced with permission from [58]. Copyright Elsevier, 2016.

4 Future perspectives

The previous section was an ample evidence that there exist a wide range of possibilities for nanoscale dielectric materials and compositions aimed at high-performance low-voltage organic electronics. Here, we discuss some of the remaining challenges and opportunities for future developments.

Firstly, the engineering of kdiel is a classical yet highly important topic for OFETs. In order to produce a large ID at a small VG, achieving a high kdiel is simply beneficial, in view of the transistor equations (e.g. (3)). However, the increased static polarity in the dielectric medium caninduce the broadening of density of states (DOS) at the surface region of the active layer [59]. Therefore, the charge transport inside the organic semiconductor, which already has a relatively broad DOS [60,61], can be seriously degraded by a high kdiel interface, causing a compatibility issue with organic materials, in addition to a high temperature annealing of oxides. Therefore, an optimum kdiel has to be found in terms of dielectric materials and/or nanostructures, in relation to a specific semiconductor and device layout. This may include a spatial variation of kdiel [62], by which a high-k base material for the low-voltage operation is covered with an ultra-thin low-k surface dielectric for the improved semiconducting property at the channel region.

Secondly, charge-carrier traps at the dielectric interface should be minimized [6365]. OFETs are interface-controlled devices, because of their operation mechanism relying on the formation of an accumulation channel at the semiconductor/dielectric interface [6]. Therefore, reducing the trap density at the dielectric level can effectively improve the overall electrical performances, including the long-term bias stability [66,67]. Importantly, this aspect is also directly associated with the promise of low-voltage driving, since the VT and switching-on characteristic of an OFET is highly sensitive to the trap density [68]. The complex surface chemistry induced by the use of nanodielectrics may inevitably increase the trap densities. In this case, secondary surface capping by an inert polymer, incorporation of the non-reacting terminating groups, or an improved physical arrangement of SAM-based nanomaterials can be a possible solution to trap-related problems.

Finally, a broader applicability of nanoscale dielectrics should be addressed. Until now, only a limited number of semiconductors and device structures have been tested in conjunction with the nanodielectric layers. As the field of organic electronics is characterized by its broad synthetic tunability, advanced nanodielectrics that can be combined with the widest possible range of materials will be desirable. These dielectrics should have an exceptional chemical stability and processability, for them to easily form an optimum film interface with various organic molecules. In term of device structure, top-gate OFETs are preferred in certain cases where a photolithographically defined narrow L and a self-passivating capability for protecting the organic semiconductor is required [69]. However, the development of nanodielectrics for that geometry would not be straightforward, as the gate dielectric should be deposited on top of a sensitive organic channel. This means that some of the dielectrics processing reported for a bottom-gate transistor may not be compatible with the top-gate counterpart, even when employing the same semiconductor. This processing issue can be solved by, for instance, the identification and demonstration of nanodielectrics exploiting the chemical solvent orthogonality [70]. The circuit-level applications should also be more broadly investigated. Still, nanodielectric OFET circuits are at the early stage of development, partly evidenced by relatively simple types of circuits demonstrated so far. Future progresses in circuit development are therefore highly demanded, which can include the improvement in dielectric patterning capability (necessary for device interconnects) and the realization of universal circuit building blocks [71].

5 Conclusion

This review summarized fundamental properties and technical advances of nanodielectric OFETs. Despite a long history, major research efforts in organic electronics have been put into the high-performance semiconducting materials. If the performance of semiconductors seems to reach a certain saturation, it might be a timely strategy and a great opportunity to take a more serious look at how to improve the dielectric, which otherwise is another integral part and performance-driving factor of OFETs [72]. In this sense, the integration of nanomaterials and nanoscale structural manipulation into an OFET dielectric is promising, especially for low-voltage circuits. Given the highly interdisciplinary nature of the science and engineering of OFETs, impressive synergies are to be foreseen when researchers with different expertise work together to figure out what can be the chemically, structurally, and functionally ideal nanodielectrics enabling next-generation OFET technologies.

Author contribution statement

S.L. and H.H. analyzed the literature and contributed to the structuring of the manuscript. C.-H.K. supervised the project and led the writing of the manuscript.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2019R1C1C1003356).

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Cite this article as: Seunghyuk Lee, Heesung Han, Chang-Hyun Kim, Nanodielectrics approaches to low-voltage organic transistors and circuits, Eur. Phys. J. Appl. Phys. 91, 20201 (2020)

All Figures

thumbnail Fig. 1

The predictive calculation of ID in OFETs under a number of different combinations of (VGVT) and tdiel. The semi-transparent band between 1 and 10 μA indicates the relevant current level to be produced in technological applications.

In the text
thumbnail Fig. 2

(a) Reaction scheme illustrating the cross-linking of PMMA insulator. (b) Current densities as a function of electric field measured on an MIM capacitor made of c-PMMA with different thicknesses. Reproduced with permission from [25]. Copyright Nature Publishing Group, 2007. (c) Transfer characteristics of the n-type TIPS-TPDO-tetraCN transistors with the c-PMMA dielectric showing a low gate-leakage current (IG) and a mechanical bending stability. Reproduced with permission from [26]. Copyright American Chemical Society, 2016. (d) Transfercharacteristics of the TIPS-pentacene: PS OFET incorporating the TAAC-synthesized polymer dielectric. Reproducedwith permission from [27]. Copyright Royal Society of Chemistry, 2014. (e) Output characteristics of the DNTT-based OFET comprising the poly(MSEMA-co-GMA) dielectric. Reproduced with permission from [28]. Copyright Royal Society of Chemistry, 2017.

In the text
thumbnail Fig. 3

(a) Schematic illustration of the OFETs incorporating self-assembled nanodielectrics with different molecular structures. (b) Leakage current density versus applied voltage in MIS capacitors. Reproduced with permission from [34]. Copyright National Academy of Sciences, 2005. (c) Transfer characteristics of the p-type OFETs based on HC14 -PA and FC18-PA SAM dielectrics. Reproduced with permission from [35]. Copyright Wiley-VCH, 2014. (d) Photograph of the low-voltage OFET circuits on a banknote. (e) Signal delay per stage performances measured on the 11-stage complementary ring oscillators. Reproduced with permission from [36]. Copyright Wiley-VCH, 2018.

In the text
thumbnail Fig. 4

(a) Illustration of the solution-based sol-gel synthesis of TiO2 dielectric film on the OFET substrate. (b) Cross-sectional TEM image of the TiO2 film annealed at 250 °C (scale bar: 10 nm). Reproduced with permission from [54]. Copyright American Chemical Society, 2015. (c) Chemical structure of the BTMH cross-linker. (d) Structure of the complementary organic inverter based on the hybrid CZB dielectric. Reproduced with permission from [57]. Copyright Institute of Physics, 2015. (e) XRD pattern of the pentacene thin film grown on the surface of sol-gel Al2 O3 (inset: AFM image). (f) Voltage-transfer characteristics of the organic complementary inverter at different supply voltages (VDD) (inset: circuit diagram). Reproduced with permission from [58]. Copyright Elsevier, 2016.

In the text

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