Effect of Polymer Gate Dielectrics on Charge Transport in Carbon Nanotube Network Transistors: Low-k Insulator for Favorable Active Interface

Solution-processable low-voltage carbon nanotube field-effect transistors with high-krelaxor ferroelectric polymer gate insulator

Achieving energy-efficient and high-performance field-effect transistors (FETs) is one of the most important goals for future electronic devices. This paper reports semiconducting single-walled carbon nanotube FETs (s-SWNT-FETs) with an optimized high-krelaxor ferroelectric insulator P(VDF-TrFE-CFE) thickness for low-voltage operation. The s-SWNT-FETs with an optimized thickness (∼800 nm) of the high-kinsulator exhibited the highest average mobility of 14.4 cm2V-1s-1at the drain voltage (ID) of 1 V, with a high current on/off ratio (Ion/off>105). The optimized device performance resulted from the suppressed gate leakage current (IG) and a sufficiently large capacitance (>50 nF cm-2) of the insulating layer. Despite the extremely high capacitance (>100 nF cm-2) of the insulating layer, an insufficient thickness (<450 nm) induces a highIG, leading to reducedIDand mobility of s-SWNT-FETs. Conversely, an overly thick insulator (>1200 nm) cannot introduce sufficient capacitance, resulting in limited device performance. The large capacitance and sufficient breakdown voltage of the insulating layer with an appropriate thickness significantly improved p-type performance. However, a reduced n-type performance was observed owing to the increased electron trap density caused by fluorine proportional to the insulator thickness. Hence, precise control of the insulator thickness is crucial for achieving low-voltage operation with enhanced s-SWNT-FET performance.

Impact of Dielectric Environment on Trion Emission from Single-Walled Carbon Nanotube Networks

Trions are charged excitons that form upon optical or electrical excitation of low-dimensional semiconductors in the presence of charge carriers (holes or electrons). Trion emission from semiconducting single-walled carbon nanotubes (SWCNTs) occurs in the near-infrared and at lower energies compared to the respective exciton. It can be used as an indicator for the presence of excess charge carriers in SWCNT samples and devices. Both excitons and trions are highly sensitive to the surrounding dielectric medium of the nanotubes, having an impact on their application in optoelectronic devices. Here, the influence of different dielectric materials on exciton and trion emission from electrostatically doped networks of polymer-sorted (6,5) SWCNTs in top-gate field-effect transistors is investigated. The observed differences of trion and exciton emission energies and intensities for hole and electron accumulation cannot be explained with the polarizability or screening characteristics of the different dielectric materials, but they show a clear dependence on the charge trapping properties of the dielectrics. Charge localization (trapping of holes or electrons by the dielectric) reduces exciton quenching, emission blue-shift and trion formation. Based on the observed carrier type and dielectric material dependent variations, the ratio of trion to exciton emission and the exciton blue-shift are not suitable as quantitative metrics for doping levels of carbon nanotubes.

Charge Transport in and Electroluminescence from sp3-Functionalized Carbon Nanotube Networks

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp³ defects leads to additional narrow and tunable photoluminescence features in the near-infrared and even enables single-photon emission at room temperature, thus strongly expanding their application potential. However, the successful integration of sp³-functionalized SWCNTs in optoelectronic devices with efficient defect state electroluminescence not only requires control over their emission properties but also a detailed understanding of the impact of functionalization on their electrical performance, especially in dense networks. Here, we demonstrate ambipolar, light-emitting field-effect transistors based on networks of pristine and functionalized polymer-sorted (6,5) SWCNTs. We investigate the influence of sp³ defects on charge transport by employing electroluminescence and (charge-modulated) photoluminescence spectroscopy combined with temperature-dependent current-voltage measurements. We find that sp³-functionalized SWCNTs actively participate in charge transport within the network as mobile carriers efficiently sample the sp³ defects, which act as shallow trap states. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states that can be tuned by the defect density.

Conformally Gated Surface Conducting Behaviors of Single-Walled Carbon Nanotube Thin-Film-Transistors

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have gathered significant interest in various emerging electronics due to their outstanding electrical and mechanical properties. Although large-area and low-cost fabrication of s-SWCNT field effect transistors (FETs) can be easily achieved via solution processing, the electrical performance of the solution-based s-SWCNT FETs is often limited by the charge transport in the s-SWCNT networks and interface between the s-SWCNT and the dielectrics depending on both s-SWCNT solution synthesis and device architecture. Here, we investigate the surface and interfacial electro-chemical behaviors of s-SWCNTs. In addition, we propose a cost-effective and straightforward process capable of minimizing polymers bound to s-SWCNT surfaces acting as an interfering element for the charge carrier transport via a heat-assisted purification (HAP). With the HAP treated s-SWCNTs, we introduced conformal dielectric configuration for s-SWCNT FETs, which are explored by a carefully designed wide array of electrical and chemical characterizations with finite-element analysis (FEA) computer simulation. For more favorable gate-field-induced surface and interfacial behaviors of s-SWCNT, we implemented conformally gated highly capacitive s-SWCNT FETs with ion-gel dielectrics, demonstrating field-effect mobility of ~8.19 cm2/V⋅s and on/off current ratio of ~105 along with negligible hysteresis.

Ultra-high drivability, high-mobility, low-voltage and high-integration intrinsically stretchable transistors

Realizing intrinsically stretchable transistors with high current drivability, high mobility, small feature size, low power and the potential for mass production is essential for advancing stretchable electronics a critical step forward. However, it is challenging to realize these requirements simultaneously due to the limitations of the existing fabrication technologies when integrating intrinsically stretchable materials into transistors. Here, we propose a removal-transfer-photolithography method (RTPM), combined with adopting poly(urea-urethane) (PUU) as a dielectric, to realize integratable intrinsically stretchable carbon nanotube thin-film transistors (IIS-CNT-TFTs). The realized IIS-CNT-TFTs achieve excellent electrical and mechanical properties simultaneously, showing high field-effect-mobility up to 221 cm2 V-1 s-1 and high current density up to 810 μA mm-1 at a low driving voltage of -1 V, which are both the highest values for intrinsically stretchable transistors today to the best of our knowledge. At the same time, the transistors can survive 2000 cycles of repeated stretching by 50%, indicating their promising applicability to stretchable circuits, displays, and wearable electronics. The achieved intrinsically stretchable thin-film transistors show higher electrical performance, higher stretching durability, and smaller feature size simultaneously compared with the state-of-the-art works, providing a novel solution to integratable intrinsically stretchable electronics. Besides, the proposed RTPM involves adopting removable sacrificial layers to protect the PDMS substrate and PUU dielectric during the photolithography and patterning steps, and finally removing the sacrificial layers to improve the electrical and mechanical performance. This method is generally applicable to further enhance the performance of the existing transistors and devices with a similar structure in soft electronics.

Guiding Charge Transport in Semiconducting Carbon Nanotube Networks by Local Optical Switching

Photoswitchable, ambipolar field-effect transistors (FETs) are fabricated with dense networks of polymer-sorted, semiconducting single-walled carbon nanotubes (SWCNTs) in top-gate geometry with photochromic molecules mixed in the polymer matrix of the gate dielectric. Both hole and electron transport are strongly affected by the presence of spiropyran and its photoisomer merocyanine. A strong and persistent reduction of charge carrier mobilities and thus drain currents upon UV illumination (photoisomerization) and its recovery by annealing give these SWCNT transistors the basic properties of optical memory devices. Temperature-dependent mobility measurements and density functional theory calculations indicate scattering of charge carriers by the large dipoles of the merocyanine molecules and electron trapping by protonated merocyanine as the underlying mechanism. The direct dependence of carrier mobility on UV exposure is employed to pattern high- and low-resistance areas within the FET channel and thus to guide charge transport through the nanotube network along predefined paths with micrometer resolution. Near-infrared electroluminescence imaging enables the direct visualization of such patterned current pathways with good contrast. Elaborate mobility and thus current density patterns can be created by local optical switching, visualized and erased again by reverse isomerization through heating.

Composite Interlayer Consisting of Alcohol-Soluble Polyfluorene and Carbon Nanotubes for Efficient Polymer Solar Cells

We report the synthesis of composite interlayers using alcohol-soluble polyfluorene (ASP)-wrapped single-walled carbon nanotubes (SWNTs) and their application as electron-transport layers for efficient organic solar cells. The ASP enables the individual dispersion of SWNTs in solution. The ASP-wrapped SWNT solutions are stable for 54 days without any aggregation or precipitation, indicating their very high dispersion stability. Using the ASP-wrapped SWNTs as a cathode interlayer on zinc oxide nanoparticles (ZnO NPs), a power conversion efficiency of 9.45% is obtained in PTB7-th:PC₇₁BM-based organic solar cells, which is mainly attributed to the improvement in the short circuit current. Performance enhancements of 18 and 17% are achieved compared to those of pure ZnO NPs and ASP on ZnO NPs, respectively. In addition, the composite interlayer is applied to non-fullerene-based photovoltaics with PM6:Y6, resulting in a power conversion efficiency of up to 14.37%. The type of SWNT (e.g., in terms of diameter range and length) is not critical to the improvement in the charge-transport properties. A low density of SWNTs in the film (∼1 SWNTs/μm² for ASP-wrapped SWNTs) has a significant influence on the charge transport in solar cells. The improvement in the performance of the solar cell is attributed to the increased internal quantum efficiency, balanced mobility between electrons and holes, and minimized charge recombination.

Polymer Encapsulants for Threshold Voltage Control in Carbon Nanotube Transistors

Although carbon nanotube transistors present outstanding performances based on key metrics, large-scale uniformity and repeatability required in printable electronics depend greatly on proper control of the electrostatic environment. Through a survey of polymer dielectric encapsulants compatible with printing processes, a simple correlation is found between the measured interfacial charge density and the onset of conduction in a transistor, providing a rational route to control the electrical characteristics of carbon nanotube transistors. Smooth and continuous balancing of the properties between unipolar p-type and n-type transport is achieved using a molar fraction series of poly(styrene-co-2-vinylpyridine) statistical copolymers combined with an electron-donating molecule. We further demonstrate the easy fabrication of a p-n diode which shows a modest rectification of 8:1.

Spectroscopic methods for the study of energetic characteristics of the normal and photoproduct forms of 3-hydroxyflavones

We report spectroscopic properties of 3-hydroxyflavone (3-HF) and 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in acetonitrile and ethyl acetate at different temperatures in the range from 10 °C to about 67 °C. These compounds are characterized by excited-state intramolecular proton transfer (ESIPT) which leads to occurrence of two forms of these molecules. For this reason their fluorescence spectra have two bands which correspond to emission of normal and photoproduct (tautomer) forms. The correlation between ratio of integrated intensity of these two bands and inverse absolute temperature (the Arrhenius plot) have been applied to estimate energetic properties, such as difference between energy levels of excited states as well ground states for normal and tautomer forms for each molecule.

Performance enhancement of carbon nanotube thin film transistor by yttrium oxide capping

Carbon nanotube thin film transistors (CNT-TFTs) are regarded as promising technology for active matrix pixel driving circuits of future flat panel displays (FPD). For FPD application, unipolar thin film transistors (TFTs) with high mobility (μ), high on-state current (I_ON), low off-current (I_OFF) at high source/drain bias and small hysteresis are required simultaneously. Though excellent values of those performance metrics have been realized individually in different reports, the overall performance of previously reported CNT-TFTs has not met the above requirements. In this paper, we found that yttrium oxide (Y₂O₃) capping is helpful in improving both I_ON and μ of CNT-TFTs. Combining Y₂O₃ capping and Al₂O₃ passivation, unipolar CNT-TFTs with high I_ON/I_OFF (>10⁷) and low I_OFF (∼pA) at -10.1 V source/drain bias, and relatively small hysteresis in the range of -30 V to +30 V gate voltage were achieved, which are capable of active matrix display driving.

Investigating Limiting Factors in Stretchable All-Carbon Transistors for Reliable Stretchable Electronics

Stretchable form factors enable electronic devices to conform to irregular 3D structures, including soft and moving entities. Intrinsically stretchable devices have potential advantages of high surface coverage of active devices, improved durability, and reduced processing costs. This work describes intrinsically stretchable transistors composed of single-walled carbon nanotube (SWNT) electrodes and semiconductors and a dielectric that consists of a nonpolar elastomer. The use of a nonpolar elastomer dielectric enabled hysteresis-free device characteristics. Compared to devices on SiO₂ dielectrics, stretchable devices with nonpolar dielectrics showed lower mobility in ambient conditions because of the absence of doping from water. The effect of a SWNT band gap on device characteristics was investigated by using different SWNT sources as the semiconductor. Large-band-gap SWNTs exhibited trap-limited behavior caused by the low capacitance of the dielectric. In contrast, high-current devices based on SWNTs with smaller band gaps were more limited by contact resistance. Of the tested SWNT sources, SWNTs with a maximum diameter of 1.5 nm performed the best, with a mobility of 15.4 cm²/Vs and an on/off ratio >10³ for stretchable transistors. Large-band-gap devices showed increased sensitivity to strain because of a pronounced dependence on the dielectric thickness, whereas contact-limited devices showed substantially less strain dependence.

Epitaxial Growth of MOF Thin Film for Modifying the Dielectric Layer in Organic Field-Effect Transistors

Metal-organic framework (MOF) thin films are important in the application of sensors and devices. However, the application of MOF thin films in organic field effect transistors (OFETs) is still a challenge to date. Here, we first use the MOF thin film prepared by a liquid-phase epitaxial (LPE) approach (also called SURMOFs) to modify the SiO₂ dielectric layer in the OFETs. After the semiconductive polymer of PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b']dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]) was coated on MOF/SiO₂ and two electrodes on the semiconducting film were deposited sequentially, MOF-based OFETs were fabricated successfully. By controlling the LPE cycles of SURMOF HKUST-1 (also named Cu₃(BTC)₂, BTC = 1,3,5-benzenetricarboxylate), the performance of the HKUST-1/SiO₂-based OFETs showed high charge mobility and low threshold voltage. This first report on the application of MOF thin film in OFETs will offer an effective approach for designing a new kind of materials for the OFET application.