Bottom-contact Field-effect Transistors Research Articles

Minority carrier diffusion length in interface-confined PbS colloidal quantum dot (CQD): Effect of CQD surface modification on diffusion length in ultrathin PbS CQD films

Photocurrent microscopy (PCM) is a versatile tool for probing charge carrier transport associated with energy band bending at an interface. For the first time, we report the ligand-dependent electronic properties of interface-confined colloidal quantum dots (CQDs) near a metal electrode and gate dielectric layers using ultrathin lead sulfide (PbS) CQD films embedded in field effect transistors (FETs). We have compared the minority carrier (electron) diffusion length (MCDL) of ultrathin PbS CQD films treated with 1,2-ethanedithiol (EDT) and ammonium sulfide ((NH4)2S) using a bottom-contact FET structure in combination with PCM. We found that the MCDL was greater in the EDT-treated PbS CQDs than in the (NH4)2S-treated PbS CQDs. The MCDL at the 2D carrier accumulation layer was observed to depend on the concentration of the majority carrier (hole). During effective electrical channel formation by electrical gating, the MCDL decreased with hole accumulation (decreasing gate voltage) in the p-channel (NH4)2S-treated PbS CQD FET, whereas it remained similar in the EDT-treated PbS CQD FET. The effect of the majority carrier concentration on the MCDL was more pronounced in the (NH4)2S-treated sample than in the EDT-treated sample. The difference in the MCDL is attributed to a difference in the surface trap density caused by different ligand treatments.

Electrochemical tuning of reduced graphene oxide in printed electrolyte-gated transistors. Impact on charge transport properties

In this work, a surfactant-free formulation of inkjet-printable graphene oxide (GO) is described. The morphology of the printed layers on gold bottom-contact field-effect transistors is studied using atomic force and optical microscopies. An in-situ approach of “green” electrochemical reduction of non-conducting GO into its conductive reduced form, i.e. rGO, is developed. A very significant effect of the reduction degree is observed on the electrical characterizations of rGO in the electrolyte-gated transistor configuration (EGFET), notably on its Dirac point (charge neutrality point) which can be shifted on a range of 1 V. This tuning of the doping level of rGO as a function of the reduction time goes hand in hand with the modulation of charge carriers' mobility over more than one order of magnitude.

Anisotropy of Charge Transport in a Uniaxially Aligned Fused Electron‐Deficient Polymer Processed by Solution Shear Coating

Precise control of the microstructure in organic semiconductors (OSCs) is essential for developing high-performance organic electronic devices. Here, a comprehensive charge transport characterization of two recently reported rigid-rod conjugated polymers that do not contain single bonds in the main chain is reported. It is demonstrated that the molecular design of the polymer makes it possible to achieve an extended linear backbone structure, which can be directly visualized by high-resolution scanning tunneling microscopy (STM). The rigid structure of the polymers allows the formation of thin films with uniaxially aligned polymer chains by using a simple one-step solution-shear/bar coating technique. These aligned films show a high optical anisotropy with a dichroic ratio of up to a factor of 6. Transport measurements performed using top-gate bottom-contact field-effect transistors exhibit a high saturation electron mobility of 0.2 cm2 V-1 s-1 along the alignment direction, which is more than six times higher than the value reported in the previous work. This work demonstrates that this new class of polymers is able to achieve mobility values comparable to state-of-the-art n-type polymers and identifies an effective processing strategy for this class of rigid-rod polymer system to optimize their charge transport properties.

Correlating Charge Transport with Structure in Deconstructed Diketopyrrolopyrrole Oligomers: A Case Study of a Monomer in Field-Effect Transistors.

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 Å, b = 13.02 Å, c = 5.85 Å, α = 101.4°, β = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with a = 24.92 Å, b = 25.59 Å, c = 5.42 Å, α = 80.3°, β = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm2/V s and an on/off ratio of 106 are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph.

A Delamination Strategy for Thinly Layered Defect-Free High-Mobility Black Phosphorus Flakes.

Extraordinary electronic and photonic features render black phosphorus (BP) an important material for the development of novel electronics and optoelectronics. Despite recent progress in the preparation of thinly layered BP flakes, scalable synthesis of large-size, pristine BP flakes remains a major challenge. An electrochemical delamination strategy is demonstrated that involves intercalation of diverse cations in non-aqueous electrolytes, thereby peeling off bulk BP crystals into defect-free flakes comprising only a few layers. The interplay between tetra-n-butylammonium cations and bisulfate anions promotes a high exfoliation yield up to 78 % and large BP flakes up to 20.6 μm. Bottom-gate and bottom-contact field-effect transistors, comprising single BP flakes only a few layers thick, exhibit a high hole mobility of 252±18 cm2 V-1 s-1 and a remarkable on/off ratio of (1.2±0.15)×105 at 143 K under vacuum. This efficient and scalable delamination method holds great promise for development of BP-based composites and optoelectronic devices.

Understanding charge trapping/detrapping at the zinc oxide (ZnO)/MAPbI3 perovskite interface in the dark and under illumination using a ZnO/perovskite/ZnO test platform.

We fabricated a zinc oxide (ZnO)/methylammonium lead iodide (MAPbI3) perovskite/ZnO field effect transistor (FET) test platform device through which ZnO/perovskite interfacial contact properties can be probed in the dark and under illumination. Using pulsed laser deposition, highly conductive (0.014 Ω cm) ZnO source and drain electrodes were fabricated allowing for the investigation of the interfacial charge transfer properties through current-voltage characteristics of a ZnO/perovskite/ZnO FET. With a bottom-contact FET device, gate voltage dependent current hysteresis in the drain current-gate voltage curves was probed at low temperature to minimize the effect of ion migration on electronic charge transport in the perovskite layer. Under illumination, importantly, ZnO/perovskite electrical contact properties were significantly altered due to electronic energy barrier change at the interface arising from the detrapping of electrons from the ZnO/perovskite interface, resulting in an enhanced dark current and a suppressed photocurrent. The origin of current hysteresis in the ZnO/perovskite/ZnO FET device is discussed relating it to interfacial charging/discharging associated with ultraviolet (UV)-induced oxygen adsorption/desorption. The results presented herein demonstrate that interfacial electronic properties at the donor (perovskite)/acceptor (ZnO) interface can be altered by photoinduced carrier trapping/detrapping, providing insights that UV-induced persistent photoconduction in transition metal oxide electron transport layers including ZnO may be contributing to the current hysteresis observed in the perovskite photovoltaic devices.

Field-effect-induced transport properties of Zn1−xMgxO nanocrystal solid solution

We report electrical properties of Zn1−xMgxO nanocrystal solid solution (NCSS) depending on the composition of Mg using a bottom-contact field-effect transistor. In the Zn1−xMgxO NCSS, as the composition of Mg increases, the field-effect mobility decreases with the threshold voltage shifting to a more positive value. The decrease in the field-effect mobility is attributed to the decrease in the size of the Zn1−xMgxO NCSS. The increase in the electron trap density in the Zn1−xMgxO NCSS with the addition of Mg caused a more positive threshold voltage shift. Change in the trap density as a function of Mg composition was demonstrated through comparison of the photoluminescence intensity.

Enhanced ambipolar charge transport in staggered carbon nanotube field-effect transistors for printed complementary-like circuits

Semiconducting single-walled carbon nanotubes (semi-SWNTs) are attractive as they can be used enable high-performance nano-electronic devices. We report the enhanced ambipolar charge transport characteristics of the semi-SWNT field-effect transistors (FETs) based on a top-gated staggered device structure. SWNTs were selectively dispersed in non-polar organic solvents and sorted by a π-conjugated polymer wrapping method. The sorted semi-SWNTs solution was used simply to form a well-percolated CNT-network, and the top-gate and bottom-contact FETs showed relatively high and equivalent electron and hole mobilities with very high on/off-current ratios and steep subthreshold slopes. The equivalent ambipolar charge transport behavior of semi-SWNTs was used to demonstrate a reliable complementary-like electronic circuits. The inverters showed a good switching threshold near the ideal point at half the driving bias, high gain, low hysteresis, and stability under repeatable operating conditions. They can thus be broadly applied as a fundamental circuit element in printed and flexible electronics.

Effect of heterocycles on field-effect transistor performances of donor-acceptor-donor type small molecules

Two D–A–D small molecules comprising triphenylamine and diketopyrrolopyrrole were synthesized having either furan or thiophene connected to the fused lactam ring. In this design, furan/thiophene diketopyrrolopyrrole acts as an acceptor and triphenylamine acts as a donor. Propeller shaped triphenylamine has its effect on packing, processability and plays a vital role in determining the π-π molecular orbital stacking in such compounds and thus the mobility of charge carriers. With TDPPT and FDPPT, maximum hole carrier mobility obtained is 2.88×10−3cm2V−1s−1 and 1.60×10−3cm2V−1s−1, respectively using bottom gate bottom contact field-effect transistor.

Influence of ligand exchange on the electrical transport properties of PbS nanocrystals

In this work, FETs are used as a research tool to study charge carrier mobilities in PbS nanocrystals (NCs) thin-films employed as semiconducting layer in bottom-gate bottom-contact (BGBC) field-effect transistors (FETs). The as-synthesised NCs are surrounded by long alkyl chain ligands which act as electrical insulators. Therefore, a ligand exchange process with shorter molecules is necessary to enhance the free charges generation and transport. We used two different ligands: 1,2 ethandithiol (EDT) and 1,2,3,4-tetrabutylammonium iodide (TBAI) and studied the charge mobility of PbS NCs comparing the electrical characteristics of FETs made by exchanged NCs. We analysed also the contemporary presence of both exchanged nanocrystals on the device. All the transistors showed p-type transport behaviour, enhanced by an annealing process at 100 °C for 10 min. After this, only the TBAI-treated NCs devices showed a n-type transport, resulting in an ambipolar behaviour. Inkjet printing deposition techniques was also successfully used to deposit PbS-(TBAI) NCs and ambipolar devices were obtained. In addition, for printed devices it was found that it is possible to modulate the charge transport properties by applying surface treatment to the substrate with a pentafluorothiophenol (PFTP). Indeed in this case, the p-type transport was suppressed while n-type behaviour was induced.

Polymorphism in Crystalline Microfibers of Achiral Octithiophene: The Effect on Charge Transport, Supramolecular Chirality and Optical Properties

Polymorphic crystalline microfibers from an achiral octithiophene with one S‐hexyl substituent per ring are separately and reproducibly grown with the same characteristics on various solid surfaces, including the interdigitated electrodes/SiO2 surface of a bottomcontact field‐effect transistor. The arrangement of the same molecule in two diverse supramolecular structures leads to markedly different electronic, optical, and charge mobility properties. The microfibers—straight and yellow emitting or helical and red emitting—exhibit p‐type charge transport characteristics, with the yellow ones showing a charge mobility and on/off current ratio of one and three orders of magnitude, respectively, greater than the red ones. Both forms show circular dichroism signals with significant differences from one form to the other. DFT calculations show that the octithiophene exists in two different quasi‐equienergetic conformations aggregating with diverse orientations of the substituents. This result suggests that the observed polymorphism is conformational in nature. The self‐assembly, driven by sulfur–sulfur non‐bonding interactions, amplifies the small property differences between conformers, leading to quite different bulk properties.

Blending effect of 6,13-bis(triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric

Solution processible poly(4-vinylphenol) is employed as a transistor dielectric material for low cost processing on flexible substrates at low temperatures. A 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene–graphene hybrid semiconductor is drop cast to fabricate bottom-gate and bottom-contact field-effect transistor devices on flexible and glass substrates under an ambient air environment. A few layers of graphene flakes increase the area in the conduction channel, and form bridge connections between the crystalline regions of the semiconductor layer which can change the surface morphology of TIPS pentacene films. The TIPS pentacene–graphene hybrid semiconductor-based organic thin film transistors (OTFTs) cross-linked with a poly(4-vinylphenol) gate dielectric exhibit an effective field-effect mobility of 0.076 cm2 V−1 s−1 and a threshold voltage of −0.7 V at V gs = −40 V. By contrast, typical TIPS pentacene shows four times lower mobility of 0.019 cm2 V−1 s−1 and a threshold voltage of 5 V. The graphene/TIPS pentacene hybrids presented in this paper can enhance the electrical characteristics of OTFTs due to their high crystallinity, uniform large-grain distribution, and effective reduction of crystal misorientation of the organic semiconductor layer, as confirmed by x-ray diffraction spectroscopy, atomic force microscopy, and optical microscopy studies.

Charge Transport in Ordered and Disordered Regions in Pristine and Sonicated-Poly(3-hexylthiophene) Films

Spectral trap density of states (DOS) and intrinsic mobilities in pristine and sonicated P3HT films were calculated and compared using temperature-dependent field-effect charge transport measurements. With sonication of P3HT solution, the field effect transistor (FET) hole mobility in its film state was improved remarkably in comparison with that in a pristine-poly(3-hexylthiophene) (P3HT) film. Spectral trap DOS were calculated through gate voltage-dependent activation energy measurements using the bottom-contact field effect transistors (FETs). For the pristine-P3HT films, the width of the shallow trap DOS was increased, indicating increased disorder in comparison with that for the sonicated-P3HT films. Further, the intrinsic mobility, 0.155 cm2/(V s), in the sonicated-P3HT film was far larger than that, 0.018 cm2/(V s), in the pristine one, suggesting that the structural properties of the P3HT films in the ordered region close to the gate dielectric are different. This reveals that a higher mobility in the sonicated-P3HT film originates from more efficient hole transport in the ordered P3HT region as well as less trap densities in the disordered region.

Synthesis of tetraselenophenoporphyrazine and its application in transistor devices

We report the first synthesis of a soluble selenophene containing porphyrazine derivative, tetraselenophenoporphyrazine (SePz). Thin films of SePz exhibit a broad and strong absorption in the visible to near-IR region, which is red shifted compared to the thiophene analogue, and demonstrate p-type carrier mobilities of 10−2 cm2 V−1 s−1 in bottom gate, bottom contact field effect transistors.

Dynamic Infrared Electro-optic Response of Soluble Organic Semiconductors in Thin Film Transistors

ABSTRACTWe use a frequency-dependent electro-optic technique to measure the hole mobility in small molecule organic semiconductors, such as 6,13 bis(triisopropylsilylethynyl)-pentacene. Measurements are made on semiconductor films in bottom gate, bottom contact field-effect transistors (FETs.) Because of the buried metal layer effect the maximum response, due to absorption in the charge layer, will be for a dielectric film ∼ 1/4 of a wavelength (in the dielectric) (e.g. ∼ 1 micron thick in the infrared.) Results are presented for FETs prepared with both spin-cast polymer and alumina dielectrics prepared by atomic layer deposition. At low frequencies the results are fit to solutions to a non-linear differential equation describing the spatial dependence of flowing charge in the FET channel, which allows us to study multiple crystals forming across one set of drain-source contacts. FETs prepared on alumina dielectrics show interesting deviations from the model at high frequencies, possibly due to increased contact impedance.

Fullerene/Cobalt Porphyrin Hybrid Nanosheets with Ambipolar Charge Transporting Characteristics

A novel supramolecular nanoarchitecture, comprising C(60)/Co porphyrin nanosheets, was prepared by a simple liquid-liquid interfacial precipitation method and fully characterized by means of optical microscopy, AFM, STEM, TEM, and XRD. It is established that the highly crystalline C(60)/Co porphyrin nanosheets have a simple (1:1) stoichiometry, and when incorporated in bottom-gate, bottom-contact field-effect transistors (FETs), they show ambipolar charge transport characteristics.

Synthesis and properties of small band gap thienoisoindigo based conjugated polymers

Using Stille and Suzuki polymerization reactions we incorporate thienoisoindigo (TII) as an acceptor co-monomer into a series of alternating π-conjugated copolymers with combinations of benzene, thiophene and carbazole as donor co-monomers. By changing the nature and length of the donor segments, the optical band gap of these soluble TII copolymers can be varied over a large range from 1.52 eV down to 0.87 eV. The semiconducting properties of the TII copolymers were established in bottom-gate bottom-contact field-effect transistors that provide hole mobilities for these materials in the range of 10−3 to 10−2 cm2 V−1 s−1.

Synthesis of a Novel Fused Thiophene‐thieno[3,2‐b]thiophene‐thiophene Donor Monomer and Co‐polymer for Use in OPV and OFETs

The synthesis of a novel fused hexacyclic electron rich monomer incorporating thieno[3,2-b]thiophene is reported and characterized by single crystal X-ray diffraction. Suzuki co-polymerization with benzothiadiazole (BT) afforded a novel low band-gap polymer P4TBT with high molecular weights and good solution processability. Bulk heterojunction solar cell devices using the P4TBT and [70]PCBM gave power conversion efficiencies of 2.5%. Top-gate, bottom-contact field effect transistors (FETs) using P4TBT displayed high hole mobilities of 0.07 cm(2) · Vs(-1) demonstrating the suitability of the novel monomer and polymer for use in high performing organic electronic devices.

Solution-Processed Molecular Bis(Naphthalene Diimide) Derivatives with High Electron Mobility

Top-gate bottom-contact field-effect transistors based on solution-processed films of molecules in which two naphthalene-1,8:4,5-bis(dicarboxdiimide)s are bridged by thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene and dithieno[3,2-b:2′,3′-d]pyrrole exhibit electron mobility values of up to 1.5 cm2 V−1 s−1.

Polymer infrared photo-detector with high sensitivity up to 1100 nm

We reported a low band-gap conjugated polymer, poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4- b]pyrazine] (PDTTP), was studied for the near infrared (NIR) photo-detector application. PDTTP shows intense absorption in NIR wavelength (to 1000 nm) and the estimated optical and electrochemical band-gaps of PDTTP are quite small around 1.15 eV and 1.08 eV, respectively. The low band-gap and the extended long wavelength absorption originates from the introduction of alternating TP units when its parent polythieno[3,4- b]pyrazine shows excellent narrow band-gap properties. Therefore, the relatively low band-gap and intense absorption in long wavelength of PDTTP make itself a promising candidate for near-infrared photo-detector. The hole mobility of the PDTTP measured from the bottom contact field effect transistor is around 1.40 × 10 −3 cm 2/V s with a on/off ratio of 2100. The photo-detector based on bulk hetero-junction PDTTP and (6,6)-phenyl-C61-butyric acid methyl ester blend (PCBM) has the incident photon-to-electron conversion efficiency 28.9% at 1000 nm (−5 V) and 6.2% at 1100 nm (−5 V). This photo-detector can be operated at a high-speed of 1 MHz. The experimental result suggests the potential applications of low band-gap conjugated polymers on near-infrared photo-detectors.