Organic Field-effect Transistor Structure Research Articles

Impact of Narrowing Density of States in Semiconducting Polymers on Performance of Organic Field-Effect Transistors.

To improve the performance of organic field-effect transistors (OFETs) employing π-conjugated polymers, a basic understanding of the relationships between the material properties and device characteristics is crucial. Although the density of states (DOS) distribution is one of the essential material properties of semiconducting polymers, insights into how the DOS shape affects the mobility (µ), subthreshold swing (S), and contact resistance (RC ) in OFETs remain lacking. In this study, by combining sensitive DOS measurements and multilayered OFET structures, it is experimentally demonstrated that narrower DOS widths in the polymer channels lead to higher µ, smaller S, and lower RC . By contrast, variation of the DOS in the bulk layer does not affect the performance. These results demonstrate a direct relationship between the polymer properties and OFET performance and highlight the importance of controlling the DOS width in π-conjugated polymers.

Fabrication and Characterization of P3HT/MoS₂ Thin-Film Based Ammonia Sensor Operated at Room Temperature

The composites of polymers and two-dimensional (2D) materials show promising applications in the area of gas sensing. In this work, we report an efficient ammonia gas (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) sensor based on poly(3-hexylthiophene)/ molybdenum disulfide (P3HT/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanocomposite. The sensing device has been fabricated in bottom-gate top contact organic field-effect transistor (OFET) assembly using P3HT/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as active channel material. The changes in the electrical response of OFET has been measured and analyzed for various ammonia gas concentration at room temperature. The sensing device in the form of an OFET structure is preferred due to multi-parameter characteristics to explore gas sensing applications. The active sensing layer has been fabricated via a self-assembled, cost-effective floating film transfer (FTM) technique. An optimized uniform sensing film of thickness 25±3 nm is used to analyze the change in the electrical characteristic of the device in terms of I/O characteristics, mobility, threshold voltage, trapped charge density, etc., for various ammonia concentrations. The OFET with nanofiber morphology of mobility 0.147 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s shows the threshold voltage of −3.78 V (in the air) and changes to −10.71 V after 100 ppm ammonia gas exposure. The device has shown a limit of detection (LOD) of 904 ppb and a sensing response of 63.45% at 100 ppm ammonia concentration. The extraction of multi-parameter sensing characteristics of the device has been conducted in a closed chamber with an ambient environment. The fabricated sensor is therefore having potential applications in high-performance ammonia sensing applications.

The Quinonoid Zwitterion Interlayer for the Improvement of Charge Carrier Mobility in Organic Field-Effect Transistors.

The interface between the semiconductor and the dielectric layer plays a crucial role in organic field-effect transistors (OFETs) because it is at the interface that charge carriers are accumulated and transported. In this study, four zwitterionic benzoquinonemonoimine dyes featuring alkyl and aryl N-substituents were used to cover the dielectric layers in OFET structures. The best interlayer material, containing aliphatic side groups, increased charge carrier mobility in the measured systems. This improvement can be explained by the reduction in the number of the charge carrier trapping sites at the dielectric active layer interface from 1014 eV−1 cm−2 to 2 × 1013 eV−1 cm−2. The density of the traps was one order of magnitude lower compared to the unmodified transistors. This resulted in an increase in charge carrier mobility in the tested poly [2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT)-based transistors to 5.4 × 10−1 cm2 V−1 s−1.

Investigation of charge carrier mobility and recombination in PBDTTPD thin layer structures

Abstract In this paper we present investigation of hole transport properties in sandwich and OFET structures with single active layer of PBDTTPD (Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]]). Sandwich structures were investigated by photo-CELIV and TOF techniques, obtained results showed strongly time dependent hole mobility and non-Gaussian DOS tail. Photogenerated charge carrier decay experiment demonstrated that bimolecular recombination coefficient is smaller than calculated Langevin recombination coefficient and this was explained by faster holes escaping recombination area and not participating in Langevin recombination process. Organic field-effect transistor structure was investigated by current transients technique to find hole mobility near the dielectric layer and to study OTS treatment influence on hole transport. The study of hole mobility dependence on temperature was performed in order to evaluate energetic disorder of interface DOS in the channel of OFET structures.

Light-Sensitive Material Structure-Electrical Performance Relationship for Optical Memory Transistors Incorporating Photochromic Dihetarylethenes.

Photoswitchable organic field-effect transistors (OFETs) with embedded photochromic materials are considered as a promising platform for development of organic optical memory devices. Unfortunately, the operational mechanism of these devices and guidelines for selection of light-sensitive materials are still poorly explored. In the present work, a series of photochromic dihetarylethenes with a cyclopentenone bridge moiety were investigated as a dielectric/semiconductor interlayer in the structure of photoswitchable OFETs. It was shown that the electrical performance and stability of the devices can be tuned by variation of the substituents in the structure of the photochromic material. In particular, it was found that dihetarylethenes with donor substituents demonstrated the best light-induced switching effects (wider memory windows and higher switching coefficients) in the devices. The operation mechanism of the light-triggered memory devices was proposed based on the differential in situ Fourier transform infrared (FTIR) spectroscopy data and regression analysis of the threshold voltage-programming time experimental dependencies. The established relationships will facilitate further rational design of new photochromic materials, thus paving a way to fast and durable organic optical memories and memory transistors (memristors).

Ferrocene on Insulator: Silane Coupling to a SiO2 Surface and Influence on Electrical Transport at a Buried Interface with an Organic Semiconductor Layer.

A silane coupling-based procedure for decoration of an insulator surface containing abundant hydroxy groups by constructing redox-active self-assembled monolayers (SAMs) is described. A newly synthesized ferrocene (Fc) derivative containing a triethoxysilyl group designated FcSi was immobilized on SiO2/Si by a simple operation that involved immersing the substrate in a toluene solution of the Fc silane coupling reagent and then rinsing the resulting substrate. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the Fc group was immobilized on SiO2/Si in the Fe(II) state. Cyclic voltammetry measurements showed that the Fc groups were electrically insulated from the Si electrode by the SiO2 layer. The FcSi on SiO2/Si structures were found to serve as a good scaffold for formation of organic semiconductor thin films by vacuum thermal evaporation of C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), which is well-known as an organic field-effect transistor (OFET) material. The X-ray diffraction profile indicated that the conventional standing-up conformation of the C8-BTBT molecules perpendicular to the substrates was maintained in the thin films formed on FcSi@SiO2/Si. Further vacuum thermal evaporation of Au provided an FcSi-based OFET structure with good transfer characteristics. The FcSi-based OFET showed pronounced source-drain current hysteresis between the forward and backward scans. The degree of this hysteresis was varied reversibly via gate bias manipulation, which was presumably accompanied by trapping and detrapping of hole carriers at the Fc-decorated SiO2 surface. These findings provide new insights into application of redox-active SAMs to nonvolatile OFET memories while also creating new interfaces through junctions with functional thin films, in which the underlying redox-active SAMs play supporting roles.

Anisotropy of charge carrier transport in PCPDTBT field-effect transistor structures

In this paper we are analyzing anisotropy of hole mobility in the organic field-effect transistor structures with an active layer of PCPDTBT (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]). Hole mobility and its dependence on the temperature was measured by i-CELIV and current transients methods. The combination of both techniques allowed us to obtain results of hole mobilities in perpendicular and parallel to the substrate directions. In the same temperature range differential scanning calorimetry (DSC) measurements were performed to detect morphological changes in the active layer of the field-effect transistor structure. From obtained results of mobility dependence on temperature, energetic disorder parameter of Bässler’s Gaussian Disorder Model (GDM) was estimated. DSC data and hole transport data demonstrated that the change in PCPDTBT’s morphology is responsible for the abrupt decrease of hole mobility on temperature in the field-effect transistor structure.

Static Polystyrene Gate Charge Density Modulation of Dinaphthothienothiophene with Tetrafluorotetracyanoquinodimethane Layer Doping: Evidence from Conductivity and Seebeck Coefficient Measurements and Correlations

We relate the static charging of organic field effect transistor (OFET) gate dielectric polymers, the resulting change in threshold voltage shift, and the modulation of thermoelectric properties of the transistor semiconductor. We utilize a bottom gate OFET structure with cross-linkable polystyrene and plain polystyrene (XLPS/PS) bilayer dielectrics and dinaphthothienothiophene (DNTT) organic molecular semiconductor with tetrafluorotetracyanoquinodimethane (F4TCNQ) doping layer. The conductivity and Seebeck coefficient are measured before and after charging. We find that the conductivity increases significantly, while the Seebeck coefficient decreases correspondingly in a trend of S ∝ ln(σ) after the dielectrics were negatively charged, resulting in an average of 5-fold increase in power factor at the maximum charging level. Additionally, temperature-dependent conductivity measurements show that the activation energy decreases with the increasing conductivity. We calculate the Seebeck coefficient based on the activation energy and find the result is consistent with the Mott mobility edge model. This work confirms the charging mechanism in our previous published work, including the location of static charges in the bulk of the dielectric, and demonstrates modulation and enhancement of the thermoelectric properties of organic thermoelectric materials by adjacent chargeable dielectrics without changing the molecular microstructure or applying external gate voltage during operation.

High-Performance Organic Field-Effect Transistors Fabricated with High-k Composite Polymer Gel Dielectrics

This study presents a composite gel-like dielectric material for organic field-effect transistors (OFET) applications. Poly(methyl methacrylate) (PMMA) gelled with propylene carbonate was used as gel dielectric material. Copper phthalocyanine was used as active layer in the OFET structures. In order to enhance the performance of the PMMA-gel dielectric material, silicium dioxide (SiO2) was used as an additive material. Various ratios of SiO2 were added to the gel dielectric and the effect of SiO2 on the OFET performance was investigated. It was clearly observed that SiO2 enhanced the performance and source-drain current of the fabricated OFETs. SiO2 was added to the PMMA-gel with different doping ratios of 0%, 10%, 30%, 50% and 100% by using a solution-processing method. The dielectric properties of the PMMA-gel:SiO2 composite materials were analyzed with impedance spectroscopy in terms of their effective capacitance ©I), tangent factor (tan(δ)), phase angle and complex dielectric constant (e′ and e″). The hole mobility of the OFETs was enhanced by 50% SiO2 nanoparticles in PMMA-gel dielectric materials from 6.83 × 10−1 cm2 V−1 s−1 to 4.66 × 100 cm2 V−1 s−1 (at VDS = − 0.5 V). The time-dependent IDS curves were analyzed for OFETs fabricated with PMMA-gel:SiO2 composite dielectric layers. It was found that all the devices worked stably under bias stress and gave fast responses for all gate voltages.

A method for direct contact resistance evaluation in low voltage coplanar organic field-effect transistors

In this paper, a method for the extrapolation of contact resistance in organic field-effect transistors (OFETs) from a single transfer characteristic curve in the linear regime is proposed. The method, namely DIrect Contact Resistance Extrapolation (DICRE), is based on the idea of making the current dependent only on contact resistance by setting the device in large over-threshold conditions. Constant contact resistance with respect to gate-to-source voltage is considered as an acceptable approximation, as confirmed by other examples in the literature. The effectiveness of the method is demonstrated by extrapolating the contact resistance of two different OFET structures (self-aligned and not self-aligned) and comparing the results with standard reference techniques, namely the Modified Transmission Line Method (M-TLM) and the Y function method. The results demonstrate that the DICRE method can be applied to low voltage devices without any damage to the gate insulator, even if the applied gate-to-source voltage drop is well beyond the values normally employed for transistor operation. The proposed method allows extrapolating a value of contact resistance comparable with the ones derived by TLM, with restrained variability. Moreover, the capability of properly recognizing the differences in contact resistance values between OFET structures with different features in terms of source/drain-gate overlap is reported. Finally, the possibility of correctly deriving the contact resistance dependence on drain-to-source voltage using DICRE is discussed.

An Inkjet‐Printed, Ultralow Voltage, Flexible Organic Field Effect Transistor

A very simple procedure for fabricating inkjet‐printed organic field effect transistors (OFETs) is reported. A reliable process for the deposition of a thin and uniform polymeric dielectric film of poly(4‐vinylphenol) (PVP) is established as a key factor for obtaining high performance devices operating at low voltages. To this aim, ink formulations, printing parameters, and cross‐linking processes are investigated. Morphological characterization of the fabricated films by means of contact profilometry and atomic force micro­scopy is provided, as well as capacitive measurements proving ideal dielectric properties. OFET structures based on PVP gate dielectric are reported: in particular, inkjet‐printed devices operated at voltages below 1 V with remarkable transistor performances such as high charge carrier mobility and low subthreshold swing are presented.

ChemInform Abstract: Proteins as Functional Interlayer in Organic Field‐Effect Transistor

AbstractReview: 29 refs.

Field-effect transistor structures on the basis of poly(3-hexylthiophene), fullerene derivatives [60]PCBM, [70]PCBM, and nickel nanoparticles

Organic field-effect transistor (OFET) structures with the active layers on the basis of composite films of semiconductor polymer poly(3-hexylthiophene) (P3HT), fullerene derivatives [60]PCBM, [70]PCBM, and nickel (Ni) nanoparticles are obtained, and their optical, electrical, and photoelectrical properties are studied. It is shown that introducing Ni nanoparticles into P3HT: [60]PCBM and P3HT: [70]PCBM films leads to an increase in the absorption and to quenching of photoluminescence of the composite in the 400–600 nm spectral band due to the plasmon effect. In P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFET structures at the P3HT: [60]PCBM and P3HT: [70]PCBM concentrations of ~1: 1 and Ni concentrations of ~3–5 wt %, current–voltage (I–V) characteristics typical of ambipolar OFETs with the dominant hole conduction are observed. The charge-carrier (hole) mobilities calculated from the I–V characteristic at VG =–10 V were found to be ~0.46 cm2/(V s) for P3HT: [60]PCBM: Ni and ~4.7 cm2/(V s) for P3HT: [70]PCBM: Ni, which means that the mobility increases if [60]PCBM in the composition is replaced with [70]PCBM. The effect of light on the I–V characteristics of P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFETs is studied.

Proteins as functional interlayer in organic field-effect transistor

Abstract The paper summarizes and discusses the recent advances of proteins as functional interlayers in organic field-effect transistors (OFETs). Specific focus is given on the proteins integrated into the device structure, either to act as dielectric materials or to perform as the functional interlayer between the dielectric and the organic semiconductor (OSC). The main emphasis is give to the location and the specific effect of protein layers in the structure of OFETs. Besides, the possibility of amyloid serving as useful building blocks for OFET is discussed.

Poly(9-vinylcarbazole)–graphene oxide composite field-effect transistors with enhanced mobility

We report on a solution processed hybrid organic field-effect transistors (OFETs) incorporating both organic semiconductor – poly(9-vinylcarbazole) (PVK) and graphene oxide (GO) flakes. We have found that PVK:GO hybrid OFETs with Au–Al source–drain electrodes exhibit the mobility as high as 3.8–6.7cm2V−1s−1 under concentration of the GO flakes varies from 3 up to 9.5wt.% respectively. The on/off ratio of PVK:GO hybrid OFETs is found to be ∼102–103 at source–drain and gate operating voltages below ∼5V. The reversible switching of the source–drain current (from low to high ISD and back) is found at positive and negative VSD∼12–13V in the PVK:GO OFET structures (GO∼3wt.%) with Au–Al source–drain electrodes. The possible mechanism of enhancing field-effect mobility in the PVK:GO OFET structures is discussed.

Electronic transduction of proton translocations in nanoassembled lamellae of bacteriorhodopsin.

An organic field-effect transistor (OFET) integrating bacteriorhodopsin (bR) nanoassembled lamellae is proposed for an in-depth study of the proton translocation processes occurring as the bioelectronic device is exposed either to light or to low concentrations of general anesthetic vapors. The study involves the morphological, structural, electrical, and spectroscopic characterizations necessary to assess the functional properties of the device as well as the bR biological activity once integrated into the functional biointerlayer (FBI)-OFET structure. The electronic transduction of the protons phototranslocation is shown as a current increase in the p-type channel only when the device is irradiated with photons known to trigger the bR photocycle, while Raman spectroscopy reveals an associated C═C isomer switch. Notably, higher energy photons bring the cis isomer back to its trans form, switching the proton pumping process off. The investigation is extended also to the study of a PM FBI-OFET exposed to volatile general anesthetics such as halothane. In this case an electronic current increase is seen upon exposure to low, clinically relevant, concentrations of anesthetics, while no evidence of isomer-switching is observed. The study of the direct electronic detection of the two different externally triggered proton translocation effects allows gathering insights into the underpinning of different bR molecular switching processes.

Electric field dependence of charge carrier hopping transport within the random energy landscape in an organic field effect transistor

We extended our analytical effective medium theory [Phys. Rev. B 81, 045202 (2010)] to describe the temperature-dependent hopping charge carrier mobility at arbitrary electric fields in the large carrier density regime. Special emphasis was made to analyze the influence of the lateral electric field on the Meyer--Neldel (MN) phenomenon observed when studying the charge mobilities in thin-film organic field-effect transistors (OFET). Our calculations are based on the average hopping transition time approach, generalized for large carrier concentration limit finite fields, and taking into account also spatial energy correlations. The calculated electric field dependences of the hopping mobility at large carrier concentrations are in good agreement with previous computer simulations data. The shift of the MN temperature in an OFET upon applied electric field is shown to be a consequence of the spatial energy correlation in the organic semiconductor film. Our calculations show that the phenomenological Gill equation is clearly inappropriate for describing conventional charge carrier transport at low carrier concentrations. On the other hand a Gill-type behavior has been observed in a temperature range relevant for measurements of the charge carrier mobility in OFET structures. Since the present model is not limited to zero-field mobility, it allows a more accurate evaluation of important material parameters from experimental data measured at a given electric field. In particular, we showed that both the MN and Gill temperature can be used for estimating the width of the density of states distribution.

Dimensionality of charge transport in organic field-effect transistors

Application of a gate bias to an organic field-effect transistor leads to accumulation of charges in the organic semiconductor within a thin region near the gate dielectric. An important question is whether the charge transport in this region can be considered two-dimensional, or whether the possibility of charge motion in the third dimension, perpendicular to the accumulation layer, plays a crucial role. In order to answer this question we have performed Monte Carlo simulations of charge transport in organic field-effect transistor structures with varying thickness of the organic layer, taking into account all effects of energetic disorder and Coulomb interactions. We show that with increasing thickness of the semiconductor layer the source-drain current monotonically increases for weak disorder, whereas for strong disorder the current first increases and then decreases. Similarly, for a fixed layer thickness the mobility may either increase or decrease with increasing gate bias. We explain these results by the enhanced effect of state filling on the current for strong disorder, which competes with the effects of Coulomb interactions and charge motion in the third dimension. Our conclusion is that apart from the situation of a single monolayer, charge transport in an organic semiconductor layer should be considered three-dimensional, even at high gate bias.

Influence of an Applied Electric Field on the Conduction Characteristics of a Bithienyl-Capped Biphenylene and Biphenyl-Capped Oligothiophenes in Organic Field-Effect Transistor Structures

ABSTRACTThin films (50 nm) of 2,5-di-4-biphenylthiophene (PPTPP), 5,5´-di-4-biphenylyl-2,2´-bithiophene (PPTTPP) and 4,4´-di-2,2´-bithienylbiphenyl (TTPPTT) were vapor-deposited on microstructured gold (source- and drain-) electrode arrays on thermal SiO2 as gate dielectric with underlying Si serving as gate electrode. The films were studied in their field-effect characteristics during film growth and subsequent to it. A decay of specific conductivity and of charge carrier mobility was observed in subsequent measurements. During annealing without an applied field the films recovered but showed a second decay as soon as an electric field was applied again for repeated characterization. Induced dipoles and subsequent structural changes as well as chemical interactions with the SiO2 interface are discussed as possible origins of these observations.

Nonvolatile memory based on pentacene organic field-effect transistors with polystyrene para-substituted oligofluorene pendent moieties as polymer electrets

We report the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFET) using polystyrenepara-substituted with π-conjugated oligofluorenes (P(St-Fl)n (n = 1–3)) as chargeable polymer electrets. Effects of fluorene conjugated length on the surface structure and memory characteristics of pentacene OFET were investigated. Among these polymer electrets, the device with the P(St-Fl) exhibited the highest field-effect mobility of 0.47 cm2 V−1 s−1 due to the largest grain size of pentacene growth. The device with P(St-Fl)3 revealed the largest hysteresis window of 76 V due to it having the longest fluorene conjugation length among the studied electrets. The smallest difference of the HOMO energy level between pentacene and P(St-Fl)3 facilitated the charge transfer from pentacene to the polymer electret. The shifts on the transfer curves in both positive and negative directions could be reversibly controlled when applied an external gate bias of ±100 V for a short time (1 μs), indicating the fast trapping-detrapping ability of the polymer electrets. The devices showed excellent nonvolatile behaviors for bistable switching. The ON and OFF states were maintained over 104 s with the Ion/Ioff current ratio of 105–106. The write-read-erase-read (WRER) cycles could be operated over 100 cycles. This study suggested that surface characteristics, charge transport, and memory characteristics of pentacene-based OFET can be manipulated by polymer electrets with different pendent conjugation length.