Semiconductor Pentacene Research Articles

Evolution of the pentacene exciton band width in pentacene-tetracene blends.

Pentacene is one of the most investigated organic semiconductors. It is well known that the motion of excitons in pentacene and other organic semiconductors is determined by inter-molecular exciton coupling based on charge-transfer processes. In the present study, we demonstrate the impact of the admixture of tetracene, which has a larger band gap and interrupts the pentacene-pentacene interaction, on the exciton behavior in pentacene. Using a combination of optical absorption and electron energy-loss spectroscopy, we show that both the Davydov splitting and the exciton band width in pentacene strongly decrease with increasing tetracene concentration, while the decrease of the exciton band width is substantially larger.

Mathematical modelling of tri-layer dielectric OTFT based on pentacene semiconductor for enhancing the electrical characteristics

Mathematical modelling of tri-layer dielectric OTFT based on pentacene semiconductor for enhancing the electrical characteristics

Strong exciton bandwidth reduction in pentacene as a function of temperature

The motion of excitons in organic semiconductors represents a key contribution to the performance of organic solar cells. It is determined by intermolecular exciton coupling. In momentum space, exciton coupling results in the exciton dispersion and defines the exciton bandwidth. We demonstrate that the exciton bandwidth in pentacene as measured using electron energy-loss spectroscopy is substantially temperature dependent, and it is drastically reduced going from 20 to 380 K. Following recent theoretical developments, we rationalize this reduction by a huge reduction of the (effective) charge carrier transfer integrals by about 60%. Our results indicate a strongly temperature-dependent energy transport in pentacene and related organic semiconductors.

Ferromagnetism above Room Temperature in a Ni-Doped Organic-Based Magnetic Semiconductor.

Ferromagnetic semiconductors with structural flexibility are an indispensable feature for future flexible spin-electronic applications. In this case, we introduce magnetic ingredients into an organic semiconductor, namely, pentacene, to form a ferromagnetic organic semiconductor (FOS). The first observation for ferromagnetic Ni-doped pentacene semiconductors at room temperature in the field of semiconductor spintronics is reported in this article. To date, the mechanism of FOSs with ferromagnetism is not understood yet, especially when their Curie temperature is enhanced above room temperature. Here, we demonstrate dopants of Ni atoms and the modulation of the growth temperature in the FOS films to achieve room-temperature ferromagnetic properties in a series of FOS films, one of which has a maximum coercivity of 257.6 Oe. The spin-exchange interaction between a Ni atom and a pentacene molecule is detected through the magnetic hysteresis obtained using a superconducting quantum interference device magnetometer. We verify the effectiveness of this spin coupling through magnetic force microscopy, Raman spectroscopy, scanning Kelvin probe microscopy, and theoretical simulation. A model for the indirect spin coupling between Ni atoms is proposed for the mechanism of room-temperature ferromagnetic ordering of spins due to the exchange force indirectly. We believe that the π-electrons of pentacene molecules at the triple state for this model can support the spin coupling of electrons of Ni atoms. Our findings facilitate the development of brand-new spintronic devices with structural flexibility and room-temperature ferromagnetism.

Extraction of source resistance in thin film transistors using a single gated-probe outside the channel

Extraction of source resistance in top contact thin film transistors (TFTs) is described using a gated three probe device structure. The three probes include a source terminal, a drain terminal and an electrode deposited outside the channel and kept open-circuited leading to unperturbed transistor operation. A measurement of floating potential of this probe together with a family of universal curves is presented, which allows estimation of source resistance from dc measurements in the linear mode of operation on a single device. Simulation results obtained for TFTs with different semiconductor technologies, like organic semiconductor pentacene, indium-gallium-zinc-oxide and amorphous silicon demonstrate the applicability of this technique. Experimental results obtained from organic TFTs with pentacene semiconductor confirm that the method gives estimates that are close to the values obtained from the relatively more complex transfer length measurement (TLM) technique. It is also shown that unlike the TLM method, the outside gated probe technique is much less sensitive to errors in device parameters and variations in device dimensions, such as the channel length.

Structural modification of poly(4-vinylphenol) insulators in pentacene transistors by using dimethyl ketone

We demonstrate a dimethyl ketone treatment for structural modification of cross-linked poly(4-vinylphenol) (c-PVP) gate insulator in pentacene thin-film transistors (TFTs). Here, the dimethyl ketone treatment was performed immediately after spin-coating the insulator solution, followed by thermal annealing for cross-linking of PVP molecules. It is found that the dimethyl ketone treatment lowered the surface energy of the PVP gate insulator and made the PVP film thinner. The results of X-ray diffraction and atomic force microscopy analyses showed that the dimethyl ketone-treated c-PVP insulator contributes to enhancing the crystallinity of pentacene semiconductor films and reducing the density of lamellar grains and bulk phase crystallites in pentacene films. Consequently, the performance improvement of the pentacene TFT is achieved by structurally modifying the c-PVP gate insulator through the dimethyl ketone treatment.

Stimuli-responsive polymer as gate dielectric for organic transistor sensors

Abstract Temperature is a characteristic often correlated with environmental and health issues. This paper presents an organic thin film transistor (OTFT) based temperature sensor having a detection range of 30–45 °C, which, therefore, encompasses the human body temperature. The OTFT sensor featured thermosensitive poly(N-isopropylacrylamide) (PNIPAM) and pentacene as the gate dielectric and semiconductor, respectively. The PNIPAM film possessed a dielectric constant of 4.2 with very low leakage current density. The OTFT exhibited high electrical performance, with a hole mobility (μ) of 0.90 ± 0.04 cm2 V−1 s−1, a threshold voltage (Vth) of −15.4 ± 1.16 V, and an on/off ratio of 104. Significant changes in the drain current and the values of Vth and μ occurred when the temperature of the device was varied within the range 30–45 °C at an interval of 0.5 °C. The operating principle for this temperature sensor was based on the structural transformation of the PNIPAM dielectric and the enhanced charge transport of the pentacene semiconductor upon varying the temperature. Flexible OTFTs fabricated on polyethylene terephthalate substrate displayed hole mobilities as high as 0.39 ± 0.01 cm2 V−1 s−1, values of Vth of −18.6 ± 0.45, and on/off ratios of 102, and were workable for over 100 bending cycles.

Transparent Flexible Nanoline Field-Effect Transistor Array with High Integration in a Large Area.

Transparent flexible transistor array requests large-area fabrication, high integration, high manufacturing throughput, inexpensive process, uniformity in transistor performance, and reproducibility. This study suggests a facile and reliable approach to meet the requirements. We use the Al-coated polymer nanofiber patterns obtained by electrohydrodynamic (EHD) printing as a photomask. We use the lithography and deposition to produce highly aligned nanolines (NLs) of metals, insulators, and semiconductors on large substrates. With these NLs, we demonstrate a highly integrated NL field-effect transistor (NL-FET) array (105/(4 × 4 in2), 254 pixel-per-inch) made of pentacene and indium zinc oxide semiconductor NLs. In addition, we demonstrate a NL complementary inverter (NL-CI) circuit consisting of pentacene and fullerene NLs. The NL-FET array shows high transparency (∼90%), flexibility (stable at 2.5 mm bending radius), uniformity (∼90%), and high performances (mobility = 0.52 cm2/(V s), on-off ratio = 7.0 × 106). The NL-CI circuit also shows high transparency, flexibility, and typical switching characteristic with a gain of 21. The reliable large-scale fabrication of the various NLs proposed in this study is expected to be applied for manufacturing transparent flexible nanoelectronic devices.

Stability of pentacene-based top gate thin film transistor with thick parylene as dielectric under humid environment

This study presents the influence of humidity effect in top gate transistors using pentacene as active layer. The devices are made on a planarized kapton substrate, and 1 μm thick parylene has been used both as gate dielectric and encapsulating layer. Two series of measurement conditions have been used. First, the humidity rate was varied from 0 to 90% in a cell by setting exposure times of 15 min and device characterized, followed to 15 min vacuum. Secondly, humidity rate is fixed at 90% during 144 min and the electrical characteristics recorded successively. The main electrical parameters such as threshold voltage (VT), mobility (μ), on-current (Ion), depletion current (Ioff) and subthreshold slope (S) have been investigated. The results showed that only the electrical parameters in subthreshold region have been affected at different relative humidity (RH) levels for the first series. Under vacuum, the device exhibited stable electrical performances whereas in RH ranging from 24% to 90% we observe an increase of depletion current, and this Ioff increase was found to be reversible. After exposing the device with 90% of RH (second series), from 0 to 108 min in humid environment, threshold voltage, Ion current, VT and charge carrier mobility remain unchanged while Ioff and S are highly affected. Beyond 108 min, Ioff current keeps increasing with a noticeable Ion current degradation. Two effects have to be taken into account: (i) Ioff current increase without any change in on-state regime is the consequence of cumulative effect of Leakage current increase in the parylene dielectric and electrical conductivity increase in pentacene semiconductor, (ii) Ion current decrease is the consequence of absorbed water molecule diffusing and interacting with pentacene. All these parameters are reversible under vacuum after pumping down.

Functional gate-dielectrics containing naphthyl for organic thin film transistors

Abstract Here, we copolymerize a monomer containing naphthyl group with 2-hydroxyethyl methacrylate (HEMA) and glycidyl methacrylate (GlyMA) in various molar ratio to obtain copolymers as functional gate dielectric layers of thin-film transistors (TFTs). Using naphthyl-containing molecule to have similar chemical structure with pentance to adjust the grain size of semiconductor. The results indicated that the semiconductor grains size decreased as molar percentage of naphthyl-containing compound in the polymer structure increased, which could also affect the charge carrier mobility of the pentacene thin-film transistors (TFTs). Therefore, this can be a promising manner to fabricate pentacene TFTs with a high charge carrier mobility by using functionalized dielectric layers to obtain uniform grain size of the pentacene semiconductor.

Improved ferroelectric properties of P(VDF-TrFE) and P(VDF-HFP) blends for organic memory FETs

The electronic device industry has recently focused its attention towards the development of organic memory devices. This research highlights the development of organic memory devices by utilizing pentacene as a semiconductor and ferroelectric polymers, such as poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], as a gate dielectric. 10 wt% solutions of P(VDF-HFP) and P(VDF-TrFE) were prepared individually and blended (50:50). The resultant solutions were coated on the gate electrode and subsequently annealed for 2 h at 90 °C. The fabricated field-effect transistors (FETs) were studied in terms of their hysteresis characteristics with particular emphasis on the memory window. The results show that the FETs fabricated using the P(VDF-HFP)/P(VDF-TrFE) blend have significantly improved memory characteristics. The enhancement in the memory characteristics is explained through the interface properties between the pentacene semiconductor and dielectric layers. This work provides a design of organic FETs with excellent memory characteristics using low-cost technology.

Guided Formation of Large Crystals of Organic and Perovskite Semiconductors by an Ultrasonicated Dispenser and Their Application as the Active Matrix of Photodetectors.

The crystallization of organic or perovskite semiconductors reflects the intermolecular interactions and crucially determines the charge transport in opto-electronic devices. In this report, we demonstrate and investigate the use of an ultrasonicated dispenser to guide the formation of crystals of organic and perovskite semiconductors. The moving speed of the dispenser affects the match between the concentration gradient and evaporation rate near the three-phase contact lines and thus the generation of various crystallization morphologies. The mechanism of crystallization is given by a relationship between the calculated concentration gradient profile and the degree of crystal alignment. Highly ordered, aligned crystals are achieved for both organic bis(triisopropylsilylethynyl)-pentacene and perovskite MAPbI3 semiconductors. Absorption spectra, Raman scattering spectroscopy analysis, and grazing incidence wide-angle X-ray scattering measurement reveal the strong anisotropy of the crystalline structures. The aligned crystals lead to remarkably enhanced electrical performances in an organic thin-film transistor (OTFT) and perovskite photodetector. As a demonstration, we combine the OTFT with photodetectors to achieve an active matrix of normally off, gate-tunable photodetectors that operate under ambient conditions.

Preliminary characterization of the response of an organic field effect transistor to ionizing radiation

Organic thin film transistors (OTFT) were investigated as a novel radiation detector. The OTFTs were fabricated on a flexible PET substrate with a PMMA dielectric layer and a pentacene semiconductor. OTFTs were irradiated up to 400 Gy using kilovoltage (100 and 180 kVp) and megavoltage (6 and 18 MV) photon beams. One OTFT was irradiated to 1000 Gy using 6 MV to observe the longevity of the device. Irradiating the devices caused a positive threshold voltage shift in each device. The magnitude of the threshold voltage shift per unit dose decreased with accumulated dose until it stabilized after approximately 200 Gy. The sensitivity ranged from 2 to 10 mV/Gy at low accumulated dose and decreased to 0.5–1.5 mV/Gy after 200 Gy of accumulated dose across the various OTFTs. After 400 Gy all of the devices were still functional with a loss in mobility of about 15, 14, 12 and 9% for beam qualities of 100 kV, 180 kV, 6 MV, and 18 MV, respectively. After 1000 Gy using 6 MV the OTFT was still functional with a sensitivity of 0.8 ± 0.1 mV/Gy after 300 Gy. This study showed that an OTFT on a flexible substrate shows a measureable response to photon irradiation of various qualities.

Flexible Organic Thin Film Transistors Incorporating a Biodegradable CO2-Based Polymer as the Substrate and Dielectric Material

Employing CO2-based polymer in electronic applications should boost the consumption of CO2 feedstocks and provide the potential for non-permanent CO2 storage. In this study, polypropylene carbonate (PPC) is utilized as a dielectric and substrate material for organic thin film transistors (OTFTs) and organic inverter. The PPC dielectric film exhibits a surface energy of 47 mN m−1, a dielectric constant of 3, a leakage current density of less than 10−6 A cm−2, and excellent compatibility with pentacene and PTCDI-C8 organic semiconductors. Bottom-gate top-contact OTFTs are fabricated using PPC as a dielectric; they exhibits good electrical performance at an operating voltage of 60 V, with electron and hole mobilities of 0.14 and 0.026 cm2 V−1 s−1, and on-to-off ratios of 105 and 103, respectively. The fabricated p- and n-type transistors were connected to form a complementary inverter that operated at supply voltages of 20 V with high and low noise margins of 85 and 69%, respectively. The suitability of PPC as a substrate is demonstrated through the preparation of PPC sheets by casting method. The fabricated PPC sheets has a transparency of 92% and acceptable mechanical properties, yet they biodegraded rapidly through enzymatic degradation when using the lipase from Rhizhopus oryzae.

Negative output differential resistance effect in organic thin film transistors based on pentacene: Characterization and modeling

A series of p-channel organic thin film transistors (OTFTs) based on small molecule pentacene semiconductor was fabricated to characterize for developing analytical modeling. In this present work, the effect of channel length variation (2.5–20μm) on the electrical parameters of the OTFTs was emphasized. As the channel length (L) of the pentacene-TFTs is decreased, some effects such as negative output differential resistance, the threshold voltage shift, an increase in the drain current (transconductance) and an improvement in other parameters may be observed. The highest device performance with the mobility value of 8×10−3cm2V−1s−1 was obtained from a short channel device (L=2.5μm). Total resistances including contact and channel resistances were extracted and discussed in detail. The results showed that the contact resistance significantly affected the performance of the OTFTs with channel lengths of 2.5μm and 5μm. A negative differential resistance (NDR) behavior was obtained from the saturation region of the output characteristics for each device in the case while channel length was decreased from 20μm to 2.5μm. This NDR effect is attributed to the trapping and de-trapping mechanism of the mobile charges at the pentacene–metal electrode interface. Finally, an analytical compact model of organic thin film transistor was developed including the effect of contact resistance and gate voltage dependent mobility, simultaneously. The proposed model was validated by comparing the results obtained from the model with those measured. The results show that the proposed model is a good agreement with the experimental transfer data for devices with the channel length value of 2.5μm, 5μm, 10μm and 20μm.

Multi-level non-volatile organic transistor-based memory using lithium-ion-encapsulated fullerene as a charge trapping layer

We report on multi-level non-volatile organic transistor-based memory using pentacene semiconductor and a lithium-ion-encapsulated fullerene (Li+@C60) as a charge trapping layer. Memory organic field-effect transistors (OFETs) with a Si++/SiO2/Li+@C60/Cytop/Pentacene/Cu structure exhibited a performance of p-type transistor with a threshold voltage (Vth) of −5.98 V and a mobility (μ) of 0.84 cm2 V−1 s−1. The multi-level memory OFETs exhibited memory windows (ΔVth) of approximate 10 V, 16 V, and 32 V, with a programming gate voltage of 150 V for 0.5 s, 5 s, and 50 s, and an erasing gate voltage of −150 V for 0.17 s, 1.7 s, and 17 s, respectively. Four logic states were clearly distinguishable in our multi-level memory, and its data could be programmed or erased many times. The multi-level memory effect in our OFETs is ascribed to the electron-trapping ability of the Li+@C60 layer.

Traps density and temperature effects on the performance of organic rectifying diode based on pentacene

In this paper, we present an approach to determinate the traps density in pentacene semiconductor. The influences of both traps distribution (exponential, gaussian) and temper-ature have been investigated. The defects density associated with pentacene was extracted by fitting the simulated and experimental curves. Current-voltage characteristics are performed using a physically based device simulator. Simulation results show a good agreement with experimental.

Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors.

The preparation of uniform large-area highly crystalline organic semiconductor single crystals remains a challenge in the field of organic field-effect transistors (OFETs). Crystal densities in the channel regions of OFETs have not yet reached sufficiently high values to provide efficient charge transport, and improving channel crystal densities remains an important research area. Herein we fabricated densely well-aligned single crystal arrays of the 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS_PEN) semiconductor using a straightforward scooping-up (SU) methodology to quickly produce a large-area self-assembled semiconductor crystal layer. The resulting crystalline TIPS_PEN strip arrays obtained using the SU method revealed a packing density that was 2.76 times the value obtained from the dip-coated channel, and the mean interspatial distance between the crystal strips decreased from 21.5 to 7.8 μm. The higher crystal packing density provided efficient charge transport in the FET devices and directly yielded field-effect mobilities as high as 2.16 cm(2)/(V s). These field-effect mobilities were more than three times the values obtained from the OFETs prepared using dip-coated channels. Furthermore, the contact resistance between the source/drain electrodes and the TIPS_PEN crystals decreased by a factor of 2. These contributions represent a significant step forward in improving semiconductor crystal alignment for the fabrication of large-area high-performance organic electronics.

Photo-Cross-Linkable Organic–Inorganic Hybrid Gate Dielectric for High Performance Organic Thin Film Transistors

In this study, we have demonstrated a novel organic–inorganic hybrid gate dielectric material, zirconium tetraacrylate (ZrTA). ZrTA gate dielectric, where inorganic Zr elements are embedded in organic acrylate matrix, takes advantage of the complementary properties of single organic or inorganic gate dielectrics. A simple spin-coating and UV-assisted cross-linking reaction of acrylate moieties allowed ZrTA film to be photopatterned. The cross-linked ZrTA film by UV and heat treatments (UV, 365 nm for 3 min; heat, 120 °C for 30 min) showed high dielectric strength (10–7 A/cm2 at 2 MV/cm), and dielectric constant (5.48). In addition, surface properties of the ZrTA film (surface energy, surface roughness) were favorable for the growth of overlying pentacene organic semiconductor. Consequently, the organic thin-film transistor composed of a pentacene semiconductor and a cross-linked ZrTA gate dielectric displayed a moderately high field-effect mobility of 0.50 cm2/(V·s) with a negligible hysteresis transfer characteristic.

A novel polymer as a functional dielectric layer for OTFTs to improve the grain size of the pentacene semiconductor

A series of novel polymers as functional dielectric layers for pentacene thin-film transistors was synthesized and investigated to explore the relationship between the grain size and the charge carrier mobility with a single variable.