Pentacene Layer Research Articles

Bioinspired Flexible and Low-Voltage Organic Synaptic Transistors for UV Light-Driven Vision Systems.

Neuromorphic vision systems, particularly those stimulated by ultraviolet (UV) light, hold great potential applications in portable electronics, wearable technology, biological analysis, military surveillance, etc. Organic artificial synaptic devices hold immense potential in this field due to their ease of processing, flexibility, and biocompatibility. In this work, we have fabricated a flexible organic field-effect transistor (OFET) that utilizes chitosan-silver nanoparticles (AgNPs) composite material as the active dielectric material. During UV light illumination, both silver nanoparticles and the pentacene layer generate a large number of charge carriers. The photogenerated carriers lead to a more significant hole accumulation at the pentacene interface, resulting in a current rise. In the absence of light, the trapped electron in the silver nanoparticles persists for a longer duration, preventing the instant recombination with holes. This extended retention of electrons leads to the observed synaptic performance of the transistor. The use of aluminum oxide (Al2O3) as one of the dielectric layers enables the device to operate effectively at low voltage (<1 V). The device mimics various crucial synaptic properties of the brain, including short-term potentiation and long-term potentiation (STP and LTP), paired-pulse facilitation (PPF), spike-duration dependent plasticity (SDDP), spike-number dependent plasticity (SNDP), and spike-rate dependent plasticity (SRDP), etc. This work introduces an approach to develop flexible organic synaptic transistors that operate efficiently at low voltages, paving the way toward high-performance, UV light-driven neuromorphic vision systems.

Modulation of charge transfer at the pentacene/graphene quantum dot interface by localized surface plasmonic resonance

We, for the first time, quantified localized surface plasmon resonance (LSPR)-induced charge transfer, and tuned charge carrier concentration in field-effect transistors (FETs) by incorporating silver nanoparticles (Ag NPs) at the pentacene/graphene quantum dot (GQD) interface. This resulted in a significant threshold voltage shift under illumination at a resonance frequency of 450 nm, primarily due to LSPR-induced charge transfer at the Ag NP/GQD interface. Further, the formation of an Ag NP-induced interface dipole in the dark accumulated holes in the pentacene layer in the absence of the gate electric field. Our research not only contributes to a deeper understanding of the interplay between plasmonics and semiconductor physics but also paves the way for new applications in electronics and photonics.

The Effect of Intermolecular Interactions between Aromatic Ring‐Containing Insulators and Semiconductors in Pentacene Thin‐Film Transistors

AbstractMobility is an important parameter to estimate the performance of organic thin‐film transistors, one of the key factors affecting the mobility is the first several semiconductors molecular layers near the insulator surface. A strategy is proposed to introduce aromatic‐rings in the gate insulating layer to modulate the growth of pentacene, thus a series of aryl group containing polymers by reversible‐addition‐fragmentation chain‐transfer polymerization is synthesized for the preparation of organic thin‐film transistor insulators. The semiconductor layer is evaporated on the polymer insulators at room temperature. As the inducing function of the aromatic ring, greater weak interactions between dielectric materials and semiconductors by simulation, which makes the initial pentacene layer weak epitaxial growth, therefore pentacene could form better morphology, especially larger crystal domains and uniform thin‐film continuity. The results show that precision design and synthesis of polymer insulator layers provide new methods to achieve high performance of organic thin‐film transistors with easier fabricating process.

Role of Graphene Topography in the Initial Stages of Pentacene Layer Growth.

Using atomic force microscopy, we probed the growth of pentacene molecules on graphene that was fabricated by chemical vapor deposition and transferred onto 300 nm-thick SiO2 substrates. The topography of such graphene has two important properties. First, its surface is comprised of folds that have different orientations, and second, it has several multilayer-graphene regions distributed over the monolayer-graphene surface. On such folded graphene features, we vacuum evaporated pentacene and observed three-dimensional islands with an average height of ∼15 nm. They are oriented either parallel or perpendicular to the folds, and they are also predominantly oriented along the symmetry axes of graphene. Orientation of pentacene islands on graphene evaporated at room temperature has a wide distribution. On the contrary, most of the pentacene islands evaporated at 60 °C are oriented at 30° with respect to the fold direction. We observed that the folds act as a potential barrier for the surface transport of pentacene molecules. In addition, we interpret the 3D growth of pentacene islands on graphene in terms of reduced polar components of the surface energy on graphene investigated with contact angle measurements.

MOF-Assimilated High-Sensitive Organic Field-Effect Transistors for Rapid Detection of a Chemical Warfare Agent.

The selective and rapid detection of trace amounts of highly toxic chemical warfare agents has become imperative for efficiently using military and civilian defense. Metal-organic frameworks (MOFs) are a class of inorganic-organic hybrid porous material that could be potential next-generation toxic gas sensors. However, the growth of a MOF thin film for efficiently utilizing the material properties for fabricating electronic devices has been challenging. Herein, we report a new approach to efficiently integrate MOF as a receptor through diffusion-induced ingress into the grain boundaries of the pentacene semiconducting film in the place of the most adaptive chemical functionalization method for sensor fabrication. We used bilayer conducting channel-based organic field-effect transistors (OFETs) as a sensing platform comprising CPO-27-Ni as the sensing layer, coated on the pentacene layer, showed a strong response toward sensing of diethyl sulfide, which is one of the stimulants of bis (2-chloroethyl) sulfide, a highly toxic sulfur mustard (HD). Using OFET as a sensing platform, these sensors can be a potential candidate for trace amounts of sulfur mustard detection below 10 ppm in real time as wearable devices for onsite uses.

On/Off Ratio of a Pentacene Field-Effect Transistor with a Discontinuous MoO&lt;sub&gt;3&lt;/sub&gt; Layer

Improvement of the on/off ratio in organic field-effect transistors through the use of pentacene and molybdenum trioxide (MoO3) layers was attempted via the preparation of a discontinuous MoO3 layer using a mesh mask. We prepared three types of devices. Device A had a conventional top-contact structure with an n-type Si wafer and a 200-nm-thick SiO2 film onto which we deposited a 70-nm-thick pentacene film and a 30-nm-thick layer of Au top electrodes. Devices B and C had a similar structure to device A but received a continuous and a discontinuous MoO3 layer, respectively. The off current in Device B was remarkably high; in contrast, the off current in Device C was reduced and dependent on the separation of the MoO3 layer. It was deduced that the high resistance of the area without MoO3 contributed to the reduced off current.

Carrier Dynamics of Efficient Triplet Harvesting in AgBiS2 /Pentacene Singlet Fission Solar Cells.

Singlet fission is a process by which an organic semiconductor is able to generate two triplet excitons from a single photon. If charges from the triplets can be successfully harvested without heavy losses in energy, then this process can enable a single-junction solar cell to surpass the Shockley-Queisser limit. While singlet fission processes are commonly observed in several materials, harvesting the resulting triplets is difficult and has been demonstrated with only a few transport materials. Here, transient absorption spectroscopy is used to investigate singlet fission and carrier transfer processes at the AgBiS2 /pentacene (AgBiS2 /Pc) heterojunction. The successful transfer of triplets from pentacene to AgBiS2 and the transfer of holes from AgBiS2 to pentacene is observed. Further singlet fission in pentacene by modifying the crystallinity of the pentacene layer and have fabricated the first singlet fission AgBiS2 /Pc solar cell is enhanced. Singlet fission devices exhibit higher external quantum efficiency compared with the control devices, and thus demonstrating the significant contribution of charges from the singlet fission process.

Molybdenum disulfide field-effect transistors with enhanced charge-injection by inserting ultrathin pentacene layer under source/drain electrodes

We report a method to improve charge-injection properties of molybdenum disulfide (MoS2)-based field-effect transistors (FETs) by introducing ultrathin pentacene film as the buffer layer under the source/drain electrodes. Compared with conventional devices with metal-only electrodes, the electrical characteristics of MoS2 FETs with the organic buffer layer exhibit a ten-fold enhancement in on-state current and a four-fold increase in field-effect mobility. These improvements are attributed to the introduction of organic buffer layer with hole-type that result in optimized contact properties. The Schottky barrier height is lowered by approximately 0.3 eV for the electron-injection in MoS2 FETs. Furthermore, electrons in metal and MoS2 could be effectively attracted to the surfaces by holes in pentacene film resulting in a tunnelling probability, which is responsible for the improved charge-injection. Current studies will be helpful in the development of high performance MoS2 optoelectrical devices.

Low-power flexible organic memristor based on PEDOT:PSS/pentacene heterojunction for artificial synapse.

Organic synaptic memristors are of considerable interest owing to their attractive characteristics and potential applications to flexible neuromorphic electronics. In this work, an organic type-II heterojunction consisting of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) and pentacene was adopted for low-voltage and flexible memristors. The conjugated polymer PEDOT:PSS serves as the flexible resistive switching (RS) layer, while the thin pentacene layer plays the role of barrier adjustment. This heterojunction enabled the memristor device to be triggered with low-energy RS operations (V < ± 1.0 V and I < 9.0 μA), and simultaneously providing high mechanical bending stability (bending radius of ≈2.5 mm, bending times = 1,000). Various synaptic properties have been successfully mimicked. Moreover, the memristors presented good potentiation/depression stability with a low cycle-to-cycle variation (CCV) of less than 8%. The artificial neural network consisting of this flexible memristor exhibited a high accuracy of 89.0% for the learning with MNIST data sets, even after 1,000 tests of 2.5% stress-strain. This study paves the way for developing low-power and flexible synaptic devices utilizing organic heterojunctions.

Ultrafast orbital tomography of a pentacene film using time-resolved momentum microscopy at a FEL

Time-resolved momentum microscopy provides insight into the ultrafast interplay between structural and electronic dynamics. Here we extend orbital tomography into the time domain in combination with time-resolved momentum microscopy at a free-electron laser (FEL) to follow transient photoelectron momentum maps of excited states of a bilayer pentacene film on Ag(110). We use optical pump and FEL probe pulses by keeping FEL source conditions to minimize space charge effects and radiation damage. From the momentum microscopy signal, we obtain time-dependent momentum maps of the excited-state dynamics of both pentacene layers separately. In a combined experimental and theoretical study, we interpret the observed signal for the bottom layer as resulting from the charge redistribution between the molecule and the substrate induced by excitation. We identify that the dynamics of the top pentacene layer resembles excited-state molecular dynamics.

Photon harvesting and light trapping in pentacene and PTCDI-C13H27 for organic solar cell application

The deep understanding of optical and spectroscopic properties of organic semiconductors is essential for designing the organic photovoltaic solar cells (OSCs) and estimating their prospect performance. In this work, the optoelectronic properties of pentacene and PTCDI-C13H27 conjugated molecules were estimated using density function theory (DFT) calculations. Hence, the parameters of energy gaps, charge transfer (CT), open circuit voltage (VOC), reorganization energy and light harvesting efficiency (LHE) have been extracted using the DFT at B3LYP/6-311G(d,p) level. The results predicted that pentacene and PTCDI-C13H27 molecules possess an efficient broad-band photon harvesting over the visible spectrum with absorption maximum at 643 and 514.5 nm respectively. Moreover, the light trapping inside the pentacene and PTCDI-C13H27 layers integrated in a proposed OSC have been studied using 3D finite difference time domain (FDTD) model. The optimum thickness of the OSC layers were evaluated based on their absorption capability.

Low-Energy Electron Irradiation Damage in Few-Monolayer Pentacene Films.

Crystalline films of pentacene molecules, two to four monolayers in thickness, are grown via in situ sublimation on silicon substrates in the ultrahigh vacuum chamber of a low-energy electron microscope. It is observed that the diffraction pattern of the pentacene layers fades upon irradiation with low-energy electrons. The damage cross section is found to increase by more than an order of magnitude for electron energies from 0 to 10 eV and by another order of magnitude from 10 to 40 eV. Close to 0 eV, damage is virtually nil. Creation of chemically reactive atomic centers after electron attachment or impact ionization is thought to trigger chemical reactions between neighboring molecules that gradually transform the layer into a disordered carbon nanomembrane. Additionally, diminishing spectroscopic features related to the unoccupied band structure of the layers, accompanied by loss of definition in real-space images, and an increase in the background intensity of diffraction images during irradiation point to chemical changes and formation of a disordered layer.

Unveiling the Photoinduced Recovery Mystery in Conjugated Polymer-Based Transistor Memory.

A straightforward mechanism for the photorecovery behavior of photoresponsive nonvolatile organic field-effect transistor (OFET) memories is proposed by employing a commercially available conjugated polymer, the poly(9,9-dioctylfluorene) (PFO), the conjugated monomer fluorene (FO), and the nonconjugated poly(vinyl alcohol) (PVA), as charge storage layers beneath the semiconducting pentacene layer. As photoexcitons are generated upon light exposure, the respective charges recombine with the trapped charges in electrets and neutralize the memory device. However, whether the excitons are generated in the semiconducting layer or the electret part, the origin that mainly governs the photorecovery behavior remains unclear. In this study, we show that when PVA, a nonphotoactive electret, replaces PFO the photorecovery behavior is totally absent, and it confirms the photorecovery behavior dominated by the excitons in situ generated in a charged electret. Moreover, PFO as a photoactive electret, exhibiting an excellent hole-trapping ability over 24 h in the dark and high Ion/Ioff current ratio of 108, has successfully demonstrated rapid photoinduced recovery under UV light. The devices also display a reliable switching ability between electrical charge trapping and optical recovery cycles for optical-recording application. This report presents a clear understanding behind photorecovery phenomena that demonstrates useful guidance to boost the development of photoactive OFET memories based on conjugated polymer electrets.

Enhanced surface passivation with TIPS pentacene and additional interfacial layer for MIS solar cells

Enhanced surface passivation with TIPS pentacene and additional interfacial layer for MIS solar cells

Organic small-molecule heterointerface for use in transistor-type non-volatile memory

A chargeable layer is an essential element for charge transfer and trapping in a transistor-based non-volatile memory device. Here we demonstrate that a heterointerface layer comprising of two different small molecules can show electrical memory characteristics. The organic heterointerface layer was fabricated with a pentacene and tris(8-hydroxyquinoline) aluminum (Alq3) layers by sequential vapor deposition without breaking the vacuum state. Pentacene was adopted as the active layer on the top, and Alq3 was used as the bottom layer for charge trapping. The bottom-gate top-contact transistor with an organic heterointerface layer showed distinct non-volatile memory behaviors and showed high air stability and reliability. We investigated the energy structure of the pentacene/Alq3 heterointerface layer to reveal the operation mechanism of the non-volatile memory and suggested that the writing/erasing gate bias-dependent energy barrier originating from the difference between the energy levels of the pentacene and Alq3 layers controls the charge transfer at the heterointerface layer. Our approach suggests a simple way to fabricate heterointerface layers for organic non-volatile memory applications with high air stability and reliability.

Annealing effect of pentacene and ultrathin metal anode buffer on the performance of small-molecule organic solar cells

The annealing effect of thin pentacene and ultrathin gold/silver anode buffer layers on both the initial performance and durability against repetitive illumination stress in small-molecule organic solar cells (OSCs) was experimentally investigated. The OSC had a structure of indium tin oxide (ITO, anode)/anode buffer/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (cathode buffer)/Ag (cathode). OSCs with a 3-nm-thick pentacene anode buffer layer provided a 15% increase in the power conversion efficiency compared to OSCs with no pentacene layer. OSCs with an ultrathin 0.3-nm-thick Au anode buffer layer provided an increase of 63% in durability compared to OSCs without the Au layer. OSCs with the Au layer did not show apparent leakage current, while those with the Ag layer did. OSCs with a combinational anode buffer of the pentacene layer/Au layer were mainly examined. The spatial distribution of Au atoms on pentacene was uniform even after annealing, which helped to improve the quality of CuPc crystals. The ultrathin Au layer brought both a large absorption of ultraviolet (UV) light and highly dense nucleation sites for overlaid CuPc film formation. These were possible reasons for durability improvement. The Au layer thickness of 0.3 nm on pentacene was optimal to achieve the condition with both less attenuation of visible light necessary for electric power generation and larger absorption of UV light causing degradation. Annealing treatment before CuPc evaporation brought both preferable initial performance and durability for OSCs with a combinational structure of the ITO/pentacene layer/Au layer. An appropriate region of annealing temperature had, however, a small window around 100°C.

Pentacene memory transistors using ligand-exchanged and energy-level-controlled PbS colloidal nanodots for charge-trapping layers

We fabricated pentacene memory transistors using a monolayer of ligand-exchanged and energy-level-controlled PbS colloidal nanodots (NDs) as charge-trapping layers, and it was demonstrated that the writing and retention times were reduced and increased, respectively. This result is explained with a model that the memory effect is due to the electrons tunneled from the pentacene layer into the PbS NDs when a writing voltage was applied. According to this model, the writing and retention characteristics were improved because of the lowered electron energy levels in the NDs.

Synthesis of SnO&lt;sub&gt;2&lt;/sub&gt; Nanoparticles Using &lt;i&gt;Ficus religiosa &lt;/i&gt;Leaf Extract and their Application in Fabrication of OFETs for Glucose Monitoring

Organic field effect transistors (OFETs) as a sensor have gained interest of researchers due to its portable size and less expensive design in the field of medical diagnostic, food monitoring, chemical detection, wearable sensors, etc. In this present research work, we demonstrate the fabrication of OFETs from organic-inorganic SnO2 nanoparticles tailored pentacenefor glucose detection. SnO2 nanoparticles were synthesized bygreen method using Ficus religiosa leaf extract. The as-synthesized SnO2 NPs with cassiterite crystal structure was analysed using X-Ray Diffraction (XRD) and the energy bandgap of ~3.8 eV was calculated using Tauc relation with absorption spectra so obtained from UV-vis spectroscopy (UV-vis). The structure and morphological analysis of SnO2 NPs with size of ~15 nm was confirmed by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analysis. The sensor characteristics of OFET device fabricated using pentacene layer (soluble pentacene precursor of DMP) and as-synthesized SnO2 nanoparticles confirmed the aqueous glucose (glucose in water) detection at room temperature (27 °C). The extracted electrical parameters such as mobility (μ), On-current (Ion), saturation current (IDsat) and the sensor response were discussed to support the sensor characteristics.

Morphology, Structure, and Dynamics of Pentacene Thin Films and Their Nanocomposites with [C2 C1 im][NTf2 ] and [C2 C1 im][OTF] Ionic Liquids.

In this study, a homogeneous thin film growth of pentacene onto indium tin oxide (ITO) coated glass surfaces is explored using a high-resolution and reproducible vapor deposition methodology. Moreover, vacuum thermal evaporation of ionic liquids (ILs) ([C2 C1 im][NTf2 ] and [C2 C1 im][OTF]) onto ITO, gold/palladium (AuPd) and pentacene surfaces were performed. A greater wettability behavior of ILs is observed for surfaces containing AuPd. Sequential and simultaneous depositions of ILs and pentacene were explored. Simultaneous depositions lead to the formation of nanocomposites films, consisting of IL micro- and nanodroplets covered by pentacene layers. Plasma surface treatment was used to induce the ILs droplets coalescence and explore the dynamics and phase separation of the nanocomposites. The [C2 C1 im][OTF] droplets were found to be completely covered with pentacene, which suggests a great affinity between cation-anion pairs and the aromatic moiety. Pentacene films and their nanocomposites with ILs exhibit a typical optical band gap of Egap =1.77 eV, indicating that the nanocomposite phase domains are large enough to behavior as the bulk.

Engineering the Doping Efficiency in Pentacene Thin Films for High Thermoelectric Performance.

Because of the high mobility and Seebeck coefficient, pentacene (PEN) is a promising candidate for organic small-molecule thermoelectric (TE) materials. However, the low intrinsic conductivity impedes its application in thermoelectricity. In this work, hexacyano-trimethylene-cyclopropane (CN6-CP) is employed as the dopant for PEN via constructing bilayer-structured thin films. The almost intact crystallinity and high charge carrier generation efficiency of these interface-doped PEN films ensure their high conductivity. Time of flight secondary ion mass spectrometry was applied to demonstrate the diffusion of dopant molecules into the PEN layer. UV-vis spectral analysis reveals that integral charge transfer happens between the PEN and CN6-CP molecules. The doping process is further characterized by electron spin-resonance, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy analysis. Under optimized conditions, the conductivity of the PEN film deposited on the SiO2/Si substrate can reach up to 10.1 S cm-1. To the best of our knowledge, this is the highest conductivity ever reported for doped PEN thin films. The optimal TE performance with a power factor of 36.4 μW m-1 K-2 can be achieved in the PEN/CN6CP thin film with a Seebeck coefficient and conductivity of 199 μV K-1 and 9.2 S cm-1, respectively. This result shows that interface doping with a strong electron acceptor is a promising approach for optimizing the TE performance of small molecular organic semiconductors.