Performance Of Organic Devices Research Articles

The Influence of a Microstructural Conformation of Oriented Floating Films of Semiconducting Polymers on Organic Device Performance.

Extended π-conjugation with backbone-planarity-driven π-π stacking dominates charge transport in semiconducting polymers (SCPs). The roles of SCP film morphology and macromolecular conformation concerning the substrate in influencing charge transport and its impact on device performance have been a subject of extensive debate. Face-on SCPs promote out-of-plane charge transport primarily through π-π stacking, with conjugated polymeric chains assisting transport in connecting crystalline domains, whereas edge-on SCPs promote in-plane charge transport primarily through conjugation and π-π stacking. In this work, we fabricated three different types of devices, namely, organic field effect transistors, organic Schottky diodes, and organic bistable memristors, as representatives of planar and vertical devices. We demonstrate that a planar device, i.e., an organic field effect transistor, performs well in an edge-on conformation exhibiting a field-effect mobility of 0.12 cm2V-1s-1 and on/off ratio >104, whereas vertical devices, i.e., organic Schottky diodes and organic memristors, perform well in a face-on conformation, exhibiting exceptionally high on/off ratios of ~107 and 106, respectively.

Gate Controlled Photocurrent Generation Mechanism in Air-Grown Organic Single Crystals for High-Speed Multiband Imaging.

Organic semiconductors herald new opportunities for fabricating high-performance flexible and wearable optoelectronic devices owing to their intrinsic mechanical flexibility, excellent optical absorption, and cool-free operation. The photocurrent generation mechanisms are of multiple physical origins, including photoconductive, photovoltaic, and photogating effects, and the influence of individual effects on the device figures-of-merit is still not well understood. Here we fabricated a high-performance pentacene single-crystal transistor employing graphene electrodes and demonstrated the modulation from the photogating mechanism to the photoconduction effect by controlling gate bias. Control experiments indicate that the calculation based on transfer curves tends to overestimate the responsivity due to nearby trap states. Using a high frequency-modulated light signal to suppress the trapping process, we successfully measured its intrinsic -3 dB bandwidth of 75 kHz. Finally, high-resolution and UV-NIR high-speed imaging capability was demonstrated. Our work provides new guidelines for understanding the photophysical process and intrinsic performances of organic devices and also confirms the potential of organic single crystals in high-speed imaging applications.

Corona Poling Induced Phase Transition to Highly Polar Phase in P(VDF-TrFE-CFE) Dielectric and Charge Transport of Organic Field-Effect Transistors.

Increasing the number of charge carriers flowing through the charge transport channel to improve the electrical performance of organic field-effect transistors (OFETs) is important because it leads to a low driving voltage and a high drain current value. This paper proposes a new strategy, the corona poling process, to enhance the electrical performance of OFETs using an external electric field when forming a dielectric film using a PVDF-based high-k dielectric terpolymer, P(VDF-TrFE-CFE). A corona poling process was applied to align the dipoles with high-k dielectric molecules and improve the capacitance, thereby increasing the number of charge carriers. Through this process, by observing the phase transition of a PVDF dielectric through a corona poling process in the GIWAXS data, the phase transition through an external electric field was thoroughly revealed for the first time. As a result, the capacitance of high-k dielectric films can be improved, and the amount of charge carriers can be increased by a simple corona poling process. In addition, to reduce the effect of deep trap sites caused by the dipole alignment, a thin low-k dielectric, polystyrene (PS), was introduced between the active and high-k dielectric layers to provide trap site passivation, thereby increasing the electrical performance of the OFET. Therefore, through this strategy, using a diketopyrrolopyrrole (DPP)-based donor-acceptor (D-A) copolymer as an active material of OFET, the average saturation region hole mobility was improved from 0.34 to 0.60 cm2/Vs. Thus, the electrical performances of the OFETs were improved by enhancing the capacitance through the corona poling process and reducing the charge carrier trap sites introduced by the high-k and low-k bi-layer dielectric layer. Importantly, this work offers a new strategy for the post-treatment to improve electrical performance of organic devices.

Surface-Doping-Induced Mobility Modulation Effect for Transport Enhancement in Organic Single-Crystal Transistors.

Molecular doping has conventionally been an effective way to improve the electrical-transport performances in organic field-effect transistors (OFETs), while corresponding mechanisms associated with specific doping techniques have been less investigated and discussed in detail. Here, based on ultrathin dinaphtho[2,3-b:2',3'-f]-thieno[3,2-b]thiophene (DNTT) single crystals, robust transconductance enhancements are realized in OFETs upon surface molecular doping realized via van der Waals epitaxially growing crystalline 1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) onto the single crystal's surface. It is proposed that it is the mobility modulation effect (MME) from the interactions between charge-transfer interface and gate electric field, that contributes to more weighted bulk carriers, and finally improves charge-transport performances. The evaluations are further supported by scanning Kelvin probe microscopy(SKPM) surface potential characterizations, which manifest the gate-induced more delocalized holes near the charge-transfer interfaces. Space-charge-limited current (SCLC) investigations, numerical calculations, and theoretical mobility modeling are also performed to corroborate the analysis. This study can deepen the understanding of charge transport in doped semiconductors and provide effective ways for optimizing the electrical performance of organic devices.

Organic small molecule semiconductor materials for OFET-based biosensors

Biosensors is an advanced detection and monitoring device for the development of biotechnology, and is also a rapid and microanalytical device at the molecular level. Demands for high sensitivity, high flexibility, good biocompatibility, easy chemical modification and low cost offer incentive for exploring new materials to develop the next-generation biosensors. With the vigorous development of organic electronics, the performances of organic devices have been effectively improved, leading to organic semiconductor materials with low cost, good flexibility, easy chemical modification and good biocompatibility for biosensors. Biosensors based on organic field-effect transistors (OFETs) have become one of the most advanced biosensor platforms because of their inherent ability to amplify received signals. Furthermore, OFET-based biosensors have been widely used in the detection of DNA, protein, cell, glucose and other biological substances due to its high sensitivity, fast analysis speed, label-free detection, small size and simple operation. This mini review briefly discusses the organic small molecule semiconductor materials, device configurations, basic principles and application fields of OFETs-based biosensors.

P-Type Chemical Doping-Induced High Bipolar Electrical Conductivities in a Thermoelectric Donor–Acceptor Copolymer

P-Type Chemical Doping-Induced High Bipolar Electrical Conductivities in a Thermoelectric Donor–Acceptor Copolymer

DC Characterization and Fast Small-Signal Parameter Extraction of Organic Thin Film Transistors With Different Geometries

Organic Devices offer low-cost manufacturing and better flexibility, sustainability and solution-processability than their Si-based MOS counterparts, which make them suitable for new applications where those characteristics are an advantage. However, organic device performance is still far from that provided by CMOS technology and many issues are still unclear. In this work, a performance comparison is made between Interdigitated and Corbino geometries of Organic Thin Film Transistors (OTFT), with different areas but fabricated with identical stack materials and techniques. To this end, I-V characteristics and C-V curves of the OTFTs were measured and a Common-Source circuit was proposed and implemented for extracting relevant electrical parameters of the devices, through a standard small signal analysis. The parameter extraction methodology in the frequency domain proposed allows rapid testing of the device/circuit performance of this technology, which for the first time is applied to organic devices. Results show that despite the large voltages, organic transistors exhibit similar channel dimension dependencies to MOS devices and can be also described by the same small signal model.

Aerosol jet printing of PEDOT:PSS for large area flexible electronics

We investigate and optimize the Aerosol Jet Printing of PEDOT:PSS by studying the role of parameters determining the final quality of the printed features. Printing thin films of organic materials requires a high control in the deposition process to control the morphology/structure of the films that determine the electrical and functional performance of organic devices. We studied aerosol jet printed lines of the PEDOT:PSS semiconductive polymer on flexible Kapton substrates. We choose the focus ratio, defined as the ratio between the sheath and carrier gas flow rates, as the main parameter that best characterizes the process and hence the characteristics of printed lines. A detailed analysis of the line width and thickness, as a function of the focus ratio and carrier gas flow for different nozzle sizes and sheath gas flow rates, is reported. The edge profile definition of printed PEDOT:PSS lines has been found to be affected by a combination of overspray, typical of aerosol deposition, and pinholes due to the Kapton/PEDOT:PSS interaction, while their core profile results to be almost defects free. An optimal printing window for different nozzle sizes has been determined in terms of a minimization of overspray/pinholes effects with respect to the whole line width. Finally, an array of whole-plastic, planar Organic Electrochemical Transistors, having both the channel and the gate electrode made of PEDOT:PSS, has been manufactured using the best combination of process parameters. Transistors have been characterized for different channel/gate area ratios and the transconductance analysed as a function of channel to gate area.

Isotope effect of carrier transport in organic semiconductors

Isotopic substitution can effectively tune the device performances of organic semiconductors. According to the experimental results of isotope effects in electric, light and magnetic process in organic semiconductors, we adopt the tight-binding model with strong electron-phonon coupling to study the isotope effects on carrier transport. We try to give a quantificational explanation and show the physical origin of isotope effects on mobility in organic semiconductors in this work. Using polaron transport dynamics with diabatic approach, we simulate the carrier transport in an array of small molecule crystals under weak bias. Because of strong electron-phonon coupling in organic materials, an injected electron will induce lattice distortion, and the carriers are no longer free electrons or holes, but elementary excitations such as solitons, polarons or bipolarons. Our simulation results indicate that the existence of deuterium and <sup>13</sup>C element will reduce the mobility of organic material, which means that the isotopic substitution can be utilized to manifest organic device performance. Besides, we also find that the isotope effect on mobility will increase with electron-phonon coupling increasing. This suggests that both the mass of lattice groups and electron-phonon coupling should be taken into account to understand the isotope effects in organic semiconductors. With the consideration of that, we derive the effective mass of polaron based on the continuum model, and verify that effective mass can successfully describe the isotope effect on mobility. The effective mass of carrier can be measured to represent the property of a material, which can tell us whether we need the isotopic substitution in organic layer to improve the device performance. Then we present the microcosmic movement of a polaron at the moment when it encounters isotopic substituted molecules. We come to the conclusion that the isotopic distribution will affect the instantaneous speed of the carrier, but has little effect on the mobility of the whole device when the substituted concentration remains constant. In conclusion, after simulating various possible isotope effects in materials, analyzing its physical mechanism and comparing calculation results in experiment, we provide a theoretical foundation for describing the isotope effects on mobility, which can be a basis of improving the performances of organic semiconductor devices.

Carrier behaviors of 6,13-Bis (triisopropylsilylethynyl) pentacene device with self-assembled monolayer

Abstract Since the discovery of organic semiconductors, material and device processing has been enhanced by improvements in carrier injection, carrier mobility, and device stability. However, these improvements are insufficient in the case of organic devices employing organic semiconductor materials. To improve the performance of organic devices, we need better understanding of the carrier injection property at the electrode-organic layer interface. In this paper, we studied carrier injection and accumulation in organic semiconductor by using electrical and optical measurements, i.e., I-V, C-V, AFM, and FTIR measurements. We used 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) as the organic semiconductor layer. In order to enhance the carrier injection into TIPS-pentacene, we focused on the binding effect of the self-assembled monolayer, i.e., (3-Mercaptopropyl) trimethoxysilane (MPTMS) and pentafluorobenzenethiol (PFBT), deposited on the surface of the substrate.

Fracture Mechanics and Oxygen Gas Barrier Properties of Al₂O₃/ZnO Nanolaminates on PET Deposited by Atomic Layer Deposition.

Rapid progress in the performance of organic devices has increased the demand for advances in the technology of thin-film permeation barriers and understanding the failure mechanisms of these material systems. Herein, we report the extensive study of mechanical and gas barrier properties of Al2O3/ZnO nanolaminate films prepared on organic substrates by atomic layer deposition (ALD). Nanolaminates of Al2O3/ZnO and single compound films of around 250 nm thickness were deposited on polyethylene terephthalate (PET) foils by ALD at 90 °C using trimethylaluminium (TMA) and diethylzinc (DEZ) as precursors and H2O as the co-reactant. STEM analysis of the nanolaminate structure revealed that steady-state film growth on PET is achieved after about 60 ALD cycles. Uniaxial tensile strain experiments revealed superior fracture and adhesive properties of single ZnO films versus the single Al2O3 film, as well as versus their nanolaminates. The superior mechanical performance of ZnO was linked to the absence of a roughly 500 to 900 nm thick sub-surface growth observed for single Al2O3 films as well as for the nanolaminates starting with an Al2O3 initial layer on PET. In contrast, the gas permeability of the nanolaminate coatings on PET was measured to be 9.4 × 10−3 O2 cm3 m−2 day−1. This is an order of magnitude less than their constituting single oxides, which opens prospects for their applications as gas barrier layers for organic electronics and food and drug packaging industries. Direct interdependency between the gas barrier and the mechanical properties was not established enabling independent tailoring of these properties for mechanically rigid and impermeable thin film coatings.

Investigation of Improved Performance for Organic Rectifying Diodes via Electrical Annealing

Electrical annealing (EA) is one of the post treatments to enhance the electrical performances of organic devices. To date, the improvements using EA have only been reported for the solution-processed devices because its mechanism has been known as the alignments of ionic impurities or polymer chains. In this paper, we applied EA to thermally evaporated organic diodes which not have ionic impurities or polymer chains. After EA, the turn-on voltage of the diode was reduced, and the forward-bias current of the diode was increased without changing the reverse-bias current, resulting in an improvement of the cutoff frequency of the rectifier. In addition, we proposed a new mechanism to explain why the EA can be applied to the thermally evaporated organic devices. Based on time-of-flight secondary ion mass spectrometry and impedance spectra, we suggest that this improvement is due to the creation of a MoO3:pentacene mixed layer, leading to ease of charge injection. We believe that our finding will be helpful to understand change at the organic/metal interfaces and useful to apply a wide range of organic devices such as organic photovoltaics, organic light-emitting diodes, and organic thin-film transistors.

Properties of NTCDA Thin Films on Ag(110): Scanning Tunneling Microscopy, Photoemission, Near-Edge X-ray Fine Structure, and Density Functional Theory Investigations

It is well proven that the properties of organic/metal interfaces play an utmost role in the performance of organic devices. Here we present a study on structural and electronic properties of high-quality 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) films grown on an Ag(110) surface. High-resolution scanning tunneling microscopy and low-energy electron diffraction show the presence of two molecular domains. Density functional theory calculations indicate that the most stable location of NTCDA corresponds to anhydride oxygen attached to the Ag atoms along the [110] direction. Photoemission results of the C 1s and O 1s core levels demonstrate a strong interfacial bonding, inducing a charge transfer from the Ag metal to the molecular monolayer. An angular-dependent study of the C K-edge near-edge X-ray fine structure spectra provides detailed information concerning the evolution of the NTCDA orientation with the film thickness.

High barrier performance of the multilayer film based on epoxy and montmorillonite

A flexible organic device needs a protective encapsulating layer to resist degradation of the device due to environmental oxygen and moisture. The performance of organic devices degrades with time due to a self-process of delamination and oxide formation of the cathode by environmental oxygen and moisture for electrochemical processes. Here, a flexible multilayer barrier film, which is a sandwiched composite layer with organic layers, has been fabricated in an inexpensive way under atmospheric conditions using an applicator on PET substrate. An organic layer (POL) consists of bisphenol A-type phenoxy resin and epoxy resin. For the middle composite layer (MMT), a composite with montmorillonite has been used as an interactive barrier. The multilayer film with ball-milled montmorillonite 2% composite layer shows lower moisture permeation rate of 2.850 × 10−2 gm−2day-1 than only POL on PET substrate. Fabrication of flexible multilayer barrier film from solution has promising industrial prospects due to facile processing at low temperatures.

A focus on polystyrene tacticity in synthesized conductive PEDOT:PSS thin films

Poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) is a transparent conductive material and a good candidate for being employed as substitute for indium tin oxide (ITO) in reducing the production costs of organic solar cells. To enhance the performance of organic devices, an improving in the conductivity of PEDOT:PSS is crucial and using the solvent additive rises the electrical conductivity by the optimization of the film morphology. The studies have only focused on the relationship between the electrical conductivity of thin films and the crystallinity of PEDOT, and it is also found that the high conductivity is observed in the highly crystalline samples. This study focused on the effect of tacticity of PS on the conductivity of PEDOT:PSS films. First, atactic and isotactic polystyrenes were sulfonated and the complexes of PEDOT:PSS were synthesized. The N-methylpyrrolidone (NMP), as a secondary dopant, was then added to the complexes and conductivity enhancement was investigated in various annealing times. The obtained films were characterized by atomic force microscopy, X-ray diffraction, four point probe resistivity measurement system, UV–visible spectroscopy, FT-IR, and cyclic voltammetry. The electrical conductivity of PEDOT:iPSS films synthesized by the isotactic polystyrene was ~ 0.68 S/cm and by adding 5 wt% NMP into PEDOT:PSS solution, the conductivity of the annealed thin layers increased more than 10-folds (~ 7.73 S/cm) at an appropriate temperature.

Unraveling giant Cu(110) surface restructuring induced by a non-planar phthalocyanine

The surface stability of coinage metals is paramount when they are used as electrode materials for functional electronic devices incorporating organic semiconductors. In this work, it is shown that the adsorption of non-planar vanadyl phthalocyanine molecules on Cu(110) drastically restructured the metal surface at room temperature, which was further enhanced upon moderate annealing. Scanning tunneling microscopy imaging demonstrated that the surface was restructured at step edges into sawtooth features that gradually replaced the (110) terraces. The edges of the modified steps were preferentially composed of chiral (1×6) kink sites decorated with vanadyl phthalocyanine molecules adsorbed in a tilted configuration with the oxygen atom pointing downwards. These results can have a strong impact on the optimization of the performance of organic devices integrated with phthalocyanine molecules.

Impact of ambient environment on the electronic structure of CuPc/Au sample

The performances of organic devices are crucially connected with their stability in the ambient environment. The impact of 24 h. Ambient environment exposure to the electronic structures of about 12 nm thick CuPc thin film on clean Au substrate have been studied employing UV photoemission spectroscopy technique. X-ray photoemission spectroscopy (XPS) was used to find out the origin of the change of the electronic structures in the sample with the exposure. The XPS study suggests that the oxidation occurs at the CuPc thin film. Due to the adsorption of oxygen in the CuPc film from the ambient air, charge carriers are formed within the CuPc film. Moreover, the XPS results imply that the CuPc film is sufficiently thinner for diffusing oxygen molecules through it and gets physically absorbed on Au substrate during the ambient exposure. Consequently, the hole injection barrier height of pristine CuPc film, grown on Au substrate, is reduced by about 0.50 eV and work-function of the pristine CuPc sample is enhanced by around 0.25 eV in the exposure. The findings will help to understand the mechanism that governs the degradation of performance of CuPc based devices in ambient environment.

Molecular dynamics simulations of graphoepitaxy of organic semiconductors, sexithiophene, and pentacene: Molecular-scale mechanisms of organic graphoepitaxy

Molecular dynamics (MD) simulations of the organic semiconductors α-sexithiophene (6T) and pentacene were carried out to clarify the mechanism of organic graphoepitaxy at the molecular level. First, the models of the grooved substrates were made and the surfaces of the inside of the grooves were modified with –OH or –OSi(CH3)3, making the surfaces hydrophilic or hydrophobic. By the MD simulations of 6T, it was found that three stable azimuthal directions exist (0, ∼45, and 90°; the angle that the c-axis makes with the groove), being consistent with experimental results. MD simulations of deposition processes of 6T and pentacene were also carried out, and pentacene molecules showed the spontaneous formation of herringbone packing during deposition. Some pentacene molecules stood on the surface and formed a cluster whose a-axis was parallel to the groove. It is expected that a deep understanding of the molecular-scale mechanisms will lead graphoepitaxy to practical applications, improving the performance of organic devices.

Transition-metal-free and halogen-free controlled synthesis of poly(3-alkylthienylene vinylene) via the Horner–Wadsworth–Emmons condensation reaction

Controlled polymerization without a transition metal or halogen.

A few-layer molecular film on polymer substrates to enhance the performance of organic devices.

In organicelectronics the functionalization of dielectric substrates with self-assembled monolayers is regarded as an effective surface modification strategy that may significantly improve the resulting device performance. However, this technique is not suitable for polymer substrates typically used in flexible electronics. Here, we report organic modifiers based on a paraffinic tripodal triptycene, which self-assembles into a completely oriented two-dimensional hexagonal triptycene array and one-dimensional layer stacking structure on polymer surfaces. Such few-layer films are analogous to conventional self-assembled monolayers on inorganic substrates in that they neutralize the polymer surface. Furthermore, the triptycene films significantly improve the crystallinity of an organic semiconductor and the overall performance of organic thin-film transistors, therefore enabling the fabrication of high-performance organic complementary circuits on polymer substrates with high oscillation speeds and low operation voltage.