Organic Semiconductor Layer Research Articles

Organic Light-Emitting Transistors Based on Pentacene and 4,5-Di(9H-carbazol-9-yl)phthalonitrile Doped onto 1,3-Bis(N-carbazolyl)benzene

In this study, multilayered, organic-based light-emitting transistors (OLETs) were produced and characterized. The neutral cluster beam deposition method was applied to successively deposit organic semiconducting layers of pentacene, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN) doped onto 1,3-bis(N-carbazolyl)benzene (mCP), and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) as the hole-transport, emissive, and electron-transport layers, respectively. The effects of drain electrodes (Au or Li/Al) and TPBi on the device performance were examined under ambient conditions. The bilayered pentacene (bottom, 50 nm)/mCP:2CzPN (top, 50 nm) and trilayered pentacene (bottom, 50 nm)/mCP:2CzPN (middle, 50 nm)/TPBi (top, 10 nm) OLETs demonstrated both electrical switching functionality and control of electroluminescence (EL) in air. In particular, the EL intensity (IEL) was significantly enhanced in the asymmetric bilayered devices adopting a low work function Li/Al drain electrode owing to the lower electro...

The effect of phenylboronic acid-based self-assembled monolayers on theperformance of organic field-effect transistors (OFETs)

The dielectric/semiconductor interface is one of the most important components strongly affecting the performance of organic field-effect transistors (OFETs). To improve OFET parameters, we applied a series of phenylboronic acid derivatives as self-assembly molecules between dielectric and organic semiconductor layers. Device performance parameters, especially threshold voltage, current on/off ratio, and subthreshold slope, were improved. To elucidate the mechanism, dielectric/semiconductor interfaces were analyzed using atomic force microscope and contact angle techniques.

Full characterization of electronic transport properties in working polymer light-emitting diodes via impedance spectroscopy

The electron and hole drift mobilities of organic semiconductor layers, localized tail state distributions, and bimolecular recombination constants in working polymer light-emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The organic light-emitting layers of these PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole). Electron and hole transit time effects are observed in the capacitance-frequency characteristics of the PLEDs, and their drift mobilities are determined over wide temperature and electric field ranges. The drift mobilities exhibit thermally activated behavior, and the localized tail state distributions from the conduction band and valence band mobility edges are then determined from analysis of the electric field dependences of the activation energies. The bimolecular recombination constants are determined from the inductive response of the impedance-frequency characteristics. The IS technique is also applicable to degradation analysis of the PLEDs; changes in the mobility balance, the localized tail state distributions, and the bimolecular recombination constant caused by aging are all shown.

Gas Sensors Based on Nano/Microstructured Organic Field-Effect Transistors.

Benefiting from the advantages of organic field-effect transistors (OFETs), including synthetic versatility of organic molecular design and environmental sensitivity, gas sensors based on OFETs have drawn much attention in recent years. Potential applications focus on the detection of specific gas species such as explosive, toxic gases, or volatile organic compounds (VOCs) that play vital roles in environmental monitoring, industrial manufacturing, smart health care, food security, and national defense. To achieve high sensitivity, selectivity, and ambient stability with rapid response and recovery speed, the regulation and adjustment of the nano/microstructure of the organic semiconductor (OSC) layer has proven to be an effective strategy. Here, the progress of OFET gas sensors with nano/microstructure is selectively presented. Devices based on OSC films one dimensional (1D) single crystal nanowires, nanorods, and nanofibers are introduced. Then, devices based on two dimensional (2D) and ultrathin OSC films, fabricated by methods such as thermal evaporation, dip-coating, spin-coating, and solution-shearing methods are presented, followed by an introduction of porous OFET sensors. Additionally, the applications of nanostructured receptors in OFET sensors are given. Finally, an outlook in view of the current research state is presented and eight further challenges for gas sensors based on OFETs are suggested.

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.

Photo-assisted recovery in ammonia sensor based on organic vertical diode

Abstract In this article, a photo-assisted gas sensor was provided to solve the slow recovering issue of the organic gas sensor. The proposed organic gas sensor can detect ammonia in parts-per-billion regime in 30 s and can be multiple uses, but the long recovery time about 5 min places difficulty to realize rapidly multiple-time sensing. Therefore, we used the photo-absorption ability in the organic semiconductor layer to generate the photocurrent and hence to compensate the current drop due to the ammonia absorption. The recovery time was significantly reduced to be only 1 min. The specific wavelength was also found to match the absorption spectrum in different organic sensing materials. As a comparison, the quartz crystal microbalance (QCM) was used to clarify the gas absorption and desorption mechanism. It is believed that this photo-assisted gas detection could be a general method to solve the slow recovery in organic-based gas sensors.

Anisotropic Magnetoresistance in NiFe-Based Polymer Spin Valves.

Precise evaluation of magnetoresistance (MR) with the identified transport mechanism is one of the key points in organic spin-valve (OSV) devices. To investigate the origin of the spin-valve-like signal in polymer spin valve with a vertical structure of NiFe/P3HT/AlO x/Co, the magnetic response measurements in the rotated magnetization direction were established in different well-designed device configurations. We identified the significant influence of anisotropic MR (AMR) and further induced tunneling AMR in NiFe electrodes contributing to the total MR signal. These findings suggest that the mechanisms responsible for the transport mode in polymer spin-valve devices also strongly depend on the interface between the ferromagnetic electrodes and organic semiconductor layer. Even for the spin-valve-resembled MR response with distinct parallel and antiparallel states, carefully controlled experiments such as temperature-dependent and angle-dependent measurements should always be performed to track down the injected spin-polarized carriers. Our thorough experiments and analyses may shed light on the effective MR signal evaluation in OSVs and spin-related parameters with transport mechanism identification.

Enhanced Seebeck Effect of a MAPbBr3 Single Crystal by an Organic and a Metal Modified Layer

AbstractThe organic–inorganic hybrid lead halide perovskite single crystal has drawn much attention of researchers for excellent characteristics, such as more excellent stability than the polycrystalline film. However, during the crystal preparation process, there are inevitably generated surface defect states, which seriously affect the charge transport properties of the single crystal. In this article, the MAPbBr3 single crystal is prepared by the inverse temperature crystallization method, the semiconductor characteristics of the crystal surface defect states are studied by the Seebeck effect method, and the crystal Seebeck coefficient is enhanced. It is confirmed that there are N‐type defect states on the surface of the MAPbBr3 single crystal. Then the organic semiconductor layer and metal surface modified layer are utilized to reduce the concentration of defects and got the enhancement of the Seebeck coefficient of MAPbBr3 single crystal. At last, the Au/MAPbBr3/PFO/Au structure device is designed and prepared to improve the thermoelectric properties of the MAPbBr3 single crystal. Results show that the thermal voltage keeps increasing, and the Seebeck coefficient is also enhanced.

Reducing contact resistance in bottom contact organic field effect transistors for integrated electronics

Organic field effect transistors (OFETs) are a promising technology for developing truly flexible, stretchable and bio-compatible integrated electronics. Understanding the contact resistance mechanisms and introducing strategies to minimize the contact resistance is vital for the continuous performance improvement of OFETs. This paper firstly discusses the suitability of various device structures for short channel OFETs and fine resolution integration. After describing the formation of the contact resistance composed of the injection and access parts, this paper comprehensively reviews approaches for reducing contact resistance with bottom contact structure OFETs, including by interface engineering and doping the organic semiconductor layer. The pros and cons of each approach are discussed in detail. It is concluded that a combination of various techniques is required to minimize the contact resistance for fine resolution integration.

A solvent-free and vacuum-free melt-processing method to fabricate organic semiconducting layers with large crystal size for organic electronic applications

We report on an improved melt-processing method to prepare organic semiconducting layers with large crystal size.

Organic Semiconductors with Carbazole and Triphenylamine Moieties for Glucose Oxidase-Based Biosensors

Organic semiconductors are among the most promising next generation materials for biosensors as the greener and cheaper alternative to transition metal-based electrodes. This work focuses on the production and analysis of organic p-type semiconductor-based glucose biosensors. The compressed graphite electrodes have electrochemically been modified with 9-ethyl-9H-carbazole (CzEt), 9-phenyl-9H-carbazole (CzPh), N,N,N-triphenylamine (TPA) and with cross-linked glucose oxidase (GOx). Response of the created biosensors to the changing concentration of glucose has been assessed by cyclic voltammetry. The electrochemical impedance of these organic semiconductor layers was investigated by using an electrochemical impedance spectroscopy method. Herein, we aim at drawing a picture of the electro-polymerized derivatives and the applicability of p-type carbazole and triphenylamine-based organic semiconductors for amperometric biosensors.

Features of Light Transmission and Stark Effect in a Plasmonic Nanostructure Comprising Organic Semiconductor and Subwavelength Aluminum Grating

Specific features of light transmission and electroabsorption in a plasmonic nanostructure with organic semiconductor (zinc phthalocyanine, ZnPc) and subwavelength aluminum grating (AlGr) have been experimentally investigated. The glass–AlGr–ZnPc–Al nanostructure was prepared using layer-by-layer deposition. First, only a part of the structure has been analyzed to check the grating quality. After deposition of an organic semiconductor (ZnPc) layer on the grating, the transmission of TE and TM polarized light through AlGr and ZnPc layers upon excitation of plasmon resonances has been studied. Afterwards, a semitransparent aluminum layer has been deposited on the ZnPc layer (an additional electrode for measuring the electroabsorption effect). For TM polarized light, a multiple increase in the electroabsorption effect has been found (in comparison with a structure without subwavelength grating). This result can be explained by the Stark effect on exciton transitions and the presence of two cavities in the structure, which are related to the excitation of plasmonic states and the Fabry–Perot effect between aluminum electrodes. The presence of the cavities leads to a decrease in the group velocity of light and, accordingly, increases the density of exciton states, which are characterized by a large difference between polarizabilities in the excited and ground states.

Fabrication, characterization, and electrical measurements of gas ammonia sensor based on organic field effect transistor

Ammonia gas sensors were fabricated based on organic field effect transistor “OFET” with n-type Si or indium tin oxide “ITO” substrate as gate electrodes and SiO2 or polymethylmethacrylate “PMMA” as gate insulator layers, respectively. Different organic semiconductor channel “OSC” layers based on polyaniline “PANI” doped with dodecylbenzene sulfonic acid “DBSA”, polyaniline–emeraldine base “PANI:EB”, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate “PEDOT:PSS” were created between source and drain electrodes. Confocal laser scanning microscopy “CLSM” was used to identify the OFET surface profiling and to measure the dimensions and thicknesses of OFET layers. The performance of the OFET ammonia sensor was evaluated based on the change in the electrical measurements such as the current–voltage “IDS–VDS” characteristic curve, saturation current, and threshold voltage at room temperature. Relative response “ΔR” and response/recovery time of the OFET ammonia sensors with different organic semiconductor layers were obtained from the change in the drain–source current “IDS” at zero gate voltage when the sensors were exposed to different consecutive cycles and evacuation of ammonia gas. The OFET sensor has PANI:DBSA as an active channel layer with Si or ITO gate exhibited ΔR to ammonia gas of ~ 63% and ~ 41.4% with response time of 2 and 4 s, respectively.

Towards high-bandwidth organic photodetection based on pure active layer polarization

Organic photodetectors offer distinct advantages over their inorganic analogues, most notably through optical transparency and flexibility, yet their figures-of-merit still lag behind those of inorganic devices, and optimization strategies generally encounter a trade-off between device responsivity and bandwidth. Here we propose a novel photodetector architecture in which an organic photoactive semiconductor layer (S) is sandwiched between two thick insulating layers (I) that separate the semiconductor from the metallic contacts (M). In this architecture a differential photocurrent response is generated purely from the polarization of the active layer under illumination. Especially for an asymmetric MISIM design, where one insulating layer is a high-k ionic liquid IIL and the other a low-k polymer dielectric Ip, the responsivity/bandwidth trade-off is broken, since the role of the IIL in efficient charge separation is maintained, while the total device capacitance is reduced according to Ip. Thus the benefits of single insulating layer differential photodetectors (MISM) using either IIL or Ip are combined in a single device. Further improvements in device performance are also demonstrated by decreasing the series resistance of the photoactive layer through semiconductor:metal blending and by operation under strong background light.

Crystallization Control of Organic Semiconductors during Meniscus‐Guided Coating by Blending with Polymer Binder

AbstractSmall molecule organic semiconductors (OSCs) suffer from their uncontrolled nucleation and growth during solution processing limiting their functionality in electronic devices. In this work, a new method is presented based on dip‐coating a blend consisting of OSC and insulating polymer to control the crystallization of the active film for organic field‐effect transistors. A small fraction of amorphous poly(methyl methacrylate) (PMMA) efficiently improves the crystallization of dip‐coated small molecule OSCs, α,ω‐dihexylquaterthiophene (DH4T) and diketopyrrolopyrrole‐sexithiophene (DPP6T). The maximum charge carrier mobilities of dip‐coated OSC:PMMA films are significantly higher than drop‐cast blend ones and comparable with OSC single crystals. The high charge carrier mobility originates from a continuous alignment of the crystalline films and stratified OSC and PMMA layers. The improved crystallization is attributed to two mechanisms: first, the polymer binder leads to a viscosity gradient at the meniscus during dip‐coating, facilitating the draw of solute and thus mass transport. Second, the polymer binder solidifies at the bottom layer, reducing the nucleation barrier height of small molecule OSC. The findings demonstrate that a small fraction of a polymer binder during dip‐coating efficiently improves the crystallization as well as the electronic properties of small molecule OSC films.

Characterisation of charge carrier transport in thin organic semiconductor layers by time-of-flight photocurrent measurements

Abstract The paper reviews recent advances in characterisation of charge carrier transport in organic semiconductor layers by time-of-flight photocurrent measurements, with the emphasis on the measurements of the samples with co-planar electrodes. These samples comprised an organic semiconductor layer whose thickness is on the order of a μm or less, and thus mimic the structures of organic thin film transistors. In the review we emphasise the importance of considering spatial variation of electric field in these, essentially two-dimensional structures, in interpretation of photocurrent transients. We review the experimental details of this type of measurements and give examples that demonstrate exceptional sensitivity of the method to minute concentration of electrically active defects in the organic semiconductors as well as the capability of probing charge transport along the channels of different mobility that reside in the same sample.

Solution‐Processed Rad‐Hard Amorphous Metal‐Oxide Thin‐Film Transistors

AbstractThe effect of 5 MeV high‐energy proton irradiation on solution‐processed metal‐oxide thin‐film transistors (TFTs) is investigated. The electrical characteristics of the devices are measured before and after proton irradiation with radiation doses of 1013, 1014, and 1015 cm−2. TFTs based on zinc oxide (ZnO) and amorphous indium gallium zinc oxide (a‐IGZO) exhibit a significant negative shift in their threshold voltage values (ΔVth ≤ −30 V) or transitioned to the conductor state as the proton radiation dose increased. For a‐ZnO and IGZO, this change in the electrical characteristics originates from the formation of proton‐irradiation‐induced oxygen vacancies in the metal‐oxide semiconductor layer. On the other hand, amorphous zinc tin oxide devices with an optimized composition exhibit relatively stable electrical characteristics when subjected to proton irradiation. Furthermore, the back‐channel passivation of oxide‐semiconductor TFTs with an n‐type organic semiconductor layer significantly improves the device stability under proton irradiation. This study demonstrates that solution‐processed metal‐oxide semiconductors have significant potential as rad‐hard large area electronic devices for nuclear and aerospace applications.

Micropatterning of organic electronic materials using a facile aqueous photolithographic process

Patterning organic semiconductors via traditional solution-based microfabrication techniques is precluded by undesired interactions between processing solvents and the organic material. Herein we show how to avoid these problems easily and introduce a simple lift-off method to pattern organic semiconductors. Positive tone resist is deposited on the substrate, followed by conventional exposure and development. After deposition of the organic semiconductor layer, the remaining photoresist is subjected to a flood exposure, rendering it developable. Lift-off is then performed using the same aqueous developer as before. We find that the aqueous developers do not compromise the integrity of the organic layer or alter its electronic performance. We utilize this technique to pattern four different organic electronic materials: epindolidione (EPI), a luminescent semiconductor, p-n photovoltaic bilayers of metal-free phthalocyanine and N,N’-dimethyltetracarboxylic diimide, and finally the archetypical conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). The result of our efforts is a facile method making use of well-established techniques that can be added to the toolbox of research and industrial scientists developing organic electronics technology.

Integrating Ultrathin Bulk‐Heterojunction Organic Semiconductor Intermediary for High‐Performance Low‐Bandgap Perovskite Solar Cells with Low Energy Loss

AbstractMixed tin (Sn)–lead (Pb) perovskite is considered the most promising low‐bandgap photovoltaic material for both pursuing the theoretical limiting efficiency of single‐junction solar cells and breaking the Shockley–Queisser limitation by constructing tandem solar cells. However, their power conversion efficiencies (PCEs) are still lagging behind those of medium‐bandgap perovskite solar cells (pero‐SCs) due to their serious energy loss (Eloss). Here, an ultrathin bulk‐heterojunction (BHJ) organic semiconductor (PBDB‐T:ITIC) layer is used as an intermediary between the hole transporting layer and Sn–Pb‐based low‐bandgap perovskite film to minimize Eloss. It is found that this BHJ PBDB‐T:ITIC intermediary simultaneously provided a cascading energy alignment in the device, facilitated high‐quality Sn–Pb perovskite film growth, and passivated the antisite defects of the perovskite surface. In this simple way, the Eloss of pero‐SCs based on (FASnI3)0.6(MAPbI3)0.4 (bandgap ≈1.25 eV) is dramatically reduced below 0.4 eV, leading to a high open‐circuit voltage (Voc) of 0.86 V. As a result, the best pero‐SC showed a significantly improved PCE of 18.03% with negligible J–V hysteresis and high stability. As far as it is known, the PCE of 18.03% and Voc of 0.86 V are the highest values among the low‐bandgap pero‐SCs to date.

Synergistic electrodeposition of bilayer films and analysis by Raman spectroscopy.

A novel methodology towards fabrication of multilayer organic devices, employing electrochemical polymer growth to form PEDOT and PEDTT layers, is successfully demonstrated. Moreover, careful control of the electrochemical conditions allows the degree of doping to be effectively altered for one of the polymer layers. Raman spectroscopy confirmed the formation and doped states of the PEDOT/PEDTT bilayer. The electrochemical deposition of a bilayer containing a de-doped PEDTT layer on top of doped PEDOT is analogous to a solution-processed organic semiconductor layer deposited on top of a PEDOT:PSS layer without the acidic PSS polymer. However, the poor solubility of electrochemically deposited PEDTT (or other electropolymerised potential candidates) raises the possibility of depositing a subsequent layer via solution-processing.