Organic Semiconductor Layer Research Articles

Facile Four-Mask Processes for Organic Thin-Film Transistor Integration Structure With Metal Interconnect

To enable an organic thin film transistor (OTFT) technology for circuit integration, the passivation layer and subsequent interconnect metallization on top of the OTFT is vital. In this work, a modified SU8 formulation is developed for forming the passivation layer on top of the OTFT with small molecule organic semiconductor (OSC). Based on the single SU8 passivation layer, via holes for interconnect metallization are easily formed, and the whole integration structure can be completed with facile four-mask processes. The OTFTs exhibit high mobility (> 2 cm $^{\textrm{2}}{/}\textrm{V}\cdot \textrm{s}$ ), low leakage ( $ A over the whole gate voltage range from 20 V to −30 V) at short channel length ( $8~\mu \text{m}$ ), excellent uniformity and stable electrical properties during operation and storage. The results well prove that the process of the modified SU8 solution for passivation is compatible with the small molecule OSC layer, and formation of the via connection is reliable.

Mobility of charge carriers in self-assembled monolayers

We present a new approach to study charge transport within 2D layers of organic semi-conductors (OSCs) using atomic force microscopy (AFM)-based lithography applied to self-assembled monolayers (SAMs), fabricated from appropriate organothiols. The extent of lateral charge transport was investigated by insulating pre-defined patches within OSC-based SAMs with regions of insulating SAM made from large band gap alkanethiolates. The new method is demonstrated using a phenyl-linked anthracenethiolate (PAT), 4-(anthracene-2-ylethynyl)benzyl thiolate. I–V characteristics of differently shaped PAT-islands were measured using the AFM tip as a top electrode. We were able to determine a relationship between island size and electrical conductivity, and from this dependence, we could obtain information on the lateral charge transport and charge carrier mobility within the thin OSC layers. Our study demonstrates that AFM nanografting of appropriately functionalized OSC molecules provides a suitable method to determine intrinsic mobilities of charge carriers in OSC thin films. In particular, this method is rather insensitive with regard to influence of grain boundaries and other defects, which hamper the application of conventional methods for the determination of mobilities in macroscopic samples.

Enhanced anisotropy and study of magnetization reversal in Co/C60 bilayer thin film

The interface between the organic semiconductor (OSC)/ferromagnetic (FM) material can exhibit ferromagnetism due to their orbital hybridization. Charge/spin transfer may occur from the FM to OSC layer leading to the formation of “spinterface,” i.e., the interface exhibiting a finite magnetic moment. In this work, the magnetic properties of the Co/C60 bilayer thin film have been studied to probe the interface between the Co and C60 layer. Polarized neutron reflectivity (PNR) measurement indicates that the thickness and moment of the spinterface are ∼2 ± 0.18 nm and 0.8 ± 0.2 μB/cage, respectively. The comparison of the magnetization reversal between the Co/C60 bilayer and the parent single layer Co thin film reveals that spinterface modifies the domain microstructure. Further, the anisotropy of the bilayer system shows a significant enhancement (∼two times) in comparison to its single layer counterpart which is probably due to an additional interfacial anisotropy arising from the orbital hybridization at the Co/C60 interface.

Polymer Light‐Emitting Transistors With Charge‐Carrier Mobilities Exceeding 1 cm2 V−1 s−1

AbstractThe vast majority of conjugated‐polymer‐based light emitting field‐effect transistors (LEFETs) are characterized by low charge‐carrier mobilities typically in the 10−5 to 10−3 cm2 V−1 s−1 range. Fast carrier transport is a highly desirable characteristic for high‐frequency LEFET operation and, potentially, for use in electrically pumped lasers. Unfortunately, high‐mobility organic semiconductors are often characterized by strong intermolecular π–π interactions that reduce luminescence. Development of new materials and/or device concepts that overcome this hurdle are therefore required. Single organic semiconductor layer based LEFETs that combine high hole mobilities with encouraging light emission characteristics are reported. This is achieved in a single polymer layer LEFET, which is further enhanced through the use of a small‐molecule/conjugated polymer blend system that possesses a film microstructure which supports enhanced charge‐carrier mobility (3.2 cm2 V−1 s−1) and promising light‐emission characteristics (1600 cd m−2) as compared to polymer‐only based LEFETs. This simple approach represents an attractive strategy to further advance the performance of solution‐processed LEFETs.

Solution-Processable Vertical Organic Light-Emitting Transistors (VOLETs) with Directly Deposited Silver Nanowires Intermediate Source Electrode.

A simple spin-coating process for fabricating vertical organic light-emitting transistors (VOLETs) is realized by utilizing silver nanowire (AgNW) as a source electrode. The optical, electrical and morphological properties of the AgNW formation was initially optimized, prior VOFET fabrication. A high molecular weight of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEH-PPV was used as an organic semiconductor layer in the VOFET in forming a multilayer structure by solution process. It was found that current density and luminance intensity of the VOLET can be modulated by a small magnitude of gate voltage. The modulation process was induced by changing an injection barrier via gate voltage bias. A space-charge-limited current (SCLC) approach in determining transistor mobility has been introduced. This preliminary and fundamental work is beneficial towards all-solution processing display devices.

Abnormal Back Channel Leakage Under Large Drain Voltage in Short Channel Organic Thin-Film Transistors

This study investigates short channel (length $ ) p-type organic thin-film transistor (OTFT) devices which exhibit off-state leakage in their ID–VG characteristics. This phenomenon is attributed to the fact that when a lower positive gate voltage is applied, gate voltage cannot control the back channel, causing back channel leakage. The behavior of back channel leakage is confirmed by the OTFT saturated-regime drain-current formula multiplied by channel length. For dual gate OTFT devices, a top gate voltage sweep and fixed different bottom gate voltages are applied, which indicates that the conduction path is distributed throughout the entire organic semiconductor (OSC) layer, and the back channel leakage can be suppressed by a positive bottom gate voltage. Finally, using dual gate sweeping for a dual gate OTFT device allows the top and bottom gate voltage to simultaneously control the front and back channels, causing a suppression of the back channel leakage, an improvement in subthreshold swing (SS) from 420 to 200 (mV/decade) and an on current increase from $\textsf {7.8}\times \textsf {10}^{-\textsf {8}}$ to $\textsf {3.3}\times \textsf {10}^{-\textsf {7}}$ A at VG-Vsub-threshold = −8 V for the W/L $=50/20~\mu \text{m}$ device.

The role of charge transfer at reduced graphene oxide/organic semiconductor interface on the charge transport properties

The effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in electron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4-TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations. • RGO FETs, exhibit a positive shift of the charge-neutrality point due to deposition of organic semiconductor layer. • The amount of shift varies with the molecule electron affinity. • The electron mobility is quenched by the presence of F4-TCNQ and TCNE, while 1-PSA don’t affect the electron mobility. • With increasing the coverage of 1-PSA, the electron mobility drops and then slightly recovers at higher coverages. • The change in the electron mobility is due to effect of teh RGO/organic-layer interface dipole field.

Recent Advances in the Bias Stress Stability of Organic Transistors

AbstractIn this progress report, recent advances in the development of organic transistors with superior bias stress stability and in the understanding of the charge traps that degrade device performance under prolonged bias stress are reviewed, with a particular focus on materials science and engineering methods. The phenomenological aspects of bias stress effects and the experimental methods for investigating charge traps are described. The recent progress in the bias stress stability of organic transistors is discussed in terms of those components that are the main focus of attempts to improve bias stress stability, i.e., organic semiconductor layers, gate dielectrics, and source/drain contacts. A brief summary of this progress is presented and the outlook for future research in this field is assessed. This report aims to summarize recent progress in this field and to provide some guidelines for studying bias stress–induced charge‐trapping phenomena.

Factors contributing to degradation of organic photovoltaic cells

Abstract The performance of organic photovoltaic (OPV) cells has considerably improved over the past decade, and now exceeds an energy conversion efficiency of 10%, the benchmark for practical use. One factor contributing to this increase is the development of π-conjugated linear acceptor molecules as alternatives to fullerene derivatives. Although conversion efficiency is an important issue for practical applications, device stability is also necessary for commercialization of OPV technologies. To date, long-term stability has been examined in OPV devices based on fullerenes. However, the factors involved in performance degradation remain poorly understood. It is difficult to address this problem because of the multilayer nature of the devices and many factors involved in degradation. In recent OPV devices based on π-conjugated molecules as acceptors, there have been few reports on stability under continuous light irradiation. Ensuring the stability of the π-conjugated molecules under illumination and electrical operation is critical for achieving practical use of OPV devices. In this paper, we comprehensively analyzed OPV devices based on EH-IDTBR as an acceptor material. We identified a decrease in OPV performance of approximately 50% under 100-h light irradiation, which we attribute to increased resistance of the organic semiconductor layer. The increase in resistance was caused by a decrease in the number of carriers, suggesting that structural changes in the EH-IDTBR molecule are an important factor affecting degradation.

Self‐assembled core‐shell structured organic nanofibers fabricated by single‐nozzle electrospinning for highly sensitive ammonia sensors

AbstractElectrospinning is a unique method to prepare 1‐dimentional nanofiber for large‐scale manufacturing. Here in, we demonstrated chemical sensors based on the semiconducting nanofibers with core‐shell structure by simple single‐nozzle electrospinning with the spontaneous phase separation. The core‐shell structure nanofiber has large active sites which make it highly sensitive to chemical analytes. The thickness of the sensing shell can be tuned by controlling the mass ratio of core and shell components. As a demonstration, the nanofiber‐based sensor exhibits high sensitivity to low‐concentration ammonia (ppb level), as well as stability and reversibility. This unique fast single‐nozzle electrospinning technique used to fabricate semiconducting nanofibers with core‐shell structure provides a facile and designable process for the integration of multifunctional organic semiconducting layer into the organic electronic system.

High‐Performance Proximity Sensors with Nanogroove‐Template‐Enhanced Extended‐Gate Field‐Effect Transistor Configuration

AbstractIn recent years human–machine interaction has become increasingly important in industrial applications and daily life. Proximity sensors are expected to become an important part of such systems. The mechanisms of these sensors are usually based on ultrasound, capacitance, triboelectric effect, optical imaging or semiconducting devices. The fabrication and sensing performance of solution‐based organic transistor proximity sensors is reported. To enhance electrical performance, nanogroove templates are introduced to guide the oriented growth of organic semiconducting layer. The templates are realized by friction‐transferring polytetrafluoroethylene thin layers onto SiO2/Si substrates. An extended gate structure for proximity sensing is designed, in which one end of a silver wire is electrically connected with the gate of the transistor and the other end serves as the sensing end. Proximity sensing is characterized by bringing various charged stimuli close to the sensor and recording the resulting change in drain current. The sensor showed good repeatability during the approach and withdrawal of a stimulus. A maximum current response of 2.2 μA between distances of 3 mm and 53 mm and distance sensitivity of 2.5 nA mm‐1 at a distance of 13 mm are obtained when a charged polytetrafluoroethylene rod is moved up and down above the sensing end.

Flexible Pressure-Sensitive Contact Transistors Operating in the Subthreshold Regime.

Organic thin-film transistor (TFT)-based pressure sensors have received huge attention for wearable electronic applications such as health monitoring and smart robotics. However, there still remains a challenge to achieve low power consumption and high sensitivity at the same time for the realization of truly wearable sensor systems where minimizing power consumption is significant because of limited battery run time. Here, we introduce a flexible pressure-sensitive contact transistor (PCT), a new type of pressure-sensing device based on organic TFTs for next-generation wearable electronic skin devices. The PCT consists of deformable S/D electrodes integrated on a staggered TFT. The deformable S/D electrodes were fabricated by embedding conducting single-walled carbon nanotubes on the surface of microstructured polydimethylsiloxane. Under pressure loads, the deformation of the electrodes on an organic semiconductor layer leads modulation of drain current from variation in both the channel geometry and contact resistance. By strategic subthreshold operation to minimize power consumption and increase the dominance of contact resistance because of gated Schottky contact, the PCT achieves both ultralow power consumption (order of 101 nW) and high sensitivity (18.96 kPa-1). Finally, we demonstrate a 5 × 5 active matrix PCT array on a 3 μm-thick parylene substrate. The device with ultralow power consumption and high sensitivity on a biocompatible flexible substrate makes the PCT promising candidate for next-generation wearable electronic skin devices.

Dynamic Charging Mechanism of Organic Electrochemical Devices Revealed with In Situ Infrared Spectroscopy

The steps responsible for device charging in organic electrochemical transistors were investigated using in situ infrared spectroscopy. Metal–electrolyte-dielectric–organic semiconductor capacitor structures were fabricated on infrared waveguides and measured using the total internal reflection sampling method. Upon the application of a voltage, charges were induced in the device, creating polaron absorption features in the mid-infrared region. The dynamics of the device charging were investigated by varying the channel length, dielectric layer thickness, and organic semiconductor layer thickness. Device charging was independent of the channel length but depended strongly on the semiconductor and dielectric layer thicknesses, indicating that the movement of ions is the primary determining factor for device charging kinetics. A quantitative model is developed combining an resistor-capacitor (RC) circuit model for the dielectric layer and a mixed ion–carrier diffusion model for the organic semiconductor lay...

In situ fabrication of organic electrochemical transistors on a microfluidic chip

Microfluid chips integrating with organic electrochemical transistors (OECTs) are useful for manufacturing biosensors with high throughput and large-scale analyses. We report here the utilization of alternating current (AC) electrodeposition to fabricate OECTs in situ on a microfluid chip. With this method, the organic semiconductor (OS) layer with a channel length of 8 ώm was readily prepared without requiring the post-bonding process in the conventional construction of microfluidic chips. Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/graphene quantum dots (PEDOT:PSS/GQDs) composites with different morphologies, such as microfilms, nanodendrites and nanowires were electropolymerized. The mass transfer process of the electropolymerization reaction was evidenced to be diffusion limited. Morphologies, growth directions, and chemical structures of OS layers could be tuned by the amplitude and the frequency of the AC voltage. Transfer and output characteristic curves of OECTs were measured on the microfluidic chip. The maximum transconductance, on/off current ratio and threshold voltage measured in the experiment was 1.58 mS, 246, and 0.120 V, respectively.

Importance of Blade-Coating Temperature for Diketopyrrolopyrrole-based Thin-Film Transistors

In this work, the effect of blade-coating temperature on the electrical properties of a conjugated donor–acceptor copolymer containing diketopyrrolopyrrole (DPP)-based thin-film transistors (TFTs) was systematically analyzed. The organic semiconductor (OSC) layers were blade-coated at various blade-coating temperatures from room temperature (RT) to 80 °C. No remarkable changes were observed in the thickness, surface morphology, and roughness of the OSC films as the blade-coating temperature increased. DPP-based TFTs exhibited two noticeable tendencies in the magnitude of field-effect mobility with increasing blade-coating temperatures. As the temperature increased up to 40 °C, the field-effect mobility increased to 148% compared to the RT values. On the contrary, when the temperature was raised to 80 °C, the field-effect mobility significantly reduced to 20.9% of the mobility at 40 °C. These phenomena can be explained by changes in the crystallinity of DPP-based films. Therefore, the appropriate setting of the blade-coating temperature is essential in obtaining superior electrical characteristics for TFTs. A blade-coating temperature of 40 °C was found to be the optimum condition in terms of electrical performance for DPP-based TFTs.

Bistaggered Contact Geometry for Symmetric Dual-Gate Organic TFTs

This transaction proposes a symmetric bistaggered dual-gate organic thin-film transistor (TFT) to minimize the difference between the top and bottom channel current characteristics and demonstrates it through experiment and 2-D simulation. In dual-gate organic TFTs, asymmetric top and bottom channel currents are mainly a result of their asymmetric contact geometry. To address this geometric limitation, the contact electrodes were sandwiched between the two organic semiconductor layers to be placed in a staggered position to both top and bottom gates. Two-way current crowding at the bistaggered contact electrodes leads to high-charge injection efficiency in both channels.

Formation of aligned periodic patterns during the crystallization of organic semiconductor thin films.

Self-organizing patterns with micrometre-scale features are promising for the large-area fabrication of photonic devices and scattering layers in optoelectronics. Pattern formation would ideally occur in the active semiconductor to avoid the need for further processing steps. Here, we report an approach to form periodic patterns in single layers of organic semiconductors by a simple annealing process. When heated, a crystallization front propagates across the film, producing a sinusoidal surface structure with wavelengths comparable to that of near-infrared light. These surface features initially form in the amorphous region within a micrometre of the crystal growth front, probably due to competition between crystal growth and surface mass transport. The pattern wavelength can be tuned from 800 nm to 2,400 nm by varying the film thickness and annealing temperature, and millimetre-scale domain sizes are obtained. This phenomenon could be exploited for the self-assembly of microstructured organic optoelectronic devices.

Contact patterning by laser printing for flexible electronics on paper

The desire for cost-effective strategies for producing organic electronic devices has led to many new methods for the organic semiconductor layer deposition; however, manufacturing contacts remains an expensive technique due to the high cost of both the materials used and the processing necessary for their patterning. In this work, we present a method for contact deposition and patterning, which overcomes these limitations and allows fabrication of all-printed organic thin-film transistors on paper. The method relies on depositing contacts using aerosol spray and patterning them with a digitally printed mask from an office laser printer, at ambient temperature and pressure. This technique, which we have denoted aerosol spray laser lithography, is cost-effective and extremely versatile in terms of material choice and electrode geometry. As the processing temperature does not exceed 155 °C, it is compatible with a variety of substrates, including plastic or paper. The success of this method marks an opportunity for a rapid, scalable, and low-cost alternative to current electrode-manufacturing techniques for development of flexible, large-area, electronic applications.

(Invited) Laser Printed Organic Field-Effect Transistors on Paper

Laser printing is a low-cost, environmentally friendly and high-throughput technique that is compatible with flexible substrates. This coating method is ubiquitous in many areas, but, in spite of its clear advantages, it has not been adopted for manufacturing electronic devices. In this presentation, I will introduce the use of laser printing for deposition and patterning of organic thin-film transistors. This solvent-free processing method allowed for simultaneous deposition, purification, and patterning of the organic semiconductor layer and definition of contacts in transistors realized on flexible substrates such as plastic and paper. We performed electrical characterization of the devices and related the results with the structural features extracted from grazing incidence wide angle x-ray scattering. To define the electrodes, we used aerosol spray laser lithography in conjunction with different metal inks; the pattern was created using a regular toner, which was subsequently selectively removed. We created a grid of transistor devices with variable channel lengths and widths, and obtained good charge carrier mobilities and an excellent tolerance to extreme bending. The success of this method marks an opportunity for a rapid, scalable, environmental friendly and low-cost alternative to current manufacturing techniques for development of flexible, large-area, electronic applications.

Organic pn homojunction solar cell

Formation of pn homojunction in a single semiconductor material through doping is key for photocurrent generation in inorganic solar cells (SCs). In contrast, a donor/acceptor heterojunction is generally utilized in organic SCs in order to split coulombically bound exciton forming in organic semiconductors. Here, an organic pn homojunction SC showing high internal quantum efficiency of about 30% is reported. pn Homojunction was created by co-deposition of p or n type dopants and an organic semiconductor. An increase of the photocurrent density by more than 8 times, from the undoped device to pn homojunction device with 5% doping, was observed. The increase of photocurrent due to charge separation was assisted by the built-in potential near the pn homojunction interface that was controlled by doping concentration. The result demonstrated that the pn homojunction interface in a single organic semiconductor layer formed by doping can achieve efficient charge separation and provide a suitable alternative to the donor/acceptor interface that has been considered necessary for photoconversion in organic SCs.