Flexible Organic Electronic Devices Research Articles

Understanding mechanical behavior and reliability of organic electronic materials

Abstract

Superlyophilicity-Facilitated Synthesis Reaction at the Microscale: Ordered Graphdiyne Stripe Arrays.

As a new member of carbon allotropes, graphdiyne is a promising material with excellent electronic performance and high elasticity, indicating the possibility of graphdiyne to serve as the building blocks in flexible electronics. However, precise positioning/patterning of graphdiyne is still a challenge for the realization of large-area and flexible organic electronic devices and circuits. Here, the direct in situ synthesis of patterning graphdiyne stripe arrays dominated by the superlyophilic grooved templates is reported, whereas the superlyophilicity of grooved templates plays a key role in allowing continuous mass transport of raw reactants into the microscale spacing. After the completion of cross-coupling reaction procedure, precisely patterned graphdiyne stripes can be generated accordingly. The size of graphdiyne stripe arrays is depending on the silicon substrate size (1 cm × 1.5 cm), and the layer thickness can be manipulated from just several nanometers to hundreds of nanometers by varying the primary concentration of hexaethynylbenzene monomers. As a proof-of-principle demonstration, a stretchable sensor based on the graphdiyne stripe arrays is performed to monitor the human finger motion. It is expected that this wettability-facilitated strategy will provide new insights into the controlled synthesis of graphdiyne toward promising flexible electronics and other optoelectronic applications.

Characteristics of silicon nitride deposited by VHF (162 MHz)-plasma enhanced chemical vapor deposition using a multi-tile push–pull plasma source

To prevent moisture and oxygen permeation into flexible organic electronic devices formed on substrates, the deposition of an inorganic diffusion barrier material such as SiNx is important for thin film encapsulation. In this study, by a very high frequency (162 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push–pull plasma source, SiNx layers were deposited with a gas mixture of NH3/SiH4 with/without N2 and the characteristics of the plasma and the deposited SiNx film as the thin film barrier were investigated. Compared to a lower frequency (60 MHz) plasma, the VHF (162 MHz) multi-tile push–pull plasma showed a lower electron temperature, a higher vibrational temperature, and higher N2 dissociation for an N2 plasma. When a SiNx layer was deposited with a mixture of NH3/SiH4 with N2 at a low temperature of 100 °C, a stoichiometric amorphous Si3N4 layer with very low Si–H bonding could be deposited. The 300 nm thick SiNx film exhibited a low water vapor transmission rate of 1.18 × 10−4 g (m2 · d)−1, in addition to an optical transmittance of higher than 90%.

Fabrication of bottom-emitting organic light-emitting diode panels interconnected with encapsulation substrate by Au[sbnd]Au flip-chip bonding and capillary-driven filling process

We present fabrication and testing of bottom-emitting organic light-emitting diode (OLED) panels based on flip-chip assembly and non-destructive scanning. In this method, the OLED and electric circuits are fabricated on separate substrates and interconnected by low temperature assembly to create a high-performance bottom-emitting OLED including other functions such as thin-film transistor (TFT) circuits. The low temperature assembly process consists of two steps. First, an OLED substrate and encapsulation glass with circuits are bonded at 100°C via AuAu bond. Encapsulation glass is utilized for the functional substrate with circuits. Next, the bonded panel is sealed by capillary underfill within and by applying and curing seal materials to the panel edge. The fabricated OLED was non-destructively evaluated by scanning acoustic microscopy (SAM). The SAM image shows all φ500μm Au bumps were bonded to Au pads, indicating that OLED and encapsulation substrates were assembled. Electroluminescence of the OLED was demonstrated by applying voltage. Stable current-luminance characteristics were obtained for the fabricated OLED with an operating voltage of 3.25V. The results indicate that the proposed fabrication is available for bottom-emitting OLED controlled by TFT circuits. In future, the assembly process can be widely applied to other flexible organic electronic devices with roll assembly by altering OLED or circuits with other devices because this is a simple pressure method with low temperature, achieving encapsulation at same time.

Optical Enhancement in Optoelectronic Devices Using Refractive Index Grading Layers.

We enhanced the optical transmittance of a multilayer barrier film by inserting a refractive index grading layer (RIGL). The result indicates that the Fresnel reflection, induced by the difference of refractive indices between Si(x)N(y) and SiO2, is reduced by the RIGL. To eliminate the Fresnel reflection while maintaining high transmittance, the optimized design of grading structures with the RIGL was conducted using an optical simulator. With the RIGL, we achieved averaged transmittance in the visible wavelength region by 89.6%. It is found that the optimized grading structure inserting the multilayer barrier film has a higher optical transmittance (89.6%) in the visible region than that of a no grading sample (82.6%). Furthermore, luminance is enhanced by 14.5% (from 10,190 to 11,670 cd m(-2) at 30 mA cm(-2)) when the grading structure is applied to organic light-emitting diodes. Finally, the results offer new opportunities in development of multilayer barrier films, which assist industrialization of very cost-effective flexible organic electronic devices.

Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices

Molecular modeling methods are used to understand the interfacial properties between the hole-transport and active layers in organic photovoltaic (OPV) devices. The hole-transport layer (HTL) consists of a blend of poly(styrene-sulfonate) and poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), whereas the active layer (AL) consists of a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Simulation results on the HTL confirm the interpenetrating lamellar structure with alternating PSS and PEDOT domains as observed in experiments. In addition, interfacial results show high PCBM interactions with the HTL, which result in PCBM migration to the HTL surface. The observed PCBM concentration profile is discussed from the perspective of attractive interactions, and it is shown that these interactions are governed by the side chain of PCBM. Calculations also suggest that OPV device performance could be improved by, for example, increasing the number of benzene rings and backbone −CH2– groups in the PCBM side chain, which would be expected to reduce PCBM concentration at the HTL surface. The results yield important insights into molecular interactions associated with the HTL and AL interfaces that contribute to final device morphology and thus provide guidelines toward materials design approaches for optimized device performance.

Recent progress on thin-film encapsulation technologies for organic electronic devices

Among the advanced electronic devices, flexible organic electronic devices with rapid development are the most promising technologies to customers and industries. Organic thin films accommodate low-cost fabrication and can exploit diverse molecules in inexpensive plastic light emitting diodes, plastic solar cells, and even plastic lasers. These properties may ultimately enable organic materials for practical applications in industry. However, the stability of organic electronic devices still remains a big challenge, because of the difficulty in fabricating commercial products with flexibility. These organic materials can be protected using substrates and barriers such as glass and metal; however, this results in a rigid device and does not satisfy the applications demanding flexible devices. Plastic substrates and transparent flexible encapsulation barriers are other possible alternatives; however, these offer little protection to oxygen and water, thus rapidly degrading the devices. Thin-film encapsulation (TFE) technology is most effective in preventing water vapor and oxygen permeation into the flexible devices. Because of these (and other) reasons, there has been an intense interest in developing transparent barrier materials with much lower permeabilities, and their market is expected to reach over $550 million by 2025. In this study, the degradation mechanism of organic electronic devices is reviewed. To increase the stability of devices in air, several TFE technologies were applied to provide efficient barrier performance. In this review, the degradation mechanism of organic electronic devices, permeation rate measurement, traditional encapsulation technologies, and TFE technologies are presented.

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers.

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9-dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led to significantly lower contact resistance (Rc) and increased mobilities for both holes and electrons. The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced ambipolar transport properties with a hole mobility of 0.5 cm2V-1S-1 and an electron mobility of 0.5 cm2V-1S-1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated polymers are not critical to improving charge-injection properties. We found that the improvements caused by the interlayer were due to the better charge injection at the metal/organic semiconductor contact interface and the increase in the charge concentration through a detailed examination of charge transport with low-temperature measurements. Finally, we successfully demonstrated complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Improved Electrical Performance of Poly(3‐hexylthiophene) Induced by Stable Doping with Polymer Dopants

A series of brominated polystyrenes (BPSs) are readily synthesized and blended as polymer dopants with the p‐type semiconducting polymer, poly(3‐hexylthiophene) (P3HT), to improve theirelectrical performance. The obtained P3HT/BPS blend films exhibit increased carrier concentration resulting from doping of P3HT by BPS, which leads to excellent electrical performance, including enhanced hole mobility and conductivity, and the conductivity or mobility increases with the bromination degree of BPS. Compared with conventional small molecular dopants, the polymer dopant, BPS, shows not only excellent doping stability, but also good solution processibility, which makes it a promising materials for fabrication of low cost, flexible, transparent, and high performance solution processable organic electronic devices. image

Thermal inkjet printing of copper tetrasulfonated phthalocyanine (CuTsPc) as a semiconducting layer on flexible MIS capacitors

Thermal inkjet-printing of copper tetrasulfonated phthalocyanine (CuTsPc) films containing polyvinyl alcohol (PVA) plasticizer were used to fabricate flexible metal-insulator-semiconductor (MIS) capacitors using anodic aluminum oxide (Al2O3) as the insulating layer. The Al2O3 layer and printed CuTsPc+PVA films were characterized individually and in a MIS structure by electrical impedance spectroscopy. Capacitance-frequency and capacitance-voltage measurements revealed distinct contributions from CuTsPc and PVA on the device electrical response. From these measurements the semiconductor charge carrier mobility, ∝, of 1.22x10- 4 cm² V- 1 s- 1 and the doping density, N A, of 3.1x10(16) cm- 3 were calculated. It was observed a low-frequency electrical response, which was attributed to diffusional processes within the printed film arising from Na+ ions dissociated from the CuTsPc molecule. A simple equivalent circuit model was proposed to take this effect into account. Finally, the results shown here demonstrate the possibility to produce flexible organic electronic devices exploiting the semiconducting properties of water-soluble CuTsPc molecules using low-cost commercial printers.

Permeation barrier performance of Hot Wire-CVD grown silicon-nitride films treated by argon plasma

In this work SiNx thin films have been deposited by Hot-Wire Chemical Vapor Deposition (HW-CVD) technique to be used as encapsulation barriers for flexible organic electronic devices fabricated on polyethylene terephthalate (PET) substrates. First results of SiNx multilayers stacked and stacks of SiNx single-layers (50nm each) separated by an Ar-plasma surface treatment are reported. The encapsulation barrier properties of these different multilayers are assessed using the electrical calcium degradation test by monitoring changes in the electrical conductance of encapsulated Ca sensors with time. The water vapor transmission rate is found to be slightly minimized (7×10−3g/m2day) for stacked SiNx single-layers exposed to argon plasma treatment during a short time (2min) as compared to that for stacked SiNx single-layers without Ar plasma treatment.

Stretchable organic memory: toward learnable and digitized stretchable electronic applications

A stretchable organic digital information storage device has been developed, which potentially advances the development of future smart and digital stretchable electronic systems. The stretchable organic memory with a buckled structure was configured by a mechanically flexible and elastic graphene bottom electrode and polymer compound. The current–voltage curve of the wrinkled memory device demonstrated electrical bistability with typical write-once-read-many times memory features and a high ON/OFF current ratio (∼105). Even under repetitive stretching, the stretchable organic memory exhibited excellent electrical switching functions and memory effects. We believe the first proof-of-concept presentation of the stretchable organic nonvolatile memory may accelerate the development of information storage device in various stretchable electronic applications, such as stretchable display, wearable computer and artificial skin. Stretchable and foldable electronic devices are very attractive, not only for their practicality but also for their potential in as-yet-undeveloped applications, such as artificial electronic skin. Now, Yang-Fang Chen from the National Taiwan University and co-workers have constructed a stretchable organic memory device. Although a variety of flexible organic electronic devices have already been built, which include transistors or solar cells, building stretchable memory devices has remained a challenge. This is because they typically contain a brittle metal electrode, and their fabrication also involves processes — such as spin coating — that are incompatible with flexible substrates. The researchers circumvented these issues by buckling both a graphene, rather than metallic, electrode and the active memory layers over a pre-stretched poly(dimethylsiloxane) elastomer. When the pre-strain was released, the materials adopted a wrinkled structure that endowed them with flexibility. The resulting device showed good electrical switching behavior and memory effects after several stretch/release cycles. A stretchable wrinkled organic memory has been successfully demonstrated. The stretchable organic memory with a graphene bottom electrode possesses rippled structures. The stretchable organic memory exhibits excellent electrical switching behaviors and memory effects even under repetitive stretching. It is believed that this stretchable organic memory may be beneficial for digital information storage in future stretchable electronic systems.

Progress of flexible organic non-volatile memory field-effect transistors

Flexible organic non-volatile memory field-effect transistors (ONVMFETs) are promising candidates in the field of flexible organic electronic devices, which can be used in flexible radio frequency tags, memories, integrated circuits and large-area displays, because of their remarkable advantages such as flexibility, lightweight, low cost and large-area organic electronics. On the basis of the introduction of the development of flexible ONVMFETs in terms of substrates, structures and characteristics, the classification of flexible ONVMFETs is summarized. Meanwhile, we discuss the effects of mechanical stress and temperature on the performance of flexible ONVMFET. Finally, some prospects as well as the challenges are pointed out.

Low temperature, solution-processed alumina for organic solar cells

In this work, we have reported for the first time a facile route for developing solution-processed Al2O3 film at a greatly reduced processing temperature and studied its applications in producing inverted polymer solar cells (PSCs). These PSCs using Al2O3 thin film as the electron-extraction layer demonstrated improved diode characteristics and achieved a 20% higher power conversion efficiency than devices using the conventional ZnO buffer layer. Furthermore, the low temperature processing of the Al2O3 film makes it compatible with fabrication of flexible organic electronic devices based on plastic substrates.

29.1: Ultra‐High Barriers for Encapsulation of Flexible Displays and Lighting Devices

AbstractA roll‐to‐roll manufactured ultra‐high permeation barrier film based on a multilayer stack of sputtered oxide layers and ORMOCER® interlayers is presented. The reproducible large‐area water vapor transmission rate is less than 8·10−5 g/(m2d) at ambient condition. The film has been evaluated as substrate for and encapsulation of flexible organic electronic devices.

3-D nanorod arrays of metal–organic KTCNQ semiconductor on textiles for flexible organic electronics

We present a facile and low temperature approach for the fabrication of flexible organic electronic devices by growing high aspect ratio nanorod arrays of potassium 7,7,8,8-tetracyanoquinodimethane (KTCNQ), a crystalline organic semiconductor with charge transfer capabilities, on cotton threads interwoven within the three-dimensional (3-D) matrix of a cotton textile. We demonstrate the capability of this material in developing opto-electronic switches and gas sensors. The ability to grow KTCNQ nanorod arrays in a radial symmetry directly on textiles as a versatile 3-D microtemplate can be extended to the synthesis of a variety of metal–organic charge transfer complexes onto different flexible substrates that can find applications in electronics, catalysis and sensing.

Enhanced storage/operation stability of small molecule organic photovoltaics using graphene oxide interfacial layer

Graphene and graphene oxide (GO) have been applied in flexible organic electronic devices with enhanced efficiency of polymeric photovoltaic (OPV) devices. In this work, we demonstrate that storage/operation stability of OPV can be substantially enhanced by spin-coating a GO buffer layer on ITO without any further treatment. With a 2nm GO buffer layer, the power conversion efficiency (PCE) of a standard copper phthalocyanine (CuPc)/fullerene (C60) based OPV device shows about 30% enhancement from 1.5% to 1.9%. More importantly, while the PCE of the standard device drop to 1/1000 of its original value after 60-days of operation-storage cycles; those of GO-buffered device maintained 84% of initial PCE even after 132-days. Atomic force microscopy studies show that CuPc forms larger crystallites on the GO-buffered ITO substrate leading to better optical absorption and thus photon utilization. Stability enhancement is attributed to the diffusion barrier of the GO layer which slow down diffusion of oxygen species from ITO to the active layers.

Polypyrrole top-contact electrodes patterned by inkjet printing assisted vapor deposition polymerization in flexible organic thin-film transistors

Highly conductive polymer, polypyrrole (PPy) was successfully patterned as source and drain (S/D) electrodes for flexible pentacene thin film transistors in top-contact structure by combining inkjet printing and vapor deposition polymerization. Facile inkjet printing of initiator and subsequent exposure of pyrrole monomers resulted in selective absorption and polymerization of pyrrole monomers on the patterned initiator region. Pentacene transistors based on printed PPy electrodes exhibited higher electrical characteristics than that of the devices with thermally evaporated Au electrodes. Improved performance of the devices based on PPy electrodes could be attributed to the reduction of contact resistance at the interface between polymer and organic semiconductor. For the replacement of metal electrodes, vapor deposition polymerization assisted inkjet printing technique can provide a versatile method to utilize highly conductive polymer as a functional electrode of flexible organic electronic devices.

Electronic Properties of Transparent Conductive Films of PEDOT:PSS on Stretchable Substrates

Despite the ubiquity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a transparent conducting electrode in flexible organic electronic devices, its potential as a stretchable conductor has not been fully explored. This paper describes the electronic and morphological characteristics of PEDOT:PSS on stretchable poly(dimethylsiloxane) (PDMS) substrates. The evolution of resistance with strain depends dramatically on the methods used to coat the hydrophobic surface of PDMS with PEDOT:PSS, which is cast from an aqueous suspension. Treatment of the PDMS with an oxygen plasma produces a brittle skin that causes the PEDOT:PSS film to fracture and an increase in resistivity by four orders of magnitude at only 10% strain. In contrast, a mild treatment of the PDMS surface with ultraviolet/ozone (UV/O3) and the addition of 1% Zonyl fluorosurfactant to the PEDOT:PSS solution produces a mechanically resilient film whose resistance increases by a factor of only two at 50% strain and retains significant conductivity up to 188% strain. Examination of the strained surfaces of these resilient PEDOT:PSS films suggests alignment of the grains in the direction of strain. Wave-like buckles that form after the first stretch >10% render the film reversibly stretchable. Significant cracking (∼2 cracks mm–1) occurs at 30% uniaxial strain, beyond which the films are not reversibly stretchable. Cyclic loading (up to 1000 stretches) produces an increase in resistivity whose net increase in resistance increases with the value of the peak strain. As an application, these stretchable, conductive films are used as electrodes in transparent, capacitive pressure sensors for mechanically compliant optoelectronic devices.

Organic light-emitting diode with indium-free metallic bilayer as transparent anode

Bilayer Cu–Ni transparent electrodes were room-temperature grown on glass by sputtering technique and used as anodes for polymer light-emitting diodes (PLEDs). The bilayer electrode structure allows combining the low sheet resistance and high transparency of Cu with the excellent stability and high work function of Ni. We demonstrate that Cu–Ni bilayer based PLED devices exhibit comparable efficiency and lifetime decay behavior to indium tin oxide (ITO) based device, with potentially significant advantages, such as easy processing, low cost and mechanical flexibility. In addition, the ductile nature of the metal electrode and its low thickness (<10 nm) make the proposed structure particularly suitable for flexible organic electronic and optoelectronic devices.