An inverted,organic WORM device based on PEDOT:PSS with very low turn-on voltage

An organic Write-Once-Read-Many (WORM) device based on poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as the active layer was fabricated with an inverted architecture. Insertion of an ultrathin layer of poly(methylmethacrylate) (PMMA) between the bottom electrode and the PEDOT:PSS resulted in a systematic and substantial decrease in turn-on voltage, from 7.0 V to less than 1.0 V. An optimal thickness of the PMMA layer was found to yield the lowest consistent turn-on voltage of ∼0.8 V, with 0.5 V being the lowest value of all fabricated devices. The switching mechanism was attributed to filamentary doping of the PEDOT:PSS. Insertion of the PMMA acted to protect the underlying ZnO from being etched by the acidic PEDOT:PSS as well as to improve its wetting properties. Devices were demonstrated on both ITO and aluminum bottom electrodes, with aluminum yielding the highest ON/OFF ratios in the study. Owing to their inverted architecture, the devices demonstrated good stability, and the retention time of the ON-state was determined to be greater than twenty months while stored in air for devices with ITO bottom electrodes. In addition to deposition via spin-coating, blade-coating was demonstrated as a viable processing technique for applications requiring rapid or large-area manufacturing.     

Flexible rewritable organic memory devices using nitrogen-doped CNTs/PEDOT:PSS composites

Nonvolatile rewritable organic memory devices based on poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and nitrogen doped multi-walled carbon nanotube (NCNT) nanocomposites were fabricated on glass and PET substrates.Organic memory devices with bistable resistive switching were obtained using very low NCTN concentration (∼0.002 wt%) in the polymeric matrix. The memory devices exhibited a good ON/OFF ratio of approximately three orders of magnitude, a good retention time of 10⁴ s under operating voltages ≤ |4V| and a few hundredths of write-read-erase-read cycles. The bistable resistive switching is mainly attributed to the creation of oxygen vacancies. These defects are introduced into the thin native Al oxide (AlOx) layer on the bottom electrode during the first voltage sweep. The well-dispersed NCNTs immersed in PEDOT:PSS play a key role as conductive channels for the electronic transport, hindering the electron trapping at the AlOx-polymer interface and inducing a soft dielectric breakdown of the AlOx layer. These PEDOT:PSS + NCNTs memory devices are to easy to apply in flexible low-cost technology and provide the possibility of large-scale integration.     

Modeling thin film formation by Ultrasonic Spray method: A case of PEDOT:PSS thin films

Recent improvements in electronic and optoelectronic devices based on solution processable polymers have motivated development of scalable processing techniques like Ultrasonic Spray technique. Including potential for roll to roll fabrication, it has many other strengths. However, with spray coating it can be difficult to prepare films with a smooth surface. Here, we present model for Ultrasonic Spray deposition of thin films, which establish a clear correlation between process parameters and the film formation process, which ultimately decide the structural features of the thin films. Based on the time to cover the spray deposition area by the sprayed droplets and the time for droplet evaporation, a balance parameter has been defined. It provides a mean to determine suitable process parameters for uniform film formation by Ultrasonic Spray method. The model is further modified for the region of higher solution flow rates, where non-uniformity in droplet distribution is introduced. The predictions based on the model have been experimentally verified with thin films of poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid (PEDOT:PSS). The method presented here can be used to predict proper deposition parameters for smooth film deposition by Ultrasonic Spray technique. Finally, the effect of film morphology on the sheet resistance of thin films of PEDOT:PSS is also presented.     

High performance,foldable, organic memories based on ultra-low voltage,thin film transistors

We report on the fabrication of highly flexible OTFT-based memory elements with excellent mechanical stability and high retention time. The devices have been fabricated using a combination of two ultrathin AlOx and Parylene C as dielectric, and TIPS-Pentacene as the semiconductor, obtaining high performing low voltage transistors with mobility up to 0.4 cm²/V s, and I_on/I_off ratio of 10⁵. Charge trapping in the Parylene C electret layer is the mechanism that allows employing these devices as non volatile memory elements, with retention time as high as 4 × 10⁵ s. The electromechanical characterization demonstrated that such memory elements can be cyclically bent around a cylinder with a radius of 150 μm without losing the stored data.     

Effect of high voltage electric field on structure and property of PEDOT:PSS film

Low electrical conductivity of PEDOT:PSS film is to some extent a limit for its wide application. To solve this problem, the high voltage electric field was used to improve the film’s electrical conductivity and its effects on the film’s structure and properties were investigated. The PEDOT:PSS film was prepared on quartz substrate with spin coating. Visible light transmittance of the prepared film was tested with UV–Visible spectroscopy and chemical structure was measured with Fourier transform Raman spectroscopy (FIRM). The surface morphology was characterized with AFM, and electrical conductivity was measured with Hall effects measurement. The results showed that with the increase of the electric field, the electrical conductivity of PEDOT:PSS film was boosted rapidly at first, and then improved slowly when the electric field was above 200 kV/m. The film’s electrical conductivity improved more than 17 times in total from 0.51 × 10^–3 up to 8.92 × 10^–3 S/m. However, the film’s visible light transmittance decreased only a little with the increase of the electric field, not more than 3%. In addition, despite little change in the chemical structure of the PEDOT:PSS film, its surface roughness increased significantly with the increase of the electric field intensity.     

Ultraviolet photodetectors based on low temperature processed ZnO/PEDOT:PSS Schottky barrier diodes

In this work, vertical Schottky barrier diodes (SBDs) were fabricated using a thin film of ZnO (50 nm) and PEDOT:PSS deposited by RF Sputtering and micro-drop casting, respectively. ITO and Au were used as ohmic contacts to ZnO and PEDOT:PSS films, respectively. The final structure consisted on Glass/ITO/ZnO/PEDOT:PSS/Au. The SBDs performance was characterized under dark and four different wavelengths conditions. From current–voltage characteristics, under dark and ambient conditions, a diode ideality factor of 1.4; a saturation current density of 1×10⁻⁹ A/cm²; a Schottky barrier height of 0.9 eV and a rectification ratio of 5 orders of magnitude at ±1 V were obtained. A carrier density of 5×10¹⁷ cm⁻³ for the ZnO film was estimated from capacitance–voltage measurements. For their characterization as photodiodes, the SBDs were illuminated with an ultra-bright UV (~380 nm) LED. A maximum UV responsivity of 0.013 A/W was obtained. The transient response of the SBDs was also analyzed with the UV LED connected to a pulsed signal of 0.5 Hz, demonstrating rise and fall times in the order of 200 ms. With a low temperature processing (<80 °C), visible-blind and UV photon-detection characteristics, the fabricated SBDs are candidates for flexible optoelectronics devices such as optical receivers for digital signal processing and measurement of light intensity.     

High-performance hole transport layer based on WS2 doped PEDOT:PSS for organic solar cells

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Electrically bistable memory device based on spin-coated molecular complex thin film

We present an organic electrically bistable memory device based on the molecular complex film composed of tetracyanoquinodimethane and a soluble methanofullerene derivative [6,6]-phenyl C61-butyric acid methyl ester. The device has an Al/molecules/Al sandwich-like structure. The molecular layer was formed by spin-coating technique instead of expensive vacuum deposition method. The current-voltage characteristics show that the device switches from the initial "low" conduction state to "high" conduction state upon application of external electric field at room temperature. The on/off ratio is up to 10/sup 6/. Either state has been found to remain stable for more than five months, even after the external electric field is removed. The device presented is of potential use for low-cost write-once-read-many-times memory applications.     

Air-stable,non-volatile resistive memory based on hybrid organic/inorganic nanocomposites

A non-volatile memory element based on organic/inorganic nanocomposites is presented. The device can be operated in ambient conditions, showing high retention time and long-term life time. The formation/rupture of metallic filaments in the organic matrix is investigated by HR-XPS and ToF-SIMS analysis, and is demonstrated to be the driving mechanism for the resistive switching.     

Rectified Schottky diodes based on PEDOT:PSS/InGaZnO junctions

We report the successful use of the high-work-function, high-conductivity transparent conducting polymer PEDOT:PSS as the Schottky contact to form the Schottky junction (and thus Schottky diode) with the n-type semiconductor a-IGZO. The Schottky didoes exhibited a low apparent turn-on voltage, a high rectification ratio of >10⁵ at ±1 V, and a decent ideality factor of ∼1.5–1.6. Detailed junction properties were systematically analyzed from J-V and C-V characteristics of the diodes. We also demonstrated the applications of PEDOT:PSS/a-IGZO Schottky junctions to various types of Schottky diodes, including the flexible, the transparent, and the flexible transparent PEDOT:PSS/a-IGZO Schottky diodes, by using different substrates and different counter electrodes.     

Solution processable PEDOT:PSS based hybrid electrodes for organic field effect transistors

We report high performance solution processed conductive inks used as contact electrodes for printed organic field effect transistors (OFETs). Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electrodes show highly improved very low sheet resistance of 65.8 ± 6.5 Ω/square (Ω/□) by addition of dimethyl sulfoxide (DMSO) and post treatment with methanol (MeOH) solvent. Sheet resistance was further improved to 33.8 ± 8.6 Ω/□ by blending silver nanowire (AgNW) with DMSO doped PEDOT:PSS. Printed OFETs with state of the art diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT) semiconducting polymer were demonstrated with various solution processable conductive inks, including bare, MeOH treated PEDOT:PSS, single wall carbon nanotubes, and hybrid PEDOT:PSS-AgNW, as the source and drain (S/D) electrode by spray printing using a metal shadow mask. The highest field effect mobility, 0.49 ± 0.03 cm² V⁻¹ s⁻¹ for DPPT-TT OFETs, was obtained using blended AgNW with DMSO doped PEDOT:PSS S/D electrode.     

Reverse recovery transient characteristic of PEDOT:PSS/n-Si hybrid organic-inorganic heterojunction

We study the junction behavior of poly (3,4-ethylenedioxythiophene):polystyrenesulphonate/n-Si hybrid organic/inorganic heterojunction by reverse recovery transient (RRT) characterization. RRT response for PEDOT:PSS/n-Si hybrid junction is reported for various n-Si doping concentration and forward bias current injection level. The presence of settling time of 8.3–23.5 μs in the RRT response in contradiction to Schottky junction model commonly assumed for PEDOT:PSS/n-Si hybrid structure. The decrease in the minority carrier lifetime from 126.8 μs to 39.5 μs with increased n-Si doping concentration, suggests that minority carriers are stored at n-Si side of the junction, which is consistent with a p⁺-n junction model for the hybrid structure. The minority carrier lifetime is found to depend on forward bias current injection level, attributed to trap-saturation effect of the recombination-centers at the PEDOT:PSS/n-Si junction. The DC-IV characteristics of the PEDOT:PSS/n-Si hybrid junction are also consistent with the notion of diffusion and trap assisted recombination dominated dark current. The diffusion dominated transport of PEDOT:PSS/n-Si leads to an ideal p⁺-n junction behavior that leverages on the good transport properties of Si. Our findings are important in the modeling and optimization of the characteristics of electronic devices based on the organic/Si hybrid junction.     

Conductive PEDOT:PSS on surface-functionalized chitosan biopolymers for stretchable skin-like electronics

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising alternative transparent electrode to replace conventional indium tix oxide (ITO) for flexible and stretchable electronics. For their applications in optoelectronic devices, realizing both high conductivity and transmittance for the films is of great necessity as a suitable high performance transparent electrode. Here, we demonstrate simultaneously enhanced electrical and optical properties of PEDOT:PSS films prepared on chitosan bio-substrates by using an organic surface modifier, 11-aminoundecanoic acid (11-AA). The sheet resistance of PEDOT:PSS films decreases from 1120.8 to 292.8 Ω/sq with an increase in a transmittance from 75.9 to 80.4% by 11-AA treatment on the chitosan films. The functional groups of 11-AA effectively enhance the adhesion property at the interface between the chitosan substrate and PEDOT:PSS by forming strong interfacial bondings and decrease insulating PSS from PEDOT:PSS films. The wearable heater devices and on-skin sensors based on the 11-AA-treated PEDOT:PSS on the chitosan bio-substrates are successfully fabricated, showing the excellent thermal and sensing performances. The 11-AA surface-modification approach for highly conductive PEDOT:PSS on chitosan bio-substrates presents a great potential for applications toward transparent, flexible and stretchable electronics.     

Alkyl-monosubstituted thiophene/phenylene co-oligomers: Synthesis,thin film preparation,and transistor device characteristics

A novel series of alkyl-monosubstituted thiophene/phenylene co-oligomers (TPCOs) has been synthesized and characterized. The introduction of alkyl chains to TPCOs improved the solubility in organic solvents. Thin films of these compounds were prepared by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by molecular bilayers. In particular, the skim method produced a large-sized free-standing thin film. We investigated the carrier transport of the thin films on the field-effect transistor (FET) configurations. These FET devices exhibited typical p-channel characteristics with clear saturation region. The introduction of alkyl monosubstituents is responsible for enhancement in the carrier mobility. The device characteristics are contrasted with those of alkyl-disubstituted compounds.     

Methanol treatment on low-conductive PEDOT:PSS to enhance the PLED's performance

In order to achieve high device efficiency in blue-emitting polymer light-emitting diodes (PLED) and white-emitting PLED, low-conductive PEDOT:PSS Clevios™ P CH 8000 (8000) has been used to replace widely adopted high-conductive PEDOT:PSS Clevios™ P Al 4083 (4083). In a blue PLED with poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) as the emission layer, though the 8000 device has a higher efficiency and a lower turn-on voltage than the 4083 device, the 8000 device exhibits low peak luminance, sharp efficiency roll-off, and permanent degradation of the emission material. By analyzing the hole-only device and the X-ray photoelectron spectroscopy spectra, it's revealed that the insulating PSS layer on 8000 surface is responsible for the inefficient hole injection. A simple methanol treatment on the 8000 surface effectively removes the redundant PSS. Without the insulating PSS layer, the hole injection becomes efficient which extends the recombination zone. As the result, the methanol-treated 8000 device not only retains low turn-on voltage and the high device efficiency of the untreated 8000 device, but also achieves the peak luminance, mild efficiency roll-off, and device stability of the 4083 device.     

Improved efficiency of indium-tin-oxide-free organic light-emitting devices using PEDOT:PSS/graphene oxide composite anode

We have demonstrated an indium-tin-oxide free organic light-emitting device (OLED) with improved efficiency by doping poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) with graphene oxide (GO) as a composite anode. In comparison with a pure PEDOT:PSS anode, 55% enhancement in efficiency has been obtained for the OLEDs based on the PEDOT:PSS/GO composite anode at an optimal condition. The PEDOT:PSS/GO composite anode shows a lower hole-injection barrier, which contributes to the improved device efficiency. Moreover, both high transmittance and good surface morphology similar to that of the pure PEDOT:PSS film also contribute to the enhanced efficiency. It is obvious that composite anode will generally be applicable in organic optoelectronic devices which require smooth and transparent anode.     

Simultaneous measurement of electrical characteristics and microstructure of crystallised PEDOT:PSS based OECTs under strain

Here is reported the evolution of the transconductance and resistance, along with microstructure, of organic electrochemical transistors (OECTs) under progressively greater strain. A decrease in transconductance and increase in resistance over several orders of magnitude are reported for strains up to 75%. These changes in electrical characteristics are correlated with changes in the microstructure of the PEDOT:PSS characterised by a positive shift, and reduction in amplitude, of the characteristic (100) peak of the grazing incidence wide-angle X-ray scattering (GIWAXS) profile, recorded in tandem with the electrical characteristics during stretching. The (100) peak is associated with the lamella stacking of the PEDOT and PSS chains, and it is shown that this lamella spacing is reduced by up to around 3.5 Å on application of strain up to 75%.     

具有低电压、低“迟滞效应”的C60薄膜晶体管

以C60为激活层,同时以聚合物/高K氧化物双绝缘层结构研制了N型有机场效应晶体管。结果表明,采用这种双层结构的绝缘层能够很好的将Ta2O5和PMMA的优点结合在一起,即既利用了Ta2O5的高介电常数又利用了PMMA与半导体层良好的界面接触特性。与采用单一Ta2O5或这PMMA绝缘层的器件相比,这种具有双层结构的器件性能大幅提升。最终研制了能够在10V低电压下正常工作的C60晶体管,其场效应迁移率、阈值电压和开关电流比分别为0.26 cm2/Vs, 3.2V和8.31×104。同时,利用修饰绝缘层PMMA的疏水性大大降低了这种具有双层结构的N型有机晶体管的“迟滞效应”,从而让器件工作时有较好的稳定性。     

Doped PEDOT:PSS electrodes,patterned through wettability control,and their effects on the electrical properties of polymer thin film transistors

The water−based conductive polymer, poly(3,4−ethylenedioxythiophene), doped with poly(styrene sulfonate) (PEDOT:PSS), has received much attention for its utility as a printable electrode due to its transparency, thermal stability, and processability; however, the electrical properties of devices prepared with printed PEDOT:PSS electrodes are generally inferior to those of devices fabricated with evaporated metal electrodes or their inorganic counterparts. Here, we show that the electrical performances of polymer thin film transistors could be improved by doping the PEDOT:PSS chains used as source and drain electrodes. The addition of HAuCl₄ to the PEDOT:PSS solution increased the electrical conductivity and work function of the electrodes. The PEDOT:PSS film doped with 10 mM HAuCl₄ provided a field effect mobility exceeding 0.01 cm²V⁻¹s⁻¹, a factor of 7 greater than the value obtained from the device prepared with pristine PEDOT electrodes.     

Design of highly stable and solution-processable electrochromic devices based on PEDOT:PSS

This study aimed to improve the repeatability of electrochromic devices(ECD) based on Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS); therefore, ferrocene was introduced as an anodic species. When 0.05 wt% ferrocene as compared to that of the electrolyte was contained in the electrolyte layer, the bleaching time significantly reduced from 110 s to 25 s without changing ΔT; consequently, repeatability markedly improved. However, ferrocenium cations, generated when ferrocene undergoes a reversible redox reaction, have high reactivity with oxygen, and hence, the stability of ECD is lowered over time. To overcome this problem, l-ascorbic acid, commonly known as vitamin C, was introduced as an antioxidant in the electrolyte layer. The repeatability of the ECD and the storage stability of the electrolyte solution were improved without side effects at an l-ascorbic acid ratio of 0.025 wt% as compared to that of the electrolyte. In addition, using cyclic voltammetry, it was confirmed that l-ascorbic acid did not affect the electrochemical properties of the ECD, and played only the role of an antioxidant for ferrocene. Furthermore, regarding encapsulated ECD, high repeatability could be maintained by preventing solvent evaporation and oxygen penetration into the electrolyte layer. When ECD of size 4 cm × 5 cm was glass encapsulated, there was no change in ΔT even after 5000 cycles for 139 h. We observed day-to-day changes in the response time and ΔT for 30 d; it maintained almost constant values from the beginning. Repeatability tests of 15 cm × 15 cm size large-area ECD for applications, such as smart windows, were conducted.