Organic Solid-State Devices
Bias stress effect in polyelectrolyte-gated organic field-effect transistors
One of the main factors contributing to bias stress instability in organic transistors is charge trapping of mobile carriers near the gate insulator-semiconductor interface into localized electronic states. In this paper, we report a study of the bias stress behavior in low-voltage (p-type) polyelectrolyte-gated organic field effect transistors (EGOFETs) at various temperatures. Stressing and recovery are very fast phenomena, six orders of magntiude faster than similar dielectric based OFETs. The mechanism proposed involves an electron transfer reaction between water and the charged semiconductor channel that promote the creation of extra protons diffusing into the polyelectrolyte.
Applied Physics Letters; Vol 102, 113306 (2013)
Tuning the threshold voltage of an electrolyte-gated organic field-effect transistor
Low-voltage organic field-effect transistors (OFETs) promise for low power consumption logic circuits. To enhance the efficiency of the logic circuits, the control of the threshold voltage of the transistors are based on is crucial. We report the systematic control of the threshold voltage of electrolyte-gated OFETs by using various gate metals. The influence of the work function of the metal is investigated in metal-electrolyte-organic semiconductor diodes and electrolyte-gated OFETs. A good correlation is found between the flat-band potential and the threshold voltage. The possibility to tune the threshold voltage over half the potential range applied and to obtain depletion-like (positive threshold voltage) and enhancement (negative threshold voltage) transistors is of great interest when integrating these transistors in logic circuits. The combination of a depletion-like and enhancement transistor leads to a clear improvement of the noise margins in depleted-load unipolar inverters.
Tuning the threshold voltage of an electrolyte-gated organic field-effect transistor, Proceedings of the National Academy of Sciences (PNAS), vol 109, pp 8394–8399 (2012)
Tuning the thermoelectric properties of conducting polymers in an electrochemical transistor
While organic field-effect transistor allow investigation of the interfacial charge transport at the semicondutor-dielectric interface, an electrochemical transistor truly modifies the oxidation level and conductivity through the bulk of an organic semiconductor. Here, the thermoelectric properties of the bulk of the conducting polymer polu(3,4 ethylenedioxythiophene) poly(styrene sulfonate) are controlled electrically by varying the gate voltage. In light of the growing interes in conducting polymers as thermoelectric generators, this method provieds an easy tool to study the physics behind the thermoelectric properties and to optimize polymer thermoelectrics.
Tuning the thermoelectric properties of conducting polymers in an electrochemical transistor, J. Am. Chemi. Soc, 134; 16456-16459 (2012)
Remanent polarization in a cryptand-polyanion bilayer implemented in an organic field effect transistor
We investigate the possibility to maintain an electric polarization in an organic bilayer via ion trapping, i.e. without any external bias. In the cryptand-polyanion bilayer, ions of specific size can be strongly coordinated with organic macrocyclic molecules. Cations move from the polyanion layer to the cryptand layer upon applying a bias and are trapped in this layer. As a result, the voltage dependence of the polarization displays a hysteresis. The bilayer is then advantageously used as an electronic insulating layer in an organic field effect transistor (OFET). The ions trapping and de-trapping can be followed by the amplitude of the threshold voltage (Vth) shift as well as its temporal evolution.
Remanent polarization in a cryptand-polyanion bilayer implemented in an organic field effect transistor, Appl. Phys. Lett. 100, 023305 (2012)
Spatial Control of p–n Junction in an Organic Light-Emitting Electrochemical Transistor
We developed a novel device organic light emitting electrochemical transistor, where p-n junction is spatially controlled by a PEDOT:PSS gate terminal. Specifically, the emission zone can be repeatedly moved back and forth within an interelectrode gap of 500 µm by application of a 4 V gate bias.
Spatial Control of p–n Junction in an Organic Light-Emitting Electrochemical Transistor, J. Am. Chemi. Soc., 134, 901-904 (2012)
Optimization of the Conducting Polymer Poly(3,4-Ethylenedioxythiophene) for Thermoelectricity
Large area thermogenerators capable of transforming thermal energy into electricity from waste heat (T<200C) and natural heat sources are highly desirable. These potentially wide scale applications imply the utilization of low cost, abundant, easily processible and reasonably efficient thermoelectric materials, as well as the possibility to pattern large area on flexible substrates. This study reveals that the thermoelectric power factor of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) can be significantly enhanced up to 324 μWK-2m-1 by controlling the material’s doping concentration. This optimization together with the intrisic low thermal conductivity of the polymer (0.32 Wm-1K-1) yields ZT=0.3, the highest value of dimensionless figure of merit of all previously studied air-stable organic materials at room temperature.
Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene), Nature Materials 10, 429-433 (2011).
Thermoelectric Properties of Conducting Polymers: The case of Poly(3-hexyl thiophene)
Positively charge carriers and PF6- counter-ions improve dramatically the electrical conductivity and deteriorate Seebeck coefficient of the conducting polymer P3HT from its pristine state. Increasingly heavy doping progressively mitigates the localizing effects of the counter-ions and decreases the disorder. The importance of the disorder in conducting polymer explains both the decreasing activation energy of the conductivity as well as the weak temperature dependence of the Seebeck coefficient at high doping level. As a result the thermoelectric power factor reaches a broad maximum at between 20% and 30% doping.
Thermoelectric Properties of Conducting Polymers: The case of Poly(3-hexyl thiophene), Physical Review B, vol 82, pp. 115454 (2010)
Vertical polyelectrolyte-gated organic field-effect transistors
In order to realize high current, low operation voltage in organic transistors, we have developed a low resolution photolithography process to fabricate vertical electrolyte-gated organic field-effect transistor with a short channel. A polyelectrolyte acid is used as gate insulator to promote large capacitance in the transistor, to ensure the low voltage operation (< 1 V) and suppress the short channel effect. Importantly, neither ultrathin organic layers, nor sub-micrometer lateral patterning is required during the fabrication, which promises a way to achieve fully reel-to-reel manufacturable organic short-channel transistors.
Vertical polyelectrolyte-gated organic field-effect transistors, Applied Physics Letters, vol 97, pp. 103303 (2010)
A water-gate organic field-effect transistor
Organic transistors gated via pure water are demonstrated. The transistors operate entirely in field-effect mode and at voltages lower that 1 V. The results open up for using the transistors in sensor applications in aqueous media.
A water-gate organic field-effect transistor, Advanced Materials, vol 22, pp. 2565 (2010)
Proton motion in a polyelectrolyte: a probe for wireless humidity sensors
In this work a novel concept for wireless sensing of humidity is presented, in which the humidity sensitive ionic motion in a polyelectrolyte membrane is directly translated into a shift of the resonance frequency of a resonance circuit. The sensor device itself can be integrated into a low-cost passive electronic sensor label which can be manufactured entirely using common printing techniques thanks to its simplicity.
Proton motion in a polyelectrolyte: a probe for wireless humidity sensors, Sensors and Actuators B, vol 143, pp. 482-486 (2010)
Low-Voltage Ring Oscillators Based on Polyelectrolyte-Gated Polymer Thin-Film Transistors
Organic integrated circuits based on top-gate p-channel transistors gated by polyanionic electrolytes are demonstrated. The high capacitance of the polyelectrolyte film permit supply voltages below 1.5 V. Seven-stage ring oscillators are driven with supply voltages down to 0.9 V and show signal propagation delays down to 300 µs per stage.
Low-Voltage Ring Oscillators Based on Polyelectrolyte-Gated Polymer Thin-Film Transistors, Advanced Materials, vol 22, pp. 72 (2010)
Insulator Polarization Mechanisms in Polyelectrolyte-Gated Organic Field-Effect Transistors
Here, the polarization response of a thin polyelectrolyte film, used as the gate-insulating material in a transistor, is controlled by varying the relative humidity. The formation of the transistor channel follows the polarization of the polyelectrolyte such that the drain transient current characteristics versus the time are rationalized by three different polarization mechanisms: at high frequencies dipolar relaxation, at intermediate frequencies ionic relaxation (migration), and at low frequencies the electric double-layer formation at the polyelectrolyte interfaces. The electric double-layers of polyelectrolyte capacitors are formed in approximately 1 μs at humid conditions with an effective capacitance per area of 10 μF cm–2 at 1 MHz, suggesting that this class of transistors might operate at up to 1 MHz at 1 V.
Insulator Polarization Mechanisms in Polyelectrolyte-Gated Organic Field-Effect Transistors, Advanced Functional Materials, vol 19, pp. 3334-3341 (2009)
Fiber-Embedded Electrolyte-Gated Field-Effect Transistors for e-Textiles
Electrolyte-gated organic transistors are created at the junction between textile microfibers. The fiber transistors, and circuits thereof, operate below 1 V. Current transients and impedance spectroscopy measurements reveal that the transistors operate in two modes: field-effect and electrochemical doping.
Fiber-Embedded Electrolyte-Gated Field-Effect Transistors for e-Textiles, Advanced Materials, vol 21, pp. 573 (2009)
Molecular vs. filamentary switching in memory devices
Impedance switching has been observed in many organic devices, but the mechanism is still a matter of debate. Reliable switch devices consisting of an organic layer of Rose Bengal derivatives sandwiched in between indium tin oxide and aluminum electrodes were fabricated. Modifying the chemical nature of the organic layers and visualizing the temperature distribution in the organic memory rule out several mechanisms. It is shown that the memory effect originates from filamentary switching.
On the switching mechanism in Rose Bengal-based memory devices, Organic Electronics, in press (2007)
Electrolyte-gated organic field-effect transistors
Large efforts have been devoted to reach high capacitance (per area) between the gate and the channel to allow OFETs to operate at low voltages. One way is to use an electrolyte as gate insulator. When a voltage is applied to the gate electrode, the ions in the electrolyte will redistribute and form electric double layers at the gate-electrolyte and electrolyte-semiconductor interfaces, where in principal all voltage is dropped. We have used polyanionic electrolytes for gating p-channel OFETs since the immobile anions prevents electrochemical doping of the semiconductor bulk. As a result, the OFETs are operated at low voltage (below 1 V) and show fast switch response (less than 1 ms). The capacitance of the electrolyte layer is essentially thickness independent, which is a huge advantage compared to other approaches such as high-k dielectrics and self-assembled monolayers that require very thin films, and this makes printing a promising low-cost manufacturing platform.
Low-voltage polymer field-effect transistors gated via a proton conductor, Advanced Materials vol. 19 no. 1, pp. 97-101 (2007)
Polymer field effect transistor gated via a poly(styrenesulfonic acid) thin film, Applied Physics Letters vol. 89, p. 143507 (2006)
Low work function plastic electrodes: Towards all-plastic flexible light-emitting diodes
All-plastic light emitting diodes require the design and fabrication of low work function plastic electrodes. Here, we show that the work function of poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) can be decreased from 4.8 eV to 3.9 eV by surface reaction with the strong electron-donor tetrakis(dimethylamino)ethylene (TDAE). The surface modification was characterized by photoelectron spectroscopy and optical spectroscopy. The low work function plastic electrode was used in a first prototype for all-plastic light emitting diodes.
Towards all-plastic flexible light emitting diodes, Chemical Physics Letters vol. 433, pp. 110-114 (2006)
Transparent plastic low-work-function poly(3,4-ethylenedioxythiophene) electrodes, Chemistry of Materials vol. 18 no. 18, pp. 4246-52 (2006)
Switchable charge traps in polymer diodes
A device based on a solid-state blend of a conjugated polymer and a photochromic molecule is presented. The blend, sandwiched between two electrodes, results in a polymer diode that allows reversible modulation between charge transport mechanisms via in situ charge trap switching triggered from external stimuli.
Photochromic diodes (book chapter) in Semiconducting polymers, Wiley-VCH vol. 2, pp. 579-611 (2006)
Switchable charge traps in polymer diodes, Advanced Materials vol. 17 no. 14, pp. 1798-1803 (2005)
Diodes based on blends of molecular switches and conjugated polymers, Synthetic Metals vol. 150 no. 3, pp. 217-221 (2005)
Highly conducting PEDOT:PSS plastic electrodes
The development of printed and flexible (opto)electronics requires specific materials for the device's electrodes. They must be electrically conducting, transparent, printable, and flexible. The conducting polymer poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT-PSS) is known as a promising candidate. Its conductivity can be increased by 3 orders of magnitude by the secondary dopant diethylene glycol (DEG). This "secondary doping" phenomenon is clarified in a combined photoelectron spectroscopy and scanning probe microscopy investigation. PEDOT-PSS appears to form a three-dimensional conducting network explaining the improvement of its electrical property upon addition of DEG. Polymer light emitting diodes are successfully fabricated using the transparent plastic PEDOT-PSS electrodes instead of the traditionally used indium tin oxide.
The Origin of the High Conductivity of Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) (PEDOT-PSS) Plastic Electrodes, Chemistry of Materials vol. 18 no. 18, pp. 4354-4360 (2006)
Memory devices based on impedance switching
The addressability of organic impedance switch devices as the memory elements of cross-point matrices is improved by introducing nonlinear behavior in their current-voltage characteristics. This is realized by adding a semiconducting layer of copper (II) phthalocyanine (CuPc) on top of the switch layer of Rose Bengal (RB) sodium salt. Leakage currents from unaddressed cells in a matrix are reduced by a factor of 17; thus improving the signal-to-background ratio, reducing driver currents and limiting the potential drop along the addressing lines. The matrix size of RB switch devices, using organic conductors, is predicted to be increased by 32 times when using the additional CuPc layer.
Towards addressable organic impedance switch devices, Applied Physics Letters vol. 87, p. 063503 (2005)
High carrier mobility in low band gap polymer-based field-effect transistors
A conjugated polymer with a low band gap of 1.21 eV, i.e., absorbing infrared light, is demonstrated as active material in field-effect transistors (FETs). The material consists of alternating fluorene units and low band gap segments with electron donor-acceptor-donor units composed of two electron-donating thiophene rings attached on both sides of a thiadiazolo-quinoxaline electron-acceptor group. The polymer is solution-processable and air-stable; the resulting FETs exhibit typical p-channel characteristics and field-effect mobility of 0.03 cm2 V–1 s–1.
High carrier mobility in low band gap polymer-based field-effect transistors, Applied Physics Letters vol. 87, p. 252105 (2005)
Near infrared light-emitting diodes
A LED based on a donor-acceptor-donor conjugated polymer yields photo- and electroluminescence peaking at about 1 micrometer wavelength.
1 micron wavelength photo- and electroluminescence from a conjugated polymer, Applied Physics Letters vol. 84 no. 18, pp. 3570-3572 (2004)
Last updated: 2013-03-27