Dago M. de Leeuw

Organic Nonvolatile Memory Devices Based on Ferroelectricity

A memory functionality is a prerequisite for many applications of electronic devices. Organic nonvolatile memory devices based on ferroelectricity are a promising approach toward the development of a low-cost memory technology. In this Review Article we discuss the latest developments in this area with a focus on three of the most important device concepts: ferroelectric capacitors, field-effect transistors, and diodes. Integration of these devices into larger memory arrays is also discussed.

Organic non-volatile memories from ferroelectric phase-separated blends

New non-volatile memories are being investigated to keep up with the organic-electronics road map. Ferroelectric polarization is an attractive physical property as the mechanism for non-volatile switching, because the two polarizations can be used as two binary levels. However, in ferroelectric capacitors the read-out of the polarization charge is destructive. The functionality of the targeted memory should be based on resistive switching. In inorganic ferroelectrics conductivity and ferroelectricity cannot be tuned independently. The challenge is to develop a storage medium in which the favourable properties of ferroelectrics such as bistability and non-volatility can be combined with the beneficial properties provided by semiconductors such as conductivity and rectification. Here we present an integrated solution by blending semiconducting and ferroelectric polymers into phase-separated networks. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor-metal contact. The combination of ferroelectric bistability with (semi)conductivity and rectification allows for solution-processed non-volatile memory arrays with a simple cross-bar architecture that can be read out non-destructively. The concept of an electrically tunable injection barrier as presented here is general and can be applied to other electronic devices such as light-emitting diodes with an integrated on/off switch.

Low-voltage polymer field-effect transistors for nonvolatile memories

We demonstrate organic nonvolatile memories based on transistors, made from spin-coated polymers, that have programming voltages of 15V and good data retention capabilities. The low-voltage programmable ferroelectric field-effect transistors were obtained by an optimized ferroelectric polymer deposition method using cyclohexanone as a solvent from which films can be obtained that are thin, smooth and defect free. The data retention characteristics were measured for 3h under constant read conditions. Extrapolation predicts that the data retention capability exceeds 10years.

All-polymer ferroelectric transistors

We demonstrate thin-film ferroelectric transistors, made entirely from organic materials that are processed from solution. The devices consist of thin ferroelectric poly(vinylidene fluoride/trifluoroethylene) films sandwiched between electrodes made of conducting poly(3,4-ethylenedioxythiophene) stabilized with polystyrene-4-sulphonic acid. On top of this stack, an organic semiconductor is applied. The ferroelectric transistors, constructed using unipolar p- or n-type semiconductor channels, have remnant current modulations of ∼103 with a retention time of hours. They can be switched in 0.1–1ms at operating voltages less than 10V.

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.

The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride)

Piezoelectricity describes interconversion between electrical charge and mechanical strain. As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exceptions are poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)), which exhibit a negative longitudinal piezoelectric coefficient. Reported explanations exclusively consider contraction with applied electric field of either the crystalline or the amorphous part of these semi-crystalline polymers. To distinguish between these conflicting interpretations, we have performed in situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure.

Origin of the drain current bistability in polymer ferroelectric field-effect transistors

The authors present measurements that elucidate the mechanism behind the observed drain current bistability in ferroelectric field-effect transistors based on the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) as the gate dielectric. Capacitance-voltage measurements on metal-insulator-semiconductor diodes demonstrate that the bistability originates from switching between two states in which the ferroelectric gate dielectric is either polarized or depolarized. Pulsed charge displacement measurements on these diodes enable a direct measurement of the accumulated charge in the polarized state of 40±3mC∕m2.

Organic ultra-thin film transistors with a liquid gate for extracellular stimulation and recording of electric activity of stem cell-derived neuronal networks

Electronic transducers of neuronal cellular activity are important devices in neuroscience and neurology. Organic field-effect transistors (OFETs) offer tailored surface chemistry, mechanical flexibility, and high sensitivity to electrostatic potential changes at device interfaces. These properties make them attractive for interfacing electronics with neural cells and performing extracellular recordings and stimulation of neuronal network activity. In this work we operate pentacene ultra-thin film (9 nm thick) transistors with a liquid gate both as transducers and electrical stimulators of neuronal network activity. These devices are highly sensitive to small potential changes in cell medium and exhibit sufficient stability under standard cell culture conditions for nine days. We show that murine neural stem cells can be adhered on top of functional devices without the need for an additional layer of cell-adhesive molecules, and then differentiated into neuronal networks. OFET response is monitored during the different phases of the neuronal differentiation process up to nine days. Only when stem cells are differentiated into neurons, it is possible to measure electrical signals in the OFET current following the stimulation. Due to the large sensing area of our device, which accommodates from hundreds to thousands of interconnected neurons, the OFET electrical signals arise from the collective electrophysiological response of the neuronal population. The maximum extracellular potential change in the cleft region adjacent to the transistor surface amounts to 350 μV. This demonstrates that pentacene ultra-thin film OFETs enable good cellular adhesion and efficient coupling of the ionic currents at the biological-organic semiconductor interface with the OFET current.

Polarization fatigue of organic ferroelectric capacitors

The polarization of the ferroelectric polymer P(VDF-TrFE) decreases upon prolonged cycling. Understanding of this fatigue behavior is of great technological importance for the implementation of P(VDF-TrFE) in random-access memories. However, the origin of fatigue is still ambiguous. Here we investigate fatigue in thin-film capacitors by systematically varying the frequency and amplitude of the driving waveform. We show that the fatigue is due to delamination of the top electrode. The origin is accumulation of gases, expelled from the capacitor, under the impermeable top electrode. The gases are formed by electron-induced phase decomposition of P(VDF-TrFE), similar as reported for inorganic ferroelectric materials. When the gas barrier is removed and the waveform is adapted, a fatigue-free ferroelectric capacitor based on P(VDF-TrFE) is realized. The capacitor can be cycled for more than 108 times, approaching the programming cycle endurance of its inorganic ferroelectric counterparts.

Nanoscale Design of Multifunctional Organic Layers for Low-Power High-Density Memory Devices

We demonstrate the design of a multifunctional organic layer by the rational combination of nanosized regions of two functional polymers. Instead of relying on a spontaneous and random phase separation process or on the tedious synthesis of block copolymers, the method involves the nanomolding of a first component, followed by the filling of the resulting open spaces by a second component. We apply this methodology to fabricate organic nonvolatile memory diodes of high density. These are built by first creating a regular array of ferroelectric nanodots by nanoimprint lithography, followed by the filling of the trenches separating the ferroelectric nanodots with a semiconducting polymer. The modulation of the current in the semiconductor by the polarization state of the ferroelectric material is demonstrated both at the scale of a single semiconductor channel and in a microscopic device measuring about 80,000 channels in parallel, for voltages below ca. 2 V. The fabrication process, which combines synergetically orthogonal functional properties with a fine control over their spatial distribution, is thus demonstrated to be efficient over large areas.