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.

Revisiting the delta-phase of poly(vinylidene fluoride) for solution-processed ferroelectric thin films Li, Mengyuan; Wondergem, Harry J.; Spijkman, Mark-Jan; Asadi, Kamal; Katsouras, Ilias; Blom, Paul W. M.; de Leeuw, Dago M.

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.

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 exception is poly(vinylidene-fluoride) (PVDF), which exhibits a negative longitudinal piezoelectric coefficient. Reported explanations consider exclusively contraction with applied electric field of either the crystalline or the amorphous part of this semi-crystalline polymer. 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. 3 Piezoelectricity describes the conversion of electrical charge to mechanical strain and vice versa. The direct piezoelectric effect is observed as a change in surface charge density of a material in response to an external mechanical stress. The effect is reversible; the thermodynamic equivalent is a change in dimension upon applying an electric field.A large piezoelectric coefficient, describing the change in spontaneous electrical polarization with applied mechanical stress, is obtained for ferroelectric materials. When an electric field is applied in the direction of the polarization most ferroelectric materials will expand. However, there is one well-known exception. The ferroelectric polymer poly(vinylidene-fluoride) (PVDF) and its copolymers with trifluoroethylene P (VDF-TrFE) show an unusual negative longitudinal piezoelectric effect. Counterintuitively, these polymers contract in the direction of an applied electric field. The two opposite behaviours are schematically represented in Fig. 1.It has been shown that the strain in PVDF varies with the polarization squared. [1] Hence the origin of piezoelectricity is electrostriction biased by the spontaneous polarization. A negative piezoelectric coefficient was extracted. Presently, two contradicting microscopic models have been proposed; the piezoelectric response is attributed to either the crystalline or the amorphous part of the semi-crystalline polymer.Quantum chemical calculations for the ferroelectric β−phase of PVDF have shown that for a single-crystal the piezoelectric effect is negative.[2] When an electric field is applied perpendicularly to the PVDF chain, the backbone stretches and its height is compressed. The lattice constant is reduced. The calculated coefficient agrees with the value experimentally determined on bulk samples, imp...

The transition rate for a single hop of a charge carrier in a semiconducting polymer is assumed to be thermally activated. As the temperature approaches absolute zero, the predicted conductivity becomes infinitesimal in contrast to the measured finite conductivity. Here we present a uniform description of charge transport in semiconducting polymers, including the existence of absolute-zero ground-state oscillations that allow nuclear tunnelling through classical barriers. The resulting expression for the macroscopic current shows a power-law dependence on both temperature and voltage. To suppress the omnipresent disorder, the predictions are experimentally verified in semiconducting polymers at high carrier density using chemically doped in-plane diodes and ferroelectric field-effect transistors. The renormalized current-voltage characteristics of various polymers and devices at all temperatures collapse on a single universal curve, thereby demonstrating the relevance of nuclear tunnelling for organic electronic devices.