Ferroelectric polymers for non‐volatile memory devices: a review

Based on intensive research, organic field‐effect transistors (OFETs) will likely remain at the zenith in information science for many years due to their numerous potential applications in portable and smart electronic devices. Elucidating the intrinsic charge‐transport behavior and realizing functional OFETs with superior device characteristics have been the cornerstones for the sustainable advancement in organic electronics. The emerging 2D molecular crystals (2DMCs) have become prosperous in the past few years, because they offer a revolutionary opportunity to overcome these obstacles. In particular, 2DMCs, which display a high uniform morphology, can be prepared using low‐cost and scalable solution‐based techniques. More importantly, their highly tunable molecular arrangement at a 2D limit is extraordinarily beneficial in property exploration and device optimization. In this article, the recent breakthroughs in material preparation, transistor applications, and physics of solution‐processed 2DMCs are reviewed, providing a promising avenue toward high‐performance, multifunctional, and diversiform organic electronic devices.

Solution-Processed 2D Molecular Crystals: Fabrication Techniques, Transistor Applications, and Physics

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Tuning Charge Transport in PVDF-Based Organic Ferroelectric Transistors: Status and Outlook.

Despite the increasing understanding of charge transport in organic field-effect transistors (OFETs), charge injection from source/drain electrodes into organic semiconductors remains crucial for improving device performance and lowering power consumption. The analysis of contact resistance is generally carried out without clearly distinguishing the Schottky barrier and access resistance. Here we show that the access resistance through the organic semiconductor bulk can significantly influence the Schottky barrier evaluation and affect the charge-transport exploration. Indeed, access resistance plays a leading role in the contact resistance, whereas the Schottky barrier (expressed as the interface resistance) determines the charge injection at the metal/semiconductor interface. The Schottky barrier evaluation strongly depends on the access resistance and bias voltage. After eliminating the access resistance effect, the intrinsic Schottky barrier appears to be very coincident and weakly dependent on the work function of the contact metal. This work provides clues to understanding the Schottky barrier and charge injection in OFETs to optimize OFETs for high-performance and advanced applications.

Role of Schottky Barrier and Access Resistance in Organic Field-Effect Transistors.

Metal-oxide-semiconductor field-effect transistors (MOSFET) were fabricated using organic semiconductor pentacene. The humidity dependence of drain current gate voltage (I D -V G ) characteristic and drain current drain voltage characteristic (I D -V D ) will be explained. Firstly, the thermal oxidation method was used to grow SiO 2 gate insulator with thickness of 11 nm. Secondly, the thermal evaporation method was used to form Au source and drain electrodes with thickness of 28 nm. The channel width and length of the transistors were 500 mm and 200 mm, respectively. By the same method, organic semiconductor material pentacene was deposited with thickness of 50 nm at vacuum of 7.8x10 -6 Torr. The hole mobility decreased from 0.035 cm 2 /(Vs) to 0.006 cm 2 /(Vs), while the threshold voltage increased from 0.5 V to 2.5 V and gate leakage current also increased from 5.8x10 -10 A to 3.3x10 -9 A when the relative humidity increased from 20% to 70%.

The Humidity Dependence of Pentacene Organic Metal-Oxide- Semiconductor Field Effect Transistor

The reduction of contact resistance in ferroelectric organic field-effect transistors (Fe-OFETs) by buffering the interfacial polarization fluctuation was reported. An ultrathin poly(methyl methacrylate) layer was inserted between the ferroelectric polymer and organic semiconductor layers. The contact resistance was significantly reduced to 55 kΩ cm. By contrast, Fe-OFETs without buffering exhibited a significantly larger contact resistance of 260 kΩ cm. Results showed that such an enhanced charge injection was attributed to the buffering effect at the semiconductor/ferroelectric interface, which narrowed the trap distribution of the organic semiconductor in the contact region. The presented work provided an efficient method of lowering the contact resistance in Fe-OFETs, which is beneficial for the further development of Fe-OFETs.

Reducing contact resistance in ferroelectric organic transistors by buffering the semiconductor/dielectric interface

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Reducing contact resistance in ferroelectric organic transistors by buffering the semiconductor/dielectric interface