A simplified model is developed to understand the field and potential distribution through devices based on a ferroelectric film in direct contact with an electrolyte. Devices based on the ferroelectric polymer polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE) were produced – in metalferroelectric-metal, metal-ferroelectric-dielectric-metal, and metal-ferroelectric-electrolyte-metal architectures – and used to test the model, and simulations based on the model and these fabricated devices were performed. From these simulations we find indication of progressive polarization of the films. Furthermore, the model implies that there is a relation between the separation of charge within the devices and the observed open circuit voltage. This relation is confirmed experimentally. The ability to polarize ferroelectric polymer films through aqueous electrolytes, combined with the strong correlation between the properties of the electrolyte double layer and the device potential, opens the door to a variety of new applications for ferroelectric technologies, e.g., regulation of cell culture growth and release, steering molecular self-assembly, or other large area applications requiring aqueous environments.
Funding agencies: Swedish Governmental Agency for Innovation Systems (VINNOVA) [2010-00507]; Knut and Alice Wallenberg Foundation; Advanced Functional Materials Center at Linkoping University; Onnesjo Foundation
The field of organic electronics emerged in the 1970s with the discovery of conducting polymers. With the introduction of plastics as conductors and semiconductors came many new possibilities both in production and function of electronic devices. Polymers can often be processed from solution and their softness provides both the possibility of working on flexible substrates, and various advantages in interfacing with other soft materials, e.g. biological samples and specimens. Conducting polymers readily partake in chemical and electrochemical reactions, providing an opportunity to develop new electrochemicallydriven devices, but also posing new problems for device engineers.
The work of this thesis has focused on organic electronic devices in which aqueous electrolytes are an active component, but still operating in conditions where it is desirable to avoid electrochemical reactions. Interfacing with aqueous electrolytes occurs in a wide variety of settings, but we have specifically had biological environments in mind as they necessarily involve the presence of water. The use of liquid electrolytes also provides the opportunity to deliver and change the device electrolyte continuously, e.g. through microfluidic systems, which could then be used as a dynamic feature and/or be used to introduce and change analytes for sensors. Of particular interest is the electric double layer at the interface between the electrolyte and other materials in the device, specifically its sensitivity to charge reorganization and high capacitance.
The thesis first focuses on organic field effect transistors gated through aqueous electrolytes. These devices are proposed as biosensors with the transistor architecture providing a direct transduction and amplification so that it can be electrically read out. It is discussed both how to distinguish between the various operating mechanisms in electrolyte thin film transistors and how to choose a strategy to achieve the desired mechanism. Two different strategies to suppress ion penetration into, and thus electrochemical doping of, the organic semiconductor are presented.
The second focus of the thesis is on polarization of ferroelectric polymer films through electrolytes. A model for the interaction between the remnant ferroelectric charge in the polymer film and the mobile ionic charges of the electrolyte is presented, and verified experimentally. The reorientation of the ferroelectric polarization via the electric double layer is also demonstrated in a regenerative medicine application; the ferroelectric polarization is shown to affect cell binding, and is used as a gentle method to nondestructively detach cells from a culture substrate.
The discovery of conductive polymers in 1977 opened up a whole new path for flexible electronics. Conducting polymers and organic semiconductors are carbon rich compounds that are able to conduct charges while flexed and are compatible with low-cost and large-scale processes including printing and coating techniques. The conducting polymer has aided the rapidly expanding field of flexible electronics, leading to many new applications such as electronic skin, RFID tags, smart labels, flexible displays, implantable medical devices, and flexible sensors.
However, there are several remaining challenges in the production and implementation of flexible electronic materials and devices. The conductivity of organic conductors and semiconductors is still orders of magnitude lower compared to their inorganic counterparts. In addition, non-flexible inorganic semiconductors still remain the materials of choice for high frequency applications; since the charge carrier mobility and thus operational speed of the organic materials are limited. Therefore, there remains a high demand to combine the high frequency operation of inorganic semiconductors with the flexible fabrication methods of organic systems for future electronics.
In addition to the challenges in the choice of materials in flexible electronics, the upscaling of the flexible devices and implementing them in circuits can also be complicated. Lack of non-linearity is an issue that arises when flexible devices with linear behavior need to be incorporated in an array or matrix form. Non-linearity is important for applications such as displays and memory arrays, where the devices are arranged as matrix cells addressed by their row and column number. If the behavior of cells in the matrix is linear, addressing each cell affects the adjacent cells. Therefore, inducing non-linearity and, consequently, addressability in such linear devices is the first step before scaling up into matrix schemes.
In this work, non-linear organic/inorganic hybrid devices are produced to overcome the limitations mentioned above and leverage the advantages of both organic and inorganic materials. Two novel methods are developed to incorporate non-flexible inorganic semiconductors into ultra-high frequency (UHF) flexible devices. In the first method, Si is ground into a powder with micrometer-sized particles and printed through standard screen printing. For the first time, allprinted flexible diodes operating in the GHz range are produced. The energy harvesting application of the printed diodes is demonstrated in a flexible circuit coupling an antenna and the display to the diode.
A second and simpler room-temperature method based on lamination was later developed, which further improves device performance and operational frequency. For the first time, a flexible semiconducting composite film consisting of Si micro-particles, glycerol, and nano-fibrillated cellulose is produced and used as the semiconducting layer of the UHF diode.
The diodes fabricated through both mentioned processes are demonstrated in energy harvesting applications in the GHz range; however, they can also serve as rectifiers or non-linear elements in any other flexible and UHF circuit.
Furthermore, a new approach is developed to induce non-linearity and hence addressability in linear devices in order to make their implementation in flexible matrix form feasible. This is accomplished by depositing a ferroelectric layer on a device electrode and thus controlling charge transfer through the electrode. The electrode current becomes limited to the charge displacement current established in the ferroelectric layer during polarization. Thus, the current does not follow the voltage linearly and non-linearity is induced in the device. The polarization voltage is dictated by the thickness of the ferroelectric layer. Therefore, the switching voltage of the device can be tuned by adjusting the ferroelectric layer thickness. In this work, the organic ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) is used due to its distinctive properties such as stability, high polarizability and simple processability. The polarization of P(VDF-TrFE) through an electrolyte and an electrophoretic liquid is investigated. In addition, a simple model is presented in order to understand the field and potential distribution, and the ferroelectric polarization, in the P(VDF-TrFE)-electrolyte contact. The induction of non-linearity through P(VDF-TrFE) is successfully demonstrated in novel addressable and bistable devices and memory elements such as non-linear electrophoretic display cells, organic ferroelectrochromic displays (FeOECDs), and ferroelectrochemical organic transistors (FeOECTs).