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).
Linköping: Linköping University Electronic Press, 2016. , 58 p.
Berggren, Magnus, ProfessorEngquist, Isak, Dr.