title: Modelling and Fabrication of Organic Semiconductor Devices for RFID Tags
Creators: name: Joseph Myers, Robert
Publisher: name: University of Liverpool Repository
Year: 2023

description: As organic semiconductor materials advance in both performance and stability, opportunities to integrate them into commercial applications increase considerably. The main benefits of organic semiconductor-related technology are the realisations of large area, flexible, cheap electronics. Already, organic light emitting diodes (OLEDs) are being integrated into small screens for mobiles phones, MP3 players, digital cameras, and high-resolution micro-displays. Such portable devices favour the high light output of OLEDs for easier readability in sunlight as well as their low power drain. However, at this time, the issue of low field effect mobilities still haunt these materials, limiting their application into low speed circuits. More recently, the introduction of the next generation of small molecule-based organic materials has increased the possibility of attaining high field effect mobilities. The majority of the focus is on pentacene as this has been shown to demonstrate effective mobilities of up to 1.5 cm2y-1s-l. Unfortunately, this type of pentacene was evaporated which does not seem feasible for large scale manufacturing. More recently, modified pentacene materials have surfaced allowing them to be dissolved in common solvents, one such example being triisopropylsilyl-pentacene (TIPSpentacene). Thin film transistors (TFTs) made with this material have been reported to have mobilities as high as 1.2 cm2y-ls-1 and onloffratios of 108. Materials such as TIPS-pentacene are now preferred as they can be integrated into low-cost manufacturing techniques such as inkjet printing and roll-to-roll processing. One of the major prospective applications for these organic materials is the integration into radio frequency identification tags (RFID); these operate at 13.56 MHz. This is a great challenge as the rectification stage will require devices with high mobilities to enable carriers to follow the signal, thus gaining the maximum amount of energy from an inductively coupled magnetic field. It is not clear as to whether Schottky diodes or gated-transistors will be required here. The advantage of gated-transistors is the simple incorporation into the fabrication process. Schottky diodes with these materials require thicker films which are incompatible with spin coated thin film transistors. This thesis focuses on three of the main components for a potential organic RFID tag: the tag antenna, the schottky diode and the organic thin film transistor. These are all vital components in the successful operation of an RFIO tag. The antenna is imperative for the power supply as it absorbs energy due to inductive coupling with the RFID reader antenna. The Schottky diode is important for the frontend/rectification stage, converting AC power to a DC supply voltage for an organic chip. The thin film transistor is hugely important as it is the backbone for logic and memory. The fundamental background into inductive coupling based RFID systems is explored and discussed. Major components such as the reader, tag and control system are introduced, while their role and importance are also looked into. Operational principles such as near field inductive coupling for systems functioning at 13.56 MHz are featured, involving the issues of data transfer and power supply. From here, the concept of mutual inductance is explored in detail, as well as highlighting the fact that most RFID systems of this nature comprise lower coupling, as low as I %. The core theory behind predicting the chip voltage on the tag is also explained, to illustrate just how many design parameters are involved and how they affect the performance of an RFID system. The challenges presented to organicbased RFID tags are also summarised and discussed. The numerous charge transport models proposed so far to represent conduction in organic semiconductors are assessed. These models include variable range Miller Abraham hopping and Poole-Frenkel mechanisms. Currently, an outright universal understanding of carrier transport is yet to be widely agreed. An analytical model is developed to demonstrate the carrier density dependence of mobility that is generally observed in organic semiconductors. An empirical relationship between mobility and carrier density, known as the Universal Mobility Law (UML) is recognised. The polycrystalline-based theory which consists of deriving expressions for quasi-drift and quasi-diffusion regions of operation is explained. TIPS-pentacene is utilised here for the first time to test the model. The fabrication procedure for creating bottom-gate bottom-contact organic thin film transistors is covered, with aluminium as the gate material, a high-K alumina gate dielectric and gold for the source and drain contacts. The transistors were fabricated and characterised in a clean non-vacuum environment. The effect of solvent choice is also investigated. comparing tetralin and toluene solvents. The field effect mobility of the charge carriers calculated were approximately 0.02 cm2Ns with threshold voltages ranging from -1V to +1V depending on the chosen solvent. The on/off current ratio estimated from the transfer characteristics were found to be six order of magnitude. Schottky diodes made with TIPS-pentacene show onloff current ratios three to four order magnitude higher than the P3HT devices, suggesting they are much more suitable for rectification circuits. The introduction of mixing the TIPSpentacene material with binders such as poly( a-methyl styrene ) (PAMS) and polystyrene (PS) produced some intriguing results. Both types of binders produced much smoother drop cast films than TIPS films without any binder applied, indicating that the binders were definitely improving the film morphology. Different surface treatments were also employed to help further increase the performance of the devices. It appeared that applying oxygen plasma to the bottom contact definitely helped adhesion. However, chemical treatments such as pentafluorobenzenethiol (PFBT) and I-hexadecanethiol (HDT) either did not affect performance or severely inhibited it.