Polyelectrolyte-Gated Organic Field Effect Transistors – Printing and Electrical Stability
Abstract: The progress in materials science during recent decades along with the steadily growing desire to accomplish novel functionalities in electronic devices and the continuous strive to achieve a more efficient manufacturing process such as low?cost robust high?volume printing techniques, has brought the organic electronics field to light. For example, organic field effect transistors (OFETs) are the fundamental building blocks of flexible electronics. OFETs present several potential advantages, such as solution processability of organic materials enabling their deposition by various printing methods at low processing temperatures, the possibility to coat large areas, and the mechanical flexibility of polymers that is compatible with plastic substrates. Employing polyelectrolytes as gate insulators in OFETs allows low?voltage operation in the range of 1 V, suppresses unintended electrochemical doping of the semiconductor bulk, and provides tolerance to thicker gate insulator layers and to the gate electrode alignment over the channel which eases the design and manufacturing requirements. These features place polyelectrolyte?gated OFETs (EGOFETs) as promising candidates to be realized in lowcost, large?area, light?weight, flexible electronic applications.The work in this thesis focuses on EGOFETs and their manufacturing using the inkjet printing technology. EGOFETs have been previously demonstrated using conventional manufacturing techniques. Several challenges have to be overcome when attempting to achieve a fully printed EGOFET, with the incompatible wetting characteristics of the semiconductor/polyelectrolyte interface being one of the main problems. This issue is addressed in paper I and paper II. Paper I presents a surface modification treatment where an amphiphilic diblock copolymer is deposited on the surface to enable the printability of the semiconductor on top of the polyelectrolyte. Paper II introduces an amphiphilic semiconducting copolymer that can switch its surface from hydrophobic to hydrophilic, when spread as thin film, upon exposure to water. Moreover, characterization of the reliability and stability of EGOFETs in terms of bias stress is reported. Bias stress is an undesired operational instability, usually manifested as a decay in the drain current, triggered by the gradual shift of the threshold voltage of the transistor under prolonged operation. This effect has been extensively studied in different OFET structures, but a proper understanding of how it is manifested in EGOFETs is still lacking. Bias stress depends strongly on the material, how it is processed, and on the transistor operating conditions. Papers III and IV report bias stress effects in EGOFET devices and inverters, respectively. The proposed mechanism involves an electron transfer reaction between adsorbed water and the charged semiconductor channel, which promotes the generation of extra protons that subsequently diffuse into the polyelectrolyte. Understanding and controlling the mechanism of bias stress in EGOFETs is crucial for further advancements and development towards commercially viable organic transistor circuits.
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