Polyelectrolytes are promising electronically insulating layers for low-voltage organic field effect transistors. However, the polyelectrolyte–semiconductor interface is difficult to manufacture due to challenges in wettability. We introduce an amphiphilic semiconducting copolymer which, when spread as a thin film, can change its surface from hydrophobic to hydrophilic upon exposure to water. This peculiar wettability is exploited in the fabrication of polyelectrolyte-gated field-effect transistors operating below 0.5 V. The prepared amphiphilic semiconducting copolymer is based on a hydrophobic regioregular poly(3-hexylthiophene) (P3HT) covalently linked to a hydrophilic poly(sulfonated)-based random block. Such a copolymer is obtained in a three-step strategy combining Grignard metathesis (GRIM), atom transfer radical polymerization (ATRP) processes, and a postmodification method. The structure of the diblock copolymer was characterized using FT-IR, 1H NMR spectroscopy, and gel permeation chromatography (GPC).

We report a method for inkjet-printing an organic semiconductor layer on top of the electrolyte insulator layer in polyelectrolyte-gated OFETs by using a surface modification treatment to overcome the underlying wettability problem at this interface. The method includes depositing an amphiphilic diblock copolymer (P3HT-b-PDMAEMA). This material is designed to have one set of blocks that mimics the hydrophobic properties of the semiconductor (poly(3-hexylthiophene) or P3HT), while the other set of blocks include polar components that improve adhesion to the polyelectrolyte insulator. Contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy confirm formation of the desired surface modification film. Successful inkjet printing of a smooth semiconductor layer allows us to manufacture complete transistor structures that exhibit low-voltage operation in the range of 1 V.

Prolonged gate bias application causes undesirable operational instabilities in organic transistors involving threshold voltage shift and drain current degradation; an effect known as bias stress. In this paper, we report how this instability is manifested in inverter circuits based on polyelectrolytegated p-type organic field effect transistors (EGOFETs) operating at low voltage. We find that bias stress causes a significant, but recoverable, shift in inverter switching threshold voltage. Measurements with two different polyelectrolytes reveal significant differences in the stressing and recovery behaviour, which is ascribed to the distinct nature of the ion conductive groups in the polyelectrolyte. Moreover, we report a large influence of illumination on the recovery process for one of the polyelectrolytes but not for the other, which demonstrates the need to characterize bias stress behavior for each new materials combination.

A main factor contributing to bias stress instability in organic transistors is charge trapping of mobile carriers near the gate insulator-semiconductor interface into localized electronic states. In this paper, we study the bias stress behavior in low-voltage (p-type) polyelectrolyte-gated organic field effect transistors (EGOFETs) at various temperatures. Stressing and recovery in these EGOFETs are found to occur six orders of magntiude faster than typical bias stress/recovery reported for dielectric-gated OFETs. The mechanism proposed for EGOFETs involves an electron transfer reaction between water and the charged semiconductor channel that promotes the creation of extra protons diffusing into the polyelectrolyte.

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.