Stretchable organic electrochemical transistors: from material design to device characterization

Abstract

Stretchable electronics are rapidly emerging as a key technology for wearable healthcare applications, driving intense research efforts in the field. Among these, stretchable organic electrochemical transistors (OECTs) have gained considerable attention as mixed-conductor devices capable of transporting both electronic and ionic charges. This unique property allows them to seamlessly interface with biological systems, making them highly promising for biomedical applications. The conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) plays a pivotal role in these devices due to its dual charge transport capability, excellent mechanical properties, biocompatibility and commercial availability. Despite their promising future, stretchable electronics and OECTs in particular face notable challenges. This includes the development of reliable fabrication techniques for stretchable systems and addressing the trade-off between stretchability and conductivity, which is a major bottleneck in the field. Furthermore, little is known about how strain-induced morphological changes in PEDOT:PSS affect the coupled ionic and electronic transport in stretchable OECTs. Addressing these obstacles and deepening the understanding of stretchable OECTs was the focus of this doctoral thesis. Accordingly, this doctoral thesis is structured into three main sections: fabrication, electrical understanding and integration into stretchable OECTs. As a foundational step for this thesis, a fast, reliable and versatile transfer-printing method is developed to deposit PEDOT:PSS onto flexible, transparent and biodegradable poly(vinyl alcohol) (PVA):glycerol substrates (Chapter 3). This approach avoids harsh chemical treatments and is compatible with established techniques to enhance PEDOT:PSS’s stretchability and conductivity. The transfer-printed films are validated through electrical and morphological studies, and a simple O2-plasma patterning method is introduced to pattern PEDOT:PSS and create structures, which are also successfully transfer-printed onto stretchable substrates. These methodologies pave the way towards stretchable OECTs. Before utilizing a conducting material in stretchable OECTs, it is essential to understand its electrical and morphological response to strain. While PEDOT:PSS is a widely used material in stretchable electronics, its inherent brittleness limits its ability to withstand significant stretching. To address this, Chapter 4 examines the effects of organic plasticizers on the morphology and electrical properties of PEDOT:PSS thin-films using a two-layer system of transfer-printed PEDOT:PSS on stretchable PVA substrates infused with glycerol (15 - 55 wt%). Within this system, glycerol is found to diffuse into PEDOT:PSS, causing reorganization of PEDOT and PSS. This process, modeled using multicomponent diffusion, is found to cause a plasticizer-dependent conductivity increase due to more interconnected PEDOT domains. This reorganization improves the material's response to strain and enables crack-free elongation, allowing pristine PEDOT:PSS to stretch up to 160 %. The improved response to strain becomes evident in an increased conductivity with strain, which is attributed to PEDOT chain alignment. Chapter 5 builds upon the transfer-printing process, O₂-plasma-patterning and insights into electrical behavior under strain to incorporate these findings into stretchable OECTs. Two device configurations are investigated, applying strain either parallel or perpendicular to the current flow. The impact of geometric and morphological changes—both essential for optimal OECT performance—are studied systematically. Geometric changes affect the transfer and output characteristics, electrolyte resistance, turn-off voltage and charge time, revealing opposing trends between the two configurations across all parameters. Focusing on hole mobility and ion uptake, strain-induced morphological changes are found to cause anisotropic charge transport and a reduction in volumetric capacitance, with the latter being independent of the device configuration. However, the parallel arrangement compensates this loss by enhancing charge carrier mobility. These findings underscore the critical role of geometric and morphological considerations in optimizing OECT performance under strain.