Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors

Abstract

voltages. For this reason, these devices have entered successfully the fields of biosensors and bioelectronics, unfolding their innovative potential in biomolecules detection, [2][3][4][5] electrophysiology, [6][7][8] in vitro and in vivo cellular recordings. [9][10][11] Currently, intensive research effort is focused on novel OECT channel materials [12] and architectures [13] to gain faster response time, increased sensitivity, and lower power consumption targeting new applications in wearable technology and health care. [14][15][16][17][18] Despite their well-established relevance in the development of novel biotechnologies, the device physics still lacks a comprehensive understanding. Operational models able to accurately define and interpret the device working principle are required, as they would reveal well-aimed strategies for metrics refinement and guide targeted optimization of novel materials. Therefore, an improved knowledge of OECTs operation would impact on applications requiring higher sensitivity and amplification of the output signal, fast response time as well as selectivity.In a typical OECT configuration, an organic film deposited between two metal electrodes, that is, source (grounded) and drain collectors, and a gate electrode are in contact with the same electrolyte solution. The organic material constituting the transistor channel is usually poly(3,4-ethylenedioxythioph ene):poly(styrene sulfonate) (PEDOT:PSS), that is, a polyelectrolyte complex containing a degenerately p-doped semiconductor (PEDOT) and a polyelectrolyte counterion (PSS). The presence of multiple phases in thin films of PEDOT:PSS at the nanoscale has been revealed by scanning probe microscopy. Pancake-shaped PEDOT-rich islands are separated by lamellas of PSS-rich domains, thus forming a blend of electronic and ionic phases. [19] Mixed ionic and electronic transport is crucial in OECT operation and originates from the reversible doping/ dedoping mechanism of the organic film. It is well known that doping and dedoping of conjugated polymers like PEDOT rely on reversible electrochemical redox processes (oxidation and reduction, respectively). [20] Electrochemical doping is the phenomenon that rules OECTs operation and is driven by the applied potential between the polymer in the channel and the gate material. Concurrently, cations from the electrolyte are needed to compensate the charge excess of PSS − when PEDOT is dedoped and therefore the speed of electrochemical doping depends on the rate of the ionic flux across the electrolyte. [21] A comprehensive understanding of electrochemical and physical phenomena originating the response of electrolyte-gated transistors is crucial for improved handling and design of these devices. However, the lack of suitable tools for direct investigation of microscale effects has hindered the possibility to bridge the gap between experiments and theoretical models. In this contribution, a scanning probe setup is used to explore the operation mechanisms of organic electrochemical transistors b...

Finding the equilibrium of organic electrochemical transistors

“…agreement with a recent study of Mariani et al 24 , who used scanning electrochemical microscopy to determine the electrochemical potential in the OECT channel. In this microscopic study, a linear dependency of the electrochemical potential inside the channel is found, with abrupt potential changes at the source and drain contacts.…”

Charge Carrier Mobility in Organic Mixed Ionic–Electronic Conductors by the Electrolyte‐Gated van der Pauw Method

“…Typically values measured in OECTs are on the order of 1-10 cm 2 V -1 s -1 . [10][11][12][13][14][15] In general, high mobilities for transport in OECT are reasonable, as the mobility is extracted in the high carrier density regime, were carrier trapping at band-edge states has only limited importance. At the same time, the accumulation does not exceed a critical limit at which energetic and structural disorder set-in due to PEDOT overoxidation.…”

“…[8] At the same time, the mixed conductivity renders the experimental characterization of individual ionic or electronic carrier mobilities difficult as both processes are intrinsically entangled and both can lead to the screening of electric fields or introduce local contact resistances. [9,10] Progress on all fronts is predicated on an advance in understanding the interrelations between ionic transport, electronic transport and ionic-electronic coupling and their dependence on processing, synthetic structure, microstructure/morphology, and electrolyte choice. [11] The extraction of the charge carrier mobility is crucial for the analysis of the transport properties of OMIECs.…”

Nanoscale Mapping of the Conductivity and Interfacial Capacitance of an Electrolyte‐Gated Organic Field‐Effect Transistor under Operation

“…[ 18 ] However, this technique does not provide relevant information when applied to organic semiconductors in which ion uptake does not take place, as in most EGOFETs. Finally, microscale local potential measurements by scanning electrochemical microscopy have been demonstrated also on OECTs, [ 19 ] but they are based on the good conducting properties of OECTs, which are not generally met in EGOFETs.…”