Investigation and Modeling of the Electrical Bias Stress in Electrolyte‐Gated Organic Transistors

Abstract

is coupled with the gate electrode via an electrolyte. The application of a bias to a polarizable gate electrode may either induce the formation of two electrical double layers or allow the intercalations of ions in the OSC film. In the first case, the transistors are operating in the field-effect mode and they are called electrolyte-gated organic field-effect transistors (EGOFETs), while in the second case, an electrochemical doping occurs and the devices are named organic electrochemical transistors (OECTs). EGOTs can operate with voltages below 1 V and are extremely sensitive to any change that occurs either on the gate electrode or on the OSC. For this reason, these devices are exploited in biosensing applications for the detection of specific binding events occurring at the gate surface, which is functionalized to match the targeted analyte. Indeed, EGOTs can be used as biosensors to detect low concentrations of relevant biomolecules (proteins, [1][2][3] nucleic acids, [4,5] drugs [6][7][8] ), but the manufacturing of these devices for scale production and commercial applications is still far to be achieved. In fact, one of the main disadvantages of EGOTs is their poor operational stability, whose origins are still under investigation. [9][10][11] Several works in the literature concerning organic transistors address the instability related to the electrical bias stress (EBS), but most of the proposed models deal with solid-state organic field-effect transistor (OFET). [12,13] For this kind of devices, the EBS is responsible for mobile charges trapping either in the OSC material, or in the insulating dielectric, or at the OSC/dielectric interface. This mechanism is usually related to high-voltageinduced water electrolysis phenomena affecting adsorbed adventitious water molecules present on the oxide surface. [12,14] The EBS in EGOTs should be ascribed to different causes since the low operating voltages prevent any water electrolysis to occur. In addition to this, the different nature of the dielectric material avoids any charge accumulation in the dielectric, being the ions in the electrolyte free to move. Concerning the operational stability of EGOTs, only a limited number of works are found in the literature addressing the issue. For instance, Blasi et al. investigated the long-term stability of EGOFETs based on poly(3hexylthiophene) (P3HT), showing that the maximum current drift over time could be modeled with a biexponential function, in which two processes with different timescales occur. [15] Electrolyte-gated organic transistors (EGOTs) are emerging as an important tool in advanced biosensing applications. However, their widespread exploitation is still limited by their poor operational stability. In order to understand the causes of this unreliability, the proposed study focuses on the influence of electrical bias stress (EBS) on EGOTs operating in aqueous electrolytes. Poly(3-hexylthiophene) (P3HT)-and poly[3-(5-carboxypentyl)thiophene)] (P3CPT)-based transistors are studied under the application o...

Organic Bioelectronics Development in Italy: A Review

“…The application of different materials, geometries and fabrication techniques, together with a deeper understanding of device physics, allows for the tailoring of the device performances to the specific applications, reaching unprecedented results. The Italian production in this field reflects a generalized trend, with the exploration of materials ranging from small molecules and polymeric OSC [ 39 , 53 , 54 , 55 , 56 ], to polymeric OMIECs [ 57 , 58 , 59 ]. The use of OSCs or OMIECs gated via an electrolyte has proven to be a successful configuration for applications in bioelectronics [ 60 ].…”

“…From a technological point of view, fabrication techniques derived from silicon technology are exploited [ 39 , 57 , 63 , 64 , 65 , 66 ], as well as 2D and 3D printing technologies [ 7 , 54 , 67 , 68 , 69 , 70 , 71 ]. Moreover, increasingly accurate models have been developed in order to better understand the physical mechanisms governing the device operations, both in terms of the electrolyte-gating working mechanisms and in terms of the biosensor’s response to a given analyte [ 55 , 56 , 59 , 72 ]. In the following subsections, the cutting-edge progress achieved by the Italian Organic Bioelectronics community concerning the fabrication and modelling of novel organic transistor-based sensors will be described.…”

“…The S/D fabrication starts with a thin film deposition (Cr or Ti are used as an adhesion layer prior the Au deposition), followed by a photolithographic step and wet etching [ 39 ]. Alternatively, a lift-off process may be employed, starting with photolithography followed by thin film deposition and lift-off of the photoresist [ 59 ].…”

“…This is fundamental to prevent short circuits during the operation of the transistor in contact with an electrolyte. A passivation layer may be deposited via chemical vapor deposition (CVD) [ 83 ], e-beam evaporation [ 59 ] or spin-coating [ 66 ], depending on the material of choice. A lithography/etching process (or alternatively a lift-off process) is necessary in this step in order to open windows in the passivation.…”

“…These are needed to allow for the OSC/OMIEC to contact the electrodes, and to connect the contact pads with the external world. The described process is summarized in Figure 4 , where a double lift-off process was employed to define S/D electrodes and passivation windows (in this case, Al 2 O 3 deposited via e-beam evaporation) [ 59 ].…”