有机电化学晶体管的加工

Abstract 

Since the advent of Organic Electrochemical Transistors (OECTs) back in the 80s, research focus has shifted from understanding the working  mechanism and expanding the materials library to fnding new applications and building larger integrated circuits. Given the strong dependency of  these devices’ performance on their geometrical dimensions and considering the increasing need for larger scale and low cost fabrication, research  on novel processing methods is paramount. Here, we review the most common processing techniques used for OECT fabrication, starting from classic  methods such as spin coating and electropolymerization to more recent and complex ones like orthogonal lithography and 3D printing. We also provide  a brief outlook on how these techniques are enabling integrated circuits and large scale circuitry in general.

Introduction

In 1984, White, Kittlesen and Wrighton presented the frst  Organic Electrochemical Transistor or OECT.Ever since  then, OECTs have developed rapidly, especially because of  their advantages for bio-electronic applications. Their hightransconductance enables an efcient transduction of ionic  into electronic currents, making the device an ideal mediator  between biochemical redox reactions and microelectronics.Furthermore, OECTs are promising for sensing applications,and their bio-compatibility makes them ideal candidates for  implantable or even edible electronics.

The depiction of an OECT can be seen in Fig. 1(a). These  devices consist of drain electrode, source electrode, gate electrode, electrolyte and channel. Commonly, organic materials  called Organic Mixed Ionic-Electronic Conductors or OMIECS  are used as channel materials. As the name implies, mixed conductors permit both ionic and electronic fuxes in their volume.  Generally, for OMIECs applied in OECTs, electronic fuxes  occur through the polymer backbone while ionic fuxes take  place in the free space between polymeric chains.

To exemplify the OECT doping mechanism, a p-type or  hole conducting channel is considered in the following. In  the absence of a gate bias ( VG ) and presence of a constant  drain voltage ( VD ), both cations and anions tend to stay in the  electrolyte, thus resulting in an undisturbed drain current ID [cf. Fig. 1(b)]. Upon application of a positive VG , cations are  pushed into the p-type channel [cf. Fig. 1(c)]. As the cation  density increases in this region, holes are extracted through the  drain electrode, thus reducing ID (de-doping) and confguring  the depletion mode of operation [cf. Fig. 1(d)].Conversely,  when a negative VG is applied, anions enter the channel [cf.  Fig. 1(e)]. With increasing presence of these negative charges,  more holes are injected into the OMIEC to achieve charge neutrality. As a consequence, ID increases (doping), confguring the  accumulation mode of operation [cf. Fig. 1(f)]. A representation of n-type OECT operation can be seen in the Supplementary Information (cf. Fig. S1).

Figure 1

These challenges sparked research into alternative  deposition and structuring methods to process and integrate  OECTs. One of the most promising methods was developed  by A. Zakhidov.He proposed to use fluorine based  solvents and resists that not only are orthogonal to standard  resist systems, but are benign to organic semiconductors for  structuring, and was able to reach sub-micrometer feature  sizes in one of the most heavily used mixed conductors,  PEDOT:PSS.

A few more promising results have been reported for  n-type small-molecules OMIECs through ethylene glycol  (EG) side chains modifcation. This strategy has been used  for derivatives of fullerene, perylene diimide (PDI), and  naphthalene bis-isatin (NB), to cultivate the following novel  molecules, namely 2-(2,3,4-tris(methoxtriglycol) phenyl) fulleropyrrolidine (C60-TEG) for fullerene derivatives, PDI- 3EG, 4Cl-PDI-3EG, and 4Cl-PDI-4EG for PDI derivatives,  and gNR and hgNR for NB derivatives. Except for C60-TEG,  the use of these molecules in OECTs lead to satisfying drain  current modulation with ON-OFF ratio in the range between  10³  and 8 × 10⁴.

Electropolymerization 

The deposition of electrically conductive conjugated polymers  can also be done via electrochemical polymerization. In this  method, a monomer is dissolved in a suitable solvent containing  a doping anion salt. The monomer is oxidized at an electrically  conductive electrode surface by applying an anodic potential  resulting in the growing of a polymer flm.

The most famous conducting polymers grown by  electropolymerization are polypyrrole (PPy) and  poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)  (PEDOT:PSS).The main advantage of the  electropolymerization method is that the coating of the  polymer takes place only locally at the conductive electrode  surface, thus an additional structuring of the deposited flm is  not needed. The disadvantage is, however, that this deposition  method requires a conductive surface, where the formation of  the polymer can start. Therefore, the method is often used for additional coatings of electrodes e.g. for neural interfaces to  increase signifcantly the electrode capacity, which enables  electrical neural stimulation.

Recently, we established a microfabrication process to create vertical OECTs using electropolymerized PEDOT:PSS  thin flms. Therefore, a 300 nm thin polyimide insulation  layer is sandwiched between the circular source and ringshaped drain electrode, both made of sputtered gold [cf.  Fig. 5(a)]. After defning the area for electropolymerization  by coating and structuring another polyimide insulation layer  on top, the PEDOT:PSS flm is deposited by applying an  anodic potential at the source and drain electrodes, which  are short-circuited. Figure 5(b) shows an SEM image of the  structure after electropolymerization. The small gap between the source and drain electrodes is bridged by the PEDOT:PSS  flm, which is creating the efective OECT channel. With  these devices we measured max. transconductance values up  to 90 mS, which are limited by the parasitic serial and contact  resistances.

Figure 5

Conclusions

Organic Electrochemical Transistors have come a long way  ever since their invention.In the past, most progress was  made in optimization of materials and individual device  performance, alongside a furthering of understanding the  working mechanism and modeling of the device.

With growing maturity, translation of this progress into realworld applications has shifted into the focus of research. Integration of OECTs, however, is a balance of several, sometimes  competing requirements. OECT behavior depends strongly  on its geometric dimensions, in particular the thickness of  the transistor channel, which requires a high flm homogeneity and structuring fdelity. Furthermore, organic mixed conductors, as used in OECTs, are fragile and often damaged by  conventional lithography. Lastly, OECTs are often labeled as low-cost technology, and in order not to compete with other  technologies, have to be prepared without involving cost intensive processes.