Enhanced Charge Injection Properties of Organic Field‐Effect Transistor by Molecular Implantation Doping

Abstract

memories, and organic field-effect transistors (OFETs), have various advantages including mechanical flexibility, low cost, solution-processed fabrication, and tunable material functionalities by molecular design compared with silicon-based materials. [1][2][3][4][5][6][7][8][9][10][11][12][13] However, the contact resistance problem arising between organic materials and metal electrodes has been one of the dominant obstacles for adopting organic semiconducting devices instead of silicon-based devices. Diverse attempts, for instance, self-assembled monolayer (SAM) treatment on metal electrodes, [14][15][16][17][18][19] inserting a charge injection layer between OSC and metals, [20][21][22][23][24][25][26][27] choice of metals for better injection properties, [28,29] adopting carbon-based conductor like graphene as electrodes, [30] have been introduced to improve carrier injection across typically a non-ohmic contact. Especially, considering large operation voltages required for OFETs, improving contact properties of organic/metal interface is an essential step for practical applications of OSCs.Contact doping is one of the most effective techniques to reduce contact resistance and has been widely employed in silicon-based devices and recently in OSCs to reduce the contact resistance. [31][32][33][34][35][36] In order to avoid undesirable OFF currents, it needs to be performed selectively, i.e., in localized regions at the source-drain contacts only and not in the channel region. The doped regions have been usually confined to the top surface of the OSC film by depositing a small amount of dopants on the top of the organic film by thermal evaporation. As a result, the position of the gate dielectrics was normally restricted to the top side of devices (i.e., FETs in a top-gate structure) in order to enhance the charge injection from metal electrodes to the accumulation layer formed on the top surface of the polymer. [31,32] Recently, the combination of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) as host and dopant material, respectively, has produced a highly conducting polymer that has been studied as a candidate for a synthetic metal and high power-factor thermoelectric material. [37][38][39][40][41] Interestingly, this combination achieved an efficient bulk-doping of PBTTT by solid-state diffusion which implied that the F 4 -TCNQ Organic semiconductors (OSCs) have been widely studied due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. However, OSC devices still have to overcome contact resistance issues for better performances. Because of the Schottky contact at the metal-OSC interfaces, a non-ideal transfer curve feature often appears in the low-drain voltage region. To improve the contact properties of OSCs, there have been several methods reported, including interface treatment by self-assembled monolayers and introducing charge injection layers. Here, a selectiv...

Highly Stable Contact Doping in Organic Field Effect Transistors by Dopant‐Blockade Method

“…In order to suppress the dopant diffusion in the channel region of the PBTTT OFETs, we adopted dopant-blockade molecules in the F 4 -TCNQ-doped-contact PBTTT OFETs, which are fabricated in the same way as our previous work. [44] The dopant-blockade molecules should be selected to avoid the charge transfer reaction with PBTTT molecules for maintaining the electrical properties of the doped-contact PBTTT OFETs (denoted as "DC-FET") after introducing dopant-blockade molecules in the PBTTT channel. Figure 1b,c shows the values of the highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level of F 4 -TCNQ and TCNQ.…”

“…Recently, we have developed a surface etching treatment for suppressing the dopant diffusion in doped-contact poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno [3,2b]thiophene) (PBTTT) OFETs. [44] This system exploited a facile and efficient bulk-doping of PBTTT via solid-state diffusion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) which resulted in high-conductivity and high carrier-concentration regions in spatially selected regions in OFETs. [45] This doping method effectively reduced the contact resistance (by a factor of 5) and showed its potential by demonstrating the low-voltage operation organic transistor.…”

Electrode and dielectric layer interface device engineering study using furan flanked diketopyrrolopyrrole–dithienothiophene polymer based organic transistors

“…This in turn thereby decreases I on /I off ratio upon PFBT treatment (Fig. 6 ) 12 .

Figure 6 PDPPF-DTT based OFETs band diagram ( a ) without and ( b ) with PFBT treatment on Au surface.

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“…The essential surge, in the performance of the OFETs can be modulated via thin film interface engineering using chosen organic semiconductor material 5 . Accordingly, there are various ways to enhance the overall OFETs performance, including the development of new organic donor–acceptor semiconductors 6 – 10 , relevant semiconductor doping 11 , implant doping 12 , use of high dielectric constant materials 13 , 14 , various material deposition processes 15 , 16 , and use of self-assembled monolayers (SAM) treatments 4 , 5 , 17 , 18 . In most of the above-mentioned strategies, it has been highlighted that charge carrier transport in polymer thin films is largely influenced by the structural and energetic disorders.…”

Recent Advances on Conductive 2D Covalent Organic Frameworks

“…Despite these significant progresses, the electrical performances of FETs based on the conductive 2D-COFs cannot match those made from traditional organic conductive polymers, such as poly(3-hexylthiophene), copper Pc, TTF, tetracyanoquinodimethane, and so on. [133][134][135][136][137][138][139][140][141][142] Although singlelayer 2D-COFs are theoretically predicted to offer a charge carrier mobility up to 95 cm 2 V −1 s −1 , [143] the experimental studies often rely on the crystalline 2D networks with micropowder or multi-layer morphologies. [18,92,118] In spite of the fact that some few-layer conductive 2D-COFs have charge carrier mobilities measured up to 20.6 cm 2 V −1 s −1 , [57,93,94] great efforts are required to improve the performance of COFs based FETs for large-scale electronics.…”