Abstract
Monolayers of six alkylphosphonic acids ranging from C8 to C18 were prepared by vacuum evaporation and incorporated into low-voltage organic field-effect transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). Similar to solution-assembled monolayers, the molecular order for vacuum-deposited monolayers improved with increasing length of the aliphatic tail. At the same time, Fourier transform infrared (FTIR) measurements suggested lower molecular coverage for longer phosphonic acids. The comparison of FTIR and vibration frequencies calculated by density functional theory indicated that monodentate bonding does not occur for any phosphonic acid. All monolayers exhibited low surface energy of ∼17.5 mJ/m(2) with a dominating Lifshitz-van der Waals component. Their surface roughness was comparable, while the nanomechanical properties were varied but not correlated to the length of the molecule. However, large improvement in transistor performance was observed with increasing length of the aliphatic tail. Upon going from C8 to C18, the mean threshold voltage decreased from -1.37 to -1.24 V, the field-effect mobility increased from 0.03 to 0.33 cm(2)/(V·s), the off-current decreased from ∼8 × 10(-13) to ∼3 × 10(-13) A, and for transistors with L = 30 μm the on-current increased from ∼3 × 10(-8) to ∼2 × 10(-6) A, and the on/off-current ratio increased from ∼3 × 10(4) to ∼4 × 10(6). Similarly, transistors with longer phosphonic acids exhibited much better air and bias-stress stability. The achieved transistor performance opens up a completely "dry" fabrication route for ultrathin dielectrics and low-voltage organic transistors.
Highlights
The focused improvement of organic field-effect transistors (OFETs) has allowed a whole host of novel demonstrations including radio frequency identification tags,[1,2] analog and digital circuits,[3−5] active matrix displays,[6] and various sensor systems.[7−9] when the application of OFETs in forthcoming areas such as wearable or disposable electronics is considered, low power consumption and low-voltage operation are necessary features, especially for applications powered by batteries or energy-harvesting devices
Reduction in the gate dielectric thickness to 10 nm or less typically involves a bilayer, where a medium-k to high-k inorganic layer is covered with an organic monolayer whose function is to suppress the leakage current, inhibit the surface OH groups, and reduce the energy of the dielectric surface.[12−17] In such a case, the transistor operating voltage can be as low as 1.5 V, while the transistor is in the off state at 0 V.18−20
In this paper we report bottom-gate OFETs based on various Aluminum oxide (AlOx)/CnPA dielectrics and an air-stable[24,25] organic semiconductor, dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT)
Summary
The focused improvement of organic field-effect transistors (OFETs) has allowed a whole host of novel demonstrations including radio frequency identification tags,[1,2] analog and digital circuits,[3−5] active matrix displays,[6] and various sensor systems.[7−9] when the application of OFETs in forthcoming areas such as wearable or disposable electronics is considered, low power consumption and low-voltage operation are necessary features, especially for applications powered by batteries or energy-harvesting devices. Aluminum oxide (AlOx) functionalized with alkylphosphonic acids (CnPA) is an established bilayer dielectric for low-voltage organic transistors.[12] Such transistors have a bottom-gate structure where the aluminum oxide is commonly prepared by oxidation of the aluminum gate electrode and the assembly of the organic monolayer is performed in solvent-based solutions. In this paper we report bottom-gate OFETs based on various AlOx/CnPA dielectrics and an air-stable[24,25] organic semiconductor, dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT). This was prompted by previous research on solution-assembled CnPA that observed better transistor performance if longer-chain CnPA were used. The achieved transistor performance opens up a completely “dry” fabrication route for ultrathin dielectrics
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