Reducing Trap States in Printed Indium Zinc Oxide Transistors by Doping with Benzyl Viologen

Abstract

the slow formation of self-assembled monolayers. [13] The contact resistances before and after doping are compared by the transmission-line method. In addition to typical current-voltage characteristics, the devices are characterized through capacitance and admittance measurements as a function of frequency and applied bias, to gain insights into charge transport affected by trap sites. Analyses of the cutoff frequency and the parallel conductance reveal the effective mobility and the density of trap states, respectively, for comparisons of undoped and BV-doped TFTs. The Cole-Cole plot, based on the real and imaginary components of admittance, provides additional graphical analyses to understand the effect of BV treatment on interfacial states. Figure 1a shows the device structure in a bottom-gate, topcontact configuration with doped source/drain electrodes with BV molecules (shown in Figure 1b). The IZO semiconductor was deposited by inkjet, and the channel region was subsequently covered by an insulator polymer poly-4-vinylphenol (PVP). A BV solution was spin-coated onto the exposed areas, followed by a rinse with toluene to remove excess, not-ionized BV molecules. The BV treatment did not add measurable thickness, but the doping process increased wetting between the polar Ag ink and the IZO surface with ionized BV molecules. Silver ink on untreated IZO showed a contact angle of ≈30°, whereas the same Ag ink completely dispersed on the BV-treated surface, as shown in the photographs of Figure 1c. Figure 2a,b shows typical transfer and output characteristics of undoped and BV-doped TFTs. The BV-doped TFT shows a more negative threshold voltage and a higher off-current than the undoped TFT. If the entire channel is doped, the IZO becomes very conductive, and the applied electric field is not sufficient to modulate and deplete the channel. Hence, we restricted BV doping to areas underneath the source and drain contacts. There is a possibility that our process may allow the BV solution to wick underneath the PVP barrier and partially dope the channel region. However, the more likely explanation for the negative shift in threshold voltage is that the IZO layer is relatively thick at around 30 nm, and as such the BV doping leads to a very conductive area underneath the contacts that are difficult to deplete.The field-effect mobility μ is determined from the slope of the transfer curve at saturation bywhere I DS is the source-drain current, W is the channel width, L is the channel length, C g is the gate dielectric capacitance, An air-stable, strongly reducing molecule benzyl viologen (BV) is used to induce charge-transfer doping of the indium zinc oxide semiconductor in inkjet-printed thin-film transistors. The device mobility is improved from 5.8 ± 1.4 cm 2 V −1 s −1 in the undoped devices and reached up to 8.7 ± 1.0 cm 2 V −1 s −1 after BV treatment. Through measurement of frequencydependent admittance and capacitance, this work quantifies the density of interface states and shows that interfacial trap density is f...

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