Solution-Processed Inorganic Thin Film Transistors Fabricated from Butylamine-Capped Indium-Doped Zinc Oxide Nanocrystals

Abstract

Indium-doped zinc oxide nanocrystals (IZO NCs), capped with stearic acid (SA) of different sizes, were synthesized using a hot injection method in a noncoordinating solvent 1-octadecene (ODE). The ligand exchange process was employed to modify the surface of IZO NCs by replacing the longer-chain ligand of stearic acid with the shorter-chain ligand of butylamine (BA). It should be noted that the ligand-exchange percentage was observed to be 75%. The change of particle size, morphology, and crystal structures were obtained using a field emission scanning electron microscope (FE-SEM) and X-ray diffraction pattern results. In our study, the 5 nm and 10 nm IZO NCs capped with stearic acid (SA-IZO) were ligand-exchanged with butylamine (BA), and were then spin-coated on a thermal oxide (SiO 2 ) gate insulator to fabricate a thin film transistor (TFT) device. The films were then annealed at various temperatures: 350 °C, 400 °C, 500 °C, and 600°C. All samples showed semiconducting behavior and exhibited n-channel TFT. Curing temperature dependent on mobility was observed. Interestingly, mobility decreases with the increasing size of NCs from 5 to 10 nm. Miller-Abrahams hopping formalism was employed to explain the hopping mechanism insight our IZO NC films. By focusing on the effect of size, different curing temperatures, electron coupling, tunneling rate, and inter-NC separation, we found that the decrease in electron mobility for larger NCs was due to smaller electronic coupling.

Newly Observed Temperature and Surface Ligand Dependence of Electron Mobility in Indium Oxide Nanocrystals Solids

“…Remarkably, the TFT In 2 O 3 NC-2AP channel shows the excellent mobility achieved to date of 9.5 cm 2 /(V·s); I on / I off is above 10 7 , with a threshold voltage of −12 V; the subthreshold swing, which is a critical metric for how well the device turns on and off, is of 2.3 V/decade, which is comparable to the as-synthesized In 2 O 3 NC-OA. We can use the Einstein–Smoluchowski equation − to express the tunneling rate, Γ, which is proportional to the mobility, as follows: where m *, Δ E , and Δ x are the effective mass of the electrons, the height of the tunneling barrier, and the interdot distance between the NCs, respectively. Figure a presents the average mobility in the linear regime for the TFT devices fabricated using our method with 2AP, BA, and 4ABA organic ligands.…”

“…A negative shift in V T with an increase in temperature is explained by the generation of thermally activated carriers from traps within the band gap of the channel. Miller Abrahams provided the basis to calculate the mobility of the charge carrier in disordered colloidal NC solids at any temperature, ,, which is a product of the exponential distance and energy terms for which charge transport in NC solids occurs by a nearest-neighbor-hopping (NNH)-activated process, and a series of incoherent tunneling transitions take place between adjacent NCs. The relation σ = en μ, − indicates that the activation energy of such a thermally activated process is described by the following equation …”