Amorphous In–Ga–Zn Oxide Semiconducting Thin Films with High Mobility from Electrochemically Generated Aqueous Nanocluster Inks

Solution processing is a scalable means of depositing large-area electronics for applications in displays, sensors, smart windows, and photovoltaics. However, solution routes typically yield films with electronic quality inferior to traditional vacuum deposition, as the solution precursors contain excess organic ligands, counterions, and/or solvent that leads to porosity in the final film. We show that electrolysis of aq. mixed metal nitrate salt solutions drives the formation of indium gallium zinc oxide (IGZO) precursor solutions, without purification, that consist of ∼1 nm radii metal–hydroxo clusters, minimal nitrate counterions, and no organic ligands. Films deposited from cluster precursors over a wide range of composition are smooth (roughness of 0.24 nm), homogeneous, dense (80% of crystalline phase), and crack-free. The transistor performance of IGZO films deposited from electrochemically synthesized clusters is compared to those from the starting nitrate salt solution, sol–gel precursors, and, as a control, vacuum-sputter-deposited films. The average channel mobility (μ AVE ) of air-annealed cluster films (In:Ga:Zn = 69:12:19) at 400 °C was ∼9 cm 2 V –1 s –1, whereas those of control nitrate salt and sol–gel precursor films were ∼5 and ∼2 cm 2 V –1 s –1, respectively. By incorporating an ultrathin indium–tin–zinc oxide interface layer prior to IGZO film deposition and air-annealing at 550 °C, a μ AVE of ∼30 cm 2 V –1 s –1 was achieved, exceeding that of sputtered IGZO control films. These data show that electrochemically derived cluster precursors yield films that are structurally and electrically superior to those deposited from metal nitrate salt and related organic sol–gel precursor solutions and approach the quality of sputtered films.