Cobalt oxide (Co 3 O 4 ) thin films were fabricated by the drop-dry deposition (DDD) method using an aqueous solution containing Co(NO 3 ) 2 and NaOH. DDD was performed by dropping the solution on a substrate, heating-drying, and rinsing in water. The as-deposited film was Co(OH) 2 , and it was converted to Co 3 O 4 by annealing in air. The Raman and x-ray diffraction peaks of Co 3 O 4 were observed for the annealed films. We also fabricated Co 3 O 4 /ZnO heterojunction solar cells with ZnO prepared by the electrochemical deposition method. The heterojunction had rectification properties and a weak photovoltaic response. Thus, DDD is promising for fabricating Co 3 O 4 layers, and the Co 3 O 4 /ZnO heterojunction can be used as a solar cell.
Magnesium hydroxide (Mg(OH)2) thin films were deposited by the drop-dry deposition (DDD) method using an aqueous solution containing Mg(NO3)2 and NaOH. DDD was performed by dropping the solution on a substrate, heating-drying, and rinsing in water. Effects of different deposition conditions on the surface morphology and optical properties of Mg(OH)2 thin films were researched. Films with a thickness of 1−2 μm were successfully deposited, and the Raman peaks of Mg(OH)2 were observed for them. Their transmittance in the visible range was 95% or more, and the bandgap was about 5.8 eV. It was found that the thin films have resistivity of the order of 105 Ωcm. Thus, the transparent and semiconducting Mg(OH)2 thin films were successfully prepared by DDD.
Drop–dry deposition (DDD) is a method of depositing thin films by heating and drying the deposition solution dropped on a substrate. We prepared Ni(OH)2 precursor thin films by DDD and annealed them in air to prepare NiO thin films. The appropriate deposition conditions were found by changing the number of drop–dry cycles and the concentrations of chemicals in the solution, and the Ni(OH)2 precursor film with a thickness of 0.3 μm and optical transmittance of more than 95% was successfully deposited. Raman and X-ray diffraction measurements were performed, and it was found that the NiO film was successfully fabricated after annealing at 400 °C. The p-type conductivity of the annealed film was confirmed by photoelectrochemical measurements. In addition, we prepared n-type ZnO by electrochemical deposition on NiO thin films. The current–voltage measurement results show that the ZnO/NiO heterojunction had rectification properties.
The drop-dry deposition (DDD) is a simple chemical technique of thin film deposition, which can be applied to metal oxides. The deposition solution is an aqueous solution including a metal salt and an alkali. However, some metal ions react spontaneously with water and precipitate. This work is the first attempt to use complexing agents in DDD to suppress the precipitation. SnO2 thin films are fabricated using DDD with Na2S2O3 as a complexing agent and via annealing in air. The results of the Auger electron spectroscopy measurement show that the O/Sn composition ratio of the annealed films approached two, indicating that the annealed films are SnO2. The photoelectrochemical measurement results show that the annealed films are n-type. Co3O4/SnO2 heterojunction is fabricated using p-type Co3O4 films which are also deposited via DDD. The heterojunction has rectification and photovoltaic properties. Thus, for the first time, a metal oxide thin film was successfully prepared via DDD using a complexing agent, and oxide thin film solar cells are successfully prepared using only DDD.
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