A p-type semiconductor CuO film with a bandgap energy of 1.4 eV has been prepared by anodic electrodeposition in a basic aqueous solution containing copper nitrate hydrate and ammonium nitrate at 297 K, and the structural, optical, and electrical characterizations were carried out. The randomly-oriented CuO film prepared on a transparent conductive glass substrate showed electrical characteristics of a 1.26 × 10₃ Ω cm in resistivity, 2.11 × 10₁₆ cm₋₃ in carrier concentration, and 0.234 cm₂ V₋₁ s₋₁ in mobility, and a slight photocurrent generation could be observed during light irradiation. The (002)-oriented CuO film could be prepared on the (111)-oriented Au/Si wafer substrate and possessed an excellent photoactivity of a large photocurrent density and quick response compared to those for the randomly oriented CuO film.

The concept of bandgap science of organic semiconductor films for use in photovoltaic cells, namely, high-purification, pn-control by doping, and design of the built-in potential based on precisely-evaluated doping parameters, is summarized. The principle characteristics of organic solar cells, namely, the exciton, donor (D)/acceptor (A) sensitization, and p-i-n cells containing co-deposited and D/A molecular blended i-interlayers, are explained. ‘Seven-nines’ (7N) purification, together with phase-separation/cystallization induced by co-evaporant 3^rd molecules allowed us to fabricate 5.3% efficient cells based on 1 µm-thick fullerene:phthalocyanine (C₆₀:H₂Pc) co-deposited films. pn-control techniques enabled by impurity doping for both single and co-deposited films were established. The carrier concentrations created by doping were determined by the Kelvin band mapping technique. The relatively high ionization efficiency of 10% for doped organic semiconductors can be explained by the formation of charge transfer (CT)-complexes between the dopants and the organic semiconductor molecules. A series of fundamental junctions, such as Schottky junctions, pn-homojunctions, p⁺, n⁺-organic/metal ohmic junctions, and n⁺-organic/ p⁺-organic ohmic homojunctions, were fabricated in both single and co-deposited organic semiconductor films by impurity doping alone. A tandem cell showing 2.4% efficiency was fabricated in which the built-in electric field was designed by manipulating the doping.

The (111)-p-Cu₂O/(0001)-n-ZnO heterostructure was successfully fabricated using a low-temperature electrodeposition method on an Au(111)/Si(100) substrate. The structural, optical and electrical characterizations were carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, transmission electron microscope-energy dispersive X-ray analysis, optical absorption, photoluminescence, and electrical measurements. Isolated hexagonal columnar ZnO grains deposited at a low cathodic current density and the interface between the ZnO and Cu₂O film were clearly observed. The (0002)-oriented continuous ZnO film was deposited on the (111)-Cu₂O film at higher cathodic current density, but the metallic Cu layer was formed between the ZnO and Cu₂O films by reducing Cu₂O to Cu₀ during the ZnO deposition. The Cu₂O/isolated-ZnO heterostructure showed an ohmic feature, and the (111)-Cu₂O/Cu/(0002)-ZnO heterostructures showed an excellent rectification with the rectification ratio of ∼10₅.

Electrochemical processes in aqueous solutions are widely used for preparation of metals, alloys, composites, metal oxides and compounds. For understanding and designing an electrochemical process, it is crucial to study its soluble chemical species, potential-pH diagram, and solubility curves drawn based on thermodynamics. In this review, equilibrium electrode potentials, critical pH values, and dissolved chemical species related to the oxidation-reduction, acid-base, and ligand-exchanging reactions, in addition to the calculation based on standard Gibbs free energy are first briefly mentioned. This is followed by the description of the change in equilibrium electrode potentials of metal and metal compounds as demonstrated in the electrochemical preparation of the Cu–In–Se precursor for the Cu(In,Ga)Se₂ solar cell application. Additionally, the advantages and usefulness of soluble chemical species, potential-pH diagram, and solubility curves are discussed, by giving examples of direct electrodepositions of metal oxides, the chemical introduction of impurities into ZnO enabling characteristic control, the chemical bath deposition process (CBD) for Zn(S,O,OH) buffer layer in Cu(In,Ga)Se₂ solar cell, and lastly, the design of the electrochemical process for fabricating CuO/Cu₂O bilayers.