1. Field of the Invention
The present invention relates to a method of producing a zinc oxide film, a method of producing a photovoltaic element, and a method of producing a semiconductor element substrate.
2. Related Background Art
A zinc oxide film is used in a photovoltaic element for converting sunlight to electric energy, a liquid crystal display and the like, as a transparent conductive film.
A conventional photovoltaic element composed of hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystalline silicon, polycrystalline silicon or the like, has utilized a reflection layer at the back of a semiconductor layer so as to improve the efficiency of collection in a long wavelength range. It is desirable that such a reflection layer exhibit reflection characteristics effective in a wavelength range of 800 nm to 1200 nm. This is because such a wavelength range is so near the end of a semiconductor material band that light in the range is slightly absorbed. Metals such as gold, silver, and copper meet this requirement.
Light is contained in a predetermined wavelength range in a semiconductor layer by providing an optically transparent irregular layer. In general, the transparent irregular layer is provided between the reflection layer and the semiconductor layer to effectively utilize reflected light and improve the short circuit current density Jsc.
To prevent a deterioration in characteristics due to a shunt path, a layer made of a light-transmissive material which has conductivity, that is, a transparent conductive layer is formed between the metal layer and the semiconductor layer. These layers generally formed by vacuum deposition or sputtering exhibit improvement in the short-circuit current density (Jsc) by 1 mA/cm2 or more.
As the examples of the above, the paper entitled xe2x80x9cEffects of Light Containment in a-SiGe Solar Cell on 29p-MF-22 Stainless Steel Substratexe2x80x9d (Compilation of Draft Papers for Lectures at the 51st Applied Physics Conference, p. 747, 1990), the paper entitled xe2x80x9cP-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cell with Bandgap Profilingxe2x80x9d (Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990), and the like discuss the reflectance and texture structure of reflection layers consisting of silver atoms. In the above papers, a short circuit current has been increased under the influence of light containment by a combination of an irregular reflection layer consisting of two silver layers deposited at different substrate temperatures, and a zinc oxide layer. The papers also discuss how to reduce a production time by depositing a film of good quality at a high speed and how to stabilize film production by forming an uniform film on a substrate, in consideration of productivity of a zinc oxide film.
The conventional photovoltaic element with a light containment layer as described above, which has excellent photoelectric conversion characteristics, has room for improvement because it does not fully utilize light. In other words, the reflectance for light in a long wavelength range of 800 nm to 1200 nm is not zero, so that a part of light to be contained is reflected into the air, thus causing losses.
When an irregular-shaped light containment layer utilizing crystalline irregularity is formed, a film consisting of grains of sizes that are effective in light containment is poor in adhesion to a base member, while a dense film which gives good adhesion to a base member does not fully function as a light containment layer.
By vacuum deposition and sputtering methods, commonly used for film formation, high speed deposition is carried out in order to increase productivity. For example, electric power input is increased to increase the number of active species for forming a transparent conductive layer. In this case, film crystallinity may decrease, so that crystals do not grow adequately. Thus grains become small, and the surface of the transparent conductive layer becomes flat, thereby resulting in insufficient scattering to contain light.
A transparent layer used for light containment is formed by the vacuum deposition method using resistor heating or electron beams, the sputtering method, or the CVD method. For example, in the CVD method, a space where active species are present is controlled with difficulty so that the shape of film on a substrate varies, thereby reducing a production efficiency. For the sputtering method, costs of producing sputtering target material and repayments of vacuumizing equipment are high, and the material using efficiency is low. Thus, the considerable costs of producing a photovoltaic element by these methods is a barrier for industrial applications of solar cells.
Japanese Patent Application Laid-Open No. 7-23775 and the paper entitled xe2x80x9cElectrolyte Optimization for Cathodic Growth of Zinc Oxide Filmsxe2x80x9d (Masanobu Izaki, Takashi Omi, Journal of Electrochemical Soc. Vol. 143, No. 3) report that to solve the problems described above, a transparent zinc oxide film was electrochemically deposited by applying current to a counter electrode immersed in a zinc nitrate solution. This method eliminates the need for expensive vacuumizing equipment and targets, thereby significantly reducing costs of producing a zinc oxide film. Since the method allows zinc oxide to deposit even on a substrate with a large area, it is promising for producing a large-area photovoltaic element, such as a solar cell.
A zinc oxide film formed by the above method is inexpensive but has the following problems.
(1) Abnormal growth of needlelike, spherical, and dendritic shapes in the order of microns or more are liable to form on deposits, especially when current density or solution concentration is increased. It is considered that when a zinc oxide film with such abnormal growths is used as a part of a photovoltaic element, it causes the shunt of a photovoltaic element.
(2) It is likely to locally vary the grain size distribution of zinc oxide. Therefore, there is a problem in film uniformity when a zinc oxide film is deposited on a large area.
(3) A zinc oxide film deposited by this method is inferior in adhesion to a base member to films deposited by the vacuum deposition using resistor heating or electron beams, sputtering, ion plating, and CVD methods.
(4) This method produces only a smooth film, not a deposited film with irregularities having the light containing effect.
The present invention has been accomplished so as to solve the problems as described above. It is an object of the present invention to stably form a zinc oxide film having an excellent light containment effect in a shorter time, compared with conventional methods, and produce a highly efficient solar cell at low cost by using a photovoltaic element containing such a zinc oxide film.
As the results of the intensive study of the present inventor for solving the above problem, the present inventor has found the fact that in the production of a zinc oxide film by electrodeposition, an electrodepositing bath is maintained at a temperature of 50xc2x0 C. or more and has a temperature profile such that the temperature of the electrodepositing bath is lower in the final stage of electrodeposition than in the initial stage of electrodeposition, thereby reducing a burnout voltage in the initial stage to make irregularities of a film surface larger. The present invention has been accomplished based on this fact.
The present invention provides a method of producing a zinc oxide film, which comprises applying current between a conductive base member immersed in an electrodepositing bath and a counter electrode immersed in the electrodepositing bath to form a zinc oxide film on the conductive base member, wherein the electrodepositing bath is maintained at a temperature of 50xc2x0 C. or more and has a temperature profile such that the electrodepositing bath is lower in the final stage of electrodeposition than in the initial stage of electrodeposition.
The present invention further provides a method of producing a photovoltaic element, which comprises the steps of: forming a zinc oxide film on a conductive base member by applying current between the conductive base member immersed in an electrodepositing bath and a counter electrode immersed in the electrodepositing bath; and forming a semiconductor layer, wherein the electrodepositing bath is maintained at a temperature of 50xc2x0 C. or more and has a temperature profile such that the electrodepositing bath is lower in the final stage of electrodeposition than in the initial stage of electrodeposition.
The present invention still further provides a method of producing a semiconductor element substrate, which comprises applying current between a conductive base member immersed in an electrodepositing bath and a counter electrode immersed in the electrodepositing bath to form a zinc oxide film on the conductive base member, wherein the electrodepositing bath is maintained at a temperature of 50xc2x0 C. or more and has a temperature profile such that the electrodepositing bath is lower in the final stage of electrodeposition than in the initial stage of electrodeposition.