1. Field of the Invention
The present invention relates to a photoelectric conversion device having on an anodized substrate a laminated structure constituted by a lower electrode, a photoelectric semiconductor layer, and an upper electrode. The present invention also relates to a solar cell using such a photoelectric conversion device.
2. Description of the Related Art
Currently, photoelectric conversion devices having a laminated structure constituted by a lower (underside) electrode, a photoelectric semiconductor layer, and an upper electrode are used in various applications including the solar cell. The photoelectric semiconductor layer generates electric current when the photoelectric semiconductor layer absorbs light.
Conventionally, the Si-based solar cell using bulk monocrystalline Si, bulk polycrystalline Si, or thin-film amorphous Si have been the mainstream of the solar cells. However, research and development semiconductor compound-based solar cells not using silicon are proceeding. The currently known semiconductor compound-based solar cells include the bulk type solar cells such as the GaAs-based solar cells and the thin-film type solar cells such as the CIS (Cu—In—Se)-based or CIGS (Cu—In—Ga—Se)-based solar cells. The CIS-based or CIGS-based solar cells use semiconductor compounds composed of a Group Ib element, one or more Group IIIb elements, and a Group VIb element, and are reported to exhibit high optical absorptance and high energy conversion efficiency. In this specification, the numbering of the groups of elements is in accordance with the short-period form of the periodic table.
Although, currently, the glass substrate is mainly used as the substrate in the solar cell, use of a flexible metal substrate is being considered. In the case where a metal substrate is used, it is necessary to form an insulation film on a surface of the substrate for preventing short circuiting between the substrate and an electrode or a photoelectric conversion layer formed over the substrate.
In order to suppress warpage and the like caused by thermal stress, it is preferable that the difference in the thermal expansion coefficient between the substrate and each layer formed above the substrate be small. Further, from the viewpoints of the difference in the thermal expansion coefficient between the substrate and each of the photoelectric conversion layer and the lower electrode, the cost, the characteristics necessary for the solar cell, and the like, it is preferable that the metal substrate contain aluminum as a main component.
Japanese Unexamined Patent Publication No. 2000-349320 (which is hereinafter referred to as JP2000-349320A) proposes use of an anodized substrate, which is produced by forming an anodized (Al2O3) film on an Al substrate. (See claim 9 in JP2000-349320A.) According to the technique disclosed in JP2000-349320A, even in the case where the area of the substrate is large, it is possible to easily form an insulation film over the entire surface of the substrate without a pinhole.
It is known that the crystallinity and the photoelectric conversion efficiency of the photoelectric conversion layers in the photoelectric conversion devices such as the CIS-based and CIGS-based photoelectric conversion devices can be improved by diffusing an alkali metal element or an alkaline earth metal element (preferably sodium) into the photoelectric conversion layers. Conventionally, sodium is diffused into the photoelectric conversion layers by using a substrate of soda lime glass (which contains sodium).
In the case where the anodized substrate disclosed in JP2000-349320A is used, the substrate contains neither the alkali metal element nor the alkaline earth metal element. Therefore, Japanese Unexamined Patent Publication Nos. 10 (1998)-074966, 10 (1998)-074967, 9 (1997)-055378 and 10 (1998)-125941, U.S. Patent Application Publication No. 20050056863, Japanese Unexamined Patent Publication Nos. 2006-210424, 2003-318424 and 2005-086167, U.S. Pat. No. 5,626,688, Japanese Unexamined Patent Publication Nos. 2004-158556 and 2004-079858, and U.S. Pat. No. 5,994,163 propose arrangement of a layer supplying an alkali metal element between a substrate and a photoelectric conversion layer. For example, arrangement of an alkali-metal supply layer on a lower electrode of molybdenum is proposed.
Nevertheless, even in the case where the alkali-metal supply layer is simply arranged between a substrate and a photoelectric conversion layer, it is difficult to diffuse the alkali metal element into the photoelectric conversion layer with satisfactory stability, efficiency, and repeatability, since the alkali metal element in the alkali-metal supply layer is diffused into the substrate in processes before formation of the photoelectric conversion layer. In addition, when an alkali metal element such as sodium is diffused into the anodized film, the anodized film can deteriorate, so that distortion of the photoelectric conversion device can increase after the formation of the photoelectric conversion layer, and microcracking or film exfoliation can occur in the photoelectric conversion layer.
Japanese Patent No. 4022577 (which is hereinafter referred to as JP4022577) discloses a process for producing a photoelectric conversion device using a glass substrate. In the process, a photoelectric conversion layer is doped with an alkali metal element before or during formation of the photoelectric conversion layer, and a diffusion barrier layer is arranged between a substrate and the photoelectric conversion layer for suppressing additional diffusion of the alkali metal element from the substrate to the photoelectric conversion layer during the process. (See claim 1 in JP4022577.) Further, JP4022577 discloses as a preferable diffusion barrier layer an insulating diffusion barrier layer formed of one of Al2O3, SiO2, Si3N4, ZrO2, and TiO2 (in claim 8 of JP4022577), and a conductive diffusion barrier layer formed of one of TiN, Pt, and Pd (in claim 9 of JP4022577).
However, the purpose of the provision of the diffusion barrier layer in JP4022577 is suppression of the additional diffusion of the alkali metal element from the glass substrate in the photoelectric conversion device. In other words, the diffusion barrier layer in JP4022577 is not provided for suppression of diffusion of the alkali metal element from an alkali-metal supply layer to the substrate.
In addition, conventionally, provision of a diffusion barrier layer for suppression of diffusion of an alkali metal element or an alkaline earth metal element in a photoelectric conversion device using an anodized substrate as disclosed in JP2000-349320A has not been reported.
Further, since the difference in the thermal expansion coefficient between the diffusion barrier layer having the composition disclosed in claims 8 or 9 in JP4022577 and the Mo electrode (which is commonly used as a lower electrode) is great, stress distortion occurs in the photoelectric conversion device, and can cause microcracking, film exfoliation, and the like. However, it is known that when the lower electrode is not a Mo electrode, no ohmic contact is realized, so that the photoelectric conversion efficiency is lowered.