The use of solar energy provides a promising alternative source of energy to conventional fossil fuels. In recent years, therefore, great attention has focused on the development of devices capable of converting solar energy to electricity, such as photovoltaic devices (also known as solar cells). Some different types of mature photovoltaic devices have been developed, some examples of which include silicon-based devices, III-V and II-VI PN junction devices, copper-indium-gallium-selenium (CICS) thin-film devices, organic sensitizer devices, organic thin-film devices, and cadmium sulfide/cadmium telluride (CdS/CdTe) thin-film devices. Publications show these devices in more detail (see, for example, Non-Patent Document 1). Unfortunately, there is still room to improve the photoelectric conversion efficiency of many of these devices. Problems being addressed by many researchers are to develop techniques for improving the efficiency.
In order to improve the conversion efficiency, a solar cell having a wavelength-converting function is studied, in which a certain wavelength of incident light (e.g., in the ultraviolet region) not contributable to photoelectric conversion is converted to a wavelength contributable to photoelectric conversion (see, for example, Patent Document 2). Such a study proposes a method of mixing a fluorescent material powder with a resin material to form a luminescent panel.
Wavelength-converting inorganic media for use in photovoltaic devices and solar cells have been disclosed so far, whereas there are few studies reported on the use of photoluminescent organic media to improve the efficiency of photovoltaic devices. In contrast to the use of inorganic media, the use of organic media is attracting attention because organic materials are typically more inexpensive and easier to use and therefore provide a more economical choice.
In addition, conventional wavelength-converting inorganic media have an insufficient degree of wavelength conversion (wavelength conversion efficiency or wavelength shift width between before and after conversion). For example, two or more wavelength-converting medium layers may be simply combined, or two or more wavelength-converting media may be simply mixed in a single layer. However, it has been found that in such a case, the wavelength-converting media themselves absorb even a wavelength that could otherwise be absorbed and used for photoelectric conversion by other wavelength-converting media or solar cells, due to their wavelength absorption properties, so that almost no improvement in photoelectric conversion efficiency can be obtained or even a negative effect such as a reduction in photoelectric conversion efficiency can be obtained.
In addition, some inorganic and organic fluorescent materials conventionally used are degraded and significantly reduced in wavelength-converting function when exposed to sunlight. Absorption and emission also need to be made at wavelengths suitable for the characteristics of solar cells, but it is difficult to provide optimal wavelengths.