It is well known that photovoltaic cells use specially prepared semiconductor junctions to convert energy from sunlight to electricity.
It is also known that a specific photovoltaic semiconductor junction utilizes, for conversion to electricity, only a portion of the spectrum of energy available in the available light. For example, the conversion of sunlight to electrical energy using well known silicon photovoltaic cells is strongly dependent upon the conversion of light with energy at or above 1.1 electron volts while the lower energy light also present is, at least in part, converted to heat instead of electricity. The heat generated can reduce the efficiency of the silicon cell for the conversion of the higher energy light to electricity.
Some of the sunlight which penetrates the solid light transmitting member of a conventional photovoltaic array is lost such as by passing through an interstice between adjacent photovoltaic cells, reflection back out of the member, absorption, and the like.
Accordingly, current photovoltaic arrays receive more energy input from incident light than they retain for conversion into electrical output, and it is highly desirable to increase the amount of light an array retains for such conversion.
One known approach for increasing the capacity of photovoltaic arrays to convert light energy to electricity is to employ one or more solid luminescent agents in the solid light transmitting member. Such agents, when exposed to sunlight, take in light from one direction and emit lower energy light in numerous directions. Examples of such agents are organic dyes such as the dyes heretofore employed in scintillation counters, lasers, and the like.
The particular luminescent agent or agents employed in conjunction with an array of specific photovoltaic cells are chosen, inter alia, for their ability to emit light in an energy level range which suits the conversion characteristics of that cell. This way, a portion of the light that would otherwise be lost for electrical generation by transmission, reflection, and the like is transformed by the luminescent agent into multi-directional light that is more readily retained in the light transmitting member and which the photovoltaic cell or cells can readily convert into electricity, thereby increasing the overall efficiency of the array.
A photovoltaic array which employs this approach is referred to as a luminescent photovoltaic array. Such an array usually employs fluorescent dyes, fluorescence being that species of luminescence wherein the emitted light is usually in the visible spectrum. However, other species of luminescence exist. Phosphoresence (light emission continues after the stimulating light has stopped) is one such species.
It should be understood that this invention covers all species of luminescence, as well as all types of luminescent agents.
Heretofore the prior art has required the photovoltaic cell or cells employed to be mounted on the thin edge of a solid, plastic or glass, light transmitting luminescent member. Such a device is fully and completely disclosed in Applied Optics, Volume 15, No. 10, Pages 2299 and 2300, October, 1976, the disclosure of which is incorporated herein by reference.