GB1562994A - Photovoltaic device - Google Patents

Description

(54) PHOTOVOLTAIC DEVICE (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to photo-electric semiconductor devices, and in particular to a semiconductor photo-voltaic cell provided with a phosphor adapted to extend the spectral range of the cell.

Semiconductor devices for converting light, e.g. solar, energy directly to electricity suffer from the disadvantage that they can utilize only a fraction of the solar spectrum.

Materials which have a relatively large energy band-gap respond to the blue end of the spectrum. Materials with a relatively small band-gap absorb at the red or infrared portion of the spectrum, but are temperature sensitive and are usually unresponsive to short wavelengths as absorption then takes place too far from the junction to produce free carriers.

The object of the invention is to minimize or to overcome this disadvantage.

According to the invention there is provided a photo-voltaic arrangement, including a semiconductor device having at least one surface junction and adapted to generate an electric current in response to incident light, and a phosphor material in contact with or adjacent said surface, and in which said phosphor material, in response to light irradiation in first frequency bands, emits light in one or more further frequency bands, said further frequency band or bands corresponding to the spectral sensitivity of the semiconductor device, in which said phosphor material includes a mixture of upconverting and down-converting phosphors, and in which the absorption characteristic of the phosphor material corresponds to those regions of the spectrum to which the semiconductor device is relatively insensitive.

Many phosphors, both organic and inorganic, phosphoresce or fluoresce under short wavelength radiation, re-emitting at longer wavelengths. Other phosphors upconvert by absorbing at long wavelengths and re-emiting at a short wavelength. A particularly useful property of the up-converting phosphors is that their conversion efficiency generally increases with radiation intensity.

The absorption bands of the phosphor material are in those regions of the spectrum to which the semiconductor is insensitive, and the emission band in the spectral region to which the semiconductor is sensitive. The phosphor material does not of course emit at a single wavelength. Its function is to convert wavelengths that cannot be absorbed, or cannot be absorbed efficiently, by the cell, to a band or bands of wavelengths that are more readily absorbed.

An embodiment of the invention will now be described with reference to the drawing accompanying the Provisional Specification which is a schematic of a solar cell energy conversion arrangement. The arrangement may be employed in the solar energy conversion systems described in our co-pending application No. 1960/76 (Serial No. 1,556,381), No. 23995/76 (Serial No. 1,552,671) and No.

23272/76 (Serial No. 1,562,912).

Referring to the drawing, the arrangement includes a photovoltaic device 11, e.g. a silicon or gallium arsenide solar cell, with or without an optical system 12 for directing light on the device junction. In the optical path is placed a wavelength conversion cell 13 comprising a phosphor material 14, which is a mixture of up-converting and down-converting phosphors, as a solution in a liquid or transparent solid, or in particulate form and dispersed in a liquid or solid 15 of matching refractive index so as to minimize unwanted light scattering from the phosphor particles 14. In response to radiation received via the optical system 12 the phosphor material emits light at a wavelength suitable for absorption by the photovoltaic device 11.

In some applications it is advantageous to separate most of the emitted fluorescent light from the incident light that is not absorbed in the phosphor material. The incident light is either roughly parallel if it arrives at the phosphor direct from the sun, or convergent if it is focussed by a lens.

By contrast, the fluorescent radiation is emitted isotropically. This difference in ray directions can be used to separate the two components by optically piping one of them making use of critical angle reflections at the surfaces of the phosphor medium, or by suitably positioned optical elements. In this way the fluorescent light can be directed to a second solar cell, which cell need not be of the same material as the first. If the spectral response of the second solar cell is matched to the phosphor output, the cell will be largely protected against excessive heating, thereby widening the choice of usable solar cell materials.

WHAT WE CLAIM IS:- 1. A photo-voltaic arrangement, including a semiconductor device having at least one surface junction and adapted to generate an electric current in response to incident light, and a phosphor material in contact with or adjacent said surface, and in which said phosphor material, in response to light irradiation in first frequency bands, emit light in one or more further frequency bands, said further frequency band or bands corresponding to the spectral sensitivity of the semiconductor device, in which said phosphor material includes a mixture of upconverting and down-converting phosphors, and in which the absorption characteristic of the phosphor material corresponds to those regions of the spectrum to which the semiconductor device is relatively insensitive.

2. An arrangement as claimed in claim 1 and in which the semiconductor device is a silicon or gallium arsenide solar cell.

3. An arrangement as claimed in claim 1 or 2, and in which the phosphor material is in particulate form dispersed in a transparent liquid or solid.

4. An arrangement as claimed in any one of claims 1 to 3, and which includes an optical system adapted to direct light on to the device junction.

5. A photovoltaic arrangement substantially as described herein with reference to the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. In some applications it is advantageous to separate most of the emitted fluorescent light from the incident light that is not absorbed in the phosphor material. The incident light is either roughly parallel if it arrives at the phosphor direct from the sun, or convergent if it is focussed by a lens. By contrast, the fluorescent radiation is emitted isotropically. This difference in ray directions can be used to separate the two components by optically piping one of them making use of critical angle reflections at the surfaces of the phosphor medium, or by suitably positioned optical elements. In this way the fluorescent light can be directed to a second solar cell, which cell need not be of the same material as the first. If the spectral response of the second solar cell is matched to the phosphor output, the cell will be largely protected against excessive heating, thereby widening the choice of usable solar cell materials. WHAT WE CLAIM IS:-

1. A photo-voltaic arrangement, including a semiconductor device having at least one surface junction and adapted to generate an electric current in response to incident light, and a phosphor material in contact with or adjacent said surface, and in which said phosphor material, in response to light irradiation in first frequency bands, emit light in one or more further frequency bands, said further frequency band or bands corresponding to the spectral sensitivity of the semiconductor device, in which said phosphor material includes a mixture of upconverting and down-converting phosphors, and in which the absorption characteristic of the phosphor material corresponds to those regions of the spectrum to which the semiconductor device is relatively insensitive.

2. An arrangement as claimed in claim 1 and in which the semiconductor device is a silicon or gallium arsenide solar cell.

3. An arrangement as claimed in claim 1 or 2, and in which the phosphor material is in particulate form dispersed in a transparent liquid or solid.

4. An arrangement as claimed in any one of claims 1 to 3, and which includes an optical system adapted to direct light on to the device junction.

5. A photovoltaic arrangement substantially as described herein with reference to the drawing accompanying the Provisional Specification.