GB2483445A

GB2483445A - Solar cell with luminescent material

Info

Publication number: GB2483445A
Application number: GB1014823.7A
Authority: GB
Inventors: Tomas Markvart
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 2010-09-07
Filing date: 2010-09-07
Publication date: 2012-03-14
Also published as: GB201014823D0

Abstract

A solar cell comprises a layer of photoconductive material 2, between two reflective layers 5,10; the reflective layers include a first wavelength-specific layer 10 for reflecting light of wavelengths greater than a cut-off value and a second fully reflective layer 5; a luminescent material 3 is provided within the structure for wavelength-shifting the incident radiation to wavelengths longer than the cut-off value of the first reflective layer 10, but below the absorption edge of the photoconductive material 2, where light is trapped between the two reflective layers 5,10 within the photoconductive material 2. The photoconductive material 2 is preferably silicon. The present invention allows for the path length of light within the photoconductive material 2 to be much greater for a relatively smaller thickness of material 2.

Description

SOLAR CELL

BACKGROUND

Technical Field of the Invention

The present invention relates to solar cells. More specifically, the present invention is concerned with solar cells which are able to trap incident radiation within the solar cell structure.

Description of Related Art

The scale of solar cell manufacture has increased substantially to the point that several square kilometres are produced every year. A challenge that remains is the cost of solar cells which is due largely to the large volume of semiconductor material involved, in most solar cells, crystalline silicon. The thickness of solar cells and the amount of material can be reduced with the use of light trapping structures that induce multiple passages of light through the solar cell and increase light absorption. Most commonly, light trapping structures are created by texturing of the front or rear surface of the solar cell. The texture may consist of a variety of elements which are ideally of random nature. In crystalline silicon solar cells, such structures are often achieved in the form of small pyramids etched chemically at the surface. Such schemes aim to randomise the directions of light rays inside the semiconductor so that a large number of rays are reflected by Total Internal Reflection and are trapped inside the semiconductor layer. However, a substantial proportion of reflected light can still escape from the structure.

SUMMARY OF THE INVENTION

The present invention therefore provides a structure including a layer of photoconductive material between two reflective layers. The reflective layers include a first wavelength-specific layer for reflecting light of wavelengths greater than a cut-off value and a second fully reflective layer. Luminescent material is provided within the structure for wavelength-shifting the incident radiation to wavelengths longer than the cut-off value of the first reflective layer, but below the absorption edge of the photoconductive material. In this way light is trapped between the two reflective layers and the path-length of the radiation within the photoconductive material increased over structures of similar thickness which do not have means to trap radiation. The luminescent material and photoconductive material are arranged to maximise the optical coupling between both materials, so that light emitted from the luminescent material is able to couple directly into the photoconductive layer.

More specifically, in a first aspect a solar cell for converting incident solar radiation to electrical energy comprising; a first wavelength-specific reflective layer for reflecting solar radiation of wavelength generally greater than a cut-off value, and a second reflective layer for reflecting all wavelengths of solar radiation, and a layer of photoconductive material provided between the first and second reflective layers, whereby the first and second reflective layers are able to trap radiation of wavelengths greater than said cut-off value within the photoconductive material, the solar cell further comprising a luminescent material also provided between the first and second reflective layers, wherein said luminescent material is capable of shifting the wavelength of the incident radiation to a wavelength generally greater than the cut-off wavelength of the first reflective layer.

The photoconductive material may have a generally planar structure comprising first and second opposing faces having the largest surface area of the faces and wherein the luminescent material is provided as a layer in contact with the first or second opposing faces of the photoconductive material. This enables the luminescent material to be applied directly, simplifying the production process. Furthermore the luminescent material will not interfere with the electronic properties of the photoconductive material in this position.

The layer of luminescent material may be optically coupled to the face of the photoconductive material so that reflection between the two layers is minimised. Optical coupling means matching the refractive indexes of the two layers.

Alternatively the luminescent material may be embedded in the photoconductive material as inorganic luminescent centres such as lanthanide ions, quantum dots or wells or as organic dye centres. In this way the light emitted from the luminescent material stands the greatest chance of being absorbed by the silicon before being reflected by one of the reflective layers.

A light-absorbing material may be provided between the two reflective surfaces.

At least one face of the photoconductive material may have a textured surface which can add to the probability that emitted or reflected light will undergo total internal reflection.

The textured surface may be an etched pyramid structure.

An air gap may be provided between photoconductive layer and first reflective surface. The luminescent material may be a luminescent dye or a semiconductor. The first reflective layer may be a dielectric mirror, dichroic mirror, photonic crystal or wavelength filter. The thickness of the photoconductive layer is in the range 0.5 -20 pm or more preferably in the range I -2 pm. The photoconductive material is crystalline silicon or multicrystalline silicon.

The luminescent material may comprise a single type of material or may comprise a mixture of at least two types of luminescent materials which emit radiation at different wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic of a light trapping scheme according to the prior art.

Figure 2 is a schematic of a solar cell showing reflective layers and luminescent layer in accordance with the first embodiment of the present invention.

Figure 3 is a schematic of a solar cell in accordance with a further embodiment of the present invention where the luminescent material is embedded in the photoconductive material.

Figure 4 is a schematic showing the path of light emitted from the luminescent material shown in Figure 3.

Figure 5 is a schematic of a solar cell in accordance with a still further embodiment of the present invention.

Figure 6 is a schematic showing the path of light emitted from the luminescent material shown in Figure 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 is a schematic of a known solar cell with light trapping, comprising a flat illuminated surface 1, reflecting rear textured surface 2, both relating to a semiconductor layer 3. 4 and are metal contacts to extract electric current. Regions 6 and 7 are highly alloyed (doped), one with a donor impurity, the other with an acceptor impurity, to separate the electron-hole pairs which are photo-generated in the semiconductor layer 3. Region 8 (sometimes called the escape cone) shows schematically the angular range of rays, with an apex angle 2O (sin = 1/n, where n is the refractive index of the semiconductor (for example, n 4.3 for silicon at the peak of the solar spectrum). Rays within the escape cone (such as ray 9) experience low reflection and are likely to escape from the layer 3. Rays outside the escape cone (such as ray 10) are reflected by total internal reflection, and are trapped in semiconductor layer 3.

This results in a large proportion of the radiation escaping from the structure.

In an embodiment of the present invention, as shown in Figure 2, the solar cell comprises a semiconductor layer 2 having a flat surface 1. The semiconductor layer is of a photoconductive material such as, but not limited to, silicon. Absorbing and luminescent centres 3 are dispersed throughout the semiconductor layer 2. The rear surface 5 of layer 2 is highly reflecting and not wavelength specific in order to reflect light of all wavelengths.

There are top contacts 6 and 7 and doped regions 8 and 9 as in prior art solar cell. The top surface I is covered with a wavelength-specific reflective layer such as a photonic mirror or photonic crystal structure 10 which reflects light of a wavelength greater than a cut-off value.

Figures 3 is a schematic diagram of another embodiment of the present invention where the absorbing and luminescent centres or substance form a layer 3 which may have a similar refractive index as semiconductor layer 2. The layer may be located adjacent to the rear reflective surface 5 or it may be located adjacent to the front reflective surface 10.

The luminescent layer centres emit light at a wavelength slightly shorter than the wavelength A9which corresponds to the absorption edge of the semiconductor which is used in the manufacture of the solar cell. For example, if the solar cell is from crystalline silicon with absorption edge near 1.1 jim, the luminescence should occur at shorter wavelength than 1.1 jim., for example, in the range 0.9 -I.09pm. The luminescent layer may consist of organic dyes such as phthalocyanides, chlorophylls or similar, inorganic luminescent centres, quantum dots, quantum wells or it may comprise another semiconductor with bandgap equal to or slightly higher than the semiconductor corresponding to the solar cell.

The solar cell may be a conventional solar cell but with contacts which do not impede greatly the transmission of light through the solar cell structure manufacture for example by the point-contact technology. The positive and negative contacts may be both on the illuminated (front) surface, or both on the rear surface, or on alternate sides of the solar cell. The solar cell may be very thin. For example, if manufactured from mono-crystalline or multi-crystalline silicon the solar cell the thickness may be in the range 0.5 -20 jim, preferably in the range I -2jim. The solar cell may be manufactured from n-type silicon or p-type silicon or intrinsic silicon. The diffused regions near the positive contact will be formed typically by diffusion or ion implantation of a acceptor impurity such a boron. The diffused regions near the negative contact will be formed typically by diffusion or ion implantation of a donor impurity such a phosphorus. The semiconductor substrate for the solar cell may be obtained by sawing of a wafer from ingot, or by deposition by physical or chemical means, or by pulling a ribbon from melt.

Figure 5 is a schematic diagram of a further embodiment of the present invention comprising flat surface I of semiconductor layer 2. An absorbing and luminescent layer 3 is adjacent at the rear of layer 2, with a textured interface 4. The rear surface 5 of layer 3 is highly reflecting. There are top contacts 6 and 7 and doped regions 8 and 9 as in prior art solar cell. The top surface 1 is covered with a photonic mirror or photonic crystal structure 10. The textured surface may be formed by anisotropic chemical etching to form small pyramids which may be regular or inverted, or by sand blasting [A. F. St. John, U.S. Patent No. 3,487,223 (December 30, 1969)] or similar. The textured surface may be formed on the opposite side of the photoconductive material layer to the luminescent layer, or both surfaces of the photoconductive layer may be textured.

In all embodiments the photonic mirror is such that it reflects ideally all light with wavelength longer than some wavelength 2, which is shorter than the wavelength 2g of the absorption edge of the solar cell semiconductor. Light with wavelengths shorter than A ideally passes through the photonic mirror without absorption or reflection. For crystalline silicon solar cells where 2g l.lim, 2 may be within the range 0.5 -1.0 m, more specifically, within the range 0.8-1.ORm. Such photonic mirrors can be manufactured by photonic crystal technology [ref.

J.D. Joannopoulos, et al, Photonic Crystals: Molding the Flow of Light, Princeton University press, 2008] The term luminescence refers to reemission of the absorbed light at a longer wavelength by fluorescence, phosphorescence or other similar light-emitting mechanisms in organic and inorganic semiconductors.

The path of light rays in the light trapping structure in the present invention is shown in Fig. 4. An incident ray 1 enters the semiconductor solar cell layer 2 with absorbing / luminescent centres. As the semiconductor absorbs light much less than the absorbing centres, the ray will in all likelihood be absorbed by one such centre. and reemitted as luminescence at a longer wavelength. By virtue of isotropic emission, the angular distribution of directions of rays emitted from the luminescent material will be nearly random, distributed over a full hemisphere.

Similarly, the path of light rays in the light trapping structure of the embodiment shown in Figure 5 are followed Figure 6. An incident ray I enters the semiconductor solar cell layer 2.

As this layer may be very thin, this ray will be in all probability transmitted and not absorbed in the layer. The ray will then enter the highly absorbing layer 3 where it will be absorbed and reemitted as luminescence at a longer wavelength. By virtue of the rear side mirror 4, the emitted ray 6 will be reflected towards the semiconductor layer 2. By virtue of the textured interface 5, the angular distribution of directions of this ray will be nearly random, distributed over a full hemisphere.

Supposing that the photonic mirror is exterior to the semiconductor, a fraction (1-1 In2) which are incident outside the escape cone (such as ray 6a in Figure 4, 6 in Figure 6), where n is the refractive index of the semiconductor solar cell material, will be reflected from the top surface of the semiconductor. A fraction 1/n2 (rays within the escape cone) will be transmitted through the interface 8 between the semiconductor solar cell material and the external space. From this transmitted fraction, a large fraction of rays (such as ray 6b, 7) will be reflected by the photonic mirror 9. Only a small fraction of rays (such as ray 6c) will escape the semiconductor layer 2. The total number of rays reflected is substantially higher than in the structure according to prior art where the wavelength of the incident ray has not been shifted by luminescence, and light at this longer wavelength not reflected by the photonic mirror.

The effective path length of light in the structure of Figure 2 is equal to Ii 2 £ 47rn2d -where n is the refractive index of the solar cell semiconductor, d is the thickness of the solar cell, kB is the Boltzmann constant (1.381 x I 23 j K1), T0 is the temperature of the absorbing layer in degrees Kelvin, frequency v0 = c/A0 is the cut-off frequency of the photonic mirror, V9 = c/A9 = E9 / h, where h is the Planck constant (6.626 x 1 O" Jsec), E9 is the bandgap of the semiconductor and Av = v0 -V9. The potentially very large term in braces is due to presence of the absorbing layer and photonic mirror, and represents the improvement in light trapping due to the present invention. The factor 4zrn2 d gives the optical path length in the

light trapping structure of prior art.

It is to be understood that various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art.

Thus, the present invention is not intended to be limited to the embodiment shown and such modifications and variations also fall within the spirit and scope of the appended claims.

Claims

CLAIMS1. A solar cell for converting incident solar radiation to electrical energy comprising; a first wavelength-specific reflective layer for reflecting solar radiation of wavelength generally greater than a cut-off value, and a second reflective layer for reflecting all wavelengths of solar radiation, and a layer of photoconductive material provided between the first and second reflective layers, whereby the first and second reflective layers are able to trap radiation of wavelengths greater than said cut-off value within the photoconductive material, the solar cell further comprising a luminescent material also provided between the first and second reflective layers, wherein said luminescent material is capable of shifting the wavelength of the incident radiation to a wavelength generally greater than the cut-off wavelength of the first reflective layer.
2. A solar cell in accordance with claim 1, wherein the photoconductive material has a generally planar structure comprising first and second opposing faces having the largest surface area of the faces and wherein the luminescent material is provided as a layer in contact with the first or second opposing faces of the photoconductive material.
3. A solar cell in accordance with claim 2, wherein the layer of luminescent material is optically coupled to the face of the photoconductive material.
4. A solar cell in accordance with claim 1, wherein the luminescent material is embedded in the photoconductive material.
5. A solar cell in accordance with any preceding claim, wherein a light-absorbing material is provided between the two reflective surfaces.
6. A solar cell in accordance with any preceding claim, wherein at least one face of the photoconductive material has a textured surface.
7. A solar cell in accordance with claim 6, wherein the textured surface is an etched pyramid structure.
8. A solar cell in accordance with any preceding claim, wherein an air gap is provided between photoconductive layer and first reflective surface.
9. A solar cell in accordance with any preceding claim, wherein the luminescent material is a luminescent dye or a semiconductor.
10. A solar cell in accordance with any preceding claim, wherein the first reflective layer is a dielectric mirror, dichroic mirror, photonic crystal or wavelength filter.
11. A solar cell in accordance with any preceding claim wherein the thickness of the photoconductive layer is in the range 0.5 -20 pm.
12. A solar cell in accordance with any preceding claim wherein the thickness of the photoconductive layer is in the range 1 -2 pm.
13. A solar cell in accordance with any preceding claim wherein the photoconductive material is crystalline silicon or multicrystalline silicon.
14. A solar cell in accordance with any preceding claim wherein the luminescent material comprises a mixture of at least two types of luminescent materials.
15. A solar cell substantially as described herein with reference to Figure 1 to 6.