WO2011062500A1

WO2011062500A1 - Method for manufacturing photovoltaic solar cell and a multifunctional photovoltaic solar cell

Info

Publication number: WO2011062500A1
Application number: PCT/NO2010/000371
Authority: WO
Inventors: Renato Bugge
Original assignee: Integrated Solar As
Priority date: 2009-10-22
Filing date: 2010-10-22
Publication date: 2011-05-26
Also published as: NO20093193A1

Abstract

The invention relates to design, growing and processing photovoltaic solar cells in several layers of AIGaInAsSb materials on a substrate. The invention also relates to multi-functional solar cell assembled by at least three layers of IIl-V materials, where at least two layers of the IM-V materials are doped with Te, Se, B or Si. At least one layer is preferably of the penternary material AIGaInAsSb and the substrate of Si. Further, described is a concentrator system so that the solar cell receives primary radiation (direct radiation from the sun) or secondary radiation (radiation from an object being heated).

Description

METHOD FOR MANUFACTURING PHOTOVOLTAIC SOLAR CELL AND A MULTIFUNCTIONAL PHOTOVOLTAIC SOLAR

CELL

The invention relates to manufacturing of photovoltaic solar cells according to the preamble of claim 1. Especially the present invention relates to designing, growing and processing of photovoltaic solar cells in several layers of AIGalnAsSb materials on a silicon substrate.

The present invention further relates to a multifunctional solar cell according to the preamble of claim 33. Especially the invention relates to photovoltaic solar cells composed of at least three layers of 11 - V materials, where at least two layers of the lll-V materials are doped with Te, Se, B or Si. At least one layer is preferably of the penternary material AIGalnAsSb and the substrate of Si.

Background

Today different semiconductor materials as silicon, Ge, GaAs and similar lll-V materials are used for solar cells. For silicon one is using thick layers of materials as this is an indirect transition semiconductor, while it for direct band gap semiconductors mainly are used thin layers. The presumption for both to work as a solar cell is that they have an un-doped or low-doped absorption area (intrinsic - i) which lies in contact with two areas which are negative (n-type) and positive (p-type) doped semiconductor material. This transition makes it possible to produce power.

For silicon solar cells the development has been focused on producing thick disks (200-500 pm) to a low cost where one to a large extent has improved the properties by reducing dark current and loss of power by optimizing contacts and surfaces, and increased the purity of the material. As the materials have indirect band gap the light will be absorbed in a large part of this material, so that one cannot use thin layers. Silicon is thus limited to solar cells having one single transition. For solar cells based on lll-V materials one has larger possibilities as these materials have indirect band gap, absorbing the light better, and accordingly produces power at smaller thickness. For thick layers of lll-V materials the light will be absorbed within a few pm of the upper layer, so that thick substrates of lll-V materials do not provide any advantage.

A p-i-n (or p-n) transition of such thin lll-V layers having a given band gap, absorbs light having energy equal to or higher than the band gap. If the energy of the light becomes much higher, only a part of the light will be absorbed and generate power. Figure 1 schematically shows how the absorption for a single p-i-n transition is; strong near the band gap, transparent in the band gap and falling over the band gap.

One possible way to increase the absorption of sunlight in lll-V solar cells has been to use several p-i-n transitions. Figure 2 shows a triple transition solar cell structure where different layers are absorbing light having different energy/wavelength. Light having highest energy is absorbed by transition 20 which is a material having a relatively high band gap. Figure 3 shows this as band gap 1. Light having energy lower than band gap 1, goes through this transition and hits transition 21 which has an average band gap (Figure 2). Figure 3 shows this as band gap 2. At the bottom of Figure 2 (nearest a substrate 23) lies transition 22 having lowest band gap (band gap 3 in Figure 3). In total, a triple transition will have high absorption of light in a larger energy area (such as shown in Figure 3) than for single p-i-n transitions.

Figure 4 shows typical incoming solar effect; the energy of the light is scattered to lie near the band gap of one single material. This results in that lll-V materials do not provide high efficiency if one use only one composition/p-i-n transition. To improve this one has chosen to use several compositions of lll-V materials to provide one or more p-i-n transitions.

Traditionally lll-V materials as GaAs, GalnAs, AIGaAs, GalnSb, AIGaAsSb, GalnAsSb or similar have been used for solar cells. These have been deposited on substrates of GaAs, GaSb, InAs, InP and similar binary substrates. Recently one has started to use substrates for depositing lll-V materials on silicon, but due to silicon and lll-V materials having different crystal structures this result in an amount of defects which make them less suitable for solar cells.

Solar cells of materials as GalnAs and GaSb have been tested with different heating sources for generating power from these. The efficiency of such single transition solar cells has not shown commercial value, and is thus not in use in the market for solar cell plants. The reason for this is that any heating source can be considered as a black radiating body, and it has thus been more efficient to use the sun as this source.

From EP 0177903 it is known a semiconductor device having gallium arsenic grown on a silicon substrate and a method for manufacturing this.

WO2009090653 describes a spectrum manipulation device and method for increased energy conversion from a solar cell.

Object

The main object of the present invention is to combine a silicon substrate of relative low costs with the lll-V solar cells higher efficiency.

An object of the invention is to provide a method for manufacturing photovoltaic solar cells which solves the above mentioned problems of prior art.

An object of the invention is to provide a method including growing a layer of monocrystalline lll-V materials directly on the silicon substrate.

An object of the invention is to use the silicon substrate as a mechanical carrier for the lll-V materials, thermal contact and/or electric contact, while the different lll-V layers effectively generates power. An object of the present invention is to provide a multifunctional solar cell which can utilize both primary radiation (directly from the sun) and secondary radiation (radiation from an object which is heated) for generating power.

An object of the invention is to utilize secondary radiation where the radiation comes from sunlight during the day and another heating source during the night, such that a large commercial plant can produce power 24 hours.

An object of the invention is to increase the efficiency of a heat-based solar cell radiation system by, in an improved manner, attending to the heating effect which radiates from the heating source, by reflecting radiation which cannot be used back to the heating source.

An object of the invention is to reduce the costs of power production by combining the high efficiency of lll-V solar cells, increasing the efficiency by using secondary radiation, reducing the costs by using the reasonable priced silicon substrate of the solar cells, and reducing the costs by utilizing large mirrors in a concentrator system. The invention

A method for manufacturing photovoltaic solar cells according to the invention is described in claim 1. Preferable features of the method are described in claims 2-32.

A multifunctional solar cell according to the invention is described in claim 33. Preferable features of the solar cell are described in claims 34-65.

The invention describes a method for manufacturing photovoltaic solar cells which includes depositing penternary lll-V materials on a silicon substrate having a low content of defects, so that two or more transitions (see triple transition, Figure 5) of AIGalnAsSb materials can be used on a cell/substrate for producing power. In Figure 5 it is shown a sketch of band gap area for

AIGalnAsSb limited by the binary extreme points, where direct band gap transitions with the exception of AIAs and AlSb are marked with *, while indirect transitions are marked with #. The dotted line intersects solid lines in different ternary materials having a grating constant of a=5.95A.

The advantage of using the penternary material AIGalnAsSb before other lll-V materials is that one can change band gap and grating constant in a large area, and at the same time one can keep the same material in several transitions. This is not possible with other types of materials and is an advantage both in the manufacturing of material and the finishing treatment of this. This is especially important for units having more than two transitions (e.g. five or seven transitions) where one must grow materials with both high band gap (i.e. AIAs-like) and low band gap (i.e. InSb-like). For manufacturing a photovoltaic solar cell according to the invention, a disk of monocrystalline silicon, preferably having a 221-surface, is arranged in a deposition plant where it is heated and exposed for damps from different furnaces of gallium, aluminium, indium, arsenic and antimony, phosphorus and/or nitrogen. For different layer are also added a small amount (<10 at%) of silicon, beryllium, selenium or tellurium which affects the resulting material, so that it can be added charge carriers for electrons (n-type doping) or holes (p-type doping). The result of the deposition process is different layers of lll-V materials where some of these consist of the compound material AIGalnAsSb, such as in the example shown in Figure 6.

Figure 6 shows that the AIGalnAsSb materials are deposited directly on silicon, but that one uses a super grating consisting of two lll-V materials as intermediate layers. Material 1 has a grating constant which is larger than material 2, so that the two materials provide alternating layers having mutual tension. This tension reduces the number of defects in the top layer.

In Figure 4 it is shown an example where one started with a low content of Al and As in the bottom layer. This layer has thus a low band gap and absorbs light with longest wavelength. In the intermediate layer a material having low content of Al, but high content of As is arranged. The material thus have higher band gap and absorbs light having intermediate energy. The last layer has approximately the same grating constant as the intermediate (see Figure 2), but having a high content of Al and As and absorbs light having highest energy. The three different layers thus absorb light within three different energy areas even though they consist of the same material. The manufacturing of the structure in the example above is not obvious, as one has silicon at the bottom, a layer with a new grating constant above this and two layers having a third grating constant above this again. The grow technique used for solving this includes growing a material having a given fraction of the mutual grating constants (e.g. 15/14 for material with 5.660A and 6.064A grating parameters). In this way one can incorporate a set of defects in the transition between the different layers which solves the fraction. In practice one will get some defects even after this, but by adjusting the growing process one can get the most to stop. Such defects which do not stop can propagate during the growing of the material. The result is then that they will act as electrical "channels" through the semiconductor structure and destroy the capability of the ^' material to work as a good solar cell. For limiting this problem one add transitions there the material composition changes gradually from one composition to another.

For preventing the remaining defects from propagating through the structure one uses a digital growing technique where one grows thin layers of AIGalnAsSb and quaternary, ternary and/or binary layers. These layers are preferably thinner than the distribution function of the electron in the material, so that the electrons experience the material as an average material. The many thin layers will of the defects be experienced as barriers and act as a defect stop grating which deflects and prevent that they propagate through the structure.

After the entire semiconductor structure is grown, the material is threated thermally for crystallizing remaining defects and possibly improving the charge carrying properties of the material. This brings the semiconductor towards thermodynamic equilibrium, so that the solar cell does not change characteristics through the operating time of the components. The process also improves the charge carrying concentration and mobility of charge carriers in the material, as shown for GaSb and GaAs in Figure 5.

A method for manufacturing photovoltaic solar cells according to the invention can be summarized as follows:

i. arranging a thin substrate of monocrystalline silicon in a vacuum deposition plant, ii. heating the substrate and exposing it to damps from different furnaces of materials from group III and V of the periodic system, and

iii. growing at least three different compounded lll-V monocrystalline semiconductor layers on the substrate, where the first grown layer on the silicon consists of

AllnASxSbj.x,

AIGaAS_xSbi.,, or AIGalnAs_xSbi._x with x>=0 and x<0.5.

The method can further include growing at least one semiconductor layer of intrinsic, p-type or n-type material, which is compounded of at least two layers of lll-V materials in an alternating super structure.

The method can further include growing at least four different compounded lll-V

monocrystalline semiconductor layers on the silicon substrate.

The method can further include growing at least five different compounded lll-V

monocrystalline semiconductor layers on the silicon substrate.

The method can further include growing the first semiconductor layer so that the transition between the silicon substrate and the first grown semiconductor layer includes a defect layer which provides a low content of defects in the grown semiconductor layer.

The method can further include growing the semiconductor layers so that the transition between two grown semiconductor layers contain a defect layer which provides a low content of defects in the last grown semiconductor layer.

The method can further include growing a second semiconductor layer consisting of AlSb, GaSb, or InSb on the Silicon substrate.

The method can further include growing a second semiconductor layer consisting of AllnSb, AIGaSb, GalnSb or AIGalnSb on the silicon substrate. The method can further include growing a second semiconductor layer consisting of AlAs_xSbi.x, GaAs_xSbi._x, InASxSbi.,,, AllnAsxSbi._x, AIGaAs_xS_bi._x, GalnAs_xSb_!._x or AIGalnAs_xSbi._x with x>=0 and x<0.5 on the silicon substrate.

The method can further include growing a third semiconductor layer consisting of AIAs, GaAs or InAs on the silicon substrate.

The method can further include growing a third semiconductor layer consisting of AllnAs, AIGaAs, GalnAs or AIGalnAs on the silicon substrate.

The method can further include growing a third semiconductor layer consisting of

GaAs_xSb_!._x, lnAs_xSbi._x, AllnAs_xSbi._x, AIGaAs_xSbi._x, GalnASxSbx._x or AIGalnASxSbj.,, with x>0.5 and x<=l on the silicon substrate.

The method can further include:

i. growing lll-V material(s) having one or more p-n, p-i-n, p-p-n or p-n-n transition(s), ii. growing the semiconductor layers so that at least two defect layer are formed between the different lll-V materials between the silicon and the lll-V transition,

iii. depositing a silicon substrate which contains both p-type and n-type material.

The method can further include using a silicon substrate having a p-n, p-i-n, p-p-n or p-n-n transition and grow semiconductor layers of lll-V materials having one or more p-n, p-i-n, p-p-n or p-n-n transitions for providing a multi-transition solar cell.

The method can further include growing the semiconductor layers so that two of the defect layers between the lll-V materials are separated by at least one p-n, p-i-n, p-p-n or p-n-n transition.

The method can further include growing the semiconductor layers so that at least one lll-V transition consist of AIAs_xSbi._x, GaAs_xSbi._x, AllnAs_xSbi._x, AIGaAs_xSb_!._x, GalnAs_xSbi._x, GalnAs.Pi._x, AlGalnAs_xPi._x and/or AIGalnASxSbi._x with x>0.5 and x<=l.

The method can further include growing the semiconductor layers so that at least one lll-V transition consists of AIAs_xSbi._x, GaAs_xSbi._x, AllnAS_xSb_!.,,, AIGaAs_xSbi._x, GalnAs^b^, GalnAs_xPi.x, AIGalnAs_xPi._x and/or AIGalnAS_xSb^ with x>0.9 and x<=l.

The method can further include doping the silicon substrate with B, P, Ga, As and/or Sb for forming intrinsic, p-type and/or n-type layers in the transition.

The method can further include using a silicon substrate having a lOO-surface of crystallographic orientation with 1-5 degrees miscut.

The method can further include using a silicon substrate having a lOO-surface of crystallographic orientation with 2-4 degrees miscut.

The method can further include using a silicon substrate having a Ill-surface of crystallographic orientation with 1-5 degrees miscut. The method can further include using a silicon substrate having a 221-surface of crystallographic orientation.

The method can further include adding silicon, beryllium, selenium or tellurium in amount(s) being less than 10 at% of the grown material.

The method can further include exposing the silicon substrate for damps of gallium, aluminium, indium, arsenic, antimony, phosphorus and/or nitrogen.

The method can further include doping/adding to at least two of the lll-V materials with silicon, beryllium, selenium or tellurium for providing n-type and p-type contact layers.

The method can further include arranging metal contacts of Pd, Sb, Ti, Pt, Au, Al, Cu and/or Ag in contact with the semiconductor layers on the silicon substrate.

The method can further include using a layer of silicon at the bottom, one layer of lll-V material having a new grating constant over this and several layers of two lll-V materials having a third grating constant over this again.

The method can further include growing lll-V materials having a given fraction of their mutual grating constants for incorporating a set of defects in the transition between the different materials solving the fraction.

The method can further include arranging transitions there the material composition changes gradually from one composition to another.

The method can further include growing thin layers of AlGalnAsSb, quaternary, ternary and/or binary layers which act as a defect stop grating which deflects the defects and prevent that they propagate through the structure.

The method can further include growing layers being thinner than the wave distribution function of the electron in the material, so that the electrons experience the material as an average material.

The method can further include treating the material thermally for crystallizing remaining defects and possibly improve the charge carrying properties of the material after the entire structure is grown.

The solar cells described above can be used for both visible, in near-infrared, infrared and mid- infrared light. In combination with a concentrator one will increase the efficiency of the solar cells, and at the same time as the area of the solar cells can be reduced in favor of increased areas of mirrors. The novel of this invention is that one in addition uses an absorber and one or more filters for generating secondary infrared heat radiation (possibly visible light if the temperature is high enough). The filters are preferably multi-layer filters having low reflection within the spectral range where the solar cell has the highest reflectivity. Outside this spectral range the filter has high reflectivity, something resulting in that the heat radiation in this area is reflected back to the absorber, and re-absorbed. The system thus attend to radiation energy which the solar cell cannot use or have low conversion efficiency for.

Further advantages and preferable features of the present invention will appear from the following example description.

Example

The present invention will below be described in detail with references to the non-limiting attached Figures, where:

Figure 1 is a schematic presentation of absorption of light from a single p-i-n or p-n transition, where the absorption layer will either be the i-layer (for p-i-n) or a low-doped layer (for p-n), Figure 2 is an example of a triple-transition lll-V solar cell on a substrate,

Figure 3 is a schematic presentation of absorption of light from a triple p-i-n transition, Figure 4 shows a typical distribution of solar incoming effect,

Figure 5 is a sketch of band gap area for AIGalnAsSb, limited by binary extreme points, where direct band gap transitions are marked with * and indirect transitions are marked with ,

Figure 6 shows a view where the AIGalnAsSb materials are deposited directly on silicon, but one uses a super grating consisting of two materials as intermediate layers,

Figure 7 is an example of a set penternary lll-V materials in a triple-transition on silicon,

Figure 8 is a view showing absorbing layers which consist of two materials 1 and 2 in a super grating structure,

Figure 9 is a schematically view of a quantum well having two AIGalnAsSb semiconductors with two different band gaps 1 and 2,

Figure 10 shows X-ray diffraction (XRD) measurements of the asymmetrical (226) top of lll-V solar cell grown on a 221-surface of silicon,

Figure 11 shows a schematic presentation of absorption from a double-transition solar cell of hetero structure material or quantum wells,

Figure 12 shows band gaps vs. grating constants for different semiconductors,

Figure 13 shows X-ray diffraction (XRD) measurements of three samples of GaAs/GaSb/AISb on silicon,

Figure 14 shows Hall mobility measurements for n-type (left) and p-type (right),

Figure 15 shows an optical microscopic picture of defect density measurements of type (Etch Pit Density),

Figure 16 shows literature values for doped GaAs of n-type (left) and p-type (right),

Figure 17 shows a secondary radiation solar cell according to a first embodiment of the invention, Figure 18 shows a secondary radiation solar cell according to a second embodiment of the invention,

Figure 19 shows a secondary radiation solar cell according to a third embodiment of the invention,

Figure 20 shows a secondary radiation solar cell according to a fourth embodiment of the invention,

Figure 21a-b show alternatives for the secondary radiation solar cell in Figure 15,

Figure 22 shows a secondary radiation solar cell according to a fifth embodiment of the invention,

Figure 23 shows a secondary radiation solar cell according to a sixth embodiment of the invention,

Figure 24 shows photoluminescence measurements of a solar cell structure given by table 1, grown on a 221-surface of monocrystalline silicon, and

Figure 25 shows photoluminescence measurements of a solar cell structure given by table 2, grown on a 221-surface of monocrystalline silicon.

Table 1 below shows a «single transition⁾) solar cell where the structure is compounded by only one p-n transition. The different layers are however more complicated than solar cells which are made of several homogenous layers as one grows «digitally» by alternating two materials(Materials 1 and 2 in table 1). The grating constants under «Material 1» is near GaAs, while the grating constants under «Material 2 » are chosen between 50 at% and 95 at% As. Layers 1, 2, 3, 6, 7 and 8 have thin super grating layers which results in that the electrons «see» the materials in these layers as homogenous materials. For layers 4 and 5 the layers are thicker so that one gets a quantum well structure; the wells being under «Material 2», while the barriers are under « Material 1». The composition of layer 4 is dependent of the indium amount «3x».

Layer Material 1 & thickness Material 2 & thickness Total layer no. thickness

7 GaAs:n+ 0,72 nm GaAsSb:n+ 1,08 nm 10 nm

6 lo.sGao ₅As:n 0,72 nm Alo.sGa₀.slnAsSb:n 1,08 nm 200 nm

5 Alo.35Ga_{0 65}As:n 3,6 nm Alo.3sGa₀.65AsSb:n 5,4 nm 100 nm

4 Alo.35Ga₀.₆₅As:p 3,6 nm Alo.3s-xGa₀.₆₅.2xl n3xAsSb:p 5,4 nm 1000 nm

3 lo.35Gao 6₅As:p+ 0,72 nm Alo -xGa_{0 6}5-2xln3xAsSb:p+ 1,08 nm 200 nm

2 GaAs:p+ 0,72 nm GaAsSb:p+ 1,08 nm 100 nm

1 GaAs 1,37 nm AIGalnAsSb 0,76 nm 300 nm

silicon substrate

Table 1.

The bottom layer in this example is compounded of the materials «GaAs» and «AIGalnAsSb», such as shown schematically in i Figure 6. This layer acts as an adhesion layer against the silicon substrate, and the two materials have a grating constant which is considerably different. This provides high tension in the material, something which helps in stopping defects during MBE- growing of the structure. The material 2 «AIGalnAsSb» in layer 1 has thus a low amount of As and/or high amount of In to provide a grating constant which is considerably higher than for GaAs. Layer no. 2 in the example is a highly Be-doped layer of digitally grown «GaAs» and «GaAsSb» which is used as contact layer against the p-contact which typically is a metal contact of Ti/Pt/Au.

Layer no. 3 in the example is a Be-doped layer of digitally grown «AIGaAs» and «AIGalnAsSb». The layer will have an average grating constant which lies in the middle of the two materials, such that the absorption layer (layer 4) has a well-defined buffer layer. Layer 2 acts also as a buffer layer, but since the materials are different from the ones used in layer 3 and 4, it always is a variance due to inaccuracies in calibration. This is avoided by that layer 3 has the same materials as layer 4.

Layer no. 4 is an absorption layer of low Be-doped materials with barriers of «AIGaAs» and wells of «AIGalnAsSb». The layer is thus not a digitally grown layer, but utilizes the quantum effect achieve a higher band gap than a thick, homogenous layer of the quantum well material. A well allows that one choose a grating constant which is larger without the absorption wavelength increasing too much (even though the band gap is reduced). The dotted line in Figure 5 shows the grating constant for the example. Increased grating constant is important to be able to get lower in band gap (by adding In), so that one stepless can change the absorption wavelength over a larger area. Larger span in the absorption wavelength results in that more of the sunlight is absorbed, and results in a more effective solar cell. There are two methods for utilizing this:

i. The band gap of the well material and/or width of the well can be changed through the absorption layer such that the solar cell absorbs more wavelengths. This then becomes a single- transition solar cell.

ii. The band gap of the well material and/or the width of the well can be changed in different absorption layers with different p-n or p-i-n transitions. This then becomes a multi-transition solar cell.

In the example in table 1 method i) is used, so that one has a single-transition solar cell.

Layers no. 5 and 6 are Te-doped digitally grown layers with «AIGaAs» and «AIGaAsSb». They act as an electron transport layer towards layer 7, which is a contact layer. Layer no. 6 acts in addition as a window as it contains a higher amount of Al (50 at%) so that the light is not absorbed by this (AIGaAs and AIGaAsSb have indirect band gap for Al-content over ca. 35 at%).

The n-contact will preferably consist of Ti/Pt/Au and be connected to layer 7.

Reference is now made to Figure 7 which shows an example of a set of penternary lll-V materials in a triple-transition on silicon. The different absorbing layers are here represented as homogenous layers of one material. Over and under each absorbing layer there is a n- and p-type doped layer which contribute to electron and hole transport, respectively. The transition between n-type and p-type layers are a layer which has a high dope concentration of n-type and/or p-type, and which thus forms a conductive layer between the different absorption layers.

Reference is now made to Figure 8 which shows an absorbing layer consisting of two materials 1 and 2 in a super grating structure. The layers 1 are thinner and has smaller band gap than the layers 2, such that the layers 1 forms quantum wells with the layers 2 as barriers.

Reference is now made to Figure 9 which shows a schematic sketch of a quantum well having two AIGalnAsSb semiconductors of different band gaps 1 and 2. Material 1 has a band gap which is smaller than material 2, but due to the quantum well structure the band gap will lie closer to material 2 than it would have done without the well structure.

Reference is now made to Figure 10 which shows X-ray diffraction (XRD) of the asymmetric (226) top. The side tops come from the quantum well structure, and reflect a grating structure of 9 nm (layers 4 and 5 in table 1).

Table 2 below shows a double-transition solar cell, where the bottom transition is similar to the one given in table 1. This is mainly to be considered as two solar cells arranged above each other, where one uses high-doped layers (layers 7 and 8) as a junction contact. This result in that the n- contact of the bottom cell (layers 2-7) is connected to the p-contact of the upper cell (layers 8-13). The structure of table 2 can thus be considered as two solar cells in series.

Table 2.

The solar cell at the top of table 2 (layers 8-13) have the same structure as the bottom cell, but has materials having a larger band gap (AIAs barriers and AIGalnAsSb wells). Especially the wells have >50 at% Al and >30 at% In, something which provides an absorbing band gap which is higher than for the bottom cell. This results in that one achieves two different absorption bands which convert sunlight of different wavelengths. Figure 11 shows this for both solar cells having hetero structure transitions and solar cells having quantum wells. Figure 11 shows that incoming sunlight 32 is absorbed by wide bands of hetero structure solar cells 34a, while quantum well solar cells 34b only absorb energies near quantum well transition. The absorption efficiency of the quantum wells are however higher than for hetero structures.

To achieve wider absorption bands for the quantum well solar cells 34a in the example one can use different well widths and/or different content of materials in the wells. A graded transition will allow higher absorption efficiency and wider absorption band for incoming light. When one is growing lll-V material directly on silicon one will get defects in the grown material, provided that these two materials do not have equal grating constant. This applies for all substrate materials, and one must thus stay within certain material compositions to be able to grow lll-V materials on commercially available substrates (Reference 1). Figure 5 shows grating constants and band gap for numerous lll-V materials. These are also shown in Figure 12 which also shows several other types of semiconductors.

By comparing the grating constant of lll-V semiconductors and silicon (a_Sr5.43A), one can see that there are only GaP, AIP og AIGaAsP materials having low amount of As which have grating constants being near enough, and which can be used for growing solar cells directly on silicon with a thick buffer (Reference 2). For other materials, as GaSb, AlSb and similar, the grating constant is considerably different, and the result of growing such a lll-V material on top of silicon is many errors in the grating structure and a high content of defects.

M. Umeno et al (Reference 3) showed that it theoretically was possible to realize a Si/AIGaAs solar cell having efficiency up to 33%, but got only 20% efficiency at practical growing and testing of the solar cell. They explained the poor efficiency by a defect density up to 2*10⁷ cm^"2, and emphasized that higher efficiency can be achieved if the defect density can be reduced.

A traditional method for lll-V growing has been to introduce thick buffers of materials where one can reduce the number of defects by increased thickness. This is not useful for Si-lll/V multi- transition solar cells, as the Si solar cell is arranged below the lll-V layers, and the light therefore would be absorbed in the buffer.

As an alternative to thick buffer layers for reducing the amount of defects, one can introduce a defect layer between the materials having a different grating constant. G. Balakrishnan et al. (Reference 4) showed that one could reduce the defect density for growing GaSb-based materials on Si by using defect layers between Si and AlSb. They achieved in this way a reduced defect density of ca. 5* 10⁵ cm^'2, something being 40 times better than what . Umeno et al. was able to do without defect layers.

According to the invention it is provided a method for and results of growing low-defect materials of GaAs-type grating constant on silicon by introducing several defect layers and several layers of lll-V material.

Reference is now made to Figure 13 which shows X-ray diffraction (XRD) of the GaAs material in the four-layer structure of table 1. The figure shows X-ray diffraction (XRD) of three samples having GaAs/GaSb/AISb on silicon. The figure shows intensity as a function of theta angle.

Reference is now made to Figure 14 which shows the result of Hall measurements of 2 urn thick Si-doped and Be-doped GaAs layers (in two different samples) which are n-type and p-type semiconductors, respectively. The figure shows Hall mobility measurements for n-type (left) and p- type (right), where the dope concentrations were n=l,0*10¹⁷ cm^"3 and p=3,8*10¹⁷ cm^"3, which resulted in a mobility of 3780 cm²/Vs and 211 cm²/Vs, respectively, in the two samples.

Reference is now made to Figure 15 which shows an optical microscopic picture of the surface at defect measurements of the type EPD (Etch Pit Density). The number of defects was measured to be 2000 cm^'2 in the best sample.

The XRD-measurement in Figure 13 shows samples without successful defect layer (wide top), with partly successful and successful defect layer (narrow top). The narrowest top has a width of 175 arc seconds, something being very low for lll-V on silicon.

The Hall measurements of Figure 14 show mobility values which lies at literature values for GaAs, which literature values are shown in Figure 16. As mentioned Figure 16 show literature values for doped GaAs of n-type (left) and p-type (right). The literature values are collected from Rode, D. L, Semiconductors and Semimetals, R. K. Willardson and A. C. Beer, eds., Academic Press, N.Y., vol. 10, 1975, p. 1. The measurements of the present invention are marked with circles and lies on the curves. The electrical properties of the material are thus very good and one can expect high power transport from solar cells grown on these samples.

Defect measurements show a defect density down to 2*10³ cm^'2. This is 250 times lower than what G. Balakrishnan (Reference 4) has reported, and is thus the lowest reported defect density of lll/V materials on silicon. The samples are thus very suitable for solar cells and other optical units of GaAs materials and lll-V materials having grating constants near GaAs.

Reference is now made to Figure 17 which shows a first embodiment of a secondary radiation solar cell according to the invention. A concentrator 31 is focusing the sunlight 32 down on an absorbing medium/absorber 33 which is heated. The medium emits infrared/visible radiation when it is heated. The infrared radiation hits a set of solar cells 34 which entirely/partly are enclosing the absorber 33 and converts this into power. The advantage with such a design is that one can make systems having secondary radiation being close to what a solar cell optimally will convert. The concentrator 31 can be parabolic either in one or two directions.

Reference is now made to Figure 18 which shows a second embodiment of a secondary radiation solar cell according to the invention. A fresnel or common optical lens 35 is focusing the sunlight 32 down on an absorbing medium/absorber 33 which is being heated. The

medium/absorber 33 emits infrared/visible radiation when it is heated. The infrared radiation hits a set of solar cells 34 enclosing the absorber 33 and converts this into power.

Reference is now made to Figure 19 which shows a third embodiment of a secondary radiation solar cell according to the invention. The sunlight is focused into a ball 36 absorbing this. A narrow aperture 37 in this ball 36 results in that a low amount of the incoming light escapes and that the amount of absorbed light therefore is high. The heat from the ball 36 hits the solar cells 34 around 38. The ball 36 can also be a cylinder or another enclosing chamber having an aperture for incoming radiation.

Reference is now made to Figure 20 which shows a fourth embodiment of a secondary radiation solar cell according to the invention. The absorber 33 emits secondary radiation 39 as a large black body or as an emitter, and which hits a filter 40. This filter 40 is covered by

antireflection/reflection coating which only lets through light/infrared radiation being within a narrow spectral range. This light hits the solar cells 34 converting the light/infrared radiation to power. A lot of the light which is not let through the filter 40 will be reflected back to the absorber 33 and contribute to heating this. Secondary radiation will in this way conserve more of the energy and become more efficient. As an alternative the filter can be a coating of the solar cells 34.

Reference is now made to Figures 21a-b which show alternative embodiments of the secondary radiation solar cell of Figure 20. Instead of a circular or spherical design, the secondary radiation cell can be segmented in different geometrical parts enclosing each other. One then gets either solar cells 34 or filters 40 enclosing the absorber 33 as a ball-like object, or with an extension as a cylinder. Different geometrical shapes can be combined to achieve a secondary radiation cell as effective as possible.

Reference is now made to Figure 22 which shows a secondary radiation solar cell according to a fifth embodiment according to the invention. The light is focused in between two plates 41 which act as an absorber. The secondary radiation from the heated plates 41 hits the filter 40 before a narrow band of this radiation hits the solar cells 34.

A concentrator 31 is focusing the sunlight inn towards an absorbing medium (absorber) 33, 37, 41 as described in the Figures 11-17. The absorbing medium 33, 37, 41 is preferably made of silicon carbide, tantalum, molybdenum or wolfram. The energy the medium is receiving can be up to 1 sun per square meter concentrator area, and the most of this will reach the absorber 33, 37, 41. When the surface absorbing is low-reflective, it is preferable to have a geometry having a small inlet aperture (as in Figures 19, 20, 21 and 22) to get the highest possible absorption.

A system can obtain an absorber temperature of 1500 K. If one assumes that the absorber 33, 37, 41 radiates as a black body, but with an emissivity of 0.7 this will have an emittance of 200 kW/m² and radiance of 64 kW/m²'Sr for all wavelengths. The temperature provides a maximum emission at 1.93 μιη wavelength with a maximum spectral radiance of ca. 21.7 kW/m² sr pm.

A system having the solution of Figure 22 and an absorber plate size 41 of 100 cm² will thus produce ca. 1000 Watt per side (total). With a band-pass filter which lets through 0.5 pm near 1.93 pm, ca. 1/6 part of the radiation will reach the solar cell, i.e. ca. 166 Watt. By the use of a fresnel lens, elliptical mirror or similar optical arrangement, one will be able to concentrate this down on a 1 cm² large solar cell. By the use of a commercial GaSb based solar cell from JX Crystals Inc., the efficiency will be 75- 80 % from 1.2 μιη to 1.7 m. At 1500 K the effect which is let through the filter from 1.2 μ η to 1.7 μιη will be ca. 12 kW/m² pr. side, i.e. ca. 120 Watt for a 10 cm x 10 cm large plate. Concentrated down to 1 cm² one can assume that some effect is lost in lens/optical component, so that 70-75 % of this effect can be used to produce power.

Reference is now made to Figure 23 which shows such a possible solution by the use of a curved mirror, and shows a secondary radiation solar cell according to a sixth embodiment according to the invention. The secondary radiation from the heated plates 41 hits the filter 40 before a narrow band of this radiation hits the concentrator 31 and is focused down on the solar cell(s) 34. The secondary radiation from upper absorption plate is not included in the figure for simplification. The reflecting filter 40 will likewise give some loss in the form of absorption, so one can assume that this will provide a solution with a maximal 60-70 % utilization of the energy of incoming light in this example. This is much better than present solar cell solutions, and can also be optimized with regard to emitter, filter and solar cell.

Reference is now made to Figure 24 which shows photoluminescence measurements of a solar cell structure as in table 1, by the use of a 532 nm pump laser. Three wide tops of ~750 nm, ~880 nm and ~980 nm originate from the doped layer of Al₀.₅oGa₀ ( ln)AsSb:Te (n doped),

Al_{0 5}Gao (ln)AsSb: Be (p+ doped) og GaAsSb (n+ doped), respectively. A thin em ission line at ~800 nm is most probably from the AI₀ 35Ga₀ ( ln)AsSb quantum wells, while the emission line at ~695 nm is from the AI₀ 5oGa₀.₅o(ln)AsSb super grating in layer no.6.

Reference is now made to Figure 25 which shows photoluminescence measurement of a solar cell structure as in table 2 by the use of a 532 nm pump laser. The wide top at ~980 nm originates from the doped layer of GaAsSb (n+ doped). The structure in table 1 is arranged at the bottom of this structure, and it is therefore most likely that the tops of Figure 24 do not appear due to the pump laser is absorbed before it reaches these layers. The thin top at ~695 nm is also seen in this figure, something supporting that it comes from the Al_{0 50}Ga0.50( ln)AsSb super grating in layer no.6. The top at ~800 nm is also present, but weaker than in Figure 24.

Figures 24 and 25 thus show that one by the technique described herein can grow the actual I I I- V structures on silicon (221) substrate, and with an optical quality which is good enough for using these as solar cells.

Modifications

Absorber and filters can be constructed so that the absorber has a temperature up towards 4000-6000 K, and that the solar cell can be replaced with ordinary solar cells in the market (Si, CdTe, Ge, GaAs, etc.). Absorber, filter, solar cell and other optical components can be arranged in a vacuum to protect these from degradation from air, rain, salts, sand or similar.

The system can be provided with further optical components to increase or reduce the amount of radiation from sunlight concentrator and/or the amount of secondary radiation from absorber. New layers can be added in the solar cell structure for improving the efficiency at infrared (1-3 pm), near-infrared (700 nm-1 pm) and/or visible (350-700 nm) wavelengths.

Layers in the solar cell structure can be of other materials than lll-V to insulate or passivate the lll-V materials or the surfaces of these.

Si0₂, polymer, Ti0₂, Al₂0₃, Si₃N or similar materials can enclose or cover the solar cell structure to passivate and/or protect this.

The solar cell can be in contact with an active or passive thermal cooling element to reduce the temperature of the solar cell, preventing heating of this or controlling the temperature of the solar cell.

The absorber can be a gas, arranged in a suitable container, or a medium which holds a higher temperature than solid material are able to, for either increasing the absorption and/or reducing the wavelength of the secondary radiation.

An external heating source can be connected to the absorber for either controlling the temperature of this or heating it when sunlight is not available.

The absorber can be heated by a combustion process between natural gas and air/oxygen, fuel and air/oxygen, hydrogen and air/oxygen or another chemical reaction which develops heat.

The absorber can be heated from liquid salts, metals or similar materials being transporting heat from an external heating source as geothermal heat, heat pump, combustion plant or radioactive source.

An optical coating can be used on concentrator, filters, solar cell or absorber for changing reflection or transmission, possibly use this for controlling this property.

References

1. Giovanni P. Donati and Kelvin Malloy: «On Interpolation of Semiconductor Solid Solution parameters: Application to Quarternary Alloy Bandgaps», Poster, 5th International Conference on Mid-Infrared Optoelectronics Materials and Devices, Annapolis, September 2002.

2. T.J. Grassman, M.R.Brenner, A.M. Carlin, S.Rajagopalan, R.Unocic, R.Dehoff, M. Mills, H.

Fraser, S.A. Ringel: «Toward Metamorphic Multijunction GaAsP/Si Photovoltaics Grown on Optimized GaP/Si Virtual Substrates Using Anion-Graded GaAsyPl-y Buffers», 34th IEEE Photovoltaic Specialists Conference (PVSC), June 2009.

3. Masayoshi Umeno, Toshimich Kato, Mingju Yang,Yutaka Azuma, Tetsuo Soga, Takashi Jimbo: «High efficiency AIGaAs/Si tandem solar cell over 20%», IEEE First World

Conference on Photovoltaic Energy Conversion, pp. 1679-1684, vol.2, December 1994.

4. G. Balakrishnan, S.H. Huang, A. Khoshakhiagh, A. Jallipalli, P. Rotella, A. Amtout, S. Krishna, CP. Haines, L.R. Dawson and D.L. Huffaker: «Room-temperature optically-pumped GaSb quantum well based VCSEL monolithically grown on Si (100) substrate», Electronics Letters, Vol. 42 No. 6, March 2006.

Claims

1. Method for manufacturing photovoltaic solar cells by means of a vacuum deposition plant, characterized in that it includes:

i. arranging a thin substrate of monocrystalline silicon,

ii. heating the substrate and exposing it to damps from different furnaces of materials from group III and V of the periodic system, and

iii. growing at least three different compounded lll-V monocrystalline semiconductor layers on the substrate, where the first grown layer on the silicon consists of AIAS_xSb^, AllnASxSbi.x,

2. Method according to claim 1, characterized in that it includes growing at least one

semiconductor layer of intrinsic, p-type or n-type material, which is compounded of at least two layers of lll-V materials in an alternating super structure.

3. Method according to claim 1, characterized in that it includes growing at least four different compounded lll-V monocrystalline semiconductor layers on the silicon substrate.

4. Method according to claim 1, characterized in that it includes growing at least five different compounded lll-V monocrystalline semiconductor layers on the silicon substrate.

5. Method according to claims 1-4, characterized in that it includes growing the first

semiconductor layer so that the transition between the silicon substrate and the first grown semiconductor layer includes a defect layer which provides a low content of defects in the grown semiconductor layer.

6. Method according to claim 5, characterized in that it includes growing the semiconductor layers so that the transition between two grown semiconductor layers contain a defect layer which provides a low content of defects in the last grown semiconductor layer.

7. Method according to claims 1-6, characterized in that it includes growing a second

semiconductor layer consisting of AlSb, GaSb, or InSb on the silicon substrate.

8. Method according to claims 1-6, characterized in that it includes growing a second

semiconductor layer consisting of AllnSb, AIGaSb, GalnSb or AIGalnSb on the silicon substrate.

9. Method according to claims 1-6, characterized in that it includes growing a second

semiconductor layer consisting of AIAs_xSbi._x, GaAS_xSbj.x, lnAs_xSbi._x, AllnAs_xSbi._x, AIGaAs_xS_bi._x, GalnASxSb_{l <} or AIGalnASxSbj._x with x>=0 and x<0.5 on the silicon substrate.

10. Method according to claims 1-9, characterized in that it includes growing a third

semiconductor layer consisting of AIAs, GaAs or InAs on the silicon substrate.

11. Method according to claims 1-9, characterized in that it includes growing a third

semiconductor layer consisting of AllnAs, AIGaAs, GalnAs or AIGalnAs on the silicon substrate.

12. Method according to claims 1-9, characterized in that it includes growing a third

semiconductor layer consisting of AlAs_xSbi.„, GaAs_xSbi._x, InAS_xSbj.,,,

GalnASxSbi.x or AIGalnASxSbx.x with x>0.5 and x<=l on the silicon substrate.

13. Method according to claims 1-12, characterized in that it further includes:

iii. using a silicon substrate which contains both p-type and n-type material.

14. Method according to claim 13, characterized in that it includes using a silicon substrate having a p-n, p-i-n, p-p-n or p-n-n transition and grow semiconductor layers of lll-V materials having one or more p-n, p-i-n, p-p-n or p-n-n transitions for providing a multi-transition solar cell.

15. Method according to claim 14, characterized in that it includes growing the semiconductor layers so that two of the defect layers between the lll-V materials are separated by at least one p- n, p-i-n, p-p-n or p-n-n transition.

16. Method according to claim 13, characterized in that it includes growing the semiconductor layers so that at least one lll-V transition consist of AIAs_xSbi._x, GaASxSb_x._x, AllnAS_xSbi.,, AIGaAs_xSbi._x, GalnASxSbi.,,, GalnASxPi.x, AIGalnAs_xPi._x and/or AIGalnAs_xSbi.x with x>0.5 and x<=l.

17. Method according to claim 13, characterized in that it includes growing the semiconductor layers so that at least one lll-V transition consists of AIASxSbx.,,, GaAs_xSb_{l x}, AllnAs_xSbi._x, AIGaASxSbi. _x, GalnAs.Sb_!.,,, GalnAs_xP_!._x,

and/or AIGalnAs_xSb_!._x with x>0.9 and x<=l.

18. Method according to claim 1, characterized in that it includes doping the silicon substrate with B, P, Ga, As and/or Sb for forming intrinsic, p-type and/or n-type layers in the transition.

19. Method according to claim 1, characterized in that it includes using a silicon substrate having a 100-surface of crystallographic orientation with 1-5 degrees miscut.

20. Method according to claim 1, characterized in that it includes using a silicon substrate having a 100-surface of crystallographic orientation with 2-4 degrees miscut.

21. Method according to claim 1, characterized in that it includes using a silicon substrate having a Ill-surface of crystallographic orientation with 1-5 degrees miscut.

22. Method according to claim 1, characterized in that it includes using a silicon substrate having a 221-surface of crystallographic orientation.

23. Method according to claims 1-21, characterized in that it includes adding silicon, beryllium, selenium or tellurium in amount(s) being less than 10 at% of the grown material.

24. Method according to claims 1-22, characterized in that it includes exposing the silicon substrate for damps of gallium, aluminium, indium, arsenic, antimony, phosphorus and/or nitrogen.

25. Method according to any one of the claims 1-24, characterized in that it includes

doping/adding to at least two of the lll-V materials with silicon, beryllium, selenium or tellurium for providing n-type and p-type contact layers.

26. Method according to any one of the claims 1-25, characterized in that it includes arranging metal contacts of Pd, Sb, Ti, Pt, Au, Al, Cu and/or Ag in contact with the semiconductor layers on the silicon substrate.

27. Method according to any one of the claims 1-26, characterized in that it includes using a layer of silicon at the bottom, one layer of lll-V material having a new grating constant over this and several layers of two lll-V materials having a third grating constant over this again.

28. Method according to claim 27, characterized in that it includes growing lll-V materials having a given fraction of their mutual grating constants for incorporating a set of defects in the transition between the different materials solving the fraction.

29. Method according to any one of the claims 1-28, characterized in that it includes arranging transitions there the material composition changes gradually from one composition to another.

30. Method according to any one of the claims 1-29, characterized in that it includes growing a thin layers of AlGalnAsSb, quaternary, ternary and/or binary layers which act as a defect stop grating which deflects the defects and prevent that they propagate through the structure.

31. Method according to claim 30, characterized in that it includes growing layers being thinner than the wave distribution function of the electron in the material, so that the electrons experience the material as an average material.

32. Method according to any one of the claims 1-31, characterized in that it includes treating the material thermally for crystallizing remaining defects and possibly improve the charge carrying properties of the material, after the entire structure is grown.

33. Multi-functional solar cell, including one or more photovoltaic solar cells (34) which are formed on a substrate, on which substrate is deposited layers of lll-V materials, characterized in that:

- the substrate is of silicon,

- the photovoltaic solar cell(s) (34) is/are formed by at least three different compounded lll-V monocrystalline semiconductor layers on the substrate, where the first grown layer on the silicon consists of AIAs_xSbi._x, AllnAs_xSbi._x, AIGaAs.Sb_! ,, or AIGalnAs_xSbi._x with x>=0 and x<0.5.

34. Multi-functional solar cell according to claim 33, characterized in that the photovoltaic solar cell(s) (34) include(s) at least one semiconductor layer of intrinsic, p-type or n-type material, which is compounded of at least two layers of lll-V materials in an alternating super structure.

35. Multi-functional solar cell according to any one of the claims 33-34, characterized in that the photovoltaic solar cell(s) (34) include(s):

- at least four different compounded lll-V monocrystalline semiconductor layers on the silicon substrate, or - at least five different compounded lll-V monocrystalline semiconductor layers on the silicon substrate.

36. Multi-functional solar cell according to any one of the claims 33-35, characterized in that the photovoltaic solar cell(s) (34) include(s) a transition between the silicon substrate and the first grown semiconductor layer containing a defect layer which provides a low content of defects in the grown semiconductor layer.

37. Multi-functional solar cell according to any one of the claims 33-36, characterized in that the photovoltaic solar cell(s) (34) include(s) a transition between two grown semiconductor layers containing a defect layer which provides a low content of defects in the last grown semiconductor layer.

38. Multi-functional solar cell according to any one of the claims 33-37, characterized in that the photovoltaic solar cell(s) (34) include(s) a second semiconductor layer consisting of:

- AlSb, GaSb, or InSb,

- AllnSb, AIGaSb, Ga lnSb or AIGa lnSb, or

- AIASxSbj.,, GaASxSbj.x, InASxSbi.x, AllnASxSbi.x, AIGaAs_xS i._x, GalnAs_xSbi._x or AIGa lnASxSbj.x with x>0 and x<0.5.

39. Multi-functional solar cell according to any one of the claims 33-38, characterized in that the photovoltaic solar cell(s) (34) include(s) a third semiconductor layer consisting of:

- AIAs, GaAs or InAs,

- AllnAs, AIGaAs, GalnAs or AIGalnAs, or

- AIAs_xSbi._x, GaASxSbj.x, lnAs_xSbi._x, AllnAs_xSbi.x, AIGaASxSbi._x, Ga lnASxSbi.x or AIGa

with x>0.5 and x<l.

40. Multi-functional solar cell according to any one of the claims 33-39, characterized in that the semiconductor layers of the photovoltaic solar cell(s) (34) include:

- one or more p-n, p-i-n, p-p-n or p-n-n transition(s),

- at least two defect layers between the different lll-V materials between the silicon substrate and the lll-V transition, and that the silicon substrate contains both p-type and n-type material.

41. Multi-functional solar cell according to any one of the claims 33-40, characterized in that the silicon substrate includes a p-n, p-i-n, p-p-n or p-n-n transition and semiconductor layers of lll-V materials having one or more p-n, p-i-n, p-p-n or p-n-n transitions.

42. Multi-functional solar cell according to claim 40, characterized in that the defect layers between the lll-V materials are separated by at least one p-n, p-i-n, p-p-n or p-n-n transition.

43. Multi-functional solar cell according to any one of the claims 33-42, characterized in that:

- at least one semiconductor transition consist of AIAs_xSb_{1 )(}, GaASxSbi.,,

AIGaAs_xSbi. „, GalnAs_xSbi._x, GalnASxP_!.,,, AIGalnAs_xP_!._x and/or AIGalnAs.Sbi ,, with x>0.5 and x<=l, or

- at least one semiconductor transition consists of AIAs_xSbi._x, GaAs_x5bi._x, Al lnASxSbj.x, AIGaAs_xSb_!. _x, GalnAs_xSbi._x, GalnAs_xPi._x, AIGalnASxPj.* and/or

with x>0.9 and x<=l.

44. Multi-functional solar cell according to claims 33, characterized in that the silicon substrate is doped with B, P, Ga, As and/or Sb for forming intrinsic, p-type and/or n-type layers in the transition.

45. Multi-functional solar cell according to claims 33, characterized in that the silicon substrate is provided with

- a 100-surface of crystallographic orientation with 1-5 degrees miscut,

- a 100-surface of crystallographic orientation with 2-4 degrees miscut,

- a Ill-surface of crystallographic orientation with 1-5 degrees miscut, or

- a 221-surface of crystallographic orientation.

46. Multi-functional solar cell according to any one of the claims 33-45, characterized in that one or more of the semiconductor layers are added silicon, beryllium, selenium or tellurium in amount(s) being less than 10 at% of the grown material.

47. Multi-functional solar cell according to claims 33, characterized in that the silicon substrate at manufacturing has been exposed to damps of gallium, aluminium, indium, arsenic, antimony, phosphorus and/or nitrogen.

48. Multi-functional solar cell according to any one of the claims 33-47, characterized in that at least two of the semiconductor layers are doped or added with silicon, beryllium, selenium or tellurium for providing n-type and p-type contact layers.

49. Multi-functional solar cell according to any one of the claims 33-48, characterized in that the photovoltaic solar cell(s) (34) is/are provided with metal contacts of Pd, Sb, Ti, Pt, Au, Al, Cu and/or Ag in contact with the semiconductor layers on the silicon substrate.

50. Multi-functional solar cell according to any one of the claims 33-49, characterized in that on the silicon substrate is arranged a semiconductor layer of lll-V material having a new grating constant over this and several semiconductor layers of two lll-V materials having a third and fourth grating constant over this again.

51. Multi-functional solar cell according to any one of the claims 33-50, characterized in that the semiconductor layers are arranged to each other with a given fraction of their mutual grating constants for incorporating a set of defects in the transition between the different materials solving the fraction.

52. Multi-functional solar cell according to any one of the claims 33-51, characterized in that the photovoltaic solar cell(s) (34) include(s) transitions there the material composition changes gradually from one composition to another.

53. Multi-functional solar cell according to any one of the claims 33-52, characterized in that the photovoltaic solar cell(s) (34) include(s) thin layers of AIGalnAsSb, quaternary, ternary and/or binary semiconductor layers which act as a defect stop grating which deflects the defects and prevent that they propagate through the structure.

54. Multi-functional solar cell according to any one of the claims 33-53, characterized in that the photovoltaic solar cell(s) (34) include(s) semiconductor layers being thinner than the wave distribution function of the electron in the material, so that the electrons experience the material as an average material.

55. Multi-functional solar cell according to any one of the claims 33-54, characterized in that the photovoltaic solar cell(s) (34) has/have been treated thermally for crystallizing remaining defects and possibly improving the charge carrying properties of the material, after the entire structure is grown.

56. Multi-functional solar cell according to any one of the claims 33-55, characterized in that it includes one or more of the following: - absorbing medium/absorbers (33, 36, 41),

- concentrator system (31) arranged for receiving either primary radiation/solar radiation (32) or secondary radiation (39) for generating electric effect,

- optical filters (40) arranged for collecting secondary radiation (39) which is reflecting and/or transmitting chosen wavelengths to absorbing medium/absorber (33, 36, 41) and photovoltaic solar cell (34), respectively, or vice versa.

57. Multi-functional solar cell according to any one of the claims 33-56, characterized in that the photovoltaic solar cell(s) (34) and/or the absorber(s) are provided with one or more optical antireflection/reflection coatings to collect secondary radiation reflecting and transmitting chosen wavelengths back towards absorber (33, 36, 41) and/or towards/into the solar cell(s) (34), respectively.

58. Multi-functional solar cell according to any one of the claims 54-57, characterized in that: - the absorbing medium/absorber (33, 36) has a circular or spherical design, such as a ball, cylinder or other enclosing chamber, and is provided with a narrow aperture (37) for incoming solar radiation (32),

- the absorbing medium/absorber (41) is plates,

- the medium/absorber (33) is segmented in different geometrical parts which enclose each other, so that solar cells (34) and filters (40) are formed enclosing the absorber (33) as a spherical object, or as an extension as a cylinder,

- the absorbing medium/absorber (33, 37, 41) is made of and/or covered with silicon carbide, tantalum, molybdenum or wolfram, or

- the absorber being a gas, arranged in a suitable container, or a medium holding a higher temperature than what solid material are able to, for either increasing the absorption and/or reducing the wavelength of the secondary radiation (39).

59. Multi-functional solar cell according to any one of the claims 54-58, characterized in that the concentrator system (31) includes one or more frensel lenses (35), optical lenses, elliptical mirrors or similar optical arrangements.

60. Multi-functional solar cell according to any one of the claims 33-59, characterized in that absorber (33, 36, 41), filter (40), photovoltaic solar cell(s) (34) and other optical components (35) are arranged in a vacuum for protection against environmental impact, such as degradation from air, rain, salts, sand or similar.

61. Multi-functional solar cell according to any one of the claims 33-60, characterized in that layers in the photovoltaic solar cell (34) include other materials than lll-V for insulation or passivation of the lll-V materials or the surfaces of these.

62. Multi-functional solar cell according to any one of the claims 33-61, characterized in that the photovoltaic solar cell (34) is/are enclosed or coated of Si0₂, polymer, Ti0₂, Al₂0₃, Si₃N₄ or similar materials for passivation and/or protection of this.

63. Multi-functional solar cell according to any one of the claims 33-62, characterized in that the solar cell is arranged in contact with an active or passive thermal cooling element for reduction of the temperature of the solar cell, preventing heating of this or controlling the temperature of the solar cell.

64. Multi-functional solar cell according to any one of the claims 33-63, characterized in that: - an external heating source is connected to the absorber (33) for either controlling the temperature of this or heating it when solar radiation (32) is not available,

- the absorber (33) is heated by a combustion process between natural gas and air/oxygen, fuel and air/oxygen, hydrogen and air/oxygen or another chemical reaction developing heat, or

- the absorber is heated by liquid salts, metals or similar materials transporting heat from an external hating source, such as geothermal heat, heat pump, combustion plant or radioactive source.

65. Multi-functional solar cell according to any one of the claims 33-64, characterized in that concentrator (31), filters (40), photovoltaic solar cell (34) or absorber (33) is provided with an optical coating for changing reflection or transmission, possibly used for controlling this property.