HK1231253A1 - Nanowire-based solar cell structure - Google Patents

Description

Nanowire-based solar cell structures

The present application is a divisional application of a patent application having an application date of 2008/19/6, and an application number of 200880103566.X (PCT/SE2008/050734), entitled "nanowire-based solar cell structure".

Technical Field

The present invention relates to a solar cell structure. In particular, the present invention relates to solar cell structures comprising nanowires as active components.

Background

Interest in solar cell technology has increased over the last few years. The ever-increasing energy costs and environmental concerns are factors behind this interest. Technological breakthroughs that represent the possibility of mass production of high-efficiency solar cells are also important factors.

The most efficient existing solar cells are made of group III-V semiconductors such as GaInP or GaInAs in the form of multijunction cells with several layers, each absorbing a different part of the solar spectrum. The advantages of this concept are illustrated by fig. 1, which fig. 1 shows the portion of the solar AM1.5 spectrum that a typical silicon Photovoltaic (PV) cell can convert into electrical energy, in contrast to a GaInP/GaInAs/Ge stack structure (distance structure).

The theoretical limit of the energy conversion efficiency of a solar cell based on a single semiconductor material is 31%. Multijunction photovoltaic cells (MJPV) can raise this limit to 43% for double-junction solar cells and 49% for triple-junction solar cells. However, the production of all the necessary different material combinations (material combination) is challenging and a high material quality of the crystals is necessary to achieve high efficiency.

Many advances have occurred and in 2006, 12, Boeing/Spectrolab announced (http:// www.spectrolab.com/com/news/news-detail. asp. This technology was originally developed for space applications as mentioned in "High-efficiency solar cells from III-V compounds semiconductors" by f.dimroth in phys.sat. sol (c)3,373(2006), where germanium (Ge) is a suitable substrate material. The availability of Ge in the crust is limited and it is expensive, which may be a limitation if such high efficiency tandem solar cells are used in large numbers in the world. For this reason, the development of multijunction solar cells based on crystalline Si or even more common substrates will open new opportunities for terrestrial applications, taking advantage of the higher multijunction efficiency, lower cost and higher availability of Si substrates compared to Ge. Prior art multi-junction photovoltaic cells comprising a lattice matching layer grown on a Ge substrate are discussed in J electric spectra Rel Phen 150,105(2006) "solartholtotalics R & D at the tiling point: a 2005 technology overview," l.l. kazmerski. In the case of the use of a concentrator (concentrator), such an MJPV cell achieves an efficiency of more than 40%.

However, technical hurdles can be identified for planar III-V multijunction solar cells. Efficiencies above 50% would be very difficult to achieve due to physical limitations. Conventional III-V materials for multijunction solar cells require perfect lattice matching over large substrate areas to avoid dislocations. Good device functionality will also require a very high degree of compositional uniformity across the wafer (wafer). This makes up-scaling (up-scaling) of large area substrates very challenging, even if such substrates are available at a reasonable price. Even if these problems can be overcome, the limited number of materials that have both a suitable bandgap and a more or less lattice matched makes it very difficult to produce more than three junctions in a planar solar cell, which is necessary to achieve very high efficiencies.

In addition to the above-described technical challenges associated with prior art multi-junction cells, both cost and scaling (scaling) present challenges. For example, multijunction cells grown on Ge or III-V substrates are very expensive due to high substrate cost and small wafer size. In addition, the high cost of the raw materials that are grown epitaxially (epitaxially) today in advanced MOCVD or even MBE reactors with low productivity and precious necessitates the use of optical concentrators to improve cost performance at the system level. Even if the cost can be reduced, the condenser is still necessary to obtain the saturation voltage even at full daylight intensity (full sun).

Disclosure of Invention

The solar cell devices of the prior art need to be improved to obtain the desired or "theoretical" advantages with respect to efficiency and production costs.

The object of the present invention is to overcome the drawbacks of the prior art. This object is achieved by a solar cell structure and a solar cell module as defined in the independent claims.

The solar cell structure according to the present invention comprises nanowires constituting the light absorbing part of the solar cell structure and a passivating shell surrounding at least a part of the nanowires. Preferably, the nanowire protrudes from the substrate.

In a first aspect of the invention, the passivating shell of the solar cell structure comprises a light guiding shell adjacent to the nanowire. Preferably, the light guiding shell is made of a material having a higher bandgap than the nanowire and preferably the light guiding shell also has an indirect bandgap.

In a second aspect of the invention, a solar cell structure comprises a plurality of nanowires positioned with a maximum spacing between adjacent nanowires that is shorter than the wavelength of light that the solar cell structure is intended to absorb. Thus, the incident light will experience a so-called "effective medium" defined by the plurality of nanowires.

In one embodiment of the invention, the nanowire comprises at least one segment forming a bandgap adapted to absorb light in a wavelength range of the solar spectrum. The solar cell structure may also be equipped with a plurality of segments, wherein each segment is adapted to absorb light in a different wavelength range of the solar spectrum. The plurality of segments are preferably arranged such that the band gap of each segment decreases in a direction away from the intended incident light and along the longitudinal axis of the nanowire (205).

The plurality of segments may be connected in series by Esaki diodes (Esaki diodes) or metal segments.

Thanks to the invention it is possible to produce high efficiency solar cells at acceptable cost.

An advantage of the present invention is that the solar cell allows heterostructures without lattice matching, allowing a large degree of freedom in the choice of material combinations. In principle, there is no limit to the different band gaps, i.e. the number of segments in the nanowire, giving the possibility to absorb all or selected parts of the solar spectrum.

Due to the small growth area for each individual wire, extremely uniform growth across the entire wafer is not required, which relaxes the requirements on the growth system. Likewise, due to the small area, the substrate may be polycrystalline or thin film silicon, or the like.

One advantage of the solar cell structure according to the first aspect of the invention is that the light guiding shell directs light through the regions of decreasing band gap in an orderly manner, allowing for sequential light collection.

In addition, the light guiding structure provides intrinsic concentration of photons to the nanowire, giving a saturation voltage even under diffuse light conditions.

A further advantage provided by the present invention is the possibility of using metal segments to connect segments of nanowires. This is not possible in prior art planar devices because the metal layer is opaque. However, in the present invention, the opacity will have a limited negative impact due to the narrow light-absorbing nanowires surrounded by the light-guiding shell.

By placing the nanowires close enough to the substrate according to the second aspect of the invention, the advantages of using nanowires are combined with an efficient absorption of light, since the incident light "sees" densely packed (close packed) nanowires as a continuous effective medium.

Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings and the claims.

Drawings

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

fig. 1 schematically shows part of the AM1.5 solar spectrum which theoretically could be used by silicon-based solar cells and GaInP/GaInAs/Ge-based solar cells, respectively, where the black areas represent the efficiency loss due to carrier (charge carrier) thermalization or photon transport;

FIG. 2a schematically illustrates a solar cell structure according to one embodiment of the invention;

figure 2b schematically shows a solar cell structure comprising a nanowire having a plurality of segments according to one embodiment of the present invention;

fig. 3 schematically shows a solar cell structure according to the present invention, wherein the top of the nanowires protrude outwards from the light guiding shell;

fig. 4a-b schematically show an embodiment of the invention, where in 4a the substrate is equipped with a diode, and in 4b the nanowire is terminated at the top end of the light guiding shell.

Fig. 5a-b schematically show an embodiment of the invention, wherein esaki diodes are used in 5a and metal segments are used in 5b to interconnect the segments of the nanowire.

FIG. 6 schematically illustrates a solar structure comprising a plurality of nanowires, each nanowire comprising a plurality of pn-junctions, according to one embodiment of the present invention;

FIG. 7 schematically illustrates a solar cell structure with closely spaced nanowires according to the present invention, which are adapted to absorb light in an efficient medium manner;

FIG. 8 schematically illustrates a solar cell structure comprising a single pn junction according to one embodiment of the present invention;

FIG. 9 schematically shows a solar cell structure comprising a plurality of pn-junctions according to another embodiment of the present invention, an

Fig. 10 schematically illustrates a solar cell structure containing multiple nanowires placed closely together, where each nanowire is surrounded by a high and indirect band gap material.

Detailed Description

Fig. 2a schematically shows an embodiment of a solar cell structure according to the invention. The nanowire 205 constitutes a light absorbing part of the solar cell structure and the passivating shell 209 surrounds at least a portion of the nanowire 205. Preferably, the nanowires protrude from the substrate 220. The nanowires may protrude substantially perpendicular to the substrate 220 or at an angle.

Incident (solar) light 201 is coupled into the nanowires 205 of the solar cell structure. Incident light generates electron-hole pairs and preferably the light absorbing part of the solar cell structure, i.e. the nanowire 205, is configured as a pn-junction to establish an electric field that pushes (promotate) current through the nanowire 205 between the front contact 203 and the back contact 202 in one direction only. For example, the front contact 203 and the back contact 202, as schematically shown in fig. 2a, are electrically connected to the top and bottom of the nanowire 205, respectively, and the light 201 is coupled into the top of the nanowire 205.

One purpose of passivating the shell 209 according to the present invention is to reduce the number of mid-gap surface states (mid-gap surface states) on the circumferential surface of the nanowire 205. The surface states can be removed from the conductive nanowires by using a passivating shell. Another purpose is to insulate the nanowire 205 from the surroundings. In addition, the passivating shell may play a more positive role in the solar cell structure in some structures. Due to compressive or tensile strain, the band gap may be increased or decreased or the band may be bent to radially separate holes from electrons. The function of the passivation shell 209 and the above-mentioned objects are more or less important or relevant in various configurations of the solar cell structure.

In one embodiment of the invention, the passivating shell 209 comprises a light guiding shell adjacent to the circumferential surface of the nanowire 205. Preferably, the nanowire 205 is made of a direct bandgap material and the light guiding shell 210 is made of a material having a high and indirect bandgap. The light guide shell may constitute the entire passivation shell 210 or may be in the form of an inner shell surrounded by an outer shell having the characteristics described above. Since the light guiding shell is made of an indirect high bandgap material, no light will be absorbed in the shell and the light guiding shell directs the light along the nanowire 205.

Referring to fig. 2b, in the nanowire 205 comprising a semiconductor material, photons having energies greater than the band gap of the semiconductor material may be absorbed. However, photons having energies substantially exceeding the band gap will generate not only electron-hole pairs but also heat, which causes thermalization losses and thus has a negative impact on the efficiency of the solar cell. In one embodiment of the invention, the internal structure of the nanowire 205 of the solar cell according to the invention may comprise one or more segments 215, each forming a bandgap adapted to absorb light in a predetermined wavelength range of the solar spectrum. The high energy portion of the light will then be absorbed in segment 215, which segment 215 forms a bandgap suitable for absorbing light in a predetermined range of wavelengths including the high energy portion, while photons having energies below the bandgap of that segment will experience that segment as they would experience a transparent waveguide.

In one embodiment of the invention, the solar cell structure comprises a light-absorbing nanowire 205, the nanowire 205 having a plurality of segments 215 distributed along the nanowire 205, wherein each segment 215 is adapted to absorb light in a different wavelength range of the solar spectrum. The incident light is used to be coupled into the top of the nanowire 205. The plurality of segments 215 are arranged such that the band gap of each segment 215 decreases in a direction from the top of the nanowire 205 towards the bottom of the nanowire 205. In this way, stepwise selective absorption and transmission of light is achieved, wherein light having an energy higher than the band gap of one of the plurality of segments 215 is absorbed and light having a lower energy is transmitted to the next segment 215. The next segment will then provide the same selective absorption and transmission with its lower bandgap, and so on. Thus, a large part of the solar cell spectrum can be effectively utilized, with limited thermalization losses, which gives high efficiency.

Fig. 2b schematically shows an embodiment of the solar cell structure of the invention comprising a nanowire 205 constituting the light absorbing part of the solar cell structure and a light guiding shell 210 surrounding at least a part of the nanowire 205. Preferably, the nanowire 205 protrudes from the substrate 220. Optionally, the nanowire 205 protrudes from the substrate 220 and comprises a plurality of segments 215 distributed along the nanowire 205, wherein each segment 215 is adapted to absorb light in a different wavelength range of the solar spectrum. The front contact 224 and the back contact are electrically connected to the top and bottom of the nanowire, respectively. As shown in fig. 2b, the front contact 224 may surround the top of the nanowire 205 and the back contact 225 may be arranged on the substrate 220 on the opposite side of the nanowire 205. In order to efficiently absorb light coupled into the solar cell structure at the top, the plurality of segments 215 are arranged such that the band gap of each segment 215 decreases in a direction from the top of the nanowire 205 towards the bottom of the nanowire 205. The light guiding shell 210 is made of a material having a higher bandgap than the light absorbing part of the nanowire 205, and preferably the bandgap is indirect. Thus, the light guiding shell 210 guides incident light in a direction from the top to the bottom of the nanowire 205 without absorption therein. Thus, incident light having longer and longer wavelengths is absorbed successively in each segment 215. Optionally, the solar cell structure comprises a dielectric layer covering the substrate surface with a wrap-around structure around the bottom of the nanowire 205. Furthermore, the solar cell structure shown in fig. 2b may comprise an outer shell layer having passivation and insulation properties, which surrounds the light guiding shell 210, as described above. Thus, the light guiding shell and the outer shell layer together constitute a passivating shell of the solar cell structure.

Fig. 3 schematically shows an embodiment of the solar cell structure of the invention, wherein the light absorbing part is a nanowire 205, which nanowire 205 protrudes from a substrate 220 and is partially surrounded by a light guiding shell 210. The top portion 240 of the nanowire 205 extends outward from the light guiding shell 209. The front contact 224 and the back contact 225 are electrically connected to the top 240 and the bottom of the nanowire 205, respectively. As shown in fig. 3b, the back contact 225 may be arranged on the substrate 220 on the opposite side of the nanowire 205 and the front contact 224 surrounds the top 240. The front contact 224 may be a metal grid (metal grid) contacting the top 240 of the nanowire 205 or a transparent contact covering the entire solar cell structure. In addition, the top 240 of the nanowire 205 extending over the light guiding shell 210 may be doped to further enhance the contact properties. Preferably, the nanowire 205 comprises a direct bandgap material and the light guiding shell 210 is made of at least one indirect bandgap material having a higher bandgap than the direct bandgap material of the nanowire 205 to obtain a light guiding function from the light guiding shell 210. The nanowire 205 comprises a plurality of segments 215, each segment forming a bandgap adapted to absorb light in a predetermined wavelength range of the solar spectrum. Preferably, the plurality of segments 215 are arranged such that the band gap formed by the segments 215 decreases successively in a direction from the top 240 of the substrate 220 and along the nanowire 205 towards the bottom. In use, incident light is coupled into the solar cell structure and high energy photons are absorbed first, followed by photons of successively lower energy being successively absorbed in the segments 215 as they propagate towards the bottom of the nanowire 205. The nanowires may be partially opaque as light is guided by the light guiding shell 210. The segments 215 may be connected in series by, for example, esaki diodes 216 or short metal segments.

The nanowire technology allows the formation of heterostructures, such as the internal structure of the nanowire 205 formed from multiple segments 215, without the need for lattice matching, which gives a large degree of freedom in material combination. A band gap that absorbs almost any wavelength range of the solar spectrum can thus be achieved in the nanowire 205 (this cannot be easily obtained by using prior art planar techniques). In principle, there is no limit to the number of different band gaps of the segments 215 of the nanowire 205 according to the invention and thus light from a large part of the solar cell spectrum can be absorbed.

Preferably, the light guiding shell 210 is epitaxially connected to the nanowire 205 by radial growth of the light guiding shell onto the nanowire 205.

In one embodiment of the invention, the solar cell structure comprises a nanowire 205, preferably in the center of the light guiding shell 210. The light guiding shell 210 is made of an indirect high bandgap material and is narrow enough to allow only single mode light propagation and is small compared to nanowires. The solar cell structure according to this embodiment functions as follows: at the top 240 of the nanowire 205 light is coupled into the solar cell structure. Since the light guiding shell 210 is an indirect high bandgap material, no light will be absorbed here, and since the light guiding shell is single-mode, the field is strongest at the core, i.e. at the location of the nanowire 205. As light travels downward, higher energy is efficiently absorbed, while photons with energies below the bandgap will only experience a transparent waveguide. As the energy bands are sequentially removed in the nanowire 205, the photons generate a photovoltage in each segment 215 that is equal to the bandgap in that segment. Ideally, the structure would be so efficient that only low energy light penetrates to the substrate. However, the substrate may also contain standard photodiodes to collect scattered higher energy photons and generate photovoltage.

The substrate 220 of the solar cell structure of the present invention may function as only a mechanical support and electrical contact, as shown in fig. 3, or it may also contain one or more electrically active components, such as a standard photodiode structure. One embodiment of such a solar cell structure with a photodiode, which is realized by counter-doped regions 222, 223 in the substrate 220, for example a p-doped region 222 and subsequently an n-doped region, is schematically illustrated in fig. 4 a.

Fig. 4b shows another embodiment of a solar cell structure according to the present invention, wherein the nanowire 205 ends at or close to the top end of the light guiding shell 210. Possibly, but not necessarily, the nanowire 210 ends with a cap 250 consisting of catalytic particles (catalytic particles), which is typical for some nanowire growth methods. This arrangement is most suitable for use on flat, preferably transparent front contacts.

The light guiding shell 210 may be considered a waveguide, although it is not limited to operating as a single mode waveguide. The light guiding shell 210 directs or guides light through the regions of decreasing band gap in an orderly manner, which enables sequential light collection. Furthermore, the light guiding shell 210 prevents losses caused by absorption at the circumferential surface of the nanowire 205 and by light coming out of the solar cell structure.

Fig. 5a schematically shows an enlargement of the nanowire 205, showing a segment 215 and an esaki diode 216, with p-type and n-type regions within the segment. Fig. 5b schematically shows an embodiment of the invention in which the esaki diode, typically used in prior art planar series cells, is exchanged for a metal segment 217. This is possible due to the reduced need for transparency of the nanowires 205 in the solar cell structure according to the invention.

A solar cell module or solar panel according to the invention typically comprises a plurality of the above-described solar cell structures, which are preferably densely packed on a substrate or wafer to cover a substantial part of the substrate or wafer surface (substential part). The solar cell module may comprise one wafer, but it is also possible that a plurality of wafers are interconnected to give the required electrical energy production.

One advantage of solar cell structures according to the present invention over prior art solar cells fabricated using planar technology is that these structures can be grown in a much simpler system than typical MOCVD. Furthermore, in principle materials with a band gap throughout the entire solar spectrum may be incorporated into the nanowires. Thus, the substrate may be used only as a support structure. Since each nanowire 205 requires a small growth area, extremely uniform growth across the entire wafer is not required, which relaxes the requirements for the growth system. Also, due to the small area, the substrate may be polycrystalline or thin film silicon, or some simpler material.

The light guiding shell layout provides an intrinsic concentration of photons into the core, which can give a saturation voltage even under diffuse light conditions.

Referring to fig. 6, a solar cell structure is provided comprising a nanowire 205 having a plurality of vertical pn junctions, wherein an upper pn junction forms a high bandgap section and a lower pn junction forms a lower bandgap section, in accordance with an embodiment of the present invention. These sections are preferably separated by esaki tunnel diodes. The light guiding shell 210 surrounds the nanowires 205 and the passivating and insulating material preferably fills the volume between the nanowires (volumn). For example, the tunnel diode layer may be heavily doped AlGaAs, GaAsP, or GaInP.

Combinations of materials with different lattice constants are difficult to achieve using planar techniques, where lattice matching is required. Since in the present invention the lattice match is of no consequence (since it would otherwise prevent this type of development when using conventional planar epitaxial growth methods), the method can be extended to more junctions in the future. For a double junction solar cell, the bandgap of the top end segment (subcell) should ideally be in the range of 1.6-1.8eV and the bandgap of the bottom end segment (subcell) in the range of 0.9-1.1 eV. These band gap energies can be achieved by using GaAsP or GaInP for the top segment and GaInAs or InAsP for the bottom segment. The entire energy range spanned by these material combinations for energy harvesting covers 0.4ev (inas) to 2.24ev (gainp).

In the light guide arrangement according to the invention, the width d of the light guide shell, as shown in fig. 6, is larger than the wavelength λ divided by its refractive index n. Preferably, the width d is greater than 500 nm. The light guiding shell 209 directs light along the nanowires by reflection. As shown, the passivating shell 209 can be a matrix (matrix) that fills the volume between the nanowires.

One example of a particular embodiment of a solar cell structure according to the present invention has a photonic light guide design, produced via radial growth of a fully transparent high refractive index shell (e.g., A1N), which functions as a full light guide structure having a diameter of about 0.5 microns, the light guide structure having about 100 nanometers being a multiple band gap core structure. Due to the dense layout of the elongated nanowires, the top portion of the nanowire 205 (about 0.5 microns) will capture the incident light flux, which is then transmitted downward in such a way that the high-energy portion will be captured in the top segment, which appears as a completely transparent waveguide to all photon energies below its bandgap. The same selective absorption and transmission will be provided by the next segment with its lower bandgap, and so on. Above the top, the selected band gap segments are long, heavily n-doped GaN segments used for contacts. The bottom end section can be made of InN and the intermediate section contains a growing Ga fraction up to about Ga_0.7In_0.3Top end of composition (composition) of N. In this case, the substrate will provide support and a back contact since the lowest bandgap will be at the bottom end of the nanowire. Other possible material combinations are AlGaInAsP. In this material system, there are direct band gap materials with values between 0.4eV up to 2.25eV, thus perfectly comparable to the state of the art of multi-junction cells. In this case, the lower segment may be in the well established InAs_1-xP_xFormed in a system and the upper segment may be, for example, in Ga_xIn_1-xThe top segment, formed in the P system, consisting of Ga-rich (70%) GaInP has a direct band gap of 2.25 eV. These are material combinations that have not been available using conventional planar techniques in which lattice matching is required.

Controlling the absorption of (solar) light using the nanowire-based solar cell structure according to the present invention may also be obtained in other ways, which may be a concept referred to as "effective medium". An "effective medium" is generally described as a structure containing a different material that is substantially smaller than the wavelength of the incident light on a length-scale (length-scale). This concept can be seen as replacing the absorption(s) typically used in continuous films by a dense arrangement of spaced, preferably parallel nanowires of substantially smaller distance than the wavelength of the incident light (which it is intended to absorb) with a dense arrangement of the preferably parallel nanowires through the optical effect of absorption(s) (this confined can be seen as a displacement of the commonly used absorption in the continuous films by the optical effect of absorption of the absorption by the dense arrangement of the absorption by the absorption of the absorption by the transparent substrate.

One embodiment of a solar cell structure according to the present invention comprises a plurality of nanowires that constitute the light absorbing part of the solar cell structure. The nanowires optionally protrude from the substrate and are provided with a maximum spacing between adjacent nanowires that is less than the wavelength of light that the solar cell structure is intended to absorb to achieve an "effective medium" effect. Preferably, the passivating shell, which is composed of a material having a high and indirect bandgap, contains at least a part of the nanowire. The passivating shell may completely fill the spacing between the nanowires.

The internal structure of the nanowire may comprise one or more segments, each segment forming a band gap adapted to absorb light in a predetermined wavelength range of the solar spectrum. By providing segments with different band gaps, each segment is adapted to absorb light in a different wavelength range of the solar spectrum.

In one embodiment of the invention, the solar cell structure comprises a plurality of nanowires provided on the substrate with a maximum spacing between adjacent nanowires that is shorter than the shortest wavelength of said different wavelength ranges.

Fig. 7 schematically illustrates the "effective medium" concept, wherein a) schematically illustrates a conventional multi-junction photovoltaic device prepared by using planar technology, wherein the plurality of layers 741, 742, 743, 744, 745, 746 form segments absorbing different parts of the incident light, indicated with thick arrows. As described in the background, forming such a multilayer structure with a suitable combination of materials is quite difficult and requires the use of an expensive III-V substrate 720. Fig. 7b schematically shows a solar cell structure according to an embodiment of the invention comprising a matrix (matrix) of densely packed nanowires 705 with a maximum nanowire pitch D, i.e. a center-to-center distance, which is smaller than the shortest wavelength for which the device is designed to absorb. Incident photons will "see" the dense array as a sequence of quasi-continuous (quasi-continuous) absorbing layers, and the generated electrons and holes will be collected precisely by the vertical nanowire structures. This approach allows for standard geometries for PV cell illumination (irradiation), ensuring the sequential absorption characteristics required for maximum PV efficiency.

The maximum spacing D between adjacent nanowires is less than 400 nm, preferably less than 200 nm and even more preferably less than 150 nm. The nanowire width in this embodiment is typically about 100 nanometers. The maximum separation D may also be related to the wavelength of light λ and the effective refractive index n of the nanowire material_effAnd (4) correlating. Preferably, the maximum distance D is smaller than λ/n_eff. The substrate 720 is preferably a silicon substrate and the nanowires 705 are preferably grown from the substrate.

Referring to fig. 8, a solar cell structure comprising nanowires 705 with a vertical single pn-junction is provided according to an embodiment of the invention. Substrate 720 may be a p-type group III-V wafer such as an InP or GaAs substrate as schematically shown, but a silicon substrate is preferred in many cases. To contact the n-conducting (n-reducing) region of the top-side top, a conductive transparent film may be deposited over the entire structure, since the regions between the n-doped nanowire regions are insulated and the dielectric mask (e.g., SiO) is surface passivated₂) Covering it, which constitutes a passivating shell 709 surrounding the nanowire 705.

Referring to fig. 9, a plurality of pn junctions forming a segment 715 are provided in the form of an effective dielectric framework, according to one embodiment of the invention. The figure schematically shows a tandem photovoltaic cell with embedded esaki tunnel diode 716 and a surrounding passivating shell 709. By selecting the length, width (diameter) and density of the nanowire 705 to be sufficiently high, this geometry will ensure that substantially all incident radiation will be absorbed by the nanowire 705. On top of the double junction, a segment of indirect bandgap material can be grown to enhance the light absorption efficiency of the light guiding method. Nanowire absorption of photons with energies exceeding the band gap of the selected material can be high and a wavelength dependent penetration depth can be expected.

The passivation shell 709 is mainly used for passivation and insulation in embodiments based on the active medium concept. However, the passivating shell 709 can include a light guiding shell, as described in other embodiments of the present description. Fig. 10 schematically illustrates an embodiment of a solar cell structure according to the present invention comprising a plurality of nanowires 705 protruding from a substrate 720. The nanowire 705 comprises a plurality of pn junction forming segments 715 separated by esaki tunneling diodes 716. The nanowires 705 are densely packed, i.e. with a maximum spacing D between the nanowires that is shorter than the wavelength of the light that the solar cell structure is intended to absorb. Preferably, the top 740 of the nanowire comprises a highly doped segment in order to obtain a low contact resistance for the front contact. As described above, the passivating shell 709, which completely fills the volume between the nanowires, contains the light guiding shell 709.

For the realization of high efficiency multi-junction photovoltaic cells based on nanowires, the light absorption is provided to occur in the proper order, and thus random absorption in different material sections should be avoided. In the above described embodiments, this sequential absorption is achieved by using a core-shell structure through which light is guided from the top end of the nanowire to the bottom end of the nanowire.

Although the solar cell structure of the present invention has been described as being suitable for coupling light into the nanowires through the front contacts or the top, the present invention is not limited thereto. Incident light may also be transmitted into the nanowires through the substrate. In this case, the segment should be arranged to absorb the highest energy closest to the substrate. Furthermore, the substrate may be thinned or even removed.

Embodiments including multiple segments 215, 715 are not limited to segments 215, 715, each of which is adapted to absorb light in a different wavelength range of the solar spectrum. The nanowires 205, 705 of the solar cell structure may comprise two or more segments 215, 715 adapted to absorb light in the same predetermined wavelength range of the solar spectrum. This can be used to step up the voltage output of the solar cell structure.

Although the present invention is described in the context of multijunction PV applications, it is expected to find application in other optoelectronic fields, such as for photo detectors. As will be appreciated by those skilled in the art, the embodiments of the invention described herein may be combined in various ways.

Suitable materials for the substrate include, but are not limited to: si, GaAs, GaP Zn, InAs, InP, GaN, Al₂O₃SiC, Ge, GaSb, ZnO, InSb, SOI (silicon-on-insulator), CdS, ZnSe, CdTe. Materials for nanowires and segments of nanowires include, but are not limited to: GaAs, InAs, Ge, ZnO, InN, GaInN, GaNAlGaInN, BN, InP, InAsP, GaAsP, GaInP, GaInAs, AlInP, GaAlInP, GaAlInAsP, GaInSb, InSb, Si. Possible donor dopants are Si, Sn, Te, Se, S, etc., and acceptor dopants are Zn, Fe, Mg, Be, Cd, etc. Suitable materials for the passivation and light guide shell include, but are not limited to: AlN, GaN, InN, AlGaInN, BN, SiC, GaP, GaAsP, AlAs, AlP, AlSb, AlAsP, GaAlAs, GaAlAsP, AlInP, SiO₂、Al₂O₃、ZnO、SiN、HfO₂、ZrO₂ZnCdTeSeS, glass, organic polymers, and the like. It should be noted that the use of nanowire technology described herein makes it possible to use nitrides such as GaN, InN and AlN.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims (5)

1. A solar cell structure on a substrate (220), comprising a layer comprising a plurality of nanowires (205), the plurality of nanowires (205) constituting light absorbing parts of the solar cell structure, the plurality of nanowires (205) being oriented perpendicular to a surface of the substrate (220) and comprising at least one PN junction in the nanowire (205),

wherein the substrate (220) comprises a photodiode structure formed by counter-doped regions (222, 223) in the substrate (220) below the nanowire (205), and at least a portion of incident light is directed to and absorbed by the photodiode structure.

2. The solar cell structure of claim 1, wherein at least one PN junction in the nanowire (205) is connected in series with a photodiode structure formed by a counter-doped region in the substrate (220).

3. The solar cell structure of claim 1, further comprising a light guiding shell (210), the light guiding shell (210) surrounding at least a portion of each of the plurality of nanowires (205) and being adapted to guide incident light along the nanowires (205) and to penetrate light absorbing portions of the solar cell structure, wherein the light guiding shell (210) has a higher band gap than the nanowires (205) or the light guiding shell (210) comprises a dielectric layer.

4. The solar cell structure of claim 3, wherein the light guiding shell (210) has a higher band gap than the nanowire (205).

5. The solar cell structure of claim 3, wherein the light guiding shell (210) comprises a dielectric layer.