SE533522C2 - Nanowire-based effective medium solar cell - Google Patents

Description

R 53nium is a suitable substrate material. The availability of Ge in the earth's crust is limited and expensive, and if such tandem solar cells with high efficiency were to be used on a large scale on earth, this would be a limitation. For this reason, the development of MJ solar cells based on crystalline Si or even simpler substrates would open up new possibilities for applications on earth, which take advantage of the MJ cells' higher efficiencies and lower costs and greater availability of Si substrates. compared to Ge. An MJ PV cell according to the prior art is shown schematically in ñg. 2. The cross section shows lattice-adapted growth on Ge-substrate3 which reaches efficiencies of more than 40% with concentrators.

Technical limitations of planar III-V multiple transition solar cells: Efficiencies of over 60% are very difficult to achieve due to physical limitations and require more than three transition elements.

Conventional III-V materials for multiple transition cells require perfect lattice fitting on large substrate surfaces to avoid dislocations.

Good functionality of the device will also require a very high degree of homogeneous composition on an entire board.

Lattice adjustment for epitaxial growth and / or thick buffer layers are necessary to avoid defect formation, which makes it difficult to manufacture devices with more than a few transitions.

In addition to the above-mentioned technical challenges associated with the SOA multiple transition cell, issues related to cost and scaling present problems such as: v Multiple transition cells grown on Ge or III-V substrates are very expensive due to high substrate costs and small size disks. v IIl-V materials are today epitazrially grown in high-grade MOCVD or even low-flow MBE reactors.

The high cost of expensive materials makes the use of optical concentrators necessary to improve the cost ratio for execution at the system level. 2 F. Dímroth, “High-efficiency solar cells from III-V compound serniconductors” Phys. State. Solar. (c) 3, 373 (2006) and http: Uwwwsgectrolab.com/com/news/news-detail.asp? id = l 72 3 Figures courtesy of LL. Kazmerski, "Solar photovoltaics R&D at the tipping point: A 2005 technology overviewv" I Electr Specter Rel Phen 150, 105 (2006) l5 20 25 30 533 522 3 ~ Even if the cost could be reduced, concentrators would need to achieve a saturation voltage even during full sunlight, which limits the maximum power output.

Disclosure of the invention It is obvious that solar cell devices according to the state of the art can be improved in order to achieve the mentioned advantages, which relate to efficiency and production cost.

The object of the present invention is to provide a solar cell structure which can overcome the disadvantages of devices according to the prior art. This is achieved by means of a solar cell structure as defined in claim 1.

The solar cell structure according to an embodiment of the present invention comprises a plurality of nanowires, which form the light-absorbing part of the solar cell structure.

The nanowires are provided in a matrix on a substrate, preferably silicon, with a pitch between adjacent nanowires that is shorter than the light wavelength that the solar cell structure is designed to absorb. The nanowires form a quasi-continuous efficient medium.

The solar cell structure according to an embodiment of the present invention comprises a nanowire, which forms the light-absorbing part of the solar cell structure, and a waveguide enclosing at least a part of the nanowire. The nanowire is made of a material with direct bandgap and the waveguide is word in a material with a large and indirect bandgap.

The light absorption is achieved through and limited to the nanowire by the provision of a segment in the nanotree which constitutes a bandgap adapted to absorb a part of the solar spectrum. Preferably, the nanowires comprise a plurality of segments, each of which has individual band gaps corresponding to different parts of the solar spectrum.

According to an embodiment of the present invention, the segments are separated by being connected in series by means of Esaki diodes.

According to an embodiment of the present invention, the segments are separated by couplings in series by means of metal segments.

Thanks to the present invention, it is possible to produce solar cells with high efficiency at reasonable costs. An advantage of the solar cell structure according to the present invention is that the structure according to the invention allows heterostructures which do not require lattice adaptation, which gives a large degree of freedom with respect to material combinations. In principle, there is no limit to the number of different bandgaps, ie. segments of the nanowire that provide the ability to absorb all or a selected portion of the solar spectrum. By placing the nanowires sufficiently close to each other on the substrate, the advantages of using nanowires are combined with an efficient absorption of the light because the incident light "sees" a continuous effective medium.

Thanks to the small plant area for each wire, no extremely homogeneous growth is needed on an entire disc, which reduces the requirements for plant systems. In addition, thanks to the small area, the substrate can be polycrystalline or thin-silicon or an even simpler material.

Embodiments of the invention are defined in the independent 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 accompanying drawings and claims.

Description of the Figure The invention will be described in more detail below in connection with the accompanying drawings, in which Fig. 1 schematically shows a part of the AMLS solar spectrum which can be used theoretically by a silicon solar cell compared to a GalnP / GaInAs / Ge solar cell. Slightly shaded areas show efficiency losses due to thermal excitation of charge carriers or transmission of photons, Fig. 2 shows a prior art MJPV cell in a cross-sectional view where lattice-adapted growth on Ge-substrate 4 reaches efficiencies of more than 40% with concentrators, Fig. 3 schematically shows the solar cell structure according to an embodiment of the present invention, 4 Figures with the permission of LL. Kazmerski, “Solar photovoltaics R&D at the tipping point: A 2005 technology overview” In Electr Specter Rel Phen 150, 105 (2006) 10 20 25 30 533 522 5 Fig. 4 schematically shows embodiments of the present invention, where in fi g. Fig. 4a shows the nanowire above the waveguide and the substrate provides a diode and where in Fig. 4b the nanowire ends at the upper end of the waveguide, Fig. 5 schematically shows embodiments of the present invention, where in Fig. 5a Esaki diodes are used and where in ñg. Fig. 5b shows metal segments used to connect the segments in the nanowire, Fig. 6 schematically shows a solar cell structure according to an embodiment of the present invention, Fig. 7 schematically shows a solar cell structure according to an embodiment of the present invention, Fig. 8 schematically shows a solar cell structure according to an embodiment of the present invention, and Fig. 9 a] schematically shows a solar cell structure according to the prior art, and b) schematically shows a solar cell structure according to an embodiment of the present invention. Detailed description The solar cell structure 300 according to the present invention is shown schematically in fi g. 3, where the light absorbing member is a nanowire 305, which is at least partially enclosed by a waveguide 305. The nanowire 305 usually comprises a material with a direct bandgap. The waveguide 3 10 consists of a material with a large and indirect band gap. The bottom of the structure is in contact with a substrate 320. The nanowire 305, which is preferably located in the middle of the waveguide 310, consists of several segments 315 with larger band gaps closer to the top, to effectively absorb different parts of the solar spectrum. The segments are connected in series with Esaki diodes 316 or short metal segments. The waveguide 3 should preferably be narrow enough to allow only light propagation in single mode, and the nanowire should be comparatively small.

The structure is contacted from the bottom by a bottom contact 325, e.g. through the substrate 320 and from above by means of a top contact, which may be a metal grid which contacts the upper wire or a transparent contact which covers the whole structure. As shown in fi g. 4a, the nanowire 305 may have an upper portion 340 extending above the wave clearer 310 to facilitate contacting properties. In addition, the portion of the nanowire extending across the waveguide may be doped to further improve the contacting properties. In this embodiment, the contacting means preferably covers only the nanowire.

In an alternative embodiment, as shown in Fig. 4b, the nanowire ends at the upper part of the guide. Possibly, but not necessarily, the nanowire ends with a cup 350 of the catalyst particle common in some nanowire plant growing methods.

The device is most teachable for flat, preferably transparent contacts.

The substrate may either function only as a mechanical support and electrical contact, as shown in Fig. 3, or it may comprise one or more electrically active components, for example, a conventional photodiode structure. An embodiment with a photodiode formed with a p-doped region 322 and the subsequent n-doped region 323 is shown in Fig. 4a.

Fig. 5a schematically shows an enlargement of the nanowire 305, with the segments and the Esaki diodes 316, and with a p-type and n-type region between the segments. Fig. 5b schematically shows an embodiment of the invention, in which the Esaki diodes are replaced by metal segments 317. This is possible, since the thin nanowire 305 in the solar cell structure according to the invention does not have to be transparent. As an alternative to the metal segment, a degenerate doped semiconductor can be used instead.

On a solar panel, a large number of the structures described above are usually tightly packed on the disk, to cover the main part of the surface with the waveguides. The solar panel can comprise a disk, but also a number of interconnected disks can provide the desired energy production.

The solar cell structure according to the invention can also be used as a detector.

The function of the solar cell structure according to the invention is as follows. At the upper part of the structure, the (solar) light is directed into the waveguide. Since the waveguide 310 is of a material with an indirectly large bandgap, no light will be absorbed here, and since the waveguide is single mode, the field is strongest in the core, i.e. where the nanowire 305 is located. As the light for fl moves downwards, higher energies are absorbed more efficiently, at the same time as photons with a lower energy content than the band gap will only be affected by a transparent waveguide. When the energy bands are sequentially separated in the nanowire 305, the photons give rise to photoelectricity in each segment 315, as large as the band gap in that segment. Under ideal conditions, the structure will be so efficient that only light with a low energy content penetrates the substrate. However, the substrate may also include an ordinary photodiode for collecting scattered photons with a high energy content.

Embodiments of solar cells according to the invention have the following features: The waveguide directs the light in a regular manner through areas of decreasing bandgap, which enables the light to collect sequentially.

Allows heterostructures where grid adjustment is not required, which provides a high degree of freedom with respect to material combinations.

In principle, there is no limit to the number of different bandgaps, ie. segment in the nanowire.

These structures can possibly grow in much simpler systems than standard MOCVD.

Materials that contain bandgaps in the entire solar spectrum can be incorporated into the wire, under ideal conditions the substrate then becomes only a support structure.

Thanks to the small plant area for each wire, no extremely homogeneous waxing is needed on an entire disc, which reduces the demands on the plant sister.

In addition, thanks to the small area, the substrate can be polycrystalline or thin-silicon or an even simpler material.

The waveguide structure provides an inherent concentration of photons to the nanowire, which provides a saturated voltage even under indirect light conditions.

The esaki diodes in planar tandem cells can be replaced with metallic or degenerate doped semiconductor segments, since the nanowire does not have to be transparent.

Example of specific embodiment: A photonic waveguide design, created by radial growth of a completely transparent shell with a high refractive index, such as AlN, functions as a whole waveguide structure of about 0.5 μm diameter, of which about 100 nm is a multiple bandgap core structure. The upper part (approx. 0.5 μm) of the nanowire will, thanks to the dense "grass-like" arrangement of long nanowires, capture the incoming flow, which is then led downwards in such a way that the high energy content component 10 20 25 25 30 533 522 8 will be captured in the upper segment, which looks like the completely transparent waveguide for all the photo energy below its bandgap. The same selective absorption and transmission will then be offered by the next segment with its lower bandgap etc. Above the top selected bandgap segment there is a long, hard n-doped GaN segment used for contacting.

The bottom segment may be made of lnN and with increasing fractions of Ga up to the top segment and consist of a composition of approximately GavlngN. In this case, the substrate will only provide support and bottom contact, since the smallest band gap will be at the bottom of the nanowire. Since the growth of different compositions of GalnN in the axial direction is not yet fully proven, the use of the AlGalnAsP NW system can be more practical. In this system there are materials with direct bandgap with values between 0.4 eV up to 2.25 eV, which consequently can very well compete with the best known multiple transition cells. In this case, the three bottom segments may consist of the well-established InAstxPx system, and the top two in e.g. The GaxlnLXP system, with the upper segment of Ga-rich (80%) GalnP having a direct bandgap of 2.25 eV, combinations that have not been accessible using conventional methods where grid fitting has been required.

According to one embodiment of the present invention, which is shown schematically in Figs. 6, a solar cell structure is provided which comprises nanowires with a vertically arranged simple pn junction. The substrate may be p-type III-V disks, such as, for example, lnP or GaAS substrates, which are shown in the figure, but silicon substrate is in many cases a preferred choice. To contact the top n-conducting areas, a conductive transparent ñlm will be deposited over the entire structure since the areas between the n-doped nanowire areas are covered by an insulating and surface passivating mask, such as SiO 2.

According to an embodiment of the present invention, which is shown schematically in Fig. 7, a solar cell structure is provided which comprises nanowires with a plurality of vertically arranged pn junctions, of which the upper pn junctions form a large band gap section and the lower pn junctions forms a section with a small band gap. These sections are preferably separated by Esaki tunnel diodes. For example, the tunnel diode bearings may be heavily doped with AlGaAs, GaAsP or GaInP. This combination of materials with different lattice constants would never have been available using conventional methods where lattice adjustment is required. Since lattice adaptation is of minor importance (since this otherwise hinders this type of development when traditional planar epitaxy methods are used), this approach may be extended to more transitions in the future. For a dual transition solar cell, the band gap for the top subcell should ideally be in the range 1.6-1.8 eV and the bottom subcell should be in the range 0.9-1.1 eV. These bandgap energies can be achieved by using GaAsP, or GaInP, for the upper cell and GalnAs, or lnAsP, for the lower cell. The total energy range covered by these material combinations for the purpose of collecting energy covers 0.4 eV (lnAs) to 2.24 eV (GaInP45). The Pn junctions will be surrounded by barrier layers with higher bandgap energy to enclose minority charge carriers in the nanowires, and to passivate the surface of the nanowires.

According to one embodiment, a plurality of pn junctions are provided inside a light guide structure. Fig. 8 shows an electromotive tandem (solar} cell with embedded Esquitine diodes and ambient light guides. By selecting the length of the nanowires, thickness and density high enough, this geometry ensures that all incident radiation will be absorbed by the nanowires. segments of indirect material are grown to improve the light absorption efficiency of the light guide approach, similar to a “grass ~ ñeld.” The nanowire absorption of photons with energies exceeding the bandgap of the selected material can be high and According to one embodiment, which is further described below, the nanowires are grown in predetermined patterns with subwavelength division.

For the realization of electronotoric multi-junction (solar) cells based on nanowires, it is required that the light absorption takes place in a correct sequence. Consequently, random absorption in the various materials must be avoided. In the embodiments described above, the sequential absorption is accomplished by means of core-shell structures. Thereby the light is conducted from the upper end of the nanowires to their lower end.

Another approach to achieve this controlled absorption, which represents a further embodiment of the present invention, can be described as an "effective medium" -like concept. An "effective medium" is usually described as structures containing different materials in length scales that are substantially less than the wavelength of the incident light. This concept can be seen as a replacement for the commonly used absorption in continuous films by the optical absorption effects by a dense arrangement of parallel nanowires, with division between the wires that is substantially less than the wavelength. This is illustrated in Figs. 9a-b, where a) illustrates a conventional electromotive multi-transition (solar) cell in which a plurality of layers 901, 902, 903, 904, 905, 906 form segments which absorb different parts of the incident light, as indicated by the thick arrow. As described in the background, it is extremely difficult to form such multilayer structures with suitable material combinations and this requires the use of Ill-V substrate 911. Figs. 9b schematically shows the solar cell structure according to an embodiment of the present invention, comprising a matrix of tightly packed nanowires 910, with a pitch D between the nanowires which is smaller than the shortest wavelength the device is adapted to absorb. Incident photons will “see” the dense array as a result of quasi-continuous absorption layers while the generated electrons and holes will be strictly collected through the vertical nanowire structures. This approach enables standard geometries for illuminating electromotive (solar) cells, thus ensuring the sequential absorption characteristics required for the highest electromotive yields.

The pitch D between adjacent nanowires is typically below 300 nm, preferably below 200 nm and more preferably below 150 nm. The size of the nanowires in this embodiment is typically on the order of 100 nm. The substrate 920 is preferably a silicon substrate and the nanowires 910 are grown from the substrate. For example, the nanowires may have the internal structure shown in Figures 6-8. The present invention has also been described in the context of electromotive multi-junction (sub-cell style adaptations), it is expected that it will find use in other fields of optoelectronics, such as photodetectors. Those skilled in the art will appreciate that the embodiments of the present invention may be combined in various ways.

Suitable materials for the substrate include, but are not limited to: Si, GaAs, GaP, GaPzZn, GaAs, lnAs, lnP, GaN, AlgOg, SiC, Ge, GaSb, ZnO, lnSb, SO1 (silicone insulator), CdS , ZnSe, CdTe. Suitable materials for the nanowires and nanowire segments include, but are not limited to: GaAs (p), lnAs, Ge, ZnO, InN, GalnN, GaN AlGalnN, BN, lnP, lnAsP, GalnP, InGaP: Si, lnGaPzZn, GaInAs AlInP, GaAllnP, GaAlInAsP, GalnSb, lnSb, Si. Possible donor dopants are Si, Sn, Te, Se, S, etc, and acceptor dopants are Zn, Fe, Mg, Be, Cd, etc. It should be noted that nano-tree technology makes it possible to use nitrides such as GaN, InN and AIN.

Other combinations of interest include, but are not limited to, GaAs, GaInP, GaAIInP and GaP systems. Although the invention has been described in connection with what are currently considered to be the most practical and preferred embodiments, it is to be understood that upp »finnirigeii is not limited to the embodiments shown. On the contrary, it is intended to cover various modifications and corresponding arrangements within the scope of the appended claims.

Claims (1)

A patent cell structure comprising a number of nanowires (910) constituting the light absorbing part of the solar cell structure, characterized in that the nanowires (910) comprise a number of segments (315), each adapted to absorb light in different wavelength ranges; and the nanowires are arranged vertically on a substrate with a pitch between adjacent nanowires, which pitch is shorter than the wavelength of the light that the solar cell structure is intended to absorb and less than the shortest wavelength of the wavelength regions, the nanowires and intermediate material acting as an effective medium. light will be conducted sequentially through the nanowires and the different wavelength ranges will be selectively absorbed in each segment (315). A solar cell structure according to claim 1, wherein the pitch between adjacent nanowires is less than 400 nm. A solar cell structure according to claim 2, wherein the pitch between adjacent nanowires is less than 150 nm. A solar cell structure according to any one of the preceding claims, wherein the nanowire (305) comprises fl your segments (315), each segment being adapted to absorb a part of the solar spectrum and arranged so that the segment with the largest band gap is closest to the upper spirit of the solar cell structure, i.e. opposite end of the end connected to the substrate. Solar cell structure according to claim 4, wherein the segments are connected in series by means of Esaki diodes (3 16). A solar cell structure according to any one of the preceding claims, wherein the nanowires (910) are grown on a silicon substrate (920). A photovoltaic panel comprising a number of photovoltaic structures according to one of the preceding claims.