WO2010018490A2

WO2010018490A2 - A photovoltaic cell and a method of manufacturing the same

Info

Publication number: WO2010018490A2
Application number: PCT/IB2009/053408
Authority: WO
Inventors: Yukiko Furukawa; Frank Pasveer; Johan Klootwijk; Jinesh Kochupurackal
Original assignee: Nxp B.V.
Priority date: 2008-08-12
Filing date: 2009-08-05
Publication date: 2010-02-18
Also published as: WO2010018490A3

Abstract

A photovoltaic cell (100) which comprises a semiconductive matrix (102) of a first type of conductivity, and a plurality of semiconductive nanoclusters (104) of a second type of conductivity which differs from the first type of conductivity which are at least partially embedded in the semiconductive matrix (102) so that a pn-junction (106) is formed between the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102).

Description

A photovoltaic cell and a method of manufacturing the same

FIELD OF THE INVENTION

The invention relates to a photovoltaic cell. Moreover, the invention relates to a method of manufacturing a photovoltaic cell. BACKGROUND OF THE INVENTION

A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. Photovoltaics may be denoted as the field of technology related to the application of solar cells as solar energy. The term solar cell may be used for devices intended specifically to capture energy from sunlight, whereas the term photovoltaic cell may be used when the source is unspecified.

There are several types of solar cells. The majority of commercially available solar cells are silicon based solar cells using a pn junction diode. Since energy conversion efficiency for the solar cell is relatively low (12-16% in the case of poly crystalline silicon solar cell), a large silicon area is required in order to gain enough electricity to supply power. Po Iy crystalline silicon and its processing are still rather expensive and a shortage of a silicon supply due of consumption by IC sector is a limitation. Thus, many studies have been carried out to find a cheaper and more efficient solar cell than the currently manufactured one. US 2008/0092946 discloses photovoltaic structures for the conversion of solar irradiance into electrical free energy. In particular implementations, the photovoltaic structures can be fabricated using low cost and scalable processes, such as magnetron sputtering. In a particular implementation, a photovoltaic cell includes a photoactive conversion layer comprising one or more columnar semiconductor and oxide layers with nanometer-size semiconductor grains surrounded by a matrix of oxide. The semiconductor and oxide layer can be a disposed between electrode layers. In some implementations, multiple semiconductor and oxide layers can be deposited. These so-called semiconductor and oxide layers absorb sun light and convert solar irradiance into electrical free energy.

However, the manufacture of conventional photovoltaic cells may be cumbersome, since it is necessary to have a seed layer and an inter layer before depositing electrode and/or pn junction, further losing area of adsorbing light due to oxide surrounding electrode semiconductive materials, which suppress efficiency of the photovoltaic cells. OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a photovoltaic cell architecture which can be manufactured with reasonable effort.

In order to achieve the object defined above, a photovoltaic cell and a method of manufacturing a photovoltaic cell according to the independent claims are provided.

According to an exemplary embodiment of the invention, a photovoltaic cell (particularly a photovoltaic structure for the conversion of solar irradiance into electrical free energy, such as a solar cell) is provided which comprises a semiconductive matrix of a first type of conductivity and a plurality of semiconductive nanoclusters (particularly spontaneously nucleated), of a second type of conductivity which differs from (particularly is complementary or opposite to) the first type of conductivity, which are at least partially (that is which are surrounded partially or entirely by matrix material) embedded (or dispersed) in the semiconductive matrix so that a pn-junction is formed (particularly at a boundary) between the plurality of semiconductive nanoclusters and the semiconductive matrix (wherein the pn- junction may be configured in a manner to allow for the generation of electron-hole pairs upon irradiation of the pn-junction with electromagnetic radiation such as light). Particularly, the nanoclusters may be bound to an electrode of the photovoltaic cell.

According to another exemplary embodiment of the invention, a method of manufacturing a photovoltaic cell is provided, wherein the method comprises at least partially embedding a plurality of semiconductive nanoclusters of a second type of conductivity in a semiconductive matrix of a first type of conductivity which differs from the second type of conductivity so that a pn-junction is formed between the plurality of semiconductive nanoclusters and the semiconductive matrix (particularly, nanoclustering and pn-junction formation may be performed simultaneously). The term "semiconductive" may particularly denote a solid material (such as silicon or germanium or gallium arsenide) that has electrical conductivity in between a conductor (such as copper) and an insulator (such as plastic). Semiconductors and insulators differ primarily in that insulators have larger band-gaps, that is energies that electrons must acquire to be free to move from atom to atom. In semiconductors at room temperature, just as in insulators, very few electrons gain enough thermal energy to leap the band gap from the valence band to the conduction band, which is necessary for electrons to be available for electric current conduction. Hence, semiconductive may mean having characteristics of a semiconductor, that is having electrical conductivity greater than insulators but less than conductors. In the context of this application, a semiconductive material may be denoted as a material having a bandgap of less than 4 eV.

The term "matrix" may particularly denote a physical medium that surrounds and holds nanoclusters of an other material. A matrix may be a three-dimensional material block which has recesses or holes which are basically entirely filled with the nanoclusters, consequently holding the nanoclusters in place. A matrix may thus denote a principal phase of a semiconductive material in which another constituent is embedded. In an embodiment, the volume of the matrix material may be larger than the volume of the nanoclusters. A matrix may be a binding, surrounding substance within which nanoclusters develop or are contained. The term "nanoclusters" may particularly denote physical particles being surrounded at least partially by a matrix so that a direct physical contact is formed between matrix and nanocluster, particularly allowing for the formation of a pn-junction at a boundary between matrix and nanoclusters. Such nanoclusters or nanoparticles may have dimensions in the order of magnitude between 0.5 nm and 100 nm, particularly between 1 nm and 50 nm. These dimensions may be average values averaged over the nanoclusters of a thin film. Such nanoclusters may have various shapes such as a spherical shape or shapes with lower degrees of order. Examples for such nanoclusters are material inclusions in a surrounding material of another phase or chemical composition formed spontaneously during production, nanotubes (such as carbon nanotubes) or nanowires, particles which are supplied to another material and become solidified, etc. In an embodiment, nanoclusters are particles of multiple atoms or molecules formed by a deposition technique so as to be surrounded by another medium. Preferably, the nanoclusters are spontaneously crystallized nano-crystalline semiconductor particles.

The term "pn-junction" may particularly denote a junction formed by combining a p-type semiconductor and an n-type semiconductor together in very close contact. The term junction may refer to the area where the two regions of the semiconductor meet. It can be thought of as the border region between the p-type and n-type blocks.

The term "type of conductivity" may denote that current is carried by positive or negative mobile charge carriers. One type of conductivity is therefore current carried by positively charged charge carriers, another type of conductivity is therefore current carried by negatively charged charge carriers. Conduction by p-type charge carriers may be denoted as a complementary conduction mechanism as compared to conduction by n-type charge carriers. The term "first type of conductivity" and "second type of conductivity" may denote that current is carried by positive or negative mobile charge carriers, or vice versa.

According to an exemplary embodiment of the invention, a photovoltaic cell is provided which does not comprise two completely spatially separated planar layers of n-doped material and p-doped material, but in contrast to this is formed of multiple grains or nanoclusters of semiconductive material in the order of magnitude of nanometers of the p- or n-type which is embedded within a surrounding matrix of a semiconductive material of the opposite type of conductivity, namely n- or p-type, respectively. By taking this measure, a solar cell may be provided which has a very high area of pn-junction at a direct connection between the nanoclusters and the surrounding portions of the matrix. Consequently, the efficiency of energy conversion between electromagnetic radiation such as light and an electric current may be significantly improved, so that the degree of efficiency of a solar cell may be improved.

Advantageously, it is possible to manufacture such a photovoltaic cell with reasonable effort, since the formation of nanoclusters and surrounding matrix may be performed in one and the same deposition procedure, with a direct controllability of the volume ratio between nanoclusters and matrix by adjusting a precursor composition. Moreover, conventional deposition or growth procedures from thin film technology may be applied. This may particularly allow for the spontaneous formation of nucleation nanoclusters in an interior of the matrix. According to an exemplary embodiment, a nanocluster-matrix solar cell prepared by thin film technology may be provided. Particularly, a photovoltaic device may be provided comprising an active layer or active layers of semiconductor nanoclusters. Such semiconductor nanoclusters may be polycrystalline or single crystalline nanocrystal grains grown by CVD (chemical vapour deposition) or ALD (atomic layer deposition), etc. In an embodiment, the grains may be located directly adjacent the surrounding matrix to thereby automatically form pn-junctions between grains and matrix without any additional effort. Thus, in an embodiment, the boundary between grains and matrix may be free from any further component such as an additional layer. Such pn-junctions may provide an intrinsic mechanism to separate electrons and holes upon absorption of a light quantum. Thus, a nanocluster-matrix pn-junction solar cell may be provided which has separated contacts for p- and n-semiconductors. Nanoclustering and pn-junction deposition may be performed simultaneously. During such a deposition procedure, the formation of spontaneously crystallized nanocrystalline semiconductors may be possible. Thus, a spontaneous nucleation may occur which may separate the phases of nanoclusters and matrix.

According to an exemplary embodiment, a thin film structure may be provided constituted of inorganic nanoclusters within an inorganic matrix. Both p- and n- semiconductors may be deposited simultaneously, so that a pn-junction may be formed in a smooth way and a limited amount of defects can be expected. Particularly using ALD deposition may allow the formation of nanoclusters with the matrix in one step processing.

Next, further exemplary embodiments of the photovoltaic cell will be explained. However, these embodiments also apply to the method. One of the first type and the second type of conductivity may be p-type, and the other one of the first type and the second type of conductivity may be n-type. Hence, in one embodiment, the nanoclusters are n-type and the matrix is p-type. In another embodiment, the nanoclusters are p-type and the matrix is n-type. N-type may denote a doping with a dopant such as arsenide. P-type may be denoted as a doping with a dopant such as gallium. For example, the n-type semiconductor material in the embodiment can be any of n-type semiconductor materials, for instance, semiconductive oxide, specially transition metal oxide with excess amount of metal or oxygen vacancies such as Tiθ2, ZnO, Snθ2, SrTiO₃, III- VI semiconductor oxide and IV, III-V, H-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants. The p-type semiconductor material in the embodiment can be any of p-type semiconductor. A preferable choice for the p-type material may be semiconductive oxide, specially transition metal oxide with excess amount of oxygen or metal vacancies such as Cu₂O, CuO, CuAlO₂, LaAlO₃, SnO, NiO, ZnO and IV, III-V, II-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants.

The photovoltaic cell may further comprise a first electrode on which the plurality of semiconductive nanoclusters and the semiconductive matrix are formed. Thus, such a first electrode may serve as some kind of substrate on which the deposition of the nanoclusters and the matrix may be performed, preferably in a common growth procedure. Such an electrode may then be used as one of two electrodes of the photovoltaic cell between which an electric current as a result of the conversion of light energy into electric energy may be generated. The first electrode, in turn, may be formed or fastened on a substrate. The first electrode can but need not be optically transparent.

The photovoltaic cell may further comprise a second electrode positioned or deposited on the plurality of semiconductive nanoclusters and the semiconductive matrix. Thus, the second electrode and the first electrode may sandwich the nanoclusters embedded in the semiconductive matrix. Such a second electrode may be formed on a planar arrangement of the nanoclusters and the embedding matrix. The second electrode may comprise a transparent electrically conductive material. Such a transparency may be a transparency for electromagnetic radiation used for generating electric energy by the photovoltaic cell, particularly optical light. An example of an optically transparent but electrically conductive material is indium tin oxide (ITO).

The nanoclusters in the matrix can be deposited either on the first non transparent electrode or on the second transparent electrode, thus processing to make the photovoltaic cell can be reversed.

In an embodiment, at least a part of the plurality of semiconductive nanoclusters may contact one another (for instance forming a continuous structure) to prevent the pn-junctions from being short-circuited with an electrode. In other words, such an architecture may allow to electrically separate the p-portion from the n-portion thereby ensuring a proper performance of the photovoltaic cell. By taking such a measure, any undesired electric bypassing between an electrode and the pn-junction may be safely prevented. Thus, the nanoclusters may contact one another (form an uninterrupted structure) in such a manner as to prevent any electrically conductive path which does not go through the nanoclusters. Additionally or alternatively, the photovoltaic cell may comprise an electrically insulating layer between the second electrode on the one hand and the plurality of semiconductive nanoclusters and the semiconductive matrix on the other hand, wherein the electrically insulating layer may comprise a plurality of through holes (assigned to each of the nanoclusters) through which the plurality of semiconductive nanoclusters are contacted (for instance by a via formed in each of the holes) with the second electrode in such a manner to prevent the pn-junctions from being short-circuited with the second electrode. Hence, a patterned or perforated electrically insulating layer (which can be made of an optically transparent material such as silicon oxide to allow transmission of light through the electrically conductive layer thereby ensuring a sufficient efficiency of the solar cell) may be formed to define at which positions electric current generated by photons having produced electron-hole pairs close to the pn-junction can be conducted to the first electrode.

The plurality of semiconductive nanoclusters may all be made of the same material. In other words, it may be sufficient that all semiconductive nanoclusters have the same material composition. This ensures a very simple manufacture, since it allows to manufacture the entire structure in a common process by controlling only a small number of precursors in a deposition procedure such as CVD, which is possible with high accuracy.

Alternatively, different ones of the plurality of semiconductive nanoclusters may be made of two or more different kinds of material. In other words, different groups of semiconductive nanoclusters may be provided wherein semiconductive nanoclusters of a common group may be made of the same material, whereas different groups of semiconductive nanoclusters may differ regarding their material composition. Different materials may have different electric current generation properties, such as different light absorption efficiency, different bandgaps, etc. In such an embodiment, two, three or even more different kinds of nanoclusters of different materials may be provided in the matrix each of which being capable of absorbing light (or more generally electromagnetic radiation) in a specific range of wavelengths. Therefore, the energy conversion efficiency of the photovoltaic cell may be further increased, since a larger range of wavelengths may be used by the system for generating electric energy.

The photovoltaic cell may comprise a backscatter layer for backscattering electromagnetic radiation, wherein the first electrode may be arranged between the backscatter layer on the one hand and the plurality of semiconductive nanoclusters and the semiconductive matrix on the other hand. Alternatively (particularly when the first electrode is not transparent), the backscatter layer may be arranged between the first electrode on the one hand and the plurality of semiconductive nanoclusters and the semiconductive matrix on the other hand The backscatter layer may be arranged at a surface of the photovoltaic cell opposing another surface of the photovoltaic cell which is to be brought in direct interaction with the light being the source of the energy generated by the photovoltaic cell. Such a backscatter layer may be capable to reflect light which has propagated through the matrix and the nanoclusters without having been absorbed to form electron hole pairs. After the reflection at the backscatter layer, the electromagnetic radiation can propagate again through the nanoclusters embedded in the matrix, thereby further increasing the probability of absorption and thus the efficiency. At least a part of the nanoclusters may have a dimension of about 0.5 nm to about 100 nm, particularly may have a dimension of about 1 nm to about 50 nm. Such a dimension may be a spatial extension along one, two or three coordinate axes. The plurality of semiconductive nanoclusters and the semiconductive matrix together form a thin film, particularly form a layer having a thickness between about 10 nm and about 10 μm, more particularly having a thickness between about 10 nm and about 100 nm. With such a small thickness in a direction perpendicular to a surface of the solar cell to be exposed to light, a high efficiency can be obtained. In the following, further exemplary embodiments of the method will be explained. However, these embodiments also apply to the photovoltaic cell.

The plurality of semiconductive nanoclusters and the semiconductive matrix may be deposited simultaneously in a common procedure. By manufacturing these two components in a common step, it is possible that the manufacture time is kept short and the costs for manufacture are kept small as well.

The plurality of semiconductive nanoclusters and the semiconductive matrix may be manufactured by thin film technology, particularly by CVD, more particularly PECVD (plasma enhanced chemical vapour deposition), or ALD. Particularly with ALD, it is possible to define the thickness of the matrix with the embedded nanoclusters with very high accuracy. MBE (molecular beam epitaxy) and sputtering are other alternatives for appropriate manufacture methods.

Since embodiments of the invention allow the combination of a large number of different materials, design freedom is achieved allowing for optimizing the material combination of nanoclusters and matrix in accordance with requirements of a specific application. In this context, the band gap of the corresponding materials may be selected to adjust the light absorbing capabilities.

For instance, the following materials may be used for forming the matrix and/or the nanoclusters: IV, III-V, H-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants such as silicon, germanium, SiC, AlP, AlAs, AlSb, AlN, C (diamond), GaP, GaAs, GaN, GaS, GaSb, InP, InAs, InN, InSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, PbSe, PbTe, HgS, HgSe, HgTe, or any other semiconductive material. It is also possible to use semiconductive metal oxide such as TiO₂, CuO, Cu₂O, CuAlO₂, LaAlO₃, NiO, ZnO Fe₂O₃, WO₃, SnO₂, SnO, Cr₂O₃, NiO, As₂O₃, Nb₂O₅, In₂O₃, CeO₂, SrTiO₃, PbTiO₃, Bi₄Ti₃Oi₂, Bii₂TiO₂₀, MgTiO₃, CaTiO₃, PbZrO₃, LiNbO₃, SrBi₂Tr₂O₉, KTaO₃, or GaAsO₄.

In an embodiment of the invention, an efficiency of the excitation of excitons (electron hole pairs) per photon may be more than 10 %, for instance in a full sunlight condition. For instance, 100 W/m² or more may be generated. The photovoltaic cell may be monolithically integrated in semiconductor technology, particularly comprising one of the group consisting of a group IV semiconductor (such as silicon or germanium), or a III-V semiconductor (such as gallium arsenide).

For any method step, any conventional procedure as known from semiconductor technology may be implemented. Forming layers or components may include deposition techniques like CVD, PECVD, ALD, oxidation or sputtering. Removing layers or components may include etching techniques like wet etching, plasma etching, etc., as well as patterning techniques like optical lithography, UV lithography, electron beam lithography, etc.

Embodiments of the invention are not bound to specific materials, so that many different materials may be used. For conductive structures, it may be possible to use metallization structures, suicide structures or polysilicon structures. For semiconductor regions or components, crystalline silicon may be used. For insulating portions, silicon oxide or silicon nitride may be used.

The system may be formed on a purely crystalline silicon wafer or on an SOI wafer (Silicon On Insulator). Any process technologies like CMOS, BIPOLAR, BICMOS may be implemented.

The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to these examples of embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Fig. 1 illustrates a photovoltaic cell according to an exemplary embodiment which has an insulating passivation layer between nanoclusters and an electrode, each nanocluster being contacted with the electrode through an assigned via.

Fig. 2 illustrates a photovoltaic cell according to another exemplary embodiment in which nanoclusters completely cover one electrode. Fig. 3 shows a TEM cross-section image of ZrO₂ nanoclusters in La₂C^ matrix layer between a silicon layer and an aluminum layer.

Fig. 4 and Fig. 5 shows diagrams indicating that, from the solar spectrum, only a part of the spectra can be used for solar cells (compare F. Dimroth et al., MRS Bulletin, March 32 (2007), p.230).

Fig. 6 illustrates a schematic drawing of a photovoltaic cell according to an exemplary embodiment of the invention showing a series of multi-junction nanocluster solar cells.

DESCRIPTION OF EMBODIMENTS

The illustration in the drawing is schematical. In different drawings, similar or identical elements are provided with the same reference signs.

Fig. 1 shows a photovoltaic cell 100 according to an exemplary embodiment of the invention. The photovoltaic cell 100 comprises a semiconductive matrix 102 of a p-type semiconductor. A plurality of semiconductor nanoclusters 104 being n-doped are embedded in the p-type semiconductor matrix 102 so that a pn-junction 106 is formed between each of the plurality of semiconductor nanoclusters 104 and the semiconductive matrix 102.

When a light 120 (such as sun light) having the energy hf (wherein h is Planck's constant, and f is the frequency of the light 120) impinges on the photovoltaic cell 100, an electron hole pair indicated schematically with reference numeral 122 (the electron-hole pair is generated at the boundary of pn-junction 106, and e- will drift to n-side and h+ to p-side) may be generated which may be separated so that one of the two charge carriers 122 will flow to a first electrode 108 and the other one of the two charge carriers 122 will flow to a second electrode 114. Consequently, an electric current may be generated between the electrodes 114, 108 which can supply an electric load 130 connected between the two electrodes 114, 108 with electric power.

A layer 150 of the semiconductor nanoclusters 104 and the semiconductive matrix 102 is deposited on the first electrode 108 which, in turn, is formed on a backscatter layer 116 which, in turn, is formed on a supporting substrate 140. The backscatter layer 116 is adapted for backscattering the light 120, wherein the first electrode 108 is arranged between the backscatter layer 116 on the one hand and the semiconductor matrix 102 on the other hand. If the first electrode 108 is a transparent electrode, the backscattering layer 116 may be arranged as indicated in Fig. 1, but if the first electrode 108 is not transparent for light, backscattering layer 116 should be arranged between the electrode 108 and pn-junction. Many metallic electrode can provide themselves the function of a backscattering layer.

The second electrode 114 is deposited above a planar surface of the semiconductive nanoclusters 104 and the semiconductor matrix 102 forming the common thin film layer 150. The second electrode 114 is a thin layer made of an optically transparent and electrically conductive material such as indium tin oxide (ITO) to allow the light 120 to propagate through the electrode 114 without being absorbed.

An electrically insulating silicon oxide passivation layer 110 is arranged between the second electrode 114 on the one hand and the plurality of semiconductive nanoclusters 104 on the other hand, wherein the silicon oxide layer 110 comprises a plurality of through holes 112 through which the semiconductive nanoclusters 104 are electrically contacted with the second electrode 114 in a manner to prevent the pn-junctions 106 from being electrically short-circuited with the second electrodes 114. In the embodiment of Fig. 1, the plurality of semiconductive nanoclusters 104 are all made of the same material.

Furthermore, a protecting cover layer 160 is shown on the second electrode 114 and may be made of an optically transparent material such as silicon oxide, glass.

Fig. 2 shows a photovoltaic cell 200 according to another exemplary embodiment. In this embodiment, the various nanoclusters 104 contact one another directly to form a continuous chain to prevent the pn-junctions 106 from being short-circuited with the second electrode 114.

Thus, Fig. 2 shows another embodiment of a solar cell consisting of the nanoclusters 104 of the n-type semiconductor dispersed in the p-type semiconductor matrix 102. Although not shown in Fig. 2 for the sake of simplicity, many of the additional components and measures shown in Fig. 1 may also be provided in the embodiment of Fig. 2

(particularly the elements denoted with reference numerals 116, 130, 140, 160).

Fig. 3 shows a TEM photo 300 having an aluminium layer, a silicon layer and a LaO_x layer with Zrθ2 nanoclusters embedded therein. Fig. 3 shows an example of a Zrθ2 nanocluster-LaO_x matrix structure deposited by ALD. Fig. 4 shows a diagram 400 having an abscissa 402 along which a wavelength is plotted. Along an ordinate 404, the spectral irradiance is plotted in W/m²,μm showing that a Si solar cell is only capable of absorbing light in a specific wavelength range. Fig. 5 shows a similar diagram 500.

More precisely, Fig. 4 shows the AM 1.5 solar spectrum and a part of the spectrum that can be used by silicon solar cells. Fig. 5 shows the situation for GalnP/GalnAs/Ge solar cells. Fig. 4 shows solar spectrum and theoretically calculated spectrum for silicon and multi-junction solar cells as an example to choose materials to absorb a wide range of solar spectrum. Making nanoclusters of different material composition in a matrix, it is possible to provide series of multi-junction solar cells in a thin film.

In view of the considerations of Fig. 4 and Fig. 5, a photovoltaic cell 600 shown in Fig. 6 according to another exemplary embodiment is provided which is capable of absorbing light from a broader wavelength range. This may be achieved by embedding first semiconductor nanoclusters 602, second semiconductor nanoclusters 604 and third semiconductor nanoclusters 606 made of different materials in a matrix 102. Thus, with the embodiment of Fig. 6, a series of multi-junctions 608, 610, 612 is provided for a multi- junction nanocluster solar cell 600. Although not shown in Fig. 6 for the sake of simplicity, many of the additional components and measures shown in Fig. 1 may also be provided in the embodiment of Fig. 6 (particularly the elements denoted with reference numerals 110, 112, 130, 140, 160).

Embodiments of the invention provide a new concept of a pn-junction solar cell, which consists of n-type nanoclusters dispersed in a p-type matrix, or vice-versa. Particularly, a solar cell is provided comprising or consisting of nanoclusters of n/p-type semiconductor dispersed in p/n-type semiconductor matrix deposited by thin film technologies such as ALD and CVD. In such a nanocluster-matrix solar cell, nanoclusters of n-type semiconductor such as semiconductive oxide with excess amount of metal or oxygen vacancies such as TiO₂, ZnO, SnO₂, SrTiO₃ and IV, III-V, H-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants and a matrix of p-type semiconductor such as semiconductive oxide with excess amount of oxygen or metal vacancies such as Cu₂O, CuO, CuAlO₂, LaAlO₃, SnO, NiO, ZnO and IV, III-V, II-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants may be grown on a transparent electrode (ITO, extremely thin transition metal) simultaneously. A boundary between an n-type nanocluster and a p-type matrix is a pn-junction. When light comes across the boundary electron-hole pairs will be generated.

Compared with a multi-junction solar-cell, both n-type nanocluster and p-type matrix phases may grow and crystallize simultaneously during the deposition. Thus, defects between a pn-junction can be minimal due to the fact that limited amount of stress and mismatch will be formed in the film, which enhances the efficiency of the solar cell. Moreover, the limited amount of stress and mismatch present in the solar cell will also result in a better resistance to higher temperature. This is an important advantage because heating of solar cells may reduce the efficiency.

Also the amount and size of the nanoclusters can be controlled by the ratio between two element precursors and the deposition conditions of the nanocluster-matrix film. This is an advantage because it is possible to tune an optimal distance between the pn-junction and electrodes, so that carriers can drift to reach a contact electrode efficiently and also adsorb light sufficiently. For a solar cell, a certain thickness of semiconductor material, depending on refractive index of the material, is necessary to adsorb incident light. In the case of silicon solar cells, a thick silicon layer may be required because of poor adsorption of light for silicon and further at large angle (around 40 degree) light is reflected on the silicon surface rather than absorbed, thus the surface of a silicon solar cell has been patterned in such a way to get maximum light absorption. A front part of the silicon solar cell, where light is adsorbed, consists of a glass or plastic cover with adhesion layer and antireflective layer. According to an embodiment of the invention, a wide variety of semiconductor materials can be deposited, thus it is possible to choose materials, which have a high adsorption of light and/or suitable band gap to adsorb a certain wavelength of light.

Embodiments of the invention may involve, inter alia?, the following advantages: - Many material combinations are available for p-type and n-type semiconductor materials. Pn-junction can be prepared by one step processing, additional processing such as diffusion and annealing are not necessary after deposition of a nanocluster- matrix arrangement. Thus, it is easy to be integrated with ICs or the other devices. The price of resulting photovoltaic cells may be low when using CVD, as is possible to extend such a manufacture to large scale (outdoor) solar cells with high temperature resistance. - The amount of defects between a pn-junction may be reduced or minimized because both phases grow and crystallize simultaneously during the deposition.

- The size of nanoclusters can be controlled, which enables to tune an optimal distance between a pn-junction and electrodes, so that carriers can drift to reach a contact electrode efficiently and also adsorb light sufficiently.

- The solar cell can be very thin, flexible and light.

Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The words "comprising" and "comprises", and the like, do not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. In a device claim enumerating several means, several of these means may be embodied by one and the same item of software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A photovoltaic cell (100), wherein the photovoltaic cell (100) comprises: a semiconductive matrix (102) of a first type of conductivity; a plurality of semiconductive nanoclusters (104) of a second type of conductivity which differs from the first type of conductivity, which plurality of semiconductive nanoclusters (104) are at least partially embedded in the semiconductive matrix (102) such that a pn-junction (106) is formed between the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102).

2. The photovoltaic cell (100) of claim 1, wherein the first type of conductivity is n-type and the second type of conductivity is p- type, or the first type of conductivity is p-type and the second type of conductivity is n- type.

3. The photovoltaic cell (100) of claim 2, wherein the n-type material comprises one of the group consisting of a semiconductive oxide with an excess amount of metal or oxygen vacancies, TiO₂, ZnO, SnO₂, SrTiO₃, III- VI oxide and IV, III-V, H-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants.

4. The photovoltaic cell (100) of claim 2, wherein the p-type material comprises one of the group consisting of a semiconductive oxide with an excess amount of oxygen or metal vacancies such as Cu₂O, CuO, CuAlO₂, LaAlO₃, SnO, NiO, ZnO and IV, III-V, II-VI, I-VII, IV-VI, H-V, V-VI semiconductors with or without dopants.

5. The photovoltaic cell (100) of claim 1, comprising a first electrode (108) on which the plurality of semiconductive nanoclusters (104) are spontaneously created in the semiconductive matrix (102) during the deposition of the layers.

6. The photovoltaic cell (100) of claim 1, comprising a second electrode (114) arranged on the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102).

7. The photovoltaic cell (100) of claim 6, wherein the second electrode (114) comprises a transparent electrically conductive material, particularly comprises indium-tin- oxide.

8. The photovoltaic cell (200) of claim 6, wherein at least a part of the plurality of semiconductive nanoclusters (104) contact the second electrode (114) and contact one another in a manner to prevent the pn-junctions (106) from being short-circuited with the second electrode (114).

9. The photovoltaic cell (100) of claim 6, comprising an electrically insulating layer (110) between the second electrode (114) on the one hand and the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102) on the other hand, wherein the electrically insulating layer (110) comprises a plurality of through holes (112) through which the plurality of semiconductive nanoclusters (104) are contacted with the second electrode (114) in a manner to prevent the pn-junctions (106) from being short- circuited with the second electrode (114).

10. The photovoltaic cell (100) of claim 1, wherein the plurality of semiconductive nanoclusters (104) are made of the same material.

11. The photovoltaic cell (100) of claim 1, wherein different ones of the plurality of semiconductive nanoclusters (104) are made of different materials, the different materials being configured to absorb electromagnetic radiation of different wavelengths.

12. The photovoltaic cell (100) of claim 5, comprising a backscatter layer (116) for backscattering electromagnetic radiation.

13. The photovoltaic cell (100) of claim 1, wherein each of the nanoclusters (104) has a dimension of 0.5 nm to 100 nm, particularly has a dimension of 1 nm to 50 nm.

14. The photovoltaic cell (100) of claim 1, wherein the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102) together form a thin film, particularly form a layer having a thickness between 10 nm and 10 μm, more particularly having a thickness between 10 nm and 100 nm.

15. A method of manufacturing a photovoltaic cell (100), wherein the method comprises: at least partially embedding a plurality of semiconductive nanoclusters (104) of a second type of conductivity in a semiconductive matrix (102) of a first type of conductivity which differs from the second type of conductivity so that a pn-junction (106) is formed between the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102).

16. The method of claim 15, wherein the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102) are deposited simultaneously in a common deposition procedure.

17. The method of claim 15, wherein the plurality of semiconductive nanoclusters (104) and the semiconductive matrix (102) are manufactured by one of the group consisting of thin film technology, Chemical Vapour Deposition, Plasma Enhanced Chemical Vapour Deposition, Molecular Beam Epitaxy, sputtering, and Atomic Layer Deposition.

18. The method of claim 15, wherein the plurality of semiconductive nanoclusters (104) are formed by spontaneous nucleation.