1. Technical Field of the Invention
The present invention relates to an asymmetric MIM type absorbent nanometric structure exhibiting wide-band light absorption, particularly in the visible range, and a method for producing such a structure. More particularly, it is applicable to ultra-thin solar cells.
2. Prior Art
Attempts are constantly being made to reduce the thickness of the active (absorber) layer of solar cells, or photovoltaic cells, in particular to reduce the transit time (time taken by electrons to reach the electrodes, which is generally more than a picosecond) and thus the recombining of photo-induced charges. Attempts are also being made to decrease the thickness of the active layer in order to reduce the costs associated with the material, both the cost of the material itself and the manufacturing cost incurred by processing a greater or smaller quantity of material. Furthermore, limiting the quantity of material enables greater scope for plans to use rare materials. In seeking to reduce the thickness of the active layer, one of the difficulties encountered is in producing a structure in which light can be confined within the active layer long enough (typically it is intended that the photon travels along an optical path several times greater than the thickness of the absorbing material) to enable maximum conversion efficiency. One method for confining electromagnetic waves (solar light) within the active layer is to attempt to excite surface plasmon resonances in structures on a subwavelength scale.
The article by Atwater et al. (‘Plasmonics for improved photovoltaic devices’, Nature Materials, 9, 205-213 (2010)) suggests integrating recent techniques implemented in the production of photovoltaic devices involving ‘plasmonics’. The purpose of plasmonics is to benefit from the resonant interaction between electromagnetic radiation (especially light) and free electrons at the interface between a metal and a dielectric material (such as air or glass) under certain conditions. This interaction generates electron density waves, exhibiting wave-like behaviour and called plasmons or surface plasmons. The article describes various types of metal nanostructures which enable the generation of surface plasmons, with the aim of trapping light in very thin semi-conductor layers, causing a large increase in absorption. In particular, the article describes the use of nanometric particles used as diffusing elements to promote coupling, the use of nanometric particles as nano-antennae, the use of a striated mirror behind a semi-conductor layer enabling the generation of surface plasmons at the mirror-semi-conductor interface.
Among the references cited by Atwater et al., for example the article by Ferry et al. (‘Improved red-response in thin film a-Si : H solar cells with soft-imprinted plasmonic black reflectors’, Appl. Phys. Letters 95, 183503 (2009)), describing the structuring of materials for solar-cell type applications, is particularly noteworthy. A structured metal layer on the back of the absorber layer enhances the absorption of longer wavelengths. However, in this article, the wide band absorption is obtained using a relatively thick active layer (500 nm) In Linguist et al. (‘Plasmonic nanocavity arrays for enhanced efficiency in organic photovoltaic cells’, Appl. Phys. Letters 93, 123308 (2008)), an array of nanocavities is formed between a structured metal anode and a (non-structured) cathode. This plasmonic structure enables the confinement of electromagnetic energy and an increase in the absorption of wavelengths greater than 700 nm, due to the existence of surface plasmons between the structured anode and the cathode.
Le Perchec at al. (‘Plasmon-based photosensors comprising a very thin semiconducting region’, Appl. Phys. Letters 94, 181104 (2009)) describes an infrared detection system comprising a very thin active layer. As in the solar cell applications, the mechanism for confining the light in the semi-conductor layer relies on the generation of plasmon resonances in a horizontal cavity formed by an MSM (metal-semi-conductor-metal) type structure in which the semi-conductor layer is sandwiched between a metal mirror underneath and a metal array on a subwavelength scale on top. It is shown that such a plasmon resonance can be modelled by a longitudinal Fabry-Pérot type resonator which verifies the relationship kneffL=π where k is the wave vector (k=2 π/λ where λ is the wavelength of the incident wave), neff is the effective index of the guided plasmon mode in the MSM multi-layer structure, L is the width of an element of the array.
These plasmon resonances shown in the articles cited above are generated at wavelengths greater than 650-700 nm. This is explained by the very nature of the surface plasmon at the metal-dielectric interface which cannot exist at short wavelengths (for example, see A. V. Zayats et al., ‘Nano-optics of surface plasmon polaritons’, Physics reports 408, 131-314, 2005).
Therefore, there is a necessity to produce ultra-thin structures exhibiting an increased wide band absorption in the visible range 500-800 nm.
The invention introduces an asymmetric MIM (metal-insulator-metal) type absorbent nanometric structure, the particular geometry of which enables in particular the generation of an increased absorption at wavelengths over the entire visible spectrum.
According to a first aspect, the invention relates to an asymmetric MIM type absorbent nanometric structure intended for receiving a wide-band incident light wave the absorption of which is to be optimised within a given spectral band in the near-infrared visible range, characterised in that it comprises an absorbent dielectric layer in said spectral band, of subwavelength thickness, arranged between a metal array formed from metal elements periodically arranged with a subwavelength period and a metal reflector, and in that:
the elements forming the metal array exhibit at least one dimension (w) suitable for forming a plasmonic resonator between the metal array and the metal reflector, under the elements of the array, which plasmonic resonator forms a Fabry-Pérot type longitudinal cavity resonating at a first wavelength of the aimed-for absorption spectral band,
the absorber layer exhibits, between the metal array and the metal reflector, at least one first thickness suitable for forming at least one first Fabry-Pérot type vertical cavity, resonating at a second wavelength of the aimed-for absorption spectral band.
According to a variant embodiment, said at least first thickness of the absorber layer is less than the absorption length of the dielectric material of which said absorber layer is made on the aimed-for absorption spectral band.
According to a variant embodiment, said at least first thickness of the absorber layer is between subtantially once and two times the thickness of the metal skin forming the metal array.
According to a variant embodiment, the absorber layer exhibits a first thickness under the elements of the array and a second thickness under the spaces between the elements of the array, which thicknesses are suitable for forming a first and a second Fabry-Pérot type vertical cavity resonating at two distinct wavelengths of the aimed-for absorption spectral band.
According to a variant embodiment, the first and second thicknesses are substantially identical.
According to a variant embodiment, the width of the elements of the metal array is suitable for obtaining a plasmon mode of the order m=3.
According to a variant embodiment, the structure further comprises a non-absorbing dielectric layer in the aimed-for absorption spectral band, arranged between said absorber layers and the metal array and/or encapsulating the metal array, enabling the thickness between the metal array and the metal reflector to be adjusted.
According to a variant embodiment, the period of the metal array is less than half the minimum wavelength of the aimed-for absorption spectral band.
According to a variant embodiment, the metal array is one-dimensional, of a period between 150 and 250 nm and formed from strips with a width between 80 and 120 nm and a thickness between 10 and 30 nm.
According to a variant embodiment, the metal array is two-dimensional, of a period according to one or other of the dimensions between 150 and 250 nm, and is formed from square or rectangular pads of sides between 80 and 120 nm, and thickness between 10 and 30 nm.
According to a second aspect, the invention relates to a solar cell comprising a nanometric structure according to the first aspect, deposited on a substrate and in which the aimed for absorption spectral band is within the visible-near-infrared range.
According to a variant embodiment, the solar cell further comprises a transparent conductive layer disposed between the metal reflector and the absorber layer.
According to a variant embodiment, the solar cell also comprises a transparent conductive layer disposed between the absorber layer and the metal array or on the metal array and the absorber layer.
According to a variant embodiment, the transparent conductive layer is made of ZnO, ITO or SnO.
According a variant embodiment, the metal reflector is multi-layer, comprising a lower layer for adhesion to the substrate and an upper layer made of noble metal, such as gold, silver or aluminium.
According to a variant embodiment, the metal array is made of noble metal, such as gold, silver or aluminium.
According to a variant embodiment, the absorber layer comprises a semi-conductor material of type III-V, such as gallium arsenide or indium phosphide.
According to a variant embodiment, the absorber layer comprises a material from among amorphous silicon, CIGS and cadmium telluride.
According to a variant embodiment, the absorber layer comprises an organic material.
According to a variant embodiment, the solar cell comprises an asymmetric MIM type absorbent nanometric structure comprising:
a silver metal reflector;
an absorber layer made of GaAs less than 50 nm thick;
a metal array made of silver less than 30 nm thick, formed from pads or strips arranged periodically, the width of said pads or strips being between 80 and 120 nm, the linear filling factor being between 0.5 and 0.7.
According to a variant embodiment, the solar cell comprises an asymmetric MIM type absorbent nanometric structure comprising:
a silver metal reflector;
an absorber layer made of GaSb less than 50 nm thick;
a metal array made of silver less than 30 nm thick, formed from pads or strips arranged periodically, the period being between 270 nm and 330 nm and the linear filling factor being between 0.5 and 0.7;
a layer made of conducting transparent material arranged on the metal array.
According to a variant embodiment, the solar cell comprises an asymmetric MIM type absorbent nanometric structure comprising:
a silver metal reflector;
an absorber layer made of CIGS less than 50 nm thick;
a metal array made of silver less than 30 nm thick, formed from pads or strips arranged periodically, the period being between 500 and 550 nm and the linear filling factor being between 0.5 and 0.7;
a layer made of conducting transparent material arranged on the metal array.
For example, the layer made of transparent conductive material is ZnO:Al, less than 50 nm thick. The Applicant has shown an enhancement of the absorption in the near infrared visible spectrum associated with resonances in the layer of transparent conductive material deposited on the metal array.
According to a third aspect, the invention relates to a method for producing a solar cell according to the second aspect comprising:
depositing one or more layers of metal on the substrate to form the metal reflector,
depositing the absorber layer onto said metal reflector,
depositing a layer of resin and structuring the layer of resin to form elements of the array,
depositing metal forming the metal array and dissolving the resin.
According to a variant embodiment, the resin is structured by nano-imprint.
Other advantages and characteristics of the invention will become apparent from reading the description, illustrated by the following Figs.:
In this type of structure, it is known to have a propagation of modes called ‘plasmon modes’ or surface plasmons, solutions to Maxwell equations at the metal/dielectric interfaces. The excitation of various plasmon modes under the effect of an incident light wave may occur if the resonance and coupling conditions are combined, these conditions depending on the geometry of the structure, and particularly those of the metal array (for example, see J. A. Schuller et al., “Plasmonics for extreme light concentration and manipulation”, Nature Materials 9, 193-204, 2010).
Each of the structures according to the invention comprises a layer of dielectric material 10 of refraction index na, of thickness ta, arranged between a metal array 11 and a metal reflector 12. In the example of the one-dimensional structure of
In a preferred application of the invention, in particular for application to solar cells, the structure is designed to receive sunlight and the aim is to optimise the absorption of the structure between around 500 and 800 nm. The layer is chosen in a material that is absorbent in this spectral band. Advantageously, as will be explained in more detail in what follows, a high-index material (typically greater than 3) will be chosen, enabling the thickness of the layer to be reduced. For example, the dielectric material is a semi-conductor material of type III-V (comprising one element in column III and one element in column V of the periodic table of elements), with a gap in the near-infrared, for example gallium arsenide (GaAs) or indium phosphide (InP). Although they have a lower index (typically between 1.5 and 2), organic polymers, for example based on fullerene derivatives, are similarly promising materials. Other materials such as cadmium telluride, amorphous silicon, microcrystalline silicon or polycrystalline silicon are also envisaged.
The metal array and metal reflector are made, for example, of silver, gold or aluminium, noble metals with low absorbency in the visible range. According to a preferred variant embodiment, the metal array may be made of gold and the metal, air-sealed reflector of silver, silver being liable to deterioration in contact with the atmosphere (sulfuration).
The Applicant has shown that, for solar cell application, with an aimed for absorption band in the visible range between 500 and 800 nm, advantageously, the width of strips (or pads) of the metal array will advantageously be chosen to be less than 150 nm, advantageously between around 80 and 120 nm, and the period less than 250 nm, advantageously between around 150 and 250 nm. The thickness of the absorber layer will be chosen as less than 100 nm, and the thickness of the metal array less than 30 nm, advantageously between around 15 and 25 nm. The metal reflector has a thickness greater than that of the metal skin, typically a thickness greater than 50 nm in the case that gold or silver is used. More details on the optimisation of the dimensions and choice of materials of the structure will be given in what follows.
In this example, the absorber layer is made of gallium arsenide (GaAs), the real part of the index of which is around 3.5 (see E. D. Palik, Handbook of Optical Constants of Solids Academic, Orlando, 1985). Its thickness is 25 nm. The layer is arranged between a silver metal array the period of which is 200 nm, and the strip width of which is 100 nm. The thickness of the array is 15 nm. The metal reflector is made of silver.
The model used for these simulations is based on the exact modal method (for example, see S. Collin et al., ‘Efficient light absorption in metal-semiconductor-metal nanostructures, Appl. Phys. Letters 85, 194, 2004), with a TM polarised wave (magnetic field parallel to strips of the array).
The Applicant has provided evidence for a remarkable absorption in three spectral bands centred on 560 nm (resonance labelled E), 675 nm (resonance labelled D) and 760 nm (resonance labelled C) respectively. Curve 21 depicts the absorption calculated in the GaAs while curve 22 depicts the absorption calculated in the total structure. These two curves are compared with the standardised solar spectrum 20 (AM1.5G, here plotted in number of photons/m2/s/nm−1 ). GaAs is advantageous, particularly in that it exhibits a gap of 1.42 eV at ambient temperature well suited for solar photovoltaic applications (for example, see T. Markwart and L. Castaner, Practical handbook of Photovoltaics, Elsevier, 2003), the record efficiency obtained experimentally for a single junction being 26.1% (theoretical maximum efficiency 32%). It is observed that the maximum absorption coincides with the maximum emission of the solar spectrum. The slight difference (less than 13%) between curves 21 and 22 over the entire visible range shows a very weak absorption by the metal array over the entire visible range, revealing excellent confinement of light in the GaAs active layer (and slight ohmic losses in the metal array). On average, 70% of incident photons are absorbed in the spectral range 500-800 nm, and 55% of photons, the energy of which is greater than the gap (embodied by the dotted line 23) are absorbed in the cell. A theoretical solar energy conversion efficiency of 17% (conversion efficiency of solar energy into electrical energy) is deduced, for a cell the external quantum efficiency of which, independent of polarisation, is given by curve 21 of
The Applicant has shown that this remarkable absorption in an ultra-thin structure (less than 50 nm in this example) can be explained by a combination of resonances, the physical principles of which differ, and which can therefore be adjusted by changing independent parameters of the structure. Consequently, by adapting the various parameters of the structure, it is possible to optimise the position of the wavelengths around which are centred the absorption peaks in order to obtain the structure response for the intended application.
The multi-resonant structure according to the invention therefore enables a paradox to be resolved, which is that, in general, if the time for the photon to pass into the structure (that is, the optical path) is increased, the spectral width of the resonance is reduced.
The Applicant has shown that each resonance can be modelled by a Fabry-Pérot resonator. The resonance condition of this resonator is generally written 1−r1r2e2ikh=0 where k=2π(neff+ikeff)/λ is the wave vector, λ the wavelength, (neff+ikeff) the complex effective index of the mode and r1 and r2 are the reflection coefficients at the ends of the resonator. Noting φ1 and φ2, the phases induced by these two reflections, the resonance condition is written 4 πhneff/λ+φ1+φ2=2 πp where p is an integer (p=±0, ±1, ±2, . . . ). For a given wavelength, the size h of the resonator is then given by the general equation:
In the case of a conventional, symmetrical Fabry-Pérot resonator, φ1=φ2=0 in the case of a dielectric surrounded by air, or φ1=φ2=±π in the case of a reflection on a metal with strong permittivity, for example silver, aluminium or gold in the infrared. The resonance condition is then simply written:
The Applicant has shown that the resonances labelled A, B and C can be described by a plasmon mode resonance under the metal fingers (or strips) in the plane of the solar cell. The MIM structure therefore plays the role of a Fabry-Pérot resonator for a plasmon wave propagating along the x axis (parallel to the plane of the mirrors) and reflecting at the ends of the elements of the array. The Applicant has thus shown a ‘horizontal’ or ‘longitudinal’ resonant cavity, that is, one parallel to the plane of the array, the length of which is given by the width w of the element of the metal array and the index by the effective index neff of the mode propagating. The change of phase may be disregarded in a first approximation at the ends of the resonator, and the wavelengths of the first three resonances approximately follow the equation:
with p=1, 2, 3 for A, B, C, respectively.
The plasmon modes have the particular feature of propagating with an effective index greater than that of the dielectric medium (see, for example, A. V. Zayats et al., ‘Nano-optics of surface plasmon polaritons’, Physics reports 408, 131-314, 2005). This effect is reinforced by coupling between a plurality of surface plasmons, as in the case of an MIM guide (here Ag—GaAs—Ag). This effect is illustrated in
Thus, the Applicant has shown that with a width of the strip of the metal array w=100 nm, three resonance peaks A, B, C at 1590 nm, 945 nm and 760 nm, respectively, are obtained, corresponding to modes m=1, m=2 and m=3. The optimisation of the parameters of the structure to obtain a resonance corresponding to mode m=3 is particularly advantageous as it enables a resonance at a wavelength of the visible spectrum with a low value of the width w of an element of the metal array (around 100 nm).
The Applicant has shown, for resonances D and E, a mechanism different from that shown in the case of resonances A, B and C.
Considering equation (2), it appears that with an index of the order of 3.5 (index of GaAs), the smallest resonator at 700 nm has a size of 100 nm and the following order resonates at λ=350 nm. It is therefore not possible to get multiple resonance in a resonator of a size less than 100 nm. The Applicant has shown that in an asymmetric MIM structure, with an absorber layer of a given index and by adequately choosing the thickness of the layer, it is possible to obtain one or even more resonances in the visible range. Indeed, it appears that the coefficient of reflection on an interface between a high-index semi-conductor (around 3 or more) and a metal such as silver, aluminium or gold, for example, deviates from its usual values for wavelengths of 600 nm. This is shown in
where na is the index of the absorbent material (such as GaAs).
For an index na=4, there therefore exists a ‘vertical’ Fabry-Pérot cavity (that is, a cavity perpendicular to the plane of the array) exists between the metal array and the metal reflector which resonates at a wavelength of 640 nm for an absorption layer thickness ta=20 nm. This resonance exists at the fundamental Fabry-Pérot order (order p=0 in equation (1)), contrary to the longitudinal cavity shown for plasmon resonances A, B and C.
Moreover, it can be shown that curve 601 exhibits few variations depending on the metal used and the absorbent material.
As can be seen in
The Applicant has shown that in an asymmetric MIM plasmonic structure of the type of
The change of vertical resonance as a function of thickness ta of the absorber layer is illustrated in
Curve 7B also depicts the curves calculated from the energy as a function of the thickness of the absorber layer, but for lower thickness values. Curves 701 and 702 of the energy for the vertical Fabry-Pérot cavity of order 0 and also curves 706 to 708 of the longitudinal plasmonic cavity energy of orders m=1 to 3, respectively, are also shown. It can be seen that for the very low thicknesses, the effective index coming into play for plasmon resonances (resonances A, B, C) decreases when the thickness of the absorber layer increases, this being associated with a decrease in coupling between the plasmons of the two Ag/GaAs interfaces. A slight spectral shift of these resonances results, which are mixed with vertical Fabry-Pérot resonances for the weakest energies, near the gap (around 1.4-1.6 eV).
Moreover, the Applicant has shown that the width of the elements of the array has little effect on the resonance at the fundamental order of the vertical Fabry-Pérot cavity. This becomes apparent in particular in
These curves have been obtained with a layer of SiO2 (silicon dioxide) with a thickness of 20 nm, deposited on a glass substrate covered with a layer of gold. The gold metal array, is manufactured by a technique called nano-imprint described, for example, in S. H. Ahn and L. J. Guo, ‘High-Speed Roll-to-Roll Nanoimprint Lithography on Flexible Plastic Substrates’ (Advanced Materials 20, 2044-2049, 2008). The geometric parameters of the array (period of 400 nm, width of elements w=200 nm, and thickness 20 nm) have been optimised to exhibit two resonant modes between 600 and 1800 nm. Although silicon dioxide is not absorbent in the visible range, and is thus not suitable for the solar-cell application, these experimental curves show the geometric conditions enabling a plasmonmode resonance with almost perfect absorption, within a wide-angle band. Reflection measurements have been taken between 3° and 60°, with TM polarisation (magnetic field along the y axis). Excitation of the fundamental mode (m=1) of the MIM structure shows an almost perfect absorption at λ=1280 nm (>98%) whatever the angle of incidence. This insensitivity to the angle of incidence is linked to the symmetry of the mode in relation to a plane of symmetry of the structure (also true for m=3). The MIM type nanostructure acts as a Fabry-Pérot resonator for the plasmonic wave propagating along the x axis, and reflects at the ends of the elements of the array. Here, the high effective index of the plasmon mode is due to very strong coupling between the very thin metal array and the semi-infinite metal reflector. The Applicant has shown that the resonance wavelength is determined primarily by the width w of the elements (see
These curves show, as in the example of a one-dimensional structure, a wide band absorption in the visible range with the presence of three absorption peaks (corresponding to the peaks C, D, E previously described). Furthermore, if the absorption peaks due to the double resonance of the ‘vertical’ Fabry-Pérot cavity are only slightly variable depending on the geometry of the metal array elements, a variation of the absorption peak of the plasmonic resonator is observed, as a function of wavelength and as a function of amplitude simultaneously, linked to the dimension of the resonator (w) and the coupling quality in the cavity depending on its geometry. In particular, an increase in absorption due to the plasmon resonance is observed with the increase of filling rate apparently due to a better coupling while the absorption due to the under-space ‘vertical’ resonance (resonance D) decreases, apparently due to the reduction in space between the elements.
The Applicant has shown that in this example the best efficiency of a solar cell which would be produced with this structure is obtained for a filling rate f=0.6 and a period of d=180 nm.
Thus, by choosing an asymmetric MIM structure comprising a GaAs layer less than 50 nm thick, for example between 20 and 30 nm, typically around 25 nm, said GaAs layer being comprised between a metal reflector, for example made of silver, and a metal array formed from strip type elements or pads also made of silver arranged periodically,
Although most of the curves simulated above have been obtained with GaAs as an absorbent dielectric material, it is evident that other materials are very promising for obtaining a multi-resonant absorbent asymmetric MIM structure such as defined above. In particular, it will be possible to seek materials such as indium phosphide (InP) or amorphous silicon (a-Si:H), which will enable structures to be designed with dielectric layers between 50 and 100 nm, making it much easier to obtain a junction for a solar cell with current technological modalities.
Whatever the dielectric material chosen, it will be preferable to choose an absorber layer thickness less than the absorption length of the dielectric material of which it is formed to obtain the aimed-for resonance of a Fabry-Pérot cavity between the metal array and the metal reflector. The absorption length of the material is defined by the depth in the material at which the intensity of an incident light wave of given wavelength is divided by e.
Advantageously, the thickness of the absorber layer is of the order of magnitude of the thickness of the metal skin forming the dielectric array or up to twice the thickness of the skin, to promote coupling of plasmon modes to metal/dielectric, dielectric/metal interfaces and to obtain elevated modal effective indices.
According to a variant embodiment, the nanometric structure further comprises a non-absorbing dielectric layer, arranged between the absorber layer and the metal array to adjust the spacing between the metal array and the metal reflector and thus to adjust the resonance wavelength. The dielectric layer may or may not encapsulate the metal array.
Although the results have been presented in one-dimensional or two-dimensional structures with elements formed from strips or square pads, the invention is not limited to these types of pattern and other patterns may be envisaged as long as a periodic structure is preserved.
The ultra-thin MIM solar cells can be manufactured on a substrate 101 covered with one or more metal layers 102 forming the metal reflector, itself covered with layers forming the absorber layer 103. The metal array 104 is deposited on the absorber layer. In the example of
The substrate 101 is arbitrary, for example formed of any material such as glass, or metal or plastic sheet or film.
In the case of a metal reflector composed of several layers, the lower layer in contact with the substrate will be able to promote adhesion (for example made of chrome or titanium), and the upper layer in contact with the absorber (case of Figs. A to C) or with the TCO layer (case of Fig. D) shall be chosen for its optical properties (preferably a noble metal of type Ag, Al, Au, etc.) and electrical properties (inferior contact for conducting the current and Schottky or ohmic contact with the absorber). These metals will be able to be deposited by vacuum evaporation assisted by electron gun, by sputtering or by electrolytic growth.
The absorber 103 is, for example, formed of a semi-conductor material having a direct gap, or behaving as a semi-conductor material having a direct gap, such as gallium arsenide (GaAs), indium phosphide (InP), copper and indium selenide (CuInGa(Se,S)2 or CIGS), cadmium telluride (CdTe) or hydrogenated amorphous silicon (a-Si:H), for example. It comprises, for example, a p/p+ doped layer, an i intrinsic layer, and an n/n+ doped layer, or even uniquely two p and n doped layers, or even a p or n layer and an intrinsic layer forming a Schottky contact with the metal (upper or lower). The p (n) layer can be the lower (upper) layer or vice versa. The absorber can also comprise a hetero structure (different materials forming, for example, the various n and p layers). The absorber can also be deposited according to known methods—for example, see A. Shah et al. ‘Photovoltaic Technology: The Case for Thin-Film Solar Cells’, Science, 285, 692-698, 1999 or J. J. Schermer et al., ‘Photon confinement in high-efficiency, thin-film III-V solar cells obtained by epitaxial lift-off’, Thin Solid Films, 511, 645-653, 2006 for depositing by the ‘lift-off’ technique.
The metal array can be manufactured by lift-off according to the procedure comprising the following steps:
deposition of a photosensitive or electrosensitive resin onto the absorber, then insolation of the resin by UV photolithography or interference lithography, or by electronic lithography.
development of the resin, dissolution of insolated parts.
deposition of the metal forming the metal array (by evaporation, by sputtering, etc.), the metal is deposited on the absorber at the locations where the resin has been insolated,
lift-off by dissolution of the resin, only the metal deposited directly onto the absorber remains, forming an array according to the insolated pattern in the resin.
According to a variant embodiment, the resin may also be structured by nano-imprint. In this case, the metal arrays are, for example, produced by soft nano-imprint assisted by UV. A PMMA resin layer of 200 nm thickness is deposited onto the metal reflector/absorber assembly, then a 10-nm thin layer of germanium, and finally a layer of photosensitive liquid resin 100 to 150 nm thick used for the nano-imprint stage. This stage of moulding, or nano-imprint, is produced in a press with a silicone mould under very low pressure, and the resin is cross-linked by UV insolation. The structures obtained are transferred into the germanium layer and the PMMA resin by reactive ion etching. This assembly of three layers is used to produce metal arrays by lift-off: a layer of gold is deposited on the sample, then the PMMA resin is dissolved in a solvent, leaving only the gold nanostructures on the surface.
The transparent conductive layer (105,
In another possible embodiment, the transparent conductive dielectric layer is deposited on the absorber, and the metal array is deposited on the transparent conductive dielectric layer (case C and D).
The collection of charges in the cell is done by the metal contact of the lower part (metal reflector) and by the array and/or the transparent conductive layer.
Apart from GaAs, the Applicant has shown remarkable results with other absorbers.
In
Thus, the Applicant has shown that an ultra-thin solar cell at very high absorption in the visible range, characterised by multi-resonances between 500 nm and 1000 nm, could be obtained owing to a multi-layer structure of the type described above comprising a GaSb layer and by choosing the characteristic parameters of the structure (mainly the width of the elements of the array, the linear filling factor, the thickness of the absorber layer and that of the upper layer in transparent conductive material). In particular, the GaSb layer will be chosen advantageously less than 50 nm, comprised between a metal reflector advantageously made of silver and a metal array also preferably made of silver, with a thickness less than 30 nm, formed from elements of the type strips or pads arranged periodically with a period advantageously comprised between 270 and 330 nm and a linear filling factor preferably comprised between 0.5 and 0.7. The layer made of transparent conductive material is advantageously made of ZnO:Al, with a thickness between 40 and 60 nm.
In
There, too, the Applicant has shown that an ultra-thin solar cell with very strong absorption in the visible range, characterised by multi-resonances between 500 nm and 1000 nm could be obtained owing to a multi-layer structure of the type previously described comprising a CIGS layer and by choosing the characteristic parameters of the structure. In particular, the CIGS layer will be chosen advantageously less than 50 nm, comprised between a metal reflector advantageously made of silver and a metal array also preferably made of silver, of thickness less than 30 nm, formed from strip- or pad-type elements arranged periodically with a period advantageously comprised between 500 and 550 nm and a linear filling factor preferentially between 0.5 and 0.7. The layer made of transparent conductive material is advantageously made of ZnO:Al, with a thickness between 40 and 60 nm.
Although described using a certain number of detailed example embodiments, the structure and method of producing the structure according to the invention comprise different variants, modifications and developments which will be obvious to the person skilled in the art, it being understood that these different variants, modifications and developments fall within the scope of the invention, as defined by the claims below.
Number | Date | Country | Kind |
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1053134 | Apr 2010 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/056028 | 4/15/2011 | WO | 00 | 1/4/2013 |