Hot-carrier multi-junction solar cells

Abstract

The present invention relates to a new type of solar cell, also referred to as a photovoltaic cell. More specifically, the present invention relates to a multi-junction solar cell further comprising an upper junction of the hot-carrier solar cell type. The invention also relates to a process for manufacturing such a solar cell. The present invention also relates to the use of such a solar cell for photovoltaic applications.

Description

[0001] HOT-CARRIER MULTI-JUNCTION SOLAR CELLS

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a new type of solar cell, also called a photovoltaic cell. More specifically, the present invention relates to a multi-junction solar cell further comprising an upper junction of the hot-carrier solar cell type. The present invention also relates to a method for manufacturing such a solar cell. The present invention also relates to the use of such a solar cell for photovoltaic applications.

[0004] STATE OF PRIOR ART

[0005] To date, solar cells are approaching their maximum theoretical efficiency, which is 33% of the power conversion efficiency under 1-sun radiation (1000 W / m 2and standard cell temperature at 25°C), also called the Shockley-Queisser (SQ) limit. One approach described in the literature to increase the efficiency of solar cells is to stack multiple junctions, also called sub-cells, with each junction consisting of a stack of different semiconductor layers forming a PN junction. The PN junctions of each sub-cell are formed so that each has a different band gap energy. Each sub-cell thus absorbs a reduced portion of the solar spectrum but with high efficiency. This results in an increased overall efficiency of such a solar cell compared to a single-junction solar cell, for example made of silicon.Such solar cells are called multi-junction solar cells (MJSCs) or tandem structures, where a top junction (or top sub-cell) is added to a bottom junction (or bottom sub-cell), allowing for better harvesting of high-energy photons. As described in the papers by Green et al. (Prog. Photovoltaics, 2021) and Geisz et al. (Nat. Energy, 2020), such multi-junction solar cells can significantly increase efficiency up to 47.1% (concentrated light, cells with 6 junctions). This is currently the absolute record for conversion efficiency achieved for a photovoltaic cell, across all technologies.

[0006] Achieving such efficiencies requires perfect current matching between the junctions when they are in series, i.e., a combination of band gaps for each sub-cell. This combination of cells in series (monolithic integration) is the majority choice for industrial applications, due to cost and ease of implementation. Identifying materials with the right combination of band gaps that can be stacked together remains a technological challenge. Moreover, the efficiency gain becomes smaller and smaller as the number of junctions increases, while the device becomes more expensive and more sensitive to spectral variations. In this context, alternative technologies are being explored with the objective of achieving better efficiencies.

[0007] A common feature of all current solar cells is that solar photons (from UV to IR wavelengths) that have energy above the semiconductor band gap generate electron-hole pairs, or excitons, with energy above the band gap. As a result of very fast carrier-carrier interactions, these carriers equilibrate according to a Fermi-Dirac distribution whose temperature is above the lattice temperature of the material, hence their name "hot carriers." In conventional solar cells, this energy, which is in kinetic form, is rapidly (on sub-picosecond time scales) dissipated as heat in the crystal. Although Shockley and Queisser calculated the theoretical maximum thermodynamic efficiency to be about 31-33% with an optimal band gap between about 1.1 eV and 1.4 eV (Shockley et al., J. Appl. Phys.1961), it was then shown that higher conversion efficiencies could be achieved if the excess energy from the hot carriers was harnessed before they cooled to lattice temperature. One approach is to extract these hot carriers before they cool. These cells are called hot-carrier solar cells (HCSCs).

[0008] Hot carrier solar cells are described in the literature. In particular, patent document WO2018 / 021966 (Nanyang University) describes a hot carrier solar cell having the objective of slowing down the cooling of hot carriers. Such a cell comprises a layer comprising one or more nanocrystals, each of said one or more nanocrystals comprising a halide perovskite material; a first electrode in contact with a first side of the nanocrystal-containing layer; and a second electrode in contact with a second side of the nanocrystal-containing layer opposite the first side; wherein the nanocrystal-containing layer has a thickness of less than 100 nm.

[0009] The development of such hot-carrier solar cells is, however, limited by the fact that the thermalization rate in the materials used is too fast to reach hot-carrier populations, even under concentrated solar radiation. The use of ultrathin absorbers could be a solution, as these ultrathin absorbers have a significantly lower thermalization rate. However, such absorbers cannot efficiently absorb low-energy photons near the band gap, which limits their efficiency.

[0010] One such document proposes an optical arrangement to direct solar energy to the layer containing the nanocrystals, but this simply refers to a concentration effect with lenses and mirrors, and not to a real increase in the absorption of the semiconductor layer, and thus relies on the fact that the material absorbs radiation very well so that a layer of 100 nm or less is sufficient, where typically several hundred nanometers minimum are generally required to achieve optimal absorption.

[0011] An object of the present invention is to provide a cell having a high conversion efficiency, including with relatively high thermalization rates compared to conventional HCSC cells, in particular at high concentration.

[0012] Another object of the present invention is to provide a solar cell capable of absorbing a wide range of the solar spectrum.

[0013] STATEMENT OF THE INVENTION

[0014] In the context of the present invention:

[0015] - the expression “between ... and ...” (for example, a range of values) must be understood as including the limits (for example, the limit values ​​of this range of values);

[0016] - any description relating to one embodiment is applicable and interchangeable with all other embodiments of the invention; and

[0017] - when an element or component is included in and / or selected from a list of elements or components, it is to be understood that this individual element or component may be selected and combined with other individual elements, or may be selected to constitute a subgroup of two or more explicitly listed elements or components; also, any element or component cited in a list of elements or components may be omitted from this list.

[0018] A first object of the present invention relates to a hot carrier multi-junction solar cell (HCMJSC), comprising: at least one lower junction; and at least one upper junction of the hot carrier solar cell (HCSC) type, comprising:

[0019] - an absorber (also sometimes called a “hot carrier absorber”);

[0020] - at least one contact layer chosen from: a / an electron contact layer, the conduction band of which is higher than the conduction band of the absorber, and / or b / a hole contact layer, the valence band of which is lower than the valence band of the absorber.

[0021] A second subject of the present invention relates to a mobile or stationary device comprising at least one HCMJSC solar cell according to the present invention, and optionally at least one optical element which is configured to direct and concentrate solar energy towards the HCMJSC solar cell (concentration).

[0022] A third subject of the present invention relates to a method of manufacturing an HCMJSC solar cell according to the invention.

[0023] A fourth subject of the present invention relates to a method for producing electricity, according to which: at least one HCMJSC solar cell or a mobile or immobile device according to the invention, is exposed to solar radiation by its upper junction side, and an electric current is collected.

[0024] PRESENTATION OF THE DRAWINGS

[0025] Other characteristics and advantages of the invention will emerge from the detailed description which follows of the non-limiting embodiments of the invention, with reference to the appended figures, in which:

[0026] Figure 1 shows the absorbed fraction of the incident photon flux (AMI .5D spectrum) as a function of the photon energy for respectively (a) an HCSC cell with a thin absorber (d = 200 nm; EG = 1.19 eV); (b) an HCMJSC cell according to the invention with a thin hot-carrier junction (d = 200 nm; EG = 1.19 eV) above a thick bottom junction (d = 2 μm; EG = 0.93 eV); (c) an MJSC cell composed of thick-film absorbers in the top (d = 2 μm; EG = 1.59 eV) and bottom (d = 2 μm; EG = 0.93 eV) junctions; and (d) an MJSC cell according to WO2018 / 021966 with hot or cold carriers in upper and lower junctions with the same materials and thicknesses as (c). The absorption coefficients of the different materials considered are obtained by extrapolation of the absorption coefficients of InP (Eg = 1.344 eV) and InGaAs (Eg = 0.74 eV). The absorption is calculated using a Beer-Lambert model.

[0027] Figure 2 represents a comparison of the efficiency of an ideal MJSC cell (line "cross", EG, top = 1.59 eV and Ec,-bot = 0.93 eV), of an optimal HCMJSC cell according to the invention (line, EG, top = 1.344 eV and Ec.bot = 0.93 eV) and of an optimal HCSC cell (line "circle", EG = 0.74 eV) as a function of the thermalization rate under a concentrated illumination of 1000 suns (AMI .5D spectrum).

[0028] Figure 3 represents the comparison of the influence of the band gaps of the upper and lower junctions on the efficiency of MJSC (a) and HCMJSC (b) cells according to the invention, for a fixed thermalization rate corresponding to the efficiency crossover point for optimal devices (referenced in Figure 2), under concentrated illumination of 1000 suns (AMI .5D spectrum).

[0029] DETAILED DESCRIPTION

[0030] The solar cell of the present invention is based on the combination of at least two distinct junctions, also called distinct sub-cells, one of which is of the hot carrier solar cell (HCSC) type, the latter being positioned as the upper junction. The combination of these two sub-cells, or junctions, makes it possible to provide equipment capable of absorbing a broad solar spectrum and with a high conversion efficiency, particularly at high concentration. The upper junction of the hot carrier solar cell (HCSC) type can in particular be of thin or ultrathin thickness, which makes it possible to avoid parasitic optical absorption, facilitates structural compatibility and reduces material costs. An ultrathin cell is more tolerant to radiation for space applications. It also has good current matching and is structurally compatible with the underlying device.The combination of the upper junction type hot carrier solar cell therefore makes it possible to improve the overall efficiency of the solar cell without degrading the performance of the lower junction(s).

[0031] Generally, within the scope of the present invention, the band gaps of the absorbers may be direct or indirect for both the upper and lower junctions.

[0032] The HCSC top junction allows the carrier cooling rate to be slowed to allow hot carriers to be collected while they are still at high energies ("hot"), thus enabling higher voltages across the junction.

[0033] The upper junction (or sub-cell) of the HCSC type is also referred to herein as a hot junction (or hot sub-cell), while the lower junction is referred to as a cold junction (or cold sub-cell).

[0034] The term "electron contact layer" means a contact layer intended to extract electrons. The electron contact layer of the HCMJSC cell according to the invention may be n-doped. The term "hole contact layer" means a contact layer intended to extract holes. The hole contact layer of the HCMJSC cell according to the invention may be p-doped. The role of the two contact layers of the upper HCSC junction is to ensure that electrons and holes are extracted separately. Doping is one possible way of ensuring this selective extraction.

[0035] The HCMJSC cell according to the present invention is such that the upper HCSC junction comprises at least one contact layer, also called a window layer, chosen from: a / an electron contact layer, the conduction band of which is higher than the conduction band of the absorber, and / or b / a hole contact layer, the valence band of which is lower than the valence band of the absorber.

[0036] According to one embodiment, the HCMJSC cell according to the present invention is such that the upper HCSC junction comprises at least two contact layers, more particularly an electron contact layer and a hole layer, at least one of which is chosen from: a / an electron contact layer, the conduction band of which is higher than the conduction band of the absorber, and / or b / a hole contact layer, the valence band of which is lower than the valence band of the absorber.

[0037] According to a preferred embodiment, the HCMJSC cell according to the present invention is such that the upper HCSC junction comprises two contact layers: a / an electron contact layer, preferably whose conduction band is higher than the conduction band of the absorber, and b / a hole contact layer, preferably whose valence band is lower than the valence band of the absorber.

[0038] According to another embodiment, the HCMJSC cell according to the present invention is such that the upper HCSC junction comprises at least two contact layers, more particularly an n-doped electron contact layer and a p-doped hole contact layer, at least one of which is chosen from: a / an n-doped electron contact layer, the conduction band of which is higher than the conduction band of the absorber, and / or b / a p-doped hole contact layer, the valence band of which is lower than the valence band of the absorber.

[0039] Thus, the HCMJSC cell according to the present invention may be such that it comprises, according to a first embodiment: a / an n-doped electron contact layer, the conduction band of which is greater than the conduction band of the absorber, and b / a p-doped hole contact layer.

[0040] According to this first embodiment, the valence band of the hole contact layer is arbitrary, in particular it can be greater than or equal to the valence band of the absorber.

[0041] According to a second embodiment, the HCMJSC cell according to the present invention is such that it comprises: a / an n-doped electron contact layer, and b / a p-doped hole contact layer, the valence band of which is lower than the valence band of the absorber.

[0042] According to this second embodiment, the conduction band of the electron contact layer is arbitrary, in particular it can be less than or equal to the conduction band of the absorber.

[0043] Preferably, the HCMJSC cell according to the present invention is such that it comprises the following two contact layers: a / an electron contact layer, which can be n-doped, whose conduction band is higher than the conduction band of the absorber, and b / a hole contact layer, which can be p-doped, whose valence band is lower than the valence band of the absorber.

[0044] According to one embodiment, the upper HCSC junction is such that it comprises at least one electron contact layer, the conduction band of which is higher than the conduction band of the absorber. For example, the conduction band of the electron contact layer is higher than the conduction band of the absorber by at least 20 meV, at least 30 meV, at least 40 meV, at least 50 meV or at least 60 meV, preferably at least 20 meV, more preferably at least 60 meV.

[0045] According to another embodiment, the HCSC upper junction is such that it comprises at least one hole contact layer, the valence band of which is lower than the valence band of the absorber. For example, the valence band of the hole contact layer is lower than the valence band of the absorber by at least 20 meV, at least 30 meV, at least 40 meV, at least 50 meV or at least 60 meV, preferably at least 20 meV, more preferably at least 60 meV.

[0046] The values ​​of the conduction and valence bands can be evaluated in particular by UV photospectroscopy, also called UV photoelectron spectroscopy (UPS).

[0047] In the context of the present invention, it may be necessary to calculate the theoretical limit of an MJSC solar cell not comprising an HCSC upper junction and compare it to the theoretical limit of a solar cell according to the invention (identical but nevertheless comprising the HCSC upper junction). These theoretical limits are evaluated using the detailed balance model which is commonly used to evaluate the theoretical limits of solar cells. The performance of hot carrier cells is then evaluated using a parameter called the thermalization coefficient Q, which indicates the capacity of a material to maintain the carriers at a high temperature. The parameter makes it possible to vary the efficiency between that of a conventional MJSC type cell (Shockley-Queisser, high Q) and the HCMJSC cell (low Q) according to the invention.Such an evaluation makes it possible to highlight the interest of the combination of sub-cells according to the invention, with a theoretical limit of the HCMJSC cell significantly higher than that of a classic MJSC cell.

[0048] According to yet another embodiment, the upper junction (HCSC) is characterized in that it comprises two contact layers: a / an electron contact layer, the conduction band of which is higher than the valence band of the absorber, and b / a hole contact layer, the valence band of which is lower than the conduction band of the absorber.

[0049] Given the possible combination of upper and lower junctions, the HCMJSC cell according to the invention allows good complementarity of the gaps of the different junctions, and constitutes a very promising and innovative solution for use in particular under highly concentrated solar flux. Indeed, the variety of junction types, the possible multiplicity of lower MJSC type junctions and thus the accessible gaps, makes it possible to envisage optimal use of the solar spectrum with a minimum number of junctions.

[0050] The HCMJSC solar cell according to the invention is such that it comprises one or more lower junctions and one or more upper junctions. For example, the cell comprises less than 10 lower junctions, less than 8 lower junctions, less than 7 lower junctions, less than 6 lower junctions, less than 5 lower junctions, preferably 1 to 4 lower junctions. The lower junctions are such that they absorb in different spectral ranges. Also, for information purposes, the solar cell may comprise from 1 to 4 upper junctions, preferably less than 4, less than 3, or less than 2 upper junctions, preferably 1 upper junction.

[0051] According to a preferred embodiment, the HCMJSC solar cell according to the invention is such that it comprises 1 upper junction and 1 to 4 lower junctions.

[0052] According to another preferred embodiment, the HCMJSC solar cell according to the invention comprises an upper junction and at least two lower junctions, said lower junctions absorbing in different spectral ranges.

[0053] The upper HCSC junction preferably comprises selective energy contact (SEC) layers, accepting only a narrow range of energies, and thus making it possible to minimize energy losses during extraction (by minimizing the increase in entropy during carrier extraction).

[0054] For information purposes, the upper HCSC junction may for example comprise in order: an electron contact layer; a first selective energy contact (SEC) layer; an absorber; a second selective energy contact (SEC) layer; and a hole contact layer.

[0055] According to a preferred embodiment, the absorber of the upper HCSC junction has a “thin or thin” thickness, less than 1 μm, for example less than 800 nm, less than 500 nm or less than 300 nm, more preferably less than 200 nm. Even more preferably, the hot absorber has a thickness of between 0.1 nm and 200 nm, for example between 0.2 nm and 150 nm.

[0056] According to another embodiment, the absorber of the upper junction HCSC has an absorption coefficient between Eg + X eV greater than 10 5 per cm -1 , X being greater than 0.1 eV.

[0057] According to yet another embodiment, the hot carriers of the absorber have a temperature rise greater than 10°C relative to the temperature of the material, under operating conditions at the maximum operating point, preferably greater than 20°C, greater than 50°C or even greater than 100°C.

[0058] According to a preferred embodiment, the lower junction is said to be "thick", or more precisely "optically thick", which means that it is thick enough to absorb the majority of photons up to its band gap energy.

[0059] Preferably, the lower junction has a thickness of between 300 nm and 300 μm, the upper junction being thinner than the lower junction. For example, it has a thickness of between 300 nm and 4 μm when it is made of a direct bandgap material of type III-V (InP, GaAs and their alloys) or perovskites; it has a much greater thickness, for example between 50 μm and 300 μm (these values ​​correspond to an absence of optical trapping, or for reasonable optical trapping) when it is made of an indirect bandgap material of type Ge or Si.

[0060] Considering similar absorption coefficients, the thickness of the upper junction is significantly less than that of the lower junction.

[0061] According to a particularly preferred embodiment of the invention, the upper junction is (optically, but in 99% of cases also physically) thinner than the lower junction.

[0062] According to one embodiment of the HCMJSC solar cell of the present invention, two neighboring junctions may be separated by a tunnel junction. For example, the solar cell of the present invention may comprise a bottom junction and an upper HCSC junction, separated by a tunnel junction. The tunnel junction allows strip connection between the junctions so as to require only 2 terminals to extract current.

[0063] It is possible, within the scope of the present invention, for all junctions, lower and upper, to be connected by tunnel junctions.

[0064] According to one embodiment of the invention, when the lower and upper junctions are separated with a tunnel junction, the sub-cells are connected in series and generate the same current, making it possible to simplify the architecture.

[0065] According to another embodiment of the present invention, the upper and lower junctions are not separated by tunnel junctions. According to this embodiment, the HCMJSC cell is devoid of tunnel junctions. According to this embodiment, the HCMJSC solar cell comprises 2 terminals per junction. Thus, for information purposes, according to this embodiment, when the HCMJSC cell comprises 2 junctions, it then comprises 4 terminals; when the HCMJSC cell comprises 3 junctions, it then comprises 6 terminals; etc.

[0066] According to yet another embodiment, the composition of the HCMJSC solar cell may be mixed with respect to the tunnel junctions and the terminals, i.e. have tunnel junctions between certain junctions and a number of terminals varying between a minimum of 2 terminals (i.e. a front terminal and a rear terminal) and a maximum of 2 times the number of junctions (e.g. 8 terminals for a cell comprising 4 junctions or 10 terminals for a junction comprising 5 junctions). Thus, according to a mixed embodiment, a solar cell comprising 1 upper junction and 4 lower junctions comprises between 2 and 10 terminals. If it comprises 2 terminals, it then comprises 4 tunnel junctions. If it comprises 6 terminals, it then comprises 2 tunnel junctions.According to yet another embodiment, beyond 2-terminal architectures (tunnel junction, and cells in series) and 4-terminal architectures (no tunnel junction, independent cells), the HCMJSC cell can have 3 terminals. The sub-cells are then either connected in series (two p / n sub-cells or two n / p sub-cells), or connected in reverse (one p / n sub-cell and the other n / p). Additionally, compared to a 2-terminal architecture, either the sub-cells share an intermediate contact, or the lower cell has two interdigitated rear contacts. The total number of possible configurations is thus very high due to the numerous possible combinations.

[0067] According to one embodiment of the present invention, at least one lower junction may have a gap energy level gradient. For example, several of the lower junctions, or even all of the lower junctions, have a gap energy level gradient.

[0068] In addition to the above, we can consider a HCMJSC solar cell which takes advantage of the complementarity of the energy levels, and comprises for example an upper junction and a lower junction, having an arrangement in which:

[0069] - the gap energy level of the upper junction varies between 0.6 eV and 1.7 eV, for example between 0.65 eV and 1.6 eV or between 0.7 eV and 1.5 eV; and

[0070] - the gap energy level of the lower junction varies between 0.6 eV and 1.5 eV, for example between 0.65 eV and 1.45 eV or between 0.7 eV and 1.4 eV.

[0071] Another example of an HCMJSC solar cell according to the invention may in particular consist of an upper junction and two lower junctions, having an arrangement in which:

[0072] - the gap energy level of the upper junction varies between 0.6 eV and 1.9 eV, for example between 0.65 eV and 1.8 eV or between 0.7 eV and 1.7 eV;

[0073] - the gap energy level of the first lower junction varies between 0.8 eV and 1.6 eV, for example between 0.85 eV and 1.55 eV or between 0.9 eV and 1.5 eV;

[0074] - the gap energy level of the second lower junction varies between 0.6 eV and 1.3 eV, for example between 0.65 eV and 1.25 eV or between 0.7 eV and 1.2 eV.

[0075] In a conventional multi-junction (MJSC) system, the gap energy level of the different junctions decreases from top to bottom. One of the many advantages of the HCMJSC cell of the present invention (having said combination of junctions including an upper junction of the HCSC type), is that the energy level of the upper HCSC junction does not necessarily have to be strictly higher than the energy level of the lower junction at its contact (or first lower junction). It may, in fact, according to one embodiment, be lower than that of the lower junction. This has the advantage that the material and composition of the upper junction and the lower junction (for example first lower junction) can be identical, which can prove advantageous in terms of manufacturing.

[0076] The upper junction absorber comprises one or more nanocrystals.

[0077] The HCMJSC solar cell according to the present invention may be such that it comprises: at least two lower junctions absorbing in different spectral ranges (different band gaps); an upper junction of the hot carrier solar cell type comprising: o an electron contact layer, which may be n-doped; o a first selective energy contact (SEC) layer; o an absorber; o a second selective energy contact (SEC) layer; and o a hole contact layer, which may be p-doped; two terminals, positioned at the ends of the cell; an anti-reflection layer positioned in contact with the upper junction; optionally at least one tunnel junction positioned between the lower junctions and / or between the upper junction and the first lower junction; preferably, tunnel junctions between each of the junctions; optionally a mirror (or a metal layer) positioned under the last junction;optionally an optical trapping element (light diffusion / diffraction) aimed at using thinner layers while maintaining absorption and positioned: o between the anti-reflection layer and the upper junction, and / or o between the upper junction and the lower junctions, and / or o under the lower junction (between the junction and the mirror).;

[0078] As indicated above, the upper junction and the lower junction can be made of the same materials (i.e. materials with the same mesh parameter) and be of the same composition, which has the aforementioned advantages. Alternatively, they can be made of the same materials and be of different compositions. Finally, they can be made of different materials. In this case, the manufacturing process will have to be adapted to these different materials.

[0079] According to one embodiment, the absorber of the upper junction comprises one or more nanocrystals. Each of these one or more nanocrystals may in particular be chosen from the group consisting of AgBiS2, perovskites, PbS and PbSe. The list of nanocrystals that may be used in the context of the present invention is nevertheless not limited to these compounds alone. According to one embodiment, the nanocrystal(s) have a characteristic hot support cooling time greater than 10 ps, ​​for example greater than 30 ps, ​​greater than 50 ps, ​​or greater than 100 ps.

[0080] According to one embodiment, said at least one lower junction is made up of at least one layer of semiconductor materials, for example at least two layers made up of two semiconductor materials. Preferably, said at least one lower junction is made up of a layer of semiconductor materials. The semiconductor materials used in the context of the present invention may be of any type. For example, they may be type IV, type III-V and type II-VI, each of these materials being doped n-type or p-type. For information, type IV semiconductor materials include Ge, Si or a combination of both (SixCei-x-); type III-V semiconductor materials include InAs, GaAs, InP, GaSb, GalnAs, GalnAsP, GaAsSb, InAIAs, AIGaAs, and diluted nitrides of type (In)GaNAs(Sb); Type II-VI semiconductor materials include CdSe and CdTe.

[0081] The tunnel junctions, which are potentially part of the configuration of the HCMJSC solar cell according to the invention, may be of any type used in HCMJSC solar cells, may, for example, each have a thickness of 15 nm per p- or n-type material.

[0082] The HCMJSC solar cell according to the invention can have a substrate configuration with solar radiation penetration via the top of the cell or a superstrate configuration for which the radiation penetrates from the bottom. In the second case, the order of the junctions remains such that the light passes through the upper junctions before the lower junctions.

[0083] A second subject of the present invention relates to a mobile or stationary device comprising at least one HCMJSC solar cell according to the present invention, and optionally at least one optical element which is configured to direct and concentrate solar energy towards the HCMJSC solar cell (concentration).

[0084] The device according to the invention may in particular be a solar panel made up of several HCMJSC cells according to the invention.

[0085] A third subject of the present invention relates to a method of manufacturing an HCMJSC solar cell according to the invention.

[0086] The HCMJSC solar cell or only part of it, in particular the lower junctions, can in particular be produced by epitaxial growth, for example molecular beam epitaxy (MBE for molecular beam epitaxy or PA-MBE for plasma-assisted molecular beam epitaxy), MOVPE (organometallic vapor phase epitaxy), or ALD (atomic layer deposition).

[0087] The HCMJSC solar cell or only part of it, in particular the upper junction, can in particular be produced by inkjet printing, with a blade or "slit-matrix" type, by chemical vapor deposition (CVD for chemical vapor deposition or PECVD for plasma-enhanced chemical vapor deposition) or by sputtering.

[0088] It is possible to adopt a sequential approach to realize the HCMJSC solar cell according to the invention. For example, it is possible to manufacture the rear part (lower junction(s)), independently of the front part (upper junction(s)) and then to combine each part together.

[0089] According to one embodiment, the HCMJSC solar cell according to the invention is in substrate configuration and comprises at least one lower junction which is configured as follows. Such a junction can in particular be manufactured by epitaxial growth. The first layer is the substrate. This can in particular be made of glass, for reasons of cost and its adequate mechanical properties. It can alternatively be composed of plastic, which is advantageous for certain specific applications such as flexible solar panels. The second layer is a metal contact layer, which can for example be composed of Mo. Mo is an ideal candidate to play the role of rear contact due, on the one hand, to its coefficient of thermal expansion close to that of glass and the absorber film and on the other hand, because it forms an ohmic contact with the absorber layer thus ensuring minimal resistivity.This layer typically has a thickness of between 0.2 μm and 1 μm, for example between 0.4 μm and 0.8 μm. The junctions that make up the solar cell are then added, i.e. the lower junction(s) and at least one upper HCSC junction, all separated by tunnel junctions. A front metal contact layer can then be deposited on top of the last layer of the upper junction. This layer can, for example, be in the form of a grid of aluminum, nickel, or an alloy of the latter two. Finally, an anti-reflection layer can be applied to reduce reflections at the top of the cell and maximize its efficiency.

[0090] A fourth subject of the present invention relates to a method for producing electricity, according to which: at least one HCMJSC solar cell or a mobile or immobile device according to the invention, is exposed to solar radiation by its upper junction side, and an electric current is collected.

[0091] EXAMPLES

[0092] Example 1: Absorption of incident solar flux (AM1.5D spectrum) as a function of photon energy for different cells

[0093] From extrapolated absorption coefficients, the absorbed fraction of the incident photon flux (AM1.5D spectrum) was estimated as a function of the photon energy for different cells:

[0094] (a) an HCSC cell with a thin absorber (d = 200 nm; EG = 1.19 eV); (b) an HCMJSC cell according to the invention with a thin hot-carrier junction (d = 200 nm; EG = 1.19 eV) above a thick bottom junction (d = 2 m; EG = 0.93 eV).

[0095] (c) an MJSC cell composed of thick-film absorbers at the top (d = 2 pm; EG = 1.59 eV) and bottom (d = 2 pm; EG = 0.93 eV) junctions; and

[0096] (d) an MJSC cell according to WO2018 / 021966 with hot or cold carriers in upper and lower junctions.

[0097] The absorption coefficients considered here are obtained by extrapolation from the absorption coefficients of InP (EG = 1.344 eV) and InGaAs (EG = 0.74 eV).

[0098] The parameters used for this calculation aim to be realistic in order to characterize the general behavior.

[0099] As illustrated by the results in Figure 1 (ad), in a “conventional” multi-junction architecture, each cell is “optically thick” so that each cell only transmits photons below the energy of its band gap to the lower cell (MJSC architecture (c) and MJSC architecture (d) according to WG2018 / 021966). Unlike the HCMJSC cells according to the invention (architecture (b)), the spectrum is thus clearly divided between the two junctions, at an energy corresponding to the band gap of the upper junction.

[0100] However, in the architecture according to the invention ((b) HCMJSC), the upper hot-carrier cell is "optically thin" (one could also say semi-transparent), so that if there were no lower (thick) junction, many photons above its band gap energy would not be absorbed, as illustrated in architecture (a) HCSC. In architecture (b), the photons not absorbed in the upper hot-carrier junction are collected in the lower junction.

[0101] Example 2: Comparison of the efficiency of different cells as a function of the thermalization rate under concentrated illumination of 1000 suns (AMI spectrum, 5D)

[0102] The efficiency of different cells was simulated as a function of the thermalization rate under concentrated illumination of 1000 suns (AMI .5D spectrum): ideal MJSC cell (line "cross", EG, top = 1.59 eV and Ec,-bot = 0.93 eV); optimal HCMJSC cell according to the invention (line, EG, top = 1.344 eV and Ec,-bot = 0.93 eV); and optimal HCSC cell (line "circle", EG = 0.74 eV).

[0103] The “thermalization rate” is a fixed parameter for a given system. Varying the thermalization rate means comparing systems that generate hot carriers more or less easily. This “thermalization rate” is directly linked to a material parameter called the “thermalization coefficient” which characterizes the ratio between the incident power and the temperature rise of the hot carriers in the linear regime. A measurement of this thermalization coefficient has been reported in several publications and for different materials, for example Bris AL et al. (Energy & Environmental Science 5, 6225-6232 (2012)) and Giteau M. et al. (J. Appl. Phys. 128, 193102 (2020)), even if it remains difficult to compare the values ​​obtained in the different publications. To put the numerical values ​​in Figure 2 into perspective, the second publication reports a thermalization rate of the order of 10 5 (W.cm lC 1) for gallium arsenide (a material known to have a high thermalization rate, therefore not optimal for hot carrier cells), assuming efficient optical trapping.

[0104] As illustrated in Figure 2, three regions can be distinguished.

[0105] The MJSC cell offers the highest efficiency for strong thermalization, because it is not possible to benefit from a hot-carrier efficiency gain. On the other hand, if the hot-carrier cell did not undergo thermalization at all, then it could be thick, and the single junction HCSC would be optimal (Fig. 2 - left part of the graph), which is not the case for real materials.

[0106] We can thus highlight that in a certain intermediate range (realistic, close to prior art systems) of thermalization rates, the HCMJSC architecture proposed by the solution according to the invention makes it possible to achieve conversion efficiencies significantly better than those of hot carrier cells (HCSC) and multi-junction cells (MJSC) (Fig.2 - central part of the graph).

[0107] The architecture according to the invention thus makes it possible to use optically thin hot carrier cells in the upper junction. This low thickness allows a reduction in the thermalization rate (proportional to the volume of the absorber), and thus makes it possible to improve the hot carrier effect as illustrated in Figure 2. These hot carriers, extracted through (semi-)selective contacts, can lead to an increase in voltage, and thus improve the efficiency of the cell.

[0108] By combining a hot-carrier cell in the upper junction with a cold thick lower junction, the problem of partial absorption is overcome by absorbing the photons that pass through the upper junction into the lower junction (Fig.l - architecture (b) HCMJSC according to the invention). The combination of the upper junction of the hot-carrier solar cell type therefore offers a synergy between hot-carrier cells and multi-junction cells.

[0109] Example 3: Comparison of the influence of the band gaps of the upper and lower junctions on the efficiency of MJSC (Fig.3a) and HCMJSC (Fig.3b) cells according to the invention, for a fixed thermalization rate corresponding to the efficiency crossover point for the optimal devices (referenced in Figure 2), under concentrated illumination of 1000 suns (AMI spectrum, 5D).

[0110] As illustrated in Figure 3, although the monomer efficiency of the two architectures is similar, the range of forbidden bands on the strand indicates that the efficiency of the HCMJSC structure according to the invention is close to its maximum and is much greater than for the MJSC structure, particularly with respect to the top cell band gap. Indeed, HCMJSC architectures with a narrow top junction band gap are less efficient in the absence of hot carriers but benefit more from the hot carrier effect because the top cell can be thinner.

[0111] As a result, the HCMJSC cells of the invention offer a much wider range of acceptable band gaps than MJSC cells, which relaxes epitaxial growth requirements.

[0112] In particular, it is possible to use two sub-cells composed of the same material. It is also possible to use a material with a narrower band gap for the upper junction than for the lower junction. Both of these realizations, largely suboptimal for the MJSC structure, can perform very well in an HCMJSC structure.

Claims

CLAIMS Hot-carrier multi-junction solar cell (HCMJSC), comprising: at least one lower junction; and at least one upper junction of the hot-carrier solar cell (HCSC) type, comprising: o an absorber; o at least one contact layer selected from: a / an electron contact layer, the conduction band of which is higher than the conduction band of the absorber, and / or b / a hole contact layer, the valence band of which is lower than the valence band of the absorber. Solar cell according to claim 1, wherein the upper junction comprises two contact layers: an electron contact layer, and a hole contact layer. Solar cell according to claim 1 or 2, wherein the upper junction comprises in the following order: an electron contact layer; a first energy-selective contact (ESC) layer; an absorber; a second energy-selective contact (ESC) layer; and a hole contact layer.A solar cell according to any one of the preceding claims, wherein the lower junction and the upper junction are separated by a tunnel junction. A solar cell according to any one of the preceding claims, comprising an upper junction and at least two lower junctions, said lower junctions absorbing in different spectral ranges. A solar cell according to any one of the preceding claims, comprising: at least two lower junctions absorbing in different spectral ranges. a hot-carrier solar cell top junction comprising: an electron contact layer; a first energy-selective contact (SEC) layer; an absorber; a second energy-selective contact (SEC) layer; and a hole contact layer; two terminals, positioned at the ends of the cell; an anti-reflective layer positioned in contact with the top junction; optionally at least one tunnel junction positioned between the lower junctions and / or between the top junction and the first lower junction; optionally a mirror or a metallic layer, positioned below the last junction; optionally an optical trapping element aimed at using thinner layers while maintaining absorption and positioned: between the anti-reflective layer and the top junction and / or between the top junction and the lower junctions and / or below the lower junction (between the junction and the mirror).A solar cell according to any one of the preceding claims, wherein the absorber of the upper junction has a thickness of less than 1 µm, preferably less than 200 nm. A solar cell according to any one of the preceding claims, wherein the hot carriers of the absorber have a temperature rise of more than 10°C relative to the material temperature, under operating conditions at the maximum operating point. A solar cell according to any one of the preceding claims, wherein the lower junction has a thickness between 300 nm and 300 µm, the upper junction being thinner than the lower junction. A device comprising at least one solar cell (HCMJSC) according to any one of claims 1 to 9 and at least one optical element configured to direct and concentrate solar energy towards the solar cell.

1. A method for manufacturing a solar cell (HCMJSC) according to any one of claims 1 to 9, by: epitaxial growth, in particular by molecular beam epitaxy (MBE for molecular beam epitaxy or PA-MBE for plasma-assisted molecular beam epitaxy), metal-organic vapor deposition (MOVPE), or atomic layer deposition (ALD); and / or by inkjet printing, with a slide or slit-matrix type, by chemical vapor deposition (CVD for chemical vapor deposition or PECVD for plasma-assisted chemical vapor deposition), or by sputtering. A method for generating electricity, wherein: at least one solar cell (HCMJSC) according to any one of claims 1 to 9 or a device according to claim 10, is exposed to solar radiation by its upper junction side, and an electric current is collected.

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