Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
  • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
  • H01L31/035236—Superlattices; Multiple quantum well structures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
  • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers
  • H01L31/072—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PN heterojunction type
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy

Definitions

• the invention relates to a semiconductor heterostructure that can be used in photovoltaic cells.
• the invention also relates to a photovoltaic cell, in particular of the thin-film type, comprising such a heterostructure as an active element, and to a solar panel comprising an association of such cells.
• Photovoltaic cells are devices that convert the light energy transported by photons - usually of solar origin - into electrical energy in the form of a direct current.
• Several photovoltaic cells are generally associated in series (to raise the voltage level) and in parallel (to increase the current intensity) so as to form a solar panel or photovoltaic module.
• Several types of photovoltaic cells exist, using different materials: solid or thin-layer cells, single-junction or multi-junction, homo-junction (s) or heterojunction (s), organic, etc.
• the active element consists of one or more PN junctions.
• the most widespread photovoltaic cells are made of bulk silicon, or deposited in the form of epitaxial layers, mono- or polycrystalline, or amorphous.
• Silicon indeed has considerable technological and economic advantages: it is a material available in large quantities, non-toxic, having a stable insulating oxide and whose implementation has been controlled for decades.
• silicon absorbs light weakly because of its indirect band structure; thus, the absorption depth of the light is relatively large (from 10 to 100 pm for wavelengths between 800 and 100 nm corresponding to the red / near infrared portion of the solar spectrum), and the carriers must broadcast on a distance of the same order of magnitude before reaching an electrical contact. This limits the maximum efficiency of photovoltaic conversion of the energy of photovoltaic cells with homo-junction in silicon. In addition, it requires the use of thick cells, and therefore expensive.
• direct band structure materials absorb light more effectively, which limits the diffusion length of the photogenerated carriers (0.1 - 1 ⁇ ) and the thickness of the cells; but the direct band structure also facilitates the recombination of the minority carriers, which goes against the goal of increasing the energy conversion efficiency.
• FIG. 1 shows, very schematically, the band structure of a doped CdSe "n" (left, reference R1) / ZnTe doped "p" heterojunction (right, reference R2).
• the alignment of the energy bands is such that:
• the upper limit E V 2 of the valence band of ZnTe lies between the upper limit E V i of the valence band and the lower limit E C i of the conduction band of the CdSe;
• the lower limit Eci of the CdSe conduction band is between the upper limit E V 2 of the valence band and the lower limit Ec2 of the ZnTe conduction band.
• the electrons "e” of the conduction band see a minimum of potential in the region R1 in CdSe, while the holes “h” of the valence band see a maximum of potential in the region R2 in ZnTe .
• the carriers remain separated, on both sides of the junction, and the recombination rate is low.
• the light absorption threshold corresponds to the transitions between the valence band states located in the ZnTe and the states of the conduction band located in the CdSe, that is to say at an energy of about 1 eV, well below the widths of the forbidden bands of the two materials considered separately.
• “Tandem” solar cells include a stack of two heterostructures with forbidden bands of different widths to better exploit the extent of the solar spectrum. See for example the article by P. Gashin et al. However, this does not make it possible to remedy the abovementioned disadvantages of heterojunction photovoltaic cells.
• Electron-hole pairs are generated in the layers of said superlattice by transitions that are direct in both the real space and the reciprocal space; the electrons diffuse rapidly in the layers with a lower energy conduction band, and the holes in those with a higher energy valence band; then these carriers diffuse layers in the plane, respectively to the region "n” and the region "p” while remaining separate, which minimizes the recombination rate.
• the upper limit of the valence band of the absorption region is aligned with that indicated by E 2 of the region R2 while the lower limit of its conduction band is aligned with that indicated by E C i, from region R1.
• the upper limit E V i of the valence band of region R1 is at an energy level lower than E V2 and the lower limit E C2 of the conduction band of region R2 is at an energy level greater than E d .
• the references E F c and E F v indicate the Fermi quasi- levels of the light-absorbed absorption region for electrons and holes, respectively.
• the photogenerated electrons drift to the left along a gradient - imperceptible in the figure - of E FC
• the photogenerated holes drift to the right along a gradient - also imperceptible in the figure - from E Fv .
• the step formed by the valence band is a barrier for the holes, while the alignment of the conduction bands ensures a fast evacuation of electrons e.
• the echelon formed by the conduction band constitutes a barrier for the electrons, while the alignment of the valence bands ensures a rapid evacuation of the holes h.
• the band structure illustrated in FIG. 2 is purely theoretical, and does not correspond to any physical system known from the prior art.
• the best known approximation of this structure consists of so-called "Grâtzel” cells, based on a porous structure of nan 2 nanoparticles (forming the R1 region) coated with a thin layer of dye (RA absorber) and bathed in a electrolyte (forming the region R2).
• RA absorber dye
• electrolyte forming the region R2
• These cells nevertheless have numerous drawbacks: the electrolyte is generally liquid requiring sealed encapsulation, the electronic transport of one nanoparticle to the other is difficult, there is little flexibility for the adaptation of the dye to the solar spectrum and, especially, the dye is easily degraded by ultraviolet light.
• the invention aims to overcome the aforementioned drawbacks of the prior art.
• An object of the invention consists of a heterostructure comprising a first region made of a first semiconductor material with n-type doping, a second region a second semiconductor material with doping. of type p and, between said first and second regions, a superlattice formed by an alternation of layers of a third and a fourth semiconductor material, the interfaces between the first region and the superlattice, between the layers of the super-network, and between the super-network and the second region being parallel to each other; characterized in that: the thicknesses of the layers are sufficiently small that the carriers are delocalized within said superlattice, forming at least a mini-electron band and a mini-band of holes; in that the upper limit of the valence band of said fourth material is between the upper limit of the valence band and the lower limit of the conduction band of said third material, and the lower limit of the conduction band of said third material is between the upper limit of the valence band and the lower limit of the conduction
• the upper limit of the valence band of said first material must be less than the lower limit of the mini-band of holes. at least 0.1 - 0.2 eV.
• the upper limit of the valence band of the first material will be less than or equal to that of the valence band of said third material.
• the lower limit of the conduction band of said second material must be greater than the upper limit of the mini-band. of electrons of at least 0.1 - 0.2 eV.
• the lower limit of the conduction band of the second material will be greater than or equal to that of the conduction band of said fourth material.
• said first material may be identical to said third material, except as regards its doping
• said second material may be identical to said fourth material, except as regards its doping
• Said layers forming a superlattice may have thicknesses of between 1 and 10 nm.
• Said superlattice may have a total thickness of between 300 nm and 1500 nm.
• Said superlattice may be formed by intrinsic semiconductor layers.
• intrinsic is meant a semiconductor material having a concentration of impurities capable of acting as dopants less than or equal to 0 16 cm -3 and preferably less than or equal to 10 -15 cm- 3 .
• Said superlattice and at least one of said first zone and said second zone may be in the form of thin layers deposited on a substrate.
• Said first, second, third and fourth materials may be inorganic semiconductors. More particularly, at least said third and fourth materials may be type II-VI or III-V semiconductors, and may especially be chosen from the following pairs: CdSe / ZnTe; CdS / ZnTe; InP / GaAs; InP / GaSb; GaN / AlAs; GaN / ZnTe; ZnO / ZnSe; ZnO / ZnTe; ZnO / CdSe; and ZnO / CdTe.
• the following pairs are particularly preferred: CdSe / ZnTe; CdS / ZnTe; InP / GaAs and InP / GaSb.
• the layers forming said superlattice, as well as their interfaces with said first and second regions, may be planar.
• Another object of the invention is a photovoltaic cell comprising a heterostructure as described above as an active element.
• Yet another object of the invention is a solar panel comprising an association of such photovoltaic cells.
• FIG. 3 a schematic representation of a heterostructure according to one embodiment of the invention.
• FIGS. 4A, 4B and 4C three representations of the band structure of the heterostructure of FIG. 3 under three different conditions, namely: in the condition of flat bands, at equilibrium and under illumination;
• FIG. 5 a photovoltaic cell according to one embodiment of the invention.
• FIG. 6 a solar panel according to one embodiment of the invention. It will be necessary first of all to define certain concepts used in the following.
• a heterostructure is a structure formed by semiconductors having forbidden bands of different widths, in contact with each other.
• the junction between two semiconductor materials having forbidden bands of different widths is a heterojunction.
• a superlattice is a periodic structure formed by the alternation of layers of different materials. With regard to the present invention, reference will always be made to the case where these materials are semiconductors having different forbidden bands.
• Such a superlattice can also be considered as a set of identical quantum wells, coupled to each other. If the coupling between the quantum wells is sufficiently strong (ie if the layers are sufficiently thin, generally of the order of a few nanometers) their discrete energy levels combine into bands of energy which, for a infinite super-network, would be continuous; we speak of "mini-bands" which correspond to electronic states delocalized on the whole volume of the super-network. It is, on another scale, the same phenomenon that leads to the formation of the band structure of the crystals.
• photovoltaic cell When speaking of a photovoltaic cell is meant a device for generating electrical energy, by photovoltaic effect, from sunlight, and in particular solar light received on the ground. Such a device must therefore be sensitive to visible and near-infrared radiation.
• a super-network behaves as an "effective material" having a structure of direct bands and bandwidth adjustable, so to speak “tailor-made”.
• Such a material may be designed to act as the absorber in the theoretical structure of FIG.
• FIG. 3 shows, very schematically, a heterostructure according to one embodiment of the invention.
• This heterostructure comprises a region R1 in CdSe doped "n", a region R2 in ZnTe doped "p" (in both cases, the doping density is of the order of 10 18 -10 20 cm -3 ) and, between two, a super-network SR formed by an alternation of layers of these two materials (Ci: CdSe layers, C 2 : ZnTe layers), without doping
• the overall thickness of the superlattice must be as low as possible, while being large enough to provide virtually total absorption of the incident sunlight, as the materials concerned have a direct band structure in reciprocal space, the ideal thickness is about 600 nm.
• R1 is made of the same material as the layers Ci (with, if necessary, a different doping), and the second region R2 is made of the same material as the layers C 2 (also with a doping which, if necessary, can be different). As will be explained in detail later, this constraint can be relaxed.
• FIG. 4A shows a representation of the band structure of the heterostructure illustrated in FIG. 3. For the sake of generality, it is indicated by:
• E C 2, Ev 2 respectively, the lower limit of the conduction band and the upper limit of the valence band of the material of the second region R2;
• E C4 , E V4 respectively, the lower limit of the conduction band and the upper limit of the valence band of the material of the layers C 2 .
• first region R1 in a material having an energy Ec1 greater than Ec3 and closer to ⁇ so as to reduce, or even eliminate, the jump potential at the interface R1 / SR.
• second region R2 of a material having an energy less than E Ev2 V4 and closer to ⁇ ⁇ so as to reduce or remove the potential jump at the SR / R2 interface.
• This can be achieved by using a first (respectively: second) material of chemical composition close to that of the third (respectively: fourth) material, but comprising an impurity by slightly changing the bandwidth.
• C i E can be increased compared to E C 3 by making the region R1 Mn x Se or Mg -x Cdi Cdi -x x Se, where x s order of a few percent.
• the band structure of FIG. 4A shows that the heterostructure of FIG. 3 is of type II.
• the layers forming the superlattice SR are small enough that the carriers are substantially delocalized; the ideal period (thickness of a layer Ci plus a thickness of a layer C 2 ) of the superlattice is approximately 4 nm; the layers Ci and C 2 may have the same thickness or different thicknesses.
• the energy levels of electrons and holes in the superlattice form "mini-bands"; in FIG. 4A, only the least energy mini-band of electrons (MBe) and the most energetic mini-band of holes (Mbh) have been represented.
• the upper limit E M Bh of the mini-band MBh and the lower limit E M Be of the mini-band MBe define an effective band gap EGSR whose width is adjustable in the range of 1 to 2 eV by varying the thicknesses layers of the super-network and therefore well adaptable to the solar spectrum.
• E M Bh of the mini-band MBh and the lower limit E M Be of the mini-band MBe define an effective band gap EGSR whose width is adjustable in the range of 1 to 2 eV by varying the thicknesses layers of the super-network and therefore well adaptable to the solar spectrum.
• E MB h is the upper limit of the valence band
• E M Be the lower limit of the valence band.
• the structure of The resulting band is very similar to Figure 2 except that the Eci and E M Be levels are not exactly aligned, as are the levels E V 2 and E M Bh-
• the levels of the mini- electron band are resonant with energy levels (with wave vector k ⁇ 0) of the conduction band of the region R1; similarly, the levels of the mini-band of holes are resonant with energy levels (with wave vector k ⁇ 0) of the valence band of the region R2. This ensures a very fast transfer of photogenerated electrons to R1 and photogenerated holes to R2.
• FIG. 4A The diagram of FIG. 4A is of the "flat band” type, ignoring the effect of load carrier distribution in the structure. On the other hand, this effect is taken into account in FIGS. 4B and 4C.
• FIG. 4B relates to the case where the heterostructure is in equilibrium conditions, in the absence of illumination, without current flow and without voltage applied across the terminals.
• the level of Fermi E F must be constant throughout the structure, which leads to a reorganization of the charges and consequently to an inclination of the bands in the superlattice (represented here as an effective material ).
• the region R2 may be constituted by a p-doped ZnTe substrate, on which alternate layers of intrinsic CdSe and ZnTe are deposited to form the SR superlattice, and then a thicker layer of n-doped CdSe to form the region. R1.
• the layer R2 may in turn be an epitaxial layer deposited on a conductive substrate, or a conductive layer serving as an electrode, deposited in turn on an insulating substrate.
• pairs of materials can be used instead of CdSe and ZnTe; in particular, they may be pairs of type II-VI or III-V inorganic semiconductors, advantageously with a direct band structure to benefit from the high absorption coefficient characterizing these materials.
• the main conditions to be respected are the following:
• type II heterostructures Neither type I heterostructures (characterized by the conditions: Ec3> E C4; ev3 ⁇ E V4) nor those of type III (ev3> Ec 4> EV4) are suitable for photovoltaic applications. Care should be taken that the terminology used in the scientific literature is not perfectly uniform, and that some authors use the term "type II" extensively, also covering structures that are actually type III.
• the forbidden band at the interface of the two materials must be compatible with a good exploitation of the spectrum of the incident light.
• this bandgap must be less than 1, 8 eV.
• the mesh parameter deviation must be small enough to allow the formation of heterojunctions with few defects (typically less than 10%, although larger deviations may be tolerated in some cases).
• the optimal width of the superlattice depends on the materials considered, as well as the nominal spectrum of the incident light. As a rule, this thickness may be between 300 nm and 1500 nm (1.5 ⁇ m).
• the doping density in the regions R1 and R2 will depend on the application considered. It is not essential that the super-network be intrinsic.
• the following table presents a nonlimiting list of pairs of materials that can be used to produce a heterostructure according to the invention.
• the first four pairs are particularly preferred because of their small gap in mesh parameters.
• the widths of the forbidden bands are indicative, drawn from the scientific literature in the field; which explains why, in some cases, a range is indicated instead of a single value.
• GaN / AlAs 20% 1, 73 eV GaN / ZnTe 29% 1, 42 eV
• the first region R1 and the layers Ci (respectively: the second region R2 and the layers C 2 ) consist of the same material.
• the first region R1, the second region R2, the layers C and C two layers are formed of respectively a first, second, third and fourth semiconductor material with E C band limits i / Evi; E C 2 Ev 2; E C i / E C 2; E C 3 Ev3 and E C4 / E V 4 respectively.
• E C band limits i / Evi E C 2 Ev 2
• E C i / E C 2 E C 3 Ev3 and E C4 / E V 4 respectively.
• the lower limit of the conduction band of said first material is between that of the conduction band of said third material and that of the least energy mini-band of said superlattice: E C 3 ⁇ Eci ⁇ E M Be ; and
• the upper limit of the valence band of said second material is between that of the valence band of said fourth material and that of the most energetic mini-band of holes of said superlattice: E V4 ⁇ E V 2 ⁇ E M Bh -
• the upper limit of the valence band of said first material (E V i) is lower than the lower limit of the mini-band of holes by at least 0.1 - 0.2 eV, and that the lower limit of the conduction band of said second material (E C 2) is greater than the upper limit of the electron mini-band of at least 0.1 - 0.2 eV.
• Evi ⁇ E V 3 and Ec2 ⁇ Ec4- Figure 5 shows a sectional view of a photovoltaic cell CP according to one embodiment of the invention.
• This cell comprises, as active region (responsible for the absorption of light and the generation of carriers), a heterostructure of the type of FIG. 3.
• a metal layer CM forms an ohmic contact with the substrate R2 in ZnTe.
• a CT layer of a transparent conductive material (eg, zinc oxide or tin oxide) deposited above the CdSe region R1 forms the opposite electrode.
• a RP glass coating protects the whole.
• the R2 region may in turn be a thin layer, having a thickness of a few micrometers or less, deposited on the CM layer, deposited in turn on an insulating substrate, for example glass.
• a photovoltaic cell according to another embodiment may also have a "tandem" structure, formed by a superposition of two heterostructures of the type of Figure 3 adapted to exploit different parts of the solar spectrum.
• FIG. 6 shows a solar panel, or photovoltaic module, formed by a series and parallel association of photovoltaic cells of the type illustrated in FIG. 5. With the exception of the cells themselves, the structure of such a panel or module is conventional.

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