Perovskite-based pin-type tandem photovoltaic structure comprising a buffer layer made of alo x

Abstract

The invention relates to a pin-type tandem photovoltaic structure comprising, from the front side (1a) to the back side (1b): o an upper electrode layer (20); o an upper photovoltaic cell (30), comprising: a buffer layer (31) made of an aluminium oxide with a thickness of between 1 nm and 5 nm; • an electron transport layer (32), • an active layer (33) based on a perovskite material; • a hole transport layer (34); o a lower photovoltaic cell (50).

Description

[0001] PIN-TYPE TANDEM PHOTOVOLTAIC STRUCTURE BASED ON PEROVSKITE WITH AN ALOx BUFFER LAYER

[0002] DESCRIPTION

[0003] TECHNICAL FIELD

[0004] The field of the invention is that of tandem type photovoltaic structures, and in particular pin type photovoltaic structures whose upper photovoltaic cell is made from perovskite.

[0005] STATE OF THE PRIOR ART

[0006] Solar cells convert part of the spectral range of solar radiation into electrical energy. To increase conversion efficiency, it is possible to manufacture photovoltaic structures with a tandem architecture that includes two photovoltaic cells stacked on top of each other and absorbing in different spectral ranges.

[0007] A pin-type tandem photovoltaic structure (also called inverse type) generally comprises, from the front face (face which receives the incident light radiation) to the rear face: o an upper electrode layer; o an upper perovskite-based photovoltaic cell, formed of an electron transport layer, a perovskite active layer, and a hole transport layer; o a recombination layer; o a lower cell, for example based on silicon; o a rear electrode layer.

[0008] The upper photovoltaic cell may also have a buffer layer, located between the upper electrode layer and the electron transport layer (n-type). The buffer layer and the n-doped layer can also be considered two sub-layers of the same electron transport layer.

[0009] The buffer layer eliminates charge accumulation between the upper electrode layer and the n-type layer, facilitates charge carrier extraction, reduces charge carrier recombination at the interface between the upper electrode layer and the n-type layer, and protects the n-type layer during deposition of the upper electrode layer.

[0010] In the field of perovskite-based photovoltaic cells, the buffer layer can be made of bathocuproine (BCP) deposited by physical vapor deposition (PVD), or of tin oxide SnO2 deposited by cathodic sputtering. Indeed, these two materials allow a good compromise between stability and efficiency to be obtained.

[0011] However, there is a need for such a pin-type tandem photovoltaic structure that exhibits improved performance, especially better stability under illumination.

[0012] STATEMENT OF THE INVENTION

[0013] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a pin-type tandem photovoltaic structure, the upper photovoltaic cell of which is based on perovskite, which has better stability under illumination.

[0014] For this, the object of the invention is a pin-type tandem photovoltaic structure, comprising, from the front face to the rear face: o an upper electrode layer; o an upper photovoltaic cell, comprising: a buffer layer; an electron transport layer; • an active layer based on a perovskite material;

[0015] • a hole transport layer; o a lower photovoltaic cell, comprising:

[0016] • an n-type layer;

[0017] • an active layer;

[0018] • a p-type layer;

[0019] According to the invention, the buffer layer is made of an aluminum oxide with a thickness of between 1 nm and 5 nm.

[0020] Some preferred but non-limiting aspects of this tandem photovoltaic structure are as follows.

[0021] The buffer layer can have a thickness between 2.5 and 4 nm.

[0022] The buffer layer can be made of an aluminum oxide AIOx, where the value x corresponds to the number of oxygen atoms per 1 aluminum atom, x being between 0.8 and 1.5.

[0023] The buffer layer can be located in contact with the upper electrode layer and the electron transport layer.

[0024] The upper electrode layer can be made of a transparent conductive oxide.

[0025] The upper electrode layer can be made of TCO such as ITO, IZO or 10:1-1.

[0026] The tandem photovoltaic structure may comprise an upper metal contact resting on and in contact with the upper electrode layer, and made of Ag, Cu, Ni, and / or Al.

[0027] The electron transport layer can be an n-type layer made of C60, PC61BM, PC71BM, PCBM / C60, or PCBM / C.

[0028] The lower photovoltaic cell can be made from silicon, perovskite, or Cu(ln,Ga)(S,Se)2. The invention also relates to a method for manufacturing a tandem photovoltaic structure according to any one of the preceding characteristics, comprising a step of producing the buffer layer by depositing atomic thin layers at a deposition temperature of between 25°C and 150°C, preferably between 40°C and 75°C.

[0029] The precursor of aluminum can be TMA, and the precursor of oxygen can be water or ozone.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: Figure 1 is a schematic and partial view, in cross-section, of a tandem photovoltaic structure of the pin type based on perovskite according to one embodiment; Figures 2A to 2D illustrate the stability under illumination of different tandem photovoltaic structures which differ from each other by the material and / or the thickness of the buffer layer:

[0032] • fig.2A: stability under illumination of the current density of the upper photovoltaic cell,

[0033] • fig.2B: stability under illumination of the open circuit voltage of the upper photovoltaic cell,

[0034] • fig.2C: stability under illumination of the form factor of the photovoltaic structure,

[0035] • fig.2D: stability under illumination of the tracking of the maximum power point of the photovoltaic structure. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0036] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0037] The invention relates to a pin-type tandem photovoltaic structure whose upper photovoltaic cell is made from pervoskite.

[0038] The photovoltaic structure is called "tandem", in the sense that it comprises two photovoltaic cells superimposed on each other, and adapted to absorb incident light radiation in different spectral ranges.

[0039] It is pin type (or inverse type), in the sense that the electron transport layer (ETL) of the upper photovoltaic cell is located on the front side (receiving the incident light radiation), and therefore the hole transport layer (HTL) is located on the back side.

[0040] In the photovoltaic structure according to the invention, a buffer layer, located between an upper electrode layer and the electron transport layer, is made of an aluminum oxide AIOx with a thickness of between 1 nm and 5 nm, and preferably between 2.5 and 4 nm. The value x of the aluminum oxide AIOx, which corresponds to the number of oxygen atoms per 1 aluminum atom, is preferably between 0.8 and 1.5. The stoichiometric value x=1.5 corresponds to alumina AI2O3.

[0041] Thus, as detailed below, it appears that such a photovoltaic structure presents better performances, in terms of stability under illumination, than in the case where the buffer layer is made with materials such as BCP and SnO2.

[0042] In the remainder of the description, a tandem photovoltaic structure of the Si / PK type is considered, in the sense that the lower photovoltaic cell is made from silicon. However, other types of lower photovoltaic cell may be suitable, such as a perovskite-based photovoltaic cell, or a Cu(ln,Ga)(S,Se)2-based photovoltaic cell, among others. In addition, the active silicon layer is here made from a double-sided polished silicon substrate, but a textured substrate may also be suitable.

[0043] Figure 1 is a schematic and partial cross-sectional view of a pin-type tandem photovoltaic structure 1 according to one embodiment.

[0044] It comprises, from the front face 1a to the rear face 1b: an upper metal contact 10, a transparent upper electrode layer 20, an upper photovoltaic cell 30 based on perovskite, a recombination layer 40, a lower photovoltaic cell 50, here based on silicon, a lower electrode layer 60 and a lower metal contact 70.

[0045] The upper 10 and lower 70 metal contacts may be made of a metallic material, for example silver Ag, copper Cu, nickel Ni, and / or aluminum Al, among others, with a thickness for example between 50 and 500 nm. They may be made by thermal evaporation, or by electron beam evaporation (EBPVD, for Electron Beam Physical Vapor Deposition, in English). They may also be made by screen printing.

[0046] The upper 20 and lower 60 electrode layers are made of a conductive oxide. The upper electrode layer 20 is made of a material transparent to the radiation to be absorbed. They can be made, for example, of ZnO possibly doped with aluminum or another element (boron, gallium, etc.), of ITO or IZO, among others, with a thickness, for example, between 20 and 250 nm. They can in particular be made by atomic layer deposition (ALD), for example in the case of ZnO, or by cathode sputtering for ZnO:AI, ITO, IZO.

[0047] The upper photovoltaic cell 30 is a perovskite-based photovoltaic cell. It comprises a buffer layer 31, an electron transport layer 32 (ETL), an active layer 33 made of a perovskite material, and a hole transport layer 34 (HTL).

[0048] The buffer layer 31 is made of an aluminum oxide AIOx with a thickness of between 1 and 5 nm, and preferably between 2.5 and 4 nm. It may be alumina AI2O3, but the value x is, more broadly, preferably between 0.8 and 1.5. This buffer layer 31 is produced by ALD deposition, at a temperature for example between 25°C and 150°C, and preferably between 40°C and 75°C. Such a deposition temperature makes it possible to limit the risks of degradation of the perovskite active layer. The buffer layer 31 is a continuous thin layer of uniform thickness (the ALD deposition is a conformal deposition). It is here located between and in contact with the upper electrode layer 20 and the n-type layer 32.

[0049] In addition to the fact that such a buffer layer 31 makes it possible to improve the stability under illumination of the photovoltaic structure 1 (as explained below), it also makes it possible to prevent the diffusion of iodine ions from the active perovskite layer 33 to the upper electrode layer 20, and therefore to prevent the decomposition of the active perovskite layer 33 on the one hand, and the corrosion of the upper electrode layer 20 on the other hand.

[0050] The electron transport layer 32 is an n-type layer that can be made, for example, of C60, PC61BM, PC71BM, PCBM / C60 or PCBM / C, with a thickness of, for example, between 5 and 50nm, and deposited in particular by thermal evaporation or by spin coating. It can also be made of a tin oxide SnOx with x between 1 and 2, with a thickness of, for example, between 5 and 30nm, and deposited in particular by sputtering or by spin coating. It can also be made of ZnO, possibly doped with aluminum, with a thickness of, for example, between 5 and 30nm, and deposited in particular by sputtering or by spin coating, or even of ITO or IZO deposited by sputtering.

[0051] The active layer 33 is made of a perovskite material of type APbX3 with A representing methylammonium, dimethylammonium, formamidinium, guanidinium or cesium cations (the aforementioned cations can be used alone or in a mixture), and X representing halide anions, preferably chlorine, bromine, iodine anions (alone or in a mixture). The active layer 33 has a thickness for example between 200nm and 1.5pm. It can be formed in different ways, for example by thermal evaporation or co-evaporation, by chemical vapor deposition, for example of the ALD type, by pulsed laser deposition (PLD for Pulsed Laser Deposition, in English).

[0052] The hole transport layer 34 is a p-type layer. It may be made of PEDOT or PTAA, with a thickness of, for example, between 5 and 50 nm, and produced in particular by spin coating. It may also be made of SAM, with a thickness of, for example, between 1 and 5 nm, and produced in particular by spin coating or by thermal evaporation. It may also be made of MoOx with a thickness of, for example, between 5 and 30 nm, and produced by thermal evaporation. It may also be made of NiOx with a thickness of, for example, between 1 and 30 nm, and produced by ALD deposition or by cathode sputtering. Other materials are also possible.

[0053] The recombination layer 40 may be a conductive layer made of a transparent conductive oxide (TCO), for example ITO, IZO, IO:H, among others.

[0054] The lower photovoltaic cell 50 is a photovoltaic cell which is here based on silicon. However, as mentioned previously, one can also consider a cell based on perovskite, or even based on other materials, such as for example Cu(ln,Ga)(S,Se)2. Here it comprises a layer 51 of n-type doped amorphous silicon, then a layer 52 of unintentionally doped amorphous silicon, an active layer 53 of crystalline silicon, a layer 54 of unintentionally doped amorphous silicon, and a layer 55 of p-type doped amorphous silicon.

[0055] For example, the photovoltaic structure 1 can be formed in the following manner, from the front face 1a to the rear face 1b: o the upper contact 10 in silver Ag, produced by thermal evaporation under vacuum; o the upper electrode layer 20 in ITO, produced by cathode sputtering; o the upper photovoltaic cell 30, formed of:

[0056] • the buffer layer 31 in AI2O3 with a thickness of between 1 nm and 5 nm, for example 2 or 3 nm, produced by ALD deposition at a temperature of between 25°C and 150°C;

[0057] • the electron transport layer 32, in Ceo deposited by thermal evaporation;

[0058] • the active layer 33 of perovskite (Cs,FA)Pb(l,Br)3, produced by centrifugal coating in a glove box under nitrogen N2;

[0059] • the hole transport layer 34 in 2PACz, produced by spin coating in a glove box under nitrogen N2; o the recombination layer 40 in ITO, produced by physical vapor deposition (PVD); o the lower photovoltaic cell 50, formed of:

[0060] • layer 51 in n-type doped amorphous silicon, produced by PECVD;

[0061] • layer 52 in amorphous silicon not intentionally doped, produced by PECVD;

[0062] • the active layer 53 in crystalline silicon;

[0063] • layer 54 in unintentionally doped amorphous silicon, produced by PECVD; layer 55 in p-type doped amorphous silicon, produced by PECVD; o the lower electrode layer 60 in ITO, produced by cathode sputtering; o the lower contact 70 in silver Ag, produced by thermal evaporation under vacuum.

[0064] The AlOx buffer layer 31 is preferably made by atomic thin film deposition (ALD), using two different precursors: TMA (trimethylaluminum Al2(CH3)6) and water H2O or ozone as oxidant. The precursor gases are here introduced into the deposition chamber of the ALD reactor sequentially, one after the other, separated by an inert gas such as nitrogen N2 to purge the deposition chamber. Thus, each precursor reacts with the surface of the material, and not with each other.

[0065] An example of a method for producing the buffer layer 31 in AI2O3 with a thickness of 3nm by ALD deposition is now described. The deposition temperature is between 25°C and 150°C, and preferably between 40°C and 75°C, here 50°C.

[0066] The deposition chamber is preheated to a temperature of 50°C, then the photovoltaic structure 1 under manufacture is introduced so that it thermalizes at this temperature. The pressure is 0.7mbar.

[0067] The deposition cycle consists of four steps, and is repeated until the buffer layer has the desired thickness.

[0068] The first step is the deposition of aluminum Al. The precursor gas TMA is injected into the deposition chamber. The injection time (pulse) is 500ms, and the gas flow is 300sccm.

[0069] The second step is a purge step, during which the inert gas N2 is injected for 5s at a flow of 300sccm.

[0070] The third step corresponds to the deposition of oxygen O. The precursor gas H2O is injected into the deposition chamber for a duration of 6s and a flow rate of 300sccm. The fourth step is a purge step, during which the inert gas N2 is injected again for 6s at a flow rate of 300sccm.

[0071] A repetition of 30 deposition cycles allows to obtain a deposit of AI2O3 with a thickness of 3nm. Finally, the TMA fluidic line is purged with nitrogen, the deposition chamber is exposed to the atmosphere, and the resulting stack is removed.

[0072] Figures 2A to 2D illustrate a comparison of the performance, in terms of stability under illumination, of tandem photovoltaic structures 1 which have the configuration of the concrete example indicated previously, and which differ from each other only by the buffer layer.

[0073] Four photovoltaic structures are considered here: the first where the buffer layer is made of BCP deposited by evaporation (symbol: solid squares), the second where the buffer layer is made of SnO2 deposited by cathodic sputtering (see open squares); the third where the buffer layer is made of 2nm AI2O3 deposited by ALD (see solid circles), and the fourth where the buffer layer is made of 3nm AI2O3 deposited by ALD (see open circles).

[0074] The photovoltaic structures were placed in a pressurized glove box under an inert atmosphere (under N2), with humidity and oxygen levels below lppm. They were illuminated for 72 hours continuously.

[0075] Fig.2A illustrates the current density J of the upper cell (before illumination Jinit and after illumination Jfin); Fig.2B illustrates the open circuit voltage Voc of the upper cell (Vocinit and Vocfin); Fig.2C illustrates the form factor FF of the tandem photovoltaic structure (FFinit and FFfin); and Fig.2D illustrates the maximum power point tracking P of the tandem photovoltaic structure (Pinit and Pfin).

[0076] With reference to Fig. 2A, it appears that the stability under illumination of the current density J of the upper cell is of the same order for the different structures, whether the buffer layer is made of BCP, SnO2, or AlOx. With reference to Fig. 2B, it appears that the stability under illumination of the open circuit voltage Voc is better when the buffer layer is made of AI2O3 with a thickness of 2nm or 3nm than when it is made of BCP. Moreover, it is noted that, in the case where it is made of AI2O3, it is better with a thickness of 3nm (which is of the same order as in the case of SnO2) than with a thickness of 2nm.

[0077] Referring to Fig. 2C, it appears that the stability under illumination of the FF form factor is better when the buffer layer is made of AI2O3 with a thickness of 2nm or 3nm than when it is made of SnO2. Moreover, it is similar in the cases of a 2nm or 3nm AI2O3 buffer layer.

[0078] With reference to Fig. 2D, it appears that the stability under illumination of the tracking of the maximum power point P is better when the buffer layer is made of AI2O3 with a thickness of 3nm than when it is made of SnO2, BCP, or even AIOx with a thickness of 2nm.

[0079] It therefore emerges that such a tandem photovoltaic structure of the Si / PK pin type has improved performance, in terms of stability under illumination, when the buffer layer is made of AIOx with a thickness of between 1 nm and 5 nm, here 2 nm or 3 nm, than when it is made of BCP or SnO2. In addition, these performances are better when the buffer layer is made of AIOx with a thickness of 3 nm than when the thickness is 2 nm. Also, the buffer layer 31 according to the invention is made of AIOx with a thickness of between 1 nm and 5 nm, and preferably between 2.5 nm and 4 nm.

[0080] Particular embodiments have just been described.

[0081] Various variations and modifications will occur to those skilled in the art.

Claims

CLAIMS 1. Tandem photovoltaic structure (1) of pin type, comprising, from the front face (la) to the rear face (lb): o an upper electrode layer (20); o an upper photovoltaic cell (30), comprising: • a buffer layer (31); • an electron transport layer (32), • an active layer (33) based on a perovskite material; • a hole transport layer (34); o a lower photovoltaic cell (50), comprising: • an n-type layer (51); • an active layer (53); • a p-type layer (55); characterized in that the buffer layer (31) is made of an aluminum oxide with a thickness of between 1 nm and 5 nm.

2. Tandem photovoltaic structure (1) according to claim 1, in which the buffer layer (31) has a thickness of between 2.5 and 4 nm.

3. Tandem photovoltaic structure (1) according to claim 1 or 2, in which the buffer layer (31) is made of an aluminum oxide AIOx, where the value x corresponds to the number of oxygen atoms per 1 aluminum atom, x being between 0.8 and 1.

5.

4. Tandem photovoltaic structure (1) according to any one of the preceding claims, wherein the buffer layer (31) is located in contact with the upper electrode layer (20) and the electron transport layer (32).

5. Tandem photovoltaic structure (1) according to any one of the preceding claims, in which the upper electrode layer (20) is made of a transparent conductive oxide.

6. Tandem photovoltaic structure (1) according to the preceding claim, in which the upper electrode layer (20) is made of TCO such as ITO, IZO or IO:H.

7. Tandem photovoltaic structure (1) according to any one of the preceding claims, comprising an upper metal contact (10), resting on and in contact with the upper electrode layer (20), and made of Ag, Cu, Ni, and / or Al.

8. Tandem photovoltaic structure (1) according to any one of the preceding claims, wherein the electron transport layer (32) is an n-type layer made of Ceo, PCeiBM, PC71BM, PCBM / Ceo, or PCBM / C.

9. Tandem photovoltaic structure (1) according to any one of the preceding claims, in which the lower photovoltaic cell (50) is made from silicon, perovskite, or Cu(ln,Ga)(S,Se)2.

10. Method for manufacturing a tandem photovoltaic structure (1) according to any one of the preceding claims, comprising a step of producing the buffer layer (31) by deposition of atomic thin layers at a deposition temperature of between 25°C and 150°C, preferably between 40°C and 75°C.

11. Manufacturing method according to the preceding claim, in which the aluminum precursor is TMA, and the oxygen precursor is water or ozone.

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