Multi-junction photovoltaic cells

Abstract

A process for the preparation of a multi-junction photovoltaic cell (1) comprising at least a first photovoltaic cell (2), a second photovoltaic cell (4) having a band gap greater than that of said first photovoltaic cell (2) by a value between 0.4 and 0.9 eV and a recombination layer (3) arranged between said first (2) and said second (4) photovoltaic cell. The recombination layer (3) is made by means of sputtering deposition of a material deriving from a single target, whose composition is constituted by 90% to 100% by weight of In2O3 and of 10% to 0% by weight of SnO2, on a substrate constituted by the first (2) or the second (4) photovoltaic cell. The sputtering deposition is carried out (a) by means of a plasma generation power between 8, 000 W and 13, 000 W, (b) in an 02 / Ar atmosphere whose volume ratio is between 1% and 5% and (c) with a speed of the substrate between 250 and 1000 cm / min.

Description

[0001] "MULTI -JUNCTION PHOTOVOLTAIC CELLS"

[0002] Cross-Reference To Related Applications

[0003] This Patent Appl ication claims priority from Italian Patent Application No . 102023000016008 filed on July 28 , 2023 , the entire disclosure of which is incorporated herein by reference .

[0004] Field of the Art

[0005] The present invention relates to a method for the reali zation of multi- j unction photovoltaic cells .

[0006] State of the Art

[0007] For some time now, photovoltaic devices , thanks to the continuous decrease in the costs o f the current produced by them, have played a key role in reducing CO2 emis sions . In particular, the development of high-ef ficiency photovoltaic technologies has led to a reduction in plant costs in terms of occupied area .

[0008] To date , technology based on crystalline silicon cells dominates the photovoltaic module market thanks to the low costs for manufacturing the modules as well as the high reliability of the technology itsel f . For this type of cells , the theoretical limit is approximately 29% and is due to factors such as , for example , the transparency of the absorbing layer to sub-gap photons and thermali zation losses in the case of high-energy photons .

[0009] One solution to overcome these limiting factors is to combine various absorber materials with dif ferent band gaps in multi- j unction devices . The use of high gap energy materials allows to reduce thermal i zation losses , while the collection of the remaining part of the light can take place with lower band gap absorber layers . The tandem photovoltaic cell , which combines two component cells , represents the simplest multi- j unction configuration .

[0010] In tandem cells , silicon is an excellent choice for the rear component cell thanks to the gap of the material ( 1 . 1 eV) , the open circuit voltage that can reach up to 750 mV and the possibility of using low-cost manufacturing processes . In the literature , ef ficiency simulations close to 40% are reported for tandem cells made starting from a silicon cell as a base . With regards to the front component of the tandem device , perovskite is one of the materials currently most studied and precisely its use has made it possible to create a perovskite / silicon tandem cell with a record ef ficiency equal to 33 . 2 % , a value that is already higher than the e f ficiency record of the single- j unction c- Si cell .

[0011] In particular, the technology based on organometall ic- halide perovskite cells has greatly attracted the interest of the scienti fic community thanks to important characteristics , such as direct band-gap, high absorption coef ficients , excellent properties in terms of electrical transport .

[0012] Perovskite is actually a mineral first found in 1839 in the Ural Mountains and composed of an oxide of Calcium and Titanium - CaTiOs . Today, the term perovskite refers to all those compounds that have the same ABX3 crystallographic structure , in which A is an atomic or molecular cation at the center of a cube , B are cations placed at the vertices of the cube and X smaller negatively charged atoms placed on the faces of the cube and composing octahedral structures in B on each vertex of the cube . Depending on the type of atoms or molecules chosen, materials with peculiar and very interesting characteristics can be obtained, such as superconductivity, photoluminescence , which allow their use in many fields .

[0013] From what is reported above it is clear that tandem solar cells represent a key technology for reducing the costs of the current generated by photovoltaic sources .

[0014] The industriali zation of this type of cell , however, has drawbacks . In fact , while the photovoltaic industry is aiming at achieving 21x21 cm-si zed cells , currently the production of tandem photovoltaic cells is stuck at 1x1 cm- si zed devices . In particular, the transition to industrial dimensional standards does not guarantee the expected ef ficiency of the cell .

[0015] In this regard, it is important to point out that tandem cells have a shorter li fe than other photovoltaic cells and that , therefore , their added value is represented precisely by the ef ficiency . This means that a loss of ef ficiency of the tandem cells represents a really important limit for this type of cell and, conversely, an increase in ef ficiency would instead make their use even more interesting .

[0016] It was found that the decrease in ef ficiency is mainly due to the so-called shunt currents , which consist of "preferential lanes" for the current flows in the device and therefore cause a decrease in the ef ficiency of the device itsel f . In addition, it was also found that the physical characteristics of the recombination layer play an important role in the presence of shunt currents .

[0017] In particular, the inventors of the present invention have identi fied the presence of "holes" in the recombination layer and excessive surface roughness thereof as the primary causes of the presence of shunt currents .

[0018] For the purposes of the present invention, here and in the following, a "hole" is defined as a material- free zone within the recombination layer, visible with the TEM technique and having an extension greater than or equal to 2 nm2.

[0019] The need was therefore felt to have a solution that would guarantee the production of multi- j unction photovoltaic cells , whose technical characteristics were such as to guarantee even on an industrial scale the ef ficiency obtained on a small scale .

[0020] The inventors of the present invention have realised a method for the preparation of a recombination layer able to avoid the presence of shunt currents and, therefore , to guarantee the ef ficiency of the tandem photovoltaic cell also for industrial dimensional standards .

[0021] Disclosure of the Invention

[0022] The obj ect of the present invention is a process for the preparation of a multi- unction photovoltaic cell comprising at least a first photovoltaic cell , a second photovoltaic cell having a band gap greater than that of said first photovoltaic cell by a value between 0 . 4 and 0 . 9 eV and a recombination layer arranged between said first and said second photovoltaic cel l ; said process being characteri zed in that said recombination layer is made by means of sputtering deposition of a material deriving from a single target , whose composition is constituted by 90% to 100% by weight of In2O3 and of 10% to 0% by weight of SnCh , on a substrate constituted by said first or said second photovoltaic cell ; said sputtering deposition being carried out ( a ) by means of a plasma generation power between 8 , 000 W and 13 , 000 W, (b ) in an atmosphere of Ch / Ar whose volume ratio is between 1 % and 5% and ( c ) with a speed of the substrate between 250 and 1350 cm / min .

[0023] Preferably, the target has a composition constituted by 95% to 99% by weight of In2O3 and of 5% to 1 % by weight of SnO2.

[0024] Preferably, said sputtering deposition is carried out by means of a plasma generation power between 8 , 000 W and 13 , 000 W

[0025] Preferably, said sputtering deposition is carried out in an atmosphere Ch / Ar whose volume ratio is between 2 . 5% to 4 % .

[0026] The inventors of the present invention have found that % of O2 in the deposition atmosphere af fects the roughness , transparency and conductivity of the realized recombination layer .

[0027] Preferably, said sputtering deposition is carried out with a speed of the substrate between 400 and 600 cm / min.

[0028] Preferably, the recombination layer has a thickness between 3 and 25 nm, more preferably less than 15 nm.

[0029] Preferably, said first photovoltaic cell constitutes the substrate for the sputtering deposition of the recombination layer.

[0030] Preferably, an electron carrier layer of said first photovoltaic cell constitutes the substrate for the sputtering deposition of the recombination layer.

[0031] Preferably, said recombination layer constitutes the substrate for the deposition of a layer of material dedicated to the transport of the holes of said second photovoltaic cell .

[0032] With the above-reported conditions of sputtering deposition, a recombinant layer having (i) a thickness between 3 and 25 nm, (ii) a roughness measured as "Root mean square roughness" between 0.3 and 1 nm and (iii) an absence of holes is obtained.

[0033] A further object of the present invention is a multijunction photovoltaic cell comprising at least a first photovoltaic cell, a second photovoltaic cell having a band gap greater than that of said first photovoltaic cell by a value between 0.4 and 0.9 eV and a recombination layer arranged between said first and said second photovoltaic cell; said multi- j unction photovoltaic cell being characterized in that said recombination layer is made of ITO and has: (a) a thickness between 3 and 25 nm, (b) a roughness measured as "Root mean square roughness" between 0.3 and 1 nm and (c) an absence of holes.

[0034] Preferably, said second photovoltaic cell has a band gap greater than that of said first photovoltaic cell by a value between 0.5 and 0.7 eV.

[0035] Preferably, said recombination layer has a crystalline structure, which by means of X-ray diffraction measures a peak at greater intensity centered between 30.3° and 31.2° of 2Q obtained by exploiting the K-cx emission of a copper source .

[0036] Preferably, said first photovoltaic cell uses silicon as the absorbing material .

[0037] Preferably, said second photovoltaic cell uses a material with perovskite structure as the absorbing material .

[0038] Brief Description of the Drawings

[0039] For a better understanding of the present invention, an embodiment thereo f will be described below for illustrative and non-limiting purposes with the aid of the accompanying figures , in which :

[0040] - figure 1 illustrates in extremely schematic form a tandem solar cell according to the present invention;

[0041] - figure 2 is a TEM image of the I TO layer constituting the recombination layer according to the present invention;

[0042] - figure 3 is a SEM image of the I TO layer constituting the recombination layer according to the present invention;

[0043] - figure 4 is an AFM image of the ITO layer constituting the recombination layer according to the present invention;

[0044] - figure 5 is an X-ray di f fraction pattern of the ITO layer constituting the recombination layer according to the present invention .

[0045] Preferred Embodiment of the Invention

[0046] In Figure 1 , a tandem cell measuring 21x21 cm is indicated as a whole with 1 .

[0047] The tandem cell 1 comprises a first photovoltaic cell 2 (bottom cell ) , a recombination layer 3 and a second photovoltaic cell 4 ( top cell ) .

[0048] The first photovoltaic cell 2 comprises a layer 5 relative to "bottom cell selective contact A" , a layer 6 relative to "bottom cell absorber" and a layer 7 relative to "bottom cell selective contact B" .

[0049] The second photovoltaic cell 4 comprises a layer 8 relative to "top cell selective contact A" , a layer 9 relative to "top cell absorber" , a layer 10 relative to "top cell selective contact B" .

[0050] The indication o f "A" or "B" indicates that the layer 8 has the same function (HTL or ETL ) as the layer 5 and that the layer 10 has the same function as the layer 7 (HTL or ETL ) .

[0051] As is known to a person skilled in the art, the abbreviation ETL indicates an electron carrier layer, while the abbreviation HTL indicates a hole carrier layer .

[0052] With the exception of the recombination layer 3 , the layers of the tandem cell 1 have been indicated with this wording since it is in common use and, therefore , easier to understand for a person skilled in the art .

[0053] The recombination layer 3 is ITO-based and has a thickness between 3 and 35 nm . The recombination layer 3 is made by means of sputtering deposition on the layer 5 (bottom cell selective contact A) , which i s either dedicated to the extraction of holes (HTL ) or, preferably, dedicated to the extraction of electrons (ETL ) .

[0054] Depending on what is reported above for the layer 5, it follows that the layer 7 (bottom cell selective contact B ) is either dedicated to the extraction of electrons (ETL ) or, preferably, dedicated to the extraction of holes (HTL ) .

[0055] In particular, preferably, the layer 5 is a layer of amorphous silicon or micro- or nanocrystalline silicon with n-type characteristics due to doping with phosphorus .

[0056] Above the recombination layer 3 there is arranged a layer 10 ( top cell selective contact B ) of material dedicated to the transport of the holes , which has a thicknes s between 1 and 20 nm and is a layer of organic molecules derived from carbazole and / or a layer of Nickel oxide - NiOx .

[0057] The layer 9 ( top cell absorber ) is a layer of hybrid organic / inorganic halide perovskite or inorganic halide perovskite or an organic absorber and has a band gap between 1 . 5 and 1 . 9 eV . The layer 8 (top cell selective contact A) is dedicated to the extraction of electrons (ETL) .

[0058] The difference in the band gap of the absorbers between the bottom cell and the top cell is between 0.4 and 0.9 eV and, preferably, between 0.5 and 0.7 eV.

[0059] As mentioned above, perovskite is a crystalline structure compound of the ABX3 type and in a preferred embodiment :

[0060] A is Cs or Formamidinium (FA) or methylammonium (MA) or a mixture of the three: Structure of the type (CsxFAyMAzBX3) , and

[0061] B is composed of bivalent metals. Preferably one between Pb2+or Sn2+or a mixture of the two. X is a halide or a mixture of halides (e.g. I3, or Brs or IxBri-xor IxBryClz) .

[0062] By way of example, perovskite may have a structure of the type CSxFAyMAzPbaSni-al^Br Clt? (with x+y+z=l and (5 = 3) (hybrid perovskite) or CsPbl^Br Clt? (Inorganic Perovskite) .

[0063] In the description of the tandem cell above, some technical characteristics of the layers have not been detailed as they concern common knowledge of an average person skilled in the art.

[0064] Below is reported the procedure used to realize the recombination layer 3.

[0065] Specifically, the recombination layer 3 is constituted by an ITO film having a thickness of 6 nm.

[0066] The deposition was carried out using a single target of Indium and Tin Oxide. In particular, the target is composed of 97% of In2O3 and of 3% of SnO2.

[0067] The sputtering was carried out using a pulsed DC generator that produces a plasma generation power equal to 11, 000 W.

[0068] An Ar and O2 atmosphere was created in the deposition chamber. In particular, an Ar flow and an O2 flow equal to 3.2% of the Ar flow are continuously blown into the deposition chamber .

[0069] As mentioned above , the amount of O2 in the deposition chamber is relevant in terms of roughness , transparency and conductivity of the recombination layer that is being reali zed .

[0070] The tray on which the substrate is arranged moves with a speed equal to 500 cm / min . In particular, the substrate is constituted by a silicon cell with exposed layer 5 which is constituted by a layer of amorphous silicon or micro or nanocrystalline silicon with n-type characteristics due to doping with phosphorus .

[0071] The deposition process lasted four minutes and produced an ITO layer with a thickness equal to 20 nm . In particular, the time indicated above also included the times necessary for vacuuming the chambers , and the time exclusively relative to the deposition is about 30 seconds .

[0072] As it may be clear to a person skilled in the art , the use of a single target necessarily involves longer production times , but at the same time it ensures the formation of a recombination layer capable of guaranteeing the sought-after ef ficiency of the resulting tandem cell .

[0073] The processes for reali zing the first and second photovoltaic cell are not relevant for the purposes of the present invention .

[0074] In this regard, the reali zation of the first and second photovoltaic cell follows techniques that have long been available to persons skilled in the art .

[0075] In particular, for the first photovoltaic cell (bottom cell ) the formation of a selective layer of charges ( P or N doped amorphous silicon) can be reali zed by means of the PECVD technique , while the formation of a transparent conductive oxide ( TCO) layer can be reali zed by means of the PVD technique .

[0076] Still for the f irst electrochemical cell , texturing ( formation of pyramids on the surface on which the recombination layer is deposited) is carried out by means of basic (wet) chemical attacks (KOH / NaOH)

[0077] Differently, for the second photovoltaic cell (top cell) , the deposition of the various layers can be carried out by means of solution techniques (such as slot die or deep coating) or by means of vacuum techniques such as thermal evaporation techniques that comprise CSS (Close Space Sublimation) , which consists of an evaporation of the materials while keeping the crucibles extremely close to the sample (Close space) ; ALD (Atomic The layer Deposition) which consists of the use of highly reactive precursors inserted into the chamber alternating them over time (Precursor 1- Purge- Precursor 2- Purge- Precursor 1- Purge- Precursor 2- Purge) and CVD (Chemical vapour deposition) . In addition, the TCO of the second photovoltaic cell (top cell) can be realized by means of the Physical Vapor Deposition (PVD) technique .

[0078] Finally, the perovskite layer can also be deposited with mixed technology using a vacuum technique such as evaporation to deposit the inorganic precursor (e.g. Pbl2) and then a solution, deposited by Deep coating or slot die, containing the organic part to obtain the final perovskite (e.g. FAPbI3)

[0079] Finally, the tandem cell is closed with a conductive transparent oxide (TCO) such as, for example, a layer of ITO (indium tin oxide) , a layer of IZO (Indium and Zinc oxide) or a layer of IWO.

[0080] At the end of the realization, the tandem cell is subjected to a heat treatment of 100°C for a time of 15 min.

[0081] The recombination layer 3 produced was subjected to TEM, SEM, AFM and X-ray analysis to verify the structure thereof .

[0082] As supported by Figures 2 - 5, the analyses have demonstrated that the recombination layer 3 of ITO has a roughness measured as "Root mean square roughness" with values contained between 0 . 3 and 1 nm, a crystalline structure , whose measurement by means of X-ray di f fraction shows a peak of greater intensity centered between 30 . 3 ° and 31 . 2 ° of 2Q obtained by exploiting the K-a emission of a copper source , and does not have material- free zones with an extension greater than or equal to 2 nm2.

[0083] It has been experimentally demonstrated that the recombination layer 3 described above guarantees a correct homogeneity of the overlying layers and the absence of shunt currents typical of the prior art , with the consequence of guaranteeing an ef ficiency of the device even for industrial scale dimensions .

[0084] In other words , it has been demonstrated that the presence of the recombination layer according to the present invention allows the transition to the industrial dimensional standards of the multi- j unction photovoltaic cell without the loss of ef ficiency of the prior art occurring .

Claims

CLAIMS1. Process for the preparation of a multi- j unction photovoltaic cell (1) comprising at least a first photovoltaic cell (2) , a second photovoltaic cell (4) having a band gap greater than that of said first photovoltaic cell (2) by a value between 0.4 and 0.9 eV and a recombination layer (3) arranged between said first (2) and said second (4) photovoltaic cell; said process being characterized in that said recombination layer (3) is made by means of sputtering deposition of a material deriving from a single target, whose composition is constituted by 90% to 100% by weight of In2O3 and of 10% to 0% by weight of SnCh, on a substrate constituted by said first (2) or said second (4) photovoltaic cell; said sputtering deposition being carried out (a) by means of a plasma generation power between 8,000 W and 13,000 W, (b) in an atmosphere of Ch / Ar whose volume ratio is between 1% and 5% and (c) with a speed of the substrate between 250 and 1350 cm / min.

2. Process according to claim 1, characterized in that the target has a composition constituted by 95% to 99% by weight of In2O3 and 5% to 1% by weight of SnO2.

3. Process according to claim 1 or 2, characterized in that said sputtering deposition is carried out by means of a plasma generation power between 8,000 W and 13,000 W4. Process according to one of the preceding claims, characterized in that said sputtering deposition is carried out in an atmosphere Ch / Ar whose volume ratio is between 2.5% to 4% .

5. Process according to one of the preceding claims, characterized in that said sputtering deposition is carried out with a speed of the substrate between 400 and 600 cm / min.

6. Process according to one of the preceding claims, characterized in that said first photovoltaic cell (2) constitutes the substrate for the sputtering deposition of the recombination layer (3) .

7. Process according to claim 6, characterized in that an electron carrier layer (5) of said first photovoltaic cell (2) constitutes the substrate for the sputtering deposition of the recombination layer (3) .

8. Process according to one of the preceding claims, characterized in that said recombination layer (3) constitutes the substrate for the deposition of a layer of material dedicated to the transport of the holes of said second photovoltaic cell (4) .

9. Multi- j unction photovoltaic cell (1) comprising at least a first photovoltaic cell (2) , a second photovoltaic cell (4) having a band gap greater than that of said first photovoltaic cell (2) by a value between 0.4 and 0.9 eV and a recombination layer (3) arranged between said first (2) and said second (4) photovoltaic cell; said multi- j unction photovoltaic cell being characterized in that said recombination layer (3) is made of ITO and has: (a) a thickness between 3 and 25 nm, (b) a roughness measured as "Root mean square roughness" between 0.3 and 1 nm and (c) an absence of material-free zones, visible with the TEM technique and each having an extension greater than or equal to 2 nm2.

10. Multi- j unction photovoltaic cell according to claim 9, characterized in that said second photovoltaic cell (4) has a band gap greater than that of said first photovoltaic cell (2) by a value between 0.5 and 0.7 eV.

11. Multi- j unction photovoltaic cell according to claim 9 or 10, characterized in that said recombination layer (3) has a crystalline structure, which by means of X-ray diffraction measures a peak of greater intensity centered between 30.3° and 31.2° of 2Q obtained by exploiting the K- cx emission of a copper source.

12. Multi- j unction photovoltaic cell according to one of claims 9 to 11, characterized in that said first photovoltaic cell (2) uses silicon as the absorbing material.

13. Multi- j unction photovoltaic cell according to one of claims 9 to 12, characterized in that said second photovoltaic cell (4) uses a material with perovskite structure as the absorbing material.

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