WO2010058283A1

WO2010058283A1 - Method for producing thin-film multilayer solar cells

Info

Publication number: WO2010058283A1
Application number: PCT/IB2009/007531
Authority: WO
Inventors: Stefano Rampino; Edmondo Gilioli; Francesco Bissoli; Francesco Pattini
Original assignee: Consiglio Nazionale delle Richerche CNR
Current assignee: Consiglio Nazionale delle Richerche CNR
Priority date: 2008-11-21
Filing date: 2009-11-20
Publication date: 2010-05-27
Anticipated expiration: 2011-05-21
Also published as: ITMI20082091A1; IT1391802B1

Abstract

The invention regards a method for producing thin- film multilayer solar cells, wherein deposited on a substrate is a plurality of, functional layers, amongst which at least one absorber layer set between a bottom conductive layer and a top conductive layer, and optionally one or more buffer layers set between the absorber layer and the top conductive layer; one or more of these layers, in particular at least the absorber layer, are deposited via a pulsed-electron deposition process.

Description

METHOD FOR PRODUCING THIN-E^ILM MULTILAYER SOLAR CELLS

TECHNICAL FIELD

The present invention relates to a method for producing thin- film multilayer solar cells via a pulsed-electron deposition process .

BACKGROUND ART

Known to the art are multilayer solar cells basically formed by a substrate, deposited on which is a plurality of functional layers, in particular a bottom conductive layer (positive ohmic layer that' constitutes the bottom electrical contact or "back contact" of the cell) , an absorber layer (i.e., a layer made of a material, for example a semiconductor material, that absorbs photons of the solar spectrum), one or more buffer layers, and a top conductive layer (negative ohmic layer that functions as top contact) .

Here and in what follows, the terms "conductive layer", "absorber layer", and "buffer layer" are used with the meaning common in the sector of thin- film solar cells; namely:

"conductive layer" designates a layer of material that conducts electric current/charge, which constitutes an electrical contact of the cell; "absorber layer" designates an active layer of the cell, made of a material that absorbs photons with photovoltaic capacity (for example, a semiconductor) , which is able in particular to absorb the photons of the solar spectrum; "buffer layer" designates a layer set between other layers of the cell with function of separation, for example made of a resistive material.

DISCLOSURE OF INVENTION

The methods for producing these structures are not altogether satisfactory, and an aim of the present invention is to provide a method that will enable production of thin- film multilayer solar cells of high quality and high performance in a relatively simple, fast, and inexpensive way.

The present invention hence regards a method for producing thin-film multilayer solar cells, which comprises a step of providing a substrate and a step of depositing on the- substrate a plurality of functional layers, amongst which at least one absorber layer having a composition of a CIS, CIGS or CIGASS type and set between a bottom conductive layer and a top conductive layer, and optionally one or more buffer layers set between the absorber layer and the top conductive layer,- wherein the absorber layer is deposited via a pulsed-electron deposition process (in what follows also referred to, for brevity, as "PED process"), by causing with a pulsed-electron beam the ablation of a target containing an excess of Cu with respect to the stoichiometric composition of the absorber layer to be formed.

According to preferred embodiments, all the layers (and consequently the entire structure of the cell), or else all the layers except the bottom conductive layer in contact with the substrate, are obtained by means of PED processes.

Advantageously, two or more layers of the cell are deposited individually and in sequence in respective pulsed-electron deposition stages, carried out by means of one and the same pulsed-electron source and replacing the targets on which the pulsed-electron beam impinges between one stage and the next and/or modifying the operating parameters of the pulsed- electron deposition process between one stage and the next.

Basically, in accordance with the invention, the cell is obtained entirely or at least in part by pulsed-electron deposition process.

As is known, the pulsed-electron deposition (PED) technique is a technique of a physical type for producing thin layers (with a thickness of between a few tenths and a few tens of microns) of conductive and dielectric materials, used in particular for producing functional devices in the sector of electronics, magnetism, sensors, generation and transport of energy and in general in nanotechnology.

Said technique is based upon the generation of a pulsed beam of high-energy electrons (indicatively, an energy of 1 to 25 keV) and the subsequent collimation of the latter towards a target made of a material with appropriate composition. The interaction between the electron beam and the atoms of the target causes the rapid evaporation (ablation) of material from the surface of the target; the vapours form a flow of evaporation particles (referred to also as "plume"), the dimensions, rate, and density of which depend upon the parameters of acceleration and collimation of the electron beam, as well as upon the nature of the material of the target. The interposition of a substrate on the path of the vapour plume causes condensation of the vapours on the surface of the substrate and consequent formation of a layer.

According to the invention, at least one of the functional layers of the cell, and specifically the absorber layer, and optionally also one or more from among the buffer layer or layers, the top conductive layer, and the bottom conductive layer are obtained via pulsed-electron deposition processes, each of which is carried out by sending an electron beam on a target, having appropriate composition (chosen on the basis of the composition of the corresponding layer to be deposited) and set in the proximity of a substrate, in such a way as to cause ablation of the target and emission by the target of particles in the vapour phase that deposit on the substrate to form the desired layer.

The absorber layer, which has typically (but not necessarily) a thickness of between 1 and 4 μm, is constituted by a semiconductor material with doping of a p type, in particular CIS or, preferably, CIGS or CIGASS.

The deposition of the absorber layer is carried out by causing, via a pulsed-electron beam, ablation of a target containing an excess of Cu with respect to the stoichiometric composition of the absorber layer to be formed, and specifically having composition:

Cu_t ( Ini-_x__yGa_xAl_y) ₂-t (Sei-_zS_z) ₂

with 0≤x<0.30; 0<y<0.40; 0<z≤0.5 and l<t<1.50.

As is known, CIS (i.e., CuInSe₂) is a material typically used in thin-film solar cells. In order to optimize the absorption of photons of the solar spectrum, CIS is doped with Ga and Al (which are substituents of In) , and sulphur (S, which is substituent of Se) obtaining the phase Cu (Ini__x._yGa_xAly) (Sei-_zS₂) ₂ or CIGASS with O≤x≤O.30, 0<y<0.40 and 0<z<0.5.

In order to obtain the deposition of an absorber layer having the desired composition, in accordance with the invention, instead of a target having stoichiometric composition (i.e., exactly the same composition as that of the layer to be formed) , a target containing an excess of Cu is used. By increasing the percentage of Cu with respect to the sum of In and Ga, the deposition of an absorber layer is obtained having the optimal composition to obtain the maximum efficiency of the cell .

For the other layers, instead, it is possible to use stoichiometric targets, namely, ones having substantially the same composition as that of the corresponding layer to be deposited.

The technique of the invention makes it possible amongst other things to obtain the absorber layer and then deposit thereon the further functional layers of the cell without the absorber layer being subjected to post-treatments such as steps of selenization and/or sulphurization, i.e., thermal treatments in hydroselenic acid (H₂Se) and/or .hydrosulphuric acid (H₂S) .

The method of the invention further comprises a possible step of modulation of the composition of the absorber layer, in particular the ratio between the concentration of Cu and In, using a target with an excess of Cu with respect to the stoichiometric composition of the absorber layer to be formed, and varying the energy conditions of the electron beam sent onto the target in the pulsed-electron deposition process. In particular, the stage of deposition of the absorber layer is carried out by varying the energy conditions of the electron beam to vary the ratio between In and its substituents Ga and Al and create a composition gradient along the thickness of the absorber layer.

As substrate on which the functional layers are to be deposited it is possible to use typical substrates for producing thin-film solar cells, such as for example substrates of vitreous material, in particular glass of the "soda-lime" type or quartz, Corning, etc.; the substrate can in any case be made of other types of materials, for example plastic/polymeric materials (e.g., polyimide) , ceramic materials, cementitious materials, metal materials (in the latter case, the substrate of a metal material performs not only the function of structural support of the cell, but also that of back contact) .

The bottom conductive layer is a conductive layer of metal material set in direct contact with the substrate and having, typically, a thickness of 500 nm to 2 μm.

Preferably, but not necessarily, the bottom conductive layer is Mo, or else Ni.

The bottom conductive layer is deposited on the substrate via a further pulsed-electron deposition stage, or else via a different process of metallization of the substrate.

The top conductive layer constitutes the n region of the cell and is formed by a material chosen from amongst transparent conductive oxides (TCOs) , in particular ZnO optionally doped with Al and B.

The material of the class commonly known as "transparent conductive oxides" (TCOs) are at the same time highly conductive (p ~ 10^"3 - 10^"4 Ωcm) and highly transparent to visible light (which is able to transmit on average more than

80% of the incident visible light) . Layers obtained with these materials are hence used simultaneously as window layers

(exploiting the high bandgap value, which is higher than 3 eV) and as ohmic contacts of the cell. The material that best guarantees simultaneously high values of electrical conductivity and optical transparency is ZnO, possibly doped with Al and B for modifying its optical and electrical properties . An optimal amount of dopant increases the conductivity by several orders of magnitude (from 10^"5 to 10⁴ 0^"1Cm^'1) , leaving the high value of optical transparency typical of ZnO unvaried, since the density of carriers of an n type increases considerably. The top conductive layer of ZnO optionally doped (with Al, B) and the CIGASS absorber layer hence give rise to the n-p junction on which the photovoltaic effect of the cell is based.

The thickness of the layer of ZnO (Al, B) is usually of between 200 and 1000 run. The deposition of the layer occurs by PED ablation in an environment poor in oxygen (in a mixture with Ar) or in a reducing environment (mixture of H₂ and Ar in a volumetric ratio of between 5:95 and 3:97) and at temperatures of between 400⁰C and 550⁰C. The starting target is obtained by- mixing and pressing of powders of ZnO and M₂O₃ at 1 to 5 wt% (M = Al or B) and subsequent sintering in air at T = 900 -

1000⁰C.

In one embodiment, the cell comprises, between the absorber layer and the top conductive layer, one or more buffer layers without cadmium, which are deposited by means of one or more respective pulsed-electron deposition stages.

Advantageously, as already highlighted, these further layers are deposited on the absorber layer without the absorber layer being previously subjected to steps of selenization and/or sulphurization, i.e., thermal treatments in hydroselenic acid (H₂Se) and/or hydrosulphuric acid (H₂S) .

In a preferred embodiment, the top conductive layer and a top buffer layer without cadmium, which is set in direct contact with the top conductive layer, are constituted respectively by a layer of conductive ZnO with n doping or other transparent conductive oxide (TCO), and by a layer of resistive ZnO (i- ZnO) purposely not doped, and are obtained in two consecutive pulsed-electron deposition stages, both carried out using a single target and modifying between one stage and the next the operating parameters of the deposition process, in particular the atmosphere in which the deposition process occurs and/or the energy conditions of the electron beam. The top buffer layer has indicatively a thickness of 20 to 80 nm and a resistivity p~10⁵ Ωcm.

Advantageously, the method of the invention envisages modulation of the electrical properties of the ZnO layers (top conductive layer and top buffer layer) by varying the parameters of the pulsed-electron deposition process. Starting from a pure-ZnO target, it is possible to vary the resistivity of the deposited layer between 0.1 and 10¹⁴ Ωcm by modifying the concentration of gaseous oxygen inside the reactor for the pulsed-electron deposition process. Starting from a target of ZnO doped with Al or B, it is possible to reduce the resistivity of the layer from 10⁵ to 10^"4 Ωcm by reducing the concentration of oxygen in the reactor and simultaneously varying the energy conditions of the electron beam, hence exploiting the mechanism of selective ablation to modulate the amount of dopant to be inserted in the ZnO matrix. This mechanism enables use of a single target of doped ZnO both for the deposition of the buffer layer of highly resistive ZnO and for that of the top conductive layer of conductive ZnO with a heavy n doping.

Optionally, set between the top buffer layer (set in contact with the top conductive layer) and the absorber layer (and hence in contact with the absorber layer) is a bottom buffer layer, which is also without cadmium, indicatively with a thickness of 20 to 120 nm, and a composition Zni-_xIn_xSei-_yS_y with O≤x≤l and O≤y≤l, or Zni._x._y-zMg_xAl_yB_zO with O≤x≤l , O≤y≤O .1 , O≤z≤O .1.

This buffer layer is advantageously deposited on the absorber layer via a pulsed-electron deposition process, starting from a polycrystalline target obtained by synthesis under pressure of the elementary precursors. The temperatures of deposition of these layers are between 100⁰C and 300⁰C.

In order to modify the surface morphology of the absorber layer thus improving the effect of adhesion of the buffer layer to the absorber layer, the absorber layer is optionally subjected, before deposition of the buffer layer, to a step of surface treatment, for example via chemical etching, which eliminates spurious surface phases that could adversely affect the growth of the buffer layer.

An example of effective chemical etching consists in wetting the surface of the absorber layer with an aqueous solution of potassium bromide (KBr) with a concentration of 0. IM and bromium (Br₂) with a concentration of 0.02M.

The top buffer layer (preferably made of resistive ZnO) is deposited, via a pulsed-electron deposition process, in an oxygen-rich atmosphere at temperatures of between approximately 400⁰C and approximately 550⁰C, on the bottom buffer layer and is hence set between the bottom buffer layer and the top conductive layer.

In a particularly advantageous embodiment (which enables use of a single buffer layer) , the cell includes a CIGASS absorber layer and a buffer layer of resistive ZnO, deposited via respective pulsed-electron deposition stages carried out with operating parameters such as to minimize the reaction between the species interdiffused by the absorber layer, i.e. Ga and In, and by the buffer layer, i.e. 0, in such a way as to couple directly the CIGASS absorber layer and the buffer layer in contact with one another.

In particular, the temperature of the stage of deposition of the buffer layer of resistive ZnO is significantly lower than that of the stage of deposition of the CIGASS absorber layer in such a way as to reduce the diffusion of the metals coming from the absorber layer. In this way, since the most important function of the buffer layers is that of preventing the reaction of the oxygen (used for the growth of the top conductive layer, for example, of conductive ZnO) with the absorber layer, there is a minimization of the simultaneous interdiffusion of the metals present in the CIGASS absorber layer (in particular Ga) and of the oxygen coming from ZnO, the reaction of which gives rise to undesirable buried insulating layers, and the use of the bottom buffer layer becomes unnecessary. In the case where this solution is not practicable (for example because, to optimize the performance of the cell, deposition of the ZnO conductive layer must come about at temperatures higher than those required for deposition of the CIGASS absorber layer) , a solar cell without bottom buffer layer can be obtained by reversing the deposition geometry: deposited in succession on the substrate, having a high optical transmittance, is the top conductive layer of conductive ZnO (n-Zno) and the top buffer layer of resistive ZnO (i-ZnO) , by operating at high temperature; next, the temperature is reduced for depositing the CIGASS absorber layer and then the bottom conductive (metal) layer. This type of cell must be logically turned upside down during its operation, in such a way that the light will enter the cell initially passing through the substrate and subsequently through the ZnO layers reaching the absorber layer . The substrate is in this case made of a vitreous or plastic materials with high optical transmittance.

According to one embodiment, in order to speed up further the process of production of the solar cell, the bottom conductive

(metal) layer is deposited on the substrate by means of metallization techniques alternative to pulsed-electron deposition (for example, DC sputtering or thermal evaporation or other vapour-phase deposition process) in such a way that the metallized substrate can be appropriately treated, for example via scribing or via other surface treatments, before being introduced into a reactor for pulsed-electron deposition processes, and is then introduced within the latter only when its surface presents the necessary pattern for deposition of the absorber layer.

According to this embodiment, the method of the invention hence comprises the steps of: depositing on the substrate the bottom conductive layer by means of a metallization process, in particular via thermal evaporation or vapour-phase deposition and specifically DC sputtering, to form a metallized substrate; treating the surface of the metallized substrate for providing the surface with a pattern suitable for deposition of the absorber layer; and introducing the treated metallized substrate into a reactor for pulsed- electron deposition processes and depositing the absorber layer and optionally other layers_. by pulsed-electron deposition process.

From what has been set forth above, the advantages afforded by the invention emerge clearly:

- the method of the invention enables production, in a relatively simple, fast, and inexpensive way, of solar cells of high structural and functional quality; - the method of the invention envisages deposition of the CIGASS absorber layer by means of a single pulsed-electron deposition stage so that post-deposition treatments such as steps of selenization and/or sulphurization, i.e., thermal treatments in hydroselenic acid (H₂Se) and/or hydrosulphuric acid (H₂S) , are not necessary; consequently, the rate of production of the solar cells increases, and risks and costs linked with the use of toxic gases such as hydroselenic and hydrosulphuric acid decrease;

- it is possible to deposit buffer layers alternative to CdS and without cadmium, thus eliminating any environmental problem linked to the use of cadmium;

- the depositions of the various layers can be performed sequentially using a single high-energy pulsed-electron deposition system, alternating exclusively the targets on which the electron beam impinges between one deposition stage and the next and/or modifying the operating parameters of the deposition process (energy of the electron beam, atmosphere and temperature in the deposition chamber, etc.); it is possible to modulate the cationic composition of the absorber layer (Cu, In, Ga, Al) starting from a target containing an excess of Cu with respect to the stoichiometric composition of the absorber layer to be formed and varying the energy conditions of the beam; in particular, it is possible to modulate the ratio between the concentrations of Cu and In, which markedly affects the efficiency of the solar cell, as well as to vary the ratio between In and its substituents (Ga, Al)^' during deposition, thus creating a composition gradient along the thickness, useful for improving the performance of the absorber layer,-

- it is possible to use a single doped-ZnO target both for the deposition of the top conductive layer of conductive ZnO with heavy n doping and for the deposition of the buffer layer of highly resistive ZnO, varying the parameters of the deposition process to modulate the electrical properties of the layers;

- it is possible to avoid deposition of the bottom buffer layer and hence drastically reduce the stages for production of the entire cell.

BRIEF DESCRIPTION OF THE DRAWINGS AND BEST MODE FOR CARRYING OUT THE INVENTION Further characteristics and advantages of the present invention will emerge clearly from the description of the ensuing non-limiting examples of embodiment, with reference to the annexed Figures 1, 2 and 3, which are schematic representations of respective structures typical of thin-film multilayer solar cells obtained with the method according to the invention (and in which indicated by an arrow is the direction of the light incident upon the cell in use) .

EXAMPLE 1 Figure 1 shows a thin- film multilayer solar cell 100 obtained in accordance with the invention.

The cell 100 comprises a substrate 101 (for example, made of glass, plastic, ceramic, cement, etc.), deposited on which is a plurality of superimposed functional layers, in particular an absorber layer 103 set between a bottom conductive layer 102 (positive ohmic layer that constitutes the bottom electrical contact or "back contact" of the cell) , set in direct contact with the substrate 101, and a top conductive layer 102 (negative ohmic layer that functions as top contact) ; the cell further comprises a bottom buffer layer 104 and a top buffer layer 104 that separate the absorber layer

103 from the top conductive layer 102 (the bottom buffer layer

104 is set between the absorber layer 103 and the top buffer layer 104, and the latter is set between the bottom buffer layer 104 and the top conductive layer 102).

The cell 100 is obtained, in accordance with the method of the invention, by inserting the substrate into a deposition chamber of a reactor for pulsed-electron deposition processes, and then depositing the layers via respective pulsed-electron deposition stages carried out with as many interchangeable targets .

For said purpose, housed in the deposition chamber is a target-carrier structure, set in the proximity of an output end of a device for generation of the electron beam; advantageously, the target-carrier structure carries a plurality of interchangeable targets and is movable by means of an actuator for bringing selectively each target in a position of use in which the target is impinged upon by the electron beam.

The cell 100 is obtained by depositing on the substrate 101, via five successive steps of pulsed-electron deposition, the layers 102-106. Each step is carried out with a different target and with appropriate operating conditions of the PED process (as described previously) . In summary:

- 1st PED step: the bottom conductive layer 102 is made of Mo or Cu and is deposited via a PED process from a target constituted by the respective elementary precursor;

- 2nd PED step: the absorber layer 103 is made of CIGASS and is deposited via PED process from a target having composition

with 0<x<0.30; 0<y<0.40; O≤z≤O .5 and l<t<1.50;

- 3rd PED step: the bottom buffer layer 104 (in direct contact with the absorber layer 103) is made of Zn₁-JjIn_xSe_I-_YSy or else

Zni-_x-y-_zMg_xAl_yB_zO and is deposited via a PED process from a polycrystalline target formed by the elementary precursors;

- 4th PED step: the top buffer layer 104 (set above the buffer layer 104 and in direct contact with the top conductive layer 102) is made of high-resistivity ZnO, deposited with PED process from pure-ZnO target;

- 5th PED step: the top conductive layer 102 is made of low- resistivity n-ZnO, deposited via PED process and target made of ZnO-M₂O₃ (with M = Al or B) .

EXAMPLE 2

According to a different embodiment, the cell 100 is obtained in a way similar to what has been described in Example 1, hence via five PED steps, but only four interchangeable targets are used: the buffer layer 105 and the top conductive layer 106, made respectively of resistive ZnO and conductive n-ZnO, are in fact obtained via respective PED processes carried out with one and the same target of ZnO-M₂O₃ (M = Al or B) and varying the operating parameters of the PED process; in particular, for the deposition of the buffer layer 105 a low- energy electron beam in O₂ atmosphere is used, whereas for the deposition of the top conductive layer 102 a high-energy electron beam is used (higher than the energy of the beam used for the deposition of the buffer layer 105) in O₂/Ar or H₂/Ar atmosphere.

EXAMPLE 3

According to a different embodiment, a metallized substrate

101 is used, on which the bottom conductive layer 102 of Mo or Cu is preliminarily deposited via sputtering or co-evaporation technique; next, the substrate 101 provided with of the layer 102 is introduced into the reactor for PED processes, and the other layers 103-106 are deposited via respective PED processes, using three interchangeable targets with modalities similar to those already described in the previous examples : - 1st PED step: the CIGASS absorber layer 103 is deposited;

- 2nd PED step: the buffer layer 104 made of Zni-_xIn_xSei-_yS_y or else Zni-_x-y._zMg_xAlyB_zO is deposited;

- 3rd PED step: the buffer layer 105 of resistive ZnO is deposited from a target of ZnO-M₂O₃ (M = Al or B) , via low- energy electron beam in O₂ atmosphere;

- 4th PED step: the top conductive layer 102 of conductive n- ZnO (with low resistivity) is deposited once again from the target of ZnO-M₂O₃ (M = Al or B) , via a high-energy electron beam in atmosphere of O₂/Ar or H₂/Ar.

EXAMPLE 4

According to a different embodiment, a metallized substrate 101 is used (i.e., one preliminarily provided with the bottom conductive layer 102) as described in Example 3, deposited on which are then the layers 103-106 with the modalities described in Example 1: each layer 103-106 is deposited with a respective PED process carried out with a respective target

(hence four interchangeable targets are used in as many PED processes) .

EXAMPLE 5

Figure 2 shown a cell 100 in which only a buffer layer, and specifically the top buffer layer 104, is provided; the bottom buffer layer 104 is absent.

The cell 100 is obtained in a way similar to what has been described in the previous examples:

- preliminarily applied to the substrate 101 is the bottom conductive layer 102 of Mo or Cu via a process of metallization of the substrate 101, for example via sputtering or co-evaporation technique (or else, in a variant, via a PED process from a target constituted by the respective elementary precursor) ;

- deposited on the metallized substrate 101 (provided with the layer 102), via PED process, is the CIGASS absorber layer 103; - the top buffer layer 104 of resistive ZnO is directly deposited on the absorber layer 103 via PED process from a pure-ZnO target; the top conductive layer 102 of conductive n-ZnO is deposited via PED process from a target of ZnO-M₂O₃ (with M=Al or B) .

EXAMPLE 6

According to a variant, the cell 100 with a single buffer layer is obtained in the following way: - preliminarily applied to the substrate 101 is the bottom conductive layer 102 of Mo or Ni via a process of metallization of the substrate 101;

- the CIGASS absorber layer 103 is deposited via PED process;

- deposited directly on the absorber layer 103 is the buffer layer 105 of resistive ZnO from a target of ZnO-M₂O₃ (M = Al or

B) , via low-energy electron beam in O₂ atmosphere; the top conductive layer 102 of conductive n-ZnO is deposited once again from the target of ZnO-M₂O₃ (M = Al or B) , via high-energy electron beam in O₂/Ar or H₂/Ar atmosphere.

EXAMPLE 7

Figure 3 shows a cell 100 in which the substrate 101 functions in effect as superlayer (i.e., in use, it faces the incident light) ; the cell 100 is obtained, in the following way. - deposited on the substrate 101 (which is a substrate with high optical transmittance, for example made of vitreous or transparent plastic materials) is the top conductive layer 102 of low-resistivity n-ZnO, via a PED process from a target of ZnO-M₂O₃ (with M=Al or B) with high-energy electron beam in O₂/Ar or H₂/Ar atmosphere;

- deposited on the layer 106 is the buffer layer 105 of resistive ZnO, via PED process from the same ZnO-M₂O₃ (M = Al or B) target, with low-energy electron beam in O₂ atmosphere;

- the absorber CIGASS layer 103 is deposited via PED process from a target with composition Cu_t (Ini-_x-_yGa_xAl_y) ₂-t (Sei-_zS_z) ₂ with 0<x<0.30; 0<y≤0.40; 0<z<0.5 and l<t<1.50;

- the bottom conductive layer 102 of Mo or Cu is deposited via PED process from a target constituted by the respective elementary precursor.

Clearly, the modalities described in above individual examples can be used in combination with one another.

Finally, it is understood that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the invention as defined in the annexed claims.

Claims

1. A method for producing thin-film multilayer solar cells, comprising a step of providing of a substrate and a step of deposition on the substrate of a plurality of layers, among which at least one absorber layer, having a CIS, CIGS or CIGASS type composition and set between a bottom conductive layer and a top conductive layer; the method being characterized in that at least the absorber layer is deposited via a pulsed-electron deposition process, causing, via a pulsed-electron beam, ablation of a target containing an excess of Cu with respect to the stoichiometric composition of the absorber layer to be formed.

2. The method according to Claim 1, wherein the absorber layer is deposited in a single pulsed-electron deposition stage.

3. The method according to Claim 1 or 2 , wherein the absorber layer is deposited via a pulsed-electron deposition process carried out with ablation of a target having composition: Cut (Ini-_x._yGa_xAl_y) ₂-t (Sei-_zS_z) ₂ with O≤x≤O.30; 0<y<0.40; 0<z<0.5 and l<t<1.50.

4. The method according to any one of the preceding claims, wherein the composition of the absorber layer, in particular the ratio between the concentration of Cu and the concentration of In, is modulated by varying the energy conditions of the electron beam sent onto the target.

5. The method according to any one of the preceding claims, wherein the process of deposition of the absorber layer is carried out by varying the energy conditions of the electron beam in order to vary the ratio between the elementary components in the absorber layer, in particular the ratio between In and its substituents Ga and/or Al, and create a composition gradient along the thickness of the absorber layer .

6. The method according to any one of the preceding claims, wherein two or more layers are deposited individually and in sequence in respective pulsed-electron deposition stages, carried out by means of one and the same source of pulsed electrons and replacing the targets on which the pulsed- electron beam impinges between one stage and the next and/or modifying the operating parameters of the pulsed-electron deposition process between one stage and the next.

7. The method according to any one of the preceding claims, wherein two or more layers selected from among the absorber layer, the top conductive layer, the bottom conductive layer, and one or more buffer layers set between the absorber layer and the top conductive layer are deposited in respective pulsed-electron deposition stages carried out in sequence.

8. The method according to any one of the preceding claims, comprising the steps of: depositing onto the substrate the bottom conductive layer by means of a metallization process, in particular via thermal evaporation or vapour-phase deposition and specifically DC sputtering, to form a metallized substrate,- treating the surface of the metallized substrate to provide the surface with a pattern suitable for deposition of the absorber layer; and introducing the treated metallized substrate into a reactor for pulsed-electron deposition processes and depositing the absorber layer and optionally other layers via pulsed-electron deposition process.

9. The method according to any one of the preceding claims, wherein the cell comprises, between the absorber layer and the top conductive layer, one or more buffer layers without cadmium, which are deposited by means of one or more respective pulsed-electron deposition stages.

10. The method according to Claim 9, wherein the top conductive layer and a first buffer layer, which is set in contact with the top conductive layer, are constituted, respectively, by a layer of conductive ZnO with n doping or other transparent conductive oxide (TCO) , and by a layer of resistive ZnO (i-ZnO) purposely not doped, and are obtained in two consecutive pulsed-electron deposition stages, both carried out using a single target and modifying between one stage and the next the operating parameters of the deposition process, in particular the atmosphere in which the deposition process occurs and/or the energy conditions of the electron beam.

11. The method according to Claim 9 or Claim 10, wherein set between the first buffer layer and the absorber layer is a second buffer layer without cadmium having composition Zn_3.. _xIn_xSei-ySy with O≤x≤l and O≤y≤l, or Zni-_x-_y-_zMg_xAl_yB_zO with O≤x≤l , O≤y≤O.l, O≤z≤O.l.

12. The method according to any one of the preceding claims, wherein the cell includes a CIGASS absorber layer and a buffer layer of resistive ZnO, deposited via respective pulsed- electron deposition stages carried out with operating parameters such as to minimize the reaction between the species interdiffused by the absorber layer, i.e. Ga and In, and by the buffer layer, i.e. 0, in such a way as to couple directly the CIGASS absorber layer and the buffer layer in contact with one another.

13. The method according to Claim 12, wherein the temperature of the stage of deposition of the buffer layer of resistive ZnO is significantly lower than that of the stage of deposition of the CIGASS absorber layer in such a way as to reduce the diffusion of the metals coming from the absorber layer.

14. The method according to Claim 12, wherein the deposition of the ZnO buffer layer is carried out at a temperature higher than the temperature of deposition of the CIGASS absorber layer, and deposited in succession on the substrate, having high optical transmittance, is the top conductive n-ZnO layer, the buffer layer of resistive . i-ZnO, the CIGASS absorber layer, and finally the bottom conductive layer.