WO2011148251A1

WO2011148251A1 - Method of making solar cells

Info

Publication number: WO2011148251A1
Application number: PCT/IB2011/001118
Authority: WO
Inventors: Carlo Taliani; Libuse Nozarova
Original assignee: Organic Spintronics S.R.L.
Priority date: 2010-05-24
Filing date: 2011-05-24
Publication date: 2011-12-01
Also published as: ITBO20100326A1; IT1400038B1; WO2011148251A8

Abstract

Method of making solar cells, the method provides that on a substrate (4) there are deposited: a substantially isolating layer (17) of ZnO of approximately 100nm, a layer (18) of CdS of approximately 100µm, a layer (19) of mixed selenides, and metal layer (20) by means of a Pulsed Plasma Deposition (PPD); the deposition of the mixed selenides takes place by hitting targets (3, 3') of different compositions at the same time by means of a plurality of fluxes of electrons.

Description

Method of making solar cells

Technical field The present invention is related to the method for the deposition of a material on a substrate, to the method of making a solar cell and to a solar cell.

Background of the invention CIS(CIGS)

becomes an ideal photoactive material for the construction of solar cells due to its suitable electron band gap of cca. 1.45 eV. The thin films of polycrystalline CIS(CIGS) can be deposited simply by different deposition processes (PVD, CVD, CSS, screen printing, spray methods, etc.). However, the previously mentioned methods generally require a special post-deposition treatment to optimize the properties of the layers.

Moreover, the known methods require to deposit a material in excess, the employment of costly substrates and relatively high temperatures and deposition energies.

It should be noted, moreover, that solar cells obtained through mentioned methods are relatively inefficient and expensive.

The aim of this invention is to deliver a deposition method for a thin layer of a material, the method for a construction of a solar cell and a solar cell itself which permit to overcome at least partially the shortcomings of the state-of-the-art methods and are simultaneously simple and cheap to execute. Summary

According to the present invention are delivered a method for the deposition of a layer of a material, the method for the construction of a solar cell and a solar cell itself as it is presented in subsequent independent claims and, preferably, in any of the claims dependent directly or indirectly on independent claims.

Brief description of the drawings

The invention will be subsequently described referring to attached figures which illustrate some examples of embodiments non-restraining, from which:

- figure 1 schematically illustrates a partial section of the device which can be employed by the method according to the present invention;

- figure 2 schematically exhibits a different form of an embodiment of the device shown in figure 1 ; and

- figure 3 shows a detail section of a product example according to the present invention.

Embodiments of the present invention

The method for the deposition of the first material of the composition Cu(lni_-yGay)(Sei- zS_z)2+x where 0≤x≤0,25, 0≤y≤0,75, 0≤z≤1 on a substrate, is provided according to a first aspect of the present invention; the deposition is performed using the Pulsed Plasma Deposition (PPD).

It should be noted that depositing the first material using PPD a first particularly

homogeneous layer is obtained (only small amount of the material is lost); moreover, such layer does not require further finishing treatments.

In the figure 1 the complete apparatus for the deposition of a material by PPD is indicated by number 1. The apparatus comprises a device 2 for plasma generation (i.e. an ionization at least partial of the rarefied gas) and guiding an electron flux towards the target 3 which contains (specifically is composed of) the given material in the way that at least part of the given material is separated from the target 3 and is deposited on a support (or substrate) 4. According to alternative embodiments, the given material can be composed of one homogeneous compound or of the combination of different compounds.

Advantageously, the target 3 is grounded. In this way the target does not repulse (on the contrary - attracts) the electron flux even if the electrons already hit the target 3.

The device 2 comprise a hollow element 5 which acts as a hollow cathode and has a cavity 6; and a main electrode 7, which comprises (especially is composed of) metallic conducting material and is placed inside the cavity of an ampoule 8 made of the dielectric material (for example glass or ceramics).

According to the embodiment illustrated in figure 1 , the main electrode 7 extends through a wall of ampoule 8.

The device 2 comprises an electronic device 9 with the function of a switch discharging to ground with adjustable frequency.

A rarefied gas is present inside the cavity 6 (and the ampoule 8). According to some embodiments, the cavity 6 (and the ampoule 8) contains the rarefied gas at the pressure lower than or equal to 10^"2 mbar. Advantageously, the rarefied gas contained inside the cavity 6 (and the ampoule 8) exhibits a pressure form 10^~2 mbar to 10^"6 mbar.

In this regard, it should be noted that the device 2 comprise a gas-feeding system (per se known and not shown) to feed the dry gas (non-restrictive examples of such gas are:

oxygen, nitrogen, argon, helium, xenon ecc.) inside the cavity 6 (and the ampoule 8); and the pumping system (per se known and not shown) contains a vacuum pump for the gas rarefaction in the cavity 6 (in other words, to lower the gas pressure inside the cavity 6). The same conditions (described in the case of the cavity 6) are substantially maintained in a chamber 10, inside which is placed the target 3 and the substrate 4.

The hollow element 5 comprises (especially, is composed of) a metallic conducting material and is connected to the power supply 12, which is designed to deliver a potential difference (up to cca. 20 kV) between the hollow element 5 and annular element 13, which acts as an anode. The hollow element 5 and the annular element 13 are connected through a capacitor 14. The apparatus 1 , furthermore, comprises of a tubular element 15 (capillary) which extends from the hollow element 5 through the annular element 13. The tubular element 15 is made of dielectric material (especially glass or ceramics) and has a diameter from cca 5 mm to cca. 7 mm. The lumen of the tubular element 15 has a diameter from cca. 2 mm to cca. 4 mm.

The deposition of the given material on the substrate is performed in an analogous way to the process described in the patent application with the publication number

WO2006/105955.

The apparatus 1 has been described for the purpose of an example, only. The device for the deposition by the PPD method can have the different structure and different working principle.

For example, according to the further embodiments, the apparatus 1 could follow the one described in the patent application number PCT/IB2010/000644.

The deposition of the material of target 3 on the substrate is made substantially according to the following manner. At the event of the main electrode discharge to the ground, the plasma and the electrons are generated between the main electrode 7 and the hollow element 5. The electrons and the plasma generated near to hollow element 5 are taken away and accelerated by the potential difference (up to 20 kV) between the hollow element 5 and the annular element 3. The electrons and plasma enter the tubular element 15 and then pass the equipotential space between annular element 13 (anode) and the target 3. The energy of the electron packet (impulse) is transferred to the target material through the electron packet impact on the target 3 surface and induces it's ablation, in other words, the explosion of the surface in the form of a plasma of the target 3 material (defined also as a "plum") propagates in the direction of the substrate 4 where it is deposited.

The ionic conductivity of the rarefied gas ensures the electrostatic screening of the space charge generated by the electrons. Consequently to this effect, the self-sustained electron and plasma beams with high energy density and power can be accelerated and directed against the target 3 which is kept at the ground potential. In such a way the beams create the explosions under the target 3 surface, which explosions generate the expulsion of the target material (the ablation process or "the explosive sublimation") forming the "plum" which propagates from the target 3 surface.

The ablation depth is determined by the beam energy density, the impulse life-time, the specific heat of evaporation and thermal conductivity of the target 3 material and by the density of the target itself.

The plum material interacts with the low pressure (from 10^"6 mbar up to 10^"2 mbar) gas carrier present in the chamber 10 during its passage from the target 3 surface to the substrate 4 and can be partially (or totally) oxidised (in the case the gas carrier is or contains or is oxygen), partially (or totally) reduced (for example in the case of argon or hydrogen as the gas carrier), or doped (for example the nitrogen present in the deposition chamber is ionised through the interaction with the "plum" and can substitute the oxygen in the structure of the metallic oxides changing the valence state of the metal) or it remains unchanged. It has been shown recently [Krasik Ya. E., Gleizer S., Chirko K., Gleizer J. Z., Felsteiner J., Bemshtam V., Matacotta F. C, J. Appl. Phys., 99, 063303, (2006)] that only small part (cca. 1%) of the electrons is accelerated by the full potential difference between the cathode (hollow element 5) and the anode (annular element 13). The energy of the main part of electrons does not exceed 500 eV. The deposition rate of the material (the growth rate of the film) can be controlled through the electron packet generation frequency (repetition rate); through the corresponding average electrical current (cca. 3 - 50 mA); through the distance between the target 3 and the substrate 4; and through the

temperature of the substrate.

Advantageously, in the first material 0,01≤x≤0,03 (in particular, x=0,02) and 0<y≤0,25 (in particular, y=0,25). According to some embodiments, z=0. According to alternative embodiments, z=1.

According to some embodiments, the target 3 is made by powder material pressing. In particular, the powder material is compressed inside a die applying the force of approx. 15 tons for ca. 30 min. The target 3 obtained by such process exhibits the diameter from ca. 28 mm up to ca. 36 mm (in particular, ca. 32 mm) and a thickness from ca. 5 mm up to ca. 6mm.

Advantageously, the target 3 (during the ablation end the deposition) is rotated with the frequency from ca. 0,8 up to ca.1 ,2 Hz (specifically, ca. 1 Hz). The target 3 (during the ablation end the deposition) is heated up to the temperature of 950 °C.

According to some embodiments, the device 2 is operated at a frequency (number of discharges in the time unit) from ca. 2 Hz up to ca. 100 Hz and a potential difference (between cathode and anode) from ca. 9 kV up to ca. 18 kV.

Advantageously, (to facilitate the homogeneous deposition) the substrate 4 is rotated (specifically, at the frequency of ca. 0,5 Hz). The substrate 4 is heated up to the

temperature from ca. 25 °C up to ca. 700 °C (advantageously, from ca 250 °C up to ca. 350 °C; particularly, at ca. 300 °C).

According to some embodiments, the chamber 10 contains the gas mixture composed of Argon and, possibly, oxygen in the amount from 0 % up to 10 % (in volume with respect to the total gas volume). Alternatively, the chamber 10 contains the gas mixture composed of Argon and up to 1 % (in volume with respect to the total gas volume) of some further gas selected from the group of H₂, He, N₂.

Advantageously, the chamber 10 contains the gas at the pressure from ca. 1^*10^"* mbar up to ca. 4^*10^"3 mbar (in particular, ca.1 ^*10^'3 mbar).

According to some embodiments, a single target 3 is used which consists of the first material.

According to some embodiments, the first material is deposited by means of at least two pulsed electron fluxes which hit at least two targets, respectively. The targets have two different compositions. The particular targets can be prepared and/or have the properties and/or be treated according to the description of the target 3. Advantageously, the pulsed electron fluxes are generated by two or more guns which demonstrates the structure and/or operation analogous to the device 2.

In some cases, two different targets (with different composition each) are employed. In other case.s three different targets (with different composition each) are used. In some even more different cases, four different targets (with different composition each) are employed.

For the purpose of example only, figure 2 shows schematically an apparatus 1 which comprises two devices 2 and 2' for plasma generation, two targets 3 and 3' of different composition and only one substrate 4. P and P' show schematically the "plumes" which are generated at the surfaces of the targets 3 and 3', respectively.

It should be noted that the "plumes" produced at different targets are combined

surprisingly in the way which permits the reaction among different components and to create at the substrate 4 the mentioned first material. In such a way it is possible to omit (or substantially simplify) the preparation of the target 3. Moreover, it is possible in this way to change very easily the composition of the first material changing the different

operational parameters.

According to some particular embodiments, at least one (specifically, any of) target is composed of one starting material selected from the group: Cu₂Se, CuSe, GaSe, ln₂Se₃, Cu₂S, CuS, GaS, ln₂S₃.

According to a specific embodiment, the first target is composed of CuSe (and/or Cu₂Se) and the second target is composed of ln₂Se₃.

According to a specific embodiment ,a first target is composed of CuSe (and/or Cu₂Se) and a second target is composed of ln₂Se₃ and the third target is composed of GaSe. In particular, the first target is composed of (Cu₂Se)i_-n(CuSe)_n wherein 0≤n<0,25 (more in particular, n is ca. 0.02).

According to a specific embodiment ,the first target is composed of CuS (end/or Cu₂S) and the second target is composed of ln₂S₃.

According to a specific embodiment the first target is composed of CuS (and/or Cu₂S) and the second target is composed of ln₂S₃ and the third target is composed of GaS. In particular, the first target is composed of (Cu₂S)i-_n(CuS)_n where 0<n≤0,25 (more in particular, n is ca. 0.02). According to one version, at least one (especially, each) target is composed of one of the components of the first material in the elementary form. According to a specific

embodiments, at least one (especially, each) target is composed of the element selected from the group: Cu, In, Ga, Se, S. In particular, at least one (in particular, each) target is composed of the element selected from the group: Cu, In, Ga, Se. According to a some embodiments, at least one (especially, each) target is composed of the element selected from the group: Cu, In, Se.

According to a some embodiments, the first material has the composition Cu (lr>i- _yGay)Se₂₊x. In particular, the first material has the composition Cu lnSe_2+x.

According to alternative embodiments, the first material has the composition Cu(lni- yGa_y)S₂₊x. Specifically, the first material exhibits the composition CulnS_2+x.

During the deposition the first layer (of the first material) is made - this layer shows a thickness of up to 8μπι (in particular, from ca. 1 pm to ca. 8pm). Advantageously, the first layer shows a thickness of 2 m to 6μηι (in particular, ca. 4pm).

It should be noted that by applying the deposition method according to the present invention a surprisingly efficient deposition is achieved, using the quantity necessarily needed of the first material and low cost substrates.

According to some embodiments, the substrate is of an essentially dielectric material (for example glass). In some cases the substrate can be covered with one layer of ITO (Indium Tin Oxide).

According to some embodiments, the substrate is of a metallic material (for example Copper, Molybdenum, stainless steel or a combination of those).

In accordance with a second aspect of the present invention, a method for production of a solar cell is provided. The method comprises an application step, during which a plurality of layers is deposited on a substrate and which comprises a first step of deposition, which is defined as the deposition as described in accordance with the first aspect of the present invention.

Furthermore, the application step comprises a second deposition step (which can precede or succeed the first deposition step) which is performed by pulsed plasma (Pulsed Plasma Deposition - PPD) and during which ZnO is deposited, producing a second layer showing a thickness of less than or equal to ca. 150nm.

Advantageously, the second layer shows a thickness of more than or equal to 80nm. It should be noted that it has been experimentally observed that with a second layer of the thickness indicated above it is surprisingly possible to obtain particularly efficient solar cells.

The conditions under which the second deposition phase is performed are such that allow the second layer (formed by ZnO) to be essentially isolating (specifically, it shows a resistivity of more than 1 x10³0hm - advantageously, more than 1 x10⁵Ohm).

Advantageously, the second layer shows a resistivity of more than 1x10m¹⁰Ohm.

In particular, ZnO is deposited in the chamber 10 which contains gas containing at least

25% (of volume in respect to the total volume of gas) of oxygen.

Advantageously, the application step comprises a third deposition step, which is between the first and the second, and during which a third layer containing (in particular, composed of) CdS is deposited. Advantageously, the third deposition phase is carried out through

Pulsed Plasma Deposition (PPD). The third layer enables the creation of a p-n transition separating the electronic holes and the electrons at the interface.

According to some embodiments, the third layer has a thickness of up to 150nm

(specifically, from ca. 80nm up to 150nm).

Advantageously, during the application step the substrate is rotated. Thus, it is possible to obtain essentially homogeneous depositions.

According to some embodiments, the first layer is deposited directly on (in contact with) the substrate. In particular, in this case the substrate is of metallic material (for example Copper, Molybdenum, stainless steel or a combination of those). Thus, on the first layer the third layer is deposited (on contact), on which third layer the second layer is deposited (on contact). Thus, on the second layer a layer of an essentially conductive material (having a resistivity of less than IxlO^Ohm) is deposited (on contact). Carbon paste is deposited on(in contact) the essentially conductive material.

Advantageously, the essentially conductive material shows a resistivity of less than 1x10^" ³Om (specifically, less than 1x10^"5Om). According to some embodiments, the essentially conductive material is ZnO that is deposited in an oxygen-free atmosphere through PPD starting from one ZnO target.

According to some embodiments, the second layer is deposited (directly on contact) upon the substrate. In particular, in this case, the substrate is from an essentially dielectric material (for example glass). In some cases, the substrate can be covered with one layer of ITO (Indium Tin Oxide). Thus, on the second layer the third layer is deposited (on contact), on which the first layer is deposited (on contact). Thus, on the first layer a metallic contact (for example Copper, Silver, Aluminum, Gold, Molybdenum or a

combination of those) is deposited (preferably through PPD).

In accordance with a third aspect of the present invention, a solar cell comprising of substrate and a first layer is provided. According to some embodiments, the solar cell includes a second layer. Advantageously, the solar cell includes a third layer (deposited on contact between the first and the second layer). Advantageously, the solar cell includes an essentially conductive material layer, a metallic contact and/or a carbon contact.

The first and/or the second and/or the third layer are defined in accordance with the first and/or second aspect of the present invention.

The substrate, the essentially conductive material, the metallic contact and/or the carbon contact are defined in accordance with the first and/or second aspect of the present invention.

Unless the opposite is explicitly stated, the content of the references (articles, books, patent applications etc.) cited in this text is referred to in its entirety. In particular, the references listed are herein incorporated by reference.

Further characteristics of the present invention will emerge from the following description of two merely illustrative, not limiting examples. Example 1

Deposition of single elements The layers of the first material were deposited from single targets of Cu, In, Ga and Se using deposition system and four independently controlled PPD guns. All the targets were prepared from starting materials (source: Sigma-Aldrich, purity 99.9+ or 99.99%) in powder form. All the powders were pressed under vacuum in a die of 32 mm diameter applying force of 15 tons for ca. 30 minutes.

The thickness of so obtained pellets is ca. 5-6 mm. The targets rotated during the ablation with the frequency of 1 Hz.

The ablation velocity of different materials varies substantially (v_Cu > vi_n >VG₃ = vs_e), thus, the efficiency of the PPD guns was adjusted through control of the discharge frequency f (2Hz≥ f > 100Hz) acceleration voltage V (9Kv > V > 18Kv) of each cannon according to the material of its target so that the resulting film would manifest the correct stoichiometry. The plumes produced by the ablation of different targets were directed towards a substrate (which can be heated to different stabilized temperatures up to 950°C) so as to overlap homogeneously upon surface of 5.08cm diameter. The substrate rotated with the frequency of ca. 0.5Hz during the deposition. The work atmosphere in the deposition chamber was constituted by Ar (or Ar + x% 0₂ 0% > x > 10%, Ar + 1 % H₂, He and/or N₂), held under pressure of ca. 1x10^"3 mbar. The layers of CIS (CIGS) were deposited on glass substrates (Pyrex, "Alcali-free" glass, quartz) or metal (Cu, Mo, stainless steel) heated to various temperatures T (25°C > T > 700°C, preferably 300°C). The substrate temperature was controlled using an IR-thermometer, the thickness observed by using a laser thickness monitor.

Example 2 Selenides deposition

Targets were produced from starting materials (Cu₂Se, CuSe, GaSe, ln₂Se₃, source:

Sigma-Aldrich) in powder form. All the powders were pressed under vacuum in a die of 32 mm diameter applying force of 15 tons for ca. 30 minutes. The composition of the target from Copper selenide was varied in the following limits: (Cu₂Se)i_-x(CuSe)_x where 0≥ x≥ 0.25. The thin layer deposition procedure took place following the procedure described in example 1. Example 3

Deposition from single target

For this deposition process various targets with already the desired composition were prepared from the first material through solid state synthesis using reactions between the powders at elevated temperatures in closed environment. The group of targets can be described with the formula: CulnS_2+x where 0 > x > 0.25 and with the formula Cu(lni- _yGa_y)S₂₊x where 0 > x≥ 0.25, 0 > y > 0.75. The thin layer deposition procedure took place following the procedure described in example 1.

Example 4

Solar cells deposited on glass layer For the production of these cells it has been used the "alcali-free" glass with a layer of ITO (Indium Tin Oxides) already pre-deposited. The substrates 16 (figure 3) were cleaned through chemic methods (bath in solvents and bi-distilled water) and then treated in DC plasma from Ar (power of 25W) for two minutes. Subsequently, the substrates were moved into the ZnO deposition chamber and a layer 17 of (ca. 100nm) isolating ZnO was deposited in oxygen atmosphere (gas pressure p = 4x10^"3 mbar). At this point the substrates were moved in the CdS/CIS (CIGS) deposition chamber and a layer 18 of CdS (ca. 80nm) was deposited on the ZnO surface and subsequently a layer 19 of the first material was grown (thickness between 1 and 8 μιη, specifically 4 m). On the layer 19 of the first material the metallic contact 20 was prepared (with magnetron "sputtering" of Cu, Ag, Al, Au or Mo). Figure 3 illustrates schematically what was obtained.

Example 5

Solar cells deposited on metal layer

Flexible solar cells were constructed on metallic substrates (Cu, Mo and/or stainless steel). These substrates were cleaned by the same procedure as described in the previous example. On the cleaned substrates placed in the CdS/CIS (CIGS) deposition chamber first a thin layer of the first material was deposited (thickness between 1 and 8 μιτι, specifically 4 μηη) and upon this layer a CdS film was applied (thickness ca. 80nm).

Subsequently, the substrates were moved into the ZnO deposition chamber and a layer of (ca. 100nm) isolating ZnO was deposited in oxygen atmosphere (gas pressure p = 4x10^"3 mbar). Subsequently, the device was moved into the ZnO deposition chamber and a ZnO layer (thickness ca. 10nm) was deposited in atmosphere of oygen (gas pressure p = 4x10^" ³ mbar) and thus conductive (thickness ca. 200-300 nm). Finally, upon the ZnO surface an electric contact was created by applying a carbon paste.

Claims

1. - Method of making a solar cell; the method comprising an application step, during which a plurality of layers are deposited on a substrate and which comprises a first deposition step of a first material having composition Cu(ln_1-yGay)(Sei_-zSz)2+x wherein 0< x ≤0.25, 0≤ y <0.75, 0< z < 1 ; the method being characterised in that the first deposition step is carried out by means of Pulsed Plasma Deposition (PPD).

2. - Method according to claim 1 , wherein the first material is deposited directing at least two pulsed flows of electrons each against a respective target having a different composition.

3. - Method according to claim 1 or 2, wherein at least a target is made of a material selected in a group consisting of: Cu₂Se, CuSe, GaSe, ln₂Se₃, Cu₂S, CuS, GaS, ln₂S₃.

4. - Method according to claim 1 or 2, wherein at least a target is made of one of the components of the first material in elementary form.

5.- Method according to Claim 4, wherein the first material has a composition

Cu(lni.yGa_y)Se_2+x; at least a target is made of an element selected in the group consisting of: Cu, In, Ga, Se.

6. - Method according to one of the preceding claims, wherein during the first deposition step a first layer, which first layer has a thickness from approximately 1 μιτι to approximately δμητι, is prepared.

7. - Method according to one of the preceding claims, wherein the application step comprises a second deposition step (which can be before or after the first deposition step), which is carried out by means of Pulsed Plasma Deposition (PPD) in the presence of oxygen and during which ZnO is deposited so as to obtain a second layer having a thickness smaller than or equal to approximately 150nm.

8. - Method according to claim 7, wherein the second layer has a thickness greater than or equal to 80nm.

9. - Method according to claim 7 or 8, wherein the application step comprises a third deposition step, which is between the first and the second deposition steps and during which a third layer, which consists of CdS and has a thickness from approximately 80nm to approximately 150nm, is deposited.

10.- Method according to one of the preceding claims, wherein the substrate is made to rotate during the application step.

11.- Solar cell comprising a substrate and a first layer having a thickness smaller than or equal to 8μτη and having a composition Cu(ln _-yGay)(Sei_-zS_z)2+x wherein 0< X <0.25, 0< y <0.75, 0< z < 1.

12. - Solar cell according to claim 1 1 , and comprising a second layer of ZnO substantially isolating and having a thickness smaller than or equal to approximately 150nm.

13. - Solar cell according to claim 12, wherein the first layer has a thickness greater than or equal to 1μτη and the second layer has a thickness greater than or equal to 80nm.

14. - Solar cell according to one of claims from 1 1 to 13, and comprising a third layer located between the first and the second layer, which consists of CdS and has a thickness from approximately 80nm to approximately 150nm.