Method for producing the P-N junction of a thin-film photovoltaic cell and corresponding method for producing a photovoltaic cell

Abstract

A method for producing a P-N junction in a thin film photovoltaic cell comprising a deposition step in which are carried out successively: a layer of precursors of a photovoltaic material of type P or N, a barrier layer and a layer of precursors of a semiconducting material of type N or P, this deposition step being followed by an annealing step carried out with a supply of S and/or Se, this annealing step leading to the formation of an absorbing layer of the type P or N and of a buffer layer of type N or P and of a P-N junction at the interface between said layers.

Claims

1. A method for producing a P-N junction in a thin film photovoltaic cell comprising a deposition step in which are successively carried out: a layer of precursors of a photovoltaic material of type P or N, a barrier layer, and a layer of precursors of a semiconducting material of type N or P, this deposition step being followed by a single annealing step carried out with a supply of S or Se or both S and Se, this annealing step leading to the formation of an absorbing layer of type P or N and of a buffer layer of type N or P and of a P-N junction at the interface between said absorbing and buffer layers, said annealing step allowing transformation of said precursors of a semiconductor material of type N or P into a material forming said buffer layer, the barrier layer preventing chemical diffusion and oxidation at the interface between the absorbing layer and the buffer layer during this annealing step.

2. The method according to claim 1, wherein the precursors of a photovoltaic material of type N or P comprise metal precursors of Cu, Zn, Sn or of Cu, Ga and In.

3. The method according to claim 1, wherein a layer of Se or S or both S and Se is deposited on the layer of precursors of a photovoltaic material of type P or N, before depositing the barrier layer.

4. The method according to claim 1, wherein the precursors of a photovoltaic material also comprise selenium or sulfur compounds or compounds of both selenium and sulfur of metal precursors.

5. The method according to claim 1, wherein the precursor of a semiconducting material of type N or P is indium.

6. The method according to claim 1, wherein, during the annealing step, the sulfur and selenium are provided in gaseous form.

7. The method according to claim 1, wherein that the annealing step is carried out at a temperature comprised between 400 and 650 C., with temperature raising ramps comprised between 1 C./s and 15 C./s.

8. The method according to claim 1, wherein the barrier layer is formed with a nitride or an oxide.

9. A method for producing a thin film photovoltaic cell comprising: producing a rear-face electrode on a substrate; applying the method according to claim 1; and producing a transparent and conductive front-face electrode.

10. The method according to claim 9, wherein the rear-face electrode is a molybdenum layer, while the front-face electrode is a ZnO layer doped with Al.

11. The method according to claim 9, wherein an intrinsic ZnO layer is deposited after applying the method for producing the P-N junction and before producing the front-face electrode.

Description

(1) The invention will be better understood and other objects, advantages and features of the latter will become more clearly apparent upon reading the description which follows and which is made with reference to the appended drawings, wherein:

(2) FIG. 1 is a sectional view of a stack of layers obtained after the deposition step of the method according to the invention,

(3) FIG. 2 is a sectional view of a stack of layers obtained after the annealing step of the method according to the invention,

(4) FIG. 3 is a graph giving the current density versus the voltage for a solar cell obtained by the method according to the invention, and

(5) FIG. 4 is a graph giving the current density versus the voltage for a solar cell similar to the one which is the subject of the graph of FIG. 3 but obtained by a conventional method.

(6) The elements common to various figures will be designated with the same references.

(7) With reference to FIG. 1, the method for producing a photovoltaic cell according to the invention first of all consists of depositing on a substrate 1, a metal layer 11 forming a rear-face electrode for the photovoltaic cell which will be obtained by the method according to the invention.

(8) The substrate 1 may be made in various materials, conventionally in glass, in plastic or in metal. Generally, this substrate is made in a sodalime glass and has a thickness comprised between 1 and a few millimeters of thickness. Flexible, metal or plastic substrates may also be contemplated. The layer 11 is for example made in molybdenum and its thickness is comprised between 100 nm and 2 m and it is notably equal to 500 nm.

(9) The deposition of the molybdenum layer may notably be achieved by cathode sputtering.

(10) On the layer 11, are then deposited, as a layer 12, the precursors which will lead to the formation of the photovoltaic layer.

(11) These are metal precursors of Cu, Zn, Sn and optionally at least one element taken from Se and S.

(12) As an example, the ratios of the Cu, Zn and Sn elements are conventionally selected so that
0.75Cu/(Zn+Sn)0.95 and 1.05Zn/Sn1.35.

(13) This layer 12 may essentially include metal precursors. In this case, the sulfur and/or selenium are then provided in gaseous form.

(14) They may also be provided in the form of a layer 13 deposited on the layer of metal precursors.

(15) Moreover, the layer 12 may be a layer of metal precursors and/or of their corresponding selenium and/or sulfur compounds. Thus, the layer 12 may be a layer of Cu, Zn and Sn and/or of their selenium or sulfur compounds such as CuSe, CuS, ZnSe, ZnS, SnSe, SnS, or any combination of these compounds. In this case, it is not necessary to provide a layer 13 consisting of sulfur and/or selenium.

(16) Generally, within the scope of the invention, provision of selenium and/or sulfur is preferred in the form of a deposit in the stack. Indeed, it is then easier to control the final composition of the photovoltaic layer than with a provision in gaseous form.

(17) Thus, as an example, the layer 12 may include three successive layers, a ZnS layer, a copper layer and a tin layer, the order of these layers may be modified.

(18) The thickness of the ZnS layer may then be of about 350 nm, that of the copper layer of about 120 nm and that of the tin layer of about 160 nm.

(19) In this case, the layer 13 may be a selenium layer and have a thickness of about 2 m.

(20) Many deposition methods suitable for forming thin layers may be used for producing the layers 12 and 13.

(21) These may be vacuum processes, such as evaporation or cathode sputtering or processes applied at atmospheric pressure, such as electrodeposition, screen printing, doctor-blading, inkjet or slit-coating.

(22) A barrier layer 14 is then directly deposited on the layer of precursors 12 or on the layer 13 of S and/or Se when it is provided. This layer is continuous.

(23) As this will be explained subsequently, this barrier layer gives the possibility of avoiding chemical diffusion of components and oxidation.

(24) This barrier layer is formed with a compound based on a nitride or oxide. It may notably be made in Si.sub.3N.sub.4, SiO.sub.2 or TiO.sub.2. Its thickness is from a few nanometers and for example equal to 4 nm.

(25) Finally, on layer 14, is then deposited a layer 15 of at least one precursor of a semiconducting material for which the doping will be of type n if the obtained photovoltaic material from layers 12 and 13 is of type p and vice versa.

(26) Thus, this layer 15 may notably be made in indium or zinc and its thickness will be from a few tens of nanometers and for example of about 30 nm.

(27) Once this stack is made, it is subject to an annealing step, carried out with a provision of S and/or Se.

(28) The chalcogen S or Se may then be provided as an elementary gas or as a gas of the H.sub.2S or H.sub.2Se type.

(29) This annealing step leads to the formation of an absorbing layer 22 in photovoltaic material and of a buffer layer 25.

(30) This annealing step does not modify the barrier layer 14.

(31) This annealing step, conventionally designated as a selenization/sulfurization anneal, is carried out at a high temperature.

(32) This annealing step may notably be carried out by using a temperature raising ramp comprised between 1 C./s and 15 C./s so as to obtain a temperature comprised between 500 and 650 C. and for example equal to 550 C.

(33) The duration of the plateau at a high temperature may be comprised between 30 seconds and 30 minutes, and it is advantageously equal to 3 minutes.

(34) This temperature allows crystallization of the metal precursors in CIGS or CZTS, in order to obtain the layer 22.

(35) This annealing step also allows transformation of the precursor(s) present in the layer 15 into a material forming the buffer layer 25.

(36) With the examples of materials given earlier, the layer 25 is then formed in In.sub.2(S, Se).sub.3 or further in Zn(O, S) or Zn(O, Se).

(37) Moreover, with the thickness values of the different layers given in the previous example, the annealing step leads to a layer 22 in CZT(S, Se) with a thickness of 1.2 m, while the layer 25 in In.sub.2(S, Se).sub.3 is of about 50 nm.

(38) During this annealing step, the barrier layer 14 prevents the diffusion of the components present in the layer 12, notably of indium or zinc, into the photovoltaic material of the layer 22. It therefore gives the possibility of avoiding oxidation between the absorbing layer and the buffer layer during annealing.

(39) However, it should be noted that this annealing step may be accompanied by the formation of a layer of Mo(S, Se).sub.2 at the interface between the molybdenum layer 11 and the layer 22 in a photovoltaic material.

(40) The thickness of the Mo(S, Se).sub.2 layer depends on many parameters, notably on the amount of provided S/Se. It may be estimated to be comprised between 5 and 100 nm. It is known that its presence promotes contact of the ohmic type at the interface with the absorbing layer 22, notably in CIGS.

(41) After this annealing step, the formation of a photovoltaic cell further requires the deposition of a transparent and conductive front-face electrode (not shown in the figures).

(42) This front-face electrode is made in a transparent and conductive material.

(43) It may notably consist of ZnO doped with Al.

(44) The thickness of this layer is typically comprised between 100 and 800 nm and preferably equal to about 500 nm.

(45) The material forming this transparent and conductive electrode may notably be deposited by cathode sputtering.

(46) It should further be noted that before the deposition of this layer forming the front-face electrode, it is possible to deposit on the buffer layer 25, a layer of a material transparent in the solar spectrum and strongly resistive.

(47) This layer is typically made in intrinsic ZnO and may have a thickness of a few tens of nanometers, and notably 50 nm for the solar cell given as an example earlier.

(48) The constitutive material of this layer may typically be deposited by cathode sputtering.

(49) This layer is optional and it may be omitted without causing any significant degradation of the electrical performances of the photovoltaic cell.

(50) Now reference is made to FIGS. 3 and 4 which are graphs giving the current density (in mA/cm.sup.2) versus voltage (in V).

(51) These graphs result from simulations made with the SOAPS piece of software of the solar cell given as an example earlier, for FIG. 3 and of a similar cell but obtained by a conventional method, for FIG. 4.

(52) These graphs give the possibility of illustrating the operation of each of these cells.

(53) Thus, for FIG. 3, this is a solar cell for which the lower or rear-face electrode is a molybdenum layer, for which the thickness is 500 nm, an absorbing layer in CZT (S, Se) for which the thickness is of about 1.2 m, a barrier layer in Si.sub.3N.sub.4 for which the thickness is of about 4 nm, a buffer layer in In.sub.2(S, Se).sub.3 with a thickness of about 50 nm, a ZnO layer for which the thickness is of about 50 nm and finally a ZnO layer doped with Al for which the thickness is of about 500 nm.

(54) This solar cell is obtained with the method according to the invention.

(55) For FIG. 4, the solar cell includes the same layers, except as regards the barrier cell which is absent.

(56) Comparison of FIGS. 3 and 4 shows that each solar cell according to the invention operates in the same way as a similar solar cell, obtained by a conventional method and not including any barrier layer.

(57) Thus, FIG. 3 shows that it is possible to obtain separation of the charges even if a barrier layer is provided at the p-n junction. In practice, this separation of the charges is possible since the electrons present in the conduction band are able to cross the barrier layer by the tunnel effect.

(58) It should further noted that with the cell obtained by the method according to the invention and with a similar cell obtained according to a conventional method and without any barrier layer, the two following quantities remain unchanged: the open circuit voltage Voc (in V) and the short-circuit current density Jsc (in mA/cm.sup.2). Indeed, for Voc, a value of 0.5263 V is measured for the cell of FIG. 3 and a value of 0.5228 V for the cell of FIG. 4. Moreover, for Jsc, a value of 45.2830 mA/cm.sup.2 is measured for the cell of FIG. 3 and a value of 45.3024 mA/cm.sup.2 for the cell of FIG. 4.

(59) As regards the form factor FF and the energy conversion yield, the corresponding values for the cell of FIG. 3 obtained by the method according to the invention is 63.27% and 15.08%. These values are respectively 78.46% and 18.58% for the cell of FIG. 4.

(60) Thus, the energy conversion yield of the solar cell of FIG. 3 is greater than the one obtained for solar cells having a structure of the superstrate type or for which the p-n junction is obtained by a vacuum process, all these cells avoiding oxidation of the absorbing layer.

(61) Moreover, as compared with all the known methods, and notably the one used for producing the solar cell of FIG. 4, the method according to the invention gives the possibility of producing in a single annealing step both the absorbing layer and the buffer layer of a solar cell.

(62) This shows that the method according to the invention actually gives the possibility of obtaining a solar cell operating satisfactorily, while avoiding the risks of oxidation and chemical diffusion at the interface between the absorbing layer and the buffer layer and the use of cadmium and by being of a simplified application.

(63) The reference symbols inserted after the technical characteristics appearing in the claims have the sole purpose of facilitating the understanding of the latter and cannot limit the scope thereof.