FR2499316A1 - Silicon solar cell metallisation with chemically sculptured surface - uses metallisation of contacts post surface doping as mask for chemical etch to remove surface traps and reduce absorption in face layer - Google Patents

Abstract

The photocell consists of a plane substrate with an active surface layer (11) of opposite types of conductivity through which the cell is irradiated and the active region (5) lies at the boundary between this layer and the substrate. The cell is covered with a network of conductors over the surface layer which is of uniform thickness. The surface layer is then chemically etched to reduce its thickness between the conductors (111) which act as a protective mask. The silicon substrate is cut parallel to the (111) plane of the crystal and in the case of a back surface field cell the front and rear faces are doped by vapour phase diffusion simultaneously with phosphine and boron hydride, the boron diffusing more slowly. A triple metallisation using fine titanium and palladium powder followed by coarser silver powder. The cell is etched using a 10 to 30 percent by volume aq. soln. of KOH at 80 to 100 deg. C for 2 to 3 minutes. The surface layer is reduced to 0.5 pm and about 90 Ohms per square resistance with only a slight undercut of the metallisation. The edges of the cell are deeply etched completely eroding the surface region (13) and surface traps.

Description

"IMPROVEMENT TO THE PRODUCTION OF A SOLAR CELL
IN VIEW, NOT ADM.ENT, TO MODEL THE THICKNESS OF THE LAYER
ACTIVE, AND CELL SO OBTAINED "
The present invention relates to a method for producing a solar cell, constructed from a flat wafer of a semiconductor material of a first conductivity type, this wafer having in particular, on the side of its anterior face intended to receive the radiation. solar, a surface layer or active layer of the second type of conductivity at the base of which is located the driving junction of the cell and on which rests a network of electroconductive lamellae forming one of the electrodes of the cell, said surface layer being relatively thicker under said lamellae between their intervals, wherein said surface layer, initially formed of uniform thickness, is then chemically etched to locally reduce the thickness thereof.

 The invention relates more particularly, but not exclusively, to solar cells of the type called "BSF" (of the English "Back Surface Field") provided with the side of their posterior face -the opposite to the anterior face de fi ned ci above-another surface layer of the first type of conductivity more doped than the material of the wafer and whose presence can significantly improve the performance of the cells.

 The conversion efficiency, electrical power delivered incident light power, of a solar cell is even higher than its active layer is less absorbent for the luminous flux. And it is well known that the light absorption is even lower than this active layer is thin on the one hand, and relatively low-doped on the other hand.

In particular, it is necessary to avoid the "high density dopant layer" in the beds immediately adjacent to the surface from which the active layer was created by diffusion treatment; this dead layer is, in fact, a photon trap, especially for those of short wavelength,
But we also know how difficult it is to achieve a good electrical contact between the electroconductive lamellae and the layer if it is very thin.

Especially if, moreover, this layer is moderately doped.

 A method for producing a solar cell which makes it possible to reconcile the antagonistic requirements of fineness of the surface layer in its active ranges and of greater thickness of this layer at the points of contact is disclosed in US Pat. No. 4,029,518. the second embodiment of the cell corresponding to the structure according to Figure 3, and initially, it was formed on the anterior face of the cell an active layer of uniform thickness, this by diffusion.

Then, localized etching of the deposited layer through the openings of a mask reproducing the pattern of the contact electrode. It was only after this pickling that the aluminum contact electrode was created.

 The desired result of obtaining a thin active layer and cleared of its "dead layer" where it receives light, as well as thicker and highly doped surface contact locations is certainly achieved by the implementation of the method described. But the stripping operation sequence has the disadvantage of requiring a special masking operation which increases the time and cost of manufacture of the cell. In addition, when creating the network of contact strips, it is necessary to work on an uneven surface flatness, which affects the regularity of the width of the strips obtained.

 The method according to the present invention makes it possible to avoid the disadvantages of the method previously described.

 According to the invention, a method for producing a solar cell comprising a phase of chemical etching of the active layer as provided for in the preamble is notably remarkable in that said chemical etching is carried out after the realization of said network. electroconductive lamellae, these then constituting a protective mask for the parts of the superficial layer that they overhang.

 An advantage of the process according to the invention appears immediately: this process does not require the formation of a specific etching mask. As a result, the thinning operation of the active layer, which has become easier to use, becomes very beneficial in view of the significant gain it affords on the quality of the cell, in particular on the conversion efficiency.

 On the other hand, the network of electroconductive lamellae, established on an initially flat active layer, has all the necessary finesse and geometric precision.

This is very important because stripping the active layer to eliminate the most doped parts causing an increase in the average resistivity of this layer and requires, by compensation, to tighten at the same time as refine the network of contact strips. So it is better that they are created on a flat surface rather than bumpy.

 According to one embodiment of the method according to the invention, applicable to silicon solar cells provided with electroconductive lamellae, superficially comprising at least one layer of silver, the chemical etching is carried out in a potassium hydroxide solution (10). at 30% by volume) in water at a temperature between 80 and 1000 C.

 The method according to the invention, in the case of the manufacture of silicon solar cells provided with an overdoped surface layer of the first type of conductivity ("BSF" layer) on their rear face, has an additional advantage. It is, in fact, very difficult in such cells to prevent diffusions of opposite conductivity types made on each of the faces of the wafer from interfering at the edges of this wafer and creating parasitic junctions, which leads to The chemical attack intended to reduce the thickness of the active layer of the cell extends, moreover, to the other bare areas of the material of the wafer and, in particular, to the place edges of this wafer, which has the result of erasing interference interference areas and very substantially lowering the corresponding leakage currents. The posterior face of the wafer and, consequently, the surface layer deposited from said face are protected during the attack by the silver surface electrode which covers this posterior surface and which has been created, as the first electrode of the anterior face, before this attack takes place.

 In the case of cells of this category made of silicon, the use of the previously designated potash pickling solution has proved particularly favorable.

It is known, in fact, that the etch rate varies according to the crystalline orientation of the etched plane. If, as is customary for solar cells, the platelet faces are parallel to <111> planes, the etch rate is relatively slow (~ 0.17 to 0.25 # m / min) and, this fact, quite suitable for thicknesses to be stripped of the order of 0.5 pm as is the case presently. But, on the other hand, on the edges where the crystalline orientation is different and the material very dislocated, this speed is much higher and one is thus assured to obtain a perfect cleaning of the zones of interference.

 The description which follows with reference to the appended drawings will make it possible to specify the characters of the invention.

 Figures 1, 2, 3 illustrate, at three stages of its construction and in sectional views, a solar cell made in the context of a process employing the method of the invention.

 The solar cell chosen for example is a cell of the BSF type defined above.

 At a first stage of manufacture of this cell, which corresponds to the drawing of FIG. 1, a silicon wafer 10 of a first conductivity type has, on the side of its anterior face 10A intended to receive the solar radiation, a layer surface 11 of the second type of conductivity. This layer 11 will be the active layer of the cell t at its base is located the motor junction J.

 The wafer 10 has, on its other face, or posterior face 10B, a second superficial layer 12 of the first type of conductivity more doped than the material of said wafer. As we know, the layer 12 devolves the double rible on the one hand to create a rear electric field allowing the return to the junction J minority carriers that were generated deep in the wafer and which, in the absence of this layer 12, recombine on the face 10B, on the other hand to form the contact layer for the rear electrode of the cell.

 The layers 11 and 12 have been created, as is conventional for solar cells, by simultaneous or successive diffusion of appropriate dopants, either from the gas phase, or from doped oxide deposits, or again by combining these two techniques. Whatever the technique or techniques employed, it is practically impossible to prevent the wafer edge from forming a marginal zone where the N and P diffusions interfere; this parasitic interfering zone 13 appears in dashed lines in FIGS. 1 and 2.

 At a later stage of its manufacture corresponding to the representation of Figure 2, the cell received its electrodes. On the anterior face 10A, in contact with the active layer 11, has been created the electroconductive lamellae network 111. The rear surface 10B has been covered over practically all of its surface, that is to say at the excluding a narrow edge region 14, an electroconductive deposit 112 resting on the surface layer 12.

 The cell of Figure 2, although provided with all its elements excluding an antireflection layer, however, can work properly. To achieve this result, it is necessary to eliminate the highly doped "dead layer" of the surface of the active layer 11, in the separating spaces of the lamellae 111; it is necessary, at the same time, to reduce the thickness of the layer 11; It is also necessary to make the parasitic zone 13 disappear. All this is obtained by applying the method according to the invention which provides for a chemical etching carried out subsequent to the making of said network of the electroconductive lamellae 111, these then constituting a protective mask. for the parts of the surface layer 11 which they overhang.

 The cell having undergone the final chemical treatment is shown in FIG. 3. On the one hand, the layer 11 appears hollowed out between the lamellae 111 and at its periphery -the dead layer is now eliminated-and reduced in thickness in these spaces compared with at the initial thickness, that still present under said lamellae 111. It should be noted that the dead layer remains under the lamellae and that, consequently, the contacts established on highly doped semiconductor beds are of good quality. On the other hand, the parasitic zone 13 has been removed. In the present case of a "BSF" type cell, chemical etching is provided after removal of the two electrodes 111 and 112, and the deposit 112 thus serves as a protective mask for the surface layer 12 that it protects; this layer 12 is only attacked on its edges.

 After the chemical treatment and to finish the cell, only the anti-reflection layer remains. This is not shown in Figure 3.

 A concrete case of implementation of the method according to the invention relates to a silicon solar cell. The plate is of conductivity type P, doped to have a resistivity of 1 to 10 Ωcm; this wafer was cut parallel to a plane <111> of the original crystal. The surface layers 11 and 12, respectively N and P # + type, were obtained by simultaneous diffusion from the vapor phase (phosphine) for the layer 11 and from a silica deposition doped with hydride. boron for layer 12.

 To obtain a layer 12 fully performing its role of photon reflector, this layer must be given a thickness of at least 0.5 μm and a doping level corresponding to a square resistance of 50 R. However, under temperature conditions and of equal time, the diffusion rate of boron is lower than that of phosphorus. Also the layer 11, for a thickness of 0.5 μm of the layer 14 initially reaches 1.1 μm; its surface doping corresponds to a square resistance of 12 Q. Such a thickness of 1.1 pm is too great. It should be reduced to 0.5 pm.

 The electrodes 111 and 112, which are conventionally made of a triple metal layer composed successively from its base, titanium, palladium and these two metals in the form of very thin films (~ 0.1 μm), then silver in greater thickness (- 5 Vm).

 It is then that the wafer is immersed in a potassium hydroxide solution (10 to 30% by volume) in water, which solution is maintained at a temperature of between 80 and 1000 C. platelet, in its regions where silicon is exposed, is attacked; this at a mean speed of between 0.17 and 0.25 # m / min for the silicon of the faces 10A and 10B. On the edge of the wafer, the attack speed is substantially higher (0.5 to 1 pm / min), which allows to obtain the complete stripping of the zone 13 during the two to three minutes that the treatment lasts . On the other hand, the silver remains unscathed as well as, practically, the semiconducting regions which it covers; only slight under-engraving of these regions occurs.

 After the etching treatment, the layer 11, reduced to the desired thickness of 0.5 μm, has a mean square resistance of between 80 and 100 Ω.

du produit VM 1M des valeurs de la tension et du courant pour une puissance fournie maximum, au produit VCO de la tension en circuit ouvert par le courant ICc de court-circuit), de l'ordre de 80%, comparé à celui, 708 environ, de cellules classiques. Cette amélioration du facteur de forme est liée notamment au très faible niveau des courants de fuite.The solar cells obtained by the process of the invention are remarkable by their conversion efficiency, on average 14%, substantially greater than that of 11 to 12% of silicon cells of conventional manufacture. Moreover, these cells are distinguished by the high value of their form factor (ratio

of the product VM 1M values of the voltage and the current for a maximum power supplied, to the product VCO of the open-circuit voltage by the current ICc of short-circuit), of the order of 80%, compared to that, 708 approximately, of conventional cells. This improvement of the form factor is related in particular to the very low level of the leakage currents.

 The method according to the invention is also applicable to solar cells manufactured from N-type silicon wafers in which the active layer 11 is of the P + type and the reflective layer 12 is of the N + type. But then the stripping conditions, particularly the time, may be different, because the active layer is comparatively thinner than the reflective layer after the common diffusion treatment and is there only need to eliminate the only one. dead layer ".

Claims (3)

- Claims  1. A method for producing a solar cell, constructed from a plane wafer (10) made of a semiconductor material of a first conductivity type, this wafer having, in particular, on the side of its anterior face (10A) intended to receive the solar radiation, a surface layer or active layer (11) of the second conductivity type at the base of which is located the motor junction (J) of the cell and on which rests a network of electroconductive lamellae (111) forming a electrodes of the cell, said surface layer being relatively thicker under said lamellae than between their intervals, whereby said surface layer, initially created with a uniform thickness, is then chemically etched so as to reduce the thickness locally, characterized in that said chemical etching is carried out subsequent to the realization of said network of electro lamellae conductive (111), these then constituting a protective mask for the parts of the superficial layer (11) they overhang.  2. A process according to claim 1, characterized in that the semiconductor material of the wafer being silicon and the electroconductive lamellae having, at least superficially, a layer of silver, the etching is carried out in a solution of potassium hydroxide ( 10 to 30% by volume) in water at a temperature between 80 and 1000 C.  3.- silicon solar cell comprising, on a first face, a surface layer or active layer (11) on which rests a network of electroconductive strips (111) made, at least on the surface of silver, said surface layer having said slats a greater thickness than in its other regions, characterized in that it comprises a second surface layer (12) disposed on its second face (10B) opposite to the previous one and covered, except at the edge (14), d an electrode (112) made, at least on the surface, of silver, which surface layer (12) has, beneath said electrode (112), a greater thickness than at the edge (14), as a result of a treatment chemical imposed on this layer according to the method of claim 2, said electrode (112) then serving as a mask.