FR2464565A1 - Laser treatment of semiconductor material - uses low and high power dosages to crystallise or anneal controlled thickness of amorphous structure, grown on non-crystalline substrate - Google Patents

Abstract

THE INVENTION CONCERNS AMORPHIC SILICON SOLAR BATTERIES. IT RELATES TO A HETEROJUNCTION SOLAR BATTERY, OF THE TYPE INCLUDING AN N-TYPE LAYER, AN INTRINSIC LAYER AND A P-TYPE LAYER. ACCORDING TO THE INVENTION, AT LEAST ONE OF THE LAYERS IS FORMED OF AMORPHIC SILICON CARBIDE. THE PENETRATION OF LIGHT IS THUS EASIER IN THE BATTERY. APPLICATION TO PHOTOVOLTAIC SOLAR BATTERIES.

Description

 The present invention relates to structures suitable for the manufacture of solar cells and based on the implementation of heterojunctions formed between amorphous materials having different forbidden bands, for example amorphous silicon and amorphous alloys of silicon and carbon.

Published documents indicate that an improvement in photovoltaic properties can be obtained by using heterostructures. An example is a device in which the front layer (window layer) is formed of a material having a relatively wide band gap and thus relatively transparent to radiation of relatively short wavelengths. Another advantage of this device configuration is the reduction of the saturation reverse current and thus the improvement of the open circuit voltage. Other improvements in the properties of the devices can also be obtained by using a heterojunction formed at the rear junction and creating a reverse field which increases the collection efficiency of the carriers. Another configuration is such as the band gap of one or more several of the layers of the device vary gradually and facilitate the reduction of the detuning at the interface of the junction with, moreover, the formation of a potential gradient in parts of the stack, so that the collection of carriers is further improved.

 Photovoltaic solar cells using amorphous silicon have been described in detail in the literature. Devices use, for example, Schottky barrier or p-i-n junction structures as shown in Figures 1 and 2 of the accompanying drawings which are schematic sections of known devices. In these two cases, a drawback of the batteries is the short diffusion distance which limits the collection of carriers to the regions of the device having an electric field. Another restriction is due to the loss of radiation by absorption in the upper layer p or n in the pin cells, or in the metal layer in the Schottky barrier cells. The layers of the front window of these devices have been produced. in a very thin form, so that these losses are reduced, but the transverse resistance is then high.

 The invention overcomes these drawbacks by using device structures based on heterojunctions of amorphous silicon and of amorphous silicon and carbon alloy. The amorphous silicon carbide # Six, in which x is included between 0 and 1, forms a continuous series of solid solutions having a forbidden band which increases when x decreases, this forbidden band reaching at most 2.6 eV for a value of x of the order of 0.38. This material is thus more transparent to visible and near infrared light than hard amorphous silicon and is therefore very suitable for the formation of a window, when it is suitably doped; it also forms a heterojunction of structure C1 Si a / Sia.

Thus, the invention relates to an amorphous silicon solar cell having a first p-type layer, an intrinsic amorphous silicon layer, and an n-type layer, the p or n type layers being formed of amorphous silicon carbide.

 The invention also relates to an amorphous silicon solar cell, comprising an upper layer of amorphous silicon carbide C1 xSix p-type, an intermediate layer of undoped (intrinsic) amorphous silicon and a lower layer of n-type amorphous silicon, on a suitable substrate.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given with reference to the appended drawings in which, FIGS. 1 and 2 have already been described.
- Figure 3 shows a first heterostructure of solar cell according to the invention; and
- Figures 4 to 7 show structures of solar cells according to the invention.

 The device represented in FIG. 3 and successively comprising a layer of C, xSixa of type p, a layer of intrinsic silicon a and a layer of silicon of type n on a glass, metal or plastic substrate, can be manufactured in the following manner.

 A suitable glass or plastic substrate can be used with a conductive coating applied for example by vacuum deposition, for example by sputtering or by chemical vapor deposition on a metal structure.

 A n-type doped amorphous silicon layer is deposited by glow discharge, at a temperature between 250 and 4000C. An advantageous doping material is phosphorus. Alternatively, arsenic or nitrogen can also be used. The thickness of the layer is preferably less than 0.1 micron. This layer is preferably doped so that its conductivity is at least 10 3 S / cm.

The second layer is formed of undoped amorphous silicon, also deposited by glow discharge at a temperature between 250 and 4000C. An advantageous thickness is between 0.5 and 1 micron.

The top layer is a p-type doped amorphous silicon carbide layer. It is deposited by glow discharge with a mixture of silene, a hydrocarbon such as ethylene, and diborane
B2H6, in a discharge in a plasma at a temperature between 250 and 4000C. Other suitable doping materials, ensuring the formation of p-type material, are also suitable. The operation is intended to form a layer which is conductive and yet more transparent than silicon doped with boron, and also, given the larger forbidden band, a heterostructure is formed which increases the voltage in open circuit.

The gas mixture is preferably chosen so that it gives, in the finished device, a band gap as close as possible to the optimal band gap of the silicon carbide alloy system, while being compatible with effective doping. An advantageous range of composition is such that the parameter x is between 0.8 and 0.95, the thickness of the layer being between 100 and
o 1000 A.

 Another process for manufacturing the upper layer uses a gaseous mixture of a silene and a hydrocarbon with boron trifluoride BF3.

The doping yield is increased by the introduction of fluorine.

 A third possibility is the growth of a layer of non-doped silicon carbide by luminescent discharge and implantation of boron ions in the layer, before thermal or laser annealing.

Alternatively, carbon tetrafluoride CF4 can be used in place of SiR2 for the introduction of fluorine into the system.

 FIG. 5 shows a device in which the layer of silicon carbide is deposited on a transparent conductive layer T formed for example of tin and indium oxide deposited on a glass or plastic substrate. The intrinsic (intrinsic silicon a) and n (n +) layers formed by amorphous silicon are deposited as described above on the silicon carbide layer and a layer M forming a metal electrode is deposited on the n type layer. The device is sensitive to light passing through the transparent substrate.

 This device is advantageous because it allows the growth of the p-type silicon carbide layer at a higher temperature and the finished device is sealed against the atmosphere.

The coating of tin and indium oxide can be used for the formation of a reflective layer.

A suitable plastic for the film is "Kapton".

 A variant of the above device can be produced by crystallization of the deposited layer of p-type silicon carbide by thermal annealing or by laser. In this embodiment, the layer of polycrystalline silicon carbide can be such that the parameter x is of the order of 1 and it can even be pure polycrystalline silicon.

FIG. 6 represents another device for a solar cell of construction similar to that of the device in FIG. 3.

The n-type amorphous silicon layer is replaced by a n-type amorphous silicon carbide layer placed on the metal layer M. In this way, in addition to a certain reverse field, the device has a transparent window which allows the light to be reflected by the rear metal layer M.

 A variant of the device of FIG. 6 is such that the transition between the layer of n-type silicon carbide and the layer of intrinsic silicon has a composition varying progressively so that the junction is less concentrated and that the field is widened.

 In addition, another variant can also be produced by progressive combination of typep silicon carbide and intrinsic amorphous silicon so that the junction is more regular. Similar variants can also be used with the other devices described.

Figure 7 shows another device suitable for forming a solar cell. The lower layer, above the metallization layer, is either formed of p-type amorphous silicon, or formed of p-type amorphous silicon carbide, and is followed by an intermediate layer of intrinsic silicon and then a n-type amorphous silicon layer or it extends into a n-type amorphous silicon carbide layer.

 The addition of carbide to the upper layer allows better penetration and an increase in the reverse field, so that current collection is facilitated.

 Other applications of the invention relate to structures of the C1xSix / Cl ySiy / Cl xSix type, on a glass, metal or plastic substrate, ensuring visible light emission and which may have applications in flat panels. display.

Claims (7)

1. Amorphous silicon solar cell, characterized in that it comprises a first p-type layer, an intrinsic amorphous silicon layer, and an n-type layer, the p or n type layers being formed of amorphous silicon carbide or polycrystalline silicon. 2. Battery according to claim 1, characterized in that the upper layer is formed of p-type amorphous silicon carbide, the intermediate layer is formed of intrinsic amorphous silicon, and the lower layer is formed of n-type amorphous silicon deposited on a metallized substrate of glass or plastic. 3. Amorphous silicon solar cell, characterized in that it comprises a glass or plastic substrate on which is mounted a solar cell structure which comprises a transparent metallic layer, a first layer of p-type amorphous silicon carbide , an intermediate layer of intrinsic amorphous silicon, a third layer of n-type amorphous silicon and a final metallic layer. 4. Battery according to claim 1, characterized in that it comprises a glass or plastic substrate with a metallized layer separating the substrate from a first layer of n-type amorphous silicon carbide, a second layer of amorphous silicon intrinsic and a third layer of p-type amorphous silicon carbide.  5. Battery according to claim 1, characterized in that it comprises a glass or plastic substrate having a metallized layer separating the substrate from a first layer of p-type amorphous silicon or p-type silicon carbide, an intermediate layer of intrinsic amorphous silicon and a third layer of n-type amorphous silicon carbide. 6. Battery according to any one of the preceding claims, characterized in that the transition from doping between two of the layers is gradual and gives a gradient effect. 7. Amorphous silicon solar cell, characterized in that it comprises several layers, at least one of which is formed of amorphous silicon carbide so that light penetrates better through the cell.