Photovoltaic cell and method of producing that cell

Details Photovoltaic cell and method of producing that cell Photovoltaic cell and method of producing that cell

1998-06-30

2001-10-30

Whitehead, Jr., Carl (Department: 2822)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S071000, C438S093000, C438S096000, C438S485000, C257S431000, C257S063000, C257S458000

Reexamination Certificate

active

06309906

ABSTRACT:

The present invention relates on the one hand to a process for the deposition of at least one layer of intrinsic microcrystalline or nanocrystalline hydrogenated silicon on a substrate, this process comprising at least one stage consisting of cleaning the substrate, placing this substrate in a deposition chamber and introducing at least one deposition gas into the said chamber.
It further concerns a device for depositing at least one layer of intrinsic microcrystalline or nanocrystalline hydrogenated silicon on a substrate, said device comprising at least one deposition chamber containing two electrodes and a support to hold said substrate, means for eliminating gases from the chamber, and means for introducing at least one deposition gas into the chamber.
In addition, it concerns a photovoltaic cell consisting of at least one substrate, one layer of transparent conductive oxide, at least one layer of hydrogenated silicon consisting of a sub-layer of positively-doped hydrogenated silicon, a sub-layer of intrinsic hydrogenated silicon and a sub-layer of negatively-doped hydrogenated silicon.
BACKGROUND OF THE INVENTION
In manufacturing photovoltaic cells, in particular, layers of amorphous silicon currently are used as active photovoltaic layers. One problem with such layers is that they degrade when exposed to light. This degradation is known as Straebler-Wronski degradation. The output of solar cells using this material deteriorates quickly.
Experiments using microcrystalline silicon, also called nanocrystalline silicon, in such solar cells nearly always yielded poor results. The high rate of defects in this type of layer effectively prohibits its use, and only a portion of the microcrystalline silicon layer participates in collecting charge conductors. Thus, the resulting photocurrent is weak and the cell is not very useful. These flaws have been attributed to the material itself, which is often considered useless.
Certain studies published in
Appl. Phys. Lett
. 65(7), p. 860, Aug. 15, 1994 entitled “Complete Microcrystalline p-i-n Solar Cell—Crystalline or Amorphous Cell Behavior?” by M. M. Meier, Flückiger, Keppner and Shah, demonstrate an involuntary doping phenomena. Depositing a layer of silicon using a conventional method such as vapor deposition known as “CVD” (Chemical Vapor Deposition) or plasma deposition in the presence of silane results in a slightly negatively-doped layer. This negative doping results in a layer that is of little or no use in a photovoltaic cell.
Experiments have been performed with a deposition method which would eliminate this negative doping. They consist of adding to the deposition gas a gas that produces positive doping to compensate for the involuntary negative doping of the microcrystalline silicon layer. This method is described in the document “IEEE 1994,” p. 409-412, entitled “Intrinsic Microcrystalline Silicon—A Promising New Thin Film Solar Cell Material” by M. M. Meier, Dubail., Flückiger, Fischer, Keppner and Shah. While it produces particularly interesting results, it is difficult to apply in industry. The amount of doping gas to be introduced into the deposition chamber depends upon a certain number of parameters that are difficult to master. These parameters consist of the quantity of desorption of gas in the deposition chamber, the flux speed of the gas, and the speed at which the layer is deposited. This method has demonstrated the feasibility of a layer using this type of material. It also demonstrates the interest that such a layer has for manufacturers of solar cells but, because of difficulties encountered in controlling the deposition parameters, there has been no industrial application of the method and it remains uniquely a laboratory procedure.
SUMMARY OF THE INVENTION
The present invention proposes to overcome these difficulties with an industrial method for depositing a layer of intrinsic microcrystalline or nanocrystalline hydrogenated silicon with very few flaws, as well as a device using the method of the invention. The present invention further proposes the industrial manufacture of components such as solar cells with electrical qualities and stability far superior to similar prior art components. In addition, the invention offers a means of attaining relatively high deposition speed to improve manufacturing methods substantially.
These objects are achieved by a process as defined in the precharacterizing clause and characterized in that a purification stage of at least one of the gases which make up the deposition gas is performed before its introduction into the deposition chamber.
According to the preferred embodiment of the process, the purification stage of at least one of the gases which make up the deposition gas comprises at least one stage for the partial elimination of the molecules including oxygen and contained in this gas.
According to a first embodiment a plasma is formed in the deposition chamber, this plasma being formed by the deposition gas containing the said purified gas.
The plasma Formation step advantageously includes a stage consisting of generating radiofrequencies of frequency f determined in the deposition gas containing the said purified gas.
The said radiofrequencies are advantageously generated at a frequency f of greater than 40 MHz.
According to a second embodiment, the plasma formation stage includes a stage consisting of generating microwaves in the deposition gas containing the said purified gas.
According to a third embodiment, the said layer of intrinsic microcrystalline or nanocrystalline hydrogenated silicon is deposited by chemical vapor deposition in the presence of the deposition gas containing the said purified gas.
According to various embodiments, the deposition gas may contain at least hydrogen and silane, argon and silane or hydrogen, argon and silane.
All the gases making up the deposition gas are advantageously purified.
According to a preferred embodiment the deposition gas contains a proportion of silane of between 0.5% and 15%.
These objects are also achieved by a device as defined in the precharacterizing clause and characterized in that it also comprises a purification means of at least one of the gases which make up the deposition gas. Said means advantageously comprises at least one device which partially eliminates molecules comprising oxygen and contained in said gas.
According to one advantageous embodiment, the device further comprises means for forming a plasma in said deposition chamber said plasma being formed by the deposition gas containing said purified gas.
According to another preferred embodiment, the device includes means of generating a radiofrequency of a frequency f determined within the plasma. Said means preferably generate a frequency higher than 40 MHz.
According to one variation, the device comprises means for generating microwaves in said plasma.
According to various embodiments, the deposition gas may contain at least hydrogen and silane, argon and silane, or hydrogen, argon and silane.
The purification means preferably purify all the gases which constitute the deposition gases.
According to one preferred embodiment, the proportion of silane in the deposition gas ranges from 0.5% to 15%.
The objects of the invention are also achieved by a photovoltaic cell such as that described in the preamble and characterized in that the layer of hydrogenated silicon consists of microcrystalline or nanocrystalline hydrogenated silicon, and further in that the sub-layer of intrinsic microcrystalline or nanocrystalline silicon contains less than 2·10
19
atoms of oxygen per cm
3
.
According to one advantageous embodiment, the photovoltaic cell further comprises a rear contact layer and a rear reflective layer, said rear contact layer forming a contact with the layer of negatively-doped microcrystalline hydrogenated silicon.
According to one preferred embodiment, the photovoltaic cell comprises a layer of amorphous hydrogenated silicon consisting of one sub-layer of positively-doped amorphous hydrogenated silicon, one sub-layer of intrinsic am

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