AU2002358050A1

AU2002358050A1 - Baking oven for photovoltaic devices

Info

Publication number: AU2002358050A1
Application number: AU2002358050A
Authority: AU
Inventors: Amerigo Romagnoli; Mariano Zarcone
Original assignee: Eurosolare SpA
Current assignee: Eni Tecnologie SpA
Priority date: 2001-12-13
Filing date: 2002-11-27
Publication date: 2003-07-09
Anticipated expiration: 2022-11-27
Also published as: ITMI20012628A1; WO2003054975A2; EP1454367A2; WO2003054975A3; CN100356589C; JP2005513406A; JP4511186B2; AU2002358050B2; CN1602554A

Description

WO 03/054975 PCT/EPO2/13372 5 BAKING OVEN FOR PHOTOVOLTAIC DEVICES The present invention relates to a baking oven for photovoltaic devices starting from semiconductor material, 10 characterized in that the entrainment of the semiconductor material inside the oven is effected by means of a series of rolls made of a ceramic material which rotate on their own axis. It is known that for the production of photovoltaic 15 devices, commonly known as solar cells, slices of semicon ductor material of varying thicknesses, are used. The production of these photovoltaic devices involves, among the different types of treatment, various baking pro cesses of the material, whose function is to fix doping 20 elements, dispersed in solutions or in serigraphic pastes, or metallic contacts, onto the semiconductor. In order to effect these baking processes, continuous ovens are used, i.e. ovens in which the material is charged and processed without any interruption. 25 The introduction of the semiconductor material into - I - WO 03/054975 PCT/EPO2/13372 these ovens is typically carried out using a chain which, pulled by two rolls outside the muffle, passes through the oven. The apparatus described above, however, has various 5 disadvantages, of which the most important are: (1) the chain in normally made of a metallic material which can cause contaminations on the part of metallic impurities in the semiconductor material treated;. (2) the temperature profiles at which the semiconductor ma 10 terial is subjected, depend on the transport chain. This generally has a thermal inertia higher than that of the semiconductor material to be baked and consequently when moving, carries with it a considerable quantity of heat. Strongly varying profiles (for example 10-200C/s) often 15 necessary for optimal processes, can only be obtained with great difficulty. Patent application EP 1010960 relates to a baking oven for photovoltaic devices which is based on the non interaction of the entrainment system with the heating sys 20 tem of the oven. In particular, the rolls, which are made of quartz, are moved by means of a chain or belt, not indi vidual, by friction. Operating according to this patent, baking profiles are obtained by means of lamp heating, which can cause 25 problems of stability and consequently uniformity; the en - 2 - WO 03/054975 PCT/EPO2/13372 trainment, moreover, can be subjected to non-uniformity as the movement of the rolls is not individual. Furthermore, the quartz rolls transparent to radiation selected for the heating of the semiconductors, are extremely expensive. 5 It has now been found that it is possible to overcome the above drawbacks by using an oven in which the trans porting of the semiconductor material through said oven, is effected by means of entrainment with ceramic rolls which rotate around their own axis by means of a shaft and belt 10 drive. The use of high purity ceramic material for the pro duction of the rolls drastically reduces metallic contami nation. Furthermore, as the rolls which move the semicon ductor material are fixed in their position in the oven and 15 the movement is transmitted by their rotation on their own axis, there are no temperature entrainment effects. In this way, even very sheer temperature profiles can be obtained. In accordance with this, the objective of the present invention relates to an oven for the preparation of photo 20 voltaic devices from semiconductor material comprising: (a) a housing delimited by curved or flat lateral walls, a top, a bottom and side passages which allow the insertion of rolls; (b) an electric engine which by means of a shaft and belt 25 drive allows the contemporaneous movement of the rolls; - 3 - WO 03/054975 PCT/EPO2/13372 (c) a device for keeping the temperature inside the housing (a) constant; (d) an insulation system of the housing (a); (e) a removal system of the gases formed during the baking 5 process. The housing preferably consists of ceramic material and the distance between one roll and another inside the housing, is such that the semiconductor material, during its rotating movement, rests on at least three of these. 10 The ceramic material used in the manufacturing of the oven is selected from sintered silica, quartz and other ma terial with a poor chemical interaction with the silicon. The semiconductor material which can be used for the preparation of photovoltaic devices is selected from slices 15 of silicon, with thicknesses of 100-500 microns, or semi conductor materials in thin layers (films) which thick nesses varying from a few microns to 100 microns. The heating of the oven can be effected according to the known methods, by the use of electric resistances or 20 infrared lamps. The temperatures inside the oven typically range from 100 0 C to 12000C. A description is provided of a typical embodiment of the invention with reference to figure 1. The embodiment is 25 illustrative and should in no way be considered as limiting - 4 - WO 03/054975 PCT/EPO2/13372 the scope of the invention. Figure 1 illustrates an oven consisting of a certain number of rolls (1) in series made of sintered silica and the contemporaneous movement of all the rolls is due to an 5 electric engine (16) by means of a shaft and belt drive (13, 11). The muffle (6) and its top (5), also made of sin tered silica, are heated by a series of electrically-fed resistances (3). The temperature is controlled with an electronic device to keep it constant. The oven is 10 equipped with a frame (9) which encloses the insulation system (2) of the muffle, and has a removal system of the process fumes (18). (1) Roll (2) Insulation 15 (3) Resistances (4) Resistance support (5) Muffle top (6) Muffle (7) Ball-bearing 20 (8) Bearing support (9) Frame (10) Shaft support (11) Crossed belt (12) Pulley 25 (13) Shaft - 5 - WO 03/054975 PCT/EPO2/13372 (14) Pinion (15) Chain (16) Engine (17) Crown 5 (18) Stack The devices obtained using the oven according to the present invention are free of metallic contaminants and are characterized by an excellent quality, allowing them to be used, for example, for the production of photovoltaic 10 cells. An example is provided hereunder for a better illus tration of the functioning of the apparatus according to the invention. EXAMPLE 1 15 An oven was produced for the diffusion of impurities of the phosphorous type with an entrainment system consist ing of 300 rolls in series, made of sintered silica. The contemporaneous movement of all the rolls is due to an electric engine (16) by means of a shaft and belt drive 20 (13, 11). The muffle (6) made of sintered silica is heated by a series of electrically-fed resistances (3). The temperature is controlled with an electronic de vice for maintaining a constant value. On selecting a set point of 900oC, the variation observed was less than 0.50C. 25 Slices of silicon on which a phosphorous doping agent - 6 - WO 03/054975 PCT/EPO2/13372 had been deposited, were passed into the oven. The passage lasted about twenty minutes. The silicon slices thus obtained had an excellent dif fusion of phosphorous impurities and could be used for the 5 production of high-quality photovoltaic cells. 10 15 20 25 7--

Claims

1. A baking oven for photovoltaic devices starting from semiconductor material characterized in that the en trainment of the semiconductor material inside the 5 oven is effected by means of a series of rolls made of a ceramic material, which rotate on their own axis.

2. The baking oven for photovoltaic devices according to claim 1, comprising: (a) a housing delimited by curved or flat lateral 10 walls, a top, a bottom and side passages which allow the insertion of rolls; (b) an electric engine which, by means of a shaft and belt drive, allows the contemporaneous movement of the rolls; 15 (c) a device for keeping the temperature inside the housing (a) constant; (d) an insulation system of the housing (a); (e) a removal system of the gases formed during the baking process. 20

3. The oven according to claim 2, wherein the housing (a) is a muffle consisting of ceramic material.

4. The oven according to claim 1, wherein the ceramic ma terial is selected from sintered silica, quartz and other material with a low chemical interaction with 25 the silicon. - -- WO 03/054975 PCT/EPO2/13372

5. The oven according to claim 1, wherein the distance between one roll and another is such that the semicon ductor material, in its rotating movement, rests on at least three of these. 5

6. The oven according to claim 1, wherein the heating is effected with electric resistances or with infrared lamps.

7. The oven according to claim 1, wherein the semiconduc tor material which can be used for the preparation of 10 photovoltaic devices is selected from slices of sili con, with thicknesses of 100-500 microns, or semicon ductor materials in thin layers (films) with thick nesses varying from a few microns to 100 microns. 15 20 -9-