Abstract:
A roll-to-roll or reel-to-reel reactor, which is assembled with a heating section, a cooling section, plus a series of modular buffer sections between them, is provided. The length of this apparatus is controlled by adding or reducing numbers of the modular buffer sections on the basis of required reaction time and the delivery speed of a continuous flexible workpiece. The whole reactor contains a heating oven, included inside a main vacuum chamber, which is assembled with a series of modular thermal control components. One or more vacuum chambers can be put outside the main vacuum chamber for the purposes of heat insulation and gas leaking protection. A thermal control component has a segment cross-section shape with some heating elements, thermocouples and cooling elements installed inside. The reactor can also be designed for annealing and/or reaction of a precursor at a surface facing down position. A modular thermal control component is fabricated with a top and a bottom piece to form a rectangle cross-section shape. Some heating modules are arranged on the top and underneath of this thermal control component. The present reactor can be used for annealing and reaction of various precursor films on a flexible substrate under vacuum and in an inert or reactive gas ambient at a temperature ranging from a room temperature to a thousand of Celsius degrees. This reactor is particularly useful in manufacturing Group IB-IIIA-VIA and Group IIB-VIA thin film solar cells such as CIGS and CdTe solar cells on flexible substrates through a continuous roll-to-roll process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a reactor that can be assembled with modular sections to fabricate multiple component semiconductor materials that can be used in thin film solar cells as absorbers or other fields of applications. With a high vacuum design and a precise temperature control, this apparatus is invented for the reaction of flexible substrates through roll-to-roll or reel-to-reel processes. 
       BACKGROUND 
       [0002]    Photovoltaic devices have recently been dramatically developed due to serious global warming and expectation of fossil fuel exhausting in the near future. The traditional photovoltaic devices, or solar cells, are based on silicon semiconductors such as single or multiple crystal silicon materials. However, these silicon materials are expensive, difficult for fabrication and in short supply in semiconductor industry. To overcome these problems, some substitutes such as thin film solar cells have been developed as the second generation of solar cells. At present, there are three main types of second generation thin film solar cells: amorphous silicon, CIGS and CdTe. In this thin film solar cell family, the CIGS solar cells possess the highest conversion efficiency that is as high as 20%, higher than 16% efficiency of the CdTe ones. In the periodic table of the elements, the elements of a CIGS absorber are located in Group IB-IIIA-VIA and the ones of CdTe absorber in Group IIB-VIA. These absorber materials all belong to multi-component p-type semiconductors. For such a semiconductor material, the distribution of different components, stoichiometry and crystal style may determine the quality of the materials. 
         [0003]    Both of CIGS and CdTe solar cells contain a stack of absorber/buffer thin film layers to create an efficient photovoltaic heterojunction. A metal oxide window containing a highly resistive layer, which has a band gap to transmit the sunlight to the absorber/buffer interface, and a lowly resistive layer to minimize the resistive losses and provide electric contacts, is deposited onto the absorber/buffer surface. This kind of design significantly reduces the charge carrier recombination in the window layer and/or in the window/buffer interface because most of the charge carrier generation and separation are localized within the absorber layer. In general, CIGS solar cell is a typical case in Group IB-IIIA-VIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) elements of the periodic table, which are excellent absorber materials for thin film solar cells. In particular, compounds containing Cu, In, Ga, Se and S are generally referred to as CIGS(S), or Cu(In, Ga)(S, Se) 2  or CuIn 1-x Ga x  (S y Se 1-y ) n , where 0≦x≦1, 0≦y≦1 and n is approximately 2, and have already been applied in the solar cell structures that gave rise to conversion efficiencies approaching 20%. Here, Cu(In, Ga)(S, Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. It should be noted that the molar ratios of Ga/(Ga+In) and Cu/(Ga+In) are very important factors to determine the compositions and the conversion efficiencies of the CIGS solar cells. In general, a good solar cell requires a ratio of Cu/(Ga+In) between 0.75 and 0.95, and Ga/(Ga+In) between 0.3 and 0.6. In comparison with CIGS, the composition of a CdTe solar cell is much simple. The content of Cd is usually close to 50% in the CdTe films. However, the Cd content may change after the deposition of a CdS layer and the subsequent annealing procedure. Close to the interface of the p-n-junction, for example, a CdS x Te 1-x  layer is formed with x usually not exceeding 0.06. However, x has a range changing from 0 to 1, which results in a compound from CdTe (x=0) to CdS (x=1). 
         [0004]    Both CIGS and CdTe films has to be annealed to form a uniform stoichiometric compound. A CIGS film is usually annealed at a temperature between 350 and 600° C. in a typical two-stage fabrication procedure. For a CdTe solar cell, a CdS film may firstly be annealed in a superstrate configuration and a CdS/CdTe bilayer may be annealed in a substrate configuration. The importance of annealing is not only for the formation of a stoichiometric semiconductor material, but also for the determination of the crystal and boundary structures that may seriously affect the semiconductor properties. Therefore, the annealing should be well controlled in a carefully designed apparatus. For example, a CIGS film is usually going through a Rapid Thermal Processing (RTP) to approach a high temperature quickly at the beginning, followed by a reaction at the raised temperature in a super-pure inert, H 2 Se, H 2 S or Se atmosphere. After annealing, an n-type semiconductor buffer layer such as CdS, ZnS, or In 2 S 3  should be deposited onto a CIGS semiconductor absorber. After then, transparent conductive oxide (TCO) materials, i.e., ZnO, SnO 2 , and ITO (indium-tin-oxide), should be deposited to form the solar cells. 
         [0005]    The annealing process is sensitive to any impure species. For example, any residue oxygen or water inside a reactor may oxidize a CIGS absorber and destroy this semiconductor material. Therefore, an annealing reactor has to be totally isolated from the outside atmosphere. A vacuum apparatus should be helpful to clean the interior chambers at the beginning through some vacuum-inert gas cycles and remain a good sealing for the equipment during the reactions. However, most of high temperature reactors may not be designed as high vacuum apparatus. For these equipments, lots of inert gas has to be used to remain the annealing under a pure inert atmosphere. Especially for a roll-to-roll or reel-to-reel continuous process, more attention has to be paid to design a qualified reactor. Some high vacuum apparatus has recently been patented and assigned to SoloPower as a roll-to-roll reactor to anneal CIGS absorbers. For example, a few patent applications have been presented to provide methods and a high vacuum reactor to selenize and anneal CIGS absorbers in a roll-to-roll process, as shown in patent applications with publication numbers US2009/0183675A1, US2009/0148598A1 and US2010/0139557A1. With their designs, the reactor comprises a primary gap defined by a peripheral wall and an insert is placed within the gap to process the reaction of a continuously traveling workpiece. This insert processing gap is vacuum-tight and an inert or reactive gas can be introduced during the reaction under a certain temperature controlled by some heating elements surrounding the peripheral wall. 
         [0006]    The main drawback for this reactor is that its integrated design requires a continuous piece of the peripheral wall. When the reaction needs time, the peripheral wall has to be designed very long or the travelling speed of the workpiece has to be very slow. The long peripheral wall may significantly increase the cost and the difficulty of fabrication, transportation and maintenance. In addition, the heating elements surrounding the body also significantly increase the temperature of the working environment. To solve these problems, the present invention provides a vacuum-tight reactor comprised with a series of modular sections. Moreover, the heating elements have been incorporated inside the reactor chamber that may be isolated with a vacuum chamber. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a reactor that can be assembled with some modular sections to fabricate thin film solar cell absorbers in a roll-to-roll or reel-to-reel process. This apparatus features a vacuum tight sealing and a precise temperature control. A flexible substrate is continuously delivered through this reactor that can be assembled to obtain various lengths according to requirements of the reaction time and the substrate delivery speeds. 
         [0008]    This apparatus can be used for annealing materials deposited on the flexible substrates under a vacuum or inert atmosphere and/or carrying out the material reactions in different reaction gases. In particular, it may be used to anneal and/or conduct reaction of semiconductive absorber layers in Group IB-IIIA-VIA and Group IIB-VIA thin film solar cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  shows an apparatus with two modular buffer sections in the center for reaction of a precursor layer coated on a flexible foil substrate to fabricate an absorber layer in a Group IB-IIIA-VIA or IIB-VIA solar cell with a roll-to-roll process. 
           [0010]      FIG. 2  is the cross-section view of a modular buffer section of the apparatus shown in  FIG. 1 . 
           [0011]      FIG. 3  shows a simplified apparatus of  FIG. 1 , here the vacuum chamber below the bottom of the apparatus has been removed. 
           [0012]      FIG. 4  is the cross-section view of a modular section of the simplified apparatus shown in  FIG. 3 . 
           [0013]      FIG. 5  is a 3D illustration of a thermal control component with arrangement of heating elements and thermocouples in a modular buffer section. 
           [0014]      FIG. 6  shows an apparatus with two modular buffer sections in the center and an upside-down precursor layer for its reaction on a flexible foil substrate to fabricate an absorber layer in a Group IB-IIIA-VIA or IIB-VIA solar cell with a roll-to-roll process. 
           [0015]      FIG. 7  is a 3D illustration of a modular thermal control component to anneal or make a reaction of a precursor on an upside-down delivered flexible substrate with arrangement of heating modules containing the heating elements and some thermocouples in a modular buffer section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The apparatus in the present invention can be used for annealing or reaction of a single or multiple layer thin film coating on a flexible foil substrate, which is useful in preparation of semiconductor absorbers in Group IB-IIIA-VIA or Group IIB-VIA thin film solar cells. It can also be employed in the annealing and reaction processes in other applications. In general, such annealing and reactions may be divided into three groups, under vacuum, under an inert gas environment, or in a reaction gas atmosphere. The present invention meets all of these conditions. The apparatus is designed for operation under a high vacuum environment. With several vacuum and inert gas cycles, the whole process can be conducted under a very pure inert environment. If a toxic gas is applied in a reaction, such as H 2 S or H 2 Se in preparation of Group IB-IIIA-VIA thin film solar cells, this apparatus can provide a double protection from leaking. Moreover, a combination of heating elements and thermocouples in the thermal control component can provide a precise temperature for both constant temperature reaction and a rapid thermal process (RTP). The apparatus in the present invention can be manufactured in different widths to adapt to the substrate widths. 
         [0017]      FIG. 1  shows an apparatus with a combination of the roll unwinding chamber plus the heating section S 1 , two modular buffer sections M 1  and M 2  in series, and the cooling section S 2  plus the roll winding chamber. The flexible substrate roll  100  is delivered from the unwinding chamber ( 100 A), through the rollers  101 A and  101 B, to the winding chamber ( 100 B) along the arrow direction with a certain speed. The unwinding chamber is directly connected to the heating section S 1  where the substrate is quickly heated up to a constant annealing or reaction temperature with a RTP process. Then the substrate goes through the annealing/reaction oven that is constituted with a series of thermal control components in modular buffer sections M 1 , M 2  . . . before it passes the cooling section S 2  that is directly connected to the winding chamber. The cooling elements in S 2  may be constituted from some stainless steel tubing loaded with cold water or a cold inert gas. The whole annealing/reaction oven  105  expended from S 1  to S 2  may be fabricated from the materials of graphite, ceramic or quartz crystal. The heating elements  102  and the cooling elements  103  can be installed inside a thermal control component or arranged above and underneath it, combined with the thermocouples  104 . The heating and the cooling elements combined with the thermocouples are densely arranged along this thermal control component to guarantee that the whole precursor film on the flexible substrate  100  is annealed or reacted at a constant temperature ±1° C. 
         [0018]    Before the RTP and the reaction start, the substrate roll  100  is loaded in the reactor. The whole system is then carried out at least three cycles of vacuum-inert gas to remove any impurity from the apparatus. During these cycles, the valves  107 A and  107 B are closed, and  109 A and  109 B are opened. One of the valves  109 A and one of  109 B are used as the vacuum outlets and the rest two as the inert gas inlets. The present apparatus is designed to obtain a high vacuum down to 10 −3  Pa. When the system is ready, the annealing/reaction oven  105  starts to heat the temperature up to a certain degree. During this process, the valve  108 A and/or  108 B are opened to a vacuum system. Then the roll  100 A starts to move along the arrow direction and the gas valve  107 A and  107 B are opened to introduce the inert or reaction gas into  107 A and out of  107 B if the annealing or the reaction is not conducted under a vacuum. The gas can penetrate the hole inlet  110 A and escape out of the hole outlet  110 B before it reaches the cooling section. For an annealing or a reaction of a CIGS precursor layer under an inert gas environment, the gas escaped from  110 B may contain lots of Se vapors. If a formation of the CIGS absorber is required in a H 2 S or H 2 Se atmosphere, the escaped gas is very toxic, especially for the fatal H 2 Se. The reaction gas is firstly controlled inside the reaction chambers. A pair of shutters  111 A and  111 B are installed between the unwinding chamber and the heating section S 1  and between the winding chamber and the cooling section S 2 . During the reaction, they are closed to leave narrow slits for the roll movement. The width of these slits may be set from 1 to 10 mm, preferrably around 2-4 mm. The inert gas with a positive pressure from  109 A and  109 B is introduced through these slits to avoid the toxic gas leaking into the unwinding and winding chambers. The escaped gas from the valve  107 B is finally introduced to a treatment system. The chamber  106  is remained under a vacuum during the whole annealing or reaction process. It has two main functions. On one hand, it is a thermal insulation buffer space to resist too much heat released to the air and stabilize the temperature in the reaction chambers. Since the vacuum space blocks the heat transfer through a heat conduction and a thermal convection but not a thermal radiation, the more vacuum or insulation layers outside the chamber  106  may be necessary to obtain a better heat insulation. On the other hand,  106  is a protection chamber for any possible gas leaking from the main chamber because the leaked gas can be vacuumed and led to the chemical treatment system. 
         [0019]      FIG. 2  shows a cross-section view of a modular buffer section. The flexible substrate  100  is illustrated inside the thermal control component  105  that contains the heating element and the thermocouple combination  201  (bottom) and  202  (top). The chamber  203  represents the reaction chamber and  106  is the vacuum chamber, as shown in  FIG. 1 . 
         [0020]      FIG. 3  stands for a simplified apparatus of the one shown in  FIG. 1 . The main difference is that the part below the annealing/reaction oven of the vacuum chamber  106  has been removed. Furthermore, the oven is more close to the bottom of the apparatus, which requires the bottom of the whole device to be constituted with excellent heat insulation materials. With this modification, the original valves  108 A and  108 B in  FIG. 1  have to be removed to the top of the apparatus, as shown in the valves  208 A and  208 B in  FIG. 3 . This modified device can also be seen in  FIG. 4  from a cross-section view. 
         [0021]      FIG. 5  shows a three-dimension illustration of an examplery thermal control component used in the present invention. It possesses an arc-shaped rather than a flat ceiling. An arc-shaped ceiling has some benefit over a flat one. If this reactor is used for the reaction of a CIGS absorber layer at a temperature ranging from 400 to 600° C., the unreacted Se will be evaporated into the gas atmosphere. If too much Se evaporates, its local concentrations above the substrate may be over-saturated and condensed even at such a high temperature. The over-saturated liquid Se may condense on the flat-shaped ceiling and drop down onto the reacted CIGS absorber surface to damage some areas of the film. If the liquid Se is condensed on an arc-shaped ceiling, however, it may flow down along the arc to the side grooves  504 . On the other hand, a flat-shaped ceiling will result in right-angled corners. The gas stream may be eddy-flowing or back-flowing around these corners, which may be another reason to cause the Se condensation. An arc-shaped ceiling may avoid this problem since it does not have a dead corner. The disadvantage of this arc-shaped ceiling is that it may generate non-uniform temperature distribution for the cross-section segment area because the distance from the center is different from the sides. Since the main heat transfer mechanisms are the heat conduction and the thermal convection in a gas atmosphere, however, the thermal radiation non-uniformity arising from the different distance may be ignored if this arc is not too sharp. 
         [0022]    In  FIG. 5 , the heating elements are represented with  501  that consists of a top piece  501 A and a bottom piece  501 B. Similarly, the thermocouple  502  is constituted from a top piece  502 A and a bottom piece  502 B. Every pair of a heating element and a thermocouple can be independently controlled to adjust the local temperature. A pair of grooves  503 A and  503 B are cut in to tightly hold the teeth from the matched thermal control component in another modular section. The distance between the substrate and the center of the ceiling may be designed from 2 to 10 mm, preferably 4-7 mm to hold sufficient Se vapor pressure inside the thermal control component, if a substrate moving speed is 1 meter per minute. 
         [0023]      FIG. 6  illustrates another different design of the reactor. In this design, the precursor layer on a flexible substrate has been turned to an upside-down position. Since most of the flexible substrates going through a high temperature reaction are made from metals, they can support a large tension for the substrate delivery. Therefore, it should not have a significant gravity sag in the center of the substrate if it transports over 10 meters without any support between the unwinding and the winding chambers. As a result, a long substrate roll can be delivered through the entire oven combined with a series of thermal control components in different modular sections without touching the bottoms. 
         [0024]    As shown in  FIG. 6 , the substrate roll  100  starts from  100 A in the unwinding chamber connected with the heating section S 1 , passes the roller  601 A and goes through the annealing/reaction oven that consists of several pieces of thermal control components, then passes the roller  601 B, and ends as roll  100 B in the winding chamber connected with the cooling section S 2 . The other structure and mechanisms are similar to the apparatus shown in  FIG. 1 , except for some modifications for the upside-down requirements and the thermal control components. In these modifications, the gas inlet  107 A and outlet  107 B in  FIG. 1  are turned down as  607 A and  607 B in  FIG. 6 , and the gas holes  110 A and  110 B in the annealing/reaction oven are opened in the bottom side of the oven as  610 A and  610 B as well. Moreover, the inlet  108 A and the outlet  108 B of the vacuum chamber  106  in  FIG. 1  are turned up as  608 A and  608 B in the vacuum chamber  606  of  FIG. 6 . Two pairs of shutters  611 A and  611 B possess the same function as their analogues  111 A and  111 B in the  FIG. 1  device. The functions of the valves  609 A and  609 B in this figure are also the same as their analogues  109 A and  109 B in  FIG. 1 . The whole operation procedure for this apparatus is similar to the one shown in  FIG. 1 , as described above. 
         [0025]    The main difference in this apparatus shown in  FIG. 6  is its thermal control component  605 . The heating elements and the thermocouples are not arranged inside the oven material but above and underneath it. The oven shape is changed from a segment style to a rectangle shape with round internal corners. Some heating modules  602 A and  602 B that contain heating elements and thermocouples to independently control temperatures are arranged closely above and underneath the rectangle shaped oven materials, as shown in  FIG. 7  for its details. The cooling section is still the same as the one shown in  FIG. 1 . It contains the thermocouples  603  and the cooling elements  604 . 
         [0026]    As shown in  FIG. 7 , this rectangle thermal control component contains an upper piece  605 A and a bottom piece  605 B, which makes it much easy to fabricate with a low cost. Between the contact areas of  605 A and  605 B, pairs of teeth and dent cut are made along the whole sides, respectively, in  605 B and  605 A, as  701  marked. This prevents the gas inside the thermal control components from escaping out of the oven. Similarly, the same structures (not shown in  FIG. 7 ) are fabricated on the end sides of every modular thermal control component to remain the gas inside the oven as more as possible.  FIG. 7  also shows some upper heating modules  602 A and some bottom heating modules  602 B. These modules contain the heating elements and the thermocouples. Every one of them can independently control the temperatures. If the oven materials are well heat conductive materials such as graphites and conductive ceramics, these heating modules can control the oven temperature very well. Because these thermal control components are working in corrosive atmosphere, the material to make these heating modules shall be corrosive resistant, such as stainless steel, molybdenum, titanium or ceramics. If they are made from stainless steel, it is suggested to coat Mo on the surfaces to prevent any possibility of iron release at a high temperature. The round internal corners of this thermal control component are designed to reduce the eddy gas flowing, as shown in  FIG. 7 . The width of the reaction oven depends on the web width, and the internal height of the oven is between 10 and 80 mm, preferably 30-50 mm. 
         [0027]    There are some advantages for this upside-down design. If this reactor is used to prepare CIGS absorbers under an inert gas atmosphere and excessive amounts of Se is pre-coated on the top of the precursor film, for example, lots of Se will be evaporated from the top during a high temperature heating since the Se evaporation has an opposite direction from the Se thermal diffusion into the CIGS precursor. An upside-down design can significantly reduce the Se evaporation because directions of the Se evaporation and its thermal diffusion into the precursor are the same. As a result, less excessive Se consumption shall be expected, and a better process control and a more stable reaction shall be predicted since less Se is remained inside the inert gas. If this reaction needs to be carried out under a H 2 S or H 2 Se gas, this upside-down design should be benefit for the gas diffusion into the CIGS precursor coating. Another advantage for this upside-down arrangement is that the over-saturated Se vapor is not easy to condense on a moving substrate. Any possibly condensed Se liquid can only drop down to the back side of the substrate but not damage the absorber material. 
       EXAMPLE 1 
     Post-Treatment of a CdTe Film in CdCl 2 :Ar:O 2  Atmosphere 
       [0028]    A CdTe solar cell deposited on a flexible substrate can only be a substrate configuration. In this example, a stainless steel roll coated with Mo may be used as a substrate and CdTe and CdS layers can be deposited with the methods of a vacuum evaporation and a chemical bath deposition (CBD), respectively. After CdTe and CdS deposition, a CdCl 2  anneal is usually required to increase the solar cell efficiency because the anneal increase the open circuit voltage V oc  and the fill factor FF. Several mechanisms for these improvements have been proposed, such as the elimination of fast-recombination centers in the CdTe film, reduction of recombination centers in the junction, and the elimination of small grains at the grain boundaries. 
         [0029]    An apparatus shown in  FIG. 1  with 10 meter long oven, assembled with 10 pieces of 1 meter long modular buffer sections, may be used o anneal CdTe absorber/CdS window layers. The roll coated with CdTe/CdS may be loaded and the whole equipment may be vacuumed down to a level of 10 −3  Pa and remained for about 15 minutes with the valves  107 A and  107 B closed. Then turn off the vacuum pumps and close one of the vacuum valves  109 A and  109 B, followed by filling Ar to about 10 5  Pa from one of the gas valves  109 A and  109 B. Repeat this cycle twice before heating the oven up to 420° C. Then the substrate may be delivered at a speed of 1 meter per minute. Meanwhile, a gas mixture of CdCl 2 :Ar:O 2  may be introduced from the valve  107 A with the partial pressures p(CdCl 2 ) from 0.65 to 1.2 Pa and p(O 2 ) from 1.5×10 3  to 2×10 4  Pa. The valve  107 B may be partially turned on to remain a gental gas flowing. The escaped gases should be led to a well sealed chemical treatment system. During annealing, one of the valves  108 A and  108 B is opened to remain the chamber  106  under vacuum. After the annealing completes, the system should be vacuumed again to remove the residue gases before opening the chambers. 
       EXAMPLE 2 
     Selenization of a CIG Precursor Layer in H 2 Se/H 2 S Gas Atmosphere to Form CIGS Absorber 
       [0030]    In a CIGS thin film solar cell, formation of the CIGS absorber may need several steps. In a traditional method, a CIG precursor was generated firstly through some different methods, vacuum or non-vacuum, then the CIG precursor was selenized under H 2 Se or H 2 Se/H 2 S atmosphere at an enhanced temperature to form a stoichiometric CIGS absorber. Because H 2 Se is a kind of extremely toxic gas, it has been much less used today. However, it is still used sometimes because this gas-solid selenization reaction may generate a better quality CIGS absorber. 
         [0031]    Due to its high toxicity, H 2 Se is hard to prepare and store. Therefore, a H 2 Se gas cylinder is very expensive. A more economical H 2 Se gas source may be from an in-situ generation. For example, the solid Se or S can be reduced by H 2  at an enhanced temperature to generate H 2 Se and H 2 S gases which can be introduced into the present apparatus for a selenization reaction. This reactor is not shown in the current invention. In this example, a flexible substrate roll coated with CIG precursor layer is loaded into an apparatus described in Example 1 and the vacuum-inert gas cycles are applied as described in Example 1. During the vacuum-inert gas cycles, a longer time, i.e., 20-30 minutes, to remain the system under a high vacuum may be necessary. Fill the working chambers with ultra-pure Ar. The oven temperature may be set between 500 and 550° C. After the oven temperature becomes stable, the substrate may be delivered at a speed of 1 meter per minute. Make sure one of the valves  108  to be opened to vacuum and the possible residues are delivered to a sealed chemical treatment system. Make sure enough H 2 Se and H 2 S gas sensors are installed around the working space. The shutters  111 A and  111 B may be closed to leave a narrow slit between 2 and 4 mm and some Ar from one of the valves  109 A and  109 B is gentally released into the oven through these two shutter slits. The reaction gas, H 2 Se or H 2 Se/H 2 S mixture, may be introduced into the oven through the valve  107 A at a pressure about 10 3  Pa. This gas may be diluted with about 5% Ar flowing from the slit  111 A. The valve  107 B should be opened and the escaped toxic gas should be delivered into the sealed chemical treatment system. After the reaction completes, the gas supply is stopped and the valve  107 A is turned off After the heating is stopped, the inert gas should be purged and the vacuum-inert gas cycle should be applied again to remove the residue gas before the chambers are opened. 
         [0032]    If the reaction has to be carried out in two different gas atmospheres, i.e., H 2 Se and H 2 S, the modular buffer sections can be modified with a gas outlet followed by a gas inlet for different gas ambient. Another gas outlet may be opened in this transition zone between these gas outlet and inlet to remove the mixed gases. 
       EXAMPLE 3 
     Selenization of a CIGS Precursor Layer Under an Inert Gas Ambient 
       [0033]    As described in Example 2, the selenization under a H 2 Se ambient is not common today. Commonly, the selenization is carried out in two ways. On one hand, Cu, Ga, In and Se are co-deposited and selenized, or Se is continuously supplied during a vacuum deposition of Cu, In and Ga. The selenization may have been completed during these processes. If not, the CIGS precursors may need anneal in a reactor as shown in the present invention. On the other hand, a CIGS precursor film may be constituted from multiple layers of Cu, Ga, In and Se. The different elements are not reacted during their depositions. Under this circumstance, the CIGS precursor must be reacted and selenized to form a high quality stoichiometric CIGS absorber. The present invention is well suitable for this application. 
         [0034]    The apparatus shown in  FIG. 1  and  FIG. 3  are both good for the selenization reaction. Inside these equipments, the precursor layer faces up. The Se amount in these precursors should be excessive because some Se will evaporate during the reaction at an enhanced temperature, especially if it is deposited on the top. During the selenization reaction, partial Se thermally diffuses down to mix with the other elements and partial Se evaporates up into the ambient. In order to reduce the Se evaporation, the apparatus shown in  FIG. 6  is employed as the reactor in this example. 
         [0035]    The substrate roll  100 A shall be loaded upside down in the unwinding chamber. This roll  100  is delivered over the roller  601 A, passing the heating section S 1 , going through the oven that consists of 10 modular buffer sections from M 1  to M 10 , cooled down in the cooling section S 2 , adjusted the position over the roller  601 B, and finally ending as  100 B inside the winding chamber. At least three vacuum-inert gas cycles are performed before heating up. During vacuuming, the vacuum level should be down to 10 −3  Pa and remained at this level for at least 20 minutes. The moving speed of the substrate may be still selected at 1 meter per minute. The oven can be heated up to 500-550° C. The temperature can be controlled precisely at ±1° C. with the heating modules. The reaction may be carried out in ultra-pure Ar or N 2  ambient. The gas may be delivered through the valve  607 A and diffuse into the oven through the holes  610 A, and then escape out of the hole  610 B and the valve  607 B. During the reaction, more and more Se may be evaporated into the atmosphere. Therefore, the escaped gas may be led to a Se recovery device or a treatment system. Because the precursor faces down, there is no condensed Se dropping onto the CIGS absorber. 
         [0036]    As described above, this apparatus can be extensively used to anneal and/or make the reaction of Group IB-IIIA-VIA or Group IIB-VIA solar cell absorbers on the flexible continuous substrates with different widths. It can precisely control the temperatures to obtain very uniform temperature distribution on the reacted precursors. It is vacuum tight to reach a high vacuum. It can make the reactions under different ambient, i.e., vacuum, inert gas or toxic gas.