Patent Publication Number: US-2007116893-A1

Title: Low-hydrogen photovoltaic cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional application of co-pending application Ser. No. 11/282,934 filed on Nov. 18, 2005 [attorney docket no. 2606.002], the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to methods and apparatus for exposing a material or work piece to a vaporous element. Specifically, the present invention provides methods and apparatus for treating a photovoltaic precursor with a vaporous element, for example, selenium or sulfur, to produce thin film CIGS or CIGSS solar cells.  
     BACKGROUND OF THE INVENTION  
      The limited availability of and environmental concerns about fossil fuels make them increasingly less attractive as a means to produce electricity. As a result of this trend, alternative energy sources, particularly solar energy, are becoming more popular. While solar energy is a reliable and dependable energy source, the costs associated with solar energy production have traditionally limited its availability and desirability as a substitute for fossil fuels. However, recent technological advances in solar cell manufacturing show promise to lower the cost of solar energy.  
      Solar energy proponents and researchers state that higher solar cell efficiency and lower production costs are two ways to reduce the overall cost of solar energy. In particular, solar cells with absorber materials comprised of copper, indium, gallium, and selenium and/or sulfur [hereinafter Cu(In,Ga)(S,Se) 2  or CIGS] show promise in higher efficiency, lower production costs, and long operational lifetimes. These absorber materials are the result of innovative thin-film manufacturing technologies that further reduce manufacturing costs by lowering raw material costs and increasing throughput and efficiencies.  
      As is commonly practiced in the art, these CIGS cells are manufactured in either a one-stage thermal co-evaporation process or a two-stage process. The single stage thermal co-evaporation process consists of depositing all of the CIGS elements onto a substrate and simultaneously heating that substrate temperature to approximately 450° C. to 600° C. to allow the constituent materials to form a crystal matrix in the absorber.  
      Although the one-step co-evaporation process is of interest to CIGS manufacturing, the two-step process may be more manufacturable and poses unique challenges of its own. In the first step of the two-step process, a material is deposited upon a substrate. The material deposited on the substrate is referred to as the “precursor.” The precursor may comprise one or more of copper (Cu), indium (In), gallium (Ga), and/or selenium (Se) and/or sulfur (Se). Usually the precursor is a mixture of copper, indium, and gallium. In the second step of the two-stage process of CIGS manufacturing, selenium or sulfur is introduced into the precursor by a process known in the art as “selenization.” Selenization typically includes heating the precursor in a selenium-rich (or sulfur-rich) environment until the elements react to make a crystal matrix to form the chalcopyrite CIGS material that becomes known as the “absorber.” Common sources of selenium or sulfur in CIGS manufacturing include vaporizing powdered selenium or sulfur, hydrogen selenide, hydrogen sulfide, or organic compounds of selenium or sulfur with low evaporation points. This process has been accepted by researchers in solar cell manufacturing methods as an acceptable means of introducing selenium or sulfur into the absorber material; however, this technique also poses substantial risks and costs. Further, as some researchers may blend certain elements of the one-stage and two-stage process, these challenges may apply to the one-stage process as well.  
      Selenization is usually practiced by two methods. In one prior art method, selenium pellets are placed in a receptacle, or “boat,” in a chamber and then the selenium and precursor are heated to release a selenium-containing vapor which interacts with the precursor. In the other prior art method the treatment chamber is filled with selenium or sulfur vapor or with hydrogen selenide (H 2 Se) or hydrogen sulfide (H 2 S) gas. Sometimes a process will involve placing hydrogen (H 2 ) gas in the treatment chamber while heating the Se or S pellets to form H 2 Se or H 2 S in situ. These two methods are essentially the only methods of selenizing photovoltaic precursors.  
      Due to the nature of the chemical reactions, an excess amount of Se or an over-pressure of Se is desirable during the selenization process. An excess of Se is typically necessary since the reaction of the Cu, In, Ga, and Se tends to “push” at least some of the Se out of the precursor at elevated temperature. Therefore, it is believed that, without excess Se present, any deposited Se will tend to evaporate out of the precursor matrix and not bind to the matrix as desired. Aspects of the present invention overcome this barrier by providing sufficient Se to minimize the escape of Se from, for example, the Cu—In—Ga matrix.  
      With regards to thermal co-evaporation, some prior art co-evaporation processes “hint” that selenization may be used to “fix” a film that might not be quite right stoichiometrically. That is, after co-evaporation, the precursor may lack sufficient Se whereby further Se addition is required to provide the desired stoichiometric quantity of Se. This further selenization is typically practiced by one of the methods discussed above.  
      Current CIGS manufacturing techniques also have serious health and environmental implications. As discussed below, various manufacturing techniques have been used to introduce selenium or sulfur into the absorber material matrix with varying success. Although some manufacturing methods are more reliable, the health or environmental concerns, especially in large-scale production volumes, make them undesirable for long-term use. More specifically, the use of the highly toxic hydrogen selenide and its derivatives is expensive because of needed safety precautions. While CIGS solar cells show great promise in solar cell manufacturing to reduce raw material costs, safe, reliable, and repeatable methods to introduce selenium or sulfur into the matrix are needed.  
      Prior art also suggests that CIGS solar cells produced by selenization processes have performance problems that may be unique to the manufacturing method. Recent studies by P. K. Johnson and A. E. Delahoy showed that solar cells produced by selenization had higher defect densities, “light-inhibited” degradation of cell efficiency of up to 97%, and a 13% reduction in Voc×FF over a 30 to 45 day period. In contrast, solar cells produced by thermal co-evaporation showed lower defect densities, lower cell efficiency reduction, and less than a 2% reduction in Voc×FF over a 30 to 45 day period. The key distinguishing feature of most selenization processes and thermal co-evaporation is that selenization usually uses a hydrogen-containing species, H 2 Se. Although some of the decreased product performance of selenized solar cells is due to encapsulation method of the module and migration of sodium from the soda lime substrate into the absorber matrix, a good portion of the discrepancies in cell performance have to do with manufacturing method. While the enhanced product performance factors make thermal co-evaporation more desirable, selenization process methods are more suited to manufacturing high efficiency cells on large area substrates.  
      Additionally, H 2 Se is incompatible with stainless steel and other metals that have the potential to replace soda lime glass as a substrate material. This distinction is increasingly important as solar cell manufacturers look to lower manufacturing costs while increasing the number of form factors available for “finished” solar cell devices. Thus, in addition to the safety and environmental concerns, a solar cell manufacturing method that comprises 1) the low hydrogen advantages of thermal co-evaporation on long term cell performance, 2) the manufacturing capability high efficiency solar cells on large area substrates, and 3) compatibility with stainless steel and other metals is also needed.  
     DESCRIPTION OF THE RELATED ART  
      Within the art of CIGS manufacturing, the selenization process is often completed in a chamber. These chambers are either rectangular, square, or round and may or may not have shelves. An exemplary embodiment of a typical chamber is disclosed in U.S. Pat. No. 6,787,485 by Probst [herein “Probst”] and a typical selenization method is disclosed in U.S. Pat. No. 5,045,409 by Eberspacher, et al. [herein “Eberspacher”]. In particular, Probst discloses a “stack oven” with an adjustable gas atmosphere capable of operating in vacuum. Additionally, the Probst apparatus comprises heating elements and shelves that contain the process items in an arrangement that interleaves the process items and the energy sources, with at least one energy source per process item. The heating sources are arranged in a quartz glass envelope, with a liquid or gas coolant flowing through the envelope. Probst focuses on thermal uniformity of the substrates, however, unlike aspects of the present invention, Probst does not disclose 1) a high utilization rate of selenium of at least 90%, 2) a solid source of the processing vapor, 3) an enhanced means to control delivery of selenium or sulfur vapor, 4) a condenser/evaporator to deliver vapor from a solid source, 5) a vapor tight inner chamber space, nor 6) an independent thermal control of the substrates, a condenser/evaporator, and the walls of the chamber. Aspects of the present invention may provide one or more of these advantages over Probst.  
      Eberspacher discloses a method to form a CIS or CIGS film without using H 2 Se, which is a highly toxic gas and is thus unsuitable for large scale manufacturing because of safety concerns. In the disclosed method of Eberspacher, a mixture of copper and indium or copper, indium, and gallium are deposited on a substrate by sputtering. A selenium film is then deposited by thermal evaporation. The substrate is then heated in the presence of hydrogen, H 2 Se, or H 2 S to form the crystalline matrix for the solar cell absorber material. While the inventor&#39;s aim was to totally eliminate the use of H 2 Se, the inventor admits in the specification that a low concentration of H 2 Se is needed to improve cell performance. Eberspacher further discloses a “conventional thermal evaporation method” which takes place in an oven with a gas inlet and outlet, but, unlike aspects of the present invention, does not disclose 1) a solid source of the processing vapor, 2) a high utilization rate of selenium of at least 90%, 3) a means to control delivery of selenium or sulfur vapor, 4) a condenser/evaporator to deliver vapor from a solid source, 5) a vapor tight inner chamber space, nor 6) an independent thermal control of the substrates, a condenser/evaporator, and the walls of the chamber. Aspects of the present invention may provide one or more of these advantages over Eberspacher.  
      U.S. Pat. Nos. 6,092,669 and 6,048,442 Kushiya, et al. [collectively herein “Kushiya”] discloses a method and apparatus for producing a thin-film solar cell. Specifically, Kushiya discusses processing the solar cells by heating them in an atmosphere of selenium or sulfur. According to Kushiya, the substrates are heated in an electric furnace with an undisclosed reactive gas at a temperature not higher than 600° C. Unlike aspects of the present invention, Kushiya does not disclose 1) a solid source of the processing vapor, 2) a high utilization rate of selenium of at least 90%, 3) a means to control delivery of selenium or sulfur vapor, 4) a condenser/evaporator to deliver vapor from a solid source, 5) a vapor tight inner chamber space, nor 5) an independent thermal control of the substrates, a condenser/evaporator, and the walls of the chamber. Aspects of the present invention may provide one or more of these advantages over Kushiya.  
      U.S. Pat. No. 6,518,086 of Beck, et al. [herein “Beck”] discloses a two-stage process to produce a CIGS or CIGSS film on a substrate for semiconductor applications. While referencing the prior art, Beck discusses the exemplary inventions of the two best known methods of selenization: (1) vapor deposition of the constituent elements followed by heating (see U.S. Pat. No. 5,141,564 issued to Chen, et al. (herein “Chen”)) and (2) a two-stage process wherein selenium or sulfur is added to the absorber crystal matrix by heating copper indium alloys with H 2 Se or Se vapor (see U.S. Pat. No. 4,798,660 issued to Ermer, et al. (herein “Ermer”) and U.S. Pat. No. 4,915,745 issued to Pollock, et al.) Beck distinguishes the first CIGS process of the Chen species as undesirable for industrial scale manufacturing because of high temperatures to form the absorber matrix. Beck further distinguishes the second selenization process of the Ermer species as undesirable because of the use of highly toxic H 2 Se, low selenium utilization, and poor adhesion to molybdenum coated substrates.  
      Beck discloses depositing a precursor layer of copper, indium, gallium, selenium, or sulfur in some combination to a substrate. These substrates are then heated in either an inert atmosphere comprising argon, xenon, helium, or nitrogen or under a selenium or sulfur vapor. The selenium vapor can come from evaporating selenium from a “boat” inside the chamber, H 2 Se, or diethylselenide. Similar to recognized methods of selenization, Beck selenizes its precursor by heating the selenium-containing boat and precursor in the treatment chamber to produce the Se vapor and holds the boat and precursor at temperature until selenization is complete. Contrary to aspect of the present invention, Beck does not disclose 1) a high utilization rate of selenium of at least 90%, 2) a means to control delivery of selenium or sulfur vapor, 3) a condenser/evaporator to deliver vapor from a solid source, 4) a vapor tight inner chamber space, nor 5) an independent thermal control of the substrates, a condenser/evaporator, and the walls of the chamber. Aspects of the present invention may provide one or more of these advantages over Beck.  
      While the disclosed prior art is not exhaustive, it is representative of what is known and practiced for selenization. In particular, in contrast with aspects of the present invention, prior art vacuum chamber apparatuses and treatment methods do not disclose 1) an independent control of substrate temperature, 2) a high utilization rate of selenium of at least 90%, 3) high throughput capability with enhanced thermal management, 4) controlled release and capture of selenium to the same place repeatedly, 5) independent control of the vapor pressure delivery of sulfur or selenium, 6) vacuum compatible selenium delivery and temperature control, 7) distinct temperature zones and valves to allow use of traditional elastomer seals and vacuum gauges, and 8) future process automation upgrade capability. Aspects of the present invention provide these other advantages and benefits not found in the prior art.  
     SUMMARY OF ASPECTS OF THE INVENTION  
      The present invention provides methods and apparatus for treating materials with vaporous elements and compounds that enhances the versatility and adaptability of the treatment process. Though aspects of the invention may be utilized in the manufacture and processing of photovoltaic material, aspects of the invention are not limited to processing photovoltaic material, but can be applied to the treatment of any material where the control and regulation of treatment temperature impacts the cost, quality, or performance of the product produced.  
      One aspect of the invention is a method for treating a work piece, for example, a CIG photovoltaic precursor, with one or more vaporous element. The method includes introducing the work piece and an element-containing material to an enclosure; heating the work piece to a first temperature; independent of the heating of the work piece, heating the element-containing material to a temperature sufficient to volatilize the element and release an element-containing vapor into the enclosure; reacting at least some of the element-containing vapor with the work piece; regulating the temperature of the element-containing material at a temperature sufficient to condense at least some of the element from the element-containing vapor on the element-containing material; and cooling the work piece to provide an element-treated work piece. In one aspect, the element comprises elemental sulfur or selenium or combinations of sulfur, selenium, tellurium, indium, gallium, or sodium. In another aspect, cooling the element-containing material may comprise cooling the element-containing material wherein substantially all of the unreacted element-containing vapor condenses on the element-containing material.  
      Another aspect of the invention is an apparatus for treating a work piece with a vaporous element, the apparatus including an enclosure; means for supporting the work piece in the enclosure; means for varying the temperature of the work piece; an element-containing material in the enclosure; means for varying the temperature of the element-containing material to produce an element-containing vapor, the means for varying the temperature of the element-containing material being independent of the means for varying the temperature of the work piece; and means for exposing at least some of the work piece to the element-containing vapor. In one aspect, the enclosure comprises an inner enclosure, and wherein the apparatus further comprises an outer enclosure enclosing the inner enclosure.  
      Another aspect of the invention is a method for preparing a treatment chamber for treating a work piece with a volatilizable element, the method including introducing a solid element-containing material to the treatment chamber, the treatment chamber comprising an internal cavity; heating the solid element-containing material to volatilize the element and produce an element-containing vapor in the internal cavity; regulating the temperature of a surface exposed to the internal cavity to a temperature at which the element-containing vapor condenses; and condensing at least some of the volatilized element from the element-containing vapor onto the surface. In one aspect, the method further includes regulating the temperature of the solid element-containing material to a temperature below the volatilization temperature of the element, for example, whereby at least some of the element from the element-containing vapor condenses onto the solid element-containing material.  
      A still further aspect of the invention is a treatment chamber isolation apparatus, the treatment chamber having an opening, the isolation apparatus including a sealing assembly having a support structure; at least one cover plate adapted to engage the treatment chamber opening; a plurality of rods having a first end adapted to engage the support structure and a second end adapted to engage the at least one cover plate; and means for compressing the sealing assembly against the treatment chamber wherein the at least one cover plate engages the treatment chamber opening to provide at least some isolation of the treatment chamber. In one aspect, the first ends of the plurality of rods are resiliently mounted to the support structure, for example, by means of springs and/or flexures.  
      Another aspect of the invention is a treatment chamber isolation apparatus, the treatment chamber having a cylindrical enclosure having an open first end, a closed second end, and an internal flange mounted between the first end and the second end, the apparatus including a sealing plate adapted to engage the internal flange; a support rod having a first end and a second end mounted to the sealing plate; a plate mounted to the second end of the support rod; and at least one actuator adapted to displace the plate whereby the sealing plate engages the internal flange of the cylindrical enclosure and substantially isolates at least part of the cylindrical enclosure. In one aspect, the treatment chamber isolation apparatus may further comprise a cylindrical extension mounted to the open first end of the cylindrical enclosure.  
      Another aspect of the invention is a material delivery device including a cylindrical body having at least one outer surface; a volatilizable material applied to the at least one outer surface; means for varying the temperature of the at least one outer surface to regulate the volatilization of the volatilizable material. In one aspect, the means for varying the temperature comprises a heat exchanger having a working fluid passing through it.  
      Another aspect of the invention is a method of delivering a volatile material including providing a cylindrical body having at least one outer surface; applying a volatilizable material to the at least one outer surface; and regulating the temperature of the at least one outer surface to vary the amount of material volatilized. In one aspect, applying a volatilizable material to at least one outer surface of the cylindrical body may comprise exposing the at least one outer surface to a vapor containing a volatilized material; and cooling the at least one outer surface to condense at least some of the volatilized material from the vapor on to the at least one outer surface. In one aspect, regulating the heating of the at least one surface may comprise regulating the flow and/or temperature of a coolant though a passage in the cylindrical body.  
      A further aspect of the invention is a treatment chamber valve actuation device including an actuation plate; at least one actuation rod mounted to the actuation plate, the at least one actuation rod adapted to penetrate a wall of the chamber and engage a valve mechanism within the chamber; an actuator adapted to displace the actuation plate wherein the plurality of actuation rods are displaced and actuate the valve mechanism; and at least one flexible plate mounted between the actuation plate and the treatment chamber. In one aspect, the at least one flexible plate may comprise a plurality of flexures mounted between the actuation plate and the treatment furnace.  
      A still further aspect of the invention is a CIGS photovoltaic cell having a low-hydrogen content or substantially no hydrogen content. In one aspect, the photovoltaic cell may comprise a substrate; and an absorber deposited on to the substrate, the absorber comprising copper, indium, gallium, and less than 5% hydrogen. In one aspect, the absorber contains less than  1 % hydrogen, or is even hydrogen free. In another aspect, the substrate may be a metallic substrate, for example, a steel, stainless steel, or titanium substrate.  
      These and further aspects of the invention are illustrated in described with respect to the attached figures. 
    
    
     BRIEF DESCRIPTION OF FIGURES  
      The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention will be readily understood from the following detailed description of aspects of the invention taken in conjunction with the accompanying figures in which:  
       FIG. 1  is a schematic block diagram of a process for treating a work piece according to one aspect of the invention.  
       FIG. 2  is a plot of a heating schedule for the treated work piece and the treatment material according to one aspect of the invention.  
       FIG. 3  is a perspective view of a treatment furnace according to another aspect of the invention.  
       FIG. 4  is a front elevation view of the furnace shown in  FIG. 3 .  
       FIG. 5  is a right side elevation view of the furnace shown in  FIG. 3 .  
       FIG. 6  is a left side elevation view of the furnace shown in  FIG. 3 .  
       FIG. 7  is a rear elevation view of the furnace shown in  FIG. 3 .  
       FIG. 8  is a top plan view of the furnace shown in  FIG. 3 .  
       FIG. 9  is a right side elevation view of the furnace shown in  FIG. 3  with the right side door removed  
       FIG. 10  is a detailed side elevation view of a tube assembly as shown as Detail  10  in  FIG. 9 .  
       FIG. 11  is a detailed side elevation view of a treatment chamber isolation mechanism shown as Detail  11  in  FIG. 10 .  
       FIG. 12A  is a right-hand perspective view of the treatment chamber isolation mechanism shown in  FIG. 11 .  
       FIG. 12B  is a left-hand perspective view of the treatment chamber isolation mechanism shown in  FIG. 11 .  
       FIG. 13  is a perspective view of the valve actuating assembly shown in  FIG. 9 .  
       FIG. 14  is a side elevation view of the valve actuating assembly shown in  FIG. 9 .  
       FIG. 15  is a detailed side elevation view of a heat exchanger shown as Detail  15  in  FIG. 10 .  
       FIG. 16A  is a perspective view of the tube and heat exchanger assembly as shown in  FIG. 10 .  
       FIG. 16B  is a detailed cross section of a conduit mounting shown in  FIG. 16A .  
       FIG. 17  is an exploded view of the heat exchanger shown in  FIGS. 15 and 16 A.  
       FIG. 18A  is a perspective view of a furnace assembly according to another aspect of the invention.  
       FIG. 18B  is a detailed view of one aspect of the furnace assembly shown in  FIG. 18A .  
       FIG. 19  is a left-hand perspective view of a tube furnace assembly shown in  FIG. 18A .  
       FIG. 20  is a front elevation view of the tube furnace shown in  FIG. 18A .  
       FIG. 21  is a right side elevation view of the tube furnace shown in  FIG. 18A .  
       FIG. 22  is a left side elevation view of the tube furnace shown in  FIG. 18A .  
       FIG. 23  is a cross sectional view of the tube furnace shown in  FIGS. 18A-22   
       FIG. 24  is a schematic block diagram of a process for charging the treatment element to the enclosure according to one aspect of the invention  
       FIG. 25  is a plot of treatment element vapor pressure as a function of temperature.  
       FIG. 26  is a plot of heat exchanger temperature as a function of coolant flow according to one aspect of the invention. 
    
    
     DETAILED DESCRIPTION OF ASPECTS OF THE INVENTION  
      The present invention comprises systems, apparatus, and methods that provide improved means for fabricating photovoltaic material that overcome many of the disadvantages of prior art systems and methods. Though aspects of the invention are particularly applicable to the handling and treatment of photovoltaic materials, aspects of the invention may be applied to many different photovoltaic and non-photovoltaic materials.  
       FIG. 1  is a schematic block diagram of a process  10  for treating a material according to one aspect of the invention. The material may comprise any material that is treated with a gas or vapor, for example, an element-containing vapor. In one aspect, the material comprises a photovoltaic precursor, for example, a precursor deposited on a substrate. The treatment gas may comprise any vaporous material. However, in one aspect, the treatment vapor comprises a chalcogen-containing vapor, for example, a sulfur-, selenium-, or tellurium-containing vapor; or an indium-, gallium-, or sodium-containing vapor. Though the material being treated may comprise any material or substance, to facilitate the disclosure of the invention, in some aspects, the material being treated may be referred to as a “work piece.” However, the use of the expression “work piece” is not intended to limit the scope of the materials to which aspects of the invention may be applied.  
      Process  10  includes a series of steps starting with step  12  of introducing the work piece to an enclosure, for example, into a treatment oven or furnace, such as treatment furnace illustrated in  FIGS. 3 through 9  or  FIGS. 18A through 23 , though any type of appropriate treatment furnace may be used. The work piece introduced to the enclosure may comprise any material that may be treated with a gas, for example, an element-containing vapor. According to one aspect of the invention, step  12  may be practiced by simply positioning the work piece on to the bottom surface of an enclosure, on to a support structure (or “boat”), or on to a shelf positioned in an enclosure. However, as will be discussed below, step  12  may be practiced by positioning one or more work pieces, for example, photovoltaic precursors deposited on substrates, into one or more individual, isolated enclosures, for example, into one or more quartz tubes. These individual, isolated enclosures permit the operator to individually regulate the treatment conditions within the individual enclosures, for example, tube, to, among other things, allow the operator to vary or control the treatment conditions within the enclosure.  
      In step  14 , the work piece is heated to a first temperature for treatment. One heating schedule that may be used to heat the work piece is illustrated in  FIG. 2 . The work piece may be heated to a first temperature to raise the work piece at least partially to treatment element temperature.  FIG. 2  is a plot  30  of a heating schedule curve  32  for a work piece and the heating schedule curve  34  and the log of the partial pressure curve  35  for a treatment element, for example, Se, according to one aspect of the invention. The abscissa  36  of plot  30  is the time of treatment, typically, in minutes; the left-hand ordinate  38  of plot  30  is the temperature, typically, degrees C; and the right-hand ordinate  39  is the log of the partial pressure of the treatment element. According to heating schedule  32 , the temperature of the work pieces may be increased from ambient temperature, T M∞ , for example, room temperature, for instance, about 20 degrees C., to a first treatment temperature, T M1 , for example, to a temperature greater than 100 degrees C. For example, when the work piece being treated is a Cu—In—Ga precursor, and treatment element is Se, temperature T M1  may be between about 100 to about 400 degrees C. This rate at which the temperature may be increased from T M∞  and T M1  may vary. For example, when the work piece being treated is a photovoltaic precursor deposited on a substrate, a slow rise in temperature may prevent the precursor from cracking and delaminating from the substrate. The rate of temperature increase may typically be between about 5 degrees C. per minute (° C./m) to about 100 degrees C. per second (° C./s), for example, about 20° C./m. The work piece to be treated may typically be held at temperature T M1  for at least about 30 seconds to about 90 minutes, for example, at least about 30 minutes.  
      After heating the work piece, the element-containing material, for example, selenium or sulfur, is heated per step  16 , for example, by means of the heating schedule illustrated in  FIG. 2 , to release treatment-material-containing vapor into the enclosure. According to one aspect of the invention, the treatment material is heated independently of the heating of the work piece being treated. The temperature of the treatment material, for example, Se, is elevated to a temperature at or above the temperature at which treatment material vapor is released, for example, at or above the vapor temperature at the prevailing pressures.  FIG. 2  also illustrates a typical heating schedule  34  for heating the treatment material. The treatment material may be any material that can be volatilized upon heating, including materials having multiple elements, that is, compounds. However, to facilitate the disclosure of the invention, in the following discussion the treatment material may be referred to as “the element-containing material” or “the element containing gas or vapor.” It is to be understood that in one aspect of the invention an element-containing material or element containing gas or vapor may comprise more than one element, for example, the element-containing material may be a mixture of elements or a compound.  
      With reference to  FIG. 2 , according to one aspect of the invention, the temperature of the treatment element, as indicated by curve  34 , is increased from an initial temperature T E0  to T E1 . As shown in  FIG. 2 , initial temperature T E0  may typically be less than or equal to ambient temperature T M∞ . For example, as will be described more fully below in the discussion of the heat exchanger, at the start of the treatment sequence, the temperature of the treatment element may be maintained at the temperature of the heat exchanger. Therefore, in one aspect of the invention, T E0  may be less than 60 degrees C., for example, about 50 degrees C. In one aspect of the invention, the initial temperature of the treatment element, T E0 , is kept below the temperature at which the element begins to volatilize. For example, when the treatment element is Se, which begins to volatilize at about 100 degrees C., the initial treatment element temperature may be kept below 100 degrees C., for example, about 50 degrees C. The temperature T E0  may vary broadly, for example, depending upon the vapor pressure of the treatment element. For example, for materials having high vapor pressures at lower temperatures, the temperature T E0  may be less than room temperature, for example, even less than 0 degrees C. The rate of temperature increase from T E0  to T E1  may vary, but may typically be between about 5° C./m to about 100° C./s, for example, about 20° C./m. The treatment element may typically be held at temperature T E1  for at least about 30 seconds to about 90 minutes, for example, at least about 15 minutes.  
      According to aspects of the present invention, the timing of the initiation of the changes in temperature shown in  FIG. 2  may vary depending upon the work piece being treated, the treatment element, and the treatment device being used, among other factors. Specifically, the relative time frame and time sequences of the increases and decreases in temperature may deviate from the relative time frames shown in  FIG. 2 . For example, though in one aspect shown in  FIG. 2 , the temperature of the work piece  32  may be increased before the temperature of the treatment element  34  is increased, in one aspect, the temperature of the treatment element  34  may be increased before or substantially at the same time as the temperature of the work piece  32  is increased. In one aspect of the invention, the temperature of the treatment element  34  is maintained below the temperature of the work piece  32 .  
      In one aspect, after holding the treatment element at temperature T E1  for about  15  minutes, the temperature of the treatment element may be increased to a temperature T E2 , for example, for Se, T E2  may be about 100 to about 400 degrees C. The rise in temperature from T E1  to T E2  may be between about 5° C./m to about 100° C./s, for example, at least about 20° C./m. The treatment element may typically be held at temperature T E2  for at least about 30 seconds to about 90 minutes, for example, at least about 30 minutes.  
      With reference again to  FIG. 1 , after heating the treatment element to release a treatment element-containing vapor into the enclosure per step  16 , the one or more work pieces are exposed to the treatment element vapor, per step  18 , whereby at least some of the work pieces are treated with the treatment element. Treating the work piece with the element-containing vapor may comprise reacting the element with the work piece or providing an overpressure of the element-containing vapor to the work piece. In one aspect, providing an overpressure comprises providing a vapor pressure of the element-containing vapor, for example, Se vapor, that is greater than the vapor pressure of the element, for example, Se, present in the work piece. This overpressure may minimize or prevent the volatilization and the net loss of element from the work piece. Step  18  may simply be practiced by allowing the work pieces positioned in the enclosure to be exposed to the treatment element vapor for a predetermined time periods, for example, 5 seconds to 5 hours. The treatment time for which the work piece is exposed to the element containing vapor may typically range from about 30 seconds to about 90 minutes.  
      As shown in  FIG. 2 , while the treatment element is held at temperature T E2 , the temperature of the work piece being treated, as indicated by curve  32 , may be increased, for example, rapidly, from temperature T M1  to T M2 , for instance, to a temperature where the treatment element begins to react with the treated material, for example, at a temperature of about 400 to about 600 degrees C. Se reacts rapidly with a Cu—In—Ga matrix to form a Cu—In—Ga—Se matrix. The rise in temperature of the treated material from T M1  to T M2  may be at a rate of between about 5° C./m to about 100° C./s, for example, about 20.0° C./m. Shortly thereafter, the temperature of the treatment element, for example, Se, is increased from T E2  to T E3  to enhance the volatilization of the treatment element and release sufficient vaporous treatment element to complete the reaction. For example, for Se, the temperature T E3  may be about 200 to about 550 degrees C. The rise in temperature of the treatment element from T E2  to T E3  may be at a rate of between about 5° C./m to about 100° C./s, for example, about 20° C./m. The treated work piece may be held at temperature T M2  to provide for sufficient reaction of the treatment element with the treated work piece. This treatment time may be at least about 30 seconds to 90 minutes.  
      With reference to  FIG. 1 , in step  20 , upon completion of the treatment of the work piece, the temperature of the treatment element is reduced, for example, by active cooling, for instance, by means of a cooling heat exchanger. According to one aspect of the invention, the reduction of the treatment element temperature substantially terminates the release of treatment-element-containing vapor from the treatment element, for example, the Se. The reduction of the treatment element temperature may also allow at least some of the treatment-element-containing vapor to condense onto the treatment element. In one aspect, substantially all of the treatment element vapor may condense onto the treatment element, for example, whereby the loss of treatment element to, for example, condensation onto the enclosure, is minimized. According to one aspect of the invention, due to this condensation or “recapture” of treatment element, the utilization rate of the treatment element, for example, Se or S, is very high. For example, in one aspect utilization rate is at least about 90% or more, in some instances at least about 95% or more. This temperature reduction of the treatment element is shown as curve  34  in  FIG. 2 .  
      Again, with reference to  FIG. 1 , before, during, or after the practice of cooling the treatment element per step  20 , the temperature of the work piece being treated may be reduced as indicated by step  22  in  FIG. 1 . Upon cooling, the treated work pieces may be further handled or processed as desired. The cooling of the work pieces may be practiced actively, for example, by means of a cooling heat exchanger and/or forced convection, or through unforced, natural convection and/or radiation.  
      This decrease in the temperature of the treated work piece is also shown in  FIG. 2 . As indicated by curve  32 , the temperature of the treated work piece is cooled, for example, slowly, from T M2  to about room temperature. This cooling may be practiced to prevent damage to the work piece; for example, when the work piece is a photovoltaic material, cooling is carried out relatively slowly to prevent cracking of the work piece or delamination from the substrate. The rate of cooling may range from between about 5° C./m to about 100° C./s, for example, about 5.0° C./m.  
      As shown in  FIG. 2 , before, during, or after the treated work piece is being cooled to room temperature, the treatment element is cooled from temperature T E3  to a lower temperature per curve  34 , for example, to a temperature below the temperature at which the element volatilizes. When the treatment element is Se, the Se is cooled to a temperature below 100 degrees C., for example, to a temperature of about 50 degrees C. The rate of cooling may range from between about 5° C./m to about 100° C./m, for example, about 15.0° C./m. Again, the curves in  FIG. 2  are representative of the invention, for example, sulfur volatilizes at a lower temperature, and tellurium volatilizes at a higher temperature.  
      In one aspect of the invention, the rapid cooling of the treatment element typically causes the vaporous element to recondense upon the cooled element whereby loss of the element to condensation on the surfaces of the furnace or associated surfaces is minimized or prevented. Thus, by controlled cooling of the treatment element, more of the treatment element is retained, for example, for further treatment.  
       FIG. 2  also displays a typical corresponding variation in the log of the partial pressure curve  35  of a treatment element as the temperature of the treatment element  34  varies. As shown, the partial pressure  35  tracks the changes in the temperature  34  very closely. For example, for a Se treatment element, at a temperature T E2  of the Se of about 100 to about 400 degrees C., the partial pressure of the Se is about 5.0 Torr at about 400 degrees C. Also, for a Se temperature T E3  of about 200 to about 550 degrees C., the partial pressure of the Se can be as high as about 80 Torr. The partial pressure curve  35  is a useful parameter in controlling the operation of the cooling heat exchanger or condenser/evaporator, as will be discussed more thoroughly below.  
      As shown in and described above with respect to  FIG. 2 , in one aspect of the invention, the temperature of a work piece to be treated and the temperature of a volatile element with which the work piece is to be treated with are independently controlled to optimize the reaction, for example, to improve reaction selectivity and/or reaction kinetics, and also to minimize the loss of treatment element, for example, sulfur or selenium. According to process  10  shown in  FIG. 1  and the heating schedules shown in  FIG. 2 , a work piece may be treated with treatment element in a controlled environment whereby the release of treatment element is regulated to optimize the treatment and minimize the loss of treatment element. In one aspect, method  10  or sub-sequences of method  10  of  FIG. 1  may be practiced repeatedly. For example, steps  14 ,  16 ,  18 , and  20  may be practiced at least twice, possibly three or more times, to effect the desired treatment of the work piece. Also, steps  18  and  20  may be practiced at least twice (possibly three or more times), for example, before proceeding with step  22 , to effect the desired treatment of the work piece.  
      The process  10  shown in  FIG. 1  may also include the optional steps of charging the enclosure with treatment element  24  and cooling the treatment element  26  prior to or during the heating step  14 . These optional steps are shown in phantom in  FIG. 1 . According to one aspect of the invention, the treating element may be introduced, or “charged,”  24  to the enclosure before the work piece to be treated is introduced to the enclosure  12 . This charging of the treatment element may be practiced by the steps illustrated in  FIG. 24  and will be discussed below. The cooling step  26  may be practiced to prevent the element-containing material from volatilizing prematurely, that is, before the work piece is ready to be treated.  
      The method of the invention may be practiced in any suitable enclosure that can be adapted to regulate the temperature of the work pieces and the treatment material, for example, independently regulated. One enclosure that may be used to practice aspects of the invention is illustrated in  FIGS. 3 through 9  and  FIGS. 18A through 23 . To those of skill in the art, the term “furnace” is sometimes reserved for devices in which work pieces are heated to temperatures of at least 1200 degrees C. while the term “oven” is sometimes reserved for devices in which work pieces are heated to temperatures of between about 200 degrees C. to about 300 degrees C. However, the use of the terms “furnace” or “oven” in the following discussion is not intended to limit the scope of the invention to these temperature ranges. Aspects of the present range may be used to heat work pieces in these and other temperature ranges, for example, as low as room temperature, for example, 20 degrees C., to as high a temperature that does not impact the performance or integrity of the disclosed devices, for example, at least 2000 degrees C.  
       FIG. 3  is a perspective view of a treatment furnace  50  according to one aspect of the invention. Treatment furnace  50  may be used to practice the invention illustrated in and described with respect to  FIGS. 1 and 2 .  FIG. 4  is a front elevation view of furnace  50  shown in  FIG. 3 .  FIG. 5  is a right side elevation view of the furnace  50  and  FIG. 6  is a left side elevation view of furnace  50 .  FIG. 7  is a rear elevation view of furnace  50  and  FIG. 8  is a top plan view of furnace  50  shown in  FIG. 3 . According to aspects of the invention, furnace  50  (and furnace  200  discussed below) are specially designed to operate under a broad range of operating conditions. For example, furnace  50  (and  200 ) may be operated under a broad range of temperatures, for example, from 0 to 2000 degrees C., and a broad range of pressures, for example, super-atmospheric pressures to sub-atmospheric pressures. For instance, furnace  50  (and  200 ) may be specially designed to operate under a vacuum ranging from just below about 1 standard atmosphere to 10 −6  Torr.  
      As shown in  FIGS. 3 through 8 , furnace  50  includes a front door assembly  52 , a right side door assembly  54 , a left side door assembly  56 , a rear panel assembly or back  58 , a top  60 , and a bottom  62 . Furnace  50  may be mounted on a plurality of support legs  64 . As shown in  FIGS. 3 and 4 , front door assembly  52  may be pivotally mounted to the furnace  50  by hinge assembly  66  and may comprise a plate  53  having appropriate reinforcing elements  55 , for example, structural tubing or angles. Reinforcing elements  55  may also comprise conduits through which a heating or cooling medium may be passed to either heat or cool furnace  50 . Plate  53  may also be heated and cooled by other means. Front door assembly  52  may include valve-actuating assembly  68  mounted to plate  53 . Valve actuating assembly  68  may be adapted to actuate one or more isolation valves mounted within furnace  50 . (See  FIGS. 13 and 14  and their description for details of valve-actuating assembly  68 .) Front door assembly  52  may typically provide means for opening furnace  50  to service furnace  50  or load work pieces, for example, photovoltaic material precursors, into furnace  50  for treatment. Front door assembly  52  may typically include a handle and locking assembly  57  for opening, closing, and securing front door assembly  52 .  
      As shown in  FIGS. 3 and 5 , right side door assembly  54  of furnace  50  may comprise a plate  57  having appropriate reinforcing elements  59 , for example, structural tubing or angles. Reinforcing elements  59  may also comprise conduits through which a heating or cooling medium may be passed to either heat or cool furnace  50 . Plate  57  may also be heated and cooled by other means. Though right side door assembly  54  may be rigidly mounted to furnace  50  (that is, not adapted to be opened), as shown in  FIG. 3 , right side door assembly  54  may be removably mounted to furnace  50 , for example, pivotally mounted to furnace  50  by means of hinge assembly  70 . Right side door assembly  54  may be secured to furnace  50  by conventional means, for example, by means of mechanical fasteners or welding, such as, clamps  72 . Clamps  72  may be conventional clamps adapted to secure right side door assembly  54 . Right side door assembly  54  may typically provide means for opening furnace  50  for servicing, for example, servicing the work piece support structures, and related treatment devices, for example, the valves, heat exchangers, or heaters, discussed below.  
      As shown in  FIGS. 3 and 6 , left side door assembly  56  of furnace  50  may be similar in construction to right side door assembly  54  and may comprise a plate  67  having appropriate reinforcing elements  69 , for example, structural tubing or angles. Again, reinforcing elements  69  may also comprise conduits through which a heating or cooling medium may be passed to either heat or cool furnace  50 . Plate  67  may also be heated and cooled by other means. Left side door assembly  56  may also be rigidly mounted to furnace  50  (that is, not adapted to be opened). However, as shown in  FIG. 6 , left side door assembly  56  may be removably mounted to furnace  50 , for example, pivotally mounted to furnace  50  by means of hinge assembly  80 . Left side door assembly  56  may be also be secured to furnace  50  by conventional means, for example, by means of mechanical fasteners or welding, such as, clamps  73 . Clamps  73  may be similar to clamps  72  provided on the right side door assembly  54  shown in  FIGS. 3 and 5 . Left side door assembly  56  may typically also provide means for opening furnace  50  for servicing, for example, servicing the work piece support structures, and related treatment devices, for example, the valves and heat exchangers, discussed below.  
       FIG. 7  illustrates the rear panel assembly  58  of furnace  50  which may comprise a plate  97  having appropriate reinforcing elements  99 , for example, structural tubing or angles. Reinforcing elements  99  may also comprise conduits through which a heating or cooling medium may be passed to either heat or cool furnace  50 . Plate  97  may also be heated and cooled by other means. Rear panel assembly  58  may also be rigidly mounted to furnace  50 , that is, not adapted to be opened, but may also be removable, for example, pivotally mounted to furnace  50  by means of hinge assembly. As shown in  FIG. 7 , rear panel assembly  58  may also be secured to furnace  50  by conventional means, for example, by means of mechanical fasteners or welding.  
      In aspects of the invention, furnace  50  may include various access ports or openings for assorted purposes, for example, for introducing or removing process fluids (that is, liquids and/or gases), introducing or removing heating or cooling fluids, applying a vacuum, or providing pathways for wiring, cabling, or instrumentation, among other reasons. As shown in  FIGS. 3 through 8 , access ports or openings may be located in front door assembly  52 , right side door assembly  54 , left side door assembly  56 , rear panel assembly  58 , top  60  or bottom  62 . As shown in  FIG. 7 , according to one aspect, rear panel assembly  58  may include a plurality of access ports, including a first row  100  of flanged ports  102  on the left side of rear panel  58  and a second row  104  of flanged ports  106  on the right side of rear panel  58 . Ports  102  in row  100  and ports  104  in row  102  may provide ports providing power, such as to heating elements  88 ; or for instrumentation wiring, such as to temperature or pressure sensors. As also shown in  FIG. 7 , rear panel  58  may also include two rows  108  of ports  110  centrally mounted on rear panel  58 . Ports  110  may be mounted in a common plate  112  and be mounted to plate  97  of rear panel  58  by a plurality of mechanical fasteners  114 , for example, bolts or screws. According to one aspect of the invention, ports  110  may provide conduits for venting, purging, or introducing process or control fluids to furnace  50 . For example, in one aspect, ports  110  may provide cooling or heating fluid (for example, liquid or gas) to an internal component, such as, to a heat exchanger (for example, heat exchanger  86  shown in  FIGS. 15 and 16 A).  
      The top assembly  60  of furnace  50  is illustrated in the top plan view of  FIG. 8 . Top assembly  60  may comprise a plate  117  having appropriate reinforcing elements  119 , for example, structural tubing or angles. Again, reinforcing elements  119  may also comprise conduits through which a heating or cooling medium may be passed to either heat or cool furnace  50 . Plate  117  may also be heated and cooled by other means. Top assembly  60  may also be rigidly mounted to furnace  50 , that is, not adapted to be opened, but may also be removable, for example, pivotally mounted to furnace  50  by means of a hinge assembly (not shown). As shown in  FIG. 8 , top assembly  60  may also be secured to furnace  50  by conventional means, for example, by means of mechanical fasteners or welding. As shown in  FIG. 8 , top assembly  60  may include a plurality of access ports, including a first row  120  of flanged ports  122  on the left side of top assembly  60  and a second row  124  of flanged ports  126  on the right side of top assembly  60 . Ports  122  in row  120  and ports  124  in row  122  may provide access ports for power, instrumentation, venting, purging, or process fluids, among other functions. As also shown in  FIG. 8 , top assembly  60  may also include one or more flanged ports  130 , for example, a centrally mounted port in top assembly  60 . Ports  130  may also provide an access for ports for power, instrumentation, venting, purging, or process fluids or simply provide a access for flushing or purging furnace  50 .  
      As shown in  FIGS. 3 through 8 , a plurality of access ports may also be located in bottom  62 . Similar to top  60 , bottom  62  may include first row  132  of flanged ports  134  on the left side of bottom assembly  62  and a second row  136  of flanged ports  138  on the right side of bottom assembly  62 . Ports  134  in row  132  and ports  138  in row  136  may also provide access to the inside of furnace  50  for providing purging, venting, process fluids, or instrumentation. Bottom  62  may also include one or more flanged ports  140  centrally mounted in bottom  62 . Port  140  may also provide an access for ports for power, instrumentation, venting, or process fluids or simply provide a access for flushing or purging furnace  50 .  
       FIG. 9  is a right side elevation view of the furnace  50  shown in  FIGS. 3 through 8  with the right side door assembly  54  and hinge assembly  70  removed to reveal the internal structure of furnace  50 . As shown in  FIG. 9 , furnace  50  includes at least one work piece treatment assembly  81 , but typically includes a plurality of work piece treatment assemblies  81  mounted in furnace  50 .  FIG. 10  is a detailed side elevation view of one work piece treatment assembly  81  as shown as Detail  10  in  FIG. 9 . In one aspect of the invention, work piece treatment assemblies  81  may be mounted front door assembly  52 , right side door assembly  54 , left side door assembly  56 , rear panel assembly  58 , top  60 , or a bottom  62 . For example, in one aspect, the work piece treatment assemblies  81  may be mounted as rack on a door or side of furnace  50  whereby one or more treatment assemblies  81  may be manipulated or handled, for example, removed or installed, by means of a door or side of furnace  50 .  
      As shown in  FIG. 10 , each treatment assembly  81  includes a treatment chamber, container, or tube  82 ; an isolation apparatus (or “flapper valve”) assembly  84 ; and a material delivery (or “condenser/evaporator”) assembly  86 . Tubes  82  are adapted to accept one or more work pieces  90 , for example, a photovoltaic precursor material, to be treated. According to aspects of the invention, work piece  90  is positioned in tube  82  and isolated from the rest of furnace  50  whereby the treatment environment within tube  82  can be controlled as desired, for example, at a desired temperature, pressure, and/or vapor content. The isolation of tubes  82  from the rest of furnace  50  minimizes the exposure of the rest of furnace  50  to gases and vapors present in tubes  82 , for example, toxic gases and vapors. According to one aspect of the invention, the use of one or more tubes or inner enclosures  82  inside an outer enclosure, for example, as provided by the walls of chamber  50 , isolates the hot treatment zone of the inner enclosure from the outer enclosure whereby low temperature sealing devices, for example, elastomeric seals, may be used to seal the outer enclosure from the ambient environment and minimize thermal damage to the sealing devices. It will be apparent to those of skill in the art that aspects of the invention provide enhanced functionality and enhanced throughput compared to prior art devices and methods.  
      As shown in  FIG. 10 , furnace  50  includes a plurality of heat sources  88  adapted to heat the one or more work pieces  90  in tube  82 . Heat sources  88  may be an infrared heat source, an inductive heat source, or a convective heat source. For example, heat source  88  may be an infrared heating lamp. Tube  82  is typically fabricated from a material that readily permits the heating of work piece  90  by means of heat sources  88 , for example, is made from a transparent or translucent material, such as, quartz, stainless steel (such as  316  stainless steel) or a corrosion-resistant alloy, such as a Hastelloy® alloy. Heat sources  88  may be mounted to the sides, roof, or floor of furnace  50  or mounted to a perforated mounting plate (not shown) having apertures sized to receive and support heat sources  88 . Tube  82  may assume any appropriate cross-sectional shape, such as round, rectangular, and square, for example, depending upon the size and shape of the work piece being treated and the size and shape of furnace  50 . A perspective view of one tube  82  that may be used in one aspect of the invention is shown in  FIG. 16A . As shown in  FIG. 10 , tube  82  may be mounted on one or more supports  92 , for example, one or more bars or support tubes mounted horizontally in furnace  50 . Supports  92  may also be fabricated from a material that readily permits the heating of work piece  90  by means of heat sources  88 , for example, is made from a transparent or translucent material, such as, glass or quartz. Tube  82  may be rigidly mounted to supports  92  or may be allowed to translate on supports  92 , for example, to permit ease of handling, for instance, removal, of tubes  82  from furnace  50 , for instance, through open rear panel assembly  58 . Supports  92  may be mounted to the sides, roof, or floor of furnace  50  or mounted to a perforated mounting plate (not shown) having apertures sized to receive and support supports  92 , for example, the same mounting plate adapted to support heat sources  88 . Temperature sensing devices, for example, thermocouples, may be mounted in tubes  82  and/or supports  92 .  
       FIG. 11  is a detailed side elevation view of a tube sealing assembly  84  shown as Detail  11  in  FIG. 10 .  FIG. 12A  is a right-hand perspective view of tube sealing assembly  84  shown in  FIG. 11 .  FIG. 12B  is a left-hand perspective view of the tube sealing assembly  84  shown in  FIG. 11 .  
      As shown in  FIGS. 11, 12A , and  12 B, sealing assembly  84  is adapted to close or seal the end of treatment chamber or tube  82  (shown in phantom). The sealing assembly  84  includes a sealing or “flapper valve” assembly  180  and means  182  for compressing the sealing assembly  180  against the treatment chamber or tube  82 . Though in some aspects the sealing assembly  180  may provide an vapor-tight cover to the one or more treatment chambers  82 , in another aspect, the engagement of sealing assembly  180  may not be vapor-tight, but may simply minimize the escape of fluids (that is, gases or liquids) from treatment chamber  82  during treatment. The sealing assembly  180  includes a support structure  184 , at least one cover plate  186  adapted to engage the treatment chamber opening, and a plurality of rods  188 . The plurality of rods  188  have a first end  190  mounted to support structure  184 , for example, resiliently mounted to support structure  184 , and a second end  195  adapted to engage cover plate  186 .  
      Support structure  184  may be any support structure adapted to support the one or more cover plates  186  and adapted to engage the means  182  for compressing the sealing assembly  180  against the treatment chamber or tube  82  while withstanding the treatment temperatures, for example, up to 800 degrees C. In one aspect, support structure  184  may be a single plate, for example, a plate having sufficient stiffness that does not require the need for additional structural support bars or ribs. In another aspect, support structure  184  may comprise a plurality of plates  192 , for example, a plurality of vertically or horizontally oriented plates. Plates  192  may be attached by means of mechanical fasteners, welding, or by common support bars or ribs. In the aspect shown in  FIG. 11 , support structure  184  may comprise a plurality of horizontal plates  192  mounted to a plurality of vertical support bars  194 . Plates  192  may be mounted to bars  194  by mechanical fasteners or welding. Plates  192  may include holes or slots  193  through which mechanical fasteners (not shown) may adjustably engage support bars  194 .  
      As shown in  FIGS. 11 and 12 A, plates  192  include a plurality of apertures  197  through which the first ends  190  of rods  188  pass through plates  192 . Rods  188  may be mounted to plates  192  by conventional means, for example, by means of mechanical fasteners. As shown in  FIGS. 11 and 12 B, in one aspect, rods  188  may mount to plates  192  by means of a flexural member  199  and one or more fasteners  201 , for example, one or more hex nuts. (Though not shown in  FIG. 12B , rod  188  may be retained to flexural member  199  by a second fastener, for example, a hex nut located behind flexural member  199 .) Flexural member  199  may comprise a “flexure” as discussed below. Flexural member  199  may include a circular disk section  203  and an elongated stem  207 . Disk section  203  may be sized to ensure that disk section  203  cannot pass through aperture  197  in plate  192 . Stem  207  may be mounted to plate  192  by conventional means, for example, by clamp plate  209  and fasteners  216 . According to one aspect of the invention, the flexibility of flexural member  199  provides for at least some alignment for the positioning of plate  186  on tube enclosure  82 . For example, flexural member  199  and pin  208  may provide a parallel flexure configuration that minimizes the misalignment of plate  186  while providing at least some resiliency or compliance in the alignment of the mating structures.  
      As shown in  FIGS. 11, 12A , and  12 B, plates  192  may include an extension  196 , for example, plates  192  and extensions  196  may comprise structural angles. Extension  196  may be solid or include a plurality of through holes  198 , for example, to facilitate assembly, to reduce the weight of support structure  184 , or to provide a purge path to eliminate virtual leaks. Plates  192 , bars  194 , and extensions  196  may be made from any metal or non-metal structural material, for example, a steel, stainless steel, titanium, nickel, or any other structural metal. In one aspect, plates  192 , bars  194 , and extensions  196 , may be made from stainless steel, for example,  304  stainless steel.  
      The one or more cover plates  186  may be metallic, but are typically made from stainless steel sheet having a thickness of from about 0.005 inches to about 0.125 inches. The size of cover plate  186  will vary depending upon the size of the treatment chamber and the treatment chambers sealing assembly  180  used to seal the treatment chamber. Cover plates  186  may be about 3 inches long to about feet long and may have a width from about 1 inch to about 1 foot. Typically cover plate  186  is about 2 feet long and about 2 inches in width. In one aspect of the invention, cover plate  186  may engage a single or a plurality of treatment chambers, for example, 2, 3, or more treatment chambers. That is, cover plate  186  may be adapted to seal a plurality of treatment chamber openings, for example, a plurality of tubes  82 .  
      Rods  188  are adapted to transmit a load from support assembly  184  to the cover plates  186 . Rods  188  may have any cross section, including square or rectangular, but are typically circular in cross section and may have a diameter of between about 0.125 inches and 0.5 inches. Rods  188  are typically about 0.375 inches in diameter and may be at least partially threaded. Rods  188  may engage plates  186  by conventional means, for example, by means of mechanical fasteners or welding. However, in one aspect, a first end  195  of rods  188  is not rigidly mounted to plates  186  but may be flexibly engaged to allow for some relative displacement between rods  188  and plates  186 . One means of providing this non-rigid engagement to the first end  189  of rod  188  is illustrated in  FIG. 12B , where first end  195  engages plates  186  by means of clips  200 . As shown in  FIG. 12B , clip  200  may comprise a central u-shaped portion  219  and at least one, typically, two, cantilevered plate sections  221 . Cantilevered plate sections  221  may comprise “flexures” as discussed below. Plate sections  221  may have at least one, typically, two, holes (not shown) adapted to engage and retain the first end  195  of rods  188 , for example, by means of one or more fasteners  217 , for example, hex nuts threaded to rods  188 . Clip  200  may be mounted to plate  186  by mechanical fasteners or welding, for example, simple resistance welding at section  219 . Clip  200  is typically also made from stainless steel, for example,  304  stainless steel.  
      As discussed above, the second end  190  of rod  188  is adapted to engage support structure  180 . As shown in  FIG. 11  and  12 B, second end  190  may pass through at least one hole  197  in plate  192  and, for example, engage flexural member  199 . As also shown in  FIG. 11  and  12 B, rod  188  may include a resilient mounting to plate  192 , for example, by means of one or more springs  204 , for example, coil springs. Springs  204  are preferably made from a temperature resistant material, for example, a high strength austenitic nickel-chromium-iron alloys, for instance, a Special Metals Corporation&#39;s Inconel® alloy, such as Inconel® 750 alloy, or its equivalent. Rod  188  may include one or more spring capturing or retaining devices, for example, a cup-like spring retaining device  206  mounted to rod  188 . In this aspect, retaining device  206  receives springs  204  to promote engagement between spring  204  and rod  188 . Retaining device may also include a sleeve  223  (see  FIG. 11 ) though which rod  188  passes. In one aspect, the end of sleeve  223  may provide a surface against which fastener  201  captures flexural member  199  when attaching flexural member  199  to rod  188 . Plates  192  may also include a recess or counter bore  222  (see  FIG. 11 ) for receiving spring  204  to facilitate assembly and ensure alignment during operation. Rods  188  and retaining devices  206  may be made from any metal or non-metal structural material, for example, a steel, stainless steel, titanium, nickel, or any other structural metal. In one aspect, rods  188  and retaining devices  206  may be made from stainless steel, for example,  304  stainless steel.  
      In one aspect, sealing assembly  84  may include additional support members for rods  188 , for example, to position rods  188  and cover plates  186  in the desired position to engage treatment chambers  82 . As shown in  FIG. 11 , sealing assembly  184  may include one or more retaining members, pins, or bars  208  to support rods  188 . In one aspect of the invention, bars  208  may comprise “flexures” as discussed below. Bars  208  may be mounted to any convenient location on support structure  180  and engage rods  188  by conventional means, for example, mechanical fasteners. In the aspect shown in  FIG. 11 , bars  208  are mounted to extension  196  (for example, to the leg of the structural angle) by means of mechanical fasteners and a clamp plate  210 . Bars  208  may be mounted to rods  188  by conventional means, including welding or mechanical fasteners. As also shown, bars  208  may be mounted to rods  188  by capture between two or more fasteners  312 , for example, hex nuts, threaded to rod  188 . It will be apparent to those of skill in the art that the threaded mounting of bar  208  to rod  188  via nuts  312  permits the assembler to vary the position of engagement whereby the elevation of rods  188  (and of plates  186 ) may be varied as desired.  
      According to aspect of the invention, the sealing assembly  84  illustrated in  FIGS. 11 and 12  is displaced into engagement with one or more treatment chambers or tubes  82  to at least partially limit the escape of fluids from treatment chamber  82  during treatment. The displacement of sealing assembly  84  into engagement with treatment chambers  82  is effected by means of valve-actuation assembly  68  (see  FIGS. 3-5  and  9 ). Though valve-actuation assembly  68  may be positioned within furnace  50  or outside of furnace  50 , in the aspect shown in  FIGS. 3-5 , valve-actuation assembly  68  is positioned outside of furnace  50  and is adapted to engage sealing assembly  84  by means of a plurality of rods extending through a wall of furnace  50 .  
       FIG. 13  is a perspective view of valve actuation assembly  68  shown in  FIG. 9 .  FIG. 14  is a side elevation view of the valve actuation assembly  68  shown in  FIG. 9 . As shown, valve actuation assembly  68  is mounted to front door plate  53  by means of a structural support  218  and is adapted to displace at least one actuation rod  220 , typically, a plurality of rods  220 . In the aspect shown, six actuation rods  220  are displaced by valve actuation assembly  68 . Valve actuation assembly  68  includes one or more linkage assemblies  224 , for example, a spherical linkage assembly, mounted to door  53  and a piston assembly  234  mounted to structural support  218  and to linkage assembly  224 . According to the present invention, piston assembly  234  displaces structural support  218  to which rods  220  are mounted to displace rods  220  and sealing assembly  84  (see  FIGS. 11 and 12 ).  
      As shown in  FIG. 14 , linkage assembly  224  includes a body  228 , a first bracket  230  by which body  228  is mounted to piston assembly  234  and a second bracket  232  mounted to plate  233  which is mounted to door plate  53 , for example, by conventional mechanical fasteners or welding. One or more pneumatic or hydraulic lines (not shown) may be provided to actuate piston assembly  234 . Hydraulic or pneumatic cylinder  234 , for example, a short-stroke cylinder, mounted to support structure  218 .  
      Support structure  218  may include a variety of structural elements for transmitting the displacement provided by piston assembly  224  to rods  220 . Piston assembly  224  is mounted to main plate  236  of support structure  218  to which the plurality of rods  220  are mounted. Main plate or actuation plate  236  may take a variety of shapes depending upon the size and number of rods  220  to which main plate  236  is mounted. In the aspect shown in  FIG. 13 , main plate  236  takes the general form of the letter “H” where the rods  220  are mounted to the uprights and the piston assembly  224  mounts to the cross beam. In one aspect, the main plate  236  may be relatively stiff, for example, at least about 0.375 inches in thickness, to promote uniform displacement of rods  220 , for example, to minimize misalignment of rods  220 . In one aspect, main plate  236  may include one or more reinforcing ribs to increase the stiffness of plate  236 .  
      Support structure  218  may also include at least one flexural plate  238 ,  240  mounted to main plate  236 . In one aspect, plates  238  and  240  comprise flexures, that is, precision flexural elements that can control the accuracy of deflection, for example, parallel flexures. (See Slocum,  Precision Machine Design  (1992), the disclosure of which is incorporated by reference herein.) Flexural plates  238  and  240  not only support the main plate  236  and rods  220 , but flexural plates  238  and  240  may also provide at least some flexibility to support structure  218  whereby rods  220  can be more uniformly displaced. Flexural plates  238  are mounted to main plate  236  by mounts  242 . Mounts  242  may assume a variety of shapes and sizes, but, as shown in  FIGS. 13 and 14 , mounts  242  may comprise a center plate  244 , a base plate  246  mounted to the center plate, and two gussets  248  mounted to the sides of the center plate. Mounts  242  may be fabricated by welding or mechanical fasteners. Flexural plates  238  may be mounted to mounts  242  by a mounting plate  250  and mechanical fasters.  
      Flexural plates  238  are also mounted to furnace  50 , for example, to the front door plate  53 , by any conventional mounting means. As shown in  FIGS. 13 and 14 , flexural plates  238  may be mounted to furnace  50  via flanged support  252 . As shown in  FIG. 14 , flanged support  252  may include a center web plate  254  and a flange plate  256 . Flanged supports  252  may be mounted to furnace  50  by conventional means, for example, mechanical fasteners or welding. Flexural plates  238  may be mounted to flanged support  252  by a mounting plate  250  and mechanical fasters.  
      Flexural plates  240  may also be mounted to main plate  236  by mounts  252 , that is, structural members similar to or identical to mounts  242  discussed above. Flexural plates  240  may be mounted to mounts  252  by a mounting plate  250  and mechanical fasteners. Flexural plates  240  are also mounted to furnace  50 , for example, to the front door plate  53 , by any conventional mounting means. As shown in  FIGS. 13 and 14 , flexural plates  240  may be mounted to furnace  50  via plates  254 . As shown in  FIG. 13 , plates  254  may be mounted to furnace  50  by conventional means, for example, mechanical fasteners or welding. Flexural plates  240  may be mounted to plates  254  by a mounting plate  250  and mechanical fasters.  
      According to one aspect of the invention, the function of valve actuation assembly  68  is to displace sealing assembly  84  against the treatment tubes  82 . This displacement of sealing assembly  84  is typically effected via rods  220 . As shown in  FIG. 14 , rods  220  are mounted to main plate  236 . Rods  220  may be mounted to plate  236  by conventional mechanical fasteners, for example, as shown in  FIG. 14 , rods  220  are mounted to plate  236  by a pair of collars  280  and  281 . Rod  220  extends into furnace  50  through flange  282  and flanged bellows assembly  284 . Bellows assembly may provide some flexibility to the insertion of rods  220  into furnace  50 . Bellows assembly  284  includes a bellows  286  and two flanged pipes  287  and  288 . Bellows assembly  284  may be a typical off-the-shelf item. According to one aspect, rods  220  may be rigidly mounted to flange  282 , for example, by mechanical fasteners or welding, whereby the displacement of rods  220  is accompanied by the displacement, for example, compression, of bellows  286 . Flanged pipe  288  is mounted to a flanged nipple  290  mounted to front door plate  53 . The flanged connections may typically comprise “conflat” flanges, for example, conflat flanges provided by the Kurt J. Lesker Company of Clairton, Pa., or their equivalent. ISO and/or ASA flange systems may also be used. After passing through front door plate  53 , rods  220  engage sealing assembly  84 . Rods  220  may engage sealing assembly  84  in any fashion effective to displace sealing assembly  84 . In one aspect, rods  220  are mounted to support structure  184  of sealing assembly  84  by mechanical fasteners, for example, by bolts or screws, to plates  192  or bars  194  (see  FIGS. 11 and 12 ) of support structure  184 . Rods  220  may also be welded to support structure  184 .  
       FIG. 15  is a detailed side elevation view of a heat exchanger or condenser/evaporator  86  shown as Detail  15  in  FIG. 10 . A perspective view of heat exchanger  86  along with tube  82  is shown in  FIG. 16A .  FIG. 16B  is a detailed cross section of the conduit mounting shown in  FIG. 16A .  FIG. 17  is an exploded view of heat exchanger  86  shown in  FIGS. 15 and 16 A. According to aspects of the invention, heat exchanger  86  (and any other heat exchanger identified herein) may be a device that exchanges heat between the body of the device and a working fluid passing through the device to vary the temperature of at least one surface of the device. In one aspect of the invention, heat exchanger  86  (and any other heat exchanger identified herein) may function as a “condenser,” that is a device having at least one surface upon which a volatilized material may condense upon, for example, by lowering the temperature of the surface. In one aspect of the invention, heat exchanger  86  (and any other heat exchanger identified herein) may function as an “evaporator,” that is, a device having at least one surface upon which a volatilizable material is applied and from which the volatilizable material may be volatilized or “evaporated,” for example, by raising the temperature of the surface. In another aspect of the invention, heat exchanger  86  (and any other heat exchanger identified herein) may function as both a “condenser” and an “evaporator,” and may be referred to as a “condovator.” 
      As shown in  FIGS. 15-17 , tube  82  comprises a main cylindrical section  83 , an open first end having a first flange  85 , and open second end having a second flange  87 . Heat exchanger  86  is mounted to flange  87  of the open second end of tube  82  wherein a surface of heat exchanger  86  is exposed to the open end of tube  82 . In one aspect of the invention heat exchanger  86  may comprise a material delivery device, that is, a device for use in regulating the delivery of a vaporous material or element, for example, vaporous Se or S, to tube  82 . As shown in  FIGS. 15-17 , heat exchanger  86  consists of an elongated cylindrical body  150  having at least one surface  151  exposed to the open end of tube  82 . According to one aspect of the invention, surface  151  is adapted to receive at least some volatilizable element, for example, by means of the “charging” process described below. The temperature of surface  151  is then regulated, for example, heated or cooled, whereby the element volatilizes and the vaporous element is released into tube  82  to treat work piece in tube  82 .  
      Cylindrical body  150  of heat exchanger  86  may be a rectangular, square, or circular cylindrical body, or any other shaped cylindrical body adapted to be mounted to a treatment tube, such as, treatment tube  82 . Cylindrical body  150  may include at least one first passage  152 , for example, a circular passage, extending the substantially the entire length of body  150 , and two smaller passages,  154  and  156 , for example, also circular, and also extending substantially the entire length of body  150 . According to one aspect of the invention, passage  152  is adapted to retain at least one heating device  158 , for example, an infrared heat source, an inductive heat source, or a convective heat source, among other devices. Passage  152  may be circular, square, rectangular, or any other shape adapted to retain a heating device  158 . According to one aspect, heating device  158  may comprise one or more heating devices positioned along one or more passages  152 . Heating device  158  typically may have a power output of at least about 200 watts, typically, at least 500 watts. For instance, heating device  158  may be an off-the-shelf infrared light tube. Heating device  158  is typically supplied with electric power by means of a wire or cable and an appropriate electrical connector not shown (for example, through port  102  shown in  FIG. 7 ).  
      Passages  154  and  156  may be coolant or heating fluid flow passages, for example, passages for transmitting a working fluid, that is, a liquid or a gas, through body  150  to heat or cool body  150  and surface  151 . The working fluid may be air; nitrogen; water; an inert gas, for example, helium; an oil; or an alcohol, for example, ethylene glycol; among other working fluids. Passages  154  and  156  are typically capped at either end by plugs  160 . Passages  154  and  156  communicate with two or more working fluid source conduits  162  and  164  adapted to receive and discharge a working fluid to and from an external source. Conduits  162  and  164  may be positioned anywhere along body  150 , and, as shown in  FIG. 16A , may be positioned in about the middle of body  150 . Conduits  162  and  164  may have about ¼-inch nominal diameter and be mounted in conduits  111 , as discussed below with respect to  FIG. 16B . Conduits  162  and  164  typically supply working fluid to heat exchanger  86  from a source outside furnace  50  (also shown in  FIGS. 7-9 ). For example, coolant flow, such as, air, may be provided to conduit  162  which passes the coolant to passage  156 . The coolant may then flow through one or more cross passages (not shown) to passage  154  and then be returned to conduit  164  at a hotter temperature when cooling (or a colder temperature when heating) than the coolant introduced through conduit  162 . The hotter coolant discharged through conduit  164  may be vented or passed through a heat exchanger for cooling (or heating) or to heat recovery, for example, the coolant may be cooled and reintroduced as coolant to conduit  162 . In one aspect, the temperature of the working fluid introduced to heat exchanger may be varied to effect the desired temperature of surface  151 . For example, the temperature of surface  151  may be regulated by varying the temperature of the working fluid introduced to heat exchanger  86  by means of an external heat exchanger (not shown).  
      As shown in  FIG. 15 , heat exchanger  86  is mounted to tube  82  whereby surface  151  is exposed to the inside of tube  82 . Since tube  82  may typically be made from a material (for example, quartz) having a different thermal expansion coefficient than the material (for example,  304  stainless steel) of the body  150  of heat exchanger  86 , the mounting of heat exchanger  86  to tube may make allowance for differences in thermal expansion. As shown in  FIGS. 15-17 , in one aspect, heat exchanger  86  may be mounted to tube  82  by means of one or more brackets or clips  166  and one or more resilient materials  168 , for example, one or more coil springs or flexures. According to this aspect, the clips  166  and coil springs  168  provide for a thermally expandable mounting of heat exchanger  86  to tube  82  while maintaining contact, for example, vapor-tight contact, between surface  151  of body  150  and tube  82 .  
       FIG. 16B  is a detailed cross section of the mounting of conduits  162  and  164  in conduit  111 . Conduit  111  comprises a cylindrical tube, for example, about ¾-inch nominal diameter, having an open first end  113  and a closed second end  115 . Conduits  162  and  164  typically extend from cylindrical body  150  of heat exchanger  86  and pass through conduit  111  and through closed end  115 . Conduits  162  and  164  may have an appropriate coupling  123 , for example, the mail pipe coupling shown, to connect to a source of coolant, for example, air. Conduits  162  and  164  may be mounted to the closed end  115  of conduit  111  by means of mechanical fasteners or welding. Conduits  111  may be mounted to chamber  50 , for example, into ports  110  in the rear wall  97  of chamber  50 , by means of an appropriate mechanical fastener. For example, port  110  may comprise an appropriate vacuum fitting, for example, an Ultra-Torr® vacuum fitting provided by the Swagelok Company, or its equivalent fitting. According to aspects of the invention, the mounting of conduits  162  and  164  in conduit  111  allows for some compliance in the mounting of tubes  82  in furnace  50 . For example, the flexibility of the mounting of conduits  162  and  164  in conduit  111  permits some adjustment in the alignment of heat exchanger  86  and tube  82  in furnace  50 .  
      As shown in  FIG. 17 , clip  166  may comprise a thin sheet metal, for example, stainless steel plate having a thickness of around 0.040 inches, bent into a U-shape. The thickness of the plate or sheet from which clip  166  is made may vary from about 0.005 inches to about 0.125 inches. Though the aspect of the invention shown in  FIGS. 15-17  includes a plurality of clips  166  retaining a plurality of springs  168 , aspects of the invention may include one or more clips  166  or clip-like structures having the function of clips  166  and one or more spring-like elements performing the function of springs  168 . As shown, the ends  167  of clip  166  may be crimped or bent to attach clip  166  to the end of tube  82 , for example, to a flange of tube  82 . Clip  166  may be mounted to body  150  by one or more fasteners  170 , for example, screws or rivets, through one or more slotted holes  172  in clip  166 . Slotted holes  172  allow clip  160  to translate with respect to body  150 , for example, due to differences in thermal expansion. The mounting of heat exchanger  86  to tube  82  may also include two or more springs  168 , for example, coil springs or Belleville springs, among others, mounted concentrically or axially with respect to each other.  
       FIG. 18A  is a right-hand perspective view of a furnace assembly  200  according to another aspect of the invention.  FIG. 18B  is a detailed view of one aspect of the furnace  200  shown in  FIG. 18A .  FIG. 19  is a left-hand perspective view of a tube furnace assembly  200  shown in  FIG. 18A  with the extraction assembly extended according to aspects of the invention.  FIG. 20  is a front elevation view of the furnace shown in  FIG. 18A .  FIG. 21  is a right side elevation view of the furnace shown in  FIG. 18A .  FIG. 22  is a left side elevation view of the furnace shown in  FIG. 18A .  FIG. 23  is a cross sectional view of the furnace assembly  200  shown in  FIGS. 18A-22 .  
      As shown in  FIG. 18A , furnace assembly  200  includes a treatment chamber  202  and a chamber isolation actuator assembly  204 . The contents of treatment chamber  202  are shown in phantom in  FIG. 18A . Treatment chamber  202  comprises a cylindrical tube  210  capped at a distal end  211  by a cover  212 . Though shown as a circular cylindrical tube in  FIG. 18A , tube  210  may comprise any cylindrical shape, for example, circular cylindrical, rectangular cylindrical, and oval cylindrical, among others. Though not shown in  FIG. 18A , one or more work pieces, for example, photovoltaic precursors, may typically be positioned within tube  210 , for example, on a support structure or “boat.” The proximal end  213  of tube  210  typically is mounted to a plate  214 , for example, for structural support and/or mounting to other fixtures. Treatment chamber  202  may also include one or more access ports  215 , such as flanged ports, for electrical power, instrumentation, or the introduction or removal (that is, purging or venting) of process fluids.  
      In a fashion similar to furnace  50  shown in  FIGS. 3-9 , according to aspects of the invention, work pieces, for example, photovoltaic material precursors, may be treated in treatment chamber  202  with vaporous elements, for example, vaporous Se or S. Furnace assembly  200  includes heating means and/or cooling means for treating work piece for example, according to predetermine temperature schedules, such as the schedule shown in  FIG. 2 . Furnace assembly  200  may include heating means  320  and cooling means  322  in the distal end  211  of tube  210 . Heating means  320  may comprise an electric heating element (for example, a concentric coil heating element) or tubing through which a working fluid, for example, heated air, water, or oil, may be passed (for example, a concentric coil tubing). Heating means  320  may be mounted to the inside or outside surfaces of cover  212  or tube  210  and the heating means may be energized by wire  323 . Cooling means  322  may also comprise tubing through which a coolant is passed, for example, one or more of the coolants referenced above. The coolant tubing may be provided in concentric coil or as one or more cooling coils  324  shown in  FIGS. 18A and 23 .  
      As also shown in  FIG. 18A , furnace  50  may also include a heating assembly  400 , that is, a heating assembly  400  mounted about cylindrical tube  210 . In  FIG. 18A , heating assembly  400  is shown in perspective cross-sectional view. Heating assembly  400  may include a cylindrical housing  402  having a first end  404  and a second end  406 . According to aspects of the invention, cylindrical housing  402  comprises some form of annular heating elements, for example, infrared, conductive, or convective heating elements. In one aspect, housing  402  includes at least one, but typically a plurality of sets of annular heating elements. In the aspect of the invention shown in  FIG. 18A  housing  402  comprises three sections of heating elements: a first section  401  adjacent first end  404  of housing  402 ; a second middle section  403 ; and third section  405  adjacent second end  406  of housing  402 . Sections  401 ,  402 , and  405  may each include a plurality of heating elements  407 , for example, a plurality of resistive heating elements power and controlled by devices not shown.  
      First end  404  includes an annular cover plate  408  having an inside diameter  409  sized to accommodate tube  210 . Plate  408  that may be mounted to housing  402  by a plurality of mechanical fasteners  410 , for example, screws. Plate  408  may be adapted to thermally isolate the heating assembly  400  from tube  210 ; for example, plate  408  may be made from an insulating material, such as a ceramic. First end  404  may also include a sealing element  412  adapted to at least partially seal the space between the outside diameter of tube  210  and inside diameter  409  of plate  408 . Sealing element  412  may be an elastomeric sealing element or a fiberglass, such as Nextel fiberglass, or its equivalent. Second end  404  may include an annular flange  416  having an inside diameter  418 . A cover plate  414  may be mounted to annular flange  416  by a plurality of mechanical fasteners  420 , for example, screws. In one aspect, cover plate  414  includes at least one aperture through which cooling means  322 , heater wire  323 , or cooling tube  324  may pass. The apertures in cover plate  414  may include a sealing element to minimize the escape of fluids.  
      Heating assembly  400  may include at least one port  422  for introducing a cooling medium to and at least one port  424  for removing a cooling medium from heating assembly  400 . Port  422  may comprise a radial hole in cylindrical housing  402  for introducing a cooling medium, for example, a gas, such as air, or a fluid, such as water, to the cavity  415  between heating assembly  400  and tube  210 . Port  424  may be adapted to remove the medium introduced. Ports  422  and  424  may be equipped with appropriate fittings (not shown) to facilitate mounting conduits, such as, tubing, to ports  422  and  424 .  
      According to aspects of the invention, heating assembly  400  may be adapted to regulate heating of tube  210  and its contents by means of individual heating zones, for example, at least two distinct heating zones. In the aspect of the invention shown in  FIG. 18A , tube  210  is heated by five (5) heating zones. Heating zone  1  may be associated with the heating means mounted to sealing plate  330 , heating zone  2  may be associated with the heating section  410  in first end  404  of housing  402 , heating zone  3  may be associated with the middle heating section  403  of housing  402 , heating zone  4  may be associated with heating section  405  of second end  406  of housing  402 , and heating zone  5  may be associated with the heating means mounted to cover plate  212 . According to aspects of the invention, the temperature of these zones may be regulated to provide the desired treatment of the work piece introduced to tube  210 , for example, to regulate the heating and/or cooling of a photovoltaic precursor to provide a solar cell with enhanced performance or reliability.  
      The isolation of treatment chamber  200  may be effected by chamber isolation actuator assembly  205  that is adapted to compress a sealing plate  330  against an internal flange  332  in cylinder  210  to isolate a volume of cylinder  210 . Isolation actuator assembly  205  includes at least, and typically two, cylinder actuator assemblies  334 , a common mounting plate  335 , and a central tube assembly  336 . Cylinder actuator assemblies  334  may each include a long stroke cylinder  338  and a short stroke cylinder  340 . Long stroke cylinder  338  and short stroke cylinder  340  may be pneumatic or hydraulic; the fluid control lines are omitted from  FIGS. 18A and 19 . Long stroke cylinders  340  are mounted at a first end to mounting plate  342  (see  FIG. 19 ), by means of bracket  344  and the second end, or working end, of long stroke cylinder  340  is mounted to short stroke cylinder  340 , for example, by means of mechanical fasteners. Door  342  may represent a portion of a housing into which furnace  200  is mounted. Short stroke cylinder  340  is mounted to mounting plate  335  by means of mechanical fasteners. Cylinders  338  and  340  displace plate  335  and rod  346  and sealing plate  330 . Long stroke cylinders  338  may be used for large displacements of sealing plate  330 , for example, during gross insertion or extraction. Short stroke cylinders  340  may be used for fine displacement of sealing plate  330 , for example, during engagement or disengagement of sealing plate  330  and internal flange  332 .  FIG. 19  illustrates an aspect of the invention in which long stoke cylinders  338  are extended. Mounting plate  335 , which may be displaced by one or more cylinder actuator assemblies  334 , is mounted to support rod or tube  346 . Support rod  346  is mounted to sealing plate  330  which is translated with the movement of mounting plate  335 . Support rod  346  is positioned inside of central tube assembly  336 . The displacement of sealing plate  330  may also be practiced manually, for example, by means of a handle and camming mechanism. The configuration of the sealing plate  330  mounted to a support rod  346 , cylinders  338 , and ball bearings (not shown) enables a pressure gradient between the treatment tube  210  and the area disposed between  214  and the back of the sealing plate  330  to be about one atmosphere.  
      Central tube assembly  336  provides a housing that, among other things, isolates the inside of tube  210  and supports rod  346 . Central tube assembly  336  includes a flanged nozzle  348  mounted to plate  342 , a dual-flanged spool  350 , a dual flanged bellows assembly  352 , and a seal plate  354 . Seal  354  may provide a vacuum-tight sealing means between the bellows assembly  352  and mounting plate  335 , for example, by means of one or more elastomeric o-rings  355 . According to one aspect of the invention, seal plate  354  and o-rings  355  are located at a distal location from the treatment zone in tube  210 , that is, between sealing plate  330  and cover plate  212 , whereby low-temperature sealing means may be used and the likelihood of thermal damage to the sealing means is minimized or prevented. This aspect of the invention further comprises an o-ring disposed between door  342  and plate  214 , which allows an additional low-temperature sealing means. Bellows assembly  352  includes rods  356  which retain the bellows assembly  352  in the compressed state when the cylinders  338  retract support rod  346 . The compression of bellows assembly  352  may be varied by means of a biasing device  358 , for example, a spring, a flexure, or a pneumatic cylinder. Central tube assembly  336  may also include a bearing support for rod  346 , for example, a low-friction bearing or roller bearing (not shown) mounted within spool  350 , for instance, centrally mounted within spool  350 . The bearing support may support tube  346  during insertion, extraction, and operation of furnace  200 .  
      As shown most clearly in the detail of  FIG. 18B , sealing plate  330  is mounted to support rod  346  by means of short mounting rod  358 , for example, by means of welding or mechanical fastener  360 . As shown in  FIG. 18B , sealing plate  330  mates with internal annular surface or flange  332  of tube  210  to provide a seal for treatment tube  210 . Due to the high temperatures under which treatment may be practiced, the mating surfaces of sealing plate  330  and flange  332  typically exhibit metal-to-metal contact with no additional sealing means there between. In one aspect, a sealing element may be provided, for example, an elasotomeric sealing element that can withstand the typical treatment temperatures expected. However, in another aspect of the invention, no elastomeric seals are needed.  
      In one aspect of the invention, sealing plate  330  may also include heating or cooling means and provide a surface upon which an element may be mounted and delivered to tube  210 . For example, sealing plate  330  may include an electric heating element or heating fluid coils  362  similar to distal end  211  of tube  210 . Also, sealing plate  330  may include tube  364  through which a working fluid can be passed. The outer surface of tube  364  may provide a surface (similar to surface  151  of heat exchanger  8 6) to which a treatment element, for example, Se, may be applied and subsequently volatilized for introducing an element-containing vapor to tube  210 . An electrical conduit  365  to heat the heating means or the cooling fluid tubing  367  may be located within support tube  346 . For example, support tube  346  may include tube connections  366  or electrical connections  368 . The tubing or wiring may access the inside of support tube  346  through an aperture  370  through plate  335  (see  FIG. 19 ).  
      According to aspects of the invention, tube furnace  200  may be used to treat work pieces, for example, CIG precursors, in a fashion similar to the operation of furnace  50 . Tube furnace  200  may first be “charged” with treatment element, for example, by introducing and heating the solid element, for example, Se, to volatilize the element, and then cooling to an internal surface of furnace  200  to cause the vaporous element to condense. In one aspect, the cover plate  212  at the distal end  211  of tube  202  may be cooled by means of cooling tube  324 , which may extend inside tube  210 , whereby the element condenses on an external surface of tube  324 . As in other aspects of the invention, after charging, the work piece to be treated may be introduced to furnace  200 , the furnace  200  may be closed by activating isolation actuator assembly  205  whereby plate  230  engages flange  232  to isolate tube  210 . The work piece to be treated and the treatment element may then be heated, for example, according to the schedule shown in  FIG. 2 , to treat the work piece and minimize the loss of treatment element.  
      According to aspects of the present invention, work piece may be treated in furnaces  50  and  200  by means of the following procedures. According to aspects of the present invention, the temperatures of multiple elements of furnaces  50  and  200  are controlled to optimize the treatment. For example, as shown in  FIG. 18A , the temperature of the sealing plate  330 , end plate  212 , tube  210  may be independently controlled. With respect to furnace  200 , shown in  FIG. 3 , the temperature of the tubes  82  and the housing walls (for example, walls  53  and  57 ) may be independently controlled. The following process may be practiced for both furnace  50  and furnace  200 , but the following discussion references furnace  50  only to facilitate the disclosure of the invention.  
      With reference to  FIG. 9 , furnace  50  is first opened and one or more treatment elements, for example, Se, is introduced to the furnace. As noted above, it is to be understood that the expression “treatment element” is used herein to facilitate the disclosure of the invention. The treatment element may comprise a treatment compound comprising two or more elements. According to aspects of the invention, the element comprises elemental sulfur or selenium or combinations of sulfur, selenium, tellurium, indium, gallium, or sodium. The introduction of the treatment element may be practiced by means of the “charging” process described below. In the following discussion, it is assumed that furnace  50  has been charged with Se on the surface  151  of heat exchanger  86  shown in  FIG. 15 .  
      The work piece to be treated with, for example, Se-containing vapor, is then introduced to the treatment tubes  82 , for example, through open door assembly  52  (See  FIG. 9 .). One or more work pieces may be introduced to treatment tube  82  on a sheet or tray to facilitate handling of the work pieces. The work piece introduced to tubes  82  may comprise any material, but in one aspect, the work piece comprises a photovoltaic cell precursor deposited on a substrate, such as the precursor on substrate shown in  FIG. 1   6 A. The substrate may be a metallic or non-metallic substrate, such as, a glass, a steel, a stainless steel, titanium, a ceramic, or a metal-coated plastic, such as a molybdenum-coated polyimide, among other substrate materials. In one aspect of the invention, where hydrogen may be present during treatment, stainless steel substrates are avoided due to stainless steel&#39;s susceptibility to hydrogen embrittlement that may cause instability in the resulting photovoltaic cell. The substrate may be provided as a thin substrate having a thickness of between about 5 microns and about 1 mm, for example, as a metallic foil. In the following discussion, reference will be made to work piece  90 , but it will be understood that in aspects of the invention any material may correspond to work piece  90 .  
      In aspects of the invention, the photovoltaic cell precursor may be any precursor material that can be treated with a vaporous element or compound. In one aspect of the invention, the precursor comprises a precursor containing one or more elements from group 11 (that is, the “coinage metals”), group 12, group 13, and group 16 (that is, the “chalcogens”) of the Periodic Table (group numbering based upon IUPAC convention; the corresponding groups in the “old” convention being  1 B,  2 B,  3 A, and  6 A, respectively). For example, in one aspect, the precursor may contain one or more of copper (Cu), indium (In), gallium (Ga), selenium (Se), sulfur (S), or sodium (Na), or combinations thereof. The precursor may be a Cu—In—Ga containing material, that is, a “CIG” material; a Cu—In—Ga—Se-containing material; or a Cu—In—Ga—Se—S-containing material.  
      After introducing work piece  90  to be treated into tubes  82 , the front door assembly  52  is closed and the furnace  50  is evacuated, for example, by applying a vacuum to one or more of the access ports, for instance a vacuum of, typically, about 10 −3  Torr gage. The furnace  50  may then be purged with a gas, for example, a dry gas, for instance, a dry inert gas, to remove as much moisture as possible. Heat may so applied to remove moisture. The inert gas may be, for example, nitrogen, argon, or helium.  
      According to one aspect of the invention, the treatment tubes  82  may be filled with a treatment gas, for example, a gas that may assist in the subsequent reaction or treatment. The treatment gas may be a forming gas, such as, hydrogen, nitrogen, or combinations thereof. A treatment gas that may also be introduced to tubes  82  may include oxygen, hydrogen selenide (H 2 Se), hydrogen sulfide (H 2 S), or an inert gas, such as argon or helium, among other treatment gases that may be used. For example, a sulfur-containing gas, such as H 2 S, may be introduced to tubes  82  whereby the H 2 S is present during the release of Se to effect a S—Se treatment, for example, to produce CIGSS. In one aspect, no forming gas may be used. The forming gas may also include hydrogen-containing gas other than H 2 Se or H 2 S, for example, water (H 2 O) vapor, ammonia (N 2 H 3 ), an alcohol, or a ketone. In one aspect, the hydrogen-containing gas may provide for the in-situ formation of H 2 Se during treatment with a Se-containing gas. Trace amounts of hydrogen, for example, in the work piece or in the chamber, for example, provided by moisture (H 2 O) in the chamber, may produce trace amounts of H 2 Se formed in situ that, for example, may react with the work piece. In one aspect of the invention, little or no hydrogen is introduced to the treatment chamber. For example, only non-hydrogen-containing gases or no forming gases at all are introduced prior to or during treatment. The gas may be introduced through one or more of the ports distributed about furnace  50 . A vacuum may also be present in tubes  82 . After introducing the gas to furnace  50 , treatment tubes  82  may be closed, for example, by activating valve actuation assembly  68  whereby sealing assembly  84  engages the openings of tubes  82 , for example, to maintain the gas and/or vacuum within tubes  82 . After isolation of tubes  82  by sealing assembly  84 , the volume between the tubes  82  and the walls of furnace  50  may be purged by an inert gas or vacuum to, for example, remove any excess gases or moisture.  
      According to aspects of the invention, upon isolation of tubes  82 , the heating of the work piece  90  can commence. Again, the heating of work piece  90  may be practiced according to the heating schedule shown by curve  32  in  FIG. 2  or another similar heating schedule. The heating of work piece  90  may be practiced by energizing heating elements  88 . The temperature of work piece  90  may be monitored by one or more temperature sensing devices mounted in furnace  50 , for example, thermocouples, resistive thermal devices (RTDs), infrared thermocouples, or a non-contact pyrometer. According to aspects of the invention, the temperature of work piece  90  is elevated to a temperature, for example, above 500 degrees C., at which the vaporous element will react with work piece  90 .  
      As shown, for example, in  FIG. 2 , before, at about the same time, or shortly after the heating work piece  90  per curve  32 , the temperature of the treatment element, for example, the selenium charged to heat exchanger  86 , is raised, for example, according to curve  34  in  FIG. 2 . As discussed above, the temperature of the treatment element may be regulated by controlling the energizing of lamp  158  in heat exchanger  86  and/or controlling the flow and/or temperature of working fluid, for example, air, through heat exchanger  86 . For example, the lower the flow of coolant through heat exchanger  86 , the hotter the element applied to the surface of the heat exchanger  86 . The temperature of the element is raised to a temperature at which the element volatilizes to form an element-containing vapor, for example, for Se, at least about 100 degrees C. However, as discussed above with respect to  FIG. 2 , for example, the temperature may be increased to accelerate the release of element-containing vapor. For example, Se may be elevated to temperature of 500 degrees C. or more to release sufficient Se-containing vapor to provide sufficient reaction with work piece  90 . Again, according to aspects of the present invention, the temperature of the treatment element may be controlled independently of the control of the temperature of work piece  90 .  
      After treatment at temperature, the treatment element and the work piece  90  may be cooled to complete the treatment, cooled prior to further treatment, or cooled for further handling. In one aspect, the temperature of the element is cooled to encourage the condensation of the vaporous element back on the element. This preferred cooling may be effected by rapidly cooling the element, for example, as shown by curve  34  in  FIG. 2  and/or maintaining the work piece  90  and other surfaces inside furnace  50  at an elevated temperature, for example, above 170 degrees C., to discourage condensation on work piece  90  or on other surfaces within furnace  50 . The element can be cooled by de-energizing or reducing the power on lamp  158  in heat exchanger  86  and/or increasing the flow of coolant through heat exchanger  86 . The cooling of work piece  90  may be effected by de-energizing lamps  88 . Typically, work piece  90  is cooled in a controlled fashion to prevent damage to work piece  90 , for example, to prevent cracking or delaminating from the substrate or damage to the substrate itself. In one aspect, furnace  50  and its contents, for example, work pieces  90 , may be rapidly cooled, for example, by forced air convective cooling. A cooling fluid my be introduced to one or more ports of furnace  50 , for example, to flanged port  130  and vented through flanged port  140 , to rapidly cool furnace  50  and its contents. The cooling fluid, for example, air, may be propelled by an air mover, such as a fan or blower, and the fluid may be passed through a cooling device, for example, a cooling heat exchanger or chiller.  
      In one aspect of the invention, the treatment or delivery of work piece  90 , for example, the selenization of work piece  90 , may comprise a steady-state treatment, a pulsed treatment, a cyclic treatment, a ramped treatment, a dual-source treatment, or a combination thereof. The treatment of work piece  90  with the element-containing vapor may be practiced with an excess amount of element-containing vapor, that is, an amount greater than the stoichiometric amount typically required. In steady-state treatment, the temperature of work piece  90  and the treatment element are elevated to treatment temperature, for example, above 400 degrees C., and maintained at the treatment temperature for the duration of treatment. In pulsed treatment, the temperature of work piece  90  is maintained at treatment temperature and the temperature of the treatment element is varied, for example, varied rapidly during treatment. In cyclic treatment, the temperature of work piece  90  is maintained at treatment temperature and the temperature of the treatment element is cyclically varied through, for example, a predetermined temperature cycle. In ramped treatment, the temperature of work piece  90  is maintained at treatment temperature and the temperature of the treatment element is ramped, for example, ramped slowly to a desired temperature during treatment. In dual source treatment or delivery, a gas containing two or more elements, for example, Se and S, may be exposed to work piece  90  at substantially the same time.  
      Dual treatment may also comprise treatment of work piece  90  with two or more vaporous elements or compounds provided by two or more heat exchangers (for example, condensers/evaporators). For example, two or more heat exchanges may be operated at different temperatures depending upon the volatilization temperature of the element or compound being delivered. In one aspect, two or more elements or compounds may be delivered by one heat exchanger, such as heat exchanger  86  shown in  FIG. 15 , for example, by depositing two or more elements or compounds on the surface  151  of heat exchanger  86 . The two or more elements or compounds deposited on the surface of a heat exchanger may typically have different volatilization temperatures, whereby species delivery may be varied by temperature. In another aspect, two or more elements or compounds may be delivered by two or more heat exchangers, such as heat exchanger  86  shown in  FIG. 15 . These two or more heat exchangers adapted to deliver two or more elements or compounds to a treatment chamber may include isolation devices that limit or prevent the release of one or more vaporous elements or compounds while one or more of other vaporous elements or compounds are being released to the treatment chamber. These isolation devices may comprise seal plate or “flapper valve” type devices, for example, devices similar to the devices shown in  FIGS. 11 and 12 . The sequence of treatment with the two or more vaporous elements or compounds may be varied depending upon the desired treatment, for example, the delivery of the two or more vaporous elements may be provided individually or substantially simultaneously. Treatment may also be practiced repeatedly or alternated from one vaporous element to another vaporous element. In one aspect, care may be taken to avoid or prevent the condensation of one vaporous element upon another vaporous element, for example, an element having a higher condensation temperature. Again, undesirable condensation on treatment elements or compounds may be avoided by use of suitable isolation means, such as the sealing devices discussed above.  
      In one aspect of the invention, dual treatment may be practiced for staged release of treatment vapors. For example, one or more heat exchangers may be used having Se and In and/or Ga compounds deposited on their outer surface. The precursor, for example, a Cu—In—Ga precursor, may first be treated with Se by raising the one or more heat exchangers to a first temperature at which Se volatilizes, but In and/or Ga compounds do not. After treatment with Se, the temperature of the one or more heat exchangers may be raised to volatilize, for example the In compound. The In compound vapor may then treat the precursor or the In compound vapor may react with the Se vapor present in the chamber to form indium selenide in situ, where the precursor may then be treated with the indium selenide vapor. A similar staged treatment may be practiced for Ga compounds, where gallium selenide may be formed in situ. Sulfur and In and/or Ga compounds may also be handled in a similar fashion to provide dual treatment with S and In and/or Ga compounds. In one aspect, this dual treatment may be an effective alternative for treating copper-rich precursors to provide effective photovoltaic materials that could not be formed otherwise. Copper-rich precursors are known to have inferior performance due to the electrical shorting effect of the excess copper compounds, for example, copper selenide. Dual treatment of copper-rich precursors according to aspects of the invention, can improve the performance of the resulting absorber.  
      The treatment of work piece  90  with the vaporous element may be practiced repeatedly, for example, three or more times, to provide the desired treatment. Upon completion of the treatment, sealing assembly  84  can be disengaged from treatment tubes  82  and the furnace purged or vented. The vented gases are typically processed to prevent release of gases to the environment.  
       FIG. 24  is a schematic block diagram  26  of a process for charging the treatment element to the enclosure according to one aspect of the invention. In one aspect of the invention, the “charging” of the element to the treatment chamber comprises the process of introducing the treatment element to the treatment enclosure whereby the treatment element can be subsequently released in a vaporous form to react with the work piece being treated. As shown in  FIG. 24 , the process of charging  26  the enclosure with the treatment element may be initiated by introducing a solid treatment element to an enclosure  40 . For example, the treatment element may be introduced as a powder, as beads, or as an ingot. The treatment element may be introduced to the enclosure by simply placing the solid element on the bottom of the enclosure, placing a container (for example, a “boat”) containing the element into the enclosure, or placing the element on an appropriate support means, for example, a shelf or cavity, located in the enclosure. The element may also be automatically fed into the furnace, for example, by means of an automated feeder, for example, an automated wire-element feeder. As noted above, it is to be understood that the expression “treatment element” is used herein to facilitate the disclosure of the invention. The treatment element may comprise a treatment compound comprising two or more elements. According to aspects of the invention, the solid element or compound introduced to the treatment chamber may be an element of group 11, 12, 13, or 16 of the Periodic Table, for example, selenium, sulfur, indium, gallium, indium selenide, indium sulfide, gallium selenide, gallium sulfide, or combinations thereof. The element or compound may also include sodium or a sodium-containing compound.  
      Next, the enclosure is closed, sealed, or otherwise isolated  41  to minimize or prevent the leakage of vaporous element from the enclosure. In one aspect, after isolating the enclosure  41 , the enclosure may be evacuated, for example, by applying a vacuum to the enclosure. In one aspect, a typical vacuum of 10 −3  Torr gage may be applied to the enclosure. The vacuum may be maintained during the charging process. When the enclosure includes an internal treatment chamber, for example, the tube  82  shown in  FIG. 16A , process  26  may include the optional step  47  of isolating the internal treatment chamber. This isolation of the internal chamber may be practiced using a chamber isolation assembly, such as sealing assembly  84  shown in  FIGS. 11 and 12 , for example, where a “flapper” valve isolates the internal chamber.  
      As shown in  FIG. 24 , according to process  26 , the treating element is then heated  42  to a temperature above which the element will volatilize at the prevailing pressure; for example, when the treatment element is Se, the Se is heated to a temperature of between about 100 and about 400 degrees C. The heating of step  42  may, for example, be effected by infrared lamps  88  shown in  FIG. 10 , heating assembly  400  shown in  FIG. 18A , or heating coils  320  shown in  FIG. 18A . At this temperature, the Se begins to volatilize to create a selenium-containing gas into the enclosure. However, to increase the volatilization, the temperature of the element may typically be increased to a temperature greater than the initial volatilization temperature to ensure a plentiful supply of the vaporous element. For example, when Se is used, the Se is typically heated to at least about 500 degrees C. to ensure an adequate supply of Se in vaporous form.  
      Prior to, during, or after the heating the treatment element  42 , at least one surface inside the enclosure is cooled  43  to provide a temperature less than the temperature at which the element volatilizes. For example, again, for Se, this temperature may typically be a temperature less than 100 degrees C., for example, a temperature of about 80 degrees C. or lower. In one aspect, the cooling is practiced to maintain the surface at a temperature below the vapor pressure temperature of the element, for example, Se or S. The cooling of a surface inside the enclosure is typically provided by some form of heat exchanger having a working fluid passing through it. One typical heat exchanger that may be used for aspects of this invention is heat exchanger  86  shown in and described with respect to  FIGS. 15-17 . According to aspects of the invention shown in  FIG. 24 , the cooled surface of the heat exchanger provides a condensation site for the condensation  44  of the vaporous element provided by heating  42 . The heating  42 , cooling  43 , and condensing  44  steps of the charging process  26  may be practiced repeatedly (for example, three or more times) or for an extended period of time (for example, at least about 20 minutes) to provide the desired content of treatment element on a surface inside the enclosure.  
      After sufficient element has been condensed upon the surface, the surface and solid element may be cooled  45  (assuming that the solid element has not completely volatilized) to terminate the volatilization. In one aspect, the cooling of the surface and the treatment element is practiced rapidly, for example, at rate of at least about 10° C./min, to allow at least some of the vaporous element released in step  42  to condense onto the solid element and/or surface. This recapture of the treatment element through controlled or rapid cooling of the solid element and/or surface minimizes the loss of the element to condensation on other surfaces of the enclosure and related structures. In one aspect, during cooling of the element for recapture, the temperature of the surfaces of the enclosure and of any surfaces within the enclosure may be maintained at an elevated temperature, for example, a temperature above 170 degrees C. for Se, to discourage condensation on surfaces other than the cooled surface or solid element. When cooling of the element  45  is completed, the element may be removed from the enclosure  46 . When an internal chamber is used, process  26  may also include the step of opening the internal chamber, for example, disengaging the sealing assembly  84  shown in  FIGS. 11 and 12 . The enclosure, and any internal treatment chambers, may be vented, for example, in preparation for subsequent treatment in the enclosure. With completion of the charging process  26 , with treatment element provided on a surface within the enclosure, the treatment of a work piece as shown and described with respect to  FIGS. 1 and 2  may commence.  
       FIG. 25  is a plot  300  of treatment element vapor pressure as a function of temperature for selenium, though a similar curve may be provided for other treatment elements.  FIG. 26  is a plot  310  of heat exchanger (for example, condenser/evaporator) temperature as a function of coolant flow according to one aspect of the invention. The curves shown in  FIG. 26  were determined for one specific heat exchanger, for example, the tube type device shown in  FIGS. 18A through 23 . Similar curves may be provided for other heat exchangers relating coolant flow to temperature. The curves for other heat exchangers may vary depending upon the size of the heat exchanger, the type of coolant used, and the thermal characteristics of the material from which the heat exchanger is made, among other things. According to one aspect of the invention, the curves shown in  FIGS. 25 and 26  may be used in conjunction with the curves shown in  FIG. 2  to control the operation of a treatment furnace, for example, to control the operation of furnace  50  shown in  FIGS. 3-9  or furnace  200  shown in  FIGS. 18-23 .  
      As shown in  FIG. 25 , the curve  302  in plot  300  represents the relationship of the vapor pressure of selenium, in Torr, as shown in the log scale on ordinate  304  in  FIG. 26 , and temperature, in degrees C, shown on abscissa  303 . Clearly, the vapor pressure of selenium increases with temperature. Plot  310  in  FIG. 26  displays three curves  312 ,  314 , and  316  that correspond to the relationship of the heat exchanger temperature (for example, evaporator/condenser), in degrees C, as shown on ordinate  318  for three furnace temperatures as a function of coolant flow, in standard cubic feet per minute (SCFM), shown on the abscissa  320 . In the aspect of the invention shown in  FIG. 26 , curves  312 ,  314 , and  315  correspond to representative furnace temperatures of 300 degrees C., 400 degrees C., and 500 degrees C., respectively. Again, the shape and magnitude of curves  312 ,  314 , and  318  may vary for other heat exchangers or other furnace operating temperatures.  
      According to aspects of the present invention, the curves that appear in  FIGS. 2, 25 , and  26  may be used to control the operation of a treatment furnace as follows.  FIG. 2  provides one desired temperature schedule for treating a work piece, for example, a photovoltaic precursor, with a vaporous element, for example, vaporous selenium or sulfur. As discussed above, to ensure an adequate supply of vaporous element, for example, Se, to obtain the desired reaction with the work piece, for example, the precursor, the temperature of the element is increased to provide a desired partial pressure of element vapor in the treatment chamber. The desired partial pressure for one aspect of the invention is shown by curve  35  in  FIG. 2 . In order to obtain this element partial pressure, the temperature of the element must be regulated according to the temperature-pressure curve  302  shown in  FIG. 25 . The temperature determined by curve  302  is then used to regulate the flow of coolant through the heat exchanger as indicated by curves  312 ,  314 , and  316  in  FIG. 26 . The flow of coolant, for example, air, to the heat exchanger is then regulated, for example, by a control valve, to obtain the desired coolant flow. In another aspect of the invention, the temperature of the coolant, for example, air, may be varied to effect the desired element temperature. For example, the temperature of the coolant may be regulated by varying the temperature of a heat exchanger adapted to heat or cool the working fluid, for example, an external heat exchanger having a working fluid passing through it.  
      For example, assuming that from curve  35  in  FIG. 2 , the desired partial pressure for treating a precursor with Se at a temperature T E3  is about 1.0 Torr. From  FIG. 25 , curve  302 , and pressure 1.0 Torr on ordinate  304 , the desired heat exchanger temperature is about 350 degrees C. By comparing a desired heat exchanger temperature of 350 degrees C. with curve  316  in  FIG. 26 , a desired heat exchanger flow rate of, for example, about 14 standard cubic feet per hour (SCFH) is obtained. Therefore, to obtain the desired temperature T E3 , the flow of coolant, for example, air, through the heat exchanger is regulated to about 14 SCFH. The flow of coolant, for example, air, through a heat exchanger according to aspects of the invention may vary from about 10 SCFH to about 25 SCFM, and is typically between about 0.1 SCFM and 3 SCFM.  
      This control of the operation of the coolant flow to regulate the heat exchanger may be practiced manually, but is preferably, practiced in an automated fashion, for example, by means of computer, programmable logic controller (PLC), temperature feedback loop, PID controller, or another automated controller. For example, in one aspect, the curves illustrated in  FIGS. 2, 25 , and  26 , may be programmed into a computer or PLC and operated to control the operation of, for example, an automated valve controller of a coolant flow valve.  
      The methods and apparatus according to aspects of the invention described above may be used to manufacture an improved photovoltaic material, for example, a material having little or no hydrogen content. Such a material has the advantage of not being prone to the deterioration in performance that characterizes prior art materials having hydrogen. For example, as discussed above, in one aspect of the invention, a precursor may be treated with a treatment gas, such as a selenium-containing vapor, with little or no presence of hydrogen. In prior art methods, selenium is typically introduced in the form of H 2 Se whereby hydrogen (H) inherently is introduced to the reaction and to the absorber matrix. According to aspects of the invention, the treatment chamber can be effectively purged of essentially all hydrogen by means of vacuum and/or non-hydrogen purge gas. As a result, the precursor, for example, the CIG precursor, can be treated with a Se— or S-containing vapor (that is, a H-free vapor) to produce an essentially H-free absorber. In one aspect of the invention, the absorber comprises a CIGS absorber having less then 5% hydrogen content, or typically less than 1% hydrogen content, or can be substantially hydrogen free. Such a low-hydrogen or hydrogen free absorber, for example, a low-H or H-free CIGS or CIGSS absorber, can provide more reliable performance without the degradation that characterizes hydrogen-containing absorbers.  
      The control and operation of furnaces  50  and  200  may be performed manually or by means of one or more automated controllers, for example, a personal computer or PLC. These control devices may include specially designed software designed to monitor and control the operation of furnaces  50  and  200 . Furnaces  50  and  200  may typically include sensors, for example, temperature sensors, pressure sensors, and flow sensors, and controllers, for example, automatic temperature, pressure, or flow controllers to regulate and control the operation of furnaces  50  and  200 . The electrical connections associated with these sensors and controllers may pass through one or more of the many access ports associated with furnaces  50  and  200 .  
      In one aspect of the invention, the methods and apparatuses disclosed herein may be practiced or utilized in a batch mode, for example, one or more work pieces may be treated in furnaces  50  or  200  and the work pieces removed for subsequent treatment of further work pieces. However, according to another aspect of the invention, the methods and apparatuses disclosed herein may be adapted for continuous treatment whereby a substantially continuous flow of work pieces may be processed according to the disclosed methods or handled by, for example, introduced and removed from, the disclosed apparatus. In one aspect, unlike the temperature variations shown  FIG. 2 , the treatment conditions of the continuous methods may be substantially stable as work pieces are introduced and removed from the treatment chambers.  
      Aspects of the present invention provide improved means of treating work pieces, especially, improved means of treating and producing photovoltaic cells. Methods and apparatus according to aspects of the present invention can assist in reducing the production costs of photovoltaic cells whereby photovoltaic energy can be a cost effective alternative to the diminishing supply of fossil fuels.  
      While several aspects of the present invention have been described and depicted herein, alternative aspects may be conceived by those skilled in the art to accomplish the same objectives. Accordingly, it is intended by the appended claims to cover all such alternative aspects that fall within the true spirit and scope of the invention.