Patent Application
20100226629
1. Field of the Inventions
The present invention relates to method and apparatus for preparing thin films of semiconductor materials for radiation detector and photovoltaic applications.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since the early 1970's there has been an effort to reduce the cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors including some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (0, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that have yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. In summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance, the Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with the Ga/(Ga+In) molar ratio varying from 0 to 1, and the Se/(Se+S) molar ratio varying from 0 to 1.
One technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where metallic components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a Culn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
Two-stage process approach may also employ stacked layers including Group VIA materials. For example, a Cu(In,Ga)Se2 film may be obtained by depositing In—Ga—Se and Cu—Se layers in an In—Ga—Se/Cu—Se stack and reacting them in presence of Se. Similarly, stacks including Group VIA materials and metallic components may also be used. Stacks including Group VIA materials include, but are not limited to In—Ga—Se/Cu stack, Cu/In/Ga/Se stack, Cu/Se/In/Ga/Se stack, etc.
Selenization and/or sulfidation (or sulfurization) of precursor layers including metallic components may be carried out in various forms of Group VIA material(s). One approach involves using gases such as H2Se, H2S or their mixtures to react, either simultaneously or consecutively, with the precursor including Cu, In and/or Ga. This way a Cu(In,Ga)(S,Se)2 film may be formed after annealing and reacting at elevated temperatures. It is possible to increase the reaction rate or reactivity by striking plasma in the reactive gas during the process of compound formation. Se vapors or S vapors from elemental sources may also be used for selenization and sulfidation. Alternately, as described before, Se and/or S may be deposited over the precursor layer including Cu, In and/or Ga and the stacked structure can be annealed at elevated temperatures to initiate reaction between the metallic elements or components and the Group VIA material(s) to form the Cu(In,Ga)(S,Se)2 compound.
Reaction step in a two-stage process is typically carried out in batch furnaces. In this approach, a number of pre-cut substrates, typically glass substrates, with precursor layers deposited on them are placed into a batch furnace and reaction is carried out for periods that may range from 15 minutes to several hours. Temperature of the batch furnace is typically raised to the reaction temperature, which may be in the range of 400-600° C., after loading the substrates. The ramp rate for this temperature rise is normally lower than 5° C./sec, typically less than 1° C./sec. This slow heating process works for selenizing metallic precursors (such as precursor layers containing only Cu, In and/or Ga) using gaseous Se sources such as H2Se or organometallic Se sources. For precursors containing solid Se, however, slow ramp rate causes Se de-wetting and morphological problems. For example, reacting a precursor layer with a structure of base/Cu/In/Se by placing it in a batch furnace with a low temperature rise rate (such as 1° C./sec) yields films that are powdery and non-uniform. Such films do not yield high efficiency solar cells.
One prior art method described in U.S. Pat. No. 5,578,503 utilizes a rapid thermal annealing (RTP) approach to react the precursor layers in a batch manner, one substrate at a time. Such RTP approaches are also disclosed in various publications (see, for example, Mooney et al., Solar Cells, vol:30, p:69, 1991, Gabor et al., AIP Conf. Proc. #268, PV Advanced Research & Development Project, p:236, 1992, and Kerr et al., IEEE Photovoltaics Specialist Conf., p:676, 2002). In the prior art RTP reactor design the temperature of the substrate with the precursor layer is raised to the reaction temperature at a high rate, typically at 10° C./sec. It is believed that such high temperature rise through the melting point of Se (220° C.) avoids the problem of de-wetting and thus yields films with good morphology.
Design of the reaction chamber to carry out selenization/sulfidation processes is critical for the quality of the resulting compound film, the efficiency of the solar cells, throughput, material utilization and cost of the process. From the foregoing, there is a need for methods and apparatus to carry out reaction of precursor layers for CIGS(S) type absorber formation, in a roll-to-roll manner. Roll-to-roll or reel-to-reel processing increases throughput and minimizes substrate handling. Therefore, it is a preferred method for large scale manufacturing of flexible solar cell structures.
The present inventions provide methods and integrated tools to form solar cell absorber layers on continuous flexible substrates. Roll-to-roll or reel-to-reel thermal or rapid thermal processing (RTP) tools (reactors) are used to react a precursor layer on a continuous flexible workpiece.
Several embodiments are given for the roll-to-roll tools that improve their versatility and the quality of the semiconductor absorber layers processed using such reactors.
An aspect of the present invention includes a reactor having multiple exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. The entrance opening and the exit opening of the process gap are open to the atmosphere. By controlling each exhaust outlet independently with valves, process times of a precursor layer in different sections of the process gap may be extended or shortened. This versatility of the reactor allows conversion of precursor layers into absorber layers having different compositions and molar ratios using the reactor of the present invention.
Another aspect of the present invention includes a reactor including multiple gas inlets and exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. The entrance opening and an exit opening of the process gap may be open to the atmosphere. A first gas inlet is connected adjacent the entrance opening and a second gas inlet is connected adjacent the exit opening of the process gap so that when an inert gas is applied through the first and second gas inlets, the inert gas flow forms a diffusion barrier, namely a first diffusion barrier and a second diffusion barrier, efficiently sealing the entrance and exit openings. A vacuum pump connected to an exhaust outlet which is placed between the first and second gas inlets and connected to the process gap establishes gas flows within the process gap from the first and second gas inlets towards the exhaust outlet such that these gas flows prevent the process gases within the process gap from leaving the process gap through the entrance opening and the exit opening.
Another aspect of the present invention includes a reactor including multiple gas inlets and exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. A supply chamber or unwind port is connected to the entrance opening of the process gap and the exit opening is open to the atmosphere. A first diffusion barrier is formed at the entrance opening by applying an inert gas flow through an external gas inlet connected to the supply chamber. The flow of this inert gas into the process gap is controlled by a gas outlet connected adjacent the entrance opening of the process gap.
Another aspect of the present invention includes a reactor including multiple gas inlets and exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. The ends of the process gap are sealed with a supply chamber or unwind chamber and a receiving chamber or rewind chamber. Accordingly, a first diffusion barrier is formed at the entrance opening of the process gap; a second diffusion barrier may be formed at the exit opening of the process gap; and a third diffusion barrier may be formed between the first and the second diffusion barriers.
Another aspect of the present invention includes an exhaust system separates the Group VIA material vapors from other gaseous species for re-cycling or easy disposal.
Yet another aspect of the present invention includes a technique and apparatus for efficient removal of moisture from the workpiece before processing precursor layer in the roll-to-roll rapid thermal processing (RTP) tool.
In a preferred aspect is provided an apparatus used to react precursor material disposed over a sheet-shaped continuous workpiece to form a solar cell absorber, the apparatus comprising: a process gap defined by a peripheral wall, wherein the sheet-shaped continuous workpiece travels between an entry opening and an exit opening of the of the process gap, wherein within the process gap a reaction process is used to form the solar cell absorber from the precursor material on the sheet-shaped continuous workpiece; an unwind port sealably attached to the entry opening of the process gap, wherein the unwind port includes an unwind chamber with a supply roll disposed therein from which the sheet-shaped continuous workpiece is advanced into the process gap through the entrance opening; a rewind port sealably attached to the exit opening of the process gap, wherein the rewind port includes a rewind chamber with a receiving roll disposed therein that receives and wraps therearound the sheet-shaped continuous workpiece from the process gap through the exit opening; and a first moisture removal unit that operates in conjunction with the unwind port and includes: a moisture desorption device to remove moisture from the sheet-shaped continuous workpiece as the sheet-shaped continuous workpiece is unwound from the supply roll disposed within the unwind chamber, wherein the moisture removed from the sheet-shaped continuous workpiece is contained within the unwind chamber, and a moisture absorption device to remove the moisture that is contained within the unwind chamber from the unwind chamber.
In another preferred aspect is provided an exhaust system to remove Group VIA material vapors from a reactor used to process precursor layers to form Group IBIIIAVIA compound thin films for solar cells, the reactor including an exhaust outlet, the exhaust system comprising: a first material collector unit, including a collector, adapted to connect to the exhaust outlet of the reactor through a first connector line to receive a first exhaust gas flow from the reactor, wherein the first exhaust gas flow includes at least one Group VIA material vapor and a carrier gas, and wherein the first connector line is maintained at a second temperature that is lower than a first temperature of the exhaust outlet so that a first amount of the Group VIA material carried by the first exhaust gas flow liquefies and flows into the collector thereby forming a first precipitate of the Group VIA material within the first material collector unit; a second material collector unit, including a condenser, connected to the first material collector unit through a second connector line to receive a second exhaust gas flow from the first material collector, wherein a second amount of the Group VIA material carried by the second exhaust gas flow is condensed by the condenser maintained at a third temperature that is lower than the second temperature so as to form a second precipitate within the second material collector unit, and wherein the second amount of the Group VIA material is less than the first amount of the Group VIA material; a third material collector unit, including a filter, connected to the second material collector unit through a third connector line to receive a third exhaust gas flow from the second material collector, and wherein a third amount of the Group VIA material carried by the third exhaust gas flow is filtered by the filter so as to collect a third precipitate in the filter, and wherein the third amount of the Group VIA material is less than the second amount of the Group VIA material; and wherein a fourth exhaust gas flow leaves the third material collector through a fourth connector line, and wherein the fourth exhaust gas flow is the carrier gas that is substantially free of the Group VIA material.
Reaction of precursors, comprising Group IB material(s), Group IIIA material(s) and optionally Group VIA material(s) or components, with Group VIA material(s) may be achieved in various ways. These techniques involve heating the precursor layer to a temperature range of 350-600° C., preferably to a range of 400-575° C., in the presence of at least one of Se, S, and Te provided by at least one of the sources such as; i) solid Se, S or Te sources directly deposited on the precursor, and ii) H2Se gas, H2S gas, H2Te gas, Se vapors, S vapors, Te vapors etc. for periods ranging from 1 minute to several hours. The Se, S, Te vapors may be generated by heating solid sources of these materials away from the precursor also. Hydride gases such as H2Se and H2S may be bottled gases. Such hydride gases and short-lifetime gases such as H2Te may also be generated in-situ, for example by electrolysis in aqueous acidic solutions of cathodes comprising S, Se and/or Te, and then provided to the reactors. Electrochemical methods to generate these hydride gases are suited for in-situ generation.
Precursor layers may be exposed to more than one Group VIA materials either simultaneously or sequentially. For example, a precursor layer comprising Cu, In, Ga, and Se may be annealed in presence of S to form Cu(In,Ga)(S,Se)2. The precursor layer in this case may be a stacked layer comprising a metallic layer containing Cu, Ga and In and a Se layer that is deposited over the metallic layer. Alternately, Se nano-particles may be dispersed throughout the metallic layer containing Cu, In and Ga. It is also possible that the precursor layer comprises Cu, In, Ga and S and during reaction this layer is annealed in presence of Se to form a Cu(In,Ga)(S,Se)2.
Some of the preferred embodiments of forming a Cu(In,Ga)(S,Se)2 compound layer may be summarized as follows: i) depositing a layer of Se on a metallic precursor comprising Cu, In and Ga forming a structure and reacting the structure in gaseous S source at elevated temperature, ii) depositing a mixed layer of S and Se or a layer of S and a layer of Se on a metallic precursor comprising Cu, In and Ga forming a structure, and reacting the structure at elevated temperature in either a gaseous atmosphere free from S or Se, or in a gaseous atmosphere comprising at least one of S and Se, iii) depositing a layer of S on a metallic precursor comprising Cu, In and Ga forming a structure and reacting the structure in gaseous Se source at elevated temperature, iv) depositing a layer of Se on a metallic precursor comprising Cu, In and Ga forming a structure, and reacting the structure at elevated temperature to form a Cu(In,Ga)Se2 layer and/or a mixed phase layer comprising selenides of Cu, In, and Ga and then reacting the Cu(In,Ga)Se2 layer and/or the mixed phase layer with a gaseous source of S, liquid source of S or a solid source of S such as a layer of S, v) depositing a layer of S on a metallic precursor comprising Cu, In and Ga forming a structure, and reacting the structure at elevated temperature to form a Cu(In,Ga)S2 layer and/or a mixed phase layer comprising sulfides of Cu, In, and Ga, and then reacting the Cu(In,Ga)S2 layer and/or the mixed phase layer with a gaseous source of Se, liquid source of Se or a solid source of Se such as a layer of Se.
It should be noted that the Group VIA materials are corrosive. Therefore, materials for all parts of the reactors or chambers that are exposed to Group VIA materials or material vapors at elevated temperatures should be properly selected. These parts should be made of or should be coated by substantially inert materials such as ceramics, e.g. alumina, tantalum oxide, titanium oxide or titania, zirconium oxide or zirconia, etc., glass, quartz, certain types of stainless steel, graphite, refractory metals such as Ta, refractory metal nitrides and/or carbides such as Ta-nitride and/or carbide, Ti-nitride and/or carbide, W-nitride and/or carbide, other nitrides and/or carbides such as Si-nitride and/or carbide, etc.
Reaction of precursor layers comprising Cu, In, Ga and optionally at least one Group VIA material may be carried out in a reactor that applies a process temperature to the precursor layer at a low rate. Alternately, rapid thermal processing (RTP) may be used where the temperature of the precursor is raised to the high reaction temperature at rates that are at least about 10° C./sec. The Group VIA material, if included in the precursor layer, may be obtained by techniques such as evaporation, sputtering and electroplating. Alternately inks comprising Group VIA nano particles may be prepared and these inks may be deposited to form a Group VIA material layer within the precursor layer. Other liquids or solutions such as organometallic solutions comprising at least one Group VIA material may also be used. Dipping into melt or ink, spraying the melt or ink, doctor-blading or ink writing techniques may also be employed to deposit such layers.
A reel-to-reel apparatus 100 or roll to roll RTP reactor to carry out reaction of a precursor layer to form a Group IBIIIAVIA compound film is shown in
Annealing and/or reaction steps may be carried out in the reactors of the present invention at substantially the atmospheric pressure, at a pressure lower than the atmospheric pressure or at a pressure higher than the atmospheric pressure. Lower pressures in reactors may be achieved through use of vacuum pumps.
The reel-to-reel apparatus 100 of
The flexible structure 106A before the reaction may be a base with a precursor film deposited on at least one face of the base. The flexible structure 106B after the reaction comprises the base and a Group IBIIIAVIA compound layer formed as a result of reaction of the precursor layer. It should be noted that we do not distinguish between the reacted and unreacted sections of the flexible structure 106 in
A Cu(In,Ga)(Se,S)2 absorber layer may be formed using the single chamber reactor design of
After loading the unreacted flexible structure 106A or web on, for example, the first spool 105A, one end of the web may be fed through the chamber 101, passing through the gaps 111 of the slits 110, and then wound on the second spool 105B. Doors (not shown) to the first port 103 and the second port 104 are closed and the system (including the first port 103, the second port 104 and the chamber 101) is evacuated to eliminate air. Alternately the system may be purged through the exhaust 112 with an inert gas such as N2 coming through any or all of the gas inlets or gas lines for a period of time. After evacuating or purging, the system is filled with the inert gas and the heater system 102 may be turned on to establish a temperature profile along the length of the chamber 101. When the desired temperature profile is established, the reactor is ready for process.
During the process of forming, for example, a Cu(In,Ga)Se2 absorber layer, a gas comprising Se vapor or a source of Se such as H2Se may be introduced into the chamber, preferably through chamber gas inlet 113. The exhaust 112 may now be opened by opening its valve so that Se bearing gas can be directed to a scrubber or trap (not shown). It should be noted that Se is a volatile material and at around the typical reaction temperatures of 400-600° C. its vapor tends to go on any cold surface present and deposit in the form of solid or liquid Se. This means that, unless precautions are taken during the reaction process, Se vapors may pass into the first port 103 and/or the second port 104 and deposit on all the surfaces there including the unreacted portion of the web in the first port 103 and the already reacted portion of the web in the second port 104. To minimize or eliminate such Se deposition, it is preferable to introduce a gas into the first port 103 through the first port gas inlet 107A and introduce a gas into the second port 104 through the second port gas inlet 107B. The introduced gas may be a Se-bearing and/or S-bearing gas that does not breakdown into Se and/or S at low temperature, but preferably the introduced gas is an inert gas such as N2 and it pressurizes the two ports establishing a flow of inert gas from the ports towards the chamber 101 through the gaps 111 of the slits 110. The velocity of this gas flow can be made high by reducing the gaps 111 of the slits 110 and/or increasing the flow rate of the gas into the ports. This way diffusion of Se vapor into the ports is reduced or prevented, directing such vapors to the exhaust 112 where it can be trapped away from the processed web. The preferred values for the gap 111 of the slits 110 may be in the range of 0.5-5 mm, more preferably in the range of 1-3 mm. Flow rate of the gas into the ports may be adjusted depending on the width of the slits which in turn depends on the width of the flexible structure 106 or web. Typical web widths may be in the range of 1-4 ft.
Once the Se-bearing gas and inert gas flows are set and the desired temperature profile of the chamber 101 is reached, the flexible structure 106 may be moved from the first port 103 to the second port 104 at a pre-determined speed. This way, an unreacted portion of the flexible structure 106 comes off the first roll 105A, enters the chamber 101, passes through the chamber 101, gets reacted forming a Cu(In,Ga)Se2 absorber layer on the base of the web and gets rolled onto the second spool 105B in the second port 104. It should be noted that there may be an optional cooling zone (not shown) within the second port 104 to cool the reacted web before winding it on the second spool 105B.
The above discussion is also applicable to the formation of absorber layers containing S. For example, to form a Cu(In,Ga)S2 layer the Se-bearing gas of the above discussion may be replaced with an S-bearing gas such as H2S. To form a Cu(In,Ga)(Se,S)2, a mixture of Se-bearing gas and S-bearing gas may be used. Alternately, a Se-bearing precursor may be utilized and reaction may be carried out in an S-bearing gas.
One feature of the system 100 of
It should be noted that in addition to the reactor temperature and the web speed, the reaction gas composition may also be changed in the multi-step reaction approach described above. For example, during the first reaction step when the web is moved from left to right a first gas such as H2Se may be used in the chamber 101 to form a selenized precursor layer. During the second reaction step when the web is moved from right to left, on the other hand, another gas such as H2S may be introduced in the chamber 101. As a result, the selenized precursor layer may be reacted with S as the web is moved from the second spool 105B to the first spool 105A and thus a Cu(In,Ga)(Se,S)2 layer may be grown by converting the already selenized precursor layer into sulfo-selenide. By selecting the gas concentrations, web speeds and reaction temperatures, the amount of Se and S in the absorber layer may be controlled. For example, S/(Se+S) molar ratio in the final absorber layer may be increased by increasing the web speed and/or reducing the reaction temperature during the first process step when reaction with Se is carried out. Similarly, the S/(Se+S) molar ratio may also be increased by reducing the web speed and/or increasing the reaction temperature during the second step of reaction where reaction with S is carried out. This provides a large degree of flexibility to optimize the absorber layer composition by optimizing the two reaction steps independent from each other.
Another embodiment of the present invention is shown in
One important feature of the design of
A Cu(In,Ga)(Se,S)2 absorber layer may be formed using the three-section chamber reactor of
The second process gas and the third process gas may be the same gas or two different gases. For example, the second process gas may comprise Se and the third process gas may comprise S. This way when a portion on the flexible structure 106 enters the section A of the three-section chamber 450 through the first gap 111A of the first slit 110A, the precursor layer on the portion starts reacting with Se forming a selenized precursor layer on the portion. When portion enters the low-volume segment 410, it gets annealed in the N2 gas (if section B is heated) within this segment until it enters section C. In section C sulfidation or sulfurization takes place due to presence of gaseous S species, and a Cu(In,Ga)(Se,S)2 absorber layer is thus formed on the portion before the portion exits the three-section chamber 450 through the second gap 111B of the second slit 110B. The S/(Se+S) molar ratio in the absorber layer may be controlled by the relative temperatures and lengths of the sections A and C. For example, at a given web speed the S/(Se+S) ratio may be increased by decreasing the length and/or reducing the temperature of section A. Alternately, or in addition, the length and/or the temperature of section C may be increased. Reverse may be done to reduce the S/(Se+S) molar ratio. It should be noted that, as in the previous example, it is possible to run the flexible structure or web backwards from right to left to continue reactions. It is also possible to change the gases introduced in each section A, B and C of the three-section chamber 450 to obtain absorber layers with different composition. The design of
A variety of different cross sectional shapes may be used for the chambers of the present invention. Two such chambers 500A and 500B having circular and rectangular cross sections, respectively, are shown in
As shown in
As shown in cross sectional view in
In the following, various embodiments of roll-to-roll or reel-to-reel RTP tools will be provided. The RTP tool of the present invention may have at least one cold zone, at least one hot zone and a buffer zone connecting these two zones. The zones in this embodiment are formed along a process gap of the RTP tool. A workpiece is processed in the process gap while it is moved in a process direction. It is understood that the terms “hot” or “warm” or “high temperature” zone and “cold” or “cool” or “low temperature” zone are intended as being conditionally relative, such that the hot/warm/high temperature zone is warmer than the cold/cool/low temperature zone, though the degree of differential does not require a maximum low temperature for the cold zone or a minimum high temperature for the hot zone.
In one embodiment, the zones are preferably placed along the process gap and form a section surrounding a portion of the process gap so that when a portion of the workpiece is advanced through a specific zone, that portion of the workpiece is treated with the thermal conditions that are assigned to that zone. In accordance with the principles of the present invention, buffer zones may be formed as part of a processing gap of the RTP tool and connect two zones which are kept in different temperatures. In this respect, a buffer zone may connect a lower temperature zone to a higher temperature zone, or a higher temperature zone to a lower temperature zone. For example, the low temperature zone may be kept at a first temperature so that a portion of a continuous workpiece is subjected to the first temperature as the portion of the continuous workpiece travels through the low temperature zone. The high temperature zone, on the other hand, may be kept at a second temperature so that the portion of the continuous workpiece is subjected to the second temperature when it travels through the high temperature zone. If the buffer zone connects the lower temperature zone to the higher temperature zone and if the portion of the continuous workpiece is made to travel from the lower temperature zone to the higher temperature zone, the temperature of the portion of the continuous workpiece is increased from the first temperature to the second temperature as it travels through the buffer zone. This, in effect, provides conditions of rapid thermal processing to the portion of the continuous workpiece. The continuous workpiece is moved at a predetermined speed through the buffer zone from the low temperature to high temperature zones of the thermal processing tool zone such that the rate of heating experienced by a portion of the continuous workpiece as it travels through the buffer zone can be easily made 10° C./second or much higher (such as 100-500° C./sec) by selecting the values for the low temperature, the high temperature, the speed of the continuous workpiece and the length of the buffer zone. In a particular embodiment, the buffer zone is less than 10% of the length of the high temperature zone, and in a preferred embodiment the length of the buffer zone is in the range of 1-5% of the length of the high temperature zone. In preferred embodiments, the specific length of the first buffer zone is less than 10 cm, and preferably less than 5 cm. This flexibility and the ability to reach very high temperature rates at low cost, keeping the processing throughputs very high are unique features of the present design.
A continuous workpiece 716 is moved with a predetermined speed in the process gap 708 during the process, in the direction depicted by arrow A. In this embodiment, a cooling system (not shown) may be used to maintain low temperature in cold zone 704, and a heating system (not shown) is used to maintain high temperature in the hot zone 706. As will be described more fully below, the buffer zone 702 is a low thermal conductivity zone connecting the cold zone to hot zone so that both zones are maintained in their set temperature ranges without any change by using a short buffer zone. It should be noted that the shorter the buffer zone is, the higher the temperature rise rate can be experienced by a portion of a workpiece moving at a constant speed through the buffer zone. In that respect, the present invention achieves buffer zone lengths in the range of 2-15 cm, making it possible to keep one end of the buffer zone at room temperature (about 20° C.) and the other end at a high temperature in the range of 500-600° C. The low thermal conductivity characteristics of the buffer zone may be provided by constructing at least one of the top wall, bottom wall and optionally side wall of the buffer zone, or at least a portion of them with low thermal conductivity materials and/or features. As shown in
As shown in
In this embodiment, the RTP tool includes a first cold zone 812A, a first buffer zone 814A, a hot zone 816, a second buffer zone 814B, and a second cold zone 812B. Accordingly, the first buffer zone 814A facilitates heating of the workpiece 804, and the second buffer zone 814B cooling of the workpiece 804. The second buffer zone 814B connects the hot zone, which is kept in a high temperature, to the cold zone, which is kept in a lower temperature. In this embodiment, in order to cause a slower rate of cooling, the second buffer zone 814B may be longer than the first buffer zone 814A, which may be kept short to facilitate rapid heating of the workpiece. A cooling system with cooling members 818 cools the cold zones 812A and 812B. An exemplary cooling system may be a cooling system using a fluid coolant such as a gas or liquid coolant. The hot zone 816 includes a series of heating members 820 placed along the hot zone 816. Heating members each may be controlled separately or in groups through use of temperature controllers and thermocouples placed near the heating members in each zone. In that respect it is possible to separate the hot zone in multiple heated zones with one or more heaters that are controlled separately. In this embodiment, the buffer zones 814A and 814B include low thermal conductivity features 821 to reduce flow of heat from the hot zone towards the cold zones.
Details of the buffer zones will be described using
In the following embodiments the roll-to-roll or reel-to-reel thermal processing or RTP tools include a reactor having an insert placed in a primary gap of the reactor. The primary gap of the reactor is defined by peripheral reactor walls including a top reactor wall, a bottom reactor wall and side reactor walls as will be further described below. The insert includes a secondary gap, also called process gap hereinafter, through which a continuous workpiece travels between an entry opening and an exit opening of the insert. The process gap is defined by insert walls including a top insert wall, a bottom insert wall and side insert walls. This process gap height and width may be varied along the process gap, and there can be separate zones as described above. The process gap, within the insert is preferably in the range of 2 mm-20 mm height and 10-200 cm width. An aspect ratio (height to width ratio) for the process gap may be between 1:50 and 1:2500. Such inserts, as well as the web valves and rollers that will be discussed later, may be used in any of the previously discussed reactors of
In one embodiment an inner space exists between at least one of the insert walls and at least a portion of the peripheral reactor walls. The width of the inner space or the distance between the at least one of the insert walls and the portion of the peripheral reactor walls may be in the range of 2-20 mm, preferably 3-5 mm. At least one gas inlet is connected to the inner space, and at least one exhaust opening connects the process gap as well as the inner space to outside and carries any gaseous products to outside the process gap and the primary gap of the reactor. Sealable doors or web valves may seal the entrance and the exit of the process gap when needed before or after the process, especially when the continuous workpiece stops moving. As the continuous workpiece with a precursor material film such as a precursor layer comprising Cu, In, Ga and Se, is continuously fed into the process gap and treated with heat and process gases (such as an inert gas, a selenium containing gas and/or a sulfur containing gas), a flushing gas such as nitrogen is delivered to the inner space through the gas inlets. Then the flushing gas, the process gas, and any other gaseous species that may be created in the process gap as a result of the heat treatment of the precursor layer within the process gap, are exhausted through the exhaust opening. During the process, at the beginning of the process or at the end of the process, movement of the continuous workpiece may be halted and the entrance and the exit doors may be sealed. In one embodiment, the bottom insert wall may include rollers on which the continuous workpiece may be moved without damaging its back surface.
As shown in
As shown in
An inner space 1112 is established between the peripheral walls 1102 and the insert 1104. Plugs 1112A are placed near the entrance 1107A and exit 1107B. Gas inlet lines 1114 provided through the peripheral walls 1102 to allow a flushing gas, depicted by arrows ‘F’, to flow into the inner space 1112. An exhaust opening 1116 is placed between the entrance 1107A and the exit 1107B, and runs through the peripheral walls 1102 and the insert 1104 to remove the exhaust gas, depicted by the arrow ‘E’, from the reactor 1100. The bottom wall 1110B of the insert 1104 may have rollers 1120 on which the continuous workpiece 1105 is moved.
During the process, the flushing gas F is flowed into the gas inlets 1114 and thereby into the inner space 1112. The gas is unable to escape near the entrance 1107A and exit 1107B because of the presence of the plugs 1112A, and it is directed towards the exhaust 1116. Process gases, depicted by the arrows ‘E’, which may be inert gases, are fed through the entrance opening 1107A and the exit opening 1107B into the process gap 1108 of the insert 1104, as a moving mechanism (not shown) moves a portion of the continuous workpiece 1105 into the process gap 1106 for reaction. The process gases P provide a barrier against discharge of selenium and sulfur vapors present in the process gap 1108 to outside of the process gap through the entrance and exit. The established process gas flow urges such vapor species to move over the top surface of the continuous workpiece 1105 towards the exhaust 1116 where they mix with the flush gas and removed as the exhaust gas E into a trap that condenses them safely. Flowing process gas moves the reactive species (such as Se and/or S) along with the continuous workpiece, keeping these species over the reacting precursor layer. This way residence time of the reacting species over the precursor layer is increased enhancing the reaction between the precursor layer the reactive species, and thus enhancing overall utilization of the volatile reactive species. For example, in batch RTP processes employed to form CIGS layers using a precursor layer comprising Cu, In, Ga and Se; an amount of selenium that is 20-100% more than what is necessary for the formation of CIGS is included in the precursor layer because these reactors loose much of the volatile Se species during the reaction process. In the present design, the volatile Se species, after they evaporate out of the precursor layer, stay over the precursor layer on other parts of the continuous workpiece and eventually get utilized. Therefore, in the roll-to-roll process of the present invention, precursor layers comprising Cu, In, Ga and Se may be prepared to have no excess Se or only up to about 10% excess Se. This is considerable savings over the prior art approaches that required 20-100% excess Se in the precursor layers. It should be noted that if the Se amount in the reactor is not adequate, the CIGS films formed under Se deficient conditions do not yield high efficiency solar cells because they typically contain low resistivity Cu-Se binary phases. During the process, the sealable doors 1109A and 1109B are kept in the open position to let the process gases P in through the entrance 1107A and the exit 1107B. However, as will be described more fully below during the processes intervals, the sealable doors 1109A and 1109B are moved into closed position or a second position, as shown with dotted lines, to seal the entrance 1107A and the exit 1107B by pressing the seal members 1111 onto the front surface 1105A and the back surface 1105B of the continuous workpiece 1105. It should be noted that the seal members against the back surface 1105B of the continuous workpiece 1105 may or may not be employed, i.e. only the top seal members may be used and the back surface of the workpiece may be supported by a flat surface.
As mentioned above, the continuous workpiece 1105 may be supplied from a supply spool adjacent the entrance opening 1107A and received by a receiving spool adjacent the exit opening 1107B of the reactor 1100. The supply spool and the receiving spool may be kept in a supply chamber (similar to the first port 103 of
There are advantages in using the reactor 1100 equipped with the sealable doors 1109A and 1109B of the present invention together with the above described vacuum sealed supply and receiving chambers to process a roll of the continuous workpiece 1105.
In one exemplary process, when processing of an entire roll of the continuous workpiece 1105 in the reactor 1100, which may be 500-1000 meters long, is almost completed in the reactor 1100, the process is halted while still a portion of the continuous workpiece 1105, which may be 2-4 meters, is still wrapped around the supply spool. Next, the sealable doors 1109A and 1109B seal the entrance opening 1107A and the exit opening 1107B by moving into the sealing position. As described above, in the sealing position, the seal members 1111 of the sealable doors 1109A, 1109B contact the front surface 1105A and the back surface 1105B of the workpiece 1105 to seal the entrance and exit openings. Once the reactor 1100 is sealed in this manner, the supply chamber is opened to atmosphere and a roll of a new continuous workpiece is loaded into the supply chamber and connected to the portion of the continuous workpiece that extends to the receiving spool. During this time, the process gap is protected from air by the sealable doors. After the supply chamber is resealed, pumped down and the sealable doors 1107A and 1107B are moved into open position, the continuous workpiece 1105 is fully advanced into the receiving chamber while pulling a leading end of the new continuous workpiece into the receiving chamber. In the following step, sealable doors are once again brought into the sealing position but this time on the front and back surfaces of the new continuous workpiece; and then the receiving chamber is unsealed and opened to detach the processed workpiece from the leading end of the new continuous workpiece and to remove the processed roll of the workpiece 1105 from the receiving chamber. Next, the leading end of the new workpiece is attached to the receiving spool; the receiving chamber is sealed and pumped down; and the sealable doors 1109A and 1109B are moved into the open position to start processing the new workpiece in the reactor 1100. Benefits of sealing the reactor in this manner especially during the workpiece loading unloading intervals are generally three fold: (1) sealing speeds up the loading a new workpiece roll and unloading the processed one; (2) sealing keeps the process gap of the reactor clean and free of oxidizing species at such intervals; and (3) sealing reduces the amount of Se in the exhaust traps since the complete removal of Se from the reactor is not required, which further enhances the utilization of Se and reduces the amount of cleaning and maintenance of the traps.
Other embodiments of reel-to-reel, or roll to roll apparatus to carry out reaction of a precursor layer on a flexible workpiece to form a Group IBIIIAVIA compound film on the workpiece will now be presented.
Accordingly, a low temperature section 2104A is located adjacent the entrance opening 2108A; a cooling section 2104C is located adjacent the exit opening 2108B of the process gap 2104; and a high temperature section 2104B is located between the low temperature section 2104A and the cooling section 2104C. Furthermore, the continuous reactor 2100 comprises a first reactor region 2100A and a second reactor region 2100B. The temperature in the low temperature section 2104A, the high temperature section 2104B and the cooling section 2104C may be in the range of 20-350° C., 400-600° C., and below 100° C., respectively. Unprocessed sections of the continuous workpiece 2105, entering the process gap 2104, may be unwrapped from a supply spool (not shown) and the processed portions, exiting the process gap, are taken up and wound around a receiving spool (not shown). During the process, inert gases such as nitrogen may be flowed into the process gap 2104 through the entrance opening 2108A and exit opening 2108B to form a diffusion barrier against volatile species such as Group VIA material containing vapors within the process gap 2104 to escape through the entrance opening 2108A and exit opening 2108B. Process gases may also be provided to the process gap 2104 by at least one gas inlet connected to the process gap 2104. Used gases and Group VIA containing vapors are removed from the process gap 2104 through an exhaust opening 2113 placed closer to the exit opening 2108B. It should be noted that other exhausts or exhaust openings located at different locations between the entrance opening 2108A and exit opening 2108B may also be utilized (see for example
In the novel reactor designs of the embodiments herein it is possible to react the precursors with more than one species in a serial manner. In this case, it is necessary to separate the more than one gaseous species from each other within the process gap 2104 as previously discussed with reference to the design of
In the design of the embodiments herein, in order to avoid any physical contact between the top layer 2107 of the continuous workpiece 2105 and the top wall 2102A of the peripheral walls 2102 when the continuous workpiece 2105 is advanced through the process gap 2104, one or more movable buffer members 2114 are placed adjacent the top wall 2102B, preferably in the high temperature section 2104B. In this embodiment the movable buffer members 2114 may be protection rollers that rotate and prevent any surface damage if the top layer 2107 touches them. The protection rollers 2114 are provided to prevent any scratching of the top layer 2107 if the continuous workpiece bows up against the top wall 2102B because of the thermal expansion caused by the entry of the continuous workpiece into the high temperature section 2104B from the low temperature section 2104A. This is a unique problem associated with continuous metallic webs or workpieces where a first portion of the web is kept at a low temperature, for example at room temperature, while the temperature of a second portion which is adjacent to the first portion is raised to an elevated temperature, such as to a temperature range of 250-600° C. In such a situation the second portion expands while the first portion stays the same. This causes the web to deform to absorb the dimensional differential (which is a width differential in the case of a thin and wide foil) between the first and second portions. Because of the low aspect ratio of the process gap 2104, the vertical distance between the top wall 2102B and the front surface 2106A of the continuous workpiece may be about 2-10 mm. Without the protection rollers 2114, in the high temperature section 2102B, the continuous workpiece may bow upwardly and the top layer 2107 may touch the top wall 2102B as it moves, resulting in damage to the absorber layer that is forming. In order to prevent this contact between the workpiece and top wall in such narrow process gaps, increasing the height of the gap to the 10-25 mm range may be considered as one of the solutions. However, as explained before, narrow gaps have attractive benefits in terms of increased materials utilization and efficient use of generated heat. Use of protection rollers results in further reduction of the height of the process gap, which further increases the efficiency of the reaction process. In one embodiment, the length of the protection rollers 2114 may be substantially equal to the width of the process gap 2104 and the protection rollers may be placed within the semi circular cavities 2115 extending along the width of the top wall 2102B. The length of the rollers may be in the range of 20-200 cm or longer depending on the width of the process gap 2104. The diameter of the rollers may be in the range of 2-20 mm, preferably in the range of 3-6 mm. They may be constructed using inert materials such as ceramics, quarts and graphite, or they may have inert coatings on their outer surfaces. The rollers may be driven (rotated by a motor) at a speed such that their surface linear velocity is about the same as the linear velocity of the moving workpiece. Alternately, the rollers may be idle rollers that rotate only when touched by the workpiece. Although in
The principles of using the movable buffer members described in the embodiments herein to avoid physical damage to the top layer 2107 of the continuous workpiece 2105 may be advantageously used for further narrowing the process gap 2104 at a selected location without concern about damage to the top layer 2107 of the continuous workpiece 2105. Referring to
The isolation function of the isolation roller 2116 may be seen in a partial view of the continuous reactor 2100 shown in
A workpiece comprising a precursor layer deposited on a flexible base may be used in the reactor of
It should be noted that more isolation rollers may be placed in various other locations within the process gap 2104 to enhance material utilization and/or improve isolation between various reactor segments or regions. For example, an additional isolation roller (not shown) may be placed to the right of the exhaust opening 2113 to improve materials utilization of gaseous species in the second reactor segment 210013 between the exit opening 2108B and the additional isolation roller. The additional isolation roller also substantially prevents the gaseous species coming from the first reactor segment 2100A towards the exhaust opening 2113, from entering the second reactor segment 2100B between the exit opening 2108B and the additional isolation roller. This way two different reactions may be carried out in the two segments of the reactor very efficiently and avoiding damage to the solar cell absorber layer surface. Isolation rollers may even be used at the entry and exit openings to reduce the inert or process gas flows through these openings while preventing the volatile species from coming out of the reactor through these openings. Also it is possible to place rollers into the bottom peripheral walls to reduce damage to the back side of the workpiece. Planarization rollers may also be used in a low temperature segment of the reactor as described in U.S. patent application Ser. No. 12/345,389, which application is expressly incorporated by reference herein.
Through control of the gas flows into and the gas flows put of the process gaps of the exemplary reactors shown in
In an embodiment of the present invention, a multi exhaust reactor comprises multiple exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. The entrance opening and the exit opening of the process gap may be connected to a supply chamber and a receiving chamber, respectively, in a sealed manner. By controlling each exhaust outlet independently with valves, the process time of a precursor layer may be extended or shortened in a segment or section of the process gap. This versatility of the reactor allows changing and controlling the composition and the molar ratio of an absorber layer obtained by reacting a precursor layer using the reactor of the present invention.
The top surface 1506A of the workpiece 1506 may comprise a precursor layer to be converted into a CIGS(S) layer. As described before there are many different types of such precursors, but a precursor layer comprising Cu, In, Ga and Se will be described below. In this exemplary process, the first gas may be an inert gas such as nitrogen (N2) establishing a first gas flow 1530 which moves in the same direction as the workpiece 1506, and the second gas may be a S containing gas such as H2S establishing a second gas flow 1531, which is in the opposite direction of the workpiece motion. Accordingly, as a portion of the workpiece is moved from the entrance opening 1507 towards the exit opening 1508, that portion of the precursor layer starts reacting, i.e. Cu, In, Ga and Se within the precursor layer start reacting with each other and then when they get exposed to S species they all start reacting with S present in the process gap 1511. The reactor 1500 shown in
If the valve V3 is open and the valves V1 and V2 are closed, then the first gas flow 1530 mainly flows over the workpiece between the entrance opening 1507 and the third exhaust opening 1503A. In this configuration the second gas flow 1531 comprising S, flows between the exit opening 1508 and the third exhaust opening 1503A. Both the first gas flow 1530 and the second gas flow 1531, along with any reaction by-products, are directed outside of the process gap by the third exhaust 1503. Since in this example the first gas is an inert gas, a portion of the precursor layer travelling between the entrance opening 1507 and the third exhaust opening 1503A is exposed to the inert gas and possibly to a vapor of Se resulting from the evaporation of Se from the portion of the precursor layer, due to the heating in the section of the process gap between the entrance opening 1507 and the third exhaust opening 1503A. Therefore, during a time period “t3” when the portion of the precursor layer is travelling between the entrance opening 1507 and the third exhaust opening 1503A, the Cu, In and Ga species in the portion of the precursor layer react with Se, forming selenide species. When the portion of the precursor layer passes the third exhaust opening 1503A and travels towards the exit opening 1508 for a time period “t4”, it gets exposed to more and more S species. Therefore, reaction with S is carried out for the time period “t4”. The resulting absorber layer of the portion may have a chemical formulation that can be represented as Cu(In,Ga)SzSew, a material with a second S/(S+Se) ratio of z/(z+w).
Since, ti<t3 and t2>t4, the compositions of the absorber layers obtained from the two exemplary cases given above would differ in relative S and Se content in a way that can be represented by the relationship x/(x+y)>z/(z+w). In other words, the location of the used exhaust opening would determine the S/(S+Se) molar ratio of the absorber layer and therefore its bandgap and electronic quality. However, one specific S/(S+Se) molar ratio, which may be good for a specific Cu/(In+Ga) ratio of a precursor layer may not be good for another precursor layer with a different Cu/(In+Ga) ratio. Therefore, a multi-exhaust tool such as the one shown in
In the reactor designs depicted in
There are various ways of establishing a diffusion barrier against the flow of volatile Group VIA materials towards the entrance and exit openings of the reactors of the present inventions. One way is to connect the entrance and exit openings to a first port (also called supply chamber or unwind chamber or port) and a second port (also called receiving chamber or rewind chamber or port), respectively (see for example
As will be described below in another embodiment of the present invention, a reactor may comprise multiple gas inlets and exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. The entrance opening and the exit opening of the process gap may be open to the atmosphere. Preferably a first gas inlet may be connected adjacent the entrance opening and a second gas inlet connected adjacent the exit opening of the process gap so that when an inert gas is applied through the first and second gas inlets, the inert gas flow forms a diffusion barrier, namely a first diffusion barrier and a second diffusion barrier, efficiently sealing the entrance and exit openings.
The reactor 1600 has a first gas inlet 1601 connected to the process gap 1616 near the entrance opening 1614 and a second gas inlet 1602 connected to the process gap near the exit opening 1615. A first process gas, which is preferably an air-free or oxygen free gas, is flowed through the first gas inlet 1601 to establish a gas flow Fl. A second process gas, which is also an air free or oxygen free gas, is flowed through the second gas inlet 1602 to establish a gas flow F2. There is an exhaust 1603 connected to the process gap 1616 between the first gas inlet 1601 and the second gas inlet 1602. An optional trap 1605 is connected to the exhaust 1603 to condense and collect the Group VIA species (Se in this example). The trap 1605 is further connected to the inlet of a flow controller 1606 such as a mass flow controller through a first pipe 1608. The outlet of the flow controller 1606 is connected to the inlet of a vacuum pump 1607 through a second pipe 1609. There may also be filters (not shown) before the mass flow controller and/or the vacuum pump 1607. The outlet of the pump 1607 discharges into the atmosphere. Since this design of the reactor 1600 operates at substantially the atmospheric pressure, control of the gas flow F3 through the exhaust 1603 is established through the use of the flow controller 1606 and the pump 1607. The pump 1607 creates vacuum or a pressure that is lower than the atmospheric pressure in the second pipe 1609 which is connected to the outlet of the flow controller 1606. The pressure of gases at the inlet (the first pipe 1608) of the flow controller 1606, on the other hand, is nearly atmospheric. This pressure differential between the inlet and outlet of the flow controller 1606 allows it to control the gas flow F3 accurately.
During processing, as the workpiece 1620 is moved through the heated process gap 1616 with a pre-set temperature profile, the gas flow F3 is set at a value that is smaller than the sum of the gas flow F1 and the gas flow F2, i.e. F3<F1+F2, and F3=F1B+F2B, wherein F1B and F2B are gas flows originating from F1 and F2 and flowing in the process gap 1616 towards the exhaust 1603. This setting assures that two other gas flows F1A and F2A are established near the entrance opening 1614 and the exit opening 1615, respectively. The gas flow F1A flows through a portion of the process gap 1616 between the first gas inlet 1601 and the entrance opening 1614, towards the entrance opening 1614. Thus the gas flow F1A establishes a barrier against air entering the process gap 1616 from outside through the entrance opening 1614. The gas flow F2A flows through a portion of the process gap 1616 between the second gas inlet 1602 and the exit opening 1615, towards the exit opening 1615. Thus the gas flow F2A establishes a barrier against air entering the process gap 1616 from outside through the exit opening 1614. As can be understood from the above discussion, while the flows F1A and F2A are used to stop diffusion of air into the process gap 1616, the flows F1B and F2B are utilized as barriers against the vaporized Group VIA material (Se in this example) to diffuse towards the entrance opening 1614 and the exit opening 1615, respectively. This is very important since Group VIA materials are poisonous and cannot be allowed to leave the process gap 1616 through the entrance opening 1614 or the exit opening 1615. Furthermore, as explained before, their condensation on the workpiece, especially after formation of the Group IBIIIAVIA absorber layer deteriorates the performance of such absorber layer as a solar cell material. The reactor 1600 is very versatile; therefore by choosing the position of the exhaust, the number of exhausts, and by selecting the values for the gas flows F1, F2 and F3, the process results can be influenced and improved. For example, for a 30 cm wide 5-10 mm high process gap: if F1 is 5 liters/min, F2 is 5 liters/min and F3 is 2 liters/min; if both the first process gas and the second process gas are nitrogen; and if the exhaust position is closer to the second gas inlet 1602 such that resistance to gas flow from the first gas inlet 1601 position to the exhaust 1603 position through the process gap 1616 is twice the resistance from the second gas inlet 1602 position to the exhaust 1603 position; the values of the various gas flows would be about: F1A=4.33 liters/min, F1B=0.67 liters/min, F2A=3.67 liters/min, and F2B=1.33 liters/min. In this example, the relatively high flows of F1A and F2A keep air out of the process gap 1616 by being effective diffusion barriers, while the relatively low flows of F1B and F2B push the Se vapors over the workpiece for their better utilization in the reaction and keep the Se vapors away from the entrance opening 1614 and the exit opening 1615. It should be noted that in this design it is preferable that the portion of the process gap 1616 around the exhaust 1603 and the exhaust 1603 itself be kept at high temperature such as at a temperature over 350° C., preferably at a temperature over 400° C. to avoid condensation of Se in such locations. It should also be noted that more than one gas inlets near the entrance opening 1614 and more than one gas inlets near the exit opening 1615 may be used. Additional gas inlets (not shown) may also be used between the first gas inlet 1601 and the second gas inlet 1602 to introduce other gases into the process gap 1616. For example, to form a CIGS(S) absorber layer a third gas inlet (not shown) may be connected to the process gap 1616 between the first gas inlet 1601 and the exhaust 1603. A sulfur bearing gas such as H2S may be introduced through this third gas inlet at a flow rate of F4. In this case the gas flow F3 needs to be adjusted such that F1B>F4 and F3=F1B+F4+F2B, and F3<F1+F2+F4. In this case the flows F1B and F2B stop both Se and S species from reaching the entrance opening 1614 and the exit opening 1615 while the precursor of the workpiece is reacted with both Se and S in the process gap 1616 to form the CIGS(S) absorber layer. It should be noted that the reactor designs described above may benefit from the rollers in the bottom wall or the top wall (i.e. movable barriers) described in
Alternatively, it is also possible to have one side of the process gap open to the atmosphere and the other side to be sealed using a chamber or port. Such a system would also have the capability to carry out reactions at around atmospheric pressure.
In operation the rewind port 1720 or receiving chamber and optionally the process gap 1706 may be evacuated and backfilled with nitrogen to eliminate air. Then the gas flows F6, F7 and F8 are established such that F8<F6+F7, and F8=F7A+F6B, where F7A=F7. In this case, there would be no flow from the second gas inlet 1702 towards the exit opening 1708 of the process gap 1706 since the rewind port 1720 is sealed and cannot be pressurized. Consequently, all of the flow F7 flows towards the exhaust 1704 as flow F7A. It should be noted that in this case the flow F6A which forms a barrier against air diffusion into the process gap 1706 through the entrance opening 1707 can be simply selected by adjusting the flows F6, F8 and F7. The flow F8 is selected to be larger than F7 so that all of the F7 flow gets exhausted without moving towards the entrance opening 1707. The flow F8 is also selected to be smaller than F7+F6 so that a barrier against air diffusion is established at the entrance opening 1707. For example, for a 30 cm wide 5-10 mm high process gap: if F6 is 5 liters/min, F7 is 5 liters/min and F8 is 6 liters/min; and if the first process gas and the second process gas are nitrogen, under these conditions, irrespective of the exhaust position it would be easy to calculate that the values of F6A and F6B would be 4 liters/min and 1 liters/min, respectively. Increasing the value of F6 without changing the values of F7 and F8 would increase the value of the flow F6A, i.e. the barrier against air diffusion, while not influencing the flow F6B which is used for reaction and as a barrier against the Group VIA material diffusion towards the entrance opening 1707. The flow F7A acts as a diffusion barrier to prevent the Group VIA vapors from entering into the rewind port 1720 through the exit opening 1708, and it can be adjusted at will in this design since it is equal to F7.
Alternatively, it is also possible to use a sealed un-wind port attached to the entrance opening and have the exit opening exposed to the atmosphere as shown in the system 1750 of
In operation, the unwind port 1760 and optionally the process gap 1752 may be evacuated and backfilled with nitrogen to eliminate air. Then the gas flows F9, F10 and F11 are established such that F11<F9+F10, and F11=F9B+F10B, where F9B=F9. In this case, there would be no flow from the first gas inlet 1755 towards the entrance opening 1753 of the process gap 1752 since the unwind port 1760 is sealed and cannot be pressurized. Consequently, all of the flow F9 flows towards the exhaust 1758 as flow F9B. It should be noted that in this case the flow F10A, which forms a barrier against air diffusion into the process gap 1752 through the exit opening 1754, can be simply selected by adjusting the flows F9, F10 and F11. The flow F11 is preferably selected to be larger than F9 so that all of the F9 flow gets exhausted without moving towards the exit opening 1754. The flow F11 is also selected to be smaller than F9+F10 so that a barrier against air diffusion is established at the exit opening 1754. For example, for a 30 cm wide 5-10 mm high process gap: if F9 is 1 liters/min, F10 is 8 liters/min and F11 is 3 liters/min; and if both the first process gas and the second process gas are nitrogen, irrespective of the exhaust position it would be easy to calculate that the values of F10A and F10B would be 6 liters/min and 2 liters/min, respectively. Increasing the value of F10 without changing values of F9 and F11 would increase the value of the flow F10A, i.e. the barrier against air diffusion through the exit opening 1754, while not influencing the flow F10B which is used for reaction and as a barrier against the Group VIA material diffusion towards the exit opening 1754. The flow F9B acts as a diffusion barrier to prevent the Group VIA vapors from entering into the unwind port 1760, and it can be adjusted at will in this design since it is equal to F9.
As indicated before, the functionality of the systems in
As will be described below in another embodiment of the present inventions, a reactor may comprise multiple gas inlets and exhaust outlets connected to a process gap of the reactor between an entrance opening and an exit opening of the process gap. In this embodiment, the ends of the process gap may be sealed with a supply chamber or unwind chamber or port and a receiving chamber or rewind chamber or port. Accordingly, a first diffusion barrier may be formed at the entrance opening of the process gap; a second diffusion barrier may be formed at the exit opening of the process gap; and a third diffusion barrier may be formed between the first and the second diffusion barriers.
The design in
It should be noted that two or more of the reactor system design aspects described with reference to
It should also be noted that roll to roll reactors may have at least one of their entrance opening and exit opening open to the atmosphere. Such reactors preferably employ vacuum pump(s) that are connected to the outlet lines or outlet ports of the gas flow controllers in their exhaust systems. The reason, as explained before, is that in such reactors, which are open to the atmosphere, it is very difficult to build pressure within the process gap and therefore at the inlet lines or inlet ports of the gas flow controllers unless large volumes of gases are flowed in the process gap and the resistance to gas flow towards the entrance opening and/or the exit opening is increased within the process gap through the use of gap lowering restrictions or barriers, etc. This represents high usage of gases, which is not attractive from economical point of view. Also, as explained before, in some processes it is preferable to use low gas flows over the workpiece surface to increase the residence time of reactive species (such as Se and/or S vapors) in the process gap. In reactors with both the entrance opening and the exit opening of the process gap sealed using sealed chambers or ports (such as a supply chamber or unwind port and a receiving chamber or rewind port shown above), vacuum pumps may or may not be used in the exhaust systems since the process gaps of such reactors may be pressurized by small process gas flows into the process gaps and since the only way for the gasses to get out of the reactor is through the exhaust and the gas flow controllers . Usage of vacuum pumps in this case improves accuracy of the gas flow controllers by increasing the pressure differential between their inlet and outlet ports. Use of vacuum pumps also reduce or prevent possible back diffusion of air from the exit port of the gas flow controller back into its inlet port and eventually back into the process gap. Such back diffusion is especially important at low process gas flows through the exhaust, and if it happens, it deteriorates the photovoltaic quality of the Group IBIIIAVIA material layer being formed.
Reactors in
The system 2000 efficiently eliminates air/oxygen from the common space in the system through the use of vacuum before the reaction process is carried out on the workpiece 2020, but at the same time reduce the time needed to unload a processed workpiece roll from the receiving chamber 2002 and load a fresh or unprocessed workpiece roll into the supply chamber 2001. For example, when the reaction or processing of a roll of a first continuous workpiece is almost completed, the process is halted while still a portion (e.g. 2-4 meters long portion) of the first continuous workpiece is wrapped around the supply spool 2021. Next, as the doors (not shown) of the supply chamber 2001 and the receiving chamber 2002 are opened to access these ports, a first gas flow F30 and a second gas flow F31 are established and flowed into the process gap 2011 through the first gas inlet 2013 and the second gas inlet 2014, respectively. At the same time, an exhaust gas flow F32 is also established through the use of a vacuum pump 2023 of the exhaust system 2012 as described before. It should be noted that at this time, the system 2000 is equivalent to the previously described systems with the two ends (i.e. the entrance opening 2026 and the exit opening 2027) of the process gap 2011 open to air atmosphere. The exhaust flow F32 and the first and second gas flows F30 and F31 are chosen so that F32=(F34+F35)<(F30+F31), and a third gas flow F33 and a fourth gas flow F36 flow towards the supply chamber 2001 and the receiving chamber 2002, respectively. The third gas flow F33 and the fourth gas flow F36 are able to flow because the doors of the supply chamber 2001 and the receiving chamber 2002 are open to the atmosphere at this time. The third gas flow F33 and the fourth gas flow F34 constitute barriers against possible diffusion of air from the atmosphere into the central part (the part between the first gas inlet 2013 and the second gas inlet 2014) of the process gap 2011 through the entrance opening 2026 and the exit opening 2027. Therefore, a new roll of a second continuous workpiece wrapped around a second supply spool may now be loaded into the supply chamber 2001 and connected to the portion of the first continuous workpiece that extends to the receiving spool 2022 in the receiving chamber 2002. Then the first continuous workpiece 1105 is fully advanced into the receiving chamber 2002 while pulling a leading end of the second continuous workpiece into the receiving chamber 2002. In the following step, the processed workpiece or the first continuous workpiece is detached from the leading end of the second continuous workpiece and removed from the receiving chamber 2002 along with the receiving spool 2022 that it is wrapped around. Subsequently a second empty receiving spool is placed into the receiving chamber 2002 and the leading end of the second continuous workpiece is attached to the second receiving spool. The system would now be ready to process the fresh roll of the second workpiece. The roll to roll reaction may be initiated after the doors to the supply chamber 2001 and the receiving chamber 2002 are closed, the first gas flow F30, which is in this example an inert gas flow, and the second gas flow F31, which is also an inert gas flow, are shut down by valves (not shown), and the common space in the system is evacuated and then back filled with an inert gas as described before. As can be seen from the above description, the design of
As mentioned above in the background, roll-to-roll or reel-to-reel processing increases throughput and minimizes substrate handling. Therefore, it is a preferred method for large scale manufacturing. Design of the reaction chamber to carry out selenization/sulfidation processes is critical for the quality of the resulting compound film, the efficiency of the solar cells, throughput, material utilization and the cost of the process. Further, the exhaust gases from such reactors contain Se and S which are environmentally unsafe materials and must be efficiently removed from the exhaust gas and recycled. Therefore, it is highly desirable to develop exhaust systems for efficient removal of Se and S from the exhaust gases of the reactors processing CIGS(S) materials. Such removed Se and/or S may also be re-used increasing materials utilization.
In another embodiment, a preferred reactor apparatus for processing precursor layers to form Group IBIIIAVIA absorber films to manufacture solar cells follows from the description of the reactors shown in
The reactor exhaust system 3220, which is placed to collect the exhaust gas from a reactor, such as exhaust 1116 of
Next, a second flow 3224B of the exhaust gas is flowed from the first material collector unit 3220A to the second material collector unit 3220B or the condenser unit through a second connector line 3226B. The second flow 3224B may include a smaller amount, for example up to about 8-10%, of the Se/S vapor originally contained in the first flow 3224A. The balance of the second flow 3224B is the carrier gas. In the condenser unit 3220B, the second flow 3224B comes in physical contact with a low temperature surface kept at temperature T3 which is less than the temperature T2. This low temperature causes the formation of a second precipitate 3228B on the low temperature surface. The second precipitate 3228B is made of Se and/or S which condenses in the condenser unit 3220B on the low temperature surface. The condenser unit 3220B may remove most of the remaining Se/S vapor, for example at least about 90% of the Se/S vapor that escaped the first material collector unit 3220A and was left in the second flow 3224B. The condenser unit may be cleaned off the collected Se/S at process intervals.
In the following step, a third flow 3224C of the exhaust gas is flowed from the condenser unit 3220B to the third material collector unit 3220C, which may be a filter unit, through a third connector line 3226C. The third flow 3224C may still include some residual Se and/or S. But since the temperature of the exhaust gas is low by this time, the Se and/or S in the third flow 3224C is typically in the form of small particles carried by the carrier gas. In the filter unit 3220C, the third flow 3224C is filtered to obtain a third precipitate 3228C from the exhaust gas. The third precipitate 3228C is made of Se and/or S solid particles and it is kept in the filter unit 3220C until emptied at process intervals.
A fourth flow 33224D of the exhaust gas leaving the filter unit 3220C through a fourth connector line 3226D is substantially free of Se and/or S and consists of almost entirely the carrier gas, e.g. N2 gas. The fourth flow 3224D of the exhaust gas may directly open to atmosphere. In a preferred embodiment the fourth flow may be directed to a vacuum pump (not shown). In another preferred embodiment the fourth flow may first be directed to the inlet of a gas flow controller or a mass flow controller (not show) and the outlet of the mass flow controller maybe connected to an inlet of a vacuum pump. Of course various valves may be included in the system to make it more serviceable and practical.
It should be noted that the carrier gas may include, in addition to an inert gas, one or more reactive gases, such as H2S and H2Se. In this case the exhaust system described collects the portion of Se and/or S that may chemically break away from their respective gases (H2Se and H2S) and form Se and/or S vapor. However, portions of these gases that stay chemically intact pass through the first, second and third material collection units and may become part of the fourth flow 3224D since H2S and H2Se remain in gas form for temperatures used in the exhaust system of
The flow chart 3230 shown in
Referring to
A second flow 3258B of the exhaust gas is flowed into the condenser unit 3250B through a second connector line 3254B. In this embodiment condenser unit 3250B includes a cooled surface 3262 for example a water cooled coil which may be kept at or below room temperature T3. Interaction of the second flow 3258B of the exhaust gas with the cooled surface 3262 causes the condensation of most (for example preferably at least 90%) of the remaining Se vapor left in the second flow 3258B on the cooled surface 3262. The cooled surface 3260 holding the solid Se particles is cleaned at process intervals.
Despite the fact that most of the Se is removed in the solidifier unit 3250A and the condenser unit 3250B, some selenium particles may still be carried with the exhaust gas through the condenser unit 3250B. Such Se particles may be fully removed in the next step in the filter unit 3250C. Accordingly, a third flow 3258C of the exhaust gas is flowed into the filter unit 3250C through a third connector line 3254C to further remove any solid particles carried with the exhaust gas. In this embodiment filter unit preferably comprises several filters. For example, the filter unit 3250C may comprise a first set of coarse filters 3263A to collect large Se particles with a size above 50μ and a second set of fine filters 3263B to collect small Se particles with a size above 1μ. The third flow 3258C is first passed through the coarse filters 3263A and then the fine filters 3263C. Even more filters with finer particle catching properties may also be utilized. Se particles held by the filters 3263A and 3263B are collected in a filter trap 3264 which is cleaned at intervals. A fourth flow 3258 of the exhaust gas consists of the carrier gas and is substantially particle free. The fourth flow 3258 is directed towards the vacuum pump 3253 through the mass flow controller unit 3252. The pump establishes a pressure differential between the inlet and outlet of the mass flow controller by forming a vacuum at the outlet of the mass flow controller. It should be noted that the inlet of the mass flow controller may be at atmospheric pressure. Cleaned exhaust gas is released to the atmosphere through an outlet 3253A of the pump 3252. The exhaust system embodiments described above efficiently remove the Se or S from the exhaust gas and allow them to be reused through a recycling process.
Solar cells may be fabricated on the compound layers formed in the reactors of the present invention using materials and methods well known in the field. Once a compound layer is formed, for example a thin (<0.1 microns) CdS layer may be deposited on a top surface of the compound layer using the chemical dip method. A transparent window of ZnO layer may be deposited over the CdS layer using MOCVD or sputtering techniques. A metallic finger pattern is optionally deposited over the ZnO layer to complete the solar cell.
As mentioned above in the background, roll-to-roll or reel-to-reel processing increases throughput and minimizes substrate handling. Therefore, it is a preferred method for large scale manufacturing. Design of the reaction chamber to carry out selenization/sulfidation processes is critical for the quality of the resulting compound film, the efficiency of the solar cells, throughput, material utilization and the cost of the process. Further, as a high temperature process, CIGS reaction is easily fouled by moisture. In roll to roll processing reactors, moisture present in between the workpiece layers in a roll may make its way into the reactor during the loading of the unprocessed roll of the workpiece into the system. Moisture absorbed on the surface of the long workpiece is the main source of this unwanted moisture. Such moisture, which may be chemisorbed or physisorbed and held on the workpiece surface can be released from the workpiece surface as the portions of the workpiece enter the heated reactor and start heating up. This released moisture may then enter the reaction zone and influence the reaction. As the unprocessed roll is unwound and advanced into the reactor, more and more moisture may be introduced into the reaction atmosphere negatively impacting the reaction process. Therefore, it is highly desirable to develop moisture removal techniques and apparatus for efficient removal of moisture from the workpieces before processing CIGS(S) forming precursor materials in the reactors.
As mentioned above, a fresh roll or unprocessed roll is the main source of unwanted moisture entry into roll to roll reactors.
Accordingly, in an embodiment, a preferred reactor system 4230 for processing precursor layers to form Group IBIIIAVIA absorber films to manufacture solar cells, follows from the description of the reactors shown in
The moisture removal unit 4232 getters the moisture, collecting it before the moisture finds its way into the reaction process gap. As shown in
Referring to
The moisture absorption device 4232B may have a container 4246 including a first opening 4247A and a second opening 4247B. The first opening 4247A of the container 4246 is sealably connected to a first opening 4248A of the supply chamber 4235A by a first duct 4249A, and the second opening 4247B is sealably connected to a second opening 4249B of the supply chamber by a second duct 4248B. During the process, a moisture containing carrier gas flows into the moisture absorption device from the supply chamber in the direction of arrow ‘A’, and a moisture free carrier gas returns to the supply chamber 4235A in the direction of arrow Gas flow may be induced by a pump (not shown) attached to the moisture removal unit 4232. A moisture absorption material such as a desiccant including, for example, one of silica gel, cobalt chloride and calcium sulfate is kept in the container 4246 to absorb the water vapor as it flows through the moisture absorption device. The desiccant may be supplied in moisture permeable bags which can be replaced with dry ones at process intervals. Alternatively the container 4246 may be filled with a desiccant material. At the process intervals, either the used desiccant or the container 4246 containing the desiccant is replenished. In this case, the first and second openings may be covered with gas and moisture permeable membranes. Passive room-temperature desiccants such as silica gel, cobalt chloride or calcium sulfate may also be used as moisture absorption materials. Other passive desiccation means include, but are not limited to titanium or other reactive metal filings which may be heated in the supply chamber 4235A. Alternatively, such heating may be located in the moisture absorption device 4232B.
It should be noted that instead of using the moisture removal unit 4232, it is possible to simply place the moisture absorption material in a permeable container 4233 (depicted with dotted lines) in the supply chamber 4235A and let moisture diffuse to the moisture absorption material to get adsorbed. In the supply chamber 4235A, the permeable container 4233 is preferably kept between the moisture desorption device 4232A and the entrance opening 4244A as shown in
Referring to
The receiving chamber moisture removal unit 4262 may include a first device 4262A or moisture desorption device which is an energy source and a second device 4262B or a moisture absorption device. The moisture desorption device 4262A is preferably located in the receiving chamber 4235B and adjacent the roll 4265 of the protective sheet material 4264. As in the above embodiment, the moisture desorption device 4262A may be located outside the receiving chamber and a window may be fitted, permitting the transfer of energy from the moisture desorption device 4262A to the workpiece 4220B through the window.
The moisture absorption device 4262B is connected to the receiving chamber externally by a first duct 4263A and a second duct 4263B. The receiving chamber moisture removal unit 4262 functions the same as the supply chamber moisture removal unit 4232. Alternatively, also in this embodiment, the moisture absorption device 4262B may be replaced with a permeable container 4233A, which is similar to the permeable container 4233 described in the above embodiment containing a moisture absorbing material, e.g., desiccant. In the above embodiment, either the moisture absorption device 4262B or the permeable container 4233A works together with the moisture desorption device to capture the water vapor released by the moisture desorption device 4262A. However, either the moisture absorption device 4262B or the permeable container 4233A may be used without having the moisture desorption device 4262A and this is within the scope of this invention.
Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
This application claims priority to U.S. Provisional Application No. 61/159053 filed Mar. 10, 2009 entitled “EXHAUST SYSTEM FOR ROLL TO ROLL REACTORS EXHAUST SYSTEM FOR ROLL TO ROLL REACTORS”, and this application claims priority to U.S. Provisional Application No. 61/161377 filed Mar. 18, 2009 entitled “MOISTURE CAPTURE IN UNWIND AND REWIND STATIONS OF ROLL TO ROLL REACTORS”, and this claims priority to U.S. Provisional Application No. 61/162235 filed Mar. 20, 2009 entitled “ROLL-TO-ROLL PROCESSING AND TOOLS FOR THIN FILM SOLAR CELL MANUFACTURING”; and this application claims priority to and is a Continuation-in-Part of U.S. patent application Ser. No. 12/344420 filed Dec. 12, 2008, entitled “REACTOR TO FORM SOLAR CELL ABSORBERS” and this application claims priority to and is a Continuation-in-Part of U.S. patent application Ser. No. 12/177007 filed Jul. 21, 2008 entitled “METHOD AND APPARATUS TO FORM THIN LAYERS OF PHOTOVOLTAIC ABSORBERS”; and this application claims priority to and is a Continuation-in-Part of U.S. patent application Ser. No. 12/345389 filed Dec. 29, 2008 entitled “METHOD AND APPARATUS TO FORM SOLAR CELL ABSORBER LAYERS WITH PLANAR SURFACE”; and this application claims priority to and is a Continuation-in-Part of U.S. patent application Ser. No. 12/642716 filed Dec. 18, 2009 entitled “REACTOR TO FORM SOLAR CELL ABSORBERS IN A ROLL-TO-ROLL FASHION”; all of the above-referenced applications are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61159053 | Mar 2009 | US | |
| 61161377 | Mar 2009 | US | |
| 61162235 | Mar 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12334420 | Dec 2008 | US |
| Child | 12721446 | US | |
| Parent | 12177007 | Jul 2008 | US |
| Child | 12334420 | US | |
| Parent | 12345389 | Dec 2008 | US |
| Child | 12177007 | US | |
| Parent | 12642716 | Dec 2009 | US |
| Child | 12345389 | US |
