Abstract:
Disclosed is a method for handling a flexible substrate of solar cell. The method includes: providing a flexible substrate; performing static electricity removal and atmospheric pressure plasma cleaning with respect to the flexible substrate; forming a first electrode on the flexible substrate; forming a first conductive semiconductor layer, an intrinsic semiconductor layer and a second conductive semiconductor layer on the first electrode; and forming a second electrode on the second conductive semiconductor layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0065987 filed on Jul. 8, 2010, the entirety of which is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for cleaning a flexible substrate of a solar cell. 
       BACKGROUND OF THE INVENTION 
       [0003]    Recently, as existing energy resources like oil and coal and the like are expected to be exhausted, much attention is increasingly paid to alternative energy sources which can be used in place of the existing energy sources. As an alternative energy source, sunlight energy is abundant and has no environmental pollution. Therefore, more and more attention is paid to the sunlight energy. 
         [0004]    A photovoltaic device, that is, a solar cell directly converts sunlight energy into electrical energy. The photovoltaic device mainly uses photovoltaic effect of semiconductor junction. In other words, when light is incident on and absorbed by a semiconductor p-n junction doped with p-type impurity and n-type impurity respectively, light energy generates electrons and holes within the semiconductor and the electrons and the holes are separated from each other by an internal electric field. As a result, a photo-electro motive force is generated between both ends of the p-n junction. Here, when electrodes are formed at both ends of the junction and connected with wires, electric current flows externally through the electrodes and the wires. 
         [0005]    In order that the existing energy sources such as oil is substituted with the sunlight energy source, it is necessary to provide a solar cell with high photovoltaic conversion efficiency. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the present invention is a method for handling a flexible substrate of solar cell. The method includes: providing a flexible substrate; performing static electricity removal and atmospheric pressure plasma cleaning with respect to the flexible substrate; forming a first electrode on the flexible substrate; forming a first conductive semiconductor layer, an intrinsic semiconductor layer and a second conductive semiconductor layer on the first electrode; and forming a second electrode on the second conductive semiconductor layer. 
         [0007]    Another aspect of the present invention is a solar cell manufacturing system including a flexible substrate. The solar cell manufacturing system includes: a roll on which the flexible substrate can be wound; at least one process chamber for forming a first conductive semiconductor layer, an intrinsic semiconductor layer and a second conductive semiconductor layer; a transfer device passes the flexible substrate through the at least one process chamber as the roll rotates; and a static electricity remover for removing static electricity of the flexible substrate placed between the roll and the at least one chamber. 
         [0008]    The manufacturing system may further comprises an atmospheric pressure plasma cleaner for cleaning the flexible substrate between the roll and the at least one chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1   a  and  1   b  show a manufacturing system for a solar cell including a flexible substrate. 
           [0010]      FIG. 2  shows a static electricity remover which can be used to remove static electricity of the flexible substrate in accordance with the embodiment of the present invention. 
           [0011]      FIG. 3  shows an atmospheric pressure plasma cleaner which can be used to clean the flexible substrate in accordance with the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    An embodiment of the present invention will be described in detail with reference to the drawings.  FIGS. 1   a  and  1   b  show a manufacturing system for a solar cell including a flexible substrate. 
         [0013]      FIG. 1   a  shows a roll-to-roll type solar cell manufacturing system.  FIG. 1   b  shows a stepping roll type solar cell manufacturing system. 
         [0014]    As shown in  FIGS. 1   a  and  1   b,  each system includes a plurality of process chambers I 0  to I 4  for forming an intrinsic semiconductor layer. Intrinsic semiconductor layers  130   a  and  130   b  of a solar cell are thicker than first conductive semiconductor layers  120   a  and  120   b  or second conductive semiconductor layers  140   a  and  140   b.  Therefore, the solar cell manufacturing system may include a larger number of the process chambers than the process chambers L 1  and L 2  that are used to form the first conductive semiconductor layers  120   a  and  120   b  and the second conductive semiconductor layers  140   a  and  140   b,  respectively. The first conductive semiconductor layers  120   a  and  120   b,  the second conductive semiconductor layers  140   a  and  140   b,  or the intrinsic semiconductor layers  130   a  and  130   b  can be formed in a process chamber in which a PECVD (Plasma Enhanced Chemical Vapor Deposition) process is performed. 
         [0015]    Here, when the first conductive semiconductor layers  120   a  and  120   b  are p-type semiconductor layers, the second conductive semiconductor layers  140   a  and  140   b  are n-type semiconductor layers. Also, when the first conductive semiconductor layers  120   a  and  120   b  are n-type semiconductor layers, the second conductive semiconductor layers  140   a  and  140   b  are p-type semiconductor layers. 
         [0016]    The roll-to-roll type solar cell manufacturing system or the stepping roll type solar cell manufacturing system can be used to manufacture a solar cell including a flexible substrate  100   a  and  100   b  such as a metal foil or a polymer substrate. In the process chambers L 1 , I 0  to I 4  and L 2 , the first conductive semiconductor layer  120   a  and  120   b,  the intrinsic semiconductor layer  130   a  and  130   b  and the second conductive semiconductor layer  140   a  and  140   b  can be formed on the flexible substrate  100   a  and  100   b.    
         [0017]    For example, when hydrogen gas, silicon-containing gas like silane gas, and group III doping gas like B 2 H 6  are introduced into the process chamber L 1 , a p-type semiconductor layer is formed on the flexible substrate  100   a  and  100   b.  Further, when hydrogen gas, silicon-containing gas, and group V doping gas like PH 3  are introduced into the process chamber L 1 , an n-type semiconductor layer is formed on the flexible substrate  100   a  and  100   b.  Hydrogen gas and silicon-containing gas are introduced into the process chamber groups I 0  to I 4  for forming the intrinsic semiconductor layer  130   a  and  130   b.  When a p-type semiconductor layer is formed in the process chamber L 1 , an n-type semiconductor layer is formed in the process chamber L 2 . When an n-type semiconductor layer is formed in the process chamber L 1 , a p-type semiconductor layer is formed in the process chamber L 2 . 
         [0018]    In the roll-to-roll type manufacturing system of  FIG. 1   a,  while a roll  400  continuously rotates, the flexible substrate  100   a  rolled in the roll  400  passes through the insides of the process chambers. As a result, a first electrode  110   a,  a first conductive semiconductor layer  120   a,  an intrinsic semiconductor layer  130   a,  a second conductive semiconductor layer  140   a  and a second electrode  150   a  are continuously formed on the flexible substrate  100   a.    
         [0019]    In the stepping roll type manufacturing system of  FIG. 1   b,  the roll  400  rotates and stops repetitively. During the rotation of the roll  400 , a gate (not shown) or a top plate (not shown) of each of the process chambers is opened and the flexible substrate  100   b  moves. During the stop of the roll  400 , the gate or the top plate is closed and the then a first electrode  110   b,  a first conductive semiconductor layer  120   b,  an intrinsic semiconductor layer  130   b,  a second conductive semiconductor layer  140   b  and a second electrode  150   b  are continuously formed on the flexible substrate  100   b  in each process chamber. 
         [0020]    As shown in  FIGS. 1   a  and  1   b,  whenever the flexible substrates  100   a  and  100   b  pass by the process chambers I 0  to I 4 , the intrinsic semiconductor layers  130   a  and  130   b  become thicker. 
         [0021]    The manufacturing systems described above include process chambers E 1  and E 2  which are used to form the first electrode  110   a  and  110   b  and the second electrode  150   a  and  150   b  respectively. However, the manufacturing systems described above may not include the process chambers E 1  and E 2  used to form the electrodes. The first electrode  110   a  and  110   b  and the second electrode  150   a  and  150   b  are formed in the process chambers E 1  and E 2  by performing a sputtering process. 
         [0022]    The first electrode  110   a  and  110   b  and the second electrode  150   a  and  150   b  are placed on the flexible substrate  100   a  and  100   b.  The first conductive semiconductor layer  120   a  and  120   b,  the intrinsic semiconductor layer  130   a  and  130   b  and the second conductive semiconductor layer  140   a  and  140   b  are placed between the first electrode  110   a  and  110   b  and the second electrode  150   a  and  150   b.    
         [0023]    The manufacturing systems shown in  FIGS. 1   a  and  1   b  can produce a single junction solar cell including the first conductive semiconductor layer  120   a  and  120   b,  the intrinsic semiconductor layer  130   a  and  130   b  and the second conductive semiconductor layer  140   a  and  140   b,  and can also produce a tandem type solar cell by further including separate process chambers that can be used to form another first conductive semiconductor layer, intrinsic semiconductor layer and second conductive semiconductor layer. 
         [0024]    Meanwhile, an integration process, such as a laser scribing process, connecting adjacent cells in series may be performed between the process chambers, or may be performed after the second electrode is formed. Further, the integration process may be performed after the first electrode is formed, or may be performed within a period from a time after the second conductive semiconductor layer is formed to a time before the second electrode is formed. The integration process may be also performed between the roll-to-roll type manufacturing systems as well. 
         [0025]    When a laser scribing process is performed on any one of the first electrode  110   a  and  110   b  and the second electrode  150   a  and  150   b,  there may remain conductive particles on the flexible substrate  100   a  and  100   b.  In the embodiment of the present invention, a cleaning process may be performed, in which an ultra sonic cleaner including a suction head removes the conductive particles. 
         [0026]    The ultra sonic cleaner purifies cooling dried air by passing the cooling dried air through a hepa filter, and then blows the cooling dried air to the flexible substrate  100   a  and  100   b  at a regular cycle by a blow unit. An ultrasonic wave is hereby generated and then the conductive particles on the flexible substrate  100   a  and  100   b  are floated. Then, the suction head sucks the floated particles and a pre-filter of the ultra sonic cleaner collects the particles. 
         [0027]    As such, in the embodiment of the present invention, the conductive particles are removed by using the ultrasonic cleaning instead of wet cleaning. As regards the wet cleaning, it costs a lot for cleaning and it may have a bad influence on the performance of the solar cell due to immersion of the substrate into a solution. Meanwhile, since the ultra sonic cleaning is performed at an atmospheric pressure without using a solution, it is possible to reduce the cost and a bad influence on the performance of the solar cell. 
         [0028]    In the embodiment of the present invention, when the flexible substrate  100   a  and  100   b  includes a metal foil, the flexible substrate  100   a  and  100   b  may include an insulation layer covering the metal foil in order to insulate the first electrode  110   a  and  110   b  from the flexible substrate  100   a  and  100   b.    
         [0029]    As such, as the flexible substrate  100   a  and  100   b  rolled in the roll  400  is unwound, the solar cell is formed. Therefore, static electricity is apt to be generated on the flexible substrate  100   a  and  100   b  by friction either between the roll  400  and the flexible substrate  100   a  and  100   b,  or between the flexible substrates  100   a  and  100   b  mutually superposed on each other. The flexible substrate may be stained by impurities attached thereto by the static electricity of the flexible substrate  100   a  and  100   b.    
         [0030]    When the flexible substrate  100   a  and  100   b  having the static electricity is transferred within the process chamber and a PECVD process or a sputtering process is performed, arcing may be generated in the process chamber due to the static electricity of the flexible substrate  100   a  and  100   b.  The arcing generated in the process chamber destroys the uniformity of a thin film formed in the process chamber, and even transforms the surface of the flexible substrate  100   a  and  100   b,  thereby having a bad influence on the performance of the solar cell. 
         [0031]    In order to remove the static electricity and impurities of the flexible substrate  100   a  and  100   b,  the embodiment of the present invention may include a step of removing the static electricity and a step of atmospheric pressure plasma cleaning for the flexible substrate  100   a  and  100   b.  To this end, as shown in  FIGS. 1   a  and  1   b,  before the flexible substrate  100   a  and  100   b  is transferred into the process chamber for forming the electrode or the semiconductor layer, a static electricity removal process and a cleaning process may be performed by a static electricity remover  200  and an atmospheric pressure plasma cleaner  300   
         [0032]    In the embodiment of the present invention, after the step of removing the static electricity is performed, and then the step of atmospheric pressure plasma cleaning may be performed. Otherwise, after the step of atmospheric pressure plasma cleaning is performed, and then the step of removing the static electricity may be performed. 
         [0033]      FIG. 2  shows a static electricity remover which can be used to remove the static electricity of the flexible substrate in accordance with the embodiment of the present invention. As shown in  FIG. 2 , the static electricity remover includes a discharge electrode  210 , a discharge electrode socket  220 , a ground electrode  230 , a high voltage generator  240 , a controller  250 , an air tank  260  and a protective resistor R. 
         [0034]    The discharge electrode  210  functions to generate corona discharge, that is, generates a positive ion and a negative ion. The discharge electrode socket  220  protects the discharge electrode  210  from the external impact and is equipped with an air nozzle (not shown) for injecting the air. The air nozzle functions as a path through which the air is injected at a certain pressure so as to transfer the ion generated by the discharge electrode  210  to the flexible substrate  100   a  and  100   b  having the static electricity to be removed. As such, the positive ion and the negative ion neutralize the static electricity of the surface of the flexible substrate  100   a  and  100   b,  thereby the static electricity of the surface of the flexible substrate  100   a  and  100   b  can be removed. 
         [0035]    The air is supplied at a certain pressure to the air nozzle through another air tank  260  and is injected through the air nozzle. In other words, air injectors  261  and  262  are respectively connected to a blower system (not shown) that generates air of a certain pressure, and always inject the air of a certain pressure to the air tank  260 . Therefore, the pressure of the air injected from the air nozzle formed in the discharge electrode socket  220  can be also maintained constant. 
         [0036]    Meanwhile, the resistor R is connected to the discharge electrode  210 . By the resistor R, corona discharge is stably generated, and the electric current capacity can be reduced. Thereby the electric shock from the contact with the discharge electrode  210  can be maximally reduced. 
         [0037]    The controller  250  controls the frequency and duty ratio of alternating voltage or controls the supplying and stopping supplying of direct voltage. The ground electrode  230  induces the voltage-applied discharge electrode  210  to generate ion. 
         [0038]    As described above, since the static electricity remover of the embodiment of the present invention removes the static electricity at atmosphere, the static electricity can be removed during the transfer of the flexible substrate without loading the flexible substrate in a vacuum chamber. As a result, manufacturing time of the solar cell can be reduced. 
         [0039]    Various static electricity removers as well as the static electricity remover shown in  FIG. 2  can be used in the embodiment of the present invention. 
         [0040]      FIG. 3  shows an atmospheric pressure plasma cleaner which can be used to clean a flexible substrate in accordance with the embodiment of the present invention. 
         [0041]    As shown in  FIG. 3 , oxygen radicals  330  generated from plasma reaction is injected to the surface of the flexible substrate  100   a  and  100   b  by a plasma generator  310  of the atmospheric pressure plasma cleaner. A power supply  340  applies an alternating voltage to the plasma generator  310 . A gas supply apparatus  350  provides gases such as nitrogen, oxygen and air and the like to the plasma generator  310  through a gas pipeline connected to the plasma generator  310 . A voltage difference is generated between both electrodes of the plasma generator  310  by the operation of the power supply  340 , and then gas plasma is generated by the voltage difference. 
         [0042]    Here, a photon, excited atoms and molecules, electrons and ions of the plasma may have energy or may be in an excitation energy state of several or several tens of electron volts. Since the excitation energy is much greater than the binding energy of the impurities on the surface of the flexible substrate  100   a  and  100   b,  the surface of the flexible substrate  100   a  and  100   b  can be cleaned by means of the plasma. 
         [0043]    A transfer device  360  transfers the flexible substrate  100   a  and  100   b  at a certain speed during the process of the atmospheric pressure plasma discharge by the plasma generator  310 . 
         [0044]    Meanwhile, in the atmospheric pressure plasma cleaning process used in the embodiment of the present invention, the surface of the flexible substrate  100   a  and  100   b  is cleaned by generating plasma at atmospheric pressure. The atmospheric pressure plasma cleaning can be hereby performed with no use of chemicals at atmospheric pressure instead of vacuum. Therefore, the atmospheric pressure plasma cleaning process can be performed at a lower cost than that of the wet cleaning process using the chemicals. 
         [0045]    Further, since the atmospheric pressure plasma cleaning process is performed at atmospheric pressure, the cleaning process can be performed during the transfer of the flexible substrate without loading the flexible substrate in a vacuum chamber. As a result, manufacturing time of the solar cell can be reduced. 
         [0046]    As such, in the embodiment of the present invention, it is possible to remove the static electricity and the impurities which are formed during the process of rolling and unrolling the flexible substrate  100   a  and  100   b  by the roll  400 . Consequently, it is possible to manufacture a stably operating solar cell. 
         [0047]    While the embodiment of the present invention has been described with reference to the accompanying drawings, it can be understood by those skilled in the art that the present invention can be embodied in other specific forms without departing from its spirit or essential characteristics. Therefore, the foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.