Patent Publication Number: US-2005121813-A1

Title: Method of forming photovoltaic device lens and method of fabricating photovoltaic panel

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a method of forming a lens on a surface of a spherical or granular photovoltaic device and a method of fabricating a photovoltaic panel having a number of spherical or granular photovoltaic devices arranged in an array.  
      2. Description of the Related Art  
      JP-A-2001-168369 and JP-A-2002-280592 disclose spherical or granular photovoltaic devices in order to improve a generating efficiency of a photovoltaic panel which converts solar energy into electric energy, for example. The spherical or granular photovoltaic device provides a substantially constant project area (an amount of received light) when viewed from every direction of incidence of sunlight. Accordingly, even when the sun altitude is low, power generation can advantageously be performed as efficiently as when the sun altitude is high.  
      JP-A-2001-168369 describes the photovoltaic panel including a light-transmissible resin casing and a number of spherical or granular photovoltaic devices arranged in an array and housed in the casing (a method of manufacturing the structure is not disclosed). A reflector is bonded to a backside of the casing so that light reflected on the reflector is received by the photovoltaic devices for the purpose of improving the power generating efficiency.  
      On the other hand, JP-A-2002-280592 describes the photovoltaic panel including a device-holding plate having one side (light receiving side) formed with a number of recesses and spherical or granular photovoltaic devices accommodated and bonded in the recesses respectively. Electrodes of the respective photovoltaic devices are formed on the other side of the device-holding plate, whereupon light incident on the photovoltaic devices is prevented from being blocked by the electrodes.  
      Regarding a method of manufacturing spherical or granular photovoltaic devices, international publication No. WO99/10935 discloses a free fall method in which drops of silicon heated to be liquefied are caused to fall free so as to be deformed by surface tension into a spherical or granular shape and solidified. JP-A-2002-60943 discloses a plasma-assisted CVD method in which Si is deposited on the entire surface of a core member in a plasma-assisted CVD machine to be formed into spherical or granular photovoltaic devices.  
      In order that a generating efficiency of a photovoltaic panel may be improved, it is effective to form a lens on a surface of the photovoltaic device so that sunlight incident around the photovoltaic device is also condensed by the lens thereby to be received by the photovoltaic devices.  
      When to be formed in the aforementioned conventional photovoltaic panels, it is considered that a lens is made from a light-transmissible resin by the injection molding. However, the spherical or granular photovoltaic devices formed by the free fall method or plasma-assisted CVD method are small in size and moreover have variations in the size (diameter) or shape (sphericity). As a result, it is difficult to make spherical convex lens suitable for the respective photovoltaic devices.  
     SUMMARY OF THE INVENTION  
      Therefore, an object of the present invention is to forming suitable spherical convex lens on the surfaces of spherical or granular photovoltaic devices in an easy and cost-effective method even when the photovoltaic devices have variations in the size (diameter) and shape (sphericity) and further, manufacturing, in an easy and cost-effective method, a photovoltaic panel provided with lens and having a high generating efficiency.  
      To achieve the object, the present invention is a method in which are carried out immersing the photovoltaic device in a liquid resin and lifting the photovoltaic device, thereby applying the liquid resin to the surface of the photovoltaic device, and hardening the liquid resin adherent to the surface of the photovoltaic device, thereby forming a resin lens on the surface of the photovoltaic device. Since a spherical convex lens is formed on the surface of the photovoltaic device by surface tension of the resin liquid, a suitable spherical convex lens can be formed on the surface of the photovoltaic device and the photovoltaic device can be positioned at the center of the lens even when the photovoltaic devices have variations in the size (diameter) and shape(sphericity). Moreover, no expensive forming machine, such as forming dies for injection molding, forming the lens is required, and the spherical convex lens can be formed on the surface of the photovoltaic device in an easy and cost-effective manner in which the photovoltaic device is immersed in the resin liquid, lifted and the resin liquid is hardened. Thus, optimization of the shape of lens and low costs can be achieved simultaneously.  
      In this case, the liquid resin applying process and the liquid resin hardening process are repeated alternately at a predetermined number of times so that the lens formed on the surface of the photovoltaic device has an increased thickness. Consequently, the thickness of the lens can be adjusted by adjustment of the number of times of immersing the photovoltaic device in the liquid resin and hardening the liquid resin and accordingly, even a thick lens can be made.  
      In this case, a light-transmissible thermosetting resin or the like may be used as the resin for forming the lens. However, it is more preferable to use a light-transmissible ultraviolet curing resin or the like. As a result, the resin can be hardened by ultraviolet radiation in a short period of time (several to several tens seconds) and accordingly, the yield can be improved.  
      Further, a method of manufacturing a photovoltaic panel having an array of a number of spherical or granular photovoltaic devices, comprises preliminarily holding the photovoltaic devices on one side of a preliminarily holding sheet with a space between each device and an adjacent one, immersing the photovoltaic devices held on the side of the preliminarily holding sheet in a liquid resin and lifting the photovoltaic devices, thereby applying the liquid resin to the surfaces of the photovoltaic devices, hardening the liquid resin applied to the surfaces of the photovoltaic devices, thereby forming resin lens on the surfaces of the photovoltaic devices respectively, applying a bonding agent to surfaces of the lens, and shrinking the preliminarily holding sheet in a direction of extension of the sheet so that the lens of each photovoltaic device is bonded to the lens of the adjacent one, thereby forming the photovoltaic panel.  
      Consequently, more preferable spherical convex lens can be manufactured collectively on the surfaces of a number of photovoltaic devices by the surface tension even when the photovoltaic devices held on the preliminarily holding sheet varies in the size and shape. Moreover, the preliminarily holding sheet is shrunk in the direction of extension of the sheet after formation of the lens, so that the lens of each photovoltaic device is bonded to the lens of the adjacent one. Consequently, the photovoltaic devices can be integrated by an easy and cost-effective method without using expensive forming equipment. Thus, a photovoltaic panel provided with lens and having a high generating efficiency can be manufactured.  
      In this case, too, the liquid resin applying process and the liquid resin hardening process are repeated alternately at a predetermined number of times so that the lens formed on the surface of the photovoltaic device has an increased thickness. Consequently, the thickness of the lens can be adjusted by adjustment of the number of times of immersing the photovoltaic device in the resin liquid and hardening the resin liquid and accordingly, even a thick lens can be made.  
      Further, after the preliminarily holding sheet has been removed from the photovoltaic panel, an electrode of each photovoltaic device may be formed on a side of the photovoltaic panel from which the preliminarily holding sheet has been removed. Consequently, since the electrodes are formed on the backside of the photovoltaic panel, light incident on the photovoltaic devices can be prevented from being intercepted by the electrodes and accordingly, the entire surface of each lens can effectively serve as a light-receiving face.  
      In this case, the electrode may be formed so as to cover a backside of each lens. Since the electrode serves as a reflecting surface of the incident light, the generating efficiency (an amount of light received by the photovoltaic device) can further be improved by the light reflecting action of the electrode.  
      On the other hand, in the preliminarily holding process, a number of photovoltaic devices may be caused to adhere to one side of the preliminarily holding sheet while a uniform space is defined between each photovoltaic device and the adjacent one. In this manner, however, the photovoltaic devices need to be aligned by some method with the uniform space between each photovoltaic device and the adjacent one before applied to one side of the preliminarily holding sheet. This can become a troublesome work.  
      In view of the aforesaid problem, a sheet made from an elastic material expandable and contractible in the direction of extension of the sheet serves as the preliminarily holding sheet. The photovoltaic devices are caused to adhere to one side of the preliminarily holding sheet in a closely massed state and thereafter, the preliminarily holding sheet is drawn out uniformly in the direction of extension of the sheet so that a uniform space is defined between each photovoltaic device and the adjacent one. Consequently, the photovoltaic devices need not be aligned with the uniform space between each photovoltaic device and the adjacent one before caused to adhere to one side of the preliminarily holding sheet. Accordingly, causing the photovoltaic devices to adhere to one side of the preliminarily holding sheet can be carried out easily. Moreover, a uniform space can be defined between the photovoltaic devices by an easy method of uniformly drawing the sheet in the direction of extension of the sheet.  
      When the resin liquid is adhered to the preliminarily holding sheet, the adherent liquid resin may serve as a bonding agent to bond the preliminarily holding sheet and photovoltaic panel. As a result, it becomes difficult to remove the preliminarily holding sheet, and the adherent liquid resin becomes an obstacle preventing the sheet from shrinking, whereupon there is a possibility that each lens cannot be bonded to the adjacent one.  
      As one countermeasure, only the photovoltaic devices are immersed in the liquid resin so that the liquid resin is prevented from adhering to the preliminarily holding sheet in the immersing process. Consequently, the aforesaid drawback due to adherence of the resin liquid to the preliminarily holding sheet can be prevented.  
      In this case, it is preferable that the bonding agent comprises a resin which is the same as forming each lens but has a lower viscosity than the resin forming each lens. Consequently, the optical characteristic of the bonding agent after hardening can correspond to that of the lens while a preferable viscosity is maintained, and the bonding agent can completely integrated as a part of the lens, whereupon an optical characteristic of the lens can be prevented from being degraded by the bonding agent.  
      Further, in the bonding agent applying process, the lens of the photovoltaic devices preliminarily held on the preliminarily holding sheet may be immersed in a liquid of said bonding agent so that the bonding agent adheres to the surface of the lens of each photovoltaic device. Consequently, the bonding agent can be applied collectively to the surfaces of the lens of a number of photovoltaic devices and accordingly, the bonding agent applying work can be carried out easily in the same manner as the lens forming (immersing method). Moreover, since the bonding agent can uniformly be applied to the lens surfaces of a number of photovoltaic devices by the immersing method, the optical characteristic of the lens can be prevented from being reduced by the bonding agent. In this bonding agent applying process, it is preferable that only the lens of each photovoltaic device is immersed in the liquid of bonding agent so that the bonding agent is prevented from adhering to the preliminarily holding sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects, features and advantages of the present invention will become clear upon reviewing of the following description of the embodiments, made with reference to the accompanying drawings, in which:  
       FIG. 1A  illustrates a process in which a number of photovoltaic devices are accommodated in a casing with a shallow bottom in a crowded state with respect to a manufacturing process for the photovoltaic panel in accordance with one embodiment of the present invention;  
       FIG. 1B  illustrates a process of adhering the photovoltaic devices to the underside of the preliminarily holding sheet in the crowded state;  
       FIG. 1C  illustrates a process of drawing the preliminarily holding sheet uniformly in the direction of extension of the sheet so that a space is increased uniformly between each photovoltaic device and the adjacent one;  
       FIG. 1D  illustrates a process of immersing the photovoltaic devices on the underside of the preliminarily holding sheet in resin liquid;  
       FIG. 1E  illustrates a process of irradiating ultraviolet light onto resin liquid adherent to the surface of each photovoltaic device on the underside of the preliminarily holding sheet to harden the resin liquid, thereby forming a lens;  
       FIG. 1F  illustrates a process of simultaneously immersing the lens of the photovoltaic devices on the underside of the preliminarily holding sheet;  
       FIG. 1G  illustrates the bonding agent adherent to the lens of the photovoltaic devices on the underside of the preliminarily holding sheet;  
       FIG. 1H  illustrates the preliminarily holding sheet shrunk in the direction of extension of the sheet so that the bonding agent adherent to each lens is brought into contact with each other;  
       FIG. 1I  illustrates a process of irradiating ultraviolet light onto the bonding agent between the lens of the photovoltaic devices so that the lens are bonded together;  
       FIG. 1J  illustrates a process of removing the preliminarily holding sheet from the backside of the photovoltaic panel;  
       FIG. 1K  illustrates a process of forming negative electrodes on the backside of the photovoltaic panel;  
       FIG. 1L  illustrates a process of forming a protecting layer (lower insulating resin layer) covering over the negative electrodes on the backside of the photovoltaic panel;  
       FIG. 1M  illustrates a sand blast process for exposing an n-type semiconductor layer of the rear end of the photovoltaic device;  
       FIG. 1N  illustrates a process of forming an insulating layer (upper insulating resin layer) over the entire backside of the photovoltaic panel;  
       FIG. 10  illustrates a process of abrading the insulating layer of the backside of the photovoltaic panel so that a p-type semiconductor layer of the rear end of the photovoltaic device is exposed from the insulating layer;  
       FIG. 1P  illustrates a process of forming positive electrodes over the entire backside of the photovoltaic panel so that the positive electrodes are adhered to the exposed faces of the p-type semiconductor devices;  
       FIG. 1Q  illustrates a process of forming a protecting insulating layer over the entire positive electrode face of the backside of the photovoltaic panel;  
       FIG. 2A  illustrates a state of an apparatus for preliminarily drawing the preliminarily holding sheet uniformly in the direction of extension of the sheet before the sheet is drawn out;  
       FIG. 2B  illustrates a state of the apparatus for preliminarily drawing the preliminarily holding sheet uniformly in the direction of extension of the sheet after the sheet has been drawn out;  
       FIG. 2C  illustrates a process of immersing, in the resin liquid, the photovoltaic devices on the lower side of the preliminarily holding sheet; and  
       FIG. 3  illustrates a data of measured relationship between projected area ratios of the lens and photovoltaic device and output current I sc . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      One embodiment of the invention will be described with reference to the drawings. First, the structure of the photovoltaic panel  27  manufactured by the method in accordance with the embodiment of the invention with reference to  FIG. 1Q .  
      The photovoltaic panel  27  comprises a number of spherical or granular photovoltaic devices  11  arranged side by side and integrated. Each photovoltaic device  11  includes a thin n-type semiconductor layer in the outer periphery thereof and a p-type semiconductor layer in the inner peripheral side. A method of manufacturing the photovoltaic device  11  should not be limited to a particular one and the free fall method described in international publication No. WO99/10935, the plasma-assisted CVD method described in JP-A-2002-60943 or another method may be employed. At a light-receiving side of each photovoltaic device  11 , a spherical convex lens  12  is made from a light-transmissible ultraviolet curing resin. Each photovoltaic device  11  is bonded to the adjacent one by a light-transmissible bonding agent so that the photovoltaic devices  11  are integrated. It is preferable that the bonding agent is the same ultraviolet curing resin as formed into each lens  12  but has a lower viscosity than that of each lens  12 . However, the material for the bonding agent should not be limited to the aforementioned.  
      A negative electrode  13  causing an outer peripheral n-type semiconductor layer of each photovoltaic device  11  to conduct is formed on the backside of the photovoltaic panel (upper side in  FIG. 1Q ) so as to cover the backside of the lens  12 . The negative electrode  13  is completely covered with two insulating resin layers  14  and  15 . The lower insulating resin layer  14  serves as a protecting layer (mask) in etching as will be described later, whereas the upper insulating resin layer  15  serves as an insulating layer insulating the negative electrode  13  and positive electrode  16 .  
      The n-type semiconductor layer is partially removed by abrasion etc. such that a portion at which the p-type semiconductor layer is exposed is formed in a rear end of each photovoltaic device  11 . A positive electrode  16  is formed so as to cause the p-type semiconductor layer to conduct. The positive electrode  16  is completely covered with a protective insulating layer  17  made from an insulating resin etc., thereby being protected and insulated.  
      A method of manufacturing the photovoltaic panel  27  constructed above will be described. As described above, the method of manufacturing the spherical or granular photovoltaic device  11  is not limited to a particular method and any method may be used to make the spherical or granular photovoltaic device  11 . One manufacturer may continuously carry out all the processes from manufacture of the photovoltaic devices  11  to manufacture of the photovoltaic panel, or one manufacturer may purchase photovoltaic devices  11  manufactured by another manufacturer and manufacture the photovoltaic panels. The following describes sequential processes for manufacturing the photovoltaic panels  27  using the spherical or granular photovoltaic devices  11  manufactured by any method.  
      [1] Preliminarily Holding Process:  
      Firstly, as shown in  FIG. 1A , a large number of spherical or granular photovoltaic devices  11  are accommodated in a shallow casing  20  to be arranged side by side in a crowded state so as not to be laid one upon another. Subsequently, as shown in  FIG. 1B , a preliminarily holding sheet  21  with an underside coated with an adhesive agent is pressed against the photovoltaic devices  11  in the casing  20  from above thereby to be adhered (preliminarily held) while the photovoltaic devices  11  are crowded without any space between the photovoltaic devices  11 . In this case, a sheet is used which is made from an elastic material, such as rubber, contractible and expandable in a direction of extension of the sheet or a direction perpendicular to the thickness of the sheet, so that the photovoltaic devices  11  are caused to adhere to the preliminarily holding sheet  21  while the sheet is in a slightly expanded state so as not to be loosened.  
      Subsequently, as shown in  FIG. 1C , the preliminarily holding sheet  21  is drawn up in the direction of extension of the sheet in a range of 360°, whereupon a uniform space is spread between the photovoltaic devices  11 . In this case, the space between the photovoltaic devices  11  adjacent to each other is increase to such a degree that the lens  12  formed in the subsequent process are prevented from being brought into contact with each other.  
      An example of apparatus for drawing the sheet  21  will be described with reference to  FIGS. 2A and 2B . The preliminarily holding sheet  21  is attached to an annular holder  22  in a slightly stretched state so as not to be loosened. An annular pusher  23  is coaxially provided in the inner peripheral side so as be moved up and down. The annular pusher  23  is pressed against the upper side of the preliminarily holding sheet  21  while the annular holder  22  is fixed in a position. Only the annular pusher  23  is descended by an air cylinder or the like so that the preliminarily holding sheet  21  is drawn along the underside of the annular pusher  23  uniformly in the direction of extension of the sheet.  
      In this case, an amount of drawing of the preliminarily holding sheet  21  is rendered large as an amount of descent of the annular pusher  23  (an amount of pressing applied to the preliminarily holding sheet  21 ) becomes larger, whereupon a space between the photovoltaic devices  11  on the underside of the preliminarily holding sheet  21  is increased. Accordingly, a descending stroke of the annular pusher  23  is adjusted when the space between the photovoltaic devices  11  is adjusted. However, when the descending stroke of the annular pusher  23  cannot be adjusted, the height of the annular holder  22  is adjusted so that a vertical space between the pusher  23  and sheet  21  before start of descent of the pusher (or a distance between the sheet  21  and the pusher  23  which is abutted against the sheet  21  after start of descent), whereby an amount of pressing applied to the sheet  21  is adjusted.  
      Alternatively, only the annular holder  22  may be raised while the annular pusher  23  is fixed in position, so that the preliminarily holding sheet  21  is drawn uniformly in the direction perpendicular to the direction of its thickness.  
      [2] Liquid Resin Applying Process:  
      The photovoltaic devices  11  on the underside of the preliminarily holding sheet  21  are immersed in the liquid resin  24  simultaneously while the sheet  21  is drawn uniformly in the direction of extension of the sheet by the above-described method such that a uniform space is defined between the photovoltaic devices  11 , as shown in  FIG. 1D . A light-transmissible ultraviolet curing resin is preferably used as the liquid resin  24 . In this case, the preliminarily holding sheet  21  may be descended so that the photovoltaic devices  11  on the underside thereof are immersed in the liquid resin  24 . However, as shown in  FIG. 2C , a resin liquid reservoir  25  may be raised while the preliminarily holding sheet  21  is fixed in position, so that the photovoltaic devices  11  are immersed in the liquid resin  24 , instead.  
      In the liquid resin attaching process, only the photovoltaic devices  11  needs to be immersed in the liquid resin  24  so that the liquid resin  24  is prevented from adhering to the preliminarily holding sheet  21 . The reason for this is that if applied to the preliminarily holding sheet  21 , the liquid resin  24  inadvertently serves as a bonding agent boding the sheet  21  to the photovoltaic panel  27 , whereupon it becomes difficult to remove the sheet  21  or the adherent resin becomes an obstacle preventing shrinkage of the sheet  21 . Thus, there is a possibility that the lens  11  of the respective photovoltaic devices  11  may not be bonded together.  
      However, since a desirable thicker lens  12  can be made as an amount of liquid resin  24  adherent to each photovoltaic device  11  becomes large. Accordingly, it is desirable to increase an amount of liquid resin  24  adherent to each photovoltaic device  11  as much as possible. For this purpose, it is preferable to immerse each photovoltaic device  11  in the liquid resin  24  as deep as possible without the liquid resin  24  adherent to the preliminarily holding sheet  21 . An amount of liquid resin  24  adherent to each photovoltaic device  11  can also be adjusted by the viscosity of the liquid resin  24  or the composition of resin.  
      After having been immersed in the liquid resin  24 , the photovoltaic devices  11  on the underside of the preliminarily holding sheet  21  are ascended out of the liquid resin  24 . As a result, as shown in  FIG. 1E , the liquid resin  24   a  adherent to each photovoltaic device  11  is formed into the shape of a spherical convex lens by the surface tension thereof.  
      [3] Resin Hardening Process:  
      After completion of the liquid resin attaching process, the manufacturing sequence advances to a resin hardening process so that ultraviolet rays are irradiated onto the liquid resin  24   a  adherent to the surface of each photovoltaic device  11  on the underside of the preliminarily holding sheet  21  thereby to harden the liquid resin  24   a,  so that the resin lends  12  are formed on the surfaces of the photovoltaic devices  11  respectively.  
      The manufacturing sequence advances to a subsequent bonding agent applying process when each of the liquid resin applying process and the resin hardening process are carried out once such that the lends  12  with target thickness is obtained. On the other hand, when the target lens  12  cannot be obtained in this case, the liquid resin applying process and the resin hardening process are carried out alternately at a predetermined number of times so that the lens  12  formed on the surface of each photovoltaic device  11  reaches the target thickness. Thus, the number of times of immersing the photovoltaic devices  11  in the liquid resin  24  and hardening the resin is adjusted, whereby the thickness of each lens  12  can be adjusted. A thick lends  12  can be manufactured.  
      [4] Bonding Agent Applying Process:  
      Each of the liquid resin applying process and the resin hardening process is carried out at a suitable number of times so that the lens  12  having a target thickness are obtained. Subsequently, the manufacturing sequence advances to a bonding agent applying process. As shown in  FIG. 1F , the lens  12  of the photovoltaic devices  11  on the underside of the preliminarily holding sheet  21  are simultaneously immersed in a liquid bonding agent  26  and then ascended so that the bonding agent  26   a  is caused to adhere to the surfaces of the lens  12  as shown in  FIG. 1G . In the bonding agent applying process, too, only the lens  12  of the photovoltaic devices  11  need to be immersed in the liquid bonding agent  26  so that the liquid bonding agent  26   a  is prevented from adhering to the preliminarily holding sheet  21 .  
      In this case, the bonding agent  26  comprises the same light-transmissible ultraviolet curing resin as of each lens  12 , which resin has a lower viscosity. Consequently, the bonding agent  26   a  can collectively be caused to adhere to the surfaces of the lens  12  in the same method as forming the lens  12  (immersion). Thus, the bonding agent applying work is exceedingly easy. Moreover, optical characteristics of the post-hardened bonding agent  26   a  can be caused to correspond with those of the lens  12  completely, and the bonding agent  26   a  can be integrated completely into a part of the lens  12 . Consequently, the bonding agent  26   a  can prevent the optical characteristics of the lens  12  from being worsened or reduced. Alternatively, the bonding agent  26   a  may be applied to the surface of each lens  12  and thus, the bonding agent  26   a  may be applied to the surface of each lens  12  by a method other than the immersion, for example painting.  
      [ 5 ] Bonding Process:  
      After completion of the bonding agent applying process, the manufacturing sequence advances to a bonding process. As shown in  FIG. 1H , in consideration of the thickness of each lens  12 , the preliminarily holding sheet  21  is shrunk in the direction of extension of the sheet such that the bonding agent  26   a  of the lens  12  of each photovoltaic device  11  is brought into contact with the bonding agent  26   a  of the lens  12  of the adjacent photovoltaic device  11 . Shrinking the preliminarily holding sheet  21  is an operation in the opposite direction to drawing the sheet as described in the preliminarily holding process.  
      While the bonding agent  26   a  of each photovoltaic device  11  is in contact with the bonding agent  26   a  of the adjacent photovoltaic device  11 , ultraviolet rays are irradiated onto the bonding agent  26   a  so that the bonding agent  26   a  is hardened, as shown in  FIG. 1I . As a result, the lens  12  of each photovoltaic device  11  is bonded to the lens  12  of the adjacent photovoltaic device  11  by the bonding agent  26   a,  whereby the photovoltaic panel  27  is formed. When the preliminarily holding sheet  21  is loose during hardening the bonding agent  26   a  such that an array of the photovoltaic devices  11  is curved, a curved photovoltaic panel  27  would be formed. Accordingly, the preliminarily holding sheet  21  needs to be held in a stretched straightforward state.  
      [6] Preliminarily Holding Sheet Removing Process:  
      After completion of the bonding process, the manufacturing sequence advances to a preliminarily holding sheet removing process. As shown in  FIG. 1J , the preliminarily holding sheet  21  is removed from the backside of the photovoltaic panel  27 . In this case, if the liquid resin  24  or the bonding agent  26  isn&#39;t adherent to the preliminarily holding sheet  21 , it can easily be removed from the backside of the photovoltaic panel  27 .  
      [7] Negative Electrode Forming Process:  
      After completion of the preliminarily holding sheet removing process, the manufacturing sequence advances to a negative electrode forming process. As shown in  FIG. 1K , a negative electrode  13  is formed on the entire backside of photovoltaic panel  27  using a conductor forming technique such as vapor deposition, metal plating, coating, chemical vapor deposition (CVD) or sputtering. It is preferable that a conductor having a small electric resistance value and tending to reflect light (to serve as a reflecting surface for incident light), such as Ag or an Ag-containing conductor. The negative electrode  13  causes an outer peripheral n-type semiconductor layer of each photovoltaic device  11  to conduct and covers the backside of the lens  12  to serve as a reflecting surface for incident light.  
      [8] Protecting Layer (Lower Insulating Resin Layer) Forming Process:  
      After completion of the negative electrode forming process, the manufacturing sequence advances to a protecting layer forming process. As shown in  FIG. 1L , an insulating resin such as an epoxy resin is applied to the entire negative electrode  13  on the backside of the photovoltaic panel  27  and hardened thereby to be formed into a protecting layer  14  (lower insulating resin layer). Thus, the entire negative electrode  13  is covered by the protecting layer  14 . A resin formed into the protecting layer  14  may be a thermosetting resin, ultraviolet curing resin, anaerobic curing resin or the like but needs to have suitable insulation and resistance to etching (to be used as a mask in the etching).  
      [9] Sandblasting Process:  
      After completion of the protecting layer forming process, the manufacturing sequence advances to a sandblasting process. As shown in  FIG. 1M , the protecting layer  14  and the negative electrode  13  on the rear end of each photovoltaic device  11  are partially removed by sandblasting, whereupon the negative type semiconductor layer on the rear end of each photovoltaic device  11  is exposed. Abrasion, laser processing, electrical discharge machining or the like may be employed to remove parts of the protecting layer  14  and the negative electrode  13 , instead of sandblasting.  
      [10] Etching Process:  
      After completion of the sandblasting process, the manufacturing sequence advances to an etching process to remove the n-type semiconductor on the rear end of the photovoltaic device  11  exposed from the protecting layer  14  using the protecting layer  14  as a mask (etching resist) by chemical etching. As a result, the inside p-type semiconductor layer is exposed. Dry etching may be employed instead of chemical etching.  
      [11] Insulating Layer (Upper Insulating Resin Layer) Forming Process:  
      After completion of the etching process, the manufacturing sequence advances to an insulating layer (upper insulating resin layer) forming process. As shown in  FIG. 1N , an insulating resin such as epoxy resin is applied to the entire backside of the photovoltaic panel  27  and hardened thereby to be formed into an insulating layer  15  (upper insulating resin layer). As a result, the negative electrode  13  which has partially been exposed in the sandblasting process is completely covered with the insulating layer  15 . The resin formed into the insulating layer  15  may be the same as or different from the lower protecting layer  14  and may be a thermosetting resin, ultraviolet curing resin, anaerobic curing resin or the like.  
      [12] Polishing Process:  
      After completion of the insulating layer forming process, the manufacturing sequence advances to a polishing process. As shown in  FIG. 10 , the insulating layer  15  of the backside of the photovoltaic panel  27  is polished by a polishing machine and flattened. Further, the p-type semiconductor layer of the rear end of each photovoltaic device  11  is exposed from the insulating layer  15 , and an exposed face of the p-type semiconductor layer is flattened. Alternatively, the insulating layer  15  may be polished by the sandblast.  
      [13] Positive Electrode Forming Process:  
      After completion of the polishing process, the manufacturing sequence advances to a positive electrode forming process. As shown in  FIG. 1P , a positive electrode  16  is formed on the entire backside of the photovoltaic panel  27  so as to adhere closely to the exposed face of the p-type semiconductor layer of each photovoltaic device  11 . A conductor composing the positive electrode  16  may be the same as or different from the conductor composing the negative electrode  13 . A method of forming the positive electrode  16  may be the same as or different from the method of forming the negative electrode  13 . For example, a conductor such as Al may be rubbed against the entire backside of the photovoltaic panel  27  so that a frictional force and frictional heat cause the conductor to adhere to the exposed face of the p-type semiconductor layer and insulating layer  15 , so that the positive electrode  16  is formed.  
      [14] Laser Sintering Process:  
      After completion of the positive electrode forming process, the manufacturing sequence advances to a laser sintering process, in which process laser beams are spot-irradiated onto a central junction of the positive electrode  16  and the p-type semiconductor layer of the rear end of each photovoltaic device  11 . As a result, the central junction is spot-heated so that the positive electrode  16  is heat-treated so as to be formed into an ohmic contact.  
      [15] Protective Insulating Layer Forming Process:  
      After completion of the laser sintering process, the manufacturing sequence advances to a protective insulating layer forming process. As shown in  FIG. 1Q , an insulating resin is applied to the entire positive electrode  16  of the backside of the photovoltaic panel  27  and hardened to be formed into a protective insulating layer  17 , which covers the entire positive electrode  16 . The resin formed into the protective insulating layer  17  may be a thermosetting resin, ultraviolet curing resin, anaerobic curing resin or the like. Manufacture of the photovoltaic panel  27  is completed when the foregoing processes 1 to 15 have been carried out thoroughly.  
      According to the foregoing embodiment, a number of spherical or granular photovoltaic devices  11  are preliminarily held on the preliminarily holding sheet  21  and immersed in the resin liquid  24  and ascended. The resin liquid  24   a  adherent to the surface of each photovoltaic device  11  is hardened so that the resin lens  12  is formed on the surface of each photovoltaic device  11 . Accordingly, the spherical convex lens  12  can be formed on the surface of each photovoltaic device  11  by the surface tension of the resin liquid  24   a.  Consequently, the suitable spherical convex lens  12  can collectively be formed on the surfaces of the photovoltaic devices  11  by the surface tension of the resin liquid  24   a  respectively even when the photovoltaic devices  11  have variations in the size (diameter) and shape (sphericity). Further, each photovoltaic device  11  can be positioned on the center of the lens  12 . Moreover, no expensive forming machine, such as forming dies for injection molding, forming the lens  12  is required. Thus, optimization of the shape of lens  12  and low costs can be achieved simultaneously. Further, the preliminarily holding sheet  21  is shrunk in the direction of extension of the sheet after formation of the lens  12 , so that the lens  12  of each photovoltaic device  11  is bonded to the lens  2  of the adjacent one by the bonding agent  26 . Consequently, the photovoltaic devices  11  can be integrated by an easy and cost-effective method without using expensive forming equipment. Thus, a photovoltaic panel  27  provided with lens  12  and having a high generating efficiency can be manufactured.  
      Further, since the electrode  13 ,  16  of each photovoltaic device  11  is formed on the backside of the photovoltaic panel  27 , light incident on the photovoltaic devices  11  can be prevented from being intercepted by the electrode  13 ,  16  and accordingly, the entire surface of each lens  12  can effectively serve as a light-receiving face.  
      Further, since the negative electrode  13  is formed so as to cover the backside of each lens  12 , it can serve as a reflecting surface of incident light and accordingly, the generating efficiency (an amount of received light of each photovoltaic device  11 ) can further be increased by the light reflection of the negative electrode  13 .  
      In order to evaluate the condensing effect of each lens  12  of the photovoltaic panel  27  manufactured by the method of the embodiment, the inventor made samples of some photovoltaic panels  27  having respective lens  12  with different sizes and measured output current I sc  in the case where the same intensity of sunlight was irradiated onto each photovoltaic panel  27 .  FIG. 3  shows a graph of measurement data. In  FIG. 3 , an axis of abscissas indicates a projected area ratio of the lens  12  to photovoltaic device  11 , whereas an axis of ordinates indicates a ratio (I sc /I o ) of output current I sc  of the photovoltaic panel  27  with lends  12  to output current I o  of the photovoltaic panel without lens  12 . The lens  12  has a larger condensing effect as the aforementioned output current ratio is increased.  
      In the test, the output current ratios were measured with respect to five samples having respective projected area ratios of 3.0, 6.0 and 8.0. As a result, values approximate to 3.0, 6.0 and 8.0 respectively were obtained. The results of the test show that the output current I sc  can be increased substantially in proportion to the increase in the projected area ratio of the lens  12  to photovoltaic device  11 . Consequently, it can be confirmed that a photovoltaic panel  27  having a high generating efficiency can be manufactured at lower costs using a smaller number of photovoltaic devices  11  than in the conventional arrangement.  
      Further, in the foregoing embodiment, a sheet made from an elastic material expandable and contractible in the direction of extension of the sheet serves as the preliminarily holding sheet  21 . As shown in  FIGS. 1B and 2A , a number of photovoltaic devices  11  are caused to adhere closely to one side of the preliminarily holding sheet  21  and thereafter, the sheet  21  is drawn uniformly in the direction of extension of the sheet. As a result, since each photovoltaic device  11  is uniformly spaced from the adjacent one, the photovoltaic devices  11  need not be arranged while being spaced from each other before photovoltaic devices  11  are caused to adhere to one side of the preliminarily holding sheet  21 . Thus, it is advantageous that the photovoltaic device  11  can be caused to adhere to one side of the preliminarily holding sheet  21  easily. Moreover, it is advantageous that a uniform space can be defined between the photovoltaic devices  11  by an easy method of uniformly drawing the sheet in the direction of extension of the sheet.  
      Alternatively, in the present invention, the preliminarily holding sheet  21  may be drawn uniformly in the direction of extension of the sheet before the photovoltaic devices  11  are caused to adhere to one side of the preliminarily holding sheet  21 . In this state, the photovoltaic devices  11  may be caused to adhere to one side of the preliminarily holding sheet  21  while being spaced uniformly from one another.  
      Further, the preliminarily holding sheet may be made from a heat-shrinkable material, instead of the elastic material such rubber. In this case, the photovoltaic devices  11  may be caused to adhere to one side of the preliminarily holding sheet without being drawn in the direction of extension of the sheet while being spaced uniformly from one another. Upon completion of the bonding agent applying process, the preliminarily holding sheet may be heated thereby to be shrunk in the direction of extension of the sheet.  
      Further, since the light-transmissible ultraviolet curing resin is used as the resin formed into the lens  12  and the bonding agent  26  in the embodiment, the resin and the bonding agent  26  can be hardened by ultraviolet radiation in a short period of time (several to several tens seconds) and accordingly, the yield can be improved. Moreover, in the embodiment, the same type of resin (with only different viscosity) as formed into the lens  12  is used as the bonding agent  26 . The bonding agent  26   a  is caused to adhere uniformly to the surface of the lens  12  of each photovoltaic device  11  by the immersion method. Accordingly, optical characteristics of each lens  12  can be prevented from being reduced by the bonding agent  26   a.    
      Alternatively, a bonding agent having composition different from the resin formed into each lens  12  may be used in the present invention. Further, a light-transmissible thermosetting resin, a light-transmissible anaerobic curing resin or the like may be used as the material for either one or both of the resin formed into each lens  12  and the bonding agent  26 , instead of the light-transmissible ultraviolet curing resin.