Patent Publication Number: US-2011056544-A1

Title: Solar cell

Description:
This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0083567 filed in the Korean Intellectual Property Office on Sep. 4, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The described various implementations relate to a solar cell. 
     2. Description of the Related Art 
     Recently, as existing energy sources such as petroleum and coal are expected to be depleted, interest in alternative energy sources for replacing the existing energy sources is increasing. Among the alternative energy sources, solar cells for generating electric energy from solar energy have been particularly spotlighted. 
     A solar cell generally includes a substrate and an emitter layer, each of which is formed of a semiconductor, and electrodes respectively formed on the substrate and the emitter layer. The semiconductors forming the substrate and the emitter layer have different conductive types, such as a p-type and an n-type. A p-n junction is formed at an interface between the substrate and the emitter layer. 
     When light is incident on the solar cell, a plurality of electron-hole pairs are generated in the semiconductors. The electron-hole pairs are separated into electrons and holes by the photovoltaic effect. Thus, the separated electrons move to the n-type semiconductor (e.g., the emitter layer) and the separated holes move to the p-type semiconductor (e.g., the substrate), and then the electrons and holes are collected by the electrodes electrically connected to the emitter layer and the substrate, respectively. The electrodes are connected to each other using electric wires to thereby obtain electric power. 
     SUMMARY OF THE INVENTION 
     In one aspect, there is a solar cell including a crystalline substrate containing first impurities of a first conductive type. The solar cell also includes a first non-crystalline layer containing second impurities of a second conductive type, the first non-crystalline layer having a first portion that includes a first concentration of the second impurities and a second portion that includes a second concentration of the second impurities, the second portion having a minimum distance from the crystalline substrate that is greater than a minimum distance of the first portion from the crystalline substrate, the second concentration being greater than the first concentration. The solar cell also includes a first electrode and a second electrode electrically connected to the first non-crystalline layer and electrically isolated from the first electrode. 
     The solar cell may include a second non-crystalline layer containing third impurities of a third conductive type, the second non-crystalline layer having a first portion that includes a first concentration of the third impurities and a second portion that includes a second concentration of the third impurities, the second portion having a minimum distance from the crystalline substrate that is greater than a minimum distance of the first portion from the crystalline substrate, the second concentration being greater than the first concentration, wherein the third conductive type is opposite of the second conductive type. The second non-crystalline layer may be positioned on a non-incident surface of the crystalline substrate upon which light is not incident. 
     In the solar cell, the first non-crystalline layer may be positioned on an incident surface of the crystalline substrate upon which light is incident. The first non-crystalline layer may be positioned on the non-incident surface of the crystalline substrate upon which light is not incident. The first conductive type may be the same as the third conductive type. The first concentration of the second impurities of the first portion of the first non-crystalline layer may be approximately zero. A concentration of the second impurities may increase at a predetermined rate between the first portion and the second portion. 
     In the solar cell, the first portion of the first non-crystalline layer may be an intrinsic semiconductor portion, and the second portion of the first non-crystalline layer may be an extrinsic semiconductor portion. The first portion of the first non-crystalline layer may be positioned proximate the crystalline substrate, and the second portion of the non-crystalline layer may be positioned proximate a surface of the non-crystalline layer opposite the crystalline substrate. The first non-crystalline layer has a single-layer structure. The first non-crystalline layer and the crystalline substrate may form a heterojunction. The first concentration and the second concentration of the second impurities may be from approximately 0 cm −3  to approximately 1×10 23  cm −3 . 
     In another general aspect, there is a semiconductor structure positioned over a first surface of a crystalline semiconductor substrate of a solar cell, the crystalline semiconductor substrate being a first conductive type. The semiconductor layer may include a first non-crystalline layer having a first concentration of impurities, and a second non-crystalline layer having a second concentration of impurities, the second concentration being different than the first concentration. The first non-crystalline layer and the second non-crystalline layer may each be non-intrinsic layers. 
     In the semiconductor structure, the first non-crystalline layer may have a minimum distance from the crystalline substrate that is greater than a minimum distance of the second non-crystalline layer from the crystalline substrate. The first concentration of impurities may be greater than the second concentration of impurities. The second concentration of impurities may be greater than the first concentration of impurities. 
     In another general aspect, there is a method that includes providing a crystalline substrate containing first impurities of a first conductive type. The method may also include forming a non-crystalline layer containing second impurities of a second conductive type on the crystalline substrate. Forming a non-crystalline layer may include forming a first portion of the non-crystalline layer that includes a first doping concentration of the second impurities and forming a second portion of the non-crystalline layer that includes a second concentration of the second impurities, the second portion having a minimum distance from the crystalline substrate that is greater than a minimum distance of the first portion from the crystalline substrate, the second concentration being greater than the first concentration. The method may also include providing a first electrode and providing a second electrode electrically connected to the non-crystalline layer and electrically isolated from the first electrode. 
     As a part of the method, forming non-crystalline layer may include forming the non-crystalline layer in a process chamber into which a dopant gas in injected. Additionally, forming the first portion and the second portion may include varying, at a predetermined rate, an amount of the dopant gas injected into the process chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a solar cell. 
         FIG. 2  is a graph illustrating an example relationship between an impurity doping concentration and a depth of an emitter layer or a back surface field layer. 
         FIG. 3  illustrates an energy band diagram between a substrate, an emitter layer, and a back surface field layer. 
         FIG. 4  is a graph indicating a relationship between a current density and an impurity doping concentration of an emitter layer or a back surface field layer. 
         FIG. 5  is a graph indicating another example relationship between an impurity doping concentration and a depth of an emitter layer or a back surface field layer. 
         FIG. 6  is another partial cross-sectional view of a solar cell. 
         FIG. 7  is a graph indicating an example relationship between an impurity doping concentration and a depth of an emitter layer or a back surface field layer in a solar cell. 
         FIG. 8  illustrates another energy band diagram between a substrate, an emitter layer, and a back surface field layer in a solar cell. 
         FIG. 9  shows various examples of an emitter layer and a back surface field layer. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Further, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being “entirely” on another element, it may be on the entire surface of the other element and may not be on a portion of an edge of the other element. 
     As shown in  FIG. 1 , a solar cell  1  includes a substrate  200 , an emitter layer  210  positioned on a front surface of the substrate  200  on which light is incident, and a back surface field (BSF) layer  220  positioned on a back surface of the substrate  200  opposite the front surface of the substrate  200  on which light is not incident. The solar cell  1  also includes first and second transparent conductive layers  231  and  232  respectively positioned on the emitter layer  210  and the back surface field layer  220 , a plurality of front electrodes  250  positioned on the first transparent conductive layer  231 , and a back electrode  260  positioned on the second transparent conductive layer  232 . 
     The substrate  200  is a semiconductor substrate formed of first conductive type silicon, such as n-type silicon, or another type of silicon. Silicon in the substrate  200  may be crystalline silicon, such as single crystal silicon and polycrystalline silicon. When the substrate  200  is of an n-type silicon, the substrate  200  may contain impurities of a group V element such as phosphor (P), arsenic (As), and/or antimony (Sb). Alternatively, the substrate  200  may be of a p-type, and/or include materials other than silicon. When the substrate  200  is of the p-type, the substrate  200  may contain impurities of a group III element such as boron (B), gallium (Ga), and/or indium (In). 
     The entire front and back surfaces of the substrate  200  may be textured to form an uneven surface or a surface having uneven characteristics. 
     The emitter layer  210  positioned in the front surface of the substrate  200  is an impurity region of a second conductive type (for example, a p-type) opposite the first conductive type (for example, the n-type) of the substrate  200 . The emitter layer  210  is formed of a different semiconductor from the substrate  200 , for example, a non-crystalline semiconductor, such as amorphous silicon (a-Si). In one example, the emitter layer  210  has a thickness of approximately 10 nm to 50 nm. However, other thicknesses may be used. Thus, the emitter layer  210  and the substrate  200  form not only a p-n junction but also a heterojunction between amorphous and crystalline silicon portions of the solar cell  1 . 
     The back surface field layer  220  on the back surface of the substrate  200  is an impurity region that is more heavily doped with impurities of the same conductive type as the substrate  200 . The back surface field layer  220  is formed of a different semiconductor from the substrate  200 , for example, a non-crystalline semiconductor, such as amorphous silicon, and thus forms the heterojunction along with the substrate  200 . 
     Accordingly, a movement of holes to the back surface of the substrate  200  is substantially prevented or is reduced by a potential barrier resulting from a difference between impurity doping concentrations of the substrate  200  and the back surface field layer  220 . Thus, a recombination and/or a disappearance of electrons and holes around the surface of the substrate  200  is/are substantially prevented or reduced. 
     In some implementations, each of the emitter layer  210  and the back surface field layer  220  is formed of amorphous silicon and the substrate  200  is formed of crystalline silicon (such as, microcrystalline silicon). Because the crystal structure of the emitter layer  210  and the back surface field layer  220  differ from the crystal structure of the substrate  200 , the emitter layer  210  and the back surface field layer  220  each forms a heterojunction with the substrate  200 . 
     As shown in  FIG. 1 , each of the emitter layer  210  and the back surface field layer  220  may be formed as a single film formed of amorphous silicon. 
     In a case of a comparative example of a solar cell generally having a separate passivation layer formed of, for example, intrinsic amorphous silicon between the substrate and the emitter layer and/or between the substrate and the back surface field layer, as reflected in the relationship between an impurity doping concentration and layer depth shown in  FIG. 7 , an impurity doping concentration sharply changes around a boundary between the substrate and the emitter layer and/or between the substrate and the back surface field layer. 
     As shown in  FIG. 7 , the impurity doping concentration C 1  included in the emitter layer or the back surface field layer is relatively high, and an impurity doping concentration C 2  of a passivation layer is relatively low. Further, the impurity doping concentration C 1  of the emitter layer or the back surface field layer is kept at a generally constant level. In the comparative example, the passivation layer formed of amorphous silicon does not have enough thickness to stably perform a passivation operation that converts unstable bonds, such as a dangling bond, existing around the surface of the substrate into stable bonds to thereby prevent or reduce a recombination and/or a disappearance of carriers moving to each of a front surface and a back surface of the substrate resulting from the unstable bonds. Thus, the passivation layer performs the passivation operation along with the emitter layer or the back surface field layer on the passivation layer. 
     In other implementations, an impurity doping concentration of each of the emitter layer  210  and the back surface field layer  220  linearly or nonlinearly changes depending on a depth from a surface of the emitter layer  210  and the back surface field layer  220 . In other words, as a distance from the surface of each of the emitter layer  210  and the back surface field layer  220  increases towards the surface of the substrate  200 , the impurity doping concentration of each of the emitter layer  210  and the back surface field layer  220  changes. 
     For example, as the distance from the surface increases, the impurity doping concentration of the emitter layer  210  gradually decreases at a predetermined rate. Thus, the impurity doping concentration of the emitter layer  210  around the contact surface between the substrate  200  and the emitter layer  210  is lower than the impurity doping concentration of the emitter layer  210  around the upper surface of the emitter layer  210 . As a result, the emitter layer  210  has a relative minimum impurity doping concentration at or near the contact surface between the substrate  200  and the emitter layer  210  and has a relative maximum impurity doping concentration at or near the upper surface of the emitter layer  210 . 
     Further, similar to the emitter layer  210 , the impurity doping concentration of the back surface field layer  220  gradually increases at a predetermined rate as a function of distance from the substrate  200 . Thus, the impurity doping concentration of the back surface field layer  220  at or near the contact surface between the substrate  200  and the back surface field layer  220  is lower than the impurity doping concentration of the back surface field layer  220  at or near the upper surface of the back surface field layer  220 . As a result, the back surface field layer  220  has a relative minimum impurity doping concentration around the contact surface between the substrate  200  and the back surface field layer  220  and may have a relative maximum impurity doping concentration around the upper surface of the back surface field layer  220 . 
     In other examples, as the emitter layer  210  and the back surface field layer  220  extend from the contact surfaces between the emitter layer  210  and the back surface field layer  220  and substrate  200 , the impurity doping concentration of each of the emitter layer  210  and the back surface field layer  220  may gradually decrease. In these examples, the emitter layer  210  and the back surface field layer  220  may have a relative maximum impurity doping concentration at or near the contact surfaces between the substrate  200 , and may have a relative minimum impurity doping concentration at or near the upper surfaces of the emitter layer  210  and the back surface field layer  220 . Additionally, a relationship between the impurity doping concentration of the emitter layer  210  and a distance from the upper surface of the emitter layer  210  may be different than a relationship between the impurity doping concentration of the back surface field layer  220  and a distance from the upper surface of the back surface field layer  220 . For example, the relationship between the impurity doping concentration of the emitter layer  210  and the distance from the upper surface of the emitter layer  210  may be the opposite of the relationship between the impurity doping concentration of the back surface field layer  220  and the distance from the upper surface of the back surface field layer  220 . 
     In some implementations, the impurity doping concentration of each of the emitter layer  210  and the back surface field layer  220  at or near the substrate  200  may be at least 0 cm −3 , and the impurity doping concentration of each of the emitter layer  210  and the back surface field layer  220  at or near the upper surfaces of the emitter layer  210  and the back surface field layer  220  may be at most approximately 1×10 23  cm −3 . 
     With regard to the production of the solar cell  1 , after an initial stage of the formation of the emitter layer  210  and/or the back surface field layer  220  is started, an amount of dopant gas present in the atmosphere of a process chamber is gradually increased from a state of substantially no dopant gas as the formation of the emitter layer  210  and/or the back surface field layer  220  progresses. Hence, the emitter layer  210  and/or the back surface field layer  220  each formed having a gradually changing impurity doping concentration. As shown in  FIG. 2 , the impurity doping concentration inside the emitter layer  210  and/or the back surface field layer  220  is indicated by a linear graph CV 1  indicating a linear change or a curvilinear graph CV 2  indicating a nonlinear change. 
       FIG. 2  is a graph illustrating a reduction in the impurity doping concentration of the emitter layer  210  and/or the back surface field layer  220  as a position within the emitter layer  210  and/or the back surface field layer  220  is close to the substrate  200  and an increase in the impurity doping concentration of the emitter layer  210  and/or the back surface field layer  220  as a position within the emitter layer  210  and/or the back surface field layer  220  is close to the upper surface of the emitter layer  210  and/or the back surface field layer  220 . 
     As above, the solar cell  1  shown in  FIG. 1  does not require a separate passivation layer capable of performing a passivation operation that converts unstable bonds, such as dangling bonds, existing between the substrate  200  and the emitter layer  210 , between the substrate  200  and the back surface field layer  220 , and around the surface of the substrate  200  into stable bonds to thereby prevent or reduce a recombination and/or a disappearance of carriers moving to each of the front surface and the back surface of the substrate resulting from the unstable bonds. 
     In some implementations, when the emitter layer  210  and/or the back surface field layer  220  have an impurity doping concentration that generally decreases as distance from the surface (for example, the upper surface) of the emitter layer  210  and/or the back surface field layer  220  increases, the upper surface of the emitter layer  210  and/or the back surface field layer  220  exhibits an extrinsic semiconductor characteristic, and a portion of the emitter layer  210  and/or the back surface field layer  220  at or near the substrate  200  exhibits an intrinsic semiconductor characteristic. On the contrary, when the emitter layer  210  and/or the back surface field layer  220  has an impurity doping concentration that increases with distance from the upper surface of the emitter layer  210  and/or the back surface field layer  220 , the upper surface of the emitter layer  210  and/or the back surface field layer  220  exhibits an intrinsic semiconductor characteristic, and a portion of the emitter layer  210  and/or the back surface field layer  220  at or near the substrate  200  exhibits an extrinsic semiconductor characteristic. 
     Although each of the emitter layer  210  and the back surface field layer  220  illustrated in  FIG. 1  has a single-layered structure, each of the emitter layer  210  and the back surface field layer  220  may perform the passivation operation as well as the above-described operations. More specifically, an intrinsic semiconductor portion of the emitter layer  210  and/or the back surface field layer  220  having a low impurity doping concentration converts unstable bonds existing around the surface of the substrate  200  into stable bonds to thereby prevent a loss of carriers and also reduces a damage (for example, a loss of carriers) resulting from a combination between impurities and carriers because of its low impurity doping concentration. Additionally, an extrinsic semiconductor portion of the emitter layer  210  and/or the back surface field layer  220  having a high impurity doping concentration forms the p-n junction with the substrate  200  or form the potential barrier along with the substrate  200  to thereby perform operations of the emitter layer  210  and/or the back surface field layer  220 . 
     With regard to the solar cell  1  of  FIG. 1 , the intrinsic semiconductor portion has thickness sufficient to stably perform the passivation operation. In some implementations, the intrinsic semiconductor portion has a thickness of for example, at least 6 nm. As mentioned above, and as shown in  FIG. 2 , a slope of the graph indicating the impurity doping concentration may increase as the emitter layer  210  and/or the back surface field layer  220  extends from the intrinsic semiconductor portion at or near the substrate  200  to the extrinsic semiconductor portion at or near the upper surface. In other words, the impurity doping concentration within the emitter layer  210  and/or the back surface field layer  220  increases to a concentration level capable of performing the passivation operation after transitioning from the substrate  200 , and then increases further, and to a greater degree, before transitioning to the first and second transparent conductive layers  231  and  232 . Hence, the conductivity and the contact characteristic of the solar cell  1  are improved. 
     In  FIG. 2 , the portion “A” indicates an intrinsic semiconductor portion where the intrinsic semiconductor characteristic is exhibited and the passivation operation is performed, and the portion “B” indicates an extrinsic semiconductor portion where the extrinsic semiconductor characteristic is exhibited and the emitter operation or the back surface field operation is performed. 
     The extrinsic semiconductor portion B includes a portion B 1  where an emitter operation and/or a back surface field operation is performed and a contact portion B 2 . An impurity doping concentration of the contact portion B 2  is higher than an impurity doping concentration of the portion B 1 , and a thickness of the portion B 2  is less than a thickness of the intrinsic semiconductor portion A associated with the passivation operation. 
     Accordingly, because the a separate passivation layer (for example, an amorphous silicon layer such as an intrinsic amorphous silicon layer) is not necessary if the emitter layer  210  and/or the back surface field layer  220  include the intrinsic semiconductor portion A, a separate chamber forming the passivation layer is not necessary. The manufacturing cost and time of the solar cell  1  are reduced by formation of the emitter layer  210  and/or the back surface field layer  220  including the intrinsic semiconductor portion A. Further, because detrimental changes in characteristics of the substrate  200  or other layers generated in a formation process of the passivation layer are substantially prevented, the efficiency of the solar cell  1  is improved by formation of the emitter layer  210  and/or the back surface field layer  220  including the intrinsic semiconductor portion A. Additionally, because the passivation operation is performed in the emitter layer  210  and/or the back surface field layer  220  at or near the substrate  200  without a separate passivation layer, an open-circuit voltage of the solar cell  1  is improved and the efficiency of the solar cell  1  is improved. 
     In some implementations, the first and second transparent conductive layers  231  and  232  are respectively positioned on the entire surface of the emitter layer  210  and the entire surface of the back surface field layer  220  and are formed of transparent conductive oxide (TCO) such as indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO). In some implementations, the second transparent conductive layer  232  on the back surface of the substrate  200  on which light is not incident may be formed of an opaque or translucent conductive material. In this case, light passing through the substrate  200  is reflected by the second transparent conductive layer  232  and then is again incident on the substrate  200 . Hence, the efficiency of the solar cell  1  can be improved by selecting an opaque or translucent conductive material for the second conductive layer  232 . 
     The first and second transparent conductive layers  231  and  232  each have good conductivity. Thus, light incident on the front surface of the substrate  200  is incident inside the substrate  200  through the first transparent conductive layer  231 . Moreover, carriers (e.g., holes) moving to the emitter layer  210  are transferred to the front electrodes  250  through the first transparent conductive layer  231  and carriers (e.g., electrons) moving to the back surface field layer  220  are transferred to the back electrode  260  through the second transparent conductive layer  232 . 
     The front electrodes  250  on the first transparent conductive layer  231  extend substantially parallel to one another in a fixed direction and are electrically connected to the emitter layer  210  through the first transparent conductive layer  231 . Thus, the front electrodes  250  collect the carriers (e.g., holes) moving to the emitter layer  210 . 
     The solar cell  1  shown in  FIG. 1  may further include a plurality of front electrode current collectors (not shown) that extend substantially parallel to one another in a direction crossing an extending direction of the front electrodes  250 . The plurality of front electrode current collectors are positioned on the same level layer as the front electrodes  250  and are electrically and physically connected to the front electrodes  250  at each of crossings of the front electrode current collectors and the front electrodes  250 . Thus, the front electrodes  250  and the front electrode current collectors are positioned on the front surface of the substrate  200  in a lattice shape. The front electrode current collectors collect carriers moving to the front electrodes  250 . The front electrode current collectors may be attached to a conductive tape connected to an external device and may output the collected carriers to the external device through the conductive tape. In some implementations, other configurations of the front electrodes  250  and/or the front electrode current collectors can be used or included. 
     The back electrode  260  is positioned on substantially the entire surface of the second transparent conductive layer  232  and is electrically connected to the back surface field layer  220  through the second transparent conductive layer  232 . Thus, the back electrode  260  collects carriers (e.g., electrons) moving to the back surface field layer  220 . 
     Further, the solar cell  1  may include a plurality of back electrode current collectors on the back electrode  260  or the second transparent conductive layer  232 . The back electrode current collectors are positioned opposite the front electrode current collectors with the substrate  200  interposed therebetween. Similar to the front electrode current collectors, the back electrode current collectors may collect carriers moving to the back electrode  260 , may be attached to a conductive tape connected to an external device, and may output the collected carriers to the external device through the conductive tape. 
     The front electrodes  250  and the back electrode  260  may be formed of at least one conductive material selected from the group consisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), alloys of these, and combinations thereof. However, other conductive materials may be used. 
     The front electrode current collectors and the back electrode current collectors transferring carriers to the external device may contain a conductive material. Conductivity of the conductive material used in the front electrode current collectors and the back electrode current collectors may be better than conductivity of the electrodes  250  and  260 , if necessary or desirable. 
     The front electrodes  250  and the back electrode  260  (in addition, the front electrode current collectors and the back electrode current collectors) may be formed having desired patterns on the first and second transparent conductive layers  231  and  232  using a photomask or a screen printing method and then performing a thermal process on the patterns. In this case, the back electrode current collectors may be formed on the back electrode  260 . 
     In use, when light irradiated to the solar cell  1  is incident on the substrate  200  through the first transparent conductive layer  231 , multiple electron-hole pairs are generated in the substrate  200 . Loss of light incident on the substrate  200  due to reflection away from the substrate and back through the first transparent conductive layer  231  is reduced due to a texture of a surface of the substrate  200 . Moreover, a light absorption increases because the textured surface of the substrate  200  causes incident light to be reflected into the substrate  200 . Hence, the efficiency of the solar cell  1  is improved. 
     The electron-hole pairs are separated into electrons and holes by the p-n junction of the substrate  200  and the emitter layer  210 . The separated holes move to the p-type emitter  210  and then are collected by the front electrodes  250 . The separated electrons move to the n-type back surface field layer  220  and are collected by the back electrode  260 . When the front electrodes  250  are connected to the back electrode  260  using electric wires (not shown), current flows therein to thereby enable use of the current for electric power. 
     As mentioned previously with regard to  FIG. 1 , a separate (intrinsic) amorphous silicon layer (i.e., the passivation layer) is not formed between the substrate  200  and the emitter layer  210  or between the substrate  200  and the back surface field layer  220 . Therefore, as shown in  FIG. 3 , energy band gap differences around an interface between the substrate  200  and the emitter layer  210  and around an interface between the substrate  200  and the back surface field layer  220  are reduced. Hence, the energy band gap gently changes in the interface between the substrate  200  and the emitter layer  210  and the interface between the substrate  200  and the back surface field layer  220 . 
     In the case of the comparative example of forming the separate passivation layer (for example, an intrinsic amorphous silicon layer) between the substrate and the emitter layer and/or between the substrate and the back surface field layer, an energy band diagram illustrated in  FIG. 8  is obtained. As shown in  FIG. 8 , the substrate is n-type crystalline silicon indicated by n-c-Si(n), the emitter layer is p-type amorphous silicon indicated by a-Si:H(p), the back surface field layer is n-type amorphous silicon indicated by a-Si:H(n + ), and the passivation layer is intrinsic amorphous silicon indicated by a-Si:H(i). Because a relatively large band offset (i.e., a difference between energy band gaps of the substrate and the passivation layer) is generated by including the separate passivation layer, smooth connections between energy band gaps of the layers are not achieved. 
     In other words, there are relatively large energy band gap differences between the substrate and the emitter layer and between the substrate and the back surface field layer when the separate passivation layer is included. The energy band gap difference adversely affects the movement of electrons “e − ” (corresponding to majority carriers) moving to the back surface field layer and the movement of holes “h + ” (corresponding to minority carriers) moving to the emitter layer. 
     In addition, when the separate passivation layer is formed having a relatively large thickness, the thick passivation layer disturbs or impedes a tunneling effect of carriers and disturbs or impedes the movement of carriers. Particularly, movement of the carriers is disturbed or impeded as they pass through the passivation layer due to poor conductivity of the amorphous silicon. Hence, the inclusion of a separate passivation layer reduces the efficiency of the solar cell. Additionally, the thickness of the separate passivation layer cannot be reduced due to a reduced affect on the ability of the separate passivation layer to perform a passivation function associated with reduced thickness. 
     With regard to  FIG. 3 , because a separate passivation layer is not included between the substrate  200  and the emitter layer  210 , or between the substrate  200  and the back surface field layer  220 , the energy band gap differences between the substrate  200  and the emitter layer  210  and between the substrate  200  and the back surface field layer  220  are smaller compared to the energy bad gap differences associated with the separate passivation layer. Accordingly, as described above, the energy band gap changes gradually or smoothly across the interface between the substrate  200  and the emitter layer  210  and across the interface between the substrate  200  and the back surface field layer  220 . Thus, carriers h +  and e −  easily move to the emitter layer  210  and the back surface field layer  220 . 
     Further, because a distance between the substrate  200  and the emitter layer  210  and a distance between the substrate  200  and the back surface field layer  220  is reduced when a separate passivation layer is not included, carriers may easily move and an amount of loss of carriers during the movement of carriers may be reduced. 
     As above, when passivation is achieved by varying the impurity doping concentrations of the emitter layer  210  and the back surface field layer  220 , each of which is formed of amorphous silicon, the thickness of the solar cell  1  may be reduced compared to solar cells that include one or more separate passivation layers. Further, carriers may easily move because the contact surface between the substrate  200  and the emitter layer  210  and the contact surface between the substrate  200  and the back surface field layer  220  have the impurity doping concentrations that are suitable for carrier conduction. 
     In other words, with regard to the carrier movement, implementations where the portions of the emitter layer  210  and the back surface field layer  220  performing the passivation operation contain relatively small concentrations of impurities may be more advantageous than implementations where the portions of the emitter layer  210  and the back surface field layer  220  performing the passivation operation do not contain any impurities. Further, a current density of the solar cell  1  is improved by the inclusion of the relatively small concentrations of impurities in the portions performing the passivation operation. 
       FIG. 4  illustrates changes in a current density and a voltage depending on an impurity doping concentration of an amorphous silicon layer including 6 graphs. As shown in  FIG. 4 , an intrinsic a-Si layer scarcely containing impurities has a minimum current density and a minimum voltage, and an a-Si layer having a maximum impurity doping concentration has a maximum current density and a maximum voltage. As above, as the impurity doping concentration increases, a magnitude of the voltage increases. Hence, a magnitude of an output power (i.e., P=V×I) increases. 
     However, in the comparative example, because only the intrinsic a-Si passivation layer performs the passivation operation, the passivation layer performs the passivation operation along with the emitter layer or the back surface field layer positioned on the passivation layer. Thus, when the impurity doping concentration of the emitter layer or the back surface field layer increases, the passivation effect decreases. Hence, a magnitude of an output voltage and a magnitude of an output power decrease. 
     However, with regard to solar cell  1 , because the passivation operation may be performed using only the intrinsic semiconductor portions of the emitter layer  210  and the back surface field layer  220 , the passivation effect does not decrease even if impurity doping concentrations of other portions of each of the emitter layer  210  and the back surface field layer  220  increase. As shown in  FIG. 4 , the magnitude of the output power may increase through an increase in the impurity doping concentration. 
     The impurity doping concentration of the a-Si layer used as the emitter layer  210  or the back surface field layer  220  gradually increases from 0 (in case of the intrinsic a-Si layer) to or through, 2×10 16  cm −3 , 2×10 17  cm −3 , 5×10 17  cm −3 , 8×10 17  cm −3 , and 2×10 18  cm −3  as the a-Si emitter layer or the a-Si back surface field layer  220  is formed on the substrate  200 . Other amounts of the impurity doping concentration may be used. For example, the impurity doping concentration may linearly or nonlinearly change within an impurity doping concentration range of 0 cm −3  to 1×10 23  cm −3 . 
     Further, as shown in  FIG. 5 , the graph indicates that the impurity doping concentration of the emitter layer  210  and the back surface field layer  220  may linearly or nonlinearly increase within the range of 0 cm −3  to 1×10 23  cm −3  as the emitter layer  210  and the back surface field layer  220  are formed on the substrate  200 . 
       FIG. 5  is a graph indicating changes in the impurity doping concentration of the emitter layer and/or the back surface field layer. As shown in  FIG. 5 , the impurity doping concentration inside the emitter layer  210  and/or the back surface field layer  220  nonlinearly changes similar to the graph of  FIG. 2 . 
     As described above, in  FIG. 5 , an impurity doping concentration of a contact portion B 1  between the substrate  200  and the emitter layer  210  and/or between the substrate  200  and the back surface field layer  220  is higher than an impurity doping concentration of the remainder A 1  of the emitter layer  210  and/or the back surface field layer  220 . The contact portion B 1  may include a contact surface between the substrate  200  and the emitter layer  210  and/or between the substrate  200  and the back surface field layer  220 . Hence, the impurity doping concentration of the emitter layer  210  and/or the back surface field layer  220  decreases from the contact portion B 1  to the upper surface of the emitter layer  210  and/or the back surface field layer  220 . The passivation effect is generated in a portion of the emitter layer  210  and/or the back surface field layer  220  that does not contain impurities or that has a low impurity doping concentration. Further, because an impurity doping concentration of the surface of the emitter layer  210  corresponding to a light incident surface is low, a reduction in an incident amount of light resulting from impurities is avoided compared to an implementation where the impurity doping concentration of the surface of the emitter layer  210  is relatively high. Hence, the efficiency of the solar cell  1  is improved by providing a relatively low impurity doping concentration at the surface of the emitter layer  210 . 
     In  FIG. 5 , graphing a doping concentration at varying positions within the emitter layer  210  and/or the back surface field layer  220 , illustrates that the impurity doping concentration of the emitter layer  210  and/or the back surface field layer  220  sharply decreases within the contact portion B 1  approaching the substrate  200 . Thus, the intrinsic semiconductor characteristic appears in the sharply decreasing portion of the impurity doping concentration. The sharply decreasing portion performs the passivation operation around the interface between the substrate  200  and the emitter layer  210  and/or between the substrate  200  and the back surface field layer  220 . 
     Alternatively, the emitter layer  210  and the back surface field layer  220  may have a linearly changing impurity doping concentration within the contact portion B 1  and/or the remainder A 1 . 
     Accordingly, as described above, because the passivation effect is generated in the emitter layer  210  and/or the back surface field layer  220  by changing the impurity doping concentration of the emitter layer  210  and/or the back surface field layer  220  without including a separate passivation layer, the open-circuit voltage of the solar cell  1  is improved and the efficiency of the solar cell  1  is improved. 
     The efficiency of a heterojunction solar cell depending on changes in an impurity doping concentration of an emitter layer is described with reference to the following Table 1, which indicates simulated results of the efficiency of a solar cell depending on changes in an impurity doping concentration of a p-type emitter layer when the p-type emitter layer (for example, an amorphous silicon layer) was formed on an n-type crystalline silicon substrate. 
     Moreover, Table 1 illustrates result under the assumption that there is no increase in defect formation associated with varying the impurity doping concentrations. More specifically, because only an intrinsic semiconductor portion performs the passivation operation in the same manner as solar cell  1  of  FIG. 1 , the passivation effect is not adversely affected even if an impurity doping concentration of an extrinsic semiconductor portion increases. 
     In the following Table 1, an impurity doping concentration of the substrate is approximately 5×10 15 /cm −3 , and resistivity of the substrate is approximately 0.99850Ω·cm. As indicated in the following Table 1, as an impurity doping concentration of the emitter layer increases, an open-circuit voltage Voc and a fill factor FF increases. Hence, the efficiency of the emitter layer increases as the impurity doping concentration of the emitter layer increases. Because the conductivity of the emitter layer increases as the impurity doping concentration of the emitter layer increases, a magnitude of activation energy for solving the energy band gap difference greatly decreases. 
     In solar cell  1  of  FIG. 1 , because a junction portion serving as the emitter layer and/or the back surface field layer is very thinly formed using a layer with a relatively high impurity doping concentration, a shallow junction is induced. Additionally, because the surface passivation of the silicon substrate requires a minimum thickness of the a-Si layer, the sufficient junction may be formed, and a reduction in the passivation effect resulting from the defect may be minimized. Further, because a heavily doped region is locally formed, a short-circuit current density Jsc was very slightly reduced because of very low light transmission. When the above conditions are applied to the back surface field layer rather than the emitter layer, the back surface field layer may have a minimum thickness capable of maintaining the passivation operation while locally inducing a strong reflection of minority carriers. Hence, a parallel resistance of the solar cell may be reduced. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Impurity doping 
                 5.00E+15 
                 5.00E+15 
                 5.00E+15 
                 5.00E+15 
                 5.00E+15 
               
               
                 concentration 
               
               
                 of substrate 
               
               
                 Resistivity of substrate 
                 0.99850 
                 0.99850 
                 0.99850 
                 0.99850 
                 0.99850 
               
               
                 (Ω · cm) 
               
               
                 Impurity doping 
                 1.25E+20 
                 5.00E+19 
                 2.50E+19 
                 1.25E+19 
                 5.00E+18 
               
               
                 concentration 
               
               
                 of emitter layer 
               
               
                 (#/cm −3 ) 
               
               
                 Activation Energy (eV) 
                 0.28 
                 0.36 
                 0.43 
                 0.46 
                 0.48 
               
               
                 Voc (V) 
                 0.645 
                 0.638 
                 0.631 
                 0.621 
                 0.615 
               
               
                 Jsc (mA/cm 2 ) 
                 36.320 
                 36.300 
                 36.410 
                 36.580 
                 36.710 
               
               
                 FF (%) 
                 76.310 
                 76.850 
                 76.640 
                 72.760 
                 63.560 
               
               
                 Efficiency (%) 
                 17.860 
                 17.810 
                 17.590 
                 16.530 
                 14.340 
               
               
                   
               
            
           
         
       
     
     The emitter layer  210  and/or the back surface field layer  220  can be formed such that the desired distributions of impurities are included. For example, the emitter  210  can be continuously formed in a single process chamber as a single layer. In this example, a concentration of the impurities present in the process chamber are controlled over time such that a portion of the emitter layer  210  formed at a first time includes a first concentration of impurities, and a second portion of the emitter layer  210  formed at a second time, which is different than the first time, includes a second concentration of impurities. As discussed above, as the emitter layer  210  is formed, the concentration of impurities contained in the emitter layer  210  can be controlled to vary linearly or non-linearly, and the concentration can be increased and decreased over time as desired to form the emitter layer  210  with a desired profile of impurity concentration from the substrate  200  to the upper surface of the emitter layer  210 . The back surface field layer  220  can be formed by a similar process in a separate process chamber, or both the emitter layer  210  and the back surface field layer  220  can be formed in the single process chamber at different times. In any case, the layers formed according to this example include impurity concentrations within the layers that vary according to varying concentrations of impurities present in the process chamber which are controlled during the formation of the layers. 
     Additionally, in this example, the impurity concentration within the emitter layer  210  at a given depth from the upper surface and/or at a given distance above the substrate  200  is substantially constant across a length and width of the emitter layer  210 . However, the concentration of impurities within the emitter layer  210  (as is also true of the back surface field layer  220 ) can vary in a controlled manner, or can vary due to random or uncontrolled factors that affect the formation process of the layer. 
     In another example, a layer, such as the emitter layer  210 , can be formed in one or more chambers during two or more separate formation processes. For example, a first portion of the emitter layer can be formed in a first process chamber at a first time and a second portion of the emitter layer  210  can be formed in a second process chamber or the first process chamber at a second time. The concentration of impurities present in the first process chamber (and in the second process chamber if used) can be maintained substantially constant during the formation of each of the first and second portions of the emitter layer  210 . However, the concentration present in the first process chamber is different than, such as less than, a concentration present during formation of the second portion. Additionally, or alternatively, the separate first and second portions of the emitter layer  210  can be formed while varying the concentration of the impurities in the process chamber linearly or non-linearly. 
     In another example, three or more distinct portions of a layer, such as the emitter layer  210  can be formed, either separately or integrally using varying impurity concentrations, constant impurity concentrations, or combinations of both. The distinct portions, if separately formed, can be formed in one or more process chambers. For example, each separate portion can be formed in a separate process chamber, if desired. Similarly, other layers can be added in the same or different process chambers, and/or various treatments or other processes can also be performed before, during, or after formation of the emitter layer  210  and/or the back surface field layer  220 . 
     The principles described above with regard to solar cell  1  of  FIG. 1  may be applied to not only a heterojunction solar cell but also a back contact solar cell, as illustrated in  FIG. 6 . Particularly,  FIG. 6  is a partial cross-sectional view of another solar cell  11 . Unlike the solar cell  1  illustrated in  FIG. 1 , a plurality of front electrodes (and a plurality of front electrode current collectors) are positioned on a back surface of a substrate  300  on which light is not incident. 
     More specifically, the solar cell  11  includes the substrate  300 , a passivation layer  340  positioned on a front surface of the substrate  300  on which light is incident, an anti-reflection layer  400  positioned on the passivation layer  340 , one or more emitter layers  310  positioned on the back surface of the substrate  300 , one or more back surface field layers  320  that are positioned on the back surface of the substrate  300  and are separated from the plurality of emitter layers  310 , one or more first electrodes  410  respectively positioned on the one or more emitter layers  310 , and one or more second electrodes  420  respectively positioned on the one or more back surface field layers  320 . 
     The substrate  300  is substantially the same as the substrate  200  illustrated in  FIG. 1 , and is formed of first conductive type crystalline silicon (for example, n-type crystalline silicon). 
     The passivation layer  340  is formed of intrinsic amorphous silicon and performs a passivation operation that converts unstable bonds generally existing around the surface of the substrate  300  into stable bonds, as described above. Because the passivation layer  340  is formed of intrinsic amorphous silicon scarcely containing impurities, a defect such as a loss of carrier resulting from the impurities is prevented or reduced. The passivation layer  340  may be formed of a silicon containing semiconductor such as silicon nitride (SiNx) and amorphous silicon nitride (a-SiNx), a non-conductive layer such as amorphous silicon dioxide (SiO 2 ), amorphous silicon oxide (a-SiO), and titanium dioxide (TiO 2 ), non-conductive polymer, or a paste containing these materials, in addition to amorphous silicon. 
     The anti-reflection layer  400  reduces a reflectance of light incident on the solar cell  11  and increases a selectivity of a predetermined wavelength band, thereby increasing the efficiency of the solar cell  11 . The anti-reflection layer  400  may have a proper refractive index so as to increase an anti-reflection effect. The anti-reflection layer  400  may be formed of SiNx, SiO 2 , SiNx:H, or SiO 2 :H. 
     As illustrated in  FIG. 6 , the anti-reflection layer  400  has a single-layer structure. However, the anti-reflection layer  400  may have a multi-layered structure such as a double-layer structure. Alternatively, the anti-reflection layer  400  may be omitted, if desired. The anti-reflection layer  400  performs the passivation operation in the same manner as the passivation layer  340 . Hence, an amount of carriers that disappear resulting from the unstable bonds is reduced by the passivation effect of the passivation layer  340  and the anti-reflection layer  400  on the front surface of the substrate  300 . As a result, the efficiency of the solar cell  11  is improved. 
     The one or more emitter layers  310  on the back surface of the substrate  300  are separated from one another and extend substantially parallel to one another in a fixed direction. Because each of the emitter layers  310  is of a conductive type opposite a conductive type of the substrate  300 , in the same manner as the emitter layer  210  of  FIG. 1 , each emitter layer  310  and the substrate  300  form a p-n junction. Each emitter layer  310  is of a p-type and is formed of amorphous silicon in the same manner as the emitter layer  210  of  FIG. 1 . 
     The one or more back surface field layers  320  are separated from the emitter layers  310  and extend on the substrate  300  substantially parallel to the emitter layers  310 . Each of the back surface field layers  320  is formed of amorphous silicon containing impurities of the same conductive type as the substrate  300  in the same manner as the back surface field layer  220  of  FIG. 1 . 
     An impurity doping concentration of each emitter layer  310  and an impurity doping concentration of each back surface field layer  320  linearly or nonlinearly changes in the same manner as the impurity doping concentrations of the emitter layer  210  and the back surface field layer  220  of  FIG. 1 . In other words, the impurity doping concentration and the characteristics of each emitter layers  310  and each back surface field layers  320  are substantially the same as those of the emitter layer  210  and the back surface field layer  220  of  FIG. 1 , except for their formation location and a shape. Accordingly, in each emitter layer  310  and each back surface field layer  320 , a passivation operation is performed in a portion with a low impurity doping concentration, and an emitter operation and a back surface field operation, respectively, are performed in a portion where an impurity doping concentration is higher than a set concentration. 
     The first and second electrodes  410  and  420  are formed of a conductive material and overlie the emitter layers  310  and the back surface field layers  320 . The first and second electrodes  410  and  420  output carriers moving to and through the emitter layers  310  and the back surface field layers  320  respectively from the substrate  300  to an external device. 
     As described above, low impurity doping concentration portions of the emitter layers  310  and the back surface field layers  320  perform the passivation operation without a separate passivation layer. 
     Further, because light is incident on the entire front surface of the substrate  300 , an amount of light incident on the substrate  300  increases. Hence, the efficiency of the solar cell  11  is improved. In addition, because the anti-reflection layer  400  reduces a reflection loss of light incident on the substrate  300 , an amount of light incident on the substrate  300  is not reduced. 
     Referring to  FIG. 9 , an example of the solar cell further includes a separate intrinsic amorphous silicon layer  510  between the substrate  300  and the emitter layer  310  and/or between the substrate  300  and the back surface field layer  320 . The intrinsic amorphous silicon layer  510  is a passivation layer performing the passivation operation. The impurity doping concentration of the emitter layer  310  and the back surface field layer  320  is linearly or nonlinearly changed from approximately 0 cm −3  to approximately 1×10 23  cm −3 . The passivation function is performed by the intrinsic amorphous silicon layer  510  as well as the emitter layer  310  and/or the passivation layer  320 , thereby improving the passivation effect. 
     In addition, as shown in  FIG. 9 , another example of the solar cell also includes a separate intrinsic amorphous silicon layer  510  between the substrate  300  and the emitter layer  310  and/or between the substrate  300  and the back surface field layer  320 . As described above, the intrinsic amorphous silicon layer  510  performs the passivation operation. However, as shown in  FIG. 9 , the solar cell includes a first emitter layer  311  positioned on the intrinsic amorphous silicon layer  510 , in which impurities are doped, and a second emitter layer  312  positioned on the first emitter layer  311 , and includes a first back surface field layer  321  positioned on the intrinsic amorphous silicon layer  510 , in which impurities are doped and a second back surface field layer  322  positioned on the first back surface field layer  321 . The impurity doping concentrations of the first emitter layer  311  and the first back surface field layer  321  are less than the impurity doping concentrations of the second emitter layer  312  and the second back surface field layer  322 , respectively. 
     The first emitter layer  311  mainly functions as the passivation layer, and the second emitter layer  312  function as an emitter layer forming the p-n junction with the substrate  300 . However, the first emitter layer  311  also forms the p-n junction along with the second emitter layer  312 . Similarly, the first back surface field layer  321  mainly functions as the passivation layer and the second back surface field layer  322  mainly performs the potential barrier. However, the first back surface field layer  321  also forms the potential barrier. In this implementation, the passivation function is performed by the intrinsic amorphous silicon layer  510  as well as the first emitter layer  311  and/or the first passivation layer  321 , thereby improving the passivation effect. 
     A thickness of the emitter layer  310  can be approximately 10 nm and a thickness of the back surface field layer  320  can be approximately 15 nm. When the emitter layer  310  has a thickness of about 10 nm, the emitter layer  310  stably performs the p-n junction and further performs the passivation operation. When the back surface field layer  330  has a thickness of about 15 nm, the back surface field layer  320  stably forms the potential barrier and further performs the passivation operation. Additionally, the first emitter layer  311  can have a thickness of approximately 6 nm and the total thickness of the first and second emitter layers  311  and  312  can be approximately 10 nm. The first back surface field layer  321  can have a thickness of about 6 nm and the total thickness of the first and second back surface field layers  321  and  322  can be approximately 15 nm. The substrate  300  is made of crystalline silicon, and the emitter layers  310 ,  311  and  312  and the back surface field layers  320 ,  321  and  322  are made of a non-crystalline silicon. 
     Although solar cells have been described with reference to a number of illustrative implementations, it should be understood that numerous modifications and other implementations can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure. More particularly, many variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure. For example, a location, number, and arrangement the emitter layers and the back surface field layers may be varied. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.