Abstract:
A method and apparatus provide for a roughened back surface of a semiconductor absorber layer of a photovoltaic device to improve adhesion. The roughened back surface may be achieved through an etching process.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/536,151 filed on Sep. 19, 2011, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention relate to a photovoltaic (PV) device with a semiconductor absorber layer and a manufacturing method thereof. 
       BACKGROUND OF THE INVENTION 
       [0003]    A PV device converts the energy of sunlight directly into electricity by the photovoltaic effect. The PV device can be, for example, a PV cell, such as a crystalline silicon cell or a thin-film cell. PV modules can include a plurality of PV cells or devices. PV cells can include multiple layers created on a substrate (or superstrate). For example, a PV device can include a transparent conductive oxide (TCO) layer, a buffer layer, and semiconductor layers formed in a stack on a substrate. The semiconductor layers can include a semiconductor window layer formed on the buffer layer and a semiconductor absorber layer formed on the semiconductor window layer. Additionally, each layer can cover all or a portion of the device and/or all or a portion of the layer or substrate underlying the layer. For example, a “layer” can include any amount of any material that contacts all or a portion of a surface. 
         [0004]      FIG. 1  is a cross-sectional view of a conventional PV device  10  typically formed sequentially in a stack on a substrate  110 , e.g., soda-lime glass. Because substrate  110  is not conductive, PV device  10  can include a front contact  120 , which can include a multi-layered TCO stack with several functional layers including a barrier layer to protect the semiconductor layers from potential contaminants, a TCO layer, and a buffer layer to mitigate potential irregularities during the formation of the semiconductor layers, for example. The semiconductor layers can include an n-type semiconductor window layer  130 , such as a cadmium sulfide (CdS) layer, formed on the front contact  120  and a p-type semiconductor absorber layer  140 , such as a cadmium telluride (CdTe) layer, formed on the semiconductor window layer  130 . The window layer  130  can allow the penetration of solar energy to the absorber layer  140 , where the optical energy is converted into electrical energy. Specifically, the n-type window layer  130  may contact the p-type absorber layer  140  to form a p-n junction. As a result of diffusion across the junction, negative acceptor ions are formed on the p-type side and positive donor ions are formed on the n-type side. The presence of the ions creates a built-in electric field across the junction. When a photon is absorbed within the p-n junction, an electron-hole pair is formed. Movement of the electron-hole pairs are influenced by the built-in electric field, which produces current flow between the front contact  120  and a back contact  150 . Back contact  150  is formed over absorber layer  140 . Back contact  150  may be a low-resistance ohmic contact. Front and back contacts  120 ,  150  may serve as electrodes for transporting photocurrent away from PV device  10 . Back support  160 , which may be glass, is formed over back contact  150  to protect PV device  10  from external hazards. 
         [0005]    During fabrication, absorber layer  140  can be formed (or deposited) on window layer  130  by a vapor transport deposition (VTD) distributor system, for example, and then heat treated. After deposition of absorber layer  140 , a back surface  149  of absorber layer  140  may have a surface roughness quantified by the arithmetic mean value (Ra, a measurement of the average roughness of a surface calculated based on the height variations of the surface) that is smooth, e.g., less than about 10 nm, which could affect adhesion of the back contact  150  to the absorber layer  140 . Additionally, a smooth back surface  149  of absorber layer  140  could potentially inhibit CdTe grain growth and doping uniformity during post-deposition heat treatment. CdTe grain growth can produce a larger CdTe grain size in the CdTe layer which may increase carrier (e.g., electron, hole) mobility within the p-n junction and thus may boost the electrical output of PV device  10 . 
         [0006]    A semiconductor absorber layer of a PV device having a greater back surface roughness is desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional view of a conventional PV device; 
           [0008]      FIG. 2A  is a cross-sectional view of a PV device according to an embodiment of the invention at an initial stage of processing; 
           [0009]      FIG. 2B  is a cross-sectional view of the PV device of  FIG. 2A  at a stage of processing subsequent to that of  FIG. 2A ; 
           [0010]      FIG. 3  is a cross-sectional view of the PV device of  FIG. 2B  at a stage of processing subsequent to that of  FIG. 2B ; and 
           [0011]      FIG. 4  is a cross-sectional view of the PV device of  FIG. 3  at a stage of processing subsequent to that of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. It should be understood that like reference numbers represent like elements throughout the drawings. Embodiments are described in sufficient detail to enable those skilled in the art to make and use them, and it is to be understood that structural, material, electrical, and procedural changes may be made to the specific embodiments disclosed, only some of which are discussed in detail below. 
         [0013]    The various embodiments disclosed herein entail etching a semiconductor absorber layer of a PV device. Etching the semiconductor absorber layer can increase the surface roughness of the back surface (i.e., non-sunny side) of the absorber layer, which can improve adhesion of a back contact to the back surface of an absorber layer for better long-term efficiency and reliability of the PV device. In addition, etching the absorber layer increases its porosity, which enhances doping uniformity and grain growth during heat treatment after the etching step. 
         [0014]    Fabrication of semiconductor layers of a PV device may involve transport of vapor from a source to a substrate, or sublimation of a solid in a closed system, for example. An apparatus for manufacturing PV devices can include a conveyor, for example a roll conveyor for conveying a substrate through a deposition chamber. The conveyor transports the substrate, e.g. a soda-lime glass plate, into a series of deposition stations for sequentially depositing layers of material on the exposed surface of the substrate. The deposition chamber can be heated to reach a processing temperature in the range of about 450° C. to about 700° C. and includes a deposition distributor connected to a deposition vapor supply. The distributor can be connected to multiple vapor supplies for deposition of various layers or the substrate can be moved through multiple and various deposition stations, each with its own vapor distributor and supply. The distributor can be in the form of a spray nozzle with varying nozzle geometries to achieve uniform distribution of the vapor supply. 
         [0015]      FIG. 2A  illustrates a cross-sectional view of a portion of a PV device  20  at an initial stage of processing, prior to the etching step. PV device  20  includes a substrate  110 , e.g., soda-lime glass, a front contact  120 , which can include a multi-layered TCO stack with several functional layers, such as a barrier layer, a TCO layer, and a buffer layer, for example. A semiconductor window layer  130  is formed over the front contact  120 . Window layer  130  can be any suitable material, for example, a binary semiconductor such as Group II-VI or III-V semiconductors, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InS, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, or mixtures thereof. 
         [0016]    Semiconductor absorber layer  240  may be formed adjacent to semiconductor window layer  130  using any suitable deposition technique. For instance, absorber layer  240  may be deposited using a VTD system, or more specifically, atmospheric pressure chemical vapor deposition (APCVD), sputtering, atomic layer epitaxy (ALE), laser ablation, physical vapor deposition (PVD), close-spaced sublimation (CSS), electrodeposition (ED), screen printing (SP), spray, or metal organic chemical vapor deposition (MOCVD). Absorber layer  240  can be any suitable semiconductor material, for example, CIGS, CdTe, or amorphous Si. 
         [0017]    In one embodiment, window layer  130  and absorber layer  240  are CdS and CdTe, respectively. 
         [0018]    The surface roughness of the back surface  249  of absorber layer  240  can be quantified by the arithmetic mean value (Ra) and the root mean-square-average (Rq). Surface roughness Ra can be measured by optical reflectometry using a surface sensor that provides a flash of light and measures the reflected and scattered light, which determines surface roughness. Prior to the processing steps described below in connection with  FIG. 2B , the back surface  249  of absorber layer  240  can have a first surface roughness mean value (first Ra) of less than 10 nm, which is relatively smooth. This could undermine adhesion of back contact  150  to absorber layer  240 , which protects against delamination, and may result in decreased porosity of absorber layer  240 , which could inhibit doping uniformity and grain growth during heat treatment after the deposition of absorber layer  240 . 
         [0019]    To mitigate such potential hazards, and now referring to  FIG. 2B , the back surface  249  of the absorber layer  240  is etched to increase surface roughness. Accordingly, after formation of the absorber layer  240  as described with respect to  FIG. 2A , at least a portion of the back surface  249  (i.e., non-sunny side) of the absorber layer  240  is contacted with an etchant.  FIG. 2B  illustrates a cross-sectional view of a portion of PV device  20  following the etching step. 
         [0020]    The back surface  249  of the absorber layer  240  can be etched by any suitable known etching technique, such as wet or dry chemical etching or mechanical scrubbing. The etching may also include spraying at least a portion of back surface  249  of absorber layer  240  with an etchant or immersing at least a portion of absorber layer  240  in the etchant. The etchant can be a liquid or a powder, for example. 
         [0021]    According to one embodiment, the etchant includes an acid, such as hydrochloric acid (HCl), an oxidizer, such as hydrogen peroxide (H 2 O 2 ), and water. The etchant may include about 1% to about 10% HCl, about 0% to about 10% H 2 O 2 , and at least about 80% water, by volume, where HCl is about a 37% solution and H 2 O 2  is about a 32% solution, by volume. In another embodiment, the etchant may include about 1.5% HCl and about 0.75% H 2 O 2 , by volume. Back surface  249  of absorber layer  240  can be etched for a period of about 10 seconds to about 50 seconds or for another period of time suitable to increase the Ra of the back surface  249  of absorber layer  240 . Generally, the etching time will be shorter with higher concentrations of the acid and the oxidizer. The etching may occur at room temperature. Absorber layer  240  is rinsed in deionized water and air dried after etching. The etching step may be repeated one or more times. In one embodiment, the etching step is repeated if the surface roughness mean value (Ra) of the back surface  249  of absorber layer  240  is less than about 10 nm. 
         [0022]    The etching step modifies at least a portion of back surface  249  of absorber layer  240  such that back surface  249  has a second surface roughness mean value (second Ra) that is greater than the first Ra after the initial formation of absorber layer  240  ( FIG. 2A ). Accordingly, the second Ra is greater than about 10 nm. The increased Ra of back surface  249  can improve the efficiency of PV device  20  by improving the adhesion between absorber layer  240  and a back contact, which may be formed adjacent to (or over) and in contact with absorber layer  240  later in the manufacturing process, described with respect to  FIG. 4 . 
         [0023]    Referring to  FIG. 3 , after the etching step, the absorber layer  240  can be heat treated with a chloride compound, e.g., CdCl 2  flux.  FIG. 3  illustrates a cross-sectional view of PV device  20  following heat treatment. The chloride compound may be applied by various techniques, such as by solution spray, vapors, or atomized mist. Post-etching chloride heat treatment can repair any defects in the absorber layer  240  caused by the etching step and can enlarge CdTe grains. For example, chloride heat treatment can heal shunts or shunting paths caused by etching. During heat treatment, recrystallization can occur, thereby making a more uniform doping (e.g. CdCl 2 ) distribution within the absorber layer  240  possible. In one embodiment, absorber layer  240  is heat treated with CdCl 2  for about 20 minutes. In one embodiment, heat treatment is conducted at a temperature in the range of about 380° C. to about 450° C. 
         [0024]    Generally, the efficiency of the PV device  20  is determined by measuring open-circuit voltage, short-circuit current, and fill factor. Open-circuit voltage (Voc) is the voltage for which the current in the external circuit is zero. Short-circuit current (Jsc) is the current that flows out of the solar cell at zero bias. The fill factor (FF) is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Jsc). Etching the absorber layer  240  as described above can also increase the porosity of absorber layer  240  over that of an unetched absorber layer (e.g., absorber layer  140 ,  FIG. 1 ). This can promote CdCl 2  doping uniformity and grain growth during the heat treatment of absorber layer  240 . Such doping uniformity and grain growth can reduce the occurrence of electrical defects within absorber layer  240  having so-called “dangling bonds” capable of producing mid-gap energy levels which promote the recombination of electrons from the conduction band with holes from the valence band which can, in turn, reduce Voc. 
         [0025]    Dopants can be applied by any suitable technique. Dopants can be introduced during heat treatment, deployed simultaneously with a CdTe source powder in a vapor transport deposition (VTD) distributor system when forming the CdTe layer  240 , or deposited in the form of an outer layer on an absorber layer  240 , for example. A dopant may also be introduced as a powder which may be accompanied by an inert carrier gas to facilitate transport of the powder in the VTD system. P-type dopants such as Group I (for example, Li, Na, K, Rb, Cs, Cu, Ag or Au) and Group V (for example, N, P, As, Sb or Bi) elements can be effective absorber layer dopants. Group I and Group V species such as chloride compounds, can be added to the flux solution applied to the back surface  249  of an absorber layer  240  prior to heat treatment, such as CdCl 2 . Applicants have also observed through visual inspection better doping uniformity with an etched CdTe absorber layer  240  as compared to an unetched CdTe absorber layer  140  ( FIG. 1 ). 
         [0026]    Referring to  FIG. 4 , after the heat treatment step, a back contact  450  is formed over and in contact with the back surface of the absorber layer  240  and a back support  160 , e.g. glass, is formed over back contact  450 .  FIG. 4  illustrates a cross-sectional view of PV device  20  following formation of back contact  450  and back support  160 . 
         [0027]    Back contact  450  can be formed by any suitable technique and can include electrically conductive material, such as metallic silver, nickel, copper, aluminum, titanium, palladium, chrome, molybdenum or any practical combination thereof. Back contact  450  may also be a TCO multi-layered stack similar to front contact  120 . Back support  160  may be any suitable material formed by a suitable technique. In one embodiment, back support  160  is a soda-lime glass. 
         [0028]    Applicants have observed and measured improved adhesion between back contact  450  and etched absorber layer  240  (having a back surface roughness mean value Ra of greater than about 10 nm) as compared to an unetched absorber layer (e.g., layer  140  of  FIG. 1  having a back surface roughness mean value Ra of less than about 10 nm) which, as discussed above with respect to  FIGS. 2A-2B , improves adhesion of absorber layer  240  to back contact  450  and improves the efficiency of PV device  20 . 
         [0029]    While disclosed embodiments have been described in detail, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the disclosed embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described.