Patent Publication Number: US-2009229666-A1

Title: Smoothing a metallic substrate for a solar cell

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate generally to the field of photovoltaic technology. 
     BACKGROUND 
     In the drive for renewal sources of energy, photovoltaic technology has assumed a preeminent position as a cheap renewable source of clean energy. In particular, solar cells based on the compound semiconductor copper indium gallium diselenide (CIGS) used as an absorber layer offer great promise for thin-film solar cells having high efficiency and low cost. In efforts to obtain thin-film solar cells based on CIGS with lower cost, technological development has pursued a goal of using substrates having a large areal footprint, on the order of 1 meter in width, and equal or greater length. Recently, manufacturing schemes employing in-line coating processes on substrates provided from roll sheet stock have been investigated to achieve this goal. 
     However, unlike the small form-factor substrates used in the past to fabricate laboratory demonstrations of thin-film solar cells, these new substrate materials present a number of engineering challenges. One such challenge is conditioning these new substrates to receive the layers deposited upon the substrates during the solar-cell fabrication process while maintaining: high yields for the process, a defect-free substrate that produces high performance, and high solar-cell efficiency, as a figure of merit. 
     SUMMARY 
     Embodiments of the present invention include a method for smoothing the surface of a metallic substrate. In one embodiment, the method includes providing a metallic substrate and smoothing a surface of the metallic substrate by irradiating the surface with a high-intensity energy source, such that the surface is smoothed to remove defects from the surface by creating an altered surface layer. The altered surface layer is configured to receive at least one layer in a fabrication process of an electronic device. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the embodiments of the invention: 
         FIG. 1A  is a cross-sectional elevation view of a layer structure of a solar cell, in accordance with an embodiment of the present invention. 
         FIG. 1B  is a schematic diagram of a model circuit of a solar cell, electrically connected to a load, in accordance with an embodiment of the present invention. 
         FIG. 2A  is a cross-sectional elevation view of a metallic substrate prior to deposition of layers in fabrication of a solar cell illustrating various types of defects at a surface of the metallic substrate having potentially deleterious effects on solar-cell efficiency, upon which embodiments of the present invention may be implemented. 
         FIG. 2B  is an expanded view of a portion of the cross-sectional elevation view of  FIG. 2A  after depositing layers to fabricate a solar cell on the metallic substrate illustrating a portion of photocurrent being lost to a shunt defect associated with a defect at the surface of the metallic substrate, upon which embodiments of the present invention may be implemented. 
         FIG. 3A  is a cross-sectional elevation view of a metallic substrate after irradiating a surface of the metallic substrate with a high-intensity energy source, in accordance with an embodiment of the present invention. 
         FIG. 3B  is an expanded view of a portion of the cross-sectional elevation view of  FIG. 3A  after irradiating a surface of the metallic substrate with a high-intensity energy source and depositing layers to fabricate a solar cell, the layers disposed on an altered surface layer of the metallic substrate, in accordance with an embodiment of the present invention. 
         FIG. 4  is an elevation view of a roll-to-roll surface smoother for smoothing the surface of a substrate in roll form from a roll of material, in accordance with an embodiment of the present invention. 
         FIG. 5  is flow chart illustrating a method for smoothing the surface of a metallic substrate, in accordance with an embodiment of the present invention. 
         FIG. 6  is flow chart illustrating a method for fabricating a solar cell, in accordance with an embodiment of the present invention. 
         FIG. 7  is flow chart illustrating a method for roll-to-roll smoothing the surface of a substrate, in accordance with an embodiment of the present invention. 
     
    
    
     The drawings referred to in this description should not be understood as being drawn to scale except if specifically noted. 
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to the various embodiments of the present invention. While the invention will be described in conjunction with the various embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Furthermore, in the following description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it should be appreciated that embodiments of the present invention may be practiced without these specific details. In other instances, well known methods, procedures, and components have not been described in detail as not to unnecessarily obscure embodiments of the present invention. 
     Physical Description of Embodiments of the Present Invention for a Solar Cell 
     With reference to  FIG. 1A , in accordance with an embodiment of the present invention, a cross-sectional elevation view of a layer structure of a solar cell  100  is shown. The solar cell  100  includes a metallic substrate  104 . A surface of the metallic substrate  104  is smoothed by irradiating the surface of the metallic substrate  104  with a high-intensity energy source, wherein the surface is smoothed to remove defects from the surface by creating an altered surface layer  104   b  of the metallic substrate  104  on a supporting portion  104   a  of the metallic substrate  104 . In accordance with the embodiment of the present invention, an absorber layer  112  is disposed on the altered surface layer  104   b ; the absorber layer  112  may include a layer of the material copper indium gallium diselenide (CIGS) having the chemical formula Cu(In 1-x Ga x )Se 2 , where x may be a decimal less than one but greater than zero that determines the relative amounts of the constituents, indium, In, and gallium, Ga. 
     As shown, the absorber layer  112  includes a p-type portion  112   a  and an n-type portion  112   b . As a result, a pn homojunction  112   c  is produced in the absorber layer  112  that seives to separate charge carriers that are created by light incident on the absorber layer  112 . To facilitate the efficient conversion of light energy to charge carriers in the absorber layer  112 , the composition of the p-type portion  112   a  of the absorber layer  112  may vary with depth to produce a graded band gap of the absorber layer  112 . Alternatively, the absorber layer  112  may include only a p-type CIGS material layer and a pn heterojunction may be produced between the absorber layer  112  and an n-type layer, such as cadmium sulfide, CdS, zinc sulfide, ZnS, or indium sulfide, InS, disposed on its top surface in place of the n-type portion  112   b  shown in  FIG. 1A . However, embodiments of the present invention are not limited to pn junctions fabricated in the manner described above, but rather a generic pn junction produced either as a homojunction in a single semiconductor material, or alternatively as a heterojunction between two different semiconductor materials, is within the spirit and scope of embodiments of the present invention. 
     In accordance with an embodiment of the present invention, on the surface of the n-type portion  112   b  of the absorber layer  112 , a transparent electrically conductive oxide (TCO) layer  116  is disposed, for example, to provide a means for collection of current flow from the absorber layer  112  for conduction to an external load. The TCO layer  116  may include zinc oxide, ZnO, or alternatively a doped conductive oxide, such as aluminum zinc oxide, Al x Zn 1-x O y , and indium tin oxide, In x Sn 1-x O y , where the subscripts x and y indicate that the relative amount of the constituents may be varied. These TCO layer materials may be sputtered directly from an oxide target, or alternatively the TCO layer may be reactively sputtered in an oxygen atmosphere from a metallic target, such as zinc, Zn, Al—Zn alloy, or In—Sn alloy targets. For example, the zinc oxide may be deposited on the absorber layer  112  by sputtering from a zinc-oxide-containing target, alternatively, the zinc oxide may be deposited from a zinc-containing target in a reactive oxygen atmosphere in a reactive-sputtering process. The reactive-sputtering process may provide a means for doping the absorber layer  112  with an n-type dopant, such as zinc, Zn, or indium, In, to create a thin n-type portion  112   b , if the partial pressure of oxygen is initially reduced during the initial stages of sputtering a metallic target, such as zinc, Zn, or indium, In, and the layer structure of the solar cell  100  is subsequently annealed to allow interdiffusion of the zinc, Zn, or indium, In, with the CIGS material of the absorber layer  112 . Alternatively, sputtering a compound target, such as zinc sulfide, ZnS, indium sulfide, InS, or cadmium sulfide, CdS, may also be used to provide the n-type layer, as described above, on the p-type portion  112   a  of the absorber layer  112 . 
     With further reference to  FIG. 1A , in accordance with the embodiment of the present invention, a conductive backing layer  108  may be disposed between the absorber layer  112  and the altered surface layer  104   b  of the metallic substrate  104  to provide a diffusion barrier between the absorber layer  112  and the metallic substrate  104 . The conductive backing layer  108  may include molybdenum, Mo, or other suitable metallic layer having a low propensity for interdiffusion with the absorber layer  112  composed of CIGS material, as well as a low diffusion coefficient for constituents of the substrate. Moreover, the conductive backing layer  108  may provide other functions in addition to, or independent of, the diffusion-barrier function, for example, a light-reflecting function, for example, as a light-reflecting layer, to enhance the efficiency of the solar cell, as well as other functions. The embodiments recited above for the conductive backing layer  108  should not be construed as limiting the function of the conductive backing layer  108  to only those recited, as other functions of the conductive backing layer  108  are within the spirit and scope of embodiments of the present invention, as well. 
     With reference now to  FIG. 1B , in accordance with an embodiment of the present invention, a schematic diagram of a model circuit  150  of a solar cell that is electrically connected to a load is shown. The model circuit  150  of the solar cell includes a current source  158  that generates a photocurrent, i L . The photocurrent, i L , is produced when a plurality of incident photons, light particles, of which one example photon  154  with energy, hv, is shown, produce electron-hole pairs in the absorber layer  112  and these electron-hole pairs are separated by the pn homojunction  112   c , or in the alternative, by a pn heterojunction as described above. It should be appreciated that the energy, hv, of each incident photon of the plurality of photons should exceed the band-gap energy, E g , that separates the valence band from the conduction band of the absorber layer  112  to produce such electron-hole pairs, which result in the photocurrent, i L . 
     The model circuit  150  of the solar cell further includes a diode  162 , which corresponds to recombination currents, primarily at the pn homojunction  112   c , that are shunted away from the connected load. In addition, the model circuit  150  of the solar cell includes two parasitic resistances corresponding to a shunt resistor  166  with shunt resistance, R sh , and to a series resistor  170  with series resistance, R s . The solar cell may be connected to a load represented by a load resistor  180  with load resistance, R L . Thus, the circuit elements of the solar cell include the current source  158 , the diode  162  and the shunt resistor  166  connected across the current source  158 , and the series resistor  170  connected in series with the load resistor  180  across the current source  158 , as shown. As the shunt resistor  166 , like the diode  162 , are connected across the current source  158 , these two circuit elements are associated with internal currents within the solar cell shunted away from useful application to the load. As the series resistor  170  connected in series with the load resistor  180  are connected across the current source  158 , the series resistor  170  is associated with internal resistance of the solar cell that limits the current flow to the load. 
     With further reference to  FIG. 1B , it should be recognized that the shunt resistance may be associated with surface leakage currents that follow paths at free surfaces that cross the pn homojunction  112   c ; free surfaces are usually found at the edges of the solar cell along the side walls of the device that define its lateral dimensions; such free surfaces may also be found at discontinuities in the absorber layer  112  that extend past the pn homojunction  112   c . The shunt resistance may also be associated with shunt defects which may be present that shunt current away from the load, as will subsequently be described in  FIG. 2B . A small value of the shunt resistance, R sh , is undesirable as it lowers the open circuit voltage, V OC , of the solar cell, which directly affects the efficiency of the solar cell. Moreover, it should also be recognized that the series resistance, R s , is associated with: the contact resistance between the p-type portion  112   a  and the conductive backing layer  108 , the bulk resistance of the p-type portion  112   a , the bulk resistance of the n-type portion  112   b , the contact resistance between the n-type portion  112   b  and TCO layer  116 , and other components, such as conductive leads, and connections in series with the load. A large value of the series resistance, R s , is undesirable as it lowers the short circuit current, I SC , of the solar cell, which also directly affects the efficiency of the solar cell. 
     With reference now to  FIG. 2A , a cross-sectional elevation view of an example metallic substrate  204  prior to deposition of layers in fabrication of a solar cell is shown that illustrates various types of defects at a surface of example metallic substrate  204  having potentially deleterious effects on solar-cell efficiency. In an embodiment of the present invention, example metallic substrate  204  has numerous defect types on its surface in the as-received state, which should be removed prior to deposition of layers in fabrication of the solar cell. Examples of the defect types at a surface of example metallic substrate  204  include, without limitation: pit  208 , carbonaceous residue  212 , protrusion  216 , inclusion  220 , and rolling groove  224 . For example, pit  208  may include a left over-hanging portion  208   a  and a right over-hanging portion  208   b , which may result from metallic flakes and protrusions being rolled onto the surface of example metallic substrate  204  during a rolling operation for reduction from billet stock down to rolled sheet stock. Pit  208  may further include a recessed portion  208   c , which forms a bottom to pit  208 , and a cavity portion  208   d  enveloped by the left and right over-hanging portions  208   a  and  208   b , and recessed portion  208   c . Carbonaceous residue  212  may originate from oil used to lubricate the roll bearings, or adventitious sources of contamination of the rolled sheet, during the rolling operation. Protrusion  216  may be generated by material extruded from the interior of the billet during the rolling operation. Inclusion  220  may be generated by surface oxides rolled under the surface of example metallic substrate  204  during the rolling operation. These oxides may originate from the oxidized layers, so called “scale,” a metallurgical term of art, that are natively present on the surface of billets, or may originate from foreign oxide particles such as alumina, silicates and alumina silicates that have an adventitious origin, which, during the rolling operation, are rolled under the surface of billets, which are used to produce the rolled sheet stock of example metallic substrate  204 . Rolling groove  224  may be generated by direct interaction of the surface of the billet with the surface of the roll during the rolling operation in reducing the billet down to rolled sheet stock. 
     With reference now to  FIG. 2B , an expanded view of a portion of the cross-sectional elevation view of  FIG. 2A  is shown as indicated by lines of projection  246  and  248 .  FIG. 2B  illustrates a shunt portion  288   a  of photocurrent  280  being lost through a shunt defect associated with a defect, pit  208 , at the surface of example metallic substrate  204  after layers have been deposited on example metallic substrate  204  to fabricate a solar cell. To simplify the discussion,  FIG. 2B  shows the solar cell structure more generically without a conductive backing layer, as may be the case, for example, in an embodiment of the present invention. A discontinuous absorber layer is shown in two portions: portion  262   a  disposed on the left over-hanging portion  208   a  of pit  208 ; and, portion  262   b  disposed on the recessed portion  208   c  of pit  208 , which forms the bottom of the pit. The cavity portion  208   d  of the pit  208  is shown partially filled with material from the deposited layers of the solar cell structure. On portions  262   a  and  262   b  of the discontinuous absorber layer are disposed, respectively, three portions of an anomalous TCO layer: portion  266   a  disposed on portion  262   a  over the left of pit  208 ; portion  266   b  disposed on portion  262   b  at the bottom, recessed portion  208   c , of pit  208 ; and, portion  266   c  disposed on a side-wall of portion  262   a  of the discontinuous absorber layer located at a discontinuity associated with the pit. The shunt defect is composed of a complex of the following structures: portion  266   c  of the anomalous TCO layer that bridges between the portion  266   a  and the top of portion  266   b  that makes electrical contact with the portion of the substrate shown as the bottom of the left over-hanging portion  208   a  of pit  208 . As shown, the shunt defect provides a low-resistance current path between the example metallic substrate  204  and portion  266   a  of the anomalous TCO layer. 
     With further reference to  FIG. 2B , a representative portion of the photocurrent  280  generated in the portion  262   a  of the discontinuous absorber layer is shown passing from the left over-hanging portion  208   a  of the pit to the portion  266   a  of the anomalous TCO layer. The photocurrent  280  divides into two separate portions: a load portion  284   a , which passes to the left through the portion  266   a  of the anomalous TCO layer; and the shunt portion  288   a , which passes to the right through the portion  266   a  of the anomalous TCO layer. The load portion  284   a  of the photocurrent  280  corresponds to a current flowing in circuit loop containing the load resistor  180  with load resistance, R L , of  FIG. 1B , described above, and completes the circuit through return load current  284   b , which passes to the right through a portion of the example metallic substrate  204  shown as the left over-hanging portion  208   a  of pit  208 . The shunt portion  288   a  of the photocurrent  280  corresponds to a current flowing in a circuit loop containing the shunt resistor  166  with shunt resistance, R sh , of  FIG. 1B , and completes the circuit through return shunt current  288   b , which passes to the left from the shunt defect found at the discontinuity in portion  262   a  of the discontinuous absorber Layer adjacent to entrance to the cavity portion  208   d  of the pit  208 . Such shunt defects short circuit current that would otherwise pass to the load, which leads to loss of solar cell efficiency, and generate hot spots that can eventually lead to catastrophic shorts that break down the pn junction of the solar cell. Therefore, it is desirable to have some means for eliminating various types of defects at the surface of example metallic substrate  204  prior to deposition of layers in the fabrication of the solar cell. 
     Notwithstanding the problems attending the use of metallic substrates, such as example metallic substrate  204 , it should be recognized that it is desirable to use such rolled sheet stock because of its low cost. However, removal of the defects at the surface of example metallic substrate  204  should be provided to preclude the costs attending yield losses of solar-cell production associated with these defects. Low-cost, rolled sheet stock suitable for use as example metallic substrate  204  may include stainless steel, aluminum, titanium, alloys of aluminum or titanium, any metallic foil, or even a metallized non-metallic substrate. Examples of aluminum and titanium alloys include aluminum-silicon alloy and titanium-aluminum alloy, respectively; an example of a metallized non-metallic substrate is a flexible, non-conductive substrate, such as a polymer substrate, with a sputtered metallic layer; and an example of a stainless steel is 430-alloy stainless steel. The defective surface region may include a peak-to-valley roughness  240  of about 5 μm, as shown in  FIG. 2A . Therefore, in accordance with an embodiment of the present invention, it is desirable to have some means for treating example metallic substrate  204  to remove defects up to about 5 μm below the surface of example metallic substrate  204 . 
     With reference now to  FIG. 3A , in accordance with an embodiment of the present invention, a cross-sectional elevation view of a metallic substrate  304  after irradiating a surface of the metallic substrate  304  with a high-intensity energy source is shown. The metallic substrate  304  includes a supporting portion  304   a  and an altered surface layer  304   b . The surface of the metallic substrate  304  is smoothed by irradiating the surface of the metallic substrate  304  with a high-intensity energy source, in which the surface is smoothed to remove defects from the surface by creating the altered surface layer  304   b  of the metallic substrate  304  on the supporting portion  304   a  of the metallic substrate  304 . In one embodiment, the altered surface layer  304   b  has a thickness  324  of less than about 5 μm; alternatively, the altered surface layer  304   b  may be less than about 25 μm. Smoothing may be accomplished with a single pass of irradiation from the high-intensity energy source over the surface of the metallic substrate, or alternatively with a plurality of passes of irradiation from the high-intensity energy source over the surface of the metallic substrate. For example, two passes of irradiation from the high-intensity energy source may be used: a first, to remove inclusions from the surface, for example, by vaporization of the inclusions; a second, to further smooth the surface, for example, by reflowing vestigial craters at the location of inclusions vaporized in the first pass. Within the spirit and scope of embodiments of the present invention, additional passes beyond two may even be used with increasing improvement of the surface topography, although the accrued improvements may come with diminished returns. 
     After the metallic substrate  304  is smoothed, in accordance with an embodiment of the present invention, the metallic substrate  304  is suitable for further fabrication of an electronic device including, for example, a solar cell. An absorber layer  362  of the solar cell may be disposed on the altered surface layer  304   b , as shown in  FIG. 3B ; the absorber layer  362  may include a layer of CIGS material. The smoothing may include a laser smoothing, wherein the laser smoothing further includes a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. Similarly, the high-intensity energy source may include a laser selected from a group including a Q-switched laser, a Q-switched neodymium-doped, yttrium-aluminum-garnet (Nd:YAG) laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, a carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser. As described above in embodiments of the present invention, lasers have been identified as one type of high-intensity energy source, but this does not preclude other high-intensity energy sources outside of lasers that are within the spirit and scope of embodiments of the present invention. In addition, prior to irradiating the surface of the metallic substrate  304  with the high-intensity energy source, a surface-treatment layer may be deposited on the metallic substrate  304 . The deposition process for depositing the surface-treatment layer may be selected from a group including physical vapor deposition (PVD), chemical vapor deposition (CVD), sol-gel deposition, sputtering, sputtering in a reactive atmosphere, cladding, laser cladding, electroplating, and electroless plating. 
     In accordance with an embodiment of the present invention, a Q-switched Nd:YAG laser may be used having a peak intensity of about 2 MW during a Q-switched pulse duration of about 40 ns; otherwise, in non-Q-switched, continuous mode operation, the Nd:YAG laser may an average power of 50 W. The laser beam delivered at the sample is homogenized by passing it through a beam homogenizer including an optical fiber having a square cross-section and a stepped index of refraction along its length to produce a large square spot of uniform intensity at the metallic substrate with a dimension of about 1.5 mm by 1.5 mm. In one embodiment of the present invention, the spot may be rastered across the surface of the sample in a raster pattern with a speed of about 4 m/s using a laser galvanometer scanner to produce an overall rate of laser smoothing of about 100 cm 2 /s. 
     With further reference to  FIG. 3A , in accordance with the embodiment of the present invention, a portion  308  of the metallic substrate  304  corresponding to the pit  208  of  FIG. 2A  is shown after irradiating the surface of the metallic substrate  304  with a high-intensity energy source, such as a laser. The altered surface layer  304   b  of the metallic substrate  304  fills in the cavity portion  208   d  of the pit  208  leaving a gently undulating surface topography suitable for further fabrication of an electronic device, such as a solar cell. The other defects: the carbonaceous residue  212 , the protrusion  216 , the inclusion  220 , and the rolling groove  224 , have been removed from the surface of the metallic substrate  304  having either been ablated from the surface or incorporated into the altered surface layer  304   b  as alloying constituents, for example, the inclusion  220 . The roughness of the surface after irradiating the metallic substrate  304  with a laser is substantially less than the peak-to-valley roughness  240 , given by distance between the top of the protrusion  216  and the bottom of the rolling groove  224  shown in  FIG. 2A , before irradiating the metallic substrate  304  with a laser. 
     With reference now to  FIG. 3B , an expanded view of a portion of the cross-sectional elevation view of  FIG. 3A  is shown as indicated by lines of projection  346  and  348 . In accordance with an embodiment of the present invention, a cross-sectional elevation view of a layer structure of a solar cell is shown as it would appear after irradiating the surface of the metallic substrate  304  with a high-intensity energy source, such as a laser, and depositing layers to fabricate the solar cell with the layers disposed on the altered surface layer  304   b  of the metallic substrate  304 . The solar cell includes the metallic substrate  304  with the surface of the metallic substrate  304  smoothed by irradiating the surface with a high-intensity energy source, so that the surface is smoothed to remove defects from the surface by creating the altered surface layer  304   b  and the absorber layer  362  disposed on the altered surface layer  304   b  of the metallic substrate  304 . The absorber layer  362  of the solar cell may include CIGS. A conductive backing layer  358  may be disposed between the absorber layer  362  and the altered surface layer  304   b  of the metallic substrate  304 . On the surface of the absorber layer  362 , a TCO layer  366  is disposed. As shown in the expanded view of  FIG. 3B , the location corresponding to the cavity portion  208   d  of the pit  208  has a gently undulating surface topography. Therefore, the shunt defect associated with the defect, pit  208 , shown in  FIG. 2A , is absent, as well as other shunt defects, so that the number of shunt defects and density of shunt defects is reduced. In addition, the altered surface layer  304   b  has a thickness of less than about 5 μM sufficient to remove defects within 5 μm of the top of the original surface of the metallic substrate  304 ; alternatively, the altered surface layer  304   b  may have a thickness of less than about 25 μm depending on the power delivered to the surface of the metallic substrate  304  by the high-intensity energy source. Moreover, after smoothing the surface of the metallic substrate  304 , the altered surface layer  304   b  has a gently undulating topography. The smoothing may include a laser smoothing which may also include a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. In addition, the high-intensity energy source may include a laser selected from a group including a Q-switched laser, a Q-switched Nd:YAG laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser. 
     With reference now to  FIG. 4 , in accordance with an embodiment of the present invention, an elevation view of a roll-to-roll surface smoother  400  for smoothing the surface of substrate in roll form is shown. The substrate is provided to roll-to-roll surface smoother  400  in roll form from a roll of material  414 . The roll-to-roll surface smoother  400  includes an unwinding spool  410  upon which the roll of material  414  including the substrate in roll form is mounted. As shown, a portion of the roll of material  414  is unwound and passes over a series of idler rollers  426 , shown as five small circles in the center of  FIG. 4 , which provide a roller-platform upon which the unwound portion of the roll of material  414  may be transported. The unwound portion of the roll of material  414  passes to the right and is taken up on a take-up spool  418  upon which it is rewound as a smoothed roll of material  422  after the substrate has been smoothed. The arrows adjacent to the idler rollers  426 , the unwinding spool  410 , and the take-up spool  418  indicate that these are rotating components of the roll-to-roll surface smoother  400 ; the idler rollers  426 , the unwinding spool  410 , and the take-up spool  418  are shown rotating in clockwise direction, as indicated by the arrow-heads on the respective arrows adjacent to these components, to transport the unwound portion of the roll of material  414  from the unwinding spool  410  on the left to the take-up spool  418  on the right. 
     With further reference to  FIG. 4 , in accordance with an embodiment of the present invention, the roll of material  414  provides the substrate as a sheet having a width (not shown), as great as about 1 m, and a thickness  450 , as great as about 125 μm. As provided the untreated surface  454  of the roll of material  414  passes under a surface treatment station on the way to the take-up spool  418 . The surface treatment station includes a high-intensity energy source  430  from which a high-intensity energy beam  434  emanates to irradiate the untreated surface  454  of the roll of material  414  to smooth the untreated surface  454 , such as shown in  FIGS. 2A and 2B , producing a smoothed surface  458 , such as shown in  FIGS. 3A and 3B , on the substrate; in this way, the surface is smoothed to remove defects from the surface by creating the altered surface layer  304   b . The high-intensity energy beam  434  may have a range 438 over which the high-intensity energy beam  434  irradiates the surface of the unwound portion of the roll of material  414 . The range 438 may be provided by homogenizing the beam to produce a wide spot with a beam homogenizer, or by rastering a focused spot back and forth along the direction of transport as indicated by the double-headed arrow corresponding to the range 438. As the substrate also has a width, the high-intensity energy beam  434  may be rastered in the width direction, perpendicular to the direction of transport (not shown), to smooth the full surface of the substrate. As shown in  FIG. 4 , the untreated surface  454  is the outer surface of the roll of material  414 . Alternatively, by disposing a treatment station on the opposite, or bottom, side of the unwound portion from that shown, the inner surface of the roll of material  414  may be smoothed (not shown). 
     With further reference to  FIG. 4 , in accordance with an embodiment of the present invention, after the surface has been smoothed, the altered surface layer is configured to receive at least one layer in a fabrication process of an electronic device, for example, as described above in  FIG. 3B . In accordance with an embodiment of the present invention, the substrate may be selected from a group including a metallic substrate and a metallized substrate, for example, a metallized non-metallic substrate including a flexible, non-conductive substrate, such as a polymer substrate, with a sputtered metallic layer. In addition, the electronic device may include a solar cell having absorber layer  362  made of, for example, CIGS material. In accordance with an embodiment of the present invention, the high-intensity energy source may include a laser selected from a group consisting of a Q-switched laser, a Q-switched Nd:YAG laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, a carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser. Moreover, smoothing may include a laser smoothing including a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. 
     With further reference to  FIG. 4  in conjunction with  FIG. 3B , in accordance with embodiments of the present invention, the roll-to-roll surface smoother  400  may be used in fabricating a solar cell. The solar cell may include a substrate  304 , a surface of the substrate  304  smoothed by irradiating the surface with a high-intensity energy source  430 , wherein the surface is smoothed to remove defects from the surface by creating an altered surface layer  304   b ; and an absorber layer  362  disposed on the altered surface layer  304   b . The absorber layer  362  of the solar cell may further include copper indium gallium diselenide (CIGS). In further embodiments of the present invention, the substrate  304  of the solar cell may be selected from a group consisting of a metallic substrate and a metallized substrate. Moreover, the substrate  304  may have a width of about 1 m and a thickness of less than about 125 μm. In an embodiment of the present invention, the altered surface layer  304   b  of the solar cell has a thickness of less than about 25 μm. 
     Description of Embodiments of the Present Invention for a Method of Smoothing a Metallic Substrate for a Solar Cell 
     With reference now to  FIG. 5 , a flow chart illustrates an embodiment of the present invention for a method  500  for smoothing the surface of a metallic substrate. At  510 , a metallic substrate is provided. At  520 , a surface of the metallic substrate is smoothed by irradiating the surface with a high-intensity energy source, such that the surface is smoothed to remove defects from the surface by creating an altered surface layer, in which the altered surface layer is configured to receive at least one layer in a fabrication process of an electronic device. In one embodiment, the altered surface layer produced by the method has a thickness of less than about 5 μm; alternatively, the altered surface layer may have a thickness of less than about 25 μm depending on the power delivered to the surface by the high-intensity energy source. Also, an electronic device fabricated with the method may include a solar cell. In addition, at least one layer of an electronic device fabricated with the method may include CIGS. 
     With further reference to  FIG. 5 , it should be recognized that rough substrate surfaces can result in diode shunt sites that result in loss of output power from the solar cell, for example, as described above in  FIGS. 2A and 2B . Laser smoothing by surface melting locally smoothes the surface by melting and reflowing the surface features without fully penetrating the substrate with the laser melt zone. Therefore, the smoothing includes a laser smoothing. The use of laser smoothing facilitates the fabrication of the solar cell by allowing the subsequent deposition of continuous and un-interrupted thin-film layers of solar-cell materials, for example, the absorber layer, on a smoothed metallic substrate. In an example laser-smoothing process, the laser preferentially heats regions of the surface having lesser heat capacity than the base portion of the metallic substrate, for example, regions with the topography of a protrusion or pit. In addition, such features can be removed by laser smoothing based on laser ablation. Therefore, the laser smoothing may also include a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. 
     In accordance with an embodiment of the present invention, the latter process, laser-induced, surface-alloying, can be accomplished by a variety of methods, including without limitation: applying a material to the surface of the metallic substrate before or during the laser-smoothing process to form a thin-film barrier layer, for example, chromium, Cr, which blocks the out-diffusion of impurities, e.g. iron, Fe, or nickel, Ni, from the metallic substrate that may have a deleterious effect on solar-cell performance; or, exposing the surface to reactive gases such as nitrogen or oxygen during the laser-smoothing process to form a nitrided, or oxidized, thin-film layer, for example, a thin-film, surface nitride or oxide layer. In the alternative to exposing the surface to a reactive gas, the surface may be shrouded in an envelope of inert gas, for example, argon, Ar, during the laser-smoothing process to maintain surface cleanliness. Moreover, the application of material to the surface of the metallic substrate, before or during the laser-smoothing process, may also include depositing a surface-treatment layer on the metallic substrate. Thus, in accordance with an embodiment of the present invention, depositing a surface-treatment layer may also include a deposition process selected from a group including physical vapor deposition (PVD), chemical vapor deposition (CVD), sol-gel deposition, sputtering, sputtering in a reactive atmosphere, cladding, laser cladding, electroplating, and electroless plating. Also, in the case of laser cladding, the cladding material may be provided from a variety of sources, including without limitation: powder, wire, liquid, as well as others within the scope and spirit of embodiments of the present invention. 
     With further reference to  FIG. 5 , in accordance with an embodiment of the present invention, various laser scanning techniques can be employed to deliver energy from the laser to the surface of the metallic substrate. For example, in an embodiment of the present invention, a laser galvanometer scanner may be used to scan a laser beam across the surface of the metallic substrate; or alternatively, a linear laser source may be used to irradiate a line, rather than a spot, on the surface of the metallic substrate. Moreover, the high-intensity energy source may also include a laser selected from a group including a Q-switched laser, a Q-switched Nd:YAG laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, a carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser, as embodiments within the spirit and scope of embodiments of the present invention. 
     With reference now to  FIG. 6 , a flow chart illustrates an embodiment of the present invention for a method  600  for fabricating a solar cell. At  610 , a metallic substrate is provided. At  620 , a surface of the metallic substrate is smoothed by irradiating the surface with a high-intensity energy source, wherein the surface is smoothed to remove defects from the surface by creating an altered surface layer, and wherein the altered surface layer is configured to receive at least one layer in a fabrication process of a solar cell. At  630 , an absorber layer is deposited on the metallic substrate. In one embodiment, the altered surface layer produced by the method has a thickness of less than about 5 μm; alternatively, the altered surface layer may have a thickness of less than about 25 μm depending on the power delivered to the surface by the high-intensity energy source. In an embodiment of the present invention, the absorber layer fabricated with the method includes CIGS. 
     With further reference to  FIG. 6 , in the embodiment of the present invention for the method  600 , the smoothing further includes a laser smoothing. The laser smoothing further includes a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. In addition, the high-intensity energy source of the method may also include a laser selected from a group including a Q-switched laser, a Q-switched Nd:YAG laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, a carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser, as embodiments within the spirit and scope of embodiments of the present invention. 
     With reference now to  FIG. 7 , a flow chart illustrates an embodiment of the present invention for a method  700  for roll-to-roll smoothing the surface of a roll of material. At  710 , a substrate in roll form from a roll of material is provided. At  720 , a surface of the roll of material is smoothed by irradiating the surface with a high-intensity energy source, such that the surface is smoothed to remove defects from the surface by creating an altered surface layer, in which the altered surface layer is configured to receive at least one layer in a fabrication process of an electronic device. In an embodiment of the present invention, the substrate is selected from a group including a metallic substrate and a metallized substrate, for example, a metallized non-metallic substrate including a flexible, non-conductive substrate, such as a polymer substrate, with a sputtered metallic layer. In one embodiment, the altered surface layer produced by the method has a thickness of less than about 5 μm; alternatively, the altered surface layer may have a thickness of less than about 25 μm depending on the power delivered to the surface by the high-intensity energy source. Also, an electronic device fabricated with the method may include a solar cell. In addition, at least one layer of an electronic device fabricated with the method may include CIGS. 
     With further reference to  FIG. 7 , it should be recognized that rough substrate surfaces can result in diode shunt sites that result in loss of output power from the solar cell, for example, as described above in  FIGS. 2A and 2B . Laser smoothing by surface melting locally smoothes the surface by melting and reflowing the surface features without fully penetrating the substrate with the laser melt zone. Therefore, the smoothing includes a laser smoothing. The use of laser smoothing facilitates the fabrication of the solar cell by allowing the subsequent deposition of continuous and un-interrupted thin-film layers of solar-cell materials, for example, the absorber layer, on a smoothed substrate. In an example laser-smoothing process, the laser preferentially heats regions of the surface having lesser heat capacity than the base portion of the substrate, for example, regions with the topography of a protrusion or pit. In addition, such features can be removed by laser smoothing based on laser ablation. Therefore, the laser smoothing may also include a process selected from a group including a laser ablation process, a laser melting-resolidification process, and a laser-induced, surface-alloying process. 
     In accordance with an embodiment of the present invention, the latter process, laser-induced, surface-alloying, can be accomplished by a variety of methods, including without limitation: applying a material to the surface of the substrate before or during the laser-smoothing process to form a thin-film barrier layer, for example, chromium, Cr, which blocks the out-diffusion of impurities, e.g. iron, Fe, or nickel, Ni, from the substrate that may have a deleterious effect on solar-cell performance; or, exposing the surface to reactive gases such as nitrogen or oxygen during the laser-smoothing process to form a nitrided, or oxidized, thin-film layer, for example, a thin-film, surface nitride or oxide layer. In the alternative to exposing the surface to a reactive gas, the surface may be shrouded in an envelope of inert gas, for example, argon, Ar, during the laser-smoothing process to maintain surface cleanliness. Moreover, the application of material to the surface of the substrate before or during the laser-smoothing process may also include depositing a surface-treatment layer on the substrate. Thus, in accordance with an embodiment of the present invention, depositing a surface-treatment layer may also include a deposition process selected from a group including physical vapor deposition (PVD), chemical vapor deposition (CVD), sol-gel deposition, sputtering, sputtering in a reactive atmosphere, cladding, laser cladding, electroplating, and electroless plating. Also, in the case of laser cladding, the cladding material may be provided from a variety of sources, including without limitation: powder, wire, liquid, as well as others within the scope and spirit of embodiments of the present invention. 
     With further reference to  FIG. 7 , in accordance with an embodiment of the present invention, various laser scanning techniques can be employed to deliver energy from the laser to the surface of the substrate. For example, in an embodiment of the present invention, a laser galvanometer scanner may be used to scan a laser beam across the surface of the substrate; or alternatively, a linear laser source may be used to irradiate a line, rather than a spot, on the surface of the substrate. Moreover, the high-intensity energy source may also include a laser selected from a group including a Q-switched laser, a Q-switched Nd:YAG laser, a Q-switched fiber laser, a Q-switched disc laser, a Q-switched slab laser, a carbon-dioxide laser, a pulsed laser, a continuous-wave laser, and a diode laser, as embodiments within the spirit and scope of embodiments of the present invention. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments described herein were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.