Patent Application
20160087577
The present disclosure relates to solar cells and, more specifically, to flexible solar cells comprising thick and thin absorber regions.
Solar cells, which are able to use energy from light to generate electricity, are increasingly an important source for renewable power. Solar cells are frequently formed from a semiconductor material such as silicon. Light is absorbed by the silicon which causes excitation of the electrons of the silicon. Some of these excited electrons then travel though the solar cell until it reaches an electrode, thereby causing current to flow.
Solar cells are rigid and this rigidity limits the ability of the solar cells to be applied to surfaces that are to be exposed to sunlight. The semiconductor region of solar cells also tend to be relatively thick and this thickness increases cost of manufacture, which can have a large impact on the practicality of using solar cells as an alternative to conventional power sources.
A solar cell includes a p-type semiconductor substrate including a plurality of thin absorption regions and a plurality of thick absorption regions. The plurality of thin absorption regions and the plurality of thick absorption regions are coplanar on a bottom side thereof. An n-type semiconductor layer is disposed over a top side of the p-type semiconductor substrate. The n-type semiconductor layer has a substantially uniform thickness. Metallurgy is disposed on top of the n-type semiconductor layer. The plurality of thin absorption regions are sufficiently thin to render the semiconductor substrate flexible.
The thick absorption regions may each be 20-70 microns thick and the thin absorption regions may each be 0-20 microns thick.
The thick absorption regions may each be at least five times as thick as each of the thin absorption regions.
A method for fabricating a solar cell includes disposing an adhesion or seed layer on a bottom surface of a semiconductor absorption substrate. A stressor layer is applied onto the adhesion layer or seed layer. The stressor layer has an intrinsic tensile stress and a pattern of thick and thin regions. The semiconductor absorption substrate is spalled such that a pattern of thick and thin absorption regions corresponding to the pattern of thick and thin stressor regions are formed therefrom. The stressor layer is removed from the bottom surface of the spalled semiconductor absorption substrate. A backing layer is applied to the bottom surface of the spalled semiconductor absorption substrate. A semiconductor emitter layer is disposed over a top surface of the spalled semiconductor absorption substrate. The semiconductor emitter layer has a substantially uniform thickness. Metallurgy is applied over the semiconductor layer.
The semiconductor absorption substrate may include a p-type semiconductor and the semiconductor emitter layer may include an n-type semiconductor.
The metallurgy may be a plurality of metallic fingers or bus contacts.
The thick absorption regions may each be 20-70 microns thick and the thin absorption regions may each be 0-20 microns thick.
The thick absorption regions may each be at least five times as thick as each of the thin absorption regions.
The plurality of thin absorption regions may be sufficiently thin to render the semiconductor substrate flexible.
A passivation layer may be disposed over the semiconductor emitter layer.
An antireflective coating may be disposed over the semiconductor emitter layer.
The step of applying the backing layer to the bottom surface of the spalled semiconductor absorption substrate may include disposing an aluminum layer to the bottom surface of the spalled semiconductor absorption substrate.
The aluminum layer may be disposed in blanket contact with the bottom surface of the spalled semiconductor absorption substrate.
The aluminum layer may be in contact with the bottom surface of the spalled semiconductor absorption substrate between a pattern of dielectric reflectors.
A back-surface-field layer may be disposed between the bottom surface of the spalled semiconductor absorption substrate and the aluminum layer.
An insulating tape carrier may be disposed under the aluminum layer.
Applying a stressor layer onto the adhesion layer or seed layer may includes, sputtering a first nickel layer on the adhesion layer or seed layer, plating a second nickel layer on the sputtered nickel layer or seed layer, disposing a resist pattern on the second nickel layer or seed layer, plating a third nickel layer on the second nickel layer through the resist pattern, and removing the resist pattern.
The stressor layer may have a tensile stress of greater than 100 megapascals.
A method for forming a mixed thickness substrate includes disposing an adhesion layer or a seed layer on a bottom surface of a semiconductor substrate. A first nickel layer is sputtered on the adhesion layer or seed layer. A second nickel layer is plated on the sputtered nickel layer or seed layer. A resist pattern is disposed on the second nickel layer or seed layer. A third nickel layer is plated on the second nickel layer through the resist pattern. The resist pattern is removed. The semiconductor substrate is spalled such that a pattern of thick and thin regions corresponding to the arrangement of the third nickel layer disposed through the resist pattern are formed therefrom.
The plurality of thin absorption regions may be sufficiently thin to render the semiconductor substrate flexible.
A more complete appreciation of the present disclosure and many of the attendant aspects thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In describing exemplary embodiments of the present disclosure illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner.
Exemplary embodiments of the present invention provide solar cells that are flexible and methods for fabricating flexible solar cells. The solar cells so described may also utilize less material and may be less expensive to manufacture. Exemplary embodiments of the present invention may utilize a spalling technique to provide a relatively thin semiconductor layer in the solar cell. The semiconductor layer may be thin enough to render the resulting solar cell substantially flexible.
Spalling is a technique used to thin a substrate in which a metallic adhesion layer may be deposited on a substrate. A spall-inducing stressor layer may then be applied to the adhesion layer. The stressor layer may have a relatively high tensile stress. The thickness and measure of tensile stress of the stressor layer may be selected to produce the desired to induce the desired spalling phenomenon. The tensile stress of the stress layer may make the substrate prone to separate at a particular depth that may be controlled by the thickness and tensile stress of the stress layer that was applied. Then, spalling may be induced by beginning to sheer the substrate along the desired depth, for example, by starting a peeling at a corner of the wafer, or by quickly changing the temperature of the substrate, for example, by submerging the substrate in liquid nitrogen. The substrate will then split cleanly at the depth that has been made prone to splitting at by the presence of the stress layer.
Exemplary embodiments of the present invention may use spalling to thin the silicon layer of the solar cell to the point at which it is flexible. However, as silicon has a relatively poor absorption of light in the near infra red (IR) range, thinning the silicon layer may produce solar cells that are less efficient at generating electricity from light than solar cells using relatively thick silicon layers. Moreover, using spalling to create a thin silicon layer may result in cracking, especially where the area being spalled is relative large. It may not be practical to tile together multiple small regions of spalled silicon due to the high cost of managing the multiple independent pieces.
Exemplary embodiments of the present invention utilize a modified spalling technique to produce a solar cell having an absorber layer, for example, a semiconductor layer, that is made up of a pattern of thick and thin absorption regions. The thick absorption regions may provide adequate power generating efficiency while the thin regions absorption regions may provide flexibility. The semiconductor layer, so spalled, may still remain a single contiguous layer thereby simplifying subsequent processing and handling thereof.
As described above, spalling is a technique used to cleanly break a substrate along a desired plane. Exemplary embodiments of the present invention provide a modified approach to spalling that does not break the substrate along a single plane, but rather breaks the substrate in such a way as to produce a pattern of thick and thin regions. Techniques for performing this modified spalling and the resulting structures are described in detail below.
As discussed above, solar cells, in accordance with exemplary embodiments of the present invention, may include a plurality of thick absorber regions and a plurality of thin absorber regions. The thick regions may be approximately 20-70 microns thick. However, the upper limit need not be material and the thick regions may be greater than or equal to 20 microns thick. For example, the thick regions may be at least 30 microns thick, at least 40 microns thick, or at least 50 microns thick.
In contrast, the thin absorber regions may be approximately 1-20 microns thick. However, the lower limit need not be material and the thin regions may be less than or equal to 20 microns thick. For example, the thin regions may be no greater than 20 microns thick, no greater than 10 microns thick, or no greater than 5 microns thick. According to some exemplary embodiments of the present invention, the thin regions may be substantially zero microns thick, for example, there might not be any absorber at all in the thin absorber regions. However, even in these cases, the solar cell may remain a single contiguous unit held together by other structural layers and would not be divided into a set of separate structures.
While the above-described thickness ranges for the thick and thin absorption regions may at least partially overlap, for example, at 20 microns; exemplary embodiments of the present invention may have thick regions that are at least twice as thick as the thin regions. For example, the thick regions may be at least 5 times as thick as the thin regions, may be at least 15 times as thick as the thin regions, may be at least 30 times as thick as the thin regions, or may be at least 75 times as thick as the thin regions. According to some exemplary embodiments of the present invention, the thick regions may be between 10 and 100 times as thick as the thin regions.
Moreover, the absorber may include multiple thick regions and/or multiple thin regions, each of which has a substantially different thickness. For example, a first thin region may be a fixed thickness no greater than 10 microns thick while a second thin region may be zero microns thick.
Exemplary embodiments of the present invention may provide a range of different geometric configurations for thick and thin absorption regions, several examples of which are discussed in detail below. Solar cells utilizing these arrangements may be referred to herein as mixed thickness solar cells.
Mixed thickness solar cells may be made from mixed thickness solar cell substrates with any of the features known to be used for single-thickness, thick solar cells. A solar cell back surface may include a blanket conductive layer contact. A solar cell front surface may include an emitter (or double/selective emitter), passivation and/or antireflection coatings, and metallic finger and bus contacts.
In particular,
While only certain solar cell layers have been shown for the purposes of simplifying the disclosure, it is to be understood that many other designs and material layers may be utilized on these flexible mixed-thickness solar cell substrates, such as Sanyo-type HIT (heterojunction with intrinsic thin layer) cells and materials, interdigitated back contact cells, and emitter wrap through cells (EWT).
The optimum arrangement of the different thickness regions with respect to the finger/bus wiring and any other structures in the cell may depend on the requirements and priorities of the solar cell being manufactured.
As described above, mixed thickness solar cells in accordance with exemplary embodiments of the present invention may utilize a modified spalling technique in which a semiconductor substrate is converted into a pattern of thick and thin absorption regions. This may be accomplished, for example, by depositing a thin adhesion layer on the semiconductor substrate. The thin adhesion layer may be metallic, but need not have tensile stress. According to one example, the thin adhesion layer may be a titanium layer having a thickness of between 10 and 50 nanometers, for example, 40 nanometers. The stressor layer may be deposited on the adhesion layer. The stressor layer may also be metallic. The stressor layer may have a desired degree of tensile stress that may be relatively large. According to one exemplary embodiment of the present invention, the stressor layer may include a combination of sputtered and plated nickel with a combined thickness of up to 40 microns and a tensile stress of between 100 and 1000 megapascals (MPa), for example, between 200 and 500 MPa, for example between 300 and 400 MPa. A stressor layer so described may be used in conjunction with a silicon absorption layer to spall a layer of silicon that is about twice the thickness of the nickel stressor layer. A backing layer, which may include tapes, may be applied to the stressor layer either before or after semiconductor spalling to help preserve the mechanical integrity of the spalled layer and/or to serve as handles for the structure. As described above, spalling may be induced by starting a peel at the wafer edge or immersing in liquid nitrogen. At some stage after spalling, some or all of the high-stress stressor layer is removed by a process such as wet etching.
As stated above, the mixed thickness substrates for flexible solar cells may include well-defined pluralities of thick and thin absorber region features. More generally, the mixed thickness substrates for flexible solar cells may include well-defined pluralities of absorber region features with each plurality containing features with a characteristic thickness. The optimum values for the number of pluralities, the relative area of each plurality, and the thickness, surface texture, size, and shape of the absorber region features comprising each plurality may depend on the specific application.
Then, as can be seen in
Then, as can be seen in
Together, the sputtered nickel layer 42, the plated nickel layer 43 and the plated nickel rectangles 45 may form the stressor layer that has a tensile stress as described above.
As can be seen in
Then spalling may be performed. The spalling may serve to fracture the semiconductor substrate 40 along the spalling surface 47, as is depicted in
As described above, in accordance with exemplary embodiments of the present invention, spalling may be used to create mixed-thickness substrate layers that include zero-thickness semiconductor regions. This can be done with the use of a two-thickness stressor layer similar to the one described above, modified so that the blanket portion of the stressor layer has a stress and thickness below the threshold for spalling. Spalling with such a stressor layers may be done using a tape backing. However, some zero-thickness regions (particularly via-shaped ones used for contacts) may be formed by starting with mixed thickness substrates having thin and thick regions and then removing the thin-thickness regions in selected areas by processes such as etching, laser scribing, etc. Both spalling and etching methods for creating zero-thickness regions may be used to create electrically isolated thick-thickness regions when the conductive layers connecting the thick-thickness regions are be removed down to the insulating (tape) backing layer, for applications in which the isolated thick-thickness cells may be connected in series (after cell fabrication) to achieve higher voltage devices.
When these spalling processes are used to form mixed thickness solar cells, at least some portion of the stressor layer may be left on the spalled semiconductor layer. In such cases, the back portion of the cell may be fabricated before the stressor layer is applied. Examples of various back cell structures that might be disposed at the solar cell back under a generic patterned stressor layer are shown and described above with reference to
Many possible materials and processes may be used to form the components of the stressor layer stack and the choice of how to arrange the stressor layer stack may be guided by reliability and cost considerations. The order in which the one or more blanket stressor layers and the one or more patterned stressor layers are formed may also be varied, and the materials of these one or more blanket and patterned stressor layers may be the same or different. While the examples described herein primarily describe how metals might be incorporated into a two-thickness stress layer stack, the stress layer stack may include materials other than metals. For example, various blanket or patterned layers may be inserted into the stress layer stack for reduced contact resistance, passivation layer modification, improved oxidation resistance, adhesion promotion, diffusion barrier function, or etch stop function. Methods of deposition include plating, physical vapor deposition (PVD) techniques such sputtering, evaporation, reactive sputtering; chemical vapor deposition (CVD), atomic layer deposition (ALD), chemical solution deposition (CSD) techniques (spray, spin-on, lamination, etc.). Methods of stressor patterning include through-mask plating, through-shadow-mask deposition, lift-off, etching through a mask, and various imprinting and/or stamping techniques. Plating, typically with a sputtered seed layer, is a preferred technique due to its low cost and compatibility with efficient-material-use through-mask plating processes. Sputtering may be higher cost but it is compatible with patterned deposition with the use of shadow masks.
According to some exemplary embodiments of the present invention, aluminum may be used instead of titanium as the back surface metallic reflector and contact. To create a blanket aluminum that is thin enough not to interfere with spalling, an aluminum adhesion-etch stop layer and a nickel stack may be used as a seed layer for plating, as shown in
An example of this process is shown in
Dual function stressor/seed layers may be provided when the desired thickness of the blanket nickel stressor is thin enough to be conveniently deposited by sputtering. This stands in contrast to the cases described above where the blanket nickel stressor is a separate layer plated onto a nickel seed layer.
In particular,
Exemplary embodiments described herein are illustrative, and many variations can be introduced without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different exemplary embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.
