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    • 专利摘要:
    Semiconductor III-V multi-junction solar cell containing graphene and obtaining method. The invention relates to a multi-junction solar cell comprising: a semiconductor structure incorporating one or more pn photovoltaically active junctions made of semiconductor III-V, metal front and rear contacts, and one or more layers of graphene deposited between the semiconductor structure and the front contacts. For many applications of this invention, it would also be necessary to deposit antireflective layers on the graphene. Also, the present invention relates to the method of obtaining the multi-junction solar cell, characterized in that it comprises depositing at least one layer of graphene by transfer on the front semiconductor surface of the solar cell, prior to depositing the front metal contacts. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2665809A1
    申请号:ES201731223
    申请日:2017-10-17
    公开日:2018-04-27
    发明作者:Carlos Algora Del Valle；Laura BARRUTIA PONCELA；Ignacio Rey-Stolle Prado；Mario OCHOA GOMEZ
    申请人:Universidad Politecnica de Madrid；
    IPC主号:
    专利说明:

    Object and technical sector of the invention The invention is directed to a new multi-junction solar cell structure that incorporates graphene to improve its efficiency, reducing the lateral resistance to the passage of the photogenerated current, as well as to a method of obtaining them. This invention is part of the photovoltaic sector, specifically in the field of manufacturing
    10 very high efficiency multi-junction solar cells such as concentration cells, solar cells for space, etc. It is also included in the field of manufacturing, transfer and graphene applications.
    Background of the invention
    15 In recent years, efforts in the photovoltaic sector and, specifically, high-efficiency photovoltaic solar energy, have focused on the search for new materials and configurations for multi-junction solar cells commonly used in electricity generation, with the in order to increase its efficiency and reduce its cost. One of the fundamental limitations of these multi-union cells is the harmful effect that
    20 exerts the series resistance to the photogenerated lateral current flowing to the front contacts. The losses associated with the series resistance depend on the square of the level of light intensity (also called concentration), which makes the efficiency, which increases at low and medium concentrations, begin to decrease for high concentration levels. This harmful effect of the series resistance is also
    25 manifests with the increase in the size of the solar cell, as is the case in space applications that require III-V semiconductor multi-junction solar cells with sizes of tens of square centimeters.
    An alternative to reduce lateral series resistance could be to use graphene
    30 in the frontal surface, so that in the case that the deposited graphene had a high electrical conductivity, it could contribute to the lateral extraction of current towards the contacts. The simplest and most direct way to integrate graphene into concentration solar cells would be to deposit graphene on the front surface of a previously manufactured solar cell in a conventional manner (that is, including the
    35 front metal mesh that makes contact). This procedure has been used by Jieun Chang, Myoung-Gyun Suh, Jinseong Heo, Dongho Kim, Joosung Kim, Sang-Moon Lee, Hyun-Jong Chung, and Yungi Kim in "Transparent Graphene Electrodes for Highly Efficient III-V Multijunction Concentrator Solar Cells "; Energy Technol 2013, 1, 283 286, where an increase in efficiency is described thanks to the decrease in series resistance. In this regard, the authors of the present invention have proven that if the same way of integrating graphene into the solar cell as described in the cited reference is followed, an improvement of the series resistance is experienced but at the cost of a decrease in the current (due to the optical absorption of graphene). The end result is a very slight increase in efficiency that would not compensate for the increase in complication and manufacturing cost of the solar cell by the inclusion of graphene.
    In addition, the authors of the present invention have found that when the integration of graphene into the concentration solar cell simply consists of the deposition of graphene on the front surface of the solar cell manufactured in a conventional manner (i.e., on the front contacts) , as presented in the Jieun Chang reference, the following problems appear:
    one. After depositing graphene on the solar cell, it is not possible to apply a heat treatment (with the necessary time and temperature parameters) to improve its adhesion and its optical and electrical properties. The optimum range of this heat treatment is around 300-400 ° C for several hours, temperature and time values that would irretrievably damage the properties of the metal contacts of the solar cell on which graphene is deposited, which typically undergo degradation for temperatures above 300 ° C.
    2. The graphene layer deposited on the front surface of the solar cell reproduces its orography, which in this case presents several microns from peak to valley due, fundamentally, to the thickness of the frontal metal mesh. As a consequence, the graphene layer is not in contact with the entire front surface of the cell or the contacts. This makes the efficiency of current extraction from the semiconductor structure towards graphene adversely affected.
    3. The layer of graphene deposited without more on the front surface of the cell makes a very weak contact with the frontal mesh of metallization, which hinders the passage of current between the metal and graphene.
    Four. For the above reasons, the transfer of graphene over the cell would be a little repeatable process if one wanted to manufacture industrially, so its reliability would be affected.
    5. Because Jieun Chang et al. they use graphene grown in their experiment on
    5 a substrate of Si / Si02 / Ni by the inductively coupled chemical vapor deposition method (Inductively coupled plasmaenhanced chemical vapor deposition -ICP-CVD-), the number of graphene layers to be incorporated or their optical or electrical properties, so its impact on the efficiency of the solar cell is questionable and unpredictable.
    In order to solve the above problems detected in the field of the art, which would also result in an improvement in the efficiency of the multi-junction solar cell, an improved graphene structure is proposed in the present invention, as well as an alternative method of integration thereof in said structure, in which the transfer of graphene on the solar cell is carried out prior to the metallization of the frontal metal contacts (unlike the method described by Jieun Chang et al.). In essence, in the present invention graphene is transferred to (integrated in) the front (upper) part of the solar cell directly on the semiconductor structure, which allows a desired number of graphene layers 20 of high quality to be transferred in a controlled manner. Optoelectronics before placing the front metal contacts. The invention thus proposes a new configuration comprising the structure of semiconductor layers that constitute the solar cell, graphene transferred on the semiconductor surface and finally the placement of both front and rear contact as well as anti-reflective layers (if necessary), as step 25 after the transfer of graphene to the cell. In this way, graphene is deposited on a practically flat (and more homogeneous) surface in the entire semiconductor area of the solar cell, which allows: a) a contact with better morphology between the semiconductor and graphene, b) a improvement of the electrical contact between the semiconductor and graphene, c) that the graphene is well adhered to the semiconductor structure 30 preventing the possible "dettachment" or lifting of graphene, since the placement of the frontal metallization is carried out on the graphene surface . This form of integration is compatible with planar technology, since graphene is embedded between the semiconductor and the front mesh, which results in high reliability. Another advantage of the invention is that it allows thermal treatments to be carried out at higher temperatures (-300-400 oC) for cleaning the surface of the
    graphene and eliminate possible organic remains, since such temperature would not be limited
    Due to the presence of frontal metallization, which, as stated above, typically should not exceed 200 oC -300 oC and whose heat treatment times are a few minutes.
    General description of the invention The present invention relates to a multi-junction semiconductor solar cell III-V comprising:
    a semiconductor structure that incorporates one or more pn junctions
    10 photovoltaically active constituting the body of the solar cell, front and rear metal contacts, the front contacts being constituted by a metal mesh,
    characterized in that it comprises one or more layers of graphene deposited between the upper part of the semiconductor structure and the front metal contacts (see Figure 15 1a and 1b).
    The upper layer of the semiconductor structure, that is, its upper surface where the graphene layer or layers are deposited, may be constituted in a particular embodiment by a window layer, or alternatively in another embodiment by a layer
    20 window in some areas of the surface and a contact layer in other areas.
    By pn junction, the junction formed by a semiconductor type p and another n must be understood, regardless of what type of semiconductor is on top of the other.
    In essence, graphene, previously grown in layers, is transferred to the semiconductor structure of interest (in this case, the upper layer of the solar cell without the metal contacts), before depositing said front metal contacts, so that graphene layers are embedded between them (semiconductor structure and metal contacts).
    Although there are various methods to grow graphene, the chemical deposit from the vapor phase (in English Chemical Vapor Deposition, whose acronym, eVD, is used from now on for simplicity) is an example of a preferred method, because it has significant advantages over the others for the objective pursued: it produces both optical and electronic properties of high quality graphene, allows graphene to grow to a great extent
    scale and also allows to control and deposit the number of layers of graphene desired. These advantages make graphene manufactured by evo optimal and preferred for the configuration of the claimed solar cells, although any graphene layer growth method is valid for the scope of the present invention.
    The authors of the present invention have proven that, due to the excellent optical and electronic properties that graphene presents, as well as the way of integration into the solar cell described here (between the semiconductor surface and the front contacts), its presence in the front part of said structure as a transparent electrode, efficiently contributes to the extraction of the photocurrent towards the front metal contacts, effectively reducing the lateral series resistance, which is responsible for these devices to decrease their efficiency at high levels of concentration (see Figure 2), when solar cells have sizes of several square centimeters even if they do not work in concentration.
    The structure described for the concentration solar cell that integrates graphene, which produces a significant improvement in its efficiency, is the result of an exhaustive analysis by the authors of multiple factors, which have not been contemplated in similar experiments such as the one carried out by Jieun Chang et al., Mainly: a) control of the number of graphene layers deposited on the semiconductor surface and their effect, b) the conditions of graphene heat treatment (temperature, time) once deposited in the solar cell, c) possible graphene attack to electrically isolate the solar cells of a wafer, and d) doped from the solar cell window layer and the work function of graphene once deposited on it, among the most important. The results of these analyzes indicate that there are ways to improve the efficiency of the solar cell but that clash with the way of integrating the graphene used in the prior art, and that they are resolved with this invention.
    It should be considered that, unless otherwise specified in the text, the limits of any interval described herein are included within the scope of the present invention.
    The invention is also directed to a method of obtaining a multi-junction solar cell of semiconductor III-V containing graphene, characterized in that it comprises:
    deposit at least one layer of graphene on the surface of the structure
    semiconductor;
    prior to the deposit of the front metal contacts.
    5 It should be noted that in the upper part of the semiconductor structure where the graphene layer is deposited is the contact layer in some areas and the window layer in other areas (Figure 1 a), or alternatively as a second alternative, Find the window layer covering the entire front surface as the last semiconductor layer (Figure 1b).
    Detailed description of the invention
    Semiconductor multi-junction solar cell / l / -V
    The structure of the multi-junction solar cell ultimately consists of multiple layers of III-V semiconductors superimposed on one another, constituting photovoltaically active pn junctions 15, for example, in the usual concentration solar cells in the field of technology, so that any known variant is applicable in this invention, since this aspect of the device does not interfere with the advantageous effect of graphene between said support and the metal contacts. The multi-junction solar cell in which graphene is incorporated can be generically described as a set of sub cells connected to each other and formed by various layers of semiconductors. The different semiconductor layers that constitute the multi-junction solar cell are formed, following the known patterns for this type of cells, by different semiconductors III-V such as GaAs, GalnP, AllnP, GalnAs, AIGalnP, GalnNAs or the like. This set is what is called here
    Semiconductor structure invention, in which various metal processing steps, cell size, etc. are defined by various technological processing steps. Semiconductor layers are normally grown epitaxially on a substrate, for example, of Germanium (Ge), Gallium Arsenide (GaAs) or silicon (Si).
    The upper layers of said semiconductor structure or support, on which graphene is deposited and which will be in direct contact with it, can be the first option of the contact layer in some areas and the window layer in other areas of the surface (Figure 1a) or, as an alternative to this first option, the window layer may be the last semiconductor layer present in the entire front surface (Figure 1b).
    35 In the first option, the contact layer has been removed from areas that will not be
    metal coated The contact layer has a very high doped (> 1018_10 19 cm-J) that guarantees a good ohmic contact with the metals to deposit on top. The graphene regions that are in contact with the contact layer (usually but not exclusively of GaAs) is where the deposit of the metal contacts 5 subsequently takes place, while the so-called active zone is the graphene zone that is in direct contact with the window layer and that has been exposed once the contact layer has been removed in certain areas of the surface. The window layer is a semiconductor layer that is optically very transparent (high bandgap), which may be of AIGalnP, AllnP or the like (the latter case being more preferable) and whose thickness 10 is of the order of 25-40nm. For example, in a preferred triple junction network-fitted multi-junction cell structure, the sub cells that form the entire solar cell (from top to bottom) are GalnP, GalnAs and Ge. Ge's own substrate acts as a subcell (the one with the lowest bandgap). Figure 3 shows the simulation of a triple junction cell structure that is formed by a window layer (in this case of AllnP) 15 on which graphene is deposited. These simulations evaluate how the current conduction mechanism between the graphene / window layer interface is based on the window doped and the work function of the deposited graphene. The results of these simulations indicate that for doped levels of the AllnP layer of the order of
    J J
    7 · 1018cm · _1 · 1019cm-, the passage of current through the 20 graphene / AllnP interface is guaranteed by means of a tunnel effect ("tunneling") that is the most effective.
    Within the scope of the present invention, graphene previously obtained can be used in any way that allows it to be deposited in layers, such as by any of the usual techniques in the field, namely exfoliation, epitaxial obtaining, reduction of Graphite oxide or chemical vapor deposition (CVO). As an example of a preferred method, graphene layers are grown and deposited on a support by means of this last cited technique (CVO). This technique, unlike other techniques such as that described by Jieun Chang et al. (ICP-CVO), allows to control the number of layers to deposit, their homogeneity and thickness, as well as the optoelectronic properties 30 of graphene. Thus, graphene grown by evo has a high optical transmittance (between 97-98%) and a high lateral electrical conductivity (between 200-400 O / frame) that are important for the function it plays in the cell, as well as a homogeneous thickness of about -0.34 nm. This technique is also capable of generating graphene sheets with areas large enough to completely cover semiconductor wafers such as those used in the manufacture of solar cells (for example, 2, 4.6 inches -5.08; 10.16 and 15, 24 cm in diameter, respectively-, etc.). Another advantage of the CVD lies in the fact that the graphene layers, in addition to being homogeneous and covering the entire semiconductor area, do not have reliefs when deposited on the semiconductor surface of the window layer (Figure 1b) or in any case the relief 5 formed by depositing both on the semiconductor surface of the contact layer and the window layer (Figure 1 a), which makes this process compatible with planar technology and thus allowing better contact with the semiconductor. Similarly, the contact between graphene and the frontal metal contact is good, both morphologically and electrically, to be properly adhered and allow the passage of
    10 current in the graphene / semiconductor interface (ohmic contact).
    Preferably, the solar cell contains between 1 and 5 layers of graphene, this number being preferably 1-2 because this ensures that the optical transparency is high. The number of layers depends on the light intensity under which the solar cells will work, their size, etc. The compromise for the optimal choice of graphene layers is based on the fact that the greater the number of layers, the lower the sheet resistance and therefore a greater electrical conduction but also a greater unwanted optical absorption. For this reason, 5 layers of graphene are established as an upper limit since it has been seen that too much light is absorbed above. Each layer has a thickness of
    20 0.34 nm by the nature of graphene.
    The metallized front contacts of the multi-junction solar cell consist of a metallic front mesh that acts as an ohmic contact. Although metal and usual treatments in the field can be used for the metallization of the frontal contacts,
    25 such as the one based on Au-Ge with silver regrowth, other metallizations recommended in graphene applications are based on alloys of the Ti / Au, Ti / Pd / Au, Ti / PI, Ti / Pd / Ag type (recommended by good properties of Ti in terms of acting as an adherent layer), Ni / Au, Pd / Au, Cr / Au, etc. which produce an adequate ohmic result.
    We have already commented that the III-V semiconductor multi-junction solar cells include a window layer on the semiconductor surface, which is usually also made of a semiconductor material that has a high gap and a thickness of 25-40 nm to optimize its transparency. More preferably, the window layer is from AllnP, although the present cell may contain a window layer of other known materials. This window layer is the upper semiconductor layer of the entire front surface of the structure (Figure 1b) on which graphene is deposited, unless in an alternative embodiment it is desired that there is a contact layer in certain areas of the surface of The semiconductor structure, in which case the two layers, contact and window, are the outermost (Figure 1a). In developing the present invention, the authors have verified the importance of the level of doping of said window layer (of AllnP) that is necessary to guarantee the passage of current between the graphene interface / window layer of AllnP. Since this invention achieves excellent morphological contact between graphene and the semiconductor structure, it is key to achieve a good ohmic contact between graphene and window layer. In fact, the simulations performed (see Figure 3) indicate the importance of both the graphene's work function and the dopedness of the window layer to guarantee the passage of current through that interface. Thus, for graphene work function values of 3.78, 4.86 and 4.95 and levels
    J
    of doped of the window layer (preferably of AllnP) of the order of 7 · 1018cm-J
    15 1 · 1019cm-, the passage of current through the grafenolcapa window interface through tunnel effect (tunneling) begins to be effective. More preferably, the level of doping of the window layer is equal to greater than 1.1019 cm · 3, since in these conditions the interface behaves as an ohmic contact (which is intended) instead of as a Schottky contact (a avoid). Therefore, in order to achieve the current flow
    20 from the semiconductor structure (through the window layer) to graphene, it is key that the doping of the window layer be as high as possible.
    Preferably, the solar cell comprises an anti-reflective coating above graphene with, typically, two layers that are different depending on the
    25 application to which the solar cell is dedicated. Typical combinations of anti-reflective layers are MgF2 / ZnS, TiOx / AIOx, etc.
    Method of obtaining the solar cell
    To obtain the multi-junction solar cell object of the invention, it is possible to start from a
    The semiconductor structure already made up of the semiconductor layers on whose surface graphene is transferred, or alternatively it is possible to include a previous stage of growth of said semiconductor layers on a substrate as part of the process. Preferably, the growth is epitaxial, and more preferably even the step of growth of the semiconductor layers is performed by the technique of
    35 Steam phase epitaxy from metalorganic precursors (Metal Organic Vapor Phase Epitaxy -MOVPE-), molecular beam epitaxy (Molecular Beam Epitaxy MBE-), Hydride vapor-phase epitaxy (HVPE), etc. or a combination of them.
    In a particular embodiment of the method, this may include a previous stage of
    5 growth of the graphene layer or layers, being more preferably still grown on a support by the CVD technique. This stage is carried out following usual procedures in the field of graphene monolayers growth by means of CVD (Li, X., W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, 1. Jung, E. Tutuc, SK Banerjee, L. Colombo and RS Ruoff "Large-Area Synthesis of
    10 High-Quality and Uniform Graphene Films on Copper Foils ". Science, (2009). 324 (5932): 1312-1314).
    Before transferring graphene to the upper semiconductor surface of the cell, it is possible and convenient to chemically treat said surface to remove rust residues and ensure adequate graphene / semiconductor contact. Preferably, in the case of depositing graphene both in the window layer and in certain parts of the contact layer (Figure 1a), said chemical treatment consists of the selective attack of the contact layer of the areas of interest (except in the areas where metal contacts will be deposited later) by wet attack (such as by means of
    20 NH40H: H20 2: H20 (2: 1: 10) for 30-40s. In the case of depositing graphene directly on the window layer on the entire surface (Figure 1b), the contact layer is completely removed by selective wet attack (such as by NH, OH: H, O ,: H, O (2: 1: 10) lasting 30-405) exposing the window layer where the graphene is to be transferred.
    To transfer the graphene layer or layers to the surface of the semiconductor substrate, any usual technique known in the field can be used. A more preferred embodiment is by wet transfer (or in English, wet transfer method). Unlike the method described by Chang et al. , the wet graphene transfer method 30 not only allows a process control as to the number of graphene monolayers that are to be transferred and deposited on the semiconductor surface, but also allows the graphene deposit to be made over the entire surface, covering 100% of the active area of the cell in question. Thus, starting with graphene, the following wet transfer procedure is performed. Some 35 references about the wet transfer technique are: Reina, A., X. Jia, J. Ho, D. Nezich, H. Son,
    V. Bulovic, M. S. Dresselhaus and J. Kong "Large Area, Few-Layer Graphene Films on Arbitrary Substrates by ehemical Vapor Deposition". Nano Letters, (2009). 9 (1): 30-35; Li, X., Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo and R. S. Ruoff "Transfer of Large-Area Graphene Films fer High-Performance Transparent Conductive
    5 Electrodes ". Nano Letters, (2009). 9 (12): 4359-4363.
    In general, the wet transfer comprises the steps of: -coating the upper part of the graphene previously grown in layers on a support with PMMA resin, -applying a chemical attack of the support obtaining a new configuration
    10 PMMAlgrafeno, -clean the structure PMMAlgrafeno eliminating any remainder that comes from the attack of said support, -transfer graphene on the upper semiconductor layer of the multi-junction solar cell to form,
    15 - remove the PMMA resin, and - apply heat treatment at a temperature between 300 ° C and 400 ° C for a time between 2 and 4 hours.
    In short, wet transfer can be done as follows
    In a particular preferred method: first, by defining the area of graphene grown to be deposited, by cutting a surface of said graphene and its growth support (for example, graphene / copper) according to the solar cell sample area in the one that you want to transfer (for example in wafers of 2 inches -5.08cm-), and then performing the relevant steps of preparation of the materials and of
    25 transfer. In a particular preferred embodiment of the wet transfer technique, graphene grown by CVD of its support is taken, taking as reference the measurements of the particular solar cell semiconductor sample (for example 2 inches) in which it is desired to perform the transference. The graphene of said structure (graphene / Cu) is coated with PMMA resin, exposing the copper support.
    30 It is subjected to heating so that the resin is properly adhered to graphene. Next, the Cu support is attacked for 30 minutes with FeCh. The new PMMAlgrafeno structure is submerged in several water passages. To clean any remaining FeCI3, the sample is immersed in HCI: H20 (1: 3) for 5 min. Again, several water passages are performed to ensure good cleaning.
    35 With the PMMAlgrafeno sample being submerged in water, the attack of
    the contact layer of the GaAs solar cell with NH40H: H20 2: H20 (2: 1: 10) for 3040s. After this attack, the AllnP window layer on which the fishing of PMMAlgrafeno is made is revealed. The new sample PMMAJgrafeno / semiconductor structure is dried with N2 avoiding leaving traces of water and making sure to obtain a flat surface. The sample is then heated at BOoe for 15 min and at 1200e for another 15 minutes to better adhere the PMMAlgrafen to the semiconductor surface and release any water molecule. The next step is the removal of the PMMA resin by introducing the sample into acetone for at least two hours. After that time, it is introduced into methanol or isopropanol and dried again.
    10 with N2. To remove any remaining organic particles or residues derived from the transfer process, a heat treatment is carried out in an H2 or H2 + Ar atmosphere (-200 sccm) for -3 hours at about -350oe. After this last step the transfer process is finished.
    15 The transfer process is repeated as many times as layers of graphene you wish to deposit. In this way, it is achieved that the transfer of graphene is carried out in a controlled manner regarding the area of graphene deposited and its number of layers.
    Similarly, after the transfer of the graphene layer or layers onto the surface
    20 of the semiconductor structure it is convenient and advisable to thermally treat the graphene surface to eliminate organic debris produced during said transfer and, as a consequence, improve its optical properties and its electronic quality before the deposit of the frontal metal contacts. Normally, this heat treatment is carried out at a temperature above 2000e and, preferably,
    25 between 350 ° C and 400 ° C. One of the advantages offered by the process of the present invention is that said heat treatment does not affect or worsen the quality of the metal contacts, since it is carried out prior to their deposition on graphene. In the most preferred embodiment of all, the heat treatment is done at a temperature between 350 ° C and 400oe, including both limits, during a
    30 time between 2 and 4 hours, more preferably 3 hours, under H2 + Ar atmosphere.
    After the transfer of graphene over the semiconductor structure, the frontal metal contacts are deposited. Said front contact reservoir, which is in the form of a metallization mesh, is made by usual metallization techniques, such as the evaporation technique. The metallization process consists in defining a pattern on the graphene surface and depositing the metallization in said pattern, so that the frontal metallization is deposited on the graphene surface that is transferred over the contact layer (usually of GaAs) if it is 5 of the structure of figure 1a or on the window layer if it is of figure lb. In both cases, the metal contact reservoir helps keep the graphene adhered on the deposited surface preventing the possible "detachmenf 'or graphene removal. This mode of integration allows the use of planar technology in industrial manufacturing, and the fact that graphene is embedded between the semiconductor and the mesh
    10 frontal, results in high reliability of the resulting solar cell.
    The preferred technique for the definition of the mesh capable of being metallized is to apply a photoresist sensitive to ultraviolet light that covers the front part of the cell where the graphene is located, defining the pattern of the metallization mesh
    15 front. Subsequently, a photolithographic mask with the metallization pattern is applied, on which an ultraviolet light is projected to eliminate the photoresist in those parts where metal is to be deposited and protecting the rest of the surface, following usual procedures known in the field of The technique.
    In this phase of the procedure, it is advisable to clean the organic remains of the photoresist on the graphene surface prior to evaporation metallization, thus guaranteeing a better graphene / metal interface. This cleaning is preferably carried out by application of oxygen plasma for a time between 60 and 90 seconds or by "ultra saw / ozone" treatment of about 10-16 minutes
    25 according to the parameters used. These methods are described in the prior art, as are the following references: "UV / Ozone treatment to reduce metal-graphene contact resistance"; W. Li, Y. Liang, D. Yu, L. Peng, K P. Pernstich, T. Shen, AR Hight Walker, G. Cheng, and A. Hacker, and A. Ritcher, Q. Li, D J. Dundlach and X. Liang. App /. Phys. Lett. 102, 183110 (2013); And "Contacting graphene" J. A. Robinson, M. LaBella, M.
    30 Zhu, M. Hollander, R. Kasarda, Z. Hughes, K. Trumbull, R. Cavalero, and D. Snyder. Appl. Phys. Leff. 98,053103 (2011).
    Then, the evaporation for the deposit of the metallization on the mesh is also done by means of usual techniques in the field, such as thermal evaporation, by 35 electron guns, etc. After the evaporation of the frontal metallization, the "Lift
    off 'O removed by acetone and methanol to remove all the photoresist along with the metal evaporated on top of it, leaving only the metal contacted directly on the graphene. In this last step it may be advisable to apply a heat treatment. The treatment conditions preferably consist of an atmosphere of
    5 H2 at 200 ° C-375 oC for a few minutes depending on the metallization to be used, in such a way that it does not adversely affect graphene but helps you to detach any trapped remains during the different technological steps described.
    Once the graphene has been transferred and the metallization deposited, the solar cells are electrically isolated within the semiconductor wafer.
    In this regard, it should be noted that, due to the size of the designed solar cells that can vary from 0.1mm2 to 100 cm2 (depending on the application such as concentration cells or space cells), it is usual to manufacture from one to 15 thousand of these multi-junction solar cells during the same process on a semiconductor wafer. For this reason, the method of obtaining numerous multi-junction solar cells as previously described is also object of this invention. In this case, several cells are prepared simultaneously in the same wafer of semiconductor material, the graphene is transferred, the front metal contacts 20 are deposited by the steps indicated above, and then the part of the transferred graphene and the part of the layers are removed semiconductors that separate (remain between) the different solar cells that are prepared in the wafer. Thus, to electrically isolate the different solar cells contained in the processed wafer, a series of chemical treatments are carried out that attack graphene and each semiconductor layer until
    25 reach the semiconductor substrate (the so-called table attack). Because graphene is inert to the wet chemical attacks used in the other layers, it is necessary first of all to attack graphene in areas of interest by a different method. For this, it is necessary:
    1. a second photolithographic step that protects both areas with photoresist
    30 active cells that contain graphene such as metallization and, at the same time, expose the graphene to be attacked (in this case the graphene existing between one cell and another);
    2. Attack graphene using the Reactive Ion Etching (RIE) technique that uses a plasma
    of oxygen This technique attacks graphene (in the areas of the future table) that has not been previously protected by the photoresist. RIE attack parameters
    they vary depending on the power used, the flow of oxygen and the number of graphene layers to be attacked; Y
    3. Once graphene is attacked, use the same photoresist that has protected the areas of interest (both the active zone of the cell and the metallization) to perform the
    5 relevant table attack. This table attack is based on a series of chemical processes that selectively attack each semiconductor layer until it reaches the substrate (see Figure 5).
    After cleaning the photoresist, again with acetone and methanol, they are deposited, if
    10 require in a particular embodiment, the anti-reflective layers necessary to maximize sunlight that reaches the device and reduce reflection losses. The number of anti-reflective layers, materials and thicknesses should be optimized for each application taking into account the optical properties of graphene.
    Brief description of the figures Figure 1: Brief description of the different parts that make up the III-V semiconductor multi-junction solar cell of this invention (the figure is not made to scale). 1) Semiconductor substrate with thicknesses that can typically vary from 60-500 m. 2) Semiconductor layers grown epitaxially on the semiconductor substrate.
    20 Figure 1 a: First option with contact layer and window layer as upper layers on which graphene is deposited. Figure 1 b: Second option, window layer as the only top layer on which graphene is deposited. 3) Semiconductor structure that integrates both the epitaxially grown semiconductor layers and the semiconductor substrate on which they have been grown. 4) Layer window. 5) Contact layer. 6)
    25 Monolayer / s of graphene transferred / s on the semiconductor structure (-0.34nm each layer). 7) Front and rear metal contacts. 8) Anti-reflective layers (optional). Figure 2: Evolution of the Form Factor (FF) as a function of concentration (in soles) for triple junction solar cells without graphene (represented by a circle), with a graphene monolayer (star) and with two graphene monolayers ( square). The analysis
    30 of the series resistance has been studied through the evolution of the form factor (FF) as a function of concentration. This experimental measure obtained by the inventors clearly demonstrates the improvement of this type of cells when they operate at high concentrations thanks to the use of graphene. The series resistance responsible for extracting the current decreases due to the incorporation of one and two monolayers
    35 graphene resulting in a better FF. A similar behavior is expected if the size of the solar cell was set on the horizontal axis while keeping the concentration constant. Figure 3: Simulation of the dark IV curves of a triple junction structure (GalnP / GalnAs / Ge considering a current density of approximately 1415mAlcm2 at a sun) with graphene in contact with the AllnP window layer depending on the doped of it and of the work function (WF) of graphene. These simulations try to illustrate the importance of both the graphene work function and the dopedness of the window layer to guarantee the passage of current through that interface. For the simulated values of the graphene work function of 3.78 eV, 4.8 6eV and 4.95 eV and levels of
    J J
    Doped from the AllnP window layer of the order of 7 · 1018cm · _1 · 1019cm ·, the passage of current through the graphene / AllnP interface through a tunnel effect ("tunneling") begins to be effective. For dopates greater than 1 · 1Q19cm · J, the interface behaves as an ohmic contact (which is intended) instead of as a Schottky contact (to avoid). Therefore, in order to achieve the passage of current from the semiconductor structure (through the window layer) to graphene, it is key that the doping of the window layer be as high as possible. Figure 4: 4.a) Scheme of a preferred manufacturing method of the solar cell object of the invention (the figure is not made to scale) based on a structure like the Figure
    1st 4.b) Scheme of a preferred manufacturing method of the semiconductor multi-junction solar cell III-V object of the invention (the figure is not made to scale) based on a structure like Figure 1.b. Steps G), H), 1) and J) discussed in the following section are not represented in the figure:
    A) Growth of the semiconductor structure (1) on the semiconductor substrate. In the case of Figure 4.a, the partial attack of the contact layer is also shown.
    B) Transfer of graphene (2) in the form of layers on the front surface of the semiconductor structure (1) And heat treatment to ensure its adhesion.
    C) Photoresist tank (3) and photolithographic process to define the pattern of metallization: incorporate a photolithic mask (5), where dark areas prevent the passage of ultraviolet light.
    D) Cleaning of organic remains from the graphene surface where they will be deposited
    the metal contacts (6).E) Deposit of (4) of the metal contacts.F) Uft-off and heat treatments for improved metallic contact.
    G) Resin tank in which windows will open to subsequently attack graphene between cells by RIE. Then, perform the attack of tables with wet chemical attack.
    H) Cleaning of the resin remains with acetone and methanol or isopropanol.
    1) Deposit of anti-reflective layers (optional). Figure 5: Graphical representation of the fundamental stages of preparation of multi-junction solar cells: a) transfer of graphene (1) and deposit of frontal contacts (2); b) graphene attack of the table area by RIE, leaving an area without graphene (3); and c) creation of tables (4) by chemical attacks of the semiconductor layers.
    DESCRIPTION OF A PREFERRED EMBODIMENT Next, a detailed and preferred but non-limiting method for manufacturing the III-V multi-junction solar semiconductor cell with integrated graphene object of the invention is described, which consists of the following steps (see diagram of Figure 4).
    A. Preparation of the semiconductor structure of the multi-junction solar cell. Starting from a wafer of a semiconductor material such as those commonly used in the field and described herein (Ge, GaAs, InP, etc ...), the epitaxial growth of the different semiconductor layers that constitute a multi-junction solar cell is performed. The techniques commonly used for such growth, which are also applicable in the present invention, are, for example, vapor phase epitaxy from metalorganic precursors (Metal Organic Vapor Phase Epitaxy -MOVPE-), molecular beam epitaxy (Molecular Beam Epitaxy -MBE-), hydride vapor phase epitaxy (Hydride vapor phase epitaxy -HVPE-), combinations thereof, etc.
    After epitaxial growth, the surface of the upper semiconductor layers where graphene is to be transferred is prepared. Preferably, in the case of depositing graphene both in the window layer and in certain parts of the contact layer (Figure 1 a) said preparation consists in the selective attack of the contact layer of the areas of interest by wet attack ( such as through NH40H: H20 2: H20
    (2: 1: 10) for 30-40 s. In the case of depositing graphene directly on the window layer (Figure 1 b) the contact layer is completely eliminated by means of selective wet attack (for example by NH40H: H20 2: H2 0 (2: 1: 10) during 3040 s) exposing the entire window layer where the graphene is to be transferred.
    Since the structure / method proposed in this invention achieves a
    5 Excellent adhesion between graphene and the semiconductor structure, through the window layer, it is key to achieve good ohmic contact between graphene and the outermost window layer of the semiconductor structure. The inventors have carried out simulations (see Figure 3) that indicate the importance of both the graphene's work function and the dopedness of the window layer to guarantee the passage of current through that
    10 interface For the simulated values of the graphene work function of 4.86 and 4.95 eV and
    doping levels of the AllnP window layer of the order of 7 · 1018cm · 3 .1 · 1019cm-, the passage of current through the graphene / AllnP interface through tunneling ("tunneling")
    Begins to be effective For dopates greater than 1 · 1019cm ·, the interface behaves as an ohmic contact (which is intended) instead of as a Schottky contact.
    15 Therefore, in order to achieve the passage of current from the semiconductor structure (through the window layer) to graphene, it is key that the doping of the window layer be as high as possible.
    A. Deposit by transfer of previously grown graphene on the semiconductor surface of the multi-junction solar cell
    After epitaxial growth of the semiconductor layers, the transfer of graphene grown by CVD is performed on the front surface of the structure (i.e., on the outermost or uppermost layers of the semiconductor structure, namely the contact layer and the layer window, depending on whether the final configuration is that of figure 1a or 1 b). Graphene is transferred by means of the wet transfer technique on said surface, which has been previously chemically treated to remove any oxide present on the surface of interest, guaranteeing a good graphene / semiconductor interface. The different steps of the wet transfer transfer technique are performed following the particular embodiment described in the present patent: Starting from graphene grown on a copper sheet, graphene is transferred in such a way that its area is determined by the measurements of the sample solar cell semiconductor in particular (for example a 2-inch wafer). The graphene of said structure (graphene / Cu) is coated with PMMA resin, exposing the copper plate. Heating is performed so that the resin is adhered to graphene. Next, the Cu 35 sheet is attacked for 30 minutes with FeCI3. The new PMMAlgrafeno structure is immersed in several
    water passages To clean the remains of FeCI3, the sample is immersed in HCI: H20 (1: 3) for 5 min. Again, several water passages are used to ensure good cleaning. In parallel, the attack of the contact layer of GaAs with NH40H: H20 2: H20 (2: 1: 10) is carried out for 30-40 S. After this attack, the AllnP window layer on which the fishing of PMMAlgrafeno is made is revealed. The new sample PMMAlgrafeno / semiconductor structure is dried with N2 for two minutes avoiding leaving water particles and making sure to obtain a flat surface. The sample is then heated at 80 ° C for 15 min and at 120 ° C for another 15 minutes to better adhere the PMMAlgrafen to the semiconductor surface and release unwanted debris. The next step is the removal of the PMMA resin by introducing the sample into acetone for at least two hours. After that time, it is introduced into methanol or isopropanol and dried again. To eliminate any remaining organic particles or residues derived from the transfer process, a heat treatment is carried out in an atmosphere of H2 or H2 + Ar (-200 sccm) for -3 hours at -350 ° C. After this last step the transfer process ends.
    The transfer process is repeated as many times as graphene layers are desired to deposit and that in the present invention is circumscribed between 1 and 5 layers.
    Finally, once transferred, the graphene layers are subjected to a heat treatment to eliminate organic debris produced during said transfer and, as a consequence, improve their optical properties and electronic quality, as well as guarantee their correct adhesion. This heat treatment is carried out at a temperature between 300 oC and 350 oC for a time of 3 hours under H2 + Ar atmosphere.
    B C D. E. Metallic front contact reservoir
    Once the graphene has been transferred to the front surface of the solar cell, the frontal metal contacts are deposited. To do this, the definition of the frontal metallization mesh pattern is first performed by depositing in the front part of the structure (which was previously determined by the surface of the semiconductor layer and which already contained graphene) of a sensitive photoresist in ultraviolet light. With the use of a photolithography mask that has said metallization pattern defined, the photoresist is eliminated thanks to the use of ultraviolet and posterior light
    development of the areas of interest where the metal is going to be deposited and, simultaneously,
    protecting the rest of the areas where you do not want to deposit said metal.
    Prior to metallization, the graphene surface is cleaned to eliminate possible
    5 organic remains of the photoresist where the metal contacts were to be deposited, to
    guarantee a better graphene / metal interface. This cleaning is carried out by
    application of oxygen plasma for a time between 60 and 90 seconds
    or with "UV / ozone ~ for 10-15 minutes depending on the parameters used.
    1 o That is when the deposit of metallization is carried out by means of
    thermal evaporation, by electron gun, etc. After evaporation of metallization
    frontal, is carried out of withdrawal ("Uft-off ') by acetone and methanol, eliminating all the
    photoresist together with the metal evaporated on top of it and leaving only the metal
    contacted directly about graphene. In this last step, it is necessary to apply a
    fifteen heat treatment to ensure good contact between graphene and used metal
    how do I contact you. Said treatment consists of an atmosphere of forming gas or of H2 at 200 oC
    -375 oC (depending on the metallization to be used), so that it does not adversely affect the
    graphene but allows to eliminate any remaining photoresist or trapped water
    during the different technological steps described.
    twenty
    F G H l. Final preparation of solar cells: electrical insulation cleaning v
    deposit of anti-reflective layers.
    Due to the size of the designed solar cells (0.1 mm2-100 cm 2) in many
    Sometimes a considerable number can be manufactured during the same process.
    25
    In this way, to electrically isolate the solar cells manufactured on it
    semiconductor substrate, a series of chemical treatments that attack each
    semiconductor layer until reaching the substrate (table attack). Because the
    Graphene is inert to such chemical attacks, it is necessary to attack graphene beforehand
    30 of the areas of interest by another method. For this, a second step is necessary
    photolithographic that protects the active areas of the cell that contain with photoresist
    graphene and at the same time exposes the graphene that wants to be attacked (in this case the
    graphene between an adjacent cell and another). For graphene attack the
    Reactive Ion Etching (RIE) technique consisting of an oxygen plasma.
    35
    Once graphene is attacked, the same photoresist that protects the areas of interest (both active area of the cell and metallization) is used to perform the relevant table attack. After cleaning the photoresist, again with acetone and methanol, as a last step prior to the individual encapsulation of each cell, the cells are optionally deposited.
    5 anti-reflective layers to maximize the sunlight that reaches the device and reducereflection losses. In this way, in a wafer solar cells are obtainedmulti-junction of semiconductors III-V (from one to several thousand) that incorporategraphene as described in this invention.
    权利要求:
    Claims (14)
    [1]
    1. A multi-junction solar cell of semiconductors III-V, comprising: a semiconductor structure of materials III-V incorporating one or more
    5 photovoltaically active pn junctions that make up the body of the solar cell,front and rear metal contacts, front contacts beingconstituted by a metal mesh,
    characterized in that it comprises at least one layer of graphene deposited between the upper part of the semiconductor structure and the frontal metal contacts.
    [2]
    2. The solar cell according to claim 1, wherein the upper layer of the semiconductor structure is a window layer or a combination of a window layer in some areas of the surface and a contact layer in the remaining areas.
    The solar cell according to claim 1 or 2, wherein the graphene layer is previously grown by chemical vapor deposition.
    [4]
    4. The solar cell according to any one of the preceding claims, which
    It comprises between 1 and 5 layers of graphene, each with a thickness of about 0.34 nm. twenty
    [5]
    5. The solar cell according to any of the preceding claims, comprising at least one anti-reflective layer deposited on the graphene layer.
    [6]
    6. The solar cell according to any one of the preceding claims,
    25 where the frontal metal contact is made by alloys selected within the group consisting of: Au-Ge, Ti / Au, Ti / Pd / Au, Ti / Pt, Ti / Pd / Ag, Pd / Au, Ni / Au and Cr (Cr / Au).
    [7]
    7. The solar cell according to any one of claims 2 to 6, wherein the window layer is AllnP with a level of doping equal to or greater than 1 · 1 019cm · 3.
    [8]
    A method of obtaining the semiconductor multi-junction solar cell III-V defined in any one of the preceding claims, characterized in that it comprises:
    deposit at least one layer of graphene on the surface of the semiconductor structure 35,
    prior to depositing the frontal metal contacts on said graphene layer.
    [9]
    9. The method according to the preceding claim, wherein the graphene layer is
    5 deposits on the surface of the semiconductor structure by transfer indamp.
    [10]
    10. The method according to any one of claims 8 or 9, comprising a previous stage of epitaxial growth of the semiconductor layers which
    10 form the semiconductor structure by means of a technique selected from vapor phase epitaxy using metalorganic precursors (MOVPE), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE) or combinations thereof.
    The method according to any one of claims 8 to 10, which comprises a previous stage of growth of the graphene layer by vapor phase deposition (eVO).
    [12]
    12. The method according to any one of claims 8 to 11, which
    20 comprises chemically treating the surface of the semiconductor structure before transferring the graphene layer.
    [13]
    13. The method according to any one of claims 8 to 12, which
    it comprises heat treating the surface of the graphene layer after the transfer stage and before the deposition of the front metal contacts.
    [14]
    14. The method according to any one of claims 8 to 13, wherein the front metal contacts are deposited on the graphene by the technique of thermal evaporation metallization or electron gun evaporation.
    [15]
    fifteen. The method according to the preceding claim, which comprises a stage for cleaning organic residues of the areas where the metal contacts are to evaporate prior to the evaporation stage, by a technique selected from application of oxygen plasma or UV / ozone .
    [16]
    16. The method according to any one of claims 14 or 15, which comprises applying a heat treatment after evaporation in the atmosphere of forming gas or H2 at a temperature between 200-450 ° C.
    The method according to any one of claims 8 to 16, whichcomprises obtaining more than one multi-junction solar cell simultaneously, and thatcomprises, after the deposit of the front metal contacts on the layer ofgraphene, electrically isolate each solar cell by treatment removalchemical of graphene and the part of the semiconductor layers surrounding each cell
    10 solar, in the following sub-stages: -through photolithographic treatment, cover graphene with a protective photoresin both in the active areas of the cell and in the frontal metallization and at the same time expose the graphene that can be eliminated; -remove graphene in the region surrounding each solar cell using the technique
    15 Reactive Ion Etching with oxygen plasma; -apply attack of tables by means of chemical wet process the part of the semiconductor layers where graphene has been previously removed, until reaching the substrate.
    The method according to any one of claims 8 to 17. which comprises depositing at least one anti-reflective layer on the graphene layer.
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    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    US20130164888A1|2010-07-01|2013-06-27|Egypt Nanotechnology Center|Graphene Solar Cell|
    CN106449790A|2016-12-09|2017-02-22|中国科学院微电子研究所|Graphene/gallium arsenide solar battery|EP3787041A1|2019-08-29|2021-03-03|AZUR SPACE Solar Power GmbH|Stacked multi-junction solar cell with metallization comprising a multilayer system|
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