WO2023026188A1

WO2023026188A1 - MXene-SILVER CONDUCTIVE PASTE, ELECTRODE, AND METHOD

Info

Publication number: WO2023026188A1
Application number: PCT/IB2022/057890
Authority: WO
Inventors: Husam Alshareef; Erkan Aydin; Stefaan DE WOLF; Jehad EL-DEMELLAWI; Atteq ur REHMAN
Original assignee: King Abdullah University of Science and Technology KAUST
Current assignee: King Abdullah University of Science and Technology KAUST
Priority date: 2021-08-24
Filing date: 2022-08-23
Publication date: 2023-03-02
Anticipated expiration: 2024-02-24

Abstract

A MXene-Ag paste (320) for forming electrical contacts includes Ag particles (402), glass frits (404), and MXene flakes (110). The MXene is given by M_n+1X_nT_x with n = 1, 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen.

Description

MXene-SILVER CONDUCTIVE PASTE, ELECTRODE, AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/236,412, filed on August 24, 2021, entitled “PREPARATION OF MXENE/SILVER CONDUCTIVE PASTE FOR ELECTRONIC DEVICE APPLICATIONS,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a paste, electrode, and method for forming the electrode for electronic devices, and more particularly, to mixing a silver (Ag) paste with a MXene paste for reducing the amount of silver used in the final paste for making the electrode.

DISCUSSION OF THE BACKGROUND

[0003] The spread of electronic devices, from communication devices to sensors for anything and everything, has increased in the past years at an unexpected pace. The Internet of Things (loT) relies on the myriad of electronic devices and sensors that automize our lives and monitor all aspects of residential, public, and industrial activity. All the devices that make the loT require one or more electronic modules, for example, a communication module to exchange data with a server, a memory to store the data, a processor to process the data and control the communication with the server, and a sensor to monitor a parameter or collect information.

[0004] Each such electronic module is typically made of electronic and/or electric components, i.e., integrated circuits, transistors, resistors, capacitors, inductors, etc., which are attached to a printed circuit board (PCB). The attachment between some of these components and the PCB is frequently achieved using a silver (Ag) paste. Metal contacts made of Ag paste are the most established way for state-of-the-art solid-state electronic and semiconductor device applications. They have been applied in the photovoltaic and semiconductor industries for many years. Their very high conductivity, minimal contact resistivity with most other semiconductor layers, and relatively higher chemical stability confer the Ag paste a unique value in several electronic and semiconductor technologies.

[0005] The amount of Ag paste used for the devices of loT adds up to a substantial number, which means that the Ag demand is increasing. This has recently made a substantial increase in the price of Ag. Compounding this problem is the witnessed explosion in solar cell use. In this regard, both the residential and industrial sectors have recently started to demand large amounts of solar panels to offset carbon-based energy generation. Each solar cell in a solar panel heavily relies on Ag paste to establish electrodes for collecting the generated electrical current. Thus, the large amount of new solar panels being manufactured today require a proportionally large amount of Ag, further putting pressure on the Ag price. Whereas the price of solar panels is expected to go down to appeal to a larger spectrum of the population, the increasing cost of Ag would remain a pressing problem for this goal.

[0006] Typically, the Ag paste is composed of three ingredients, i.e., (a) micron or nanoscale Ag flakes/particles, which provide the paste with superior conductivity; (b) glass frit, for adherence of the Ag contact to a substrate (e.g., silicon); and (c) an organic medium, which provides the desired rheological properties that keep Ag particles and glass frit intact and retains the contacts with an ideal aspect ratio. However, Ag remains the main constituent (more than 90 wt% of the entire paste mass), making the overall price quite high. Hence, to realize cost- effective semiconductor technologies, the substitution/reduction of Ag content in the paste with cheaper material is highly desirable.

[0007] Thus, there is a need for a new paste with electrical properties similar to the Ag paste yet cheaper, readily available, and can be manipulated as easily as the Ag paste.

BRIEF SUMMARY OF THE INVENTION

[0008] According to an embodiment, there is a MXene-Ag paste for forming electrical contacts, and the MXene-Ag paste includes Ag particles, glass frits, and MXene flakes. The MXene is given by M_n+iX_nT_x with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen.

[0009] According to another embodiment, there is a solar cell for transforming solar energy into electrical energy, and the solar cell includes a crystalline Si layer having opposite first and second surfaces, a first-type amorphous Si layer located on the first surface, an second-type amorphous Si layer located on the second surface, a first electrode formed on the first-type amorphous Si layer, and a second electrode formed on the second-type amorphous Si layer. The first electrode includes Ag particles and MXene flakes. MXene is given by Mn+iXnT_x with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen. The first- and second-type corresponds to n- and p-type or p- and n-type.

[0010] According to yet another embodiment, there is a tandem solar cell that includes an n-type base having first and second surfaces, opposite to each other, a p-type layer formed on the second surface of the n-type base, wherein the p-type layer and the n-type base form a first pn junction, a perovskite layer formed on the first surface of the n-type base, wherein the perovskite layer and the n-type base form a second pn junction; a first electrode formed on the perovskite layer, and a second electrode formed on the p-type layer. The first electrode includes Ag particles and MXene flakes. MXene is given by Mn+iXnTx with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0012] Figures 1A and 1B are schematic diagrams of a process for forming MXene flakes from a corresponding MAX phase power;

[0013] Figure 2 is a flow chart corresponding to the method illustrated in Figures 1A and 1B and including an additional step;

[0014] Figure 3A shows the MXene paste placed over the Ag paste, Figure 3B shows the two pastes being mixed, and Figure 3C shows the homogeneous mix that results in the MXene-Ag paste;

[0015] Figure 4A schematically illustrates the structural composition of the Ag paste, while Figure 4B schematically illustrates the structural composition of the MXene-Ag paste;

[0016] Figure 5 illustrates a semiconductor device having electrodes formed with the MXene-Ag paste;

[0017] Figure 6A is a scanning electrode microscope, SEM, image of the MXene-Ag paste applied on the top of the device shown in Figure 5, while Figure 6B is an SEM image of a cross-section of the MXene-Ag paste-based electrode formed on the semiconductor device in Figure 5;

[0018] Figure 7 is a table that illustrates the line resistance and bulk resistivity of the MXene-Ag paste versus the concentration of the MXene paste; [0019] Figure 8 illustrates how the conductor resistance of an electrode made with the MXene-Ag paste changes as the Ag paste blend ratio in wt% changes;

[0020] Figure 9A illustrates a silicon heterojunction solar cell having an electrode formed from the MXene-Ag paste, and Figure 9B shows a solar module including plural silicon heterojunction solar cells installed at rooftop of a residential house;

[0021] Figure 10 illustrates a semiconductor device having a single pn junction and one or more electrodes formed of the MXene-Ag paste;

[0022] Figure 11 illustrates an npn transistor having electrodes formed of the MXene-Ag paste;

[0023] Figure 12 illustrates a tandem solar cell having electrodes formed of the MXene-Ag paste; and

[0024] Figure 13 is a flow chart of a method for forming electrodes with the

MXene-Ag paste.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a solar cell that uses a MXene-Ag paste for forming a top electrode. However, the embodiments to be discussed next are not limited to a solar cell or the top electrode but may be applied to other electronic devices or faces of the solar cell.

[0026] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0027] According to an embodiment, a MXene-Ag paste is manufactured and used for any semiconductor or electronic device that previously used the pure Ag paste. MXenes are a new class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides. They adopt the general formula Mn+iX_nT_x (n = 1 , 2, 3, or 4), where M represents early transition metals (e.g., Ti, Zr, Hf, V, Nb, Ta, Mo), X is carbon and/or nitrogen, and T_x denotes the surface-terminated species, i.e. , hydroxyl, fluorine, and/or oxygen. Members of the MXene family, e.g., TisC2Tx, Ti2CT_x, V2CTX, Nb2CT_x, M02CTX, Mo2TiCT_x, as well as many others, have a unique set of properties that are appropriate for semiconductor elements, for example, high electrical conductivity, good adherence to semiconductors. Hence, pastes of several members of the MXene family can be admixed with Ag paste, resulting in several MXene-Ag pastes with distinctive collections of well-controlled properties. It is noted that the MXene pastes can also be mixed with other types of metallic pastes, e.g., Al paste, which may be used for the back end of the solar cell.

[0028] Among the MXene family, TisC2Tx MXene stands out for its metal-like conductivity, superior hydrophilicity, exceptional solution processibility, and widely tunable properties, making it highly compatible with solution-based metallic pastes. In general, MXenes are synthesized by selectively removing the metallic layer from their parent MAX phases, i.e., a family of layered ternary ceramics, through cheap and straightforward top-down etching techniques. MAX phases are layered, hexagonal carbides and nitrides which have the general formula: Mn+iAXn, (MAX) where n = 1 to 4 and A is an A-group (mostly I IIA and IVA, or groups 13 and 14) element. In this regard, Figures 1A and 1 B schematically represent the synthesis route of one of the MXenes, i.e., TisC2Tx, by removing the aluminum (Al) layer from the parent TisAIC2 MAX phase using a mixture of lithium fluoride (Li F) and hydrochloric acid (HCI). More specifically, a method for synthesizing the MXene-Ag paste, which is illustrated in Figure 2, includes a step 200 of selecting a MAX powder 102 (e.g., TisAIC2). The chemical structure 104 of the TisAIC2 MAX powder 102 is also illustrated in Figure 1A, with layers 106 of Al intercalated with layers 108 of TisC2. In step 202, the TisAIC2 MAX powder 102 is etched with LiF and HCI to remove the Al layer 106. This step results in the exfoliated TisC2T_x nanosheets or flakes 110, as shown in Figure 1 B. The exfoliated TisC2T_x nanosheets or flakes 110 are then delaminated in step 204 using several rounds of washing in water followed by sonication so that a TisC2T_x MXene solution 114 is obtained. Afterward, the MXene TisC2T_x solution 114 is centrifuged in step 206 for multiple rounds while gradually mixed with one or more organic compounds, e.g., dimethyl ether (CH3OCH3), until the entire water is almost replaced with the organic compound, e.g., dimethyl ether (CH3OCH3), forming a MXene paste 120 as sediment. Likewise, other MXene pastes may also be obtained using fluoride-containing top-down etching methods. Similar to the process illustrated in Figure 2, other MXene pastes may be obtained based on different MXene materials. In step 208, the MXene paste 120 is mixed with an Ag paste 310, as shown in Figure 3A, to form the MXene-Ag paste 320, shown in Figure 3C. Note that the two pastes are mixed as shown in Figure 3B, until the resulting paste 320 is homogeneous. This can be achieved manually or with the help of a mechanical mixer. In one application, the MXene, Ag particles, and the glass frits that form the paste 320 represent at least 99% of the total mass of the paste.

[0029] The MXene (e.g., TisC2T_x)-Ag paste 320 may be made with different weight percentages. For example, in one embodiment, the percentage of MXene paste 120 mass is between 20 and 30 by weight of the total mass of the resulting MXene-Ag paste 320. In one application, the percentage of MXene paste mass may be as low as 10% by weight. Figures 4A and 4B schematically illustrate the crosssection of metal contacts 400 and 410 formed with Ag-only paste 310 and MXene-Ag paste 320, respectively. Note that a metal contact may be formed between a semiconductor layer and an electrode to attach the electrode to the semiconductor layer. It is observed in Figure 4B that the MXene flakes 110 replaced a portion of the Ag particles 402, reducing the overall weight percentage (wt%) of Ag in the paste 320. Figures 4A and 4B also show the presence of glass frits 404. Furthermore, the Ag paste has shown good compaction with MXenes, with widely adjustable rheology according to the wt% of the added MXene paste.

[0030] In one embodiment, the MXene admixed with commercial Ag paste was used for screen printed contacts of silicon solar cells. Note that the Mxene paste can be mixed with various types of pastes, and the final paste mixture can be printed successfully by a screen printer through a mesh opening in the screen or stencil; other deposition techniques may also be used, such as inkjet printing, stamping, 3D printing, etc. The commercially available Ag paste shows good compaction with MXenes, and the rheology of the paste can be adjusted according to the MXene percentage added. Example MXene-Ag paste 320 was formed with good rheological properties, which is suitable for screen printing. The appropriateness of the resultant blend was verified during the screen-printing process, where the MXene-Ag paste 320 was easily squeezed through a stencil screen onto a silicon dioxide (SiC>2)- coated Si substrate 502 (corresponding to a semiconductor device 500, which may be any of those discussed herein), as illustrated in Figure 5. The insulating SiC>2 layer 504 was used to obtain the accurate bulk resistivity of the printed MXene-Ag metal lines 506. Figure 5 illustrates this arrangement, and Figures 6A and 6B are the scanning electron microscopy (SEM) micrographs of the top and cross-section of the printed MXene-Ag paste 320 after annealing at 200 °C. Figure 6A displays the MXene flakes (6 - 7 pm) 110 and Ag particles (400 - 500 nm) 402, while Figure 6B shows the screen- printed electrode 506 having an average height H of about 35 pm (the height range is between 25 and 50 pm) and an average width W of about 50 pm (the width range is between 20 and 100 pm). The fact that the viscosity of the MXene-Ag paste 320 is tunable by simply changing the percentage of the two ingredient pastes makes it helpful in defining the mechanical dimensions of the printed metallic lines, given that the height to width ratio (aspect ratio) plays a significant role in printed electrodes for device applications. Such tunable viscosity allows for a low cost and mass printing technique with a further decrease in material cost resulting from the partial replacement of Ag with MXene flakes.

[0031] The MXene-Ag paste 320 has been found to have very good compaction between the MXene flakes and Ag nanoparticles, allowing for easy printing of the final MXene-Ag paste using matured screen-printed technology or similar technologies. The paste has the ability to successfully print continuous lines with microscale resolution owing to the high rheological quality of the MXene-Ag paste. Moreover, due to the exceptional rheological properties of the pastes, printed lines with higher aspect ratios (height/width) and easily tunable thicknesses can be achieved. The process of forming the contacts 410 requires only a low-temperature annealing process (-200 °C), making it suitable for temperature-sensitive electronic/optoelectronic devices. The resistivity of the MXene-Ag paste 320 can be tuned according to the desired conductivity of the contacts of various electronic devices and hence can reduce the overall Ag usage. By adding up the MXene in weight percent for metal lines (e.g., L = 80 mm X W = 10 mm) with a screen-printer, according to an embodiment, it is possible to tune the electrical properties of the printed metal lines 506 for different applications. The change in wt% in the prepared MXene-Ag pastes has resulted in distinctive line and bulk resistivities, as shown in the table in Figure 7. The conductor resistance of the MXene-Ag paste 320 as a function of the Ag past blend ratio is illustrated in Figure 8. Thus, it is possible to select the appropriate Ag paste blend ratio of a line, electrode, or contact for a specific application, if the desired conductor resistance for that application is known, by using either the graph shown in Figure 8 and/or the data in the table in Figure 7. [0032] As discussed above, the MXene-Ag paste 320 may be used with several devices for renewable energies and semiconductor industries. For example, the paste 320 can be used to fabricate solar cells, lasers, sensors, photodetectors, transistors, electrochromic devices, smart windows, electromagnetic shielding, LEDs, capacitors, and batteries, where metal electrodes are part of such devices. The use of the paste 320 for solar cells is now discussed in more detail. Note that the paste 320 may be similarly used for the other applications noted above.

[0033] For the solar cells, the MXene-Ag paste 320 can be used to form electrodes and/or as a contact point between a substrate and electrodes. More specifically, the paste 320 may be used as the main ingredient in forming the electrodes for a silicon heterojunction (SHJ) solar cell and related technologies such as tandem solar cells, including those using perovskite and silicon subcells. SHJ solar cells require low temperature (200 °C) processing to prevent any kind of degradation. The MXene-Ag paste is best suited for this job as it can be annealed at such low temperatures. An example of a SHJ solar cell 900 is shown in Figure 9A, and this cell 900 is part of a solar module 910, as shown in Figure 9B. The solar module 910 includes plural solar cells 900, and one or more of these modules are shown in Figure 9B being installed on a structure 912.

[0034] The solar cell 900 combines two different technologies into one cell: a crystalline silicon cell 920 sandwiched between two layers of amorphous “thin-film” silicon (a-Si) 922 and 924. This allows an increase in the efficiency of the panels and more energy to be harvested when compared to conventional silicon solar panels. In one application, the layer 920 is made with crystalline silicon - either monocrystalline or polycrystalline, while amorphous silicon is used for the thin-films 922 and 924.

[0035] With heterojunction solar cells, a conventional crystalline silicon wafer has amorphous silicon deposited on its front and back surfaces. This results in a couple of layers of thin-films that absorb extra photons that would otherwise not get captured by the middle crystalline silicon wafer. The a-Si thin-layer 922 on top captures some sunlight before it arrives at the crystalline layer 920, and it also collects some sunlight that reflects off the layers below. Because it is very thin, it is capable of collecting much of the sunlight as it passes right through, and the sunlight that passes through the crystalline layer 920 is absorbed by the thin a-Si layer 924 underneath. For carrier collection transparent conductive oxide (TCO) as transport layer 926 on top of the a-Si thin film layer 922 and at the rear with a-Si 924 is needed, as shown in Figure 9A. [0036] Finally, one or more electrodes 506 are formed on the top surface

900A of the solar cell 900 to collect the corresponding electrical carriers. In one application, a grid pattern is formed on the top surface 900A, and this grid pattern may include plural fingers, 20 to 200 pm in width, placed 1 - 5 mm apart. The resistivity of silicon is too high to conduct away all the generated current, so a lower resistivity metal grid is placed on the surface to conduct away the current. The metal grid (electrodes or fingers 506) shades the cell from the incoming light, so there is a compromise between light collection and resistance of the metal grid. However, in this embodiment, the metal grid is formed by the MXene-Ag paste 320.

[0037] Similar or different electrodes 906 (also as a grid pattern) may also be formed on the bottom surface 900B of the solar cell to collect electrical carriers.

Electrodes 506, as discussed above, can be made exclusively with the MXene-Ag paste 320. While Figure 9A shows the entire electrode 506 being made of the paste 320, in one embodiment, it is possible to attach a metallic electrode, e.g., electrode 906, which is made of Al particles, with the paste 320, to the surface 900B of the solar cell 900, i.e. , the paste 320 is not the electrode, it only serves as a “glue” to attach the electrodes 906 to the semiconductor device 900. The paste 320 may be deposited on the surface of the solar cell by using, for example, a screen-printing process or similar processes.

[0038] For the solar module 910, plural solar cells 900 need to be electrically connected to each other. Thus, electrical contacts 930 that use the paste 320 may be employed to connect each solar cell to another and to the receiver of the produced current, as illustrated in Figure 9B. The contacts must be very thin (at least in the front) so as not to block the incoming sunlight to the cell.

[0039] Similarly, this paste can be used to contact/pattern lasers to enable ultrafast broadband operations. Moreover, the paste can be used as electrode materials for photodetectors or transistors. Two electrodes (anode and cathode), 506 and 1020 are required to contact the photodetectors (pn junction shown in Figure 10 made with semiconductor layers 1010 and 1012) based upon their structure (either vertical or lateral). These photodetectors 1000 are based upon photo-diodes, photomultiplication (or gain), and photo-conductors as schematically illustrated in Figure 10 or based on photo-transistors 1100 (three electrodes 506, 1020, and 1030 are required in this case to contact the gate layer 1012, source 1010 and drain 1014) as schematically illustrated in Figure 11 . The paste 320 can also be adapted for other classes of transistor families having three terminal contacts (gate, source, and drain), such as bipolar junctions (npn or pnp), metal-oxide semiconductor field effect transistors (MOSFET), or thin film transistors (TFT), etc.

[0040] The ability to print micropatterns and microcontacts from these pastes is highly applicable for various sensing applications such as biosensors, humidity sensors, gas sensors, etc. For large area applications, such printed contacts can serve as an electrode to drive the voltage, as in the case of smart windows, electrochromic devices, etc. Moreover, MXene-Ag pastes can also be used to fabricate LEDs as top and bottom electrodes to drive the LEDs with respect to the direction of the light illumination. Due to their superior conductivity, such MXene-Ag pastes can be used as excellent electrode material for electrochemical capacitors

(supercapacitors) and various kinds of batteries.

[0041] The application of the paste 320 to a tandem solar cell is now discussed with regard to Figure 12. Figure 12 shows perovskite-based tandem solar cell 1200 having an n-doped float-zone (FZ) Si wafer 1202 with a thickness of 260- 280 pm, which was used for the Si bottom cell fabrication. The Si wafer 1202 has a top surface 1202A, and a bottom surface 1202B and was processed to have doubleside textured top and bottom surfaces with randomly distributed top and bottom pyramids 1204A and 1204B, which was obtained by using an alkaline solution. In other words, the top and bottom surfaces of the Si wafer 1202 are not flat. Note that a flat-sided Si wafer may be used instead of the textured Si wafer to obtain the tandem solar cell. The size of the pyramids 1204A and 1204B is controlled by adjusting the alkaline concentration and the process temperature. The wafer 1202 was dipped into a hydrofluoric acid to remove any native oxide solution followed by a cleaning process before being transferred into a plasma-enhanced chemical vapour deposition (PECVD) cluster for a-Si deposition. Next, a 8 nm intrinsic (i) layer 1206 was grown on the top, and bottom faces of the wafer 1202, a 6 nm n-doped a-Si layer 1210 was grown on the top face, and a 13 nm p-doped a-Si layer 1208 was grown on the bottom face using the PECVD cluster tool, as illustrated in Figure 12. Then, a 150 nm indium tin oxide (ITO) layer 1212 and a 250 nm Ag layer 1214 were sputtered on the backside of the wafer 1202. A 15 nm ITO recombination layer 1218 and a 2-PACz layer 1220 were sputtered on the front side of the wafer 1202. In order to recover sputtering damage, an annealing step was performed at 200 °C for 10 min. The layers 1202/1206/1208 form a first pn junction on the bottom surface of the device.

[0042] The top pyramids 1204A of the Si wafer 1202 were subjected to UV- Ozone treatment for 15 min before being transferred into the glovebox for processing. One of two materials may then be deposited to act as a hole transport layer, Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) or [2-(9H-carbazol-9- yl)ethyl]phosphonic acid (2PACz). For the PTAA deposition, 2 mg/mL PTAA solution in anhydrous chlorobenzene (CB) was used. For 2PACz deposition, 1 mg/mL 2- PACz in ethanol was used. The PTAA or 2PACz material, which acts as the hole transport layer (HTL) 1220, was spin-coated (other processes may be used for this step) on the ITO-coated substrate 1202 at 5000 rpm for 50 s, followed by the step of drying at 100 °C for 10 min.

[0043] A perovskite precursor ink (any perovskite may be used) was then spin-coated at 2000 rpm for 50 s on the layer 1220, then followed with 7000 rpm for 10 s, to form the perovskite layer 1224. Chlorobenzene may be dropped in the center of the substrate 12 s before the end of the spin-coating process. After the rotation ceased, the substrate was immediately transferred onto a hotplate of 100 °C and was annealed for 15 min to form the perovskite layer 1224. The layers 1210/1224 form a second pn junction on the top surface of the device.

[0044] After the perovskite deposition, 1 nm LiF 1226 and 15 nm Ceo 1228 were subsequently deposited by thermal evaporation on the top of the solar cell. A 20 nm SnC>2 layer 1230 was then deposited by atomic layer deposition (ALD). The substrate 102 temperature was maintained at 100 °C during the ALD deposition with TDMASn precursor source at 80 °C and H2O source at 18 °C. A 70 nm indium zinc oxide (IZO) layer 1232 was sputtered on top of the SnC>2 layer 1230 through a shadow mask. The paste 320 was then screen printed over the IZO layer (or any other transparent conductive oxide) 1230 to form the desired shape of fingers of the future top electrode, with a thickness of about 350 nm, and was then annealed to make the top electrode 506. Finally, a 120 nm MgF2 layer 1236 was thermally evaporated on top of the IZO layer 1230 as an anti-reflection layer. The thicknesses of the LiF, Ceo, IZO, and MgF2 layers can vary from the values given herein by up to 20%.

[0045] A method for making MXene-Ag electrodes on the surface of a semiconductor device is now discussed with regard to Figure 13. The method includes a step 1300 of providing the semiconductor device, for example, solar cell, transistor, photodetector, laser device, etc. In step 1302, depending on the semiconductor device, the electrical conductivity of an electrode to be formed on the semiconductor device is determined, for example, based on a potential work that exists at the boundary between the semiconductor device and the future electrode. Based on the desired electrical conductivity, a percentage of the MXene paste 120 to the total MXene-Ag paste 320 is selected in step 1304, for example, based on the graph shown in Figure 8, and a corresponding paste is mixed up in step 1306. Then, in step 1308, stencil printing of the paste 320 on the semiconductor device is performed, and in step 1310, the printed paste is annealed at a given temperature, for example, 200 Celsius, for forming either an electrode or a metal contact. The stencil printing step may be replaced with similar methods that are used for adding the traditional Ag paste to such devices.

[0046] The disclosed embodiments provide an electrically conductive paste that uses less Ag than the existing ones for providing electrical contacts and/or electrodes for electronic devices. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications, and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0047] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0048] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

1. A MXene-Ag paste (320) for forming electrical contacts, the MXene-Ag paste (320) comprising:

Ag particles (402); glass frits (404); and

MXene flakes (110), wherein MXene is given by M_n+iX_nT_x with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen.

2. The paste of Claim 1 , wherein a mass of the MXene flakes is between 20 to 30% of a total mass of the paste.

3. The paste of Claim 1 , wherein the MXene flakes include TisC2T_x.

4. The paste of Claim 1 , wherein the MXene flakes, Ag particles, and the glass frits represent at least 99% of a total mass of the paste.

5. A solar cell (900) for transforming solar energy into electrical energy, the solar cell (900) comprising: a crystalline Si layer (920) having opposite first and second surfaces; a first-type amorphous Si layer (922) located on the first surface; an second-type amorphous Si layer (924) located on the second surface; a first electrode (506) formed on the first-type amorphous Si layer (922); and a second electrode (906) formed on the second-type amorphous Si layer (924), wherein the first electrode (506) includes Ag particles (402) and MXene flakes (110),

MXene is given by Mn+iXnT_x with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and T_x denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen, and wherein the first- and second-type corresponds to n- and p-type or p- and n- type.

6. The solar cell of Claim 5, wherein the first electrode further includes glass frits.

7. The solar cell of Claim 5, wherein a mass of the MXene flakes is between 20 and 30 % of a total mass of the first electrode.

8. The solar cell of Claim 5, wherein the MXene flakes include TisC2Tx.

9. The solar cell of Claim 5, wherein the entire first electrode is made of Ag particles, MXene flakes, and glass frits.

10. The solar cell of Claim 5, wherein the first electrode further includes glass frits, and the MXene flakes, Ag particles, and the glass frits represent at least 99% of a total mass of the first electrode.

11. The solar cell of Claim 5, wherein the second electrode does not include the MXene flakes, and the second electrode is attached to the solar cell with a paste that includes the MXene flakes and Ag particles.

12. A tandem solar cell (1200) comprising: an n-type base (1202) having first and second surfaces (1202A, 1202B), opposite to each other; a p-type layer (1208) formed on the second surface (1202B) of the n-type base (1202), wherein the p-type layer (1208) and the n-type base (1202) form a first pn junction; a perovskite layer (1224) formed on the first surface (1202A) of the n-type base (1202), wherein the perovskite layer (1224) and the n-type base (1202) form a second pn junction; a first electrode (506) formed on the perovskite layer (1224); and a second electrode (1214) formed on the p-type layer (1208), wherein the first electrode (506) includes Ag particles (402) and MXene flakes

(110), and MXene is given by Mn+iX_nT_x with n = 1 , 2, 3, or 4, where M represents early transition metals, X is carbon and/or nitrogen, and Tx denotes surface-terminated species including hydroxyl, fluorine, and/or oxygen.

13. The tandem solar cell of Claim 12, wherein the first electrode further includes glass frits.

14. The tandem solar cell of Claim 12, wherein a mass of the MXene flakes is between 20 and 30 % of a total mass of the first electrode.

15. The tandem solar cell of Claim 12, wherein the MXene flakes include TisC2Tx.

16. The tandem solar cell of Claim 12, wherein the entire first electrode is made of Ag particles, MXene flakes, and glass frits.

17. The tandem solar cell of Claim 12, wherein the first electrode further includes glass frits, and the MXene flakes, Ag particles, and the glass frits represent at least 99% of a total mass of first electrode.

18. The tandem solar cell of Claim 12, wherein the second electrode does not include the MXene flakes.

19. The tandem solar cell of Claim 12, wherein the first and second surfaces of the n-type base are structured to have corresponding pyramids.

20. The tandem solar cell of Claim 19, further comprising: a layer of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid, 2PACz, or a layer of Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, PTAA, formed between the n-type base and the perovskite layer.