Patent Application
20190148574
The present invention generally relates to superstrates and, more specifically, to superstrates for solar cell applications.
Photovoltaics refer to a class of methods for converting light into electricity using the photovoltaic effect. Due to technological advances in recent years, photovoltaics are becoming a more viable, carbon-free source of electricity generation. A photovoltaic system typically employs an array of solar cells to generate electrical power. Solar cells can be made of a variety of semiconductors, typically a silicon based structure, acting as a substrate and can include front and rear contacts that are used to conduct current out of the solar cell. The conversion process involves the absorption of light rays by what can be referred to as the active region of the solar cell, which can excite electrons in the substrate into a higher state of energy. The excitation allows the electrons to move as an electric current that can then be extracted to an external circuit and stored.
Superstrates incorporating effectively transparent contacts in accordance with various embodiments of the invention can be implemented in many different ways. One embodiment includes an optoelectronic device including a photoabsorbing surface and a polymer layer including a first surface and a second surface, wherein the first surface defines a plurality of triangular grooves filled with a conductive material, wherein the filled triangular grooves form three-dimensional contacts that includes at least one surface such that at least a portion of radiation incident on the surface is redirected onto the photoabsorbing surface.
In another embodiment, the photoabsorbing surface includes a material that is one of: a III-V material, GaAs, CdTe, GICS, perovskite, and silicon. III-V material, GaAs, CdTe, GICS, perovskite, and silicon.
In a further embodiment, the optoelectronic device further includes a plurality of existing metallic contacts on the photoabsorbing surface.
In still another embodiment, the optoelectronic device further includes solder material in contact with at least one of the existing contacts and the conductive material of at least one of the plurality of triangular grooves.
In a still further embodiment, the optoelectronic device further includes a layer of transparent conductive oxide in contact with the photoabsorbing surface and the polymer layer.
In yet another embodiment, the layer of transparent conductive oxide includes a transparent conductive oxide material that is one of: indium tin oxide and fluorine doped tin oxide.
In a yet further embodiment, the layer of transparent conductive oxide has a thickness of less than 200 nm.
In another additional embodiment, the polymer layer includes a material that is one of: ethylene-vinyl acetate, polydimethylsiloxane, polyurethane, and polymethylmethacrylate.
In a further additional embodiment, the conductive material includes silver nanoparticle ink.
In another embodiment again, the conductive material is a composite including a triangular core in contact with at least two reflective surfaces.
In a further embodiment again, at least one of the plurality of triangular grooves have a height-to-width aspect ratio of at least 2:1.
In still yet another embodiment, at least one of the plurality of triangular grooves have a height of approximately 15 μm and a width of approximately 5 μm.
In a still yet further embodiment, the plurality of triangular grooves is in a grid pattern.
In still another additional embodiment, the polymer layer has a thickness of less than 500 μm.
In a still further additional embodiment, the optoelectronic device further includes a sub silicon solar cell.
In still another embodiment again, the optoelectronic device further includes a lamination layer in contact with the second surface of the polymer layer.
A still further embodiment again includes a method of manufacturing a superstrate integrated with an optoelectronic device, the method including providing a layer of transparent polymer, forming a plurality of grooves within the layer of transparent polymer, infilling the plurality of grooves with a conductive material, and integrating the layer of transparent polymer with an optoelectronic device.
In yet another additional embodiment, the plurality of grooves is infilled using an electroplating process.
In a yet further additional embodiment, the optoelectronic device includes a layer of transparent conductive oxide and the layer of transparent polymer is in contact with the layer of transparent conductive oxide after integration with the optoelectronic device.
A yet another embodiment again includes an optoelectronic device including a photoabsorbing surface including perovskite, a layer of polydimethylsiloxane in contact with the photoabsorbing surface, and a layer of indium tin oxide in contact with the photoabsorbing surface and the layer of polydimethylsiloxane, wherein the layer of polydimethylsiloxane includes a first surface and a second surface, the first surface defines a plurality of triangular grooves filled with silver nanoparticle ink, and at least one of the plurality of triangular grooves have a cross-section with a height-to-width ratio of at least 2:1.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.
The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention.
Turning now to the drawings, effectively transparent and highly conductive superstrates for optoelectronic applications are illustrated. Superstrates in accordance with various embodiments of the invention can be utilized for a variety of applications. For example, in solar cell applications, a superstrate incorporating effectively transparent contacts (“ETCs”) can be implemented to boost solar cell power output. In conventional solar cells, metal contacts are typically required for charge extraction from solar cells. Traditional screen-printed metal contact grids typically cover up to ˜5% of the cell front surface, blocking sunlight from reaching the photovoltaic absorber below. These shading losses are among the largest causes of performance loss in most solar cells. For certain solar cells, another major optical loss mechanism emerges from the transparent conductive oxide (“TCO”) needed to provide low loss lateral charge transport. These TCOs can exhibit parasitic absorption that leads to significant loss in current density. Furthermore, in large-scale devices, the high photocurrents can require metal grid fingers in order to achieve low resistance. These metal fingers can lead to further optical losses due to geometric shading.
Superstrates containing ETCs (“ETC superstrates”) in accordance with various embodiments of the invention can be implemented to reduce optical losses by decreasing the thickness of the TCO and by reducing or eliminating shading losses of metal grid fingers. The superstrates can incorporate effectively transparent contacts (“ETCs”) that enable a significant reduction in the TCO thickness required for current extraction with a high fill factor. By reducing the thickness of the TCO layer in solar cells, the short circuit current density can be enhanced by more than 1 mA/cm2 due to decreased parasitic absorption and optimized antireflection properties. However, in order to provide low-loss lateral charge transport, decreased TCO thickness requires the introduction of metal grid fingers. Effectively transparent contacts in accordance with various embodiments of the invention are microscale fingers capable of redirecting incoming light towards the active area of the solar cells. As such, the contacts can be placed closely together without introducing shading losses. In some embodiments, integrating ETCs in solar cell superstrates can lead to high conductivity (<5 Ω/sq sheet resistance). For some applications, such as in the case of perovskite solar cells, the absorption within the active layer can even exceed the absorption of cells without metal grids due to light trapping. As discussed above, ETCs in accordance with various embodiments of the invention can also be utilized to reduce shading losses. In many embodiments, the ETCs are triangular shaped, high-aspect-ratio silver gridlines. When sunlight impinges the ETC gridlines, the incident rays can be efficiently reflected towards the active area of the solar cell, rather than being reflected away and lost. Such techniques can substantially reduce and/or eliminate the shading loss problem and boosts the solar cell power output. In some embodiments, solar cell power output can be increased by ˜5%. In a number of embodiments, ETCs can achieve effective optical transparency of greater than 99% even at relatively dense grid spacing and over a wide range of angles of incidence. In many cases, the integration of ETCs can allow for a transparency of ˜99.9%.
Superstrates containing ETCs can be constructed in many different ways. In many embodiments, ETC superstrates include a transparent material with grooves, which can be infilled with reflective, conductive material(s) such as but not limited to silver and aluminum. In further embodiments, the grooves are triangular-shaped. In some embodiments, the ETC superstrate incorporates one or more transparent conductors on the side of the ETCs, such as but not limited to TCOs such as indium tin oxide (“ITO”), conductive polymers such as PEDOT:PSS, nanowire meshes such as silver nanowires. ETC superstrates can also incorporate transparent layers on the side opposite the ETCs, such as but not limited to glass layers and antireflective coatings The ETC superstrate can further incorporate other elements of benefit depending on the given application. For example, in solar cell applications, bus bars or tabbing pads can also be incorporated. Additionally, features to aid in attaching the ETC superstrate to the solar cell, such as but not limited to voids and indentations can be incorporated to provide clearance around bus bars or tabbing areas. ETC superstrates in accordance with various embodiments of the invention can be incorporated in many different applications. In many embodiments, the ETC superstrate can be applied to the front surface of a solar cell, such as but not limited to a crystalline Si and a III-V solar cell, by aligning the ETC gridlines with existing conventional gridline contacts on the solar cell. The composite can then be laminated or mechanically pressed. Some types of solar cells, such as amorphous heterojunction Si cells, may have a transparent conductive layer, such as an ITO layer, as the existing contact surface instead of conventional gridlines. In such cases, the ETC superstrate can be aligned using another point of reference. For example, the alignment can be performed by centering the ETC superstrate on the solar cell. In either case, the ETC superstrate can incorporate pre-treated surfaces, adhesives, and/or solder pastes to enable reliable mechanical attachment between the superstrate and the solar cell and/or to enable reliable electrical contact between the ETC gridlines and the solar cell's existing contacts during the lamination process. In the case of bifacial solar cells, which receive sunlight from either or both sides, ETC superstrates can, in the manner described above or any other method, be applied to either or both sides of the solar cell. As can readily be appreciated, the specific manner in incorporating ETC superstrates can depend on the specific application. For example, some embodiments include a solar cell with a CdTe layer that is deposited via vapor transport deposition (“VTD”), a process that occurs at temperatures around 400 C. In such embodiments, the ETC superstrate can be applied to the front surface of the solar cell, rather than as a platform onto which the CdTe is deposited via VTD since the ETC Superstrate cannot withstand the high temperatures at which the VTD process takes place.
In many embodiment, the ETC superstrate can be used as a platform onto which additional layers are deposited in order to fabricate the solar cell. For example, a perovskite solar cell can be fabricated on an ETC superstrate having an ITO-coated surface by sequentially depositing: a hole transport layer (“HTL”) such as but not limited to nickel oxide, a perovskite layer such as but not limited to methylammonium lead iodide, an electron transport layer (“ETL”) such as but not limited to PCBM, and a back contact electrode such as but not limited to silver. Such layers can be deposited by a variety of methods including but not limited to solution processing, spin coating, doctor blading, slot die casting, spraying, spray pyrolysis, vacuum deposition, evaporation, sputtering, and/or atomic layer deposition. Additionally, other types of thin-film solar cells can be deposited onto ETC superstrates, including but not limited to CdTe, CIGS, organic, dye-sensitized, and tandem combinations of thin-film solar cells. Further discussions of superstrates, computational simulations, and related methods of integration and fabrication in accordance with various embodiments of the invention can be found in the sections below.
The above described approach constitutes an effective and scalable way of enhancing the short-circuit current density in perovskite solar cells and incorporates materials that are widely used in the photovoltaic industry. Using such techniques, the area fraction devoted to macroscopic grid fingers and busbars can be further reduced on large-scale solar cells and modules as compared with conventional designs. Furthermore, ETC superstrates may find application in thin film tandem solar cell architectures as well as in other optoelectronic devices. In addition, ETC superstrates can be fabricated as thin and lightweight membranes that are particularly interesting for space, aviation, and mobile applications. In many embodiments, sol-gel ETC membranes with a thickness of 40-80 μm and a specific weight of 2.5±0.1 mg/cm2 can be implemented. Compared to a standard glass substrate, such ETC membranes can be 1000 times lighter. As can readily be appreciated, any of a variety of different membrane materials, such as but not limited to PDMS and space resistant polymers, can be utilized.
Certain types of solar cells contain existing metal contacts on their front surfaces. Such conventional, flat metal contacts can cause shading optical losses. ETC superstrates in accordance with various embodiments of the invention can be implemented to address these shading losses and to improve the electrical conductivity of the front grid. ETC superstrates can be formed in many different configurations. In many embodiments, the ETC superstrate includes a polymer layer with embedded effectively transparent contacts. The polymer layer can be made of various materials, including but not limited to PDMS, ethylene-vinyl acetate (“EVA”), polyurethane, polymethylmethacrylate (“PMMA”), or any other suitable polymer and materials. Effectively transparent contacts can also be fabricated and implemented in many different ways. In some embodiments, the ETCs are micro-scale metal contacts. In further embodiments, the ETCs are silver contacts. ETCs can also be implemented in a variety of different shapes. In a number of embodiments, the ETCs have triangular cross-sections. By incorporating ETCs in the polymer, charge conduction can be enabled while maintaining the optical transparency of the polymer layer.
ETC superstrates can be implemented as a drop-in replacement for III-V, thin-film, and silicon solar cell manufacturers by either replacing the conventional EVA/PDMS encapsulant material or replacing the transparent conductive oxide layer used for thin-film solar cell technologies that leads to unwanted parasitic absorption. Such ETC superstrates can be constructed to be compatible with current solar cell manufacturing methods, allowing for the integration of the ETC superstrates in the existing production line of solar cell manufacturers. The strength of the ETC superstrate is that it can provide 99.9% optical transparency by incorporating ETCs that redirect the incoming sunlight. In addition, the ETCs can be electrically connected to the existing contacts on the solar cell, which improves the finger resistance and sheet resistance. This allows for optimized grid-layouts, optimized doping levels, and new cell designs that can maximize the use of ETC superstrates. For example, by placing the ETCs close together, the doping levels in the emitter layer can change, reducing the parasitic absorption.
As the industry moves toward finer metal grid lines, higher aspect ratio metal contacts can be needed to provide sufficient charge extraction, which scales with the cross-section of the metal contact. However, just reducing the finger line width will not be sufficient to boost the efficiency of solar cells since the sheet resistance will also increase. By incorporating ETCs, finer line widths can be enabled, reducing or eliminating shading losses while maintaining a sufficient electrical charge extraction since the ETC can be designed to have aspect ratio that is a factor of 3-6× larger than the conventional screen-printed or flat metal contacts.
ETC superstrates in accordance with various embodiments of the invention can be fabricated in many different ways. In some embodiments, machining of a master mold and then soft-imprinting polymer replication processes are utilized to form the polymer layer of the ETC superstrate.
The silicon master mold 202 can be used to make a copy via a two-step copying process. First, a negative mold 204 can be formed from the master mold by casting a forming a suitable material, such as PDMS, against the master mold. From the negative copy, a positive imprint 206 can then be formed by casting or forming the ETC superstrate polymer material against the negative mold 204. Such copies can be formed in a number of different ways using different materials. In many embodiments, uncured PDMS resin can be applied on the surface of soda-lime glass, and then the desired mold can be pressed into the PDMS layer such that, after curing, the thin PDMS layer features triangular cross-section grooves. The thickness of the PDMS layer can vary depending on the specific requirements of a given application. In some embodiments, the PDMS layer has a thickness of approximately 40 μm outside of the grooves. The positive imprint 206 can then be infilled 208 and encapsulated or integrated 210 for various purposes. Although
Instead of working with a negative master, a positive master can be fabricated with upstanding triangular features on the silicon substrate. Master molds with triangular cross-section lines can be prepared with a variety of different methods. In many embodiments, the positive master is fabricated using a two-photon lithography process. In such embodiments, the copying process no longer includes the negative copy step. In the illustrative embodiment, the copy is made of EVA. However, any of a number of different materials can also be utilized. Such materials can include but not limited to PDMS, polyurethane, PMMA, PET, and various suitable polymers.
In many embodiments, the fabrication process can include a step where one polymer layer is used as a mold or tool to cast or emboss another polymer layer of the same material. However, it can be critical that the polymer layers do not adhere to one another. In such embodiments, a surface treatment step can be performed to prevent unwanted adhesion. Different surface treatments can be used depending on the type of polymers used. For example, in embodiments including a PDMS copy, the PDMS stamp can be surface functionalized the with an oxygen plasma treatment and subsequent coating with a (3-(N-Ethylamino)isobutyl)trimethoxysilane (4 wt % in methanol, Gelest SIE4886.0) self-assembled monolayer. In EVA-PDMS steps, the surface can be treated with oxygen plasma followed by a TEFLON™ or silane treatment.
After the polymer layer of the superstrate is fabricated, the grooves can be filled with a conductive ink, such as but not limited to silver nanoparticle ink, to form the contacts. In many embodiments, the contacts are formed with aluminum. In some embodiments, the contacts are formed with a core and outer layer. In further embodiments, the core is formed with copper. As can readily be appreciated, various materials can be utilized to form ETCs, and the choice of which can depend on the specific requirements of a given application. After the infilling step, a curing step can be performed. In some embodiments, a two-step curing process is performed. First, the solvent from the ink can be removed using a variety of different methods, including but not limited to vacuum treatment, annealing, applying a voltage, HCl, and photocuring. Afterwards, a secondary curing step can be performed in order to reduce the resistance of the conductive ink and provide a sufficient sheet resistance of the ETC superstrate. The secondary curing step can be performed with any of the methods mentioned above.
The filling process can be achieved using any of a number of different methods. For example, the process shown in
Depending on the surface energy of the polymer material utilized, the filling profile of the ink can differ. In many embodiments, an oxygen plasma surface treatment step can be performed to improve surface wetting. In a number of embodiments, the oxygen plasma treatment was conducted for approximately 36 seconds. In some embodiments, capillary action was utilized in order to prevent ink spilling outside of the groves. The oxygen plasma treatment can render the polymer surface hydrophilic to facilitate the filling process. Such properties can allow for a capillary flow of more than 1 cm from one side and therefore a distance of more than 2 cm in between the ink infilling area. This length scale can be comparable with the distance of busbars in macroscopic devices. Changes to the channel geometry and surface treatment can result in capillary flow over longer or shorter distances.
Due to the strong wetting of ink inside the grooves, a concave ink surface typically forms. This parabolic profile can depend on the different surface energies of the materials used and on the type of filling techniques used.
Another method for changing the filling profile includes a secondary filling step. In such embodiments, the capillary flow process to infill the grooves is repeated twice. First, the grooves are infilled with a conductive ink via capillary flow. The conductive ink can then be cured to remove solvent from the ink to prevent the ink from changing shape. Afterwards, the filling step is repeated, allowing for filling profile to change. In many embodiments, the second filling step is utilized to completely fill the grooves, allowing the formed ETCs to form a good electrical contact with the layer that on which the superstrate will be attached, or providing a suitably smooth surface onto which deposit layers of a solar cell.
The need for changing the filling profile due to parabolic filling profiles can be circumvented by utilizing different filling methods. One such method includes the use of a removable surface. By closing the triangular shaped channel with a removable surface, the capillary flow can be enhanced. The entire channel can also be filled to prevent the formation of a parabolic filling profile.
Another alternative method for infilling the grooves includes the use of electroplating. In many embodiments, electroplating can be used the core of a core-outer layer ETC construction. In such embodiments, an outer layer can first be formed using capillary flow, and electroplating can be used to form the core. Using this fabrication process, the outer layer can be formed with a material, such as silver, that results in a highly reflective surface (allowing for the ETCs to redirect incoming light and attaining high optical transparency), and the core can be formed with a good conductive material, such as copper. The core material can also be selected to provide support for the structure, allowing for a higher tolerance of mechanical stress and the ability to retain the structure's original shape. The composite can be more inexpensive to manufacture compared to an ETC made entirely of silver while maintaining sufficient performance standards. As can readily be appreciated, any of a variety of materials can be plated and utilized with the electroplating process.
Another alternative method for infilling of the grooves is depicted in
A variation on the approach described above is shown in
Another alternative method for infilling of the grooves includes the use of pressure to distribute the conductive ink throughout the grooves. Substrates used in this filling process can be prepared with larger busbar areas that can be infilled with ink. Afterwards, pressure can be applied to the softer busbar areas on the side of the ETC superstrate. The applied pressure can cause the ink to flow throughout the micro-channel. Such a process is conceptually illustrated in
Another approach for the infilling of the grooves includes processing the polymer such that the surfaces of the grooves is hydrophilic while the remaining surfaces of the polymer is hydrophobic. This can be achieved using various techniques known in the art. After the polymer is processed, ink can be deposited to cover and fill at least the grooves. Any of a variety of deposition techniques can be used, including but not limited to inkjetting and spraying. The ink on the remaining surfaces of the polymer can then be removed using any of a variety of different techniques, such as but not limited to doctor blading processes.
ETC superstrates in accordance with various embodiments of the invention can be formed as a multilayered composite. In many embodiments, a thin layer superstrate with embedded ETCs is constructed on top of a second layer. In some embodiments, the superstrate layer is formed with PDMS while the second layer is formed with EVA. In such embodiments, the PDMS allows for a high annealing temperature in order to reduce the resistance of the conductive ink while the EVA layer allows the superstrates to be manufactured at a lower cost. In such embodiments, the superstrate layer can be formed to be as thin as possible to further reduce cost. As can readily be appreciated, the materials used for the two layers can vary and can depend on the specific requirements of a given application.
In order to make the ETC superstrate compatible with certain solar cell grid designs, a busbar can be integrated into the ETC superstrate. In many embodiments, a single-conductor busbar layout is used. In other embodiments, a multi-conductor busbar layout is used. The busbar conductor(s) can be of any shape, including triangular shaped, such that light incident on the busbar from vertically above the solar cell is reflected directly at the solar cell surface, in the same manner as described for the ETC gridlines.
For a sufficient single-conductor triangular busbar, the size of the busbar will typically be larger than the size of individual ETC gridlines, owing to the greater current carried by the busbar; however, the width of the busbar will typically be smaller than traditional busbars, owing to the high aspect ratio of the triangular bus bar. In some embodiments, the size of the busbar is on the order of several 100s of micrometers. In further embodiments, the busbars are on the scale of 100-300 micrometers in width and 300-900 micrometers in height. Because of the size discrepancy between the smaller ETC conductors and the larger busbar conductors, in some embodiments, the two types of conductors can be fabricated by different methods, for example, the ETC conductors can be fabricated using the capillary silver ink inflow process described above, while the triangular busbar conductor may be fabricated by pressing a triangular-shaped bar of silver-plated copper into the superstrate groove.
A multi-conductor busbar layout permits the use of smaller busbar conductors, including the case of triangular busbar conductors having similar approximate dimensions as the ETC gridlines. Examples of a single-conductor busbar and a multi-conductor busbar ETC grid pattern in accordance with various embodiments of the invention are conceptually illustrated in
In many embodiments, additional layers and/or materials can be added to the superstrate to change the surface that will be integrated with other structures. In some embodiments, a soldering layer is added to the ETCs.
In addition to the methods described above, ETC superstrates in accordance with various embodiments of the invention can be fabricated using techniques compatible with large-scale fabrication. In many embodiments, the fabrication includes the mechanically removing material from a polymer sheet to form triangular grooves. The removal process can be achieved in a variety of different ways. In some embodiments, the removal process is performed using laser(s). In other embodiments, a diamond scribe is used to remove the material. In these embodiments, the process can be performed over a large area, allowing for high throughput manufacturing.
ETC superstrates in accordance with various embodiments of the invention can be implemented in a variety of solar cell applications. In many embodiments, the ETC superstrate is integrated with a perovskite solar cell.
In many embodiments, the ETC superstrate is integrated with a solar cell having existing contacts. In such cases, the integration process can include an alignment step. Alignment can be performed using several methods. In some embodiments, an alignment arm, which can be computer controlled, is used to position the ETC superstrate to align with the solar cell. In other embodiments, the alignment system positions the solar cell to align with the ETC superstrate. After the integration of individual solar cells with TC superstrates, a lamination layer can be added to hold the modules together while providing protection from environmental disturbances.
ETC superstrates in accordance with various embodiments of the invention can be formed with a layer of TCO or transparent conductors in general.
In addition to the integration of ETC superstrates as a replacement for the top contact/encapsulant material, ETC superstrates can also serve as a starting material on which a solar cell can be grown. In many embodiments, the ETC superstrate can be formed on top of the front glass of the solar module. For example, for a perovskite solar cell, the ETC superstrate can be placed on top of a glass substrate, and the perovskite solar cell stack can then be integrated on top of the superstrate.
In embodiments where the ETC superstrate is integrated with a silicon PERC solar cell, the ETCs can form an electrical contact with the existing contacts on the solar cell. In such cases, the parabolic filling profile as discussed above can be beneficial since the space created by the parabolic curve can be filled with the existing contact.
In some embodiments, the ETC superstrate is integrated in a tandem solar cell architecture.
In many embodiments, the ETC superstrate can be integrated such that only the ETCs remain. In such embodiments, the ETC superstrate is first placed on top of the solar cell. A polymer removal process, such as through the use of a solvent or the application of heat, can be performed to remove the polymer layer, leaving behind the ETCs.
In addition to the many different ways ETCs can be integrated with various applications, these integration schemes can occur during the module manufacturing process. As discussed above, ETC superstrates can serve as a replacement for EVA module encapsulation. In such cases, the ETC superstrate simply replaces the encapsulant during the solar module manufacturing process. In some embodiments, the ETC superstrate is introduced in an intermediate step prior to module encapsulation. ETC superstrates can be implemented to align individually to solar cells during or after tabbing. In some embodiments, ETC superstrates can be implemented in a whole-module encapsulation process. In a number of embodiments, module-size superstrates can be aligned with busbar-less solar cells in a shingled configuration and laminated in a single step.
Wavelength-dependent transmission and reflection measurements of ETC superstrates can be performed using a chopped and monochromated white light source. Transmitted and reflected light can be directed onto a photodiode connected to a lock-in amplifier. The direct photodiode signal can be used as reference for the measurements.
Computational optical simulations and experiments can be performed to investigate the effects of decreased indium tin oxide thickness on the external quantum efficiency of perovskite solar cells. In many embodiments, simulations and experiments were performed on solar cells with a soda-lime glass superstrate, ITO with different thicknesses, 10 nm NiO, 375 nm Formamidinium cesium lead iodide (FACsPbI3) perovskite, 10 nm phenyl-C61-butyric acid methyl ester (PCBM), and 300 nm silver. Optical simulations can be performed using appropriate software, such as PV Lighthouse's Module Ray Tracer. Complex refraction indices of the individual materials can be obtained by ellipsometry or by transmission/reflection measurements. The wavelength-dependent absorption within the perovskite layer can be simulated and weighted with the AM 1.5G solar spectrum. Integrating over the wavelength can lead to the generated photocurrent density.
In many embodiments, the internal quantum efficiency of the device is assumed constant over the whole wavelength regime.
The external quantum efficiency (“EQE”) and the reflection can also be measured on the above-described FACsPbI3 perovskite solar cells with different ITO thicknesses.
Between 350 nm and 400 nm, the discrepancy can be explained by the underestimated reflection loss. Furthermore, it is likely that an internal quantum efficiency lower than 1 is causing the difference in other wavelength regimes.
Optical simulations can be performed in order to quantify the effective transparency and light trapping properties of ETC superstrates depending on the geometry of the ETCs. Similar simulation software and materials as described in the previous section can be used. In many embodiments, triangular cross-section silver lines were added within the glass superstrate. In further embodiments, a 60 nm thin layer of ITO1 was used in addition. The light absorption within the perovskite layer can be determined and weighted with the solar spectrum.
Furthermore, the sheet resistance of superstrates with the same ETC geometries as used for the optical studies can be calculated. An ink conductivity of 6 μΩ cm can be used. A conductivity of 195 μΩ·cm was measured for ITO1, which at 60 nm thickness corresponds to a sheet resistance (RS_ITO) of 32.5 Ω/sq. The ETC sheet resistance (RS_ETC) can be calculated as described previously, and the overall sheet resistance (RS) can be determined by: RS=(RS_ETC×RS_ITO)/(RS_ETC+RS_ITO).
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. It is therefore to be understood that the present invention may be practiced in ways other than specifically described, without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
The current application claims the benefit of and priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/586,591 entitled “Transparent, Conductive and Lightweight Superstrates for Perovskite, Thin Film and Tandem Solar Cells,” filed Nov. 15, 2017 and U.S. Provisional Patent Application No. 62/742,069 entitled “Transparent, Conductive and Lightweight Superstrates for Perovskite, Thin Film and Tandem Solar Cells,” filed Oct. 5, 2018. The disclosures of U.S. Provisional Patent Application Nos. 62/586,591 and 62/742,069 are hereby incorporated by reference in their entireties for all purposes.
This invention was made with government support under Grant No. DE-EE0004946/T-114930 awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62586591 | Nov 2017 | US | |
| 62742069 | Oct 2018 | US |
