Solar cells, methods for their preparation and uses thereof

Abstract

The present disclosure relates, for example, to solar cells comprising an absorber layer and a surface passivation layer bearing nanometer-scale point contact openings on a surface of the absorber layer, to methods for their preparation and uses thereof, for example, in tandem solar cells. The absorber layer may, for example comprise CuIn(S,Se)2 and may, for example, be deposited via a method comprising a solution-based process.

Description

SOLAR CELLS, METHODS FOR THEIR PREPARATION AND USES THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of priority from co-pending U.S. provisional application no. 63 / 538,616 filed on September 15, 2023, the contents of which are incorporated herein by reference in their entirety.FIELD

[0002] The present disclosure relates, for example, to solar cells comprising an absorber layer and a surface passivation layer bearing nanometer-scale point contact openings on a surface of the absorber layer, to methods for their preparation and uses thereof, for example, in tandem solar cells. The absorber layer may, for example comprise CuIn(S,Se)2 and may, for example, be deposited via a method comprising a solution-based process.BACKGROUND

[0003] Photovoltaic (PV) technologies can convert abundant solar energy into electricity without inducing a substantial environmental burden. For the continued deployment of PV technologies on a global scale, low manufacturing costs and enhanced power conversion efficiencies (PCEs) are urgently needed. For the latter, stacking solar cells with cascading bandgaps (Eg) presents the prospect of overcoming the single-junction Shockley-Queisser efficiency limit of about 29.4% (Richter et al., 2013). Theoretically, PCEs of about 46% can be achieved by two-junction tandem devices when the rear, and front sub cell exhibit bandgaps of 0.94 eV and 1.6 eV, respectively (De Vos, 1980). Here, for the rear cell, the well- established chalcogenide thin-film CuIn(S,Se)2 (CISSe) PV technology offers tunable bandgaps (from 1.0 to 1.53 eV by adjusting the [Se] / [S]+[Se] ratios), low carbon footprint, and device efficiencies up to 19.2% (vacuum processed) (Feurer et al., 2019). Notably, CISSe PV films can be prepared by solution processing methods that disruptively reduce capital equipment expense and energy consumed per square meter of absorber deposited (Suresh & Uhl, 2021). When integrated with wide bandgap, solution processable perovskite solar cell technologies, all thin-film perovskite / CISSe tandem devices - attractive, for example, as they can be manufactured on thin, lightweight, and flexible substrates in a cost-effective manner - promise a path towards high-performance photo voltaics at affordable costs.

[0004] Compared to a standard absorber film of about 2 pm, the use of submicron CISSe absorber layers further increases the production throughput (decreases deposition duration),reduces material use (e.g. In, and Ga), and lowers manufacturing costs (Vermang et al., 2013). Additionally, using submicron thick absorber fdms increases the mechanical flexibility of the resulting solar cell device, allowing for integration in a broad range of consumer-oriented applications including building integrated PV and wearable electronics. However, issues surrounding a highly recombinative rear interface (increased absorber surface-to-volume ratio), and reduced solar spectrum utilization (i.e. thinner absorber films combined with a mediocre reflective Mo back contact), limit device PCEs to less than 10% (Vermang et al., 2013). Moreover, the thickness of submicron absorber films is in the same order of magnitude as the minority carrier diffusion length, causing the photo-generated carriers to be more exposed to the highly recombinative rear interface (Lopes et al., 2023).

[0005] Record PCEs of about 26.8% have been reported for c-Si solar cells, attributed to innovations such as surface and contact passivation - that is, by reducing recombination of charge carriers (Green et al, 2023). Similarly, for chalcogenide solar cells, rear surface recombination losses can be mitigated by the application of an AI2O3 surface passivation layer at the Mo / CISSe rear interface. For a p-type CISSe absorber film, AI2O3 has been shown to reduce rear interface recombination by chemical (reducing the interface trap density) and field-effect passivation (high density of negative charges repel and reduce the surface minority charge carrier concentration) (Kotipalli et al., 2015).SUMMARY

[0006] The present disclosure includes a method of preparing a solar cell, the method comprising: depositing an absorber layer on a surface passivation layer bearing nanometerscale point contact openings, the surface passivation layer being positioned between the absorber layer and a back contact layer of the solar cell, the back contact layer being in communication with a substrate layer of the solar cell; depositing a buffer layer on the absorber layer; and depositing a transparent contact layer on the buffer layer.

[0007] In an embodiment, the absorber layer comprises CuIn(S,Se)2.

[0008] In an embodiment, the absorber layer is deposited via a method comprising a solution-based process.

[0009] In an embodiment, the absorber layer has a thickness of less than 1 pm.

[0010] In an embodiment, the surface passivation layer is prepared by a method comprising: depositing a layer comprising nanoparticles on the back contact layer;depositing a layer comprising a surface passivation material on the layer comprising the nanoparticles, thereby forming a stack; and removing the nanoparticles from the surface of the stack, thereby forming the surface passivation layer bearing nanometer-scale point contact openings.

[0011] In an embodiment, the nanoparticles comprise cadmium sulfide, zinc sulfide, zinc oxide, molybdenum metal or silver metal. In another embodiment, the nanoparticles comprise cadmium sulfide. In a further embodiment, the layer comprising the nanoparticles is deposited via a method comprising chemical bath deposition. In another embodiment, the nanoparticles are removed from the surface of the stack via a method comprising agitating the stack.

[0012] In an embodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm.

[0013] In an embodiment, the surface passivation material comprises a metal oxide. In another embodiment, the metal oxide comprises AI2O3, HfOx, GazCh, SiCh, TiCh or combinations thereof. In a further embodiment, the metal oxide comprises AI2O3.

[0014] In an embodiment, the layer comprising the surface passivation material is deposited by a method comprising atomic layer deposition.

[0015] In an embodiment, the layer comprising the surface passivation material has a thickness of about 2 nm to about 10 nm.

[0016] In an embodiment, the absorber layer comprises CuIn(S,Se)2 and the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

[0017] In an embodiment, the preparation of the molecular ink further comprises addition of a source of sodium ions to the solvent. In another embodiment, the source of sodium ions comprises NaCl. In an embodiment, the source of sulfide comprises thiourea, the source ofcopper comprises copper (II) chloride, the source of indium comprises indium (III) chloride and the solvent comprises dimethylformamide.

[0018] In an embodiment, the method further comprises etching the absorber layer prior to depositing the buffer layer.

[0019] In an embodiment, the buffer layer comprises a metal sulfide. In another embodiment, the metal sulfide is cadmium sulfide. In a further embodiment, the metal sulfide is deposited on the absorber layer by a method comprising chemical bath deposition.

[0020] In an embodiment, the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer.

[0021] In an embodiment, the method comprises depositing the intrinsic zinc oxide layer on the buffer layer by a method comprising sputtering and depositing the indium tin oxide layer on the intrinsic zinc oxide layer by a method comprising sputtering.

[0022] In an embodiment, the substrate layer comprises soda lime glass and the back contact layer comprises molybdenum.

[0023] In an embodiment, the method further comprises depositing a current collection grid on the transparent contact layer.

[0024] In an embodiment, the method further comprises scribing the deposited layers down to the substrate layer.

[0025] The present disclosure also includes a solar cell prepared by a method of preparing a solar cell of the present disclosure.

[0026] The present disclosure also includes a solar cell comprising a surface passivation layer bearing nanometer-scale point contact openings prepared by a method comprising depositing a layer comprising nanoparticles as described herein; and an absorber layer prepared by a method comprising a solution-based process as described herein, for example, wherein the absorber layer comprises CuIn(S,Se)2, and the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

[0027] The present disclosure also includes a solar cell comprising: an absorber layer; and a surface passivation layer bearing nanometer-scale point contact openings on a surface of the absorber layer.

[0028] In an embodiment, the surface passivation layer is on the rear surface of the absorber layer.

[0029] In an embodiment, the absorber layer has been deposited via a method comprising a solution-based process.

[0030] In an embodiment, the absorber layer comprises CuIn(S,Se)2 and the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

[0031] In an embodiment, the preparation of the molecular ink further comprises addition of a source of sodium ions to the solvent. In another embodiment, the source of sodium ions comprises NaCl. In an embodiment, the source of sulfide comprises thiourea, the source of copper comprises copper (II) chloride, the source of indium comprises indium (III) chloride and the solvent comprises dimethylformamide.

[0032] In an embodiment, the absorber layer comprises CuIn(S,Se)2.

[0033] In an embodiment, the CuIn(S,Se)2 has a [Cu] / [In] ratio of about 1.0.

[0034] In an embodiment, the absorber layer is doped with sodium ions.

[0035] In an embodiment, the absorber layer has a thickness of less than 1 pm.

[0036] In an embodiment, the surface passivation layer comprises a metal oxide. In another embodiment, the metal oxide comprises AI2O3, HfOx, Ga2Os, SiCh, TiCh or combinations thereof. In a further embodiment, the metal oxide comprises AI2O3.

[0037] In an embodiment, the surface passivation layer has a thickness of about 2 nm to about 10 nm.

[0038] In an embodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm.

[0039] In an embodiment, the solar cell comprises: a substrate layer; a back contact layer in communication with the substrate layer, wherein the surface passivation layer is positioned between the back contact layer and the absorber layer; a buffer layer on the absorber layer; and a transparent contact layer on the buffer layer.

[0040] In an embodiment, the substrate layer comprises soda lime glass and the back contact layer comprises molybdenum.

[0041] In an embodiment, the buffer layer comprises cadmium sulfide.

[0042] In an embodiment, the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer.

[0043] In an embodiment, the solar cell further comprises an anti-reflective coating.

[0044] In an embodiment, the solar cell has been scribed.

[0045] In an embodiment, the solar cell further comprises a current collection grid on the transparent contact layer.

[0046] The present disclosure also includes a use of a solar cell of the present disclosure or prepared by a method of the present disclosure in a tandem solar cell. The present disclosure also includes a tandem solar cell comprising a solar cell of the present disclosure or prepared by a method of the present disclosure. In an embodiment, the tandem solar cell is a printed thin-fdm tandem solar cell. In another embodiment, the solar cell of the present disclosure or prepared by a method of the present disclosure is the rear sub cell. In another embodiment, the front sub cell comprises a perovskite solar cell.

[0047] The present disclosure also includes a use of a surface passivation layer bearing nanometer-scale point contact openings for passivation of an absorber layer in a solar cell,wherein the absorber layer has been deposited by a method comprising a solution-based process. In an embodiment, the surface passivation layer is a rear surface passivation layer for the absorber layer in the solar cell. In an embodiment, the absorber layer is as described herein, for example, in embodiments relating to the solar cells of the present disclosure or has been prepared by a method as described herein, for example, embodiments relating to the methods of preparing a solar cell of the present disclosure; and / or wherein the surface passivation layer is as described herein, for example, in embodiments relating to the solar cells of the present disclosure or has been prepared by a method as described herein, for example, embodiments relating to the methods of preparing a solar cell of the present disclosure.

[0048] Other features and advantages of the present disclosure will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the disclosure, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should rather be given the broadest interpretation consistent with the description as a whole.BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The embodiments of the disclosure will now be described in greater detail with reference to the attached drawings, in which:

[0050] FIG. 1 shows a photograph of / V, / V-dimethylformamide-thiourea (DMF-TU) precursor molecular ink with [Cu] / [In]=1.0 M, and NaCl (0. 1 at. %) (left) and a ball and stick representation of the Cu-based and In-based complex (right).

[0051] FIGs. 2-3 show the contact angle made by DMF molecular inks ([In]=0.80 M) on the surface of a soda lime glass (SLG) / Mo surface (FIG. 2) and SLG / Mo / about 6 nm AI2O3 surface (FIG. 3). Low contact angles (<10°) were detected in both cases, indicating excellent wettability.

[0052] FIG. 4 shows a photograph of precursor films obtained after ink deposition and annealing on SLG / Mo substrates (right) and on SLG / MO / AI2O3 (left). The precursor films are placed in a graphite box, with Se pellets for further processing.

[0053] FIG. 5 shows a schematic representation of the process used to create nanosized openings through the surface of the AI2O3 passivation layer according to an embodiment.

[0054] FIG. 6 shows a plain light microscopic image of CdS NPs on a SLG / Mo substrate. The particles are randomly dispersed over the surface of the SLG / Mo substrate and have an average diameter of about 445 nm. Scale bar shows 5 pm.

[0055] FIG. 7 shows microscopic images of the surface of CdS-particle rich SLG / Mo surfaces coated with about 6 nm AI2O3 (left) and about 15 nm AI2O3 (right), after sonication in a 10 v / v% HC1 solution. The CdS nanoparticles embedded in the thicker AI2O3 layer could not be removed from the fdm surface. Scale bars in each image show 5 pm.

[0056] FIG. 8 shows a microscopic image of the surface of SLG / Mo / 15 nm AI2O3 fdm stack after being selenized at 550 °C in a tube furnace for 30 mins. Most of the AI2O3 surface appears to have blistered or ruptured. Scale bar shows 5 pm.

[0057] FIG. 9 shows a laser scanning confocal microscopic image of the nanosized openings (circles) through the AI2O3 layer (grey background). Scale bar shows 5 pm.

[0058] FIG. 10 shows depth and lateral profde of an exemplary nanosized opening showing the nanosized opening to have an average diameter of about 415 nm.

[0059] FIG. 11 shows the Mo, Al, and O distribution maps from EDX measurements using Mo-Ma (left), Al-Ka (center), and O-Ka (right) X-ray lines, respectively.

[0060] FIG. 12 shows side-view (upper), and top-view (lower) scanning electron microscope images of untreated, about 0.55 pm (left), and about 0.75 pm thick (right) CISSe absorber fdms. Scale bars show 500 nm (upper images) and 1 pm (lower images).

[0061] FIG. 13 shows time resolved photoluminescence data of CdS-coated, 0.55 (U), 0.55 (A), 0.75 (U), and 0.75 (A) samples.

[0062] FIG. 14 shows photoluminescence spectra of CdS-coated, 0.55 (U), 0.55 (A), 0.75 (U), and 0.75 (A) samples.

[0063] FIGs. 15-18 show, for the completed solar cell device: a comparison of depth profiles of apparent charge carrier density (Nc-v) extracted from CV measurements (FIG. 15); extraction of activation energies from temperature dependent open-circuit voltage measurements for completed solar cell devices (FIG. 16); extraction of minimum bandgap from external quantum efficiency (EQE) for reference (0.55 (U) (♦ ), 0.75 (U) (•)), and rear passivated (0.55 (A) (A). 0.75 (A) (*) devices) (FIG. 17); and J-V characteristics of 0.75 pm-thick CISSe solar cell devices, with different device architectures (FIG. 18).

[0064] FIG. 19 shows box plots of the average total area device parameters of the 0.55 (U), 0.55 (A), 0.75 (U), and 0.75 (A) solar cell devices: total area efficiency (upper left); Voc(upper right); Jsc (lower left); and fill factor (lower right). The mean values of the respective parameters are provided in the box plots and highlighted as dark grey boxes.

[0065] FIG. 20 shows J-V characteristics of the reference (0.55 (U) (♦ ), 0.75 (U) (•)) and rear passivated (0.55 (A) (^-), 0.75 (A) ( *) devices).

[0066] FIG. 21 shows EQE measurements of the reference (0.55 (U) (*), 0.75 (U) (**)) and rear passivated (0.55 (A) (***), 0.75 (A) (****) devices).DETAILED DESCRIPTIONI, Definitions

[0067] Unless otherwise indicated, the definitions and embodiments described in this and other sections are intended to be applicable to all embodiments and aspects of the disclosure herein described for which they would be understood to be suitable by a person skilled in the art.

[0068] As used herein, the words “comprising” (and any form thereof, such as “comprise” and “comprises”), “having” (and any form thereof, such as “have” and “has”), “including” (and any form thereof, such as “include” and “includes”) or “containing” (and any form thereof, such as “contain” and “contains”), are inclusive or open-ended and do not exclude additional, unrecited elements or process / method steps. As used herein, the word “consisting” and its derivatives are intended to be close-ended terms that specify the presence of the stated features, elements, components, groups, integers and / or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and / or steps. The term “consisting essentially of’, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and / or steps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers and / or steps.

[0069] Terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the term it modifies.

[0070] As used in this disclosure, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise.

[0071] The term “and / or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of’ or “one or more” of the listed items is present or used.

[0072] The term “suitable” as used herein means that the selection of specific reagents and / or conditions will depend on the reaction being performed and the desired results, but nonetheless, can generally be made by a person skilled in the art once all relevant information is known.II. Solar Cells. Methods of Preparation and Uses

[0073] An V, / V-dimethylformamide and thiourea-based route was used to fabricate exemplary submicron (0.55 and 0.75 pm) CuIn(S,Se)2 (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. Reducing the absorber film thickness below 1 pm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly recombinative, and mediocre reflective Mo / CISSe rear interface. In contrast, for the first time, to mitigate the rear recombination losses, a new rear contacting structure involving a surface passivation layer, and point contact openings is reported herein for solution-processed CISSe films and demonstrated in tangible devices. For this example, an atomic layer deposited AI2O3 film was used to passivate the Mo / CISSe rear surface and nanosized spherical precipitates formed via chemical bath deposition of CdS were used to generate nanosized point openings. The AI2O3 passivated CISSe solar cells showed an increase in the open-circuit voltage (Voc) and short-circuit current density when compared to reference cells with equivalent CISSe film thicknesses. Notably, a Voc increase of about 62 mV resulted in active area efficiencies of 14.2% for rear passivated devices with an about 0.75 pm thick absorber layer, the highest reported value for submicron-based solution processed, low bandgap CISSe solar cells.

[0074] Accordingly, the present disclosure includes a method of preparing a solar cell, the method comprising: depositing an absorber layer on a surface passivation layer bearing nanometerscale point contact openings, the surface passivation layer being positioned between the absorber layer and a back contact layer of the solar cell, the back contact layer being in communication with a substrate layer of the solar cell; depositing a buffer layer on the absorber layer; and depositing a transparent contact layer on the buffer layer.

[0075] In an embodiment, the absorber layer comprises CuIn(S,Se)2. In some embodiments the CuIn(S,Se)2 is deposited via a method comprising a solution-based process. However, in alternative embodiments, the CuIn(S,Se)2 may be deposited by any suitable other means, the selection of which can be readily made by a person skilled in the art.

[0076] Void-free, large grained (about 500 nm), smooth absorber fdm surfaces (<jrms less than 30 nm) were obtained in the examples of the present disclosure, challenging the traditional conceptions of inherently inferior material properties for solution processed absorber fdms. Solution-based approaches may also be advantageous, for example, for the preparation of flexible substrates. Accordingly, in an embodiment, the absorber layer is deposited via a method comprising a solution-based process. In such embodiments, the absorber layer may comprise any material or combinations thereof suitable for use in an absorber layer of a solar cell and capable of being deposited via a solution-based process, the selection of which can be readily made by a person skilled in the art. In an embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcogenide-based absorber material. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcopyrite-structured semiconductor of the CIS-based family ((Ag,Cu)(In,Ga,Al)(Se,S)2), a kesterite or combinations thereof. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcopyrite-structured semiconductor of the CIS-based family ((Ag,Cu)(In,Ga,Al)(Se,S)2). In another embodiment, the absorber layer deposited via a method comprising a solutionbased process comprises CuIn(S,Se)2, Cu(In,Ga)Se2 Cu2ZnSn(S,Se)4, CdTe, (Cu,Ag)(In,Ga)(S,Se)2, Sb2Se3 or Sb2S3. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises CuIn(S,Se)2.

[0077] The solution-based process can comprise any suitable method and / or means, the selection of which can be readily made by a person skilled in the art. In an embodiment, the deposition comprises screen printing, doctor blading, spin coating or combinations thereof.

[0078] In some embodiments wherein the absorber layer comprises CuIn(S,Se)2, the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer;optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

[0079] In some embodiments, the method comprises adding a suitable dopant (e.g., sodium ions) to the absorber layer. Accordingly, in an embodiment, the preparation of the molecular ink further comprises addition of a source of sodium ions to the solvent. In another embodiment, the source of sodium ions comprises a suitable sodium halide. In a further embodiment, the source of sodium ions comprises NaCl.

[0080] The source of sulfide, the source of copper and the source of indium comprise any suitable source or combinations thereof, the selection of which can be made by a person skilled in the art. In an embodiment, the source of sulfide comprises thiourea. In another embodiment, the source of copper comprises copper (II) chloride. In a further embodiment, the source of indium comprises indium (III) chloride. The solvent can comprise any suitable solvent or combinations thereof, the selection of which can be made by a person skilled in the art. In an embodiment, the solvent comprises dimethylformamide. In an embodiment, the sulfide comprises thiourea, the source of copper comprises copper (II) chloride, the source of indium comprises indium (III) chloride and the solvent comprises dimethylformamide.

[0081] In an embodiment, the molecular ink is prepared by adding the source of sulfide, the source of copper, optionally the source of dopant, and the source of indium to the solvent in the foregoing sequence, wherein the method comprises agitating (e.g., stirring) until dissolution, prior to adding the subsequent reagent.

[0082] The selenization can comprise any suitable method and / or means, the selection of which can be readily made by a person skilled in the art. In an embodiment, the selenization comprises exposing the annealed layer to a suitable source of selenium (e.g., selenium pellets) at a suitable temperature (e.g., a temperature of about 450 °C to about 650 °C or about 550 °C) for a time suitable to obtain the CuIn(S,Se)2 absorber layer (e.g, about 10 minutes to about 2 hours or about 30 minutes) under a suitable atmosphere, e.g., a flow of suitable inert gas (e.g., argon).

[0083] In an embodiment, the absorber layer has a thickness of less than 1 pm. In another embodiment, the absorber layer has a thickness of from about 0.4 pm to about 0.9 pm. In a further embodiment, the absorber layer has a thickness of about 0.55 pm. In another embodiment, the absorber layer has a thickness of about 0.75 pm.

[0084] The surface passivation layer bearing nanometer-scale point contact openings can be prepared by any suitable method and / or means, the selection of which can be made by a person skilled in the art having regard to the present disclosure. For example, methods comprising lithography can be used to generate well-controlled grids of nanometer-scale point contact openings. Accordingly, in an embodiment, the nanometer-scale point contact openings in the surface passivation layer are prepared by a method comprising lithography. However, in some embodiments, it is desirable to use an alternative method, which may, for example, afford advantages such as ease of fabrication and / or reduced processing costs. Accordingly, in another embodiment, the surface passivation layer is prepared by a method comprising: depositing a layer comprising nanoparticles on the back contact layer; depositing a layer comprising a surface passivation material on the layer comprising the nanoparticles, thereby forming a stack; and removing the nanoparticles from the surface of the stack, thereby forming the surface passivation layer bearing nanometer-scale point contact openings.

[0085] The nanoparticles can comprise any suitable material or combinations thereof, the selection of which can be made by a person skilled in the art. In an embodiment, the nanoparticles comprise a suitable metal sulfide, metal oxide, elemental metal or combinations thereof. In another embodiment, the nanoparticles comprise cadmium sulfide, zinc sulfide, zinc oxide, molybdenum metal or silver metal. In a further embodiment, the nanoparticles comprise cadmium sulfide. In another embodiment, the nanoparticles comprise a non-toxic material. In an embodiment, the nanoparticles are comprised of a material other than cadmium sulfide.

[0086] The layer comprising the nanoparticles may be deposited by any suitable means, which may, for example, depend on the identity of the nanoparticles. In an embodiment, the layer comprising the nanoparticles is deposited via a method comprising chemical bath deposition. In an embodiment, the nanoparticles are prepared via a method comprising reacting a source of a suitable metal (e.g., cadmium (II)) and a source of sulfide in an aqueous environment to prepare an aqueous suspension comprising the nanoparticles. The source of metal (e.g., cadmium (II)) and source of sulfide can be any suitable source, the selection of which can be readily made by a person skilled in the art. In another embodiment, the source of cadmium (II) comprises CdS04, the source of sulfide comprises thiourea and the aqueous environment further comprises NH4OH. In a further embodiment, the depositing the layer comprising the nanoparticles comprises immersing the substrate into the aqueous suspension.

[0087] The nanoparticles can be removed from the surface of the stack via any suitable method and / or means, the selection of which can be made by a person skilled in the art. In an embodiment, the nanoparticles are removed from the surface of the stack via a method comprising agitating the stack. In another embodiment, the agitation is carried out via a method comprising ultrasonic agitation in a suitable solvent or combinations thereof, for example, in an aqueous solution comprising a suitable acid such as hydrochloric acid.

[0088] In an alternative embodiment, the nanometer-scale point contact openings are prepared by a method comprising depositing a layer of an alkali fluoride on a layer comprising the surface passivation material then thermal treatment to create randomly spaced nanometerscale point contact openings. In another embodiment, the layer of the alkali fluoride has a thickness of about 15 nm to about 45 nm, about 25 nm to about 35nm or about 30 nm. The alkali fluoride can be any suitable alkali fluoride. In another embodiment, the alkali fluoride comprises sodium fluoride, potassium fluoride, rubidium fluoride or combinations thereof.

[0089] The suitable surface passivation materials such as AI2O3 are desirably an electrical insulator, and consequently, point openings are desirable for electrical connection for the solar cell. The nanometer-scale point contact openings can have any suitable diameter, the selection of which may depend, for example, on the identify of the absorber layer. A person skilled in the art having regard, for example, to the present disclosure could readily calculate a suitable diameter. For example, the size and distance (pitch) between the point openings may be determined by, among other parameters, the minority carrier diffusion length (see, for example: Repins et al., 2010). When compared to c-Si solar cells, thin fdm CISSe solar cells have short minority carrier diffusion lengths (about 1 pm) and lifetimes (about a few nanoseconds) (see, for example: Vermang et al., 2014). Thus, scaling from the Si solar cell technology, for carrier diffusion lengths of 0.5 to 1 pm openings with about 300 to about 500 nm in diameter and pitches between 1.5 to 2.5 pm may be desirable for efficient carrier collection (Repins et al., 2010). A person skilled in the art would also appreciate that although the average diameter of the point contact openings is desirably in the nanometer-scale, the surface passivation layer may also have minor amounts of point contact openings outside of that scale e.g., the point contact openings in the surface passivation layer may consist essentially of those in the nanometer-scale.

[0090] Accordingly, in an embodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm. In anotherembodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 350 nm to about 450 nm or about 400 nm to about 420 nm.

[0091] The surface passivation material can comprise any material or combination thereof suitable for surface passivation of the absorber layer, the selection of which can be made by a person skilled in the art. In an embodiment, the surface passivation material comprises a metal oxide. In another embodiment, the metal oxide comprises AI2O3, HfOx, Ga2Os, SiCh. TiCh or combinations thereof. In a further embodiment, the metal oxide comprises AI2O3.

[0092] The layer comprising the surface passivation material can be deposited via any suitable method and / or means, the selection of which may depend, for example, on the identity of the surface passivation material, but can be readily made by a person skilled in the art. In an embodiment, the layer comprising the surface passivation material is deposited by a method comprising atomic layer deposition.

[0093] The thickness of the layer comprising the surface passivation material may depend, for example, on the method for its preparation. For example, in methods comprising the deposition of a layer comprising nanoparticles as described herein, thinner layers of surface passivation material may be advantageous, for example, to facilitate removal of the nanoparticles. Accordingly, in an embodiment, the layer comprising the surface passivation material has a thickness of about 2 nm to about 10 nm. In another embodiment, the layer comprising the surface passivation material has a thickness of about 4 nm to about 8 nm or about 6 nm.

[0094] In some embodiments, the method further comprises etching the absorber layer prior to depositing the buffer layer. The conditions for etching can comprise any suitable conditions, the selection of which can be made by a person skilled in the art. In an embodiment, the etching comprises KCN etching. For example, in an embodiment, the etching comprises contacting the absorber layer with an aqueous solution comprising a suitable concentration of KCN (e.g., from about 5 wt.% to about 15 wt.%) for a time suitable to etch the absorber layer (e.g., for about 30 seconds to about 2 minutes).

[0095] The buffer layer can comprise any material or combinations thereof suitable for use in the solar cells of the present disclosure, the selection of which can be made by a person skilled in the art. In an embodiment, the buffer layer comprises a metal sulfide. In another embodiment, the metal sulfide is cadmium sulfide. The buffer layer such as the metal sulfide can be deposited by any suitable method and / or means which may, for example, depend on theidentity of the buffer layer material, but can be readily selected by a person skilled in the art. In an embodiment, the metal sulfide (e.g., the cadmium sulfide) is deposited on the absorber layer by a method comprising chemical bath deposition. In an embodiment, the chemical bath deposition comprises reaction of a source of suitable metal (e.g., cadmium (II)) and a source of sulfide in an aqueous environment. The source of metal (e.g., cadmium (II)) and source of sulfide can be any suitable source, the selection of which can be readily made by a person skilled in the art. In another embodiment, the source of cadmium (II) comprises CdSCti, the source of sulfide comprises thiourea and the aqueous environment further comprises NH4OH.

[0096] The transparent contact layer can comprise any material or combinations thereof suitable for use in the solar cells of the present disclosure, the selection of which can be made by a person skilled in the art. For example, a person skilled in the art would readily appreciate that such solar cells may desirably comprise n-type semiconductor materials, with a resistive layer of the transparent contact layer being positioned between the buffer layer and a conductive layer of the transparent contact layer. In an embodiment, the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer. The method for depositing the transparent contact layer can comprise any suitable method and / or means which may depend, for example, on the identity of the materials comprising the transparent contact layer, but can be readily selected by a person skilled in the art. In an embodiment, the method comprises depositing the transparent contact layer via a method comprising sputtering. In another embodiment, the method comprises depositing the intrinsic zinc oxide layer on the buffer layer by a method comprising sputtering and depositing the indium tin oxide layer on the intrinsic zinc oxide layer by a method comprising sputtering.

[0097] The substrate layer can comprise any suitable material or combinations thereof, suitable for use in the solar cells of the present disclosure. In an embodiment, the substrate layer comprises soda lime glass. The back contact layer can comprise any suitable material or combinations thereof, suitable for use in the solar cells of the present disclosure. In an embodiment, the back contact layer comprises molybdenum. In another embodiment, the substrate layer comprises soda lime glass and the back contact layer comprises molybdenum.

[0098] In some embodiments, the method further comprises depositing a current collection grid on the transparent contact layer. In an embodiment, the current collection gridcomprises Ni / Al. In an embodiment, the current collection grid is deposited on the transparent contact layer by a method comprising electron-beam evaporation.

[0099] In an embodiment, the method further comprises scribing the deposited layers down to the substrate layer.

[0100] The present disclosure also includes a solar cell prepared by a method of preparing a solar cell of the present disclosure. The present disclosure also includes a solar cell comprising a surface passivation layer bearing nanometer-scale point contact openings prepared by a method comprising: depositing a layer comprising nanoparticles on the back contact layer; depositing a layer comprising a surface passivation material on the layer comprising the nanoparticles, thereby forming a stack; and removing the nanoparticles from the surface of the stack, thereby forming the surface passivation layer bearing nanometerscale point contact openings; and an absorber layer prepared by a comprising a solution-based process, for example, wherein the absorber layer comprises CuIn(S,Se)2, and the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer. A person skilled in the art would readily appreciate that embodiments relating to such solar cells may be varied, as appropriate, as described herein in respect to the methods of preparing solar cells of the present disclosure.

[0101] The present disclosure also includes a solar cell comprising: an absorber layer; and a surface passivation layer bearing nanometer-scale point contact openings on a surface of the absorber layer.

[0102] In an embodiment, the surface passivation layer is on the rear surface of the absorber layer. However, in some embodiments, the surface passivation layer is on the front surface of the absorber layer. In another embodiment, the surface passivation layer is on the front surface of the absorber layer and the rear surface of the absorber layer.

[0103] In an embodiment, the absorber layer comprises CuIn(S,Se)2. In some embodiments the CuIn(S,Se)2 is deposited via a method comprising a solution-based process.However, in alternative embodiments, the CuIn(S,Se)2 may be deposited by any suitable other means, the selection of which may be readily made by a person skilled in the art.

[0104] In an embodiment, the absorber layer has been deposited via a method comprising a solution-based process. In such embodiments, the absorber layer may comprise any material or combinations thereof suitable for use in an absorber layer of a solar cell and capable of being deposited via a solution-based process, the selection of which can be readily made by a person skilled in the art. In an embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcogenide-based absorber material. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcopyrite-structured semiconductor of the CIS-based family ((Ag,Cu)(In,Ga,Al)(Se,S)2), a kesterite or combinations thereof. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises a chalcopyrite-structured semiconductor of the CIS-based family ((Ag,Cu)(In,Ga,Al)(Se,S)2). In another embodiment, the absorber layer deposited via a method comprising a solutionbased process comprises CuIn(S,Se)2, Cu(In,Ga)Se2 Cu2ZnSn(S,Se)4, CdTe, (Cu,Ag)(In,Ga)(S,Se)2, Sb2Se3 or Sb2S3. In another embodiment, the absorber layer deposited via a method comprising a solution-based process comprises CuIn(S,Se)2.

[0105] The solution-based process can comprise any suitable method and / or means, the selection of which can be readily made by a person skilled in the art. In an embodiment, the deposition comprises screen printing, doctor blading, spin coating or combinations thereof.

[0106] In some embodiments wherein the absorber layer comprises CuIn(S,Se)2, the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

[0107] In some embodiments wherein the absorber layer is doped, the method comprises adding a suitable source of dopant (e.g., sodium ions) to the molecular ink. Accordingly, in an embodiment, the preparation of the molecular ink further comprises addition of a sourceof sodium ions to the solvent. In another embodiment, the source of sodium ions comprises a suitable sodium halide. In a further embodiment, the source of sodium ions comprises NaCl.

[0108] The source of sulfide, the source of copper and the source of indium comprise any suitable source or combinations thereof, the selection of which can be made by a person skilled in the art. In an embodiment, the source of sulfide comprises thiourea. In another embodiment, the source of copper comprises copper (II) chloride. In a further embodiment, the source of indium comprises indium (III) chloride. The solvent can comprise any suitable solvent or combinations thereof, the selection of which can be made by a person skilled in the art. In an embodiment, the solvent comprises dimethylformamide. In an embodiment, the sulfide comprises thiourea, the source of copper comprises copper (II) chloride, the source of indium comprises indium (III) chloride and the solvent comprises dimethylformamide.

[0109] In an embodiment, the molecular ink is prepared by adding the source of sulfide, the source of copper, optionally the source of dopant, and the source of indium to the solvent in the foregoing sequence, wherein the method comprises agitating (e.g., stirring) until dissolution prior to adding the subsequent reagent.

[0110] The selenization can comprise any suitable method and / or means, the selection of which can be readily made by a person skilled in the art. In an embodiment, the selenization comprises exposing the annealed layer to a suitable source of selenium (e.g., selenium pellets) at a suitable temperature (e.g., a temperature of about 450 °C to about 650 °C or about 550 °C) for a time suitable to obtain the CuIn(S,Se)2 absorber layer (e.g, about 10 minutes to about 2 hours or about 30 minutes) under a suitable atmosphere, e.g., a flow of suitable inert gas (e.g., argon).

[0111] In an embodiment, the CuIn(S,Se)2 has a [Cu] / [In] ratio of about 0.95 to about 1.05. In another embodiment, the CuIn(S,Se)2 has a [Cu] / [In] ratio of about 1.0.

[0112] In an embodiment, the absorber layer is doped with a suitable dopant. In another embodiment, the absorber layer is doped with sodium ions.

[0113] In an embodiment, the absorber layer has a thickness of less than 1 pm. In another embodiment, the absorber layer has a thickness of from about 0.4 pm to about 0.9 pm. In a further embodiment, the absorber layer has a thickness of about 0.55 pm. In another embodiment, the absorber layer has a thickness of about 0.75 pm.

[0114] The surface passivation layer can comprise any material or combination thereof suitable for surface passivation of the absorber layer, the selection of which can be made by aperson skilled in the art. In an embodiment, the surface passivation layer comprises a metal oxide. In another embodiment, the metal oxide comprises AI2O3, HfOx, Ga2Os, SiCh. TiCh or combinations thereof. In a further embodiment, the metal oxide comprises AI2O3.

[0115] The thickness of the surface passivation layer may depend, for example, on the method for its preparation. For example, in solar cells in which the surface passivation layer is prepared by a method comprising deposition of a layer comprising nanoparticles as described herein, thinner layers may be advantageous, for example, to facilitate removal of the nanoparticles. Accordingly, in an embodiment, the surface passivation layer has a thickness of about 2 nm to about 10 nm. In another embodiment, the surface passivation layer has a thickness of about 4 nm to about 8 nm or about 6 nm.

[0116] The nanometer-scale point contact openings can have any suitable diameter, the selection of which may depend, for example, on the identify of the absorber layer. A person skilled in the art having regard, for example, to the present disclosure could readily calculate a suitable diameter. A person skilled in the art would also appreciate that although the average diameter of the point contact openings is desirably in the nanometer-scale, the surface passivation layer may also have minor amounts of point contact openings outside of that scale e.g., the point contact openings in the surface passivation layer may consist essentially of those in the nanometer-scale. In an embodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm. In another embodiment, the nanometer-scale point contact openings have an average diameter in the range of from about 350 nm to about 450 nm or about 400 nm to about 420 nm.

[0117] In an embodiment, the solar cell comprises: a substrate layer; a back contact layer in communication with the substrate layer, wherein the surface passivation layer is positioned between the back contact layer and the absorber layer; a buffer layer on the absorber layer; and a transparent contact layer on the buffer layer.

[0118] The substrate layer can comprise any suitable material or combinations thereof, suitable for use in the solar cells of the present disclosure. In an embodiment, the substrate layer comprises soda lime glass. The back contact layer can comprise any suitable material or combinations thereof, suitable for use in the solar cells of the present disclosure. In anembodiment, the back contact layer comprises molybdenum. In another embodiment, the substrate layer comprises soda lime glass and the back contact layer comprises molybdenum.

[0119] The buffer layer can comprise any material or combinations thereof suitable for use in the solar cells of the present disclosure, the selection of which can be made by a person skilled in the art. In an embodiment, the buffer layer comprises a metal sulfide. In another embodiment, the metal sulfide is cadmium sulfide.

[0120] The transparent contact layer can comprise any material or combinations thereof suitable for use in the solar cells of the present disclosure, the selection of which can be made by a person skilled in the art. For example, a person skilled in the art would readily appreciate that such solar cells may desirably comprise n-type semiconductor materials, with a resistive layer of the transparent contact layer being positioned between the buffer layer and a conductive layer of the transparent contact layer. In an embodiment, the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer.

[0121] In an embodiment, the solar cell further comprises an anti-reflective coating.

[0122] In an embodiment, the solar cell has been scribed.

[0123] In an embodiment, the solar cell further comprises a current collection grid on the transparent contact layer. In an embodiment, the current collection grid comprises Ni / Al.

[0124] The present disclosure also includes a use of a solar cell of the present disclosure or prepared by a method of the present disclosure in a tandem solar cell. The present disclosure also includes a tandem solar cell comprising a solar cell of the present disclosure or prepared by a method of the present disclosure. It will be appreciated by a person skilled in the art that the solar cell can be any suitable solar cell described in embodiments herein, the selection of which may depend, for example, on the bandgap of the respective sub cells, but could be selected by a person skilled in the art having regard to the present disclosure. Means and / or methods of determining a bandgap of a particular solar cell are also known to the skilled person.

[0125] As described herein, void-free, large grained (about 500 nm), smooth absorber fdm surfaces (<Jrms less than 30 nm) were obtained in the examples of the present disclosure, challenging the traditional conceptions of inherently inferior material properties for solution processed absorber fdms, which may, for example, pave the way, for the development ofhigh-efficiency, and cost-competitive, printed thin-film tandem solar cells. Accordingly, in an embodiment, the tandem solar cell is a printed thin-film tandem solar cell.

[0126] In an embodiment, the solar cell of the present disclosure or prepared by a method of preparing a solar cell of the present disclosure is the rear sub cell. However, in other embodiments, the solar cell of the present disclosure or prepared by a method of preparing a solar cell of the present disclosure is the front sub cell.

[0127] In some embodiments, the solar cell of the present disclosure or prepared by a method of preparing a solar cell of the present disclosure is the rear sub cell and the front sub cell comprises a perovskite solar cell. A person skilled in the art having regard to the present disclosure could readily select a suitable perovskite solar cell for use in such tandem cells.

[0128] The present disclosure also comprises a use of a surface passivation layer bearing nanometer-scale point contact openings for passivation of an absorber layer in a solar cell, wherein the absorber layer has been deposited by a method comprising a solution-based process. A person skilled in the art would readily appreciate that embodiments relating to the surface passivation layer, the absorber layer, the solution-based process and other features relating to the solar cell may be varied, as appropriate, as described herein in respect to the embodiments relating to the solar cells and methods of their preparation. In an embodiment, the surface passivation layer is a rear surface passivation layer for the absorber layer in the solar cell.

[0129] The following non-limiting examples are illustrative of the present disclosure:EXAMPLESExample 1: Rear Surface Passivation for Ink-Based, Submicron CuIn(S,Se)2 Solar Cells

[0130] In the present study, submicron thick CISSe absorber films were fabricated using an V. V-dimethy I formamide (DMF) and thiourea (TU) - based molecular-ink processing route. An ultrathin AI2O3 film with nanosized point openings between the rear contact and the CISSe absorber layer was used to reduce the rear interface recombination losses. The AI2O3 rear passivated devices with 0.75 pm thick absorber films demonstrated a four-fold increase in the photoluminescence (PL) peak intensity, an average minority lifetime (T) of 14 nm, and an opencircuit voltage (Voc) of 578.4 mV. Notably, active area efficiencies of 14.2%, and about 12.0% were obtained by rear-passivated devices with 0.75 pm and 0.55 pm thick absorber, respectively, which represents the highest reported values for low bandgap CISSe solar cells with absorbers of equivalent thicknesses and ink-based deposition processes.I. Experimental Section

[0131] Molecular Precursor-Ink Preparation: Molecular precursor ink preparation, deposition, and annealing were carried out in a fume hood, under ambient air conditions. The [Cu] / [In]=1.0 precursor ink was prepared by adding 4.8 M TU (99%, Aldrich), 0.8 M CuCl (99.995%, Aldrich), 0.0032 MNaCl (99.99%, Aldrich), InCl3(99.99%, Aldrich), and 0.8 M InCl3(99.99%, Aldrich) to the DMF (99.8%, Aldrich) solvent (FIG. 1). The reagents were added in the sequence mentioned and stirred until complete dissolution before adding the next reagent.

[0132] Solar Cell Fabrication: To prepare the precursor films, molecular inks were filtered with 0.45 pm PTFE filters, spin-coated at 1500 rpm for one minute on SLG / Mo substrates (reference, FIG. 2) or SLG / Mo / nanopatterned-AECL. (passivated, FIG. 3), annealed on a hotplate at 260 °C for 2 minutes, and cooled for 2 minutes. This ink-coating-annealing- cooling process was repeated 2 and 3 times, to prepare 0.55 pm, and 0.75 pm absorber layers, respectively (FIG. 4). For the passivated samples, AhO3was deposited by using a plasma- enhanced atomic layer deposition (ALD) system (GEMStar™ XT-R, Arridance, USA), using alternative purges of trimethylaluminum (TMA) and H2O. The thickness of the film was controlled by adjusting the ALD cycle number based on a growth rate of about 0. 11 nm / cycle. The thickness was calibrated by subj ecting a silicon wafer to 268 cycles of ALD, which yielded about 30 nm of AI3O3on the clean silicon wafer (confirmed by profilometry measurements). The precursor films were subsequently placed in a graphite box with Se pellets (about 330 mg, 99.99%, Aldrich) and selenized at 550 °C in a tube furnace for 30 mins under constant Ar (99.99%) flow, and atmospheric pressure (about 1 bar) to obtain reference or rear passivated CISSe absorber films. After selenization, the films were KCN-etched (10 wt.% in H2O, 60 s). Next, devices were finalized with a CdS buffer layer (about 30 nm) by chemical bath deposition (0.015M CdSCL (99.99%, Aldrich), 1.9 M NH4OH (ACS grade, Aldrich), and 74 mM TU (99%, Aldrich)), iZnO / ITO (50 nm / 250 nm) transparent contact layers by sputtering, and Ni / Al (50 nm / 1000 nm) grid via e-beam evaporation. To obtain a particle-rich CdS deposition condition, the following process was used: after preparing the CBD CdS solution, Mo / SLG substrates were immersed after the CBD solution reacted for 5 (“A”) min — during which time CdS nanoparticles were formed within the solution. Thereafter, the substrates were immersed for 3 (“B”) min and a thin particle rich CdS film was grown on the SLG / Mo. By varying time intervals, A and B, the CdS particle-density on the SLG / Mo surface wascontrolled. After mechanical scribing to a cell size of around 0.11 cm2, devices were characterized. No antireflection coating was used in any of the devices.

[0133] Characterization of CISSe Absorber Films: Film morphology, and elemental composition analysis was collected on a Tescan™ Mira 3 XMU scanning electron microscope with a field emission gun. Elemental distribution maps via energy-dispersive X-ray were acquired at 7 keV and evaluated by the Aztec software suite. Secondary electrons were used for surface, and cross-section imaging at 5 keV acceleration voltage. Further, the surface roughness of the absorber films was measured with a Bruker™ Dektak XT Stylus Profiler with a stylus radius of 2 pm, and stylus force of 10 mg. Photoluminescence and time resolved PL measurements were performed on CdS-coated absorber layers using a Hamamatsu C12132 near-IR fluorescence lifetime measurement system. A diffuse spot of 30 mm2was illuminated on the absorber layers with a 15 kHz, 1.2 ns pulsed, 532 nm laser having an average laser power of 1 mW for the charge carrier lifetime measurements. A Keyence™ Laser confocal microscope was used to obtain laser confocal microscopic images at 150x magnification.

[0134] Characterization of CISSe Solar Cells: CISSe solar cells were measured with a Keithley™ 2401 source meter using a 4-point probe under simulated AM1.5G spectrum (incident power calibrated using a reference silicon photodiode) from a Science Tech AAA Solar Simulator System. Temperature dependent measurements were performed by using liquid nitrogen. Prior to the measurements, the samples were kept in the dark for 1 h to ensure a relaxed state. External quantum efficiency measurements were performed using a grating monochromatic-based dual-beam setup under chopped light from a Xenon lamp. CV measurements were acquired at room temperature using an Agilent™ 4980A Precision LCR meter with the DC bias range varying from -2.0 V to 0.5 V. The apparent charge carrier profile was extracted at 100 kHz to prevent signal contribution from defect charge carrier states.II. Results and Discussion

[0135] The effects of rear surface passivation on ink-based CISSe solar cells were studied by utilizing a modified rear interface device architecture, i.e. a soda lime glass (SLG) / Mo / AhO3 film stack compared with a standard SLG / Mo. Nanosized openings are required to facilitate electrical contact while adequately passivating the rear surface (Suresh et al., 2019). For this, a multistep approach involving 1) deposition of a nanoparticle (NP)-rich CdS layer by chemical bath deposition (CBD) on SLG / Mo substrate, 2) atomic layer deposition (ALD) of an AI2O3 layer on the CdS NP-rich SLG / Mo substrate, and 3) CdS NP removal from thesurface of SLG / MO / AI2O3 film stack by ultrasonic agitation in 10 v / v% HC1 was utilized (FIG. 5). Referring to FIG. 5, in the exemplary process shown therein, a CdS nanoparticle (NP)-rich (grey spheres) layer is grown on the surface of SLG / Mo, which is subsequently coated with an ultrathin layer of AI2O3 (white layer). The particles are then removed (ultrasonic agitation) to create nanosized openings in the AI2O3 layer.

[0136] FIG. 6 depicts the laser scanning confocal microscope image of a NP-rich CdS film deposited on a SLG / Mo substrate. Using this technique, an average CdS NP diameter of 445±50 nm was calculated. Consecutive experiments were conducted to confirm satisfactory NP removal after being coated by an ALD-AI2O3 layer. The highly conformal nature of ALD limited the AI2O3 layer thickness to less than 10 nm, as the NPs embedded in thicker layers could not be removed by this method (FIG. 7). In addition, greater than 15 nm AI2O3 layers were observed to blister (FIG. 8) due to outgassing (Vermang et al., 2013), when subjected to harsh film processing conditions (500-600 °C, under Se vapors). Thus, an ALD-AI2O3 thickness of about 6±1 nm was selected for the rear passivated solar cell devices. FIG. 9 presents the laser scanning confocal microscopic image of the surface of a SLG / Mo / about 6 nm AI2O3 film stack, after CdS NP removal. As seen in FIG. 9, the point openings are spaced randomly and are spherical in shape, having an average diameter of about 405±20 nm, consistent with previously deposited CdS NPs. The corresponding depth profile of a single nano-sized opening obtained along the indicated line is depicted in FIG. 10. From equivalent analysis of similar samples, the depth of nanosized openings was determined to be 12±4 nm. A similar sample was analyzed by scanning electron microscopy (SEM)-energy-dispersive X-ray (EDX), and the top-view Mo-M, Al-K, and O-K images (FIG. 11) corroborate the formation of the nanosized openings through the AI2O3 surface passivation layer.

[0137] Following previous results on comparing the efficiency of DMF-TU based CISSe devices with different [Cu] contents (Suresh et al., 2023), a [Cu] / [In] ratio of 1.0 which was shown as the optimum ratio for CISSe devices, was adopted here. Stoichiometric ([Cu] / [In] of about 1.0) DMF-TU based inks were coated on the standard SLG / Mo, and rear modified SLG / MO / AI2O3 film stacks, and thermally treated to build the desired absorber layer thicknesses of 0.55 pm and 0.75 pm. Notably, NaCl (about 0.1 at.%) was added to molecular precursor inks to avoid the “roll-over” effect typical of devices with insufficient sodium in the absorber film, since AI2O3 layers act as a barrier for Na diffusion from the SLG substrate. In the present disclosure, CISSe samples are sometimes denoted by the film thickness, incombination with the nanopattemed passivation layer used. For example, AI2O3 passivated 0.55 pm absorber films are denoted by 0.55 (A), and the untreated reference sample by 0.55 (U). Unless stated otherwise, both the untreated reference, and the AI2O3 passivated samples were prepared from the NaCl (0.1 at.%) doped DMF-TU molecular precursor inks.

[0138] SEM imaging (FIG. 12) determined the average CISSe absorber layer thicknesses for the 0.55 (U), and 0.75 (U) to be about 0.56±0.02 pm, and about 0.78±0.02 pm, respectively. Furthermore, large-grained (about 0.55 pm) absorber layers with a smooth film surface (root mean square roughness (<jmis) <30 nm, as determined by profilometry measurements) were observed in both the absorber films. Notably, grains for the 0.55 (U) films extended along the thickness of the absorber film. Furthermore, the grain size was not affected by the thickness of the absorber film, which pointed to sufficient Se supply.

[0139] To examine whether the AI2O3 rear passivation mediated a reduction in the rear surface recombination, time-resolved photoluminescence measurements (TRPL) were conducted on CdS-coated rear surface passivated (0.55 (A), and 0.75 (A)), and reference samples (0.55 (U), and 0.75 (U)). In thin CISSe absorber films (<800 nm), charge carriers are generated throughout the whole absorber thickness, and consequently, photoluminescence measurements are sensitive to the recombination at the Mo / CISSe interface (Bose et al., 2018). FIG. 13 shows that all the films had a characteristic bi-exponential PL decay, with varying time scales. For the samples with 0.55 pm absorbers, the average minority carrier lifetime increased from 6 ns (extracted following the method in Ohnesorge et al., 1998) to about 12 ns, from reference to passivated devices. Likewise, increased minority carrier lifetimes of 14 ns were obtained by the 0.75 (A) device (vs. 9 ns for 0.75 (U)), which indicated that the rear surface passivation schemes indeed improved the carrier lifetimes of the CISSe absorber layer. A fourfold increase in the photoluminescence peak intensity was obtained for the passivated devices (FIG. 14) when compared with the respective reference devices. First principles calculation by Hsu et al. (2012) showed that AI2O3 passivates a Cu(In,Ga)(S,Se)2 surface by 1) reducing about 35% of the interface defect density, and 2) reducing the rear surface minority carrier concentration due to field effect passivation. Thus, a significant reduction in the surface recombination velocity (and thus rear interface recombination losses) is expected for the AI2O3 rear-passivated CISSe surfaces, when compared with the standard Mo / CISSe interface (Vermang et al., 2013). The enhanced TRPL lifetimes, in combination with increased PL peak intensities would thus point to the effective rear surface passivatingcapabilities of the AI2O3 layer (Hsu et al., 2012). Further, PL data showed emission peaks at about 0.999 eV for all the absorber film samples (FIG. 14).

[0140] Motivated by these favorable results, reference solar cell devices were fabricated in the Mo / CISSe / CdS / i-ZnO / ITO / Ni / Al device architecture, with the passivated devices additionally exhibiting a nanopattemed AI2O3 layer between the Mo and CISSe layer. Measurements of the charge carrier density and width of the space charge region were obtained from capacitance-voltage (CV) profiling. FIG. 15 shows the apparent charge carrier densities from capacitance-voltage measurements, extracted using conventional methods (Heath et al., 2004). A widely reported, U-shaped charge carrier concentration profile was observed for all CISSe solar cell devices (Salome et al., 2013). With passivation, the charge carrier densities decreased from 7.8 x 1015cm'3to 7.1 x 1015cm'3and from 6.2 x 1015cm'3and 5.2 x 1015cm'3for the 0.55 (U) and 0.75 (U) devices, respectively. Consistent with the decreased free carrier densities for the 0.55 (A) and 0.75 (A) devices, the depletion width of the reference device increased from about 293 to 296 nm, and 282 to 297 nm, for the 0.55 (U) and 0.75 (U) devices, respectively. A wider space charge region improves the long wavelength charge carrier collection; however, lower charge carrier densities result in reduced electric fields across the space charge region (Rau & Schock, 1999). While not wishing to be limited by theory, the [Na] content may affect the carrier density in the CISSe thin films, however, similar charge carrier densities concentrations for the passivated and reference solar cell devices may suggest identical [Na] in the absorber films (Stanbery et al., 2000). Additionally, none of the solar cell devices were fully depleted, as the width of the space charge region was lower than the thickness of the corresponding absorber layers.

[0141] FIG. 16 depicts the temperature dependence of Voc values for reference and rear passivated solar cell devices. A linear fit was employed to extrapolate the recombination activation energy (Ea) of the dominant recombination mechanism (following the method in Blood & Orton, 1992). The differences between PL peaks (indicative of the bandgap energies), and Eawere 36.8, 6.9, 27.7, and 8.9 mV for the 0.55 (U), 0.55 (A), 0.75 (U), 0.75 (A) devices, respectively. While not wishing to be limited by theory, a larger difference between Egand Eafor the reference devices when compared to the passivated devices may indicate a reduction in the trap states in the passivated absorber films (Eisenbarth et al., 2010). Due to the design of the experiment that focused on the alteration of the rear contact only, the inventors posit that the reduced recombination suggested by the TRPL, PL, andtemperature dependent Voc measurements, is indicative of the successful rear surface passivation of the CISSe absorbers with the AI2O3 passivation layers. Furthermore, the PL- determined bandgap energies were corroborated by external quantum efficiency (EQE)- determined bandgap energies estimated as 1.001, 1.001, 1.004, and 1.002 eV for the 0.55 (U), 0.55 (A), 0.75 (U), and 0.75 (A), respectively (FIG. 17).

[0142] FIG. 18 shows representative current density-voltage (J-V) curves for an un- pattemed AI2O3 rear surface passivated CISSe solar cell device (absorber thickness about 0.75 pm). While not wishing to be limited by theory, the low fill factor (FF) (about 16.1%), and high series resistance (51.3 Qcm2) suggested inadequate back contacting (AI2O3 is an insulating layer). FIG. 18 also shows a representative J-V curve of a CISSe device with an about 0.75 pm thick, undoped (no NaCl added to the molecular precursor ink) absorber film, and a nanopattemed AI2O3 layer at the rear interface. The devices exhibited low FF (about 38.1%), and a “roll-over” effect, typical of solar cells with insufficient sodium in the absorber film (KotipaHi et al., 2015). This confirmed that the AI2O3 surface passivated CISSe solar cell devices required 1) an array of point openings through the AI2O3 for appropriate contacting, and 2) extra supply of sodium.

[0143] FIG. 19 presents the average device parameters for the reference (0.55 (U), 0.75 (U)), and nanopattemed AI2O3 rear passivated devices (0.55 (A), 0.75 (A)), including short circuit current density (Jsc), Voc, fill factor, and PCE. Unless otherwise stated, the J-V curves and PCE are based on total area From 0.55 (U) to 0.55 (A) devices, the average PCE increased from 8.9% to 11.2%, with the main enhancements gained from Voc increase (504.5 mV to 565.0 mV). Furthermore, the 12% (rel.) improvement in the Voc contributed to higher fill factors (from 64.4% to 65.5%) with comparable series resistances (aboutl.4 Qcm2, for 0.55 (U), and 1.9 Qcm2for 0.55 (A)). Comparable series resistance values suggest a well-defined and adequate contacting area for the passivated devices and that the generated charge carriers were not significantly affected by the lateral distance between point openings. Average PCEs of 10.9% were obtained by the 0.75 (U) devices, a 22.0% (rel.) increase when compared to 0.55 (U) devices.

[0144] In addition to insufficient light absorption (optical losses), reducing the absorber layer thickness increases the impact of the mediocre reflective and passivated Mo / CISSe interface closer to the space charge region of the device (Mitzi et al., 2004). Consequently, the 0.55 (U) devices had an 18.0% (rel.) lower Jsc, followed by Voc (2.2% deficiency), and FF (1.3% deficiency). On the other hand, when compared to 0.75 (U), increased Voc (565.0 mV vs. 516.0 mV), and FF (65.5% vs. 65.3%) for the 0.55 (A) devices led to higher PCEs (11.2%vs. 10.9%). For the 0.75 (A) devices, the average Voc and FF of reference devices increased from 515.0 mV and 65.3%, to 573.4 mV and 66.3% respectively. Combined with improvements to the Jsc, the PCEs increased from 10.9% to 13.3%, a 22% (rel.) increase when compared with the 0.75 (U) devices. FIG. 20 shows the J-V curves of the best-performing CISSe devices in this study with varying absorber fdm thicknesses. We report a highest PCE of 13.7%, obtained by a 0.75 (A) device having a Jsc, Voc, and FF of 34.9 mA / cm2, 578.4 mV, and 67.9%, respectively. The 0.55 (A) device demonstrated a highest PCE of 11.5%, Jsc of 30.6 mA / cm2, VOC of 566.1 mV, and FF of 66.5%. Capacitance-voltage measurements (above) suggested the passivated devices to have similar, or marginally lower charge carrier concentration when compared with the respective reference devices. Hence, the Voc improvements were not ascribed to the changes in the charge carrier density values.

[0145] To better understand the improved Voc for the passivated solar cell devices, the reverse saturated current (Jo) was calculated (using methods described in Hegedus & Shafarman, 2004) to be 1.3xl0'7, and 4.8xl0'7Acm'2for the 0.55 (A), and 0.75 (A) devices, respectively. This indicated a drastic reduction by 1 order of magnitude when compared with the 3.4xl0'6, and Ll xlO'6Acm'2values for the 0.55 (U), and 0.75 (U) devices, respectively. This reduction in the reverse saturation current indicated a reduction in the recombination losses, which is well in line with significant increases in the minority lifetime and a fourfold increase in the PL response for the passivated device (see TRPL and PL results above). The EQE data for the highest efficiency devices with varying absorber film thicknesses is shown in FIG. 21. Calculated Jsc values from EQE were 29.0, 30.6, 32.6 and 34.7 mAcm2, for the 0.55 (U), 0.55 (A), 0.75 (U), and 0.75 (A) device, respectively, consistent with the J-V results (FIG. 20). Compared to 0.75 (U), while not wishing to be limited by theory, the EQE response decrease in the near-infrared region for the 0.55 (U) may stem from incomplete absorption of incident light due to thinner absorber films. Notably, the spectral response improved in the 500 to 1200 nm wavelength range for the passivated devices when compared with the respective reference devices. The improvements to the Jsc were thus ascribed to increased average minority carrier diffusion length (due to field effect passivation), and a reduction in the rear interface defect density (chemical passivation), rather than to marginal optical benefits (ultrathin AI2O3). These results are further corroborated by the increased minority carrier lifetimes and space charge region width from TRPL and CV measurements, respectively, that indicated improved carrier collection.

[0146] The cell Voc of 578.4 mV exceeds values of the currently reported highest efficiency DMF-TU-based device (active area PCE about 14.2%, 36.1 mAcm’2Jsc, 578 mV Voc, 67.9% FF, see Table 1), pointing to the excellent charge carrier management in our rear surface passivated devices (Jiang et al., 2020). To the best of the inventors’ knowledge, the active area efficiencies of 12.0%, and 14.2% are the highest reported efficiencies for submicron (0.56 pm and 0.78 pm, respectively) DMF-TU-based CISSe devices (see Table 5.1 of Suresh & Uhl, 2021). Further improvements are expected by applying well-order rear contacting structures (e.g. using lithography to generate well -controlled grids of nanosized point contact openings), bandgap gradients, and application of anti-reflective coatings.Table 1. Solar cell performance (active area) for devices with solution processed CuIn(S,Se)2 absorbers various film thicknesses.* Devices with anti-reflective coatings.III. Conclusions

[0147] In conclusion, a facile rear surface passivation scheme was developed and implemented for solution processed CISSe solar cell devices for the first time. A 6 nm layer of ALD-AI2O3 was used to passivate the Mo / CISSe rear interface, while nanosized spherical precipitates formed during CBD of CdS were used to generate point contact openings. By carefully tailoring the AI2O3 film thickness, and optimizing the contacting area at the rear interface, record active area efficiencies of 14.2% (without MgF2) were obtained for solution processed CISSe devices with 0.75 pm absorber films, with Vocs of up to 578.4 mV. Furthermore, solution-based routes for CISSe film fabrication are typically associated with the formation of additional porous, fine-grained layers due to the incorporation of impurities in the film prior to selenization and high absorber roughness (Suresh et al., 2022). Consequently, impurities in the layer effects the film morphology, light absorption (reduced absorber thickness), carrier collection, and reduces control over the overall absorber filmcomposition. Notably, a limiting factor in development of perovskite / CISSe tandem solar cells has been the relatively rough (Grms >100 nm) CISSe top surface, with lateral feature sizes in the order of about 500 nm. In this work, void-free, large grained (about 500 nm), smooth absorber film surfaces (<Jrms less than 30 nm) were obtained, challenging the traditional conceptions of inherently inferior material properties for solution processed absorber films, and thus, paving the way, for example, for the development of high-efficiency, and cost- competitive, printed thin-film tandem solar cells (e.g., CISSe / perovskite).

[0148] While the disclosure has been described with reference to what are presently considered to be the preferred examples, it is to be understood that the disclosure is not limited to the disclosed examples. To the contrary, the present disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0149] All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. Where a term in the present application is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term.FULL CITATIONS FOR DOCUMENTS REFERRRED TO IN THE DESCRIPTIONP. Blood and J. W. Orton, “The electrical characterization of semiconductors : majority carriers and electron states” Techniques of Physics 1992, 14, i-xxiii.S. Bose, J. M. V. Cunha, S. Suresh, J. De Wild, T. S. Lopes, J. R. S. Barbosa, R. Silva, J. Borme, P. A. Fernandes, B. Vermang and P. M. P. Salome, “Optical lithography patterning of SiO2 layers for interface passivation of thin film solar cells” Solar RRL 2018, 2, 1800212.J. A. Clark, A. Murray, J.-m. Lee, T. S. Autrey, A. D. Collord and H. W. Hillhouse, “Complexation chemistry in N.N-dimethylformamide-based molecular inks for chalcogenide semiconductors and photovoltaic devices” J Am. Chem. Soc. 2019, 141, 298-308.A. De Vos, “Detailed balance limit of the efficiency of tandem solar cells” Journal of Physics D: Applied Physics 1980, 13, 839-846.T. Eisenbarth, T. Unold, R. Caballero, C. A. Kaufmann and H.-W. Schock, “Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells” Journal of Applied Physics 2010, 107, 034509.T. Feurer, R. Carron, G. Torres Sevilla, F. Fu, S. Pisoni, Y. E. Romanyuk, S. Buecheler and A. N. Tiwari, “Efficiency improvement of near-stoichiometric cuinse2 solar cells for application in tandem devices” Advanced Energy Materials 2019, 9, 1901428.M. A. Green, E. D. Dunlop, M. Yoshita, N. Kopidakis, K. Bothe, G. Siefer and X. Hao, “Solar cell efficiency tables (version 62)” Progress in Photovoltaics: Research and Applications 2023, 31, 651-663.J. T. Heath, J. D. Cohen and W. N. Shafarman, “Bulk and metastable defects in CuIni-xGaxSe2 thin films using drive-level capacitance profiling” Journal of Applied Physics 2004, 95, 1000-1010.S. S. Hegedus and W. N. Shafarman, “Thin-film solar cells: device measurements and analysis” Progress in Photovoltaics: Research and Applications 2004, 12, 155-176.W.-W. Hsu, J. Y. Chen, T.-H. Cheng, S. C. Lu, W.-S. Ho, Y.-Y. Chen, Y.-J. Chien and C.W. Liu, “Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited AI2O3” Applied Physics Letters 2012, 100, 023508.J. Jiang, R. Giridharagopal, E. Jedlicka, K. Sun, S. Yu, S. Wu, Y. Gong, W. Yan, D. S. Ginger, M. A. Green, X. Hao, W. Huang and H. Xin, “Highly efficient copper-rich chalcopyrite solar cells from DMF molecular solution” Nano Energy 2020, 69, 104438.R. Kotipalli, B. Vermang, J. Joel, R. Rajkumar, M. Edoff and D. Flandre, “Investigating the electronic properties of AhO3 / Cu(In,Ga)Se2 interface” AIP Advances 2015, 5, 107101.T. S. Lopes, J. P. Teixeira, M. A. Curado, B. R. Ferreira, A. J. N. Oliveira, J. M. V. Cunha, M. Monteiro, A. Violas, J. R. S. Barbosa, P. C. Sousa, I. Caha. J. Borme, K. Oliveira, J. Ring, W. C. Chen, Y. Zhou, K. Takei, E. Niemi, F. L. Deepak, M. Edoff, G. Brammertz, P. A. Fernandes, B. Vermang and P. M. P. Salome, “Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technology” npj Flex Electron 2023, 7, 4.D. B. Mitzi, L. L. Kosbar, C. E. Murray, M. Copel and A. Afzali, “High-mobility ultrathin semiconducting films prepared by spin coating” Nature 2004, 428, 299-303.B. Ohnesorge, R. Weigand, G. Bacher, A. Forchel, W. Riedl and F. H. Karg, “Minoritycarrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells” Applied Physics Letters 1998, 73, 1224-1226.U. Rau and H. W. Schock, “Electronic properties of Cu(In,Ga)Se2 heterojunction solar cells-recent achievements, current understanding, and future challenges” Applie d Physics A 1999, 69, 131-147.I. L. Repins, W. K. Metzger, C. L. Perkins, J. V. Li and M. A. Contreras, “Correlation Between Measured Minority-Carrier Lifetime and Cu(In,Ga)Se2 Device Performance” IEEE Transactions on Electron Devices 2010, 57, 2957-2963.A. Richter, M. Hermle and S. W. Glunz, “Reassessment of the limiting efficiency for crystalline silicon solar cells” IEEE Journal of Photovoltaics 2013, 3, 1184-1191.P. M. Salome, A. Hultqvist, V. Fjallstrom, M. Edoff, B. Aitken, K. Vaidyanathan, K. Zhang, K. Fuller and C. K. Williams, “Cu(In,Ga)Se2 Solar Cells With Varying Na Content Prepared on Nominally Alkali-Free Glass Substrates” IEEE Journal of Photovoltaics 2013, 3, 852-858.Y. Siddique, K. Son, T. R. Rana, S. D. H. Naqvi, P. M. Hoang, A. Ullah, H. Tran, S. M. Lee, S. Hong, S. K. Ahn, I. Jeong and S. Ahn, “Air-processable high-efficiency CISSe solar cells from DMF molecular solution and their application to perovskite / CISSe tandems” Energy & Environmental Science 2022, 15, 1479-1492.B. J. Stanbery, S. Kincal, S. Kim, T. J. Anderson, O. D. Crisalle, S. P. Ahrenkiel and G. Lippold, “Role of sodium in the control of defect structures in CIS [solar cells]” Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036), Anchorage, AK, USA, 2000, 440-445.S. Suresh, J. de Wild, T. Kohl, D. G. Bui du, G. Brammertz, M. Meuris, J. Poortmans, O. Isabella, M. Zeman and B. Vermang, “A study to improve light confinement and rearsurface passivation in a thin-Cu(In, Ga)Se2 solar cell” Thin Solid Films 2019, 669, 399-403.S. Suresh and A. R. Uhl, “Present status of solution-processing routes for Cu(In,Ga)(S,Se)2 solar cell absorbers” Advanced Energy Materials 2021, 11, 2003743.S. Suresh, D. J. Rokke, A. A. Drew, E. Alruqobah, R. Agrawal and A. R. Uhl, “Extrinsic doping of ink-based Cu(In,Ga)(S,Se)2-absorbers for photovoltaic applications” Advanced Energy Materials 2022, 12, 2103961.S. Suresh, D. Abou-Ras, T. H. Chowdhury and A. R. Uhl, “Over 13% efficient, ambient airprocessed CuIn(S,Se)2 solar cells via compositional engineering of molecular inks” Solar RRL 2023, 7 2300437.A. R. Uhl, A. Rajagopal, J. A. Clark, A. Murray, T. Feurer, S. Buecheler, A. K. Y. Jen and H. W. Hillhouse, “Solution-processed low-bandgap CuIn(S,Se)2 absorbers for high- efficiency single-junction and monolithic chalcopyrite-perovskite tandem solar cells” Advanced Energy Materials 2018, 8, 1801254.B. Vermang, V. Fjallstrom, J. Pettersson, P. Salome and M. Edoff, “Development of rear surface passivated Cu(In,Ga)Se2 thin film solar cells with nano-sized local rear point contacts” Solar Energy Materials and Solar Cells 2013, 117, 505-511.B. Vermang, J. T. Watjen, V. Fjallstrom, F. Rostvall, M. Edoff, R. Kotipalli, F. Henry and D. Flandre, “Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se2 solar cells” Progress in Photovoltaics: Research and Applications 2014, 22, 1023-1029.

Claims

CLAIMSWhat is claimed is:

1. A method of preparing a solar cell, the method comprising: depositing an absorber layer on a surface passivation layer bearing nanometerscale point contact openings, the surface passivation layer being positioned between the absorber layer and a back contact layer of the solar cell, the back contact layer being in communication with a substrate layer of the solar cell; depositing a buffer layer on the absorber layer; and depositing a transparent contact layer on the buffer layer.

2. The method of claim 1, wherein the absorber layer comprises CuIn(S,Se)2.

3. The method of claim 1 or 2, wherein the absorber layer is deposited via a method comprising a solution-based process.

4. The method of any one of claims 1 to 3, wherein the absorber layer has a thickness of less than 1 pm.

5. The method of any one of claims 1 to 4, wherein the surface passivation layer is prepared by a method comprising: depositing a layer comprising nanoparticles on the back contact layer; depositing a layer comprising a surface passivation material on the layer comprising the nanoparticles, thereby forming a stack; and removing the nanoparticles from the surface of the stack, thereby forming the surface passivation layer bearing nanometer-scale point contact openings.

6. The method of claim 5, wherein the nanoparticles comprise cadmium sulfide, zinc sulfide, zinc oxide, molybdenum metal or silver metal.

7. The method of claim 6, wherein the nanoparticles comprise cadmium sulfide.

8. The method of any one of claims 5 to 7, wherein the layer comprising the nanoparticles is deposited via a method comprising chemical bath deposition.

9. The method of any one of claims 5 to 8, wherein the nanoparticles are removed from the surface of the stack via a method comprising agitating the stack.

10. The method of any one of claims 1 to 9, wherein the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm.

11. The method of any one of claims 1 to 10, wherein the surface passivation material comprises a metal oxide.

12. The method of claim 11, wherein the metal oxide comprises AI2O3, HfOx, Ga2Os, S i O2- TiO? or combinations thereof.

13. The method of claim 11, wherein the metal oxide comprises AI2O3.

14. The method of any one of claims 1 to 13, wherein the layer comprising the surface passivation material is deposited by a method comprising atomic layer deposition.

15. The method of any one of claims 1 to 14, wherein the layer comprising the surface passivation material has a thickness of about 2 nm to about 10 nm.

16. The method of any one of claims 1 to 15, wherein the absorber layer comprises CuIn(S,Se)2 and the method of depositing the absorber layer comprises: adding a source of sulfide, a source of copper and a source of indium to a solvent to prepare a molecular ink; coating the surface passivation layer with the molecular ink; annealing the molecular-ink coated surface passivation layer; optionally repeating the coating and annealing until a desired thickness has been reached; and selenization to obtain the CuIn(S,Se)2 absorber layer.

17. The method of claim 16, wherein the preparation of the molecular ink further comprises addition of a source of sodium ions to the solvent.

18. The method of claim 17, wherein the source of sodium ions comprises NaCl.

19. The method of any one of claims 16 to 18, wherein the source of sulfide comprises thiourea, the source of copper comprises copper (II) chloride, the source of indium comprises indium (III) chloride and the solvent comprises dimethylformamide.

20. The method of any one of claims 1 to 19, wherein the method further comprises etching the absorber layer prior to depositing the buffer layer.

21. The method of any one of claims 1 to 20, wherein the buffer layer comprises a metal sulfide.

22. The method of claim 21, wherein the metal sulfide is cadmium sulfide.

23. The method of claim 21 or 22, wherein the metal sulfide is deposited on the absorber layer by a method comprising chemical bath deposition.

24. The method of any one of claims 1 to 23, wherein the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer.

25. The method of claim 24, wherein the method comprises depositing the intrinsic zinc oxide layer on the buffer layer by a method comprising sputtering and depositing the indium tin oxide layer on the intrinsic zinc oxide layer by a method comprising sputtering.

26. The method of any one of claims 1 to 25, wherein the substrate layer comprises soda lime glass and wherein the back contact layer comprises molybdenum.

27. The method of any one of claims 1 to 26, wherein the method further comprises depositing a current collection grid on the transparent contact layer.

28. The method of any one of claims 1 to 27, wherein the method further comprises scribing the deposited layers down to the substrate layer.

29. A solar cell prepared by a method as defined in any one of claims 1 to 28.

30. A solar cell comprising a surface passivation layer bearing nanometer-scale point contact openings prepared by a method as defined in any one of claims 5 to 15 and an absorber layer prepared by a method as defined in any one of claims 3 and 16 to 19.

31. A solar cell comprising: an absorber layer; and a surface passivation layer bearing nanometer-scale point contact openings on a surface of the absorber layer.

32. The solar cell of claim 31, wherein the surface passivation layer is on the rear surface of the absorber layer.

33. The solar cell of claim 31 or 32, wherein the absorber layer has been deposited via a method comprising a solution-based process.

34. The solar cell of any one of claims 31 to 33, wherein the absorber layer has been prepared by a method as defined in any one of claims 16 to 19.

35. The solar cell of claim 31 or 32, wherein the absorber layer comprises CuIn(S,Se)2.

36. The solar cell of claim 35, wherein the CuIn(S,Se)2 has a [Cu] / [In] ratio of about 1.0.

37. The solar cell of any one of claims 31, 32, 35 or 36, wherein the absorber layer is doped with sodium ions.

38. The solar cell of any one of claims 31 to 37, wherein the absorber layer has a thickness of less than 1 pm.

39. The solar cell of any one of claims 31 to 38, wherein the surface passivation layer comprises a metal oxide.

40. The solar cell of claim 39, wherein the metal oxide comprises AI2O3, HfOx, Ga2Os, S i O2- TiO? or combinations thereof.

41. The solar cell of claim 39, wherein the metal oxide comprises AI2O3.

42. The solar cell of any one of claims 31 to 41, wherein the surface passivation layer has a thickness of about 2 nm to about 10 nm.

43. The solar cell of any one of claims 31 to 42, wherein the nanometer-scale point contact openings have an average diameter in the range of from about 300 nm to about 500 nm.

44. The solar cell of any one of claims 31 to 43, wherein the solar cell comprises: a substrate layer; a back contact layer in communication with the substrate layer, wherein the surface passivation layer is positioned between the back contact layer and the absorber layer; a buffer layer on the absorber layer; and a transparent contact layer on the buffer layer.

45. The solar cell of claim 44, wherein the substrate layer comprises soda lime glass and the back contact layer comprises molybdenum.

46. The solar cell of claim 44 or 45, wherein the buffer layer comprises cadmium sulfide.

47. The solar cell of any one of claims 44 to 46, wherein the transparent contact layer comprises an intrinsic zinc oxide layer and an indium tin oxide layer, the intrinsic zinc oxide layer being positioned between the buffer layer and the indium tin oxide layer.

48. The solar cell of any one of claims 31 to 47, further comprising an anti-reflective coating.

49. The solar cell of any one of claims 31 to 48, that has been scribed.

50. The solar cell of any one of claims 31 to 49, wherein the solar cell further comprises a current collection grid on the transparent contact layer.

51. Use of a solar cell as defined in any one of claims 29 to 50 in a tandem solar cell.

52. A tandem solar cell comprising a solar cell as defined in any one of claims 29 to 51.

53. The use of claim 51 or tandem solar cell of claim 52, wherein the tandem solar cell is a printed thin-film tandem solar cell.

54. The use or tandem solar cell of any one of claims 51 to 53, wherein the solar cell as defined in any one of claims 29 to 50 is the rear sub cell.

55. The use or solar cell of claim 54, wherein the front sub cell comprises a perovskite solar cell.

56. Use of a surface passivation layer bearing nanometer-scale point contact openings for passivation of an absorber layer in a solar cell, wherein the absorber layer has been deposited by a method comprising a solution-based process.

57. The use of claim 56, wherein the surface passivation layer is a rear surface passivation layer for the absorber layer in the solar cell.

58. The use of claim 56 or 57, wherein the absorber layer is as defined in any one of claims 35 to 38 or has been prepared by a method as defined in any one of claims 16 to 19; and / or wherein the surface passivation layer is as defined in any one of claims 39 to 43 or has been prepared by a method as defined in any one of claims 5 to 15.

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