WO2023018641A1

WO2023018641A1 - Methods for forming perovskite photovoltaic devices

Info

Publication number: WO2023018641A1
Application number: PCT/US2022/039685
Authority: WO
Inventors: Le Chen; Tze-bin SONG; Vera STEINMANN; Hsinhan Tsai; Xueping YI; Zhibo Zhao
Original assignee: First Solar, Inc.
Priority date: 2021-08-09
Filing date: 2022-08-08
Publication date: 2023-02-16

Abstract

Methods and compositions for forming perovskite absorber layers for use in manufacturing photovoltaic devices are described.

Description

METHODS FOR FORMING PEROVSKITE PHOTOVOLTAIC DEVICES

BACKGROUND

[0001] The present specification generally relates to photovoltaic devices, and, more specifically, to methods and compositions for forming perovskite absorber layers for use in manufacturing photovoltaic devices.

[0002] Photovoltaic devices generate electrical power by converting light into electricity using semiconductor materials that exhibit the photovoltaic effect. Perovskites are a class of materials which may form an active layer in photovoltaic devices. Perovskite compounds have an ABX3 structure, where A and B are cations and X is a halogen anion. Materials including lead halide and tin halide perovskite compounds have been studied for use in photovoltaic devices. In these structures, the A site may be composed of one or more cations, such as methylammonium (MA), formamidinium (FA), cesium (Cs⁺), or rubidium (Rb⁺). The B site may be occupied, for example, by one or more of lead (Pb⁺²), tin (Sn⁺²), or germanium (Ge⁺²) cations. And the X site may be occupied by one or more halogen anions, such as iodine (T), bromine (Br‘), or chlorine (Cl’). In a photovoltaic device, the perovskite material is positioned in contact with and between an electron charge transport layer and hole charge transport layer.

[0003] Methods of making perovskite materials may include a solution-based process performed by sequentially coating to form a metal halide film, followed by application of an organic cation solution on the metal halide film to form a bilayer of BX2 and AX films, and then the two layers are reacted to form the ABX3 structure. While solution-based methods have shown some improvements, existing deposition processes and compositions may yield perovskite absorbers having undesirable grain boundary defects, bulk defects, and surface defects. Such defects can lead to durability issues related to thermal instability and/or optical instability of the perovskite absorber layer. Specifically, thermal and optical instability can make perovskite absorber layers impractical for use in photovoltaic devices.

[0004] Methods are needed for making stable and highly efficient devices for large-scale manufacturing production. It would be advantageous to provide improved methods, systems, and structures for efficient and scalable fabrication of perovskite absorbers from perovskite precursors having superior perovskite absorber durability.

SUMMARY

[0005] The embodiments provided herein relate to methods for forming photovoltaic devices having a perovskite absorber layer. These and additional features provided by the embodiments described herein will be more fully understood in view of the following detailed description, in conjunction with the drawings.

DRAWINGS

[0006] The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

[0007] FIG. 1 schematically depicts a cross-sectional view of a N-I-P photovoltaic device according to one or more embodiments shown and described herein;

[0008] FIG. 2 schematically depicts a cross-sectional view of a P-I-N photovoltaic device according to one or more embodiments shown and described herein; and

[0009] FIGS. 3A and 3B schematically depict a two-step method for forming perovskite absorber layer according to one or more embodiments shown and described herein.

DETAILED DESCRIPTION

[0010] Embodiments of the present disclosure relate to photovoltaic devices with a perovskite absorber layer and methods of forming the same. Generally, the photovoltaic devices provided herein can include partly-formed or fully-formed photovoltaic modules. Various embodiments of the perovskite absorber layer, the partly-formed photovoltaic structure, and the photovoltaic device, as well as systems and methods for forming the layers, structures, and devices, will be described in more detail

[0011] Photovoltaic devices may contain several material layers deposited sequentially over a substrate. Steps for manufacturing a photovoltaic device may include sequentially disposing functional layers or layer precursors in a “stack” of layers through one or more deposition processes, including, but not limited to, spin coating, spray coating, slot-die coating, blade coating, dip coating, sputtering, evaporation, molecular beam deposition, pyrolysis, closed space sublimation (CSS), pulsed laser deposition (PLD), chemical vapor deposition (CVD), electrochemical deposition (ECD), atomic layer deposition (ALD), thermal evaporation, or vapor transport deposition (VTD). Manufacturing of photovoltaic devices can further include the selective removal of portions of certain layers of the stack of layers, such as by scribing, to divide the photovoltaic device into a plurality of photovoltaic cells.

[0012] Organic-inorganic or inorganic metal halide perovskite materials can be used to absorb light energy in photovoltaic devices. For example, perovskite absorber compounds can be formed from organic-inorganic perovskite materials having an ABX3 structure, where A can be an organic cation, B can be a metal cation and X can be a halogen anion. Specific materials and compounds have been studied for use in photovoltaic devices. In example perovskite structures, the A site may be occupied by one or more of MA, FA, Cs, or Rb. The B site may be occupied by one or more metal cation such as, for example, Pb, Sn, Ge, or other group 14 element. The X site may be occupied by one or more halogen such as, for example, I, Br, or Cl. In a photovoltaic device, the perovskite material is positioned in contact with and between an electron charge transport layer and hole charge transport layer. Perovskite photovoltaic devices may be configured in either a N- I-P or P-I-N orientation, with either the electron or hole charge transport layer towards the lightincident side of the device.

[0013] FIG. 1 schematically depicts a perovskite photovoltaic device 100 having an N-I-P structure, i.e., the electron charge transport layer is proximate the light-incident side. The hole transport layer of the N-I-P structure is further from the light-incident side relative to the electron transport layer. FIG. 2 schematically depicts a perovskite photovoltaic device 200 having an P-I- N structure, i.e., the hole transport layer is proximate the light-incident side. The electron transport layer of the P-I-N structure is further from the light-incident side relative to the hole transport layer.

[0014] Referring collectively to FIGS. 1 and 2, photovoltaic device 100 and photovoltaic device 200 can be configured to receive light and transform light into electrical signals, e.g., photons can be absorbed from the light and transformed into electrical signals via the photovoltaic effect. Accordingly, the photovoltaic device 100 and photovoltaic device 200 can define an energy, light-incident, or front side 102 configured to be exposed to a light source such as, for example, the sun. The photovoltaic device 100 and photovoltaic device 200 can also define an opposing side 104 offset from the light-incident side 102 such as, for example, by a plurality of material layers. It is noted that the term “light” can refer to various wavelengths of the electromagnetic spectrum such as, but not limited to, wavelengths in the ultraviolet (UV), infrared (IR), and visible portions of the electromagnetic spectrum. “Sunlight,” as used herein, refers to light emitted by the sun.

[0015] Photovoltaic device 100 and photovoltaic device 200 can include a plurality of layers disposed between the light-incident front side 102 and the opposing side 104. As used herein, the term “layer” refers to a thickness of material provided upon a surface. Each layer can cover all or any portion of the surface. In some embodiments, the layers of photovoltaic device 100 and photovoltaic device 200 can be divided to form a plurality of photovoltaic cells. For example, the photovoltaic cells can be scribed according to a plurality of serial scribes and a plurality of parallel scribes.

[0016] The layers of photovoltaic device 100 and photovoltaic device 200 can include a substrate 110 configured to facilitate the transmission of light into the one or more active layers. The substrate 110 can be disposed at the front side 102. The substrate 110 can have a first surface 112 substantially facing the front side 102 and a second surface 114 substantially facing the opposing side 104. One or more layers of material can be disposed between the first surface 112 and the second surface 114 of the substrate 110.

[0017] The substrate 110 can be substantially transparent. In some embodiments, the substrate comprises a substantially transparent material such as, for example, glass. Suitable glass can include soda-lime glass, a glass with reduced iron content, or a glass with about 90% transmittance. Optionally, the substrate 110 can include a performance coating applied to form the exterior or front side 102. The performance coating can be configured to interact with light or to improve durability of the substrate 110 such as, but not limited to, an antireflective coating, an antisoiling coating, or a combination thereof.

[0018] Photovoltaic device 100 and photovoltaic device 200 can optionally include a barrier layer 130 configured to mitigate diffusion of contaminants from the substrate 110, which could result in degradation or delamination. The barrier layer 130 can have a first surface 132 substantially facing the front side 102 and a second surface 134 substantially facing the opposing side 104. In some embodiments, the barrier layer 130 can be provided adjacent to the substrate 110. For example, the first surface 132 of the barrier layer 130 can be provided upon the second surface 114 of the substrate 110. The phrase "adjacent to," as used herein, means that two layers are disposed contiguously and without any intervening materials between at least a portion of the layers.

[0019] The barrier layer 130, when present, may be substantially transparent, thermally stable, with a reduced number of pin holes, having sodium-blocking capability, and/or having good adhesive properties. Alternatively or additionally, the barrier layer 130 can be configured to apply color suppression to light. The barrier layer 130 can include one or more layers of suitable material, including, but not limited to, tin oxide, silicon dioxide, aluminum-doped silicon oxide, silicon oxide, silicon nitride, or aluminum oxide. The barrier layer 130 can have any suitable thickness bounded by the first surface 132 and the second surface 134, including, for example, more than about 100 A in one embodiment, more than about 150 A in another embodiment, or less than about 200 A in a further embodiment. In some embodiments, the barrier layer 130 can be omitted.

[0020] Photovoltaic device 100 and photovoltaic device 200 can include an electrode layer 140 configured to provide electrical contact to transport charge carriers generated by photo electric conversion of light into electrical power such as, for example, electrons (negative charge carriers). Accordingly, the electrode layer 140 may function as an anode. The electrode layer 140 can have a first surface 142 substantially facing the energy side 102 and a second surface 144 substantially facing the opposing side 104. In a P-I-N structure of photovoltaic device 200, the electrode layer 140 may be referred to as a back contact. In a N-I-P structure of photovoltaic device 100, the electrode layer 140 is substantially transparent and may be referred to as a front contact or transparent electrode layer. In some photovoltaic devices 100 with a N-I-P structure, the electrode layer 140 can be provided adjacent to the barrier layer 130 or adjacent to the substrate 110. The electrode layer 140 can be formed from one or more layers of an n-type semiconductor material. The electrode layer 140 may have a wide band gap. Specifically, the wide band gap can have a larger energy value compared to the energy of the photons of the light, which can mitigate undesired absorption of light. The electrode layer 140 can include one or more layers of suitable material, including, but not limited to, tin oxide, tin dioxide, indium tin oxide, or cadmium stannate, and the material may be doped.

[0021] Photovoltaic device 100 and photovoltaic device 200 can include an electron transport layer (ETL) 150. The electron transport layer (ETL) may also be called a negative charge transport layer, a n-type contact, an e" selective contact, or an electron-selective layer. The ETL 150 can have a first surface 152 substantially facing the front side 102 and a second surface 154 substantially facing the opposing side 104. The ETL 150 may be positioned adjacent to the transparent electrode layer 140. For example, in a N-I-P structure of photovoltaic device 100, the first surface 152 of the ETL 150 can be provided in contact with the second surface 144 of the transparent electrode layer 140. The ETL 150 may have any suitable thickness between the first surface 152 and the second surface 154, including, for example, more than about 100 A in one embodiment, between about 100 A and about 800 A in another embodiment, or between about 150 A and about 600 A in a further embodiment.

[0022] Photovoltaic device 100 and photovoltaic device 200 can include a perovskite absorber layer 160 comprising a perovskite material configured to cooperate with adjacent layers to form a N-I-P or P-I-N junction for conversion of optical energy. Accordingly, absorbed photons of light can free electron-hole pairs and generate carrier flow, which can yield electrical power.

[0023] Lead halide and other metal halide perovskite compounds may be used in the perovskite absorber layer of a photovoltaic device. The metal halide perovskite compounds have an ABX3 structure, where A and B are cations and X is a halogen anion. In examples, the A site may be occupied by one or more organic or inorganic cations. For example, the A site may be composed of one or more of: methylammonium (MA), formamidinium (FA), cesium (Cs), or rubidium (Rb) cations. The B site may be occupied by one or more metals, such as lead (Pb) or tin (Sb). And the X site may be occupied by one or more halides, such as iodine (I), bromine (Br), or chlorine (Cl). In a photovoltaic device, the perovskite absorber layer of perovskite material may be positioned in contact with and between a negative charge transport layer and positive charge transport layer.

[0024] The perovskite absorber layer may be formed by selecting and reacting one or more A-type cations or A-X-type compounds with one or more B-X metal halide compounds. Metal halide materials suitable for use in forming a perovskite compound for the perovskite absorber layer include iodides, bromides, and/or chlorides, in combination with a metal, alkali metal and/or combinations thereof. Metal halide materials suitable for use in a perovskite compound include, but are not limited to, lead iodide (Pbb), cesium iodide (CsI), lead bromide (PbBn), cesium bromide (CsBr), cesium lead iodide (CsPbb), cesium tin iodide (CsSnF), lead chloride (PbCh), tin iodide (Snh), tin bromide (SnBn), and/or tin chloride (SnCh).

[0025] The perovskite absorber layer 160 can have a first surface 162 substantially facing the energy side 102 and a second surface 164 substantially facing the opposing side 104. A thickness of the perovskite absorber layer 160 can be defined between the first surface 162 and the second surface 164. In an example device, the thickness of the perovskite absorber layer 160 can be between about 250 nm to about 5,000 nm such as, for example, between about 400 nm to about 2,000 nm in one embodiment, or between about 500 nm to about 1,500 nm in another embodiment.

[0026] Photovoltaic device 100 and photovoltaic device 200 can include a hole transport layer 180. The hole transport layer 180 may also be called a positive charge transport layer, p-type contact, hole transport material, h⁺ selective contact, or a hole-selective layer. It may be positioned in contact with a conducting layer which functions as a cathode. The hole transport layer 180 provides electrical contact to the perovskite absorber layer 160. The hole transport layer 180 can have a first surface 182 substantially facing the front side 102 and a second surface 184 substantially facing the opposing side 104. A thickness of the hole transport layer 180 can be defined between the first surface 182 and the second surface 184. The thickness of the hole transport layer 180 can be between about 2 nm to about 200 nm, or, between about 2 nm to about 100 nm. In an N-I-P structure of photovoltaic device 100, the hole transport layer 180 may be provided adjacent to the perovskite absorber layer 160 on the back side, such that the first surface 182 of the hole transport layer 180 contacts the second surface 164 of the perovskite absorber layer 160. In aP-I-N structure of photovoltaic device 200, the hole transport layer 180 may contact the perovskite absorber layer 160 on the front or light-facing side, such that the second surface [0027] In some embodiments, the hole transport layer 180 can include nickel oxides (NiO_x). In some devices, a hole transport layer 180 can include an organic compound, such as 2, 2', 7,7'- Tetrakis [N,N-di (4-methoxyphenyl) amino] -9,9'-spirobifluorene (spiro-OMeTAD), poly[bis(4- phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and/or poly(3-hexylthiophene-2,5-diyl) (P3HT).

[0028] Photovoltaic device 100 and photovoltaic device 200 can include a conducting layer 190, which can operate as a cathode. The conducting layer 190 can have a first surface 192 substantially facing the energy side 102 and a second surface 194 substantially facing the opposing side 104. In embodiments of the photovoltaic device 100 having a N-I-P structure, as depicted in FIG. 1, the conducting layer 190 may function as a back electrode and be provided adjacent to the hole transport layer 180 such that the first surface 192 of the conducting layer 190 contacts the second surface 184 of the hole transport layer 180.

[0029] In embodiments the photovoltaic device 200 having a P-I-N structure, as shown in FIG. 2, the conducting layer 190 is provided between the substrate 110 and absorber 160. Optionally, the conducting layer 190 can be adjacent to the hole transport layer 180 such that the second surface 194 of the conducting layer 190 contacts the first surface 182 of the hole transport layer 180. In photovoltaic devices 200 with a P-I-N structure, both the hole transport layer 180 and the conducting layer 190 are substantially transparent.

[0030] In some embodiments, the conducting layer 190 can comprise a conducting material such as, for example, one or more layers of a composition such as a metal, a metal oxide, or nitrogen-containing metal, silver, nickel, copper, aluminum, titanium, palladium, chrome, molybdenum, gold, or the like. Examples of a nitrogen-containing metal layer can include aluminum nitride, molybdenum nitride, nickel nitride, titanium nitride, tungsten nitride, selenium nitride, tantalum nitride, or vanadium nitride. An example metal oxide conducting material includes tin oxide.

[0031] Photovoltaic device 100 and photovoltaic device 200 can include a back support 196 configured to cooperate with the substrate 110 to form a housing for the one or more photovoltaic cells. The back support 196 can be disposed at the opposing side 104 of photovoltaic device 100 and photovoltaic device 200. As an example, in photovoltaic devices 100 having an N-I-P structure, as depicted in FIG. 1, the back support 196 can be formed adjacent to the conducting layer 190. Alternatively, in photovoltaic devices 200 having a P-I-N structure, as depicted in FIG. 2, the back support 196 may be formed adjacent to the electrode layer 140. The back support 196 can include any suitable material, including, for example, glass (e.g., soda-lime glass). [0032] Referring collectively to FIGS. 2 and 3A, a two-step method 210 for forming a perovskite absorber layer 160 on a charge transport layer of the photovoltaic device 200 having a P-I-N configuration is depicted. For embodiments formed in the P-I-N configuration, the perovskite absorber layer 160 can be formed over the hole transport layer 180. For the purpose of defining and describing the embodiments provided herein, the term “two-step” defines a process for forming a perovskite absorber layer 160 that includes the formation of a metal halide film 230 and the conversion of the metal halide film 230 into the perovskite absorber layer 160 using an organic cation solution 214. The two-step method 210 is distinguished from a “one-step” process for forming perovskites, whereby the perovskite is created from a single solution without formation of a metal halide film.

[0033] The two-step method 210 can include a process 220 for applying a metal halide solution 222 to the hole transport layer 180. During process 220, the metal halide solution 222 can be applied directly to the second surface 184 of the hole transport layer 180. Accordingly, the metal halide solution 222 can directly contact the second surface 184 of the hole transport layer 180. Suitable application techniques include, but are not limited to, spin coating, roll coating, spraying, or the like. The metal halide solution 222 can include one or more metal halide materials dissolved with a solvent at a molarity of between about 0.5 M and about 5 M such as, for example, between about 0.75 M and about 3 M in some embodiments, and between about 1 M and about 2 M in other embodiments. As noted above, suitable metal halide materials for use in the metal halide solution 222 include, but are not limited to, lead iodide (Pbb), cesium iodide (CsI), lead bromide (PbBn), cesium bromide (CsBr), cesium lead iodide (CsPbb), cesium tin iodide (CsSnF), lead chloride (PbCh), tin iodide (Snb), tin bromide (SnBn), and/or tin chloride (SnCh). Suitable solvents include high polarity solvents, i.e., polar solvents having a Snyder polarity of greater than 4.5, such as, for example, Dimethylformamide (DMF), Dimethylsulfoxide (DMSO), N- Methylformamide (NMF), gamma-butyrolactone (GBL), acetonitrile (ACN), and combinations thereof.

[0034] According to the embodiments provided herein, the metal halide solution 222 can include a pseudohalide salt configured to improve the durability of the perovskite absorber layer 160. Pseudohalide salts can include an organic cation combined with various organic or inorganic anions. The negative charge anion can operate to passivate halide vacancies in metal halides such as, for example, iodide vacancies. Without being bound to theory, it is believed that passivating such vacancies can reduce defects present at the bulk, surface or the grain boundaries of the perovskite absorber layer 160.

[0035] Applicant has discovered that the energy needed to insert psuedohalide salts into the crystalline structure of metal halides and perovskite materials is large enough to inhibit diffusion. To avoid the diffusion process, pseudohalide salts can be added to the metal halide solution 222. The pseudohalide salt can be provided as an additive to the metal halide solution 222 at a relatively lower amount compared to the metal halide. Specifically, the molarity of the pseudo halide salt in the metal halide solution 222 can be between about 0.01% M and about 25% M such as, for example, between about 0.02% M and about 5% M in one embodiment, or between about 0.02% M and about 1% M.

[0036] Psuedohalide salts include compounds having similar chemistry to halide ions such as, for example, BF4- class compounds, ammonium acids, BF6- class compounds, SCN- class compounds, OCN- class compounds, and HCOO- class compounds. Suitable BF4- class compounds can include l-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), Tetrabutylammonium Tetrafluoroborate, Formamidinium Tetrafluoroborate (FABF4), Methylammonium Hexafluorophosphate, Methylammonium Tetrafluoroborate (MABF4), Benzylammonium Tetrafluoroborate (BABF4), n-Propylammonium Tetrafluoroborate (PABF4), and Guanidinium Tetrafluoroborate (GABF4). Suitable ammonium acids can include 5- Ammonium valeric acid iodide (5-AVAI), 5-Ammonium valeric acid bromide (5-AVAB), 5- Ammonium valeric acid chloride (5-AVAC), 2-Furanemethylammonium iodide, 2- Furanemethylammonium bromide, 2-Furanemethylammonium chloride, Biphenylammonium bromide, Biphenylammonium iodide, Biphenylammonium chloride, 1 -Naphthylammonium chloride, 2-Naphthylammonium bromide, and 1 -Naphthylammonium bromide. Suitable SCN- class compounds can include Methylamine Thiocyanate, Formamidine Thiocyanate, and Guanidine Thiocyanate. Suitable OCN- class compounds can include NaOCN, KOCN, 1,4- Diisocyanatobutane, Tetrabutylammonium cyanate, 1,8-Diisocyanatooctane. Suitable HCOO- class compounds can include Formamidine Formate.

[0037] Referring still to FIGS. 2 and 3A, the two-step method 210 can include a process 224 for forming a metal halide film 230 from the metal halide solution 222. In some embodiments, the metal halide solution 222 can be heated to remove the solvent. For example, the metal halide solution 222 can be heated at temperature in a range of about 60° C to about 80° C for about 1 minute to about 5 minutes. As noted above, the metal halide solution 222 includes psuedohalide salts. Accordingly, the psuedohalide salts can be pre-incorporated into the metal halide film 230 such that the pseudohalide salt operates to passivate halide vacancies within the metal halide film 230. The metal halide film 230 can have a first surface 232 substantially facing the substrate 110 and a second surface 234 offset from the first surface 234 by a thickness of the metal halide film 230. In an example embodiment, the thickness of the metal halide film 230 can be between about 50 nm to about 5,000 nm such as, for example, between about 100 nm to about 2,000 nm in one embodiment, or between about 150 nm to about 500 nm in another embodiment.

[0038] The two-step method 210 can include a process 236 for applying an organic cation solution 238 to the metal halide film 230. During process 236, the organic cation solution 238 can be applied directly to the second surface 234 of the metal halide film 230. Accordingly, the organic cation solution 238 can diffuse from the second surface 234 of the metal halide film 230 towards the first surface 232 of the metal halide film 230. Suitable application techniques include, but are not limited to, spin coating, roll coating, spraying, or the like. The organic cation solution 238 can include one or more A-site cations dissolved with a solvent at a molar concentration of between about 0.1 M and about 4.5 M such as, for example between about 0.25 M and about 3M in some embodiments, or between about 0.5 M and about 2 M in other embodiments. As noted above, organic cation materials for use in the organic cation solution 238 include organic A-site cations suitable as perovskite precursors such as, for example, methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or the like. Optionally, the organic cation solution 238 can further include inorganic A-site cations suitable as perovskite precursors such as, for example, cesium (Cs) cations, rubidium (Rb) cations, potassium (K) cations, or the like. Suitable solvents include low polarity solvents, solvents having a Snyder polarity of 4.5 or less such as, for example, proponal, n-butanol, ethanol, Isobutyl alcohol, and combinations thereof.

[0039] The two-step method 210 can include a process 240 for converting the organic cation solution 238 and the metal halide film 230 into the perovskite absorber layer 160. Conversion can include heat treatment performed at temperatures lower than 200° C such as, for example between about 90° C and about 160° C in one embodiment, or between about 100° C and about 150° C in another embodiment. The heat treatment may be performed for durations of less than about 120 minutes, for example, less than about 45 minutes in one embodiment.

[0040] Referring collectively to FIGS. 1, 2, 3A and 3B, the two step method 210 can be utilized to form the perovskite absorber layer 160 of photovoltaic devices 100 having an N-I-P structure, as depicted in FIG. 1, and the perovskite absorber layer 160 of photovoltaic devices 200 having a P-I-N structure, as depicted in FIG. 2. The two step method 210 is substantially similar when utilized for the N-I-P structure and the P-I-N structure, i.e., the process parameters, composition of the metal halide solution 222, the organic cation solution 238 and composition of the metal halide film 230 can be the same. The primary differences in application to the N-I-P structure and the P-I-N structure are related to the order of layer formation, which is described in further detail herein below.

[0041] With specific reference to FIGS. 1 and 3B, the two step method 210 a two-step method 210 for forming a perovskite absorber layer 160 on a charge transport layer of the photovoltaic device 100 having a N-I-P configuration is depicted. The two-step method 210 can include process 220 for applying the metal halide solution 222 to a charge transport layer. In the N-I-P configuration, the metal halide solution 222 can be applied directly to the second surface 154 of the electron transport layer 150. Accordingly, the metal halide solution 222 can directly contact the second surface 154 of the electron transport layer 150.

[0042] Next at process 224, the metal halide film 230 can be formed from the metal halide solution 222. Accordingly, the psuedohalide salts can be pre-incorporated into the metal halide film 230 such that the pseudohalide salt operates to passivate halide vacancies within the metal halide film 230. The two-step method 210 can further include process 236 for applying the organic cation solution 238 to the metal halide film 230. Next at process 240, the organic cation solution 238 and the metal halide film 230 can be converted into the perovskite absorber layer 160.

[0043] Examples

[0044] Multiple comparative examples were prepared using a two-step method for formation of the perovskite absorber layer. The metal halide solution included 1.5 M solution of PbL and 0.15 M Cs dissolved in a DMF/DMSO solution (900/100pL). The metal halide solution was deposited at room temperature by spin coating at 1500 rpm for 30, and then heated at temperature in a range of 60-80 C for 1-5 minutes. The organic cation solution was prepared with 90 mg/mL of FAI, 6 mg/mL of MAI, 4 mg/mL MAC1, and 9 mg/mL of DMA with an alcohol solvent. The organic cation solution was applied at room temperature onto the metal halide film by spin coating for 30 seconds at 2000 rpm. The metal halide films and organic cation solution were heat treated in air at about 150° C for 25 minutes at ambient pressure. After thermal annealing, conversion of the precursor materials to the perovskite layer was complete. The formed perovskite absorber layer 160 had a substantially uniform thickness of about 700 nm.

[0045] Examples of the embodiments described herein were formed similarly to the comparative examples, except 0.03% M of BMIMBF4 was added to the metal halide solution. Each of the comparative example and the example cells were formed from 2X2 inch coupons that were sub-divided into cells having NIP structures with a Glass/FTO/NiOx/PTAA/perovskite absorber layer/PCBM/BCP/Ag stack. The cells were measured for initial efficiency and efficiency after stressing under light soak conditions. The efficiency measurements were conducted in air using a solar simulator with a forward seep (F) and a reverse sweep (R). The light soak was conducted in an N2 environment at 55° C performed with continuous 1 sun of illumination. The results of the forward sweep are summarized below in Table 1. The results of the reverse sweep are summarized below in Table 2. [0046] Table 1

[0047] Referring above to Table 1, the forward scan efficiency measurements are normalized to the efficiency of the Comparative Example prior to light soak. For measurements that failed to collect an “X” is indicatied. Generally, the Examples demonstrated more stability relative to the Comparative Example throughout the 14 day light soak test. Although each of the Examples had lower initial normalized efficiency than the Comparative Example, all but Example 1 and Example 10 (missing data) achieved greater normalized efficiency than the Comparative Example after 14 days of light soak. However, both Example 1 and Example 10 appear to be more stable than the Comparative Example. Example 1 lost 10% of its initial normalized efficiency over the 14 day light soak compared to 20% loss from the Comparative Example. Similarly, Example 10 demonstrated 3% loss of initial normalized efficiency over a7.1 day light soak compared to 5% loss from the Comparative Example.

[0048] Table 2

[0049] Referring above to Table 2, the reverse scan efficiency measurements are normalized to the efficiency of the Comparative Example prior to light soak. For measurements that failed to collect an “X” is indicatied. Generally, the Examples demonstrated more stability relative to the Comparative Example throughout the 14 day light soak test. Although the Examples, except for Example 9, had lower initial normalized efficiency than the Comparative Example, all but Example 10 achieved greater normalized efficiency than the Comparative Example after 14 days of light soak. However, Example 10 appears to be more stable than the Comparative Example.

[0050] It should now be understood that defects can be ameliorated by adding pseudohalide salts into the metal halide solution of a two-step process for forming perovskite absorber layers. The control of such defects can improve the thermodynamic stability of perovskite absorber layers and operational stability of perovskite solar cells.

[0051] According to the embodiments provided herein, a two-step method of forming a perovskite absorber layer can include applying a metal halide solution to stack of layers comprising a charge transport layer. The metal halide solution can include a psuedohalide salt. The metal halide solution can be heated. A metal halide film can be formed on the stack of layers. The pseudohalide salt can be preincorporated into the metal halide film. An organic cation solution can be applied to the metal halide film. The organic cation solution can include an organic A-site cation perovskite precursor. The organic solution and the metal halide film can be converted into the perovskite absorber layer.

[0052] It is noted that the terms "substantially" and "about" may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0053] While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications may be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of the claimed subject matter.

Claims

CLAIMS What is claimed is:

1. A two-step method of forming a perovskite absorber layer comprising: applying a metal halide solution to stack of layers comprising a charge transport layer, wherein the metal halide solution comprises a psuedohalide salt; heating the metal halide solution, whereby a metal halide film is formed on the stack of layers, wherein the pseudohalide salt is preincorporated into the metal halide film; applying an organic cation solution to the metal halide film, wherein the organic cation solution comprises an organic A-site cation perovskite precursor; and converting the organic solution and the metal halide film into the perovskite absorber layer.

2. The two-step method of claim 1, wherein the metal halide solution has a lower amount of the psuedohalide salt relative to a metal halide of the metal halide solution.

3. The two-step method of claim 2, wherein the metal halide solution comprises between 0.01% M and 25% M of the psuedohalide salt.

4. The two-step method of claim 3, wherein the metal halide is in the metal halide solution at a molarity between 0.5 M and 5 M.

5. The two-step method of claim 1, wherein the pseudohalide salt is a BF4- class compound.

6. The two-step method of claim 5, wherein the BF4- class compound is l-Butyl-3- methylimidazolium tetrafluoroborate (BMIMBF4), Tetrabutylammonium Tetrafluoroborate, Formamidinium Tetrafluoroborate (FABF4), Methylammonium Hexafluorophosphate, Methylammonium Tetrafluoroborate (MABF4), Benzylammonium Tetrafluoroborate (BABF4), n-Propylammonium Tetrafluoroborate (PABF4), or Guanidinium Tetrafluoroborate (GABF4).

7. The two-step method of claim 1, wherein the pseudohalide salt is an ammonium acid.

8. The two-step method of claim 7, wherein the ammonium acid is 5-Ammonium valeric acid iodide (5-AVAI), 5-Ammonium valeric acid bromide (5-AVAB), 5-Ammonium valeric acid chloride (5-AVAC), 2-Furanemethylammonium iodide, 2-Furanemethylammonium bromide, 2- Furanemethylammonium chloride, Biphenylammonium bromide, Biphenylammonium iodide, Biphenylammonium chloride, 1 -Naphthylammonium chloride, 2 -Naphthyl ammonium bromide, or 1 -Naphthylammonium bromide.

9. The two-step method of claim 1, wherein the pseudohalide salt is a SCN- class compound.

10. The two-step method of claim 9, wherein the SCN- class compound is Methylamine Thiocyanate, Formamidine Thiocyanate, or Guanidine Thiocyanate.

11. The two-step method of claim 1, wherein the pseudohalide salt is an OCN- class compound.

12. The two-step method of claim 11, wherein the OCN- class compound is NaOCN, KOCN, 1,4-Diisocyanatobutane, Tetrabutylammonium cyanate, or 1,8-Diisocyanatooctane.

13. The two-step method of claim 1, wherein the pseudohalide salt is an HCOO- class compound.

14. The two-step method of claim 13, wherein the HCOO- class compound is Formamidine Formate.

15. The two-step method of claim 1, wherein the metal halide solution is applied directly to the charge transport layer.

16. The two-step method of claim 1, wherein the organic A-site cation perovskite precursor is methylammonium (MA), formamidinium (FA), or dimethylammonium (DMA).

17. The two-step method of claim 1, wherein the organic cation solution comprises an inorganic A-site cation perovskite precursor.

18. The two-step method of claim 1, wherein the inorganic A-site cation perovskite precursor is a cesium (Cs) cation, rubidium (Rb) cation, or potassium (K) cation.

19. The two-step method of any of claims 1-2, wherein the metal halide solution comprises between 0.01% M and 25% M of the psuedohalide salt.

20. The two-step method of any of claims 1-2 and 19, wherein the metal halide is in the metal halide solution at a molarity between 0.5 M and 5 M.

21. The two-step method of any of claims 1-2 and 19-20, wherein the pseudohalide salt is a BF4- class compound.

22. The two-step method of claim 21, wherein the BF4- class compound is l-Butyl-3- methylimidazolium tetrafluoroborate (BMIMBF4), Tetrabutylammonium Tetrafluoroborate, Formamidinium Tetrafluoroborate (FABF4), Methylammonium Hexafluorophosphate, Methylammonium Tetrafluoroborate (MABF4), Benzylammonium Tetrafluoroborate (BABF4), n-Propylammonium Tetrafluoroborate (PABF4), or Guanidinium Tetrafluoroborate (GABF4).

23. The two-step method of any of claims 1-2 and 19-20, wherein the pseudohalide salt is an ammonium acid.

24. The two-step method of claim 23, wherein the ammonium acid is 5-Ammonium valeric acid iodide (5-AVAI), 5-Ammonium valeric acid bromide (5-AVAB), 5-Ammonium valeric acid chloride (5-AVAC), 2-Furanemethylammonium iodide, 2-Furanemethylammonium bromide, 2- Furanemethylammonium chloride, Biphenylammonium bromide, Biphenylammonium iodide, Biphenylammonium chloride, 1 -Naphthylammonium chloride, 2 -Naphthyl ammonium bromide, or 1 -Naphthylammonium bromide.

25. The two-step method of any of claims 1-2 and 19-20, wherein the pseudohalide salt is a SCN- class compound.

26. The two-step method of claim 25, wherein the SCN- class compound is Methylamine Thiocyanate, Formamidine Thiocyanate, or Guanidine Thiocyanate.

27. The two-step method of any of claims 1-2 and 19-20, wherein the pseudohalide salt is an OCN- class compound.

28. The two-step method of claim 27, wherein the OCN- class compound is NaOCN, KOCN, 1,4-Diisocyanatobutane, Tetrabutylammonium cyanate, or 1,8-Diisocyanatooctane.

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29. The two-step method of any of claims 1-2 and 19-20, wherein the pseudohalide salt is an HCOO- class compound.

30. The two-step method of claim 29, wherein the HCOO- class compound is Formamidine Formate.

31. The two-step method of any of claims 1-2 and 19-30, wherein the metal halide solution is applied directly to the charge transport layer.

32. The two-step method of any of claims 1-2 and 19-31, wherein the organic A-site cation perovskite precursor is methylammonium (MA), formamidinium (FA), or dimethylammonium (DMA).

33. The two-step method of any of claims 1-2 and 19-32, wherein the organic cation solution comprises an inorganic A-site cation perovskite precursor.

34. The two-step method of any of claims 1-2 and 19-33, wherein the inorganic A-site cation perovskite precursor is a cesium (Cs) cation, rubidium (Rb) cation, or potassium (K) cation.

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