Perovskite photovoltaic devices and methods for fabrication by solution processing

Abstract

The present invention relates to a carbon ink composition for use in printing a conductive carbon layer onto a perovskite solar device laminate. Additionally, a perovskite solar device laminate comprising a conductive carbon layer and a method for manufacturing such a device by solution processing methods are also provided.

Description

[Technical Field]

[0001] The present invention relates to a carbon ink composition for use in printing a conductive carbon layer onto a perovskite solar device laminate. Additionally, a perovskite solar device laminate comprising a conductive carbon layer and a method for manufacturing such a device by solution processing methods are also provided. [Background technology]

[0002] Interest in perovskite photovoltaics has grown significantly, with significant improvements in the performance of laboratory-based devices being reported. [1-3] Since the first report of perovskite-based devices, device efficiency has increased dramatically, achieving power conversion efficiencies (PCEs) of over 25%. In addition to impressive device performance improvements in laboratory-scale fabrication, perovskite photovoltaics as solution-processed devices promise high scalability through roll-to-roll coating processes.

[0003] Among the available coating technologies, the feasibility of scaling up perovskite photovoltaics (PV) has been largely explored through slot-die coating processes. This industrially scalable process promises fast roll-to-roll (R2R) processing of perovskite PV devices, and when combined with R2R encapsulation, it offers an attractive proposition: large quantities of photovoltaic devices can be manufactured to completion in a single process. Research in this area is ongoing with feasibility demonstrations performed in benchtop deposition tools for both pin and nip device stacks.

[0004] More recently, perovskite PV devices have been demonstrated that are partially R2R coated. In one study, a two-step continuous process for fabricating R2R nip devices by slot-die coating with PCE as high as 13% was demonstrated.

[23] Othman et al. first applied triple cation mixed halide perovskites in inverted pin stacks using R2R coating and achieved a PCE of 12%.

[24] In their work, the lower hole transport layer was also modified with guanidine iodide additive to form millimeter-sized perovskite clusters. The inventors of the present invention have previously demonstrated a PET / ITO / PEDOT:PSS / MAPI / PCBM / BCP pin device structure, in which all layers except the conductive top electrode were coated sequentially with R2R, achieving a PCE of 12.2%.

[25] Recently, the feasibility of laminated electrodes composed of multiple silver and carbon laminates has been demonstrated by applying the electrode to the laminate with a roll-to-roll coating using a calendar press.

[26] .

[0005] These represent important advances toward the bright prospects of large-area, mass-production-scale solution-processed perovskite devices and are summarized in Table 1. However, in almost all R2R perovskite photovoltaic work to date, the deposition of the final electrodes is achieved by evaporation of precious and resource-limited metals such as gold or silver.

[0006] While the evaporation of gold or silver as contact electrodes has enabled researchers to fabricate devices efficiently and consistently, this slow, expensive and wasteful process remains a significant barrier to perovskites leaving the lab and finding real-world applications.

[0007] It is therefore an object of the present invention to provide a solution to this slow and costly manufacturing process by producing photovoltaic devices with printed solution-based top electrodes, optionally by roll-to-roll processing.

[0008] Printed carbon electrodes have proven effective in triple mesoporous structures, but require several high-temperature (>450 °C) processing steps that are not compatible with the polymer film substrates typically required for R2R processing, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN).

[35] Although carbon electrodes have been successfully utilized in laboratory-scale planar device architectures, several major issues exist that prevent carbon-electrode perovskite solar cells (PSCs) from achieving high performance.

[0009] Zhang et al. overcome the solvent compatibility issue by solvent exchange and hot pressing a dry carbon layer onto the finished device.

[41] An alternative strategy is to protect the perovskite layer using copper thiocyanate.

[42] or interlayer - a vapor deposited interlayer to enable integration of the carbon electrode

[43] Another innovative research project is to use complex or high-temperature processes such as

[44] used spray-coated graphene electrodes on both sides of a modular stacked top-contact PSC design, whose sheet resistance and thickness significantly decreased under applied pressure. However, due to the complexity, high temperature, or carbon electrode formulation, these solutions cannot be scaled up by R2R processing.

[0010] To overcome both the scalability and compatibility issues, Gaveshana et al. introduced a novel bicomponent electrode comprising a modified aluminum foil and a carbon layer.

[45] Their electrodes are compatible with the underlying layers and can be easily pressed or laminated, but aluminum foil, on the other hand, cannot be applied for continuous roll-to-roll operation.

[0011] It is therefore a further object of the present invention to provide a carbon ink formulation suitable for the deposition of a carbon top electrode onto a perovskite device stack by solution-based processes such as slot-die coating, screen printing, flexographic printing, blade coating, gravure printing, slide coating, curtain coating, inkjet printing, roll coating, curtain coating, and / or wire bar coating that are compatible with R2R.

[0012] As one skilled in the art will readily appreciate, formulation of a carbon ink that is compatible with the other perovskite device layers is critical for the realization of such solution-based carbon electrodes, and solvent orthogonality with the device stack is a limiting factor. Furthermore, to enable coating of the entire perovskite device stack, including the electrodes, the device architecture must be designed for process compatibility, i.e., low-temperature processing, layer orthogonality, layer-to-layer alignment, and solvent compatibility.

[0013] Thus, disclosed herein for the first time are unique carbon ink formulations and all-roll-printable perovskite photovoltaic device structures that use such formulations to deposit a perovskite-compatible back electrode in place of conventional evaporated metal contacts, overcoming interlayer incompatibilities and recombination losses to provide device performance comparable to devices using evaporated gold electrodes. Furthermore, such roll-to-roll printed devices surprisingly exhibit superior long-term stability compared to typical PSC devices with conventional evaporated metal top electrodes. Summary of the Invention

[0014] The invention, in its various aspects, is as set out in the appended claims.

[0015] According to a first aspect of the present invention, there is provided a carbon ink composition for use in printing a conductive carbon layer onto a perovskite solar device stack, which ink composition is fully compatible with the underlying device stack layers and is rheologically compatible with solution processing methods such as roll-to-roll slot die coating.

[0016] The ink composition of the present invention comprises (a) one or more binders, (b) one or more populations of conductive carbon particles, and (c) one or more solvents, wherein the solvent is selected from ethanol, ethyl acetate, 1-butanol, 2-methylanisole, xylene, toluene, cyclohexanone, indane, or any isomer thereof.

[0017] Each of the above solvents is expected to be compatible with perovskite materials and therefore can be used in solution processing of perovskite solar device stacks without compromising PCE. Furthermore, each of these solvents advantageously has a suitable boiling point between about 70°C and about 250°C, preferably between about 150°C and about 200°C, thus allowing for low temperature cure while remaining wet during solution-processed carbon deposition.

[0018] In a preferred embodiment, the solvent component of the ink composition is 2-methylanisole or 1-butanol, most preferably 2-methylanisole. This solvent has been shown to not cause degradation of the photoluminescence of the perovskite layer even after 24 hours of immersion. Furthermore, 2-methylanisole has a boiling point of 170°C, making it well suited for low-temperature cure solution processing. Furthermore, 2-methylanisole is not significantly toxic, and therefore has no associated workplace exposure limits, making it suitable for scale-up and mass use.

[0019] In contrast to the above solvents, water is well known to be incompatible with perovskite materials, which have been shown to rapidly decompose when exposed to water. Therefore, to maximize the stability of solution-processed perovskite solar device stacks and thus maintain the PCE, the ink formulations of the present invention are preferably substantially water-free.

[0020] As used herein, the term "substantially water-free" refers to a formulation or composition that contains about 10% by weight or less of free or non-associated water, preferably about 5% by weight or less, more preferably about 1% by weight or less, and even more preferably about 0.5% by weight or less. Those skilled in the art will understand that to the extent any of the individual components of the composition are hydrophilic, they may contain water in associated, non-free, or absorbed form, and / or may absorb water from the atmosphere. In particularly preferred embodiments, such compositions contain no free or non-associated water.

[0021] The ink composition of the present invention includes one or more conductive carbon particles that form a conductive carbon layer after deposition and drying of the ink. As will be readily understood by those skilled in the art, the type of carbon particles included in the ink is not particularly limited, and appropriate particle type(s) can be selected depending on, for example, the conductivity and viscosity requirements. However, in a preferred embodiment, the ink includes or consists of a mixture of carbon black and graphite particles, more preferably a mixture having a weight ratio of graphite to carbon black of about 1.5:1 to about 4:1. A mixture of graphite to carbon black of about 2.6:1 has been shown to provide excellent PCE when deposited as an electrode layer in a perovskite device stack.

[0022] The ink composition of the present invention includes one or more binders. Such binders provide adhesion to the substrate and, depending on the concentration, provide the necessary rheological properties for the ink composition. As those skilled in the art will readily appreciate, the type of binder is not particularly limited, so long as the binder is soluble in the selected solvent and allows the ink formulation to be printed onto a substrate, such as a perovskite solar device laminate. However, in a preferred embodiment, the binder includes or consists of one or more hydrophilic polymers, such as polyvinyl alcohol, polyvinylpyrrolidone, gelatin, cellulose, pectin, polyoxazoline, polyvinylacetamide, partially hydrolyzed polyvinyl acetate / vinyl alcohol, polyacrylic acid, polyacrylamide, polyalkylene oxide, sulfonated or phosphated polyester and polystyrene, casein, zein, albumin, and derivatives thereof. More preferably, the binder includes or consists of one or more celluloses.

[0023] Preferably, the cellulose is a cellulose ether, and even more preferably, ethyl cellulose. Particularly preferred cellulose, cellulose ether, and carboxymethyl cellulose binders have an Mw of about 10,000 to about 200,000 Da.

[0024] A good balance of rheological properties and electrical conductivity, enabling the use of inks to deposit carbon electrodes in perovskite solar device laminates, is achieved using inks containing about 15 wt% to about 35 wt% carbon particles and about 5 wt% to about 15 wt% binder in the solvent. However, for compatibility with roll-to-roll processes such as slot die coating, the amounts of binder and carbon particles are typically reduced to lower the ink viscosity. For example, in a preferred embodiment, a low-viscosity formulation is provided that retains excellent electrical conductivity and contains about 7 wt% to about 18 wt% carbon particles and about 3 wt% to about 10 wt% binder in the solvent.

[0025] As mentioned above, the binder and carbon particles are provided in an amount sufficient to control the rheological aspects of the ink composition. In a preferred embodiment, the ink composition is a shear thinning composition. Furthermore, when the ink is used for stencil or screen printing, the ink preferably has a shear thinning property of 0.1 to 1000 s. -1 However, in a preferred embodiment, the ink is used in a slot die coating process compatible with R2R, and the ink preferably has a shear viscosity of between 0.1 and 1000 s. -1 The shear viscosity is between about 15 Pa.s and about 1.5 Pa.s over the shear range.

[0026] Additionally, the ink formulation may be doped with one or more conventional dopants to improve the efficiency of the perovskite device. For example, in certain preferred embodiments, the ink may be doped with about 0.5 mg / ml to 20 mg / ml of a dopant such as bathocuproine.

[0027] Exemplary ink compositions of the present invention are shown in Tables 3-5.

[0028] The ink of the first aspect of the present invention is configured to be used to deposit a perovskite-compatible carbon back electrode in place of traditional evaporated metal contacts in perovskite solar device stacks, overcoming interlayer incompatibility and recombination losses to provide device performance comparable to devices using evaporated gold electrodes. Thus, according to a second aspect, the present invention provides a planar structure perovskite solar device stack comprising: (i) a transparent conductive layer; (ii) an electron transport layer; (iii) a perovskite layer; (iv) a hole transport layer; and (v) a conductive carbon layer.

[0029] As will be appreciated by those skilled in the art, such layers may be arranged in a nip device configuration, where the electron transport layer is in contact with the transparent conducting layer ((i)-(ii)-(iii)-(iv)-(v) configuration), or such layers may be arranged in an inverted pn device configuration, where the hole transport layer is in contact with the transparent conducting layer ((i)-(iv)-(iii)-(ii)-(v) configuration). However, in a preferred embodiment, the perovskite solar device stack has a nip device configuration.

[0030] As can be easily understood by those skilled in the art, the selection of the transparent conductive layer is not particularly limited. For example, if flexibility is not required, this layer may be formed from glass coated with indium tin oxide (ITO). However, if a flexible substrate is required, for example in roll-to-roll processing, this layer is preferably formed from a polymer such as a PET or PEN film coated with ITO.

[0031] Likewise, the choice of electron transport layer is not particularly limited, however, in a preferred embodiment, this layer is formed from tin oxide (SnO2) because it can be processed at low temperatures and is suitable for slot die coating.

[0032] Likewise, the choice of perovskite layer is not limited, however, in a preferred embodiment, this layer comprises methylammonium lead halide, more preferably methylammonium lead iodide (MAPI), a solid compound having a perovskite structure and the chemical formula CH3NH3PbI3.

[0033] Furthermore, the choice of hole transport material is not particularly limited and may include, for example, Spiro-MeOTAD. However, the use of such a hole transport layer in combination with a conductive carbon layer has been shown to significantly reduce efficiency and degrade J-V characteristics compared to similar stacks with gold electrodes. However, this efficiency loss is overcome by preferably using at least one hole transport layer comprising nickel oxide, copper thiocyanate, copper indium sulfide, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)], poly(3-hexylthiophene-2,5-diyl (P3HT), poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) polymer blends, or any combination thereof. Multiple hole transport layers may be provided; in such embodiments, the perovskite solar device stack preferably comprises two or three hole transport layers, each comprising nickel oxide, copper thiocyanate, copper indium sulfide, PTAA, P3HT, and / or PEDOT polymer blends.

[0034] In a particularly preferred embodiment, a PEDOT polymer blend containing a sulfonated block copolymer as a counterionomer is used as the hole transport layer; such counterionomers counter the instability issues associated with sulfonated homopolymer counterionomers, such as sodium polystyrene sulfonate (PSS), and the resulting device architecture has surprisingly been found to outperform similar gold-evaporated based devices. PEDOT polymer blends containing sulfonated block copolymers as counterionomers are commercially available from a variety of sources, such as Clevios™ HTL Solar 3 (Haraeus Group, Hannau, Delaware).

[0035] Without wishing to be bound by any particular theory, it is believed that PEDOT can not only act as a hole transport layer that is compatible with the carbon ink solvent, but its inherent conductivity also aids charge transfer to the printed carbon electrode, thus minimizing recombination losses.

[0036] Particularly when the hole transport material comprises PEDOT, the hole transport layer preferably has a thickness of about 40 to about 200 nm, ideally about 90 nm, at which thickness the perovskite is effectively covered with low defect generation at this thickness.

[0037] The conductive carbon layer is formed by printing the ink of the first aspect, and therefore preferred features relating to this carbon component are as described above for the first aspect of the invention.

[0038] Each layer of the planar structure perovskite solar device laminate of the second aspect, including the conductive carbon layer, can be deposited by a solution processing method, such as by slot die coating. Thus, according to a third aspect, the present invention provides a method of manufacturing the planar structure solar device laminate of the second aspect, in which an electron transport layer, a perovskite layer, a hole transport layer and a conductive carbon layer are coated sequentially on a transparent substrate by a solution processing method.

[0039] In a preferred embodiment, the layers are coated onto a transparent substrate by slot die coating. In such a process, each of the electron transport layer, perovskite layer, hole transport layer, and conductive carbon layer is suitably coated at a coating speed of about 0.1 m / min to about 1000 m / min, preferably about 0.2 m / min to about 500 m / min, more preferably 0.5 m / min to about 5 m / min, and most preferably about 1 m / min.

[0040] Furthermore, after application of each layer to the laminate and before application of any subsequent layers, the coated substrate is dried, preferably by exposure to heat at between about 120°C and about 140°C for a time period between about 30 seconds and about 2 minutes.

[0041] In a preferred embodiment, the transparent substrate is preferably a flexible substrate such as PET or PEN, and may be coated with a tin-doped indium oxide (ITO) film. In such an embodiment, roll-to-roll processing can produce device stacks that are free of destructive defects as demonstrated using electroluminescence testing.

[0042] Throughout this description and the claims, the terms "comprise" and "contain," and variations of these terms such as "comprising" and "comprises," mean "including but not limited to" and do not exclude other moieties, additives, ingredients, integers, or steps. Throughout this description and the claims, the singular includes the plural unless the context requires otherwise. In particular, where the indefinite article is used, the specification should be understood as contemplating the plural as well as the singular unless the context requires otherwise. Throughout this description and the claims, the term "about" means ±5%, alternatively ±2%, unless the context requires otherwise.

[0043] All references, including any patents or patent applications, cited herein are hereby incorporated by reference. No reference is admitted to constitute prior art. Further, no prior art is admitted to form part of the common general knowledge in the art.

[0044] Preferred features of each aspect of the invention may be as described in relation to any of the other aspects.

[0045] Other features of the present invention will become apparent from the following examples. Generally speaking, the present invention extends to any novel one or any novel combination of the features disclosed in this specification (including the accompanying claims and drawings). Accordingly, any feature, integer, property, compound, or chemical moiety described in connection with a particular aspect, embodiment, or example of the present invention should be understood to be applicable to any other aspect, embodiment, or example described herein, unless incompatible.

[0046] Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

[0047] The invention will now be described, by way of example only, with reference to the following figures and tables: [Brief explanation of the drawings]

[0048] [Figure 1a] PL measurements - ITO / MAPI samples were immersed in various solvents for 24 hours. [Figure 1b] PL measurements - glass / MAPI / carbon with various carbon ink formulations. Carbon inks containing methylanisole, 1-butanol, and 1-butanol-BCP solvents have ethyl cellulose as the binder, while carbon inks containing diacetone alcohol have a tripolymer (poly(vinyl chloride-co-vinyl acetate-co-vinyl alcohol) resin). [Figure 2a] Schematic of the stencil coating method and photographs of PSCs with carbon and gold top electrodes. [Figure 2b] Steady-state PL spectra of perovskite / carbon, perovskite / spiro-MeOTAD / carbon, and perovskite / spiro / gold samples. [Figure 2c]J-V curves for perovskite / carbon, perovskite / spiro-MeOTAD / carbon, and perovskite / spiro / gold samples. [Figure 2d] Cross-sectional SEM image of stencil-coated carbon. [Figure 2e] JV curves of perovskite / carbon and perovskite / PEDOT / carbon. [Figure 2f] Steady-state PL spectra of perovskite / carbon and perovskite / PEDOT / carbon. [Figure 3a] Statistical distribution of PCE for PSCs with MAPI / C, MAPI / spiro-MeOTAD / C, and MAPI / spiro-MeOTAD / Au structures. [Figure 3b] Statistical distribution of Voc for PSCs with MAPI / C, MAPI / spiro-MeOTAD / C, and MAPI / spiro-MeOTAD / Au structures. [Figure 3c] Statistical distribution of Jsc for PSCs with MAPI / C, MAPI / spiro-MeOTAD / C, and MAPI / spiro-MeOTAD / Au structures. [Figure 3d] Statistical distribution of FFs for PSCs with MAPI / C, MAPI / spiro-MeOTAD / C, and MAPI / spiro-MeOTAD / Au structures. [Figure 4a] Statistical distribution of PCE for flexible and rigid PSCs with MAPI / PEDOT / C and MAPI / PEDOT / Au structures. [Figure 4b] Statistical distribution of Jsc for flexible and rigid PSCs with MAPI / PEDOT / C and MAPI / PEDOT / Au structures. [Figure 4c] Statistical distribution of Voc for flexible and rigid PSCs with MAPI / PEDOT / C and MAPI / PEDOT / Au structures. [Figure 4d] Statistical distribution of FFs for flexible and rigid PSCs with MAPI / PEDOT / C and MAPI / PEDOT / Au structures. [Figure 5a] Nyquist plots for device structures SnO2 / MAPI / carbon, SnO2 / MAPI / PEDOT / C, and SnO2 / MAPI / PEDOT / gold. [Figure 5b] EQE spectra of device structures SnO2 / MAPI / carbon, SnO2 / MAPI / PEDOT / C, and SnO2 / MAPI / PEDOT / gold. [Figure 5c] XPS spectral mapping of rigid PSCs with various thicknesses of PEDOT. [Figure 6a] Statistical distribution of PCE for PSCs with PEDOT layer of various thicknesses. [Figure 6b] Statistical distribution of Jsc for PSCs with PEDOT layer of various thicknesses. [Figure 6c] Statistical distribution of Voc for PSCs with PEDOT layers of various thicknesses. [Figure 6d] Statistical distribution of FF for PSCs with PEDOT layers of various thicknesses. [Figure 7a] Steady-state PL spectra of MAPI / PEDOT / C, MAPI / PEDOT / Au, and MAPI / Spiro-MeOTAD / Au structures. [Figure 7b] J-V curves of MAPI / PEDOT / C, MAPI / PEDOT / Au, and MAPI / Spiro-MeOTAD / Au structures. [Figure 7c] Development pathway of photovoltaic performance of PSCs with carbon top electrodes. [Figure 7d] Humidity stability test of carbon top electrode device and gold top electrode device at 25°C and 70% RH. [Figure 7e] Thermal / humidity stability testing of carbon top electrode devices and gold top electrode devices at 65°C and 85% RH. [Figure 8] Schematic visualization of the R2R slot die coating method for fabricating PSCs with carbon top electrodes. [Figure 9a] Electroluminescence photo of small-scale flexible PSC. [Figure 9b] Photographs of EL emission from a R2R carbon electrode PSC. The center image is a superposition of the left and right images. The left image was captured under illumination with no bias applied, while the right image was taken in the dark under bias. [Figure 10a] XRD spectra of MAPI (control), carbon stencil coated, and R2R coated samples. [Figure 10b] Cross-sectional FE-SEM. [Figure 10c] XPS spectra of PSCs with R2R v stenciled carbon electrodes. [Figure 10d] JV curves of gold-deposited r2r PSCs and all-R2R PSCs. [Figure 10e] Jt curve of r2r PSC with gold deposition. [Figure 10f] Jt curves for PSCs with all R2R structures. [Figure 11a] Statistical distribution of PCE for PSCs with R2R slot die coating and all-R2R slot die coating with evaporated gold. [Figure 11b] Statistical distribution of Jsc for PSCs with R2R slot die coating and all-R2R slot die coating with evaporated gold. [Figure 11c] Statistical distribution of Voc for PSCs with R2R slot die coating and all-R2R slot die coating with evaporated gold. [Figure 11d] Statistical distribution of FF for PSCs with R2R slot die coating and all-R2R slot die coating with evaporated gold. [Figure 12] Photographs of perovskite samples soaked in various solvents for 24 hours. [Figure 13a] Photoluminescence measurements of perovskite samples coated with carbon ink formulations. [Figure 13b] Photoluminescence measurements after 24 hours of solvent immersion. [Figure 14] X-ray diffraction analysis of perovskites after printing with different carbon ink formulations. [Figure 15] Device efficiency of cells fabricated using 1-butanol inks with different carbon contents. [Figure 16] Device efficiency of cells fabricated using 1-butanol carbon ink with various concentrations of bathocuproin dopant. DETAILED DESCRIPTION OF THE INVENTION

[0049] [Table 1] Photovoltaic parameters of previously published studies of perovskite solar cells with R2R coating. [Table 2] XRD results for MAPI as a control, MAPI samples with stencil-coated carbon, and MAPI samples with R2R carbon coating. [Table 3] Optimized 1-butanol ink composition. [Table 4] Optimized 2-methylanisole composition.

[0050] material and method Unless otherwise stated, all chemicals and solvents, including lead iodide (PbI, TCI chemicals, 99.99%), methylammonium iodide (MAI, greatcell solar, >99% anhydrous), tin(iv) oxide colloidal dispersion (Alfa Aesar, 15% in HO), PEDOT (HTL SOLAR3, Ossila), carbon black and graphite (Imerys), 1-butanol (Scientific Laboratory Supplies, 99.8% anhydrous), methylamine solution (Fisher Scientific, 33% in ethanol), and 2-methylanisole (Alfa Aesar, >99%), were purchased and used without further purification. All other chemicals and solvents, including spiro-MeOTAD powder (HPLC, 99%), lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI, 99.99%), 4-tert-butylpyridine (t-BP, 96%), FK209 Co(iii)TFSI salt, ethyl cellulose, acetonitrile (99.8% anhydrous), and chlorobenzene (99.8% anhydrous), were purchased from Sigma-Aldrich.

[0051] Characterization A Keithley 2400 source meter and a solar simulator (Newport Oriel Sol3A) were used to measure the photovoltaic performance of the entire device. JV curves and stabilized current (Jt) curves (300 s period) were measured at 1 sun after calibration. The active area was 0.09 cm for all devices. 2and was determined by a mask on top of the measurement module. EQE curves were measured as a function of wavelength in the 300–800 nm range in a custom-built system consisting of a xenon arc and quartz halogen white light source and a Bentham TMc300 monochromator. Steady-state PL spectra were obtained using an FS5 spectrofluorometer (Edinburgh Instruments) with an excitation wavelength of 430 nm and a 496 nm long-pass filter in the emission path. Layer thicknesses were measured using a KLA Tencor D-600 profilometer. To measure layer thicknesses on flexible substrates, an air table was used to avoid vibrations during measurements and reduce noise in the resulting profiles. The morphology of films grown on glass substrates was investigated using a Hitachi TM3000 desktop SEM and a JEOL-JSM-7800F field-emission scanning electron microscope (FE-SEM) at an accelerating voltage of 5 kV, a working distance of 10 mm, and a magnification of 1500x. Cross sections of films grown on PET substrates were obtained using a Zeiss Crossbeam 550 FIB FEGSEM. The Ga source FIB was operated in the standard configuration with the sample surface perpendicular to the ion beam and tilted 54 degrees relative to the electron beam column. After roughly trenching the area using a high-current 30 kV, 15 nA probe, the 30 kV probe was reduced to 30 kV, 700 pA, and the cross section was developed using a 55.4 degree tilt. Images were then recorded using an InLens secondary electron detector, and backscattered electron images were obtained using a filtered ESB detector. XPS analysis was performed using a Kratos axis supra XPS with a Minibeam 6 gas cluster ion source. XRD scans were performed using a Bruker D8 advance instrument using a copper source in Bragg-Brentano geometry, between 10° and 50°, with a 0.02° step size and 1 second per step. Electrochemical impedance spectroscopy was performed using a Zahner CIMPs-X photoelectrochemical workstation to obtain Nyquist plots of the equivalent circuits used for impedance and capacitance data. Measurements were performed in the dark over a frequency range of 1 MHz to 1 Hz with a potentiostat DC voltage of 0.8 V.Electroluminescence images were taken in the dark with a modified Canon EOS 600D camera without an IR filter, while a constant voltage was applied to the device with an RS Pro Bench Power Supply.

[0052] Example 1: Device Architecture To achieve roll-to-roll processed PSCs, a low-temperature solution-based process is needed to deposit each layer of the device that can be easily coated onto a flexible substrate. Tin oxide is used for slot-die coating in combination with the perovskite and Spiro-MeOTAD layers of the PSC.

[28] It has been demonstrated as an effective low-temperature processed electron transport layer suitable for R2R coatings. Together with tin oxide, perovskite precursors dissolved in acetonitrile have been successfully used as excellent low-toxicity inks for R2R coatings. [25,28] However, the main challenge remains the coating of the top electrode in a continuous sequential roll-to-roll process. Therefore, to obtain a carbon ink formulation intended for such use, the compatibility, conductivity, and scalability of such carbon-containing inks were evaluated.

[0053] The carbon ink formulation contains three components: a solvent whose compatibility with perovskites is a challenge; a binder that controls rheology, an important consideration for scalability; and a conductive carbon that alters both the rheology and the conductivity of the ink after drying.

[0054] First, solvent compatibility was assessed by immersing the coated perovskite layer in a series of candidate solvents for 24 hours, after which time the samples were visually inspected and analyzed using photoluminescence spectroscopy (Figure 1a). The quenching capabilities of carbon inks formulated from these solvents are shown in Figure 1b. 2-Methylanisole exhibits no colorimetric change, has a PL consistent with that of the unimmersed perovskite control, and has a favorable boiling point of 170 °C, allowing for low-temperature curing while remaining wet during processing. Furthermore, 2-methylanisole has no significant toxicity and can therefore be considered an environmentally friendly solvent with no relevant workplace exposure limits, making it suitable for scale-up and large-scale use. Therefore, an exemplary ink was formulated by combining 2-methylanisole, an ethyl cellulose binder, carbon black, and a graphite conductive component. Details regarding the methods for preparation, development, and optimization of the carbon ink formulation are provided in the Appendix.

[0055] The potential of the optimized carbon ink formulation was then evaluated by first assessing the performance of small-scale spin-coated perovskite devices containing a carbon top electrode. Devices were fabricated using spin-coating of planar nip structures of ITO / SnO2 / MAPI / carbon and ITO / SnO2 / MAPI / Spiro / gold (control device), with the gold electrode evaporated and the carbon electrode applied by stencil coating of a viscous paste. Figure 2a shows a schematic of the carbon coating method and a photograph of the fabricated device. To coat the carbon using the stencil method, a PET-patterned stencil was laser-cut to the same size and shape as the shadow mask used for Au deposition.

[0056] The resulting devices reached an excellent efficiency of 18.9% with gold electrodes, whereas carbon electrode devices yielded only approximately 3% efficiency and exhibited poor J-V characteristics (Figures 2c and 3). These results suggest that direct charge transfer between the perovskite and carbon poses significant problems. The carbon layer has a low sheet resistance of 5.28 Ω / sq., and SEM images of its morphology (Figure 2d) do not show significant voids, suggesting that the charge transfer problem may have been related to the interface between these two layers. Therefore, we spin-coated spiro-MeOTAD, the most common hole-transport material, to fabricate an ITO / SnO2 / MAPI / Spiro / carbon structure. However, due to its incompatibility with the carbon ink, the results were the same.

[0057] Compared to the spiro-MeOTAD / gold device, steady-state photoluminescence (SSPL) measurements in Figure 2b show poor quenching, further revealing the poor charge transfer between spiro-MeOTAD and carbon. This indicates poor compatibility between the hole transport layer and the carbon ink formulation. The similarity of the SSPL and JV curves for the samples with and without spiro-MeOTAD suggests that the incompatibility of the carbon ink resulted in the dissolution / removal of the spiro layer underneath the carbon. Subsequently, commercially available and easy-to-coat PEDOT was used as the hole transport material to replace spiro-MeOTAD. The JV measurements in Figure 2e demonstrate that the replacement with PEDOT played a significant role in device performance due to its compatibility with the carbon ink. While device efficiency decreased with evaporated gold electrodes, it was significantly improved with stencil-printed carbon electrodes, reaching a maximum of 13.25% (14.57% after stabilization). Full statistical data are shown in Figure 4, demonstrating the consistency of the results. Furthermore, SSPL measurements showed efficient quenching (Figure 2f), suggesting the need to use PEDOT or any other equivalent compatible hole-transporting material between the perovskite and the carbon top electrode. The results confirmed that PEDOT not only acts as a hole-transporting layer compatible with the carbon ink solvent, but is also inherently conductive, aiding charge transfer to the printed carbon electrode and minimizing recombination losses.

[0058] The compatibility of the carbon electrodes with the device structure was further demonstrated by electrochemical impedance spectroscopy (Figure 5a), where the series resistance values ​​derived from the Nyquist plot were significantly lower for the gold and carbon electrodes (7.5 and 14.8 ohms, respectively), indicating good charge transfer at the electrode interface.

[0059] The corresponding external quantum efficiency (EQE) spectra of the three devices are shown in Figure 5b. As expected, due to its low charge extraction, the perovskite / carbon stack without an intercalation layer has a low EQE% (maximum approximately 30%) across the entire wavelength range. The overall QE curve improvement is usually caused by higher charge extraction / lower recombination, and coating PEDOT between the perovskite / carbon or perovskite / gold layers significantly increases charge transfer from the absorbing layer to the contact electrode. In the wavelength range of 400–600 nm, both the PEDOT / carbon and PEDOT / gold devices have relatively similar EQE%. However, at 600–700 nm, a decrease in EQE is observed for the PEDOT / carbon stack. This decrease explains the expected higher-quality interface and reflection between the PEDOT / gold and PEDOT / carbon stacks, as the long-wavelength response is related to the backside of the device.

[0060] Considering scaling up device structures to R2R coatings, understanding the optimal thickness of the PEDOT interlayer is important because thin coating layers are susceptible to defects, while more robust thicker layers are detrimental to series resistance. Therefore, we coated PEDOT at thicknesses ranging from 60 to 140 nm and found 90 nm to be optimal (Figure 6). X-ray photoelectron spectroscopy (XPS) lead elemental mapping of 28 x 28 mm ITO / SnO2 / MAPI / PEDOT-coated substrates revealed a narrow range of atomic lead (up to 0.83 wt%), suggesting that the perovskite was effectively coated with few defects at a thickness of 90 nm. With thicker depositions (130-140 nm), the lower defect rate did not result in improved efficiency, despite fewer perovskite deposits being observed (Figure 5C).

[0061] EIS indicates that the charge transfer at the interface between PEDOT and carbon is comparable to PEDOT / gold, as confirmed by both the SSPL graph and JV data, which show similar quenching and PV performance (Figure 7a, b). This confirms that the carbon electrode used in this study is a superior alternative to gold, but this can only be achieved through the development of optimized device structure, formulation, and processing (Figure 7c). Initially, device efficiencies of approximately 3% were achieved with carbon-only (no HTL) and spiro-MeOTAD-based cells, with some improvement achieved by optimizing the carbon electrode formulation. However, by introducing a PEDOT HTL and then optimizing the PEDOT thickness (Figures 5 and 6), the efficiency was increased to 12.5%. Final parameter optimization of the carbon electrode by stencil coating resulted in an optimized device efficiency of 14.57%.

[0062] Typically, conventional PEDOT:PSS as an intercalation layer is known to be hygroscopic, potentially leading to reduced device stability. However, stability studies have revealed that the commercially available HTL Solar 3 PEDOT polymer blend overcomes this instability issue when used in PEDOT / carbon systems. Under 70% RH / 25 °C conditions, unencapsulated PEDOT / carbon and PEDOT / gold devices have long-term stability, retaining 80% efficiency over 1000 h compared to conventional spiro-MeOTAD samples, which lost 50% of their efficiency over the same time period (Figure 7d).

[0063] The additional protection provided by the carbon electrode becomes apparent under high humidity / temperature conditions. A heat / humidity test (ISOS D3) at 65 °C / 85% RH in the dark demonstrates the high stability of the non-encapsulated carbon electrode device compared to the evaporated gold electrode (Figure 7e). The device architectures with evaporated gold electrodes outperformed the PEDOT / carbon electrode, as the spiro / Au and PEDOT / Au devices dropped to 20% and 40% of their original PCE within the first 15 minutes (T80 193 s and 236 s), respectively, while the PEDOT / carbon device retained over 85% of its original efficiency for the duration of the test (60 minutes).

[0064] Example 2: Roll-to-roll processed device Despite the fact that most research in the field of nanostructured solar cells has focused on device performance development, several promising breakthrough reports in recent years have focused on scaling up these devices, particularly through R2R coating (Table 1). However, as of the time of writing, the fabrication of devices with an all-R2R structure, including the top electrode, has not been reported. The incorporation of solution-processed R2R electrodes is an important advancement because, when combined with R2R encapsulation, it enables the fabrication of large quantities of complete perovskite photovoltaic devices in a single, rapid process, making it a key enabler for translating many advances in perovskite research from the lab to the fab (lab-to-fab) stage.

[0065] Given that the prototype carbon electrode device in Example 1 showed the same performance results as the gold electrode, the carbon ink was modified by reducing its viscosity for compatibility with slot-die coating and evaluated for its suitability for use in fabricating PSCs by roll-to-roll coating. Flexible substrate-based nip devices were fabricated by using pre-patterned 100 mm wide PET rolls coated with ITO (sheet resistance of 50 Ω / sq).

[0066] Figure 8 shows a schematic of the roll-to-roll coating process performed using Coatema's SmartCoater. The procedure for each slot-die coating layer involves three successive steps: coating, annealing, and rewinding the substrate in preparation for the next layer coating. All device layers were slot-die coated at a common coating speed of 1 m / min and passed through a 1-m-long oven. Upon completion of coating, no further processing was required to complete the device structure. The definitions of the coating parameters for tin oxide, perovskite, PEDOT, and carbon inks are provided in the Appendix.

[0067] The functionality of R2R coated devices was investigated using electroluminescence (EL) light emission testing. In a defect-free structure, electrical phenomena are converted to light emission in response to the passage of electricity through the device, with brighter areas indicating better performance zones of the device. However, defects in some areas / points of each layer can lead to the cessation of illumination in that area, or even an electrical disconnection between the two sides of the circuit, causing the entire device to darken. Figure 9a shows a flexible stencil-coated carbon electrode device. As can be seen, three of the four stripes of the small-scale stencil-coated device performed well, while one had corner issues, indicating a coating defect. Furthermore, a photograph of a randomly cut device sample from our slot-die-coated R2R solar cell is shown in Figure 9b. Illumination along the length of the electrodes indicates the absence of destructive defects along them.

[0068] XRD analysis was used to ensure that the R2R developed carbon ink was fully compatible with the underlying perovskite layer. For a proper comparison, three device structures were used. Complete device stacks containing PEDOT and carbon applied using both stencil coating and R2R slot die coating were then compared with perovskite (MAPI) coated on tin oxide as a pristine control. Relative peak intensities in the XRD spectra indicate that the stencil coated carbon sample and the R2R device have similar PbI2 concentrations relative to the MAPI peak, slightly lower than the pristine MAPI sample (Figure 10a). This suggests that the carbon inks used for stencil coating and the R2R slot die are not only compatible with MAPI, but also exhibit protective properties against its degradation (Table 2). Furthermore, the continuous R2R drying process in the oven did not degrade the perovskite, which has a crystallite size of 50–90 nm, another indication of the successful carbon coating.

[0069] Cross-sectional FE-SEM (Figure 10b1) and FIB-SEM (Figure 10b2) images of the R2R slot-die coated device show that the carbon ink forms a compact layer without voids that can form due to solvent entrapment, and the carbon electrode makes intimate conformal contact with the underlying PEDOT layer. XPS elemental lead mapping of the R2R coated SnO2 / MAPI / PEDOT (Figure 10c) shows that the coverage is effective, covering high spots to prevent pinhole / short defects. The lead % in the XPS map is lower than that identified in the optimal spin-coated device (Figure 5c), highlighting the increased requirement for defect coverage in the R2R slot-die compared to spin-coating.

[0070] To test a 20-m-long R2R-printed device, the substrate was manually diced into segments of similar size to the small-scale device in Example 1, and randomly selected samples were measured conventionally. Sections without printed carbon electrodes were also collected and gold electrodes were evaporated, but the measurement protocol was maintained for both gold and carbon electrode devices, and a light soaking effect was observed for the carbon electrodes. The R2R perovskite device using evaporated gold electrodes had a high PCE of 13.21% (12.87% stabilized PCE), comparable to the best-performing evaporated-electrode R2R device reported. In the all-printed carbon electrode cell, a PCE of 9.79% (10.84% ​​stabilized PCE) was achieved.

[0071] Based on the JV curve of the hero cell in Figure 10d and the boxplot statistics shown in Figure 11, both the fully R2R device and the R2R / gold device have very similar values ​​for current density and open-circuit voltage. However, the fully R2R device exhibited a lower fill factor, leading to a lower PCE, compared to the evaporated gold sample. Furthermore, it should be noted that while the performance of the PEDOT / carbon device is improved compared to the spiro-MeOTAD / carbon, it still suffers from a lack of adequate fill factor compared to the conventional spiro-MeOTAD / gold device. While PEDOT is not the only suitable carbon electrode interlayer candidate, all have proven effective for the first demonstration of R2R-coated perovskite devices. This strategy can be further extended to even better hole transport / intercalation materials, particularly nickel oxide, copper thiocyanate, copper indium sulfide, PTAA, and / or P3HT, in combination with a compatible carbon ink formulation, potentially achieving higher performance all-roll-to-roll coated perovskites.

[0072] Example 3: Additional Solvent Testing The suitability of various ink formulation solvents was tested through various compatibility tests.

[0073] For initial evaluation, perovskite samples were immersed in the solvent of interest. Specifically, a layer of MAPI perovskite was coated onto a glass substrate. The coated substrate was then placed in a Petri dish filled with solvent, completely submerging the coated substrate. The sample was then left undisturbed for 24 hours. The results of these initial tests are shown in Figure 12. The development of a yellow color indicates that the perovskite is degrading, with the original dark brown MAPI color being replaced by a yellow lead iodide color. Significant perovskite degradation was observed in the M-cresol and terpineol solvent tests.

[0074] Subsequently, photoluminescence studies were performed on perovskite layers printed using carbon inks formulated with different solvents, along with solvent-immersed perovskite layers. Signal quenching indicated improved charge transfer and therefore improved performance, i.e., performance closer to that of a control perovskite sample without carbon ink or solvent immersion. Photoluminescence results are shown in Figure 13a and Figure 13b, where carbon inks formulated from diacetone alcohol (A), 1-butanol (Buta), BCP-doped 1-butanol (Buta BCP), terpineol (HH), o-xylene, and 2-methylanisole (MA) were used.

[0075] Furthermore, inks formulated with different solvents were coated onto perovskite and subjected to XRD analysis, with the results shown in Figure 14. The characteristic peaks at 14° and 12.6° represent perovskite and lead iodide, respectively, and the large peak at 26.5° represents the carbon in the ink. The perovskite formulation used contained 6% excess lead iodide, so the small lead iodide peak was expected in all samples. This ratio was maintained for 1-butanol and 2-methylanisole solvents, but it decreased for diacetone alcohol and terpineol solvents, suggesting decomposition of the perovskite.

[0076] Example 4: Optimization of 1-butanol ink A preliminary optimization study was conducted on 1-butanol ink compositions with various carbon contents, and the results are shown in Figure 15. All ink formulations contained 12.5% ​​ethyl cellulose binder with 1-butanol. The higher content was 29.4% total carbon by weight, and the lower content was 21.7% total carbon by weight, with the ratios specified representing the graphite:carbon black ratio in the ink formulations tested. This figure shows the optimal performance of the formulations disclosed in Table 3.

[0077] Furthermore, Figure 16 shows the results of efficiency testing of perovskite devices doped with bathocuproine in 1-butanol ink at levels between 1 mg / ml and 10 mg / ml, showing optimal performance at 10 mg / ml.

[0078] Example 5: Optimization of 2-methylanisole ink Preliminary optimization studies were also conducted on 2-methylanisole ink compositions, testing the efficiency of tin oxide / perovskite (MAPI) / PEDOT / carbon nip device stacks for formulations made for stencil coating and roll-to-roll slot die coating, with optimal performance observed using the formulations disclosed in Table 4.

[0079] Abstract This work presents an extremely low-cost and scalable carbon ink formulation that can replace conventional and expensive laboratory-scale metal top electrodes. Furthermore, this application demonstrates the effect of a PEDOT layer on the modification of the perovskite / HTL / carbon interface. Benefiting from the combination of this HTL and a low-viscosity slot-die compatible carbon ink, we have fabricated the world's first fully roll-to-roll flexible perovskite solar cell.

[0080] A prototype carbon electrode small-scale device on a rigid glass substrate achieved a PCE of 13.25% and was then expanded to a fully roll-to-roll coating system with a 20 m long flexible substrate, yielding a stabilized PCE of 10.8%. More importantly, devices fabricated with carbon electrodes provided photovoltaic performance similar to that of conventional evaporated gold electrodes, with hero device PCEs of 13.26% and 13.61%, respectively. Furthermore, while carbon electrode devices exhibit better long-term stability than gold, these devices also exhibited significantly greater stability under harsh thermal and humidity conditions.

[0081] The use of PEDOT / carbon electrodes in R2R perovskite devices has been demonstrated to be a viable alternative to the traditional expensive and difficult-to-scalable spiro-MeOTAD / gold electrodes, a breakthrough that marks a leap towards reaching the promise of perovskite photovoltaics in printing millions of meters of solar cells worldwide.

[0082] [Table 1]

[0083] [Table 2]

[0084] [Table 3]

[0085] [Table 4]

[0086] appendix (A) Manufacturing method for rigid devices ITO glass substrates (sheet resistance ≈ 15 Ω / sq) were cleaned / cleaned in an ultrasonic cleaner for 10 min using Hellmanex cleaning concentrate (2% in DI water), DI water, acetone, and 2-propanol, respectively. After drying with N2 blowing, the substrates were treated in a UV ozone cleaner for 15 min to improve surface energy and wettability. To deposit the electron transport layer, a 15% tin(IV) oxide colloidal dispersion in HO was diluted with water (4.2 wt% SnO2), then spin-coated at 4000 rpm for 30 s and annealed at 150 °C for 30 min.

[0087] After this, the 0.73 M perovskite solution was filtered through a 0.2 μm PTFE syringe filter and deposited at 3000 rpm for 60 s, followed by annealing at 120 °C for 10 min. Details of the preparation of the perovskite MAPI solution in acetonitrile are given in our previous work.

[25] Briefly, perovskite ink was prepared using a methylamine gas bubbling method and acetonitrile solvent. Typically, 14.23 g of lead iodide and 4.63 g of methylammonium iodide were weighed into a flask, and 40 ml of anhydrous acetonitrile was added under a nitrogen atmosphere. The flask was sealed and removed to the ambient environment, and a stream of nitrogen gas was bubbled through a methylamine solution (33% in ethanol) while stirring. The methylamine in the gas stream was carried through a dryerite-filled drying tube into the perovskite precursor solution. As the methylamine was bubbled into the solution, the precursor dissolved, changing from a black suspension to a pale yellow solution. At this point, the bubbling was stopped, and the perovskite precursor ink was recovered. Later, solutions were formulated using a commercially available ACN / MA solution (TCI Chemicals), avoiding the need to bubble MA gas through the precursor solution. In both cases, the solutions were refrigerated for storage and used within 8 weeks of preparation. Next, a PEDOT solution (in toluene) was spin-coated on top of the perovskite absorber layer at 3000 rpm for 40 seconds and annealed at 110 °C for 10 minutes. For devices with spiro-MeOTAD as the HTL, 90 mg / ml of spiro powder was dissolved in chlorobenzene. 20 μl of LiTFSI (520 mg / ml in ACN), 30 μl of 4-tert-butylpyridine, and 10 μl of FK209 (300 mg / ml in ACN) were then added as additives, and the final solution was stirred at 60 °C. The solution was then spin-coated at 4000 rpm for 30 seconds without further annealing. Finally, Au was evaporated onto the hole transport layer as the top electrode. Alternatively, for devices with carbon electrodes, carbon ink was coated by stencil coating and then dried at 110 °C for 10 minutes.

[0088] (B) Preparation of an exemplary carbon ink Ethyl cellulose and 2-methylanisole were weighed and mixed twice for 5 minutes at 3500 rpm under high shear using a Speedmixer. The solution was then left for 24 hours to ensure complete dissolution of the binder. Then, both components—carbon black (0.77 g for stencil ink, 0.34 g for slot-die ink) and graphite (2 g for stencil ink, 0.88 g for slot-die ink)—weighed into the resin and mixed again in a Speedmixer (2 x 3500 rpm, 5 minutes) before being left for 24 hours to allow solvent absorption by the carbon and achieve the final viscosity of the ink. Finally, the ink was mixed again twice for 5 minutes at 3500 rpm to break up any remaining agglomerates and ensure a smooth, well-mixed ink.

[0089] (C) Roll-to-roll device manufacturing method Using a Cotema Smartcoater roll-to-roll coating system, each layer of the device was sequentially slot-die coated onto 50 Ω / sq ITO at a common speed of 1 m / min. Tin oxide (Alpha Aesar) diluted to 1.2 wt% in DI water and 10% 1-butanol was coated at a width of 90 mm and a wet film thickness of 5-11 μm (optimum 7 μm) with a 1 mm meniscus guide, a 200 μm gap, and a drying temperature of 140 °C. The next perovskite solution (used in the fabrication of rigid devices) was coated at a width of 90 mm and a wet film thickness of 3-9 μm (optimum 7 μm) with a 1 mm meniscus guide, a 200 μm gap, and an air knife with a nitrogen flow of 50 L / min immediately after coating, followed by drying at an oven setpoint of 150 °C. PEDOT (HTL solar 3) was coated as supplied with a wet film thickness of 3-6 μm (optimum 4 μm) and width of 90 mm using a 0.25 mm meniscus guide, a 150 μm gap, and a drying temperature of 140 °C. The electrode pattern was defined by coating carbon ink in 2 x 3 mm stripes. No meniscus guide was used, the coating thickness was 250 μm wet film, and the oven set point was 140 °C. The oven dwell time for all coating layers was 1 min.

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Claims

1. a. one or more binders; b. one or more populations of conductive carbon particles; c. one or more solvents; It contains A carbon ink formulation, wherein the solvent is selected from ethanol, ethyl acetate, 1-butanol, 2-methylanisole, xylene, toluene, cyclohexanone, indane, or any isomer thereof.

2. The formulation of claim 1, wherein the solvent is 2-methylanisole or 1-butanol.

3. 3. The formulation of claim 1 or 2, wherein the formulation is substantially free of water.

4. The formulation according to any one of claims 1 to 3, wherein the conductive carbon particles are selected from carbon black, graphite particles, and mixtures thereof.

5. 5. The formulation of claim 4, wherein the conductive carbon particles comprise a mixture of carbon black and graphite particles, the weight ratio of graphite to carbon black being from about 1.5:1 to about 4:

1.

6. 6. The formulation of any one of claims 1 to 5, wherein the binder comprises one or more hydrophilic polymers, which may be selected from polyvinyl alcohol, polyvinylpyrrolidone, gelatin, cellulose, pectin, polyoxazoline, polyvinylacetamide, partially hydrolyzed polyvinyl acetate / vinyl alcohol, polyacrylic acid, polyacrylamide, polyalkylene oxide, sulfonated or phosphated polyesters and polystyrene, casein, zein, albumin, and derivatives thereof.

7. The formulation of claim 6, wherein the binder comprises a cellulose ether, a carboxymethyl cellulose ether that may have a Mw of from about 10,000 to about 200,000 Da.

8. The formulation of any one of claims 1 to 7, comprising about 15% to about 35% by weight of carbon particles and about 5% to about 15% by weight of binder.

9. 0.1 to 1000 seconds －１ 9. The formulation of any one of claims 1 to 8, having a shear viscosity of between about 15 Pa.s and about 1.5 Pa.s over a shear range of 15 Pa.s to 1.5 Pa.s.

10. 10. The formulation of any one of claims 1 to 9, doped with one or more dopants, optionally doped with about 0.5 mg / ml to about 20 mg / ml of said dopant(s).

11. 11. The formulation of claim 10, wherein the dopant is bathocuproine.

12. a. a transparent conductive layer; b. at least one electron transport layer; c. at least one perovskite layer; d. at least one hole transport layer; e. A conductive carbon layer; 1. A planar structure perovskite solar device stack comprising:

13. a. the conductive layer comprises an ITO coated polymer; b. The electron transport layer is made of SnO ２ Including, c. the perovskite layer comprises methylammonium lead halide; d. the at least one hole transport layer comprises nickel oxide, copper thiocyanate, copper indium sulfide, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA), poly(3-hexylthiophene-2,5-diyl (P3HT), poly(3,4-ethylenedioxythiophene) polymer blend, or any combination thereof; The device stack of claim 12.

14. 14. The device stack of claim 13, wherein the hole transport layer has a thickness of about 40 to about 200 nm.

15. 15. A method for manufacturing a device stack according to any one of claims 12 to 14, wherein the electron transport layer(s), perovskite layer(s), hole transport layer(s) and conductive carbon layer(s) are coated sequentially onto a transparent substrate by solution processing methods.

16. 16. The method of claim 15, wherein the layers are coated onto the transparent substrate by slot die coating, and each of the electron transport layer, perovskite layer, hole transport layer, and conductive carbon layer may be coated at a coating speed of from about 0.1 m / min to about 1000 m / min.

17. 17. The method of claim 15 or 16, wherein after application of each layer to the laminate and before application of any subsequent layer, the coated substrate is dried by exposure to heat, optionally to a temperature of between about 120°C and about 140°C, and / or for a time of between about 30 seconds and about 2 minutes.

18. 18. The method of any one of claims 15 to 17, wherein the transparent substrate is a flexible substrate such as PET or PEN, optionally coated with a tin-doped indium oxide (ITO) film, and the device stack is formed by roll-to-roll processing.

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