FI130053B

FI130053B - Perovskite precursor ink formulation for solar cells

Info

Publication number: FI130053B
Application number: FI20195267A
Authority: FI
Inventors: Syed Farhan Hashmi; Syed Ghufran Hashmi; Taina Lamminmäki
Original assignee: Aalto Univ Foundation Sr
Priority date: 2019-04-02
Filing date: 2019-04-02
Publication date: 2023-01-13
Also published as: WO2020201631A1; FI20195267A1

Abstract

The present invention relates to a printable formulation of a perovskite precursor ink, comprising one or more organic or inorganic cation(s), one or more metal cation(s) of the carbon family and one or more halogen anion(s), as well as one or more surfactants, preferably selected from liquid surfactants, and a solvent or solvent mixture. The formulation is prepared by mixing the components, followed by heating to a temperature of 50-100°C, thus evaporating both solvent and surfactant.

Description

PEROVSKITE PRECURSOR INK FORMULATION FOR SOLAR CELLS

Background of the Invention Field of the Invention

[001] The present invention concerns a novel inkjet-printable perovskite precursor ink formulation suitable for use in carbon-based printed perovskite solar cells.

[002] Particularly, the invention concerns such ink formulations, which have been prepared using liquid surfactants.

Description of Related Art

[003] A perovskite solar cell is a type of solar cell, where the light-harvesting active layer is based on a structured organic-inorganic lead halide or tin halide material, common alternatives including a methylammonium lead trihalide or a formamidinium lead trihalide.

[004] Common perovskite application techniques include spin coating, vapor deposition, or other vapor-assisted techniques. A recently developed alternative technique is inkjet printing, which is simple and has the additional advantage of providing the option of manufacturing the entire solar cell with the help of printing techniques. However, it requires a smooth and flowing precursor solution. oO > [005] Remaining problems related to perovskite solar cells are related to stability, 3 as degradation of common perovskite materials has been observed in standard

N environmental conditions.

I 30 = [006] Hashmi et al. (2016) describes a perovskite precursor ink formulated from

S lead iodide (Pbl,), methyl ammonium iodide (MAT) and 5-ammonium valeric acid (5- 3 AVAI), suitable for use in manufacturing printed perovskite solar cells having the desired

N high stability and reproducibility.

[007] Also Liu et al (2018) describe a perovskite with improved thermal stability, obtained by incorporating cesium-5-aminovaleric acetate into the perovskite.

[008] Similarly, the two publications by Hashmi et al., published in 2017, describe the preparation of printed perovskite solar cells using a precursor solution containing lead iodide (Pbl,), methyl ammonium iodide (MAI) and 5-ammonium valeric acid (5-AVAI), in gamma butyrolactone (GBL) as solvent, thus giving the solar cells high durability and reproducibility, as well as long-term stability.

[009] Bi et al. (2016) describes a photovoltaic cell based on the use of metal halide perovskite films, which have been produced from a solution containing formamidinium iodide (FAT), lead iodide (Pbl,), methylammonium bromide (MABr) and lead bromide (PbBr,) in a mixed solvent containing dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). This solar cell has been described as particularly efficient, with high solar-to- — electric power-conversion efficiency and intense electroluminescence.

[0010] Aitola et al. (2016) also describes a perovskite solar cell with high power conversion efficiency, where the perovskite layer is a formamidinium lead iodide- methylammonium lead bromide applied by spin coating.

[0011] High efficiencies, with operational stability, have been reported also by Arora et al. (2017), using a cesium formamidinium methylammonium lead iodide/bromide perovskite.

[0012] These perovskites have several advantages, including the fact that they can be manufactured in a traditional lab environment, instead of the clean room facilities reguired = by traditional silicon solar cell materials.

N

3 [0013] Despite the developments described in these publications, there is still a need

S 30 for improvements, particularly relating to the scalability of the process, and to preparing

E fully printable perovskite precursor ink formulations. © [0014] Deng et al. (2018) describes the use of surfactants to facilitate the coating of > smooth and uniform layers of perovskite.

N 35

[0015] However, these perovskites are simple, and lack the precipitation retarding compound that has been found by the present inventors to be essential. Further, these known perovskites have been applied using blade coating. This is a common technique used when the solar cells to be manufactured are thin, and a direct deposition of the perovskite as a coating onto the cell is sufficient.

[0016] Thus, there is still a need for printable inks that provide smooth and homogeneous infiltration even over large areas, while also providing homogeneous crystallization.

Summary of the Invention

[0017] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0018] According to a first aspect of the present invention, there is provided a printable perovskite ink formulation including one or more surfactant(s).

[0019] According to a second aspect, there is provided a method for preparing such a perovskite precursor solution.

[0020] Thus, the present invention relates to a novel inkjet-printable perovskite precursor ink formulation suitable for use in carbon-based printed perovskite solar cells, particularly to a formulation including one or more surfactants, preferably being liquid surfactants, which can be removed from the formulation in connection with the o 25 — evaporation of the solvent, at which point also the surfactant has become unnecessary. & UU + [0021] Several advantages are achieved using the present invention. Among others, = the ink formulations of the invention can be printed onto the solar cell substrate in fully

S dissolved form, whereafter a homogeneous crystal growth can be achieved.

E: 30

N [0022] Further, the surfactants used in the invention provide the ink formulations

S with low surface tensions, which is a reguirement of the inkjet printing technigue.

R

Brief Description of the Drawings

[0023] FIGURE 1 illustrates the J-V curves of carbon-based printed perovskite solar cells (CPSCs) fabricated using traditional ink without surfactant, and ink with added surfactant, selected from ethylene glycol and Triton X-100, the Figure indicating the results both before and after Humidity Assisted Thermal Treatment (THE).

Embodiments of the Invention

[0024] Definitions

In the present context, the term “surfactant” encompasses chemical additives that lower the surface tension of chemical formulations, typically by increasing the attraction of the molecules of the formulation to each other.

They also increase the smoothness, uniformity and homogeneity of the dispersions and solutions.

The term “perovskite” is used to name a class of compounds that have the same crystal structure as CaTiOs. In the context of solar cells, these “perovskite” compounds are typically based on lead or tin halides, such as trihalides. In the present invention, lead halides are preferred, particularly lead iodides. oO > 25 The carbon-based printed perovskite solar cells (CPSCs) as described herein, x mean solar cells having a perovskite-based active light-harvesting layer and a

N carbon-based counter-electrode, fabricated by a printing technigue, such as z inkjet printing. Using the herein described perovskite formulation, it is a

N possible to manufacture the entire solar cell by inkjet printing. & 30 2 [0025] The present invention thus relates to a novel inkjet-printable perovskite

N precursor ink formulation suitable for use in carbon-based printed perovskite solar cells.

The formulation includes one or more surfactant(s) to facilitate the inkjet printing, and to form a smooth and uniform perovskite.

[0026] The surfactants of the invention function by reducing the surface tension of the ink formulation, and have in the present invention been found to provide also a smooth and homogeneous infiltration of the ink into the substrate, as well as to provide 5 homogeneous drying and crystallization of the perovskite crystals. This will help in achieving high-performance perovskite solar cells.

[0027] The surfactants are preferably selected from surfactants of the liquid type, more preferably of the non-ionic liquid type, suitable examples being the surfactants of the

Triton X series (octylphenol ethoxylates), such as Triton X-100, as well as surfactants based on glycerol or ethylene glycol. The most suitable surfactants for use in the present invention are Triton X-100, glycerol and ethylene glycol, either alone or as mixtures of two or all three of them, with ethylene glycol and Triton X being preferred, particularly Triton

X-100.

[0028] Gel-type surfactants are useful in forming a homogeneous ink formulation, but remain in the formulation after evaporation of the solvent, whereby they have an inhibitory effect on the crystallization of the perovskite. — [0029] For providing the ink formulation in solution form, one or more solvents are added to the formulation. This will facilitate an even infiltration of the ink into the solar cell substrate. The solvent or solvent mixture is typically selected from one or more of the solvents gamma-butyrolactone (GBL), gamma-valerolactone (GVL), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) or N-methyl-2-pyrrolidone (NMP), preferred solvents — being gamma-butyrolactone (GBL), dimethyl formamide (DMF) and dimethyl sulfoxide = (DMSO), used either alone or as a solvent mixture.

N

3 [0030] The amounts of precursor components are typically adjusted to give

N essentially eguivalent amounts of each, while the amount of solvent is typically determined

E 30 — based on the smallest amount required to dissolve the active materials upon heating.

S [0031] The surfactant(s) are typically added to the ink formulation in amounts of 3 0.1-0.6 vol-% calculated from the volume of solvent, preferably 0.20.5 vol-%, and most

N suitably about 0.4 vol-%.

[0032] The above listed surfactants have similar boiling points as the herein mentioned solvents, whereby the process step of solvent evaporation that typically follows the step of applying the precursor ink formulation onto the solar cell substrate will cause also the surfactant to evaporate, thus allowing perovskite crystal growth.

[0033] According to an embodiment, the perovskite precursor ink is prepared by mixing the active materials with the surfactant in a solvent or a solvent mixture, followed by heating to a temperature of 50-100°C, such as a temperature of 70 °C. The increased temperature is preferably maintained for 15-60 minutes, most suitably for about 30 — minutes. The thus obtained dissolved ink can then be cooled down to room temperature, and stored until use with no precipitation showing at least within 2 weeks of storage.

[0034] The heating will cause evaporation of the solvent as well as the surfactant, leaving no excess additives in the final product, while still providing a stabile and smooth — perovskite in the final product.

[0035] In the perovskite precursor solution/ink of the present invention, the perovskite is typically formed of a mixture of organic and inorganic precursor components, which can include for example a cation selected from the methyl-ammonium (CH;NH; ,

MA), ethyl-ammonium (CH;CH,NH; , EA), formamidinium (NH,CH=NH,", FA) or cesium (Cs") ions, a metal cation of the carbon family selected from the germanium (Ge), tin (Sn”*) and lead (Pb*") ions, as well as a halogen anion selected from the fluoride (F), chloride (CT), bromide (Br) and iodide (I') ions, and further comprises a solvent for dissolving the components of the perovskite as well as any additives. o [0036] In the present invention, the methyl-ammonium (MA) and the > formamidinium (FA) cations have been found particularly advantageous alternatives, 3 although also mixtures of two or more cations have also been found to have advantages.

N Likewise the cesium cation (Cs) has advantages. Generally, the MA cation is considered

E 30 to be more humidity-resistant than the other mentioned cations, while the FA and Cs ions

NS are considered to be thermally more stable.

LO o [0037] Of the other precursors components, the lead ion is typically used as the

S metal cation of the carbon family. The preferred halogen anion is, in turn, the iodide ion, optionally in combination with the bromide ion.

[0038] Said inorganic/organic cation, metal cation and halogen ion form the light- harvesting component of the perovskite formulation of the invention.

[0039] The ink formulation can include, in addition to the light-harvesting components, a compound or component that is suitable to inhibit, slow down, reduce and/or prevent one or more selected from: nucleation rate, crystal growth and precipitation of perovskite, perovskite crystals and/or a perovskite intermediate phase, while allowing crystallization upon deposition of the ink formulation onto the substrate, e.g. after an additional process step, such as heating. Said compound is referred to as "precipitation retarding compound".

[0040] In the ink formulation of the present invention, particularly when preparing an ink formulation for application using ink jet printing, the use of a suitable precipitation retarding compound is essential, since that compound will provide a homogeneous perovskite layer in the final product.

[0041] The "precipitation retarding compound" may be part of the perovskite to be formed or may be separate. A preferred "precipitation retarding compound" comprises an anchoring group, suitable to anchor the compound to the surface of a metal oxide material.

[0042] Anchoring groups are preferably selected from the group consisting of: -

COOH, -CONH,, -PO;H,, -PO,H,R', -PO4H,, -SO;H), -CONHOH, combinations thereof, salts thereof, deprotonated forms thereof, and other derivatives thereof, where R'is selected from organic substituents comprising 1-20 carbon atoms and 0-10 heteroatoms, preferably 1-10 carbon atoms and 0-8 heteroatoms, most suitably 1-5 carbon atoms and 0-3 o heteroatoms, and is optionally halogenated, independently of said heteroatoms. Preferred > heteroatoms are O, S, Se, Te, N, B and P.

S

A [0043] A preferred "precipitation retarding compound" comprises a positively 7 30 charged group, which may be any stable, positively charged group of an organic or

E organometallic compound.

O io [0044] Another preferred "precipitation retarding compound" comprises a nitrogen > atom, preferably a positively charged nitrogen atom, such as the nitrogen atom in the — groups selected from -NH;", -NHC(NH; )=NH, and -N=CH-NH;".

[0045] In a particularly preferred embodiment, said "precipitation retarding compound" comprises a linker connecting said anchoring group and said positively charged group, for example said positively charged nitrogen atom.

[0046] A preferred linker moiety is an organic moiety comprising 1-20 carbons and 0-10 heteroatoms, preferably 2-12 carbons and 0-7 heteroatoms, more preferably 3-10 carbons and 0-5 heteroatoms, and most preferably 4-8 carbons and O heteroatoms. Said organic moiety may be totally or partially halogenated, independently of said heteroatoms.

[0047] A particularly preferred linker moiety is selected from the group consisting of a C1-C20 alkanediyl, C2-C20 alkynediyl, C4-C20 heteroaryldiyl, and C6-C20 aryldiyl, preferably C2-C15 alkanediyl, C2-C15 alkynediyl, C4-C15 heteroaryldiyl, and C6-C15 aryldiyl, more preferably C3-C12 alkanediyl, C3-C20 alkynediyl, C4-C12 heteroaryldiyl, and C6-C12 aryldiyl, most preferably C4-C8 alkanediyl, C4-C8 alkynediyl, C4-C8 — heteroaryldiyl, and C6-C8 aryldiyl.

[0048] In a preferred embodiment, the "precipitation retarding compound" is a cation selected from the cations of formulae (1)-(3):

HO

St I NZ yt In Hj HO e + Faw © (1) O (2) O (3). and the salts thereof, wherein n is an integer of 1-10, preferably 2-7, most preferably 3-6. o > 25 [0049] Suitable examples of the "precipitation retarding compound" include 5- <r ammoniumvaleric acid (5-AVA), S-aminopentanamide (5-APAC), 4-

N aminobutylphosphonic acid (4-ABPAC) and 5-ammonium valeric acid iodide (5-AVAI),

I particularly being selected from an aminoacid, an amino acid hydrohalide, a + formamidinium halide, and an imidazolium halide. A particularly preferred precipitation 8 30 retarding compound for use in the present invention is S-ammonium valeric acid iodide (5- > AVAN).

N

[0050] In an embodiment, the perovskite precursor solution comprises components in addition to said "precipitation retarding compound". In the event that the retarding compound is added in the form of a cation, this cation may also be referred to as a “first organic cation”. As indicated, the perovskite precursor solution preferably comprises at least a further or “second organic cation”, which is required for perovskite formation.

Preferably, said second organic cation is comprised in said organic inorganic perovskite and/or said first organic cation is preferably associated with or comprised in said organic inorganic perovskite.

[0051] It is noted that said "second organic cation" may and preferably is present in higher amounts compared to said "first organic cation". Preferably, the "second organic — cation" is present in higher amounts and thus preferably a more important constituent of the organic-inorganic perovskite to be deposited. Without wishing to be bound by theory, the inventors believe that the "first organic cation", on the other hand, may be located at the interfaces between the perovskite and other materials, for example with the n-type semiconductor or possibly said porous insulating or spaceholder layer as described — elsewhere in this specification.

[0052] In the event that the "precipitation retarding compound" is not added in the form of a cation in said precursor solution, but as an uncharged molecule or as an anion, said "second organic cation" may be considered to be the only organic cation present.

[0053] Based on the above, one embodiment of the invention encompassed an ink formulation prepared by mixing methyl ammonium iodide (MAI) with lead iodide (Pbl,) as light-harvesting precursor components, along with 5-ammonium valeric acid iodide (5-

AVAI) to increase the stability of the final perovskite, along with one or more surfactant, — to increase the printability of the formulation. Further, a solvent is added. o > [0054] According to a second embodiment, the ink formulation of the invention is 3 prepared by mixing formamidinium iodide (FAI), lead iodide (Pbl,), methylammonium

N bromide (MABr) and lead bromide (PbBr;) as light-harvesting precursor components,

E 30 along with one or more surfactant to increase the printability of the formulation, in a

NS solvent or solvent mixture, preferably being a mixed solvent containing dimethyl io formamide (DMF) and dimethyl sulfoxide (DMSO).

N [0055] According to a third embodiment, the surfactant(s) are mixed with cesium acetate or cesium iodide, either with or without MAI, and lead iodide, as light-harvesting precursor components, along with S-AVAI to increase the stability of the final perovskite,

as well as one or more surfactant(s) to increase the printability of the formulation, in a solvent or solvent mixture.

[0056] According to a fourth embodiment, the surfactant(s) used to increase the printability of the formulation are mixed with a mixed-cation precursor ink, the mixed cations being the methylammonium and the formamidinium cations, along with 5-A VAT to increase the stability of the final perovskite, in a solvent or a solvent mixture.

[0057] Since the perovskite precursor solution of the invention preferably comprises — all components that are required to provide said perovskite, the perovskite is preferably deposited in a one-step deposition process, where all components of the perovskite are contained in a single solution that is deposited during said step of depositing a perovskite precursor solution.

[0058] However, the invention may also encompass printing the perovskite by sequential deposition. Deposition in a single-step deposition of a solution containing all components of the perovskite is still preferred.

[0059] In an embodiment of the invention, the perovskite precursor solution is an ink — solution, which can be deposited by printing techniques such as inkjet printing. For preparing a printable ink, the use of a solvent is advantageous.

[0060] Thus, in an embodiment of the method of the invention, said perovskite precursor ink or solution is deposited per printing or manually infiltrated, or infiltrated via programmable multi-channel pipetting robot onto a porous substrate layer, such as a o porous carbon back contact layer, so as to infiltrate said substrate. For example, said > porous carbon back contact layer can be a porous carbon electrode. The carbon electrode x may also be deposited by screen printing. s

E 30 [0061] It is to be understood that the embodiments of the invention disclosed are not

N limited to the particular structures, process steps, or materials disclosed herein, but are & extended to equivalents thereof as would be recognized by those ordinarily skilled in the = relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0062] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” — in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0063] As used herein, a plurality of items, structural elements, compositional — elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unigue member. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0064] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In this description, — numerous specific details are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details. > [0065] While the forgoing examples are illustrative of the principles of the present > 25 invention in one or more particular applications, it will be apparent to those of ordinary x skill in the art that numerous modifications in form, usage and details of implementation

N can be made without the exercise of inventive faculty, and without departing from the z principles and concepts of the invention. Accordingly, it is not intended that the invention

N be limited, except as by the claims set forth below.

S 30 = [0066] The following non-limiting examples are intended merely to illustrate the

N advantages obtained with the embodiments of the present invention.

EXAMPLES

Example 1 — Perovskite ink formulations

[0067] A perovskite ink precursor base solutions was prepared using the following procedure:

[0068] A mixture was prepared from 0.53 g of Pbl, (TCI-Chemicals), 0.19 g of methyl ammonium iodide (MAI, Dyesol) and 0.0176 g of 5-ammonium valeric acid iodide (5-AVAI Dyesol) in 1 ml of gamma-butyrolactone (Sigma Aldrich) in a glass vial under a laboratory fume hood. The glass vial was sealed and placed for stirring for 30 min on a preheated (70 °C) hot-plate. The ingredients were completely dissolved and a clear yellow solution was obtained that was allowed to cool down to room temperature.

[0069] Different inks were prepared by either using the above solution, as such, or by mixing the solution with 4 ul of a surfactant selected from ethylene glycol and Triton

X-100.

[0070] The surfactants were found to be completely dissolved in the solvent of the — solution, thus forming an identical appearance for the obtained surfactant-containing solution as for the ink without surfactant.

[0071] Surface tension measurements (in 65%RH) clearly indicated lower results for the surfactant-containing solutions as compared to the ink without surfactant (see Table 1), o 25 — with Triton X providing the most promising results.

S

+ Table 1. Surface tension values of the different inks = Surface tension, mN/m Density,

E Water 71.62 0.52 0.989

S Ink without surfactant 46.83 0.22 1.634 3 Ink with ethylene glycol 46.40 0.16 1.628

O

N Ink with Triton X-100 45.74 0.14 1.606

[0072] The J-V curves (energy/voltage) of carbon-based printed perovskite solar cells (CPSCs) fabricated using said inks are shown in Fig. 1.

[0073] Based on the results of Fig. 1, it is clear that the addition of surfactants does not affect the inherent properties of the ink, whereby surfactant-containing ink can be used to infiltrate over a larger area, using scalable techniques, such as inkjet infiltration/printing.

Example 2 — Manual, semi-automated and inkjet infiltration of the perovskite precursor ink

[0074] The perovskite precursor ink as prepared above was infiltrated (6-9 microlitres) either manually at room temperature (from a commonly available rubber based dropper) or with a drop-on-demand Dimatix materials inkjet printer (DMP-2831, Dimatix-

Fujifilm Inc., USA) on a carbon electrode at a printing temperature ranging from room — temperature to 30 °C printing temperature with 15 um drop spacing and by applying a customized waveform with 18V amplitude and 1-8 kHz frequency. Nevertheless the ink can also be infiltrated by using a programmable multi-channel pipetting robot as described in the publication by Hashmi et al. (J. Mater. Chem. A, 2017, 5, 12060-12067).

[0075] Steady conditions were maintained (room temperature), whereas the relative humidity inside the printer hood was ~ 30 -32%. The non-active area of the samples was first masked with adhesive polyimide cut out shapes before perovskite precursor ink dispensing to limit the presence of perovskite only in the active area. The wet substrates were allowed to dwell for 3-8 minutes in order to let the liquid sip into the porous > 25 — structures. After that, the carbon based perovskite solar cells were kept in a closed plastic

N box and were placed in a preheated oven at 50 °C for 30 minutes. After that, cover of the = plastic box was removed and the carbon based perovskite solar cells were further heated

S for 1-3 hours at 60 *C in the oven to ensure the complete growth of the perovskite absorber

E layer, and were then removed from the box and were kept in vacuum prior to

S 30 measurements or use.

O

N

Example 3 — Fabrication of a perovskite solar cell

[0076] Fluorine doped tin oxide (FTO) coated glass substrates (10x10 cm”, Rs = 7

O/Sg, Product code: TCO22-7, Solaronix) were first etched with an automated fiber laser, and cleaned by sequential sonications in Hellmanex 1% aqueous solution, acetone, and isopropanol (15 min each). Then a compact layer of TiO; (30-40 nm) was deposited by spray pyrolysis over the etched glass substrates placed on a hot-plate set to 550 °C, of a diluted solution of titanium diisopropoxide bis(acetylacetonate) (75% in isopropanol,

Sigma-Aldrich) in absolute ethanol (1:80) using nitrogen as a carrier gas. Areas of the substrate had been masked with glass strips to prevent the coating in the subsequent contact (silver) areas. After cooling down to room temperature, a silver paste (Sun

Chemical CRSN2442) was screen-printed and dried at 150 °C for 15 min to obtain silver contacts for anode and cathode. The 500 nm thick mesoporous TiO; layer was obtained by screen-printing (diluted Ti-Nanoxide T/SP in terpineol, Solaronix) on the compact TiO, layer followed by drying at 150 °C for 5 min, and sintering at 500 °C for 15 min. The substrates were cooled down to room temperature. Similarly, the insulating mesoporous

ZrO, layer was also obtained by screen-printing (Zr Nanoxide ZT/SP, Solaronix) on the aforementioned TiO, layer followed by drying at 150°C for 5 min, and sintering at 500°C for 30 min. The substrates were again cooled down to room temperature. The conductive — porous carbon electrode was fabricated by screen-printing a carbon paste (Elcocarb B/SP,

Solaronix), drying at 150 °C for 5 min, and firing at 400 °C for 30 minutes. The substrates were again cooled down to room temperature before the infiltration of the perovskite precursor solution in the porous stack. 2

S 25 x Industrial Applicability

N z [0077] The present ink formulations can be used as the active light-harvesting layers a

N of solar cells, particularly for carbon-based printed perovskite solar cells. & 30 2 [0078] In particular, the present material is useful in improving the flowability of

N known ink formulations, thus improving the performance of the solar cells fabricated using said inks.

Citation List

Patent Literature

FD0185024

Non-Patent Literature

Aitola, K., Sveinbjörnsson, K., Correa-Baena, J.-P., Kaskela, A., Abate, A., Tian, Y.,

Johansson, EM J, Grätzel, M., Kauppinen, E.I., Hagfeldt, A., Boschloo, G., Carbon nanotube-based hybrid hole-transporting material and selective contact for high efficiency perovskite solar cells, Energy Environ. Sci., 2016, 9, 461-466

Arora, N., Dar, M.I., Hinderhofer, A, Pellet, N., Schreiber, F., Zakeeruddin, SM.

Grätzel, M., Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%, Science 2017, 358, 768-771

Bi, D., Tress, W., Dar, M.I., Gao, P., Luo, J., Renevier, C., Schenk, K., Abate, A_, — Giordano, F., Baena, J-P.C., Decoppet, J-D., Zakeeruddin, S.M., Nazeeruddin, MK ,

Grätzel, M., Hagfeldt, A., Efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2016, vol. 2, No. 1, e1501170

Deng, Y., Zheng, X., Bai, Y., Wang, Q., Zhao, J., Huang, J., Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic — modules, Nature Energy 2018, vol. 3, 560-566

Hashmi, S.G., Martineau, D. Li, X., Ozkan, M., Tiihinen, A., Dar, M.I., Sarikka, T.,

Zakeeruddin, S.M., Paltakari, J., Lund, P.D., Grätzel, M., Air processed inkjet infiltrated carbon based printed perovskite solar cells with high stability and reproducibility, Adv.

Mater. Technol. 2016, 1600183, 1-6 = 25 Hashmi, S.G., Martineau, D., Dar, M.I., Myllymäki, T.T.T., Sarikka, T., Vainio, U, 3 Zakeeruddin, S.M., Grätzel , M., High performance carbon-based printed perovskite solar

N cells with humidity assisted thermal treatment, J. Mater. Chem. A, 2017, 5, 12060-12067 7 Hashmi, S.G., Tiihinen, A., Martineau, D., Ozkan, M., Vivo, P., Kaunisto, K., 2 Vainio, U., Zakeeruddin, S.M., Grätzel, M., Long term stability of air processed inkjet

S 30 infiltrated carbon-based printed perovskite solar cells under intense ultra-violet light 3 soaking, J. Mater. Chem. A, 2017, 5, 4797-4802 & Liu, X., Zhang, Y., Hua, J., Peng, Y., Huang, F., Zhong, J., Li, W., Ku, Z., Cheng,

Y .-B., Improving the intrinsic thermal stability of the MAPDI; perovskite by incorporating cesium 5-aminovaleric acetate, RSC Adv. 2018, 8 14991

Claims

1. A printable formulation of a perovskite precursor ink, comprising one or more organic or inorganic cation(s), one or more metal cation(s) of the carbon family and one or more halogen anion(s), characterized in that it includes one or more surfactant(s) selected from ethylene glycol —based surfactants and surfactants of the Triton X series (octylphenol ethoxylates), and also contains 5-ammonium valeric acid or a halogen salt thereof, as a stabilizer.

2 The formulation of claim 1, wherein the organic or inorganic cation is selected from methyl-ammonium (CH3NH3", or MA) ethyl-ammonium (CH3CH:NH3”, or EA), formamidinium (NH;CH=NH>", or FA), or cesium (Cs) ions, or mixtures thereof, preferably being the methyl-ammonium or formamidinium ion, optionally mixed with the cesium ion.

3. The formulation of claim 1 or 2, wherein the metal cation of the carbon family is selected from germanium (Ge”*), tin (Sn?*) and lead (Pb**), preferably being lead.

4. The formulation of any preceding claim, wherein the halogen anion is selected from fluoride (F), chloride (CI), bromide (Br") and iodide (I'), or mixtures thereof, preferably being iodide, optionally mixed with the bromide ion.

5. The formulation of any preceding claim, which contains a halogen salt of 5- ammonium valeric acid, being 5-ammonium valeric acid iodide, as a stabilizer.

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6. The formulation of any preceding claim, wherein the surfactant(s) is/are selected O from surfactants of the Triton X series (octylphenol ethoxylates), e.g. Triton X-100. 3 o

7. The formulation of any preceding claim, which includes one or more solvents, - 30 preferably selected from the solvents gamma-butyrolactone (GBL), gamma-valerolactone E (GVL), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) or N-methyl-2- S pyrrolidone (NMP), or a mixture thereof, more preferably selected from gamma- 3 butyrolactone (GBL), dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO), or a N mixture thereof.

8. The formulation of any of claims 1 to 7, which comprises methyl ammonium iodide (MAD), lead iodide (Pblz), and 5-ammonium valeric acid iodide (5-AV AI), as well as said one or more surfactant(s), in a solvent or a solvent mixture.

9 The formulation of any of claims 1 to 7, which comprises formamidinium iodide (FAI), lead iodide (PbI2), methylammonium bromide (MABr) and lead bromide (PbBr,), s well as said one or more surfactant(s), in a solvent or solvent mixture.

10. The formulation of any of claims 1 to 7, which comprises cesium acetate or cesium iodide, lead iodide and 5-ammonium valeric acid iodide (5-AVAT), either with or without methyl ammonium iodide (MAI), as well as said one or more surfactant(s), in a solvent or a solvent mixture.

11. The formulation of any of claims 1 to 7, which comprises a mixed cation precursor — ink, the mixed cations being the methylammonium and the formamidinium cations, and 5- ammonium valeric acid iodide (5-AVAT), as well as said one or more surfactant(s), in a solvent or solvent mixture.

12. A process for preparing the printable formulation of any of claims 1 to 11, characterized by mixing the components of the formulation, including one or more surfactant(s) selected from ethylene glycol —based surfactants and surfactants of the Triton X series (octylphenol ethoxylates), S-ammonium valeric acid or a halogen salt thereof, as a stabilizer, and one or more solvent(s), followed by heating to a temperature of 50-100°C, and then cooling it down to room temperature.

13. The process of claim 12, wherein the mixed components of the formulation are N heated, after mixing, to a temperature of 70 °C. N LÖ <Q O N I Ao a N © N LO O O N