Process for the preparation of low-dimensional perovskite with preferential orientation

Abstract

The process for the preparation of low- dimensional perovskite with preferential orientation comprises the phases of: supply of at least a first precursor (A) comprising inorganic iodide; supply of at least a second precursor (B) comprising organic iodide; supply of at least a third precursor (C) selected from at least one of methylammonium iodide and methylammonium chloride; mixing the first precursor (A), the second precursor (B) and the third precursor (C) with at least one solvent selected from the list comprising: N- dimethylformamide (DMF), dimethyl sulfoxide (DMSO), gamma- butyrolactone (GBL) and ethanol (Et-OH) to obtain a precursor mixture; heating of the precursor mixture to a temperature below 100°C; cooling of the precursor mixture; deposition and accretion of the precursor mixture to obtain low-dimensional perovskite with preferential orientation.

Description

[0001] PROCESS FOR THE PREPARATION OF LOW-DIMENSIONAL PEROVSKITE WITH PREFERENTIAL ORIENTATION

[0002] Technical Field

[0003] The present invention relates to a process for the preparation of low-dimensional perovskite with preferential orientation.

[0004] Background Art

[0005] Over the past decade, hybrid perovskite materials have attracted significant technological and industrial interest due to their unique properties that make them a photoactive material for photovoltaic systems, registering values of power conversion efficiency (PCE) comparable to crystalline silicon.

[0006] Perovskites are inexpensive materials that are easy to fabricate because they allow them to be machined at low cost and temperature.

[0007] Moreover, such materials can be deposited on rigid or flexible substrates using established industrial techniques, providing a practical alternative to the more established technologies on the market.

[0008] In addition, perovskites are very versatile materials. With simple chemical manipulation, it is possible to obtain different classes of perovskites with different optical, physical and structural properties which can be adapted for different applications.

[0009] One of these classes are the so-called low- dimensional perovskites (LDP).

[0010] The latter are characterized by a layered structure wherein octahedral phases are separated by organic cations arranged in confined slabs.

[0011] This type of structure gives great stability and unique properties to the material such as excitonic absorption confined to a limited region of the visible spectra, color tunability and semitransparency.

[0012] On the other hand, due to the inherent anisotropy of the material structure, the charge transport in the material strongly differs in the parallel direction (dir.A) and perpendicular direction (dir.B) to the confined plane, affecting the photocurrent produced and thus the efficiency of photovoltaic devices.

[0013] Since charge transport in the direction A is significantly higher than that in the direction B, controlling the growth of material in this preferential direction is critical in order to maximize charge extraction at the solar cell electrodes and to improve the efficiency thereof.

[0014] As a result, devices without control over crystal growth are limited to the photocurrent produced and a maximum PCE value reached around 1.5%.

[0015] In contrast, however, higher values are known around 10% achieved in the case of well-oriented material.

[0016] It should be pointed out that such materials are widely used within solar cells. Perovskite-structured solar cells are among the emerging technologies that have marked the greatest development in recent years, due to their potential high efficiency, low production cost and simple processability, making them potentially very attractive from an industrial perspective.

[0017] Perovskite-structured solar cells represent the newest and most promising photovoltaic cell technology by showing a solar energy -to-electricity conversion efficiency rate of up to 25.6% per single junction on a laboratory scale and touching 15-20% on a module scale.

[0018] In comparison, silicon-based cells generally used in common photovoltaic panels are provided with an average conversion rate of between 15% and 20% and can reach a maximum of 26.7% on a laboratory scale (as estimated by the Oxford PV company, Oxford OX5 1QU, UK and reported by the official NREL chart - https: / / www.mel.gov / pv / cell-efficiency.htinl).

[0019] However, the transition to industrial production of photovoltaic panels provided with perovskite-structured solar cells has not yet been possible because such solar cells have significant degradation issues, reaching efficiency values of less than 80% of initial efficiency in the first 1,000 hours of use (under accelerated aging conditions).

[0020] To remedy at least part of the above drawbacks, the use of special coating materials for encapsulating solar cells is known.

[0021] The development of innovative encapsulants is critical to providing excellent durability for photovoltaic panels and enabling commercial-scale production of new perovskite solar cell-related technologies.

[0022] In fact, encapsulation has been shown to play a key role in avoiding degradation and / or improving stability for a variety of PV system types (J. Phys. Energy 2 (2020) 031002).

[0023] When properly designed, encapsulation films or coatings can act as barrier layers by limiting the diffusion of oxygen and moisture, thus preventing the penetration of UV radiation, thus reducing sensitivity to strong thermal fluctuations and even inhibiting the irreversible escape of volatile decomposition products that may have formed from the solar cell components, resulting in the protection of the electrode and active layer interface.

[0024] Ideally, encapsulating materials should have good processability, excellent chemical inertness and high barrier properties for oxygen and moisture. In addition, encapsulants should have high total light transmission (>90% of incident light) and excellent resistance to UV degradation and thermal oxidation. Finally, other important characteristics are good mechanical strength, excellent adhesion to the solar cell to minimize the risk of delamination, thermal expansion coefficients close to those of PSC components to avoid mechanical damage during stability testing and high flexibility to accommodate angular stress changes during bending or related to temperature cycling (J. Phys. Energy 2 (2020) 031002).

[0025] When properly designed, encapsulation films or coatings can act as barrier layers by limiting the diffusion of oxygen and moisture, thus preventing the penetration of UV radiation, reducing sensitivity to strong thermal fluctuations and even inhibiting the irreversible escape of volatile decomposition products that may have formed from the solar cell components, resulting in the protection of the electrode and active layer interface.

[0026] Ideally, encapsulation materials should have good workability, excellent chemical inertness and high barrier properties for oxygen and moisture.

[0027] In addition, encapsulants should have high total light transmission (>90% of incident light) and excellent resistance to UV degradation and thermal oxidation. Description of the Invention

[0028] The main aim of the present invention is to devise a process for the preparation of low-dimensional perovskite with preferential orientation which allows inducing controlled crystalline orientation of perovskite in thin-film form so as to exploit its anisotropy properties. Orientation of the material in the preferential direction of charge transport can induce an increase in the performance of devices wherein low-dimensional perovskite is used, such as in the case of photovoltaic devices wherein preferential growth in the direction of charge extraction allows for a significant increase in the photocurrent produced by the device under conditions of exposure to sunlight.

[0029] Within this aim, one object of the present invention is to devise a process for the preparation of low-dimensional perovskite with preferential orientation which allows ensuring exact atomic-scale control of nucleation and crystal growth of LDP.

[0030] A further object of the present invention is to devise a process for the preparation of low-dimensional perovskite with preferential orientation which allows stringent control of the orientation of the low-dimensional perovskite thin film. Still one object of the present invention is to devise a process for the preparation of low-dimensional perovskite with preferential orientation which is implementable directly in high-efficiency solar cells.

[0031] Another object of the present invention is to devise a process for the preparation of low-dimensional perovskite with preferential orientation which can overcome the aforementioned drawbacks of the prior art within the framework of a simple, rational, easy and effective to use as well as low cost solution.

[0032] The above objects are achieved by this process for the preparation of lowdimensional perovskite with preferential orientation having the characteristics of claim 1.

[0033] Other characteristics and advantages of the present invention will become more apparent from the description of a preferred, but not exclusive, embodiment of a process for the preparation of low-dimensional perovskite with preferential orientation, illustrated by way of an indicative, yet non-limiting example, in the accompanying tables of drawings wherein:

[0034] Figure la is a schematic representation of the structure of an LDP material wherein the octahedra represent the inorganic part of the LDP material itself, confined in slabs separated by the black organic molecules. The A and B directions are indicated by their respective arrows;

[0035] Figure lb is a schematic representation of a perovskite-based solar cell. The path that the charges must follow so as to be collected is the vertical path represented by the double arrow; to maximize charge extraction from the LDP material, the direction A of the crystals must be oriented in the same direction as the path of the charges;

[0036] Figure 2a is a representative graph of an absorption spectrum of UV-vis light;

[0037] Figure 2b is a representative graph of an X-ray diffraction spectrum of the synthesized materials with different content of additive methylammonium chloride (MAC1);

[0038] Figures 3a-3c are representative of the comparative study of short-circuit current (Figure 3a), power conversion efficiency (Figure 3b) and open-circuit voltage values (Figure 3c) respectively, based on the use of perovskite LDP with different additive content MAC1 in solar cells. Each point in the graph refers to a different device, to get statistics on about 40 different devices tested.

[0039] Embodiments of the Invention

[0040] In a first aspect, the present invention relates to a process for the preparation of low-dimensional perovskite (LDP) with preferential orientation.

[0041] The process comprises the phases of: supply of at least a first precursor (A) comprising inorganic iodide; supply of at least a second precursor (B) comprising organic iodide; supply of at least a third precursor (C) selected from at least one of methylammonium iodide and methylammonium chloride; mixing of the at least first precursor (A), the at least second precursor (B) and the at least third precursor (C) with at least one solvent selected from the list comprising: N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), gamma-butyrolactone (GBL) and ethanol (Et-OH) to obtain a precursor mixture; heating of the precursor mixture to a temperature below 100°C; cooling of the precursor mixture; deposition and accretion of the precursor mixture to obtain low-dimensional perovskite with preferential orientation.

[0042] It is specified that within the scope of this disclosure the expression “with preferential orientation” relates to the fact that the resulting perovskite has a controlled crystalline orientation so as to exploit its anisotropic properties.

[0043] In other words, the preferred orientation corresponds to the most efficient charge extraction path (dir.A) of solar cells comprising low-dimensional perovskite according to the present invention.

[0044] Advantageously, the first precursor (A) comprises lead iodide.

[0045] In detail, organic iodide comprises one iodide of an ammonium derivative.

[0046] Such a derivative comprises one or more amines or, e.g., ammonia.

[0047] Preferably, organic iodide comprises at least one linear aliphatic chain C1-C12. The above aliphatic chain has at least one end conjugated with one of: benzene, thiophene or pyrrole.

[0048] By way of example, according to a preferred embodiment of the process according to the invention, the aforementioned second precursor (B) comprises thiophene-methylammonium iodide.

[0049] Stringent control of the orientation of the perovskite crystals is achieved by an equally specific check of the stoichiometric ratios among the precursors used. For this purpose, during the phase of mixing, the first precursor (A), the second precursor (B) and the third precursor (C) are mixed together in accordance with the following stoichiometric ratio A:B:C=n:2:n-l wherein n is comprised between 1 and 5.

[0050] This expedient allows obtaining a low-dimensional perovskite with preferential orientation having the following brute formula: R2MAn-iPbn(Ii-xClx)3n+i wherein R is the second precursor (B) and X corresponds to the molar ratio of methylammonium iodide to methylammonium chloride, wherein the above molar ratio is comprised between 0 and 1.

[0051] In other words, the process in accordance with the present invention allows obtaining a low-dimensional perovskite with n=2. Advantageously, the first precursor (A), the second precursor (B) and the third precursor (C) are in powder form.

[0052] In this way, during the mixing phase, the powders of each precursor are dissolved according to the aforementioned stoichiometric ratio in at least one of N- dimethylformamide (DMF), dimethyl sulfoxide (DMSO), gamma-butyrolactone (GBL) and ethanol (Et-OH) at a concentration comprised between 0.5M and 1 M with respect to the moles of lead previously weighed to obtain the precursor mixture.

[0053] This precursor mixture is subjected to agitation to promote solubilization of the powders until the latter are completely dissolved, meaning that the solution looks transparent and without any residue in suspension.

[0054] Next, the process comprises a phase of heating the precursor mixture to a temperature below 150°C.

[0055] Preferably, the temperature is comprised between 40°C and 90°C. Advantageously, the phase of heating has a duration of less than 6 hours.

[0056] According to a preferred embodiment of the process according to the invention, the phase of heating has a duration of between 2 and 4 hours.

[0057] The duration of the heating phase is critical to promote the germination of perovskite crystals in solution, which will promote the orientation thereof in the next phase.

[0058] Next, the process has the cooling phase so that the precursor mixture can be subsequently manipulated as described below, that is, allowing the phase of deposition and accretion to be carried out.

[0059] In detail, the phase of deposition and accretion is carried out by means of at least one step of spreading the precursor mixture onto a rotating supporting surface. The rotating supporting surface has a rotational speed of more than 3,000 rpm. In detail, the supporting surface can be either of the rigid or flexible type.

[0060] Advantageously, the supporting surface is of the type of a smooth or mesoporous surface such as, e.g., plastic or glassy materials.

[0061] In accordance with a preferred embodiment of the process according to the invention, the supporting surface has a size of less than 10x10cm2. However, faced with the need to achieve greater extensions, the spreading step is carried out by means of different methods of the type, e.g., blade coating, slot die coating or inkjet printing.

[0062] In accordance with a preferred embodiment of the process according to the invention, the supporting surface is set in rotation with an initial acceleration of 2,000 rpm / s.

[0063] Thereafter, after 20 seconds have elapsed from the start of the rotation, the process according to this invention comprises at least one phase of dispensing a solvent selected from the list comprising: toluene, chlorobenzene, anisole, chloroform, ethyl acetate or mixtures thereof

[0064] It should be pointed out that the phase of deposition and accretion and the phase of dispensing are carried out simultaneously.

[0065] Advantageously, the solvent is dispensed along a dispensing direction substantially transverse to the supporting surface.

[0066] This process, known to the technician in the field as solvent quenching, involves dispensing the solvent in droplet form onto the precursor mixture.

[0067] The solvent is selected from the list comprising: toluene, chlorobenzene, anisole, chloroform, ethyl acetate or mixtures thereof.

[0068] This expedient makes it possible to greatly speed up the crystallization process of perovskite, thus promoting the accretion thereof according to the desired orientation.

[0069] Next, the process comprises a phase of further heating the perovskite to a temperature comprised between 60° and 120°C.

[0070] In detail, the phase of further heating has a duration comprised between 30 and 90 minutes.

[0071] Example

[0072] The process according to the present invention was tested and the lowdimensional perovskite with preferential orientation obtained was subjected to X- ray diffraction analysis, the results of which are given below.

[0073] Lead iodide, methylammonium chloride, methylammonium iodide and thiophene-methylammonium iodide are mixed together according to the stoichiometric proportions described above and solubilized in dimethylformamide until completely solubilized.

[0074] The resulting precursor mixture was subjected to pre-heating to promote the formation of nucleation sites.

[0075] Once the precursor mixture has been deposited on the rotating supporting surface, solvent quenching, i.e., the toluene dispensing phase, is carried out.

[0076] Figures 2a and 2b show the light absorption and X-ray diffraction (XRD) analyses, which showed that the process in accordance with the present invention does not change the composition of the material but only the orientation thereof. In fact, the absorption spectra remain unchanged with the addition of the additive in the material, i.e., methylammonium chloride (MAC1), while the difference in the intensity of the peaks in the XRD pattern suggests that the presence of the additive promotes crystal growth along a specific direction compared to the others (direction (202) corresponding to the peak at 28.67°).

[0077] Finally, Figures 3-5 show the performance of the PV devices in terms of PCE, short-circuit current density (Jsc) and open-circuit voltage (Voc) by increasing the content of the additive MAC1 in the material.

[0078] All these parameters reach their maximum value per MAC1 content up to 90% (compared with other sources of methylammonium in the stoichiometry of the material).

[0079] This increase in parameters must be primarily aimed at the improved charge extraction of the charges generated in the LDP material due to the controlled growth of the crystals and their preferential orientation, as demonstrated by the large increase in the value of the photogenerated current of the devices (Figure 3b).

[0080] Surprisingly, the PCE value increases from 1.5% for the material without additives, up to 10% for the best devices, consisting of more than 6 times more power generated by the solar cell.

[0081] This approach has also been tested and confirmed for other types of LDP materials with different organic composition, thus validating the process in accordance with the present invention as a path to control crystal growth and improve the efficiency of LDP-based photovoltaic devices.

[0082] In a second aspect, the present invention relates to an absorbent material, particularly for solar cells, comprising low-dimensional perovskite with preferential orientation obtained by the above-described process. Advantageously, the absorbent material is in the form of a thin film.

[0083] In a further aspect, the present invention relates to a photovoltaic panel comprising at least one solar cell, in turn, comprising at least the absorbing material.

[0084] Preferably, the solar cell is a perovskite structure solar cell. It has in practice been ascertained that the described invention achieves the intended objects.

[0085] In particular, the expedient of employing a mixture of three precursors is emphasized and results in a direct orientation corresponding to the most efficient charge extraction path (dir.A), with the inorganic slabs aligned along the preferential direction of charge extraction of a typical stack of perovskite solar cells (Figure lb).

Claims

CLAIMS1) Process for the preparation of low-dimensional perovskite with preferential orientation, comprising the phases of: supply of at least a first precursor (A) comprising inorganic iodide; supply of at least a second precursor (B) comprising organic iodide; supply of at least a third precursor (C) selected from at least one of methylammonium iodide and methylammonium chloride; mixing of said at least first precursor (A), said at least second precursor (B) and said at least third precursor (C) with at least one solvent selected from the list comprising: N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), gamma-butyrolactone (GBL) and ethanol (Et-OH) to obtain a precursor mixture; heating of said precursor mixture to a temperature below 100°C; cooling of said precursor mixture; deposition and accretion of said precursor mixture to obtain low-dimensional perovskite with preferential orientation.2) Process according to claim 1, characterized by the fact that said at least first precursor (A) comprises lead iodide.3) Process according to one or more of the preceding claims, characterized by the fact that said organic iodide comprises at least one linear C1-C12 aliphatic chain.4) Process according to one or more of the preceding claims, characterized by the fact that said organic iodide comprises one iodide of an ammonium derivative.5) Process according to one or more of claims 3-4, characterized by the fact that said aliphatic chain comprises at least one end conjugated with one of: benzene, thiophene and pyrrole.6) Process according to one or more of the preceding claims, characterized by the fact that, in said phase of mixing, said at least first precursor (A), said at least second precursor (B) and said at least third precursor (C) are mixed together in accordance with the following stoichiometric ratio A:B:C=n:2:n-l wherein n is comprised between 1 and 5.7) Process according to one or more of the preceding claims, characterized by the fact that said low-dimensional perovskite with preferential orientation has the following formula: R2MAn-iPbn(Ii-xClx)3n+i wherein R is said at least second precursor (B) and X corresponds to the molar ratio of said methylammonium iodide to methylammonium chloride, said molar ratio being comprised between 0 and 1.8) Process according to one or more of the preceding claims, characterized by the fact that said phase of deposition and accretion is carried out by means of at least one step of spreading said precursor mixture onto a rotating supporting surface.9) Process according to one or more of the preceding claims, characterized by the fact that said supporting surface has a rotational speed of more than 3,000 rpm.10) Process according to one or more of the preceding claims, characterized by the fact that it comprises, at the same time as said phase of deposition and accretion, at least one phase of dispensing a solvent selected from the list comprising: toluene, chlorobenzene, ethyl acetate, anisole, chloroform or mixtures thereof11) Process according to one or more of the preceding claims, characterized by the fact that said solvent is dispensed along a dispensing direction substantially transverse to said supporting surface.12) Process according to one or more of the preceding claims, characterized by the fact that it comprises a phase of heating said perovskite to a temperature below 150°C.13) Absorbent material, particularly for solar cells, characterized by the fact that it comprises low-dimensional perovskite with preferential orientation obtained by the process according to one or more of the preceding claims.14) Absorbent material according to claim 13, characterized by the fact that it is in the form of a thin film.15) Photovoltaic panel comprising at least one solar cell comprising at least the absorbent material according to claim 13 or 14.16) Photovoltaic panel according to claim 15, characterized by the fact that said solar cell is a perovskite structure solar cell.

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