Patent Application
20210217968
This invention relates to porous perovskite photoactive films, and more particularly, to porous perovskite films containing microgels. The present invention also relates to processes for the preparation of these films and to their use in perovskite solar cells.
Perovskite solar cells (PSCs) continue to generate enormous research interest in the field of renewable energy generation, because of their meteoric rise in power conversion efficiencies (PCEs), thereby offering the potential for low-carbon energy generation. PSCs contain a light harvesting photoactive layer, the layer being formed from a compound with an ABX3 perovskite crystal structure.
Hybrid organic-inorganic halide perovskites are an important class of perovskite compounds. This is due to the near optimum balance of perovskite material properties that suit their use as light harvesting layers in solar cells. These properties include panchromatic absorption (Kazim et al., Angewandt. Chem. Int. Ed., 2014, 53, 2812-2824), low exciton binding energies (Miyata et al., Nat. Phys., 2015, 11, 582), high exciton diffusion lengths (Stranks et al., Science, 2013, 342, 341-344) and defect tolerant performance (De Marco et al., Nano Lett., 2016, 16, 1009-1016). Pb-based perovskites provide the highest power conversion efficiencies and are based on earth-abundant materials (Frost et al., Acc. Chem. Res. 2016, 49, 528-535).
An area of future energy generation is building-integrated solar cells, which involve semi-transparent solar cells for windows or interior walls. Hence, there is considerable interest in establishing semi-transparent PSCs. One method to achieve such PSCs involves including controlled porosity within deposited perovskite photoactive films (Eperon et al., ACS Nano, 2014, 8, 591-598).
Inverse opal (IO) films consist of highly ordered monodisperse pores and have been widely studied for optical and electronic applications (Wang et al., ACS Nano, 2017, 11, 8026-8033). IO perovskite films are prepared using multi-step approaches involving colloidal particle templates which are subsequently removed (Meng et al., Nano Lett., 2016, 16, 4166-4173; Zhou et al., Adv. Mater., 2017, 29, 170368). The IO perovskite films reported to date have tuneable reflectivity and respectable PCEs, but unfortunately, existing methods to prepare IO films use delicate procedures that are time-consuming. Their future scale-up in a cost-effective manner for PSCs is a daunting challenge. In contrast, disordered inverse opal (DIO) morphologies are arguably more relevant to future large-scale applications (Neale et al., J. Phys. Chem. C, 2011, 115, 14341-14346). Such morphologies can provide strong light scattering and are of interest for enhancing solar cell performance. It is therefore desirable to establish a scalable method to give porous perovskite films with a DIO morphology.
Microgels (MGs) are crosslinked polymer colloid particles that swell in a thermodynamically good solvent (Saunders et al., Adv. Coll. Interf Sci., 1999, 80, 25). They have been used for many years to form surface coatings in the automotive industry and have unique rheological and reversible space-filling properties. In order to stabilize PSCs and reduce the amount of expensive hole transport material used, polystyrene microgels have been used as an encapsulating layer on top of perovskite films (Chen et al., Nanoscale, 2017, 9, 10126-10137); such use, however, was found to reduce the PCEs compared to control PSCs.
It is therefore an object of the present invention to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.
The present invention provides novel porous perovskite films comprising microgel particles within the photoactive perovskite layer. A micropatterning approach uses microgels as pore-forming particles to produce perovskite films, typically with a DIO morphology. The present invention also provides a scalable method for depositing such porous perovskite films in a single step. The microgels increase the perovskite capping layer thickness as well as the grain size and perovskite conversion. Advantageously, the microgels increase the extent of light harvesting and photoluminescence (PL) intensity of the perovskite phase. Furthermore, the presence of the microgels also increases the PCE of perovskite solar cells fabricated with porous perovskite films according to the present invention, compared to microgel-free control PSCs.
Therefore, in a first aspect, the present invention provides a porous photoactive layer for a perovskite solar cell, the layer comprising:
a hybrid inorganic-organic perovskite of formula ABX3, wherein:
In another aspect, the present invention provides a method of forming a porous photoactive layer as described herein, comprising the steps of:
In a further aspect, the present invention provides a perovskite solar cell comprising a porous photoactive layer as described herein.
The above and further aspects of the invention are described in further detail herein.
[The code used to denote the films referred to below is ‘MPxMGy’ where x and y are the concentrations of MAPbl3-zClz and MG used to prepare the films. Note that MA is CH3NH3+].
Particular embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:
The present invention provides a porous photoactive layer for a perovskite solar cell, the layer comprising:
a hybrid inorganic-organic perovskite of formula ABX3, wherein:
Photoactive layers are used in solar cells to absorb light. In perovskite solar cells (PSCs), a photoactive layer includes a perovskite-structured material with a crystal structure of general formula ABX3. Hybrid inorganic-organic perovskite compounds are a major class of compounds used as photoactive layers in PSCs.
The photoactive layer of the present invention comprises a hybrid inorganic-organic perovskite of formula ABX3, as described herein. A and B must balance the X− charge, so that overall A is singly-charged and B is doubly-charged. In other words, there are three X− anions, so to balance the charge, overall, even though A and B may be combinations of different cations, A must be A+ and B must be B2+. For example, A is C1-6alkyl-NH3+ and B is (Sn2+)0.3(Pb2+)0.7; or A is (Cs+)0.5(C1-6alkyl-NH3+)0.5 and B is Pb2+.
In an embodiment, A is C1-6alkyl-NH3+. C1-6alkyl refers to a branched or unbranched alkyl chain containing between 1 and 6 carbon atoms. In an embodiment, C1-6alkyl is methyl. In an embodiment, A is CH3NH3+. Formamidinium refers to the protonated form of formamidine.
In an embodiment, B is Pb2+.
In an embodiment, X is selected from one or more of Br and I−. In an embodiment, X is selected from one or more of Br and Cl−. In an embodiment, X is selected from one or more of Cl− and I−. In an embodiment, X is a combination of Cl− and I−.
In an embodiment, A is C1-6alkyl-NH3+ and B is Pb2+. In an embodiment, A is CH3NH3+ and B is Pb2+. In an embodiment, A is C1-6alkyl-NH3+, B is Pb2+ and X is selected from one or more of Cl− and I−, preferably a combination of Cl− and I−. In an embodiment, A is CH3NH3+, B is Pb2+ and X is selected from one or more of Cl− and I−, preferably a combination of Cl− and I−. In an embodiment, the porous photoactive layer comprises a hybrid inorganic-organic perovskite of formula (CH3NH3+)(Pb2+)(I−)3-z(Cl−)z where z is 0 to 3.
The hybrid inorganic-organic perovskites are typically prepared by mixing perovskite precursors in a suitable solvent. A suitable solvent, or solvent system, is one in which the precursors dissolve. Suitable solvents include polar aprotic solvents, such as dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO). Suitable solvents may be selected from γ-butyrolactone, dimethyl formamide, dimethyl sulfoxide, or a combination of these solvents. In an embodiment, the hybrid inorganic-organic perovskite is formed from perovskite precursors. The hybrid inorganic-organic perovskite precursors are compounds which, when combined, are capable of forming a hybrid inorganic-organic perovskite of formula ABX3, as defined herein. In a preferred embodiment, the precursors are of the formula AX and BX2. In a preferred embodiment, the precursors are C1-6alkyl-NH3X and PbX2, such as CH3NH3X and PbX2 (for example, CH3NH3I and PbCl2). In an embodiment, the hybrid inorganic-organic perovskite is of formula (CH3NH3+)(Pb2+)(I−)3-z(Cl−)z where z is 0 to 3; and is formed from the precursors CH3NH3I and PbCl2.
In the present invention, microgels are used as a micropatterning additive for hybrid inorganic-organic perovskite photoactive layers. The microgels act as pore-forming agents around which the hybrid inorganic-organic perovskite crystallises during layer deposition. In contrast to some previously reported methods to prepare porous perovskite layers (Meng et al., Nano Lett., 2016, 16, 4166-4173; Zhou et al., Adv. Mater, 2017, 29, 170368) where the patterning additive was removed, the porous layers of the present invention are prepared via a single-step deposition method without the need to subsequently remove the microgel particles.
In order to prepare the porous photoactive layers according to the present invention, microgels were used which dispersed in solvents suitable for hybrid inorganic-organic perovskite preparation (polar aprotic solvents such as γ-butyrolactone, dimethyl formamide, and dimethyl sulfoxide), without dissolving. It is important that the microgels are also capable of swelling in polar aprotic solvents. ‘Good’ solvents for swelling microgels as described herein are therefore polar aprotic solvents such as γ-butyrolactone, dimethyl formamide, and dimethyl sulfoxide. ‘Poor’ solvents for swelling microgels as described herein are polar protic solvents such as ethanol or methanol, or non-polar solvents such as toluene, hexane or diethyl ether.
Accordingly, the porous photoactive layer of the present invention further comprises a microgel comprising a plurality of microgel particles formed from a hydrophilic crosslinked polymeric material capable of swelling in polar aprotic solvents.
The crosslinked polymeric material should be hydrophilic to enable it to be dispersed and swelled in polar aprotic solvents (such as γ-butyrolactone, dimethyl formamide, and dimethyl sulfoxide). Hydrophilic polymers may result from the co-polymerisation of monomers bearing hydrophilic moieties (such as hydrogen bond donor/acceptor moieties). In an embodiment, the microgel particles are prepared by non-aqueous dispersion polymerisation. Preferably, the microgel particles are prepared by non-aqueous dispersion polymerisation of monomers bearing hydrophilic moieties. Preferably the monomers are vinyl monomers bearing hydrophilic moieties. Preferably, the polymerisation is a free-radical co-polymerisation initiated via a suitable free-radical source such as AIBN. In an embodiment, the polymerisation is carried out in the presence of additional vinyl co-polymer stabilisers, such as polyvinylpyrrolidone/polyvinyl acetate co-polymer (PVP-co-PVA).
The swelling capability of the polymerized microgel particles can be assessed by comparing the z-average diameters (dz) of the microgel particles (MGs) dispersed in a poor solvent (e.g. ethanol) and a good solvent (e.g. DMSO) via a suitable technique such as dynamic light scattering. This comparison can be seen in
In an embodiment, the microgel particles comprise a co-polymer of monomers (I) and (II):
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Y is:
and optionally R10 and R11 are both hydrogen.
In an embodiment monomer (I) is:
In an embodiment monomer (I) is:
and R1, R2 and R3 are hydrogen.
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Y is:
and R10 and R11 are taken together with the moieties to which they are attached to form a 4- to 9-membered lactam.
In an embodiment monomer (I) is:
In an embodiment monomer (I) is:
and R1, R2 and R3 are hydrogen.
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Y is:
and optionally R12 is NR13R14. In a preferred embodiment R13 and R14 are both hydrogen. In a preferred embodiment R13 is hydrogen and R14 is isopropyl.
In an embodiment monomer (I) is:
and R1, R2 and R3 are hydrogen.
In an embodiment monomer (I) is:
and R1, R2 and R3 are hydrogen.
In an embodiment monomer (I) is:
In an embodiment monomer (I) is:
and R1 is methyl and R2 and R3 are hydrogen.
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Z is selected from:
In an embodiment, Z is:
and L1 is selected from divalent alkyl, divalent alkylether, divalent alkylamine, divalent alkylamide and divalent alkylester linker groups.
As used herein ‘divalent alkyl’ group refers to a branched or unbranched, optionally substituted 1 to 20 carbon alkylene linker. Examples of suitable divalent alkyl groups include methylene (CH2) ethylene (CH2CH2), propylene or butylene.
As used herein ‘divalent alkylether’ group refers to a branched or unbranched, optionally substituted 1 to 20 carbon alkylene linker wherein one or more carbon atoms have been replaced by oxygen atoms. Examples of suitable divalent alkylether groups include —CH2OCH2—, —CH2CH2OCH2CH2—, —CH2OCH2CH2— and —CH2OCH2CH2OCH2—.
As used herein ‘divalent alkylamine’ group refers to a branched or unbranched, optionally substituted 1 to 20 carbon alkylene linker wherein one or more carbon atoms have been replaced by nitrogen atoms. Examples of suitable divalent alkylamine groups include —CH2NHCH2—, —CH2CH2NHCH2CH2—, —CH2NHCH2CH2— and —CH2NHCH2CH2NHCH2—. The N atoms may be optionally substituted with C1-3alkyl groups.
As used herein ‘divalent alkylamide’ group refers to a branched or unbranched, optionally substituted 1 to 20 carbon alkylene linker wherein one or more carbon atoms have been replaced by amide moieties (—NHC(O)— or —C(O)NH—). Examples of suitable divalent alkylamide groups include —CH2—C(O)NH—CH2—, —CH2CH2—C(O)NH—CH2CH2— and —CH2—NHC(O)CH2CH2—. The N atoms may be optionally substituted with C1-3alkyl groups.
As used herein ‘divalent alkylester’ group refers to a branched or unbranched, optionally substituted 1 to 20 carbon alkylene linker wherein one or more carbon atoms have been replaced by ester moieties (—OC(O)— or —C(O)O—). Examples of suitable divalent alkylester groups include —CH2—C(O)O—CH2—, —CH2CH2—C(O)O—CH2CH2— and —CH2—OC(O)CH2CH2—.
In an embodiment, Z is:
and L1 is a divalent alkyl or divalent alkylether linker group, preferably L1 is a divalent alkylether linker group.
In an embodiment, Z is
In an embodiment monomer (II) is:
and R4, R5, R6, R7, R8 and R9 are hydrogen.
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Z is:
and L2 is selected from divalent alkyl, divalent alkylether, divalent alkylamine, divalent alkylamide and divalent alkylester linker groups.
In an embodiment, Z is:
and L2 is a divalent alkyl or divalent alkylether linker group, preferably L1 is a divalent alkyl linker group.
In an embodiment, there is provided a microgel comprising a co-polymer of monomers (I) and (II), as described herein, wherein Z is:
In an embodiment monomer (II) is:
and R4, R5, R6, R7, Wand Ware hydrogen.
In an embodiment, Z is:
and n is 1 to 20, such as 1-5, or preferably n is 1.
In an embodiment monomer (II) is:
and R4, R5, R8 and R9 are hydrogen and R6 and R7 are methyl.
In an embodiment, the microgel particles comprise a co-polymer of:
This co-polymer is poly(N-vinylformamide-co-2-(N-vinylformamido)ethyl ether) (PNVF-NVEE).
In an embodiment, the microgel particles comprise a co-polymer of:
This co-polymer is poly(N-vinylcaprolactam-co-N,N′-methylenebis(acrylamide)) (PNVC-BA).
In an embodiment, the microgel particles comprise a co-polymer of:
This co-polymer is poly(acrylamide-co-N,N′-methylenebis(acrylamide)) (PA-BA).
In an embodiment, the microgel particles comprise a co-polymer of:
This co-polymer is poly(N-isopropylacrylamide-co-N,N′-methylenebis(acrylamide)) (PNIPAM-BA).
In an embodiment, the microgel particles comprise a co-polymer of:
wherein n and p are as defined herein. This co-polymer is poly(polyethylene glycol methacrylate-co-polyethylene glycol dimethacrylate) (PPEGMA-PEDGMA).
The micropatterning approach of the present invention uses microgels as pore-forming particles to produce porous perovskite films. Therefore, the average pore size in the photoactive layer is influenced by the average diameter of the swollen (e.g. swollen from a ‘good’ swelling solvent such as DMSO) microgel particles after layer deposition. In an embodiment, the microgel particles as described herein have a diameter after swelling in the range 0.1 to 5 μm (100 to 5000 nm), more preferably in the range 0.1 to 1.5 μm (100 to 1500 nm), even more preferably in the range 400 to 1200 nm. In an embodiment, the microgel particles as described herein have a diameter after swelling in the range 750 to 1500 nm, more preferably in the range 900 to 1200 nm.
When a layer of microgel particles are deposited from a thermodynamically good solvent they tend to flatten. This can be seen in
In an embodiment, in a porous photoactive layer as described herein, the pores in the layer coincide with the location of the microgel particles.
In an embodiment, a porous photoactive layer as described herein, has an average pore size in the range 100-5000 nm, such as 100-1500 nm, preferably 500-1500 nm, more preferably 500-1200 nm, such as 700-1100 nm, or 800-1000 nm. In an embodiment, a porous photoactive layer as described herein, has an average pore size in the range 400-700 nm, or 300-600 nm. A porous photoactive layer with an average pore size in the range 300-700 nm, may allow photoactive layers to be prepared which are coloured, due to scattering of visible light by the pores.
The micropatterning effect of the MGs typically produces porous perovskite layers with disordered inverse opal (DIO) morphology, as shown in
It is desirable, in terms of the photoactive properties of the deposited layer and of solar cells constructed therefrom, that the DIO morphology covers the photoactive layer surface in a substantially continuous DIO film. In an embodiment, the porous photoactive layer has a disordered inverse opal morphology which covers greater than 60% of the photoactive layer surface. In a preferred embodiment, the porous photoactive layer has a disordered inverse opal morphology which covers greater than 70% of the photoactive layer surface, greater than 75% of the photoactive layer surface, more preferably greater than 80% of the photoactive layer surface, most preferably greater than 90% of the photoactive layer surface. In an embodiment, the porous photoactive layer has a disordered inverse opal morphology which covers the photoactive layer in a substantially continuous DIO film. In an embodiment, the porous photoactive layer has a disordered inverse opal morphology with substantially no voids in the DIO film. Voids refers to regions of the layer which do not possess DIO morphology. In an embodiment, the porous photoactive layer has a disordered inverse opal morphology with substantially no areas of perovskite in the DIO film. Areas of perovskite refers to non-porous regions of the layer, i.e. areas where microgel particles have not formed pores.
In an embodiment, the porous photoactive layer comprises 600 to 1200 nm pores and has a disordered inverse opal morphology which covers greater than 60% of the photoactive layer surface. In an embodiment, the porous photoactive layer comprises 700 to 1100 nm pores and has a disordered inverse opal morphology which covers greater than 70% of the photoactive layer surface. In an embodiment, the porous photoactive layer comprises 800 to 1000 nm pores and has a disordered inverse opal morphology which covers greater than 80% of the photoactive layer surface.
In an aspect of the present invention, there is provided a method of forming a porous photoactive layer as described herein, comprising the steps of:
In step a) microgel particles as described herein, are swollen in a thermodynamically ‘good’ solvent for the microgels. A suitable solvent will swell the particles to 1.2-100 times the size of the unswollen particles. In an embodiment, the linear swelling ratio of the microgel particles is in the range 1.2 to 100. In an embodiment, in step a) the particles are swollen to 1.2-50 times, such as 1.2-25, 1.2-10 or 1.2-5 times the size of the unswollen particles. In an embodiment, the linear swelling ratio of the microgel particles is in the range 1.2 to 50, such as 1.2 to 25, 1.2 to 10, or 1.2 to 5. In an alternative embodiment, step a) comprises swelling particles of the microgel in a solvent to 1.5-10 times the volume (this is the volume swelling ratio) of the unswollen particles. Preferably, the volume swelling ratio of the microgel particles is between 1.5 and 5, between 1.5 and 3, or between 2 and 3, compared to the unswollen particles. A suitable solvent comprises polar aprotic solvents. In an embodiment, the solvent is selected from γ-butyrolactone, dimethyl formamide, dimethyl sulfoxide, or a combination of these solvents. In a preferred embodiment, the solvent is dimethyl sulfoxide. In an embodiment, the solvent in step a) comprises polar aprotic solvents or water, or a mixture thereof.
In step b) hybrid inorganic-organic perovskite precursors are added to the dispersion of swollen microgel particles from step a), so that after deposition the hybrid inorganic-organic perovskite crystallises on the substrate. The hybrid inorganic-organic perovskite precursors are compounds which, when combined, are capable of forming a hybrid inorganic-organic perovskite of formula ABX3, as defined herein. In a preferred embodiment, the precursors are of the formula AX and BX2, wherein A, B and X are as defined herein. For example, the perovskite precursors may be C1-6alkyl-NH3X and PbX2, such as CH3NH3X and PbX2 (for example, CH3NH3I and PbCl2).
In a preferred embodiment, steps a) and b) are carried out concurrently. In other words, the microgel particles and the perovskite precursors, are dispersed in a suitable solvent (such as a polar aprotic solvent), in a single step.
Step c) involves deposition of the perovskite precursor/microgel dispersion onto a substrate. A suitable substrate for forming the porous photoactive layer on, is any layer which may be conventionally used in the production of solar cells and which is stable to the solvent used in step a). Preferably, the substrate is a mesoporous TiO2 layer.
A skilled person may envisage various deposition methods suitable for achieving the coating in step c). Such methods may include casting, doctor blading, screen printing, inkjet printing, pad printing, knife coating, meniscus coating, slot die coating, gravure coating, reverse gravure coating, kiss coating, micro-roll coating, roll-to-roll coating, curtain coating, slide coating, spin coating, spray coating, flexographic printing, offset printing, rotatory screen printing, evaporative coating or dip coating. In a preferred embodiment, the coating in step c) is carried out by spin coating.
In step d) the solvent is evaporated, either passively (under ambient conditions) or actively (e.g. via the application of heat and/or vacuum to the coated dispersion). Solvent evaporation drives crystallization of the hybrid inorganic-organic perovskite. To enhance perovskite crystallization, an anti-solvent may be added during the coating or evaporating steps. In an embodiment, step c further comprises the addition of an anti-solvent during coating. Preferably the anti-solvent is added at or towards the end of the coating step. An anti-solvent is defined as a solvent in which the hybrid inorganic-organic perovskite, as described herein, has poor solubility. In an embodiment, the anti-solvent is selected from chlorobenzene, benzene, xylene, toluene, methanol, ethanol, ethylene glycol, 2-propanol, chloroform, THF, acetonitrile, and benzonitrile, or a combination thereof. In a preferred embodiment, the anti-solvent is toluene.
The DIO coverage of the photoactive layer surface can be influenced by the concentrations of both the microgel and the hybrid inorganic-organic perovskite precursors. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 1-10% w/w. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 1.5-7% w/w. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 2-5% w/w. In an embodiment, in step b) the HIOP precursors are at a concentration (CMP) of 20-60% w/w. In an embodiment, in step b) the HIOP precursors are at a concentration (CMP) of 25-50% w/w. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 1-10% w/w, and in step b) the HIOP precursors are at a concentration (CMP) of 15-70% w/w. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 1.5-7% w/w, and in step b) the HIOP precursors are at a concentration (CMP) of 20-60% w/w. In an embodiment, in step a) the microgel particles are at a concentration (CMG) of 2-5% w/w, and in step b) the HIOP precursors are at a concentration (CMP) of 25-50% w/w.
In a further aspect, there is provided a porous photoactive layer directly obtained by, obtained by, or obtainable by a method as described herein.
As already discussed, the porous perovskite layers of the present invention typically have DIO morphology. Due to their strong light scattering and tunable reflectivity, DIO films are of interest for enhancing solar cell performance. As expected, with increasing HIOP concentration, an increase in absorbance of UV-visible light is observed (
In comparison to MG-free perovskite layers, the HIOP grain size is increased in layers prepared according to the present invention, at the same concentration of HIOP precursors (
Porous photoactive layers prepared according to the present invention may also demonstrate increased HIOP conversion. Conversion refers to the % of HIOP produced by the reaction of the precursor perovskite species.
PSCs can be constructed comprising porous photoactive layers according to the present invention. Therefore, in an aspect of the invention, there is provided a perovskite solar cell comprising a porous photoactive layer as described herein.
Perovskite solar cells may be constructed using processes and techniques familiar to those in the field. A PSC is typically formed from a number of layers selected from one or more of glass, indium tin oxide (ITO), TiO2 hole-blocking layer (bl-TiO2), mesoporous TiO2 layer (mp-TiO2), perovskite photoactive layer (capping layer), hole transport layer and gold. Preferably, the PSC layers are deposited on top of one another in the order glass, ITO, TiO2 hole-blocking layer (bl-TiO2), mesoporous TiO2 layer (mp-TiO2), perovskite photoactive layer, hole transport layer and gold. In PSCs comprising porous photoactive layers according to the present invention, the porous photoactive layer is the perovskite photoactive layer (capping layer).
A process for forming a PSC may comprise coating the HIOP-MG dispersion onto a glass/ITO/bl-TiO2/mp-TiO2 substrate, and then applying a hole transport layer, followed by a gold coating (see
PSCs formed with porous photoactive layers according to the present invention may have significantly higher short-circuit current density (Jsc) values compared to analogous PSCs prepared with non-porous photoactive layers. Increased light harvesting by the porous photoactive layers as described herein, due to their increased capping layer thickness, may be contributing to the superior Jsc values.
PSCs formed with porous photoactive layers according to the present invention may have significantly higher power conversion efficiency (PCE) values compared to analogous PSCs prepared with non-porous photoactive layers. It is postulated that the collapsed microgel particles located in the pores sitting on top of the mp-TiO2 layer, act as insulation between the hole transport layer and the mp-TiO2 thus preventing short-circuits which decrease the PCE. The superior PCEs are also attributable to the increased Jsc values.
Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
The invention will now be described in more detail in relation to the following illustrative examples.
Unless stated otherwise, the following methodology was used to obtain physical measurements.
Dynamic light scattering (DLS) measurements was obtained using a Malvern Zetasizer Nano ZS instrument (via cumulants analysis). The z-average diameter (dz) is an average value from five runs.
Optical microscopy was conducted using an Olympus BX41 microscope. Fractional coverage values were calculated using Image J software.
The top view SEM was obtained using a Philips XL30 FEGSEM and the cross-section SEM was obtained using a Carl Zeiss Sigma FESEM. The samples were coated with Au or Pd.
AFM images were obtained using an Asylum Research MFP-3D operating in AC (“tapping”) mode.
UV-visible spectra were recorded using a Perkin Elmer Lamda 25 UV-Vis spectrometer. The average visible transmittance was measured between 370 and 740 nm.
Film thickness measurements were conducted using a Dektak 8 Stylus Profilometer (Bruker).
XRD patterns were conducted using a Bruker D8 Advance diffractometer (Cu-Kα). Films were scanned with a step size of 0.02°. The films were prepared under nitrogen atmosphere and measured using an airtight holder.
Photoluminescence (PL) spectra were obtained using an Edinburgh Instruments FLS980 spectrometer. The beam was incident on the film surface side and an excitation wavelength of 480 nm was used.
Unless stated otherwise, the following methodology was used to obtain device measurements.
The current density-voltage (J-V) characteristics were measured using a Keithley 2420 Sourcemeter and 100mWcm2 illumination (AM 1.5G) and a calibrated NREL certified Oriel Si-reference cell. An Oriel solar simulator (SOL3A) was used for these measurements. The active area of the devices (0.025 cm2) was determined using a square aperture within a mask. The data shown are from the reverse scan unless otherwise stated (Voc to Jsc) and the sweep rate was 100 mV s−1.
N-vinylformamide (NVF, 98%), azoisobutyronitrile (AIBN, 98%), potassium-tert-butoxide (95%), bis(2-bromoethyl)ether (BBE, 95%), dicyclohexyl-18-crown-6 (98%), anhydrous tetrahydrofuran (THF, 99.9%), and ethanol (99.9%), poly(l-vinylpyrrolidone-co-vinyl acetate) (PVP-co-PVA, Mn˜50,000 g/mol), anhydrous sodium sulphate (100%), chloroform (99.9%), sodium chloride (NaCl, 100%), toluene (99.8%), chlorobenzene (CBZ, 99.8%), isopropanol (IPA, anhydrous, 99.5%), 4-tert-butylpyridine (TBP, 96%) and lithium bistrifluoromethanesulfonimidate (LiTFSI, 99.95%) were all purchased from Aldrich and used as received. Methyl amine solution (33 wt. % in absolute EtOH) and hydroiodic acid (57 wt. %), titanium diisopropoxide bis(acetylacetonate) (TDB, 75 wt % in IPA), lead (II) chloride (PbCl2, 98%) and dimethyl sulfoxide (DMSO, 99.7%) were also purchased from Aldrich and used as received. Methylammonium iodide (MAI) was synthesised and purified using the method previously reported (Etgar et al., J. Amer. Chem. Soc., 2012, 134, 17396-17399). Titania paste (TiO2, 18 NRT) was purchased from Dyesol and used as received. Spiro-MeOTAD (Spiro, N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine, Fenglin Chemicals, 99.5%) was also used as received. Water was of ultra-high purity and de-ionised.
Method 1—Preparation of 2-(N-vinylformamido)ethyl ether (NVEE)
NVEE was synthesized in a 250 mL reactor equipped with an overhead stirrer. Firstly, a mixture of NVF (7.1 g, 100 mmol), potassium-tert-butoxide (12 g, 105 mmol) and dicyclohexyl-18-crown-6 (1 g, 2.65 mmol) were dissolved in anhydrous THF (100 mL). Then this mixture was stirred vigorously at room temperature for 45 min and was cooled to 0° C. in an ice bath for 20 min. Bis(2-bromoethyl)ether (9.3 g, 40 mmol) was then added dropwise to the mixture during cooling and the mixture was stirred at room temp for 72 h. After that, KBr was removed from the mixture by filtration and the reaction mixture was concentrated under rotary evaporator and diluted with water (100 mL). The product was repeatedly extracted with chloroform (5×40 mL) and washed twice with brine and then dried over anhydrous sodium sulphate (40 g) for 24 h. Finally, the product was recovered as a liquid after concentration using rotary evaporation.
Method 2—Preparation of poly(NVF-co-NVEE) [PNVF-9NVEE] Microgel Particles
NVF-9NVEE microgel particles were prepared by non-aqueous dispersion polymerization. The MGs nominally contained 9.0 wt % NVEE based on monomer. A mixture of NVF (6.0 g, 85.5 mmol), PVP-co-PVA (1.8 g) and NVEE (1.79 g, 8.28 mmol) were added to EtOH (86 mL) in a four-necked round bottomed flask equipped with overhead stirrer, nitrogen supply and a reflux condenser. The solution was heated to 70° C. and stirred vigorously. Then, AIBN (0.240 g, 1.45 mmol) in EtOH (2.0 mL) was added to the reactor flask and the polymerization allowed to continue for 1 h. The dispersion was filtered with a 50 μm mesh filter after cooling to 0° C. and was then purified by three centrifugation and re-dispersion steps in EtOH.
The MGs prepared according to Method 2, when deposited from ethanol and DMSO, had z-average diameters (dz) of 885 nm (PDI=0.013) and 1125 nm (PDI=0.088) respectively, as measured by dynamic light scattering (
Method 3—Preparation of poly(NVF-co-NVEE) [PNVF-9NVEE] Sub-Micrometer Microgel Particles
NVF-9NVEE sub-micrometer microgel particles were prepared by non-aqueous dispersion polymerization. The MGs nominally contained 9.0 wt % NVEE based on monomer. A mixture of NVF (3.0 g, 42.75 mmol), PVP-co-PVA (1.8 g) and NVEE (0.9 g, 4.16 mmol) were added to EtOH in a four-necked round bottomed flask equipped with overhead stirrer, nitrogen supply and a reflux condenser. The solution was heated to 70° C. and stirred vigorously. Then, AIBN (0.12 g, 0.73 mmol) in EtOH (2.0 mL) was added to the reaction flask and the polymerization allowed to continue for 1 h. The dispersion was filtered with a 50 μm mesh filter after cooling to 0° C. and was then purified by three centrifugation and re-dispersion steps in EtOH.
The MGs prepared according to Method 3, when deposited from ethanol and DMSO, had z-average diameters (dz) of 354 nm (PDI=0.012) and 495 nm (PDI=0.041) respectively, as measured by dynamic light scattering (
The MGs exhibited low polydispersity as shown by TEM images (e.g.
NVF-9NVEE microgel particles as prepared in Methods 2 and 3 were dispersed in EtOH (‘poor’ solvent) and centrifuged at 7,000 rpm and then re-dispersed in DMSO (‘good’ solvent). Then, the centrifugation speed was increased to 10,000 rpm. Finally, the sedimented particles were re-dispersed in DMSO again. The MG particles were re-dispersed in DMSO at various concentrations from 1.0 to 7.0 wt %. The MG dispersion were rapidly added (dropwise) to a clean and dry glass slide and spin coated at 3,000 rpm for 15 s using a Laurell WS-650 Mz-23NPP spin processor for the films.
The optical micrographs of the deposited Method 2 MGx films are shown in
Method 5—Preparation of (CH3NH3+)(Pb2+)(I−)3-z(Cl−)z [MAPbl3-zClz]/MG Films
Indium tin oxide (ITO)-coated glass substrates (20 Ω/sq) were cleaned by ultrasonication in a 1.0 wt % Hellmanex solution, rinsed with water, IPA, NaOH (2.5 M), water again and dried. A TiO2 hole blocking layer (bl-TiO2) (60 nm) was spin-coated at 2000 rpm for 60 s onto the ITO using TDB solution in 1-butanol (0.15 M followed by 0.30 M) and subsequent heating at 125° C. for 5 min. After that, TiO2 paste (1:5 in EtOH) was spin coated at 5000 rpm for 30 s onto the cleaned ITO substrate to form a mesoporous scaffold (mp-TiO2). The mp-TiO2 films (thickness ˜250 nm) were annealed at 500° C. for 30 min and cooled to room temperature. A MAPbl3-zClz with MGs precursor solution (100 μl)* was spin-coated onto the ITO/bl-TiO2/mp-TiO2 substrate at 4000 rpm for 25 s. During the spin coating process, toluene (500 μL) was dropped on the surface of film being fabricated in the last 15 s. The films were dried at 100° C. for 45 min. All films were stored in a desiccator over P2O5 in the dark until investigation. *The precursor solution contained MAI and PbCl2 (3:1 molar ratio) to give MAPbl3-zClz. The solution also contained MGs in DMSO at various compositions. For example, in the MP37.5MG3.0 dispersion the precursor mixture/solution contained 37.5 wt. % of perovskite precursors [i.e., MAI and PbCl2 (3:1 molar ratio)], 3.0 wt. % of MG and 59.5 wt. % of DMSO.
The perovskite/MG films are denoted in terms of the concentrations of MG (CMG) and MAPbl3-zClz (CMP) used to spin coat the film, i.e., MPxMGy. The MGs were first characterized and then the morphologies of the MPxMGy films were investigated.
The average pore size for MPxMGy films (prepared according to Method 5 using M2 MGs) containing DIO morphologies were measured and are given in Table A.
The average pore size for the MP30MG2.0, MP30MG3.0 and MP30MG4.0 films decreases as CMG increases. This decrease in size is due to closer packing (and compression) of the MGs during film deposition. Hence, the MG packing influenced the DIO pore size.
SEM images for MP30MGy films (prepared according to Method 5 using microgels prepared according to Method 3 (M3 MGs)) with y=0, 1, 3 and 4% are shown in
The effect of the MGs on the perovskite structure was investigated using XRD for M25MG3.0, MP37.5MG3.0 and MP45MG2.0 films (M2 MGs) as well as the respective MG-free films (see
The light absorption properties of the films were assessed using UV-visible spectroscopy (see
Photoluminescence (PL) spectra were measured for the films. The effect of x (or CMP) was investigated for the MPxMG3.0 films (M2 MGs)—see
The procedure to prepare the ITO/bl-TiO2/mp-TiO2/MAPbl3-zClz (MPx) and ITO/bl-TiO2/mp-TiO2/MAPbl3-zClz/MGs (MPxMGy) films was as described above in Method 5, prepared using MGs according to Method 2. A hole transport layer (spiro film −200 nm) was formed by spin-coating LiTFSI (4.8 μL, 520 mg/ml) and TBP (8.0 μL) in CBZ as solvent at room temperature at 4000 rpm for 20 s onto the MPx and MPxMGy films. Then all devices were coated with a gold layer (70 nm) by thermal evaporation.
PSC devices with MP25MG3.0, MP37.5MG3.0 and MP45MG2.0 DIO layers were constructed. Control devices were also constructed using MP25, MP37.5 and MP45 photoactive layers.
Representative J-V curves are shown in
The DIO films had significantly higher Jsc values compared to the respective control samples in all cases. This result implies that the MG particles increased photo-induced charge transport in the vertical direction and is attributed to more capping layer being retained. The UV-visible spectra clearly showed that the MGs increased the light absorption from MAPbl3-zClz, especially in the blue region (see
| Number | Date | Country | Kind |
|---|---|---|---|
| 1808461.6 | May 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2019/051429 | 5/23/2019 | WO | 00 |
