WO2019013709A1

WO2019013709A1 - Low-dimensional inorganic-organic hybrid metal halide perovskites

Info

Publication number: WO2019013709A1
Application number: PCT/SG2018/050344
Authority: WO
Inventors: Thirumal KRISHNAMOORTHY; Subodh Gautam Mhaisalkar; Nripan Mathews; Han Sen SOO
Original assignee: Nanyang Technological University
Priority date: 2017-07-11
Filing date: 2018-07-11
Publication date: 2019-01-17
Also published as: CN110869466B; CN110869466A

Abstract

Disclosed herein is a zero-dimensional hybrid organic-inorganic perovskite material of Formula (I) wherein A contains at least one aromatic or heteroaromatic ring, M is selected from Ge, Pb, Sn, Mn, Cu, Co, and Eu, and X is a halide or pseudohalide. Preferably, M is Sn, and the organic part is meta-xylenediammonium. Also disclosed herein is a method of preparing said materials and the use of said materials for white light emitting devices.

Description

Low-dimensional Inorganic-organic Hybrid Metal Halide Perovskites Field of Invention This invention relates to low-dimensional inorganic-organic hybrid metal halide perovskites, and the use of said materials for white light applications.

Background The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

There is currently strong interest in the lighting technology known as solid-state lighting (SSL), where the light emission is produced from solid-state light emitting diodes (LEDs). LEDs typically have compact structures, longer lifespans and exhibit higher energy conversion efficiencies than other light sources (e.g. see Schubert, E. F. et al. Science 2005, 308, 1274, Shirasaki, Y. et al. Nat. Photonics 2013, 7, 13, and Ponce, F. A. et al. Nature 1997, 386, 351). In particular, white LEDs (WLEDs) are especially important and widespread since white lighting has the broadest applications.

Commercially available WLEDs typically consist of a single blue gallium nitride (GaN) LED combined with a yellow-emitting yttrium aluminium garnet (YAG) phosphor. They give white light in high luminous efficacy but with relatively low colour rendering indices (CRI < 75). For example, see Pan, Y. et al., J. Phys. Chem. Solids 2004, 65, 845, Wu, J. L. et al. Chem. Phys. Lett. 2007, 441, 250, Jang, H. S. et al. J. Lumin. 2007, 126, 371 , and Ye, S. et al. Mater. Sci. Eng., R. 2010, 71, 1. These phosphors are synthesised under harsh conditions (high temperatures and pressures), which imposes limitations on the materials and substrates that can be used. Further, oxide-based phosphors are typically not processible in solution form to make thin, uniform coatings. This may make it difficult to achieve a uniform coating for better light quality.

Organic-inorganic hybrid metal halide perovskites have gained substantial attention in the last few years for applications in photovoltaics and light emitting diodes, among others (e.g. see Ye, S. et al. Mater. Sci. Eng., R. 2010, 71, 1 , Saliba, M. et al. Energy Environ. Sci. 2016, 9, 1989, Dou, L. et al. Nat. Commun. 2014, 5, 5404, Saparov, B. et al. Chem. Rev. 2016, 116, 4558, Krishnamoorthy, T. et al. J. Mater. Chem. A 2015, 3, 23829, Zhu, H. et al. Nat. Mater. 2015, 14, 636, Yuan, M. et al. Nat. Nanotechnol. 2016, 11, 872). The development of hybrid low dimensional perovskite materials for SSL is especially appealing. This is because they typically show broadband emissions which are due to a triplet state of metal-centered electronic transition that occurs in molecular zero-dimensional (0D) perovskite and the formation of self-trapped excitons in semiconducting 1 D and 2D perovskites (e.g. see Dursun, I . et al. ACS Photonics 2016, 3, 1 150, Wu, W.-L. et al. Chem. Mater. 2017, 29, 935, Wang, G.-E. et al. Chem. Sci. 2015, 6, 7222, Dohner, E. R. et al. J. Am. Chem. Soc. 2014, 136, 13154, Zhou, C. et al. Chem. Sci. 2018, 9, 586, and Thirumal, K. et al. Chem. Mater. 2017, 29, 3947).

Accordingly, several low dimensional hybrid perovskites that exhibit high photoluminescence quantum efficiencies (PLQE) have been discovered due to their high exciton binding energies. Under photoexcitation, these materials form free excitons (FE) due to the dielectric confinement by the organic ammonium layers, and these FEs becomes self-trapped excitons (STEs) when the FEs couple to phonons. This phenomenon is especially significant when the metal halide octahedra are severely distorted and the dimensionality of the perovskites is lowered (e.g. see Wu, X. ; Trinh, M. T. ; Niesner, D. ; Zhu, H. ; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J. ; Zhu, X. Y. J. Am. Chem. Soc. 2015, 137, 2089). Subsequently, the STE will broaden the emission spectrum and also produce a large Stokes-shift. For instance, white light emission from two-dimensional (2D) lead halide perovskites (EDBE)PbBr₄ (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) was first reported by Karunadasa and coworkers (Dohner, E. R. ; Jaffe, A.; Bradshaw, L. R. ; Karunadasa, H. I. J. Am. Chem. Soc. 2014, 136, 13154). Further, Ma and coworkers prepared microscale perovskites of (EDBE)PbBr₄ with improved PLQE of 18% to demonstrate that white light emitting diodes can be fabricated using the perovskite as a phosphor (e.g. see Yuan, Z.; Zhou, C; Messier, J. ; Tian, Y.; Shu, Y.; Wang, J. ; Xin, Y.; Ma, B. Adv. Opt. Mater. 2016, 4, 2009). White light emission has also been observed from the 1 D perovskite C₄H₁₄N₂PbBr₄ with a PLQE of 20% (e.g. see Teunis, M. B. ; Lawrence, K. N. ; Dutta, P. ; Siegel, A. P.; Sardar, R. Nanoscale 2016, 8, 17433 and Yuan, Z. et al. Nat. Commun. 2017, 8, 14051). Recently, a 2D perovskite, (CeHsCaH^PbCU, with white light emission was reported and it was found that the broadband emission likely originated from strong coupling of excitons to the organic framework (Thirumal, K. et al. Chem. Mater. 2017, 29, 3947).

Nonetheless, white light emission by a single material is a rare occurrence and has been observed in only less than half a dozen hybrid perovskites thus far (e.g. see Dohner, E. R.; Jaffe, A. ; Bradshaw, L. R.; Karunadasa, H. I. J. Am. Chem. Soc. 2014, 136, 13154, Yuan, Z. et al. Nat. Commun. 2017, 8, 14051 , Dohner, E. R. et al. J. Am. Chem. Soc. 2014, 136, 1718, Hu, T. et al. J. Phys. Chem. Lett. 2016, 7, 2258 and Yangui, A. et al. J. Phys. Chem. C 2015, 119, 23638). More recently, a tin based zero-dimensional perovskite was reported with yellow emission and a practically quantitative PLQE (Zhou, C. et al. arXiv: 1702.07200v1 2017).

Given the above, there remains a need for new perovskite materials for use as phosphors for white light applications. More importantly, these materials have to be easy to synthesise and to be able to demonstrate good PLQE, versatile PL properties and high optical and thermal stabilities. Such materials can potentially be used alone or in combinations with other phosphors in white light-emitting devices.

Summary of Invention

It has been surprisingly found that zero-dimensional perovskites have useful optical properties, some of these perovskites may be particularly suited to the formation of white light-emitting devices. In a first aspect of the invention, there is provided a zero-dimensional perovskite of formula I:

[AiLNR¹ R² R³ ) _n] m⁺ [ X₆]⁴- I

wherein: M is selected from Ge or, more particularly, Pb, Sn, Mn, Cu, Co, and Eu;

each X is independently selected from halo or pseudohalo;

A represents a 5- to 14-membered ring system comprising at least one aromatic or heteroaromatic ring which ring system is unsubstituted or substituted by halo, d_i ₂ alkyl, C₂_i₂ alkenyl, C_2-12 alkynyl, 0-Ci.i₂ alkyl, 0-C₂._{i 2} alkenyl, 0-C₂_i₂ alkynyl, S-Ci_i₂ alkyl, S-C₂.i₂ alkenyl and S-C₂-i₂ alkynyl, which latter nine groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, SR⁵, and Ci_-6 alkyl;

each L represents a linking group attaching NR¹ R²R³ to A and is selected from Ci_-12 alkyl, C_2-12 alkenyl, C₂.₁₂ alkynyl, 0-C_{1 -12} alkyl, 0-C_2-12 alkenyl, 0-C₂.₁₂ alkynyl, S-C_{1 -12} alkyl, S-C₂.₁₂ alkenyl and S-C₂.₁₂ alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁶, SR⁷, and Ci_-6 alkyl; each R¹ to R³ group independently represents H or Ci_-6 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo, OR⁸ and (^.3 alkyl;

each R⁴ to R⁸ group independently represents H or C1.3 alkyl;

n is 1 or 2; and

when:

n is 1 , then m is 4 and p is 1 ; and

n is 2, then m is 2 and p is 2. In a second aspect of the invention, there is also provided a zero-dimensional perovskite of formula II I:

[ACLNR^R³),,]^ [SnX₆]^2" I II wherein A, L, n, X and R¹ to R³ are as defined above for formula I and when:

n is 1 then q is 2 and r is 1 ; and

n is 2 then q is 1 and r is 2.

In a third aspect of the invention, there is also provided a zero-dimensional perovskite of formula la:

([A(LNR¹R²R³)_ni]^,⁺)₂ [M'X₆]^4" [M"X₆]⁴- la wherein A, L, n, X and R¹ to R³ are as defined above for formula I ;

M' and M" are independently selected from Ge, Pb, Sn, Mn, Cu, Co, and Eu, preferably M and M' are not the same metal;

n' is 1 or 2; and

when:

n' is 1 , then m' is 4 and p' is 1 ; and

n' is 2, then m' is 2 and p' is 2.

In a fourth aspect of the invention, there is also provided a zero-dimensional perovskite of formula lb:

([A^ R^!R^^^^^j^M"^]⁵-^'"^]³- wherein A, L, n, X and R¹ to R³ are as defined above for formula I ;

M'" is selected from In, Ga or, more particularly, Tl;

M"" is selected from As or, more particularly, Sb and Bi; n" is 1 or 2; and

when:

n" is 1 , then m" is 4 and p" is 1 ; and

n" is 2, then m" is 2 and p" is 2.

In embodiments of the above aspects:

(a) X may be selected from one or more of the group consisting of F, Br, CI, I , CN, SCN, NCS, CP, NC, OCN, OSCN, SeCN, N₃ (e.g. X may be selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, and NCS, such as Br and/or CI);

(b) A may represent a 6- to 10-membered aromatic ring system or a 5- to 10-membered heteroaromatic ring system, which ring systems are unsubstituted or substituted by halo, d.₆ alkyl, C^ alkenyl, C₂.s alkynyl, which latter three groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, and d.₃ alkyl (e.g. A may represent phenyl, thiophenyl, furanyl, pyrrolyl, pyridinyl, which five ring systems are unsubstituted or substituted by halo and C₁.3 alkyl);

(c) each L represents a linking group attaching NR¹ R²R³ to A and may be selected from Ci_-6 alkyl, C₂.₆ alkenyl, C^ alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl (e.g. each L represents a linking group attaching NR¹R²R³ to A and may be selected from d_₃ alkyl, which group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl);

(d) each R¹ to R³ group may independently represent H or Ci_-3 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo and C_1-3 alkyl (e.g. each R¹ to R³ group may independently represent H).

Specific embodiments of formula I that may be mentioned in combination with any one or more of (a) to (d) above include those in which n is 2 and m is 2. Specific embodiments of the compounds of formula I that may be mentioned herein include:

(i) mefa-xylenediammonium tin bromide

[SnBre] |4-

mefa-xylenediammonium tin chloride

[SnCle]⁴

o/ 70-xylenediammonium tin bromide

[SnBre]⁴

o/f/70-xylenediammonium tin chloride

[SnCle]⁴

para-xylenediammonium tin bromide 2+

[SnBre]⁴

para-xylenediammonium tin chloride

2+

[SnCle]⁴

(vii) mefa-xylenediammonium lead bromide

[PbBre] |4-

(viii) mefa-xylenediammonium lead chloride 2+

[PbCle]⁴

mefa-xylenediammonium germanium bromide

mefa-xylenediammonium germanium chloride

[GeCle]⁴

Particularly preferred compounds that may be mentioned herein include compound (i) to (vi) of the list above, such as compound (i).

In a fifth aspect of the invention, there is also provided a zero-dimensional perovskite of formula IV:

[BCINR^R³)._;]^ [MX₈]^6" IV wherein:

M is selected from Ge or, more particularly, Pb, Sn, Mn, Cu, Co, and Eu;

L and R¹ to R³ are as defined above for formula I;

B represents a carbon atom with 4-s R⁹ groups;

each R⁹ group independently represents H or d-₆ alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo, OR¹⁰ and Ci_₃ alkyl;

each R¹⁰ group independently represents H or Ci.₃ alkyl;

s is 1 , 2 or 3, such that when:

s is 1 then t is 6 and u is 1 ;

s is 2 then t is 3 and u is 2; and

s is 3 then t is 2 and u is 3.

In embodiments of the above aspect:

(a) X may be selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, NCS, CP, NC, OCN, OSCN, SeCN, N₃ (e.g. X may be selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, and NCS, such as Br and/or CI);

(b) each L represents a linking group attaching NR¹ R²R³ to A and may be selected from Ci-6 alkyl, C^ alkenyl, C^ alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl (e.g. each L represents a linking group attaching NR¹R²R³ to A and may be selected from Ci_-3 alkyl, which group is unsubstituted or substituted by one or more substituents selected from halo and C_1-3 alkyl);

(c) each R¹ to R³ group may independently represent H or C _-3 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl (e.g. each R¹ to R³ group may independently represent H).

Specific embodiments of formula IV that may be mentioned in combination with any one or more of (a) to (c) above include those in which s is 1 and t is 6. Specific embodiments of the compounds of formula IV that may be mentioned herein include:

(I) n-butylammonium tin bromide ((ⁿBuNH₃)₆SnBr₈);

(II) n-butylammonium tin chloride ((ⁿBuNH₃)₆SnCl₈);

(III) n-butylammonium germanium bromide ((ⁿBuNH₃)₆GeBr₈); and

(IV) n-butylammonium germanium chloride ((ⁿBuNH₃)₆GeCI₈).

In embodiments of the above aspects, the zero-dimensional perovskites may display more of the following properties:

(a) a broadband emission in the range of from 400 to 650 nm; (b) a peak maximum of from 450 to 600 nm, such as from 475 to 525 nm, such as 509 nm;

(c) full-width at half maximum as measured in the emission spectrum of from 50 to 150 nm, such as from 75 to 125 nm, such as 95 nm;

(d) a photoluminescence quantum yield of from 55 to 100%, such as from 57 to

80%, such as from 60 to 70%;

(e) a CIE value of close to (0.33, 0.33), such as any value from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(f) a colour rendering index of from 45 to 100, such as from 47 to 55, such as 51 , such as from 80 to 99, such as 85 to 95, such as close to or equal to 100; and

(g) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990. The above may particularly apply to zero-dimensional perovskites according to formula I, particularly where M is tin.

In a further aspect of the invention, there is provided a white light emitting device comprising a zero-dimensional perovskite of the first to fifth aspects of the invention and any technically sensible combination of their respective embodiments. In white light emitting devices according to the invention, the device may further comprise a red phosphor (e.g. a red phosphor that is selected from one or more of the group consisting of silicon oxides, sulfoselenides, fluorides, oxyfluorides, sulfates, nitrides, and oxynitrides, optionally wherein the sulfoselenide is BUVR03, the silicon oxide is Sr₃Si0₅:Eu²⁺ and the nitride is CaAISiN₃:Eu²⁺ and/or Sr₂Si₅N₈:Eu²⁺.). When the red phosphor is present as part of the white light emitting device, the wt:wt ratio of the zero-dimensional perovskite to red phosphor may be from 5:1 to 1 :1.5, such as 2: 1 to 1 :1. A red phosphor may be particularly preferred for use with perovskites of formula I where M is Sn (e.g. such as perovskite (i) in the list above). Phosphors of other colours (e.g. blue or violet) may be used in combination with the materials disclosed herein when necessary to do so. For example a blue phosphor may be used with the perovskite (x) in the list above.

The white light emitting device according to the above aspect and embodiments may displays one or more of the following properties:

(a) a broadband emission in the range of from 400 to 800 nm;

(b) a peak maximum of from 450 to 600 nm, such as from 475 to 525 nm, such as 518 nm for the perovskite as measures as part of the device;

(c) a colour rendering index of from 70 to 90, such as from 78 to 86; (d) a CI E value of close to (0.33, 0.33), such as any value of from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(e) a colour rendering index of from 45 to 100, such as from 47 to 55, such as 51 , such as from 80 to 99, such as 85 to 95, such as close to or equal to 100; and

(f) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990. The above may particularly apply to white light emitting devices that incorporate zero-dimensional perovskites according to formula I, particularly where M is tin. In yet a further aspect of the invention, there is also provided a method of preparing a zero- dimensional perovskite of formula I as defined above in the first aspect of the invention and any technically sensible combination of its embodiments, comprising the steps of:

(a) mixing a tin halide and/or pseudo halide and a compound of formula II:

in a solvent, where A, L, R¹ to R³, X, n, m and p are as defined above for formula I and y is equal to p; and

(b) adding an antisolvent to the mixture to precipitate the zero-dimensional perovskite of formula I.

In embodiments of the above aspect the antisolvent may be added to the mixture by vapour diffusion to provide crystals of the zero-dimensional perovskite of formula I (e.g. the antisolvent may be added directly to the mixture to provide a powdered form of the zero- dimensional perovskite of formula I). When an antisolvent is used in the process described herein, the solvent may be DMF or DMSO and the antisolvent may be a Ci to C₄ alkyl alcohol and/or a Ci to C₄ alkyl ether.

In yet further embodiments, the method may be performed at a temperature of from 10 to 50 °C, such as from 15 to 30 °C, such as from 20 to 27 °C, such as 25 °C.

Zero-dimensional perovskites according to formulae III and IV may be manufactured by analogy to the process described in the above aspect (and any technically sensible combination of its embodiments) above for making zero-dimensional perovskites according to formula I. Brief Description of Drawings

Fig. 1 Depicts the X-ray crystal structure of m-XDATB: (a) shows the projection of m-XDATB along the a-axis, while (b and c) show the isolated SnBr₆ ^4" octahedral unit with bond lengths and bond angles respectively; (d and e) show the projection of m-XDATB along the c-axis and b-axis respectively, with the isolated SnBr₆ ^4" octahedral unit and XDA cations arranged alternately in layers.

Fig. 2 Depicts the solid-state NMR spectra of m-XDATB: (a) ¹¹⁹Sn CP-MAS (12 kHz) NMR spectra with both the experimental and simulated spectra; (b) ¹H MAS (30 kHz) NMR spectrum; and (c) ¹³C CP-MAS (12 kHz) NMR spectrum of m-XDATB. The signals due to spinning sidebands in (c) were labelled with an asterisk.

Fig. 3 Depicts the (a) XPS survey scan of m-XDATB for Sn, Br, C and N; (b) narrow XPS scan for Sn 3d₃/2 and 3d₅/2 orbitals; and (c) photoemission spectroscopy in air (PESA) spectrum of m-XDATB powder.

Fig. 4: (a) TGA; and (b) DSC thermogram of m-XDATB. Fig. 5 Depicts the optical properties of m-XDATB: (a) absorption and excitation spectra; (b) steady-state PL spectrum obtained at 350 nm excitation; (c and d) temperature-dependent PL spectra and PL decay data of m-XDATB measured from 298K to 77K, using 330 nm excitation; (e) a plot of fluorescence lifetime as a function of temperatures; and (f) a plot of fluorescence intensities as a function of the reciprocal temperatures.

Fig. 6 Depicts the optical properties of m-XDATB: (a) PLE spectra measured at different temperatures; (b and d) PL spectra obtained at room temperature and at 80 K respectively, using different excitation wavelengths; and (c) PLE spectra for different wavelengths of the emission.

Fig. 7 Depicts (a) the excitation and PL spectra of powder BUVR03; (b) PL spectra of single crystals and powder samples of m-XDATB; (c) XRD spectrum of powdered m-XDATB in comparison with that of the starting material (m-XDABr₂) and the simulated spectrum of m- XDATB; and (d) FE-SEM image of ball-milled powder of m-XDATB powder.

Fig. 8 Depicts (a) PL spectra of the lights emitted by the devices containing mixed phosphors of m-XDATB/BUVR03 at different weight ratios; (b) CIE 1931 colour plot showing coordinates of the lights emitted by the m-XDALB, BUVR03, and mixture of these two phosphors at different ratios; and (c) FE-SEM image of phosphors doped polydimethylsiloxane (PDMS) films prepared with different ratios of -XDATB and BUVR03. Fig. 9 Depicts the X-ray crystal structure of m-XDALC: (a and b) show the projection of m- XDALC along a- and b-axis respectively, showing the isolated PbCl_s ^4" octahedral unit and ammonium cations arranged in alternating layers; (c) shows the projection of m-XDALC along c-axis; (d) shows the hydrogen bonding of ammonium protons to three chlorides of three different octahedral PbCl_s ^4" units; and (e and f) show the isolated PbCI₆ ^4" octahedral unit with bond lengths and bond angles respectively.

Fig. 10 Depicts the X-ray crystal structure of m-XDALB: (a and b) show the projection of m- XDALC along a- and b-axis respectively, displaying the organic ammonium cations intercalating between the "0D" lead bromide; (c) shows the projection of m-XDALB along the c-axis; (d) shows the hydrogen bonding of ammonium protons to three bromides of three different octahedral PbBr_B ^4" units; and (e) and (f) show the isolated PbBr₆ ^4" octahedral unit with bond lengths and bond angles respectively.

Fig. 11 Depicts the solid-state NMR characterisation of m-XDALB: (a) ²⁰⁷Pb Hahn echo (static) NMR spectra of m-XDALB with the experimental and simulated spectra; (b) ¹ H MAS (12 kHz) NMR spectrum; and (c) ¹³C CPMAS (12 kHz) NMR spectrum of m-XDALB. The signals due to spinning sidebands in (c) were labelled with an asterisk.

Fig. 12 Depicts (a) the comparison of thin film XRD patterns of m-XDALB with increasing amounts of excess PbBr₂; (b) thin film XRD data of m-XDALB spin-coated from a DMF solution; and (c and d) comparison of the simulated and experimental powder XRD patterns of 0D m-XDALB sample and m-XDALC respectively.

Fig. 13 Depicts the TGA thermogram of the m-XDALB powder samples.

Fig. 14 Depicts the DSC thermogram of the m-XDALB powder samples.

Fig. 15 Depicts (a) PLE spectra of m-XDALB measured at 80 K, and absorption spectra of m-XDALC, 0D m-XDALB, and 1 D m-XDALB thin film prepared with excess of 1.0 mole of PbBr₂ measured at room temperature; (b) steady-state PL spectra of 0D m-XDALB single- crystal measured at 80 K, and 1 D m-XDALB thin film prepared with an excess of 1.0 mole PbBr₂ measured at room temperature; and (c) PESA spectrum of the 0D m-XDALB. Fig. 16 Depicts the comparison of absorption spectra of m-XDALB thin films with increasing amounts of excess PbBr₂, and powder m-XDALB. Fig. 17 Depicts the (a) temperature-dependent PLE spectra of OD m-XDALB for the emission maximum at 440 nm; (b) PL spectra of OD m-XDALB obtained with excitation at 334 nm; (c) PL spectra of OD m-XDALB and (d) m-XDABr₂ measured at 80 K, with excitation at various wavelengths. Fig. 18 Depicts (a) the PL spectrum of the 1 D m-XDALB thin film synthesised using 1.0 mole excess of PbBr₂; (b) the CIE 1931 color plot showing the coordinates of the white emission from the 1 D m-XDALB thin film. The PL experiments for (a) and (b) were conducted using 360 nm pulsed laser excitation (50 fs, 1 kHz) with a fluence of ~ 5.7 pj/cm²; (c) time- resolved PL kinetics of the 1 D m-XDALB thin films prepared using various amounts of excess PbBr₂ precursor (0, 0.5, and 1 mole); and (d) PL decay kinetics of the 1 D m-XDALB thin films measured at various wavelengths (550, 550, and 600 nm). The PL lifetime experiments for (c) and (d) were conducted using 310 nm pulsed laser excitation (50 fs, 1 kHz) with a fluence of 8.5 pj/cm². Fig. 19 Depicts (a) CIE 1931 coordinates of m-XDALB white emitter at different temperatures (boxed); (b) detailed variation of the coordinates as a function of temperature. The coordinates tend towards the white region near room temperature. The experiments were conducted using 310 nm pulsed laser excitation (50 fs, 1 KHz) with a fluence of 4.2 pj/cm²

Fig. 20 Depicts temperature-dependent PL measurements of a 1 D m-XDALB thin film conducted using 310 nm pulsed laser excitation (50 fs, 1 kHz) with a fluence of 4.2 pj/cm²: (a) temperature-dependent PL spectra from 300K to 20K. The inset shows the emission peak intensity of free (400 nm) and bound excitons (600 nm). These bound excitons possess a de-trapping energy of 9.4 ± 0.7 me; (b) integrated PL intensity of the broad emission and FWHM as a function of temperature; (c) colour plot depicting the variation of PL intensity with temperature and wavelength for broad emission (~600 nm); and (d) for free exitonic (~400 nm); and (e) temperature-dependent PL decay kinetics of m-XDALB thin film synthesised with 1 mole equivalents of excess PbBr₂. A significant PL lifetime lengthening was observed at low temperatures. Fig. 21 Depicts the kinetics of self-trapped excitons in a m-XDALB thin film synthesised with 1 mole of excess PbBr₂. The PL experiments were conducted using 310 nm excitation (50 fs, 1 kHz) with a fluence of 8.5 pj/cm², while the TA measurements were conducted using 310 nm excitation (150 fs, 1 kHz) with a fluence of 23.8 pj/cm²: (a) room temperature PL Kinetics measured at 400 nm (free excitons) and 550 nm (self-trapped excitons); (b) evolution of the kinetics of free excitons (measured at 400 nm) at different temperatures; (c) pseudo-colour TA plot showing the change in absorption (ΔΑ) as a function of probe wavelength and probe delay time; (d) TA kinetics probed at 450, 500, 550, and 600 nm. The inset shows ΔΑ as a function of probe wavelength at selected delay times; (e) short timescale TA kinetics probed at 400, 420, and 440 nm; (f) proposed schematic summarising the main processes occurring in this white emitter; and (g) PL kinetics of m-XDALB synthesised with 0, 0.5, and 1.0 moles of excess PbBr₂. A short lifetime component for the white emitter (1.0 moles of excess PbBr₂ added) was observed. Fig. 22 Depicts the X-ray crystal structure of m-XDAGB and BTB: (a) projection of m- XDAGB along the c-axis; (b and c) a- and b-axes respectively, showing the isolated GeBr_B ^4" octahedral unit and XDA cations arranged alternately in layers; and (d-f) crystal structures of BTB with different projections in the: (d) b-c plane; (e) a-c plane; and (f) a-b plane. Fig. 23 Depicts the NMR spectra of m-XDAGB and BTB: (a) ¹H MAS (30 kHz), (b) ¹³C CP- MAS (12 kHz; (c) ¹¹⁹Sn CP-MAS (12 kHz) with the experimental and simulated spectra.

Fig. 24 Depicts the XPS spectra of (a and b) m-XDAGB; and (c and d) BTB. Fig. 25 Depicts the XRD and simulated spectra of (a) m-XDAGB; and (b) BTB.

Fig. 26 Depicts the (a) TGA; and (b) DSC thermogram of m-XDAGB.

Fig. 27 Depicts (a) PLE and (b) steady-state PL spectra of BTB and m-XDAGB; (c and d) PL lifetimes of m-XDAGB and BTB respectively.

Fig. 28 Depicts the morphology of (a) m-XDAGB powder; and (b) BTB microplatelets.

Fig. 29 Depicts (a) PL spectra of the white light emitted by the mixture of m-XDAGB and

at 9: 1 ratio; and (b) CIE 1931 colour plot showing coordinates of emissions of m-XDAGB, m-XDAGB and blue phosphor, fresh BTB and BTB after a week. Description

As noted above, the current invention relates to zero-dimensional perovskites with useful optical properties that make them useful in the formation of a number of optical devices, such as white light-emitting devices. Such zero-dimensional perovskites include the compounds of formulae I, III and IV described below.

As noted above, disclosed herein are zero-dimensional perovskites of formula I: [AfXNR¹ R² R³ ) _n] m⁺ [MX₆]^4" I

wherein:

M is selected from Ge or, more particularly, Pb, Sn, Mn, Cu, Co, and Eu;

each X is independently selected from halo or pseudohalo;

A represents a 5- to 14-membered ring system comprising at least one aromatic or heteroaromatic ring which ring system is unsubstituted or substituted by halo, d-12 alkyl, C2-12 alkenyl, C2-12 alkynyl, OC^ alkyl, O-C2-12 alkenyl, O-C2-12 alkynyl, S-Ci-i₂ alkyl, S-C₂.₁₂ alkenyl and S-C2-12 alkynyl, which latter nine groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, SR⁵, and alkyl;

each L represents a linking group attaching NR¹ R²R³ to A and is selected from

alkyl, C2-12 alkenyl, C₂-i₂ alkynyl,

alkyl, O-C2-12 alkenyl, 0-C₂-i₂ alkynyl, alkyl, S-C₂.i₂ alkenyl and S-C2-12 alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁶, SR⁷, and Ci_-6 alkyl;

each R¹ to R³ group independently represents H or Ci_₆ alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo, OR⁸ and C1.3 alkyl;

each R⁴ to R⁸ group independently represents H or d.₃ alkyl;

n is 1 or 2; and

when:

n is 1 , then m is 4 and p is 1 ; and

n is 2, then m is 2 and p is 2.

In embodiments herein, the word "comprising" may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word "comprising" may also relate to the situation where only the components/features listed are intended to be present (e.g. the word "comprising" may be replaced by the phrases "consists of or "consists essentially of"). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word "comprising" and synonyms thereof may be replaced by the phrase "consisting of" or the phrase "consists essentially of" or synonyms thereof and vice versa.

When referred to herein, "zero-dimensional perovskite" may refer to compositions where the metal (pseudo)halide complex (e.g. metal pseudohalide or, more particularly, metal halide octahedra) are completely decoupled in all three directions. In other words, each metal (pseudo)halide complex is not in direct contact with another metal (pseudo)halide complex in all three dimensions, as they are separated from each other by the ammonium-containing organic ligands. For the avoidance of doubt, unless explicitly otherwise stated, reference to "a perovskite" herein is intended to refer to a zero-dimensional perovskite. In the compound of formula I, it should be noted that the integer "p" is intended to refer to the charge of a single AflNR^R³),_! molecule, which is determined by the number (n) of ammonium groups present in the molecule, while the integer "m" is intended to refer to the number of such A(LNR¹R²R³)_n molecules complexed to the metal (pseudo)halide. As will be appreciated, the number of A(LNR¹R²R³)_n molecules required to balance the charge of the metal (pseudeo)halide component of the perovskite is determined by "n", which may be 1 or 2. In the situation where n is 1 , the charge "p" for a single molecule of A(LNR¹R²R³)_n is 1 + and so the number of A(LNR¹R²R )_n molecules required to balance the charge of the metal (pseudeo)halide is four (i.e. m = 4). Alternatively, when n is 2, the charge "p" for a single molecule of A(LNR¹R²R³)_n is 2+ and so the number of A(LNR¹R²R³)_n molecules required to balance the charge of the metal (pseudeo)halide is two (i.e. m = 2).

Heteroaromatic groups may be wholly aromatic or partly aromatic in character. Heteroaromatic groups that may be mentioned include acridinyl, benzimidazolyl, benzisothiazolyl, benzisoxazolyl, benzodioxanyl, benzodioxepanyl, benzodioxepinyl, benzodioxolyl, benzofuranyl, benzofurazanyl, benzo[c]isoxazolidinyl, benzomorpholinyl, 2,1 ,3-benzoxadiazolyl, benzoxazinyl (including 3,4-dihydro-2 - -1 ,4-benzoxazinyl), benzoxazolidinyl, benzoxazolyl, benzopyrazolyl, benzo[e]pyrimidine, 2,1 ,3-benzothiadiazolyl, benzothiazolyl, benzothienyl, benzotriazolyl, carbazolyl, chromanyl, chromenyl, cinnolinyl, 2,3-dihydrobenzimidazolyl, 2,3-dihydrobenzo[6]furanyl, 1 ,3-dihydrobenzo[c]furanyl, 1 ,3- dihydro-2,1-benzisoxazolyl, 2,3-dihydropyrrolo[2,3-£)]pyridinyl, furanyl, furazanyl, imidazolyl, imidazo[1 ,2-a]pyridinyl, imidazo[2,3-6]thiazolyl, indazolyl, indolinyl, indolyl, isobenzofuranyl, isochromanyl, isoindolinyl, isoindolyl, isoquinolinyl, isothiaziolyl, isothiochromanyl, isoxazolyl, naphtho[1 ,2-j ]furanyl, naphthyridinyl (including 1 ,6-naphthyridinyl or, particularly, 1 ,5- naphthyridinyl and 1 ,8-naphthyridinyl), oxadiazolyl, oxazolyl, phenazinyl, phenothiazinyl, phthalazinyl, pteridinyl, purinyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrimidinyl, pyrrolo[2,3-0]pyridinyl, pyrrolo[5, 1-jb]pyridinyl, pyrrolo[2,3-c]pyridinyl, pyrrolyl, quinazolinyl, quinolinyl, quinolizinyl, quinoxalinyl, tetrazolyl, thiadiazolyl, thiazolyl, thieno[5,1-c]pyridinyl, thiochromanyl, thiophenetyl, triazolyl, 1 ,3,4-triazolo[2,3-0]pyrimidinyl, xanthenyl and the like. Particular heteroaromatic groups that may be mentioned include the 5- to 10-membered heterocyclic groups from the list above. Further heteroaromatic groups that may be mentioned include the 5- and 8-membered (e.g. 5- to 6-membered) heteroaromatic groups from the list above.

Substituents on heteroaromatic groups may, where appropriate, be located on any atom in the ring system including a heteroatom. The point of attachment of heteroaromatic groups may be via any atom in the ring system including (where appropriate) a heteroatom (such as a nitrogen atom), or an atom on any fused carbocyclic ring that may be present as part of the ring system. Heteroaromatic groups may also be in the N- or S- oxidised form.

For the avoidance of doubt, in cases in which the identity of two or more substituents in a compound may be the same, the actual identities of the respective substituents are not in any way interdependent.

Unless otherwise stated, the term "alkyl" refers to an unbranched or branched, linear or cyclic, saturated hydrocarbyl radical, which may be substituted or unsubstituted (with, for example, one or more halo atoms). Where the term "alkyl" refers to an acyclic group, it is preferably Ci_₁₂ alkyl and, more preferably, Ci_-6 alkyl (such as ethyl, propyl, (e.g. n-propyl or isopropyl), butyl (e.g. branched or unbranched butyl), pentyl or, more preferably, methyl). Where the term "alkyl" is a cyclic group (which may be where the group "cycloalkyl" is specified), it is preferably C₃.₁₂ cycloalkyl and, more preferably, C_5-io (e.g. C₅.₇) cycloalkyl.

When used herein, the term "alkenyl" refers to an unbranched or branched, linear unsaturated hydrocarbyl radical, which may be substituted or unsubstituted (with, for example, one or more halo atoms). The alkenyl group is preferably C₂-12 alkenyl and, more preferably, C₂_₆ alkenyl (such as ethenyl, propenyl), butenyl (e.g. branched or unbranched butenyl), or pentenyl). When used herein, the term "alkynyl" refers to an unbranched or branched, linear unsaturated hydrocarbyl radical, which may be substituted or unsubstituted (with, for example, one or more halo atoms). The alkynyl group is preferably C2-12 alkynyl and, more preferably, C₂-s alkynyl (such as ethynyl, propynyl), butynyl (e.g. branched or unbranched butynyl), or pentynyl).

The term "halo", when used herein, includes fluorine, chlorine, bromine and iodine.

The term "aromatic ring" when used herein includes C₆.14 (such as C_6-13 (e.g. C₆.io)) aromatic groups. Such groups may be monocyclic, bicyclic or tricyclic and have between 6 and 14 ring carbon atoms, in which at least one ring is aromatic. The point of attachment of the aromatic groups may be via any atom of the ring system. However, when aryl aromatic are bicyclic or tricyclic, they are linked to the rest of the molecule via an aromatic ring. C₆.14 aromatic groups include phenyl, naphthyl and the like, such as 1 ,2,3,4-tetrahydronaphthyl, indanyl, indenyl and fluorenyl. Most preferred aromatic groups include phenyl.

As is inherent in formula I, the metal M is in the 2+ oxidation state. Suitable metals that may be included in the perovskites of formula I include, Ge and, more particularly, Sn. A second zero-dimensional perovskite class that is disclosed herein is a zero-dimensional perovskite of formula III:

[ACLNR^R³)^ [SnX₅] 2- wherein A, L, n, X and R¹ to R³ are as defined above for formula I and when:

n is 1 then q is 2 and r is 1 ; and

n is 2 then q is 1 and r is 2.

As will be appreciated, the discussion above with respect to "p" applies to "r" in the above formula.

A third zero-dimensional perovskite class that is disclosed herein is a zero-dimensional perovskite of formula la:

([A(LNR¹R²R³)_ni]^,⁺)₂ [M'X₆]⁴- [M"X₆]^4" la wherein A, L, n, X and R¹ to R³ are as defined above for formula I ; Μ' and M" are independently selected from Ge, Pb, Sn, Mn, Cu, Co, and Eu, preferably M and M' are not the same metal;

n' is 1 or 2; and

when:

n' is 1 , then m' is 4 and p' is 1 ; and

n' is 2, then m' is 2 and p' is 2.

As will be appreciated, the discussion above with respect to "p" applies to p' in the above formula.

A fourth zero-dimensional perovskite class that is disclosed herein is a zero-dimensional perovskite of formula lb: wherein A, L, n, X and R¹ to R³ are as defined above for formula I ;

M'" is selected from In, Ga or, more particularly, Tl;

M"" is selected from As or, more particularly, Sb and Bi;

n" is 1 or 2; and

when:

n" is 1 , then m" is 4 and p" is 1 ; and

n" is 2, then m" is 2 and p" is 2.

As will be appreciated, the discussion above with respect to "p" applies to p" in the above formula.

For the perovskites of formula I, la, lb and formula III , X may be halo or a pseudohalo. Suitable halides and pseudohalo groups that may be mentioned herein include, but are not limited to, F, Br, CI, I , CN, SCN, NCS, CP, NC, OCN, OSCN, SeCN, N₃. Particular halides and pseudohalo groups that may be mentioned herein include, but are not limited to, F, Br, CI, I, CN, SCN, and NCS. For example, X may be Br and/or CI. For the perovskites of formula I , la, lb and formula II I, A may be a 5- to 14-membered ring system comprising at least one aromatic or heteroaromatic ring that is unsubstituted or substituted as described above. In certain embodiments, A may represent a 6- to 10- membered aromatic ring system or a 5- to 10-membered heteroaromatic ring system, which ring systems are unsubstituted or substituted by halo, Ci_-6 alkyl, C₂.6 alkenyl, C₂.s alkynyl, which latter three groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, and C_1-3 alkyl. For example, A may represent phenyl, thiophenyl, furanyl, pyrrolyl, pyridinyl, which five ring systems are unsubstituted or substituted by halo and Ci-3 alkyl.

As will be appreciated, the L group in formulae I, la, lb and III above attach the ammonium group(s) (NR¹ R²R³) to A as described above. In more particular embodiments disclosed herein, each L may be selected from C_h alky!, C₂-₆ alkenyl, C₂-₆ alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl. For example, each L may be selected from Ci_-3 alkyl, which group is unsubstituted or substituted by one or more substituents selected from halo and d_₃ alkyl.

In formula I, la, lb and formula III above, each nitrogen carries groups R¹ to R³ as described above. In particular embodiments that may be mentioned herein each R¹ to R³ group may independently represent H or d_₃ alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl (e.g. each R¹ to R³ group may independently represent H).

In embodiments relating to formula I, include those in which n and m are 2 (i.e. p is also 2). Specific compounds of formula I where n and m are 2 disclosed herein include:

(i) mefa-xylenediammonium tin bromide

mefa-xylenediammonium tin chloride [SnCle]⁴

(iii) o/ 70-xylenediammonium tin bromide

[SnBre]⁴

o/f/70-xylenediammonium tin chloride

[SnCle]⁴

para-xylenediammonium tin bromide

2+

[SnBre]⁴

para-xylenediammonium tin chloride

2+

[SnCle]⁴

(vii) mefa-xylenediammonium lead bromide

[PbBre] |4-

(viii) mefa-xylenediammonium lead chloride 2+

[PbCle]⁴

mefa-xylenediammonium germanium bromide

mefa-xylenediammonium germanium chloride

[GeCle]⁴

Particularly preferred compounds that may be mentioned in embodiments of the invention include compounds (i) to (vi) of the list above, such as, more particularly, compound (i).

The perovskites of formula I and formula III fall within the general formula AA: A _/nMXs, in this context, A is a divalent organic ammonium ion and when M (a metal) is divalent, the n is 2 and when M is tetravalent, then n is 4. In the general formula M may be Pb²⁺, Sn²⁺, Sn⁴⁺, Mn²⁺, Cu²⁺, Co²⁺, Eu²⁺, Bi³⁺, Sb³⁺ and mixtures thereof. X in this general formula may be F^", CI^", B r, Γ, CN^", SCN^", NCS^", other pseudohalides and combinations thereof. The divalent ammonium anion may be derived from small aromatic compounds (e.g. benzene, thiophene, furan, pyrrole and pyridine, as well as mixed aromatic compounds) and cycloalkyl. The ammonium species may be-NH₃ ⁺, -NH₂ ⁺(R), -NH⁺(R)(R'), and -N⁺(R)(R')(R"). Where each of R, R' and R" may be alkyl, aryl, oxyalkyl, thioalkyl, halogen, OH, C0₂H, C0₂R'" (e.g. R'" may be alkyl or aryl).

A fifth zero-dimensional perovskite class that is disclosed herein is a zero-dimensional perovskite of formula IV:

[BCLNR^R³).;]^ [MX₈]⁶- IV wherein:

M is selected from Ge or, more particularly, Pb, Sn, Mn, Cu, Co, and Eu;

L and R¹ to R³ are as defined above for formula I;

B represents a carbon atom with 4-s R⁹ groups (i.e. 4 minus s);

each R⁹ group independently represents H or d.₆ alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo, OR¹⁰ and Ci_-3 alkyl;

each R¹⁰ group independently represents H or d.₃ alkyl;

s is 1 , 2 or 3, such that when:

s is 1 then t is 6 and u is 1 (and there are 3 R⁹ groups);

s is 2 then t is 3 and u is 2 (and there are 2 R⁹ groups); and

s is 3 then t is 2 and u is 3 (and there is 1 R⁹ group). In embodiments of the above formula IV:

(a) X may be selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, NCS, CP, NC, OCN, OSCN, SeCN, N₃ (e.g. X may be selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, and NCS, such as Br and/or CI); and/or

(b) each L represents a linking group attaching NR¹ R²R³ to A and may be selected from Ci-6 alkyl, C^ alkenyl, C^ alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl (e.g. each L represents a linking group attaching NR¹R²R³ to A and may be selected from Ci_-3 alkyl, which group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl); and/or

(c) each R¹ to R³ group may independently represent H or Ci_-3 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl

(e.g. each R¹ to R³ group may independently represent H). Specific perovskites of formula IV that may be mentioned in combination with any one or more of (a) to (c) above include those in which s is 1 and t is 6. Specific embodiments of the compounds of formula IV that may be mentioned herein include:

(I) n-butylammonium tin bromide ((ⁿBuNH₃)₆SnBr₈);

(I I) n-butylammonium tin chloride ((ⁿBuNH₃)₆SnCI₈);

(I II) n-butylammonium germanium bromide ((ⁿBuNH₃)₆GeBr₈); and

(IV) n-butylammonium germanium chloride ((ⁿBuNH₃)₆GeCI₈).

It has been surprisingly discovered that the zero-dimensional perovskite materials mentioned hereinabove display quantum confinement of excitons and so are expected (and are shown to do so below) to be favourable for the formation of self-trapped excited states, that is self- trapped excitons that broaden the emission spectrum. In addition, the charges are confined within each isolated metal (pseudo)halide complex (e.g. metal (pseudo)halide octahedron), which may lead to improved PLQEs. These materials are very easy to prepare at room temperature and display good processability, leading to easy film formation, making these materials attractive phosphors for WLEDs. This attractiveness is also improved by the fact that the perovskites disclosed herein also have good thermal and moisture stability.

Given the above, the perovskites of formula I, la, lb, II I, IV and AA may display one or more of the following properties:

(a) a broadband emission in the range of from 400 to 650 nm;

(b) a peak maximum of from 450 to 600 nm, such as from 475 to 525 nm, such as 509 nm;

(d) a photoluminescence quantum yield of from 55 to 100%, such as from 57 to 80%, such as from 60 to 70%;

(e) a CI E value of close to (0.33, 0.33), such as any value from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(g) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990. The above may particularly apply to zero-dimensional perovskites according to formula I, particularly where M is tin. Given the improved PLQE demonstrated for the perovskites disclosed herein, said perovskites may be particularly suited to use in the manufacture of a white light emitting device. As such, there is also disclosed herein a white light emitting device comprising a zero-dimensional perovskite of formula I, la, lb, II I, IV, and AA.

In white light emitting devices according to the invention, the device may further comprise a red phosphor (e.g. a red phosphor that is selected from one or more of the group consisting of silicon oxides, sulfoselenides, fluorides, oxyfluorides, sulfates, nitrides, and oxynitrides, optionally wherein the sulfoselenide is BUVR03, the silicon oxide is Sr₃Si0₅: Eu²⁺ and the nitride is CaAISiN₃: Eu²⁺ and/or Sr₂Si₅N₈: Eu²⁺. When the red phosphor is present as part of the white light emitting device, the wt:wt ratio of the zero-dimensional perovskite to red phosphor may be from 5: 1 to 1 : 1.5, such as 2: 1 to 1 : 1. A red phosphor may be particularly preferred for use with perovskites of formula I where M is Sn (e.g. such as perovskite (i) in the list above).

In other cases, a phosphor that emits a different wavelength of light may be preferred, such as blue or violet, for use in combination with a perovskite disclosed herein. The selection of a phosphor to pair with the perovskite will depend on the wavelength of light emitted by the phosphor. For example, the perovskite (x) in the list above emits light having a yellow colour, so a phosphor that emits light with a violet or, more particularly, blue colour may be preferred to provide a white light emitting device.

Details of how to manufacture a white light emitting device are provided in the examples below and may be adapted by analogy.

The white light emitting device may display one or more of the following properties:

(a) a broadband emission in the range of from 400 to 800 nm;

(c) a colour rendering index of from 70 to 90, such as from 78 to 86;

(d) a CI E value of close to (0.33, 0.33), such as any value from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(e) a colour rendering index of from 45 to 100, such as from 47 to 55, such as 51 , such as from 80 to 99, such as 85 to 95, such as close to or equal to 100; and (f) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990. The above may particularly apply to white light emitting devices that incorporate zero-dimensional perovskites according to formula I, particularly where M is tin.

As mentioned above, the conditions required to manufacture are very convenient and can be conducted at room temperature. With that in mind, there is also provided a method of preparing a zero-dimensional perovskite of formula I as defined above, comprising the steps of:

(a) mixing a tin halide and/or pseudo halide and a compound of formula II:

It will be appreciated that perovskites of formula la, lb, III and IV may be manufactured by analogy to the above process. When used herein "solvent" refers to a fluid that is capable of holding the starting materials and, more particularly, the product perovskite in solution when provided in a suitable amount, which is readily determined by a person skilled in processes of this type (e.g. an economically viable amount of solvent). Suitable solvents that may be mentioned herein (that may be used to manufacture perovskites of formula I, la, lb, II I, IV and AA) include, but are not limited to DMF, DMSO and combinations thereof.

In order to obtain the perovskite, the solvent may be gradually removed from the mixture of step (b) to cause the perovskite to precipitate out. Alternatively, an antisolvent may be added to the mixture of step (b) above by vapour diffusion to provide crystals of the zero- dimensional perovskites or the antisolvent may be added directly to the mixture to provide a powdered form of the zero-dimensional perovskite. When the solvent used is DMF and/or DMSO, the antisolvent may be a Ci to C₄ alkyl alcohol and/or a Ci to C₄ alkyl ether.

As noted above, the perovskites described herein may be conveniently manufactured at ambient (room) temperature. For example, the methods described above may be performed at a temperature of from 10 to 50 °C, such as from 15 to 30 °C, such as from 20 to 27 °C, such as 25 °C.

Further aspects and embodiments of the current invention and disclosure are provided by the following non-limiting examples.

Examples

Materials

SnBr₂, PbBr₂, PbCI₂, Ge0₂, xylylenediamine, hypophosphorous acid, hydrobromic acid (48%), europium doped barium magnesium aluminate (BaMgAI₁₀Oi₇:Eu) and all anhydrous solvents used in this study were purchased from Sigma-Aldrich and used without further purification. HPLC grade solvents were used in photoluminescence spectroscopic measurements. UVTOP LED was purchased from Thorlabs.

Methods

Nuclear Magnetic Resonance (NMR) Spectroscopy

All NMR experiments were carried out on a Bruker Avance III HD 600 MHz spectrometer with a Bruker 1.9 mm or 4 mm HXY MAS probe. Spectral simulation of all NMR spectra was achieved via dmfit (Massiot, D. et al., Magn. Reson. Chem. 2001 , 40, 70). The ¹ H solid state NMR experiments were completed at 14.1 T {v_Q (¹ H) = 600.18 MHz) with an MAS frequency of 30 kHz for m-xylylenediammonium tin bromide (m-XDATB) and BTB. An MAS frequency of 12 kHz was used for m-xylylenediammonium lead chloride and bromide (m-XDALC and /D-XDALB), and m-xylylenediammonium germanium bromide {m- XDAGB). A ¹ H one-pulse sequence was employed and resulting data was referenced with respect to adamantane (CioHi₆(_S); 6_IS0 = 1.82 ppm).

The ¹³C solid state NMR experiments were completed at 14.1 T («¾ (¹³C) = 150.92 MHz) with an MAS frequency of 12 KHz. A ¹³C CPMAS pulse sequence was employed and resulting data was referenced with respect to adamantane (Ci₀H₁₆(_S); <5_iso = 38.48, 40.49 ppm). The ¹¹⁹Sn solid state NMR experiments were completed at 14.1 T (z¾ (¹¹⁹Sn) = 223.81 MHz) with an MAS frequency of 12 KHz. A ¹¹⁹Sn CP-MAS pulse sequence was employed and resulting data was referenced with respect to Sn0_2(S) {5_iso = -604.3 ppm). A ¹H ^π/₂ pulse length of 2.5 /vs, determined on adamantane, and a recycle delay of 6.5 s were used in all experiments. The ¹³C and ¹¹⁹Sn CPMAS experiments both employed a contact pulse length of 5000 με and utilised high-power proton decoupling.

The ²⁰⁷Pb solid state NMR experiments were carried out at 14.1 T (z/₀(²⁰⁷Pb) = 125.55 MHz) under static conditions. A ²⁰⁷Pb Hahn echo pulse sequence was employed and resulting data was referenced with respect to 1.1M Pb(N0₃)₂₍aq₎ (¾₀ = -2965.7 ppm). [2] The ²⁰⁷Pb Hahn echo experiments utilised ^π ₂ and π pulses of 5 and 10 με, determined on Pb(N0₃)₂ (aq), with a recycle delay of 5 s and an echo delay of 76 /vs. Steady-State and Time-Resolved Photoluminescence (PL) Spectroscopy

The laser wavelengths (310 and 340 nm) used for PL measurements were obtained using a Coherent OPerA-Solo optical parametric amplifier from a 800 nm input pulse laser. The 800 nm pulse laser originated from a Coherent Libra regenerative amplifier (1 kHz, 50 fs, 800 nm) which was seeded by a Coherent Vitesse oscillator (50 fs, 80 MHz). For all measurements, the excitation laser beam was passed through a short pass optical filter to remove residual 800 nm photons in the beam.

For steady-PL measurements, the emissions were collected using a backscattered PL configuration and dispersed using a 300 g/mm spectrometer (Acton, Spectra Pro 2500i). A long pass filter was also used to remove any scattered laser radiation in the emission, which is important in preventing a second order diffracted laser peak (due to the grating) from appearing in the PL spectrum at the visible wavelengths. In addition, spectral corrections using monochromator and charge-coupled device (CCD) response functions were also carried out to obtain the final PL spectra.

For time-resolved PL measurements, the back-scattered emissions were temporally resolved using an Optronis Optoscope™ streak camera system with an ultimate ~10 ps resolution at the fastest scan speed. Field-Emission Scanning Electron Microscopy (FE-SEM)

FE-SEM imaging (JEOL, JSM-7600F, 5 kV) was carried out on the samples to determine the morphology of the perovskites.

Powder X-Ray Diffraction (XRD)

The phase purities of perovskites were confirmed by XRD measurements using a Bruker D8 Advance diffractometer equipped with a Cu Ka X-ray tube operated at 40 kV and 40 mA, scanned using a step size of 0.02° and time per step of 1 s.

Thermogravimetric analysis (TGA)

TGA was performed using a TGA Q500 V6.7 (TA Instruments) over a temperature range from 22 °C to 800 °C at a ramp of 10 °C/min under nitrogen flux of 60 mL/min.

Differential scanning calorimetry (DSC)

DSC was conducted on a Q10 V9.9 build calorimeter (TA Instruments) at a rate of 10 °C/min under nitrogen flow rate of 60 ml/min. The minimum temperature was -80 °C and the typical maximum temperature was 180 °C, which is well below the degradation temperature of the compound.

Transient Absorption (TA) Measurements

The laser wavelength (310 nm) used for the TA measurement was obtained using a Light Conversion TOPAS-C optical parametric amplifier from a 800 nm input pulse laser. The 800 nm pulse laser originated from a Coherent Legend regenerative amplifier (1 kHz, 150 fs, 800 nm) seeded by a Coherent Vitesse oscillator (100 fs, 80 MHz). The visible probe pulses were generated by focusing a small portion of the fundamental 800 nm laser pulses into a sapphire plate. Visible probe (~420 - 750 nm) femtosecond TA spectra were taken with an Ultrafast System HELIOS TA spectrometer. Importantly, any unconverted residual 800 nm pump and probe pulses were removed using an optical filter before directing at the sample. Synthesis of m-xylylenediammonium halides

The m-xylylenediammonium halides were prepared by slowly adding 2.0 moles of hydrohalic acid into 1.0 mole of m-xylylenediamine in methanol at 0 °C under vigorous stirring to give a 1 M solution. White precipitate of xylylenediammonium halides was formed and subsequently filtered, washed with diethyl ether, and dried at 50 °C under vacuum for 12 h.

Example 1. Single crystal growth and structural characterisation of m- xylylenediammonium tin bromide (m-XDATB)

Synthesis

The m-xylylenediammonium bromide (m-XDABr₂) was prepared as described above. Single crystals of m-XDATB were grown by anti-solvent vapour-assisted crystallisation. One equivalent of tin halide (SnX₂) and two equivalents of m-XDABr₂ ammonium halide were dissolved in a minimum amount of /V,/V-dimethylformamide (DMF) to give a saturated solution. The vapour of the anti-solvent, dichloromethane (DCM), was then allowed to diffuse into the perovskite solution. This resulted in the formation of single-crystals of m-XDATB (with molecular formula of (m-XDA)₂SnBr₆ for X-ray diffraction) after a few days. The crystals were washed with DCM and used for single-crystal X-ray diffraction (XRD) measurements and also for ultrafast optical spectroscopic studies.

Characterisation by single-crystal X-ray diffraction (XRD) The unit cell of m-XDATB is shown in Fig. 1a, which depicts how the XDA dications interspersed between and stabilised the isolated SnBr_s ^4" octahedra units. On the other hand, Fig. 1d and e show that the ammonium groups of the XDA cations are spatially trans to each other, and the dications stacked in a parallel manner to one another. The ammonium groups of the m-XDA dications also appear to be hydrogen bonded to three bromides of three different octahedral SnBr₆ ^4" units as observed in 0D lead bromide.

Each SnBr₆ ^4" unit is crystallographically centrosymmetric with three distinct Sn-Br bond lengths, which vary from 2.976 to 3.013 A (Fig. 1b). The variance in the octahedral elongation was calculated to be 1.0085 A which indicates that there is significant variation in the bond length of the octahedra in comparison to the variance in the octahedral elongation of perfect octahedra of methylammonium lead iodide which is 1.0001 A (Robinson, K.; Gibbs, G. V.; Ribbe, P. H. Science 1971 , 172, 567). The bond angles for the cis-halides vary from 82.35 to 97.65° (Fig. 1 c), and the trans halide bond angles are all 180° due to the crystallographically imposed symmetry. The variance in octahedral angles a²oct was found to be 31.39°^Λ2, which indicates significant distortion of the tin bromide octahedra in comparison to the variance in octahedral angles of perfect octahedra geometry of methylammonium lead iodide which is 0.1584°^Λ2.

Comparing the variances in the bond length and angle, the variance in bond angle is more pronounced in m-XDATB. Table 1 shows the relevant sample and crystal data for m-XDATB. The X-ray crystallographic information file (CIF) for m-XDATB has been deposited in the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1545198.

Table 1. Sample and crystal data for m-XDATB.

Characterisation by NMR spectroscopy

Multinuclear solid-state nuclear magnetic resonance (NMR) studies were done in order to confirm the chemical structure of the Sn-Br octahedra. The ^{1 19}Sn NMR spectrum of m- XDATB shows a single ¹¹⁹Sn component where 5_iso = -723.8 ± 0.8 ppm that was split into a multiplet via ^{1 19}Sn-^79/81 Br J-coupling (Fig. 2a). An agreeable fit of the multiplet was achieved by simulating the spectrum of a single ^{1 19}Sn component bonded to six identical ^79/81 Br sites. The details of the fit are shown in Table 2. The J-coupling in m-XDATB was measured as 1J(¹¹⁹Sn-^{79 81} Br) = 1.5 ± 0.1 kHz which is coherent with ¹J (¹¹⁹Sn-^79/81 Br) values reported on SnBr₄ (0.92 KHz).¹⁹ The clarity of the observed ¹¹⁹Sn multiplet indicates that the SnBr₆ octahedral unit is highly symmetrical in m-XDATB as splitting was not observed in other Sn- Br octahedra reported so far.

Table 2. ¹¹⁹Sn NMR parameters of m-XDATB determined via J-coupling spectral simulation

The ¹ H and ¹³C NMR spectra of m-XDATB are shown in Fig. 2b and c respectively. The ¹ H NMR spectrum shows two broad resonances, with the one at ~8 ppm assigned to the benzyl ring and ammonium hydrogens, while the one at ~3 ppm was assigned to the benzyl CH₂ hydrogens. The ¹³C NMR spectrum shows eight sharp resonances in which the six resonances at higher frequencies (138-131 ppm) were assigned to the benzyl ring group, whereas the two resonances at lower frequencies (45-43 ppm) were assigned to the benzyl CH₂ group. As such, the composition and high purity of the /D-XDATB crystal were confirmed by the above NMR characterisations.

Example 2. Oxidative and thermal stability of m-XDATB, characterised by X-ray Photoelectron Spectroscopy (XPS), Photoelectron Spectroscopy in Air (PESA), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)

The oxidative stability of m-XDATB was investigated using X-ray photoelectron spectroscopy (XPS) survey spectra in the binding energy range from 0 to 800 eV as shown in Fig. 3a_. Core level peaks for elements Sn, Br, C and N were observed which therefore confirmed all the elements present in m-XDATB. The XPS narrow scan for the binding energies of d_3/2 and d_5/2 electron of Sn confirms the existence of the Sn metal in one oxidation state (Fig. 3b). However, the oxidation state of Sn is not clear due to the close binding energies of 3d_{5 2} for Sn⁴⁺ and Sn²⁺. Since, the molecular formulae of m-XDATB was established as (m- XDA)₂SnBr₆ by single crystal X-ray crystallography, the oxidation state of Sn was therefore confirmed as +2 (tin is present in this compound as Sn²⁺).

To understand the high stability of m-XDATB in air, photoelectron spectroscopy in air (PESA) was conducted on the powder samples and the highest occupied molecular orbitals (HOMO) was determined to be -5.50 eV relative to vacuum (Fig. 3c). The high air-stability of m-XDATB bivalent Sn-perovskite is probably due to the low-lying HOMO, together with the good coverage of the organic ammonium cations surrounding the tin ions (Song, T.-B. et al. ACS Energy Lett. 2017, 2, 897; Yokoyama, T. et al. J. Phys. Chem. Lett. 2016, 7, 776; Nikol, H. et al. Inorg. Chem. 1992, 31, 3277).

The thermogravimetric analysis (TGA) thermogram of m-XDATB collected up to 800 °C, shows significant weight loss only beyond 200 °C (Fig. 4a). In addition, the cooling curve of the differential scanning calorimetry (DSC) thermogram shows no evidence of a phase transition between -50 to 175 °C (Fig. 4b), suggesting that this phosphor is stable in the working temperature range of typical LED phosphors.

Example 3. Optical properties of m-XDATB

The absorption and photoluminescence excitation (PLE) spectra have been measured using single-crystals of m-XDATB at room temperature are shown in Fig. 5a, where the absorption spectrum shows a peak at 384 nm and the optical bandgap of the material was calculated to be 2.85 eV. In addition, the PLE spectrum measured at room temperature for the 509 nm emission gave two peak maxima. The difference between absorption and PLE spectra could be due to charge transfer between the metal ion and the bromide ligand. Hence, the absorption peak at 385 nm can be assigned to metal to ligand, or ligand to metal charge transfer (MLCT/LMCT) transition involving the promotion of 5s² lone pair of the metal to empty d orbital of bromide ion or promotion of p electron of bromide to the empty p orbitals of tin ion (Kunkely, H.; Paukner, A.; Vogler, A. Polyhedron 1989, 8, 2937). Subsequently, the emission spectrum, which was measured at 350 nm excitation, showed a Gaussian-shaped emission that covered the visible region from 400 to 650 nm as shown in Fig. 5b. This PL has an emission maximum at 509 nm, full width at half maximum (FWHM) of 95 nm, and large Stokes-shift of 160 nm. The photoluminescence quantum yield (PLQY) of the A77-XDATB measured using the single-crystal was determined to be 60 %, which is one of the highest values reported so far for 0D perovskite. It has been reported that the broadband emissions from Sn²⁺ ion doped alkali halides were due to metal-cantered s-p electronic transition that involves triplet state (Jacobs, P. W. M. J. Phys. Chem. Solids 1991 , 52, 35). Therefore, the difference in the absorption and PLE spectra could be due to the metal-cantered electronic transition. However, to further understand the metal-cantered electronic transition steady-state temperature-dependent PL, PLE and time-resolved PL measurements were carried out. Steady-state temperature-dependent PL measurement shows an increase in PL intensity and decrease in FWHM with decreasing the temperature. The peak maximum shows red shift reaching 519 nm as the temperature was reduced to -150 °C. This was later blue shifted to 515 nm with further decrease of the temperature to -196 °C. (Fig. 5c). This behaviour could be due to different levels of structural deformation at different temperatures.

The PL decay of m-XDATB is shown in Fig. 5d, which indicates a single exponential decay at all the temperature investigated and therefore, strongly supports that the PL originated from radiative recombination. The longer PL lifetime also indicates that the emission is from the spin-forbidden triplet state and that the transition from the excited singlet to the triplet state was aided by spin-orbit coupling that occurs due to the heavy atoms such as tin and bromide. In addition, it was observed that the PL lifetime increased slightly from approximately 580 ns to 718 ns with a decrease in temperature from 298 K to 77 K (Fig. 5e). The dependence of the fluorescence intensities in relation to the respective reciprocal temperatures is as shown in Fig. 5f.

Generally, molecules containing metal ions with nS² electronic configurations undergo large structural deformations in their excited s-p triplet states, which lead to the decrease of the triplet energies of the molecules (Strasser, A. et al. Inorg. Chem. Commun. 2004, 7, 528; Vogler, A.; Nikol, H. Comments Inorg. Chem. 1993, 14, 245). Bromide ion, being a small and monodentate ligand, can probably allow more structural freedom for the deformation of the metal halide octahedra, therefore leading to a large Stokes-shifted PL. It has been reported that Stoke's shift is also dependent on the distortion of octahedra and the coordination number (Nikol, H.; Vogler, A. J. Am. Chem. Soc. 1991 , 113, 8988).

In addition, PLE spectra measured at room temperature and 80 K showed two peak maxima (340 nm and 305 nm) and five peak maxima (340, 318, 308, 301 , and 290 nm) respectively (Fig. 6a). Peak splitting at low temperature indicates that the high Jahn-Teller distortion of Sn-Br octahedra and these peaks can be assigned to A and B bands of Sn²⁺ ion doped in KBr as reported by Tsuboi et al (Tsuboi, T.; Oyama, K.; Jacobs, P. W. M. J. Phys. C: Solid State Phys. 1974, 7, 221). Similar emission spectra were obtained using different excitation wavelengths and the excitation spectra for the different emission peaks were also similar (Fig. 6b-d). Therefore, this indicates that the entire broadband emission arises from the same excited state. Example 4. Fabrication of a white light emitting diode (WLED) using m-XDATB with a red light-emitting sulfoselenide (BUVR03)

White LEDs were fabricated by blending the cyan light-emitting m-XDATB perovskite with a red light-emitting sulfoselenide (BUVR03) in order to shift the CIE (International Commission on Illumination - also known as Commission Internationale de I'Elcairage) coordinates of the cyan light emitted by m-XDATB closer to white. The PLE and PL spectra of BUVR03 are as shown in Fig. 7a. Typically, phosphors (30 mg) at different weight ratios of m-XDATB/BUVR03 were mixed with 1.0 g of polydimethylsiloxane (PDMS) elastomer (SYLGARD^® 184 Silicone Elastomer Kit) and 0.3 g of a curing agent (SYLGARD^® 184 Silicone Elastomer Kit). These viscous solutions were then poured into a waffle pack to control the shape and thickness. The films were removed after ageing overnight.

For practical reasons, powder samples of m-XDATB for large-scale application in the WLED were used instead of single-grown crystals. This is because the powder samples can be synthesised faster and easier than growing the single crystals. Typically, the powder sample was prepared by drying the perovskite solution (from Example 1) on a hotplate. The sample was the scooped out and was ball-milled before it was incorporated into the PDMS film. The ball-milled powder was characterised by powder XRD which confirmed the crystal structure of m-XDATB (though the sample was associated with small amount of unreacted m-XDABr₂) (Fig. 7c). In addition, the PL spectrum of the as- prepared sample was similar to that of the single-crystal in general, except that it was blue- shifted due to the reduced size of the particles from the ball-milling process (Fig 7b). The morphologies of the particles were also analysed by field-emission scanning electron microscopy (FESEM) as shown in Fig. 7d. Phosphors doped polydimethylsiloxane (PDMS) films were prepared with different ratios of m-XDATB and red emitting phosphors (BUVR03) for a remote phosphor LED configuration. A typical SEM image of the as-synthesised film single-crystals is as shown in Fig. 8c. Phosphor-based light emitting devices were then fabricated using the phosphor doped PDMS films on top of commercial UV LEDs (340 nm, operated at applied voltage of 5V) to optically excite the phosphors in the PDMS film. These LEDs exhibited broad PL emission that covered the visible region from 400 nm to 800 nm as shown in Fig. 8a. The CIE coordinates for the spectra of pure phosphors and mixed phosphors were also obtained (Fig. 8b).

The values for the mixed phosphors should lie on the line in between the CIE values of the two pure phosphors. However, they have deviated due to reabsorption of the blue emission band from m-XDATB by BUVR03. Consequently, the emission maximum of m-XDATB is shifted to 518 nm. The colours of the PDMS films under UV excitation are shown in the inset of Fig. 8b. The colour rendering index (CRI), correlated color temperature (CCT), and CIE values of the lights from these devices are summarised in Table 3. Good colour qualities for white light emission can be defined by CIE coordinates close to (0.33, 0.33), CRI and CCT values close to 100 and 5500 respectively. The mixtures of m-XDATB and BUVR03 at 2:1 and 1 : 1 ratios showed white-light emissions with good colour qualities. Table 3. Qualities of the broadband light emitted by m-XDATB, BUVR03 and combinations of both phosphors.

Example 5. Synthesis and structural characterisation of m-xylylenediammonium lead bromide and chloride (m-XDALB and m-XDALC)

Synthesis

The anti-solvent vapour assisted crystallisation method was used to grow single crystals of the m-XDALC and m-XDALB perovskites. For each sample (m-XDALC and m-XDALB), one mole of respective PbX₂ and two mole of the respective m-xylylenediammonium halides were added to DMF to give a final concentration of the perovskites at 1 M. The mixture was then ultrasonicated to produce a clear solution. Each of the solutions was then incubated in a chamber saturated with the vapour of the anti-solvent and good quality crystals of m- xylylenediammonium lead halides suitable for single crystal X-ray diffraction was obtained. Isopropanol and diethyl ether were used as the anti-solvent for the growth of the m-XDALC and m-XDALB crystals respectively. The crystals were formed after one week and were washed with the anti-solvent and subsequently characterised by single-crystal X-ray crystallography and used for ultrafast optical spectroscopic studies. Both compounds grew in the monoclinic crystal system with molecular formulae of (/77-XDA)₂PbX_s

Characterisation by single-crystal X-ray diffraction (XRD)

The crystal structures are illustrated along three different axes (a, b and c axes). Fig. 9a and 10a show how the XDA dications interspersed between and stabilised the isolated Pb s^4" octahedral units for m-XDALC and m-XDALB respectively. On the other hand, Fig. 9b and 10b show how the ammonium groups of the XDA cations are spatially trans to each other, and the dications stacked in a parallel manner to one another.

The ammonium groups of the m-XDA dications appear to be hydrogen bonded to three halides of three different octahedra PbX₆ ^4" units (Fig. 9d and 10d). Each PbX₆ ^4" unit is crystallographically centrosym metric with three distinct Pb-X bond lengths, which vary from 2.875 to 2.910 A for m-XDALC (Fig. 9e) and from 3.012 to 3.044 A for the m-XDALB system (Fig. 10e). In addition, the bond angles for the c/^'s-halides vary from 82.36 to 97.64° in the chloride (Fig. 9f) and from 81.70 to 98.30° in the bromide system (Fig. 10f). However, the trans halide bond angles are all 180° due to the crystallographically imposed symmetry. The variance in octahedral angles ² _0dL and the octahedral elongation _oct were calculated to be 33.76° and 1.0092 A for m-XDALC, and 36.38° and 1.0099 A for m-XDALB, which indicate significant distortion from the idealised octahedral geometry. Table 4 depicts the sample and crystal data, while Table 5 depicts the data collection and structure refinement for /n-XDALC and m-XDALB. The CIF data for m-XDALC and m- XDALB have been deposited with the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1531645 and 1531646.

Table 4. Sample and crystal data for m-XDALC and m-XDALB.

Sample m-XDALC m-XDALB

Identification

shs219s shs125s

code

Chemical

C₁₆H₂₈CI₆N₄Pb C₁₆H₂₈Br₆N₄Pb formula

Formula

696.31 g/mol 963.07 g/mol

weight

Temperature 153(2) K 103(2) K

Wavelength 1.54178 A 0.71073 A

Crystal size 0.010 x 0.040 x 0.060 mm 0.240 x 0.280 x 0.360 mm

Crystal habit colourless plate colourless block

Crystal system monoclinic monoclinic

Space group P 1 21/c 1 P 1 21/c 1

a = 10.5982(9) A a = 90° a = 10.713(2) A a = 90°

Unit cell

b = 13.8594(12) A β= 102.761(5)° b = 14.476(3) A β= 103.635(4)° dimensions

c = 8.3290(7) A γ = 90° c = 8.5014(16) A γ = 90°

Volume 1193.18(18) A³ 1281.3(4) A³

Z 2 2

Density

1.938 g/cm³ 2.496 g/cm³

(calculated)

Absorption

20.012 mm^"1 15.950 mm^"1

coefficient

F(000) 672 888

Table 5. Data collection and structure refinement for m-XDALC and m-XDALB.

mean Characterisation by NMR spectroscopy

The Pb NMR spectrum of m-XDALB shows a single Pb component, where 5_iso = -595 ± 1 ppm was split into a multiplet via ²⁰⁷Pb-⁷⁹'⁸¹ Br J-coupling (Fig. 11a). An agreeable fit of the multiplet was achieved by simulating the spectrum of a single ²⁰⁷Pb component bonded to six identical ^{79 81} Br sites and the details of the fit are given in Table 6. The J-coupling in m-XDALB was measured as ¹J(²⁰⁷Pb-^79/81 Br) = 2.0 ± 0.2 kHz, which is coherent with reported lead halide J-couplings (Wang, F. et a\., J. Am. Chem. Soc. 1995, 117, 6637; Wrackmeyer, B. et al., In Annual Reports on NMR Spectroscopy; Webb, G. A., Ed.; Academic Press: 1990; Vol. 22, p 249). The clarity of the observed ²⁰⁷Pb multiplet indicates that the PbBr₆ octahedral unit is highly symmetrical in m-XDALB and is analogous to the ¹¹⁹Sn NMR investigation of the related m-XDATB compound in Example 1.

Table 6. Pb NMR parameters of m-XDALB determined via J-coupling spectral simulation

The ¹H and ¹³C NMR spectra of m-XDALB and m-XDALC are shown in Fig. 11 b and c respectively. Both the ¹ H NMR spectra of m-XDALB and m-XDALC show two broad resonances, while that of m-XDALB show an additional three sharp resonances. The broad resonance at ~8 ppm was assigned to the phenyl ring and ammonium hydrogen sites, while the broad resonance at ~3 ppm was assigned to the benzyl CH₂ hydrogens. These resonances were assigned similarly to that of m-XDATB. Therefore, the sharper resonances in the ¹H NMR spectrum were probably due to organic solvent residues.

The sharper ¹³C NMR spectra show eight sharp resonances, in which the six resonances at higher frequencies (138 - 131 ppm) were assigned to the phenyl ring, whereas the two resonances at lower frequencies (45 - 44 ppm) were assigned to the benzyl CH₂ group. Therefore, the above NMR data confirms the composition of the m-XDALB and m-XDALC crystals. Example 6. Preparation and characterisation of thin films of m-XDALB synthesised with different amount of PbBr₂

Since organic-inorganic hybrid perovskites can be synthesised and processed easily in their solution forms, thin films of m-XDALB were prepared and used for subsequent characterisation. As the solubility of m-XDABr₂ was poor in DMF, dimethylsulfoxide (DMSO) was used to dissolve the ammonium salt and PbBr₂ instead.

To prepare a thin film of m-XDALB, a solution of m-XDALB was prepared by dissolving 2.0 equivalents of m-XDABr₂ and 1.0 equivalent of PbBr₂ in DMSO at 100 °C to give a 1 M solution.

For solutions with excess PbBr₂, they were prepared by dissolving excess amount of PbBr₂ in 1.0 M perovskite solutions. Thin films of these materials were prepared by spin-coating the perovskite solution at 4000 rpm for 30 seconds on a quartz plate followed by annealing at 100 °C.

Interestingly, the thin film X-ray diffraction (XRD) pattern of the film prepared using DMSO shows an intense peak at a 2Θ value of 8.9°, which is inconsistent with the simulated (100) peak of the monoclinic m-XDALB (Fig. 12a). However, the preparation of the m-XDALB thin film using DMF also gave the same unknown peak at 8.9° along with the peaks that corresponds to the formation of m-XDALB (Fig. 12b). This therefore ruled out the possibility of the diffraction peak at 8.9° originating from a DMSO-solvated complex. It was observed that the addition of excess PbBr₂ to the perovskite solution was found to stimulate formation of the crystallographically characterised phase of m-XDALB in the thin films. Thin films were prepared by systematically by adding 0 to 1 mole equivalents of excess PbBr₂ in increments of 0.16 mole and the XRD patterns of all the films are as shown in Fig. 12a.

The addition of excess PbBr₂ reduced the intensity of the unknown peak and concurrently increased the intensity of the peaks for the monoclinic m-XDALB. The unknown peak at 8.9° almost disappeared in the film with 0.83 molar equivalents of excess PbBr₂ (a total of 1.83 equivalent of PbBr₂ was present in the mixture) indicating that adding 0.83 equivalent excess of PbBr₂ is optimal for the synthesis thin film of a pure phase of monoclinic m-XDALB.

However, the amount of excess PbBr₂ was further increased to 1.0 mole and this resulted in the formation of a new and strong peak at 7.9°. This new peak may arise due to an increased d-spacing of the (100) planes of m-XDALB in order to accommodate the excess lead bromide. This strong peak is due to the formation of semiconducting higher dimensional perovskite with preferred orientation of crystal plane. The absorption spectrum of thus prepared thin film has excitonic peak similar to the peak observed for 1 D perovskite (Yuan, Z. et al. Nat. Comm. 2017, 8, 14051). In addition, this 1 D m-XDALB thin film found to show broadband emission constitute of excitonic and self-trapped excitonic peaks. Therefore, this 1 D m-XDALB thin film and the 0D m-XDALB single crystals were used for further photophysical characterisations.

Example 7. Powder XRD characterisation of 0D m-XDALB and m-XDALC, and thermal stability of 0D m-XDALB

Powder XRD analysis were carried out on powder samples that were precipitated from the solution of PbBr₂ and m-XDABr₂ in DMF with diethyl ether as an anti-solvent. The powder XRD pattern of this precipitate as shown in Fig. 12c confirmed the formation of monodinic 0D m-XDALB in reference to the simulated spectrum. Powder XRD for m-XDALC was also carried out and a good match between the experimental data and simulated data was observed (Fig. 12d).

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were conducted on the precipitated powder samples of 0D m-XDALB. The TGA thermogram of this material, collected up to 800 °C, indicated that it was stable up to at least 250 °C (Fig. 13). In addition, the DSC thermogram showed no evidence of a phase transition up to 200 °C (Fig. 14), suggesting that the thin films annealed at 100 °C should be stable.

Example 8. Optical properties of m-XDALB and m-XDALC The absorption spectra of m-XDALC single crystals and 0D m-XDALB powders exhibited narrow absorption bands with maxima at 280 and 310 nm for m-XDALC and m-XDALB respectively (Fig. 15a). In order to understand the band levels and band gap of the 0D m- XDALB, we conducted photoelectron spectroscopy in air (PESA) on powder samples and determined that the valence band maximum (VBM) was -6.06 eV relative to vacuum (Fig. 15d). By using the band gap obtained from the Tauc plot, we calculated the conduction band maximum (CBM) to be - 2.31 eV.

Notably, broadband emissions were observed for m-XDALB, as shown in Fig. 15b. Similarly, 0D m-XDALB single crystals exhibited broadband emission that covers from 400 nm to 500 nm with peak maximum at 440 nm, under the photoexcitation at 334 nm (Fig. 17b). This blue emission was very weak at room temperature so measurements were carried out at lower temperatures and the PL spectra are shown in Fig. 17b. As the temperature was lowered from 280 K to 80 K, the PL intensity improved nearly 66 times without any changes in the PL profile. Similarly, the PLE spectra also showed enhancement of intensity as the temperature was lowered (Fig. 17a). As shown in Fig. 17a, the PLE spectrum of 0D m-XDALB measured at 80 K has three peaks at 295 nm, 320 nm and 330 nm and the last two peaks merges together with new peak maximum at 334 nm as the temperature is increased to 230 K. PL and PLE measurements at 230 K shows Stokes's shift of 106 nm and FWHM of PL to be 70 nm, which are relatively small in comparison to other 0D perovskite reported so far. This could be due to the higher distortion of molecular complex in its ground state indicated by its higher variance in octahedral angles σ² _οα and the higher octahedral elongation so it has less room to exhibit structural distortion under photoexcitation. Spectra measured above 230 K were not included as they were generally clear due to the weak emission and scattering of light by the powder samples.

In addition, the PL spectrum of m-XDABr₂ at 80 K was recorded and was found to be emissive (Fig. 17d). The excitation of /77-XDABr₂ at different wavelengths gave PL emissions in the visible region from 400 nm to 650 nm with vibronic progression but the emissions from m-XDALB perovskite did not show vibronic progression (Fig. 17c). In addition, the PLE profiles of m-XDABr₂ and the 0D m-XDALB were also different, which undoubtedly indicates that the emission from the 0D m-XDALB perovskite is from the metal-centered s-p transitions of Pb-Br octahedra whereas the emission from the organic ammonium bromide crystals could be due to crystallisation-induced emission (CIE). Example 9. Optical properties of the 1 D m-XDALB thin films and its broadband emission for white light application

The broadband emission of 1 D m-XDALB thin films synthesised with 1.0 mole excess of PbBr₂ is as shown in Fig. 18a and has CIE 1931 coordinates of (0.36, 0.40), which lies in the "white" region (Fig. 18b). In addition, this emission demonstrates a good CRI of 82.9 and a CCT of 4805 K, suitable for cool white light applications. The excitonic decay kinetics at ~400nm for this white emitter is also distinctly longer lived compared to samples prepared using 0 and 0.5 equivalents of excess PbBr₂ (Fig. 18c). Interestingly, the PL kinetics at 500, 550, and 600 nm were found to be invariant (Fig. 18d), suggesting that a single species is responsible for the broad emission. Fig. 20 shows the temperature-dependent PL spectra of the thin film. Upon lowering the temperature, a steady increase in PL peak intensity was observed for the broad emission centered at ~600 nm (Fig. 20a and c). On the contrary, the free excitonic PL intensity centered at ~400 nm showed a non-monotonic behaviour with a maximum at T ~ 120 K (Fig. 20a inset and d). The initial increase could be due to reduced non-radiative recombination of free excitons, while the subsequent decrease could be due to suppressed detrapping of bound excitons into free excitons.

These PL spectral changes also resulted in deviation of the CIE 1931 coordinates from the white region at lower temperatures (Fig. 19). The variations in integrated PL intensity and FWHM with temperature are summarised in Fig. 20b.

At low temperatures, the probability of phonon absorption for indirect recombination was reduced which led to increased radiative recombination of the bound excitons, therefore resulting in an increase in the PL intensity. This reduction in non-radiative recombination at lower temperatures is also consistent with the lengthening of PL lifetime at lower temperatures (Fig. 20e). Importantly, the reduction cannot be attributed to thermal dissipation (25 meV) or tunneling (negligible activation barrier). Mechanism of the PL processes

The formation of self-trapped excitons from free excitons was reflected in both the PL decay and transient absorption (TA) kinetics. Firstly, short PL lifetime components were observed which can be assigned to self-trapping (Fig. 21 a). The short lifetime component was found to be ~5 ps, which was shorter than the time-resolution of the streak camera (~10 ps). In addition, this short component was not observed in samples with 0 and 0.5 moles of excess PbBr₂, which do not exhibit white emission (Fig. 21 g).

Secondly, the population of free excitons is temperature dependent (Fig. 21 b). Despite the highest peak PL intensity at 70K, the population of free excitons after self-trapping is still the lowest. This suggests that the detrapping probability of self-trapped excitons is greatly reduced at low temperatures.

Thirdly, there exists a broad photo-induced absorption (PIA) signal (ΔΑ > 0) in the pseudo- colour TA plot (Fig. 21c). This is consistent with indirect formation of self-trapped free excitons and a previously reported TA spectrum (Hu, T. et al. J. Phys. Chem. Lett. 2016, 7, 2258). The lifetime across this broad and featureless PIA (Fig. 21d inset) is also unchanged across different probe wavelengths (Fig. 21d). This behaviour is also consistent with invariant PL kinetics measured across the broad emission in Fig.18d. These data further confirm that the PIA signal may arise from a single self-trapped excitonic state. Fig. 21 e shows the short time scale kinetics probed at different wavelengths. There exists a short lifetime component in the TA kinetics when the sample was probed away from the featureless PIA region (self-trapped state) towards 400 nm (Fig. 21 e). This concurs with the short timescale PL kinetics in Fig. 21a. A fit of this short lifetime component gave an ultrafast self-trapping time of 170±150 fs. Based on all the above photophysical measurements, the processes occurring in this white emitter can be summarised in Fig. 21f. These processes are: (i) laser excitation to generate free excitons that recombine with lifetime of (2.77 ± 0.01) ns; (ii) ultrafast self-trapping which occurs at (170 ± 150) ps after overcoming a self-trapping barrier of -10 meV; (iii) exciton- phonon scattering which broadens the energy linewidth of self-trapped excitonic emission; and (iv) recombination of self-trapped excitons with lifetime of (2.8 ± 0.1)ns.

Example 10. Synthesis and structural characterisation of m-xylylenediammonium germanium bromide (m-XDAGB) Synthesis

Ge0₂ (0.25 g) was first dissolved in a mixture of hydrobromic acid (2.25 mL) and hypophosphorous acid (2.25 mL) at 100 °C under stirring to get clear solution. m-XDABr₂ (1.5 g) was then added to the mixture at the same temperature and was stirred to obtain a clear solution. Slow cooling of this solution in the refrigerator resulted in the growth of good quality single-crystals for X-ray diffraction analysis. The crystals were washed with anhydrous ethanol for several times and analysis by FTIR spectroscopy confirmed that the crystals were free from hypophosphorous acid. Characterisation by single-crystal X-ray diffraction

The resolved unit cell of a single crystal of /n-XDAGB is as shown in Fig. 22a-c in various projections, with formula of (A77-XDA)₂GeBr₆. The crystal system was determined to be triclinic which is different from the monoclinic crystal systems of m-XDALB and m-XDATB_. The isolated octahedral embedded in the organic matrix are clearly seen. Each Ge-Br octahedron has three different bond lengths which varies significantly from 2.8229 A to 2.9113 A. but, the angle (Br-Ge-Br) between right-angled bromides varies slightly from 87.94° to 92.059°. Quadratic elongation and bond angle variance were measured to be 1.0009 and 1.9125^Λ2, which indicate a less distorted Ge-Br octahedron.

The sample and crystal data for m-XDAGB is summarised in Table 7 and the CIF data has been deposited with the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1580601.

Characterisation by NMR spectroscopy Multinuclear solid-state NMR studies of m-XDAGB was carried out to confirm the chemical structure of the Ge-Br octahedron. The ¹ H NMR spectrum of m-XDAGB shows two broad resonances, with the one at ~8 ppm assigned to benzyl ring and ammonium hydrogen sites while the one at ~3 ppm was assigned to benzyl CH₂ hydrogens (Fig. 23a). These resonances were assigned in a similar as that of m-XDATB and m-XDALB.

The ¹³C NMR spectrum of m-XDAGB shows five sharp resonances in which the two resonances at lower frequencies of 44-42 ppm was assigned to the benzyl CH₂ group, while the three resonances at higher frequencies of 136-131 ppm was assigned to the phenyl ring (Fig. 23b). The central resonance (5_iso = 133.5 ± 0.2 ppm) shows a larger intensity as it contains four overlapping proton components. Hence, the ¹³C NMR data confirms the composition of the m-XDAGB crystals.

Example 11. Synthesis and structural characterisation of butylammonium tin bromide (BTB)

Synthesis of BTB

Similar to m-XDAGB, single-crystals of BTB were also prepared by the cooling method in which hot perovskite solution of butylammonium bromide (924 mg, 6 equivalents) and tin bromide (278 mg, 1 equivalent) in acetone (4 ml.) at 70 °C was cooled to room temperature slowly to produce good quality single-crystals for X-ray diffraction analysis.

Characterisation by single-crystal X-ray diffraction

The X-ray crystal structure of BTB shows a formula of

which has 0D perovskite structure (Fig. 22d). BTB has monoclinic crystal structure with the unit cell comprised of isolated Sn-Br octahedra and butylammonium cations for charge neutrality. In addition, the unit cell contains molecules of free-standing butylammonium bromide to separate the Sn-Br octahedra from each other and to hold the crystal structure intact. As such, the general molecular formula of BTB is A₆MBr₈ ((C₄H₉NH₃)₆SnBr₈) due to the presence of free-standing butylammonium bromide, instead of typical A₄MX_S (Cs₄PbBr₆) for 0D perovskites with monovalent A-site cation. The butylammonium cations have anti and gauche conformations along C2-C3 bonds. Each Sn-Br octahedron has three different bond lengths which vary from 2.9454 A to 3.0276 A, the angle (Br-Sn-Br) between right-angled bromides varies slightly from 87.93° to 92.44°, and the angle between the bromides that are on the same axis varies from 179.25° to 179.61°. Quadratic elongation and bond angle variance of Sn-Br octahedron were measured to be 1.0014 and 4.3024^Λ2, which indicate a less distorted Sn-Br octahedron. The sample and crystal data for BTB is summarised in Table 7 and the CIF data has been deposited with the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1837838.

Table 7. Sample and crystal data for m-XDAGB and BTB.

Characterisation by NMR spectroscopy Multinuclear solid-state NMR studies of BTB was also carried out to confirm the chemical structure of the Sn-Br octahedron. The ¹H NMR spectrum of BTB contains multiple resonances over the range of 9-0 ppm which could be due to rigid arrangement of butylammonium cations that have anti and gauche conformations in BTB crystals (Fig. 23a). Alternatively, this may be due to residual toluene and acetone solvents used in the sample preparation.

The ¹³C NMR spectrum of BTB shows four high intensity sharp resonances between 42 and 14 ppm that correspond to the butyl carbon sites (Fig. 23b). The additional resonances at higher frequencies can be assigned to the organic solvent residues (toluene and acetone) already observed in the ¹ H NMR data. It is interesting to note that the ¹³C NMR spectrum shows the resonances at ~15 and ~21 ppm, which correspond to the CH₃ and adjacent CH₂ groups respectively, were both split into two components of unequal intensity. This suggests that two distinct conformations of the n-butylammonium chain exist within the BTB structure.

The ¹¹⁹Sn NMR spectrum of BTB in Fig. 23c shows a single ^{1 19}Sn component where 6_iso = -591 ± 2 ppm. An agreeable fit of the multiplet was achieved by simulating the spectrum using a Gaussian/Lorentzian lineshape in which the details of the fit are given in Table SX. Unlike previous studies on SnBr₆ octahedra compounds, no Sn-Br J-coupling was observed which is probably due to the additional Br atoms adjacent to the octahedral.

Example 12. Characterisation of /n-XDAGB and BTB by X-ray Photoelectron Spectroscopy (XPS), X-ray Powder Diffraction (XRD), thermogravimetry (TGA) and differential scanning calorimetry (DSC)

XPS survey scan for m-XDAGB shows the core level peaks for Ge, Br, C, N and O, which confirm the presence of all the elements present in the m-XDAGB perovskite (Fig. 24a). The peak due to oxygen is from the phosphorous acid used in the preparation of the perovskite. Narrow scan for the binding energy of 2p_3/2 core level shows single peak at 1218.3 eV which supports the presence of Ge in +2 oxidation state (Fig. 24b).

Similarly, wide scan for BTB shows the core level peaks for the presence of Sn, Br, C, and N and narrow scan for the binding energies of d_{3 2} and d_5/2 core levels of Sn shows one peak for each orbital, indicating the presence of Sn in the +2 oxidation state. (Fig. 24c and d).

The single-crystals produced in a single batch were also analysed by powder XRD. As shown in Fig. 25a and b, a good match between the powder XRD spectra of finely grinded single-crystals with that of simulated pattern confirmed the purity of m-XDAGB and BTB. To verify the thermal stability of these perovskites for potential use as phosphors, TGA and DSC thermograms were collected which showed that m-XDAGB is stable at least up to 200 °C (Fig. 26a and b).

Example 13. Optical properties of m-XDAGB and BTB

The PLE and PL spectra for m-XDAGB and BTB are shown in Fig. 27a and b respectively. The PLE spectrum of m-XDAGB collected for the emission maximum at 555 nm shows a peak maximum at 330 nm (Fig. 27a). Hence, m-XDAGB was excited at 330 nm and the steady-state PL was collected which covers the visible region from 450 nm to 700 nm, with FWHM of 1 10 nm, peak maximum at 555 nm, and Stokes' shift of 225 nm (Fig. 27b). This PL emission was red-shifted as compared to the emissions of its counterparts (m-XDALB and m-XDATB) and the FWHM of m-XDAGB was also larger than that of m-XDALB and m- XDATB. The difference in the photoluminescence properties can be attributed to the distortions of different metal halide octahedra which can be quantified by quadratic elongation and bond angle variance (Robinson, K. et al., Science 1971 , 772, 567). Similarly, comparison of Sn-Br octahedra present in BTB with that of m-XDATB perovskites also indicates that the FWHM and Stokes's shift are higher for BTB which has a less distorted metal halide octahedra. The larger Stokes' shift and FWHM for BTB could be due to the availability of larger room for Jahn-Teller distortion. In addition, PL lifetimes of m-XDAGB and BTB were measured and found to be 3.54 ps and 3.06 ps respectively (Fig. 27c and d). The PLQY of /n-XDAGB was determined to be 20 %. The PLQY can potentially be influenced by metal-halide bond character, heavy-atom effect, distortion of octahedra and charge transfer between the metal and halide ions. Table 8 summarises the distortions, FWHM, Stokes's shift and PLQY between are increasing with decreasing distortion of metal halide octahedron.

Table 8. Comparison of distortions, FWHM, Stokes's shift and PLQY among the perovskites synthesised in this application.

*measured at 230 K Example 14. m-XDAGB and BTB for white light applications

To incorporate these perovskite in WLEDs, particle size of these perovskite were first reduced. Microparticles of m-XDAGB were produced by simply grinding the single-crystals of m-XDAGB using mortar and pestle and sieving through the mesh with pore size of 45 pm. The FESEM image of the powder m-XDAGB is as shown in Fig. 28a.

Microparticles of BTB were prepared by ultrasonicating the BTB suspension in toluene. High power ultrasonication was carried out at 90% power for few minutes and repeated for few times. The resultant solution was centrifuged with the supernatant removed. The remaining solid was dried and subsequently used for fabricating WLED. As shown in Fig. 25b, powder XRD of BTB microparticle showed the peaks for 001 planes due to preferred orientation of that planes. Microplatelet morphology of thus produced BTB can be seen from the FESEM image in Fig. 28b.

WLED based on m-XDAGB was fabricated by mixing powder of yellow-emitting m-XDAGB perovskite with a blue-emitting BaMgAI₁₀Oi₇:Eu phosphor at a 9:1 weight ratio. This mixture was then added with silicone elastomer (SYLGARD 184 monomer), followed by spin coating at 1000 rpm on a quartz substrate to produce a thin film. The addition of the blue emitting phosphor is to shift the CIE coordinates of the overall emission closer to white (Fig. 29). Details of the broadband emission were summarised in Table 9.

Table 9. Qualities of the broadband light emitted by m-XDAGB, BTB, BaMgAI₁₀Oi₇:Eu and combinations of the phosphors.

CIE CRI CCT

m-XDAGB (0.3833, 0.4946) 48.7 4513

9:1 0.3205, 0.3870) 64 5926

BTB (fresh) 0.5444, 0.4428 72 2034

BTB (after a week) 0.4381 , 0.5111 57.3 3675

Claims

1. A zero-dimensional perovskite of formula I:

wherein: each X is independently selected from halo or pseudohalo;

A represents a 5- to 14-membered ring system comprising at least one aromatic or heteroaromatic ring which ring system is unsubstituted or substituted by halo,

alkyl, C₂. 12 alkenyl, C2-12 alkynyl, O-C1-12 alkyl, O-C2-12 alkenyl, O-C2-12 alkynyl, S-C1.12 alkyl, S-C2-12 alkenyl and S-C₂_i₂ alkynyl, which latter nine groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, SR⁵, and Ci_-6 alkyl;

each L represents a linking group attaching NR¹ R²R³ to A and is selected from Ci.12 alkyl, C₂. 12 alkenyl, C2-12 alkynyl, O-C1-12 alkyl, O-C2-12 alkenyl, O-C2-12 alkynyl, S-CM₂ alkyl, S-C2-12 alkenyl and S-C2-12 alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁶, SR⁷, and C _-6 alkyl;

each R¹ to R³ group independently represents H or d_-6 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo, OR⁸ and d-3 alkyl;

each R⁴ to R⁸ group independently represents H or d-3 alkyl;

n is 1 or 2; and

when:

n is 1 , then m is 4 and p is 1 ; and

n is 2, then m is 2 and p is 2.

2. The perovskite according to Claim 1 , wherein X is selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, NCS, CP, NC, OCN, OSCN, SeCN, N₃.

3. The perovskite according to Claim 2, wherein X is selected from one or more of the group consisting of F, Br, CI, I, CN, SCN, and NCS.

4. The perovskite according to Claim 3, wherein X is selected from one or more of the group consisting of Br and CI.

5. The perovskite according to any one of the preceding claims, wherein A represents a 6- to 10-membered aromatic ring system or a 5- to 10-membered heteroaromatic ring system, which ring systems are unsubstituted or substituted by halo, Ci_-6 alkyl, C₂-6 alkenyl, C₂-6 alkynyl, which latter three groups are unsubstituted or substituted by one or more substituents selected from halo, OR⁴, and C _-3 alkyl.

6. The perovskite according to Claim 5, wherein A represents phenyl, thiophenyl, furanyl, pyrrolyl, pyridinyl, which five ring systems are unsubstituted or substituted by halo and d.3 alkyl.

7. The perovskite according to any one of the preceding claims, wherein each L represents a linking group attaching NR¹ R²R³ to A and is selected from Ci_-6 alkyl, C₂-6 alkenyl, C₂.6 alkynyl, which groups are unsubstituted or substituted by one or more substituents selected from halo and d.₃ alkyl.

8. The perovskite according to Claim 7, wherein each L represents a linking group attaching NR¹ R²R³ to A and is selected from Ci_-3 alkyl, which group is unsubstituted or substituted by one or more substituents selected from halo and Ci_-3 alkyl.

9. The perovskite according to any one of the preceding claims, wherein each R¹ to R³ group independently represents H or Ci_-3 alkyl, which latter group is unsubstituted or substituted by one or more substituents selected from halo and C_1-3 alkyl.

10. The perovskite according to Claim 9, wherein each R¹ to R³ group independently represents H.

11. The perovskite according to any one of the preceding claims, wherein n is 2 and m is 2.

12. The perovskite according to any one of the preceding claims selected from:

(i) mefa-xylenediammonium tin bromide [SnBre]

(ii) mefa-xylenediammonium tin chloride

[SnCle]⁴

(iii) o/f/70-xylenediammonium tin bromide

orf/?o-xylenediammonium tin chloride

[SnC ]⁴

(v) para-xylenediammonium tin bromide

[SnBre]⁴

para-xylenediammonium tin chloride

13. The perovskite according to any one of the preceding claims is meta- xylenediammonium tin bromide

[SnBre]⁴

14. The perovskite according to any one of the preceding claims, wherein the perovskite displays one or more of:

(a) a broadband emission in the range of from 400 to 650 nm;

(e) a CI E of from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(f) a colour rendering index of from 45 to 100, such as from 47 to 55, such as 51 , such as from 80 to 99, such as 85 to 95; and

(g) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990.

15. A white light emitting device comprising a zero-dimensional perovskite of formula I as defined in any one of Claims 1 to 14.

16. The device according to Claim 15, wherein the device further comprises a red phosphor.

17. The device according to Claim 16, wherein the red phosphor is selected from one or more of the group consisting of silicon oxides, sulfoselenides, fluorides, oxyfluorides, sulfates, nitrides, and oxynitrides, optionally wherein the sulfoselenide is BUVR03, the silicon oxide is Sr₃Si0₅:Eu²⁺ and the nitride is CaAISiN₃:Eu²⁺ and/or Sr₂Si₅N₈:Eu²⁺.

18. The device according to Claims 16 and 17, wherein the wt:wt ratio of perovskite to red phosphor is from 5: 1 to 1 :1.5, such as 2:1 to 1 :1.

19. The device according to any one of Claims 14 to 18, wherein the device displays one or more of:

(a) a broadband emission in the range of from 400 to 800 nm;

(c) a colour rendering index of from 70 to 90, such as from 78 to 86; (d) a CIE value of close to (0.33, 0.33), such as any value from (0.21 , 0.41) to (0.41 , 0.21), such as from (0.25, 0.38) to (0.38, 0.25), such as from (0.31 , 0.37) to (0.37, 0.31), such as around (0.33, 0.33), such as (0.35, 0.37);

(f) a colour temperature of from 1000 to 10000, such as from 5000 to 9500, such as from 5500 to 8990.

20. A method of preparing a zero-dimensional perovskite of formula I as defined in any one of Claims 1 to 14 comprising the steps of:

(a) mixing a tin halide and/or pseudo halide and a compound of formula II:

in a solvent, where A, L, R¹ to R³, X, n, m and p are as defined in any one of Claims 1 to 14 and y is equal to p; and

21. The method according to Claim 20, wherein the antisolvent is added to the mixture by vapour diffusion to provide crystals of the zero-dimensional perovskite of formula I.

22. The method according to Claim 20, wherein the antisolvent is added directly to the mixture to provide a powdered form of the zero-dimensional perovskite of formula I.

23. The method according to any one of Claims 20 to 22, wherein the solvent is DMF or DMSO and the antisolvent is a d to C₄ alkyl alcohol and/or a d to C₄ alkyl ether.

24. The method according to any one of Claims 20 to 23, wherein the method is performed at a temperature of from 10 to 50 °C, such as from 15 to 30 °C, such as from 20 to 27 °C, such as 25 °C.