Synthesis, Structure, and Tunability of Zero-Dimensional Organic-Inorganic Metal Halides Utilizing the m-Xylylenediammonium Cation: MXD2PbI6, MXDBiI5, and MXD3Bi2Br12·2H2O.

Over the past decade, the efficiency of photovoltaic devices based on CH3NH3PbI3 have dramatically increased. This has driven research efforts in all areas, from the discovery of materials to film processing to long-term device stability studies. Here, we report the synthesis and structure of three new "zero dimensional" organic-inorganic metal halides which use the meta-xylylenediammonium (MXD) cation: MXD2PbI6, MXDBiI5, and (MXD)3Bi2Br12·2H2O. The different structures of the new materials lead to compounds with a range of band gaps with MXDBiI5 having the lowest at 2.15 eV. We have explored the tunabilty of MXDBiI5 through halide substitution by preparing a series of samples with composition MXDBiI5-x Br x and determined the halide content using energy dispersive X-ray spectroscopy. A large range of solid solution is obtained in MXDBiI5-x Br x , resulting in the formation of single-phase materials for bromine contents from x = 0 to 3.71 (iodine contents from 1.29 to 5). This highlights the fact that zero-dimensional organic-inorganic halides are highly tunable, in a similar manner to the higher-dimensional perovskite counterparts. Such new materials open up the opportunity for further studies of the physics and optoelectronic properties of these materials.

Introduction

In 2009, Miyasaka and co-workers first tested CH3NH3PbI3 and CH3NH3PbBr3 in photovoltaic devices.[1] Although the initial efficiencies were modest, the resulting optimization of all aspects of device fabrication has led to groundbreaking photovoltaic power conversion efficiencies.[2−4] Power conversion efficiencies of 25.7% have now been reported for single junction photovoltaics based on CH3NH3PbI3 and 29.8% for tandem photovoltaics, based on a combination of silicon and a perovskite layer.[5]

CH3NH3PbI3 adopts the perovskite structure, (ABX3) where A = CH3NH3+, B= Pb2+, and X = I– (or another halide). The structure consists of corner-sharing PbI6 octahedra, with the organic cation being disordered on the perovskite A-site at room temperature.[6] As the size of the organic ammonium cation is increased, different perovskite-related structure types can form.[7] In particular, layered perovskites based on the Ruddlesden–Popper and Dion–Jacobson structure types have received considerable interest. A notable breakthrough came when a power conversion efficiency of 12.52% was achieved for (C4H9NH3)2(CH3NH3)3Pb4I13, which adopts the Ruddlesden–Popper structure.[8] Very recently, the “Memory Seed Effect” has been reported, which involves dissolving presynthesized crystals in solvents such as DMF, which enables the preparation of high-quality thin films for a variety of layered perovskites.[9] The solutions have been shown to retain “memory seed” crystallites, which facilitates the production of phase-pure layered perovskites.[9] The use of this “Memory Seed Effect” has led to solar cells with power conversion efficiencies of 17.1% for (C4H9NH3)2(CH3NH3)3Pb4I13.[9]

Organic–inorganic halide perovskites exhibit great compositional flexibility.[4,10] The A, B, and X sites can be doped in order to fine-tune the electronic structure and band gap for a range of optoelectronic applications.[3,10,11] In particular, as the electronic structure of organic–inorganic metal halides depends on the B-site cation and halide, halide substitution is an excellent way of tuning the band gap of these materials.[12,13] For example, halide substitution in FAPbI3-xBr (FA = formamidinium) resulted in the band gap being tuned from 1.48 to 2.23 eV.[3] Unfortunately, photoinduced halide segregation has been reported in CH3NH3PbI3–Br, yielding iodine- and bromine-rich domains in the thin films.[14] Nevertheless, such tunability of the materials is of interest, particularly in the fields of indoor photovoltaics or tandem solar cells, which can utilize two perovskites with different compositions and band gaps.[4] One method of preventing halide segregation is to use multidentate ligands which create interfaces with a low defect content.[15]

Recently, several new families of layered perovskites have been prepared which incorporate methylammonium in addition to cations which contain aromatic rings such as phenylethylammonium (PEA), 4-aminomethylpyridinium (4AMPY), 3-aminomethylpyridinium (3AMPY), or meta-phenylenediammonium (mPDA).[16−18] Several of these perovskites adopt the Dion–Jacobson structure.[16,17] Interestingly, the position of the aminomethyl group on the pyridinium ring in 4AMPY and 3AMPY influences the stacking of the inorganic layer containing lead iodide octahedra and results in a decrease in band gap when going from (4AMPY)(CH3NH3)PbI3 to (3AMPY)(CH3NH3)PbI3.[16] (3AMPY)(CH3NH3)3Pb4I13 exhibited a power conversion efficiency of 9.20%. In the (mPDA)(CH3NH3)PbI3 system, fabrication of phase pure thin films has been challenging for some values of n.[17] (PEA)2(CH3NH3)2Pb3I10 has been shown to have enhanced moisture stability with respect to CH3NH3PbI3.[18]

The term “zero-dimensional perovskite” has been coined for organic–inorganic metal halides which consist of isolated clusters of metal halide octahedra which are separated by organic cations, although strictly speaking these materials do not possess the perovskite structure. Depending on the nature of the metal and halide, different connectivities of octahedra may exist in the cluster, such as isolated MX6 (where M = metal cation and X = halide) or edge-sharing octahedra to make M2X10 or face-sharing octahedra to make M2X9. Zero-dimensional materials are already showing interesting properties; for example, (1,3-propanediammonium)2Bi2I10·2H2O has been tested as a photodetector, and reproducible photocurrents could be drawn from this material.[19] (CH3NH3)3Bi2I9 shows highly anisotropic photoluminescence and has shown evidence of quantum cutting.[20] In addition, isovalent doping in the zero-dimensional (CH3NH3)3Bi2–SbI9 has shown a band-bowing phenomena.[21] Zero-dimensional all-inorganic halides such as A4PbX6 and Cs4SnX6 (A = K, Cs, Rb and X = Cl, Br, I) are also of interest, particularly for white light emission, and work by Mohammed et al. has recently found that the local octahedral distortions enabled the formation of self-trapped states.[22]

Motivated by reports of enhanced moisture stability when aromatic organic ammonium cations are used,[18] along with the huge compositional and structural diversity of organic–inorganic halides, we have explored the synthesis and tunability of new organic–inorganic halides which use the meta-xylylenediammonium (MXD, H3NCH2C6H4CH2NH32+) cation.

Experimental Section

The synthesis of all materials was based on modifications of the method reported by Poglitsch and Weber for the preparation of CH3NH3PbI3.[23] The appropriate metal oxide was dissolved in 5 mL of HX and heated to 80 °C. Separately, 0.5 mL of HX was added to 0.4 mL m-xylylenediamine, resulting in the formation of crystals. These crystals were heated until dissolved. The two solutions were mixed and stirred for 30 min. The solution was cooled to room temperature, and the resulting crystals were filtered. The crystals were dried in a vacuum oven at 80 °C for 2 h.

For the synthesis of MXDBiI5–Br, 0.47 g Bi2O3 was added to a mixture of HX acid (X = Br, I). The volumetric ratio of HI:HBr used in the reactions are given in Table . The reaction mixture was then placed on a hot plate and heated to 80 °C, with stirring, until all reactants were dissolved. Then, an equimolar quantity of m-xylylenediamine (0.14 mL) was added to the solution, and the mixture was stirred for 30 min. After cooling to room temperature, the crystals were filtered off and left to dry in the fume hood.

Table 1

Volumetric Ratios of HBr and HI Used in the Synthesis of the MXDBiI5–Br Mixed Halide Samples

HI	0.00	0.125	0.250	0.375	0.500	0.625	0.750	0.875	1.000
HBr	1.00	0.875	0.750	0.625	0.500	0.375	0.250	0.125	0.000

Powder X-ray diffraction data were collected on a Panalytical Empyrean Diffractometer using CuKα1 radiation in Bragg–Brentano geometry. Data were collected from 5° to 70° with a step size of 0.017° and a time per step of 0.94 s. PXRD data were analyzed using Topas Academic ver. 6.[24]

Single crystal diffraction data were recorded at either 173 or 293 K using a Rigaku FR-X Ultrahigh brilliance Microfocus RA generator/confocal optics and Rigaku XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Intensity data was collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space (CrystalClear).[25] The data was processed using CrysAlisPro software.[26] Structure solution was carried out using SHELXT,[27] and structure refinement by full matrix least-squares against F2 was carried out with SHELXL (2018/3).[28] Non-hydrogen atoms were refined anisotropically, and carbon-bound hydrogens were refined using a riding model. Ammonium hydrogens were located from the difference Fourier map and refined isotropically subject to distance restraints. Selected crystallographic data are presented in Tables and 3. Deposition numbers 2151572–2151574 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Table 2

Refinement Details for MXD2PbI6, MXDBiI5, and MXD2Bi2Br12·2H2O

	MXD2PbI6	MXDBiI5	(MXD)3Bi2Br12·2H2O
CCDC Code	2151572	2151573	2151574
Formula	C16H28I6N4Pb	C16H28Bi2I10N4	C24H46Bi2Br12N6O2
Formula Weight	1245.01	1963.38	1827.55
Crystal Description	Yellow prism	Red plate	Yellow prism
Crystal Size (mm3)	0.12 × 0.05 × 0.02	0.15 × 0.06 × 0.01	0.07 × 0.06 × 0.02
Temperature (K)	173(2)	293(2)	173(2)
Crystal System	Monoclinic	Monoclinic	Triclinic
Space group	P21/c	I2/m	P1̅
a (Å)	10.9539(3)	8.5380(4)	8.0562(2)
b (Å)	15.5492(3)	11.6966(6)	14.2053(3)
c (Å)	8.8638(2)	19.2708(8)	21.0458(5)
α (deg)			102.611(2)
β (deg)	104.565(3)	99.550(4)	100.779(2)
γ (deg)			99.681(2)
Volume (Å3)	1461.20(6)	1897.82(16)	2253.38(9)
Z	2	2	2
ρ (calc, g/cm3)	2.830	3.436	2.693
μ (mm–1)	12.122	17.407	18.469
F(000)	1104	1696	1668
Reflections collected	18551	6306	39841
Independent reflections (Rint)	3423 (0.0308)	2302 (0.0324)	10230 (0.0393)
Parameters, restraints	148,48	91, 7	481, 33
Goodness-of-fit on F2	1.074	1.093	1.091
R1	0.0179	0.0371	0.0636
R1 [I > 2σ(I)]	0.0161	0.0278	0.0398
wR2	0.0356	0.0680	0.0743
wR2 [I > 2σ(I)]	0.0353	0.0645	0.0696
Largest diff. peak and hole (e/Å3)	0.781 and −1.310	0.940 and −1.512	1.706 and −1.458

Table 3

M–X Bond Lengths and Octahedral Distortions for MX6 Octahedra in MXD2PbI6, MXDBiI5, and MXD3Bi2Br12·2H2O

	MXD2PbI6	MXDBiI5	(MXD)3Bi2Br12·2H2O
M-X(1) (Å)	3.20771(17)	2.9710(5)	2.7942(8)	2.9350(8)
M-X(2) (Å)	3.20776(17)	3.0245(6)	2.8029(8)	2.8073(8)
M-X(3) (Å)	3.21169(17)	3.0565(4)	2.9389(8)	2.8203(8)
M-X(4) (Å)	3.21171(17)	3.0565(4)	2.8813(8)	2.8733(8)
M-X(5) (Å)	3.2443(2)	3.2117(6)	2.8836(8)	2.9270(8)
M-X(6) (Å)	3.2443(2)	3.2459(6)	2.8650(8)	2.7722(8)
Bond length distortion (Δd, × 10–4)[29]	0.258	10.39	8.65	12.98
Bond angle variance (σ2)[29]	31.63	13.13	18.42	7.46

Scanning electron microscopy studies were carried out using a JSM IT200 equipped with a 25 mm2 Jeol DrySD EDS detector and a Jeol JSM 5600. UV–vis diffuse reflectance spectra were collected on a Jasco V650 spectrophotometer equipped with an integrating sphere, in the wavelength range 190–900 nm. BaSO4 was used as a reference.

Results and Discussion

Crystal Structure

The reactions of MXD with either Bi2O3 and HI or HBr, along with PbO and HI, produced the crystalline products MXDBiI5, MXD3Bi2Br12·2H2O, and MXD2PbI6. Their structures were determined from single-crystal X-ray diffraction data, and selected parameters are tabulated in Table .

The structure of MXD2PbI6 consists of isolated PbI6 octahedra separated by MXD cations (Figure ). Although the structure is not strictly a layered perovskite, the structure can be thought of as a “pseudolayered” material, as the PbI6 octahedra form layers in the crystal structure but are too far apart to form the corner-sharing-octahedral connectivity required in the perovskite structure. It is likely that this is driven by the ammonium groups from the large MXD cation, which protrude into the inorganic layer. The Pb–I bond lengths range from 3.20774(17) to 3.2443(2) Å (Table ). The I–Pb–I bond angles range from 81.808(4)° to 98.192(4)° for cis I–Pb–I angles, while the trans I–Pb–I are 180°. This deviation away from the ideal octahedral angles indicates that the octahedra show some distortion. Such a distortion can be quantified in terms of the bond angle variance and the bond length distortion, as originally reported by Robinson et al.[29] Here, we calculate the bond length distortion to be 0.258 and bond angle variance to be 31.63. The bond angle variance here is much smaller than the 234.2 reported for (mPDA)PbI4 recently reported.[17] The aromatic core of the MXD cation is not directly above adjacent MXD cations but is offset. The orientations of the adjacent MXD cations result in NH3 groups pointing in three different directions to give a Y-shaped arrangement of the NH3 groups, with one pair of NH3 groups being almost eclipsed (Figure c). Hydrogen bonds between NH3 hydrogens and iodine from the PbI6 octahedra range from 3.546(2) to 3.718(2) Å. Figure S1 shows the PXRD data of the bulk sample. Pawley fits were carried out using the unit cell parameters and space group obtained from single-crystal diffraction. Twelve Chebyshev background parameters, sample displacement, and profile parameters were also refined. The resulting fit is shown in Figure S1, and this confirms the phase purity of the sample.

Figure 1

Crystal structure of MXD2PbI6 (a) viewed down the a-axis, (b) viewed down the b-axis, (c) viewed down the c-axis, and (d) to show the arrangement of adjacent MXD cations.

In contrast to the structure of MXD2PbI6, which contains isolated PbI6 octahedra, the structure of MXDBiI5 consists of BiI6 octahedra, which share edges to form Bi2I10 dimers (Figure ). This kind of structural motif has also been observed in other organic bismuth halides.[30,31] The Bi2I10 units are separated by the MXD cations. The Bi–I bond lengths range from 2.9710(5) to 3.2459(6) Å (Table ). The I–B–I angles range from 171.904(19)° to 178.914(15)° for the iodine in the trans positions and from 85.756(15)° to 94.581(17)° in the cis position. The Bi octahedra show some distortion, with a bond length distortion of 10.39 and bond angle variance of 13.13. The orientation of the MXD cation plays an important role in holding the structure together. The aromatic rings of adjacent MXD cations are strictly parallel and have a centroid–centroid distance of 3.509(5) Å. The close register of the rings forces the ammonium groups to point to the same side of the ring for each cation, so when considered for each pair of cations, all four NH3 groups point in opposite directions. The I1–I4 intercluster distance is 4.1357(6) Å, and this is of particular note, as this is comparable to the short interlayer I–I distances of 4.00 and 4.04 Å, which were reported in (H3NC6H4NH3)(CH3NH3)Pb2I7 and (H3NC6H4NH3)(CH3NH3)2Pb3I10, respectively.[17] Although these are thought to be the shortest interlayer iodine–iodine distances in Dion–Jacobson perovskites, we note that in zero-dimensional organic−inorganic halides, short intercluster distances of 3.7253(16) Å have been reported in (H3NC6H4NH3)2Bi2I10·4H2O, while short interchain distances of 3.871(10) Å have been reported in (H3NC6H4NH3)Bi2I8·I2.[17,31,32] Hydrogen bonds between NH3 hydrogens and iodine in the Bi2I10 units ranged from 3.712(5) to 3.812(6) Å. Figure S2 shows the PXRD data of the bulk MXDBiI5 sample. Pawley fits were carried out using the cell parameters and space group obtained from single crystal diffraction MXDBiI5 and the same nonstructural parameters that were described for MXD2PbI6. The resulting fit is shown in Figure S2, and this confirms the phase purity of the sample.

Figure 2

Structure of MXDBiI5 (a) viewed down the a-axis, (b) viewed down the b-axis, and (c) showing the π–π stacking arrangement of the MXD cations.

The structure of MXD3Bi2Br12·2H2O consists of isolated BiBr6 octahedra, with the asymmetric unit containing two inequivalent BiBr6 octahedra (Figure ). For the Bi(1) octahedron, the Bi–Br bond lengths range from 2.7942(8) to 2.9389(8) Å, while for the Bi(2) octahedron, the Bi–Br bond lengths range from 2.7722(8) to 2.9350(8) Å (Table ). In addition, the bond angles of Bi(1) range from 84.45(2)° to 99.54(2)° for cis bond angles and 170.57(2)° to 176.29(3)° for trans angles, while for Bi(2), the bond angles range from 174.21(2)° to 177.20(3)° for trans bond angles and 85.46(2)° to 95.13(2)° for cis bond angles. Calculation of the bond length distortion and bond angle variance show that the BiBr6 octahedra exhibit different levels of distortion, with Bi(1) showing the greater angular variance than the Bi(2), while Bi(1) shows less distortion of bond lengths than Bi(2) (Table ). The bond angle variance of the BiBr6 octahedra are much smaller than that of MXD2PbI6, which also contains isolated octahedra (PbI6) in the structure. They are also significantly smaller than the bond angle variance reported for (mPDA)PbI4, but are comparable to those reported for some quinoline and isoquinoline lead halides.[17,33] The shortest hydrogen bonds are formed between the NH3 and H2O, with N···O distances of 2.757(12) Å and 2.822(10) Å. Figure S3 shows the PXRD data of the bulk MXD3Bi2Br12·2H2O sample. Pawley fits were carried out using the cell parameters and space group obtained from single crystal diffraction and parameters described for MXD2PbI6. The resulting fit is shown in Figure S3, and this confirms the phase purity of the sample. SEM images of MXD2PbI6, MXDBiI5, and MXD3Bi2Br12·2H2O are shown in Figure .

Figure 3

Crystal structure of MXD3Bi2Br12·2H2O (a) viewed down the a-axis, (b) viewed down the b-axis, and (c) arrangement of adjacent MXD cations.

Figure 4

SEM images of (a) MXD2PbI6, (b) MXDBiI5, and (c) MXD3Bi2Br12·2H2O.

SEM images of MXD2PbI6, MXDBiI5, and MXD3Bi2Br12·2H2O are shown in Figure . MXD2PbI6 displays aggregates of crystals, with a needle-like morphology, of dimensions of approximately 600 μm by 40 μm. Crystallites of MXDBiI5 also show a needle-like morphology, although the crystals are much shorter than those reported for the MXD2PbI6, with dimensions of approximately 50–75 μm by 20 μm. The MXD3Bi2Br12·2H2O sample consists of much thinner crystallites with a narrow, plate-like morphology. A range of crystallite sizes can be observed, with typical dimensions being around 50 μm by 100–200 μm. Although the control of morphology was not considered in this study, this will be of interest in the future, when manufacturing these materials into thin films, as any pinholes in thin films as a result of surface morphology can influence factors such as device performance in photovoltaics or LEDs.

Halide Substitution

It is well-known that the valence band and conduction bands in organic–inorganic halides comprise orbital contributions from both the halide and the inorganic cation.[34] Therefore, isovalent doping on the anion site, e.g., replacing I– with Br–, is a particularly useful technique to tune the band gap of the CH3NH3PbI3 perovskites.[13,35] This is often termed halide substitution and results in the creation of materials with compositions such as CH3NH3PbI3–Br. Zero-dimensional inorganic–organic halides commonly exhibit a similar electronic structure to the 3D analogues, but with greater orbital decoupling;[36] therefore, we decided to probe halide substitution in MXDBiI5–Br, in order to determine the doping limit of Br– in the MXDBiI5 structure type. A photograph of the MXDBiI5–Br samples is shown in Figure and shows that the color of the samples can be varied from yellow (Br-rich samples) to dark red (I-rich samples). As these samples were synthesized using solution based-routes, the ratio of I to Br was determined using energy dispersive spectroscopy (EDS) using the SEM. The I:Br ratio in a sample is not the same as the I:Br ratio in its precursor solutions, but across the I:Br ratios studied, the ratio in the sample can be related directly to its ratio in solution (Figure S5).[36] We note that similar phenomena have been reported when two different cations are used in the synthesis of Ruddlesden–Popper phases, as nonstoichiometric ratios of reagents must be used to isolate the phase pure product.[37] In addition, in the synthesis of Cs2SnX6 (X = Cl, Br, I) mixed halides, the products were found to be richer in the Cl or Br than would be expected given the ratios in the precursors, and this has been attributed to the difference in solubility of halides in solution.[38]

Figure 5

Photograph of halide substituted samples, MXDBiI5–Br.

The PXRD data of MXDBiI5–Br samples are shown in Figure , and a representative SEM image of the polycrystalline sample of MXDBiI4.11Br0.89 is shown in Figure S5. As can be seen, the 100% Br sample (i.e., “x = 5”) exhibits a completely different PXRD pattern to the other samples prepared in this series, which indicates that it adopts a completely different structure type, MXD3Bi2Br12·2H2O (vide supra). However, with only a small amount of HI in the precursor solution (see Table and Figure S4), the MXDBiI5 structure type is adopted, and all peaks can be indexed to a monoclinic unit cell in space group I2/m. This structure type is adopted for all MXDBiI5–Br compositions which have a bromine content, x, between 0 and 3.71 (i.e., having iodine contents from 1.29 up to 5.0). In order to determine the extent of the solid solution in MXDBiI5–Br, Pawley refinements were carried out. During the refinements, 12 Chebyshev polynomial terms were used to fit the background, and in addition, unit cell parameters, profile parameters, and specimen displacement were all refined. The resulting variation of unit cell volume with iodine content is shown in Figure b, and the corresponding unit cell parameters are plotted in the Supporting Information (Figure S6). As can be seen from Figure b, there is a linear relationship between bromine content (x) and unit cell volume, in agreement with Vegard’s law, with the exception of a slight leveling off at the lowest bromine contents. This shows that there is a large region of solid solution in the MXDBiI5–Br system. This region of solid solution ranges from a bromine content, x, of 0 to 3.71, and it is likely that a full solid solution could be obtained with further optimization of synthetic conditions. As the bromine content increases, the β angle increases, but there is a leveling off at the highest iodine contents (which corresponds to the lowest bromine contents). The a, b, and c unit cell parameters all show a linear relationship with bromine content and exhibit a smaller leveling off at the lowest bromine contents. The decrease in unit cell parameters with increasing bromine content is expected due to the smaller size of Br– (1.96 Å) with respect to I– (2.20 Å).[39]

Figure 6

(a) PXRD data of MXDBiI5–Br and (b) variation of unit cell volume with bromine content for MXDBiI5–Br. The bromine-end member (x = 5) has a different structure type so has been excluded from part b.

The Kubelka–Munk transformations of the UV–vis diffuse reflectance spectra of the MXDBiI5–Br samples and MXD3Bi2Br12·2H2O are shown in Figure , while the MXD2PbI6 spectrum is shown in Figure S7. Two absorption features are observed in the spectra for MXDBiI5–Br. For MXDBiI5–Br, the first feature is in the 2.36–2.06 eV range (corresponding to 525–600 nm) and the second feature is centered around 1.9 eV (∼650 nm), while for MXD2PbI6, the peak occurs at ∼500 nm (2.5 eV) We note that dual features have been observed in both UV–vis spectroscopy and photoluminescence measurements for both 2D, layered materials, and zero-dimensional materials.[40−42] For example, Nag et al. noticed dual-emission in photoluminescence studies of single crystals of (PEA)2SnI4 (PEA = phenylethylammonium), which was also accompanied by two absorption features in UV–vis absorption spectra of the same material.[41] A similar behavior was also observed for (4-AMP)SnI4 (4-AMP = (4-aminomethyl)piperidinium).[41] Dual PL emission has also been observed for the zero-dimensional TPA2SbCl5 (TPA = tetrapropylammonium).[42] To date, the presence of dual features in the photoluminescence or absorption spectra has been attributed to differences in the edge or bulk of the crystals or self-trapped excitons.[41,42] As the bromide content is increased in MXDBiI5–Br, the absorption edge shifts to larger energies, which is in agreement with an increase in band gap with increasing Br content. The resulting band gaps were estimated using Tauc plots and are listed in Table . The lowest band gaps were obtained for the iodine-rich compositions, with MXDBiI5 exhibiting the lowest band gap in the series (2.15 eV) and MXD3Bi2Br12·2H2O the largest (2.86 eV). In contrast, the band gap of MXD2PbI6 was determined to be 2.33 eV. The variation of band gap with bromine content is shown in Figure b and shows an increase in band gap with increasing bromine content, although this trend is not linear. We also note that the band gap of MXDBiI5–Br can be tuned over a similar range to that observed for the FAPbI3–Br perovskites, which allowed tuning of the band gap from 1.48 to 2.25 eV.[3] The high crystallinity of these samples warrants further investigation into the photostability of MXDBiI5-xBr, as in the mixed cation, mixed halide perovskites, CsFA1–PbI3–Br, a high level of crystallinity was found to suppress halide segregation.[43]

Figure 7

(a) UV–vis diffuse reflectance spectra for MXDBiI5Br converted using the Kubelka–Munk function (α/s = (1 – R)2/2R) and (b) the variation of band gap with bromine content for MXDBiI5–Br. The x = 5 sample has a different structure type.

Table 4

Bromine Content in MXDBiI5–Br (Determined from EDS) and Corresponding Band Gap of MXDBiI5–Br Samples

Bromine Content (x) in MXDBiI5-xBrx	Band Gap (eV)
0.00	2.15
0.27	2.16
0.43	2.17
0.89	2.22
1.69	2.27
2.71	2.36
3.71	2.48
MXD3Bi2Br12·2H2O (x = 5)	2.86

Conclusions

Here, we have reported the synthesis and characterization of three new organic–inorganic metal halides, using the meta-xylylenediammonium (MXD) cation: MXDBiI5, MXD3Bi2Br12·2H2O, and MXD2PbI6. MXDBiI5 has short intercluster I–I distances and π–π stacking between the MXD cations, while also possessing the lowest band gap of the materials studied here. We explored the tunability of MXDBiI5 through halide substitution and found that a large region of solid solution exists in the MXDBiI5–Br system (where x = 0 to 3.71). This work highlights the fact that zero-dimensional organic–inorganic halides are also highly tunable semiconductors and opens up the way for further studies of the physics and the long-term stability of these materials.