Introduction

Organic-inorganic hybrid perovskite MAPbX3 (MA: methylammonium; X: halide) materials are potential candidates for use in optoelectronic devices1, including lasers2, photodetectors3, photosensitive transistors4, and light-emitting devices (LEDs)1. The power conversion efficiencies (PCEs) of organic-inorganic hybrid perovskite solar cells (PSCs) have increased from 3%5 to 23.6%6 over the past few years due to their unique features, such as broad and strong light absorption7, longer carrier lifetime8, long charge-carrier diffusion length9,10, high carrier mobility11 and small exciton binding energy12. At the same time, one attractive feature of hybrid perovskites as photovoltaic absorbers is that their bandgap (Eg) can be tuned continuously in several ways, such as substituting the central organic molecule MA with FA (formamidinium)13, replacing Pb with other metals (Sn or Ge)14, and alloying different halides into the structure.

In a few short years, the mixed-halide perovskite CH3NH3Pb(BrxI1−x)3 materials have been successfully produced via substitution of I with Br ions15,16,17,18,19,20,21, corresponding to a varying bandgap from 1.5 eV to 2.2 eV14. These properties make this class of material for use in multi-colour light-emitting16 or lasing22 applications and for the larger bandgap absorption in tandem solar cells23. Tu et al.24 modulated CH3NH3Pb(BrxI1−x)3 films, resulting in PCEs exceeding 18% at x = 0.14, which was a significant improvement of the photovoltaic performance. Additionally, the improvement of the open-circuit voltage of CH3NH3Pb(BrxI1−x)3 solar cells has a strong relationship with the Br-ion content. Although the bandgaps of perovskites can be tuned by introducing bromide ion, there have some unexpected effects also emerged, such as light-induced effects25,26,27,28. CH3NH3Pb(IxBr1−x)3 (0.1 < x < 0.8) materials undergo phase separation into iodide-rich and bromide-rich regions under prolonged illumination and revert to their original states after a few minutes in the dark, which leads to the formation of smaller-band-gap “trap states”25. Such de-mixing of the halides is detrimental to the photovoltaic performance because it leads to charge-carrier trapping in halide-rich regions, which decreases the open-circuit voltage with an increasing bromide content above 20%24. Unfortunately, many mixed-halide perovskite used in solar cells have rarely been tested for their stability regarding phase segregation.

And also to the best of our knowledge, the most common method employed to fabricate mixed-halide CH3NH3Pb(BrxI1−x)3 materials is to spin-coat solutions of stoichiometric Pb2+ and the halides (I− + Br−) on a suitable substrate, followed by annealing4,16,25. However, this method requires many kinds of organic solvents, long reaction times, or careful adjustment of the reaction conditions, etc. To date, a facile and rapid method is still needed to synthesize mixed-halide perovskite materials. Facile solvothermal methods have been considered as most promising routes ascribed to their advantages of low temperature, a single-step process, and high reproducibility29. Zhao et al.30 also demonstrated a facile synthetic approach for preparing mixed-halide perovskite CH3NH3Pb(Br1−xClx)3 crystals by solvothermal growth. However, CH3NH3Pb(BrxI1−x)3 crystals synthesized by a solvothermal method have not been investigated to date.

In this study, CH3NH3Pb(BrxI1−x)3 (0 ≤ x ≤ 1) crystals in the full composition range were synthesized for the first time by a facile solvothermal method. We used X-ray diffraction (XRD) and photoluminescence (PL) measurements to detect the transformation of the phase structure and the variations of the optical properties in the CH3NH3Pb(BrxI1−x)3 (0 ≤ x ≤ 1) system. At same time, PL was applied to study the photoinduced phase separation in the mixed-halide perovskite CH3NH3Pb(BrxI1−x)3 crystals. Therefore, more studies are required to understand the fundamental properties in mixed-halide perovskites CH3NH3Pb(BrxI1−x)3, which will be very important in optimizing related optoelectronic devices.

Results and Discussion

To analyse the phase structure transformation of the mixed-halide perovskite crystals with increasing VHBr% from 0 to 100%, the XRD patterns of CH3NH3Pb(BrxI1−x)3 crystals are shown in Fig. 1(a), and the magnified patterns in 2θ = 27.5 −31° are shown in Fig. 1(b). According to the XRD patterns in Fig. 1(a), the CH3NH3PbI3 and CH3NH3PbBr3 crystals with VHBr% for 0% and 100% have a tetragonal phase structure with the I4/mcm space group and a cubic structure phase with the \(Pm\bar{3}m\) space group, respectively, which agree with previous reports20. The diffraction peaks show a systematic shift to higher scattering angles with increasing VHBr%, which indicates a decrease of the unit cell size with increasing bromine content. Because the gradual substitution of the larger I atoms with the smaller Br atoms decreases the lattice spacing. The XRD peaks in Fig. 1(a) are also relatively sharp and no peaks of impurities were detected, thus these materials are good crystals and high purities.

Figure 1
figure 1

(a) The XRD patterns of CH3NH3Pb(BrxI1−x)3 obtained with VHBr% for 0%, 10%, 20%, 40%, 50%, 60%, 80%, and 100%, (b) The XRD patterns of CH3NH3Pb(BrxI1−x)3 magnified in 2θ from 27.5° to 31° (the subscript c is defined as cubic phase, and the subscript t is defined as tetragonal phase).

Full size image

CH3NH3PbI3 (VHBr% = 0%) has two peaks located at 28.15° and 28.44°, as shown in Fig. 1(b), which are indexed to (004) and (220) planes of the tetragonal phase. The (004) diffraction peak gradually disappears and finally merges into a single peak upon increasing VHBr% above 20%, corresponding to the (200) plane of the cubic phase, which confirms that the symmetry of phase structure improve. Further substitution of I ion with Br ions into the tetragonal phase of CH3NH3PbI3 causes the systematic shift of the (200) peak towards higher scattering angle. In other words, the tetragonal (pseudo-cubic) phase can transform into the cubic phase with increasing VHBr%. It ascribe to that the smaller halide ion radius is favourable for the formation of the cubic structure, which is accepted as a criterion for the distortion of the PbX6 octahedra16.

The lattice parameter a of the CH3NH3Pb(BrxI1−x)3 crystals indexed by pseudo-cubic or cubic symmetry as a function of VHBr% is shown in Fig. 2. The a gradually decreases from 8.89 Å to 5.94 Å with increasing VHBr%, which confirms that the lattice spacing decreases with increasing Br ions. Moreover, the slope displays an obviously abrupt change from the tetragonal to cubic phase upon increasing VHBr% from 10% to 20%. The a of CH3NH3Pb(BrxI1−x)3 exhibits a linear relationship above 20% of VHBr%, as shown in Fig. 2. This linear trend satisfies Vegard’s Law31,32, and thus the lattice parameter changes linearly with composition of the perovskite. In general, single-phase mixed-halide perovskite CH3NH3Pb(BrxI1−x)3 crystals were synthesized by the facile solvothermal method.

Figure 2
figure 2

The relationship between the lattice parameter a of the CH3NH3Pb(BrxI1−x)3 crystals and VHBr%.

Full size image

To further analyse the morphologies and compositions of the CH3NH3Pb(BrxI1−x)3 crystals, SEM images of crystals with VHBr% for 0%, 50%, and 100% and the corresponding EDS spectra are shown in Fig. 3. Furthermore, the morphologies of crystals with others VHBr% for 20%, 40%, 60%, and 80% are shown in Fig. S1. The SEM images show that all of the as-synthesized crystals retain cuboid shapes. Length of side of the cuboid shapes become gradually shorten from 3–5 μm to 1–3 μm with increasing the VHBr%, which implies the crystal structure variation. The EDS spectra with VHBr% for 0%, 50%, and 100% show that the composition ratio of I + Br to Pb is 2.796, 2.98, and 2.719, respectively, which are slightly different from the previous reports12, because the iodine or bromine atoms can possibly escape and metallic Pb can separate from the perovskite crystal under the experimental conditions of EDS33. The composition ratio of Br to I + Br is 50% in Fig. 3(b), which indicates that VHBr% agree well with the predicted × (the composition ratio of Br to I + Br).

Figure 3
figure 3

The SEM images of CH3NH3Pb(BrxI1−x)3 crystals obtained at 150 °C for 4 h with VHBr% = 0% (a), 50% (b), 100% (c) and the corresponding EDS spectra.

Full size image

To further confirm the compositions of CH3NH3Pb(BrxI1−x)3 crystals with VHBr% from 0 to 100%, we measured the XPS spectra of the crystals, the XPS full scan spectra and detailed spectra of Pb 4f, I 3d and Br 3d are provided in Fig. S2. The average compositions were calculated using the XPS peak areas of I 3d, Br 3d, the area of I 3d shorten and Br 3d increase, which confirm that the composition percent of I lacked and Br increased depending on volume ratio of HBr. Table 1 shows the elemental composition in CH3NH3Pb(BrxI1−X)3 crystals with increasing VHBr% from 0 to 100%, the composition ratio of Br to I + Br increased from 0 to 1. The results again show that the VHBr% is nearly equal to the composition ratio of Br to I + Br, that is x. It should be noted that this is the first demonstration of the synthesis of mixed-halide perovskite CH3NH3Pb(BrxI1−x)3 crystals in the entire composition range via solvothermal method.

Table 1 Elemental composition in CH3NH3Pb(BrxI1−X)3 crystals with VHBr%.
Full size table

The varied of composition could be influenced the band gap or optical properties of CH3NH3Pb(BrxI1−x)3 crystals, we measured the PL spectra of crystals with increasing VHBr%, as shown in Fig. 4(a). The PL peaks of the crystals for pure CH3NH3PbI3 and CH3NH3PbBr3 with VHBr% for 0% and 100% point at 768.09 nm and 548.82 nm, respectively, corresponding to bandgaps (Eg) of 1.61 eV and 2.26 eV. A systematic shift of the PL spectra for CH3NH3Pb(BrxI1−x)3 to shorter wavelengths was observed with increasing VHBr%, which declare that the Eg can be tuned from 1.61 eV to 2.26 eV by adjusting the halide content confirming in Table 1. And the colours of the crystals also change correspondingly from dark brown for CH3NH3PbI3 to brown-red for CH3NH3Pb(BrxI1−x)3 and then to yellow for CH3NH3PbBr3 upon increasing the Br ions, as shown in Fig. S3. Furthermore, the PL spectrum of the crystals at 50% VHBr% shows two emission peaks in Fig. 4(a), which imply that the crystal possibly comprises two phases25.

Figure 4
figure 4

(a) The PL spectra of CH3NH3Pb(BrxI1−x)3 crystals under a low illumination power of 3 mW with VHBr% from 0% to 100%; (b) the relationship between the band gaps (Eg) of CH3NH3Pb(BrxI1−x)3 and VHBr%.

Full size image

The Eg variation with VHBr% in CH3NH3Pb(BrxI1−x)3 is plotted in Fig. 4(b). According to previous studies16, the Eg nonlinear variation with the composition x (VHBr%) in the alloy can be expressed by the following quadratic equation (eq. 1):

$$\begin{array}{rcl}{{\rm{E}}}_{{\rm{g}}}{[\text{CH}}_{3}{{\rm{NH}}}_{3}{\rm{Pb}}{({{\rm{Br}}}_{{\rm{x}}}{{\rm{I}}}_{1-{\rm{x}}})}_{3}] & = & {{\rm{E}}}_{{\rm{g}}}{[\text{CH}}_{3}{{\rm{NH}}}_{3}{{\rm{PbI}}}_{3}]+{({\rm{E}}}_{{\rm{g}}}{[\text{CH}}_{3}{{\rm{NH}}}_{3}{{\rm{PbBr}}}_{3}]\\ & & -{{\rm{E}}}_{{\rm{g}}}{[\text{CH}}_{3}{{\rm{NH}}}_{3}{{\rm{PbI}}}_{3}]-{\rm{b}}){\rm{x}}+{{\rm{bx}}}^{2}\end{array}$$
(1)

where b is the bowing parameter, which depends on the properties of the inter-substitutional atoms34. The bowing parameter illustrates the fluctuation degree in the crystal field or the nonlinear effect arising from the anisotropic nature of binding35. A least-squares fit (red line) of the Eg in Fig. 4(b) yields bowing parameter of b = 0.53 eV, resulting in eq. 2.

$${{\rm{E}}}_{{\rm{g}}}(x)=1.61+0.12\,{\rm{x}}+0.53\,{{\rm{x}}}^{2}$$
(2)

The experimental values of Eg agree well with values of the least-squares fit below 50%, as shown in Fig. 4(b), which shows that CH3NH3PbI3 and CH3NH3PbBr3 have good miscibility. Above 50%, CH3NH3PbBr3 is predominant in growth of the crystals, which confirms in Table S1.

To further understand the origin of the PL feature, Fig. 5 shows the PL spectra of the crystals with VHBr% for 40%, 50%, and 60% under sequential illumination. Initially, the perovskite with 40% displays an emission peak at 728.2 nm (1.7 eV). With continuous illumination from 15 s to 240 s, a new higher-energy peak forms at 534.6 nm (2.32 eV) and grows in intensity, as shown in Fig. 5(a), which indicates that small clusters enriched in bromine emerge in the crystals and act as recombination centres. Moreover, we should point out that the position of this new peak is independent of the halide composition and bandgap.

Figure 5
figure 5

The PL spectra of CH3NH3Pb(BrxI1−x)3 obtained with VHBr% for 40% (a), 50% (b), and 60% (c) after different light-soaking times excited at 3 mW with 375 nm. The inset shows the time-resolved PL (TRPL) dynamics of the two peaks.

Full size image

Similarly, for crystals with 50%, we found that the emission peak at 549 nm grows greatly in intensity with prolonged illumination, as shown in Fig. 5(b), which ascribe to enriched bromine content. In addition, the emission from the Br-rich region is significantly weaker than that of the iodide-rich region, as shown in Fig. 5(a,b), because charge carriers caused by illumination are quickly transported and accumulated at the iodide-rich region26.

When increasing the VHBr% to 60%, however, the peak at 547.2 nm decays, and another peak at 716.1 nm grows with prolonged illumination, as shown in Fig. 5(c). The increase in intensity of the lower-band-gap PL peak suggests that these iodide-enriched regions (defects), which act as recombination centre traps, have higher luminescence efficiency than the rest of the perovskite crystals. In other words, I-rich regions serve as the primary charge-carrier recombination sites or irrespective of the carrier generation site in the mixed-halide systems. Similar arguments of charge transfer between Br-rich and I-rich regions as well as trap-initiated recombination have been proposed in earlier studies25.

According to reports by Slotcavage36 and the experimental results, we speculated that halide phase separation of CH3NH3Pb(BrxI1−x)3 in small halide-enriched domains is induced by sequential illumination, and the localized strain further promotes halide phase separation, which based on halide migration and possibly caused by photo-excited charge interactions. And the halide migration in perovskites is thought to occur through halogen vacancies37. This instability may limit the achievable voltages resulting in degraded performances of related photovoltaic devices.

It is again confirmed that halide phase separation occurs in the time-resolved photoluminescence (TRPL) measurements, as shown in the inset in Fig. 5. The TRPL spectra show that the higher-energy band (red line) decays more rapidly than the lower-energy band (black line), which indicates that the initially formed mixed-halide perovskite, with 40 to 60% bromide content, are comprised of two species or two phases. To testify the range of VHBr% for photoinduced phase separation, we have synthesized CH3NH3Pb(BrxI1−x)3 crystals with different VHBr% and treated them with continuous illumination. The results demonstrate that the crystals display phase separation with VHBr% from 40% to 60%, as shown in Fig. 5, the crystals with VHBr% = 20% and 80% did not display phase separation, as shown in Fig. S4. Moreover, we should state here that the photoinduced change of PL spectra never return to original status after keeping sample for several hours in dark at room temperature, which is inconsistent with previous reports25,36,38. It ascribe to that the crystals have the larger crystallite size and higher crystalline quality, which reduce ion migration while enhancing the stability of perovskite materials39.

Furthermore, the above experimental observations in the optoelectronic properties with various halide contents provide insight into the tuneability of mixed-halide perovskite. To clarify the relationship between the PL feature and the band structure, in this work, we using first-principle calculations study the band structure under variable doped composition conditions, based on experimental lattice parameters. Considering the composition of the unit cell, we focused on the pseudo-cubic phase of the mixed-halide materials of CH3NH3Pb(BrxI1−x)3 with x = 0.333 and 0.667, and used the band gap approximation of them to fit the result of band gap of CH3NH3Pb(BrxI1−x)3 crystals with VHBr% for 40% and 60%40,41. Figures 6 and S5 illustrate the results of the band structure, partial density of states (PDOS) and total density of states (DOS) for the CH3NH3Pb(BrxI1−x)3 with x = 0.333 and 0.667. It is found that the valence band maximum (VBM) originate mainly from the strong interaction of the Br-4p, I-5p, Pb-6s and Pb-6p states, the conduction band minimum (CBM) is mainly composed of Pb 6p states for x = 0.333 and 0.667. Moreover, the addition of Br introduces Br 4p states in the VBM whereas the VBM mixed contribution from I 5p and Br 4p in x = 0.333 and 0.667. And the Pb 6p contribution at CBM is unchanged at all systems. The results also show that the Eg of the bromine doped by 33.3% and 66.7% are 1.53 eV and 1.61 eV in Fig. 6, respectively. However, the range of band gap of the Br doped by 33.3% and 66.7% are 1.53–3.22 eV and 1.61–3.60 eV. Thus, they can be interpreting the PL patterns of the bromine doped by 40% and 60% corresponding with Eg values (1.7~2.32 eV and 1.73~2.26 eV, respectively), as shown in Fig. 5(a,c).

Figure 6
figure 6

Electronic band structures of CH3NH3Pb(I1−xBrx)3 with (a) x = 0.333 and (b) 0.667, Zero energy is set at the top of the valence bands.

Full size image

In summary, The mixed-halide hybrid perovskite CH3NH3Pb(BrxI1−x)3 (0 ≤ x ≤ 1) crystals have been systematically synthesized by a solvothermal method through adjusting concentration of Br ions. The XRD indicated that the crystalline structure transitioned from the tetragonal phase to the cubic phase with the introduction of Br ions, and the crystals have higher crystallinities and purities. The SEM showed that all the as-synthesized crystals retain cuboid shapes. Furthermore, PL peaks of the CH3NH3Pb(BrxI1−x)3 crystals could be tuned from 768 nm to 549 nm, corresponding to a variation in the bandgap from 1.61 eV to 2.26 eV. Moreover, CH3NH3PbI3 and CH3NH3PbBr3 had good miscibility below 50% about VHBr%. Notably, CH3NH3Pb(BrxI1−x)3 (0.4 ≤ x ≤ 0.6) crystals obviously appear phase separation by prolonged illumination due to the formation of small clusters enriched with lower-band-gap, iodide-rich and higher-band-gap, bromide-rich domains, which induced localized strain to promote halide phase separation. In addition, the electronic band structures of the crystals were used to explain many of peaks in the PL patterns with VHBr% about 40% and 60%. Meanwhile, modifying the perovskite morphology and crystallinity greatly improved the stability.

Experiments Methods

Synthesis

All chemical reagents (analytical grade) were directly used without further purification and were supplied by Sigma-Aldrich.

The similar experimental process used here had been reported in our previous work33. Pb(Ac)2·3H2O (60 mg, Ac− = CH3COO−, 99.9%) was completely dissolved in a mixed solution of hydroiodic acid (HI, 45% in water) and hydrobromic acid (HBr, 40% in water). Then, 30 mL of isopropanol (IPA, 99.9%) was added and stirred for 5 min, and 0.3 mL of a methylamine solution (CH3NH2, 30% in water) was added dropwise. The mixture was further stirred for 5 min and then put into 50 mL stainless steel Teflon-lined autoclave, and was sealed and heated in furnace at 150 °C for 4 h, after cooling naturally to room temperature. The precipitates were collected and washed with isopropanol by centrifugation at room temperature, and then were dried under vacuum at 60 °C for 4 h. We mixed the solution in various volume ratios of HI and HBr, and VHBr% is defined as the ratio of VHBr:VHI+HBr (the total volume of HI and HBr is 1 mL).

Characterization

The structures of the products were investigated by X-ray diffraction (XRD, X’TRA) using Cu Kα radiation (λ = 0.1542 nm). The X-ray tube voltage and current were set at 40 kV and 40 mA, respectively. The morphologies and elemental analyses of the products were observed by field-emission scanning electron microscopy (FE-SEM, JSM-7000 F) in energy-dispersive spectroscopy (EDS) mode. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe) was used to identify the elemental compositions of the products, and the resolution of the spectrometer was chosen to be 0.6 eV with a pass energy setting of 40 eV. The photoluminescence (PL) spectra of the products were recorded on a HORIBA iHR 320 fluorescence spectrophotometer with an excitation wavelength of 375 nm at room temperature. The 375 nm line of a picosecond pulsed laser diode (PicoQuant PDL 800-D) was used as the excitation light source for time-resolved PL measurements, and the PL decays were recorded by a time-correlated single-photon counting module and a picosecond event timer (PicoHarp 300)

Computational Methods

The band structures of the pseudo-cubic phase CH3NH3Pb(BrxI1−x)3 are calculated within the framework of density functional theory by using the CASTEP package. Norm-conserving pseudopotentials and Perdew-Burke-Ernzerhof (PBE) functional with the generalized gradient approximation (GGA) were used to model the electron-ion interactions and exchange-correlation potential, respectively42,43. We focused on the pseudo-cubic phase of the mixed-halide materials of CH3NH3Pb(BrxI1−x)3 with x = 0.333 and 0.667. The high cutoff energy for the plane-wave basis is set at 750 eV and the Brillouin zone is sampled by a 5 × 5 × 5 k-point sampling grid. The convergence tolerance of maximum force, maximum displacement and energy were 0.01 eV/Å, 5.0 × 10−4 Å and 5.0 × 10−6 eV/atom, respectively. These parameters were controlled to ensure convergence40.