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Improving the Photoluminescence Properties of Perovskite CH3NH3PbBr3xClx Films by Modulating Organic Cation and Chlorine Concentrations Jun Yan, Bing Zhang, Yunlin Chen, Ao Zhang, and Xiaohan Ke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01303 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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ACS Applied Materials & Interfaces

Improving the Photoluminescence Properties of Perovskite CH3NH3PbBr3-xClx Films by Modulating Organic Cation and Chlorine Concentrations Jun Yan*，Bing Zhang，Yunlin Chen*，Ao Zhang, Xiaohan Ke Institute of Applied Micro-Nano Materials, School of Science, Beijing Jiaotong University, Beijing 100044, People’s Republic of China ABSTRACT The photoluminescence (PL) properties of inorganic-organic perovskites can be drastically changed by tuning the halogen composition, especially the Cl content. However, our research demonstrated that in addition to the influence of Cl concentration, the PL emission intensity of CH3NH3PbBr3 strongly depends on the content of CH3NH3Br in the coating solution. The effects of CH3NH3Br and Cl concentrations on the PL properties of CH3NH3PbBr3-xClx were investigated. We found that a strong PL emission intensity of CH3NH3PbBr3 can be obtained from solutions with a high CH3NH3Br concentration. The PL emission intensities of CH3NH3PbBr3-xClx films were enhanced by adjusting the molar ratio of PbBr to PbCl2 only in a highly concentrated CH3NH3Br environment. Moreover, it was found that an optimum CH3NH3Br/PbBr2/PbCl2 ratio in the precursor solutions can be used to obtain the strongest PL emission intensity of CH3NH3PbBr3-xClx films. Further studies revealed that both CH3NH3Br and Cl concentrations significantly influence the CH3NH3PbBr3-xClx films evolution, which affects their PL properties. KEYWORDS: photoluminescence, perovskite, emission intensity, CH3NH3Br content,

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Cl concentration ■ INTRODUCTION Organometallic halide perovskite solar cells have rapidly developed, and their power conversion efficiencies (PCEs) have surpassed 20 %.1-3 In addition to solar cells, the inorganic-organic hybrid perovskite materials have been extensively studied for use in light-emitting devices.4-9 The reported photoluminescence (PL) properties of the hybrid perovskite materials are closely related to their inorganic component. 10-12

The emission color of perovskite light-emitting devices (PeLEDs) can be tuned

from the near-infrared to green by modulating the halide composition in the perovskite.4,

12, 13

In particular, Cl incorporation influences the morphology of

perovskite films, which improves their optoelectronic characteristics. 14, 15 Despite the chloride ions in the reaction mixture, researchers reported that it is difficult to detect the trace of Cl in the final perovskite films. 16, 17, 18 The role of Cl in the morphological evolution of perovskite thin films is still a fundamental question.15-20 Zhang et al. 21 reported that tuning the Cl/Br ratio changed the PL emission intensities of MAPbBr3-xClx (MA=CH3NH3). In addition to Cl, CH3NH3+ also has a remarkable effect on the morphology and photoelectric properties of perovskite films. Yu et al. 19 reported that the introduction of a CH3NH3+-rich environment slows down the perovskite formation process and improves the growth of the crystal. Im et al.

22

reported that the crystal size of MAPbI3 strongly depends on the concentration of MAI, which influenced the photovoltaic performance. Motta et al. 23 calculated the effect of the orientation of the organic molecules on the electronic properties of MAPbI3. They


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suggested that the organic molecular rotations would lead to a transition from a direct to an indirect band gap in MAPbI3. Our studies found that modulating the molar ratios of organic to inorganic content in MAPbI3 and MAPbBr3 significantly influenced their optical properties.24, 25 In this work, to clarify the role of Cl and CH3NH3+ in the perovskite films, the influence of MABr and Cl concentrations on the PL properties and morphology of MAPbBr3 and MAPbBr3-xClx were investigated. The effects of MABr and Cl concentrations on the PL properties of MAPbBr3-xClx were decoupled. Our research may be helpful for achieving precise control over the formation of perovskite films to improve their light-emitting performance. ■ EXPERIMENTAL SECTION

The MAPbBr3 and MAPbBr3-xClx films were fabricated by a single-step spin-coating method. The MAPbBr3 solutions were made by dissolving 3:1, 2:1 and 1:1 molar ratios of MABr and PbBr2 in N, N-dimethylformamide (DMF). For MAPbBr3-xClx, the precursor solutions consisted of MABr, PbBr2 and PbCl2, with MABr/PbBr2/PbCl2 molar ratios of 2:0.8:0.2, 2:0.5:0.5, 2:0.2:0.8, 1.5:0.8:0.2, 1.5:0.5:0.5, 1.5:0.2:0.8, 1:0.8:0.2, 1:0.5:0.5 and 1:0.2:0.8. These precursor solutions were stirred at room temperature for 30 min in a nitrogen environment. The MAPbBr3 and MAPbBr3-xClx films were fabricated on pre-cleaned glass substrates by spin-coating the precursor solutions at 5000 r.p.m. for 30 s in a nitrogen filled glovebox. Finally the compositions of the thin films are not same as that in the precursor solution. However, it is difficult to determine the real compositions of these thin films. Hence, we study the effect of changing the precursor solution on the



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properties of MAPbBr3 thin films. For all thin films, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the crystal structure and morphology, respectively. Sample thicknesses were measured using an Ambios Technology XP-2 stylus profilometer. The absorption spectra of MAPbBr3 and MAPbBr3-xClx

films

were

measured

using

a

UV-VIS

spectrometer.

The

photoluminescence (PL) emission spectra of MAPbBr3 and MAPbBr3-xClx films were measured by a photoluminescence system in reflection. The voltage of photomultiplier was 750 V. The PL emission spectra of glass substrates were measured as the background data. To eliminate the noise, we deducted the background of PL spectra of all the thin films. The time-resolved PL spectra of MAPbBr3 and MAPbBr3-xClx films were measured by Time-resolved Fluorescence Spectrometers F900 (excitation wavelength: 485 nm, powder density: 2.4 mW/cm2). ■ RESULTS AND DISCUSSION

The MAPbBr3 and MAPbBr3-xClx films were prepared with different molar ratios of organic to inorganic content and Cl to Br. Table 1 shows the sample numbers, lattice constants and optical band gaps of all of the films. The samples were divided into four groups. For group Ⅰ, the spin-coating solutions of No. 1-3 consisted of different amounts of MABr (0.8 M to 0.4 M) and 0.4 M PbBr2. As a result, the ratio of MABr to PbBr2 was varied from 2:1 to 1:1. For group Ⅱ, the precursor solutions of No. 4-6 were composed of 0.8 M MABr and different amounts of PbBr2 (0.32 M to 0.08 M) and PbCl2 (0.08 M to 0.32 M). Therefore, the MABr/PbBr2/PbCl2 ratio was


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changed from 2:0.8:0.2 to 2:0.2:0.8. For group Ⅲ and Ⅳ, the concentrations of PbBr2 and PbCl2 were changed in exactly the same way as was done in group Ⅱ; however, the MABr concentrations in group Ⅲ and Ⅳ are 0.6 M and 0.4 M, respectively. Therefore, the MABr/PbBr2/PbCl2 ratios of No. 7-9 (group Ⅲ) and No. 10-12 (group Ⅳ) were altered from 1.5:0.8:0.2 to 1.5:0.2:0.8 and 1.0:0.8:0.2 to 1.0:0.2:0.8, respectively. Figure 1 shows the XRD patterns of MAPbBr3 and MAPbBr3-xClx thin films. All of the samples show similar XRD peaks, which correspond to the cubic perovskite structure. The diffraction peaks along (100), (200), (211) and (400), as shown in Figure 1, belong to the MAPbBr3 phase, and no impurity peaks can be found in these samples. We didn’t find any impurity phases in these thin films even in high MABr and Cl concentration. Therefore, all the samples were not annealed. The peaks intensities of No. 2 and No. 3 are much larger than that of No. 1. Therefore, the vertical scales of No. 2 and No. 3 are different from that of No. 1 and there is a broken scale in Figure 1 (a). Figure S1 (a) shows the XRD peaks (100) and (200) of No.1 and No.3. The angles of (100) and (200) peaks of No.3 are larger than that of No.1, which indicates that the lattice constant of No.3 is smaller than that of No.1. Our previous study found that the lattice constant of MAPbBr3 increases with increasing MABr concentration.25 The big differences of XRD peaks intensities between No.1 and No.3 are related to the preferred orientation and the variation of crystal size. As the MABr concentration decreases, the preferred orientation of (100) becomes more obvious, and the grain size of MAPbBr3 films increases, as shown in Figure 5. Baikie et al.

26

reported that the lattice parameters of MAPbBr3 and MAPbCl3 powders are 5.93129



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(4) Å and 5.68415 (6) Å, respectively. However, the lattice parameter of thin film is different from that of powder. The growth of grains of thin films will be influenced by different substrates and sometimes have the preferred orientation. Usually, compared with powders, there are not enough X-ray diffraction peaks which can be refined to get accurate lattice parameters for thin films. For example, researchers reported that the lattice constants of MAPbBr3 and MAPbBr1.8Cl1.2 thin films are 5.91 Å and 5.79 Å, respectively. 21 Our researches show that without Cl doping, the lattice parameters of MAPbBr3 films are around 5.93 Å. For No. 10-12, the lattice constant of Cl-rich thin film (No. 12) is 5.80 Å, which is close to the reported value.21 According to Table 1, if the MABr concentration remains the same, then (except for No. 4) increasing the ratio of Cl in the mixed halides gradually decreases the lattice constant of the compounds. The XRD peak (100) shifts towards a larger angle with increasing the Cl doping ratio, as shown in Figure S1 (c)-(f). The changes in lattice constant suggest that the Cl concentration has an impact on the original crystal structure of MAPbBr3. The smaller lattice constant of the thin films with increasing the Cl concentration may be a hint at the development of a MAPbCl3 phase. However, we didn’t find any visible peak splitting at higher orders. Researchers have found that both MAI concentration and Cl content can influence the film growth of MAPbI3-xClx.15,

22

Therefore, we speculate that the abnormal deviation in the lattice constant of No. 4 may be related to a synergistic effect between the MABr concentration and the Cl content in MAPbBr3-xClx. Increasing the MABr concentration will increase the lattice constant of MAPbBr3. Introduction of Cl at high MABr concentrations could inhibit


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aggregation and improve the film quality (details will be discussed later), which may enhance the effect of MABr concentration on the lattice constant of MAPbBr3. The data in Figure 1 clearly show that all of the films have a preferred orientation along the (100) direction, no matter what effect the doping with Cl has on the crystal growth. Table 1 Sample numbers, MABr/PbBr2 and MABr/PbBr2/PbCl2 molar ratios in the precursor solutions, lattice constants and optical band gaps of MAPbBr3 and MAPbBr3-xClx films Group

Specimen

MAPbBr3

Lattice

Optical band

No.

No.

MABr/PbBr2 molar ratio

constant (nm)

gap (eV)

Ⅰ

1

2:1

5.93

2.31

2

1.5:1

5.93

1.83

3

1:1

5.92

1.2

MAPbBr3-xClx MABr/PbBr2/PbCl2 molar ratio Ⅱ

Ⅲ

Ⅳ

4

2:0.8:0.2

5.96

2.25

5

2:0.5:0.5

5.91

2.34

6

2:0.2:0.8

5.91

2.45

7

1.5:0.8:0.2

5.90

2.08

8

1.5:0.5:0.5

5.87

2.3

9

1.5:0.2:0.8

5.83

2.34

10

1:0.8:0.2

5.90

1.57



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11

1:0.5:0.5

5.85

1.54

12

1:0.2:0.8

5.80

2.5

Figure 1 X-ray diffraction patterns of MAPbBr3 and MAPbBr3-xClx. (a) MAPbBr3: Group Ⅰ , No. 1-3 (MABr/PbBr2 molar ratio changes from 2:1 to 1:1) (b) MAPbBr3-xClx: Group Ⅱ, No. 4-6 (MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.2:0.8) (c) MAPbBr3-xClx: Group Ⅲ, No. 7-9 (MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.2:0.8) (d) MAPbBr3-xClx: Group Ⅳ, No. 10-12 (MABr/PbBr2/PbCl2 molar ratio changes from 1:0.8:0.2 to 1:0.2:0.8). The photoluminescence (PL) emission spectra of MAPbBr3 and MAPbBr3-xClx are shown in Figure 2. All of the samples, which were prepared and tested under the same conditions, were measured at room temperature with an excitation wavelength of 400 nm. The emission peak position of each sample was labeled in Figure 2. Both the MABr and Cl concentrations can influence the PL intensity of MAPbBr3-xClx;


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however, the key reason for this is the unknown. To uncover this mystery, we investigated the PL properties of MAPbBr3 and MAPbBr3-xClx that were prepared with different MABr concentrations and Cl content. For group Ⅰ, the PL peak wavelengths of No. 1-3 are 536 nm. The PL emission intensity of MAPbBr3 decreases with decreasing the MABr concentration, as shown in Figure 2 (a), which is consistent with a previous study.25 Besides No. 1-3, we also study the influence of other MABr concentrations on the PL intensity of MAPbBr3. Figure 3 displays the relationship between the molar ratio of MABr to PbBr2 and the PL intensity of MAPbBr3. When the ratio is less than 2:1, an exponential function can be used to fit the data, suggesting the PL intensity of MAPbBr3 strongly correlates with the MABr concentration. The maximum PL intensity of MAPbBr3 can be obtained at the ratio of MABr to PbBr2=2:1. Further increasing the MABr concentration decreases the PL intensity of MAPbBr3. To distinguish the effect of Cl on the optical properties of MAPbBr3 films from that of MABr we induced Cl in No. 1, No. 2 and No. 3 whose molar ratio of MABr to PbBr2 are 2:1, 1.5:1 and 1:1, respectively. Figure 2 (b) shows the PL emission spectra of samples in group Ⅱ (MABr concentration=0.8 M). The MABr/PbBr2/PbCl2 ratio of No. 4-6 was changed from 2:0.8:0.2 to 2:0.2:0.8. The emission peaks of No. 4-6 shift to shorter wavelengths (528-492 nm) and their intensities gradually decrease with increasing Cl content. Researchers have reported that compared with MAPbBr3-xClx, the PL emission peaks of MAPbBr3 were almost negligible. 21 However, our research shows that MAPbBr3 prepared in a MABr rich environment (No. 1) exhibits a strong PL emission intensity. The PL intensity of No. 4



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is stronger than that of No. 1 (dashed line in Figure 2 (b)), but it is drastically reduced by further increasing the Cl content (No. 5 and No. 6). To further understand the role of MABr and Cl concentrations on the PL properties of MAPbBr3, we altered the concentrations of PbBr2 and PbCl2 in group Ⅲ and Ⅳ whose MABr concentrations are 0.6 M and 0.4 M, respectively. The PL emission peaks of the samples in group Ⅲ (No. 7-9) and Ⅳ(No. 10-12) are shown in Figure 2 (c) and (d), respectively. Compared with No. 2, whose MABr concentration is 0.6 M, the PL emission peaks of No. 7-9 shift to shorter wavelengths (526-487 nm) and their PL emission intensities gradually decrease with increasing Cl content. For No. 3 and No. 10-12, which possess the lowest MABr concentration (0.4 M), the PL emission intensities nearly disappear. These results clearly show that Cl concentration significantly changes the PL emission peak wavelength of MAPbBr3-xClx. However, the remarkable increase in the PL emissions of MAPbBr3 and MAPbBr3-xClx strongly correlates with the MABr concentration. The strong PL emissions among the samples in group Ⅰ-Ⅳ were associated with the MABr rich environment (No. 1 and No. 4). When the MABr concentration is reduced, doping with Cl does not enhance the PL emissions of MAPbBr3-xClx (group Ⅲ, Ⅳ). It is suggested that compared with the Cl concentration, the MABr content has a more potent effect on the PL emissions of MAPbBr3-xClx. To further clarify the influence of MABr rich environment on the PL properties of MAPbBr3 films, No.1 prepared with highest MABr concentration was annealed at 100

⁰C for 1 hour in vacuum. Figure S2 shows the XRD patterns and PL emission spectra of No.1 before and after heat treatment. After thermal treatment, the XRD peaks of


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No.1 change almost nothing. However, the PL intensity of No.1 annealed for 1 hour is much smaller than that of No.1 without heat treatment, as shown in Figure S2 (b). Annealing may release excess MA+ from MAPbBr3, which is not helpful for improving the PL intensities of MAPbBr3 films. The variation in PL properties of samples in group Ⅰ-Ⅳ can be visually observed in a darkroom. Figure 4 shows the emission colors of thin films in group Ⅰ-Ⅳ that were illuminated with UV light. The emission colors of No. 1-3 gradually change from bright chartreuse to little emission. This transformation in group Ⅰ is related only to the luminous intensities of MAPbBr3, as shown in Figure 4 (a), (e) and (i). For No. 4-6 (Figure 4 (b), (c) and (d)) and No. 7-9 (Figure 4 (f), (g) and (h)), the emission colors modulate from green to little emission with increasing Cl content. Although Cl concentration does tune the emission peak wavelength of the samples in group Ⅱ and Ⅲ, the blue emission is weak and barely visible. The extraordinarily bright emission colors of the thin films in group Ⅰ-Ⅳ (No. 1 and No. 4), as shown in Figure 4, were obtained from the highest MABr concentration (0.8 M). When the MABr concentration was reduced to 0.4 M, there is no visual emission from No. 3 (Figure 4 (i)) and No. 10-12 (Figure 4 (j), (k) and (l)). These results are consistent with those that are presented in Figure 2. It has been broadly observed that chlorine concentration has a remarkable effect on the morphological development of halide perovskites, and this effect strongly influences their optical properties.15-20 Therefore, we studied the influences of MABr concentration and Cl doping on the morphology of perovskite films.



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Figure 2 Steady state photoluminescence (PL) emission spectra of MAPbBr3 and MAPbBr3-xClx films with an excitation wavelength of 400 nm. (a) MAPbBr3: Group Ⅰ, No. 1-3 (MABr/PbBr2 molar ratio changes from 2:1 to 1:1) (b) MAPbBr3-xClx: Group Ⅱ, No. 4-6 (MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.2:0.8) (c) MAPbBr3-xClx: Group Ⅲ, No. 7-9 (MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.2:0.8) (d) MAPbBr3-xClx: Group Ⅳ, No. 10-12 (MABr/PbBr2/PbCl2 molar ratio changes from 1:0.8:0.2 to 1:0.2:0.8).

Figure 3 PL counts of MAPbBr3 versus molar ratio of MABr to PbBr2


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Figure 4 MAPbBr3 and MAPbBr3-xClx films illuminated with UV light in a darkroom. (a)-(d) No. 1: MABr/PbBr2 molar ratio is 2:1; No. 4-6: MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.2:0.8 (e)-(h) No. 2: MABr/PbBr2 molar is 1.5:1; No. 7-9: MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.2:0.8 (i)-(l) No. 3: MABr/PbBr2 molar is 1:1; No. 10-12: MABr/PbBr2/PbCl2 molar ratio changes from 1:0.8:0.2 to 1:0.2:0.8.



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Figure 5 The SEM images of MAPbBr3 and MAPbBr3-xClx films. (a)-(d) No. 1: MABr/PbBr2 molar ratio is 2:1; No. 4-6: MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.2:0.8 (e)-(h) No. 2: MABr/PbBr2 molar is 1.5:1; No. 7-9: MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.2:0.8 (i)-(l) No. 3: MABr/PbBr2 molar is 1:1; No. 10-12: MABr/PbBr2/PbCl2 molar ratio changes from 1:0.8:0.2 to 1:0.2:0.8. Previous studies reported that the formation mechanism and the role of ion concentration on perovskite thin films.27, 28 Here, we study the synergistic effect of MABr and Cl concentrations on the morphologies of MAPbBr3-xClx thin films. Figure 5 shows the morphologies of thin films in group Ⅰ-Ⅳ as was measured by SEM. We find that the MAPbBr3 crystal growth is strongly related to the MABr concentration. For No. 1 (0.8 M MABr), the SEM image (Figure 5 (a)) reveals a highly dense, non-uniform aggregation thin film. The average crystal size of No. 1 which is determined from Figure 5 (a) and Figure S2 is～0.4 µm. Compared with No.1, the SEM images of No. 2 (Figure 5 (e) and No. 3 (Figure 5 (i)) show cubic-shaped


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crystals. The average crystal size is determined to be ~0.75 µm for No. 2 (Figure 5 (e) and Figure S3) and ~1.49 µm for No. 3 (Figure S4). The thicknesses of these films which are measured by stylus profilometer are around 50 nm. This indicates that the growth rates of crystals along different directions are different. The different growth rates are related to the preferred orientation of MAPbBr3 films, as shown in Figure 1. The SEM studies of thin films in group Ⅰ indicate that the crystal size of MAPbBr3 increases as the MABr concentration decreases, but the spacing between individual crystals becomes bigger. The PL emission intensity of MAPbBr3 decreases with increasing the crystal size. A higher MABr concentration leads to smaller crystal sizes for MAPbBr3 and smaller spacing between the crystals. The small spacing indicates fewer voids and a compact film surface, which may play a key role in producing the strong PL emission of MAPbBr3. The variation of Cl concentration induces a morphological evolution in MAPbBr3-xClx films, which alters their PL properties. Figure 5 (b) shows the SEM image of No. 4. It is notable that the No. 4 film is continuous and uniform, and there is no aggregation on the surface. This indicates that the introduction of Cl at high MABr concentrations could inhibit aggregation and improve the film quality of No. 4. Therefore, No. 4 has a stronger PL emission intensity than No. 1. However, further increasing the Cl content, as shown in Figure 5 (c) (No. 5) and (e) (No. 6), reintroduces some scattered islands, and gaps between the islands can be observed in the films. It has been reported that a CH3NH3+ rich environment contributed to a slowdown in the perovskite formation process, and the Cl incorporation helps to remove excess CH3NH3+ at low annealing temperatures,



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which improved the film quality of MAPbI3-xClx.19 The effects of MABr and Cl concentrations on the morphology of MAPbBr3-xClx are similar to their effects on MAPbI3-xClx. These results suggest that an appropriate amount of Cl at high MABr concentrations can inhibit the formation of clusters and improve the quality of MAPbBr3-xClx films. However, introducing too much Cl lowers the CH3NH3+ concentration and produces voids between the crystals, which reduce the PL emission intensities of MAPbBr3-xClx films. Moreover, the PbBr2 framework provides nucleation sites that facilitate the growth of MAPbBr3 crystals，while substituting PbBr2 with PbCl2 gradually induces film reconstruction. Therefore, the clusters in Figure 5 (d) are different from those in Figure 5 (a). The introduction of chlorine to the precursor solutions with low MABr concentrations does not reduce the gaps between the crystals of the films, but it may lower the CH3NH3+ concentration in group Ⅲ and Ⅳ, as shown in Figure 5 (f)-(h) and Figure 5 (j)-(l). As a result, the PL emission intensities of thin films in group Ⅲ and Ⅳ decrease with increasing Cl content. Based on the above results, we can only obtain excellent PL emission intensity from MAPbBr3-xClx that is grown in a high MABr concentration environment, and the optimum MABr/PbBr2/PbCl2 ratio in the precursor solution is 2:0.8:0.2. In addition to the PL property and morphology, the substitution of Br with Cl also alters the light absorption spectra of MAPbBr3.


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Figure 6 UV-vis absorption spectra of MAPbBr3 and MAPbBr3-xClx films. (a) MAPbBr3: Group Ⅰ, No. 1-3 (MABr/PbBr2 molar ratio changes from 2:1 to 1:1) (b) MAPbBr3-xClx: Group Ⅱ, No. 4-6 (MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.2:0.8) (c) MAPbBr3-xClx: Group Ⅲ, No. 7-9 (MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.2:0.8) (d) MAPbBr3-xClx: Group Ⅳ, No. 10-12 (MABr/PbBr2/PbCl2 molar ratio changes from 1:0.8:0.2 to 1:0.2:0.8). Figure 6 shows the light absorption spectra of the films in group Ⅰ-Ⅳ. We estimated the optical band gaps for all of the samples from Tauc plots29; the band gaps are summarized in Table 1. Band gap was determined by plotting (αhv)2 against energy in eV (direct band gap) or (αhv)1/2 against energy in eV (indirect band gap), known as a Tauc plots29. For direct band gap sample, if the wavelength is smaller than the absorption band edge, the absorption profile will increase monotonically. If the band gap of sample is indirect, the absorption spectrum will show two apparent variations. The absorption profile of indirect band gap sample first shows small



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absorption amplitude and then a significantly more pronounced increase with decreasing the wavelength. However, based on the absorption spectra, we didn’t find an obvious optical band gap transition from direct to indirect for MAPbBr3-xClx thin films with changing the MABr or Cl concentration. Therefore, all the optical band gaps of thin films were estimated as direct band gaps. Figure S5-S8 show the Tauc plots of all the thin films. D’Innocenzo et al.

30

reported that the optical absorption

edge of MAPbI3 shifted to longer wavelengths (smaller optical band gap) with increasing the average crystallite size. For MAPbBr3, we got the similar results. The variation of crystallite size may also play an important role in adjusting the optical band gap of MAPbBr3. The absorption spectra of some samples such as No.2 and No.3 are different from other samples, which may stem from the variation of crystallite size and morphologies of thin films. For No. 1-3, as shown in Figure 6 (a) and Table 1, the band gaps and average crystal sizes change from 2.31 to 1.2 eV and 0.4 to 1.49 µm, respectively with decreased MABr concentration. However, the PL emission peaks wavelengths of No. 1-3 do not change by altering the MABr concentration. The optical band gap is smaller than the PL transition energy of No.2. We speculated that the PL intensity of MAPbBr3 may be closely related to the MA interstitial defects (MAi). Stoumpos et al. 31 reported that the pure and homogeneous MAPbI3 showed weak PL properties, even PL-inactive. They suggested that the defects might be the source of PL enhancement of MAPbI3. In our research, the high MABr concentration may induce MAi defect in MAPbBr3 thin films. Researchers reported that MAi defect has shallow transition energy level and low formation energy


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in MAPbI3 and MAPbBr3.32-34 Therefore, the MAi defect may act as radiative recombination center which enhance the PL intensity of MAPbBr3. The PL intensity of MAPbBr3 may depend on the number of MAi defects. The higher number of MAi defects in MAPbBr3, the higher is the ability to exhibit strong PL intensity. Hence, the strong PL intensity of MAPbBr3 exhibits in MA+ rich environment and the reduction of the PL intensity of No.1 after annealing may be related to the reduction in MAi defects. As the variation of PL properties and absorption spectra result from different reasons, the shift of absorption onset is different from those PL wavelengths of MAPbBr3 films. Further investigation on the difference between the PL emission peaks wavelengths and the tunable band gaps of No. 1-3 is currently underway. For No. 4-12, the substitution of Br with Cl leads to a blue-shift in the absorption onset, as shown in Figure 6 (b)-(d). The band gaps of No. 4-12 can be tuned from 2.25-2.45 eV for group Ⅱ, 2.08-2.34 eV for group Ⅲ and 1.54-2.5 eV for group Ⅳ by increasing the Cl content. The variation in absorption band edges, which is induced by the Cl concentration, is consistent with that for the PL emission peak wavelengths in group Ⅱ-Ⅳ. The tunability of PL intensity and wide band gap range for MAPbBr3 and MAPbBr3-xClx, which derive from modulating either the MABr content or the Cl concentration, may be helpful for use in bright visible light-emitting devices.



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Figure 7 Time-resolved PL decay detected at the peak wavelength of emission for MAPbBr3-xClx. (a) MAPbBr3: No. 1-3 (MABr/PbBr2 molar ratio changes from 2:1 to 1:1) (b) MAPbBr3-xClx: No. 4-5 (MABr/PbBr2/PbCl2 molar ratio changes from 2:0.8:0.2 to 2:0.5:0.5), No. 7-8 (MABr/PbBr2/PbCl2 molar ratio changes from 1.5:0.8:0.2 to 1.5:0.5:0.5) The recombination lifetimes of No. 1-3, No. 4-5 and No. 7-8 were confirmed by measuring PL decay at the emission peak wavelengths (λpeak), as shown in Figure 7. We use a triexponential function of time F (t) to fit the PL decay curves.21, 35 Ft ∑ /

(1)

Where is a prefactor and τ is the time constant. According to the fitted curves data, the average recombination lifetimes (τ ) of these thin films can be calculated


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by the following equation: 21, 35

τ ∑ τ / ∑ τ

(2)

Table 2 The average PL lifetimes of MAPbBr3-xClx at the emission wavelength Specimen No.

λpeak (nm)

τave (ns)

1

536

24.1

2

536

21.2

3

536

34.6

4

528

25.4

5

510

23.9

7

526

14.4

8

506

18.3

Table 2 shows the results. The maximum τ of MAPbBr3 films in our research is 34.6 ns. D’Innocenzo et al. 30 studied the relationship between the morphology and the average recombination lifetime of MAPbI3.30 They indicated that for MAPbI3 thin films, the larger crystallites presented longer photoluminescence lifetime. However, the relationship between the crystallite dimension and the PL intensities of perovskite thin films is still not clear. Zhang et al. 21 found that the PL emission intensities of MAPbBr3-xClx can be drastically changed by adjusting the Cl/Br ratio. They tuned the Cl/Br ratio by dissolving various amounts of MABr (0.75 M to 2 M) and PbCl2 (0.25 M) in DMF.21 However, in this way, the MA+ concentration also changed. It is not clear that whether the variation of MA+ concentration influences PL intensities of MAPbBr3-xClx or not. They indicated that MAPbBr2.4Cl0.6 exhibited the longest recombination lifetime of 446 ns, while MAPbBr2.25Cl0.75 exhibited the strongest PL emission intensity.21 In our research, for No. 1-3, the PL intensities drastically



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decrease with decreasing the MABr concentration, while the recombination lifetimes don’t decrease, as shown in Figure 2 (a) and Table 2. No. 4 exhibits the strongest PL intensity and its recombination lifetime is 25.4 ns. No. 3 shows the weakest PL intensity and its recombination lifetime is 34.6 ns. Therefore, the PL intensities of MAPbBr3 and MAPbBr3-xClx films are irrelevant to the recombination lifetimes. As the crystallite size increases from 0.4 µm to 1.49 µm, the PL intensities of No. 1-3 drastically decrease. The variation of PL intensities of MAPbBr3 and MAPbBr3-xClx films may be not related to the crystallite size. Zhang et al. 21 speculated that the variation of PL intensities of MAPbBr3-xClx might be associated with the orientation and vibration restraint of MA+ in the perovskite lattice as Cl/Br ratio varied. Our previous study found that tuning the MABr concentration induced the variation of the electronic states of carbon in MAPbBr3.25 In this paper, we find that the MABr content has more prominent influence on the PL intensities of MAPbBr3-xClx than Cl concentration. This may stem from the variation of the electronic states of MA+ in the perovskite film with adjusting the MABr concentration. High MABr concentration contributes to form highly dense, compact MAPbBr3 thin film with strong PL intensity but small crystal size. Low MABr concentration leads to large crystal size but poor surface coverage and weak PL intensity of MAPbBr3 thin film. The large crystallites may present long PL lifetime. In our research, No.3 has the largest crystal size 1.49 µm and the maximum PL lifetime 34.6 ns. Introducing Cl at high MABr concentrations can inhibit the formation of clusters and improve the films qualities and PL properties of MAPbBr3-xClx. However, doping too much Cl may remove MA+


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and lower the films qualities and PL intensities of MAPbBr3-xClx. ■ CONCLUSION

In conclusion, our research shows that both the MABr and Cl concentrations have significant influence over the PL emission intensities of MAPbBr3 and MAPbBr3-xClx. The strong PL emission intensities of MAPbBr3 and MAPbBr3-xClx can be obtained from solutions with a high MABr concentration. The introduction of Cl leads to a blue-shift in the PL emission peak wavelength of MAPbBr3-xClx. However, adding Cl in the precursor solution of MAPbBr3-xClx can enhance the PL emission intensity only in a high MABr concentration environment. Compared with the Cl, a high MABr concentration has a more prominent effect on increasing the PL emission intensities of MAPbBr3-xClx. Furthermore, a high MABr concentration leads to small crystal sizes but a compact surface, whereas a low MABr concentration results in large crystal sizes and many more voids in the MAPbBr3 and MAPbBr3-xClx thin films. The strong PL emission intensity of MAPbBr3-xClx stems from MA+ rich environment, dense and uniform film. An appropriate amount of Cl at a high MABr concentration can inhibit the formation of clusters and improve the quality of MAPbBr3-xClx films. We find that for excellent PL emission intensity the optimum MABr/PbBr2/PbCl2 ratio in the precursor solution of MAPbBr3-xClx is 2:0.8:0.2. This work may contribute to clarify the roles of MABr concentration and chlorine in influencing the PL properties of MAPbBr3-xClx, which is the information that can help fabricate high efficient light-emitting materials.



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■ ASSOCIATED CONTENT

Supporting information The X-ray diffraction peaks (100) and (200) of thin films, Grain size distribution, Optical band gaps of thin films ■ AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work is financially supported by the Fundamental Research Funds for the Central Universities (No.2015JBM104), National Natural Science Foundation of China (No.61178052) and Programs Foundation of Ministry of Education of China (No.20130009110008). ■ REFERENCES (1) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices, Nat. Nanotechnol. 2015, 10, 391-402. (2) Niu, G.; Guo, X.; Wang, L. Review of Recent Progress in Chemical Stability of Perovskite Solar Cells, J. Mater. Chem. A, 2015, 3, 8970–8980. (3) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High


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