Suppression and Reversion of Light-Induced Phase Separation in

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Suppression and Reversion of Light-induced Phase Separation in Mixed-halide Perovskites by Oxygen Passivation Weisheng Fan, Yongliang Shi, Tongfei Shi, shenglong chu, Wenjing Chen, Kester Ighodalo, Jin Zhao, Xinhua Li, and Zhengguo Xiao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b01383 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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Suppression and reversion of light-induced phase separation in mixed-halide perovskites by oxygen passivation Weisheng Fan,1,2,3,† Yongliang Shi,2,3,† Tongfei Shi,1 Shenglong Chu,1,2,3 Wenjing Chen,2,3 Kester O. Ighodalo,2,3 Jin Zhao,2,3,* Xinhua Li,1,* Zhengguo Xiao2,3,* 1Key

Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China

2Hefei

National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

3Department

of Physics, CAS Key Laboratory of Strongly-coupled Quantum Matter

Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Corresponding Authors *E-mail:

[email protected], [email protected], [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT: Mixed-halide perovskites (MHPs), such as methylammonium lead bromide-iodide, CH3NH3Pb(BrxI1-x)3 (0 Pb2+.33 This suggests that the most likely ion migration channels in MAPb(BrxI1-x)3 are those related to XV. The released energy for O atom filling the XV is relatively large, around 1.6 eV; O atom will therefore preferentially fill the XV. In addition, oxygen atom has tighter bonds with adjacent Pb atoms than Br and I atoms due to its larger electronegativity. Therefore, the halide ions have to overcome higher diffusion barriers, resulting in suppressed phase segregation. Base on the above results, we are able to explain the suppression and recovery of phase separation of MHP films in O2 under light illumination. As shown in Figure 7, under light illumination, the trapped charges cause band bending and the generated electric field cause halide, most likely iodide ions, drift to the crystal surface (Figure 7b). Oxygen can passivate the surface traps, so that the band bending near the surface can be reduced (Figure 7c). Therefore, the accumulated iodide ions are expected to diffusion back to the bulk due to the concentration gradient (Figure 7d), resulting in uniform MHP films under light illumination. We also examine the duration of O2 passivation effect in MAPb(Br0.4I0.6)3 film. The film was illuminated under one sun in O2 before measurement, and stored in N2 atmosphere in dark during the measurement. As shown in Figure S4, the PL peak position kept almost the same at around 660 nm for around 100 hours. The PL peak then red-shifted to 710 nm, indicating that the oxygen passivation is a reversible process. 9 ACS Paragon Plus Environment

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We finally test the passivation effect of O2 in LEDs (Figure S5). It also should be noted that the LED performances using stoichiometric MAPb(BrxI1-x)3 (0≤x≤1) are usually very poor because of the low electron-hole capture rates. Incorporation of extra bulky organoammonium halide can dramatically improve the performance, but that will affect the study of phase separation in this work. Therefore, we continue to use stoichiometric MAPb(Br0.4I0.6)3 film in this work. The perovskite LED structure is ITO/poly-TPD (40 nm)/MAPb(Br0.4I0.6)3 (40 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm) (ITO: indium tin oxide; poly-TPD: poly[N,N'-bis(4-butylphenyl)-N,N'bis(phenyl)-benzidine];

TPBi:

2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-

benzimidazole)). It should be noted that the electric field can also drive halide migration and phase separation.8 Figure S5 shows current density, external quantum efficiency and electroluminescence (EL) spectra of the LEDs. The LED shows a strong EL peak at 660 nm after O2 passivation initially, compared with 730 nm for control devices without passivation. This result further demonstrate the role of O2 passivation in suppressing light (or electric field) induced phase separation in MHPs. In summary, we have studied the O2 passivation effect and suppression of light induced phase separation in MHP films. Combining both experimental results and theoretical calculations, we demonstrate that oxygen molecule can suppress halide redistribution by passivating the traps and/or occupying halide vacancies. In addition, the phase separated films can be recovered to uniform state after O2 passivation. Our work provides a promising way to solve phase separation issue in MHPs. In addition, the switchable emission color in N2 and O2 may have potential application in light emitting field-effect transistors where the perovskite films expose to external environment.


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FIGURES b

a x=0 x=0.2 x=0.4

40

1.0 Fresh

x=0.6 x=0.8 x=1

Normalized PL intensity (a.u.)

Fresh 40 20 N2 40 20 H2O 40

1.0 N 2 0.5 1.0 H O 2 0.5 1.0 O 2 0.5

20 0

x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1

0.5

20

Absorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O2 500

600 700 Wavelength (nm)

0.0

800

500


800

Figure 1. (a) Absorption and (b) normalized PL spectrum of MAPb(BrxI1-x)3 films before illumination and after illumination in a pure nitrogen, moisture (N2+H2O, ~35% relative humidity), and pure oxygen environment. The PL measurement was done with a continuous excitation with a wavelength of 350 nm and an intensity of 6 mW cm-2.


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Absorption (%)

a

MAPb(Br0.2I0.8)3

b

Illumination time 0 min 10 min 20 min 40 min

40

0

c

MAPb(Br0.4I0.6)3

g

f

e

Illumination time 0 min 10min 20min 40min 60min 80min


500


800


d

MAPb(Br0.6I0.4)3 Illumination time 0 min 10 min 20 min 40 min 60 min 80 min

Illumination time 0 min 10 min 20 min 40 min 60 min 80 min

20

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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h Illumination time 0 min 10 min 20 min 40 min

Illumination time 0 min 10 min 20 min 40 min 60 min 80 min

800

MAPb(Br0.8I0.2)3


800


800

Figure 2. Absorption and PL spectrum of MAPb(BrxI1-x)3 films with illumination time in an oxygen environment for (a, e) x=0.2, (b, f) x=0.4, (c, g) x=0.6 and (d, h) x=0.8. The PL measurement was done in a pure oxygen environment with a continuous excitation with a wavelength of 350 nm and an intensity of 6 mW cm-2.


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20

50

Normalized PL intensity

d

0

30

60


0.5

0.0

60

Mixed-phase PL Iodide-phase PL

1.0

0.0

30 40 Time (min)

90 120 Time (min)

150

0

10

20

30 40 Time (min)

Mixed-phase PL

1.0

0.5

0.0

180

Mixed/Iodide-phase PL / 2.1 mW cm-2 / 3.8 mW cm-2 / 6.0 mW cm-2

0.5

50

60

Iodide-phase PL

Oxygen

10

1.0

Nitrogen

0

Darkness (20 min)

c

Mixed/Iodide-phase PL / 100:0 (VO2:VN2) / 70:30 (VO2:VN2) / 30:70 (VO2:VN2)

0.5

0.0

b

Excitation intensity: 10 mW cm-2

1.0

Illumination (10 min)


a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

20

40 60 Time (min)

80

100

Figure 3. Evolution of PL emission under (a) varying O2 partial pressures and (b) varying light intensities in pure O2. (c) Evolution of PL emission under light/dark circles and (d) O2/N2 circles. All measurements were performed on MAPb(Br0.4I0.6)3 films. The PL intensities of mixed-phase PL (660 nm) and Iodide-phase PL (720nm) are normalized to their maximum values respectively. The PL measurement was done with a continuous excitation with a wavelength of 350 nm and an intensity of 6.0 mW cm-2, unless otherwise stated.


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20


5h 6h 7h 8h

800

1.0

0.5

Heating time 0h 1h 2h 3h 4h 5h 6h 7h 8h

0.0 500


Absorption edge/ PL peak position (nm)

40

0 500

c

b Heating time 0h 1h 2h 3h 4h


a Absorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

720 710 700 690 680 670 660 650

Absorption edge position PL peak position

0 1 2 3 4 5 6 7 8 Time (h)

Figure 4. (a) Absorption and (b) PL spectrum of MAPb(Br0.4I0.6)3 film with different heating time in an oxygen environment. The PL measurement was done in a pure oxygen environment with a continuous excitation with a wavelength of 350 nm and an intensity of 6 mW cm-2. (c) Plot of absorption edges and PL peak positions versus heating time.


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a

Normalized XRD intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13

b

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d

c

e

MAPb(Br0.2I0.8)3 MAPb(Br0.4I0.6)3 MAPb(Br0.6I0.4)3 MAPb(Br0.8I0.2)3 MAPbI3 Fresh Fresh Fresh Fresh Fresh

f

MAPbBr3 Fresh

N2

N2

N2

N2

N2

N2

H2O

H 2O

H 2O

H 2O

H 2O

H 2O

O2

O2

O2

O2

O2

O2

14 15 14 15 2 (degree) 2 (degree)

14 15 2 (degree)

14 15 2 (degree)

15 16 14 15 14 2 (degree) 2 (degree)

Figure 5. XRD characterization of MAPb(BrxI1-x)3 films for (a) x=0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8 and (f) x=1. Black, red, blue and pink line represent before illumination and after illumination in N2, H2O (N2+H2O, ~35% relative humidity) and O2 respectively (the fitted curves are indicated by the dashed lines).


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a

b Fresh O2

Pb 4f Intensity (a.u.)

4f7/2

134

Fresh O2

O 1s

4f5/2 Pb-O

136

c

138 140 142 144 Binding Energy (eV)

DOS (a.u.)

Pbi

146 526 528 530 532 534 536 538 Binding Energy (eV) d

Total Pbi

O passivated Pbi

Total Pbi O

O passivated Iv

Total O

Trap state

e

Total

Iv

DOS (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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f

Trap state -3

-2

-1

0

1

2

3 -3

-2

-1

Br

I

Pb

O

0

1

2

3

Energy (eV)

Energy (eV) MA

Figure 6. The XPS core level spectra for (a) Pb 4f and (b) O 1s of MAPb(Br0.4I0.6)3 film before and after illumination in oxygen. The PDOS of (c, d) Pbi and (e, f) IV before and after O2 passivation. The insets show the orbital distribution of the trap states.



a

Uniform state in dark

b

Br- I Br BrI- I- I- Br I- Br- I- I I-

I- - BrI- Br- I - BrI IBr- IBrI- I-

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Light induced phase separation I- Br - Br- I- Br -I I BrBr I Br I -I I I I I Brdrift I Br- Br- I- BrII I I I

eh+

Trap states

d

c

Uniform state under light I- - BrI- Br- I - BrI IBr- IBrI- I-

O2 O2 O2 O2

Br- I Br BrI I- I- Br I- Br- I- I I-

Halide diffusion after O2 passivation under light I- Br - Br- I- Br - I O2I BrBr I Br I I O 2 I I I I I BrO2 Br- diffusion Br- I- I- O2 I- Br II I

Figure 7. Schematics of the reversion of phase separation in MHPs due to O2 passivation. (a) Uniform state in dark, (b) light induced phase separation caused by the trapped charges at the surface, (c) iodide diffusion back to the bulk due to the concentration gradient, (d) uniform state under light.


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Table 1. Composition changes of MAPb(BrxI1-x)3 films before and after illumination in different environment. Samples Fresh

N2

H2O

x values for MAPb(BrxI1-x)3 films x=0.2

x=0.4

x=0.6

x=0.8

x=0.05

x=0.29

x=0.50

x=0.71

x=0.34

x=0.51

x=0.85

x=0.89

x=0.04

x=0.36

x=0.51

x=0.66

x=0.33

x=0.57

x=0.82

x=0.86



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Method section including mixed-halide perovskite film fabrication and characterizations, theoretical calculation, additional absorption and PL spectra. AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected], [email protected], [email protected].

Author Contributions †Weisheng

Fan and Yongliang Shi contributed equally. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 51872274, 51472247, 51671182), the Joint Funds of the National Natural Science Foundation of China (No. U1632123). REFERENCES (1)

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Suppression and Reversion of Light-Induced Phase Separation in

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