Efficient Sky-Blue Perovskite Light-Emitting Devices Based on

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Efficient Sky-Blue Perovskite Light Emitting Devices Based on Ethylammonium Bromide Induced Layered Perovskites Qi Wang, Jie Ren, Xuefeng Peng, Xiaxia Ji, and Xiaohui Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07458 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Applied Materials & Interfaces

Efficient Sky-Blue Perovskite Light Emitting Devices Based on Ethylammonium Bromide Induced Layered Perovskites Qi Wang, Jie Ren, Xue-Feng Peng, Xia-Xia Ji, Xiao-Hui Yang* School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China ABSTRACT: Low-dimensional organometallic halide perovskites are actively studied for light emitting applications due to their properties such as solution-processability, high luminescence quantum yield, large exciton binding energy and tunable band gap. Introduction of large-group ammonium halides not only serves as a convenient and versatile method to obtain layered perovskites, but also allows the exploitation of energy funneling process to achieve high efficiency light emission. Herein, we investigate the influence of the ethylammonium bromide addition on the morphology, crystallite structure and optical properties of the resultant perovskite materials and report that the phase transition from bulk to layered perovskite occurs in presence of excess ethylammonium bromide. Based on this strategy, we report green perovskite light emitting devices with the maximum external quantum efficiency of ca. 3% and power efficiency of 9.3 lm/W. Notably, blue layered perovskite light emitting devices with the Commission Internationale de I’Eclairage (CIE) coordinates of (0.16, 0.23) exhibit the maximum external quantum efficiency of 2.6% and power efficiency of 1 lm/W at 100 cd/m2, representing large improvement over those of previously reported analogous devices.

Keywords: layered perovskites, surface morphology, crystallite structure, phase transition, light emitting devices 1

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1. INTROUDUCTION Organometallic halide perovskites (Peros) are emerging as a new generation of semiconducting materials, which have attracted broad attention from industry and academia.1 The main work on Peros has focused on solar cells, boosting the power conversion efficiency of such devices to more than 20% in the past several years.2 Meanwhile, other Pero optoelectronic devices such as light emitting devices3 and photodetectors4 are also actively studied. Electroluminescence (EL) from Peros was initially observed at cryogenic temperature in the 1990s.5 Tan et al.6 reported room temperature operating green and near-infrared Pero light emitting devices with the respective external quantum efficiency (EQE) of 0.1 and 0.7%. Simultaneous work of the morphology control, device configuration and interface engineering advances the EQEs of Pero light emitting devices rapidly.3-20 Cho et al. reported CH3NH3PbBr3-based light emitting devices with the maximum EQE of 8.53% by manipulating the ratio of lead (Ⅱ) bromide (PbBr2) to methylammonium bromide (MABr) in the precursor solution and using a molecular additive to restrict the crystallization of CH3NH3PbBr3.8 So far, there are scarce reports on blue Pero light emitting devices, which are nevertheless essential for display applications. Blue light emitting devices based on bulk Peros with Cl or mixed Br and Cl anions have been reported.9, 10 Due to the large dielectric constant and average distance between electrons and holes, the exciton binding energy of Peros is reported to be 2−6 meV21, which is much smaller than several hundreds meV for organic materials22. Compared to organic materials, Peros have the advantages of excellent optoelectronic properties approaching those of traditional semiconductors like GaAs via low-temperature solution-processable preparation method, as well as good color purity, simple method for material purification and large tolerance to defect formation.1-3 Low exciton binding 2


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energy of bulk Peros may impose a severe limitation on the rate of radiative transition. To resolve this issue, low-dimensional Peros with blue-shifted emission spectrum and enlarged exciton binding energy due to the dielectric and quantum confinement effects are explored. Layered Peros with formula (RNH3)2[CH3NH3PbBr3]nPbBr4 consist of n layers of stacking two-dimensional Pero sheets terminated by organic RNH3 groups, exhibiting increased band gap energy with decreasing n.3 Liang11 and Kumar12 described violet and blue layered Pero light emitting devices with the maximum EQEs of ca. 0.04 and 0.2%, respectively. Other low-dimensional Peros such as Pero quantum dots are used to construct blue light emitting devices as well. For example, Pan13 reported blue light emitting devices based on CsPbBr3 quantum dots with the maximum luminance and EQE of ca. 35 cd/m2 and 1.9%. Introduction of large-group ammonium halides not only serves as a convenient and versatile method to obtain layered Peros (quasi-2D Peros), but also allows the exploitation of energy funneling process among various layered Pero phases to achieve

high

efficiency

light

emission.14

Chen17

investigated

the

influence

of 2-

-phenoxyethylamine (PEOA): methylamine (MA) cation ratios on the properties of the derived Pero materials and reported blue layered Pero light emitting devices with the maximum EQE of 1.1% and luminance of 19.5 cd/m2. Herein, we study the effects of the ethylammonium bromide (EABr) addition on the morphology, crystallite structure and optical properties of the resultant Pero materials. Our results indicate that the phase transition from bulk to layered Pero occurs in presence of excess EABr, accompanied by a ca. 40 nm blue-shift of emission wavelength. Based on this strategy, we report blue layered Pero light emitting devices with the maximum EQE of ca. 2.6% and power efficiency of 3 lm/W.

2. EXPERIMENTAL SECTION: 3



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Materials. MABr, EABr, PbBr2 and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) were purchased from Xi’an Polymer Technology Corp (China). Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and organic solvents including dimethylformamide (DMF) and chlorobenzene (CB) were brought from Heraeus Corp. (Germany) and Sigma-Aldrich, respectively. All the materials were used as received. Preparation of Pero light emitting devices. Scheme 1 shows the configuration of the devices. Precleaned indium tin oxide (ITO) substrates were treated with UV-ozone for 30 min and used immediately for device preparation. A PEDOT:PSS layer was spin-coated onto ITO substrates from the aqueous dispersion, which was subsequently heated at 170oC for 10 min under the ambient conditions. MABr, EABr and PbBr2 with a fixed PbBr2:(MABr+EABr) molar ratio of 1:1.5 and MABr:EABr molar ratio of 1:1 or 1:1.3, denoted as 1:1 or 1:1.3 Pero in the following, were co-dissolved into anhydrous DMF at 60oC for 4 h. The solutions with the PbBr2:MABr/EABr molar ratio of 1:1.5 were made as well for the preparation of CH3NH3PbBr3 and C2H5NH3PbBr3 layers. Various Pero films were coated on top of PEDOT:PSS layer using CB as the anti-solvent inside a glove-box23, where the oxygen and moisture levels were below 5 ppm. The thickness of Pero layers was approximately 50 nm, as determined by a Vecco-Dektak surface profilometer. Without exposure to air, the samples were transferred to a thermal evaporator resided inside a glove-box, where 50 nm TmPyPB, 1 nm CsF and 150 nm Al were sequentially deposited. Characterization. Current density−voltage−luminance (J−V−L) characteristics of the devices were recorded by a Keithley 2400 Source-Measure Unit and a Konica-Minolta CS-100 Chroma Meter controlled by a homemade Labview program. The composition and crystallite structure of Pero films were analyzed with a Rigaku D/Max-B X-ray diffractometer (XRD) equipped with 4


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Cu-Kα radiation source. The morphology of Pero films was studied using a Jeon scanning electron microscope and a Hitachi atomic force microscope. The measurements of UV-Vis and photoluminescence (PL) spectra were carried out with a Shimadzu UV-2600 UV-Vis spectrophotometer and a Hitachi F4600 fluorospectrometer, respectively.

3. RESULTS AND DISCUSSION:

Scheme 1. The configuration of Pero light emitting devices. CH3NH3PbBr3 1:1 1:1.3 C2H5NH3PbBr3 4

6

8

10

2 Theta (Degree)

10

15

20

25

30

35

40

45

50

2 Theta (Degree)

Figure 1. XRD patterns of CH3NH3PbBr3, C2H5NH3PbBr3, 1:1 and 1:1.3 Pero samples; Inset shows the XRD profile of 1:1.3 Pero sample in the low angle region. For CH3NH3PbBr3 sample, the diffraction peaks at 14.9, 21.2, 30.1, 33.8, 37.0, 43.1 and 45.8o as shown in Figure 1 can be indexed to the (100), (110), (200), (210), (112), (220) and (300) planes of CH3NH3PbBr3 cubic phase24, respectively, whereas C2H5NH3PbBr3 sample shows the diffraction peaks at 11.3, 15.1, 26.8 and 30.7o, in good agreement with the results of the previous report.19 Diffractogram of 1:1 Pero sample closely resembles that of CH3NH3PbBr3 sample, 5



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indicating limited incorporation of EA cations into the Pero core. Meanwhile, the sample shows the diffraction peaks with reduced intensity and broadened full width at half maximum (FWHM) compared to CH3NH3PbBr3 sample, which reflects reduced crystallinity of 1:1 Pero sample. 1:1.3 Pero sample exhibits the diffraction peaks at 14.9 and 30.1o similar to those of CH3NH3PbBr3, as well as the diffraction peaks at 11.5, 15.3, 27.1 and 31.0o shifted toward higher degree with respect to those of C2H5NH3PbBr3, which is associated with the different sizes of MA and EA cations. Mittal19 reported that the incorporation of large EA cations into the Pero core led to the changes of the electronic coupling between Pb and Br atoms in the PbBr6- octahedral, resulting in increased band gap of stoichiometric Pero nanoparticles. The XRD analysis of 1:1.3 Pero sample is in line with the incorporation of EA cations into the Pero core. Notably, the XRD profile of 1:1.3 sample possesses sharp and distinct diffraction peaks at 3.7 and 7.4o (the inset of Figure 1), an indication of forming layered Pero phases with n=3.17

(a)

(b)

(c)

Figure 2. The top-view SEM images of CH3NH3PbBr3 (a), 1:1 (b) and 1:1.3 (c) films on top of PEDOT:PSS layer; Insets show the high-resolution images. Figure 2 shows the top-view SEM images of CH3NH3PbBr3, 1:1 and 1:1.3 Pero films. All the samples show continuous crystalline morphology with few voids or pores. Compared to CH3NH3PbBr3 sample, 1:1 and 1:1.3 Pero samples exhibit less pronounced grain boundaries as 6


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shown in the high-resolution images (insets of Figure 2). 1:1 Pero sample shows decreased grain size with respect to CH3NH3PbBr3 sample, consistent with the results of XRD measurements, probably due to the hindrance effect of a large-group ammonium bromide to the crystallite growth.25 The crystallinity of the sample increases with further increase of EABr molar ratio as a result of the formation of layered Pero phases. In contrast, the crystallinity of CH3NH3PbBr3 films decreases monotonically with increasing MABr content in the precursor, 26 which is attributed to the formation of small colloidal particles in the precursor solution due to the coordinating effect of MABr. 27 The results of AFM measurements are shown in Figure S1 in the Supporting Information. All the samples show relatively smooth and continuous morphology. The root mean square roughness value of the sample first decreases with the addition of equimolar EABr from 9.5 to 7.7 nm and then starts to increase to 12.3 nm with further increase of EABr content. The results of AFM measurements are consistent with those of SEM measurements. 1.0

200000

0.6

Intensity (a.u)

CH3NH3PbBr3 C2H5NH3PbBr3 1:1 1:1.3

0.8 Absorption (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


0.4

150000

100000

50000

0.2

0 400

0.0 400

450

500

550

600

450

500

550

600

Wavelength (nm)

Wavelength (nm)

(a)

(b)

Figure 3. The UV-Vis (a) and PL (b) spectra of 1:1 and 1:1.3 Pero films, as well as CH3NH3PbBr3 and C2H5NH3PbBr3 films. As presented in Figure 3a, CH3NH3PbBr3 sample shows the absorption on-set of ca. 540 nm and gradually increasing absorbance in the short wavelength regime, on the other hand, C2H5NH3PbBr3 sample possesses largely blue-shifted absorption onset of ca. 430 nm, in 7



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accordance with the reports in the literature.19,28 1:1 Pero films show a similar absorption profile to that of CH3NH3PbBr3 with a 20 nm blue-shift in the absorption on-set probably due to the strain or passivation effect of a large-group ammonium bromide.25 In stark contrast, the absorption spectrum of 1:1.3 Pero film features the sharp excitonic absorption peaks with the maxima at 438 and 464 nm, which can be assigned to the absorption of layered Pero phases with the main component having n=325, in good agreement with the results of XRD measurements. PL spectrum of C2H5NH3PbBr3 sample (Figure 3b) contains an emission peak with the maximum at 433 nm and relatively low intensity, which is consistent with the previous report.19 PL spectrum of 1:1 Pero sample has the maximum at 514 nm, presenting a 10-15 nm blue-shift compared to that of CH3NH3PbBr3 sample. Meanwhile, PL intensity of 1:1 Pero sample is ca. 4-5 times stronger than that of CH3NH3PbBr3 sample, which can be attributed to effective passivation of large ammonium bromides.16, 17, 25 In addition, PL spectra of CH3NH3PbBr3 and 1:1 Pero samples have a similar FWHM of ca. 20 nm, indicating that either the number of layered Pero phases or their dimensional diversity is not large, in line with the results of XRD measurements. In contrast, PL spectrum of 1:1.3 Pero sample has a non-Gaussian profile with the maxima at 473 and 485 nm as well as a much larger FWHM, reflecting that layered Pero phases with different n values are formed. While the absorption peak at 438 nm is dominant in the UV-Vis spectrum, the main peak in the PL spectrum is located at 485 nm, implying that there prevails energy funneling process among the layered Pero phases.14, 15 The area ratio of PL spectra provides a rough estimation of the relative PL efficiency of the samples with similar composition and absorbance at the excitation wavelength of 425 nm.29 Using this method, we estimate that the relative PL efficiency of 1:1.3 Pero sample is ca. 60% that of 1:1 Pero sample. 8


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4

EQE (%)

3

2

1

0 4

8

12 2

Current density (mA/cm )

(a)

(b) (0.12, 0.75) (0.13, 0.26) (0.18, 0.76)

1.0

8

EL Intensity (a.u)

Power efficiency (lm/W)

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


4

0.8 0.6 0.4 0.2

0 10

0.0 400

100 2

Luminance (cd/m )

500

600

700

Wavelength (nm)

(c)

(d)

(e) Figure 4. The J−V−L (a), EQE−current density (b), power efficiency−luminance (c) properties and EL spectra (d) of the devices as well as the energy level diagram (e) of the devices with the structure of ITO/PEDOT:PSS/MAPbBr3 (50 nm)/TmPyPB (50 nm)/CsF/Al3.

Compared to the CH3NH3PbBr3 devices, the 1:1 Pero devices shows the J−V characteristics (Figure 4a) slightly shifted to higher voltage direction, probably related to decreased crystallinity 9



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of 1:1 Pero sample as manifested in the SEM and AFM measurements or reduced leakage current. The 1:1.3 Pero devices show elevated light emission onset voltage from 2.4 to 2.7 V, which is in accordance with increased band-gap as shown in the UV-Vis spectrum due to the formation of layered Pero phases. The 1:1.3 devices show luminance saturation at 3.5−4 V, while luminance of the 1:1 and MAPbBr3 devices increases with the increase of the drive voltage. The exact mechanism is not understood at the present stage. We hypothesize that non-radiative recombination centers are formed in layered Peros under high voltage, which is in line with the recent report of strong electric field induced irreversible degradation of layered Peros.18 The 1:1 Pero devices show the maximum EQE of ca. 3% (Figure 4b) and power efficiency of 9.3 lm/W at 662 cd/m2 (Figure 4c), the former of which is more than 5 times higher than 0.55% of the analogous CH3NH3PbBr3 devices. Significant enhancement in device EQE can be attributed to improved surface morphology and enhanced PL intensity as discussed above. Table 1. Comparison of the properties including the maximum EQE[%], maximum luminance[cd/m2] and peak emission wavelength[nm] of blue layered Pero light emitting devices.

reference

layered Pero formula

EQE

luminance

peak wavelength

Liang et al.11

(PEA)2PbBr4

0.04

N.A

410

Kumar et al.12

CH3NH3PbBr3

0.2

8.5

489

Chen et al.17

(PEOA)2(MA)n-1PbnBr3n+1

1.1

19.5

480,494,508

Congreve et al.18

(C4H9)2(MA)n-1PbnBr3n+1

0.006

≈1

500

Cheng et al.20

(4-PBA)2PbBr4

0.015

186

491

This work

(EA)2(MA)n-1PbnBr3n+1

2.6

200

473, 485

The maximum EQE of the 1:1.3 Pero devices is ca. 2.6%, which is the record EQE value for blue layered Pero light emitting devices (Table 1). Figure S2 in the Support Information shows the

10


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histogram of the maximum EQEs for the 1:1.3 Pero devices. The EQE of the 1:1.3 Pero devices is decreased with increasing drive voltage probably due to electric field induced Pero degradation as discussed in Ref 18. Nevertheless, the devices show the power efficiency of 1 lm/W at 100 cd/m2. Good performances of the devices underline that the addition of a large-group ammonium halide serves as an effective approach to enhance the EQE of blue Pero light emitting devices. The devices prepared using an even larger EABr proportion show the maximum EQE of ca. 0.1%, which may be attributed to the strong electron-phonon interactions in layered Pero phases with small n values.30 As shown in Figure 4d, EL spectra of the devices are almost identical to PL spectra of Pero films (Figure 3b) in terms of peak position and lineshape, indicating that light emission solely comes from Pero layers as a direct consequence of good confinement of carrier recombination (Figure 4e). Note that the valence band maximum and conduction band minimum of layered Peros are up-shifting toward the vacuum level with decreasing n value.31 EL spectrum of the 1:1 Pero devices (Figure 4e) shows the maximum at 514 nm and a FWHM of ca. 20 nm. The CIE coordinates of the devices are (0.12, 0.75), representing color-saturated green emission. And the 1:1.3 Pero devices have the emission maxima of 473 and 485 nm in the EL spectrum and the CIE coordinates of (0.13, 0.26). Luminance of the devices is decreased to the half of the initial value (ca. 100 cd/m2) under constant current density of 10-15 mA/cm2 in the period of 10−20 min, which is similar to the reported life-time of perovskite light emitting devices15,32. Several factors including morphology integrity, material stability toward oxygen and moisture under electric field, ion migration induced interfacial degradation and the stability of charge injection layer may account for the limited life-time of the devices. 4. CONCLUSION. 11



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In summary, we report the evolution of surface morphology, crystallite structure and optical properties of perovskite materials with increasing amount of ethylammonium bromide additive. The appearance of low-angle diffraction peaks in diffractogram and sharp excitonic absorption in the UV-Vis spectrum as well as the existence of multiple blue-shifted emission peaks in the PL spectrum indicate the phase transition from bulk to layered perovskite. The addition of equimolar ethylammonium bromide increases PL intensity by a factor of 5 due to effective passivation of ethylammonium bromide, benefiting for the EQE of green perovskite light emitting devices. Notably, blue layered perovskite light emitting devices show the maximum EQE of 2.6% and power efficiency of 1 lm/W at 100 cd/m2, representing large improvement over those of previously reported analogous devices. Further work on the selection of proper ammonium halide and optimization of device structure is underlying to enhance the EQE and stability of blue layered perovskite light emitting devices. ■ ASSOCIATED CONTENTS The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.07458m. AFM images of the samples; the histogram of peak EQEs for the 1:1.3 Pero devices (PDF) ■ AUTHOR INFORMATION Corresponding author *E-mail: [email protected] ORCID

Xiao-Hui Yang: 0000-0002-0753-4385 Notes The authors declare no competing ﬁnancial interest. ■ ACKOWLEDGEMENT Financial support by the National Natural Science Foundation of China (Grant nos. 61177030 and 11474232) is acknowledged. 12


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Efficient Sky-Blue Perovskite Light-Emitting Devices Based on

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