Alternative Type 2D-3D Lead Halide Perovskite with Inorganic Sodium

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Alternative Type 2D-3D Lead Halide Perovskite with Inorganic Sodium Ions as Spacer for High Performance Light Emitting Diodes Chen Wu, Tian Wu, Yingguo Yang, John A McLeod, Yusheng Wang, Yatao Zou, Tianshu Zhai, Junnan Li, Muyang Ban, Tao Song, Xingyu Gao, Steffen Duhm, Henning Sirringhaus, and Baoquan Sun ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07632 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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Alternative Type 2D-3D Lead Halide Perovskite with Inorganic Sodium Ions as Spacer for High Performance Light Emitting Diodes Chen Wu‡1, Tian Wu‡1, Yingguo Yang2, John A. McLeod1, Yusheng Wang1, Yatao Zou1, Tianshu Zhai1, Junnan Li1, Muyang Ban1, Tao Song*1, Xingyu Gao2, Steffen Duhm1, Henning Sirringhaus*3 and Baoquan Sun*1 1Jiangsu

Key Laboratory for Carbon-Based Functional Materials & Devices, Institute

of Functional Nano & Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou 215123, People’s Republic of China 2Shanghai

Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied

Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, Shanghai 201204, China 3Cavendish

Laboratory, Department of Physics, University of Cambridge, JJ

Thomson Avenue, Cambridge CB3 0HE, UK

‡These

authors contributed equally to this work

*Corresponding Author Tao Song: [email protected] Henning Sirringhaus: [email protected] Baoquan Sun: [email protected]

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ABSTRACT Two-dimensional (2D) lead halide perovskites with long-chain ammonium halides display high photoluminescence quantum yield (PLQY), due to their size and dielectric confinement, which promise a high efficiency and low-cost light emitting diode (LED). However, the presence of insulating organic long-chain spacer cation (L) dramatically deteriorates the charge transport properties along the out-of-plane nanoplatelet direction or adjacent nanocrystals, which would limit the LED device performance. In order to overcome this issue, we successfully incorporate small alkaline ions such as sodium (Na+) to replace long organic molecule. Grazing incident X-ray diffraction (GIXRD) measurements verify the 2D layered formation with preferential crystallite orientation. In addition, the incorporated sodium salt also generates amorphous sodium lead bromide (NaPbBr3) in perovskite as spacers to form nanocrystal-like halide perovskite film. PLQY is dramatically improved in the sodium incorporated film associating with enhanced PL lifetime. With incorporating small concentration of an organic additive, this 2D-3D perovskite can achieve a compact and uniform film. Therefore, a 2D-3D perovskite achieves a high external quantum efficiency (EQE) of 15.9% with good operational stability. Our work develops a type of 2D-3D halide perovskite with various inorganic ions as spacers for high performance of promising optoelectronic devices.

KEYWORDS: perovskite, light-emitting diode, alkaline halide, two-dimensional, dielectric confinement.

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Two-dimensional (2D) Ruddlesden-Popper (RP) halide lead perovskites are a class of layered materials defined by a formula L2An-1PbnX3n+1, where L is spacer cation, A is cation for perovskite crystal formation, X is halide.1-9 The nanoplatelet thickness of perovskite can be controlled by the value of crystal-unit ,5 which forms a quantumwell-like structure due to the size quantum and dielectric confinement.1, 10 Compared with three-dimensional (3D) lead halide perovskite with a relatively low exciton binding energy (Eb) (9-60 meV), 2D perovskite can form a large Eb at room temperature because exciton wave function is tightly confined in the limited dimension.1, 11-13 There is a large rate for radiative recombination occurring in 2D perovskite materials, which promises it as an efficient emitter layer for high efficiency perovskite light emitting diodes (LEDs).2-7 By this ingenious strategy, tremendous efforts have been devoted to achieve a ground-breaking efficient LED with an external quantum efficiency (EQE) over 10%.3-5 Nevertheless, in such low-dimensional perovskites, long-chain ammonium

cations

(for

PEAX=phenylethylammonium,

example,

BAX=n-butylammonium,

NMAX=1-naphthylmethylamonium;

PBAX=phenylbutylammonium, X = I, Br) with large cation sizes cannot enter perovskite crystal structure, which act as ligands to confine the perovskite crystallite growth.4 Organic cation spacers act as insulating layer between the semiconductor nanoplatelet to enhance its quantum confinement effect.1-3 On the other hand, these organic spacer cations with relatively low conductivity inhibit its out-of-plane or adjacent nanocrystals charge transport, which restrict the optoelectronic device efficiency.10 One of urgent requirements to further improve performance of device is a 3


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method for better controlling charge transport along out-of-plane nanoplatelet direction or adjacent nanocrystals as well as enhancing radiative yields of 2D or nanocrystal-like halide perovskite materials. Here, we have developed a type of 2D-3D halide perovskite induced by small-size inorganic alkaline metal salts in order to overcome the poor charge transport properties of nanocrystal-like halide perovskite film. Inorganic spacer molar ratios dramatically affect the optical properties and crystal orientation as well as film morphology. By varying optimization inorganic salt molar ratio with additional trace of organic additive in these 2D-3D perovskite, a uniform and compact film with photoluminescence quantum yield (PLQY) of over 50% is achieved. As a result, a LED device based on this type of 2D-3D perovskite achieves a brightness of 11560 cd m-2, a current efficiency (CE) of ~50.3 cd A-1 and a maximum peak EQE of 15.9%. In addition, the LED device displays enhanced operational lifetime, which contributes excellent film stability against Joule heat. Our work shows a possible way to an alternative type of lead halide perovskite for efficient and stable optoelectronic devices such as LED, solar cell and photodetector. RESULTS AND DISCUSSION Alkaline Metal as Spacer for 2D-3D Perovskite Verification In all present 2D halide L2An-1PbnX3n+1, the size of organic ammonium spacer (L) is much larger than A one, which can act as spacer interleaving between [PbnX3n] nanoplatelets. Inspired by previous 2D metal oxide perovskite, 14 where spacer can be either large organic ammonium or small cations such as sodium (Na+), potassium (K+), 4


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rubidium (Rb+), here, Na+ is used as spacer to form 2D lead halide perovskite, as shown in Figure 1c. The value of crystal-unit determines the quantum-well thickness.2 As a result, emission efficiency can be enhanced by size and dielectric confinement.15 In order to verify a layered perovskite formation, sodium bromide (NaBr) is used as spacer ligand. The precursor solutions were spin-coated by dissolving cesium bromide (CsBr), lead bromide (PbBr2) and NaBr in dimethyl sulfoxide (DMSO) with different x from 0% to 70% (see Experimental Section). Here, the concentration of NaBr (CNaBr) is defined by the molar ratio (x%) according to an equation of [CNaBr × x+CCsBr × (1x)]/CPbBr2=1.2, where CCsBr and CPbBr2 are concentrations of CsBr and PbBr2, respectively. The specific chemical components in the precursor solution are shown in Table S1. Both -2 X-ray diffraction (XRD) (Figure S1a) and GIXRD results (Figure 1a) are conducted to determine the crystallinity and orientation of the 2D-3D films. GIXRD results shown in Figure 1a and Figure S3 reveal a series of diffraction patterns of perovskite with different NaBr molar ratios (x = 0%, 5%, 10%, 15%, 30%, 40%, 50%, 70%). As shown in Figure 1a, the crystal diffraction patterns for pristine 3D CsPbBr3 (x = 0%) film at q=10.81 nm-1 ((100) plane) and q=15.36 nm-1 ((110) plane) are half uniform rings, respectively, which indicates that the crystal is randomly orientated without any preferential direction. With increasing the NaBr molar ratio from 5% to 70%, a transformation is observed from the continuous Debye Scherer ring pattern to discrete Bragg one. Diffraction pattern signal at q=15.36 nm-1 ascribed to (110) planes gradually accumulates at vertical direction in association with (100) plane signal piling up at 45o azimuthal angle, which should be strong revelation of a highly 5


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oriented film. To obtain quantitative analysis, the diffraction intensity along the azimuthal angle arising from the (110) plane with the ring at q=15.36 nm-1 are plotted in Figure S2. With progressively increasing NaBr molar ratios, diffraction peak is piled up at 90o azimuthal angle, which confirms the preferential crystal orientations along (110) planes.10 By azimuthal integral of the GIXRD patterns, the diffraction intensity along q space curves below 10 nm-1 is plotted in Figure 1b. Beside diffraction peak from 3D CsPbBr3, there are extra peaks with q values at ~9.2 nm-1, ~7.8 nm-1, ~4.5 nm-1, which is ascribed to the lattice distance of ~6.8 Å, ~8.1 Å, ~13.9 Å, respectively. 3D CsPbBr3 crystallizes in a cubic space group with dimensions of a=b=c=5.83 Å.16 Therefore, the value of 8.1 Å should be ascribed to the repeating distance of Na2PbBr4 (=1) within unit cell along stacking condition (detailed estimation of the interlayer separation can be found in supporting text 1 (ST1). The individual [PbBr4]2nanoplatelets are separated by one layer Na+ ion with a separation distance of ~2.3 Å, which is exact Na+ diameter (radius of Na+ is 1.16 Å when it is 6-coordinate).17 Coincidently, the distance of 13.9 Å can correspond to =2, as shown in Figure 1c. According to the GIXRD measurement, the diffraction (0k0) plane peaks from the separation distance between discrete platelet layers become stronger when crystal-unit value in precursor chemical component formula approaches two, as shown in Figure 1a. q value at 9.2 nm-1 (d= ~6.8 Å) is ascribed to the (020) plane between discrete Na2CsPb2Br7 platelets. -2 XRD measurement also shows preferential orientation with progressively decreasing crystal-unit value, as shown in Figure S1a. Here, both -2 XRD and GIXRD do not show a pure 2D perovskite diffraction 6


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signal, so that we believe that the films compose of both 2D and 3D perovskite, which is consistent with previous organic ligand based 2D perovskite based on similar spincoating method from precursor solution.18 This method typically only yields a mixture with a low phase (≤3) and 3D perovskite,19 which is consistent with the GIXRD results. As shown here, the distance between nanoplatelets along out of plane direction or adjacent nanocrystals is only 2-3 Å, which is much smaller than those based on organic ligand (For BAX as spacer for 2D perovskite, d= ~7.12 Å).8 Electrical conductivity for the perovskite films with 50% NaBr, 50% PEABr, 50% NMABr and 50% BABr are estimated with identical thickness and device structures. All the films display the almost same height topography atomic force microscopy (AFM) image in Figure S4, which rule out the leakage current difference resulting in current level variation. The short distance dramatically enhances the electrical properties, as show Figure 1d. Here, for hole only device, inorganic ion of sodium as spacer for 2D-3D perovskite displays the largest current level, which is lined with short distance with adjacent crystallites. In order to rule out Na+ substitution of Cs+, -2 XRD patterns of cubic phase perovskite with various NaBr molar ratios (x =0%, 5%, 10%, 15%) are carefully explored. Evidently, as shown in Figure S1a, all the perovskite films exhibit almost the same diffraction peaks mainly including (100), (110) and (200), which are all ascribed to diffraction peaks from powder diffraction file for standard 3D CsPbBr3.20 No extra diffraction peak is detected, indicating that there is no extra crystal phase with incorporation of NaBr. The enlarged -2 XRD peaks from (110) planes are also plotted 7


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in Figure S1b. No peak shift is observed among these perovskite films with incorporating NaBr. The extent of micro-strain in the perovskite films is quantified by analyzing the peak broadening in the -2 XRD patterns according to the modified Williamson-Hall (W-H) method (see ST2).21, 22 The analysis reveals that incorporation NaBr does not result in any lattice strain, which indicates that there is no crystal imperfection and distortion in the film (Figure S1c). Therefore, the possibility of substitution of Cs+ with Na+ is ruled out according to the analysis of XRD data. Our density functional theory (DFT) calculations suggest that a strong Na-Br interaction is present in the NaPbBr3 films. As long as a substantial amount of NaBr is present, formation of 2D layered perovskite films as suggested by the GIXRD data is energetically preferable to a mixed NaPbBr3/CsPbBr3 perovskite, as shown in Figure S5a. Given a particular stoichiometry, formation of thin =1 layers above thicker layers is energetically preferred to formation of layers with more uniform thickness in Na-rich conditions, supporting the experimental observation of =1 and =2 layers at high NaBr concentrations. Using DFT to optimize the internal atomic coordinates in selected layered structures suggests that the Na atoms are pulled closer to the PbBr6 octahedra than suggested in Figure 1c (selected layered structures with optimized coordinates are shown in Figure S5b). Reassuringly, optimizing the volume of these layered structures results in almost identical distances to those reported in Figure 1c (DFT optimization suggests a negligible reduction of 0.6%). This strongly supports the interpretation of the GIXRD data presented above. Forming these layered structures requires a high concentration of NaBr to fill the 8


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interlayers (as shown in Figure S5a). In Na-poor conditions, it is difficult to disprove the existence of perovskite NaPbBr3. However, our DFT calculations suggest that due to the strong Na-Br interaction, a cubic NaPbBr3 perovskite is unlikely to form. In fact, there is an “energy plateau” for perovskite-like NaPbBr3 with a variety of structural distortions interfaced with CsPbBr3 perovskite, as shown in Figure S5c. The structural flexibility of perovskite-like NaPbBr3 and the energetic favorability of a strong interface with CsPbBr3 suggest that any Na-substitution for Cs that may likely result in amorphous structure, and consequently not be observed in XRD. Our DFT calculations therefore suggest that 2D perovskite with Na interlayers will form whenever sufficient Na is present. If sufficient Na is not present to fill the interlayer it is possible that an amorphous mixture of NaPbBr3/CsPbBr3 forms. Optical Characterization of 2D-3D Perovskite Ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectra of perovskite films with different NaBr molar ratios are investigated to find out material structure affecting on excited states, as shown in Figure 2a, 2b. With increasing NaBr molar ratio, both optical absorption exciton and PL peaks feature a red-shift. Here, according to the grain size extracted from -2 XRD data, the grain size does not dramatically change with increasing NaBr molar ratio in the range of ~21 to 29 nm, as shown in Figure S1b. All the crystal sizes are much larger than its Bohr diameter. Therefore, the reason from the grain size affecting optical properties would be ruled out.20 In a low-dimensional dielectric quantum well structure, dielectric confinement can occur arising from a quantum well sandwiched by barrier materials with a smaller 9


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dielectric constant (ε2) and a larger band gap than the well material (ε1), as shown in Figure S6.15 NaPbBr3 displays a much large band gap (3.67 eV) than CsPbBr3 (2.34 eV), extracted from absorption spectrum by Tauc plot in Figure S7. In addition, the extracted dielectric constant of NaPbBr3 (ε2=~1.5-2.8) is much smaller than that of CsPbBr3 (ε1=~4.8-8.9) according to impedance spectroscopy, as shown in Figure S8. The detailed calculation of dielectric constant is discussed in ST3. The film morphology of NaPbBr3 and CsPbBr3 is comparable as shown in Figure S9，which rules out the film morphology effect on measurement. The experimental value of dielectric constant of CsPbBr3 is comparable to theoretical one of 3.6.23 Here, CsPbBr3 displays narrower bandgap than NaPbBr3, while NaPbBr3 exhibits lower dielectric constant than CsPbBr3. Thus, the contribution from dielectric confinement would be significantly enhanced with reducing quantum well width (the crystal size of CsPbBr3).15 Here, increasing NaBr molar ratio lead to more NaPbBr3 generating and more obvious dielectric confinement is observed associated with a large Eb.24 Therefore, a reduced maximum optical band-gap (Eopt) is expected with enlarged Eb, as shown in Figure 2c. NaBr molar ratio dramatically affect the PLQY of perovskite films, as shown in Figure 2d, which is consistent with 2D perovskite with organic spacers.2, 3 3D CsPbBr3 with 0% NaBr shows a very low PLQY due to small Eb. With increasing NaBr molar ratio, the PLQY increases due to enhancement dielectric confinement with reducing dimensional distribution.15 2D-3D perovskite film with 15% NaBr yields a PLQY as high as ~51%. However, the PLQY drops with further NaBr molar ratio, which is likely due to less emitting material components as well as presence of ultrathin Na2Csn-1PbnBr3n+1 (n≤2) 10


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platelets.1, 24 Time-resolved PL decay spectra of perovskite film by spatially resolved confocal fluorescence lifetime imaging microscopy (FLIM) show that both PL intensity and lifetime progressively increases from 3D CsPbBr3 film to 2D-3D films, as shown in Figure 2e and Figure S10. The phasor plot also called the polar plot is used to analyze lifetime data.25 The extracted PL lifetime of ~4 ns for 3D CsPbBr3 progressively increase to 13 ns for 2D-3D perovskite with 15% NaBr. Temperaturedependent PL shows that PL peak displays obvious shift in NaBr-based 2D-3D perovskite, as shown in Figure S11. However, less PL peak shift is observed in pristine 3D CsPbBr3. This feature is likely correlated with a fact that there is nanoplatelet in 2D-3D perovskite with 10% NaBr, because nanoplatelet can more easily occur phase transition against temperature.26 2D-3D Perovskite Film Optimization In order to achieve high performance perovskite LEDs, a compact and pinhole-free film with high PLQY is indispensable to avoid any leakage current.27, 28 However, as shown in inset of Figure 3a, b, 2D-3D perovskite films with 0% and 10% NaBr display noncontinuous films with pinholes, which may due to poor solubility of CsBr.29,

30

Fortunately, an organic molecule 1,4,7,10,13,16-hexaoxacyclooctadecane (referred to as “crown” in the following sections, the molecular structure is shown in Figure S12) is added as a molecular additive in the solution, which can dramatically enhance the precursor solubility. The density of pinholes is largely suppressed by inserting crown molecules, as shown in Figure 3a, b. It indicates that incorporation crown can dramatically reduce the pin-hole generation. As shown in Figure S13, the Fourier 11


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transform infrared spectroscopy (FTIR) in perovskite film with 3.5 mg/ml crown also shows no vibrational peaks of -C-C- and -C-O-. Even increasing the crown addition more than ten times (50 mg/ml), the signal is rather weak. Therefore, we believe that there is likely no crown left in as-prepared perovskite film. It is worth noting that 2D3D perovskite-crown with 10% NaBr (defined 10% NaBr-crown) is compact and uniform with a root mean square (r.m.s) value as low as 1.54 nm. On the other hand, 3D CsPbBr3-crown still shows many pin-poles with high roughness up to 6.74 nm (see Figure S14). Obviously, small amount of crown can suppress the perovskite growth favoring uniform and compact film morphology. Dynamic light scattering (DLS) experiments based on precursor solution (Figure S12) provide the direct evidence that scattering particles with size decreased to several tens of nanometers, which is ascribed to the increasing precursor solubility after the presence of crown. The Scanning Electron Microscope (SEM) images of 2D-3D perovskite-crown films with different NaBr molar ratios (x = 5%, 15%, 50%, 70%) are shown in Figure S15. The morphologies of perovskite thin films become poor for 2D-3D perovskite-crown when x is larger than 15%. It is worth noting that the trace amount of crown does not dramatically change the properties, as shown UV-vis absorption and PL spectra in Figure S16, GIXRD results in Figure 3c. Both UV-vis absorption and PL peaks with and without crown do not show any peak shift. PLQY is slightly enhanced with crown incorporation, which yields a peak value of 55% for 2D-3D perovskite-crown with 15% NaBr. Azimuthal integral of the GIXRD patterns for 2D-3D perovskite-crown perovskite with different NaBr molar ratios (x = 0%, 5%, 10%, 15%, 50%, 70%) clearly 12


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show the diffraction peaks from distances nanoplatelet along out of plane direction with different values ( = 1, 2, 3), as shown in Figure 3d, Figure S16d. LED Device Performance LEDs consisting of multilayer device architecture with glass/indium tin oxide (ITO)/poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4′-(N-(4-butylphenyl)(TFB)/poly (Nvinylcarbazole)(PVK)/perovskite/2’,2’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole)(TPBi)/LiF/Al. The corresponding energy-level diagram of the multiple layers is shown in Figure 4a. In all the LED devices, it is worth noting that crown is always added to improve the film quality except it is mentioned. The energy level values are taken from references.5, 28, 31 The bilayer-structured of TFB/PVK is used as hole transport layers to realize efficient hole injection as well as achieve a compact and uniform perovskite layer.5,

32

The cross-sectional SEM image of a typical device is

shown in Figure 4b with the following order, ITO (~150 nm), TFB/PVK (~20 nm), perovskite (~40 nm), TPBi (~40 nm) and LiF/Al (~100 nm). To gain a better insight of the injection process in our devices, ultraviolet photoelectron spectroscopy (UPS) measurements (Figure S17) have been performed on different perovskite films in order to estimate the work function (WF) and valence band maximum (VBM). It shows that the perovskite films with different NaBr ratios display only neglectable offset in WF and VBM with values ~4.5 eV and ~5.6 eV, respectively, which are lined with the reported values of 4.46 eV and 5.64 eV for CsPbBr3.33 It indicates that charge injection barrier offset should be ruled out to effect the LED device performance. The current density-voltage-luminance (J-V-L) curves of LED devices based on 2D-3D 13


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perovskite-crown with different NaBr molar ratios (x = 0%, 5%, 10%, 15%) are shown in Figure 4c, respectively. The device based on 3D CsPbBr3-crown displays a high leakage current level, which is ascribed to poor film morphology. The 3D CsPbBr3 based LED shows a maximum brightness of 1342 cd m2 (Figure 4c). When 2D-3D perovskite-crown film with NaBr acts as an emitting layer, the current is significantly reduced due to the enhanced film quality as well as low charge transport properties along out-of-plane direction. The 2D-3D perovskite-crown with 10% NaBr based LED reaches a maximum brightness (Lmax) of 11560 cd m-2 at bias of 7.5 V. Further increasing NaBr molar ratio to 15% enhances brightness with a value of 11700 cd m-2 at the same bias but slightly increasing current level, which may be ascribed to reduced film quality. The electroluminescence (EL) turn-on voltages (Vt) for 2D-3D perovskite LEDs (calculated with luminance of 1 cd m−2) are slightly reduced in comparison with 3D one, which is likely correlated with their high luminescence efficiency. Figure 4d displays EQE-current density (EQE-J) curves of LEDs based on 2D-3D perovskite-crown with different NaBr molar ratios. And the electrical output characteristics are summarized in Table 1. An LED device with a size of 1 × 1 cm2 under operation is taken in Figure S18, which shows relatively large uniform emission. The EL emission of LED can be assumed as a standard Lambertian profile (see in Figure S19). Figure S20 displays EL spectra of perovskite LEDs based on 2D-3D perovskite-crown films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%). The FWHM of perovskite LEDs are 22.5 nm, 21.8 nm, 21.7 nm, 21.7 nm, respectively. It reveals that perovskite-crown films with different NaBr molar ratios (x= 0%, 5%, 10%, 14


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15%) do not change the shape and EL spectra. The current-efficiency-J (CE-J) and power-efficiency-J (PE-J) curves of devices based on 2D-3D perovskite-crown films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%) are provided in Figure S21. Accordingly, the calculated EQE for 2D-3D perovskite-crown film with 10% NaBr reaches a maximum value of ~15.9 %, which is one of a record value for perovskite LED reported to date based on our best knowledge, as shown in Table S3.2-7, 9, 27, 28, 30, 34-39.

A histogram of 2D-3D perovskite-crown with 10% NaBr based device

performance statistics for 45 devices shown an average EQE of 14.3% as shown in Figure 4f, which shows its good device reproducibility. The current efficiency is dropping with increasing of NaBr, which can be ascribed to poor film quality although PLQY is enhanced. Therefore, a continuous and pinhole-free perovskite film with high PLQY is indispensable to achieve a high LED efficiency.4 We also fabricated devices with perovskite film without crown, as shown in Figure S22a. With adding NaBr, the leakage current is suppressed due to improved film quality. The EQE of the device-based perovskite with 10% NaBr is 4.97% which is over six times in comparison with that of pristine one, and the device electrical output characteristic are summarized in Table S2. It clearly indicates that NaBr incorporation still can enhance the device performance even in the absence of crown. In addition, the concentration of crown also plays a key role on the device performance. The devices with different amounts of crown are shown in Figure S22b and Table S2. The lower efficiency of these devices indicates that 3.5 mg/ml is an optimized concentration. In order to investigate the device operational lifetime, the EQE of perovskite LED 15


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device against time encapsulated by epoxy resin is monitored, as shown in Figure 4e. The devices based on 3D CsPbBr3-crown film and 2D-3D perovskite-crown film with 10% NaBr are compared. The EQE values of 3D CsPbBr3-crown gradually decreases and drops 90% of its initial value in 30 days. However, the LED device based on 2D3D perovskite-crown film with 10% NaBr can maintained 80% of its initial value for 30 days. Time-dependent stability under constant current was also measured, as shown in Figure S23. It is clear that 10% NaBr-crown shows much longer operating life time. The luminance of 10% NaBr-crown based perovskite LED decrease to 50% after ~2.5 h while 0% NaBr-crown only shows ~0.5 h. The dramatically improved stability of 2D3D perovskite can be attributed to the compact and uniform perovskite, which have been reported in 2D solar cells.10 CONCLUSION In summary, we have developed a 2D-3D perovskite with small alkaline ions as spacers for high performance PeLEDs. Different from Cs+, alkaline ions of such as Na+ act as spacers for L2Csn-1PbnBr3n+1 instead of occupying perovskite cation site. 2D-3D perovskites display enhanced dielectric confinement. In addition, the film quality of 2D-3D perovskite films with NaBr molar ratio (x≤10%) can be dramatically improved with incorporation of organic molecule of crown. A compact and uniform 2D-3D perovskite-crown film (15% NaBr) with high PLQY of over 50% is achieved. A bright and efficient 2D-3D perovskite-crown (10% NaBr) based LED with an EQE of 15.9% is demonstrated. In addition, these 2D-3D perovskite-crown film based LEDs display an excellent stability. This 2D-3D perovskite shows a possible path to explore inorganic 16


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ions as spacers for specific properties, which may benefit other optoelectronic devices such as photodetectors, solar cells and lasers based on low dimensional perovskite. EXPERIMENTAL SECTION Materials: Lead (II) bromide (99.999%, metals basis), cesium bromide (99.999%, metals basis), sodium bromide (99.999%, metals basis) were purchased from Alfa Aesar. Hydrobromic acid (HBr, 48% weight in water) was purchased from TCI. NMA, PEA, BA, LiF and Al (evaporation slug, 99.999%) was purchased from Sigma Aldrich. DMSO (99.9%) was purchased from Innochem. Chlorobenzene (99.8% extra dry) and methylbenzene (99.8% extra dry) were purchased from Acros. TFB and PVK and TPBi were purchased from Han Feng Chemical Co. All the chemical materials were directly used without any further purifications. The synthesis of NMABr, PEABr, BABr were following the method described by literatures.2, 3, 4 Preparation of Perovskite Films: The precursor solution was obtained with different amount of NaBr according to their chemical component formula without or with crown in DMSO solution at 60 oC with 2-hour stirring. Perovskite films were obtained by spincoating precursor solution after being filtered through polytetraﬂuoroethylene filters (0.45 µm) via a two-step process (1000 r.p.m. for 5 s then 3000 r.p.m. for 55 s), respectively. Finally, we annealed the perovskite film at 100 oC for 1 min. For perovskite films with crown, the concentration of crown is 3.5 mg/ml except mentioned. Device Fabrication: ITO was cleaned by O2 plasma for 15mins and transferred to N2 filled glove box for the next steps. Then, TFB layers were subsequently spin-coated on ITO at 1000 r.p.m. for 45 s with a concentration of 8 mg ml-1 in chlorobenzene, followed 17


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by annealing at 120 oC for 20 min. PVK film was spin-coated on TFB layer at 4000 r.p.m. for 45 s with a concentration of 4 mg ml-1 in toluene, followed by annealing at 150 oC for 20 min. After that, the perovskite film was spin-coated on PVK layer. TPBi (40 nm), LiF (1.5 nm) and Al (100 nm) were deposited on perovskite film by thermal evaporation under vacuum pressure below 2× 10−6 mbar, respectively. The area of device is 0.09 cm-2. Characterization: SEM images of perovskite films were characterized by Carl Zeiss (Supra 55). AFM images of perovskite films were acquired with Cypher AFM (Cypher, S). DLS is measured by Nano-ZS90 (Malvern). UV-Vis absorption spectra were carried by a UV-Vis spectrometer (SPECORD S 600). IHR 320 (Horiba Instruments Inc.) was used to get PL spectra and PLQY. Fluorescence spectrophotometer (HORIB-FM-2015) was operated for the collection of PL decay lifetimes. For PL mapping, excited wavelength is 405 nm at 20 MHz with Nikon 60X/1.2NA WI objective lens. XRD was measured by Bruker D8 Advance X-ray diffractometer. 2D GIXRD was measured at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility. The wavelength of X-ray was 1.24 Å. MarCCD mounted vertically at a distance ≈326 mm from the sample with a grazing incidence angle of 0.1° and an exposure time of 60s was used to acquire 2D GIXRD patterns. Then, 2D GIXRD patterns were processed by FIT2D software and displayed in scattering vector q coordinates. The FILM approach was supported by Alba-STED system. FTIR spectra were recorded on Vertex-70 FTIR spectrophotometer. Hamahatsua C9920-11 was used for angle-dependent emission profile of perovskite LED device. J-V-L characteristics and EL spectra of perovskite18


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based LEDs were collected by Keithley 2400 sourcemeter and a PhotoResearch spectrometer PR670, respectively. The EQE calculation process was shown in ST4. Impedance spectroscopy was performed in the range of 103 Hz to 105 Hz using a 120MHz Precision Impedance Analyzer with an AC drive voltage of 10mV. UPS measurements were conducted in a SPECSTM photoelectron spectroscopy system with a monochromatized HeI (21.22 eV) excitation source DFT Calculations: DFT calculations were performed on perovskite CsPbBr3, NaPbBr3, layered Na4CsnPbn+2Br3n+8, layered PbBr2, and cubic NaBr and CsBr using the WIEN2k code. Internal atomic coordinates for all atoms were relaxed using the MSR1a method (to a force minimization of less than 5 mRyd/bohr). After atomic relaxation, atomic spheres were fixed at Na = 2.44 Bohr, Pb, Cs, Br = 2.50 Bohr for all compounds considered. The product of the maximum planewave (Kmax) and minimum atomic sphere (Rmin) was set at 7.0 for all compounds containing Na, it was set slightly higher at 7.17 for PbBr2, CsBr, and CsPbBr3. A k-point grid of 1000 points was used for the Brillouin zone of NaBr, an equivalently scaled grid was used for all other compounds (such that product of the number of k-points and the unit cell size was a constant). The PBE GGA exchange-correlation functional was used. Spin-orbit coupling was not considered. Convergence criteria was 0.0001 Ryd in energy, 0.001 e in charge, and 1 mRyd/Bohr in force.

Supporting Information Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org Estimation of interlayer separation of 2D perovskite and lattice strain; Calculation 19


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method of dielectric constant and EQE; -2 XRD patterns of different perovskite films; Radially integrated scattering intensity of GIXRD profiles; GIXRD patterns of perovskite films; AFM images of perovskite films with different ligands; DFT simulation results; Schematic of dielectric confinement effect; Tauc plots for CsPbBr3 and NaPbBr3 films; Capacitance versus frequency plot and SEM images of CsPbBr3 and NaPbBr3 films; Time-resolved PL decay spectra; Temperature-dependent PL of the different perovskite films; DLS measurements of perovskite solutions; FTIR spectra; AFM images of

CsPbBr3-crown and 2D-3D perovskite-crown with 10% NaBr; SEM

images of 2D-3D perovskite with different NaBr molar ratios; Absorption, PL spectra, PLQY of different perovskite films; Structural schematic of layered perovskites; UPS spectra of 2D-3D perovskite films with different NaBr molar ratios; Photograph of LED device; Angular intensity profile of a perovskite LED; EL spectra of perovskite LEDs based on 2D-3D perovskite-crown films; Performance of different devices based on 2D-3D perovskite-crown films; Electrical output of perovskite LEDs without crown and with different amounts of crown; Stability of different perovskite LEDs; Receipts of perovskite solutions; Electrical output characteristics of PeLEDs based on perovskite without and with different crown concentration in the precursor solution; A summary for reported output characteristics of representative perovskite LEDs.

Corresponding Author *Tao Song: [email protected] *Henning Sirringhaus: [email protected] *Baoquan Sun: [email protected] ORCID 20


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Tian Wu: 0000-0002-9504-3454 Yatao Zou: 0000-0002-7659-0989 Tao Song: 0000-0002-2978-8829 Henning Sirringhaus: 0000-0001-9827-6061 Baoquan Sun: 0000-0002-4507-4578 Associated Content The authors declare no competing financial interest. Author Contributions C.W. ‡ and T.W. ‡ contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2016YFA0202402), the National Natural Science Foundation of China (61674108, 11811520119), Jiangsu High Educational Natural Science Foundation (18KJA430012), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the 111 program and Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC).

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with Improved Stability for Tandem Solar Cells. J. Phys. Chem. Lett. 2016, 7, 746-751.

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Figures and Captions

Figure 1 (a) 2D GIXRD pattern images, (b) Azimuthal integration (plotted) of GIXRD patterns of the 2D-3D perovskite films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%, 50%, 70%). “*” label in the 2D GIXRD profiles in Figure (b) highlights diffraction peaks along the ring at q< 10 nm-1 from the separation distance between discrete platelet layers. (c) View of unit cell of Na2Csn−1PbnBr3n+1 perovskite with different values ( = 1, 2). (d) Current-density-voltage curves for hole-only devices based on 2D-3D perovskite films with 50% NaBr, 50% PEABr, 50% NMABr, 50% BABr, respectively. 29


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Figure 2 (a) Optical absorption for 2D-3D perovskite films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 70%). (b) PL for 2D-3D perovskite films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%, 50%, 70%). (c) Energy diagram schematically revealing the electronic band gap (Eg), the optical band gap (Eopt), and the exciton binding energy (Eb) of perovskite with increasing NaBr molar ratios. We assume that all the emission originates from 3D perovskite due to funnel effect. CBM: conduction band minimum; VBM: valence band maximum. (d) PLQY of 2D-3D perovskite films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 70%) (excited at 365 nm at ~2.3 mW cm-2). (e) PL lifetime mapping for 2D-3D perovskite films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%) based on FLIM approach. The scan area of all time-resolved confocal images is 20 µm × 20 µm.

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Figure 3 SEM images of (a) 3D CsPbBr3-crown film with 0% NaBr. Inset in Figure 3a shows SEM image of 3D CsPbBr3 film with 0% NaBr, scale bar: 200 nm. (b) 2D-3D perovskite-crown film with 10% NaBr. Inset in Figure 3b shows SEM image of 2D-3D perovskite film with 10% NaBr, scale bar: 200 nm. (c) 2D GIXRD pattern images of 2D-3D perovskite-crown films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%, 50%, 70%) (d) Azimuthal integration (plotted) of GIXRD patterns in (c). “*” label in the 2D GIXRD profiles highlights diffraction peaks along the ring at q< 10 nm-1 from the separation distance between discrete layers. 31


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Figure 4 (a) Energy band diagram of the PeLED device structure. (b) Cross-sectional SEM image of the LED device based on 2D-3D perovskite films. (c) J-V-L data and (d) EQE-J of devices based on 2D-3D perovskite-crown films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%). (e) Time-dependent EQE measurement for perovskite LEDs based on 2D-3D perovskite-crown films with 0% NaBr and 10% NaBr (The devices were tested every 24 hours at a luminance of approximately 1000 cd/m2). (f) EQE histogram of 45 LED devices for perovskite LEDs based on 2D-3D perovskitecrown films with 10% NaBr.

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ACS Nano

Table 1 Electrical output characteristics of LED devices based on 2D-3D perovskitecrown films with different NaBr molar ratios (x= 0%, 5%, 10%, 15%).

Perovskite

VT (V)

CE (cd

A-1)

PE (lm

W-1)

Lmax (cd

m-2)

EQE

EL peak

(%)

(nm)

0% NaBr-crown

~3.1

12.6

9.4

1342

3.7

516

5% NaBr-crown

~2.7

28.6

25.4

5370

6.7

518

10% NaBr-crown

~2.6

50.3

45.1

11560

15.9

518

15% NaBr-crown

~2.6

43.2

34.2

13820

12.6

518

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TOC Figure:

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Alternative Type 2D-3D Lead Halide Perovskite with Inorganic Sodium

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