English

• 1.   Introduction

  Organic-inorganic hybrid perovskites have received considerable attention due to their unique properties of high photoluminescence quantum efficiency, high mobility, tunable color and easy process [1-5]. The external quantum efficiency (EQE) of nearinfrared (NIR) and green perovskite light-emitting diodes (LEDs) has reached 3.5% [3] and 8.5% [6], respectively. Recently, Li et al. reported 5.7% EQE in red perovskite LEDs based on CsPbI₃ nanocrystals [7]. However, the efficiency of blue perovskite LEDs is still very low. By tailoring chloride in perovskites, blue electroluminescence (EL) for CH₃NH₃PbCl₃ device can be observed at 77 K [8]. Kim et al. and Kumawat et al. can achieve blue EL of perovskites at room temperature, but the EQEs are below 0.001% [9, 10], which is due to lower photoluminescence quantum efficiencies (PLQEs) and halide phase segregation of perovskite films. The highest reported EQE for blue perovskite LEDs is 0.07% based on inorganic perovskite quantum dots (QDs, CsPb (Cl/Br)₃) [11]. However, this requires careful synthesis of QDs with high optical properties, and the device turn-on voltage is high (5.1 V) [11]. Here, we demonstrate a sky-blue layered perovskite emitter that contains mixed organic (4-phenylbutylamine, 4-PBA) and inorganic (cesium, Cs) cations. We find that sky-blue layered perovskite with good film coverage can be achieved at low temperature by introducing a large organic cation. This provides efficient EL at 491 nm, leading to sky-blue perovskite LEDs with maximum EQE of 0.015% and a low turn-on voltage of 2.9 V.

  2.   Experimental

  2.1   Synthesis and materials preparation

  Colloidal ZnO nanocrystals were synthesized by a solution-precipitation process [12] with some modifications. 4-PBABr (C₆H₅C₄H₈NH₃Br) were synthesized by adding 1.8 g hydrobromic acid (45 wt% in water) into a stirring solution of 4-phenylbutylamine (10 mmol) in tetrahydrofuran (THF, 50 mL) at 0 8C for 2 h. Then the solution was evaporated at 50 8C to obtain the precipitate, which was washed three times with THF:CH₂Cl₂ (3:1) mixture and dried in a vacuum drying oven. The perovskite precursor solutions were prepared by dissolving 4-PBABr, CsBr and PbBr₂ with molar ratio of 2:1:1 in dimethyl sulfoxide (DMSO) (7 wt%) and stirred at 60 8C for 2 h in a nitrogen-filled glovebox. The precursor was used to deposit layered perovskite films, which was abbreviated as PCPbB films below.

  2.2   Device fabrication

  The device structure is indium tin oxide (ITO)/polyethylenimine ethoxylated (PEIE) modified zinc oxide (ZnO, ~20 nm)/PCPbB (~40 nm)/poly (9, 9-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine) (TFB, ~40 nm)/molybdenum oxide (MoO_x, ~7 nm)/Al (~100 nm). A detailed description of device fabrication can be found elsewhere [3]. Here, a solution of PEIE in 2-methoxyethanol (0.4 wt%) was spin-coated onto the ZnO films at a speed of 5000 rpm. The substrates were rinsed twice with DMF, leaving ultrathin layers of PEIE on top of the ZnO films. The ultrathin layer of PEIE can significantly decrease the work function of ZnO [13]. The perovskite films were prepared by spin-coating the precursor solution onto the PEIE treated ZnO films, followed by annealing on a hot plate at 80 8C.

  2.3   Characterization

  All perovskite LED device characterizations were carried out at room temperature in a nitrogen-filled glovebox. A Keithley 2400 source meter and a fiber integration sphere (FOIS-1) couple with a QE65 Pro spectrometer was used for the measurements [14]. Atomic force microscope (AFM) images were collected in noncontact mode (Park XE7). UV-vis absorbance and photoluminance (PL) spectra of the perovskite films were recorded using a UV-vis spectrophotometer (UV-1750, SHIMADZU) and a fluorescent spectrophotometer (F-4600, HITACHI) with a 200 W Xe lamp as the excitation source, respectively.

  3.   Results and discussion

  The layered perovskites with large organic and small organic/ inorganic cations are intermediates between two-dimensional (2D) and three-dimensional (3D) perovskites, which have exhibited good stability in perovskite solar cells [15, 16]. As shown in Fig. 1a, the absorbance spectrum of PCPbB film has exciton absorption peaks at 427 and 455 nm, respectively. There is no absorption peak close to 3D perovskite. When excited at 350 nm, the PL spectrum shows a weak emission peak at 435 nm which is consistent with the absorbance spectrum. Notably, there is a strong emission peak at 475 nm, which is~52 nm blue-shifted compared to that of 3D perovskite (CsPbBr₃, 527 nm) [17]. Also, a weak emission peak at 466 nm is hidden by the PL tail of the 475 nm peak. The absorption and PL measurements indicate the formation of quasi-2D perovskite in PCPbB film [15], possessing optical properties between that of the 2D perovskite ((4-PBA)₂PbBr₄, Fig. 1b) and 3D perovskite (CsPbBr₃) [17].

  图 1

  

  图 1  Absorption and PL spectra of the perovskite film. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

  Figure 1.  Absorption and PL spectra of the perovskite film. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

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  Generally, the 3D inorganic perovskite film has poor film morphology [7, 17]. The incomplete surface coverage can cause pin-holes of perovskite films, limiting device performance due to non-radiative recombination [2, 3]. The PCPbB film has a uniform surface coverage and AFM image (Fig. 2a) shows a root-meansquare roughness of 2.3 nm, which is comparable to that of the 2D (4-PBA)₂PbBr₄ perovskite film (2.2 nm, Fig. 2b) with good film morphology [18]. We suggest the good morphology of the PCPbB film benefits from the inclusion of large organic cation [15].

  图 2

  

  图 2  AFM images of the perovskite films. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

  Figure 2.  AFM images of the perovskite films. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

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  Fig. 3a shows the EL spectra of the PCPbB perovskite LED devices at different voltages. The EL spectrum shows peaks at 435, 466 and 491 nm, which is consistent with PL emission spectrum of the PCPbB perovskites. Also, the EL spectra did not change as the applied voltage increased, indicating that excitons are effectively confined in the emission layer. No electroluminescence from the ZnO or TFB layers is observed during the measurement [19, 20]. This device exhibits sky-blue emission with Commission Internationale de l’Eclairage (CIE) color coordinates of (0.09, 0.25). The current density-voltage-luminance (J-V-L) characteristics are shown in Fig. 3b. The sky-blue perovskite LEDs turns on at a low voltage of 2.9 V (defined as the voltage at the luminance of 1 cd/m²). After turn on, the luminance immediately increases as voltage increases, and reaches a maximum luminance of 186 cd/m² at 4.5 V. The peak EQE is 0.015% at 186 cd/m2 (Fig. 3c).

  图 3

  

  图 3  (a) EL spectra of PCPbB device at various voltages; (b) dependence of current density and luminance on the driving voltage; (c) EQE versus current density.

  Figure 3.  (a) EL spectra of PCPbB device at various voltages; (b) dependence of current density and luminance on the driving voltage; (c) EQE versus current density.

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  4.   Conclusion

  We have demonstrated that the addition of large organic cation in the inorganic perovskite structure can produce quasi-2D layered perovskite. The large organic cations can facilitate the formation of perovskite film with good film coverage and emission properties. This leads to an efficient sky-blue perovskite LED with low turn-on voltage and high EQE.

  5.   Acknowledgments

  This work is financially supported by the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (No. 2015CB932200), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20131413, BK20140952, BM2012010), the National Natural Science Foundation of China (Nos. 11474164, 61405091), the National 973 Program of China (No. 2015CB654901), the Jiangsu Specially-Appointed Professor program, the Synergetic Innovation Center for Organic Electronics and Information Displays.

• 1. [1]

     Deschler F., Price M., Pathak S.. High photoluminescence efficiency and optically pumped lasing in solution-processed mixed halide perovskite semiconductors[J]. J. Phys. Chem. Lett., 2014, 5:  1421-1426. doi: 10.1021/jz5005285

  2. [2]

     Tan Z.K., Moghaddam R.S., Lai M.L.. Bright light-emitting diodes based on organometal halide perovskite[J]. Nat. Nanotechnol., 2014, 9:  687-692. doi: 10.1038/nnano.2014.149

  3. [3]

     Wang J., Wang N., Jin Y.. Interfacial control toward efficient and low-voltage perovskite light-emitting diodes[J]. Adv. Mater., 2015, 27:  2311-2316. doi: 10.1002/adma.v27.14

  4. [4]

     Wang N., Si J., Jin Y., Wang J., Huang W.. Solution-processed organic-inorganic hybrid perovskites:a class of dream materials beyond photovoltaic applications[J]. Acta Chim. Sin., 2015, 73:  171-178. doi: 10.6023/A14100711

  5. [5]

     Liu T.H., Chen K., Hu Q.. Fast-growing procedure for perovskite films in planar heterojunction perovskite solar cells[J]. Chin. Chem. Lett., 2015, 26:  1518-1521. doi: 10.1016/j.cclet.2015.09.022

  6. [6]

     Cho H., Jeong S.H., Park M.H.. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes[J]. Science, 2015, 350:  1222-1225. doi: 10.1126/science.aad1818

  7. [7]

     Li G., Rivarola F.W.R., Davis N.J.L.K.. Highly efficient perovskite nanocrystal light-emitting diodes enabled by a universal crosslinking method[J]. Adv. Mater., 2016, 28:  3528-3534. doi: 10.1002/adma.201600064

  8. [8]

     Sadhanala A., Ahmad S., Zhao B.. Blue-green color tunable solution processable organolead chloride-bromide mixed halide perovskites for optoelectronic applications[J]. Nano Lett., 2015, 15:  6095-6101. doi: 10.1021/acs.nanolett.5b02369

  9. [9]

     Kim Y.H., Cho H., Heo J.H.. Multicolored organic/inorganic hybrid perovskite light-emitting diodes[J]. Adv. Mater., 2015, 27:  1248-1254. doi: 10.1002/adma.201403751

  10. [10]

      Kumawat N.K., Dey A., Narsimhan K.L., Kabra D.. Near infrared to visible electroluminescent diodes based on organometallic halide perovskites:structural and optical investigation[J]. ACS Photon., 2015, 2:  349-354. doi: 10.1021/acsphotonics.5b00018

  11. [11]

      Song J., Li J., Li X.. Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX₃)[J]. Adv. Mater., 2015, 27:  7162-7167. doi: 10.1002/adma.201502567

  12. [12]

      Qian L., Zheng Y., Choudhury K.R.. Electroluminescence from light-emitting polymer/ZnO nanoparticle heterojunctions at sub-bandgap voltages[J]. Nano Today, 2010, 5:  384-389. doi: 10.1016/j.nantod.2010.08.010

  13. [13]

      Zhou Y., Fuentes-Hernandez C., Shim J.. A universal method to produce low-work function electrodes for organic electronics[J]. Science, 2012, 336:  327-332. doi: 10.1126/science.1218829

  14. [14]

      Dai X., Zhang Z., Jin Y.. Solution-processed, high-performance light-emitting diodes based on quantum dots[J]. Nature, 2014, 515:  96-99. doi: 10.1038/nature13829

  15. [15]

      Smith I.C., Hoke E.T., Solis-Ibarra D., McGehee M.D., Karunadasa H.I.. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability[J]. Angew. Chem. Int. Ed., 2014, 53:  11232-11235. doi: 10.1002/anie.201406466

  16. [16]

      Quan L.N., Yuan M., Comin R.. Ligand-stabilized reduced-dimensionality perovskites[J]. J. Am. Chem. Soc., 2016, 138:  2649-2655. doi: 10.1021/jacs.5b11740

  17. [17]

      Yantara N., Bhaumik S., Yan F.. Inorganic halide perovskites for efficient light-emitting diodes[J]. J. Phys. Chem. Lett., 2015, 6:  4360-4364. doi: 10.1021/acs.jpclett.5b02011

  18. [18]

      Mitzi D.B., Chondroudis K., Kagan C.R.. Organic-inorganic electronics[J]. IBM J. Res. Dev., 2001, 45:  29-45. doi: 10.1147/rd.451.0029

  19. [19]

      Wang N., Cheng L., Si J.. Morphology control of perovskite light-emitting diodes by using amino acid self-assembled monolayers[J]. Appl. Phys. Lett., 2016, 108:  141102. doi: 10.1063/1.4945330

  20. [20]

      Trattnig R., Pevzner L., Jäger M.. Bright blue solution processed triple-layer polymer light-emitting diodes realized by thermal layer stabilization and orthogonal solvents[J]. Adv. Funct. Mater., 2013, 23:  4897-4905. doi: 10.1002/adfm.v23.39

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  Figure 1  Absorption and PL spectra of the perovskite film. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

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  Figure 2  AFM images of the perovskite films. (a) PCPbB film and (b) (4-PBA)₂PbBr₄ film.

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  Figure 3  (a) EL spectra of PCPbB device at various voltages; (b) dependence of current density and luminance on the driving voltage; (c) EQE versus current density.

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