Citation

Lu Cheng, Yu Cao, Rui Ge, Ying-Qiang Wei, Na-Na Wang, Jian-Pu Wang, Wei Huang. Sky-blue perovskite light-emitting diodes based on quasi-two-dimensional layered perovskites[J]. Chin. Chem. Lett., 2017,28(1): 29-31  

Lu Cheng^a, Yu Cao^a, Rui Ge^a, Ying-Qiang Wei^a, Na-Na Wang^a, Jian-Pu Wang^a, Wei Huang^a,b     

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.

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.

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.

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.

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].

Figure 1

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

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].

Figure 2

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

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).

Figure 3

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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.

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.

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.