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Growth of Nano-Sized Single Crystals for Efficient Perovskite Light-Emitting Diodes Seungjin Lee, Jong Hyun Park, Yun Seok Nam, Bo Ram Lee, Baodan Zhao, Daniele Di Nuzzo, Eui Dae Jung, Hansol Jeon, Ju-Young Kim, Hu Young Jeong, Richard H. Friend, and Myoung Hoon Song ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b09148 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018
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Growth of Nano-Sized Single Crystals for Efficient Perovskite Light-Emitting Diodes Seungjin Lee,†,∥ Jong Hyun Park,†,∥ Yun Seok Nam,† Bo Ram Lee,‡,§ Baodan Zhao,‡ Daniele Di Nuzzo,‡ Eui Dae Jung,† Hansol Jeon,† Ju-Young Kim,† Hu Young Jeong,₤ Richard H. Friend,‡ Myoung Hoon Song†,* †
School of Materials Science Engineering, Ulsan National Institute of Science and Technology,
UNIST-gil 50, Ulsan, 44919, Republic of Korea. ‡
Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom.
₤
UNIST Central Research Facilities, Ulsan National Institute of Science and Technology,
UNIST-gil 50, Ulsan, 44919, Republic of Korea. §
Department of Physics, Pukyong National University, 45 Yongso-ro, Nam-Gu, Busan 48513,
Republic of Korea. KEYWORDS: perovskite light-emitting diode, nano-sized single crystal, amine ligand, defect site, electroluminescence blinking Corresponding Author: Prof. M. H. Song *E-mail:
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ABSTRACT:
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Organic–inorganic hybrid perovskites are emerging as promising emitting
materials due to their narrow full-width at half-maximum emissions, color tunability, and high photoluminescence quantum yields (PLQYs). However, the thermal generation of free charges at room temperature results in a low radiative recombination rate and an excitation intensitydependent PLQY, which is associated with the trap density. Here, we report perovskite films composed of uniform nano-sized single crystals (average diameter = 31.7 nm) produced by introducing bulky amine ligands and performing the growth at a lower temperature. By effectively controlling the crystal growth, we maximized the radiative bimolecular recombination yield by reducing the trap density and spatially confining the charges. Finally, highly bright and efficient green emissive perovskite light-emitting diodes that do not suffer from electroluminescence blinking were achieved with a luminance of up to 55,400 cd m−2, current efficiency of 55.2 cd A−1, and an external quantum efficiency of 12.1%.
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Organic-inorganic hybrid perovskites (OIPs) are emerging as promising emitting materials for light-emitting diodes (LEDs) due to their narrow full-width at half-maximum (FWHM) emissions, color tunability, and high photoluminescence quantum yields (PLQYs).1-6 However, the low exciton binding energies of OIPs mean the excitons are thermally dissociated and so generate free charges. This results in a low electron–hole binding rate for radiative decay and a PLQY dependence on the excitation intensity.7-9 To increase the radiative bimolecular recombination rate, several strategies have recently been adopted such as multi-layered quasi-2D perovskites,4,6,10 reduction of the perovskite grain size1,5,11,12 and mixed cation perovskites.13,14 These approaches successfully enhanced external quantum efficiency (EQE) values of perovskite LEDs (PeLEDs), achieving up to 10.4% for green emissions13 and 11.7% for near-infrared emissions.6 Although these strategies significantly increase PeLED efficiency, there is still room for improvement. The reduced grain size spatially confines the charges, but with a trade-off in the increased grain boundary area that may provide defects that act as charge traps. In addition, perovskite grains processed by spin-coating unavoidably contain numerous outer (grain boundary) and inner (crystallographic) defects.15-17 These defects of perovskite grains cause charge trapping and ion movement, which degrade device efficiency and stability. For example, they contribute to such undesired phenomena as hysteresis5,18-20 in OIP solar cells and electroluminescent (EL) blinking21-26 in PeLEDs. EL blinking is a key problem that must be resolved for the use of PeLEDs in display applications. Here, we report a simple method to grow methylammonium lead tribromide (MAPbBr3) films uniformly covered with nano-sized single crystals by introducing phenylmethylamine (PMA), a bulky amine ligand, and controlling the temperature. This method enables the formation of
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defect-free nanograins with substantially reduced crystal size. With such an effective control over crystal growth, the radiative recombination rate was maximized by reducing charge trapping and confining the charges within the small grains. Finally, highly efficient and electroluminescence (EL) blinking-free PeLEDs are achieved with a luminance of 55,400 cd m−2, current efficiency (CE) of 55.2 cd A−1, and an EQE of 12.1%. RESULTS AND DISCUSSION Device Structure and Film Fabrication Method. The PeLED device architecture and corresponding cross-sectional scanning electron microscopy (SEM) image are shown in Figure 1a,c. MAPbBr3 films were fabricated by the anti-solvent dropping method. In this process, the PMA-containing chlorobenzene (CB) anti-solvent is dropped onto the precursor-coated substrate during spin-coating (Figure 1d). The anti-solvent immediately washes out the “good” solvent, a dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) mixture in which the precursors are highly soluble; this leads to the fast crystallization of MAPbBr3, which hampers the growth of highly crystalline films (Figure S1a). However, the bulky PMA ligand added to the CB lowers the critical free energy of nucleation and critical nucleus size by effectively reducing the nuclei surface energies, which enhances the nucleation of MAPbBr3. Moreover, PMA retards crystal growth by capping the MAPbBr3 nuclei (Figure 1b and Figure S1b) and leads to the growth of highly crystalline and small crystals. In contrast to the case where surface passivation is applied after film growth, our method not only grows highly crystalline crystals with few defects but also passivates the surface defects of the crystals. Morphologies and Film Analysis. The morphologies of the MAPbBr3 films fabricated with various concentrations of PMA at different temperatures were observed using SEM (Figure 2a).
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The MAPbBr3 film fabricated without PMA showed large round polycrystalline grains. The MAPbBr3 film prepared with 0.25 vol.% PMA showed a better crystallinity, with smaller and more cubic-shaped crystals. For the optimized PMA content of 0.5 vol.%, the MAPbBr3 crystals were almost perfectly cubic shape with a further reduced size. However, the MAPbBr3 film fabricated with 1.0 vol.% PMA showed additional crystal shapes, such as plates and rods. As the concentration of PMA increased further, the plate-like crystals gradually became dominant until the MAPbBr3 film was composed only of such crystals at 4.0 vol.% PMA. This is attributed to the formation of a 2D layered perovskite structure: the high concentration of the bulky amine meant that the 3D perovskite structure was limited to a 2D layered perovskite structure by steric hindrance (Figure S2). The 3D to 2D layered perovskite structural change was confirmed by Xray diffraction (XRD) and absorption measurements (Figure 2b,c). The XRD peak at 14.9° and the band edge absorption at 520 nm correspond to 3D perovskite, and the XRD peaks at 5.28° and 10.56° and the absorption peak at 399 nm correspond to 2D layered perovskite. The MAPbBr3 films fabricated with 0 vol.% to ≤0.5 vol.% PMA showed only the XRD peak at 14.9° and the band edge absorption at 520 nm. In contrast, those fabricated with >0.5 vol.% PMA showed additional XRD peaks at 5.28° and 10.56° and a second absorption peak at 399 nm. As the concentration of PMA increases, the XRD peaks at 5.28° and 10.56° and absorption peak at 399 nm increase in intensity while the XRD peak at 14.9° and absorption peak at 520 nm decrease, which indicates the structural change from the 3D to 2D layered perovskite. Finally, MAPbBr3 grown with 4.0 vol.% PMA showed only the XRD peaks at 5.28° and 10.56° and absorption peak at 399 nm, which indicates the presence of only the 2D layered perovskite. In agreement with the XRD and absorption results, this 4.0 vol.% PMA MAPbBr3 showed a 404 nm photoluminescence (PL) emission due to the large Bandgap of the 2D perovskite (Figure
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2d). Moreover, the morphologies of the MAPbBr3 films fabricated with different PMA concentrations at a low temperature (10~15°C) were also observed (Figure S3). These films possessed smaller crystals because crystal growth was further slowed at the low temperature (Figure 2a and Figure S1c), while their morphologies showed a similar trend according to PMA concentration to that of the high-temperature cases. The average crystal size of the MAPbBr3 film fabricated with 0.5 vol.% PMA reduced from 81.6 nm to 31.7 nm on decreasing the temperature (Figure S4). Transmission Electron Microscopy and Fourier Transform Infrared Spectroscopy. The 0.5 vol.% MAPbBr3 crystals also were investigated by transmission electron microscopy (TEM). These crystals were prepared by sonicating the MAPbBr3 film in CB and dispersing them in CB. The MAPbBr3 crystals had a width of approximately 100 nm and a cubic shape (Figure 3a). The electron diffraction (ED) pattern of a MAPbBr3 crystal confirmed the presence of a single crystal of high crystallinity (Figure 3b), which gave intense diffraction peaks from the {100} planes. Fourier transform infrared (FT-IR) spectroscopy was performed to investigate the existence of PMA after the MAPbBr3 film growth (Figure S5). The presence of PMA was confirmed by the apparent benzene ring absorption peaks, which include the peak at 693, 750 cm−1 from C-H outof-plane deformation, 1400–1500 cm−1 from C-C ring stretching, and 3000–3100 cm−1 from aromatic C-H stretching.27,28 The FT-IR spectrum of PMA-free MAPbBr3 did not show any absorption peaks related to the benzene ring modes, which indicates that the CB anti-solvent was vaporized during spin-coating. In contrast, the FT-IR spectrum of PMA-containing MAPbBr3 showed benzene ring absorption peaks, which indicate that PMA remains on the surface of the MAPbBr3 crystals due to their strong mutual interaction. Therefore, for the optimized PMA concentration of 0.5 vol.%, PMA ligands not only grow nano-sized single crystals with few
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defects but also passivate the surface defects of the crystals. To investigate the MAPbBr3 trap density with and without PMA, the dark current of hole-only devices was measured under applied bias (Figure S6). The MAPbBr3 with PMA has lower trap density compared with the MAPbBr3 without PMA (see details for trap density calculation in Supporting Information). Enhancement of Optical Properties and PeLED Performance. To confirm the improvement of optical properties of PMA-containing MAPbBr3 films, time-resolved PL decay and steady-state PL measurements were performed. The lifetime of MAPbBr3 was enhanced from 18.2 ns (without PMA) to 114.2 ns by the addition of 0.5 vol.% PMA, indicating a strong reduction in the non-radiative decay pathway (Figure 4a and Table S1). The 2D layered perovskite films formed with higher PMA contents showed shorter lifetimes, which reduced to 0.34 ns for 4.0 vol.% PMA. The MAPbBr3 films prepared with PMA treatment showed much higher PLQYs compared with the pristine film (Figure 4b) and the MAPbBr3 with optimized PMA concentration of 0.50 vol.% showed much enhanced external PLQY (from 10.7% to 57.8%). PeLEDs were optimized by testing different electron transport layer (ETL) thicknesses to increase the recombination rate by balancing the charge carriers (Figure S7 and Table S2). The charge injection-balance was confirmed by measuring single carrier current densities of single carrier devices with different thicknesses of 2,2 ′ ,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole) (TPBi) (Figure S8). Perovskite films were also fabricated at an optimized low temperature to reduce the grain size, which increases the recombination rate by confining the charges within the small grains. The MAPbBr3 film fabricated at the optimized temperatures showed significantly enhanced PLQY compared with the MAPbBr3 fabricated at other temperatures. Along with enhancement of PLQY, PeLEDs fabricated at the optimized temperature showed increased maximum luminance and device efficiencies (Figure S9 and Table
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S3). The voltage-dependent current density, luminance, and device efficiency characteristics were measured for PeLEDs fabricated with the optimized temperature and ETL thickness and different PMA concentrations; the results are shown in Figure 4 and Table 1. The PMA-treated devices showed lower leakage current densities than the reference device due to the reduced number of perovskite defect sites (Figure 4c). Moreover, the devices with a PMA content of
