Enhanced Performance of Red Perovskite Light-Emitting Diodes

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Enhanced Performance of Red Perovskite Light-Emitting Diodes through the Dimensional Tailoring of Perovskite Multiple Quantum Wells Jin Chang, Shuting Zhang, Nana Wang, Yan Sun, Yingqiang Wei, Renzhi Li, Chang Yi, Jianpu Wang, and Wei Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03417 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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The Journal of Physical Chemistry Letters

Enhanced Performance of Red Perovskite LightEmitting Diodes through the Dimensional Tailoring of Perovskite Multiple Quantum Wells Jin Chang1, Shuting Zhang1, Nana Wang1, Yan Sun1, Yingqiang Wei1, Renzhi Li1, Chang Yi1, Jianpu Wang1, and Wei Huang1,2 1

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China 2

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi’an 710072, China

E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Halide perovskite multiple quantum wells (MQWs) have recently shown great potential in the field of light-emitting diodes. We report a facile solution-based approach to fabricate dimensionality-tunable perovskite MQWs by introducing 1-naphthylmethyl ammonium (NMA) cations into CsPbI3 perovskites. Through the dimensional tailoring of (NMA)2Csn-1PbnI3n+1 perovskite MQWs, the crystallinity and photoluminescence quantum efficiencies (PLQEs) are significantly improved. We have obtained high performance red perovskite light-emitting diodes (PeLEDs) with a luminance of 732 cd m-2 and a maximum external quantum efficiency of 7.3%, which are among the best performing red PeLEDs. Significantly, the maximum luminance of our PeLEDs is obtained at a low applied voltage of 3.4 V, with a turn-on voltage close to the perovskite bandgap (Vturn-on ≈ 1.9 V). These outstanding performances demonstrate that dimensional tailoring of perovskite MQWs is a feasible and effective strategy to achieve high performance PeLEDs, which is attractive for the full-color display applications of perovskite.

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Halide perovskites have emerged as promising light-emitting materials in the field of lightemitting diodes, mainly due to their tunable bandgap, high color purity and low-cost solution processability.1-4 According to the dimensionality of materials, the perovskites used in perovskite light-emitting diodes (PeLEDs) mainly contain three-dimensional (3D) perovskites,1-3 twodimensional (2D) perovskites,5, 6 quasi-2D perovskites,7, 8 and so on. The 3D perovskite (such as CH3NH3PbI3) films usually possess various non-radiative recombination channels caused by defects or pin-holes, which limits the device performance.2 Compared with 3D perovskites, 2D perovskites usually possess a slower crystallization speed, which is beneficial to fabricate uniform thin films.6 However, the strong exciton-phonon interaction9 in 2D perovskite increases the exciton quenching rates at room temperature, thus leads to poor PeLED performances.5, 6, 10 By contrast, quasi-2D perovskite films possess combined advantages of both 3D and 2D perovskites, thus showing great potential in optoelectronic fields.7, 8, 11-13 It is known that quasi2D perovskite film usually is a self-assembled mixture of perovskite quantum wells (QWs) with different bandgaps, thus can be considered as multiple quantum well (MQW) structure.7 The bandgaps of perovskite QWs are determined by the layer number of metal halide octahedrons. Owing to the energy cascade,7 injected carriers can be confined within the small region of narrow bandgap perovskite QWs with high luminescence efficiency, thus boosting the external quantum efficiency (EQE) of PeLEDs up to 11.7% in the near-infrared (NIR) region.7 Generally, red light-emitting perovskites can be obtained by decreasing the dimensionality of iodide-based perovskite and/or mixing the halide anions (iodide and bromide). For lowdimensional nanocrystal-based red PeLEDs, over 5% EQE has been achieved by means of the atomic layer deposition (ALD) technique, but it is time-consuming and not suitable for scale-up production.14 By using low-temperature solution process, efficient (EQE = 3.7%) red PeLEDs


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with good stability (a lifetime of 5h under a constant current density of 10 mA cm-2) are achieved by using quasi-2D iodide-based perovskite MQWs.15 For halide-mixed red PeLEDs, phase segregation is often observed in perovskites, which leads to the emission peak shift under operation conditions.16-18 Recently, emission peak-stabilized red PeLEDs with 5% EQE are reported by using long-chain organic ammonium as capping layer at CsyMA1-yPb(I1-xBrx)3 perovskite nano-grain surfaces.19 Similarly, red PeLEDs with 7.3% EQE are obtained by using phenylbutylammonium-passivated perovskite nanoplates.20 Despite the fast improvement of the EQE value of red PeLEDs, the turn-on voltage is still high (> 3 V)19, 20 and the luminance is relatively low.14, 15, 20 In this work, we demonstrate that the overall performance of PeLEDs can be significantly enhanced by tailoring the dimensionality of perovskite MQWs through changing the ratio of large cations to small cations in precursor solutions. Through the dimensional modulation of Csbased perovskite MQWs, a maximum EQE of 7.3% is achieved for red PeLEDs with a low turnon voltage of 1.9 V and a high luminance of 732 cd [email protected] V, representing the best performing red PeLEDs.

Figure 1. a) Schematic structures of (NMA)2Csn-1PbnI3n+1 (NCPI, n is the layer number of [PbI6]4octahedral layers) perovskites with different n values, showing the dimensional evolution from 2D (n = 1) to 3D (n = ∞) perovskite. The purple, big blue, red, gray, and small blue spheres


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represent Cs, Pb, I, C and N atoms, respectively. b) XRD patterns of NCPI perovskite films prepared from precursor solutions with different NMAI/CsI/PbI2 molar ratios (2:1:2 for NCPI-1; 2:2:2 for NCPI-2; 2.2:3:2 for NCPI-3).

Cesium-based quasi-2D perovskite films are prepared by depositing precursor solutions of 1naphthylmethyl ammonium iodide (NMAI), cesium iodide (CsI) and PbI2 with various molar ratios in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The general molecular formula is referred to as (NMA)2Csn-1PbnI3n+1 (NCPI), where n is the layer number of [PbI6]4- octahedrons. Figure 1a shows the schematic structures of 2D, quasi-2D and 3D perovskites, illustrating the dimensional evolution of Cs-based perovskite. NCPI perovskite films with desired dimensionality can be obtained by tailoring the molar ratio of NMAI, CsI and PbI2 in precursor solutions, followed by spin-coating and annealing at 110 °C for 5 min. The reason for selecting NMA as the organic component is mainly because it can easily form 2D perovskite, and the conjugated structure could be beneficial for charge transfer between perovskite layers. In order to investigate the effects of dimensionality on the crystallinity of perovskite films, the crystal structures of NCPI films are characterized by the X-ray diffraction (XRD) technique. Figure 1b shows the XRD patterns of typical NCPI films (NCPI-1, NCPI-2 and NCPI-3) prepared from solutions with different NMAI/CsI/PbI2 molar ratios (2:1:2, 2:2:2 and 2.2:3:2). It is observed that all those NCPI films exhibit peaks at around 14.3° and 28.6°, which is in accordance with the cubic phase of CsPbI3, indicating the formation of large-n perovskite quantum wells (QWs) that close to the 3D structure. As increasing the dimensionality by raising the CsI/NMAI molar ratio from 1:2 to 2:2 and 3:2.2, the XRD intensity is obviously enhanced


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along with the narrowing of full width at half maximum (FWHM), which suggests the improvement of crystallinity of NCPI films and likely the formation of large-n perovskite QWs. The XRD results also demonstrate that metastable cubic CsPbI3 can be obtained at a low temperature (110 °C) at the presence of large cations, as we discussed previously.15 In addition, the atomic force microscope (AFM) characterization shows that the grain size of NCPI-3 is obviously larger than that of NCPI-1 (Figure S1), suggesting the existence of large-n perovskite QWs in the NCPI-3 film.

Figure 2. a) Absorption and PL (445 nm excitation) spectra of the NCPI-1 and NCPI-3 films deposited on quartz substrates. b) Schematic of the energy transfer process within perovskite


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MQWs. c) Time-resolved PL spectra of the NCPI-3 film probed at different emission energies. d) Excitation-intensity-dependent PLQE of the NCPI-1 and NCPI-3 films, respectively.

To investigate how the dimensionality tuning process affects the optical properties of NCPI films, both steady-state and time-resolved optical characterizations were carried out. Figure 2a shows the absorption and photoluminescence (PL) spectra of the NCPI-1 and NCPI-3 films, respectively. Both absorption and PL spectra exhibit several peaks, suggesting that each NCPI perovskite film features the MQW structure with different bandgaps.7 The absorption spectrum of NCPI-1 shows a strong exciton absorption peak at 2.44 eV, weak shoulders at 2.22 eV, 2.05 eV and 1.85 eV, which are attributed to the n = 1, n = 2, n = 3 and large-n QWs, respectively.15 By contrast, the absorption spectrum of NCPI-3 shows a strong absorption peaks at 2.22 eV, weak shoulders at 2.05 eV and 1.82 eV, which are attributed to the n = 2, n = 3 and large-n QWs, respectively. The absorption measurements indicate that the major component of NCPI-1 and NCPI-3 films is n = 1 and n = 2 QWs, respectively. The PL spectra of NCPI-1 and NCPI-3 films show emission peaks at ~2.4 eV, ~2.2 eV, ~2.0 eV and ~1.8 eV, which correspond well with their absorption peaks. The dominant PL emission peaks of NCPI-1 and NCPI-3 films are observed at 1.82 eV and 1.79 eV, respectively, which are slightly blue-shifted to that of 3D CsPbI3 perovskite (1.78 eV) and can be attributed to large-n QWs.15 The strongest exciton absorption peaks of n = 1, 2 QWs present very weak PL emissions which can only be observed in logarithmic scale (Figure 2a). This could be due to the energy transfer process (Figure 2b) in perovskite MQWs.7 The time-resolved PL spectra (Figure 2c) of NCPI-3 film shows that the PL decay lifetime increases as the probe energy decreases, suggesting that the energy transfer in MQWs is much faster than the radiative recombination.7 Compared with NCPI-1, the dominant


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PL emission peak of the NCPI-3 MQW film is relatively red-shifted, confirming that the NCPI-3 film contains more of the larger-n QWs. The weak PL emissions of the n = 1, 2 QWs suggest that not all of the small-n QWs are capable to completely transfer energy to large-n QWs. We find the photoluminescence quantum efficiencies (PLQEs) of perovskite MQW films can also be tuned through the dimensional tailing process. Figure 2d shows the excitation-intensitydependent PLQEs of the NCPI-1 and NCPI-3 MQW films under a 445 nm continuous wave laser excitation. It is observed that NCPI-1 shows high PLQE (~20%) when the excitation intensity is as low as 0.07 mW cm-2. As the increase of excitation energy, the PLQE of NCPI-1 slightly increases to 25% until the excitation is ~10 mW cm-2. By contrast, the PLQE of NCPI-3 is low (~6%) at low excitation intensity, and increases as the increase of excitation intensity. The highest PLQE of 43% is obtained for NCPI-3 when the excitation is ~30 mW cm-2. The distinct PLQE feature of NCPI-1 and NCPI-3 films is mainly attributed to their different dimensionality. Compared with NCPI-3, the relatively low-dimensional NCPI-1 has stronger exciton confinement and shorter diffusion length. These properties can suppress the trap-induced nonradiative recombination which is a main loss channel in perovskites at low excitation intensities. As a result, the NCPI-1 film shows higher PLQE at low excitation intensities.15 When the excitation intensity is increased over 10 mW cm-2, the high exciton density effect can lead to non-radiative Auger recombination in the NCPI-1 film. It is known that the Auger recombination in QWs can be suppressed by increasing the well-width of QWs.21 Compared to NCPI-1 film, the NCPI-3 film presents larger-n QWs resulting from the dimensional tailing process. Therefore, Auger recombination can be reduced in the NCPI-3 film, leading to high PLQE at high excitation intensities.


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Figure 3. Device characterizations of red PeLEDs based on NCPI MQWs. a) Flat-band energy level diagram of NCPI-based PeLEDs. The bandgaps of NCPI QWs are estimated from the optical spectra. The valance band (VB) and conduction band (CB) values of the NCPI-1 and other layers are taken from previous literature.2, 15 b) EL spectra of NCPI-based PeLEDs. Inset is


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a digital picture showing a red LED with the logo of the IAM. c) The CIE coordinate of the NCPI-3 PeLEDs. d) Dependence of current density and luminance on the driving voltage. e) EQE values versus current density. For the NCPI-3 PeLED, a maximum EQE of 7.3% is achieved at a current density of 51 mA cm-2. f) Histograms of peak EQEs for 33 devices based on NCPI-3 MQWs.

In order to investigate the effects of MQW dimensionality on the performance of PeLEDs, PeLEDs are fabricated by using solution process. The device configuration is indium tin oxide (ITO)/polyethylenimine ethoxylated (PEIE) modified zinc oxide (ZnO)/NCPI MQWs/poly(9,9dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB)/molybdenum oxide (MoOx)/gold. The detailed fabrication method is shown in the Experimental Section. Figure 3a shows the energy level diagram of NCPI MQW-based PeLEDs. The bandgaps of NCPI QWs are estimated from the optical spectra. The valance band (VB) and conduction band (CB) values of the NCPI-1 and other layers are taken from previous literature.2, 15 The electroluminescence (EL) spectra are shown in Figure 3b. The EL emission peak of NCPI-1 PeLED is observed at 684 nm, and the peak is red-shifted as the increase of MQW dimensionality (690 nm for NCPI-2 and 694 nm for NCPI-3). The NCPI-3 PeLED device presents Commission Internationale de I’Eclairage (CIE) color coordinates of (0.72, 0.27) (Figure 3c), indicating very pure red emission. All emission peaks of EL spectra are consistent with the dominant PL peaks. While the weak emission peaks from small-n QWs in PL spectra are absent in EL spectra, which is mainly because that injected charges directly recombine within the large-n QWs in the cascade energy structure.7 Notably, the EL emission peak does not change at different bias voltages (Figure S2), suggesting good color stability.


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The current density-voltage-luminance (J-V-L) and EQE characteristics of NCPI red PeLEDs are shown in Figure 3d and Figure 3e, respectively. It is observed that, as increasing the dimensionality of MQW films, the current density and luminance are gradually increased at a fixed voltage (Figure 3d). As expected, the larger-n dominated NCPI-3 MQWs exhibits the best PeLED performances. The turn-on voltage of NCPI-3 PeLED is around 1.9 V, which is 0.8 V lower than that of NCPI-1 and 0.2 V lower than that of NCPI-2 (Figure 3d). The low turn-on voltage and high current density of NCPI-3 PeLED indicate that the charge-transport property of NCPI MQWs are enhanced during the dimensional tailoring process. For lower-dimensional NCPI-1 and NCPI-2 perovskites, they have wider bandgaps and more insulating organic components, which demands higher voltage to make charges pass through the perovskite layers. A brightness of 732 cd m-2 is achieved at 3.4 V with the NCPI-3 device, which is the highest value for red PeLEDs at the same operation voltage.3, 15, 19, 20, 22, 23 The peak EQE is enhanced from 1.2% to 7.3% as the MQW dimensionality is increased from the NCPI-1 to the NCPI-3 (Figure 3e). The maximum current efficiency and luminous efficacy of NCPI-3 PeLED is ~0.36 cd A-1 and 0.47 lm W-1, respectively (Figure S3). A histogram for 33 NCPI-3 PeLEDs (Figure 3f) shows an average peak EQE of 7% and a low relative standard deviation of 2.6%, indicating a good repeatability of our high performance MQW red PeLEDs. It is noted that, as the dimensionality is further increased, the turn-on voltage and luminance are similar, but EQE is remarkably lower than that of NCPI-3 (Figure S4). This could be attributed to the increase of trap-induced non-radiative recombination in the higher-dimensional NCPI MQWs, as suggested by the time-resolved PL spectra (Figure S5). Among red PeLEDs, our EQE value is equal high with the reported most efficiency device,20 while our brightness is much higher and operation voltage is much lower than the reported one.


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In summary, high performance red PeLEDs with a maximum EQE of 7.3%, a luminance of 732 cd m-2 and a turn-on voltage of 1.9 V are achieved by tailoring the dimensionality of cesiumbased perovskite MQWs. The enhanced performance is mainly attributed to the improved film crystallinity and PLQE value during the dimensional tailoring process. Our results demonstrate that the dimensional tailoring of perovskite MQWs is a feasible and effective strategy to achieve high performance red PeLED without emission peak shift, which is attractive for the full-color display applications.

EXPERIMENTAL METHODS Materials. Colloidal ZnO nanocrystals were synthesized by a wet-chemical method with some modifications.24 NMAI was synthesized as previous report.7 NCPI precursor solutions were prepared by dissolving NMAI, CsI and PbI2 in a mixed solvent of DMF/DMSO (v/v = 9:1) with 10 wt.% concentration. The NCPI-1, NCPI-2 and NCPI-3 precursors were defined as the NMAI/CsI/PbI2 molar ratio was 2:1:2, 2:2:2 and 2.2:3:2, respectively. All precursor solutions were stirred at 60 °C for 2h in a glovebox and filtered through a 0.2 µm PTFE filter. Device Fabrication. Red PeLEDs are fabricated with the device configuration of indium tin oxide/polyethylenimine ethoxylated modified zinc oxide/NCPI MQWs/poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB)/molybdenum

oxide/gold.

A detailed

description of the device fabrication can be found elsewhere.7 Here, the TFB layers were deposited from an m-xylene solution (8 mg mL-1) at 2,000 rpm. The device area was 3 mm2 as defined by the overlapping area of the ITO films and top electrodes. Characterization. XRD measurements were performed on a Rigaku Smartlab X-ray Diffraction system operated at 40 kV and 30 mA with Cu Kα radiation (λ = 1.5406 Å). UV-vis absorption


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spectra were recorded using a UV-vis spectrometer with an integrating sphere (Cary 5000, Agilent). PL spectra were measured at room temperature using a fluorescent spectrometer (F4600, HITACHI) with a 200 W Xe lamp as the excitation source. The time-resolved fluorescence spectra were obtained by using an Edinburgh Instruments (FLS920) spectrometer. Perovskite films were excited from the quartz glass substrate side by a 405 nm pulsed diode laser (EPL-405) with a fluence around 4 nJ cm-2. The PLQE measurement was carried out using a three-step technique by combination of laser, optical fiber, spectrometer and integrating sphere.25 Atomic force microscope (AFM) images were collected in non-contact mode (Park XE7). All fabricated PeLEDs were characterized at room temperature in a nitrogen-filled glovebox. Devices were swept from zero bias to forward bias with a rate of 0.05 V s-1. ASSOCIATED CONTENT Supporting Information AFM images, EL spectra, current efficiency, luminous efficiency, photoelectric performance of PeLEDs, and time-resolved PL spectra of perovskite films. ACKNOWLEDGMENT This work is financially supported by the Major Research Plan of the National Natural Science Foundation of China (91733302), the Joint Research Program between China and European Union (2016YFE0112000), the Natural Science Foundation of Jiangsu Province, China (BK20171002, BK20170991, BK20150043), the National Natural Science Foundation of China (51703094, 61634001, 11474164, 61405091), the Natural Science Fund for Colleges and Universities in Jiangsu Province of China (16KJB430016), the National Science Fund for


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Distinguished Young Scholars (61725502), the Synergetic Innovation Center for Organic Electronics and Information Displays. REFERENCES (1) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222-1225. (2) Wang, J.; Wang, N.; Jin, Y.; Si, J.; Tan, Z.-K.; Du, H.; Cheng, L.; Dai, X.; Bai, S.; He, H.; et al. Interfacial Control Toward Efficient and Low-Voltage Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 2311-2316. (3) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; et al. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687-692. (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. Acta Chim. Sinica 2015, 73, 171-178. (5) Era, M.; Morimoto, S.; Tsutsui, T.; Saito, S. Organic-Inorganic Heterostructure Electroluminescent Device Using a Layered Perovskite Semiconductor (C6H5C2H4NH3)2PbI4. Appl. Phys. Lett. 1994, 65, 676-678. (6) Li, R.; Yi, C.; Ge, R.; Zou, W.; Cheng, L.; Wang, N.; Wang, J.; Huang, W. RoomTemperature Electroluminescence from Two-Dimensional Lead Halide Perovskites. Appl. Phys. Lett. 2016, 109, 151101.


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(15) Zhang, S.; Yi, C.; Wang, N.; Sun, Y.; Zou, W.; Wei, Y.; Cao, Y.; Miao, Y.; Li, R.; Yin, Y.; et al. Efficient Red Perovskite Light-Emitting Diodes Based on Solution-Processed Multiple Quantum Wells. Adv. Mater. 2017, 29, 1606600. (16) Slotcavage, D. J.; Karunadasa, H. I.; McGehee, M. D. Light-Induced Phase Segregation in Halide-Perovskite Absorbers. ACS Energy Lett. 2016, 1, 1199-1205. (17) Barker, A. J.; Sadhanala, A.; Deschler, F.; Gandini, M.; Senanayak, S. P.; Pearce, P. M.; Mosconi, E.; Pearson, A. J.; Wu, Y.; Srimath Kandada, A. R.; et al. Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films. ACS Energy Lett. 2017, 2, 1416-1424. (18) Braly, I. L.; Stoddard, R. J.; Rajagopal, A.; Uhl, A. R.; Katahara, J. K.; Jen, A. K. Y.; Hillhouse, H. W. Current-Induced Phase Segregation in Mixed Halide Hybrid Perovskites and its Impact on Two-Terminal Tandem Solar Cell Design. ACS Energy Lett. 2017, 2, 1841-1847. (19) Xiao, Z.; Zhao, L.; Tran, N. L.; Lin, Y. L.; Silver, S. H.; Kerner, R. A.; Yao, N.; Kahn, A.; Scholes, G. D.; Rand, B. P. Mixed-Halide Perovskites with Stabilized Bandgaps. Nano Lett. 2017, 17, 6863-6869. (20) Si, J.; Liu, Y.; He, Z.; Du, H.; Du, K.; Chen, D.; Li, J.; Xu, M.; Tian, H.; He, H.; et al. Efficient and High-Color-Purity Light-Emitting Diodes Based on In Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses. ACS Nano 2017, 11, 11100-11107. (21) Gardner, N. F.; Müller, G. O.; Shen, Y. C.; Chen, G.; Watanabe, S.; Götz, W.; Krames, M. R. Blue-Emitting InGaN-GaN Double-Heterostructure Light-Emitting Diodes Reaching Maximum Quantum Efficiency above 200A/cm2. Appl. Phys. Lett. 2007, 91, 243506.


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(22) Jaramillo-Quintero, O. A.; Sanchez, R. S.; Rincon, M.; Mora-Sero, I. Bright VisibleInfrared Light Emitting Diodes Based on Hybrid Halide Perovskite with Spiro-OMeTAD as a Hole-Injecting Layer. J. Phys. Chem. Lett. 2015, 6, 1883-1890. (23) Zhang, X.; Sun, C.; Zhang, Y.; Wu, H.; Ji, C.; Chuai, Y.; Wang, P.; Wen, S.; Zhang, C.; Yu, W. W. Bright Perovskite Nanocrystal Films for Efficient Light-Emitting Devices. J. Phys. Chem. Lett. 2016, 7, 4602-4610. (24) Qian, L.; Zheng, Y.; Choudhury, K. R.; Bera, D.; So, F.; Xue, J.; Holloway, P. H. Electroluminescence from Light-Emitting Polymer/ZnO Nanoparticle Heterojunctions at SubBandgap Voltages. Nano Today 2010, 5, 384-389. (25) De Mello, J. C.; Wittmann, H. F.; Friend, R. H. An Improved Experimental Determination of External Photoluminescence Quantum Efficiency. Adv. Mater. 1997, 9, 230-232.


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(a) 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


2D (n=1)

3D (n=)

Quasi 2D (n=2, 3, …)

···

(NMA)2Csn-1PbnI3n+1 (NCPI)


(b)

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(a)

Page 19 of 20 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

(c)

(b)


(d)


(a)

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


(b)

(c)

(d)

(e)

(f)


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Enhanced Performance of Red Perovskite Light-Emitting Diodes

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