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High Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment Takayuki Chiba, Keigo Hoshi, Yong-Jin Pu, Yuya Takeda, Yukihiro Hayashi, Satoru Ohisa, So Kawata, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

High Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment Takayuki Chiba,* Keigo Hoshi, Yong-Jin Pu, Yuya Takeda, Yukihiro Hayashi, Satoru Ohisa, So Kawata, Junji Kido* Graduate School of Organic Materials Science, Frontier Center for Organic Materials, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan E-mail: [email protected]; [email protected] Tel & Fax: +81-238-26-3595

Abstract All inorganic perovskites quantum dots (PeQDs) have attracted much attention for used in thin film display applications and solid-state lighting applications, owing to their narrow band emission with high photoluminescence quantum yields (PLQYs), color tunability, and solution processability. Here, we fabricated low driving voltage and high efficiency CsPbBr3 PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing process with an ester solvent containing butyl acetate (AcOBu) to remove excess ligands from the PeQDs. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission with a full width at half maximum of 19 nm. We also demonstrated energy level alignment of the PeQD-LED in order to achieve effective hole injection into PeQDs from the adjacent hole injection layer. The PeQD-LED with AcOBu-washed PeQDs exhibited a maximum power efficiency of 31.7 lm W–1 and EQE of 8.73%. Control of the interfacial PeQDs through ligand removal and energy level alignment in the device structure are promising methods for obtaining high PLQYs in film state and high device efficiency. Keywords: perovskites, quantum dot, light-emitting device, washing solvent, interfacial energy level alignment 1. Introduction Lead halide perovskite light-emitting devices (PeLEDs) have recently attracted considerable interest for applications in thin film displays and solid-state lighting, owing to their narrow band emissions, tunable color properties, and easy of solution processability. Organic-inorganic hybrid CH3NH3PbBr3 (MAPbBr3) PeLEDs with an external quantum efficiency (EQE) of 0.73% were reported in 2014.1 Since then, thickness, grain size and interfacial engineering of such perovskite films have been demonstrated as ways to improve the recombination rate, photoluminescence 1 ACS Paragon Plus Environment


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quantum yield (PLQY) and device efficiency.2-12 PeLEDs with green emissive MAPbBr3 have been made with a high current efficiency of 42.9 cd A–1 and an EQE of 8.53%.13 Highly efficient red emissive PeLEDs with EQE > 10% were demonstrated in 2016,14-15 indicative of rapid research progress in this field. However, control over various aspects of film morphology (such as grain size and surface roughness) and limited stability remains16 an issue for in organic-inorganic hybrid perovskites film. With this challenge in mind, inorganic CsPbX3 (X = Cl, Br, I) perovskite quantum dots (PeQDs) have been demonstrated to simultaneously achieve high PLQY, smooth surface roughness, ink stability, and device efficiency.17-35 Kovalenko et al. reported a novel and elegant synthesis method of color-tunable CsPbX3 PeQDs with high PLQYs of 50–90% and narrow PL spectra.17 Typically, PeQDs are capped by long alkyl ligands such as oleic acid (OA) and oleylamine (OAm) to achieve high PLQY and high dispersibility in nonpolar solvents such as toluene, hexane, and octane.36 However, these long alkyl ligands have insulating behavior, and prevent charge injection from adjacent hole and electron transporting layers, reducing the charge transport property of the PeQDs film. Therefore, green-emissive PeQDs light-emitting devices (PeQD-LEDs) with such ligands feature lower EQE compared to organic-inorganic hybrid MAPbBr3 PeLEDs.37-40 To overcome this drawback, Bakr et al. reported the replacement of OA and OAm by di-dodecyl dimethyl ammonium bromide (DDAB), a relatively short ligand with a bromide ion pair, to improve the charge injection properties.41-42 CsPbBr3 PeQDs capped with short DDAB ligands exhibited not only high PLQY (71%) but also smooth surface roughness compared to cases with the conventional long alkyl ligands of OA and OAm, indicating superior surface passivation and stabilization of the PeQDs. In addition, the EQE of a device with CsPbBr3 PeQDs capped with DDAB was as high as 3.0%, with a maximum luminance of 330 cd m–2. In another effort to obtain highly efficient PeQD-LEDs, Zeng et al. demonstrated the use of mixed solvents during the washing process to remove excess ligands.43 The use of hexane and ethyl acetate (AcOEt) mixed solvents for washing PeQDs led to a high EQE of 6.27%. In this manuscript, we describe an effective washing process to remove excess ligands from PeQD surfaces using various poor solvents such as butanol (BuOH), AcOEt and butyl acetate (AcOBu). The effect of washing solvent on PLQY, PL lifetime, PeQD size and surface roughness of CsPbBr3 PeQDs films were investigated. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42% due to its inhibition PeQD-PeQD energy transfer, compared to PeQDs washed with BuOH and AcOEt. The PeQD-LED based on CsPbBr3 PeQDs washed with AcOBu exhibited a narrow electroluminescence (EL) emission with a full width at half maximum (FWHM) of 17 nm and a very low turn-on voltage of 2.60 V. The maximum power efficiency and EQE reached 31.7 lm W–1 and the EQE was 8.73%. Therefore, choosing the appropriate washing solvent, and aligning the energy level of the device structure are key features for achieving high PLQY, high 2 ACS Paragon Plus Environment

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ink stability in coating solvents, and low driving voltage of such devices with high EQE. 2. Results and Discussion CsPbBr3 PeQDs were synthesized via a conventional hot-injection method with lead(II) bromide (PbBr2) and cesium oleate in octadecene (ODE) solution according to previously reported method.42 The synthesized CsPbBr3 PeQD crude solution was washed using BuOH to remove impurities. The ligand exchange process was carried out using the short alkyl ligand of DDAB to cap the PeQDs in place of their long alkyl ligands (OA and OAm). The synthesized or ligand exchanged PeQDs requires a washing process to remove excess ligands and the oiliness of the ODE reaction solvent due to unstable because without washed PeQDs is unstable due to excess ligand such as OAm and DDAB.42 In addition, PeQDs without washing process is poor film-forming properties due to containing oiliness ligands and solvent. These impurities can also inhibit film formation and reduce charge injection. Therefore, the addition of poor solvents such as alcohols into nonpolar solution of PeQDs can potentiate the precipitation of PeQDs after centrifugation. However, choosing the appropriate solvent to use in the washing PeQDs is critical because PeQDs can contain ionic species (i.e., Br–) that are sensitive to solvent polarity. We tested the addition of various poor solvents such as methanol (MeOH), ethoxyethanol (EtEtOH), BuOH, isopropanol (IPA), acetonitrile (MeCN), AcOEt, and AcOBu into solution of CsPbBr3 PeQDs in toluene. CsPbBr3 PeQDs solutions with added MeOH, IPA, and EtEtOH showed no emission under UV illumination as shown in Figure S1, indicating that the use of polar solvents causes optical quenching of the PeQDs. On the other hand, the addition of AcOEt and AcOBu into this CsPbBr3 PeQDs toluene solution led to similar brightness compared with the unaltered CsPbBr3 PeQDs toluene solution, even with after 120 h. This demonstrates the influence of the washing effect on CsPbBr3 PeQD-LEDs when using BuOH, AcOEt, and AcOBu as poor solvents on the CsPbBr3 PeQDs solution after ligand exchange. Ester solvents can be used as the washing solvent without quenching of the CsPbBr3 PeQDs as shown in Figure 1. The particles sizes after washing were estimated to be approximately ~10 nm for PeQDs washed with BuOH, and ~9 nm for AcOEt and AcOBu based on transmission electron microscopy (TEM) measurements (Figure 2a–c). The PeQDs washed with BuOH featured slightly larger particles compared to PeQD washed with AcOEt and AcOBu. This result could be attributed to ligand loss at the PeQD surface after BuOH washing, which can cause undesirable crystal growth.43 X-ray diffraction (XRD) measurements were performed on the washed PeQDs. A conventional cubic crystal phase was expressed, corresponding to previous report (Figure S2).37 In addition, Fourier transform infrared spectroscopy (FTIR) was also used to confirm the washing solvent effect on the PeQDs as shown in Figure 2d and Figure S3. All PeQDs washed with AcOBu, AcOEt, and BuOH exhibited stretching vibration modes for methylene groups at 2923 and 2854 cm–1. On the 3 ACS Paragon Plus Environment


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other hand, AcOBu-washed PeQDs film showed a clear peak at 2960 cm–1, corresponding to the C-H stretching vibration mode of the methyl group in DDAB. Such FTIR peaks were also observed for BuOH- AcOEt-washed PeQD films at 2960 cm–1, albeit at lower intensity. In addition, the FTIR spectra of BuOH- and AcOEt-washed PeQD films exhibited a C=O stretching vibration at 1710 cm– 1

, a C=C stretching vibration at 3004 and 1630 cm–1, and a N-H stretching at 3310 cm–1, indicating

the presence of the long alkyl ligands OA and OAm.44-47 These FTIR peaks were not observed in the case of the AcOBu-washed PeQDs film, due to the complete removal of OA and OAm. UV-vis absorption, PL spectroscopy and PLQY measurements of CsPbBr3 PeQDs washed with various solvents were performed to investigate the influence of surface ligand conditions on the optical properties of diluted toluene solutions and films on quartz substrates. Figure 2e shows the UV-vis absorption spectra and PL spectra of CsPbBr3 PeQDs washed with these three solvents. The optical energy gaps of CsPbBr3 PeQDs were 2.36 eV for BuOH, 2.37 eV for AcOEt, and 2.38 eV for AcOEt, indicating that washing solvent has a minimal effect on the energy gap. The PL peaks of the PeQDs films were 517 nm for BuOH washing, 512 nm for AcOEt, and 513 nm for AcOBu; these are all slightly red-shifted (1-2 nm) compared to the solution state due to the slight aggregation of PeQDs in the films. These PL peak shifts correlated with the particle sizes of the PeQDs. The FWHM values of the PL spectra were 20 nm for BuOH, 22 nm for AcOEt, and 19 nm for AcOBu. These results suggest that the PeQDs washed with AcOBu feature greater monodispersity among these three washing solvents. The PeQDs in solution showed high PLQY value of 70–80%. Meanwhile, the PeQDs film showed a lower PLQY of 30–40% compared to solution. Ultraviolet photoelectron spectroscopy (UPS) was also performed on PeQDs films with various washing solvents as shown in Figure 2f. The washed PeQDs exhibited an identical valence band (VB) of approximately 5.5 eV across all three solvent types, indicating that the washing solvent did not affected the PeQDs energy level. As a result, the PLQY of PeQDs film is a crucial factor in determining the EQE of the PeQD-LEDs. To better understand the relationship between washing solvent and surface ligand state with regards to PLQY, the time-resolved PL measurements were performed on film samples. The PL decay curves of PeQD solution in toluene and films on quartz substrate were fitted with a multi-exponential function as shown in Figure 3. The PL decay curves for solution in toluene are slower of 19–26 ns (Figure 3a) than those of film state of 11-17 ns (Figure 3b) at detection wavelength of 513 nm. This larger PL lifetime would be attributed to higher PLQY for PeQDs solutions due to suppressing non-radiative recombination or energy transfer. Meanwhile, AcOBu-washed PeQDs film showed slower PL lifetime compared with BuOH- and AcOEt-washed PeQDs films. These results indicate that the removal of excess ligands (OA and OAm) from the PeQDs by using AcOBu washing leads to suppressing non-radiative recombination and improving PLQY.43, 48 The photophysical properties of the CsPbBr3 PeQDs solution and film are summarized 4 ACS Paragon Plus Environment

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in Table S1 The effect of washing solvent on the film morphology was also investigated via scanning electron microscopy (SEM) and atomic force microscopy (AFM) as shown in Figure S4. The BuOH-washed PeQDs film exhibited uniform density (Figure S4a), but a few aggregate structures were also observed over large scanning areas (Figure S5). The PeQDs films washed with AcOEt and AcOBu exhibited a highly density as shown in Figure S4b–c compared to the PeQDs film washed with BuOH. The AFM images of these samples corroborated the same trends shown by SEM. The BuOH-washed film exhibited a roughness value of 3.7 nm, as the lower number of surface ligands caused aggregation among the PeQDs in the film (Figure S4d). Meanwhile, the morphologies of the AcOEt- and AcOBu-washed PeQDs films featured relatively low surface roughness (2.9 and 3.0 nm r.m.s., respectively) (Figure S4e–f), owing to the similar grain sizes among their constituent PeQDs. Therefore, the removal of excess ligands from PeQDs can affect both the photophysical properties (PLQYs and PL lifetime) and the film morphological properties. For additional studies, we fabricated PeQD-LEDs with the following structures; indium tin oxide (ITO) (130 nm)/ modified poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with nafion (40 nm)/poly(4-butylphenyl-diphenyl-amine) (poly-TPD) (20 nm)/CsPbBr3 PeQDs with various washing solvents/tris-[1-phenyl-1H-benzimidazole] (TPBi) (50 nm)/lithium 8-quinolate (Liq) (1 nm)/aluminum (Al) (100 nm). This modified PEDOT:PSS with nafion containing perfluoro sulfonate side chains (for hole injection layer) is well-known as an effective approach to achieve both highly efficient device performance owing to the work function control in PEDOT:PSS,4, 13 as well as drastically reduced device leakage currents. The work functions of PEDOT:PSS and modified PEDOT:PSS were 4.72 and 5.27 eV, respectively, based on UPS measurements (Figure S6a). Poly-TPD is a hole-transporting polymer with a low ionization potential energy of 5.31 eV, which corresponds to the work function of modified PEDOT:PSS (Figure S6b). Thus, the hole injection barrier between modified PEDOT:PSS and poly-TPD is quite small. After washing, CsPbBr3 PeQDs were dispersed in nonpolar octane for coating onto poly-TPD without dissolution. The other functional layers such as TPBi, Liq and Al were deposited by vacuum evaporation. The energy diagram of the PeQDLED is shown in Figure 4a. Electroluminescence (EL) spectra of the PeQD-LEDs with different washing solvents featured identical emissions as the CsPbBr3 PeQDs at current density of 25 mA cm–2. No other emissions were observed from neighboring functional layers (Figure 4b). This strongly suggests that the injected hole and electron charges appropriately converted to excitons within the CsPbBr3 PeQDs. The EL peak of the device with AcOBu-washed PeQDs was present at 512 nm, corresponding to its PE peak (513 nm), with extremely narrow FWHM (17 nm). Current density-voltage and luminance-voltage characteristics are shown in Figure 4c–d. All PeQD-LEDs exhibited minimal leakage current due to addition of nafion into the PEODT:PSS (Figure S7). The device with 5 ACS Paragon Plus Environment


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BuOH-washed PeQDs showed a current density of 26.3 mA cm–2, higher than the case of washed with AcOEt and AcOBu washing (11.7 and 9.3 mA cm–2 respectively) at 4 V (Figure 4c). This indicates that the reduced number of ligands on the PeQDs washed with BuOH enabled charge transport in the high current density region. The device with CsPbBr3 PeQDs washed with AcOBu showed a low turn-on voltage of 2.60 V at 1 cd m–2, which is close to the emission energy of the EL peak at 512 nm (2.42 eV) and the driving voltages of 3.11 and 4.15 V at 100 and 1000 cd m–2, respectively. Likewise, the device with BuOH- and AcOEt- washed PeQDs also showed low turn-on voltages of 2.63 and 2.61 V and driving voltages of 3.07 and 3.20 V at 100 cd m–2, respectively, due to the identical VB of PeQDs among the three different washing solvents. To our knowledge, these driving voltage characteristics of the PeQD-LEDs are considerably lower than those in previous reports of green PeQD-LEDs.42-43 The maximum luminance values of 1490 cd m–2 for BuOH wash, 790 cd m–2 for AcOEt wash, and 1660 cd m–2 for AcOBu wash, respectively, were also observed. Figure 4e–f show the power efficiency and EQE of the PeQD-LEDs, respectively. The device with AcOBu-washed PeQDs exhibited a higher current efficiency (18.8 cd A–1 at 100 cd m–2) than those of the BuOH (14.2 cd A–1) and AcOEt (11.5 cd A–1) devices. In addition, we achieved a remarkably high power efficiency of 18.9 lm W–1 and EQE of 6.25% at 100 cd m–2, respectively. The maximum power efficiency and EQE were 31.7 lm W–1 and EQE 8.73%, respectively, a higher in previous reports of green PeLEDs or PeQD-LEDs.13, 42-43 These results indicated that AcOBu washing CsPbBr3 PeQDs promoted both high PLQY in the thin film state and high EQE in devices, due to the removal of excess ligands and the prevention of non-radiation recombination. The PeQD-LED with BuOH- and AcOEt-washed PeQDs also exhibited high power efficiencies of 14.5 and 11.3 lm W–1, and EQEs of 4.46 and 4.59% at 100 cd m–2, respectively. These device performances characteristics are summarized in Table S2. 3. Conclusion In summary, we demonstrated that the use of BuOH, AcOEt, and AcOBu for washing of PeQDs. The CsPbBr3 PeQDs film washed with AcOBu showed a high PLQY of 42% due to suppression of non-radiation process, and the efficient removal of excess OA and OAm ligands from the PeQDs compared to the PeQDs washed with BuOH and AcOEt. The PeQD-LED based on an AcOBu-washed CsPbBr3 PeQDs film exhibited a narrow EL spectrum with a high color purity (FWHM of 17 nm) and a very low turn-on voltage (2.60 V). The maximum power efficiency and EQE reached 31.7 lm W–1 and 8.73%, respectively. Therefore, removal of excess ligands from PeQDs and energy level alignment within PeQD-LED are promising ways to develop highly efficient PeQD-LED with high PLQY in film state. 6 ACS Paragon Plus Environment

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4. Experimental Section Materials: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was purchased from Clevious. Poly(4-butylphenyl-diphenyl-amine) (poly-TPD) was purchased from American Dye Source and was used as received. Tris-[1-phenyl-1H-benzimidazole] (TPBi) and lithium 8-quinolate (Liq) were purchased from e-Ray Optoelectronics Technology Co., Ltd., and used as received. All other materials and solvents were purchased from Sigma-Aldrich, and were used as received.

Synthesis and ligand exchange of CsPbBr3 QDs: The CsPbBr3 QDs were synthesized using a previously reported method.42 Cesium carbonate (542 mg, 99.99%) was loaded into a 25 mL three-neck flask along with octadecene (ODE) (20 mL, 90%) and oleic acid (OA) (1.66 mL, 90%). The mixed solution was dried at 120 ˚C for 1 h under N2 until c Cs-oleate formed. ODE (100 mL), OA (10 mL), oleylamine (OAm) (10 mL, 90%), and PbBr2 (1.38 g, 99.99%) were loaded into a 250 mL three-neck-flask and dried under vacuum at 120 ˚C for 1 h and heated to 180 ˚C under N2. The Cs-oleate solution in ODE was then quickly injected. After 5 s, the reaction mixture solution was cooled using an ice-water bath. The crude solution was centrifuged at 12000 rpm for 15 min, and the precipitate was collected and dispersed in toluene. The toluene dispersion was centrifuged at 12000 rpm at 15 min, and the stably dispersed supernatant was collected. Butanol (BuOH) was added into the toluene dispersion with a volume ration of 1.5:1. The BuOH and toluene mixture dispersion was centrifuged again at 12000 rpm at 15 min, and the precipitate was collected and dispersed again in toluene. The ligand exchange process also followed a previously reported method.42 1 mL of the after solvents (BuOH, AcOEt and AcOBu) washed CsPbBr3 dispersion in toluene (concentration of 15 mg mL–1), 50 μL OA, and 2 mL of DDAB toluene solution (0.05 M) were added under stirring. Washing process: BuOH, ethyl acetate (AcOEt), or butyl acetate (AcOBu) was added to the ligand exchanged CsPbBr3 dispersion in a volume ratio of 2:1, and were centrifuged at 12000 rpm at 15 min. The precipitate washed with BuOH was collected and dispersed in octane with concentration of 10 mg mL–1. In a case of washed with AcOEt and AcOBu, the same process were repeated. Characterization: Transmission electron microscopy was carried out with a JEOL JEM-2100F. Fourier transform infrared spectroscopy (FTIR) was recorded using a JASCO FT/IR-4700. X-ray diffraction patterns were measured with a Rigaku SmartLab diffractometer. Photoluminescence spectra were collected were performed using a HORIBA FluoroMax-2 luminescence spectrometer. Photoluminescence quantum yield were obtained using a Hamamatsu C9920-01 integral sphere 7 ACS Paragon Plus Environment


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system. Photoluminescence lifetime ware determined using a Hamamatsu C11367 Quantaurus-Tau. UV-vis absorption spectra were measured using a Shimadzu UV-3150 UV-vis-NIR spectrophotometer. Scanning electron microscopy was measured using JEOL JSM-6700F. Surface roughness was observed from a Bruker Dimension Icon AFM by tapping mode. The valence band was determined by ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Scientific Theta probe) under high vacuum condition of ~10–6 Pa. Device fabrication: Indium tin oxide (ITO) substrates were cleaned by deionized water with ultrasonic spin cleaning and then by UV–ozone treatment for 10 min. After the UV–ozone treatment, Nafion blend with PEDOT:PSS (55 wt% containing in AI4083) were spin-coted onto ITO-coated glass substrate and annealed at 150 ˚C for 10 min resulted in a 40 nm thick layer. For the preparation of hole transporting layer with Poly-TPD dissolved in chlorobenzene with a concentration of 4 mg mL–1. Poly-TPD was spin-coated onto HIL and annealed at 100 ˚C for 10 min resulted in a 20 nm thick layer. CsPbBr3 PeQDs were spin-coated onto poly-TPD at 2000 rpm for 30 s in N2-filled glove box. Other functional layers of TPBi (50 nm), Liq (1 nm) and Al (100 nm) anode were deposited by thermal evaporated under high vacuum (~10−5 Pa). Active area of the device was 2 mm2. The PeQD-LEDs were characterized after encapsulation using epoxy glue and a glass cover. Electroluminescence spectra were recorded using Hamamatsu PMA-11 photonic multichannel analyzer. The current-density–voltage and luminance–voltage characteristics were measured using a Keithley source measure unit 2400 and a Minolta CS200 luminance meter, respectively. Acknowledgments The authors would like to thanks the “Grant-in Aid for Scientific Research A Grant Number 15H02203” of the Japan Society for the Promotion of Science (JSPS). The authors would like to also thank the Center of Innovation Program of the Japan Science and Technology Agency (JST).

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Figure 1. Scheme of the PeQDs washing process using BuOH, AcOEt, and AcOBu.

Figure 2. TEM images of PeQDs films washed with a) BuOH, b) AcOEt, and c) AcOBu; d) FTIR spectra, e) UV-vis absorption and PL spectra of PeQDs film (dash line: PL spectra of solution), and f) UPS spectra of PeQDs.


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(a)

(b)

1

Film@513nm

BuOH AcOEt AcOBu

0.1

0.01

0.001

0

50

100 Delay (ns)

150

BuOH AcOEt AcOBu

PL intensity (a.u.)

PL intensity (a.u.)

Solution@513nm

200

0

50

100 Delay (ns)

150

200

Figure 3. PL decay curves of CsPbBr3 PeQDs a) solution in toluene and b) films washed with BuOH, AcOEt, and AcOBu at detection wavelength of 513 nm.

Current density (mA cm–2)

3 (c) 10

102 101 100 10-1 10-2

104

10-4

102

101

100 2

BuOH AcOEt AcOBu

3

4 5 Voltage (V)

6

3 4 5 Voltage (V)

6

7

10

1

7

2

EQE (%)

103

1

(f) 10

(e) Power efficiency (lm/W)

(d)

BuOH AcOEt AcOBu

10-3

10-5 0

Luminance (cd m–2)

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


0.01

1

BuOH AcOEt AcOBu

0.1 1 10 –2 Current density (mA cm )

100

0.01

BuOH AcOEt AcOBu

0.1 1 10 –2 Current density (mA cm )

100

Figure 4. CsPbBr3 PeQDLEDs performance. a) energy diagram of the PeQDLED. b) EL spectra of a device at 25 mA cm-2. Inset: emission image of the device. c) Current density-voltage characteristics, d) luminance-voltage characteristics, e) power efficiency-current density characteristics, and f) external quantum efficiency-current density characteristics.



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