Purification of Perovskite Quantum Dots Using Low-Dielectric

Jul 3, 2018 - Purification of Perovskite Quantum Dots Using Low-Dielectric-Constant Washing Solvent “Diglyme” for Highly Efficient Light-Emitting ...
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Purification of Perovskite Quantum Dots Using Low-DielectricConstant Washing Solvent “Diglyme” for Highly Efficient LightEmitting Devices Keigo Hoshi, Takayuki Chiba,* Jun Sato, Yukihiro Hayashi, Yoshihito Takahashi, Hinako Ebe, Satoru Ohisa, and Junji Kido* Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan Downloaded via TUFTS UNIV on July 19, 2018 at 00:11:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite quantum dots (QDs) are known as ionic nanocrystals, and their optical properties are greatly affected by the washing solvent used during the purification process. Here, we demonstrate the purification process of CsPbBr3 perovskite QDs using lowdielectric-constant solvents to completely remove impurities, such as the reaction solvent and desorbed ligands. The use of the ether solvent diethylene glycol dimethyl ether (diglyme), having a low dielectric constant of ε = 7.23, as a poor solvent for reprecipitation allowed for multiple wash cycles, which led to high purity and high photoluminescence quantum yield for CsPbBr3 QDs. The lightemitting device constructed with the CsPbBr3 QDs and washed twice with diglyme (two-wash) showed a low turn-on voltage of 2.7 V and a peak external quantum efficiency of over 8%. Thus, the purification of perovskite QDs with multiple wash cycles using a low-dielectric-constant solvent is an effective approach for enhancing not only the optical properties but also the efficiency of perovskite quantum dot light-emitting devices. KEYWORDS: perovskite quantum dot, light-emitting device, purification, low dielectric constant

1. INTRODUCTION Cesium lead halide perovskite quantum dots (QDs) have drawn attention for their superior performance as materials used in light-emitting devices (LEDs) due to their narrow full width at half-maximum (FWHM), high photoluminescence quantum yield (PLQY), and tunable emission wavelengths.1−24 Kovalenko et al. established a simple synthesis method for CsPbX3 (X = Cl, Br, or I) QDs in 2015.1 The first reported CsPbBr3-based perovskite quantum dot light-emitting devices (QD-LEDs) exhibited an external quantum efficiency (EQE) of 0.12%.4 Afterward, a highly efficient CsPbBr3-based perovskite QD-LED was developed based on ligand exchange from conventional long ligands, such as oleic acid (OA) and oleylamine (OAM), to a short ligand didodecyl dimethyl ammonium bromide (DDAB) containing a Br anion, with an EQE of 3.0%.25 Zeng et al. demonstrated ligand density control with different washing solvents and the number of wash cycles to improve the surface passivation and carrier injection, which resulted in a high EQE of 6.27%.26 In addition, we demonstrated that a washing process with an ester solvent after ligand exchange removes impurities such as excess ligands and the reaction solvent.27 Thus, surface ligand engineering and purification process are significantly important to obtain high-quality perovskite QDs.28,29 In general, the reprecipitation method is used for the purification of perovskite QDs.4,7 When the perovskite QDs © XXXX American Chemical Society

are capped with the long alkyl ligands, OA and OAM, lowdielectric-constant solvents, such as toluene (ε = 2.24) and octane (ε = 1.95), act as good solvents, while the highdielectric-constant solvents, such as alcohols, act as poor solvents. The addition of a high-dielectric-constant solvent to a perovskite QD dispersion results in precipitated QDs, which can be removed leaving the impurities behind. However, ionic perovskite QDs are significantly sensitive to high-dielectricconstant solvents, such as methanol, ethanol, and 2-propanol.27 Therefore, the choice of a poor solvent with a low dielectric constant for reprecipitation and the number of wash cycles are both key to achieving high purity and a high yield of perovskite QDs. Herein, we demonstrate the purification of perovskite QDs using various poor solvents with low dielectric constants for reprecipitation to achieve a high PLQY and high efficiency in LEDs. The DDAB CsPbBr3 perovskite QD film, based on ligand exchange, was washed with the ether solvent diglyme, which has a low dielectric constant of ε = 7.23. The film showed a PLQY of 50% due to complete removal of desorbed ligands and the reaction solvent. The perovskite QD-LED Received: April 13, 2018 Accepted: July 3, 2018 Published: July 3, 2018 A

DOI: 10.1021/acsami.8b05954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

values. The CsPbBr3 QDs dispersed only in toluene (ε = 2.24), without a poor solvent, and exhibited a PLQY of 47.1%. When using low-dielectric-constant solvent (ε < 5), the PLQYs are similar to the initial value of the CsPbBr3 QDs dispersed only in toluene. However, the low-dielectric-constant solvents do not reprecipitate the perovskite QDs due to dispersion stability. In the dielectric constant range of ε = 5−10, the addition of the ether solvent diglyme showed the highest PLQY of 64.0% among all of the solvents in our experiments. The dielectric constant of diglyme (ε = 7.23) is lower than that of conventional BuOH; therefore, diglyme can be used in additional wash cycles without causing surface defects on the perovskite QDs. The detailed properties of solvents and PLQYs are listed in Table S1. The ligand-exchanged CsPbBr3 QDs capped by DDAB, a relatively short alkyl ligand with a Br anion, were prepared to achieve highly efficient LEDs.25 The bromine anion of DDAB leads to suppression of anion defects in the perovskite QDs.27 The ligand-exchanged CsPbBr3 perovskite QDs were purified by the addition of diglyme as a poor solvent for the reprecipitation process of the QDs to remove the OED, OA, and OAM. We demonstrated the multiple wash cycles with diglyme (one- or two-wash). A schematic of the ligand exchange and purification process for perovskite QDs is shown in Figure 2. In the 1H NMR spectra (Figure 3a,b), the DDABcapped CsPbBr3 QDs with diglyme two-wash exhibited no characteristic resonances of terminal alkyne in ODE (4.9 and 5.8 ppm) or alkyne in OA or OAM (5.3 ppm), which indicates that the impurities were completely removed. On the other hand, the diglyme one-wash QDs sample exhibited resonances of both terminal alkyne (ODE) and alkyne (OA or OAM) due to remaining impurities. Fourier transform infrared (FT-IR) spectroscopy was also performed to confirm the effectiveness of multiple wash cycles of perovskite QDs, as shown in Figure 3c. The DDAB-capped CsPbBr3 QD film with diglyme onewash exhibited a peak related to the CO stretching vibration at 1710 cm−1, indicating the presence of OA on the surface of CsPbBr3 QDs, whereas a peak related to N−H stretching at 3310 cm−1 was not observed. In the ligand exchange process, a small amount of OA was added to the QDs dispersion to desorb OAM from the surface of the CsPbBr3 QDs. However, the diglyme one-wash CsPbBr3 QDs were insufficiently purified due to remaining OA as an impurity. On the other hand, the DDAB-capped CsPbBr3 QD film with diglyme twowash exhibited no peaks related to CO (1710 cm−1) or N− H (3310 cm−1) stretching vibration, which indicates the complete removal of OA and OAM. In addition, the C−H stretching vibrations of the methyl group (CH3) at 2960 cm−1 and methylene group at 2923 and 2854 cm−1 were observed regardless of the number of wash cycles. Thus, surface ligand

purified by diglyme washing exhibited a low driving voltage of 2.7 V and a high EQE of over 8%.

2. RESULTS AND DISCUSSION The green-emitting CsPbBr3 perovskite QDs were synthesized via a conventional hot-injection method using Cs-oleate and PbBr2 precursors dissolved in a solution containing OA, OAM, and 1-octadecene (ODE). To remove impurities such as the reaction solvent ODE and unreacted OA and OAM, the synthesized CsPbBr3 QDs were dispersed in good solvent toluene and, subsequently, high-dielectric-constant butanol (BuOH, ε = 17.1) as a conventional poor solvent was added to dispersion for purification and isolation of perovskite QDs. 1H NMR analysis was performed to investigate the impurities after the purification process (Figure S1). Terminal alkene resonance peaks were clearly observed at 4.9 and 5.8 ppm, which indicated that the reaction solvent ODE is present as an impurity.30 In addition, we identified the alkene resonances of desorbed OA and OAM ligands, which also act as impurities in perovskite QDs, at 5.3 ppm. This result indicates that the use of BuOH, as a poor solvent, insufficiently purifies perovskite QDs. Moreover, poor solvents with a high dielectric constant, such as BuOH, cannot be used in repeat wash cycles because they will cause surface defects, such as ligand desorption of OA and OAM, or anion defects from perovskite QDs. We investigated 36 kinds of poor solvents with a wide range of dielectric constants from 1.84 to 48.9 for the purification of perovskite QDs. These poor solvents were added to CsPbBr3 QDs dispersed in toluene to investigate their photoluminescence properties (Figure S2). Figure 1 shows the dielectric

Figure 1. Relationship between PLQY and the dielectric constants of various washing solvents. In this graph, 36 different solvents were added to CsPbBr3 toluene solution (15 mg mL−1) after washing once with BuOH. Next, 5 mL of a toluene solution was added to this sample.

constant dependence on PLQY of CsPbBr3 QDs dispersed in toluene and poor solvent mixtures, and we found that their PLQY remarkably decreased with increasing dielectric constant

Figure 2. Schematic of the ligand exchange and washing process for CsPbBr3 perovskite QDs. B

DOI: 10.1021/acsami.8b05954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Identification of impurities and crystal structure analysis: (a) 1H NMR full spectra, (b) 1H NMR spectra ranging from 4.8 to 6 ppm, (c) FT-IR spectra, and (d) XRD spectra.

Figure 4. Characteristics of CsPbBr3 QD-LEDs washed with diglyme: (a) device structure, (b) EL spectra at a current density of 50 mA cm−2, (c) luminance−voltage characteristics, and (d) EQE−current density characteristics.

completely converted OA and OAM into DDAB in the case of diglyme two-wash CsPbBr3 QDs. On the other hand, we also

performed the purification process with ester solvent methyl acetate. The DDAB-capped CsPbBr3 QDs with methyl acetate C

DOI: 10.1021/acsami.8b05954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

diglyme two-wash showed superior performance with an EQE of 5.0% at 100 cd m−2 with a peak EQE of 8.08%. Figure S7 shows a histogram of the EQE values of 27 LEDs with diglyme one- and two-wash, which confirm the good reproducibility of the LEDs. One the other hand, the CsPbBr3 QD-LED with one-wash methyl acetate exhibited a high turn-on voltage of 3.0 V and a low EQE of 3.3%, as shown in Figure S8. The operational lifetime of CsPbBr3 QD-LEDs is rather short (several minutes), and further study is required to improve the stability and to understand the device degradation. The detailed performance of the CsPbBr3 QD-LEDs is shown in Table S3.

one- and two-wash exhibited the resonance of alkyne (OA or OAM) in the 1H NMR spectra, as shown in Figure S3a. Similarly, both one- and two-wash CsPbBr3 QDs with methyl acetate exhibited CO stretching vibration, which indicated the presence of OA as an impurity (Figure S3b). Moreover, the DDAB-capped CsPbBr3 QDs with two-wash are unstable in toluene or octane solution due to decreasing ligand density. Therefore, a low-dielectric-constant solvent with multiple wash cycles is very useful for the purification of perovskite QDs. Xray diffraction (XRD) analyses were performed on the perovskite QD films on silicon substrates, as shown in Figure 3d. Both CsPbBr3 QD films with diglyme one- or two-wash exhibited a conventional cubic crystal phase, which corresponds to a previous report.27 The UV−vis absorption, photoluminescence (PL) spectroscopy, and PLQY measurements of the CsPbBr3 QD films washed with diglyme were performed on quartz substrates to confirm the influence of the number of wash cycles (Figure S4 and Table S2). The PL spectra of diglyme two-wash exhibited a slight red shift in the peak wavelength and narrow FWHM compared to those of the diglyme one-wash QD film as the former became monodisperse with the increased number of wash cycles. The PLQY decreased as the number of wash cycles increased, and the onewash QD film showed a PLQY of 68% compared to the twowash QD film, which showed a PLQY of 50%. Moreover, the yield of CsPbBr3 QDs after the diglyme two-wash purification process was high (26%) compared to the purification process using another ester solvent, butyl acetate (10%), with a low dielectric constant of ε = 5.01. Thus, the use of diglyme as a poor solvent in the purification process enables not only high PLQY in the film but also high yield. Finally, we fabricated CsPbBr3 QD-LEDs with the following structure: indium tin oxide (ITO; 130 nm)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) doped with Nafion (modified PEDOT:PSS; 40 nm)/poly(4-butylphenyl-diphenyl-amine) (poly-TPD; 20 nm)/CsPbBr3 QDs (10 nm)/tris-(1-phenyl-1H-benzimidazole) (TPBi; 50 nm)/lithium 8-quinolate (Liq; 1 nm)/Al (100 nm), as shown in Figure 4a. The cross-sectional transmission electron microscopy (TEM) image of CsPbBr3 QD-LED is shown in Figure S5. The modified PEDOT:PSS and poly-TPD were used as a hole injection layer and hole transport layer, respectively. For the fabrication of LEDs, CsPbBr3 QDs were dispersed in octane at a concentration of 10 mg mL−1 to form a film on poly-TPD without dissolution. The electron transport layer (TPBi), electron-injection layer (Liq), and Al cathode were sequentially deposited by thermal evaporation under high vacuum. The electroluminescence (EL) spectra of the CsPbBr3 QD-LEDs with diglyme one- or two-wash exhibited identical emissions from CsPbBr3 QDs without other emission from neighboring poly-TPD or TPBi at a constant current density of 50 mA cm−1, as shown in Figure 4b. The peak wavelength and FWHM are 508 and 21 nm, respectively, for the diglyme one-wash QD-LED, and 510 and 20 nm, respectively, for the diglyme two-wash QD-LED, which correspond to the PL spectra. The luminance−voltage curves are shown in Figure 4c. The CsPbBr3 QD-LED with diglyme two-wash exhibited a lower turn-on voltage of 2.7 V compared to 2.8 V for one-wash and also showed higher luminance compared to the one-wash CsPbBr3 QD-LED, which indicates a reduction of surface ligands for two-wash CsPbBr3 QDs. Figures 4d and S6 show the EQE−current density characteristics of the CsPbBr3 QD-LEDs. The CsPbBr3 QD-LED with

3. CONCLUSIONS In summary, we demonstrated a purification process for CsPbBr3 perovskite QDs using low-dielectric-constant solvents to completely remove impurities such as the reaction solvent and desorbed ligands. The use of the ether solvent diglyme, having a low dielectric constant of ε = 7.23, as a poor solvent for reprecipitation enabled multiple wash cycles, which led to high purity, high PLQY, and high yield of CsPbBr3 QDs. The LED based on two-wash CsPbBr3 QDs with diglyme showed a low turn-on voltage of 2.7 V and a peak EQE of over 8%. Thus, purification and isolation by multiple wash cycles with a lowdielectric-constant solvent would be an effective approach for enhancing not only the optical properties but also the efficiencies of perovskite QD-LEDs. 4. EXPERIMENTAL SECTION 4.1. Materials. Cesium carbonate (Cs2CO3, 99.99%), lead(II) bromide (PbBr2, 99.99%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), and oleylamine (OAM, 90%) were purchased from SigmaAldrich. Didodecyl dimethyl ammonium bromide (DDAB) and diethylene glycol dimethyl ether (diglyme) were purchased from Tokyo Chemical Industry. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, AI4083) and poly(4-butylphenyldiphenyl-amine) (poly-TPD) were purchased from Clevious and American Dye Source. The synthesis and ligand exchange process of CsPbBr3 QDs are described in detail in the Supporting Information. 4.2. Purification Process of CsPbBr3 QDs. Diglyme was added to the ligand-exchanged CsPbBr3 QDs dispersion with toluene (1:1 v/ v), which was then centrifuged at 10 000 rpm for 10 min. The precipitated DDAB-capped CsPbBr3 QDs washed with diglyme were collected and redispersed in octane with a concentration of 10 mg mL−1. The same process was repeated for two-wash QDs. 4.3. Characterization of CsPbBr3 QDs. Ultraviolet−visible absorption spectra, photoluminescence spectra, and PLQYs were measured using a Shimadzu UV-3150, a HORIBA FluoroMax-2, and a Hamamatsu C9920-01 integral sphere system, respectively. 1H NMR spectra and Fourier transform infrared spectra were measured with a JEOL 500 and a JASCO FT/IR-4700, respectively. X-ray diffraction spectra were collected with a Rigaku SmartLab diffractometer. 4.4. CsPbBr3 QD-LED Fabrication. Indium tin oxide (ITO) substrates were cleaned with ultrasonic water rinsing and UV−ozone treatment. PEDOT:PSS with 55 wt % Nafion was spin-coated onto ITO glass substrates and baked at 150 °C for 10 min under atmospheric conditions. Poly-TPD (4 mg mL−1 in chlorobenzene) was spin-coated onto modified PEDOT:PSS and baked at 100 °C for 10 min. CsPbBr3 QDs (10 mg mL−1 in octane) were spin-coated onto poly-TPD at 1000 rpm for 30 s in a glovebox (nitrogen condition). TPBi (50 nm), Liq (1 nm), and Al cathode (100 nm) were thermally evaporated with an active area of 2 mm2. Electroluminescence spectra and current density−luminance−voltage characteristics were measured using Hamamatsu PMA-11, Keithley SMU 2400, and Minolta CS200. D

DOI: 10.1021/acsami.8b05954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05954. Experimental methods; 1H NMR spectra; photographs of different solvents added to the perovskite QDs toluene solution; UV−vis absorption; PL spectra; crosssectional TEM image; and current density−voltage, current efficiency−current density, and power efficiency−current density curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel./Fax: +81-238-263595 (T.C.). *E-mail: [email protected] (J.K.). ORCID

Takayuki Chiba: 0000-0002-6893-7874 Satoru Ohisa: 0000-0002-2916-7512 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “Grant-in-Aid for Scientific Research A” (Grant no. 15H02203) from the Japan Society for the Promotion of Science (JSPS) and “Center of Innovation Program” of the Japan Science and Technology Agency (JST). The author thanks Kazuhiro Oikawa and Hiroto Itou (Konicaminolta) for the cross-sectional TEM measurement.

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DOI: 10.1021/acsami.8b05954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX