Surface Ligand Engineering for Efficient Perovskite Nanocrystals

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Functional Nanostructured Materials (including low-D carbon)

Surface Ligand Engineering for Efficient Perovskite Nanocrystals-Based Light-Emitting Diodes Jong Hyun Park, Ah-young Lee, Jae Choul Yu, Yun Seok Nam, Yonghoon Choi, Jongnam Park, and Myoung Hoon Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20808 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Surface Ligand Engineering for Efficient Perovskite Nanocrystals-Based Light-Emitting Diodes Jong Hyun Park,a, † Ah-young Lee,a, † Jae Choul Yu,a Yun Seok Nam,a Yonghoon Choi,a Jongnam Park,a Myoung Hoon Songa,*

aSchool

of Materials Science and Engineering and Low Dimensional Carbon Center and

Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan, 44919, Republic of Korea.

†These

authors contributed equally to this work.

*To whom correspondence should be addressed. E-mail: [email protected] Corresponding Author: *Prof. M. H. Song *E-mail: [email protected] KEYWORDS: perovskite light-emitting diodes, nanocrystal, ligand engineering, nanocrystals stability, high efficiency

ACS Paragon Plus Environment

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ABSTRACT Lead halide perovskites (LHPs) are emerging as promising materials for light-emitting device applications due to the tunability of bandgap, narrow emission, solution processability, and flexibility. Typically, LHP nanocrystals (NCs) with surface ligands show high photoluminescence quantum yields (PLQYs) due to charge carrier confinement with higher exciton binding energy (Eb). However, the conventionally used oleylamine (OAm) ligands result in the low electrical conductivity and stability of perovskite NCs (PNCs) due to a long carbon chain without conjugation bonds and weak interaction with the surface of NCs. Here, we report the effect of bulkiness and chain length of ligand materials on the properties and stability of CsPbBr3 PNCs by replacing OAm with other suitable ligands. The effect of the bulkiness of quaternary ammonium bromide (QAB) ligands was systemically studied. The less bulky QAB ligands surrounded the surface of NCs effectively, and brought better surface passivation and less aggregation compared to bulky QAB ligands, and finally the optical property and stability of CsPbBr3 PNCs were enhanced. Furthermore, the electrical property of CsPbBr3 PNCs was optimized by tuning the long-chain length of QAB ligands for


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balanced charge-carrier transport. Finally, we achieved highly efficient green emissive CsPbBr3 PNCs light-emitting diodes (LEDs) by using PNCs with optimized didecyl dimethyl ammonium bromide ligands with a current efficiency of 31.7 cd A−1, and external quantum efficiency of 9.7%, which were enhanced 16-fold compared to those of CsPbBr3 LEDs using PNCs with conventional OAm ligands.


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1. INTRODUCTION

Because of properties such as tunable bandgap, narrow full-width at half maximum, solution processability, and flexibility, lead halide perovskites (LHPs) are emerging as promising materials for light-emitting device applications.1-8 However, long charge diffusion length and small exciton binding energy in LHPs is not suitable for lightemitting diode (LED) application.9,10 In addition, solution-processed LHPs inevitably have several defects, which induce trap-assisted nonradiative recombination.11-14 Recently, several approaches have been employed to solve these problems of LHPs, such as reduction of crystal sizes, energy transfer via introduction of quasi-2D structure in perovskites, and various defect-passivation methods.2-8,15-19 Moreover, several efforts have been devoted to obtain high quality of perovskite nanocrystals (PNCs), since the synthesis method of PNCs was introduced.20 Although PNCs have excellent optical properties due to efficient charge-carrier confinement with higher exciton binding energy (Eb), there are still significant obstacles such as low colloidal stability, low charge-carrier


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transport due to the use of insulating ligand materials, and the reduction of photoluminescence quantum yield (PLQY) during the film formation.21-25 It has been reported that the instability of PNCs arises from oleylamine (OAm) ligands, which can be easily detached from the surface of PNCs and induce surface traps and low colloidal stability.26-30 In addition, these long and linear chains of OAm ligands hinder sufficient carrier transport for device applications due to their insulating property.24,28 Thus, to solve these problems, the introduction of new ligand materials to achieve PNCs with enhanced optical and electrical properties has been actively studied.23,24,28,29 The use of different structures of ligands changes the key properties of PNCs, such as their optical properties, electrical properties, stability, and size. In particular, the bulkiness of ligands can affect the surface coverage of PNCs because of the steric hindrance between the ligands.30 As the optical properties of nanocrystals (NCs) are highly dependent on the surface state of PNCs due to the high surface-to-volume ratio, surface defects cause severe reduction of optical properties.31,32 In addition, the uncapped surface of PNCs can cause aggregation of PNCs and crystal structure change.23 On the other hand, the lengths of insulating ligands are well related to the


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electrical properties of NC films via changing the distance between PNCs. A larger distance between PNCs results in lower carrier transport between them.22,29,33 To realize highly efficient PNC-based LEDs (PNCLEDs), both optical and electrical properties of PNCs should be optimized along with good colloidal stability. Recently, several reports have mentioned enhanced optical properties and stability of PNCs using quaternary ammonium bromide (QAB) due to its superior surface passivation ability.21,23,34,35 Although there have been many studies replacing OAm with other ligands, the proper structures of ligands for PNCLED application have still not been systemically studied. In this study, we optimized the bulkiness and chain length of ligands in PNCs. With the post-synthetic ligand-exchange method, we successfully replaced the OAm ligands of PNCs with QAB ion pair without any significant change in shape and crystal structure. To compare the effect of bulkiness of ligands, QAB ligands with different numbers of long chains were used, and better optical properties and stability of PNCs were achieved by employing less bulky ligands. Furthermore, to achieve proper chargecarrier balance, the electrical properties of PNCs were optimized using different long-


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chain lengths of ligands. With the optimization of both optical and electrical properties of PNCs via ligand engineering, highly efficient green-emitting PNCLEDs were achieved, showing a maximum luminance of 2,269 cd/m2, current efficiency (CE) of 31.7 cd A−1, and external quantum efficiency (EQE) of 9.7%.

2. EXPERIMENTAL SECTION 2.1.

Materials. Cesium carbonate (Cs2CO3, 99.9%, Sigma aldrich), lead bromide (PbBr2, 99,998%, Alfa aesar), dioctyldimethylammonium bromide (DOAB, 97.0%, TCI), methyltrioctylammonium bromide (TrOAB, 97%, Sigma aldrich), tetraoctylammonium bromide (TeOAB, 98%, Sigma aldrich), didecyldimethyl ammonium bromide (DDeAB, 98%, TCI), didodecyldimethyl ammonium bromide (DDAB, 98%, Sigma aldrich), and ditetradecyldimethyl ammonium bromide (DTAB, 97%, TCI), oleylamine (OAm, 70%, Sigma aldrich), oleic acid (OA, 90%, Sigma aldrich), 1-octadecene (ODE, 90%, Sigma aldrich), hexane (anhydrous, 95%, Sigma aldrich), methyl acetate (anhydrous, 99.5%, Sigma Aldrich), toluene (anhydrous, 99.8%, Sigma Aldrich), Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS, Clevios), Poly[bis(4-phenyl) (4-butylphenyl) amine] (poly-TPD, OSM), and 2, 2′, 2′′ -(1,3,5-benzinetriyl)- tris(1-phenyl-1-H-benzimidazole) (TPBi, OSM), Tris(4-carbazoyl-9-ylphenyl) amine (TCTA, OSM).


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2.2.

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Cesium-oleate solution. 0.407 g of Cs2CO3, 1.25 mL of OA, and 20 mL of ODE were simultaneously loaded into the flask and degassed under vacuum at 130 ⁰C for 2 hr. After 2 hr degassing, temperature was raised to 150 ˚C under vacuum for complete reaction.

2.3.

Synthesis of perovskite nanocrystals (PNCs). 0.138 g of PbBr2 and 10 mL of ODE are loaded into the flask and degassed under vacuum at 120 ˚C for 1 hr. Under nitrogen condition, degassed 1 mL of OAm and 1 mL of OA were added into 50 mL flask of the PbBr2 solution. Further degassing was conducted at 180 ˚C for 30 min. To synthesize the PNCs, 0.8 mL of cesium-oleate is injected into the PbBr2 solution under nitrogen condition. After 5 s, cooling of the flask was conducted in the ice bath. All synthesis method was followed the modified Protesescu et al. method.20

2.4.

Purification of PNCs. To separate the as-synthesized PNCs from impurity, PNCs solution was centrifuged at 11000 rpm for 5 min, and several purification procedures were conducted with dispersing PNCs into the mixture of hexane and methyl acetate. The purified PNCs were dispersed in hexane.

2.5.

Ligand exchange. To exchange pristine ligands into QAB ligands, the purified PNCs were dispersed to toluene with concentration of 8 mg/ml. 100 l of OA and 200 l of 0.05 M QAB are injected sequentially in the 1 ml of dispersed PNCs. For purification, the exchanged PNCs solution was centrifuged with adding methyl acetate. The purified and exchanged PNCs were dispersed in hexane.


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2.6.

PNCs characterization. Transmission electron microscopy (TEM) was measured in JEM-2100 (JEOL) with 200 kV of acceleration voltage. The sample was prepared with dropping 5 μL of hexane dispersed PNCs on TEM grid (copper 300 mesh) and drying under ambient condition. Fourier transform infrared (FT-IR) spectroscopy was measured in Cary 670 (Agilent). The sample was prepared with drying the hexane dispersed PNCs on Aucoated glass substrate. Absorbance was measured on Cary 5000 (Agilent), in which the hexane-dispersed dilute PNCs solution was putted in the cuvette. For obtain X-ray photoelectron spectroscopy (XPS) spectra, Escalab 250Xi instrument (Thermo Fisher Co.) was used with an Al-Kα monochromatic X-ray source. The data analysis was conducted with Casa XPS software. Time-resolved and steady-state PL spectra were obtained by timecorrelated single-photon counting (TCSPC) setup. The sample was excited by 405 nm pulsed diode laser with continuous wave. The detail measurements condition is presented in previous report.15 PLQY measurement of PNCs solution dispersed in hexane was conducted

with a QE-2000 (Otsuka Photal Electronics) equipped with an integrating hemisphere, and samples were excited the wavelength of 365 nm. The X-ray diffraction (XRD) patterns of the glass/CsPbBr3 PNCs films were obtained by a D8 Advance diffractometer (Bruker) equipped with a Cu K radiation source ( = 1.5405 Å) with step size of 0.02°. The atomic force microscopy (AFM) images of glass/Poly-TPD/CsPbBr3 PNCs was obtained with D1-300 (Veeco Co.).

2.7.

PNCLEDs fabrication. Cleaned indium tin oxide (ITO)-coated glass substrates were prepared using ultra-sonication process in acetone and isopropanol sequentially. The CsPbBr3 PNCs LEDs were fabricated with structure of ITO/PEDOT:PSS/Poly-TPD/CsPbBr3


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PNCs/LiF/Al. A PEDOT:PSS dispersion was deposited onto an ITO substrate through spin coating at 5000 rpm for 40 s. After PEDOT:PSS deposition, substrate was annealed at 140 °C for 10 min. Poly-TPD (in chlorobenzene, 12 mg mL-1) and CsPbBr3 NCs were sequentially deposited through spin coating at 3000 rpm for 40 s. Poly-TPD and PNC layers were annealed at 100°C for 5 min. For the deposition of TPBi (50 nm) and LiF/Al (1 nm/100 nm) electrodes, thermal evaporation system was used.

2.8.

PNCLEDs characterization. PNCLEDs device characteristics was obtained with a Keithley 2400 Source Meter and a CS-2000 Konica Minolta Spectroradiometer. PNCLEDs device characteristics was measured under ambient air conditions without any encapsulation.

3. RESULTS AND DISCUSSION In this work, colloidal PNCs were synthesized using the hot-injection method,20 and the post-synthetic ligand-exchange method was used to exchange OAm ligands with various QAB ligands, as shown in Figure 1a.24 OA and QAB solution were sequentially injected into purified PNCs dispersed in toluene with vigorous stirring. As OA is injected, protonated OAm forms an acid-base complex with deprotonated OA, resulting in detachment of OAm from the PNCs’ surface.26,34 After prompt injection of QAB ligand solution, QAB ligands attach onto the surface of PNCs. To investigate the effects of


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various ligand structures of PNCs, PNCs with six different QAB ligands were synthesized through the ligand exchange process (Figure 1b). To confirm the ligand exchange on the surface of PNCs, FT-IR measurement and XPS measurement was conducted. In the FT-IR spectrums (Figure S1), the peak ranges from 2850 to 2950 cm−1 indicates the CH2 stretching mode and the peak at 1467 cm−1 represents the CH2 bending mode, which simultaneously exists in both pristine ligands and exchanged ligands.37,38 The absorption peaks at 1419 and 1531 cm−1 denote the symmetric and asymmetric vibrations of the carboxylate group from the complexed oleate anions on the NC surface,38 and these specific absorption peaks are only observed in pristine PNCs, indicating complete desorption of OAm from NC surface. In the XPS measurement results (Figure S2), significant changes were observed in high-resolution spectra of the N 1s core level. For the pristine PNCs, the N 1s core level was fitted with two components at 399.9 eV indicating amine groups (-NH2) and 401.8 eV indicating protonated amine group (-NH3+).24,28 However, for the PNCs with QAB ligands, the N 1s core level was fitted with single peak at 402.2 eV indicating tert-ammonium cations from


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QAB ligands.24,28 From the FT-IR and XPS analysis, we confirmed the complete ligand exchange from OAm to the QAB ligands. QAB ligands with different numbers of octyl chains along the methyl chains as DOAB, TrOAB, and TeOAB were chosen to elucidate the effect of bulkiness of ligands. Although we tried to use trimethyloctylammonium bromide as a ligand, which has one octyl chain and three methyl chains, the solubility of the ligand in toluene was too low to conduct the ligand exchange process. TEM was used to observe the size and morphological difference of PNCs after the ligand exchange process, as shown in Figure 2a. All PNCs with various ligands showed a monodispersed cubic shape after ligand exchange. As the number of octyl chains increased from 2 to 4, the size of PNCs became larger from 8 nm to 13 nm. Because of the ionic feature of perovskites, the low coverage of TeOAB ligands induce facile attachment of remnant precursors, resulting in crystal growth during ligand exchange process.23 Moreover, to investigate the possibility of structural distortion during the ligand exchange process, XRD spectra were obtained (Figure 2b). In the XRD spectrum, the as-synthesized and ligand-exchanged PNCs showed similar peaks around 15° and 30°, corresponding to the (101) and (202)


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directions,22 respectively which confirmed that no crystal structural change occurred during the ligand exchange process. To compare the optical properties of PNCs with those of different ligands, the photoluminescence (PL) and absorbance of PNCs with OAm, DOAB, TrOAB, and TeOAB ligands were measured. There is no significant change in the PL and absorption spectra of PNCs with different bulkiness of ligands even though slightly blue-shifted absorption and PL spectra of PNCs with OA, DOAB, and TrOAB (Figure 2c). These blue-shift of absorption and PL spectra were induced by quantum confinement effect. Not like PNCs with TeOAB (~13 nm), PNCs with OA, DOAB, and TrOAB are sufficiently small (~8 nm) to have quantum confinement effect, which results in increase of band gap.20 In addition, the PL decay of PNCs dispersed in hexane was measured, and curves were fitted by a biexponential function (Figure 2d, Table S1). Each short-lived PL lifetime (1) and long-lived PL lifetime (2) originates from the radiative exciton recombination and trapping-associated recombination.39,40 As the number of octyl long chains increased, the average PL lifetime (av) increased from 7.36 ns to 9.29 ns, and the ratio of 2 increased gradually from PNCs with DOAB to TeOAB. These results


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indicate that PNCs with bulkier ligands have more trap sites, compared to those with less bulky ligands. In addition, the PLQYs of PNCs dispersed in hexane were measured and the PNCs with DOAB ligands showed the highest PLQY value, compared to those with bulkier ligands. The PLQY decreased (from 70% to 48%) as the number of long chains increased from 2 to 4, which was in excellent agreement with the results of PL decay (Figure 2e). A large number of long chains, such as those in the TeOAB ligand, show high steric hindrance between the ligands, which reduces the chance for the binding of ligands toward the surface of PNCs during the ligand exchange process, resulting in poor surface coverage, which results in more surface trap sites.30,41 These low surface-ligand densities create numerous surface traps, as shown in Figure S3, leading to poor optical properties of PNCs with bulky ligands.23 However, all QAB ligands PNCs show higher PLQY compared with OAm (38%), which comes from halide surface passivation effects.24

Furthermore, the stability of PNCs with different number of long-chain (octyl chains) ligands was studied. All PNCs dispersed in hexane showed bright green colors at first. However, the PNCs with OAm, TrOAB, and TeOAB ligands turned yellow after six days, which may be due to the significant aggregation of PNCs, whereas the PNCs with


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DOAB ligands remained green even after six days, as shown in Figure 3a. To confirm the colloidal stability of PNCs solution further, the PLQYs of PNCs dispersed in hexane were measured over time, as shown in Figure 3b. After six days of synthesis, the PNCs with OAm and TrOAB ligands showed a 50% drop in PLQY and the PNCs with TeOAB ligands showed only 10% of the pristine PLQY value, whereas the PNCs with DOAB ligands maintained 70% of the pristine PLQY value even in air with 60% relative humidity (RH), which is consistent with the result shown in Figure 3a. Next, to evaluate the stability of PNC films, the PNC films with different ligands were fabricated on glass substrates by the spincoating method. The films were kept in air with RH up to 60%, which is the same condition as the PNC solution stability measurement. Although all PNCs showed bright and uniform PL under UV light irradiation at first, the PL brightness of PNC films weakened as time passed, as shown in Figure 3c. Finally, the PNC film with TeOAB showed almost no PL under UV light, after six days. Compared with other PNC films, the PNC film with DOAB ligands showed the most stable PL emission even after six days. In addition, XRD analysis was performed for investigating the structural stability of PNCs with different ligands, as shown in Figure 3d. The PNC films with OAm, TrOAB, and TeOAB ligands showed sharper and split XRD patterns around 31o after being kept


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in air for six days with RH up to 60%, indicating the change in crystal structure due to degradation and aggregation,42,43 whereas the PNC film with DOAB ligands showed unchanged XRD patterns without any split pattern, indicating superior structural stability than those with the other ligands. The size and shape change of PNCs were observed by TEM, depending on storage time. The PNCs with OAm, TrOAB, and TeOAB ligands were deposited on TEM grids and kept in air for three days with RH up to 60% and the size of the PNCs was found to drastically increase larger than 20 nm due to the aggregation of PNCs, as shown in Figure 3e. However, the PNCs with DOAB ligands showed less increase in size (~ 10 nm) compared to the others, which is consistent with the results of solution and film stability in Figure 3a-d. These aggregation phenomena come from the poor surface coverage of PNCs due to the bulkiness of ligands with large number of octyl chains. Although, OAm ligands have only single long chain, it easily detached from the surface of PNCs and caused severe degradation of PNCs.26 Next, PNCLEDs with OAm and the ligands having different number of octyl chains were fabricated. The typical device configuration of PNCLEDs (ITO/PEDOT:PSS/PolyTPD/CsPbBr3 PNCs/TPBi/LiF/Al) is shown in Figure 4a. In Figure S4, the film


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morphology of PNCs with different bulkiness of ligands on the poly-TPD was measured using an AFM and no significant difference was observed in surface morphology of PNCs with different ligands. However, film fabricated OAm PNCs exhibit different morphology with more roughness. Figure S5 and Table S2 shows the efficiencies of the PNCLEDs of the four PNCs with different ligands, and the luminance versus current density (L-J) characteristics. The PNCLED with DOAB ligand also showed the highest device efficiency with a CE of 19.6 cd/A and EQE of 6.09%. The result indicates that DOAB ligands effectively passivate the surface traps of PNCs, resulting in improved radiative recombination compared to OAm, TrOAB, and TeOAB ligands. All electroluminescence (EL) spectra of PNCLEDs with a narrow full-width at half maximum (FWHM) of 20 nm were observed. However, there was a slight blue shift in the emission peaks from 517 nm to 513 nm, which was the same as the result of PL. Furthermore, to enhance the properties of CsPbBr3 PNCs further, the length of carbon chain of the ligand was changed from 8 to 14, such as DOAB, DDeAB, DDAB, and DTAB. The number of long chains was fixed to two, which showed the best optical properties and colloidal stability in previous results. Firstly, TEM images of the PNCs with


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DOAB, DDeAB, DDAB, and DTAB ligands were obtained, in which all PNCs showed a monodispersed cubic shape after ligand exchange, as shown in Figure S6. The size of PNCs becomes slightly smaller (~8 nm) with longer ligands, which indicates that the increasing length of ligands suppresses crystal growth during the ligand exchange procedure. However, the size difference was not much large, compared to the case of using ligands with different bulkiness. XRD spectra of PNCs with DOAB, DDeAB, DDAB, and DTAB ligands were measured and it was confirmed that no crystal structure change from the ligand exchange process occurred (Figure S7a). The PL, absorbance, PL decay, and PLQY were measured to compare the optical properties of PNCs with DOAB, DDeAB, DDAB, and DTAB ligands, as shown in Figure S7b and Figure S8a,b. All PNCs show no significant shift in absorption and PL spectra, and almost the same PL lifetimes of 7.5 ns with similar short-lived lifetime (1) and long-lived lifetime (2) ratio (Table S3). These results indicate that the length of ligands has no considerable effect on the optical properties of PNCs due to no significant change in surface ligand density. To realize highly efficient LEDs, not only excellent optical property but also optimum electrical property of the emissive layer is essential. In advance, the morphology of


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PNCs with DOAB, DDeAB, DDAB, and DTAB ligands on the poly-TPD layers was observed with AFM and no significant difference was observed in surface morphology. (Figure S9). As it is well known that the length of ligands is highly related to the electrical property of PNCs, the electrical properties of the emissive layers of PNCs with DOAB, DDeAB, DDAB, and DTAB ligands were compared with those of an electron-only device (Figure 4g, ITO/ZnO/CsPbBr3

PNCs/TPBi/LiF/Al)

ITO/PEDOT:PSS/Poly-TPD/CsPbBr3

and

hole-only

device

PNCs/TCTA/MoO3/Al).22,32,33

(Figure Both

hole

4h, and

electron current densities increase as the long-chain length of ligands decreases from 14 to 8, which indicates that shortening the chain length of ligands improves the charge carrier transport. Next, PNCLEDs were fabricated using PNCs with DOAB, DDeAB, DDAB, and DTAB ligands; the device characteristics are shown in Figure 4b–f, and Table 1. The current densities of PNCLEDs increase as the long chain length of ligands decreases at the same voltage, which is consistent with the results of hole-only and electron-only current density, as shown in Figure 4g,h. However, the PNCLED using PNCs with DDeAB ligands shows the highest device efficiency rather than those with DOAB ligands that have the shortest chain length. As shown in the electron-only and hole-only


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devices, this is because DDeAB ligands bring better charge carrier balance for radiative recombination in our device structure compared to DOAB (Figure 4g,h). The optimized device with DDeAB ligands exhibited a maximum luminance of 2,269 cd/m2, CE of 31.7 cd/A, EQE of 9.71%, and high color purity with an FWHM of 20 nm at wavelength of 513 nm. A summary of device characteristics compared with literature is shown in Table S4. Unfortunately, this wavelength is blue-shifted from pure green (~530 nm) which can be realized through various methods.29,44,45 The enhancement of PNCLED performance was possible by choosing proper ligands that show good surface coverage ability and optimum chain length for high optical properties, stability, and balanced charge-carrier transport.

4. CONCULSION In summary, we optimized the optical and electrical properties of PNCs for PNCLEDs application by employing various QAB types of ligands. The selection of the proper bulkiness of QAB as ligands led to an improvement in the stability and optical properties of CsPbBr3 NCs. The less bulky QAB surrounded the NC surface effectively, and


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brought better surface passivation and less aggregation compared to bulky QAB ligands. Moreover, charge-carrier transport capability was improved through the tuning of the chain length of ligands. Compared to conventionally used OAm ligands, the optical and electrical properties and stability of CsPbBr3 PNCs were enhanced by using DDeAB as ligands. Finally, these NCs were used for LED operation and highly efficient PNCLEDs with a maximum luminance of 2,270 cd/m2, CE of 31.7 cd A−1, and EQE of 9.71% were achieved. The CE and EQE of PNCLEDs using PNCs with optimized DDeAB ligands are enhanced 16-fold compared to those of PNCLEDs using PNCs with conventional OAm ligands.


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Figure 1. Schematic of a) ligand exchange process and b) chemical structures of various QAB ligand materials with different bulkiness and length.


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Figure 2. a) TEM images, b) normalized XRD patterns, c) absorption and PL spectra, d) time-resolved PL spectra, and e) PLQY of the CsPbBr3 PNCs with different bulkiness of ligands.


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Figure 3. Solution and film stability of CsPbBr3 PNCs with different bulkiness of ligands. a) A photograph showing CsPbBr3 PNCs dispersed in hexane just synthesized and after


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being kept in air for six days. b) PLQYs of CsPbBr3 PNCs dispersed in hexane recorded as a function of time while being kept in air. c) A photograph showing the green PL emission of CsPbBr3 PNC films while being kept in air with RH up to 60%. d) Normalized XRD patterns of just fabricated CsPbBr3 NC films and after being kept in air for six days with RH up to 60% e) TEM images of CsPbBr3 NCs deposited on TEM grids after

being

kept

in

air

for

three

days

with

RH

up

to

60%.


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Figure 4. a) Structure of the PNCLED, b) current density versus voltage (J–V), c) luminance versus voltage (L–V), d) current efficiency versus voltage (CE–J), e) external quantum efficiency versus current density (EQE–J), f) normalized EL spectra of the PNCLEDs using CsPbBr3 PNCs with different length of ligands and photograph (inset) of PNCLEDs using CsPbBr3 PNCs with DDeAB ligands, g) electron current density of electron-only device, and h) hole current density of hole-only device fabricated with CsPbBr3 PNCs with different length of ligands and energy band diagram (inset) of electron-only device and hole-only device.


27


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Table 1. Summarized device performance of PNCLEDs with CsPbBr3 PNCs having Turn-on Luminance CE Device configuration (PNCLEDs)

max.

EQE

max.

voltage

2 max. [cd/m ]

[cd/A]

[%]

[V]

@ bias

@ bias

@ bias

@

0.1

cd/m2 ITO / PEDOT:PSS / Poly-TPD / CsPbBr3 990 @ 4.4 17.52 @ 6.97 (DOAB) /TPBi / LiF / Al

V

ITO / PEDOT:PSS / Poly-TPD / CsPbBr3 2,270 (DDeAB) /TPBi / LiF / Al

(DTAB) /TPBi / LiF / Al

4.4 V 4.0 V 4.4 V

@

4.0 V

@ 14.78 @ 4.55

5.6 V

@

4.4 V

@ 17.09 @ 6.72

5.2 V

ITO / PEDOT:PSS / Poly-TPD / CsPbBr3 1,760

4.0 V

@ 31.70 @ 9.71

5.0 V

ITO / PEDOT:PSS / Poly-TPD / CsPbBr3 2,360 (DDAB) /TPBi / LiF / Al

4.0 V

@

4.4 V

@

2.6 2.6 2.6 2.8

different length of ligand.


28

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

FT-IR spectra, XPS spectroscopy, Schematic of PNCs having different ligands, AFM, LED performance, TEM, Absorption and PL, Time-resolved PL, PLQY, and a table of experimental data.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contribution

†J.H.

Park and A-Y. Lee contributed equally to this work.


29


Page 30 of 43

ACKNOWLEDGMENT

This

study

was

supported

by

the

Mid-Career

Researcher

Program

(2018R1A2B2006198). This work was also supported by a brand project (1.180043.01) of

the

Ulsan

National

Institute

of

Science

and

Technology

(UNIST).


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Surface Ligand Engineering for Efficient Perovskite Nanocrystals

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