Surface Engineering of Room Temperature-Grown Inorganic

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Functional Inorganic Materials and Devices

Surface engineering of room temperature grown inorganic perovskite quantum dots for highly efficient inverted light emitting diodes Eric Moyen, Haeyeon Jun, Hyo-Min Kim, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15212 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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ACS Applied Materials & Interfaces

Surface Engineering of Room Temperature Grown Inorganic Perovskite Quantum Dots for Highly Efficient Inverted Light Emitting Diodes Eric Moyen, Haeyeon Jun, Hyo-Min Kim, Jin Jang*

Advanced Display Research Center (ADRC) and Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, South Korea

KEYWORDS : Perovskite quantum dots, light emitting diodes, photo activation, ligands exchange, ligands assisted reprecipitation.

ABSTRACT : Inorganic cesium lead bromide quantum dots (CsPbBr3 QDs) are usually synthesized via a high temperature process (hot injection, HI). This process is similar to that used for the synthesis of other semiconductor QDs (i.e. CdSe@ZnS), which limits their potential cost advantage. CsPbBr3 QDs can also be synthesized at room temperature (RT) in a low cost and easily scalable process which, thus, is one of the greatest advantages of the CsPbBr3 QDs. However,

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LEDs fabricated using RT-QDs exhibit poor performance compared to those of HI-QDs. In fact, QDs are surrounded by insulating ligands to maintain their colloidal stability, but those ligands need to be removed to obtain high performance LEDs. Here we show that ligands removal techniques used for HI-QDs are not sufficient in the case of RT-QDs. Additional ligands engineering and annealing steps are necessary to remove the excess of ligands from RT-QDs films while preventing the coalescence of the QDs. The eventual surface defects induced by annealing can be healed by a subsequent photo-activation step. Moreover, the use of solution processable inorganic charge transport layers (CTL) can reduce the fabrication costs of LEDs. We fabricated an inverted LED based on a metal oxide electron transport layer (ETL) and a RT-QDs emitting layer which exhibited a maximum current efficiency of 17.61 cd A-1 and a maximum luminance of 22825 cd m-2.

TEXT 1. Introduction Inorganic perovskites quantum dots (CsPbX3, with X = Cl, Br or I)1 have attracted an increasing attention of the community for their potential use as photo-active layer in optoelectronic devices2–4 such as light emitting diodes (LED),5,6 color converters,7,8 lasers,9 photo-detectors10 or solar cells.11 This is due to their outstanding optical properties, i.e. their narrow emission band (~ 20 nm) and high photo-luminescence quantum yield (PLQY ~ 90 % reported), recently attributed to their bright triplet exciton state.12 Their emission wavelength can be tuned from ~ 380 to ~ 680 nm by tuning either their size or their halide composition.13 Their stability and luminance properties can be further enhanced by replacing their cation by, for example, methyl ammonium (MA) or Rb.14,15 Not only the optical properties of perovskites are equivalent or superior to those of other semi-


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conducting quantum dots, but also they might be synthesized using cheaper precursors. Their fabrication process can be slightly simpler than that of other semiconducting QDs (i.e. CdSe@ZnS) since the secondary growth of a shell is not necessary to prevent non-radiative exciton recombination. However, perovskites quantum dots are mostly synthesized by hot injection (HI), which is similar to the technique used for other quantum dots. HI relies on the localized injection (QDs size dispersion) of precursors at a well-defined temperature (defining the QDs size) into the proper set of solvent / precursor / ligands under a controlled atmosphere, followed by a controlled cooling down to room temperature. Upon scaling-up, the difficulty is to maintain the reaction localized to avoid unwanted size dispersion and defects. Thus, the potential cost advantages of perovskite QDs are diluted by the complicated infrastructure / process involved for their growth. Consequently, the best interest of perovskite quantum dots resides in the fact that they can be synthesized in a so-called ligand-assisted reprecipitation technique (LARP). This is a simple room temperature process which necessitates almost no equipment (i.e. two beakers in a laboratory) and can be easily scaled up.16–18 However, most of the works reporting on high efficiency perovskite QDs LEDs (Pe-QLEDs) rely on QDs synthesized by hot injection (HI-Pe-QLEDs). While current efficiencies up to 18 cd A-1 have been reported for HI-Pe-QLEDs,19 the highest current efficiency reported for a LARP-synthesized CsPbBr3 QDs LED (LARP-Pe-QLED) is only 9.22 cd A-1.20 A comparative study has shown that, if all other parameters are kept constant, the overall performances of LARP-Pe-QLEDs can be 1 or 2 order of magnitudes lower than that of HI-PeQLEDs.21 QDs need to be capped with ligands to prevent their aggregation in inks22. Those ligands are usually insulating long organic chains (oleic acid and oleylamine in the case of CsPbBr3 QDs) which prevent the efficient injection of charge carriers into QD thin films and consequently limit


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the efficiency of LEDs. The most common way to remove the ligands is to evaporate them by annealing the QD thin film. However, upon annealing, perovskite QDs tend to coalesce and develop surface defects which degrade their photo/electro-luminescence properties.23 Thus, alternative strategies have been developed to improve the performance of Pe-QLEDs. Multiple washing of the QDs solution could efficiently reduce the number of insulating ligands while maintaining a stable ink. This enabled the fabrication of high performance LEDs which exhibited a current efficiency (CE) higher than 13 cd A-1 and a luminance over 10000 cd m-2.24 It has also been shown that replacing the long insulating ligands with shorter and more conducting ligands such as di-dodecyl dimethyl ammonium bromide (DDAB) resulted in highly stable films.25 The CE of LEDs based on such films increased to 18 cd A-1 with, however, a moderate luminance of 1500 cd m-2.19 A recent study has shown that oleic acid can be replaced by shorter octanoic acid to synthesize room temperature organic/inorganic QDs (FACsPbBr3). Such QDs used in combination with high mobility organic electron transport layers resulted in LEDs exhibiting current efficiencies as high as 45 cd A-1.26 Recently, most of the attention has been drawn to the use of inorganic carrier transport layers (ZnO or TiO2 for n-type, NiO for p-type) to fabricate Pe-QLEDs.27,28 Those metal oxides can be solution processed and their bandgap can be easily tuned by addition of dopants such as Li29 or Mg.30 Thus, their conduction or valence bands can be finely aligned with the ones of the perovskite emitting layer. Moreover, they are more stable than their organics / small molecules counterparts (such as PEDOT:PSS, poly-TPD, TPBi, and etc.) and they can act as a moisture / oxygen diffusion barrier layer to protect the perovskite emitting layer (EML) from the environment. This makes them interesting candidates to increase the operation lifetime of perovskite-based LEDs. Such LEDs achieved a CE of 7.96 cd A-1 and a luminance of 6093 cd m-2 in a planar configuration31 and up to


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8.736 cd A-1 and 10206 cd m-2 in a coaxial configuration.32 Note that in the case of the coaxial configuration, the electro-luminescence has been enhanced by plasmonic effects via the embedding of gold nanoparticles into the EML.33 Here, we show that ligands washing / exchange methods are not efficient enough when applied to LARP-synthesized QDs. However, a moderate annealing of the QDs thin films can remove most of the remaining ligands. DDAB ligands can hinder the coalescence of the QDs upon annealing and a subsequent photo-activation step enables the full recovery of their optical properties.34 By using an inorganic magnesium and lithium co-doped zinc oxide (MLZO) electron transport layer (ETL) in an inverted LED structure,35 we obtain high performance inverted LEDs based on LARP synthesized CsPbBr3 QDs. Those LEDs exhibit a CE of 17.61 cd A-1, a luminance of more than 22500 cd m-2 and an external quantum efficiency (EQE) of 5.7%. 2. Results and Discussion CsPbBr3 QDs were prepared via previously reported room temperature synthesis methods (Figure S1 a). Briefly, cesium bromide (CsBr) and lead bromide (PbBr2) were codissolved in DMF. After complete dilution of the precursors, a small amount (6 % v/v) of oleylamine (OAm) was added. 0.5 mL of that solution was added dropwise to a 10 mL mixture of toluene and oleic acid (10:2) under vigorous stirring. The resulting solution was first centrifuged to extract larger particles. The supernatant was mixed with methyl acetate (MeAc, 1:1). After centrifugation, the supernatant was discarded and the pellet was dissolved in toluene (10 mg mL-1). We used this ink as reference (Figure S1 b). Films coated using that reference ink will be denoted as S0 in the following (Figure 1 a).


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The same cleaning process was repeated once (MeAc : QDs = 2:1) on half of the reference ink, which corresponds to a double washing of the QDs (Figure S1 c). After discarding the supernatant, the pellet was re-dissolved in toluene (5 or 10 mg mL-1). Thin films coated from the double washed ink will be denoted as S1 after drying for 30 mins (Figure 1 b), S2 after annealing at 120°C for 15 min in N2 (Figure 1 c) and S3 after an additional 15 min exposure to UV (Figure 1 d). A 0.05M mixture of DDAB in toluene was added to the other half of the reference ink (DDAB : QDs = 2:10) and stirred at room temperature for 2 hours (Figure S1 d). The cleaning procedure was repeated twice on the resulting ink (MeAc : QDs = 2:1), which corresponds to a total of three washing steps (Figure S1 e). After discarding the supernatant, the pellet was re-dissolved in toluene (3 or 10 mg mL-1). Thin films coated with the ink exchanged with DDAB and triple washed will be denoted as S4 after drying for 30 mins (Figure 1 e), S5 after annealing at 120°C for 15 min in N2 (Figure 1 f) and S6 after an additional 15 min exposure to UV (Figure 1 g). Note that the annealing temperature of 120°C was chosen to be below the expected orthorhombic to cubic phase transition of bulk CsPbBr3.36 The UV treatment was applied in air using a 365 nm low power (150 mW cm-2) UV lamp and will simply be denoted as UV in the following. The mass of the QDs was reduced by about 50% during the double washing procedure, and by about 70% after the ligand exchange and triple washing procedure. In both cases the QDs did not aggregate; thus, the loss of mass only corresponds to ligands removal. DDAB-coated QDs can undergo a supplementary washing step due the higher stability of DDAB ligands.19,25 However, additional washing steps led to aggregation and subsequent degradation of the QDs.37 In opposition with HI CsPbBr3 QDs, it was not necessary to add protonated OA to promote the exchange with


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DDAB. This indicates that a large excess of protonated OA remains after a single wash for LARPsynthesized QDs, which is consistent with the observations of Veldhuis et al.20 For photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements, the inks were diluted to 10 mg mL-1 (S0), 5 mg mL-1 (S1, S2, S3) and 3 mg mL-1 (S4, S5, S6) in order to have a similar concentration of virtually ligand-free QDs in each ink and a similar thickness after coating. For all other measurements (i.e. TEM, XPS and LEDs), inks were diluted to 10 mg mL-1. Figure 2 a and b show the variations of the PL peak intensity and wavelength upon the various treatments for S0, S1, S2, S4 and S0, S5, S6, S7 respectively. After the second washing step (S1), the PL intensity decreased by 20% with respect to the single washed reference (S0) with no shift of the emission line. After annealing (S2) the PL intensity decreased by ~ 50 % with a 9.2 nm shift to higher wavelengths. Exposure to UV (S3) increased the PL intensity to ~ 80 % of its initial value and slightly red-shifted the emission peak of 0.4 nm. After DDAB exchange (S4), the PL peak was red-shifted by 2.4 nm and showed a slightly higher intensity than S0. Annealing (S5) decreased the PL intensity of ~ 20 % with a 4.6 nm red shift. Finally, exposure to UV (S6) increased the PL intensity back to its initial value. The red shift of the PL peak can be attributed to both the relaxation of the CsPbBr3 lattice upon ligands removal34 and the coalescence of the QDs.23 On the one hand, ligands can induce a slight compression of the CsPbBr3 lattice; the relaxation of the lattice upon ligands removal results in a narrowing of bandgap and a red-shift of the PL emission peak.38 On the other hand, the coalescence of the QDs narrows the bandgap towards its bulk value, which also results in a red shift of the QDs emission line. Since coalescence is unlikely to happen upon ligands exchange, the small PL red


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shift after ligands exchange and triple washing can be attributed to the reduction of the number of ligands around the QDs. However, after annealing, the shift was more pronounced for OA/OAmcoated QDs than for DDAB-coated QDs. This suggests that DDAB ligands are more stable upon annealing than OA/OAm ligands and that OA/OAm-coated QDs and eventually coalesced upon annealing. This will be further confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS). Figure 2 c and d present the TRPL measurements for S0, S1, S2, S3 and S0, S4, S5, S6 respectively. The PL decay times were obtained by fitting the data with a sum of three exponential decays as previously reported for CsPbBr3 QDs.39,40 The lifetime values are reported in Table 1 and the ratios between the various decay times in Table S1. In agreement with the PL results, double washing the QDs (S1) resulted in a moderate degradation of the average decay time (8.54 ns) compared to S0 (10.65 ns). After annealing (S2) the average decay time was reduced to 4.15 ns and partially recovered to 7.15 ns after exposure to UV (S3). After exchange to DDAB and further washing (S4) the average decay time decreased to 8.54 ns compared to S0 (10.65 ns). However, a closer look at the data (inset of Figure 2 d) reveals that the faster component of the decay time increased from 2.77 ns in S0 to 3.13 ns in S4, while the slower component decreased from 67 ns in S0 to 50 ns in S4. This indicates that exchange to DDAB closes the slower indirect recombination channels, and thus increases the probability of faster direct radiative recombinations.41 As shown below the average size of the QDs is ~10 nm, larger than Bohr radius for CsPbBr3, thus the direct recombinations channels might originate from interband transitions.42,43 Our results suggest that the slower channels might be related to surface halide defects (Br), which could be passivated and stabilized by the DDA+ cations. Further annealing (S5) resulted in a slight reduction of the radiative recombination rate which recovered to the pre-annealing value after exposure to UVs (S6).


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As confirmed by XPS below, the increase of the PL intensity and decay time upon exposure to UV results from the photo-activation of the QDs. Once most surface ligands have been removed by washing and annealing, water (ambient air moisture) can be photo-adsorbed on the QDs, leading to photo-oxidation and subsequent smoothing of the QDs surface. This process passivates or removes non-radiative recombination centers on the surface and consequently improves the PL properties of the QDs.34,44,45 The non-radiative traps resulting from the annealing process are partially irreversible by photo-activation for the OA/OAm-coated QDs, while they are fully reversible for the DDAB-coated QDs. Figure 3 a-e show TEM micrographs of S0, S1, S2, S4, S5 with details of a single QD in inset. Figure 3 f and Table 1 summarize the size distribution of the QDs measured over 400 QDs on each sample. As shown in Figure S2, the Fast Fourier Transform of high resolution TEM images resulted in the same set of rings. Those are typical of the orthorhombic Pnma CsPbBr3 reciprocal lattice.36 Thus, washing, annealing or ligands exchange did not induce any phase transition. Upon washing and/or ligands exchange (S0 in Figure 3 a, S1 in Figure 3 b, S4 in Figure 3 d), the average size of the QDs remained unchanged (~ 10 ± 2 nm). Upon annealing, the average size of the OA/OAm-coated QDs increased to 11.6 ± 3.9 nm (Figure 3 c, S2). Their size distribution became asymmetric (the skewness increased from 0.4 to 1.8), with a maximum still centered around 10 nm, which shows that the QDs coalesced rather than growing (Figure 1 b and c). However, the size of the DDAB-coated QDs did not change during the annealing process (Figure 3 e, S5). This suggests that, due to the strong affinity of the protonated DDA+ for the Br on the QDs surface, the DDAB ligands remained bonded to the QDs during annealing, which prevented the coalescence of the latter (Figure 1 e and f).


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Those results are consistent with the PL measurements discussed above. The large red-shift of the PL emission peak for the OA/OAm-coated QDs upon annealing can be attributed to both the relaxation of the CsPbBr3 lattice following the removal of OA and OAm and to the coalescence of the QDs (Figure 2 a). The smaller red-shift of the DDAB-coated QDs during annealing (Figure 2 b) cannot be related to an increase of the QDs size and is only the consequence of the lattice relaxation.34 Apparition of surface defects and coalescence of the QDs (non-radiative recombination centers at grain boundaries) upon annealing resulted in a large decrease of the PL decay time in S2 (Figure 2 c). The UV treatment applied on S3 could photo-activate the exposed QDs surface, but defects at grain boundaries were irreversible. Thus, the PL decay time only partially recovered from S2 to S3. In the case of S5, the moderate decrease of the PL decay time probably originated from the surface defects appearing during annealing. The UV treatment applied on S6 efficiently suppressed those defects and the PL intensity / decay time fully recovered from S5 to S6 (Figure 2 b and d). Figure 4 shows the XPS spectra obtained on S0 to S6. The relative C/Pb concentration (Figure 4 a) decreased upon washing, from 14.2 % in S0 to 8.4 % in S1 (double washing) and to 6.3 % in S4 (DDAB and triple washing). After annealing, the C content decreased to 5.8 % in both S2 and S5. Exposure to UV further decreased the amount of C to 3.6 % in S3 and to 4.9 % in S6. Those observations confirm that multiple washing is an efficient way to decrease the number of organic ligands in the ink. However, additional thermal and photo treatments of the QDs films can eventually enhance the ligands removal. The evaporation of OA and OAm from the thin films at 120°C, i.e. well below their melting points (~360°C) is due to the vapor pressure of OA and OAm (∼10 and ∼103 Pa respectively).46 The decrease of the C content upon annealing and exposure to UV was more moderate in the case of DDAB, which confirms that the bonding of DDAB to the


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QDs is stronger than that of OA or OAm. The relative content of N (in OAm and DDAB) to Pb shown in Figure S3 a follows a similar trend and confirms the cumulated reduction of surface ligands upon washing, exchange, annealing and exposure to UVs. Figure 4 b depicts the shifts of the N1s binding energy upon the series of treatments. The exchange to DDAB (S0 to S4) was accompanied by a 0.5 eV shift to higher binding energy of the N1s peak. Nitrogen can be found in a different chemical environment in OAm (amine or protonated amine) and in DDAB (tert-ammonium). Thus, the shift is the signature of the effective exchange of ligands. Further annealing and exposure to UVs did not modify the binding energy of the N1s level for DDAB-coated QDs. A closer inspection of the N1s peak (Figure 4 c, S0, S1, S2, S3) reveals that it fits a single component centered at ~ 401.3 eV for the OA/OAm coated QDs. Oleylamine is the only source of nitrogen in the film, thus the peak corresponds to the protonated amine groups (NH3+) of oleylamine. For the DDAB coated QDs (Figure 4 c, S4, S5, S6), the N1S peak became wider and fitted a linear combination of two components. The peak centered at ~ 401.3 eV corresponds to nitrogen in OAm (dashed blue in Figure 4 c). Thus, the peak centered at ~ 401.8 eV can only be attributed to nitrogen in DDAB (i.e. tert-ammonium cations from DDAB, dashed red in Figure 4 c). In the case of HI-synthesized CsPbBr3 QDs, no such trace of the initial ligands was found after exchange.19 Thus, residual OA/OAm ligands after exchange to DDAB might be characteristics of LARP-synthesized QDs. The addition of more DDAB, and eventually protonated OA, during exchange systematically resulted in the dissolution of the QDs or the formation of (OAm)PbBr4 wells described by Pan et al.25 A full removal of the original ligands could not be achieved by a simple exchange and washing of the LARP-synthesized QDs. However, upon annealing (S5) and exposure to UVs (S6) the peak associated with OAm vanished. Consequently,


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the amount of residual OAm (and supposedly OA) ligands could be significantly reduced by annealing and exposure to UVs. Figure S3 b shows the evolution of the relative oxygen to lead content upon the various QDs treatments. It reveals a slight increase of the oxygen content after exposure to UVs, for both OA/OAm and DDAB-capped QDs and consequently confirms that the increase of the PL intensity and PL decay time after exposure to UVs is the result of photo-activation.34,44 In summary, most of the ligands of OA/OAm-coated QDs could be removed by double washing and annealing. However, the QDs tend to coalesce upon annealing due to the absence of ligands to keep them apart from each other. A UV treatment could passivate the surface defects but could not heal those at grain boundaries if the QDs were coalesced. If the QDs were coated with DDAB, multiple washing only removed part of the original OA/OAm ligands. Further annealing could reduce the amount residual ligands while the DDAB ligands remained around the QDs, preventing their coalescence and degradation. Exposure to UV enabled a full recovery of their photoluminescent properties. Finally, we used the QDs layers treated in various ways as emitting layers (EML) in inverted light emitting diodes. The structure of the devices was the following: ITO/LZO/MLZO/QDs (S1S6)/TCTA/NPD/HAT-CN/Liq-Al. The energy diagram of the LED is shown in Figure 5 a. A cross sectional TEM view of the device in Figure 5 b reveals that the MOx layers was 40 nm thick, the QDs layer was 25 nm thick (i.e. 2 or 3 monolayers) and the organic hole injection/transport layers (HIL/HTL) layers were 50 nm thick. The bandgap of ZnO can be tuned with the addition of Li and/or Mg dopants, hence improving the alignment of the MOx conduction band to the QDs conduction band. The LZO (10 % Li) layer


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was annealed in air (O rich) and acts as electron injection layer, while a 5% Mg doped LZO (MLZO) was annealed in a N2 filled glovebox and acted as electron transport layer. It was recently found that the addition of a small amount of Mg in LZO reduces the amount of hydroxyl radicals on the surface, thus preventing exciton quenching at the MLZO/QDs interface.35 The performance of devices based on S1-6 as EML are reported in Figure 5 c, d and in Table 2. The corresponding JV curves are shown in Figure S4. Double washing (S1) or DDAB exchange and washing (S4) of the QDs resulted in LEDs with very modest performance, i.e. 0.04 cd A-1, and 144.4 cd m-2 for S1 (Figure 5 c), and 0.23 cd A-1 and 234.4 cd m-2 for S4 (Figure 5 d). At that point the QDs received a treatment similar to the ones used to fabricate record efficiency HI-Pe-QLEDs, i.e. multiple washing and/or ligand exchange. Indeed, those treatment was optimized (set of solvent for washing), but it cannot explain such a wide difference of performance. This is consistent with the work of Hames et al.21 where LARP-Pe-QLEDs showed performance 10 to 100 times lower than HI CsPbBr3 QDs-based LEDs and confirms that LARP-synthesized QDs require additional treatments to express their full potential. This is most probably due to the excess of insulating ligands used during synthesis (for example, 20 % v/v of OA in toluene in the case of LARP, compared to 1 % v/v in the case of HI), which prevents efficient charge injection and limits the uniformity of the spin-coated EML.21 The room temperature process also prevents the deprotonation of OA and favors the hydrogen bonding of OA to the QDs.20 Moreover, the presence of a thin (single layer) PbBr-rich shell has been reported for LARP-synthesized QDs.18 Those might result in a different interaction between the QDs and the ligands and thus, a different behavior upon washing for LARP- and HI-synthesized QDs. As shown above, annealing the EML removed part of the insulating ligands which in return resulted in a ~ 10-fold increase of the overall performance. For example, upon annealing, the CE


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increased from 0.04 cd A-1 in S1 to 0.25 cd A-1 in S2 (Figure 5 c) and from 0.23 cd A-1 in S4 to 3.70 cd A-1 in S5 (Figure 5 d). The luminance values followed similar trends. The electroluminescence (EL) spectra in Figure S5 shows a red-shift similar to the one of the PL spectra upon annealing. Additional removal of ligands by annealing efficiently improved the carrier injection into the QDs, in spite of the coalescence of the QDs and the eventual formation of surface defects during the operation. The performances of DDAB-based LEDs are superior to those of the OAm/OA-based LEDs despite a similar carbon content at the surface (c.f. XPS in Figure 3 a). This indicates that the stronger affinity of DDA+ for the negatively charged sites of the QDs surface might result in an improved charge transfer from the ligand to the QDs. Moreover, as shown above by TRPL, exchange to DDAB closes the slower indirect PL decay channel, which might in return increase the conversion efficiency. Exposing the pre-annealed EML to UV resulted in an additional 5 to 10-fold increase of the performance. The CE increased from 0.25 cd A-1 in S2 to 1.97 cd A-1 in S3, and from 3.70 cd A-1 in S5 to 11.25 cd A-1 in S6. The performance increase upon exposure to UVs might have three origins, an additional residual ligands removal (Figure 4 a), surface smoothing and defects passivation (Figure 2) and a thin surface oxide layer (Figure S3 b) which might prevent exciton quenching at the interface. We further optimized the device based on S6, the QDs concentration was increased from 10 mg mL-1 to 15 mg mL-1 and the coating speed increased from 500 rpm to 3000 rpm, resulting in a more uniform layer of similar thickness. With those adjustments the LARP-Pe-QLEDs achieved, on average over 12 devices, 15.03 ± 1.25 cd A-1, 8.39 ± 1.22 lm W-1, 16295 ± 3305 cd m-2, and EQEs of 5.29 ± 0.25 %. The performance of the best device are reported in Figure 5 e and its currentvoltage-luminance (J-V-L) characteristics in Figure 5 f. The device reached a CE of 17.61 cd A-1,


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a power efficiency of 9.70 lm W-1 and a luminance of 22825 cd m-2 and an EQE of 5.7% with a low 2.7 V turn on voltage and a low leakage current of ~ 2x10-6 mA cm-2. The EL spectra in Figure S6 shows a peak centered at 519.7 nm with an FWHM of 17.3 nm, corresponding to the CIE coordinates (0.107, 0.788) in the whole operation range. As shown in Table 3, those performances are superior or equivalent to any reported HI or LARP CsPbBr3 based devices, in particular to those using a metal oxide ETL. According to a recent report,26 those performance might be further improved by adding a some FA into the CsPbBr3 lattice. 3. Conclusion We demonstrated that, as expected, multiple washing removes insulating ligands from QDs inks and thin films. However, such a treatment is not efficient enough to obtain high performance LEDs in the case of LARP-synthesized QDs. Further ligands removal by annealing is necessary to increase the performance of LEDs. A minimal residual number of ligands is necessary to prevent QDs aggregation in inks and coalescence in thin films. Thus, it is preferable to use more stable and more conducting ligands such as DDAB, which prevent QDs coalescence upon other insulating ligands removal and help charge carrier injection into the QDs. Finally, photo-activation of the QDs can recover from the small defects induced by annealing. Based on such treatments, inverted LEDs based on LARP-synthesized CsPbBr3 QDs with an inorganic MLZO ETL can reach or outperform the performances of HI and LARP inorganic Pe-QLEDs using organic ETL. This works paves the way towards the cost reduction and scalability of high-performance perovskite LEDs. It also provides some hints to further improve the lifetime of such LEDs due the higher stability of the ligands and the use of inorganic CTLs. 4. Experimental Section


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Chemicals: Perovskite precursors were purchased from TCI (CsBr 99 %, PbBr2 99.999 %). Methyl acetate (99.5 %) was purchased from Daejung chemicals. All other chemicals (Toluene 99.8 % anhydrous, DMF 99.8 % anhydrous, Oleic acid 90 %, Oleylamine 70%, DDAB 98 %) were purchased from Sigma-Aldrich. Perovskite precursor powders were vacuum dried prior to use. CsPbBr3 QD synthesis: A mixture of CsBr (0.4 mmol), PbBr2 (0.48 mmol) and OAm (600 µL) was stirred in DMF (10 mL) for 24 hours. 0.5 mL of the previous mixture was added dropwise into a 10:2 mixture of toluene and oleic acid under vigorous stirring. After synthesis, the resulting solution was centrifuged for 5 min. at 9000 rpm to extract larger particles and aggregates. The supernatant was mixed with methyl acetate in a 1:1 volume ratio and centrifuged for 10 min. at 9000 rpm. After centrifugation, the supernatant was discarded and the pellet dissolved in toluene (10 mg mL-1). This washing step was eventually repeated multiple times, see text and fig. S1 for details. Ligands exchange: 231 mg of DDAB were dissolved in 10 mL toluene (0.05M). That DDAB solution was added to the 10 mg mL-1 single washed QDs solution in a 10:1 volume ratio and stirred for 4 hours. The resulting solution washed twice as described above and in Fig. S1. LZO and MLZO solutions: 0.5 M of zinc acetate dihydrate (Zn(C4H6O4)2·2H2O) was dissolved in 2-methoxyethanol (C3H8O2, 2-ME). Monoethanolamine (NH2CH2CH2OH, MEA) was added as a stabilizer in a 1:1 molar ratio with the precursors. Lithium acetate hydrate (CH3COOLi·H2O) and magnesium acetate tetrahydrate ((CH3COO)2Mg·4H2O) were added into the ZnO solution to obtain LZO (10 at.% Li) and MLZO (10 at.% Li and 5 at.% Mg) solutions.


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Devices fabrication: CsPbBr3 QLEDs were fabricated on UV/O3 activated patterned ITO substrates by successively spin-coating LZO (2000 rpm, annealing: 300°C, 10 min. in air), MLZO (2000 rpm, annealing: 220°C, 10 min. in N2), CsPbBr3 QDs (10 mg mL-1 in toluene, 500 rpm or 15 mg mL-1, 3000 rpm, eventually annealed in N2 at 120°C for 15 min and exposed to UVs in air for 15 min). TCTA (10 nm), NPD (20 nm), HAT-CN (20 nm) and Al (100 nm) were deposited by thermal evaporation in a vacuum (10-6 mbar) chamber. Light emitting areas were 4 mm2. Characterizations: Photo-activation was done by exposing QDs thin films to a Spectroline ENF240C 365nm lamp (in air). PL spectra were recorded using a Jasco FP-6500 spectrofluorometer at an excitation wavelength of 365 nm. TEM micrographs was taken in a FEI Titan operating at 300 kV. XPS spectra were measured with a Thermo K-alpha spectrometer (mono-chromated Al X-ray sources, Al Kα line: 1486.6 eV). LEDs were powered with a Keitley 2635A Sourcemeter and their performances measured with a Konica Minolta Chroma-Meter CS-100A colorimeter (a CS-1000 was used for EQE measurements).


17


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FIGURES

Figure 1. Schematics of the various treatments applied to the CsPbBr3 QDs thin films. a) Single washed QDs thin film (S0) – b) Double washed OA/OAm-capped QDs thin film after coating (S1) – c) After annealing at 120°C (S2) – d) After exposure to UV (S3) – e) Triple washed DDABcapped QDs thin film after coating (S4) – f) After annealing at 120°C (S5) – g) After exposure to UV (S6). OA/OAm-capped QDs tend to coalesce during the process while DDAB keep QDs away from each other. Photo-activation is efficient on the exposed QDs surfaces but not at the grain boundaries.


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Figure 2. Photo-luminescence spectra of a) OA/OAm- and b) DDAB-capped CsPbBr3 quantum dots. Time resolved PL spectra of c) OA/OAm- and d) DDAB-capped CsPbBr3 QDs thin films, details on the fast components in inset. S0 – S6 are defined in Fig. 1 and S1. Numerical values are summarized in Table 1 and Table S1. The PL peaks red-shift upon annealing as a result of ligands removal. The PL intensities and decay lifetimes increase upon exposure to UV. Photo-activation of the QDs can partially or fully heal the surface defects created upon annealing, depending if the QDs are coated with OA/OAm or DDAB.


19


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Figure 3. TEM micrographs of the CsPbBr3 QDs capped with various ligands, before and after annealing - a, b, c, d, e) Large scale images, details of a single QD in inset for S0, S1, S2, S4 and S5 respectively. Note the coalesced QDs in S2 - f) QDs edge lengths histogram. Numerical values are summarized in Table 1. S0, S1, S4 and S5 show a similar distribution of edge sizes. The distribution becomes asymmetric (to greater edge sizes) for S2, but its maximum remains the same as other samples. Double washed OA/OAm-coated QDs tend to coalesce upon annealing while DDAB coated QDs do not.


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Figure 4. XPS analysis for CsPbBr3 QDs at room temperature, after annealing and after exposure to UVs. OA/OAm films are in shades of blue and DDAB films are in shades of red. a) Evolution of the C over Pb ratio, the C content decreases upon the various treatments – b) Evolution the N1s binding energy, the N1s peak shifts of 0.5 eV upon DDAB exchange – c) Details and fits of the N1s peak. For S0, S1, S2 and S3 the N1s peaks fits a single component at ~401.3 eV attributed the amine groups of OAm. After exchange to DDAB, the N1s peak of S4, S5 and S6 shows the previous component from OA/OAm and another one at ~ 401.8 eV attributed to the tert-ammonium cations of DDAB. The OAm component decreases with further annealing in S5 and UV treatments in S6.


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Figure 5. Light emitting diodes based on CsPbBr3 QDs - a) Energy diagram of the devices – b) Cross-sectional TEM micrographs of one device. Left: large scale image, right: details of the inorganic ETL (40 nm), perovskite EML (25 nm) and the HTL (50 nm) – c) Current efficiency (CE) as a function of luminance using S1, S2 and S3 as EMLs -d) CE as a function of luminance using S4, S5 and S6 as EMLs – e) Performance of a device based on S6 with an optimized EML thickness and uniformity. The CE reaches 17.61 cd A-1 and the luminance 22825 cd m-2 – f) Corresponding current-voltage-luminance (J-V-L) characteristics showing a low 2x10-6 mA m-2 leakage current and 2.7 V turn on voltage.


22

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TABLES

Table 1: evolution of the PL peaks wavelengths, decay lifetimes and CsPbBr3 QDs sizes with various treatments applied to the QDs thin films. Sample

Ligands

Treatment

PL λmax (nm)

τ1 (ns)

τ2 (ns)

τ3 (ns)

(ns)

(nm)

σL (nm)

skew. L

S0

OA + OAm

single wash

506

2.77

12.79

67.39

10.65

9.88

1.82

1.09

S1

OA + OAm

double wash

506

2.11

12.97

90.13

8.54

9.94

1.79

0.42

S2

OA + OAm

+ 120°C 15 min

515.2

1.23

9.09

78.01

4.15

11.62

3.89

1.80

S3

OA + OAm

+ UV 15 min

515.6

1.65

11.38

91.99

7.15

-

-

-

S4

DDAB

triple wash

508.4

3.13

12.16

50.47

8.08

9.82

1.94

0.57

S5

DDAB

+ 120°C 15 min

513

2.48

11.04

58.85

6.51

9.62

1.92

0.62

S6

DDAB

+ UV 15 min

513

3.18

12.00

54.69

7.68

-

-

-

Table 2: evolution of the performances (EL peaks wavelengths, maximum current efficiencies, luminances, power efficiencies and external quantum efficiencies) of LEDs when various treatments were applied to the EML. Device

EL λmax (nm)

CEmax (Cd.A-1)

Lmax (Cd.m-2)

PEmax (lm.W-1)

EQEmax (%)

S1

508.70

0.04

144.40

0.02

0.01

S2

521.30

0.25

967.20

0.18

0.04

S3

521.30

1.97

3536.00

1.30

0.28

S4

508.40

0.23

234.40

0.28

0.15

S5

519.80

3.70

1064.00

4.68

2.03

S6

519.80

11.25

7189.00

7.97

4.53

Optim.

519.70

17.61

22825.00

9.70

5.71

Table 3: comparative table of reported perovskite quantum dots LEDs. QDs Synthesis

QDs

Device Structure

ETL

CEmax (Cd.A-1)

Lmax (Cd.m-2)

EQEmax (%)

Ref.

Hot injection

CsPbBr3

Conventional

Organic

13.30

13000.00

6.27

24

Hot injection

CsPbBr3

Conventional

Organic

18.80

1500.00

8.73

19

Hot injection

CsPbBr3

Inverted

MOx

2.29

3809.00

2.39

47

Hot injection

CsPbBr3

Inverted

MOx (NWs)

8.73

10206.00

4.63

32

Hot injection

CsPbBr3

Conventional

MOx

7.96

6093.20

3.79

31


23


Page 24 of 43

RT

FACsPbBr3

Conventional

Organic

45.5

55000

11.6

26

LARP

CsPbBr3

Conventional

Organic

9.22

3880.00

2.64

20

LARP

CsPbBr3

Inverted

MOx

17.61

22825.00

5.70

this work

ASSOCIATED CONTENT

Supporting Information. Schematics of the QDs fabrication process, ligands exchange and washing procedure. Fast Fourier Transforms of HRTEM images. Evolution of the N and O contents from XPS. Electro-luminescence spectra.

AUTHOR INFORMATION

Corresponding Author

E-mail:

* [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (10052044) and KDRC (Korea Display Research Corporation) support program for the development of future devices technology for display industry


25


Page 26 of 43

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Page 38 of 43

Table of contents graphics.


38

Page 39 of 43 OA + OAm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

UV 365nm


S1

S2

S3

5nm

Second wash

annealing @ 120°C

exposure to UVs

CsPbBr3 QD

S0

UV 365nm

First wash

DDAB

S4

S5

ACS Paragon Plus Environment DDAB + double wash

annealing @ 120°C

S6 exposure to UVs

Norm. PL intensity (a.u.)

ACS Applied Materials & Interfaces Page 40 of 43 104 4 10

Norm. TRPL counts (a.u.)

11.0 2 30.5 4 5 60.0 7 8 9 1.5 10 11 12 1.0 13 14 0.5 15 16 17 0.0 18 19

OA + OAm S0 (sgl. wash) S2 (+120°C 15m) S3 (+UV 15m) S1 (dbl. wash)

0

103

480

500 520 Wavelength (nm)

0

540

DDAB S0 (sgl. wash) S5 (+120°C 15m) S6 (+UV 15m) S4 (tpl. wash)

25

50 75 Time (ns)

104

0

103

data

500 520 Wavelength (nm)

540

100

125

104

DDAB ACS Paragon Plus10Environment 2

480

10

OA + OAm data fit

102

Norm. TRPL counts (a.u.)

Norm. PL intensity (a.u.)

1.5

0

25

10

fit

50 75 Time (ns)

100

125

S1 - OA+OAm - dbl. wash

OA + OAm

1 2 S0 - OA+OAm - sgl. wash 3 4 5 6 7 8 9 5nm 10nm 10 11 12 13 DDAB 14 15

S2 - OA+OAm - dbl. wash - anneal

OA+OAm DDAB S0 S4 S1 S5 S2

1.0 ACS Applied Materials & Interfaces

0.8

10nm

5nm S4 - DDAB - tpl. wash

10nm

5nm

S5 - DDAB - tpl. wash - anneal

Norm. counts (a.u.)

Page 41 of 43

0.6

0.4

0.2

ACS Paragon Plus Environment 10nm

5nm

10nm

5nm

0.0

5

7

9

11 13 15 QDs edge length (nm)

17

19

21

16 14

C/Pb at.%

data N1s ACS Applied Materials & Interfaces Page 42fit of 43 OA + OAm OA + OAm N1s (OAm)

S0

DDAB

DDAB

12

N1s Binding Energy (eV)

C/Pb at.%

S1

S4

(+UV 15 min)

S5 S2

DDAB exchange

sgl. wash S0

mult. wash

S1

120°C 15 min

S6

S6 (+120°C 15 min)

S3 UVs 15 min

S2

Norm. XPS counts (a.u.)

1 10 2 8 3 6 4 4 5 2 6 0 7 8 9401.2 10 11 401.4 12 401.6 13 14 401.8 15 16 402.0 17 18

S5 (tpl. wash)

S4 (+UV 15 min)

S3

S3 DDAB exchange

S4 N1s OA + OAm DDAB sgl. wash

mult. wash

N1s (DDAB)

S5

(+120°C 15 min)

S2

S6

(dbl. wash)

S1

ACS Paragon Plus Environment (sgl. wash) 120°C 15 min

UVs 15 min

S0

404

403

402 401 400 Binding Energy (eV)

399

101

100

100

OA + OAm S1 S2 S3

10nm

101

104

102 103 Luminance (cd.m−2)

10−2

102 104

100

10−1

DDAB S4 S5 S6

ACS Paragon Plus Environment 10−2 0 10

10−1

cd.A−1 lm.W−1 10−2 0 10

104

10

1

2

3


10

4

Current density (mA.cm−2)

Current efficiency (cd.A−1)

LZO/MLZO


101

TCTA/NPD/HAT-CN

CsPbBr3 QDs

101

Optimized device S6 15 mg.mL−1

10

0

103 10−2 102 Optimized device S6 15 mg.mL−1

10−4

10

mA.cm−2

−6

−5

101

cd.m−2 0

5 Voltage (V)

10

100

Luminance (cd.m−2)

5e -9.

10−2 0 10

10−1

Power efficiency (lm.W−1)

10−1


100

V


-4.

3e

V Liq/Al

8e V HAT-CN

-4.

6e

V

7e

101

V 3e

-2. 4e

V

NPD

TCTA

V 6e

-5.

MLZO

CsPbBr3 QDs

5e

V

-5.

...

V

-5.

LZO

101 Applied Materials & Interfaces ACS

-6.

-6.

-4.

2e

V ITO

1-4 2 3-5 4 -6 5 6-7 7 8 9 10 11 Al 12 13 TCTA/NPD/HAT-CN 14 QDs CsPbBr3 LZO/MLZO 15 16 ITO17 18 19 Glass 50nm 20

Energy (eV)

1e V -2. 5e V

0e V

-3.

-3.

-3

-3.

3e V

Page 43 of 43 -2

Surface Engineering of Room Temperature-Grown Inorganic

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