Engineering Color-Stable Blue Light-Emitting Diodes with Lead Halide

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Functional Inorganic Materials and Devices

Engineering Color-Stable Blue Light-Emitting Diodes with Lead-Halide Perovskite Nanocrystals Stefan Thomas Ochsenbein, Franziska Krieg, Yevhen Shynkarenko, Gabriele Raino, and Maksym V. Kovalenko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Materials & Interfaces

Engineering Color-Stable Blue Light-Emitting Diodes with Lead-Halide Perovskite Nanocrystals Stefan T. Ochsenbein, Franziska Krieg, Yevhen Shynkarenko, Gabriele Rainò, Maksym V. Kovalenko* Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, CH-8093 Zürich, Switzerland Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland

KEYWORDS: perovskite, nanocrystal, light-emitting diode, electroluminescence

ACS Paragon Plus Environment

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ABSTRACT Nanocrystalline lead-halide perovskites are promising as emissive layers for light-emitting diodes due to their bright, tunable emission with very narrow linewidth. Blue perovskite lightemitting diodes, in the wavelength range useful for display applications (460 – 470 nm), could be made with CsPb(Br/Cl)3 nanocrystals (NCs). However, mixed halide perovskites suffer from color-instability, foremost, due to segregation of halide ions. In this study, we address this issue with several measures. First, we show that thinner CsPb(Br/Cl)3 NC layers are less prone to colorinstability. Additionally, inefficient hole-injection due to the deep-lying valence band of CsPb(Br/Cl)3 NCs detrimentally affects device performance, and we mitigate this problem by stepwise hole-injection using two hole-transporting materials. Next, we employ NCs capped with zwitterionic ligands that allow for a more thorough washing of the NC solutions. Furthermore, our new device layout explores the use of polystyrene in the emitting layer to limit current leakage. Undertaking these steps, we show light-emitting diodes with a stable electroluminescence peak wavelength of 463 nm over the lifetime of the device and a peak external quantum efficiency of over 1%. The results prove that perovskite NCs are a viable contender in the development of blueemissive, active-pixel-displays.

TEXT

Lead halide perovskites (LHPs), with a general formula of APbX3 [A=Cs, formamidinium (FA) or methylammonium (MA), X=halide], are a category of solution-processable semiconductors, whose bandgap can be easily tuned by adjusting the composition, i.e. by varying the halides. This


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characteristic of LHPs has been used to optimize the bandgap of LHP absorbers for photovoltaics13

and the photoluminescence (PL) wavelength of LHP nanocrystals (NCs).4-6 The benefits that

arise from the soft nature of LHPs, such as facile syntheses of mixed-halide compositions directly or through post-synthetic halide exchange,7-9 are somewhat neutralized by the closely related lability of these materials. In practical terms, halide segregation can also be seen leading to bandgap shifts and therefore color-instability, and this had been observed in all forms of these materials – bulk crystals, thin films and colloidal NCs.10-13 Such spectral shifts have been observed under illumination (e.g. LHP photovoltaics),14-17 but also under applied bias in light-emitting diodes (LEDs).12, 18-20 For some compositions, a bandgap shift may even occur just over time, triggered exclusively by the higher thermodynamic stability of the respective monohalide phases as in FAPb(Cl/Br)3 NCs.21 With their narrow PL linewidths,4-6 LHP NCs display a high color purity, which would make them ideal emitters for light-emitting LEDs.5, 22 Improving color purity is one of the most eminent goals put forward by the International Telecommunication Union for ultra-high definition televisions, as described in their recommendation for such systems (known as BT.2020).23-24 However, for the blue pixels in displays, wavelengths between 460 and 470 nm are needed, and thus mixed-halide NCs need to be used, e.g., CsPb(Br/Cl)3. This naturally raises the question of color-instability during device operation. Also, efficient blue LHP LEDs have in general been difficult to engineer,18, 22, 25 in contrast to red and green-emissive devices.26-27 The large band-gap of blue-emitting materials hinders efficient electron and hole-injection – an issue also known for OLEDs28 and quantum-dot LEDs.29 Moreover, the deep-lying valence band of blue-emitting LHPs18, 22, 30 constitutes a major challenge for hole-injection. This challenge, if not overcome, leads to charge-imbalance, i.e. excess electrons, in the emitting layer. Excess electrons can deteriorate the electroluminescence (EL) by


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Auger quenching31 or redox chemistry.32 The large band-gap of blue emitters also facilitates nonradiative relaxation, reflected in reduced PL quantum yields,6, 22, 33 which also reduces the final, attainable LED efficiency. Here, we report LEDs with improved color-stability using CsPb(Br/Cl)3 NCs (emission peak at 463 nm). The achieved external quantum efficiency (EQE) of 1% is among the highest reported efficiencies at a wavelength below 470 nm.22, 25 We found that limiting charge accumulation on the CsPb(Br/Cl)3 NCs by adjusting the film thickness was crucial to achieving improved color stability. Additionally, in order to balance the charge transport, two hole-transporting layers (HTLs) had been employed for facilitating the hole injection. Alternatively, Congreve and coworkers, used a doped hole-transporting layer (HTL)18,

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with graded valence-band energy.

Furthermore, we have employed NCs synthesized with zwitterionic ligands that exhibit more static surface-coating than conventional mixtures of long-chain carboxylates and amines, thereby allowing for more thorough washing without losing the colloidal and chemical integrity of NCs. Their encapsulation in a polystyrene layer further minimized leakage currents with an overall boost of device performances.


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Figure 1. (a) Structure of the long-chain zwitterionic sulfobetaine molecule used as a capping ligand for the nanocrystals, (b) TEM image of CsPb(Br/Cl)3 NCs, (c) XRD pattern of CsPb(Br/Cl)3 NCs (blue line, offset for clarity) and calculated reference pattern of CsPbBr3 in the orthorhombic Pbnm space group (black line), (d) absorption (blue line) and PL (black line) spectra of CsPb(Br/Cl)3 NCs in toluene.

Synthesis and characterization of NCs. The CsPb(Br/Cl)3 NCs employed in this study were synthesized

directly

with

the

zwitterionic

sulfobetaine

ligand

(3-(N,N-

dimethyloctadecylammonio)-propanesulfonate) shown in Figure 1a by injecting mixed bromide and chloride adducts of tri-octylphosphine (7:3 Br:Cl) into a solution of Cs and Pb carboxylates and sulfobetaine in octadecene at elevated temperatures (150 C). After injection, the mixture was


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immediately cooled to room-temperature. The NCs were isolated by the addition of acetone to precipitate them, followed by centrifugation and redispersion in toluene.34 Transmission electron microscopy (TEM) reveals the typical cube-shaped perovskite NCs with edge-lengths of ~10 nm (Figure 1b). The powder X-ray diffraction (XRD) pattern (Figure 1c, referenced to bulk CsPbBr3), shows the orthorhombic perovskite lattice, with pronounced compositional shift (shrinking of the unit cell) and finite size-broadening effect. Optical characterization reveals the first absorption peak at 448 nm and a sharp PL peak at 461 nm (Figure 1d), exemplifying the band-edge absorption and excitonic emission typical for these semiconductor NCs. The PL peak of the CsPb(Br/Cl)3 NCs at 461 nm is only 13.6 nm wide (full-width-half-maximum, FWHM, 77 meV). Such narrow linewidths are a prerequisite for the BT.2020 recommendation,23-24 and are presently not attainable by other common emitters suited for LEDs such as fluorescent, phosphorescent or thermallyactivated delayed fluorescent organic molecules.28,

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The photoluminescence quantum yield

(PLQY) of the CsPb(Br/Cl)3 NCs in solution was typically around 50 – 60%. Fabrication and characterization of LEDs. The sulfobetaine-capped perovskite NCs offer better long-term stability and allow for more thorough washing with antisolvents.34 Thus, it is possible to wash away excess ligands from the NC solution. Excess ligands lead to insulating layers around the NCs and impede efficient charge-injection into the NCs, while too few ligands results in high leakage currents and unstable NCs, as was shown by Li et al. and Chiba et al.36-37 We additionally washed the sulfobetaine-capped CsPb(Br/Cl)3 NCs three times with ethylacetate after the initial isolation step, without affecting the PL wavelength. The LED, schematically shown in Figure 2a, comprised several solution and vacuum-deposited layers. First, several layers were produced by spin-coating a hole-injecting layer of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS, aqueous solution) on a pre-patterned indium-tin oxide


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(ITO)-coated glass substrate as an anode, followed by spin-coating the hole-transporting materials poly[N,N’-bis(4-butylphenyl)-N,N’-bisphenylbenzidine] (Poly-TPD, chlorobenzene solution) and 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP, chlorobenzene solution), and then spin-coating the emitting-layer (EML) from a toluene-solution of CsPb(Br/Cl)3 NCs and polystyrene. Afterwards, the devices were transferred to a high-vacuum chamber for thermal evaporation of the electrontransporting layer (ETL) of 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM, 50 nm), the thin electron-injection layer (EIL) of lithium fluoride (LiF, 1.2 nm), and the aluminum (Al, 100 nm) cathode. The work function of the indium-tin oxide (ITO) anode and the LiF/Al cathode are depicted in Figure 2b together with the valence and conduction-band energies (bottom and top of colored bars) of the transport materials and of the CsPb(Br/Cl)3 NCs.18, 38-40 The LED performance was assessed by measuring the current density and the luminance of each pixel (5 pixels per substrate) as a function of applied voltage (Figure 2c). The current density vs. voltage curve shows two distinct regions. Below ~2.5 V, the current density is rather low and increases only slightly with increasing voltage. Above 2.5 V, the current density increases, first, very steeply with increasing voltage before slowly turning over. The two regions in the current density vs. voltage curve roughly correspond to the off and on the state of the LED as the luminance vs. voltage curve shows (Figure 2c). The luminance is unmeasurable below ~2.9 V, but then increases steadily to a maximum of 318 cd/m2 at 8.25 V before decreasing slightly to 280 cd/m2 at 10 V.


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Figure 2. (a) Schematic drawing of the LED stack, (b) work function and bandgap of the different materials used in LED stack, energies vs. vacuum, (c) current density (black line) and luminance (blue line) vs. voltage for a thin LED with the device structure shown in Figure 2a, (d) EQE vs. current density for the same LED. The energy corresponding to the turn-on voltage for efficient LEDs should match the band-gap energy of the emitting material; the CsPb(Br/Cl)3 NCs emit at ~463 nm or 2.68 eV. The turn-on voltage of ~2.9 V is thus only slightly higher than expected, indicating low barriers for chargeinjection. The EQE is shown as a function of the current density in Figure 2c, and it increases from 0.7% at 0.1 mA/cm2 to a maximum of 1.2% at 4.4 mA/cm2 before slightly decreasing to ~1% at 21 mA/cm2. The EQE is above 1% between 0.4 and 21 mA/cm2 – a range of almost two orders of magnitude in current density. Over the same range in current density, the luminance increases almost linearly from 1.8 to 93 cd/m2. These results indicate that the charge-injection is fairly balanced over that region as unbalanced charges would lead to a fast efficiency roll-off.41 Experiments with only one HTL revealed lower EQE values, see Figure S1, and were thus not


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optimized. The statistical distribution of EQE values (Figure S2) shows that 1.0% is the most likely outcome in a standard device configuration as described in the Supporting information. EQEs of ~1% reported here together with the ~2% from Ref.25 are still far below the ~20% recently reported for green and red perovskite LEDs.26-27 Yet this is a significant step in the development of CsPb(Br/Cl)3 LEDs and, in general, for the blue region below 470 nm, where only Cd-based quantum dots have yielded efficient, color-pure LEDs.42 In comparison, commercial blue organic light-emitting diodes (OLEDs) containing a fluorescent molecular emitter achieve a maximum EQE of 7.5%.43 Although green and red OLEDs use phosphorescent emitters with higher EQEs than fluorescent emitters because of spin-statistics, phosphorescent blue emitters have not been stable thus far.28 Color and color stability of CsPb(Br/Cl)3 LEDs. As described above, a common problem in mixed-halide LHPs is halide segregation that results in a permanent or dynamic shift of the PL or the EL.11-13, 18 We note that in a colloidal state CsPb(Br/Cl)3 NCs retain the PL peak wavelengths over an extended period of time (at least several months) under ambient conditions. In LED devices, we measured the electroluminescence (EL) spectra of the LEDs between 4 and 10 V, and observed sharp peaks centered at ~463 nm (Figure 3a) in the whole voltage range. More detailed analysis shows that the EL peak position is indeed constant with voltage (see Figure 3b), but that the peak slightly broadens with increasing voltage as indicated by the white lines in the contour plot of Figure 3b. The FWHM increases from 14 to 16 nm between 4 and 10 V, an effect that might be due to Joule heating at higher voltages. The calculated color coordinates from the EL peak of the CsPb(Br/Cl)3 NCs at 4 V (0.138, 0.039), are very close to the blue corner-point of the BT.2020 recommendation with coordinates (0.131, 0.046), as demonstrated in the full Commission Internationale de l'Éclairage (CIE) color


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diagram, Figure 3c. Upon zooming into the blue region, Figure 3d, the small distance to the cornerpoint is emphasized. In Figures 3c and 3d, the color coordinates for commercial OLED displays are also indicated.43-44 Our LEDs are very close to the desired color coordinates for blue display applications according to BT.2020, and closer than commercial OLED materials. Blue OLED emitters have a linewidth of ~36 nm,44 more than twice the linewidth of our CsPb(Br/Cl)3 LEDs. The narrow linewidth is not only important for the color quality of a potential display, but also for the brightness: the Helmholtz-Kohlrausch effect describes how more saturated (i.e., purer) colors are perceived brighter than less saturated colors for the same luminance.45-46 Thus, with narrowlinewidth CsPb(Br/Cl)3 emitters, blue LEDs could be operated at a lower luminance than, e.g. current blue OLEDs, and thus lower power.

Figure 3. (a) EL spectra vs. wavelength of a thin CsPb(Br/Cl)3 LED at different voltages between 4.0 and 10.0 V, (b) contour-plot of normalized EL vs. voltage and wavelength with peak position of EL (thick white line) and FWHM of the EL peak (distance between thin white lines) indicated,


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(c) CIE diagram with ITU-R BT.2020 (white triangle), and color coordinates of CsPb(Br/Cl)3 LED at 4 V () and a blue OLED (), (d) zoomed version of (c). Constant voltage operation also shows a stable EL peak position of LEDs with thin EML (see Figure 4). With a constant applied voltage of 5 V, an EL spectrum was measured every second for 4 minutes. Figure 4a shows the spectra recorded between the start and the 60th second in ten-second intervals, as well as the spectra after 120 and 240 s. The EL intensity is decreasing with time, but the peak position remains the same, which can also be seen in the contour plot of the normalized EL intensity as a function of wavelength and time in Figure 4b. Over the same time-range, the current-density slightly increases from 6.5 to 7.2 mA/cm2. Although there is no observable shift in the emission energy, the EL intensity decreases over time. This indicates that the overall operational stability is a more pressing issue than color stability for these LEDs, as was also observed by Congreve and co-workers.25 As the experiments were performed in open air, it was not possible to observe recovery of EL.


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Figure 4. (a) EL spectra recorded at 5 V. Spectra at intervals of 10 s between 0 and 60 s, and spectra after 120 and 240 s are shown, (b) contour plot of normalize EL intensity vs. wavelength and time recorded at 5 V.

The results presented in Figures 2, 3 and 4 of efficient, color-stable LEDs are for thin device stacks. These results clearly show that a thin emitting layer leads to color stable LEDs. In order to better understand the reasons for the color (in)stability of mixed lead-halide perovskites (LHPs) under LED operation, we looked at the thickness-dependence of the EL spectra through varying the concentrations of the spin-coated materials. Some of the corresponding results are shown in Figure S3. We observed that thicker LED stacks formed by spin-coating from higher concentrations of NCs or polystyrene were less color-stable (Figure S3a and S3b). In these thicker, color-unstable LEDs the electric-field strength at the same voltage must be lower than in the thinner, color-stable devices, because the electrodes are farther apart. Therefore, color stability


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does not directly correlate with the applied electric field strength. The observed results of higher color-stability for thinner devices disfavor the hypothesis that the applied electric field alone leads to color-instability. Alternatively, charge accumulation could be a possible source for colorinstability. In an electric field, thicker emitting layers could favor charge accumulation within them, leading to local charge imbalances. These imbalanced charges in the emitting layer may be driving the halide segregation responsible for the color shift. This is also supported by the observed color-shift for thicker HTLs (Figure S3c), which favor unbalanced charge-injection. In summary, balanced charges in the emitting layer, a pre-requisite for higher EQE, also improve the colorstability of these LEDs. Effect of polystyrene on LED performance. The addition of polystyrene to the emitting layer was crucial to reach EQE values above 1% in a reproducible fashion. We hypothesize that the more intensely washed sulfobetaine-capped NCs allow for higher leakage currents than optimallywashed LHP NCs, whereby the optimal-wash is difficult to reproduce and adapt from material to material. Figure 5a shows a log-log plot of the current density as a function of voltage for LEDs prepared with different amounts of polystyrene in the EML. The linear portion of the current density vs. voltage plot, observed at low voltages, is due to finite parallel or shunt resistance, indicating that electrons can flow from cathode to anode while bypassing the NCs without yielding EL. The results in Figure 5a clearly demonstrate that polystyrene increases the shunt resistance,47 and thus suppresses leakage currents. Similar effects from a polymer have been observed for CdSe quantum-dot LEDs, where poly(methyl methacrylate) (PMMA) was spin-coated over the QDlayer.48 Preliminary results with green sulfobetaine-capped CsPbBr3 NCs and polystyrene yielded greatly improved EQE values compared to our previously reported values34 for this material (Figure S4).


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Examining the dependence of peak luminance and peak EQE values for different polystyrene concentrations, Figure 5b, reveals that the ideal concentration lays around 1 mg/mL. These data indicate that concentrations above 1 mg/mL may be beneficial for maximizing EQE. However, there are two effects limiting the useful polystyrene concentration: (a) peak luminance dramatically decreases above 1 mg/mL, shown in Figure 5b, and (b) the color becomes unstable at higher concentrations (see Figure S3b). It seems that above 1 mg/mL, polystyrene limits not only leakage current but generally hinders effective charge transport to the NCs. Additionally, we found that 1 mg/mL polystyrene addition does not appreciably alter the lifetime of thin films made from CsPb(Br/Cl)3 NCs (Figure S6), but slightly increases their PLQY from 29% (without polystyrene ) to 37% (with polystyrene).

Figure 5. (a) Log-log plot of current density vs. voltage for LEDs of CsPb(Br/Cl)3 NCs without polystyrene added to the EML (black line), with 0.5 mg/mL polystyrene (green line), and 1.0 mg/mL polystyrene (blue line): a clear reduction of current leakage has been observed. (b) Peak luminance and peak EQE as a function of polystyrene (PS) concentration between 0 and 1.5 mg/mL.


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In conclusion, highly efficient blue-LEDs with improved color-stability are obtained by employing a thin CsPb(Br/Cl)3 NC emitting layer. The use of zwitterionic sulfobetaine ligands allowed for better purification of the NCs without affecting their structural or colloidal integrity. A new device layout was employed that utilized two hole-transporting layers to overcome the issue of valence band alignment, and a polystyrene layer to reduce the current leakage. These blue LEDs are among the most efficient LHP-based LEDs in the wavelength range below 470 nm. The EL wavelength is stable between 4 and 10 V, and for the entire lifetime of the LEDs at 5 V. As for most LHP LEDs, long-term stability is still the most pressing issue that has to be solved before commercialization.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details, additional results on color stability, statistics of LED performance, green LEDs (PDF)

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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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013)/ERC Grant Agreement no. 306733 (NANOSOLID Starting Grant). Bogdan M. Benin is acknowledged for proofreading the manuscript.

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Engineering Color-Stable Blue Light-Emitting Diodes with Lead Halide

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