Mixing Entropy-Induced Layering Polydispersity Enabling Efficient and

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Letter

Mixing Entropy-Induced Layering Polydispersity Enables Efficient and Stable Perovskite Nanocrystal Light-Emitting Diodes SUDHIR KUMAR, Jakub Jagielski, Tian Tian, Nikolaos Kallikounis, Wan-Chi Lee, and Chih-Jen Shih ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02013 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Energy Letters

Mixing Entropy-Induced Layering Polydispersity Enables Efficient and Stable Perovskite Nanocrystal Light-Emitting Diodes Sudhir Kumar1§, Jakub Jagielski1§, Tian Tian1, Nikolaos Kallikounis1, Wan-Chi Lee1 and Chih-Jen Shih1* 1

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland

* Author to whom all correspondence should be addressed. Email: [email protected] § These authors contribute equally to this work.

Abstract Colloidal perovskite nanocrystals are emerging as one of the most promising candidates for next-generation monochromatic light sources that require precise bandgap tunability. However, the current efficiency (ηCE) and operational lifetime in their light-emitting diodes (LEDs) remain low due to impeded carrier transport and exciton quenching through the NC ligand layer. Here, we show that the fundamental limitation can be overcome in the superstructures containing polydisperse colloidal quantum wells of organic-inorganic hybrid perovskites. The mixing entropy-induced layering polydispersity promotes the delayed radiative energy transfer (DRET) that guides exciton transport with negligible nonradiative losses, boosting the thin-film photoluminescence quantum yield >96%. By using the superstructures in LEDs, we report a ηCE of 30.4 cd A-1, with an operational lifetime (LT50) of 184 minutes at a high constant driving current of 10 mA cm-2. These represent among the most high-performance colloidal perovskite nanocrystals LEDs ever demonstrated by far.

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The emergence of organic-inorganic hybrid perovskite (OIHP) semiconductors has generated considerable research effort aimed at demonstrating their optoelectronic devices, including the light-emitting diodes (LEDs) 1. Fundamental investigations of the material photophysics triggered by the unprecedented success of OIHP solar cells have further accelerated the progress

2-12

. Indeed, solar cell and LED designs share a common goal of the

reduction of nonradiative losses 13, as it has been proven that the fast motion of organic cations, coupled with transient deformation of PbX3- octahedral units (X = I, Br, and Cl), create large polarons and local ferroelectric domains that effectively protect carriers from defect and trap scattering 14. In practice, an important motivation for the development of perovskite LEDs is the need of monochromatic light sources covering the entire visible range of human eye 15, in particular in the deep green spectral region 16 (Fig. S1). This thanks the fact that the solutionprocessed OIHP materials possess extremely narrowband emission outperforming the state-ofthe-art organic and quantum dot (QD) technologies 15. Nevertheless, to date, although the bulk OIHP and quasi-2D-based LEDs with high peak external quantum efficiency (ηext > 14%) and current efficiency (ηCE > 60 cd A-1, see table S1) have been demonstrated 2, 6, 17-20, it has been very challenging to precisely tune the emission wavelength without sacrificing the device efficiency

15

, which considerably undermines the technological advantage. During the

preparation stage of this manuscript, there are a number of efficient red and green LEDs reported based on colloidal perovskite nanocrystal emitters.21-24 In addition, the operation lifetime remains on the low side, ranging from seconds to few hours, and a degree of bandgap instability upon biasing is often observed 19, 25-27. The bias-induced degradation has been attributed to ion segregation under electric fields, due to the ionic nature of OIHP lattice 25. This mechanism may be magnified in the bulk OIHPbased films, in which small grain size is generally preferable to increase the probability of carrier recombination 2, since the diffusivity of ions can be orders of magnitude higher along grain boundaries

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. To overcome the inherent challenges, current research effort has been 3 ACS Paragon Plus Environment


focused on the colloidal nanocrystals (NCs) of OIHPs

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, following the path of metal

chalcogenide semiconductors. Clearly, on one hand, the emission wavelength can be precisely tuned using anion/cation mixing as well as reduced dimensionality in colloidal chemistry; on the other hand, upon forming the NC assemblies, the surface ligands structurally isolate individual NCs, presumably hindering ion migration. However, the highest device current efficiency in literature remained low (table S1)

23, 30-31

, unable to reflect their high

photoluminescence (PL) quantum yield (ηPL up to ~90%) in solution

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. The low device

efficiencies had been attributed to impeded carrier transport and exciton quenching through the NC ligand layer

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, whereas this hypothesis seems to contradict the earlier reports of high-

efficiency LEDs based on CdSe QDs 32, in which long-chain ligands were also used to minimize self-quenching in their assemblies. In this report, we have elucidated the fundamental mechanisms governing the electroluminance (EL) performance of perovskite NC assemblies. Accordingly, the efficiency and lifetime limitations have been overcome. We synthesized a series of binary-cation perovskite NCs, FAxMA1-xPbBr3, where FA = formamidinium, CH3(NH2)2+ and MA = methylammonium, CH3NH3+, at room temperature. The organic precursors were mixed at desired stoichiometric ratios before injecting into the nonpolar phase, which contains two surfactant species that form ligands on NC surfaces (details see the methods section in the supplementary materials). As revealed by the cryo-transmission electron micrograph (cryo-TEM, Fig. 1a), the NCs are of nanoplatelet shape, with twodimensional (2D), step-like quasicontinuuum electronic structure 33. Hereafter, we refer them to the colloidal quantum wells (CQWs). Note that it has been suggested that the CQW growth is thermodynamically preferable at a relatively low temperature compared to the colloidal quantum dot (CQD) counterparts

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, in both cubic CdSe and OIHP systems. In the system

considered here, the CQWs have the formula of (C8H17NH3)2[FAxMA1-xPbBr3]nPbBr4, where n is the stacking number of perovskite unit cells, determining the optical bandgaps of individual NCs 7, 35-36. 4 ACS Paragon Plus Environment

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Fig. 1. Superstructures containing polydisperse CQWs for x = 0.5. (a) Representative cryoTEM image and photographs of solution and film under UV excitation (bottom right). (b) Schematic of the superstructure containing randomly stacked CQWs with different layer numbers. (c) The measured thin-film ηPL values as a function of time (dots), fitted with monoexponential function (solid curve), demonstrating excellent air stability (~50% RH) at room temperature. Inset: thickness distribution obtained by statistically analyzing TEM images. The superstructures containing randomly stacked CQWs were fabricated by spin coating the colloidal solutions on target substrates (schematic see Fig. 1b). For example, for x = 0.5, the thickness distribution was investigated by statistically analyzing a number of representative TEM images (Fig. 1C inset). Accordingly, we identify four major products, corresponding to n = 1, 3, 4, and 7, with a high degree of polydispersity. The absolute thin-film

ηPL values were determined at 370 nm in the samples deposited on glass (film thickness ~30 snm). Irrespective of x, near-unity absolute ηPL (> 96%) were obtained in thin films (Fig. 1C and table S2), considerably higher than those in their solutions (~85%). We have attributed the “aggregation-induced emission (AIE)” characteristics, or namely, radiative recombination is promoted in the CQW assemblies, to the more restricted motion of organic cations in the superstructures, as described in our previous work prerequisite for high-efficiency LEDs

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. Note that a high thin-film ηPL is a

. The CQW superstructures exhibit excellent air

stability at room temperature (Fig. 1c, at ~50% relative humidity). By fitting the measured ηPL versus time with monoexponential function, the time constant for ηPL decay in air is estimated to be ~140 days. By further mixing the CQWs with a small amount of PMMA, the air stability 5 ACS Paragon Plus Environment

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is even better (Fig. S3), possibly due a reduced degree of exposure to the surroundings.39 Indeed, compared to the iodide OIHPs, the bromide counterparts stabilize the organic cations due to the smaller PbBr3- octahedral

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, as reflected by their relatively large Goldschmidt's

tolerance factors (0.95 < t < 1.05) 8. The volatile nature of organic cations 41 is therefore greatly suppressed. On the other hand, the surface ligand layer appears to sterically hinder the diffusion of water and oxygen into perovskite crystal lattice. These facts form a solid basis for stable EL devices. The incorporation of FA into CQW lattices is confirmed by X-ray diffraction (XRD) (Fig. S4). The corresponding d-spacings of (00l) peaks were determined and found to follow the Vegard's law nicely, suggesting uniform cation mixing. Note that mixing cation in iodidebased OIHP system is mainly to suppress the formation of non-photoactive FAPbI3 phase 41. In the mixed-cation bromide CQWs considered here, nevertheless, since both FAPbBr3 and MAPbBr3 possess stable cubic structures at room temperature, we did not observe a notable dependence of the air stability on x; in addition, the effect of FA content on the excitonic characteristics, such as ηPL, is also minimal (table S2). In other words, on the level of individual nanocrystals, our current evidence suggests that the excitonic characteristics are not fundamentally modified upon cation mixing. Intriguingly, we find that the primary effect of cation mixing in CQWs is to change layering polydispersity in the resulting superstructures, as revealed in their absorption spectra (for example, x = 0.4 and 0.8 in Fig. 2a). Because the quantum confinement effect is significantly enhanced when n < 5 7, the two absorption peaks at ~450 nm and ~470 nm correspond to the excitonic features for n = 3 and n = 4 CQWs 7, respectively. We suppose that the peak associated with n = 1 is screened due to a low absorbance per NC. The degree of layering polydispersity as a function of x is therefore estimated by calculating the intensity of the excitonic peaks (Fig. 2B). Note that one could also observe the spectral components associated with the thin layers in the emission spectra (Fig. 2a), but their dependence on x is 6 ACS Paragon Plus Environment

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Fig. 2. Effect of FA content on layering polydispersity of synthesized CQWs and the resulting LED performance. (a) Absorption (Abs, blue dots) and PL (black curves) spectra for x = 0.4 and 0.8 superstructures, showing that mixing of organic cations changes the layering polydispersity. (b) Calculated degree of layering polydispersity and entropy of mixing (top) and peak current efficiency in fabricated LEDs (bottom) as a function of x. rather weak and noisy. Clearly, the polydispersity gradually increases with x and reaches its maximum at x ~ 0.4, and deceases again down to x = 1.0. We rationalize the observed composition-induced layering polydispersity as follows. First, we find that the polydispersity-x dependence can be nicely described by the entropy of cation mixing in lattice, ∆mixS = –k[x ln(x) + (1-x)ln(1-x)], where k is Boltzmann constant (Fig. 2B). It is therefore hypothesized that ∆mixS considerably lowers the free energy per unit volume in lattice, ∆Gv = ∆Hv – T∆mixS, where ∆Hv is the enthalpy per unit volume, upon cation mixing. This hypothesis holds when ∆Hv for MAPbBr3 (t ~ 0.95) and FAPbBr3 (t ~ 1.05) lattices are fairly close, so that the ∆mixS effect remains symmetric to x ~ 0.5 and sufficiently strong at room temperature. Next, consider the growth of individual CQW NC with the stacking layer number n, the process becomes spontaneous when the total free energy, ∆G(n) = A(nδ∆Gv + 2γ) < 0 34, where A is the area of 2D plane, γ is the surface energy, and δ is the thickness of crystal lattice. 7 ACS Paragon Plus Environment


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It follows that by increasing ∆mixS, the high surface energy penalty that prevents thin CQW growth may be overcome, thereby increasing the degree of layering polydispersity (Fig. 2b). The EL performance for the superstructures in LEDs is found to strongly correlate with the layering polydispersity (Fig. 2b bottom). With the same device architecture (details see the methods section in the supplementary materials), the peak ηCE is ~250%-fold higher by increasing the layering polydispersity from x = 0 to x = 0.5, following the trend of layering polydispersity. Note that the layering polydispersity does not decrease the emission color purity (see Fig. 2a). The PL spectral components associated with the thin CQWs (n = 3 and 4) are at ~1% level, irrespective of x; the emission wavelength is determined by the optical bandgap of n = 7 CQWs, which is slightly red-shifted upon FA doping (Fig. S5), regulated by the van der Waals interactions between organic cations and inorganic octahedra 42. Combining with the fact of strong absorption from the thin CQWs, we conclude that the cascade energy transfer (ET) between CQWs, driven by the spectral overlap between the donor (n = 1, 3, and 4) emission spectrum and the acceptor (n = 7) absorption spectrum

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, is responsible for the

polydispersity-enhanced EL efficiency. The scenario elucidates the fundamental mechanisms limiting the development of highefficiency perovskite NC LEDs. Indeed, compared to CdSe-based CQDs, perovskite NCs usually possess a small degree of quantum confinement because of the small Bohr radius

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.

This fact has resulted in two consequences hindering cascade ET in their assemblies: (i) the absorbance near the bandgap edge is weak due to a lack of excitonic features, and (ii) the emission and absorption spectra for individual NCs are nearly identical within their size inhomogeneity. Accordingly, the absorption-emission spectral overlap in typical perovskite NC assemblies is relatively small, thereby resulting in a less pronounced cascade ET 45, compared to the CdSe-based CQDs. Here we demonstrate that the limitation can be overcome in the superstructures of polydisperse CQWs, as the ET process is significantly promoted by reduced dimensionality and layering polydispersity.

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Fig. 3. Delayed radiative energy transfer in x = 0.5 CQW superstructures. (a) Relative thinfilm ηPL and absorbance as a function of excitation wavelength λex. (b) TRPL of the diluted colloidal solution using λex = 365nm (blue dots) and 470nm (red dots). The inset magnifies the early time response. (c) TRPL of the assembled superstructure using the two excitation wavelengths. The inset presents the ET-induced PL events (ET PL) (green dots), fitted with the proposed ET model (black curve). The time required to reach its maximum, tmax, is determined accordingly, longer than the time constants for radiative recombination in donor CQWs, suggesting that the ET mechanism involves real photon emission. The above analysis, however, does not completely solve the puzzle of near-unity ηPL in the superstructures. Indeed, in emitter complexes containing organic chromophores or semiconductor NCs, a high degree of ET usually compromises the thin-film ηPL, since the nonradiative quenching pathways are substantially generated in assemblies, arresting the radiative recombination

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. To gain more insights into the ET process, we investigate the

excitation-dependent PL characteristics for the x = 0.5 CQW samples (Fig. 3). The relative ηPL values were determined, together with the absorbance, as a function of the excitation wavelength, λex are shown in Fig. 3A. We find that the ηPL response is weakly dependent on the superstructure absorbance, with only a ~±2% fluctuation. Assuming emission all comes from n = 7 CQWs (see Fig. 2a), we derive the quantum yield - wavelength relation ηPL(λex) (see the supplementary materials) and determine the energy transfer efficiency from donor to acceptor CQWs, ηET, to be ~ 1.0. The dynamics of energy transfer are further studied using time-resolved PL (TRPL) spectroscopy (Figs. 3b and 3c). We compare the PL decay responses by pumping the samples 9 ACS Paragon Plus Environment

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with two wavelengths, λex = 365 and 470 nm, which optically excite all CQW species (n = 1, 3, 4, and 7) and only the acceptor one (n = 7), respectively. In the diluted CQW solution, in which the inter-particle ET is minimized, the PL decay is faster for λex = 365 nm at all times (Fig. 3b), since recombination is dominated by the thin CQWs that possess a higher degree of quantum confinement. By fitting the early-time responses (< 40 ns), we determine the radiative recombination time constants for donor (n = 1, 3, and 4) and acceptor (n = 7) CQWs, τRD and

τRA, to be 14 and 19 ns, respectively. On the other hand, in the thin-film superstructure, the PL decay for λex = 470 nm is nearly identical to that in solution (early time constant τ = 20 ns) (Fig. 3c). For λex = 365 nm, the PL decay ties with that of λex = 470 nm within the first ~30 ns, confirming the absence of ET, while beyond 30 ns, delayed emission is observed, corresponding to the transferred excitons from the donor CQWs. The ET-induced PL events (ET PL)

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is

extracted as a function of time (Fig. 3c inset) and fitted with the proposed ET model considering kinetics in series (see the supplementary materials). The model appears to describe the ET PL response nicely and allows us to determine the time required to reach its maximum, tmax = 50 ns, and ET time constant, τET = 140 ns. With all the photophysical quantities, we highlight three unique observations in the CQW superstructures: (i) the nonradiative losses during energy transfer in the CQW solids are negligible, hypothetically due to the nature of defect screening in OIHP lattices; (ii) the delayed radiative energy transfer (DRET) 47 dominates the ET process, involving real photon emission from the donor that is subsequently absorbed by the acceptor, because τET > τRD; and (iii) the fast Förster resonance energy transfer (FRET) process, on the other hand, is inhibited, due to the screened Coulombic interactions between neighboring CQWs 14. We notice that the optical pumping frequency is in the UV region, significantly beyond those of the PbBr3- longitudinal optical (LO) phonons (~ 1 THz) and the rotational motion of the dipolar organic cations (~ 0.1 THz)

14

, so in principle, the intrinsic OIHP dielectric response cannot effectively screen the 10 ACS Paragon Plus Environment

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Coulomb potential exerted by an exciton dipole in the donor CQW. It may suggest that, despite a degree of quantum confinement in 2D plane, the photo-generated electron-hole pairs in the thin CQWs still possess the characteristics of free carriers (FCs) rather than excitons, as have been observed in the bulk HIOPs 48. The physical hypothesis presented here explains the seemly controversial observation that the relatively slow ET kinetics can lead to near-unity ηPL and ηET in the CQW superstructures. Clearly, the FRET is a distance-sensitive process varying with distance as R-6, where R is the separation between a donor/acceptor dipole pair 47. The FRET rate is therefore highly dependent on the ligand length and NC ordering/orientation, reducing the ET efficiency in the ensemble. The DRET, on the other hand, varies with distance as R-2 by decoupling the interactions with the induced dipole

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, which appears to be an attractive ET mechanism to

explore when the nonradiative losses and FRET do not outpace

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, as seen in the perovskite

CQW solid system considered here. In our optimized LED devices, as will be discussed below, we even found that ηET in thin films does not decrease significantly upon mixing with a small amount of insulating poly(methyl methacrylate) (PMMA), indirectly corroborating our hypothesis proposed here. Accordingly, the limiting factor of surface long-chain ligands that impedes charge transfer has been overcome by promoting interwell DRET, a mechanism never been considered in perovskite nanocrystal solids. Comparing to the reported ET mechanisms in perovskite thin-films

6, 18, 20

, we notice

that the energy transfer (ET) time scales in the quasi-2D films are usually very small (0.5 – 50 ps) 6, presumably resulting from the ultrathin interlayer spacing. Accordingly, the ET mechanism may be categorized into the Dexter mechanism. On the other hand, the ET mechanism in the mixing perovskite quantum dot system20 might be analogous to ours; however, the ET time scale and efficiency were not quantified. It is also noteworthy that because (i) the quantum yield is approaching unity and (ii) the energy transfer time scales are relatively slow (~10 ns), in principle, TRPL and the transient absorbance (TA) should yield 11 ACS Paragon Plus Environment

ACS Energy Letters

consistent results and comparable information. We believe that the more advanced photophysical characterization techniques, including the pump-probe spectroscopy, may help to further corroborate the mechanisms proposed here. Given the layering polydispersity-induced energy transfer with near-unity ηPL and ηET, we fabricated and optimized the LED devices using x = 0.5 superstructures (device architecture see Fig. 4a and Fig. S7). We use a typical three-layered structure, which consists of a holetransport layer (HTL), an emission layer (EML), and an electron-transport layer (ETL). Extensive experimentation was carried out to optimize the HTL and ETL materials and their thickness (see the supplementary materials). We also investigated the effect of EML compositions and thin film surface morphologies (details see the supplementary materials and

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Fig. 4. Device characteristics of perovskite nanocrystal LEDs using polydisperse CQWs at x = 0.5. (a) Schematic of device architecture. (b) Current density and luminance as a function of driving voltage. (c) Current efficiency and external quantum efficiency as a function of current density. (d) EL spectrum showing a high color purity with the color coordinates of (0.170, 0.780) on the CIE 1931 color space. (e-f) Current efficiency as a function of current density in the optimized large-area (e) and ultra-flexible (f) device. Inset: Photograph of monochromatic green EL from a large-area (e) and a rolled (f) LED device (scale bar: 1 cm). 12 ACS Paragon Plus Environment

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Figure S9). The optimal device exhibits a high ηCE of 30.41 cd A-1, a ηext of 7.37%, and a peak power efficiency (ηPE) of 27.30 lm W-1 at the operational voltage, V, of 3.5 V (Figs. 4b and 4c). To our knowledge, these are among the best efficiencies ever reported in perovskite nanocrystal LEDs. A low turn-on voltage (VT) of 2.8 V is also demonstrated. Efficiency histograms based on 84 devices are shown in Fig. S8. The EL emission is centered at 528 nm with a full width at half maximum (FWHM) of 22.5±0.5 nm, reaching the color coordinates of (0.170, 0.780) on the International Commission on Illumination (CIE) 1931 color space (Fig. 4d). The color coordinates reported here represent the “purest” green color ever demonstrated by far in all LED technologies, covering 98.5% of the color gamut defined by the recommendation (Rec.) 2020 standard (Fig. 4d inset) for the first time 16. In addition, we also fabricated the LED devices using perovskite CQWs with varying x content from 0 to 1 (see detail in table S3 and Fig. S10). We also fabricated ultra-flexible LEDs based on the CQW solutions showing comparable characteristics (Fig. 4f; ηCE = 12.6 cd A-1 and ηext of 3.1% at 3.5 V), together with voltage-independent EL spectra (Fig. S11). The emission intensity remains consistent at positive and negative bending radius during test. A large-area (400 mm2) device was also

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Fig. 5. Electrical stability of the fabricated perovskite nanocrystal LEDs. (a) Comparison of the normalized luminance (L/L0) as a function of time under a constant current stress of J = 20 mA cm-2 for three CQW devices corresponding to x = 0, 0.5, and 1, showing the initial luminance L0 of 560, 1751, and 1288 cd m-2, respectively. (b) The normalized luminance as a function of time for x = 0.5 CQW device stressed at J = 10 mA cm-2, demonstrating an LT50 value of 184 minutes. 13 ACS Paragon Plus Environment


demonstrated without compromise of efficiencies (Fig. 4e; ηCE = 27.4 cd A-1 and ηext of 6.7% at 3.5 V) (Fig. S12). We also investigate the operational lifetime of the x = 0.5 CQW perovskite LEDs. We did further optimization in device architecture to achieve a longer operational lifetime (details see Supplementary Information). The time for the luminance to decay to 50% of the initial luminance, LT50, of an encapsulated device, under a constant current density J = 20 mA cm-2, corresponding to the initial luminance L0 of 1751 cd m-2, was determined to be 37.33 minutes for the high L0 (Fig. 5a). The luminance decay rate seems to be comparable to the x = 0 and x = 1 CQW devices, suggesting that the operational lifetime is not strongly dependent on the composition and layering polydispersity. These devices show a small increase in the driving current (Vdriv) < 0.7 V during the measurement (Fig. S14). We also tested another device at a lower current J = 10 mA cm-2 (L0 = 991 cd m-2), showing an LT50 of 184 minutes (Fig. 5b), which is one of the longest operational lifetimes ever reported in the perovskite LEDs based on colloidal nanocrystals. In addition, the emission instability upon electrical stress is negligible (Fig. 5b inset). We believe that the operational lifetime can be further increased by employing the air-stable organic or metal oxide transport materials49. We demonstrate high-efficiency perovskite nanocrystal LEDs with an operational lifetime reaching 184 minutes. Our results uncover the fundamental mechanisms governing EL performance in the perovskite nanocrystal LEDs, highlighting the importance of layering polydispersity that promotes the delayed radiative energy transfer in the nanocrystal superstructures. The fundamental principles presented here open an avenue towards the realization of monochromatic LEDs covering the entire visible spectrum.


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Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic procedures, characterization techniques and parameters, materials, fabrication, and characterization of perovskite LED devices, and supplementary figures including absorption and PL spectra of 2D perovskites, additional TEM images, temperature dependent PL spectra, additional TRPL spectra, molecular structures of other materials used in LEDs, AFM and SEM images for emissive layer, cross-section SEM image of LED, device performance and architecture for small and large area devices, photographs and EL characteristics of flexible perovskite LEDs, and comparative device performance tables.

Acknowledgement C.J.S., S.K., and J.J., are grateful for financial support from ETH startup funding and Swiss National Science Foundation (project number: 200021-178944). The authors thank Dr. Frank Krumeich for TEM analysis and Dr. Sarah Keller for technical support of TRPL. In addition, the technical support from the FIRST Lab in ETH Zurich is highly appreciated. Author contributions: S.K. and C.J.S. conceived the idea and designed the experiments. S.K. designed, fabricated, and characterized the LED devices. S.K. and N.K. optimized the LED devices. J.J. synthesized the perovskite CQWs and carried out the XRD analysis. S.K. and J.J. characterized the photophysical properties of CQWs. S.K. carried out TRPL spectroscopy and analyzed the results with C.J.S. T.T. and C.J.S. discussed and built the ET model. T.T. prepared the schematics of polydisperse CQWs and device architectures. W.C.L. modelled Rec. 2020 gamut coverage as a function of FWHM and emission wavelength. S.K. and C.J.S. prepared figures and co-wrote the paper. Competing interests: C.J.S., S.K., and J.J. are inventors on a patent application related to this work (application no. EP17179375.5, filed 3 July 2017). The authors declare that they have no other competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the supplementary materials.



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