Study of Perovskite QD Down-Converted LEDs and Six-Color White

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Study of Perovskite QD Down-Converted LEDs and Six-Color White LEDs for Future Displays with Excellent Color Performance Hee Chang Yoon, Heejoon Kang, Soyoung Lee, Ji Hye Oh, Heesun Yang, and Young Rag Do ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05468 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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ACS Applied Materials & Interfaces

Study of Perovskite QD Down-Converted LEDs and Six-Color White LEDs for Future Displays with Excellent Color Performance Hee Chang Yoon†, Heejoon Kang†, Soyoung Lee†, Ji Hye Oh†, Heesun Yang‡, and Young Rag Do†,*

†

Department of Chemistry, Kookmin University, Seoul 136-702, Republic of Korea

‡

Department of Materials Science and Engineering, Hongik University, Seoul 121-791,

Republic of Korea

Abstract A narrow-emitting red, green and blue (RGB) perovskite quantum dot (PeQD)-based tricolored display system can widen the color gamut over the National Television System Committee (NTSC) to 120%, but this value is misleading with regard to the color perception of cyan and yellow reproduced in the narrow RGB spectra. We propose that a PeQD-based six-color display system can reproduce true-to-life spectral distributions with high fidelity, widen the color gamut, and close the cyan and yellow gap in the RGB tri-colored display by adding cyan (Cy), yellowish green (Yg), and orange colors (Or). In this study, we

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demonstrated pure-colored CsPbX3 (X = Cl, Br, I, or their halide mixtures; Cl/Br and Br/I) PeQD-based monochromatic down-converted light-emitting diodes (DC-LED) for the first time, and incorporated PeQDs with UV-curable binders and long-wavelength-pass-dichroicfilters (LPDFs). CsPbX3 PeQD-based pure Cy-, G-, Yg-, Or-, R-emitting monochromatic DCLED provide luminous efficacy (LE) values of 81, 184, 79, 80, and 35 lm/W, respectively, at 20 mA. We also confirmed the suitability and the possibility of access to future color-by-blue backlights for field-sequential-color liquid crystal displays, using six-color multi-package white LEDs, as well as future six-colored light-emitting devices with high vision and color performance. The fabricated six-color multi-package white LEDs exhibited an appropriate LE (62 lm/W at total 120 mA), excellent color qualities (color rendering index (CRI) = 96, special CRI for red (R9) = 97) at a correlated color temperature (CCT) of 6500 K, and a wide color gamut covering the NTSC up to 145% in the 1931 Commission International de l’Eclairage (CIE) color coordinates space.

KEYWORDS: perovskite, quantum dot, cesium lead halide, down converted LED, high color performance LED, field-sequential-color LCD backlight

Introduction Since the recent development of organic-inorganic perovskite (Pe)-based solar cells, organometallic halide-based perovskites have emerged as excellent semiconductors for photovoltaic and optoelectronic devices, such as solar cells,1–4 perovskite light-emitting diodes (PeLED),5,6 lasers,7,8 photodetectors,9,10 light-emitting electro-chemical cells,11 and down-converted (DC) materials in white LEDs.12 Recently, there have been numerous attempts at fabricating PeLEDs and DC-LEDs as light emission materials using related organometallic perovskite materials, instead of using perovskites as light absorption materials


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in solar cells. Tan et al. demonstrated the first instance of high-brightness infrared and visible electroluminescence at room temperature from PeLEDs fabricated using solution-processed organometallic halide perovskites.5 Recently, inorganic CsPbX3 (X = Cl, Br, I, or their halide mixtures; Cl/Br and Br/I) perovskite quantum dots (PeQDs) were developed as alternative materials for optoelectronic devices such as solar cells, lasers, photodetectors, white DCLEDs and PeLEDs. Immediately after the first report on such systems by Protesescu et al., many groups have reported that inorganic CsPbX3 PeQDs have excellent optical properties including narrow emission lines, high quantum yields (QY = 0.50–0.90), short radiative lifetimes, and room-temperature optical amplifications.13–21 CsPbX3 PeQDs have been reported as excellent candidates for replacing environmentally harmful Cd-based colloidal QDs in PeLED and/or DC-LED applications, as well as for replacing expensive rare earthbased inorganic phosphors in white DC-LED applications. Other reports regarding anionic exchange reactions have also demonstrated viable methods of synthesizing tunable perovskite-based colloidal QDs which have photoluminescence (PL) QYs under blue excitation, based on all-inorganic CsPbX3 PeQDs tunable in emission color from green (x = Br) to blue (x = Cl) or red (x = I) via ionic exchange.20,21 In particular, red, green, and blue (RGB) CsPbX3 PeQDs also exhibit wide color gamut, covering up to ~140% of the national television standard committee (NTSC) color standard because of their narrow emission spectra (< 50 nm).13 Moreover, white DC-LEDs with tunable color temperatures were fabricated for the first time using green and red CsPbX3 PeQDs/PMMA film.22 In various display screens, a wide range of colors are produced by additive mixing using only RGB as three primary colors. The narrow RGB spectra of CsPbX3 PeQDs are considered ideal for RGB color-by-blue display technologies,23,24 RG QD-enhanced LCDs,25– 28

RGB backlight typed field-sequential-color liquid crystal display (FSC-LCD)

technology,29,30 multidirectional backlights for 3D displays,31,32 and RGB color PeLED



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displays,33,34 because of their increased color gamut in CIE color space. Moreover, conventional 2D displays using some advantages of the QDs such as wide color gamut, high efficiency, and high color reproduction including real cyan and yellow colors, are also important for the 3D display applications.35 During RGB additive mixing, a cyan color results from combining blue and green, while a yellow color results from combining green and red in RGB display screens. However, this perceived yellow color does not fall within the yellow visible spectrum when viewing a yellow color in a RGB-based display screen. There is a noticeable difference between spectral yellow light (~580 nm) perceived in daily life and a mixed yellow light of red (~650 nm) and green (~520 nm) light perceived from RGB displays (Figures 1a and b). This difference is also getting worse and worse as the bandwidth of RGB colors becomes narrower. In the human eye, visual color perception is finalized by analyzing and processing complex signals from the short (S), medium (M), and long (L) cone cells in the visual cortex of the brain.36 Due to this color perception process, both spectral yellow and mixed yellow colors stimulate our eyes in a similar manner, so we cannot discern between the two, and both appear as similar yellow light to the human eye. It can be simply considered that the combination colors of narrow-band RGB spectra in display screens increase the possibility of misleading our own color perception. It is necessary to develop alternative color mixing processes for display screens or color-by-blue typed displays in order to reproduce true-to-life spectral distributions with high fidelity. To close the cyan and yellow gap in the RGB spectrum (Figure 1c) of a RGB tri-colored display, the cyan (Cy), yellowish green (Yg), and orange (Or) colors of narrow band CsPbX3 PeQDs can be added to this RGB color mixing system. Recent findings12,13,22 pertaining to color-tunable CsPbX3 PeQDs confirmed our suggestion that the combination of narrow-band Cy/G/Yg/Or/R (CGYOR)-emitting CsPbX3 PeQDs and an InGaN B LED could close the cyan and yellow gap in RGB displays, reproducing colors similar to the original colors on display screens and maximizing the color


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gamut (Figure 1d). This color-tunability also clearly indicates that CGYOR-emitting CsPbX3 PeQDs are promising alternatives to inorganic phosphors and CdSe-based QDs in colored DC-LEDs for application to purer mono-colored LEDs for six-colored FSC-LCDs, multidirectional six-colored LED backlights for 3D displays, color-conversion materials for CGYOR-emitting PeQD-enhanced LCDs, six-colored color-by-blue QD-LED displays, and six-colored PeLED display (see Figure 2). However, all inorganic-based colloidal halide PeQDs are unstable in polar solvents such as water and alcohol, similar to the instability of general CH3NH3PbX3 organometal halidebased PeQDs against humidity and high temperature.37,38 This property restricts the available media in which these materials can be used to nonpolar or moderately polar polymers and solvents. In this study, pure CGYOR-emitting inorganic CsPb(Br0.75,Cl0.25)3, CsPbBr3, CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 PeQDs were synthesized by hot injection methods to study the possible use of monochromatic DC-LEDs in future backlight applications of six-colored FSC-LCDs and six primary color-by-blue QD-LEDs. The reaction temperature of this type of QD synthesis was tuned previously to obtain optimal optical quality in terms of high PLQY and narrow emission bandwidth or full-width at halfmaximum (FWHM).13 The thermal stability and polymer stability of the synthesized inorganic-based PeQDs were also studied to improve the optical properties of pure monochromatic DC-LEDs capped with a long-wavelength-pass-dichroic-filter (LPDF).39–43 The selection of less polar photo-polymerized resins for the synthesized PeQDs yielded less detrimental QDs in the LED package during fabrication of monochromatic DC-LEDs. To the best of our knowledge, this is the first report to show favorable luminous efficacy (LE) of CGYOR-emitting monochromatic DC-LEDs incorporated with synthesized PeQDs and a nonpolar photo-curable binder. The color purity of CsPbX3 PeQD-based CGYOR-emitting monochromatic DC-LEDs is comparable to those of PeQD materials. Herein, the successful



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fabrication of colored CsPbX3 PeQD-based CGYOR-emitting DC-LEDs demonstrates the strong potential of PL applications using colloidal PeQDs in monochromatic DC-LEDs or color-converted, color-by-blue QD-LEDs, although further studies are necessary to improve the external quantum efficiency (EQE) and device stability of colloidal CsPbX3 PeQD-based CGYOR-emitting DC-LEDs. It is also necessary to increase the suitability of the five CsPbX3 PeQD-based CGYOR-emitting monochromatic DC-LEDs and a B InGaN LED applied into six-colored LED candidates for FSC-LCDs. Additionally, the colored monochromatic LEDs with wavelengths in the “green gap” range can be used to close the “green gap or yellow gap” problem in nitride-based III-V LED technologies. Therefore, this is the first report describing the fabrication and optical properties of pure color CsPbX3 PeQD-based CGYOR-emitting monochromatic DC-LEDs and their six-color multi-package white LEDs.

EXPERIMENTAL METHODS 1. Materials and Chemicals Cesium carbonate (Cs2CO3, 99.995%, Aldrich), lead chloride (PbCl2, 99.999%, Aldrich), lead bromide (PbBr2, 99.999%, Aldrich), lead iodide (PbI2, 99.999%, Aldrich), 1-octadecene (ODE, 90%, Aldrich), oleylamine (OLA, 80-90%, Acros Organics), oleic acid (OA, 90%, Aldrich), trioctylphosphine (TOP, 90%, Aldrich), n-hexane (95%, Aldrich), and toluene (99.8%, Aldrich) were used without any further purification. InGaN B LEDs (λmax = 445 nm, Dongbu LED, Inc.) were used as excitation sources. The UV-curable binder (Norland optical adhesive 63® (NOA 63)) was utilized as an encapsulant on a cup-type InGaN B LED mold.

2. 1. Preparation of Cs-oleate solution13


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To prepare Cs-oleate solution as Cs-source, 0.08 g of Cs2CO3, 3 mL of ODE, and 0.25 mL of OA were placed in a 20 mL vial and sealed with a rubber septa and Parafilm®. The Cs-oleate solution was purged using N2 or Ar gas at 150°C until the Cs2CO3 powder was completely dissolved, and then maintained at a minimum temperature of 140 °C.

2. 2. Synthesis of CsPbX3 PeQDs13 For the synthesis of CsPbX3 PeQDs, 5 mL of ODE (as the reaction solvent) and 0.188 mmol of PbX2 (X = Cl, Br, I, or their mixtures) were placed into a 100 mL 3-neck flask and degassed for 60 min at 120°C. The reaction flask was subsequently filled with N2 gas, and 0.5 mL of OLA and 0.5 mL of OA were injected into this flask at the same temperature (120 °C). After complete dissolution of the PbX2 powder, the reaction temperature was raised to 180 or 200 °C, and 0.4 mL of the as-prepared Cs-oleate was quickly injected into the reaction flask. This reaction solution was maintained at its temperature for 5 s before the crude CsPbX3 PeQD solution was quickly cooled with an ice-water bath. The crude CsPbX3 PeQD solution was purified through centrifugation at 12,000 rpm for 5 min. The obtained CsPbX3 PeQDs were dissolved in hexane, and the CsPbX3 PeQD/hexane solution was centrifuged using a purification process identical to that described earlier. After centrifugation, the precipitate was discarded and the clear supernatant CsPbX3 PeQD/hexane solution was stored in lightblocked desiccator for further study.

2.3 Fabrication of pure-colored CsPbX3 PeQD-based monochromatic DC-LED For the fabrication of CsPbX3 PeQD-based monochromatic DC-LEDs, purified CsPbX3 (synthesized at 200 °C) PeQD/hexane solution was arranged with optical density values of



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1.5 at 495 nm for CsPb(Br0.75,Cl0.25)3 Cy, 515 nm for CsPbBr3 G, 540 nm for CsPb(Br0.65I0.35)3 Yg, 578 nm for CsPb(Br0.5I0.5)3 Or, and 635 nm for CsPb(Br0.35I0.65)3 R. The NOA 63 UV-curable binder was dissolved in toluene at 4 wt%, and 1 mL of this NOA 63/toluene solution was mixed with 0.27 mL of the as-obtained CsPbX3 PeQD/hexane solution in a 5 mL glass vial. The mixed PeQD/NOA 63 solution was placed under vacuum at ~0.1 torr for 180-240 min to evaporate solvents. The residual PeQD/NOA 63 paste was then applied on a cup-type InGaN B LED mold, and exposed under 365 nm UV-light for 30 min to form the PeQD/polymer solid matrix. In order to realize the pure-colored CsPbX3 PeQDbased monochromatic DC-LEDs, the LPDF was simply capped on the top of the fabricated DC-LED package.

2.4 Fabrication of freestanding CsPbX3 PeQD-based films In order to fabricate freestanding CsPbX3 PeQD-based films, the preparation of CsPbX3/NOA 63 paste was followed as the same process in Experimental section (2.3). A glass substrate was cleaned by sonication in ethanol and acetone (1:1) for 10 min and dried under N2 gas. A 200 µm spacer was attached on the glass substrate, and as-prepared PeQD/NOA 63 paste was uniformly printed in a 200 µm spacer. The PeQD/NOA 63 paste was then exposed to 365 nm UV-light for 30 - 40 min to harden the PeQD-based film, and the fabricated PeQD-based film was readily obtained by detaching the spacer and the glass substrate.

2.5 Characterization The absorbance and photoluminescence spectra of CsPbX3 PeQDs were obtained by utilizing a UV-Vis spectrometer (S-3100, SINCO Co., Ltd) with a Xe-lamp and a spectrophotometer


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(Darsa, PSI TRADING CO., Ltd.), respectively. The QYs of CsPbX3 PeQDs were calculated by comparison with a commercial dye (Rhodamine 6G, QY = 0.95 in ethanol). The crystal structures of the CsPbX3 PeQDs were measured using X-ray diffraction (XRD; D-max 2500, Rigaku) with Cu Kα radiation (scanning range from 10° < 2θ < 55° at 1°/min steps). The lattice, morphology, and size of CsPbX3 PeQDs were investigated using transmission electron microscopy (TEM; JEM-2100F, JEOL, Ltd.) with 200 kV. The optical properties of the CsPbX3 PeQD-based monochromatic DC-LEDs were measured through an integrated sphere by utilizing a spectrophotometer (Darsapro-5000, PSI TRADING Co., Ltd.). The current dependences of each B/Cy/G/Yg/Or/R (BCGYOR)-color converting monochromatic DCLED were measured while varying the applied current from 10 to 120 mA with a 10 mA interval.

Results and discussion To obtain the appropriate CGYOR-emitting CsPbX3 PeQDs, tunable CsPb(Br1-x,Clx)3 and CsPb(Br1-x,Ix)3 PeQDs were synthesized by controlling the halide anion contents and reaction temperatures. The detailed optimized results of various PeQDs are described in the Supporting Information in Figure S1. Based on our synthetic approaches, the 200 °C-heated CsPb(Br0.75,Cl0.25)3, CsPbBr3, CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 PeQDs were selected as CGYOR-emitting PeQDs for further examination. As described in the Experimental section, five CGYOR-emitting CsPb(X1-x,Yx)3 (X = Br, Y = Cl, or I) ternary and quaternary PeQDs were synthesized by a hot injection method with a two-step heating process and a sequential quenching process. Figures 3a and b show the UV-visible absorption and PL spectra of CsPb(Br0.75,Cl0.25)3, CsPbBr3, CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 PeQDs prepared at 200 °C. The band gap and consequent emission energies of the PeQDs are significantly blue-shifted by increasing the concentration of the



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smaller anion (I Br Cl) constituent.13,20,21 Here, the narrow bandgaps of the PeQDs can be widened by forming the alloy with smaller anions, decreasing both the unit cell and the covalency of the resulting PeQDs. As a result of controlling the bandgap, the PL emission spectra of the PeQDs are controlled by changing the anion composition ratio (I/Br or Cl/Br) of the CsPb(X1-x,Yx)3 PeQDs. As shown in Figures 3c and d, the target color purities and emission colors of five CGYOR-emitting PeQDs were selected for their application in monochromatic DC-LEDs with wide color gamut by optimizing the reaction temperature. Here, the variations of the peak wavelength, FWHM, CIE color coordinates, and PL quantum yield (PLQY) of five CGYOR-emitting PeQDs are summarized in Table 1. As shown in Figure 3 and Table 1, the peak wavelengths of the selected CGYOR-emitting PeQDs reached 499, 515, 546, 589, 643 nm with FWHM values of 18, 19, 24, 28, and 36 nm, respectively. The PLQYs of CsPb(Br0.75,Cl0.25)3 Cy, CsPbBr3 G, CsPb(Br0.65,I0.35)3 Yg, CsPb(Br0.5,I0.5)3 Or, and CsPb(Br0.35,I0.65)3 R-emitting PeQDs varied based on the composition of the anion constituents in the solid solutions from the end member of CsPbBr3 and CsPbI3 (or CsPbCl3) PeQDs. The PLQY of CsPbBr3 G-emitting PeQDs reached the highest value of 0.81 in five selected CGYOR-emitting PeQDs. There, the high exciting binding energy of ternary CsPbBr3 PeQDs and proper surface passivations account for efficient PL emissions in CsPbBr3 PeQDs.12 Unlike previously reported PLQY trends obtained from ion-exchanged PeQDs,20,21 our results show that the PLQY of directly synthesized CsPb(X1-x,Yx)3-alloyed PeQDs dropped from both end members of CsPbBr3 and CsPbI3 PeQDs to the minimum value of CsPb(Br0.65,I0.35)3 PeQDs of 0.47 when forming solid solutions at 200°C between the CsPbBr3 and CsPbI3 phases (Figure S1). The decreased PLQYs of solid solution PeQDs are possibly due to the increased number of lattice and surface defects from size mismatches between Br- and I- (or Cl-) anions when substituting Brions in CsPbBr3 with either Cl- or I-. Contrary to the low PLQY of CsPbI3 (~0.30) reported


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previously, the PLQY of our deep R-emitting CsPbI3 PeQDs (λ = 682 nm) prepared at 180°C reached 0.73 in all PeQDs prepared in this experiment after systematically optimizing the synthetic conditions (Figure S1). Therefore, both end members of a series of CsPb(Br1-x,Ix)3 solid solutions show better PLQYs and lower amounts of defects than those of the middle members in CsPb(Br1-x,Ix)3 solid solutions. Recently, highly luminescent semiconductor II-VI and III-V QDs have inspired new display applications that replace the inorganic phosphors or organic fluorescent materials in DC-LEDs and color enhancers in QD films of LCD backlights. Similar to II-VI and III-V QDs for display applications, CsPbX3 PeQDs show superior PLQY and narrower PL spectra than inorganic and organic phosphors with fine tuning of the emission peaks, and can produce saturated monochromatic colors. Figure 3c shows CIE color coordinates of a B LED and five selected CGYOR-emitting PeQDs in CIE chromaticity diagrams for color-by-blue device applications and photographs of PL emissions of five selected CGYOR-emitting PeQDs. The CIE color coordinates of CGYOR-emitting PeQDs are located close to those of the pure colored spectrum, as purer colors are achieved with narrower emission spectra at high PL intensities. Figure 3 also shows that a selected triangle of a B LED and G- and R-emitting PeQDs encompasses ~131% of the NTSC standard. Otherwise, this indicates that any display device with a selected hexagon of a B LED and five selected CGYOR-emitting PeQDs can reproduce ~165% of the NTSC RGB color gamut. This is the largest color reproduction area of real materials ever reported in any other kinds of PL materials which are applicable in color-by-blue display devices.23,24 Nearly all values in Figure 3 and Table 1 indicate that the selected CGYOR-emitting PeQDs show adjustable peak wavelengths and color coordinates with excellent or proper PLQYs and narrow FWHM values for application in CGYOR-color converting monochromatic DC-LEDs, although further significant improvements in PLQY are necessary for the middle members of solid solution PeQDs between two end members of



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CsPbX3 (X = Br, and I (or Cl)) PeQDs. The crystal structure of CsPb(Br0.75,Cl0.25)3 Cy, CsPbBr3 G, CsPb(Br0.65,I0.35)3 Yg, CsPb(Br0.5,I0.5)3 Or, and CsPb(Br0.35,I0.65)3 R-emitting PeQD samples were studied through XRD measurements and high resolution transmission electron microscopy (HR-TEM) observations. As shown in Figures S2a and b, the XRD patterns of all five PeQDs indicate that ternary pristine CsPbBr3 PeQDs is not only indexed as cubic CsPbBr3 (a = 5.874 Å, space group Pm3-m, ICSD 029073) but quaternary CsPb(Br0.75,Cl0.25)3, CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 are also indexed with a cubic phase, same as the cubic CsPbBr3 phase and in agreement with reports by Protesescu et al. and Akkerman et al.13,21 Consistent with these reports (Figure S2 in the Supporting Information), the substitution of Cl- into CsPbBr3 PeQDs shrinks the unit cell and shifts the diffraction peaks to higher angles, while I- expands the unit cell and shifts the diffraction peaks to lower angles.13 Figures S2c and d provide HR-TEM images of five selected PeQDs. The HR-TEM image of CsPbBr3 exhibits ~11 nm-diameter cubic shaped nanocrystals that are highly monodispersed with a narrow size distribution. The inset of the CsPbBr3 HR-TEM image exhibits lattice fringes throughout the whole crystal with a cubic lattice parameter of ~0.58 nm along the (110) crystal direction. The HR-TEM images of quaternary PeQDs indicate that the shapes of CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 are almost wholly maintained as the concentration of I- increases in CsPb(Br1-x,Ix)3. Their lattice fringes and crystal quality are slightly degraded with increased concentration of other halide ions in CsPbX3 PeQDs. The mixed anions and decreased crystallinity of quaternary CsPb(Br1-x,Ix)3 PeQDs may explain the lowered PLQYs of CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 PeQDs resulting from the increased lattice and surface defects in the mixed anion crystals, compared to the end members of well-crystallized CsPbBr3 and CsPbI3 PeQDs. The TEM images also show that the average size of CsPb(Br1-x,Ix)3 PeQDs remains relatively constant with


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increased I- concentration. Similar phenomena are also present in a series of CsPb(Br1-x,Clx)3 PeQDs when forming solid solutions between CsPbBr3 and CsPb(Br0.75Cl0.25)3 PeQDs. A slight irregularity in the PeQDs crystals develops and the average sizes and PLQYs are decreased with increased Cl- content in quaternary CsPb(Br1-x,Clx)3 PeQDs. In all of these inorganic PeQDs, even though the emission peaks of the narrowbandwidth PeQDs are not easily tuned to the PL wavelength through size control of the perovskite core QDs, the fine-tuning of the PL peak wavelength and the CIE color coordinates (Figure 3 a–c) of PeQDs can be successfully performed by controlling the composition of the solid solutions of quaternary PeQDs (i.e. halide ion ratios). To study the suitability of the narrow-bandwidth CGYOR-emitting PeQDs as color-conversion materials for use in highly pure color-quality monochromatic DC-LEDs, we compared a whole series of CsPb(Br1-x,Ix)3 (x = 0, 0.2, 0.35, 0.4, 0.5, 0.6, 0.65, 0.8, and 1.0) and CsPb(Br0.75,Cl0.25)3 PeQD samples (Figures S1 and S2); we then selected CGYOR-emitting PeQDs candidates with 499, 515, 546, 589, 643 nm after considering both the high PLQY and the emission peak wavelength to cover the entire visible range of wavelength when fabricating the six-colored BCGYOR multi-package white LEDs (a B LED (450 nm) and five CGYOR-emitting monochromatic DC-LEDs). As is well known, six-colored multi-chip III-V LEDs can be fabricated and used to realize a narrow-FWHM six-color multi-package white LED for FSCLCD backlighting. However, different colored III-V LEDs have different compositional material and color systems (e.g., InGaN for B, Cy, and pure G LEDs, AlInGaN for Yg LED and AlGaInP for Or and R LEDs), and they behave very differently with respects to the surrounding temperature and the applied current.44,45 A simple and inexpensive operating circuitry is needed to correct the different dependences of six-colored III-V LEDs with surrounding temperatures and applied currents. In addition, the low EQE of G, Yg, and Or III-V semiconductor LEDs (i.e. green gap, yellow gap, or amber gap) limits the application of



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these G/Yg/Or-emitting DC-LEDs in six-colored backlights for FSC-LCDs or directional backlights of 3D displays. As previously reported, one possible solution for closing the green gap of III-V LEDs is based on the use of phosphor- or QD-based DC-LEDs which convert blue into any color in the wavelength range between pure green and orange with the help of a blue-mirror-yellow-window LPDF, which can reflect the short wavelength and pass the long wavelength based on the half-band-edge wavelength (500, 515, 550 nm) of LPDFs (see Figure S3). We have developed a variety of efficient monochromatic LEDs using LPDFcapped DC-LEDs using inorganic phosphors or III-V QDs to close the green gap wavelength ranges of III-V LEDs.41,43 However, there is no concerted approach for fabricating various colored DC-LED lamps using unique and Cd-free DC-materials having PL spectra with wide spectral tenability, narrow emission, and a FWHM of < 50 nm in the entire visible range. Narrow bandwidth II-VI QDs are environmentally toxic, and nontoxic III-V QDs are limited PL spectrum bandwidth reduction (< 45 nm). Fortunately, the combination of narrow emissions with a FWHM of 20–50 nm, wide wavelength tunability of 470–700 nm, high PLQYs up to 0.81, reduced toxicity, and simple synthetic requirements provides excellent alternatives with highly desirable characteristics for monochromatic LED backlights in FSCLCDs or directional LED backlights in 3D displays.13 Therefore, in this study, we fabricate and characterize an LPDF-capped, CGYOR-emitting monochromatic DC-LED coated with binder paste and CsPbX3 PeQDs. To fabricate CsPbX3 PeQD-based monochromatic DC-LEDs, it is necessary to find an appropriate polymer binder to effectively disperse OLA and OA-capped PeQDs in the polymer matrix for fabricating DC-LED packages. The dispersibility and miscibility of the hydrophobic ligand-capped PeQDs in the polymer matrix is governed by the nature of the polymer, the surface ligands on the PeQDs, and their solubility in the desired polymer solutions.46–48 The complete dispersion of QDs in polymers at high loading levels enables the


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development of highly efficient QD/polymer hybrid paste for DC-LEDs. Herein, we compare several polymer candidates for fabricating PeQD/polymer nanohybrid pastes that can be used as high-performance monochromatic PeQD/polymer hybrids for DC-LED packages. To prepare the PeQD/polymer solid matrix, we compared two types of thermoplastic polymers, three types of photopolymers via photo-polymerization of monomers with a photosensitizer, and one kind of liquid photopolymer that cures under UV light. Due to the results of thermal, humidity, and UV stability tests of CsPbBr3 PeQDs/polymers (see Figure S4), we choose the NOA 63 UV-curable photopolymer to encapsulate the CsPbX3 PeQDs for the fabrication of the DC-LED. As shown in the schematic diagram in Figure 4, the present fabrication process for the PeQD composite film was simple, easily upscaled, flexible, and tunable with regard to controlling the color of the PeQD material type, load, and thickness of the film. The detailed curing experiments of various PeQD/polymer solid hybrids were described in the Experimental section. A thermally curable binder such as conventional silicone resin (OE6636 a/b kit, Dow Corning Co.) cannot be cured above 100°C for application to CsPbX3 PeQD/polymer pastes in a LED package, as the CsPbX3 PeQDs lose their luminescent properties above 100°C (Figures S4a and b). This was done because lattice thermal expansion in the CsPbX3 PeQD would decrease the interaction between the cation and the anion.49 Moreover, the weak polar property of a thermal curable binder induces aggregation which can degrade the luminescent properties through luminescence quenching or light-scattering.50 Instead of using thermal curing binder, two different types of UV curable polymers were used for the solidification of PeQD/polymer hybrids. First, CsPbBr3 PeQD powders were mixed with three liquid monomers (methyl methacrylate (MMA), hexyl methacrylate (HMA), and dodecyl methacrylate (DMA)). By adding a photoinitiator (Irgacure 819, BASF) to these solutions, their polymerization processes for PMMA, PHMA, and PDMA were also conducted in a blue LED cup at room temperature for 2 h with UV light. However, the



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resultant composites of all three CsPbX3 PeQD/PMMA, PeQD/PHMA, and PeQD/PDMA hybrids yielded DC-LED packages with low light emission at low and high concentrations of CsPbBr3 PeQDs in comparison with high luminescent colloidal PeQDs (Figure S5). Moreover, the PeQD/PHMA and PeQD/PDMA hybrids were not perfectly cured under a sticky condition, which does not apply to an encapsulant. We speculate that the unknown side reactions in the low and high concentrated PeQD solutions are detrimental to the stability of PeQDs in PMMA, PHMA, or PDMA. Secondly, CsPbX3 PeQD/hexane solutions were mixed with a polyurethane-based liquid photopolymer (NOA 63), which is a good binder and miscible with OLA and OA-capped PeQDs, because the use of a liquid photopolymer can eliminate the heat curable process common to other optical binders. The mixed PeQD/NOA 63 paste was dried under vacuum and cured for 30 min under UV light. The good miscibility between the PeQDs and the NOA 63 binder, combined with a simple photo-curing operation, can yield PeQD-based LED packages with excellent optical clarity and bright light emission. Figure 4b shows images of five selected CGYOR-emitting PeQD/NOA 63 composite films obtained with UV irradiation for photo-polymerization. The color emitting images clearly indicate that pure colored PeQD films can be realized using a mixture of NOA 63 polymer and a series of narrow-band CsPbX3 PeQDs. Figures S6a and b present the PL spectra and CIE color coordinates, respectively, of these pure colored PeQD films, which correspondingly cover 127 % of the three-colored and 157% of the six-colored PeQD film, compared to the NTSC standard. Figure 5a shows schematics of five LPDF-capped, CGYOR monochromatic DC-LEDs and six-color package white LEDs. Figures 5b and c present the electroluminescence (EL) spectra, 1931 CIE color coordinates, and images of the B semiconductor-type LED and five selected colors of full down-converted, LPDF-capped PeQDs -based DC-LEDs with PeQD/NOA 63 paste. As shown in Figure S7, the DC-LEDs without LPDFs show mixed


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emissions of blue from the InGaN B LED and the PeQD emission (CGYOR). However, the DC-LEDs with LPDFs show full-down converted PeQDs emissions, which enhance the emission output because the LPDF reflects and recycles a transmitted blue emission from the LED chip through the PeQDs layer.41,42 In addition, compared to conventional PeQD-based DC-LEDs (without LPDF), the LPDF-capped DC-LEDs can reduce the PeQD concentration in the NOA 63 binder and enhance the EQE by reducing the energy exchange between the PeQDs and the scattering loss from the agglomerated PeQDs.39,40 Here, a series of narrow spectral-band CGYOR-colors were obtained by varying the type of CsPbX3 PeQDs and optimizing the critical concentration of PeQDs in the polymer matrix in LPDF-capped DCLEDs. As seen in Figures 3b and 5b and Tables 1 and 2, the EL emission spectra and FWHMs of CGYOR-emitting monochromatic DC-LEDs are well matched with the PL emission spectra and FWHMs of the corresponding PeQDs as previously reported, even though the peak wavelengths are slightly red-shifted. This minor red-shift indicates that the PeQD agglomeration and energy exchange between neighboring PeQDs are not as pronounced in the PeQD/NOA 63 binder package of monochromatic LEDs. The color emitting images of DC-LEDs also confirm that any kind of pure color can be realized using the B LED as an excitation source, the LPDF as blue-mirror and yellow-window filter and a series of narrowband CsPbX3 PeQDs. The color gamut of the RGB triangle of monochromatic LEDs covers ~120% of the NTSC standard, but a BCGYOR hexagon encompasses ~145% of this standard. The combination of five CGYOR monochromatic LEDs and a B LED can produce the largest color reproduction area in display backlight systems for FSC-LCDs or 3D-displays which are well matched to the enlarged color gamut of the five PeQD and B LED hexagon. (Figures 3c and 5c) It implies that the color reproduction capability of color displays is only limited by the color reproduction regulations of broadcasting systems governed by NTSC, rather than the color reproduction capability of hexagon-colored displays. Therefore, this large color



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reproduction area has great potential to change the paradigm of color display systems. To the best of our knowledge, a few publications have reported the figures of merit for the vision performance of PeQD-based DC-LEDs with PeQD/binder paste. An LE of 48 lm/W was reported at a applied current of 4.9 mA by integrating green CH3NH3PbBr3 PeQDs/PMMA plates and red K2SiF6:Mn4+ (KSF)/silicone plates with a B-emitting InGaN chip.12 Although detailed color quality metrics such as LE and CRI were unknown, white DC-LEDs using the green and red CsPbX3 PeQDs/PMMA films were realized with tunable color temperatures.22 Herein, we report the measurable and meaningful figures of merit of five LPDF-capped PeQD-based monochromatic LEDs and six-color multi-package white DC-LEDs with PeQD/NOA 63 binder to assess their practical applicability as PeQDs for DCLEDs. Tables 1 and 2 confirm that the EQE trend of monochromatic LEDs matches the PLQY trend of the corresponding QDs in terms of emission peak wavelength. The luminous efficacy of radiation (LER) trend in the monochromatic LEDs on the emission wavelength results from the appearance of the photopic spectral LE function (photopic eye sensitivity function V(λ)). Tables 1 and 2 also indicate that the CsPb(Br0.75,Cl0.25)3, CsPbBr3, CsPb(Br0.65,I0.35)3, CsPb(Br0.5,I0.5)3, and CsPb(Br0.35,I0.65)3 PeQDs provide PLQYs of 0.63, 0.81, 0.47, 0.50 and 0.79, respectively, and their corresponding LPDF-capped, CGYOR DCLEDs provide LE values of 81, 184, 79, 80, and 35 lm/W, respectively, at an applied current of 20 mA. This LE trend matches the dependence of LE on EQE and LER (LE = EQE × LER). The challenge, therefore, comes from enhancing the PLQY of each colored PeQDs with quaternary typed PeQDs in order to improve the LE of monochromatic LPDF-capped DC-LEDs. The pure green monochromatic LED with ternary CsPbBr3 PeQDs show the highest LE of 184 lm/W at 20 mA with x = 0.151, y = 0.731, and EQE values of 0.371 at 524 nm with 18 nm spectral width. This is due to the high PLQY of 0.81 of G-emitting PeQDs with low defect concentration, and the low loss of PLQY while fabricating CsPbBr3 PeQD-


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based G LEDs. This indicates that the CsPbBr3 PeQD-based G DC-LED is a promising green monochromatic LED candidate to close the green gap in chip-typed LEDs in terms of bandwidth, color coordinates and energy efficiency, although further improvements of PLQY, thermal stability and moisture stability are necessary for all studied PeQDs. Furthermore, the detailed EL performances of the five LPDF-capped CGYOR-emitting monochromatic LEDs and an InGaN B LED were determined by measuring the current dependence as a function of the applied current. Figures 6a and b show that the LEs and normalized LEs of each of the six BCGYOR-emitting monochromatic LEDs exhibit different decreasing trends with an increase in the applied current from 10 to 120 mA. The LEs at 120 mA for the BCGYOR color monochromatic LED are 24, 41, 113, 18, 23, and 8.5 lm/W, with decreasing percentages to 0.24, 0.50, 0.39, 0.77, 0.71, and 0.76 relative to the LEs at 20 mA. The normalized LE of the G-emitting monochromatic LED is higher than that of the other solid-solution CsPbX3-based monochromatic LEDs at 120 mA. This result is evidence of the instability of Br-/I- or Br-/Cl- mixed solid-solution quaternary CsPbX3 PeQDs owing to lattice and surface defects. In the last section, RGB tri-color multi-package white LEDs and BCGYOR six-color multi-package white LEDs were prepared and characterized to compare the effects of the color reproduction capability of both triangle and hexagon-colored LEDs in display systems. RGB tri-color multi-package white LEDs and BCGYOR six-color multi-package white LEDs can realize a set of five correlated color temperatures (CCTs) specified by the American National Standards Institute (ANSI) standard by tuning the fractional applied current of the RGB or BCGYOR primary LEDs in tri-color or six-color multi-package white LEDs. Figures 7a and b show the overlapped, integrated emission spectra of the LEDs in both tri-color multi-package and six-color multi-package white LEDs for 6500, 5000, 4500, 3500, and 2700



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K CCTs. The total applied current of the tri-color multi-package LED was 60 mA, and for the six-color multi-package LED it was 120 mA; we also considered the white colors of either a RGB white LED or a BCGYOR LED with CCTs. As previously reported, the fractional intensity of the orange and red spectrum in Or- and R-emitting PeQD-based monochromatic DC-LEDs in BCGYOR six-color multi-package white LEDs gradually increase as the CCT of the white color decreases.43 In contrast, the fractional intensity of the B, Cy, and G spectra in six-color multi-package white LEDs increases as the CCT increases. Otherwise, the intensity of the Yg LED remains constant as CCT varies. Any specified white colors, and all colors in the RGB triangle area (120% of NTSC, Figure 7c) in tri-color multi-package white LEDs, as well as the BCGYOR hexagon area (145% of NTSC, Figure 7d) in six-color multipackage white LEDs in the CIE diagram, can be realized by dynamically controlling the fractional applied current in the colored DC-LED and a B LED package in the tri-color multipackage or six-color multi-package white LEDs. However, white colors at similar CCT values have different white spectra between the RGB tri-color multi-package white LED and the BCGYOR six-color multi-package white LED. In the case of tri-color multi-package white LEDs, there is a cyan gap between the blue and green peaks and a yellow gap between the green and red peaks in any white spectrum. Figures 7e and f show the emission spectra of cyan and yellow colors as reproduced with similar CIE x and y color coordinates by RGB tricolor and BCGYOR six-color white LEDs. As expected, the cyan and yellow colors are mixed colors of blue and green (BG) and green and red (GR), respectively. Otherwise, sixcolor white LED can reproduce the cyan and yellow colors in the emission spectrum such that they are similar to the actual cyan and yellow colors in nature. When compared to actual images of both cyan and yellow colors, both the RGB tri-color and BCGYOR six-color white LEDs reproduce slightly different colors. This means that people cannot perceive the actual color of objects in the RGB color mixing system of RGB-colored TVs. It indicates people


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cannot perceive actual cyan and amber colors in current RGB display systems, including TFT-LCD and AM-OLED systems. Otherwise, there are no color gaps in the wavelength range of the photopic eye sensitivity function (V(λ)) in any white lights of six-color multipackage white LEDs. People can perceive any color in the wavelength range of the V(λ) curve of visible light in a six-color multi-package white LED (see Figure 7b). This means that controlling six individual LEDs can reproduce the actual object colors in the display screen. If BCGYOR six-color multi-package LED backlights are used for FSC-LCDs, directional color LEDs for 3D displays or BCGYOR six subpixelled color-by-blue OLEDs or LCDs are possible in the future. As shown in actual images (insets of Figures 7c and d) of white colors from both the RGB tri-color multi-package white LEDs and the BCGYOR six-color multi-package white LEDs with CCTs, both white colors have different color tones and hues even at similar CCT and color coordinates, due to the different spectral power distributions (SPD) of white colors. In general, the color rendering index (CRI, Ra) and special CRI for red (R9) are used to appraise the ability of a specific light source to reproduce the actual colors of objects compared to ideal light, such as sunlight (> 5,000 K) or Planckian locus radiation (≤ 5,000 K). Table 3 summarizes the resulting optical performances of two types of white LEDs including tri-color multi-package white LEDs comprised of a B LED with G/R color-by-blue monochromatic DC-LEDs, and the six-color multi-package white LED comprised of a B LED and CGYOR color-by-blue monochromatic DC-LEDs fabricated in this experiment. This table indicates that the CRIs and R9s of the six-color multi-package white LED system with narrow-band BCGYOR LEDs have much higher values (Ras = 95 – 97, and R9s = 86 – 97) than those of the tri-color multi-package white LED with narrow-band RGB LEDs (Ras = 4 – 19 and R9s = -250 – -307) in the white CCT range between 6500 and 2700 K. The actual emission images and CRI data of white colors in the six-color multi-package white LED



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indicate that the actual colors of objects can be reproduced in the display screen using sixcolored BCGYOR spectrum having a narrow-band spectrum. Therefore, the use of BCGYOR six-colored PeQDs with narrow spectral bands is considered the simplest way to view actual colors of objects in display screens and reproduce any colors within the largest color gamut in the CIE color diagram. Table 3 also shows that LE performances (LE of 55, 54, 53, 50, and 46 lm/W for 6500, 5000, 4500, 3500, and 2700 K, respectively, total applied current of 60 mA) of RGB tri-color multi-package white LEDs are lower than those of the BCGYOR six-color multi-package white LEDs (LE of 62, 61, 61, 62, and 61 lm/W for corresponding CCTs, total 120 mA). The higher LE of BCGYOR six-color multi-package white LEDs is due to the relatively higher LERs of six-colored white LEDs than those of RGB white LEDs and the well distributed fractional currents of all six BCGYOR DC-LEDs. To further enhance the LEs and EQEs of BCGYOR six-color multi-package white LEDs, it is necessary to improve the LEs and EQEs of Cy/Yg/Or-emitting monochromatic LEDs (similar level to G-emitting monochromatic LEDs) and make the fractional applied current of each primary DC-LED more even.51 To achieve this, the number of internal and surface defects should be decreased and the crystal quality and PLQY should be increased to the levels of the G-emitting CsPbBr3 PeQDs. Although the LEs and EQEs of white LEDs are still inferior to the commercialized inorganic phosphor-based white DC-LEDs, this is the first report to show the optical performances of fully PeQD-based color-by-blue DC-LEDs, reflecting the many potential applications of fabricated wide-color gamut (~145% of NTSC) and actual-color reproducible hexagondisplay devices.

Conclusion


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We successfully synthesized narrow band-width and highly-efficient fully inorganic CsPbX3 PeQDs using a facile hot-injection method with a two-step heating process, followed by thermal quenching. To obtain variable color emissive CsPbX3 PeQDs, we employed a synthetic method of controlling the ratio of halide composition, which is a suitable means of shifting the emission wavelength of CsPbX3 PeQDs. The five CGYOR-emitting CsPbX3 PeQDs were synthesized at 200°C as ternary and quaternary CsPb(Br0.75,Cl0.25)3 Cy, CsPbBr3 G, CsPb(Br0.65I0.35)3 Yg, CsPb(Br0.5I0.5)3 Or, and CsPb(Br0.35I0.65)3 R-color converters, respectively. The optical properties of the selected CGYOR-emitting CsPbX3 PeQDs revealed color emission wavelengths 499, 515, 546, 589, and 643 nm and FWHM values of 18, 19, 24, 28, and 36 nm, respectively, with PLQYs of 0.81 – 0.47. Our result indicates that the minimum value of PLQY reaches 0.47 in Yg-emitting CsPb(Br0.65I0.35)3, obtained at 200 °C, owing to the formation of a Br-/I- mixed solid-solution that increases the number of lattice and surface defects due to the size mismatch between Br- and I-. Further synthetic approaches are required to obtain highly luminescent Br- and I- mixed solid-solution CsPbX3 PeQDs. Using the selected five CGYOR-emitting CsPbX3-based PeQDs and a UV-curable polyurethane-based NOA 63 binder, we realized pure-color and high vision performance monochromatic DC-LEDs. These could be applied to display technologies such as backlights for FSC-LEDs and 3D displays. Blue-mirror-yellow-window LPDF-capped, CsPbX3 PeQDbased pure CGYOR-emitting monochromatic DC-LEDs provide LE values of 81, 184, 79, 80, and 35 lm/W, respectively, and EQEs of 0.28, 0.37, 0.13, 0.19, and 0.31, respectively, at an applied current of 20 mA. Based on the combination of five CGYOR-emitting monochromatic DC-LEDs and an InGaN-based B LED, these CsPbX3 PeQD-based six colorby-blue BCGYOR multi-package white LEDs (with an InGaN B LED) are capable of providing actual color from displays with high color qualities. RGB triangle tri-color multipackage white LEDs and BCGYOR hexagon six-color multi-package white LEDs were



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characterized to compare the effect of the color reproduction capability with the NTSC standard, covering ~120% and ~145% respectively. Moreover, in terms of overall color properties, BCGYOR six-colored multi-package white LEDs shows moderate LE values of 60–62 lm/W and exceptional vision and color performance (CRI = 95 – 97 and R9 = 86 – 97) at 2700, 3500, 4500, 5000, and 6500 K with at total applied current of 120 mA. Accordingly, the optical performances of six-color white LEDs make it possible to realize a future BCGYOR six-colored display with excellent color performance capabilities.


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ASSOCIATED CONTENT Supporting Information The results of additional optical measurements of the CsPbX3 quantum dots, the structural properties as determined through XRD and TEM analyses, the PeQD/polymer paste stability under UV light, the moisture, and the heating and information of LPDFs are available via the ACS Publication website at http://pubs.acs.org.

The supporting Information is available via the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Phone: +82-2-910-4893.

Fax: +82-2-910-4415.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP (Ministry of Science, ICT&Future Planning)) (No. 2015M3D1A1069709 and No.2016R1A5A1p12966).



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Reference (1) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591:1−7. (2) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (4) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite Sensitized Solar Cells. Nature 2013, 499, 316-319. (5) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright Lightemitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (6) Kim, Y.-H.; Cho, H.; Heo, J. H.; Kim, T.-S.; Myoung, N.; Lee, C.-L.; Im, S. H.; Lee, T.-W. Multicolored Organic/Inorganic Hybrid Perovskite Light-Emitting Diodes. Adv. Mater. 2014, 27, 1248−1254. (7) Sutherland, B. R.; Hoogland, S.; Adachi, M. M.; Wong, C. T. O.; Sargent, E. H.; Al, S. E. T. Conformal Organohalide Perovskites Enable Lasing on Spherical Resonators. ACS Nano 2014, 8, 10947−10952.


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Figure captions

Figure 1. Perception of yellow color: (a) the yellow spectrum from an actual yellow-colored subject, (b) the combined spectrum of green and red subpixels from the yellow-colored subject on the display screen, (c) the spectrum of a conventional, tri-color RGB display, and (d) the spectrum of a six-color BCGYOR display.



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Figure 2. Schematic diagrams of (a) BCGYOR six-colored backlight types of FSC-LEDs, (b) multidirectional BCGYOR six-colored backlight for a 3D display, (c) a BCGYOR sixcolored color-by-blue micro-LED display, and (d) a BCGYOR six-colored PeLED display.


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Figure 3. (a) UV-visible absorption and (b) photoluminescence (PL) spectra, (c) color coordinates and (d) photographs of emitting solutions of CsPb(Br0.75,Cl0.25)3 Cy-, CsPbBr3 G-, CsPb(Br0.65,I0.35)3 Yg-, CsPb(Br0.5,I0.5)3 Or- and CsPb(Br0.35,I0.65)3 R-emitting PeQDs prepared at 200 °C.



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Figure 4. (a) Overall schematic of the fabrication process of the freestanding PeQD photocurable films. (a-1) Preparation of PeQD/NOA 63 Paste. (a-2) Schematic of spacer on glass substrate. (a-3) Fabrication of PeQD/NOA 63 paste film through screen printing and UV curing process. (a-4) Elimination of spacer. (a-5) Detachment of the PeQD/NOA 63 film from glass substrate. (a-6) Flexible PeQD/NOA 63 free-standing film. (b) Images of flexible emitting PeQD free-standing films of CsPb(Br0.75,Cl0.25)3 Cy-, CsPbBr3 G-, CsPb(Br0.65,I0.35)3 Yg-, CsPb(Br0.5,I0.5)3 Or-, and CsPb(Br0.35,I0.65)3 R-emitting PeQDs in NOA 63 polymer composites.


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Figure 5. (a) Schematic diagram, (b) EL spectra, and (c) color coordinates of the LPDFcapped, CsPb(Br0.75,Cl0.25)3 Cy-, CsPbBr3 G-, CsPb(Br0.65,I0.35)3 Yg-, CsPb(Br0.5,I0.5)3 Or-, and CsPb(Br0.35,I0.65)3 R-emitting PeQDs-based monochromatic DC-LEDs and a B InGaN LED, inset: photographs of BCGYOR emitting monochromatic DC-LEDs.



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Figure 6. (a) LE (lm/W) and (b) normalized LE of each six-colored BCGYOR monochromatic DC-LEDs as a function of applied current, with range between 10 and 120 mA.


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Figure 7. EL spectra of (a) the RGB tri-color and (b) the BCGYOR six-color multi-package white LED as white CCT range between 6500 K and 2700 K. Arrows mean the increase and the decrease of EL intensity through arrow’s indication in increasing the CCT. The gray color spectrum indicates photopic eye sensitivity function V(λ). CIE color coordinates and color gamut of (c) the RGB tri-color and (d) the BCGYOR six-color multi-package white LED. The gray hollow stars indicate the CIE coordinates of multi package white LED, and cyan



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and dark yellow sphere dots indicate the CIE coordinates of cyan and yellow colored multi package LED; inset of (c), (d) show photographs of the RGB tri-color and the BCGYOR sixcolor multi-package white LED from the integrated sphere with a decrease in the CCT from left to right. The emitting spectra and actual photograph images of cyan and yellow reproduced by (e) RGB tri-color white LEDs and (f) the BCGYOR six-color multi-package white LED.


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Table Captions

Table

1.

Optical

properties

of colloidal

CsPb(Br0.75,Cl0.25)3

Cy-,

CsPbBr3

G-,

CsPb(Br0.65,I0.35)3 Yg-, CsPb(Br0.5,I0.5)3 Or-, and CsPb(Br0.35,I0.65)3 R-emitting PeQDs

Table 2. Optical properties of CsPb(Br0.75,Cl0.25)3 Cy-, CsPbBr3 G-, CsPb(Br0.65,I0.35)3 Yg-, CsPb(Br0.5,I0.5)3 Or-, and CsPb(Br0.35,I0.65)3 R-emitting PeQD-based monochromatic DCLEDs and a InGaN B LED.



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Table 3. Optical properties of RGB tri-color and BCGYOR six-color multi-package white LEDs with CCTs ranging from 6500 K and 2700 K


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Table of Contents/Abstract Graphic


Study of Perovskite QD Down-Converted LEDs and Six-Color White

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