Efficient White-Light-Emitting Diodes Fabricated from Highly

Apr 25, 2012 - Yellow-emitting CIS/ZnS QDs (Cu/In = 1/4) were demonstrated to be suitable ...... the aq. phase via surface ligand exchange with dihydr...
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Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/Shell Quantum Dots Woo-Seuk Song and Heesun Yang* Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea ABSTRACT: Copper indium sulfide (CIS) quantum dots (QDs) with different Cu/In molar ratios of 1/1, 1/2, and 1/4 are synthesized via a hot colloidal route. The band gap energy of CIS QDs is observed to be dependent on Cu/In ratio, exhibiting a higher band gap from more Cudeficient QDs. The emission wavelengths of all CIS QDs belong to a deep red region (665−717 nm) with relatively low quantum yields (QYs) of 8.6− 12.7%. Compared to respective original core QDs, the absorption peaks of all CIS/ZnS QDs are blue-shifted, and their emission wavelengths move to a higher energy accordingly, showing a quite tunable emission from yellow to red. The effective surface passivation by a ZnS overlayer results in a dramatic increase in QY of CIS/ZnS QDs in the range of 68−78%. All CIS/ZnS QDs are tested as wavelength converters for the fabrication of QD-based lightemitting diodes (LEDs). QD-based white LEDs that consist of only a single type of QD are for the first time realized by applying yellow-emitting CIS/ ZnS QDs as a result of the appropriate color mixing between blue emission from a LED chip and yellow emission from QDs. Detailed electroluminescent properties including color rendering index, Commission Internationale de l’Eclairage (CIE) color coordinates, and luminous efficiency of QD-based white LEDs are evaluated as a function of forward current. KEYWORDS: copper indium sulfide, quantum dots, Cu/In molar ratio, white LED

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INTRODUCTION Significant efforts on the synthesis and optical characterization of ternary I−III−VI-type semiconductor nanocrystals (quantum dots, QDs) including CuInS2,1−13 CuGaS2,14 AgInS2,15,16 and AgGaS2 QDs17 have been made recently since they exhibit useful absorption and emission features that are well suited for the application to photovoltaic, lighting, and display devices. Moreover, Cd-free composition of I−III−VI QDs is well compatible with a global trend to regulate use of toxic, environment-malign materials such as Cd- or Pb-containing II− VI QDs. Among the I−III−VI QDs listed above, copper indium sulfide (CIS) QDs have been most extensively synthesized mainly by heating,1−6,9,11 hot-injection,8,10 and solvothermal routes.7,12,13 Photoluminescence (PL) quantum yield (QY) of CIS QDs is dramatically enhanced typically upon ZnS overcoating. For instance, Kim et al. reported the maximum PL QY of 65% from 625 nm-emitting CIS/ZnS core/shell QDs, in which the ZnS overlayer was deposited through a cation exchange process.8 Meanwhile, the same QY of 565 nmemitting CIS/ZnS QDs, which were synthesized via a facile, stepwise solvothermal growth strategy, was also reported in the literature.7 Very recently, Li et al. published an efficient synthesis of CIS QDs with a high chemical yield by an octadecene-free heating method and reported the record QY values of 67% from 671 nm-emitting CIS/ZnS and 86% from 709 nm-emitting CIS/CdS QDs.4 As seen from the above reports, the emission wavelengths of CIS QDs showing the maximum QYs seem to be quite different, depending on their © XXXX American Chemical Society

own synthetic approach and detail. Unremitting work to obtain more efficiently fluorescent CIS QDs is still underway. The radiative carrier recombination in CIS QDs is governed by intragap defect states, resulting in a substantially large Stokes-shifted emission versus absorption. Therefore, compared to the tunability of band gap energy of CIS QDs with a size variation, that of emission energy is rather limited. The broad feature of the emission band in CIS QDs is characteristic of such a defect-related transition due to the strong electron− phonon interaction15 and the distribution of distance between defect sites.15,18 Such a broad emission band of CIS QDs would be inadequate for the application to display devices, in which a high degree of color purity (i.e., narrow bandwidth) is generally required. However, in general illumination-targeted white-lightemitting diodes (LEDs), the phosphors with a broad emission band are preferred to cover a wide spectral region, suggesting that the CIS QD may be regarded as a potential candidate for the fabrication of white LEDs. Combined with a blue or near-UV LED chip, highly efficient QDs have been used as downconverters (or wavelength converters), taking advantage of the high excitation efficiency of QDs by blue or near-UV photons. CdSe or its derivative QDs have been most widely chosen for LED fabrication by adopting one19−21 or multiple types22−25 of QD emitters, Received: March 16, 2012 Revised: April 24, 2012

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dispersion having an optical density of ∼3.0 at 450 nm (corresponding to an approximate concentration of ∼43 mg/mL), and then the chloroform in the above mixture was removed by heating at 90 °C for 1 h. Subsequently, the final QD paste was prepared by adding the same amount (1.23 g) of a hardener (OE-6630 A) to the CIS/ZnS QD− silicone resin mixture and dispensed on the mold of a blue LED chip. The dispensed QD paste was placed in a two-step thermal curing process at 70 °C for 30 min and then 120 °C for 1 h in an oven. Characterization. X-ray diffraction (XRD) (Rigaku, Ultima IV) using Cu Kα radiation was utilized to analyze the crystal structure of QDs. High-resolution TEM (transmission electron microscopy) images of the QDs were obtained using a JEOL JEM-4010 electron microscope operated at an accelerating voltage of 400 kV. The actual chemical compositions of CIS and CIS/ZnS QDs were collected by an energy-dispersive spectroscopy (EDS) (EDAX Inc., Phoenix)equipped scanning electron microscope operating at 15 kV. Absorption and PL emission spectra of CIS core and CIS/ZnS core/shell QDs were obtained by UV−visible absorption spectroscopy (Shimadzu, UV-2450) and a 500 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd., Darsa Pro-5200), respectively. PL QYs of the QDs were calculated by comparing their integrated emissions with that of rhodamine 6G (QY of ∼96%) ethanol solution with an identical optical density of ∼0.05 at 450 nm. Optical properties (electroluminescent (EL) spectrum, luminous efficiency, correlated color temperature (CCT), Commission Internationale de l’Eclairage (CIE) color coordinates, and color rendering index (CRI)) of the fabricated QD-LEDs were measured under various forward currents of 5−100 mA in an integrating sphere with a diode array rapid analyzer system (PSI Co. Ltd.).

depending on the desired emission color. Among them, Jang et al. reported that QD-based LEDs, in which green- and redemitting CdSe/multishell QDs with sufficiently high QYs of >95% were combined with a blue InGaN LED for the display backlight application, exhibited a high-quality white light with a luminous efficiency of 41 lm/W at an operating current of 20 mA and a high color reproducibility of 100% relative to the National Television Systems Committee (NTSC) color space.24 In addition, the white QD-LED, consisting of three types of green, yellow, and red CdSe/multishell QDs on a blue LED chip, was also fabricated for the general lighting application, showing a luminous efficiency of 32 lm/W at 40 mA and a high color rendering index (CRI) of 88.25 Moreover, white LEDs could be also fabricated simply by adding an orange or red QD into a conventional green or yellow phosphor-based LED platform (also referred to as QD-assisted white LED).26−30 In this work, CIS QDs having different Cu/In molar ratios of 1/1, 1/2, and 1/4 are first prepared, and subsequently, ZnS overcoating is done on the respective core QDs. Absorption and emission features of CIS/ZnS QDs are compared with those of the CIS ones. Depending on the Cu/In ratio for CIS synthesis, CIS/ZnS QDs show tunable emission colors from yellow, orange, to red with sufficiently high QYs of 68−78%. As described above, white QD-LED has been typically realized by either mixing 2 or 3 types of QD emitters or adding a specific wavelength-emitting QD into a phosphor. Taking full advantage of the broad band emission nature of highly efficient CIS/ZnS QDs, however, white QD-LED that is made up of only a single type of QD and exhibits a high luminous efficiency of 63.4 lm/W and a moderate CRI of 72 at a forward current of 20 mA is for the first time demonstrated.

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RESULTS AND DISCUSSION XRD patterns of three CIS core QDs with Cu/In molar ratios of 1/1, 1/2, and 1/4 in the starting solution concentration were compared in Figure 1a, where no distinguishable difference in

EXPERIMENTAL SECTION

Materials. Cu(I) iodide (CuI, 99.999%), In acetate (In(Ac)3, 99.99%), 1-dodecanethiol (DDT, 98%), 1-octadecene (ODE, 90%), Zn stearate (10−12% Zn basis), and all other solvents were purchased from Sigma-Aldrich and used as received. Synthesis of CIS Core and CIS/ZnS Core/Shell QDs. CIS QDs with different Cu/In compositions of 1/1, 1/2, and 1/4 were prepared by fixing the amount of In precursor and varying the amount of Cu precursor. For a typical synthesis of CIS QDs with a Cu/In 1/1, 0.5 mmol (0.095 g) of CuI, 0.5 mmol (0.146 g) of In(Ac)3, and 5 mL of DDT were loaded in 50 mL of a three-neck flask. The reaction mixture was degassed during heating to 100 °C and backfilled with Ar and subsequently further heated to 230 °C within 10 min. The growth of CIS core QDs was allowed for 5 min at that temperature. CIS QDs with Cu/In ratios of 1/2 and 1/4 were synthesized in exactly the same way as above except using 0.25 and 0.125 mmol of CuI, respectively. For the ZnS shell overcoating, a ZnS shell stock solution, consisting of 4 mmol (2.528 g) of Zn stearate, 1 mL of DDT, and 4 mL of ODE, was prepared beforehand by heating the shell mixture on a hot plate at 190 °C. This shell stock solution was added dropwise at a rate of 0.9− 1.0 mL/min into the CIS core crude solution at 230 °C. Then the mixture was heated to 240 °C and maintained at that temperature for optimum shell overcoating times of 60−70 min. As-reacted CIS core and CIS/ZnS core/shell QDs were precipitated with an excess of ethanol, purified repeatedly with a solvent combination of chloroform/ ethanol by a centrifugation (7000 rpm/10 min), and finally dispersed in chloroform. Fabrication of a CIS/ZnS QD-Based LED Device. QD-LEDs were fabricated by combining a 50 × 50 mm2 surface mounting device typed InGaN-based blue-emitting LED (λpeak = 455 nm, Haewon Semiconductor, Korea) with a CIS/ZnS QD−silicone resin mixture as follows; first, 1.23 g of thermally curable silicone resin (OE-6630 B, Dow Corning Co.) was mixed with CIS/ZnS QD chloroform

Figure 1. (a) XRD patterns of CIS and CIS/ZnS QDs synthesized by using Cu/In ratios of 1/1, 1/2, and 1/4. TEM images of (b) CIS and (c) CIS/ZnS QDs with a Cu/In ratio of 1/2.

reflection peak angle was observed despite a relatively large variation of Cu content in CIS QDs. Three distinct reflection peaks with 2θ values of 28.0°, 46.5°, and 54.9° could be well indexed to (112), (204)/(220), and (116)/(312) planes of a known tetragonal chalcopyrite structure of the CuInS2 phase.1,2 These XRD results were consistent with those published in the literature, where the XRD pattern of highly off-stoichiometric B

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(Cu/In ratio of 1/7) CIS QDs was the same as that of stoichiometric ones (Cu/In ratio of 1/1).3 A certain metastable, In-rich CIS phase such CuIn3S5 might be expected to be generated under the Cu-deficient synthesis; however, the identification of such a metastable phase is practically challenging due to its structural similarity to the CuInS2 phase and XRD peak broadening. Nevertheless, based on Raman spectroscopic results on CIS QDs with various degrees of Cu deficiency analyzed by Uehara et al.,3 our offstoichiometric CIS QDs with Cu/In ratios of 1/2 and 1/4 are thought to possess the same chalcopyrite framework as stoichiometric QDs, although such Cu-deficient QDs are likely to include a high density of Cu-related defects (e.g., Cu vacancy and In interstitial at the Cu site). Overcoating of the ZnS shell was conducted under an identical condition for the above three CIS core QDs. XRD patterns of all core/shell QDs after ZnS shelling were also the same (Figure 1a), as expected. Since the lattice parameter (a = 0.5517 nm) of chalcopyrite CuInS2 is larger than that (a = 0.5345 nm) of zinc blende ZnS, the reflection peaks of CIS/ZnS core/shell QDs shifted to a larger 2θ compared to CIS core ones, closely approaching those of (111), (220), and (311) planes of the zinc blende ZnS phase and thus indicating that the ZnS overlayer was deposited appropriately on the surface of the respective CIS core QDs. TEM work on CIS QDs with different Cu/In ratios was conducted, but the difference in shape and size was not observed between those CIS QDs. Figure 1b presents a TEM image of representative CIS QDs with a Cu/In = 1/2, whose size was widely dispersed in the range of 2.0−2.5 nm. Such a size distribution is usually inherent in the noninjection-based synthetic route and can be partially attributed to a continual release of the S ion from the DDT molecule throughout the reaction. Upon ZnS shelling on the same CIS QDs (Cu/In = 1/2), the overall size of core/shell QDs increased to 3.2−4.0 nm, as seen from Figure 1c. Similar to the size distribution of CIS core QDs, that of CIS/ZnS QDs was also wide, conjecturing that the thickness of the ZnS shell deposited on different-sized CIS QDs might be roughly the same. The chemical compositions of three CIS core QDs were assessed by EDS measurements, and their compositional spectra and quantitative results are shown in Figure 2a. Actual Cu/In composition ratios of CIS QDs with Cu/In = 1/1, 1/2, and 1/4 were calculated to be 1.00, 0.49, and 0.25, respectively, which were almost identical with the solution molar ratios used for CIS QD synthesis. For all CIS QD samples, a somewhat excess content of S was detected; for instance, Cu:In:S was measured to be 0.49:1:2.2 in the case of CIS QDs with Cu/In = 1/2, being ascribed to the contribution of S from the cationcapping surface ligand of DDT. As seen from the EDS spectra of all CIS/ZnS QD samples in Figure 2b, the signals of Cu and In lines were significantly attenuated after ZnS shelling by the predominance of Zn peaks, and in particular, the Cu Lα line appeared to be buried in the Zn Lα line due to a close proximity in energy. Compared to bare CIS QDs, the Cu/In ratios of CIS/ZnS QDs were almost maintained. Assuming that the cation (Cu and In) precursors added for CIS synthesis were almost consumed and considering that the Zn/In ratio in the starting solution concentration for all CIS QDs was 8, a relatively low actual Zn content resulted for all CIS/ZnS QDs. This observation indicates that all Zn stearate added for shell overcoating did not participate in the formation of ZnS overlayer, and the remaining precursor would stay as an

Figure 2. EDS spectra and calculated atomic ratio (insets) of (a) CIS and (b) CIS/ZnS QDs synthesized under different Cu/In ratios of 1/ 1, 1/2, and 1/4.

unreacted part and/or generate byproducts, both of which were ultimately washed out during purification processing. Figure 3a presents absorption spectra of CIS QDs with different Cu/In ratios, where a blueshift in absorption from more Cu-deficient CIS QDs is evident. This blueshift is not related with a size variation, as mentioned earlier with the above TEM result. Such a variation of the Cu/In compositiondependent band gap in CIS QDs is consistent with the previous reports, where this phenomenon is generally attributable to the lowering of valence band due to the weakened repulsion between Cu d and S p orbitals in Cu-deficient material, ultimately leading to a band gap widening.3,5,6 As seen from the normalized emission spectra and fluorescent images (inset) of CIS QDs in Figure 3b, all core QDs emitted in the deep red region (with peak wavelengths of 717 nm for Cu/In = 1/1 and 665 nm for Cu/In = 1/4) with broad emission bandwidths of 128−141 nm. A systematic blueshift of emission observed from the QD sample with a higher degree of Cu deficiency should be associated with a band gap widening as described above. Large Stokes shifts of emission versus absorption wavelength up to ∼650 meV imply that the radiative decay should not stem from the carrier recombination between quantized electron−hole levels but be involved with internal and/or surface defects sites that serve as intragap states, although the accurate assignment of electron−hole recombination channels in CIS QDs still seems ambiguous. A commonly accepted transition mechanism is so-called donor−acceptor pair (DAP) recombination,2,5,7,11,31 where InCu (In substituted at the Cu site) and/ or VS (S vacancy) are likely to act as donor states with VCu (Cu vacancy) as an acceptor state.7 An alternative carrier recombination between the quantized conduction band minimum and defect (acceptor) trap level is also persuasive as proposed for CuInSe QDs by Nose et al.32 and for CIS QDs C

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size and consequently the band gap widening. If a size reduction of the CIS core occurs during the shell overcoating, a simple comparison of TEM-based size difference between CIS and CIS/ZnS QDs would not provide an accurate thickness of the ZnS shell as well as an effective core size of CIS/ZnS QDs. Moreover, very recently Kim et al. suggested that the lattice mismatch (∼3.1%) between CIS and ZnS induces a compressive stress on the CIS core (i.e., contraction of the CIS lattice), increasing its band gap.6 However, a careful comparison of full-width-at-half-maxima (fwhm's) of XRD reflection peaks in Figure 1a could provide a clue to the blueshifted absorption upon shell overcoating. The fwhm's of (112) peaks of all CIS core QDs were wider than those of the same peaks of respective CIS/ZnS QDs (e.g., the fwhm's of (112) peaks of Cu/In = 1/1-based CIS and CIS/ZnS QDs were 3.35° and 2.87°, respectively). This rather indicates that the core size of CIS/ZnS QDs became larger after shell overcoating. Considering the favorable resemblance between CIS and ZnS mentioned above, the alloying of the whole CIS core by ZnS during the overgrowth would also be highly plausible. The ZnS overlayer that sits directly on and/or near the surface of the original CIS core might participate in the alloying process through the interdiffusion with the CIS core, resulting in an increased size of alloyed core along with a band gap widening. As the band gap of the CIS core in CIS/ZnS QDs increases, emission spectra of all CIS/ZnS QDs were blue-shifted relative to their respective CIS core counterparts, showing red (623 nm), orange (598 nm), and yellow (564 nm) emissions from CIS/ZnS QDs with Cu/In = 1/1, 1/2, and 1/4, respectively (Figure 3d), accompanying the dramatic rise in emission QY. Owing to the effective surface passivation by the ZnS overlayer, the QYs of CIS/ZnS QDs with Cu/In = 1/1, 1/2, and 1/4 were dramatically enhanced to 68, 74, and 78%, respectively, and these QY values were reproducibly attainable. To the best of our knowledge, in yellow emission-capable CIS QDs that are the most useful as color converters for white LED fabrication, the QY obtained from our Cu/In = 1/4-based CIS/ZnS QDs is the record value to date, even though a higher QY of 86% from deep red-emitting CIS/CdS QDs was reported in the literature.4 It is also noted that the best QY recorded from some yellow-emitting CIS/ZnS QDs exceeded 80% (e.g., ∼85%). For the fabrication of QD-based LEDs, respective CIS/ZnS QDs with Cu/In ratios of 1/1, 1/2, and 1/4 dispersed in silicone resin were dispensed in the mold of blue LEDs having a luminous efficiency of 15.7 lm/W at 20 mA. EL spectra of QDLEDs with different types of CIS/ZnS QDs including a bare blue LED were compared in Figure 4a. When compared with PL emission spectra (Figure 3d), the QD peak wavelengths of QD-LEDs having Cu/In = 1/1-, 1/2-, and 1/4-based QDs were in EL spectra red-shifted to 653, 622, and 581 nm, respectively. Considering that the particle concentration (optical density of ∼0.05) of QD dispersion for PL was significantly lower than that of QD resin for EL, such a redshift is natural, attributable to the reabsorption of photons emitted from small QDs by large ones. In addition, due to the lack of QD solubility in the silicone resin matrix, QD agglomeration is inevitable to some extent. In the QD agglomerates, the dipole−dipole energy transfer that is strongly dependent on QD distance would be more facilitated,9,26 leading to a pronounced redshift by an efficient energy transfer event between QDs. Moreover, the difference in dielectric constants of the QD-surrounding dispersion media (chloroform for PL versus silicone resin for

Figure 3. (a,c) Absorption and (b,d) normalized PL emission spectra of CIS and CIS/ZnS QDs synthesized under different Cu/In ratios of 1/1, 1/2, and 1/4. The excitation wavelength used for PL measurement was 450 nm. The photographs of fluorescent CIS and CIS/ZnS QDs dispersed in chloroform under a 365 nm UV illumination are shown in the insets of (b,d), respectively.

by Li et al.4 The broad bandwidth of emission is characteristic of defect-related radiative transition like the case of CIS QDs. The QD size inhomogeneity might lead to a broadened feature of emission. However, our recent report suggested that through the comparison of size-selective precipitated CIS QDs an improvement of size inhomogeneity rarely modified the emission bandwidth,7 reinforcing the above defect-related recombination mechanism. Emission QYs of CIS QDs with Cu/In = 1/1, 1/2, and 1/4 were measured to be 8.6, 11.3, and 12.7%, respectively. Such an increasing trend in QY with more Cu-deficient QDs coincides with the results reported from Uehara3 and Nam et al.5 A higher density of Cu-related defect states generated by intentionally preparing Cu-deficient QDs would give rise to a higher probability of carrier recombination, resulting in an enhanced efficiency. ZnS overcoating was conducted on CIS core QDs with Cu/ In ratios of 1/1, 1/2, and 1/4. As shown in absorption spectra of the resulting CIS/ZnS QDs of Figure 3c, absorption peaks of all CIS/ZnS QDs moved to a shorter wavelength compared to their original CIS core QDs. Such a blueshift in absorption of CIS/ZnS QDs versus CIS core counterparts is commonly observed and may be explained in several manners. Surface etching of core QDs4,6 or formation of an alloyed interfacial CIS−ZnS shell layer7 during the ZnS overgrowth, resulting in the size reduction of core QDs, has been proposed previously. Cation exchange of Cu+ and In3+ ions in pregrown CIS core QDs by the Zn2+ ion also seems a quite convincing reason for the decrease of effective core size, taking into account that CIS and ZnS resemble each other considerably with respect to the crystal structure, cation radius, and Gibb formation enthalpy.8 Possible origins for a blue-shifted absorption upon ZnS shelling proposed above are all involved with a decrease of effective core D

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yellow QD-based LED in luminous efficiency relative to red and orange QD-based ones partly originates from the higher QY (78%) of yellow QDs versus red (68%) and orange QDs (74%); however, it is mainly ascribed to the above wavelengthdependent sensitivity. The QD emissions of red and orange QD-based LED were weighted excessively to the red wavelength side (i.e., toward a lower photopic sensitivity), and thus a significant portion of the QD radiation could not contribute to the luminous efficiency. The photograph of the QD-LED fabricated using Cu/In = 1/4-based yellow QDs and its strongly luminescing image under 20 mA are shown in Figure 4c. To the best of our knowledge, this work is the first demonstration of the fabrication of a white LED that consisted of only a single type of QD as a color converter, whereas several works on the fabrication of white LEDs based on the combination of multiple types of QDs22−25 or QD-added phosphors26−30 have been published earlier. EL spectra of white QD-LED with yellow QDs (Cu/In = 1/ 4) as a function of forward current from 5 to 100 mA are shown in Figure 5. With increasing forward bias, both blue and

Figure 4. (a) EL spectra and (b) CIE color coordinates of QD-LEDs combined with Cu/In = 1/1-, 1/2-, and 1/4-based CIS/ZnS QDs under a forward current of 20 mA. The EL spectrum of a bare blue InGaN LED is also included in (a) as a reference. (c) Photographs of white QD-LED fabricated with Cu/In = 1/4-based CIS/ZnS QDs before and after the application of a forward current of 20 mA.

EL) can also cause the redshift since the exciton binding energy of QDs is affected by the dielectric constant of the surrounding medium.19 However, a supplementary experiment, where the QD concentration in silicone resin was varied, showed that an increasing QD concentration led to a larger redshift in a monotonic way, ascribing the redshift to the former mechanism of energy transfer between QDs. As shown in Figure 4b, the mixed colors generated from QD-LEDs with red-emitting (Cu/ In = 1/1) and orange-emitting (Cu/In = 1/2) CIS/ZnS QDs exhibited the color coordinates of (0.270, 0.128) and (0.327, 0.182), respectively, being far from white light. Thus, their color-rendering property is not worth mentioning. In the case of QD-LEDs having yellow-emitting CIS/ZnS QDs (Cu/In = 1/4), however, the white light with a CRI of 72 and color coordinates of (0.347, 0.288) could be obtained as a result of the appropriate color mixing between blue emission from an InGaN LED chip plus yellow emission from QDs, indicating that these QDs could be good blue-to-yellow wavelength converters for the generation of white light, just like widely used YAG:Ce phosphors. The luminous efficiencies of the QD-LEDs with red-, orange-, and yellow-emitting CIS/ZnS QDs were 14.8, 27.6, and 63.4 lm/W, respectively, at a forward current of 20 mA. Although the luminous efficiency of bare blue LED used in this work was 15.7 lm/W at the same input current, the QD-LEDs can have a much higher efficiency value due to the photopic sensitivity of the human eye. In other words, although the bare blue LED emitted more radiation (i.e., larger integrated emission area in the EL spectra) than QD-LEDs, the contribution of blue radiation to the luminous flux is much less compared to green−yellow radiation, consequently exhibiting higher luminous efficiencies from orange and yellow QD-LEDs versus bare blue LED. Besides, a superiority of

Figure 5. EL spectra of white QD-LED fabricated with Cu/In = 1/4based CIS/ZnS QDs as a function of forward current from 5 to 100 mA.

QD emissions monotonically increased. The color coordinates (Figure 6a), CRI (Figure 6b), and CCT (Figure 6b) lay in

Figure 6. Variations of (a) CIE color coordinates, (b) CRI/CCT, and (c) luminous efficiency/light conversion efficiency of white QD-LED fabricated with Cu/In = 1/4-based CIS/ZnS QDs as a function of forward current from 5 to 100 mA. E

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decreasing with a higher applied current, originating mainly from the intrinsic efficiency drop of the LED chip, not from the QD saturation.

(0.334−0.348, 0.273−0.291), 72−75, and 4447−5380 K, respectively, demonstrating a quite high stability of our QDLED against the variation of forward current. The blue-sided shift of the color coordinates of white light with increasing forward current, which in turn results in the variation of CRI and CCT values, implies that QD emission would become somewhat saturated with increasing current bias. The luminous flux from the white QD-LED increased with increasing input current, as expected from Figure 5; however, its luminous efficiency and light conversion efficiency decreased in a different fashion with increasing forward bias, as shown in Figure 6c. The light conversion efficiency is defined as the ratio of the converted QD emission to the blue emission spent to be converted to QD emission in the QD-LED package. The light conversion efficiency tended to gradually drop from 75.2% at 5 mA to 62.0% at 100 mA. This observation is consistent with the above blue-sided shift of the color coordinates of white light due to the saturation of QD emission with increasing current. This excellent light conversion efficiency of 74.7% at 20 mA is even higher than the record value (72%) from the QD-LED based on green-emitting CdSe/multishell QDs with a nearly 100% QY.24 When compared to the reduction of the light conversion efficiency with increasing input current, the luminous efficiency of white QD-LED dropped more rapidly, i.e., from 79.3 lm/W at 5 mA, 63.4 lm/W at 20 mA, and down to 28.1 lm/W at 100 mA (Figure 6c). It is well-known that nitride-based LEDs themselves suffer from a reduction (drop) of the internal quantum efficiency with increasing injection current.33,34 The luminous efficiency of a bare InGaN blue chip used in this work was also measured at the identical operating current range, resulting in a similarly significant reduction from 19.9 lm/W at 5 mA to 7.6 lm/W at 100 mA. This efficiency drop of ca. ∼62% from 5 to 100 mA in the bare blue LED was comparable to that (ca. ∼65%) in a white QD-LED, suggesting that such a decreasing luminous efficiency of white QD-LED at a higher current operation is attributable mainly to the intrinsic efficiency drop of the LED chip, not to the QD saturation.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-320-3039. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0013377) and the IT R&D program of MKE/IITA (2009-F-020-01).

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REFERENCES

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CONCLUSIONS CIS core QDs having Cu/In molar ratios of 1/1, 1/2, and 1/4 were efficiently synthesized by reacting CuI and In(Ac)3 with DDT at 230 °C. As a result of a widened band gap from Cudeficient CIS QDs, a systematic blueshift of emission was observed from QDs with a higher degree of Cu deficiency. The CIS QDs exhibited defect state-related broad band emission with peak wavelength of 665−717 nm and QYs of 8.6−12.7%. Owing to the effective surface passivation by an appropriate ZnS overcoating, the resulting CIS/ZnS core/shell QDs displayed dramatically enhanced QYs of 68−78% and quite tunable emission colors from red (623 nm), orange (598 nm), to yellow (564 nm), depending on the Cu/In ratio used for the CIS core synthesis. All CIS/ZnS QDs were combined as wavelength converters with a blue LED chip. As a result, yellow-emitting CIS/ZnS QDs with a Cu/In ratio of 1/4 were for the first time demonstrated to be suitable blue-to-yellow color converters for the fabrication of white QD-LEDs. EL properties of white QD-LEDs were characterized as a function of forward current from 5 to 100 mA, showing the CIE color coordinates of (0.334−0.348, 0.273−0.291), CRI of 72−75, and CCT of 4447−5380 K. High luminous efficiency of 63.4 lm/W and light conversion efficiency of 74.7% could be obtained from our white QD-LED at a forward current of 20 mA. The luminous efficiency of the white QD-LED kept F

dx.doi.org/10.1021/cm300837z | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

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dx.doi.org/10.1021/cm300837z | Chem. Mater. XXXX, XXX, XXX−XXX