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White lighting device from composite films embedded with hydrophilic Cu(In, Ga)S2/ZnS and hydrophobic InP/ZnS quantum dots
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 225601 (http://iopscience.iop.org/0957-4484/25/22/225601) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 225601 (7pp)
doi:10.1088/0957-4484/25/22/225601
White lighting device from composite films embedded with hydrophilic Cu(In, Ga)S2/ZnS and hydrophobic InP/ZnS quantum dots Jong-Hoon Kim and Heesun Yang Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea E-mail: [email protected] Received 26 November 2013, revised 21 February 2014 Accepted for publication 11 March 2014 Published 8 May 2014 Abstract
Two types of non-Cd quantum dots (QDs)—In/Ga ratio-varied, green-to-greenish-yellow fluorescence-tuned Cu−In−Ga−S (CIGS) alloy ones, and red-emitting InP ones—are synthesized for use as down-converters in conjunction with a blue light-emitting diode (LED). Among a series of Ga-rich CI1−xGxS/ZnS core/shell QDs (x = 0.7, 0.8, and 0.9), CI0.2G0.8S/ZnS QD is chosen for the hydrophobic-to-hydrophilic surface modification via an in-situ ligand exchange and then embedded in a water-soluble polyvinyl alcohol (PVA). This free-standing composite film is utilized as a down-converter for the fabrication of a remote-type white QD-LED, but the resulting bi-colored device exhibits a cool white light with a limited color rendering index property. To improve white light qualities, another QD-polymer film of hydrophobic red InP/ ZnS QD-embedding polyvinylpyrrolidone is sequentially stacked onto the CI0.2G0.8S/ZnS QDPVA film, producing a unique dual color-emitting, flexible and transparent bilayered composite film. Tri-colored white QD-LED integrated with the bilayered QD film possesses an exceptional color rendering property through reinforcing a red spectral component and balancing a white spectral distribution. S Online supplementary data available from stacks.iop.org/NANO/25/225601/mmedia Keywords: white lighting, composite film, non-Cd quantum dots, hydrophilic, hydrophobic (Some figures may appear in colour only in the online journal) 1. Introduction
alternative visible QD candidates. Han et al reported the fabrication of a white QD-LED, consisting of green-, yellow-, and red-emitting InP/ZnS core/shell (C/S) QDs with moderate photoluminescence quantum yields (PL QYs) of 20−42%, with a color rendering index (CRI) of 89 [5]. And highly fluorescent ZnS-shelled CIS and Cu−In−Ga−S (CIGS) QDs, whose emissive properties were somewhat similar to those of the conventional Y3Al5O12:Ce3+ or Sr3SiO5:Ce3+, Li+ bulk phosphor used in white phosphor-converted (pc)-LED with respect to PL QY (100 nm), were developed and applied as down-converters for the fabrication of white QDLEDs with moderate CRI values of 72−78 [6, 7]. The white QD-LED can be simply fabricated by mimicking the white pc-LED technology, i.e., by mechanically blending one or more kinds of QDs with a polymeric
II−VI, III−V, and I−III−VI type semiconductor quantum dots (QDs) that are capable of not only highly emitting visible light, but also efficiently absorbing blue excitation light, have been regarded as promising down-converters in conjunction with a blue light-emitting diode (LED) pumping source for the fabrication of white solid-state lighting devices. To date, a large amount of white QD-LED work has proceeded with II−VI QDs owning to their exceptional fluorescent properties [1−4]. However, the presence of a noxious Cd constituent in such QD compositions is unfortunately inevitable in securing the visible spectrum, and thus would be a crucial obstacle to the future commercialization of white QD-LEDs. Hence, Cdfree InP and Cu−In−S (CuInS2, CIS), representatives of the III−V and I−III−VI groups, respectively, have emerged as 0957-4484/14/225601+07$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
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encapsulant and filling an LED mold with this QD paste. However, this archetypal fabrication approach may be inadequate for QD-LED fabrication. Typically long-chained hydrophobic QD surface ligands are not compatible in achieving a uniform QD dispersion with the commercially available encapsulant (e.g., epoxy or silicone resin), leading to a substantial QD agglomeration. In the resulting QD agglomerates, non-radiative inter-QD energy transfer as well as light scattering would take place efficiently, ultimately reducing the luminous efficacy of QD-LED device [2, 8, 9]. In addition, QD surface ligands prevent a viscous QD paste from being completely hardened even after a thorough curing processing, attributable to the so-called catalyst poisoning effect, i.e., the hindrance of the cross-linking reaction through the complexation with a platinum catalyst [10]. To overcome the above QD surface-related issues in LED packaging, we recently proposed a remote-type white QD-LED loaded with a free-standing CIS QD-polymethylmethacrylate (PMMA) composite film [9]. But this remote-type device possessed cool white lights with mediocre CRI values of 71−72 due to the insufficient white spectral coverage from use of single CIS QDs. Herein, in an attempt to enhance the light qualities of white emission from the above composite film-based, remotetype white QD-LED, we synthesize a series of green-togreenish-yellow emission-tuned CIGS with different In/Ga ratios and red InP QDs, and then prepare an optically transparent QD-polymer composite film containing the above nonCd QDs of CIGS and InP. To minimize the unwanted energy transfer between CIGS and InP QDs that will be highly probable in the case of the QD blend distributed randomly inside a single matrix, the spatial separation of CIGS from InP QDs in the film may be desirable. Thus, a unique dual coloremitting, bilayered composite film, where CIGS and InP QDs are individually embedded into two transparent polymers of polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP), respectively, is for the first time devised and applied as a down-converting material for the realization of a higherquality white lighting device.
successively injected slowly into the above CIGS core growth solution. Subsequently, the shelling reaction proceeded by further raising the temperature to, and maintaining it at, 260 °C for 80 min, and was completed by injecting 4 ml of oleic acid (OA, 90%) and holding for 20 min. For an in-situ hydrophobic-to-hydrophilic modification of QD surface, 6 ml of 3-mercaptopropionic acid (MPA, >99%) was consecutively introduced into the resulting CIGS/ZnS QD growth solution. And then the temperature was lowered to 180 °C and kept at that temperature for 40 min. The surface-modified QDs were precipitated with 10 ml of sodium tetraborate buffer solution (pH = 9.18). These QD precipitates were purified repeatedly (three times) with a solvent combination of buffer solution/ acetone via a conventional re-dissolution/precipitation protocol by centrifugation (100 00 rpm, 10 min) and finally redispersed into distilled deionized (DI) water. It is noted that unmodified hydrophobic QDs were also washed in the same fashion as above with a solvent combination of chloroform/ ethanol and then dispersed into chloroform for the optical characterization. 2.2. Synthesis of InP/ZnS QDs
In a typical preparation of red-emitting InP/ZnS C/S QDs, a mixture of 1.0 mmol of indium chloride (InCl3, 99.999%), 0.7 mmol of zinc oxide (ZnO, 99.999%) and 6 ml of oleylamine (OLA, 70%) was placed under the same degassing/ purging procedures as for CIGS/QDs, and its temperature was quickly elevated to and kept at 290 °C for 5−7 min to obtain complete solute dissolution. Subsequently, its temperature was lowered to 220 °C, and then 0.25 ml of tris(dimethylamino)phosphine (P(N(CH3)2)3, 97%) was swiftly injected and the growth of InP QDs proceeded at that temperature for 4 min. The following ZnS shelling proceeded in a consecutive two-step way as follows: first, 1 ml of DDT (as a sulfur source) was dropwisely introduced into the above hot pregrown InP core QD solution, and an additional injection of 2.5 ml of OA followed. This shelling reaction was maintained at 200 °C for 7 h. It is here noted that despite the absence of Zn source in the shelling solution (i.e., DDT only) the ZnS shell was effectively created, since plenty of pre-existing Zn species in the starting core mixture, which did not participate in core growth, were available for shell formation. Second, as an adjunctive shelling procedure, 1 mmol of Zn stearate dissolved in 5 ml of ODE was further added and reacted at 200 °C for 1−2 h. After cooling, as-prepared InP/ZnS QDs were thoroughly purified by the same route as above and redispersed into chloroform for the optical characterization and the preparation of QD-polymer composite film.
2. Experimental details 2.1. Synthesis and surface modification of CIGS/ZnS QDs
Three Ga-rich CI1−xGxS core QDs with x values of 0.7, 0.8, and 0.9 were prepared and then the identical ZnS shelling condition was applied. For a typical synthesis of CI0.2G0.8S/ ZnS C/S QD, a mixture of 0.125 mmol of Cu (I) iodide (CuI, 99.999%), 0.1 mmol of In acetate (In(Ac)3, 99.99%), 0.4 mmol of Ga acetylacetonate (Ga(acac)3, 99.99%) and 5 ml of 1-dodecanethiol (DDT, 98%) was placed in a 50 ml threeneck round flask, and then degassed and Ar-purged during heating to 120 °C. The temperature of this mixture was raised to and maintained at 250 °C for 3 min 50 s for the core growth. A ready-made ZnS shell stock solution made by dissolving 8 mmol of Zn stearate (10−12% Zn basis), 4 ml of DDT, and 8 ml of 1-octadecene (ODE, 90%) was
2.3. Preparation of QD-polymer composite films and fabrication of white QD-LEDs
To prepare QD-polymer composite films with different QD loading amounts in a polymer matrix, 0.070, 0.094, or 0.117 g of hydrophilic CI0.2G0.8S/ZnS QD was mixed with 0.266 g of PVA (Mw = 300 00−700 00 g) in 7 ml of DI water, corresponding to approximately 20, 26, or 30 wt% CI0.2G0.8S/ZnS 2
Nanotechnology 25 (2014) 225601
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QD-loaded composite film, respectively. A fraction of the resulting homogeneous aqueous blend was poured into an aluminum dish and subsequently dried slowly at 50 °C for 12 h in an oven. Another QD blend solution was prepared by mixing 0.035, 0.044, or 0.052 g of hydrophobic red-emitting InP/ZnS QD with 0.325 g of PVP (Mw = 360 000 g) in 5 ml of chloroform (i.e., approximately 10, 12, or 14 wt% InP/ZnS QD-loaded composite film, respectively). A fraction of the individual InP/ZnS QD-PVP solutions was placed on top of the above solidified CI0.2G0.8S/ZnS QD-PVA film and dried at 50 °C for 4 h, producing a bilayered film of CI0.2G0.8S/ZnS QD-PVA//InP/ZnS QD-PVP. A remote-type QD-LED was fabricated by punching the as-prepared QD-polymer composite film into a 3.5 mm-diameter disk shape, loading it on top of a 50 × 50 mm2 surface mounting device type InGaN-based blue (455 nm)-emitting LED mold and encapsulating it with epoxy resin/hardener (weight ratio of 1) that was cured thermally at 110 °C for 1 h. 2.4. Characterizations
Absorption and transmittance spectra of QDs and QD-polymer composite films, respectively, were recorded by UV-visible absorption spectroscopy (Shimadzu, UV-2450). PL spectra of QDs were collected with a 500 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd, Darsa Pro-5200). PL QY of the QDs was determined by comparing the integrated emission area of a dilute QD solution (e.g., chloroform dispersions for hydrophobic CIGS/ZnS and InP/ZnS QDs and aqueous dispersion for hydrophilic CIGS/ZnS QD) with that of rhodamine 6G (with a known QY of ∼96%) ethanol solution with the same optical density (∼0.05). High resolution-transmission electron microscopy (TEM) work with a JEOL JEM-4010 electron microscope operating at an accelerating voltage of 400 kV was performed on colloidal QDs and QD-polymer composite films cross-sectionally cut by a microtome to obtain the size distribution of QDs and spatial distribution of QDs inside polymer matrix, respectively. The chemical compositions of CIGS and CIGS/ZnS QDs were assessed by a scanning electron microscope operating at 15 kV and equipped with an energy dispersive spectrometer (EDS) (EDAX Inc., Phoenix). Various electroluminescence (EL) data including EL spectrum, luminous efficacy (LE), correlated color temperature (CCT), Commission Internationale de l’Eclairage (CIE) color coordinates, and CRI of the fabricated remote-type QD-LEDs were obtained in an integrating sphere with a diode array rapid analyzer system (PSI Co. Ltd). The blue-to-QD light conversion efficiency (CE) of QD-LED devices was also assessed by calculating the integrated spectral ratio of down-converted QD emission to the difference between the bare blue LED reference emission and the blue emission of QD-LED [6, 7].
Figure 1. (a) Absorption and (b) normalized PL spectra of green-to-
greenish-yellow-emitting CI1−xGxS/ZnS QDs (x = 0.7, 0.8, and 0.9). (c) Absorption and PL spectra of red-emitting InP/ZnS QD. Photographs of the respective fluorescent QD dispersions illuminated under a 365 nm UV lamp are shown in the insets of (b) and (c), respectively.
−Cu−In−S (ZCIS) [11−14] or CIGS alloyed QDs [7, 15−17]. Our QD synthesis in the present work focused on highly fluorescent CI1−xGxS/ZnS QDs with x = 0.7−0.9. These Garich compositions can afford green-to-greenish-yellow emissions (depending on In/Ga ratio), while In-rich CIGS/ZnS QDs emit relatively longer wavelengths. As shown in the UVvisible absorption and normalized PL spectra of 3 CIGS/ZnS QDs (figure 1(a)), with decreasing In/Ga ratio both absorption and emission peaks gradually moved to a higher energy side as a result of the intended formation of Ga-richer core QDs. It is noted that for a better clarity the background levels in absorption spectra of 3 CIGS/ZnS QDs were intentionally adjusted. The band gap energies of 3 CI1−xGxS/ZnS QDs with x = 0.7, 0.8, and 0.9 were indirectly determined due to the indistinct absorption features to be 2.89, 2.93, and 2.97 eV, respectively, through converting the respective absorption spectra in figure 1(a) into (Ahν)2−hν curves (where, A = absorbance, h = Planck’s constant, ν = light frequency) and extrapolating the linear slopes to the energy (hν) axis (see figure S1 of the online supplementary data for details, available at stacks.iop.org/NANO/25/225601/mmedia) [15, 18]. Using CI0.2G0.8S and CI0.2G0.8S/ZnS QD samples an EDSbased chemical composition analysis was conducted, revealing that the actual In/Ga ratio of as-synthesized QDs was very
3. Results and discussion The band gap and consequent emission energies of chalcopyrite ternary CIS QDs can be shifted to the blue by incorporating a Zn or Ga constituent, forming the quaternary Zn 3
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close to the In/Ga precursor ratio used for QD synthesis (figure S2 and table S1). As compared to these band gap values, the emission energies of all CIGS/ZnS QDs were appreciably Stokes-shifted, peaking at 546, 536, and 523 nm for CI1−xGxS/ZnS QDs with x = 0.7, 0.8, and 0.9, respectively (figure 1(b)), since the luminescence of I−III−VI QDs having the compositions of CIS and its derivatives (e.g., CIGS, ZCIS) stems from a non-excitonic, intra-gap defect stateinvolved recombination of band-to-band photo-excited carriers [6, 7, 13, 14]. PL QYs and emission bandwidths of all CIGS/ZnS QDs were in similar ranges of 70−73% and 100 −106 nm, respectively. A red-emitting InP/ZnS QD, which will be applied as a down-converter in conjunction with the CIGS/ZnS one for the fabrication of white QD-LED, was separately synthesized. As presented in the absorption and PL spectra in figure 1(c), InP/ZnS QD possessed a well-defined excitonic absorption peak at 589 nm and slightly Stokesshifted PL emission peak at 617 nm. Such a distinct absorption feature is an indirect indicator of a uniform size distribution of InP core. As a consequence, PL bandwidth of InP/ ZnS QD was as narrow as 60 nm. PL bandwidth is one of the key indices in evaluating the fluorescent QD quality. Taking into account that the conventional red InP QDs synthesized by a commonly used, but highly dangerous, expensive P precursor of tris(trimethylsilyl)phosphine (P(TMS)3) exhibited broad PL bandwidths of approximately 78−100 nm [5, 19 −21], our red QD may be regarded as a high-quality emitter. While InP core exhibited no detectable fluorescence, an appropriate ZnS shelling effectively eliminated the defective states of core surface, giving rise to a dramatically bright emission with a PL QY of 67%. As shown in figure S3, the sizes of CI0.2G0.8S/ZnS and InP/ZnS QDs were found to be distributed in the ranges of 3.2−4.5 and 3.0−3.3 nm, respectively. CI0.2G0.8S/ZnS QD, a representative of a series of CIGS/ ZnS QDs in figure 1, was placed in an in-situ surface treatment for hydrophobic-to-hydrophilic ligand exchange. The choice of CI0.2G0.8S/ZnS QD for the surface modification was made, bearing a high-color rendering, tri-colored white QDLED fabrication in mind. As noticed from figure 1(a), the spectral overlap between high-energy absorption of CI0.1G0.9S/ZnS QD and emission (455 nm) of a blue LED source was not sufficient, leading to an inefficient optical excitation and thus a low fluorescent efficiency of CI0.1G0.9S/ ZnS QD down-converter. Meanwhile, the white spectral coverage as widened as possible is generally desirable to realize a higher-CRI white lighting device. In this aspect, when a red InP/ZnS QD is incorporated as a co-down-converter, a shorter wavelength-emitting CI0.2G0.8S/ZnS QD should be more advantageous than a long wavelength-emitting CI0.3G0.7S/ZnS one, since the former would better fill the green spectral gap existing between blue chip emission and down-converted QD emission. Figure 2 presents the comparison of PL spectra of pristine hydrophobic versus in-situ surface-modified hydrophilic CI0.2G0.8S/ZnS QDs dispersed in chloroform and DI water, respectively, with an identical optical density of ∼0.05. The thiol group of MPA has a relatively strong anchoring capability for the cationic sites on
Figure 2. Comparison of PL spectra of hydrophobic versus
hydrophilic CI0.2G0.8S/ZnS QDs dispersed in chloroform and DI water, respectively, with an identical optical density of ∼0.05 at 420 nm. The inset presents an optically clear aqueous dispersion with hydrophilic QDs under UV irradiation.
QD surface versus the original ligand of OA [22], rendering the QD surface terminated with the carboxyl group. The resulting hydrophilic QD was completely soluble in DI water, as seen in the UV-irradiated fluorescent image of aqueous QD dispersion (inset of figure 2). Compared to the original hydrophobic QD, the hydrophilic one maintained the same PL spectral position, but exhibited a decreased QY of 52%. PL quenching to a certain extent after such a ligand exchange is a general but hardly-avoidable phenomenon, typically attributable to either the changed surface electronic structure or the population of deteriorated surface-related non-radiative trap sites of ligand-exchanged QD [1, 13, 23]. Three transparent composite films with an approximate thickness of ∼150 μm, where hydrophilic CI0.2G0.8S/ZnS QDs are embedded in PVA matrix with different QD loading concentrations of 20, 26, and 30 wt%, were prepared, as shown in figure 3(a). The individual QD composite films were mounted on top of a blue LED mold in a remote fashion and further encapsulated with a thermo-curable epoxy resin (see figure 3(b) for the device schematic). Figure 3(c) shows EL spectra of the above composite-films-loaded white QD-LEDs operated at an input current of 20 mA. Increasing QD loading led to simultaneous decreases in blue and QD emissions, resulting in reduced LE and blue-to-QD emission CE from 48.2 lm W−1 and 37.5% for 20 wt% to 42.5 lm W−1 and 30.9% for 30 wt% (table 1). The peak wavelength of QD emission also tended to be gradually red-shifted with increasing QD loading, i.e., from 539 nm for 20 wt% to 543 nm for 30 wt%. A higher degree of QD loading in the matrix would lead to a greater absorption of blue excitation light, generally leading to the expectation of a concomitant increase in down-converted QD emission. However, our results showed a rather decreased QD emission, presumably attributable to the efficient non-radiative event of Förster resonant energy transfer (FRET). In the case of the composite 4
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Table 1. Primary EL characteristics of LE, CE, CRI, CCT, and CIE color coordinates of white QD-LEDs.
QD loading 20 wt% CI0.2G0.8S 26 wt% CI0.2G0.8S 30 wt% CI0.2G0.8S 20 wt% CI0.2G0.8S/ 10 wt% InP 20 wt% CI0.2G0.8S/ 12 wt% InP 20 wt% CI0.2G0.8S/ 14 wt% InP
LE (lm/ W)
CE (%)
CRI
CCT (K)
CIE (x, y)
48.2
37.5
67
8560
(0.273, 0.343)
45.3
34.2
64
7143
(0.290, 0.379)
42.5
30.9
60
6066
(0.314, 0.426)
35.6
33.5
89
7095
(0.302, 0.333)
34.7
32.7
94
5322
(0.337, 0.356)
33.1
31.4
92
3691
(0.389, 0.367)
directly poured on top of the pre-formed CI0.2G0.8S/ZnS QDPVA solid film. Since PVA matrix is not dissolved by chloroform, two composite films of CI0.2G0.8S/ZnS QD-PVA and InP/ZnS QD-PVP could be sequentially stacked without an interlayer mixing, producing a homogeneous bilayered film. Three bilayered composite films of a total thickness of 200 μm were prepared by forming 50 μm thick InP/ZnS QDPVP films with the varied QD loading concentrations of 10, 12, and 14 wt% onto the same 20 wt% CI0.2G0.8S/ZnS QDPVA film. As shown in UV-visible transmittance spectra (figure 4(a)), all bilayered films attained a decent visible transparency. The transmittance (I) tended to decrease with a higher InP/ZnS QD loading, specifically, from 54% for 10 wt% to 47% for 14 wt% at 550 nm, attributable to an increasing volume fraction (VQD) of InP/ZnS QD inside a fixed volume (or thickness) of PVP matrix according to I ∝ exp(–VQD) [26]. It is also noted that the noticeable flections located at around 560−630 nm are attributable to the absorption of InP/ZnS QDs in the bilayered films. Figure 4(b) presents a photograph of a transparent, flexible, dual coloremitting bilayered film under UV illumination. CI0.2G0.8S/ ZnS and InP/ZnS QDs were also observed by TEM to be uniformly dispersed in the respective polymer matrices of PVA and PVP without a noticeable QD agglomeration (figures 4(c), (d)). The individual bilayered composite films were integrated with their InP/ZnS QD-PVP side facing toward a blue LED chip in the same manner as in figure 3(b). As shown in tricolored EL spectra of white QD-LEDs in operation at 20 mA (figure 5(a)), with increasing InP/ZnS QD loading, the downconverted InP/ZnS QD emission monotonically increased due to a higher extent of blue light absorption, naturally accompanying the diminished CI0.2G0.8S/ZnS QD and blue
Figure 3. (a) Photograph of hydrophilic CI0.2G0.8S/ZnS QDembedding PVA composite films (with the same thickness of 150 μm) with different QD loading concentrations of 20, 26, and 30 wt%. (b) Schematic illustration of a remote-type QD-LED. (c) EL spectra of white QD-LEDs loaded with the composite films in (a) in operation at 20 mA.
film with an excessive QD loading and consequently a very close inter-QD distance, non-radiative FRET should be highly efficient, considering that the general QD systems have a critical ET distance of only ∼5 nm and the FRET efficiency is proportional to ∼1/R6 (R = inter-QD distance) [24, 25]. While 26 and 30 wt% QD loading produced white light with low CRI values of 60−64 due to the spectral imbalance between blue and QD emissions, 20 wt% QD loading afforded a slightly better white spectral distribution but a still insufficient CRI of 67. This limited color rendering property can be ascribed to the spectral deficiencies which are inherent in bicolored white LEDs. In terms of CCT, the white light tended to become warmer from 8560 for 20 wt% to 6066 K 30 wt% owing to the decreasing contribution of the blue component in the overall white spectrum. The simple addition of a red spectral component InP/ZnS QD to the above bi-colored white QD-LED can substantially improve the white light qualities, i.e., a higher CRI and lower CCT. The hydrophobic InP/ZnS QD was well miscible in chloroform with PVP and the resulting QD-PVP solution was 5
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Figure 4. (a) UV-visible transmittance spectra of 200 μm thick
bilayered composite films, where 10, 12, and 14 wt% InP/ZnS QDloaded PVP films are formed onto the same 20 wt% CI0.2G0.8S/ZnS QD-PVA film. (b) Photograph of a transparent, flexible, dual coloremitting bilayered film consisting of 12 wt% InP/ZnS QD-PVP and 20 wt% CI0.2G0.8S/ZnS QD-PVA. TEM images showing the distributions of (c) CI0.2G0.8S/ZnS and (d) InP/ZnS QDs embedded in PVA and PVP, respectively. Figure 5. (a) Tri-colored EL spectra and EL images (insets) and (b)
CIE color coordinates of white QD-LEDs loaded with the bilayered films in figure 4(a) in operation at 20 mA. (c) Forward currentdependent EL spectral evolution of a white QD-LED loaded with a bilayered film consisting of 12 wt% InP/ZnS QD-PVP and 20 wt% CI0.2G0.8S/ZnS QD-PVA.
emissions at the same time (see the insets of figure 5(a) for EL images for the respective white QD-LEDs). Accordingly, this incremental red spectral contribution shifted CIE color coordinates more to the red side, specifically, from (0.302, 0.333) for 10 wt% to (0.389, 0.367) for the 14 wt% InP/ZnS QDloaded case (figure 5(b)), adjunctively resulting in the evolution to warmer (i.e., lower color temperature) white emissions from 7095 to 3691 K, respectively (table 1). Compared to the previous bi-colored white LED loaded with 20 wt% CI0.2G0.8S/ZnS QD-PVA film alone, the tri-colored, bilayered film-loaded ones exhibited dramatically enhanced CRI values of 89−94 owning to a more complete white spectral coverage enabled by the red QD down-converter. The CEs of bilayered film-loaded devices decreased to 31.4−33.5%, depending on InP/ZnS QD loading, from the 37.5% of the CI0.2G0.8S/ZnS QD-based monolayered film-loaded one. Luminous flux (lm), a function of the relative sensitivity of human vision to a particular spectrum, exhibits a lower value in red versus the green−yellow region. Hence, as a result of the combined factors of the reductions in both CE and luminous flux, bilayered film-loaded QD-LEDs possessed lower LEs of 33.1 −35.6 lm W−1 compared to that (48.2 lm−1W) of their monolayered film-loaded counterpart (table 1). The 12 wt% InP/ ZnS QD-loaded device having the highest CRI (94) was further operated under various forward currents of 20−80 mA. As seen from the EL spectral evolution of figure 5(c), the spectral intensities of tri-color components increased in an almost monotonic fashion with increasing forward current,
indicative of a high spectral stability against current variation. Thus, CIE coordinates, CRI, and CCT were only varied in the narrow ranges of (0.334−0.337, 0.344−0.356), 94−95, and 5322−5439 K, respectively.
4. Conclusions Highly fluorescent non-Cd QDs of green-to-greenish-yellow CIGS/ZnS and red InP/ZnS QDs with PL QYs of 70−73% and 67%, respectively, were synthesized. A 536 nm-emitting CI0.2G0.8S/ZnS QD and three Ga-rich CIGS/ZnS ones were consecutively surface-modified via in-situ hydrophobic-tohydrophilic ligand exchange and then successfully embedded in a water-soluble PVA, generating 150 μm thick free-standing composite films with varied QD loading concentrations of 20−30 wt%. Remote-type white QD-LEDs combined with CI0.2G0.8S/ZnS QD-PVA films exhibited LEs of 42.5−48.2 lm W−1, CRIs of 60−67, and CCTs of 6066−8560 K, depending on QD loading concentration in the matrix. By sequentially stacking an additional composite film, consisting of hydrophobic red InP/ZnS QD-embedding PVP, on 20 wt% 6
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J-H Kim and H Yang
CI0.2G0.8S/ZnS QD-PVA film, a 200 μm thick dual coloremitting bilayered film with a high visible transmittance (47 −54% at 550 nm) was prepared. Three bilayered composite films with different InP/ZnS QD loading concentrations of 10, 12, and 14 wt% were applied for the fabrication of tri-colored white QD-LEDs. Compared to the devices with a single color CI0.2G0.8S/ZnS QD emitter, dual color-emitting bilayered films-integrated ones showed decreased LEs of 33.1−35.6 lm W−1, but dramatically enhanced CRIs of 89−94 and lower CCTs of 3691−7095 K, depending on InP/ZnS QD loading concentration.
[8] Woo H, Lim J, Lee Y, Sung J, Shin H, Oh J M, Choi M, Yoon H, Bae W K and Char K 2013 J. Mater. Chem. C 1 1983 [9] Jang E P, Song W S, Lee K H and Yang H 2013 Nanotechnology 24 045607 [10] Weaver J, Zakeri R, Aouadi S and Kohli P 2009 J. Mater. Chem. 19 3198 [11] Zhang J, Xie R G and Yang W S 2011 Chem. Mater. 23 3357 [12] De Trizio L, Prato M, Genovese A, Casu A, Povia M, Simonutti R, Alcocer M J P, D’Andrea C, Tassone F and Manna L 2012 Chem. Mater. 24 2400 [13] Zhang W J and Zhong X H 2011 Inorg. Chem. 50 4065 [14] Chen B K, Zhong H Z, Zhang W Q, Tan Z A, Li Y F, Yu C R, Zhai T Y, Bando Y S, Yang S Y and Zou B S 2012 Adv. Funct. Mater. 22 2081 [15] Wang Y H A, Zhang X Y, Bao N Z, Lin B P and Gupta A 2011 J. Am. Chem. Soc. 133 11072 [16] Pan D C, Wang X L, Zhou Z H, Chen W, Xu C L and Lu Y F 2009 Chem. Mater. 21 2489 [17] Sun C V, Gardner J S, Long G, Bajracharya C, Thurber A, Punnoose A, Rodriguez R G and Pak J J 2010 Chem. Mater. 22 2699 [18] Zhong H Z, Lo S S, Mirkovic T, Li Y C, Ding Y Q, Li Y F and Scholes G D 2010 ACS Nano 4 5253 [19] Kim S, Kim T, Kang M, Kwak S K, Yoo T W, Park L S, Yang I, Hwang S, Lee J E, Kim S K and Kim S W 2012 J. Am. Chem. Soc. 134 3804 [20] Yang X Y, Zhao D W, Leck K S, Tan S T, Tang Y X, Zhao J L, Demir H V and Sun X W 2012 Adv. Mater. 24 4180 [21] Kim T, Kim S W, Kang M and Kim S W 2012 J. Phys. Chem. Lett. 3 214 [22] Xie R G, Rutherford M and Peng X G 2009 J. Am. Chem. Soc. 131 5691 [23] Luan W L, Yang H W, Wan Z, Yuan B X, Yu X H and Tu S T 2012 J. Nanopart. Res. 14 762 [24] Mutlugun E et al 2012 Nano Lett. 12 3986 [25] Rogach A L, Klar T A, Lupton J M, Meijerink A and Feldmann J 2009 J. Mater. Chem. 19 1208 [26] Zou W, Du Z J, Li H Q and Zhang C 2011 J. Mater. Chem. 21 13276
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068158 and 2011-0013377) and by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000182).
References [1] Jang E, Jun S, Jang H, Llim J, Kim B and Kim Y 2010 Adv. Mater. 22 3076 [2] Jun S, Lee J and Jang E 2013 ACS Nano 7 1472 [3] Nizamoglu S, Zengin G and Demir H V 2008 Appl. Phys. Lett. 92 031102 [4] Wang X B, Li W W and Sun K 2011 J. Mater. Chem. 21 8558 [5] Kim K, Jeong S, Woo J Y and Han C S 2012 Nanotechnology 23 065602 [6] Song W S and Yang H 2012 Chem. Mater. 24 1961 [7] Song W S, Kim J H, Lee J H, Lee H S, Do Y R and Yang H 2012 J. Mater. Chem. 22 21901
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White lighting device from composite films embedded with hydrophilic Cu(In, Ga)S2/ZnS and hydrophobic InP/ZnS quantum dots
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 225601 (http://iopscience.iop.org/0957-4484/25/22/225601) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 225601 (7pp)
doi:10.1088/0957-4484/25/22/225601
White lighting device from composite films embedded with hydrophilic Cu(In, Ga)S2/ZnS and hydrophobic InP/ZnS quantum dots Jong-Hoon Kim and Heesun Yang Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea E-mail: [email protected] Received 26 November 2013, revised 21 February 2014 Accepted for publication 11 March 2014 Published 8 May 2014 Abstract
Two types of non-Cd quantum dots (QDs)—In/Ga ratio-varied, green-to-greenish-yellow fluorescence-tuned Cu−In−Ga−S (CIGS) alloy ones, and red-emitting InP ones—are synthesized for use as down-converters in conjunction with a blue light-emitting diode (LED). Among a series of Ga-rich CI1−xGxS/ZnS core/shell QDs (x = 0.7, 0.8, and 0.9), CI0.2G0.8S/ZnS QD is chosen for the hydrophobic-to-hydrophilic surface modification via an in-situ ligand exchange and then embedded in a water-soluble polyvinyl alcohol (PVA). This free-standing composite film is utilized as a down-converter for the fabrication of a remote-type white QD-LED, but the resulting bi-colored device exhibits a cool white light with a limited color rendering index property. To improve white light qualities, another QD-polymer film of hydrophobic red InP/ ZnS QD-embedding polyvinylpyrrolidone is sequentially stacked onto the CI0.2G0.8S/ZnS QDPVA film, producing a unique dual color-emitting, flexible and transparent bilayered composite film. Tri-colored white QD-LED integrated with the bilayered QD film possesses an exceptional color rendering property through reinforcing a red spectral component and balancing a white spectral distribution. S Online supplementary data available from stacks.iop.org/NANO/25/225601/mmedia Keywords: white lighting, composite film, non-Cd quantum dots, hydrophilic, hydrophobic (Some figures may appear in colour only in the online journal) 1. Introduction
alternative visible QD candidates. Han et al reported the fabrication of a white QD-LED, consisting of green-, yellow-, and red-emitting InP/ZnS core/shell (C/S) QDs with moderate photoluminescence quantum yields (PL QYs) of 20−42%, with a color rendering index (CRI) of 89 [5]. And highly fluorescent ZnS-shelled CIS and Cu−In−Ga−S (CIGS) QDs, whose emissive properties were somewhat similar to those of the conventional Y3Al5O12:Ce3+ or Sr3SiO5:Ce3+, Li+ bulk phosphor used in white phosphor-converted (pc)-LED with respect to PL QY (100 nm), were developed and applied as down-converters for the fabrication of white QDLEDs with moderate CRI values of 72−78 [6, 7]. The white QD-LED can be simply fabricated by mimicking the white pc-LED technology, i.e., by mechanically blending one or more kinds of QDs with a polymeric
II−VI, III−V, and I−III−VI type semiconductor quantum dots (QDs) that are capable of not only highly emitting visible light, but also efficiently absorbing blue excitation light, have been regarded as promising down-converters in conjunction with a blue light-emitting diode (LED) pumping source for the fabrication of white solid-state lighting devices. To date, a large amount of white QD-LED work has proceeded with II−VI QDs owning to their exceptional fluorescent properties [1−4]. However, the presence of a noxious Cd constituent in such QD compositions is unfortunately inevitable in securing the visible spectrum, and thus would be a crucial obstacle to the future commercialization of white QD-LEDs. Hence, Cdfree InP and Cu−In−S (CuInS2, CIS), representatives of the III−V and I−III−VI groups, respectively, have emerged as 0957-4484/14/225601+07$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 225601
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encapsulant and filling an LED mold with this QD paste. However, this archetypal fabrication approach may be inadequate for QD-LED fabrication. Typically long-chained hydrophobic QD surface ligands are not compatible in achieving a uniform QD dispersion with the commercially available encapsulant (e.g., epoxy or silicone resin), leading to a substantial QD agglomeration. In the resulting QD agglomerates, non-radiative inter-QD energy transfer as well as light scattering would take place efficiently, ultimately reducing the luminous efficacy of QD-LED device [2, 8, 9]. In addition, QD surface ligands prevent a viscous QD paste from being completely hardened even after a thorough curing processing, attributable to the so-called catalyst poisoning effect, i.e., the hindrance of the cross-linking reaction through the complexation with a platinum catalyst [10]. To overcome the above QD surface-related issues in LED packaging, we recently proposed a remote-type white QD-LED loaded with a free-standing CIS QD-polymethylmethacrylate (PMMA) composite film [9]. But this remote-type device possessed cool white lights with mediocre CRI values of 71−72 due to the insufficient white spectral coverage from use of single CIS QDs. Herein, in an attempt to enhance the light qualities of white emission from the above composite film-based, remotetype white QD-LED, we synthesize a series of green-togreenish-yellow emission-tuned CIGS with different In/Ga ratios and red InP QDs, and then prepare an optically transparent QD-polymer composite film containing the above nonCd QDs of CIGS and InP. To minimize the unwanted energy transfer between CIGS and InP QDs that will be highly probable in the case of the QD blend distributed randomly inside a single matrix, the spatial separation of CIGS from InP QDs in the film may be desirable. Thus, a unique dual coloremitting, bilayered composite film, where CIGS and InP QDs are individually embedded into two transparent polymers of polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP), respectively, is for the first time devised and applied as a down-converting material for the realization of a higherquality white lighting device.
successively injected slowly into the above CIGS core growth solution. Subsequently, the shelling reaction proceeded by further raising the temperature to, and maintaining it at, 260 °C for 80 min, and was completed by injecting 4 ml of oleic acid (OA, 90%) and holding for 20 min. For an in-situ hydrophobic-to-hydrophilic modification of QD surface, 6 ml of 3-mercaptopropionic acid (MPA, >99%) was consecutively introduced into the resulting CIGS/ZnS QD growth solution. And then the temperature was lowered to 180 °C and kept at that temperature for 40 min. The surface-modified QDs were precipitated with 10 ml of sodium tetraborate buffer solution (pH = 9.18). These QD precipitates were purified repeatedly (three times) with a solvent combination of buffer solution/ acetone via a conventional re-dissolution/precipitation protocol by centrifugation (100 00 rpm, 10 min) and finally redispersed into distilled deionized (DI) water. It is noted that unmodified hydrophobic QDs were also washed in the same fashion as above with a solvent combination of chloroform/ ethanol and then dispersed into chloroform for the optical characterization. 2.2. Synthesis of InP/ZnS QDs
In a typical preparation of red-emitting InP/ZnS C/S QDs, a mixture of 1.0 mmol of indium chloride (InCl3, 99.999%), 0.7 mmol of zinc oxide (ZnO, 99.999%) and 6 ml of oleylamine (OLA, 70%) was placed under the same degassing/ purging procedures as for CIGS/QDs, and its temperature was quickly elevated to and kept at 290 °C for 5−7 min to obtain complete solute dissolution. Subsequently, its temperature was lowered to 220 °C, and then 0.25 ml of tris(dimethylamino)phosphine (P(N(CH3)2)3, 97%) was swiftly injected and the growth of InP QDs proceeded at that temperature for 4 min. The following ZnS shelling proceeded in a consecutive two-step way as follows: first, 1 ml of DDT (as a sulfur source) was dropwisely introduced into the above hot pregrown InP core QD solution, and an additional injection of 2.5 ml of OA followed. This shelling reaction was maintained at 200 °C for 7 h. It is here noted that despite the absence of Zn source in the shelling solution (i.e., DDT only) the ZnS shell was effectively created, since plenty of pre-existing Zn species in the starting core mixture, which did not participate in core growth, were available for shell formation. Second, as an adjunctive shelling procedure, 1 mmol of Zn stearate dissolved in 5 ml of ODE was further added and reacted at 200 °C for 1−2 h. After cooling, as-prepared InP/ZnS QDs were thoroughly purified by the same route as above and redispersed into chloroform for the optical characterization and the preparation of QD-polymer composite film.
2. Experimental details 2.1. Synthesis and surface modification of CIGS/ZnS QDs
Three Ga-rich CI1−xGxS core QDs with x values of 0.7, 0.8, and 0.9 were prepared and then the identical ZnS shelling condition was applied. For a typical synthesis of CI0.2G0.8S/ ZnS C/S QD, a mixture of 0.125 mmol of Cu (I) iodide (CuI, 99.999%), 0.1 mmol of In acetate (In(Ac)3, 99.99%), 0.4 mmol of Ga acetylacetonate (Ga(acac)3, 99.99%) and 5 ml of 1-dodecanethiol (DDT, 98%) was placed in a 50 ml threeneck round flask, and then degassed and Ar-purged during heating to 120 °C. The temperature of this mixture was raised to and maintained at 250 °C for 3 min 50 s for the core growth. A ready-made ZnS shell stock solution made by dissolving 8 mmol of Zn stearate (10−12% Zn basis), 4 ml of DDT, and 8 ml of 1-octadecene (ODE, 90%) was
2.3. Preparation of QD-polymer composite films and fabrication of white QD-LEDs
To prepare QD-polymer composite films with different QD loading amounts in a polymer matrix, 0.070, 0.094, or 0.117 g of hydrophilic CI0.2G0.8S/ZnS QD was mixed with 0.266 g of PVA (Mw = 300 00−700 00 g) in 7 ml of DI water, corresponding to approximately 20, 26, or 30 wt% CI0.2G0.8S/ZnS 2
Nanotechnology 25 (2014) 225601
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QD-loaded composite film, respectively. A fraction of the resulting homogeneous aqueous blend was poured into an aluminum dish and subsequently dried slowly at 50 °C for 12 h in an oven. Another QD blend solution was prepared by mixing 0.035, 0.044, or 0.052 g of hydrophobic red-emitting InP/ZnS QD with 0.325 g of PVP (Mw = 360 000 g) in 5 ml of chloroform (i.e., approximately 10, 12, or 14 wt% InP/ZnS QD-loaded composite film, respectively). A fraction of the individual InP/ZnS QD-PVP solutions was placed on top of the above solidified CI0.2G0.8S/ZnS QD-PVA film and dried at 50 °C for 4 h, producing a bilayered film of CI0.2G0.8S/ZnS QD-PVA//InP/ZnS QD-PVP. A remote-type QD-LED was fabricated by punching the as-prepared QD-polymer composite film into a 3.5 mm-diameter disk shape, loading it on top of a 50 × 50 mm2 surface mounting device type InGaN-based blue (455 nm)-emitting LED mold and encapsulating it with epoxy resin/hardener (weight ratio of 1) that was cured thermally at 110 °C for 1 h. 2.4. Characterizations
Absorption and transmittance spectra of QDs and QD-polymer composite films, respectively, were recorded by UV-visible absorption spectroscopy (Shimadzu, UV-2450). PL spectra of QDs were collected with a 500 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd, Darsa Pro-5200). PL QY of the QDs was determined by comparing the integrated emission area of a dilute QD solution (e.g., chloroform dispersions for hydrophobic CIGS/ZnS and InP/ZnS QDs and aqueous dispersion for hydrophilic CIGS/ZnS QD) with that of rhodamine 6G (with a known QY of ∼96%) ethanol solution with the same optical density (∼0.05). High resolution-transmission electron microscopy (TEM) work with a JEOL JEM-4010 electron microscope operating at an accelerating voltage of 400 kV was performed on colloidal QDs and QD-polymer composite films cross-sectionally cut by a microtome to obtain the size distribution of QDs and spatial distribution of QDs inside polymer matrix, respectively. The chemical compositions of CIGS and CIGS/ZnS QDs were assessed by a scanning electron microscope operating at 15 kV and equipped with an energy dispersive spectrometer (EDS) (EDAX Inc., Phoenix). Various electroluminescence (EL) data including EL spectrum, luminous efficacy (LE), correlated color temperature (CCT), Commission Internationale de l’Eclairage (CIE) color coordinates, and CRI of the fabricated remote-type QD-LEDs were obtained in an integrating sphere with a diode array rapid analyzer system (PSI Co. Ltd). The blue-to-QD light conversion efficiency (CE) of QD-LED devices was also assessed by calculating the integrated spectral ratio of down-converted QD emission to the difference between the bare blue LED reference emission and the blue emission of QD-LED [6, 7].
Figure 1. (a) Absorption and (b) normalized PL spectra of green-to-
greenish-yellow-emitting CI1−xGxS/ZnS QDs (x = 0.7, 0.8, and 0.9). (c) Absorption and PL spectra of red-emitting InP/ZnS QD. Photographs of the respective fluorescent QD dispersions illuminated under a 365 nm UV lamp are shown in the insets of (b) and (c), respectively.
−Cu−In−S (ZCIS) [11−14] or CIGS alloyed QDs [7, 15−17]. Our QD synthesis in the present work focused on highly fluorescent CI1−xGxS/ZnS QDs with x = 0.7−0.9. These Garich compositions can afford green-to-greenish-yellow emissions (depending on In/Ga ratio), while In-rich CIGS/ZnS QDs emit relatively longer wavelengths. As shown in the UVvisible absorption and normalized PL spectra of 3 CIGS/ZnS QDs (figure 1(a)), with decreasing In/Ga ratio both absorption and emission peaks gradually moved to a higher energy side as a result of the intended formation of Ga-richer core QDs. It is noted that for a better clarity the background levels in absorption spectra of 3 CIGS/ZnS QDs were intentionally adjusted. The band gap energies of 3 CI1−xGxS/ZnS QDs with x = 0.7, 0.8, and 0.9 were indirectly determined due to the indistinct absorption features to be 2.89, 2.93, and 2.97 eV, respectively, through converting the respective absorption spectra in figure 1(a) into (Ahν)2−hν curves (where, A = absorbance, h = Planck’s constant, ν = light frequency) and extrapolating the linear slopes to the energy (hν) axis (see figure S1 of the online supplementary data for details, available at stacks.iop.org/NANO/25/225601/mmedia) [15, 18]. Using CI0.2G0.8S and CI0.2G0.8S/ZnS QD samples an EDSbased chemical composition analysis was conducted, revealing that the actual In/Ga ratio of as-synthesized QDs was very
3. Results and discussion The band gap and consequent emission energies of chalcopyrite ternary CIS QDs can be shifted to the blue by incorporating a Zn or Ga constituent, forming the quaternary Zn 3
Nanotechnology 25 (2014) 225601
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close to the In/Ga precursor ratio used for QD synthesis (figure S2 and table S1). As compared to these band gap values, the emission energies of all CIGS/ZnS QDs were appreciably Stokes-shifted, peaking at 546, 536, and 523 nm for CI1−xGxS/ZnS QDs with x = 0.7, 0.8, and 0.9, respectively (figure 1(b)), since the luminescence of I−III−VI QDs having the compositions of CIS and its derivatives (e.g., CIGS, ZCIS) stems from a non-excitonic, intra-gap defect stateinvolved recombination of band-to-band photo-excited carriers [6, 7, 13, 14]. PL QYs and emission bandwidths of all CIGS/ZnS QDs were in similar ranges of 70−73% and 100 −106 nm, respectively. A red-emitting InP/ZnS QD, which will be applied as a down-converter in conjunction with the CIGS/ZnS one for the fabrication of white QD-LED, was separately synthesized. As presented in the absorption and PL spectra in figure 1(c), InP/ZnS QD possessed a well-defined excitonic absorption peak at 589 nm and slightly Stokesshifted PL emission peak at 617 nm. Such a distinct absorption feature is an indirect indicator of a uniform size distribution of InP core. As a consequence, PL bandwidth of InP/ ZnS QD was as narrow as 60 nm. PL bandwidth is one of the key indices in evaluating the fluorescent QD quality. Taking into account that the conventional red InP QDs synthesized by a commonly used, but highly dangerous, expensive P precursor of tris(trimethylsilyl)phosphine (P(TMS)3) exhibited broad PL bandwidths of approximately 78−100 nm [5, 19 −21], our red QD may be regarded as a high-quality emitter. While InP core exhibited no detectable fluorescence, an appropriate ZnS shelling effectively eliminated the defective states of core surface, giving rise to a dramatically bright emission with a PL QY of 67%. As shown in figure S3, the sizes of CI0.2G0.8S/ZnS and InP/ZnS QDs were found to be distributed in the ranges of 3.2−4.5 and 3.0−3.3 nm, respectively. CI0.2G0.8S/ZnS QD, a representative of a series of CIGS/ ZnS QDs in figure 1, was placed in an in-situ surface treatment for hydrophobic-to-hydrophilic ligand exchange. The choice of CI0.2G0.8S/ZnS QD for the surface modification was made, bearing a high-color rendering, tri-colored white QDLED fabrication in mind. As noticed from figure 1(a), the spectral overlap between high-energy absorption of CI0.1G0.9S/ZnS QD and emission (455 nm) of a blue LED source was not sufficient, leading to an inefficient optical excitation and thus a low fluorescent efficiency of CI0.1G0.9S/ ZnS QD down-converter. Meanwhile, the white spectral coverage as widened as possible is generally desirable to realize a higher-CRI white lighting device. In this aspect, when a red InP/ZnS QD is incorporated as a co-down-converter, a shorter wavelength-emitting CI0.2G0.8S/ZnS QD should be more advantageous than a long wavelength-emitting CI0.3G0.7S/ZnS one, since the former would better fill the green spectral gap existing between blue chip emission and down-converted QD emission. Figure 2 presents the comparison of PL spectra of pristine hydrophobic versus in-situ surface-modified hydrophilic CI0.2G0.8S/ZnS QDs dispersed in chloroform and DI water, respectively, with an identical optical density of ∼0.05. The thiol group of MPA has a relatively strong anchoring capability for the cationic sites on
Figure 2. Comparison of PL spectra of hydrophobic versus
hydrophilic CI0.2G0.8S/ZnS QDs dispersed in chloroform and DI water, respectively, with an identical optical density of ∼0.05 at 420 nm. The inset presents an optically clear aqueous dispersion with hydrophilic QDs under UV irradiation.
QD surface versus the original ligand of OA [22], rendering the QD surface terminated with the carboxyl group. The resulting hydrophilic QD was completely soluble in DI water, as seen in the UV-irradiated fluorescent image of aqueous QD dispersion (inset of figure 2). Compared to the original hydrophobic QD, the hydrophilic one maintained the same PL spectral position, but exhibited a decreased QY of 52%. PL quenching to a certain extent after such a ligand exchange is a general but hardly-avoidable phenomenon, typically attributable to either the changed surface electronic structure or the population of deteriorated surface-related non-radiative trap sites of ligand-exchanged QD [1, 13, 23]. Three transparent composite films with an approximate thickness of ∼150 μm, where hydrophilic CI0.2G0.8S/ZnS QDs are embedded in PVA matrix with different QD loading concentrations of 20, 26, and 30 wt%, were prepared, as shown in figure 3(a). The individual QD composite films were mounted on top of a blue LED mold in a remote fashion and further encapsulated with a thermo-curable epoxy resin (see figure 3(b) for the device schematic). Figure 3(c) shows EL spectra of the above composite-films-loaded white QD-LEDs operated at an input current of 20 mA. Increasing QD loading led to simultaneous decreases in blue and QD emissions, resulting in reduced LE and blue-to-QD emission CE from 48.2 lm W−1 and 37.5% for 20 wt% to 42.5 lm W−1 and 30.9% for 30 wt% (table 1). The peak wavelength of QD emission also tended to be gradually red-shifted with increasing QD loading, i.e., from 539 nm for 20 wt% to 543 nm for 30 wt%. A higher degree of QD loading in the matrix would lead to a greater absorption of blue excitation light, generally leading to the expectation of a concomitant increase in down-converted QD emission. However, our results showed a rather decreased QD emission, presumably attributable to the efficient non-radiative event of Förster resonant energy transfer (FRET). In the case of the composite 4
Nanotechnology 25 (2014) 225601
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Table 1. Primary EL characteristics of LE, CE, CRI, CCT, and CIE color coordinates of white QD-LEDs.
QD loading 20 wt% CI0.2G0.8S 26 wt% CI0.2G0.8S 30 wt% CI0.2G0.8S 20 wt% CI0.2G0.8S/ 10 wt% InP 20 wt% CI0.2G0.8S/ 12 wt% InP 20 wt% CI0.2G0.8S/ 14 wt% InP
LE (lm/ W)
CE (%)
CRI
CCT (K)
CIE (x, y)
48.2
37.5
67
8560
(0.273, 0.343)
45.3
34.2
64
7143
(0.290, 0.379)
42.5
30.9
60
6066
(0.314, 0.426)
35.6
33.5
89
7095
(0.302, 0.333)
34.7
32.7
94
5322
(0.337, 0.356)
33.1
31.4
92
3691
(0.389, 0.367)
directly poured on top of the pre-formed CI0.2G0.8S/ZnS QDPVA solid film. Since PVA matrix is not dissolved by chloroform, two composite films of CI0.2G0.8S/ZnS QD-PVA and InP/ZnS QD-PVP could be sequentially stacked without an interlayer mixing, producing a homogeneous bilayered film. Three bilayered composite films of a total thickness of 200 μm were prepared by forming 50 μm thick InP/ZnS QDPVP films with the varied QD loading concentrations of 10, 12, and 14 wt% onto the same 20 wt% CI0.2G0.8S/ZnS QDPVA film. As shown in UV-visible transmittance spectra (figure 4(a)), all bilayered films attained a decent visible transparency. The transmittance (I) tended to decrease with a higher InP/ZnS QD loading, specifically, from 54% for 10 wt% to 47% for 14 wt% at 550 nm, attributable to an increasing volume fraction (VQD) of InP/ZnS QD inside a fixed volume (or thickness) of PVP matrix according to I ∝ exp(–VQD) [26]. It is also noted that the noticeable flections located at around 560−630 nm are attributable to the absorption of InP/ZnS QDs in the bilayered films. Figure 4(b) presents a photograph of a transparent, flexible, dual coloremitting bilayered film under UV illumination. CI0.2G0.8S/ ZnS and InP/ZnS QDs were also observed by TEM to be uniformly dispersed in the respective polymer matrices of PVA and PVP without a noticeable QD agglomeration (figures 4(c), (d)). The individual bilayered composite films were integrated with their InP/ZnS QD-PVP side facing toward a blue LED chip in the same manner as in figure 3(b). As shown in tricolored EL spectra of white QD-LEDs in operation at 20 mA (figure 5(a)), with increasing InP/ZnS QD loading, the downconverted InP/ZnS QD emission monotonically increased due to a higher extent of blue light absorption, naturally accompanying the diminished CI0.2G0.8S/ZnS QD and blue
Figure 3. (a) Photograph of hydrophilic CI0.2G0.8S/ZnS QDembedding PVA composite films (with the same thickness of 150 μm) with different QD loading concentrations of 20, 26, and 30 wt%. (b) Schematic illustration of a remote-type QD-LED. (c) EL spectra of white QD-LEDs loaded with the composite films in (a) in operation at 20 mA.
film with an excessive QD loading and consequently a very close inter-QD distance, non-radiative FRET should be highly efficient, considering that the general QD systems have a critical ET distance of only ∼5 nm and the FRET efficiency is proportional to ∼1/R6 (R = inter-QD distance) [24, 25]. While 26 and 30 wt% QD loading produced white light with low CRI values of 60−64 due to the spectral imbalance between blue and QD emissions, 20 wt% QD loading afforded a slightly better white spectral distribution but a still insufficient CRI of 67. This limited color rendering property can be ascribed to the spectral deficiencies which are inherent in bicolored white LEDs. In terms of CCT, the white light tended to become warmer from 8560 for 20 wt% to 6066 K 30 wt% owing to the decreasing contribution of the blue component in the overall white spectrum. The simple addition of a red spectral component InP/ZnS QD to the above bi-colored white QD-LED can substantially improve the white light qualities, i.e., a higher CRI and lower CCT. The hydrophobic InP/ZnS QD was well miscible in chloroform with PVP and the resulting QD-PVP solution was 5
Nanotechnology 25 (2014) 225601
J-H Kim and H Yang
Figure 4. (a) UV-visible transmittance spectra of 200 μm thick
bilayered composite films, where 10, 12, and 14 wt% InP/ZnS QDloaded PVP films are formed onto the same 20 wt% CI0.2G0.8S/ZnS QD-PVA film. (b) Photograph of a transparent, flexible, dual coloremitting bilayered film consisting of 12 wt% InP/ZnS QD-PVP and 20 wt% CI0.2G0.8S/ZnS QD-PVA. TEM images showing the distributions of (c) CI0.2G0.8S/ZnS and (d) InP/ZnS QDs embedded in PVA and PVP, respectively. Figure 5. (a) Tri-colored EL spectra and EL images (insets) and (b)
CIE color coordinates of white QD-LEDs loaded with the bilayered films in figure 4(a) in operation at 20 mA. (c) Forward currentdependent EL spectral evolution of a white QD-LED loaded with a bilayered film consisting of 12 wt% InP/ZnS QD-PVP and 20 wt% CI0.2G0.8S/ZnS QD-PVA.
emissions at the same time (see the insets of figure 5(a) for EL images for the respective white QD-LEDs). Accordingly, this incremental red spectral contribution shifted CIE color coordinates more to the red side, specifically, from (0.302, 0.333) for 10 wt% to (0.389, 0.367) for the 14 wt% InP/ZnS QDloaded case (figure 5(b)), adjunctively resulting in the evolution to warmer (i.e., lower color temperature) white emissions from 7095 to 3691 K, respectively (table 1). Compared to the previous bi-colored white LED loaded with 20 wt% CI0.2G0.8S/ZnS QD-PVA film alone, the tri-colored, bilayered film-loaded ones exhibited dramatically enhanced CRI values of 89−94 owning to a more complete white spectral coverage enabled by the red QD down-converter. The CEs of bilayered film-loaded devices decreased to 31.4−33.5%, depending on InP/ZnS QD loading, from the 37.5% of the CI0.2G0.8S/ZnS QD-based monolayered film-loaded one. Luminous flux (lm), a function of the relative sensitivity of human vision to a particular spectrum, exhibits a lower value in red versus the green−yellow region. Hence, as a result of the combined factors of the reductions in both CE and luminous flux, bilayered film-loaded QD-LEDs possessed lower LEs of 33.1 −35.6 lm W−1 compared to that (48.2 lm−1W) of their monolayered film-loaded counterpart (table 1). The 12 wt% InP/ ZnS QD-loaded device having the highest CRI (94) was further operated under various forward currents of 20−80 mA. As seen from the EL spectral evolution of figure 5(c), the spectral intensities of tri-color components increased in an almost monotonic fashion with increasing forward current,
indicative of a high spectral stability against current variation. Thus, CIE coordinates, CRI, and CCT were only varied in the narrow ranges of (0.334−0.337, 0.344−0.356), 94−95, and 5322−5439 K, respectively.
4. Conclusions Highly fluorescent non-Cd QDs of green-to-greenish-yellow CIGS/ZnS and red InP/ZnS QDs with PL QYs of 70−73% and 67%, respectively, were synthesized. A 536 nm-emitting CI0.2G0.8S/ZnS QD and three Ga-rich CIGS/ZnS ones were consecutively surface-modified via in-situ hydrophobic-tohydrophilic ligand exchange and then successfully embedded in a water-soluble PVA, generating 150 μm thick free-standing composite films with varied QD loading concentrations of 20−30 wt%. Remote-type white QD-LEDs combined with CI0.2G0.8S/ZnS QD-PVA films exhibited LEs of 42.5−48.2 lm W−1, CRIs of 60−67, and CCTs of 6066−8560 K, depending on QD loading concentration in the matrix. By sequentially stacking an additional composite film, consisting of hydrophobic red InP/ZnS QD-embedding PVP, on 20 wt% 6
Nanotechnology 25 (2014) 225601
J-H Kim and H Yang
CI0.2G0.8S/ZnS QD-PVA film, a 200 μm thick dual coloremitting bilayered film with a high visible transmittance (47 −54% at 550 nm) was prepared. Three bilayered composite films with different InP/ZnS QD loading concentrations of 10, 12, and 14 wt% were applied for the fabrication of tri-colored white QD-LEDs. Compared to the devices with a single color CI0.2G0.8S/ZnS QD emitter, dual color-emitting bilayered films-integrated ones showed decreased LEs of 33.1−35.6 lm W−1, but dramatically enhanced CRIs of 89−94 and lower CCTs of 3691−7095 K, depending on InP/ZnS QD loading concentration.
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Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068158 and 2011-0013377) and by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000182).
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