1. Introduction

The availability of high-efficiency III−nitride light-emitting diodes (LEDs) as pumping excitation sources, which have progressed significantly through the improvements in the internal quantum efficiency [1–4] and extraction efficiency [5–7], has enabled the fabrication of high-performance white LEDs. Highly bright quantum dot (QD)-based white LEDs, analogous to phosphor-converted (pc)-white LEDs, have been emerging mainly targeting for the huge industrial market of general lighting and display backlight sources. Although cadmium (Cd)-containing QD visible emitters having exceptional fluorescent characteristics have shown great promise in white QD-LED fabrication [8–11], the presence of a toxic Cd element has always rendered their practical usage doubtful, necessitating the development of alternative QDs with non-Cd compositions such as InP and CuInS₂ (CIS). High-quality InP QDs have been synthesized by commonly selecting a highly toxic, expensive phosphorus (P) precursor of tris(trimethylsilyl)phosphine (P(TMS)₃), since its high reactivity enables a burst of QD nucleation. Thereafter, different wavelength-emitting, multiple InP QDs were co-combined with a blue LED to acquire color-mixed white spectral coverage [12] or only red InP QDs were integrated with pc-white LED [13,14] to improve color rendering property by reinforcing the red spectral deficiency. Likewise, based on significant synthetic advances in CIS QDs, they have been successfully utilized as efficient down-converters in QD-LEDs. For instance, we previously demonstrated the synthesis of highly bright yellow CIS QDs and the following fabrication of a single CIS QD-containing white LED [15,16]. In this work, two bright non-Cd core/shell QDs of green InP/ZnS and orange CIS/ZnS were synthesized and a novel combination of both QD down-converters was for the first time exploited to fabricate a high-color rendering white light QD-LED.

2. Experimental details

For a typical synthesis of green-emitting InP/ZnS QDs, 0.9 mmol of InCl₃, 1.2 mmol of ZnO and 6 ml of oleylamine (OLA) were placed in a three-neck flask, degassed/purged with argon, and quickly heated to 280°C. A clear mixture was obtained within 5 min at that temperature and then cooled down to 190°C. Then, 0.25 ml of tris(dimethylamino)phosphine (P(N(CH₃)₂)₃), a much safer and cheaper P source than P(TMS)₃, was hot-injected into the above mixture and the growth of InP core QDs was allowed for 4 min at 190°C. ZnS shelling was successively made simply by dropwisely adding 1 ml of 1-dodecanethiol (DDT) as a sulfur source to the pre-grown InP core QDs and maintaining the shell growth at 200°C for 7 h. It is noted that owing to the Zn source co-existing in the above InP core synthesis an additional Zn introduction was not necessary for the formation of ZnS shell. In a synthesis of orange-emitting CIS/ZnS QDs, 0.1 mmol of CuI and 0.5 mmol of In acetate were mixed with 4 ml of 1-octadecene (ODE) in a three-neck flask. After degassing and Ar purging, the reaction vessel was heated to 230°C for 10 min. Then, 4 ml of DDT was injected into the reaction flask and the growth of CIS core QDs proceeded for 5 min. A ZnS stock solution, prepared by dissolving 4 mmol of Zn acetate dihydrate in a mixture of 2 ml of DDT, 2 ml of oleic acid (OA), and 4 ml of ODE at 190°C, was consecutively added into the above hot reaction mixture of CIS core QDs, and the shell growth proceeded at 240°C for 2 h. As-synthesized InP/ZnS and CIS/ZnS QDs were repeatedly purified through the conventional centrifugation-based precipitation/dispersion method using a solvent combination of chloroform/ethanol, and dispersed in chloroform for the optical characterization and device fabrication.

QD-LEDs were fabricated using a 50 × 50 mm² surface-mounted device (SMD) typed InGaN-based blue-emitting LED (peak wavelength of 450 nm and bandwidth of 20 nm, Haewon Semiconductor, Korea). 3 ml of chloroform dispersion of InP/ZnS or CIS/ZnS QD having an optical density (OD) of ~3.0 at 450 nm was blended with thermo-curable epoxy resin/hardener (weight ratio of 1). The resulting QD paste was dispensed in the mold of LED and then subjected to a two-step curing process of 70°C for 30 min and 110°C for 1 h.

X-ray diffraction (XRD) (Rigaku, Ultima IV) using Cu K_α radiation was employed to analyze the crystal structure of QDs. Absorption and photoluminescence (PL) spectra of QDs were recorded by UV–visible absorption spectroscopy (Shimadzu, UV-2450) and a 500 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd., Darsa Pro-5200), respectively. PL quantum yield (QY) of the QDs was calculated by comparing the integrated emission of QD-chloroform dispersion with that of rhodamine 6G (QY of ~96%) ethanol solution with the same OD (~0.05) at wavelengths of 450 nm for CIS/ZnS and 420 nm for InP/ZnS QDs. High resolution-transmission electron microscopy (TEM) work on both QDs was performed with JEOL JEM-4010 electron microscope operated at an accelerating voltage of 400 kV. Primary electroluminescent (EL) data including EL spectrum, luminous efficacy (LE), correlated color temperature (CCT), CIE color coordinates, and color rendering index (CRI) of the QD-LEDs were collected in an integrating sphere with a diode array rapid analyzer system (PSI Co. Ltd.).

3. Results and discussion

Typical absorption and PL spectra of InP/ZnS and CIS/ZnS QDs are shown in Fig. 1. Well-resolved excitonic absorption feature of InP/ZnS QDs implies that the present P(N(CH₃)₂)₃-based hot-injection approach could effectively produce highly monodisperse QDs in size. On the other hand, CIS/ZnS QDs possessed a rather indistinct absorption peak, presumably attributable to broad distributions in size/composition and/or a unique electronic property of CIS [17]. Green InP/ZnS QDs showed a peak emission wavelength of 512 nm and an emission QY of 52%. In particular, the emission bandwidth (52 nm) of our InP/ZnS QDs was quite narrow, as expected from their absorption spectrum above, compared to the conventional P(TMS)₃-derived counterparts [12,14,18,19]. Meanwhile, orange CIS/ZnS QDs with an excellent QY of 79% exhibited a large Stokes-shifted emission peaking at 583 nm along with a substantially broad band emission with a bandwidth of 120 nm, since their radiative recombination of photoexcited carriers is involved with intra-gap defect states [15,16]. The crystallographic structures of core QDs of InP and CIS were well indexed with zinc blende (JCPDS: 32-0452) and chalcopyrite (JCPDS: 85-1575) phases, respectively (not shown here). As shown in XRD patterns of Fig. 2(a), upon ZnS shelling, the primary three reflection peaks of both QDs were similarly shifted to a larger 2θ, being close to those of ZnS phase, consistent with the suitable formation of ZnS shell on the respective core QDs. The average sizes of InP/ZnS and CIS/ZnS QDs were estimated from TEM images of Figs. 2(b),and 2(d) to be 3.2 and 2.8 nm, respectively.

 

Fig. 1 UV-visible absorption/PL spectra and UV-irradiated fluorescent images of InP/ZnS and CIS/ZnS QD dispersions.

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Fig. 2 (a) XRD patterns of InP/ZnS and CIS/ZnS QDs and TEM images of (b) InP/ZnS and (c) CIS/ZnS QDs.

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The excellent spectral overlap between absorptions of both QDs (Fig. 1) and blue (450 nm) emission of an LED chip can afford the efficient down-conversion of blue-to-QD emission. Typical EL spectra of two QD-LEDs, where the respective InP/ZnS and CIS/ZnS QDs are combined with a blue LED and a forward current of 20 mA is applied, are shown in Fig. 3. None of these devices generated a white light due to the deficient spectral coverage, resulting in no CRI and CCT values. Compared to the peak emission wavelengths of the dilute QD dispersions in Fig. 1, those of down-converted QD emission were red-shifted to 530 and 611 nm for InP/ZnS and CIS/ZnS QD-LEDs, respectively, resulting from the nontrivial, unavoidable dipole-dipole resonant energy transfer and light reabsorption events between close-spaced, different-sized QD ensembles. The conversion efficiencies (CEs) of blue-to-QD emission were estimated to be 28.9 and 56.3% for InP/ZnS and CIS/ZnS QD-LEDs, respectively. A higher CE from the latter device is evidently ascribed to a higher PL QY (79%) of CIS/ZnS QDs compared to InP/ZnS ones (52%). Meanwhile, InP/ZnS and CIS/ZnS QD-LEDs exhibited LEs of 42.1 and 47.4 lm/W, respectively. A relatively small difference in LE versus CE between both devices is natural, considering a wavelength-dependent photometric quantity of luminous flux (lm), i.e., a higher eye sensitivity factor for green versus red color emission.

 

Fig. 3 EL spectra of InP/ZnS and CIS/ZnS QD-based LEDs operated at a forward current of 20 mA. The photographs of as-fabricated and 20 mA-driven QD-LEDs are also shown in the inset.

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As recognized from Fig. 1, the spectral overlap between absorption of InP/ZnS QDs and emission of CIS/ZnS ones is insignificant in contrast to the substantial overlap between absorption of CIS/ZnS QDs and emission of InP/ZnS ones. Hence, to suppress the unwanted light reabsorption between these two QDs InP/ZnS QDs were sequentially dispensed on top of CIS/ZnS ones with a weight ratio of InP/ZnS to CIS/ZnS QDs of about 2.5, as schematically presented in the inset of Fig. 4(a). A tricolored QD-LED containing two down-converters of InP/ZnS and CIS/ZnS QDs effectively generated a spectrally wide emission. Figure 4(a) shows the evolution of EL spectra as a function of drive current up to 200 mA, and the primary EL values of LE, CE, CRI, and CCT at the respective drive currents are summarized in Table 1. The present tricolored QD-LED naturally possessed intermediate values with respect to LE and CE at 20 mA between those of the earlier two bicolored QD-LEDs. The integrated intensities of blue chip versus QD emission at 20−200 mA are compared in Fig. 4(b). While the blue chip emission exhibited an almost linear increase, a noticeable gradual quenching of the QD emission intensity with increasing drive current was observed. As a result, CE dropped slightly from 41.0% at 20 mA to 34.7% at 200 mA, attributable to the thermal quenching of QDs induced by the rise of chip temperature at a higher drive current [20]. Concomitant with this CE drop, i.e., a little higher spectral ratio of blue to QD emission with increasing drive current, CIE color coordinates slightly moved toward the blue side from (0.366, 0.307) at 20 mA to (0.355, 0.294) at 200 mA (Fig. 4(c)), whose variation is comparable to or smaller than those of the conventional white pc-white LEDs [21,22]. Meanwhile, as commonly observed elsewhere [15,16], a drive current-dependent LE reduction was rather substantial, showing 45.5 and 19.0 lm/W at 20 and 200 mA, respectively, as a combined result of the major factor of intrinsic efficacy drop of chip itself along with the minor factor of the previous slight CE drop. As sensed from a wide coverage of tricolored white spectrum (Fig. 4(a)), the white light was of sufficient high-quality in a white lighting standard, showing an excellent CRI of 90 irrespective of drive current. For one’s reference, other types of white QD-LEDs consisting of the combination of blue LED chip with CdSe- [9] or InP-based QDs [12] typically utilized a mixture of three QD emitters of green, yellow, and red to obtain a white spectrum as broad as possible, showing CRIs of 88−89. Moreover, CCT values of 3803−4134K (Table 1) were in proximity to an ideal warm white (2700−3000K according to the US DOE L-Prize CCT requirement for general lighting [23]), even though a higher current led to the gradual CCT increase as expected from the variation of CIE color coordinates (Fig. 4(c)).

 

Fig. 4 (a) Evolution of EL spectra of CIS/ZnS−InP/ZnS QD-based LED with increasing drive current up to 200 mA. The device schematic and its white EL images at 20 versus 200 mA are shown in upper middle and right insets, respectively. Variations of (b) integrated blue chip versus QD emissions and (c) CIE color coordinates of white emissions as a function of forward current.

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Table 1. Variations of primary EL values of LE, CE, CRI, and CCT of a tricolored QD-LED operated in the range of 20−200 mA.

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4. Conclusion

Two non-Cd down-converters of narrow-band green InP/ZnS and broad-band orange CIS/ZnS QDs having emission QYs of 52 and 79%, respectively, were integrated onto a blue LED with a sequential configuration of InP/ZnS-on-CIS/ZnS QDs. The resulting tricolored QD-LED possessed a wide EL spectral coverage along with a red-weighted spectrum, giving rise to a high-color rendering, warm white light. Effects of the drive current in the range of 20−200 mA on primary EL performances of white QD-LED were investigated, exhibiting an invariant CRI value of 90, but decreasing CEs (41.0→34.7%) and LEs (45.5→19.0 lm/W) and increasing CCTs (3803→4134K) with increasing drive current.