1. Introduction

Among solid-state lighting technology, phosphor-converted white light-emitting diodes (pc-WLEDs) are excellent candidates to replace incandescent lamps for their merit of high energy conservation, long lifetime, high luminous efficiency as well as polarized emissions [1]. To generate warm light for general indoor lighting, red phosphors are very necessary [2,3]. Semiconductor nanocrystals (NCs) are emerging color tunable emissive light converters [4]. It has been demonstrated that the pc-WLED devices integrated with red emissive CdSe/ZnS, CdS:Cu/ZnS NCs show improved color rendering metric [5–13]. However, cadmium based NCs have limited future owing to the well-known toxicity. Recently, CuInS₂ and InP based NCs, are investigated as desirable low toxic alternatives [14–19]. Particularly, CuInS₂ based NCs exhibit very broad emissions spectra with full width at half maximum (FWHM) of 100-120 nm, large Stokes shifts of 200~300 meV and finely-tunable emissions [14]. These notable features provide great advantages to improve the color rending as well as tune the color temperature in WLEDs applications. In this work, we firstly demonstrated the applicability of red emissive CuInS₂ based NCs as a phosphor for high color rendering warm light generating.

2. Experimental section

CuInS₂ based NCs with emission peaks of 614 nm were prepared according to the method described in our previous work [20]. CuI (0.19 g, 1 mmol), In(OAc)₃ (1.16 g, 4 mmol) were mixed with dodecanethiol (DDT, 5 mL) and 1-octadecene (ODE, 25 mL) in a 100 mL three-necked flask. Then the reaction mixture was degassed under vacuum for 20 min at 120 °C. Oleic acid (OA, 2.5 mL) was added into the reaction flask, and the solution was continuously degassed for another 20 min under nitrogen flow. The solution was then heated to 220 °C to form a deep red colloidal solution. After that, DDT (5 mL) was injected into the as-prepared core solution. Subsequently, a fixed amount of Zn stock solution (2.64 g Zn(OAc)₂, 10 mL oleylamine and 10 mL ODE) was drop by drop added into the reaction mixture in 10 batches at intervals of 15 min. Afterward, the resulting NCs solution was cooled to room temperature and precipitated by adding excess acetone and methanol. The flocculent precipitate was centrifuged at 8500 rpm for 5 min and the supernatant was decanted. This process was repeated five times and the precipitation was dried to powder for WLED fabrication. For a typical dual phosphors based WLEDs with surface mounted device (SMD) type, 0.167 g thermally curable silicone resin (OE-6551A, Dow Corning Co.) was mixed with 0.005 g CuInS₂ NCs dispersed in chloroform (ca. 1 mL) and 0.1 g YAG:Ce, then the solvent was removed by heating at 50 °C for 1 h. Subsequently, hardener (OE-6551B, 0.333 g) was added into the dual phosphors and silicone resin composite. The dual phosphors in silicone resin were coated onto InGaN LED chip (emission peaks: 455-457.5 nm, Sanan optoelectronics, China), and placed in thermal curing process at 150 °C for 1h. Three phosphors based WLEDs with SMD type were fabricated through similar procedure by using a mixture of three phosphors (0.025 g yellow emissive YAG:Ce, 0.06 g green emissive Eu²⁺ doped silicate (G2762) and 0.005 g red CuInS₂ NCs) in silicone resin. For high power (HP) devices, a mixture of 0.04 g YAG:Ce, 0.06 g G2762 and 0.01g red CuInS₂ NCs was used. The mass concentrations of phosphors in silicone resin for SMD and HP devices were 17.4% and 22% respectively.

3. Results and discussions

The red emissive CuInS₂ based NCs were synthesized through a colloidal solution method, which has been described in our previous report [20]. Figure 1(a) shows the absorption and photoluminescence (PL) spectra of typical red emissive CuInS₂ based NCs. The absolute PL quantum yield (QY) of red emissive CuInS₂ based NCs was determined to be ~75% using Quantaurus Tau QY measurement system. It is noted that the red emissive CuInS₂ based NCs can be efficiently excited by the blue chip (emission peak at 455 nm) due to their strong absorption in the wavelength region from UV to blue. The red emissive CuInS₂ based NCs have an emission peak at 614 nm and their emission spectrum extend to deep red region (>700 nm) with FWHM of ~113 nm. These properties enable them to be suitable red color converting materials for WLEDs. As shown in the TEM images in inset of Fig. 1(a), the red emissive CuInS₂ NCs have an average particle size of 5.4 nm. The green emissive Eu²⁺ doped silicate (G2762) and yellow emissive YAG:Ce phosphors are commercial products from Intematix corporation. Their emission spectra are depicted in Fig. 1(b). The emission spectra of these phosphors match well with the electroluminescence (EL) spectra of blue chips for white light generation. Especially, the red emissive CuInS₂ NCs can compensate the missing red and deep-red regions.

 

Fig. 1 (a) Absorption (black solid line), PL spectra (red solid line) excited at 450 nm and TEM image (inset) of typical red emissive CuInS₂ based NCs. (b) PL and PLE spectra of G2762, YAG:Ce phosphors and red emissive CuInS₂ based NCs excited at 450 nm, EL spectrum of blue LED chip (blue solid line); Inset: the corresponding picture of G2762, YAG:Ce phosphors and red emissive CuInS₂ NCs under UV radiation.

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A simple way to improve the color rendering index (CRI) value of commercial YAG:Ce based pc-WLEDs is to add red emissive phosphors [21]. The surface mounted devices (SMD) were fabricated by employing a blend of red emissive CuInS₂ based NCs and YAG:Ce phosphor with an thermally curable silicone resin (OE-6551AB, Dow Corning Co.). Figure 2(a) shows the EL spectrum of YAG:Ce based pc-WLEDs with (red line) and without (black line) CuInS₂ based NCs. Figure 2(b) depicts the different CIE color coordinates of the chromaticity diagram between the dual phosphor and the single phosphor based WLEDs. The color coordinate of the single phosphor based device locates at (0.334, 0.353), which is out of white light region. Obviously, by adding red emissive CuInS₂ NCs, the color coordinate of device based on dual phosphors moved to white light region (0.334, 0.328), which is quite close to pure white CIE color coordinates (0.333, 0.333). Table 1 summarized the parameters of surface mounted devices based on dual phosphors operated at 20 mA. The WLEDs based on dual phosphors exhibit improved CRI of ~82, which is much better than that of the single phosphor based WLEDs (CRI: R_a = 71). The luminous efficiency slightly decreased from ~110 lm/W to ~91 lm/W with adding CuInS₂ based NCs into silicone resin. This is a reasonable result for dual phosphor based WLEDs [21].

 

Fig. 2 (a) EL spectra of the as-fabricated surface mounted devices operated at a forward bias current of 350 mA using YAG:Ce (black line), YAG:Ce and red CuInS₂ NCs (red line). (b) CIE chromaticity diagram of the devices based on YAG: Ce phosphor with and without red CuInS₂ based NCs.

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Table 1. The luminous efficiencies (L_E), CIE coordinates (x, y), color temperatures (T_c) and color rendering index (R_a) of the dual-phosphors surface mounted devices operated at 20 mA.

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Although the incorporation of red emissive CuInS₂ based NCs into YAG:Ce based WLEDs obviously improved the CRI value of WLEDs, this strategy cannot generate warm white light with high CRI for indoor lighting. A combination of three phosphors was applied to improve the color rendering index and tune the correlated color temperature (CCT) for warm light generation. A green emissive Eu²⁺ doped silicate phosphor (G2762, PL spectrum was shown in Fig. 1(b)) was introduced. Table 2 summarized the parameters of three-phosphor based WLEDs with SMD type. The as fabricated SMD type WLEDs with a combination of green, yellow and red emissive phosphors (weight ratio of 12:5:1) exhibit excellent CRI up to ~92. A typical EL spectrum of three-phosphor based WLED with SMD type is shown in Fig. 3(a) . It is observed that the EL spectrum has three obvious emission peaks at 455, 535 and 640 nm, corresponding to blue InGaN LED chip, G2762, and red emissive CuInS₂ based NCs respectively. We further studied the influence of red emissive CuInS₂ based NCs on the CIE color coordinates and color temperature of WLEDs. Figure 3(b) shows the CIE color coordinates of various devices using dual phosphors (G2762 and YAG:Ce) or three phosphors (G2762,YAG:Ce and red emissive CuInS₂ NCs). The green dotted line presents the color coordinates of WLEDs based on G2762 and YAG:Ce phosphors. It is obvious that all the color coordinates are out of the white light region and the color coordinates shifted to white light region with adding red emissive CuInS₂ NCs. Impressively, the WLEDs based on three phosphors have CIE color coordinates of (0.334-0.390, 0.332-0.396), CRI value of ~92 and luminous efficiencies of ~45 lm/W. By varying the amount of curing silicone resin, the devices exhibit tunable correlated color temperature along the Planckian locus from 3800 K to 5400 K.

Table 2. The luminous efficiencies (L_E), CIE coordinates (x, y), color temperatures (T_c) and color rendering index (R_a) of the as-fabricated three-phosphors WLEDs with SMD type (operated at 20 mA) and high power type (operated at 350 mA).

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Fig. 3 (a) EL spectra of blue LED chip coated with G2762 and YAG:Ce phosphors, blue LED chip coated with CuInS₂ based NCs, and blue LED chip with a mixture of G2762, YAG:Ce phosphors and CuInS₂ based NCs. (b) CIE color coordinates of the WLED devices based on dual phosphors (G2762, YAG:Ce) and hybrid composites of three phosphors (G2762, YAG:Ce and CuInS₂ based NCs).

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The indoor lighting requires high color rendering warm white light. The CCT of three-phosphors WLED devices was further tuned to generate warm light (< 4000 K). By increasing the concentration of phosphors in silicone resin, we fabricated high power type devices with tunable CCT from 3402 to 11304 K (see Table 2). As shown in Fig. 4(a) , the color of the silicone resin shifted from light orange to deep orange-red with the increase of phosphors in silicone resin. Figure 4(b) shows the generated white lighting with CCT of 3402 K, 3649 K, 3903 K, 4326 K,5450 K, 6503 K and 11304 K. By visual observation, sample 7 and 6 show cool white light, sample 5, 4 and 3 show pure white light, sample 2 and 1 show warm white light. This change also correspond to the EL spectra measurements. As shown in Fig. 4(c), the red emission intensity around 620 nm greatly increased with CuInS₂ NCs adding. From Fig. 4(c), it is observed that all the devices have CIE color coordinates in the white light region along with the Planckian locus. It is worth to noted that the two devices with CIE coordinates of (0.2794, 0.2729), (0.312, 0.334) and CCTs of 3402, 3649 K generate high color rendering warm light with a CRI of ~80. In addition, the high power devices have luminous efficiency of 50~60 lm/W, which is acceptable for commercial applications.

 

Fig. 4 Photographs of (a) the as-fabricated three phosphor WLEDs and (b) lighting operated at 350 mA. (c) EL spectra and (d) CIE color coordinates of the devices with tunable color temperatures under a forward bias current of 350 mA.

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4. Conclusion

In summary, we demonstrated the potential of CuInS₂ based NCs as red emissive phosphor for WLEDs applications. The CuInS₂ based NCs with intrinsic broad emission spectra in red region not only improved the color rendering properties, but also tune the color temperature to generate warm light. The experimental results show that the CRI of YAG:Ce based WLEDs obviously improved from 71 to 82 through adding red emissive CuInS₂ based NCs. By further introducing green emissive phosphor G2762, the SMD devices of three phosphors based WLEDs have CRI up to ~92, luminous efficiency up to ~45 lm/W, and tunable CCT of 3800-5400 K. Furthermore, the high-power type three phosphor devices show CRI of ~80 and luminous efficiency of 50~60 lm/W with tunable CCT from 11304 to 3402 K along the Planckian locus by varying the amount of phosphors in silicone resin. The efficient generation of warm light with a CRI of 80 is acceptable for indoor lighting. These results suggest that CuInS₂ based NCs are competitive as the promising red emissive phosphor for warm WLEDs applications.