Warm White Light-Emitting Diodes Based on a Novel Orange Cationic Iridium(III) Complex

A novel orange cationic iridium(III) complex [(TPTA)2Ir(dPPOA)]PF6 (TPTA: 3,4,5-triphenyl-4H-1,2,4-triazole, dPPOA: N,N-diphenyl-4-(5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl)aniline) was synthesized and used as a phosphor in light-emitting diodes (LEDs). [(TPTA)2Ir(dPPOA)]PF6 has high thermal stability with a decomposition temperature (Td) of 375 °C, and its relative emission intensity at 100 °C is 88.8% of that at 25°C. When only [(TPTA)2Ir(dPPOA)]PF6 was used as a phosphor at 6.0 wt % in silicone and excited by a blue GaN (GaN: gallium nitride) chip (450 nm), an orange LED was obtained. A white LED fabricated by a blue GaN chip (450 nm) and only yellow phosphor Y3Al5O12:Ce3+ (YAG:Ce) (1.0 wt % in silicone) emitted cold white light, its CIE (CIE: Commission International de I’Eclairage) value was (0.32, 0.33), color rendering index (CRI) was 72.2, correlated color temperature (CCT) was 6877 K, and luminous efficiency (ηL) was 128.5 lm∙W−1. Such a cold white LED became a neutral white LED when [(TPTA)2Ir(dPPOA)]PF6 was added at 0.5 wt %; its corresponding CIE value was (0.35, 0.33), CRI was 78.4, CCT was 4896 K, and ηL was 85.2 lm∙W−1. It further became a warm white LED when [(TPTA)2Ir(dPPOA)]PF6 was added at 1.0 wt %; its corresponding CIE value was (0.39, 0.36), CRI was 80.2, CCT was 3473 K, and ηL was 46.1 lm∙W−1. The results show that [(TPTA)2Ir(dPPOA)]PF6 is a promising phosphor candidate for fabricating warm white LEDs.


Introduction
Due to high efficiency, long lifetime, and energy-saving and environmentally-friendly properties, white light-emitting diodes (WLEDs) have attracted significant attention and are used as the new generation solid-state light sources in general illumination, full-color displays, liquid crystal display backlights and so on [1][2][3][4]. At present, the commercial WLEDs are mainly obtained by the combination of a blue GaN (GaN: gallium nitride) chip (λ max,em ≈ 450 nm)and a yellow-emitting Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) phosphor. YAG:Ce is a blue-light excitable phosphor with high photoluminescence efficiency and good thermal stability [4][5][6]. However, because the main emission of YAG:Ce is in the greenish yellow region, aforementioned commercial WLEDs have low color rendering index (CRI) and high correlated color temperature (CCT). Due to the absence of a red light component in their spectra, the light emitted from such WLEDs is cold white light [7][8][9][10][11][12][13][14][15][16]. Two approaches have been developed to overcome this drawback. In the first approach, a red light component was emitted via some ions (such as Eu 3+ , Pr 3+ ) being doped in YAG:Ce [7][8][9]; however, the yellow emission was obviously decreased although slightly red light was obtained. In the other approach, some red phosphors were complementally used in aforementioned WLEDs [5,9,10]. Relatively better performances (including CRI, CCT and efficiency etc.) can be obtained via the second approach, because the red phosphors are mainly excited by the blue GaN chip and the emission of YAG:Ce is seldom affected. Hence, the development of new efficient red phosphors for warm WLEDs based on blue chips is very important and urgently needed. Up to now, besides many inorganic phosphors (such as CaSiAlN 3 :Eu 2+ [10], InP/GaP/ZnS quantum dots [11], Mn 4+ activated fluorides [5,12,13], and so on), many organic luminescent materials (such as organic metal complexes [14][15][16][17][18], polymers [19] and small-molecule fluorescent dyes [20,21], and so on) have also been used as red luminescent materials in WLEDs.
As a consecutive and upgraded research by us, in this work, another novel orange cationic iridium(III) complex also using dPPOA as auxiliary ligand, but using 3,4,5-triphenyl-4H-1,2,4-triazole (TPTA) instead of CBT as the main ligand was synthesized and also used in YAG:Ce-based WLEDs to obtain warm white light. Donor-acceptor bipolar units have been widely used in organic photoresponse materials for extending absorption range and increasing absorption rate [25,31,32]. Many organic cationic iridium(III) complexes can easily be excited by ultraviolet light, but cannot be effectively excited by blue light (such as the blue light of a GaN chip). In order to make this new complex effectively excited by the blue light of a GaN chip, adonor-acceptor bipolar unit of triphenylamine-oxadiazole was contained in the auxiliary ligand dPPOA; theoretically, the electron-donating or/and electron-withdrawing functional groups on the main ligand will further improve the effect of this donor-acceptor bipolar unit [31,32]. In our previous work [15], CBT containing an electron-donating carbazole group had been successfully used in the cationic iridium(III) complex [Ir(CBT) 2 (dPPOA)]PF 6 for warm WLEDs. This work tried the main ligand TPTA containing an electron-withdrawing 1,2,4-triazole group. In order to achieve higher performance in this new work, besides the replacement of the main ligand in the complex as a new attempt, 450 nm-emitting GaN blue chips with much higher luminous efficiency were also used to replace the 465 nm-emitting GaN blue chips used in previous work [15]. As expected, the cold white light of YAG:Ce-based WLEDs also gradually became warm white light with the increase of the addition of this new cationic iridium(III) complex; at the same time, such WLEDs exhibited encouraging light-emitting performances.

General Information
All chemicals and reagents were purchased from chemical reagent companies and used without further purification unless otherwise stated. 1 H NMR spectra were recorded on a Bruker AV400 spectrometer (Bruker, Fällanden, Switzerland) operating at 400 MHz; tetramethylsilane (TMS) was used as internal standard. Mass spectra (MS) were obtained on a Bruker amaZon SL liquid chromatography mass spectrometer (LC-MS, Bruker, Karlsruhe, Germany) with an electrospray ionization (ESI) interface using acetonitrile as matrix solvent. Elemental analysis (EA) was performed on a Vario EL III Elemental Analysis Instrument (Elementar, Hanau, Germany). Ultraviolet-visible (UV-vis, Agilent, Palo Alto, CA, USA) absorption spectra were measured on an Agilent 8453 UV-visible Spectroscopy System. Excitation and emission spectra of samples were documented on a Cary Eclipse FL1011M003 (Varian, Palo Alto, CA, USA) spectrofluorometer; the xenon lamp was used as an excitation source, and the temperature of solid samples was controlled by a temperature controller (REX-C110, Kaituo Compressor Parts Co., Ltd, Dongguan, China). Thermogravimetric (TG) analysis was carried out up to 600 • C in N 2 atmosphere with a heating speed of 10.0 • C·min −1 on a NETZSCH STA 449F3 thermogravimetric analyzer (NETZSCH, Selb, Germany). The electroluminescent spectra of LEDs were recorded on a high-accuracy array spectrometer (HSP6000, HongPu Optoelectronics Technology Co., Ltd, Hangzhou, China).
Synthesis of the chloro-bridged dimer (TPTA) 2 Ir(µ-Cl) 2 Ir(TPTA) 2 : A mixture of IrCl 3 ·3H 2 O (1.06 g, 3.0 mmol) and TPTA (1.81 g, 6.10 mmol) in H 2 O (10 mL) and 2-methoxyethanol (30 mL) was refluxed in Ar atmosphere for 24 h. After being cooled to room temperature, the resultant yellow precipitate was collected on a filter, washed with water and methanol alternately, and then dried in a vacuum. Yield was 75.0% (1.85 g) yellow solid. This dimer product was directly used for the next step without further purification and characterization.

Fabrication and Measurements of LEDs
Two kinds of LEDs were fabricated and measured in this work. (i) Only [(TPTA)2Ir(dPPOA)]PF6 was blended and stirred homogeneously in silicone atthe mass ratios of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 wt %, and then coated on the surface of 450 nm-emitting GaN chips until the reflective cavities were filled up; (ii) YAG:Ce was blended in silicone at a constant mass ratio of 1.0 wt %, and [(TPTA)2Ir(dPPOA)]PF6 was blended together at different mass ratios of 0.0, 0.5, 1.0 and 1.5 wt %. The silicone containing YAG:Ce and [(TPTA)2Ir(dPPOA)]PF6 was also coated on the surface of 450 nm-emitting GaN chips until the reflective cavities were filled up. In the blending process, an analytical balance with the readability of 0.01 mg was used for weighing, 0.10000-0.12000 g viscous silicone was weighed in a little bottle every time, then the quantities of [(TPTA)2Ir(dPPOA)]PF6 and/or YAG:Ce were calculated and weighed. After [(TPTA)2Ir(dPPOA)]PF6 and/or YAG:Ce were added to the silicone, a drop of CH2Cl2 was also added, then the mixture was stirred by hand with a little stainless steel rod for about one minute until all the ingredients were well combined. The LEDs were dried and solidified at 150 °C for 1 h. The LEDs were all operated at 20 mA forward current and 5 V reverse voltage, with their performances measured by an integrating sphere spectroradiometer system (Everfine PMS-50, Everfine Photo-E-Info Co. Ltd., Hangzhou, China).

UV-Vis Absorption Spectra
The normalized UV-vis absorption spectrum of [(TPTA)2Ir(dPPOA)]PF6 in CH2Cl2 solution at 1.0 × 10 −5 mol•L −1 together with the emission spectra of the blue GaN chip and YAG:Ce are shown in Figure 2. The strong absorption band between 230 nm and 380 nm can be ascribed to the spin-allowed 1 π-π* transition of the ligands; the maximum absorption wavelength (λabs, max) is 267

Fabrication and Measurements of LEDs
Two kinds of LEDs were fabricated and measured in this work. (i) Only [(TPTA) 2 Ir(dPPOA)]PF 6 was blended and stirred homogeneously in silicone atthe mass ratios of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 wt %, and then coated on the surface of 450 nm-emitting GaN chips until the reflective cavities were filled up; (ii) YAG:Ce was blended in silicone at a constant mass ratio of 1.0 wt %, and [(TPTA) 2 Ir(dPPOA)]PF 6 was blended together at different mass ratios of 0.0, 0.5, 1.0 and 1.5 wt %. The silicone containing YAG:Ce and [(TPTA) 2 Ir(dPPOA)]PF 6 was also coated on the surface of 450 nm-emitting GaN chips until the reflective cavities were filled up. In the blending process, an analytical balance with the readability of 0.01 mg was used for weighing, 0.10000-0.12000 g viscous silicone was weighed in a little bottle every time, then the quantities of [(TPTA) 2 Ir(dPPOA)]PF 6 and/or YAG:Ce were calculated and weighed. After [(TPTA) 2 Ir(dPPOA)]PF 6 and/or YAG:Ce were added to the silicone, a drop of CH 2 Cl 2 was also added, then the mixture was stirred by hand with a little stainless steel rod for about one minute until all the ingredients were well combined. The LEDs were dried and solidified at 150 • C for 1 h. The LEDs were all operated at 20 mA forward current and 5 V reverse voltage, with their performances measured by an integrating sphere spectroradiometer system (Everfine PMS-50, Everfine Photo-E-Info Co., Ltd., Hangzhou, China).

UV-Vis Absorption Spectra
The normalized UV-vis absorption spectrum of [(TPTA) 2 Ir(dPPOA)]PF 6 in CH 2 Cl 2 solution at 1.0 × 10 −5 mol·L −1 together with the emission spectra of the blue GaN chip and YAG:Ce are shown in Figure 2. The strong absorption band between 230 nm and 380 nm can be ascribed to the spin-allowed 1 π-π* transition of the ligands; the maximum absorption wavelength (λ abs, max ) is 267 nm (ε 267nm = 5.4 × 10 4 L·mol −1 ·cm −1 ). The weak absorption band from 380 nm extending to the visible region results from the overlapping absorption of the spin-allowed singlet metal-to-ligand charge-transfer ( 1 MLCT), the spin-forbidden triplet metal-to-ligand charge-transfer ( 3 MLCT) and 3 π-π* in the ligands [25,34]. The admixture absorption of 3 MLCT and 3 π-π* with higher-lying 1 MLCT is caused by the strong spin-orbit coupling induced by the heavy iridium atom [35,36]. As shown in Figure 2, there is a large overlap between the absorption spectrum of [(TPTA) 2 Ir(dPPOA)]PF 6 and the emission spectrum of the blue GaN chip (λ em, max = 450 nm), which suggests that Förster resonance energy transfer (FRET) is able to take place between them easily and [(TPTA) 2 Ir(dPPOA)]PF 6 can be well excited by the blue GaN chip [37]. The overlap between the absorption spectrum of [(TPTA) 2 Ir(dPPOA)]PF 6 and the emission spectrum of YAG:Ce can almost be ignored, which suggests that the emission of YAG:Ce would be seldom affected by [(TPTA) 2 Ir(dPPOA)]PF 6 , and [(TPTA) 2 Ir(dPPOA)]PF 6 is a practicable red luminescent additive for YAG:Ce-based WLEDs. nm (ε267nm = 5.4 × 10 4 L•mol −1 •cm −1 ). The weak absorption band from 380 nm extending to the visible region results from the overlapping absorption of the spin-allowed singlet metal-to-ligand charge-transfer ( 1 MLCT), the spin-forbidden triplet metal-to-ligand charge-transfer ( 3 MLCT) and 3 π-π* in the ligands [25,34]. The admixture absorption of 3 MLCT and 3 π-π* with higher-lying 1 MLCT is caused by the strong spin-orbit coupling induced by the heavy iridium atom [35,36]. As shown in Figure 2, there is a large overlap between the absorption spectrum of [(TPTA)2Ir(dPPOA)]PF6 and the emission spectrum of the blue GaN chip (λem, max = 450 nm), which suggests that Förster resonance energy transfer (FRET) is able to take place between them easily and [(TPTA)2Ir(dPPOA)]PF6 can be well excited by the blue GaN chip [37].

Photoluminescent Properties
The photoluminescent properties of [(TPTA)2Ir(dPPOA)]PF6 in solution and blended in silicone were investigated and the results are shown in Figure 3. Both excitation (Ex, λem =600 nm) and emission (Em, λex = 450 nm) spectra of [(TPTA)2Ir(dPPOA)]PF6 in silicone showed obvious red shift in comparison with those of [(TPTA)2Ir(dPPOA)]PF6 in CH2Cl2 solution (λem = 581 nm, λex = 450 nm). The red shift should be caused mainly by the increase of the conjugated system of [(TPTA)2Ir(dPPOA)]PF6 in solid silicone, because the steric hindrance and distortion of the phenyls in ligands TPTA and dPPOA are restrained to a certain degree. At the same time, the conjugated system of [(TPTA)2Ir(dPPOA)]PF6 can also be increased by intermolecular π-π stacking in silicone.
In general, sufficient overlap between the excitation spectra of phosphors and the emission spectra of the LED chips is necessary to realize energy transfer from the LED chips to phosphors efficiently. In this work, the excitation spectra of [(TPTA)2Ir(dPPOA)]PF6 in CH2Cl2 solution and blended in silicone lie 225−500 nm and 225−580 nm respectively, and both cover the emission spectra of the 450 nm-emitting blue GaN chip (as shown in Figure 2), which means that

Photoluminescent Properties
The photoluminescent properties of [(TPTA) 2 Ir(dPPOA)]PF 6 in solution and blended in silicone were investigated and the results are shown in Figure 3. Both excitation (Ex, λ em =600 nm) and emission (Em, λ ex = 450 nm) spectra of [(TPTA) 2 Ir(dPPOA)]PF 6 in silicone showed obvious red shift in comparison with those of [(TPTA) 2 Ir(dPPOA)]PF 6 in CH 2 Cl 2 solution (λ em = 581 nm, λ ex = 450 nm). The red shift should be caused mainly by the increase of the conjugated system of [(TPTA) 2 Ir(dPPOA)]PF 6 in solid silicone, because the steric hindrance and distortion of the phenyls in ligands TPTA and dPPOA are restrained to a certain degree. At the same time, the conjugated system of [(TPTA) 2 Ir(dPPOA)]PF 6 can also be increased by intermolecular π-π stacking in silicone.
In general, sufficient overlap between the excitation spectra of phosphors and the emission spectra of the LED chips is necessary to realize energy transfer from the LED chips to phosphors efficiently. In this work, the excitation spectra of [(TPTA) 2 Ir(dPPOA)]PF 6 in CH 2 Cl 2 solution and blended in silicone lie 225−500 nm and 225−580 nm respectively, and both cover the emission spectra of the 450 nm-emitting blue GaN chip (as shown in Figure 2), which means that [(TPTA) 2 Ir(dPPOA)]PF 6

Thermal Stability and Thermal Quenching Properties
The thermal property of [(TPTA)2Ir(dPPOA)]PF6 was characterized by thermogravimetry (TG) in nitrogen atmosphere at a heating speed of 10.0 °C•min −1 , and the resultant TG curve is shown in Figure 4. With temperature increasing, the TG curve begins to dip suddenly after about 375 °C, which means that the thermal decomposition happened and 375 °C can be regarded asits thermal decomposition temperature (Td). Such a high decomposition temperature suggests that [(TPTA)2Ir(dPPOA)]PF6 has a high thermal stability and is enough to meet the requirement of its application in LEDs, since LED devices are fabricated and usually work at a temperature below 150 °C [14].

Thermal Stability and Thermal Quenching Properties
The thermal property of [(TPTA) 2 Ir(dPPOA)]PF 6 was characterized by thermogravimetry (TG) in nitrogen atmosphere at a heating speed of 10.0 • C·min −1 , and the resultant TG curve is shown in Figure 4. With temperature increasing, the TG curve begins to dip suddenly after about 375 • C, which means that the thermal decomposition happened and 375 • C can be regarded asits thermal decomposition temperature (T d ). Such a high decomposition temperature suggests that [(TPTA) 2 Ir(dPPOA)]PF 6 has a high thermal stability and is enough to meet the requirement of its application in LEDs, since LED devices are fabricated and usually work at a temperature below 150 • C [14].

Thermal Stability and Thermal Quenching Properties
The thermal property of [(TPTA)2Ir(dPPOA)]PF6 was characterized by thermogravimetry (TG) in nitrogen atmosphere at a heating speed of 10.0 °C•min −1 , and the resultant TG curve is shown in Figure 4. With temperature increasing, the TG curve begins to dip suddenly after about 375 °C, which means that the thermal decomposition happened and 375 °C can be regarded asits thermal decomposition temperature (Td). Such a high decomposition temperature suggests that [(TPTA)2Ir(dPPOA)]PF6 has a high thermal stability and is enough to meet the requirement of its application in LEDs, since LED devices are fabricated and usually work at a temperature below 150 °C [14].   The thermal-quenching property of [(TPTA) 2 Ir(dPPOA)]PF 6 was alsoinvestigatedand the results are shown in Figures 5 and 6. Figure 5 depicts the temperature-dependent photoluminescent spectra (λ ex = 450 nm) of [(TPTA) 2 Ir(dPPOA)]PF 6 powders. From 25 • C to 200 • C, the light-emitting color of [(TPTA) 2 Ir(dPPOA)]PF 6 exhibited high thermal stability because the profile, wavelength band and the maximum wavelengths (around 601 nm) of its emission spectra at different temperatures were almost unchanged, with the exceptionof the intensity. The relative emission intensity of [(TPTA) 2 Ir(dPPOA)]PF 6 as a function of temperature is shown in Figure 6. Like most phosphors used in LEDs, the emission intensity of [(TPTA) 2 Ir(dPPOA)]PF 6 also decreases with increasing temperature (i.e., thermal quenching) [4,[38][39][40]. The relative emission intensities at different temperaturesare descending: 97.2% (at 50 • C), 93.7% (at 75 • C), 88.8% (at 100 • C), 82.3% (at 125 • C), 76.5% (at 150 • C), 68.9% (at 175 • C), 61.9% (at 200 • C). The thermal quenching of luminescent organic metal complexes is usually caused by the aggravating jiggle and wiggle of atoms, and the rotating and stretching vibration of covalent bonds as the temperature increases. The activation energy (E a ) of the thermal quenching can be described by the Arrhenius equation [38][39][40]: where I represents the emission intensity at any testing temperature (25-200 • C), I o represents the emission intensity at room temperature (25 • C), A is a constant, k B is a Boltzmann constant, and T is any testing temperature. From the Arrhenius equation, the relationship of ln(I o /I − 1) with 1/T can be obtained; the experimental data are well-fitted and shown in Figure 6 (inset); then, the value of E a for [(TPTA) 2 Ir(dPPOA)]PF 6 was calculated to be 0.2647 eV from the slope value −(E a /k B ). For phosphors used in LEDs, a high E a value means low thermal quenching. The emission intensity decay rate and E a value show that [(TPTA) 2 Ir(dPPOA)]PF 6 is an applicable phosphor for LEDs and its thermal quenching is lower than that of many orange or red phosphors reported in recent years [40][41][42]. The thermal-quenching property of [(TPTA)2Ir(dPPOA)]PF6 was alsoinvestigatedand the results are shown in Figures 5 and 6. Figure 5 depicts the temperature-dependent photoluminescent spectra (λex = 450 nm) of [(TPTA)2Ir(dPPOA)]PF6 powders. From 25 °C to 200 °C, the light-emitting color of [(TPTA)2Ir(dPPOA)]PF6 exhibited high thermal stability because the profile, wavelength band and the maximum wavelengths (around 601 nm) of its emission spectra at different temperatures were almost unchanged, with the exceptionof the intensity. The relative emission intensity of [(TPTA)2Ir(dPPOA)]PF6 as a function of temperature is shown in Figure 6. Like most phosphors used in LEDs, the emission intensity of [(TPTA)2Ir(dPPOA)]PF6 also decreases with increasing temperature (i.e., thermal quenching) [4,[38][39][40]. The relative emission intensities at different temperaturesare descending: 97.2% (at 50 °C), 93.7% (at 75 °C), 88.8% (at 100 °C), 82.3% (at 125 °C), 76.5% (at 150 °C), 68.9% (at 175 °C), 61.9% (at 200 °C). The thermal quenching of luminescent organic metal complexes is usually caused by the aggravating jiggle and wiggle of atoms, and the rotating and stretching vibration of covalent bonds as the temperature increases. The activation energy (Ea) of the thermal quenching can be described by the Arrhenius equation [38][39][40]: where I represents the emission intensity at any testing temperature (25-200 °C), Io represents the emission intensity at room temperature (25 °C), A is a constant, kB is a Boltzmann constant, and T is any testing temperature. From the Arrhenius equation, the relationship of ln(Io/I − 1) with 1/T can be obtained; the experimental data are well-fitted and shown in Figure 6 (inset); then, the value of Ea for [(TPTA)2Ir(dPPOA)]PF6 was calculated to be 0.2647 eV from the slope value −(Ea/kB). For phosphors used in LEDs, a high Ea value means low thermal quenching. The emission intensity decay rate and Ea value show that [(TPTA)2Ir(dPPOA)]PF6 is an applicable phosphor for LEDs and its thermal quenching is lower than that of many orange or red phosphors reported in recent years [40][41][42].      In order to demonstrate the application of [(TPTA)2Ir(dPPOA)]PF6 for WLEDs, a series of blue GaN-based WLEDs using YAG:Ce (1.0 wt %) and [(TPTA)2Ir(dPPOA)]PF6 (x wt %, x = 0.0, 0.5, 1.0, 1.5) as phosphors blended in silicone were fabricated and measured. The emission spectra of these WLEDs are shown in Figure 8 and their performances are listed in Table 2   In order to demonstrate the application of [(TPTA) 2 Ir(dPPOA)]PF 6 for WLEDs, a series of blue GaN-based WLEDs using YAG:Ce (1.0 wt %) and [(TPTA) 2 Ir(dPPOA)]PF 6 (x wt %, x = 0.0, 0.5, 1.0, 1.5) as phosphors blended in silicone were fabricated and measured. The emission spectra of these WLEDs are shown in Figure 8 and their performances are listed in Table 2. As mentioned previously, the emission peaks of blue GaN chips were also on the left, and their maximum wavelengths were also around 450 nm. On the right, the broad emission peaks from 500 nm to 725 nm were the mixture of emissions from YAG:Ce and [(TPTA) 2 Ir(dPPOA)]PF 6   respectively. In contrast with WLED No. g (128.5 lm•W −1 ), the luminous efficiencies decreased due to energy loss in light conversion. Besides some loss of luminous efficiencies when blue light was directly absorbed by [(TPTA)2Ir(dPPOA)]PF6 and transformed into orange light, some of the yellow light emitted from YAG:Ce was also absorbed by [(TPTA)2Ir(dPPOA)]PF6. Consequently, there was also a loss of luminous. Due to some degree of overlap between the excitation spectra of the iridium complex in silicone and the emission spectra of YAG:Ce (Figures 2 and 3), such secondary light energy conversion aggravated the loss of total luminous efficiencies of the LEDs. By comparison with [Ir(CBT)2(dPPOA)]PF6 which was also used as a red-light-source phosphor in warm WLEDs in our previous work [15], the WLEDs using [Ir(CBT)2(dPPOA)]PF6 exhibited lower loss of luminous efficiencies. Thus, the aforementioned secondary light energy conversion between YAG:Ce and the iridium(III) complex can be ignored, mainly because ligand TPTA make wider excitation spectra for   The luminous efficiencies of WLEDs No. h, i and j were85.2 lm·W −1 , 46.1 lm·W −1 and 45.3 lm·W −1 respectively. In contrast with WLED No. g (128.5 lm·W −1 ), the luminous efficiencies decreased due to energy loss in light conversion. Besides some loss of luminous efficiencies when blue light was directly absorbed by [(TPTA) 2 Ir(dPPOA)]PF 6 and transformed into orange light, some of the yellow light emitted from YAG:Ce was also absorbed by [(TPTA) 2 Ir(dPPOA)]PF 6 . Consequently, there was also a loss of luminous. Due to some degree of overlap between the excitation spectra of the iridium complex in silicone and the emission spectra of YAG:Ce (Figures 2 and 3), such secondary light energy conversion aggravated the loss of total luminous efficiencies of the LEDs. By comparison with [Ir(CBT) 2 (dPPOA)]PF 6 which was also used as a red-light-source phosphor in warm WLEDs in our previous work [15], the WLEDs using [Ir(CBT) 2 (dPPOA)]PF 6 exhibited lower loss of luminous efficiencies. Thus, the aforementioned secondary light energy conversion between YAG:Ce and the iridium(III) complex can be ignored, mainly because ligand TPTA make wider excitation spectra for [(TPTA) 2 Ir(dPPOA)]PF 6 than that of CBT for [Ir(CBT) 2 (dPPOA)]PF 6 . The excitation spectra of [Ir(CBT) 2 (dPPOA)]PF 6 have much lower overlap with the emission spectra of YAG:Ce. Even so, the remaining efficiencies were still encouraging. Moreover, the CRIs of No. h, i and j have been improved, the CCTs of No. h, i and j have been lowered significantly and warmer white lights are obtained. The light emitted from WLED No. h became neutral white light, while the light emitted from WLEDs No. i and j were both warm white light. The CIE chromaticity coordinates of WLEDs No. g, h, i and j were (0.32, 0.33), (0.35, 0.33), (0.39, 0.36) and (0.44, 0.40), respectively. Their CIE chromaticity coordinates and working state photographs are all shown in Figure 9 to visually exhibit the aforementioned changes. In order to further exhibit the changes, the CIE chromaticity coordinates and working state photograph of the orange LED (No. f, only using [(TPTA) 2 Ir(dPPOA)]PF 6 as a phosphor at 6.0 wt %) is also shown in Figure 9, because WLEDs No. h, i and j can be regarded as intermediate devices between WLEDs No. h and orange LED No. f. chromaticity coordinates and working state photographs are all shown in Figure 9 to visually exhibit the aforementioned changes. In order to further exhibit the changes, the CIE chromaticity coordinates and working state photograph of the orange LED (No. f, only using [(TPTA)2Ir(dPPOA)]PF6 as a phosphor at 6.0 wt %) is also shown in Figure 9, because WLEDs No. h, i and j can be regarded as intermediate devices between WLEDs No. h and orange LED No. f.

Conflicts of Interest:
The authors declare no conflict of interest.