Application of a novel cationic iridium(iii) complex as a red phosphor in warm white light-emitting diodes

Article abstract of DOI:10.1039/c5nj02032h

A novel red-emitting cationic iridium(iii) complex, [(L_m)₂Ir(L_a)]PF₆, (L_m: 2-(9-(2-ethylhexyl)-9H-carbazol-3-yl)benzo[d]thiazole, L_a: N,N-diphenyl-4-(5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl)aniline) was synthesized. Its ultraviolet-visible absorption and photoluminescent properties show that the complex can be efficiently excited by a 465 nm-emitting blue GaN chip, its decomposition temperature (T_d) is 340 °C, and its relative emission intensity at 100 °C is 88.3% of 25 °C. Perfect red light with a CIE value of (0.65, 0.34) was obtained when it was used as a phosphor at 6.0 wt% blending concentration in epoxy resin in a blue GaN-based LED. A 465 nm-emitting blue GaN-based LED only using yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) as a phosphor (1.0 wt% in epoxy resin) emitted cold white, and its corresponding color rendering index (CRI) was 74.1, correlated color temperature (CCT) was 6026 K, and luminous efficiency (η_L) was 25.3 lm W^-1. It became a neutral white light LED when the iridium(iii) complex was added at 0.5 wt%, the corresponding CRI was 79.5, CCT was 4004 K, and η_L was 32.6 lm W^-1. It further became a warm white LED when the complex was blended at 1.0 wt% and 1.5 wt%, the corresponding CRI were 80.0 and 79.6, CCT were 3650 K and 3133 K, η_L were 25.5 lm W^-1 and 22.8 lm W^-1, and CIE values were (0.40, 0.39) and (0.43, 0.40), respectively. This complex is a promising red phosphor candidate for red LEDs and warm white LEDs.

Full text of DOI:10.1039/c5nj02032h

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Application of a novel cationic iridium(III) complex
as a red phosphor in warm white light-emitting
diodes†
Cite this:DOI: 10.1039/c5nj02032h
Guoyun Meng, Zeyu Chen, Huaijun Tang,* Yong Liu, Liying Wei and
Zhengliang Wang*
A novel red-emitting cationic iridium(III) complex, [(L
-yl)benzo[d]thiazole, L : N,N-diphenyl-4-(5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl)aniline) was synthesized.
Its ultraviolet-visible absorption and photoluminescent properties show that the complex can be
efficiently excited by a 465 nm-emitting blue GaN chip, its decomposition temperature (T ) is 340 1C,
and its relative emission intensity at 100 1C is 88.3% of 25 1C. Perfect red light with a CIE value of
0.65, 0.34) was obtained when it was used as a phosphor at 6.0 wt% blending concentration in epoxy
resin in a blue GaN-based LED. A 465 nm-emitting blue GaN-based LED only using yellow-emitting
m 2 a 6 m
) Ir(L )]PF , (L : 2-(9-(2-ethylhexyl)-9H-carbazol-
3
a
d
(
3+
3
Y Al
5
O12:Ce (YAG:Ce) as a phosphor (1.0 wt% in epoxy resin) emitted cold white, and its corres-
ponding color rendering index (CRI) was 74.1, correlated color temperature (CCT) was 6026 K, and
ꢀ1
luminous efficiency (Z
was added at 0.5 wt%, the corresponding CRI was 79.5, CCT was 4004 K, and Z
became a warm white LED when the complex was blended at 1.0 wt% and 1.5 wt%, the corresponding CRI
L
) was 25.3 lm W . It became a neutral white light LED when the iridium(III) complex
Received (in Victoria, Australia)
ꢀ
1
1st August 2015,
L
was 32.6 lm W . It further
Accepted 22nd September 2015
ꢀ ꢀ1
1
were 80.0 and 79.6, CCT were 3650 K and 3133 K, Z were 25.5 lm W and 22.8 lm W , and CIE values
L
DOI: 10.1039/c5nj02032h
were (0.40, 0.39) and (0.43, 0.40), respectively. This complex is a promising red phosphor candidate for red
LEDs and warm white LEDs.
www.rsc.org/njc
3
,4,8,9
(yellow/orange) with blue GaN chips.
At present, commer-
1
. Introduction
cial WLEDs are mainly fabricated by the combination of blue
3
+
As next generation solid-state light sources, white light-emitting LED chips and the yellow-emitting Y Al O :Ce (YAG:Ce)
diodes (WLEDs) have attracted intense attention and have been phosphor.
3
5 12
3
–5,7–12
However, because the main emission of
developed rapidly, due to their potential applications in general YAG:Ce is in the greenish yellow region, such WLEDs show a
illumination, full-color displays, liquid crystal display backlights, low color rendering index (CRI) and high correlated color
visible light communications (VLC), automobile headlights, and temperature (CCT) due to the lack of red light component
1–7
7,13–17
optical detection in analytical instruments.
Compared with in their spectra.
There are mainly two approaches to
conventional light sources (incandescent lamps and fluorescent overcome these drawbacks. In the first approach, some other
+
10
3+ 13
3+ 14
lamps), WLEDs can provide some competitive advantages such metal ions (such as Cu , Eu , and Pr
as high efficiency, high brightness, low energy consumption, long YAG:Ce to increase the red light component, and although
lifetimes and environment-friendly properties. slightly red light was emitted by these optimized YAG:Ce, the
White light emission can be achieved by several methods, yellow emission obviously decreased. In the other approach,
) are doped in
1
5–18
such as the combination of tricolor phosphors (blue, green and red phosphors are added in YAG:Ce based WLEDs,
and
red) with UV-InGaN chips, double-color phosphors (green/ comparatively, this is a better approach because better lumi-
yellow and red) with blue GaN chips, and one-color phosphors nescent performances (such as CRI, CCT and efficiency) are
obtained. Good red phosphors are very essential as they play a
Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions,
Joint Research Centre for International Cross-border Ethnic Regions Biomass Clean
Utilization in Yunnan, School of Chemistry & Environment, Yunnan Minzu University,
Kunming 650500, China. E-mail: tanghuaijun@sohu.com, wzhl629@163.com;
Tel: +86-871-65913013
key role in the second approach, and to date, numerous red phos-
phors (such as Ce , Eu or Mn activated inorganic oxysalts;
3+
2+
4+
15,18–23
silicon nitrides; silicon oxynitrides; and thiosilicates;
16,24–26
4+
7,17,27
quantum dots;
europium(III) complexes
perfectly satisfactory red phosphors for practical applications in
Mn activated fluorides;
and organic
28–30
) have been developed for LEDs; however,
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nj02032h
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WLEDs based on YAG:Ce and blue GaN chips are still very scarce have also been used as phosphors in LEDs and some other
due to various drawbacks and problems. These drawbacks and advantages, such as broad emission spectra, good color ten-
problems include excessively strong thermal quenching even at ability and hydrophobic nature without susceptibility to humidity,
2+
3+ 19,31
low temperatures (such as CaZnOS:Eu and Sr CeO :Eu );
have been exhibited. Organic cationic iridium(III) complex lumi-
2
4
deep red and near-IR emission obtaining the low eye sensitivity nescent materials have been rapidly developed in the last decade
4
+ 18
(
such as Ca14Al10Zn
of red phosphors showing poor overlap with blue light emitting cells (LECs)
from GaN chips and therefore making them almost useless due to their high efficiency of 100% theoretical quantum
6
O35:Mn ); the excitation/absorption bands and have been widely applied in light-emitting electrochemical
43,44
45–47
and organic light-emitting diodes (OLEDs)
2
8–30
(
such as organic europium(III) complexes);
and potential efficiency, hydrophobic nature, broad emission, excellent color
degradation under high temperature and humidity conditions tunability via various ligands, and high thermal and photic
2
+
2+
2+
2+
43–47
(
such as M SiO :Eu (M = Ca , Sr , Ba ), (Sr,Ca) Si -
stability.
novel red-emitting cationic iridium(III) complex was synthe-
At present, the most active and promising red phosphors sized, characterized and used as a red phosphor in YAG:Ce
In this study, to develop new red phosphors, a
2
4
3
1ꢀx
3
+
4+ 32
Al O4+xF :Ce and K SiF :Mn ).
x
1ꢀx
2
6
1
6,24–26
systems, such as semiconductor quantum dots (QDs)
and Mn doped fluorides,
based WLEDs with 465 nm-emitting GaN blue chips.
4
+
7,17,27
are also beset with some
drawbacks. Semiconductor QDs are capable of adjusting emission
energy by utilizing the quantum size effect, and their emission
light colors are tunable by changing the particle size of the
nanocrystal during synthesis. The drawbacks of QDs include
environmental concerns because of the fact that many bright
2
. Experimental section
2.1 Synthesis and characterization of the iridium(III) complex
2.1.1 General information. All chemicals and reagents
2+ 24,25
emitting QD materials contain Cd ,
the difficulties of accu- were purchased from chemical reagent companies and used
1
rate size control during synthesis and application, QDs absorption without further purification unless otherwise stated. H NMR
of the emission from other phosphors in the blend, thermal spectra were obtained on a Bruker AV400 spectrometer operating
32
4+
quenching and cost. Mn doped fluorides can be considered at 400 MHz, and tetramethylsilane (TMS) was used as the internal
4+
as improved products of Mn doped oxysalts red materials. The standard. Elemental analyses (EA) were performed on a Vario
4
+
4+
emission of Mn doped in oxysalts (such as Mg
2
TiO
is located in the deep red region, obtained on a Bruker amaZon SL liquid chromatography mass
usually between 650 and 730 nm, which is too far red-shifted spectrometer (LC-MS) with an electrospray ionization (ESI) interface
4
:Mn and EL III Elemental Analysis Instrument. Mass spectra (MS) were
4+ 18,33
Ca Al Zn O :Mn )
14
10
6 35
7,17
for efficient warm WLEDs with a high luminous efficiency.
using acetonitrile as the matrix solvent. Ultraviolet-visible (UV-vis)
4+
4+
However, Mn doped in fluorides (such as K SiF :Mn and absorption spectra were measured on an Agilent 8453 UV-visible
2 6
4
4
+ 7,17,27
2
BaGeF
6
:Mn )
2
exhibit a ‘‘blueshift’’ of the E - A transition Spectroscopy System. Photoluminescence (PL) spectra were
relative to oxide hosts, and their emission usually occurs obtained on a Jobin Yvon FL3-21 spectrofluorometer at room
between 590 and 660 nm. This blueshift is thought to be a temperature and a 450 W xenon lamp was used as the excita-
result of the weakening of the nephelauxetic effect in highly tion source, and the temperature of solid sample was controlled
ionic fluoride crystals, which results in increased Racah para- by a temperature controller (REX-C110, Kaituo Compressor
32
4+
meter values. The narrow–red emission of Mn doped fluorides Parts Co. Ltd, Dongguan City, China). Thermogravimetry (TG)
between 590 and 660 nm can better match eye sensitivity and is and differential thermal gravimetric (DTG) analysis curves were
useful for lighting and display applications because a higher measured on a Netzsch STA449F3 thermal analyzer at a heating
ꢀ
1
CRI and efficacy can be obtained, and at the same time, thermal rate 10 1C min under N
2
. The electroluminescence of LEDs
quenching in these materials is not significant at normal working was obtained on a high accurate array spectrometer (HSP6000,
7
,17,32
temperatures (below 150 1C).
Although the advantages HongPu Optoelectronics Technology Co. Ltd, Hangzhou City,
4
+
of Mn doped fluorides are obvious, their drawbacks must China).
be paid close attention. These drawbacks include the use of
The synthetic route and chemical structure of the cationic
the hazardous and corrosive chemical HF in their synthesis, iridium(III) complex are shown in Scheme 1, and experimental
susceptibility to degradation under high temperature and humidity details and characterization data are given in the following
conditions due to their solubility in water and potential hydro- section.
4
+
ꢀ
32
lysis of Mn –F bonds. Therefore, it is still very important
and urgent to develop new efficient red phosphors as additives [d]thiazole (3). 9-(2-Ethylhexyl)-9H-carbazol (1) and 3-formyl-9-(2-
for WLEDs. ethylhexyl)-9H-carbazol (2) were synthesized according to the
2.1.2 Synthesis of 2-(9-(2-ethylhexyl)-9H-carbazol-3-yl)benzo-
47
To date, most of the phosphors used in LEDs are inorganic published procedure. Compound 2 (2.3 g, 7.49 mmol) and
down-conversion luminescent materials, which is mainly due 2-aminothiophenol (0.94 g, 7.52 mmol) were dissolved in
3
4
to their high stability and energy conversion efficiency. At dimethyl sulfoxide (DMSO, 20 mL). The mixture was stirred at
the same time, many organic down-conversion luminescent 140 1C for 8 h, then poured into water and extracted with
2
8–30
materials, such as organic rare earth complexes,
organic dichloromethane. The organic phase was washed with brine
3
5
36–38
boron complexes, organic iridium(III) complexes,
lumi- and dried by anhydrous MgSO
4
. After the solvent was removed,
3
9,40
40–42
nescent polymers
and small-molecule fluorescent dyes
the residue was purified by silica gel column chromatography using
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reaction mixture until the brown color disappeared, and it was
extracted with ethyl acetate (3 ꢂ 15 mL). The organic phase was
washed with brine (3 ꢂ 10 mL) and dried by anhydrous Na SO .
2
4
The solvent was distilled off and the residue was purified by
silica gel column chromatography using a mixture of ethyl
acetate and dichloromethane (volume rate, 1 : 20) as the eluent
to afford the desired compound 8. Yield 62.5% (0.80 g), yellowish-
1
green solid. H NMR (400 MHz, CDCl
3
, ppm), d: 8.80 (d, 1H,
3
3
J = 4.4 Hz, pyridine–H), d: 8.30 (d, 1H, J = 8.0 Hz, ArH), d: 8.02
d, 2H, J = 8.0 Hz, ArH), d: 7.89 (t, 1H, J = 8.0 Hz, ArH), d: 7.46
t, 1H, J = 6.4 Hz, ArH), d: 7.32 (t, 4H, J = 7.6 Hz, ArH), d: 7.09–
.18 (m, 8H, ArH).
.1.5 Synthesis of the iridium(III) complex. The chloro-bridged
3
3
(
(
3
3
7
2
dimer 4 (0.68 g, 0.32 mmol) and compound 8 (0.25 g, 0.64 mmol)
were added to 1,2-ethanediol (30 mL). The mixture was heated to
150 1C and kept under an argon atmosphere with stirring for 16 h.
After cooling to room temperature, an orange solution was
ꢀ1
4 6
obtained, then 10 mL aqueous solution of NH PF (1.0 mol L )
was added, and a yellow suspension appeared immediately. The
solid was filtered, washed with water and dried under vacuum. The
crude product was purified by column chromatography on silica
gel, eluting with CH
2 2
Cl /MeCN (volume rate, 10 : 1). Yield 80.0%
1
Scheme 1 Synthetic route and chemical structure of the iridium(III) complex.
(
8
0.80 g), brownish red solid. H NMR (400 MHz, CDCl
3
, ppm), d:
.60 (d, 1H, J = 7.6 Hz, ArH), d: 8.53 (s, 1H, ArH), 8.49 (s, 1H, ArH),
3
3
3
d: 8.40 (t, 1H, J = 8.0 Hz, ArH), d: 8.05 (q, 2H, J = 8.0 Hz, ArH), d:
3
dichloromethane and hexane (volume rate, 1 : 1) as the eluent. 7.87–7.95 (m, 5H, ArH), d: 7.40 (t, 2H, J = 7.6 Hz, ArH), d: 7.29–7.35
Yield 68.2% (2.1 g), brownish-yellow oil. H NMR (400 MHz, CDCl , (m, 7H, ArH), d: 7.11–7.22 (m, 13H, ArH), d: 6.99 (d, 2H, J = 8.0 Hz,
1
3
3
3
ppm), d: 8.83 (d, 1H, J = 1.6 Hz, ArH), d: 8.15–8.20 (m, 2H, ArH), ArH), d: 6.33–6.42 (m, 2H, ArH), d: 6.01 (m, 1H, ArH), d: 3.60
3
3
4
3
4
d: 8.07 (d, 1H, J = 8.0 Hz, ArH), d: 7.89 (dd, 1H, J = 8.0 Hz, J = (dd, 4H, J = 14.4 Hz, J = 6.8 Hz, –N–CH –), d: 1.25 (s, 2H, –CHo),
2
3
3
0
.4 Hz, ArH), d: 7.24–7.50 (m, 6H, ArH), d: 4.16 (dd, 2H, J = 7.6 Hz, d: 0.86–1.05 (m, 16H, alkyl-H), d: 0.72 (t, 6H, J = 5.6 Hz, –CH
3
),
] ; Anal.
2
: C, 60.92; H, 4.65; N, 7.29%. Found: C,
4
4
+
J = 2.0 Hz, J = 3.5 Hz, –N–CH
1
2
–), d: 2.01–2.11 (m, 1H, –CHo), d: d: 0.46–0.57 (m, 6H, –CH
3
); ESI-MS (m/Z): 1405.4 [M ꢀ PF
OPS
S: C, 78.60; H, 6.84; 61.57; H, 4.86; N, 6.94%.
N, 6.79%. Found: C, 78.65; H, 6.65; N, 6.81%.
.1.3 Synthesis of the chloro-bridged dimer (4). A mixture
of IrCl ꢁ3H O (0.89 g, 2.5 mmol) and compound 3 (2.10 g,
6
3
.26–1.39 (m, 8H, alkyl–H), d: 0.91 (t, 3H, J = 5.7 Hz, –CH
3
71 6
), d: 0.86 Calc. for C78H F IrN
8
3
3 28 2
(t, 3H, J = 4.4 Hz, –CH ). Calc. for C27H N
2
2.2 LEDs fabrication and measurements
3
2
5
.10 mmol) in H
2
O (8 mL) and 2-methoxyethanol (24 mL) was Two types of LEDs were fabricated and measured in this study,
refluxed under argon for 24 h. After cooling to room temperature, one type is a red LED only using the cationic iridium(III)
the resultant yellow precipitate was collected on a filter, washed complex as the phosphor and the other type is a WLED using
with water and methanol alternately, and then dried in a vacuum. the cationic iridium(III) complex together with YAG:Ce as phos-
Yield 84.3% (2.20 g), yellow solid. This dimer product was directly phors. In the case of the red LED devices, the iridium(III)
used for the next step after drying under vacuum without further complex was blended in epoxy resin homogeneously at the
purification and characterization.
concentrations of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 wt% and coated
2
.1.4 Synthesis of N,N-diphenyl-4-(5-(pyridin-2-yl)-1,3,4- on the surface of 465 nm-emitting GaN chips until the reflective
oxadiazol-2-yl)aniline (8). 1-(Pyridine-2-yl)propan-1-one (5) and cavities were just filled. The coated chips were dried and
picolinohydrazide (6) were synthesized according to the proce- solidified at 150 1C for 1 h. Then, they were completely
4
8
dure described in a previous publication. 4-(Diphenylamino)- encapsulated with a transparent epoxy resin, which was poured
benzaldehyde (7) also was prepared according to the published in semispherical molds. The WLEDs were fabricated using the
4
9
procedure. A solution of compound 6 (0.45 g, 3.28 mmol) and same procedure for the abovementioned red LEDs. In the
compound 7 (0.90 g, 3.28 mmol) in ethanol (10 mL) was fabrication of the WLEDs, the YAG:Ce phosphor was blended
refluxed under Ar atmosphere for 8 h, and then the solvent in epoxy resin at a constant mass ratio of 1.0 wt%; however, the
was removed by vacuum distillation. The resultant residue was iridium(III) complex phosphor was blended together at different
redissolved in DMSO (10 mL), and potassium carbonate (1.37 g, mass ratios of 0.0, 0.5, 1.0 and 1.5 wt%. The performances of all
9
.91 mmol) and iodine (1.2 g, 4.73 mmol) were added. The the LEDs were measured using an integrating sphere spectro-
mixture was stirred at 100 1C for 4 h. After cooling to room radiometer system (Everfine PMS-50). The LEDs were operated
temperature, a solution of Na
(5%) was dropped in the at 20 mA forward current and 5 V reverse voltage.
2 2 3
S O
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3. Results and discussion
3.1 UV-vis absorption spectrum
The UV-vis absorption spectrum of the cationic iridium(III)
complex was obtained in CH Cl solution at room temperature
2 2
and this spectrum together with the emission spectra of
the blue GaN chip and YAG:Ce are shown in Fig. 1. There are
three strong peaks on the absorption spectrum, and the corres-
ponding maximum absorption wavelengths (labs,max) are 242 nm
4
ꢀ1
ꢀ1
(
1
e2
= 6.79 ꢂ 10 L mol cm ), 324 nm (e
= 6.28 ꢂ
42nm
324nm
4
ꢀ1
ꢀ1
4
ꢀ1 ꢀ1
0 L mol cm ) and 401 nm (e
These strong absorption bands are mainly caused by the spin-
allowed p–p* transition in the ligands. The relatively weak
= 5.20 ꢂ 10 L mol cm ).
4
01nm
1
50
absorption bands at the longer wavelength region (about 420–
5
65 nm) can be attributed to an admixture of the spin-allowed
1
Fig. 2 Normalized excitation (Ex, lem = 639 nm) and emission (Em, lex
=
singlet metal-to-ligand charge-transfer ( MLCT) and spin-
3
2 2
465 nm) spectra of the cationic iridium(III) complex in CH Cl solution at
forbidden triplet metal-to-ligand charge-transfer ( MLCT) and
5
ꢀ1
1
.0 ꢂ 10ꢀ mol L and blended in epoxy resin at 6.0 wt% (coated on quartz
3
50,51
p–p* in the ligands.
MLCT and p–p* with a higher-lying MLCT is caused by the
The admixture absorption of the
plate). Inset: an image of the iridium(III) complex powder excited by blue
3
3
1
light (lex = 465 nm).
strong spin–orbit coupling induced by the heavy iridium
5
1
atom. As shown in Fig. 1, there is a large overlap between
the absorption spectrum of the iridium(III) complex and emis-
sion spectrum of the blue GaN chip (lem,max = 465 nm), which
suggests that F o¨ rster resonance energy transfer (FRET) is able
to occur between them easily and this iridium(III) complex can
be well excited by the blue GaN chip. On the contrary, the
overlap between the absorption spectrum of the iridium(III)
complex and the emission spectrum of YAG:Ce is very limited;
therefore, the emission of YAG:Ce almost would not be affected
by the iridium(III) complex and such iridium(III) complex is a
practicable red luminescent additive for YAG:Ce based WLEDs.
solution and blended in epoxy resin are very similar to each
other, which suggests that the conjugated systems of the
complex in epoxy resin almost do not enlarge probably due to
no palpable molecular interaction and p–p piles at low blending
concentrations. In general, sufficient overlap between the excita-
tion spectra of phosphors and emission spectra of LED chips is
very necessary to realize energy transfer from the LED chips to
phosphors efficiently. In this study, the excitation spectra of the
cationic iridium(III) complex in CH Cl solution and blended in
5
2
2
2
epoxy resin both lie between 245 nm to 510 nm and both cover the
emission spectra of the 465-emitting blue GaN chip (as shown in
Fig. 1), which means that this cationic iridium(III) complex can be
3.2 Photoluminescent property
The normalized excitation (Ex, lem = 639 nm) and emission efficiently excited by the blue GaN chip. At the same time, the
(
Em, lex = 465 nm) spectra of the cationic iridium(III) complex overlaps between the excitation spectra of the iridium(III) complex
ꢀ5
ꢀ1
in CH
2
Cl
2
solution at 1.0 ꢂ 10 mol L and blended in epoxy and the emission spectrum of YAG:Ce (as shown in Fig. 1) are still
resin at 6.0 wt% are shown in Fig. 2. Both the excitation and very limited, which again demonstrates that the emission of
emission spectra of the cationic iridium(III) complex in CH Cl
YAG:Ce almost would not be affected by the iridium(III) complex
and both of them can be efficiently excited by the blue GaN chip.
The emission spectra of the cationic iridium(III) complex mainly
lie between 550 nm and 840 nm with the maximum emission
wavelengths of 641 nm (in solution) and 635 nm (in epoxy resin)
respectively; therefore, it is a perfect red phosphor.
2
2
3.3 Thermal stability and thermal quenching properties
Inorganic luminescent materials are usually synthesized using
two methods: high temperature solid-state reactions (such as
4
+
YAG:Ce) and solution-based chemical synthesis (such as Mn
3
4
doped fluorides). The thermal stability of inorganic lumines-
cent materials prepared using solid-state reactions do not need
to be additionally investigated, because the materials already
have encountered high temperature during their synthesis.
However, the thermal stability of organic luminescent materials
must been studied due to the exothermic working process of
the LEDs and heat treatment in the device fabrication.
Fig. 1 Normalized UV-vis absorption spectrum of the cationic iridium(III)
ꢀ5
ꢀ1
complex in CH
2
Cl
2
solution at 1.0 ꢂ 10 mol L and emission spectra of
the blue GaN chip and YAG:Ce.
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Fig. 3 TG and DTG curves of the iridium(III) complex.
Fig. 5 Relative PL intensity of the cationic iridium(III) complex as a func-
The thermal stability of the cationic iridium(III) complex was
investigated using TG and DTG under nitrogen atmosphere,
and the results are shown in Fig. 3. A tiny mass loss (about
tion of temperature. The inset represents ln(I
o
/I ꢀ 1) versus 1/T and the
calculated activation energy (E ) for the cationic iridium(III) complex.
a
1
.0%) occurred in the TG curve between 150 1C and 210 1C due
PL spectra (lex = 465 nm) of the cationic iridium(III) complex. The
relative PL intensity of the cationic iridium(III) complex as a
function of temperature is shown in Fig. 5. From 25 1C to
to the loss of adsorptive water and organic solvent residues.
With an increase in temperature, the TG curve begins to decrease
suddenly after about 340 1C, which means that thermal decom-
position happened and 340 1C is its thermal decomposition
200 1C, the profile, wavelength band and the maximum wave-
lengths (641 nm) of the emission spectra at different tempera-
tures almost do not change, except for intensity, which suggests
that the light-emitting color of the iridium(III) complex itself has
high thermal stability. Similar to most phosphors used in LEDs,
the emission intensity of the iridium(III) complex decreases with
temperature (T ). From 210 1C to 405 1C, a big mass loss of
d
24.6% occurred mainly due to the loss of the neutral auxiliary
ligand, N,N-diphenyl-4-(5-(pyridine-2-yl)-1,3,4-oxadiazol-2-yl)aniline,
because the coordination bond is the weakest bond in such
4
7
heteroleptic complex. Then, the main anionic ligand (9-(2-
4,32
increasing temperature (i.e. thermal quenching).
The relative
ethylhexyl)-9H-carbazol-3-yl) benzo[d]thiazole began to loosen
PL intensities at different temperatures are (in decreasing order):
and decompose. This high decomposition temperature (T
d
=
1
1
6
00% (at 25 1C), 98.7% (at 50 1C), 94.3% (at 75 1C), 88.3% (at
00 1C), 81.8% (at 125 1C), 77.1% (at 150 1C), 69.0% (at 175 1C),
1.2% (at 200 1C). The activation energy (E ) of the thermal
a
3
40 1C) of the iridium(III) complex suggests that it has high
thermal stability, which is enough to meet the requirement of
its application in LEDs, because LEDs devices are fabricated
and work usually at a temperature below 150 1C.
The thermal quenching properties of the iridium(III) complex
were also investigated. Fig. 4 depicts the temperature-dependent
3
0
quenching can be described by the Arrhenius equation as
follows:
18,20,23
Io
I ¼
ꢀ
ꢁ
Ea
1
þ A exp ꢀ
kBT
where I and I
complex at the experimental temperature and room temperature
RT, 25 1C), A is a constant, and k is Boltzmann’s constant. From
o
represent the PL intensity of the iridium(III)
(
B
the Arrhenius equation, the relationship of ln(I /I ꢀ 1) with 1/T
o
can be obtained, and the experimental data are well-fitted and
shown in Fig. 5 (inset). E
(E /k ), and the calculated value of E
phosphors, a higher E value means lower thermal quenching in
a same range of rising temperature. Although the PL intensity
decay rate and E value of the iridium(III) complex show that its
a
can be calculated by the slope value of
ꢀ
a
B
a
is 0.3183 eV. For different
a
a
thermal quenching is greater than that of some red phosphors,
2
+
15
which mainly include some Eu activated silicon nitrides,
4+
7,17,18
Mn activated oxysalts and fluorides,
is much lower than many recently reported red phosphors.
This result indicates that the iridium(III) complex has the potential
its thermal quenching
1
9,23,31,32
Fig. 4 Temperature-dependent PL spectra (lex = 465 nm) of the cationic
iridium(III) complex measured with increasing temperature from 25 1C to 200 1C. as a red phosphor for WLED applications.
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.4 Performances of the LEDs
NJC
3
Table 1 Performances of the blue GaN-based LEDs obtained using only
the cationic iridium(III) complex as the phosphor at different blending
The luminescent property of GaN-based LEDs only using the
cationic iridium(III) complex as the phosphor was investigated
at first. The emission spectra of the LEDs using the complex as the
phosphor blended in epoxy resin at different concentrations at
a
concentrations
Blending
No. of concentration efficiency
Luminous
l
em,max
CRI CCT (K) (nm)
ꢀ1
LEDs (wt%)
(lm W
)
CIE (x, y)
2
0 mA forward current are shown in Fig. 6, and their detailed
a
b
c
d
e
f
1.0
2.0
3.0
4.0
5.0
6.0
7.8
5.2
4.3
3.6
2.4
1.0
8.3 100 000 470, 623 (0.26, 0.22)
10.6 19 641 468, 628 (0.29, 0.22)
performances are summarized in Table 1. The sharp blue emission
peaks with the maximum wavelengths around 469 nm obviously
originate from the emission of the blue GaN chips. The broad
emission bands from 560 nm to 780 nm with the maximum peaks
around 630 nm can be ascribed to the emission of the cationic
iridium(III) complex, because they are primarily consistent with its PL
spectra. With an increase in the blending concentrations, the
emission peaks of the GaN chip gradually decrease and eventually
disappear at 6.0 wt%. On the contrary, the emission peaks of the
cationic iridium(III) complex gradually increase in intensity. With the
change in blending concentrations, the CIE (Commission Inter-
nationale de L’Eclairage) chromaticity coordinates of the LEDs grad-
ually move from the cold white light region to the red light region. At
24.2
30.6
62.9
71.9
3984 470, 627 (0.35, 0.27)
2925 470, 629 (0.38, 0.29)
1490 471, 638 (0.54, 0.34)
1010 641
(0.65, 0.34)
a
At 20 mA forward current and 5 V reverse voltage.
6.0 wt%, only pure red light of the complex was emitted, and a
relatively high CRI of 71.9 and a low CCT of 1010 K were achieved.
The CIE became (0.65, 0.34) which was very close to the ‘‘ideal red’’
value (0.67, 0.33) of the NTSC (National Television Standard
Committee). These results show that the cationic iridium(III) complex
is a potential red phosphor that can improve the quality of YAG:Ce
based WLEDs via increasing the CRI and lowering the CCT.
To demonstrate the application of the cationic iridium(III)
complex for WLEDs, a series of blue GaN-based WLEDs were
fabricated using YAG:Ce (1.0 wt%) and the cationic iridium(III) Fig. 7 Emission spectra of the blue GaN-based WLEDs using YAG:Ce
complex (x wt%, x = 0.0, 0.5, 1.0, 1.5) as phosphors blended in (1.0 wt%) and the cationic iridium(III) complex (x wt%, x = 0.0, 0.5, 1.0, and
1
.5) as phosphors blended in epoxy resin at different concentrations.
epoxy resin at different concentrations. The emission spectra of
these WLEDs are shown in Fig. 7 and their performances are
listed in the Table 2. It can be observed that the emission peaks
on the right show an obvious red-shift after the cationic
iridium(III) complex was blended, and at the same time, the
red-shift is positively related to the blending concentration.
The maximum emission wavelengths (lem,max) of these peaks
Table 2 Performances of the blue GaN-based WLEDs obtained using
YAG:Ce and the cationic iridium(III) complex as phosphors at different
blending concentrations
a
Blending
concentration
Luminous
(wt%)
No. of
efficiency
CCT
CRI (K) (nm)
l
em,max
ꢀ1
LEDs YAG:Ce complex (lm W
)
CIE (x, y)
g
h
i
1.0
1.0
1.0
1.0
0.0
0.5
1.0
1.5
25.3
32.6
25.5
22.8
74.1 6026 470, 552 (0.32, 0.38)
79.5 4004 468, 593 (0.38, 0.38)
80.0 3650 468, 598 (0.40, 0.39)
79.6 3133 469, 604 (0.43, 0.40)
j
a
At 20 mA forward current and 5 V reverse voltage.
change from 552 nm to 593 nm, then successively to 598 nm
and 604 nm. Moreover, the intensity of the emission peaks on
the right increase with the increase of the blending concentrations,
because the blending concentration of YAG:Ce is constant; there-
fore, the increase of the emission intensity is inevitably caused by
the increase of the cationic iridium(III) complex. With the red-shift
of the emission peaks on the right, the CRI of the devices is
improved, and the CCT is lowered. The broad red emission of the
cationic iridium(III) complex is distinctly different from the sharp
Fig. 6 Emission spectra of the blue GaN-based LEDs only using the cationic
iridium(III) complex as the phosphor at different blending concentrations, at
28–30
20 mA forward current.
red emission of organic europium(III) complexes
and some
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it can be efficiently excited by a 465 nm-emitting blue GaN chip;
however, the emission of YAG:Ce would almost not be affected
by it. It has high thermal stability with a decomposition
temperature (T ) of 340 1C, and its relative emission intensity
d
at 100 1C is 88.3% of 25 1C. This cationic iridium(III) complex
can effectively improve the red light component of WLEDs, and
cold white GaN-based LEDs only using YAG:Ce (at 1.0 wt%) can
become neutral white LEDs when the complex is blended in
epoxy resin at low concentrations (such as 0.5 wt%), and can
further become warm white LEDs with the increase of the
blending concentrations (such as 1.0 wt%, and 1.5 wt%). This
cationic iridium(III) complex is a promising red phosphor
candidate for red LEDs and warm white LEDs.
Acknowledgements
This study was supported by National Nature Science Founda-
tion of China (No. 21262046 and 21261027), the Key Scientific
Research Fund of Yunnan Province Education Department
(No. 2011Z003), and the Innovation Program for postgraduate
Fig. 8 CIE chromaticity coordinates of the blue GaN-based LEDs using of Yunnan Minzu University (No. 2015YJCXY263).
YAG:Ce and the cationic iridium(III) complex as phosphors at different
blending concentrations. (f) only 6.0 wt% complex; (g) only 1.0 wt%
YAG:Ce; (h) 1.0 wt% YAG:Ce and 0.5 wt% complex; (i) 1.0 wt% YAG:Ce
and 1.0 wt% complex; (j) 1.0 wt% YAG:Ce and 1.5 wt% complex. Inset: References
Images of the LEDs (g), (h), (j) and (f) in the working state.
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