Efficient and high colour-purity green-light polymer light-emitting diodes (PLEDs) based on a PVK-supported Tb3+-containing metallopolymer

Article abstract of DOI:10.1039/c7tc02092a

Based on the free-radical copolymerization of the green-light complex monomer [Tb(eb-PMP)₃(4-VB-PBI)] (2; Heb-PMP = 4-(2-ethylbutanoyl)-1-phenyl-3-methyl-1H-pyrazol-5(4H)-one; 4-VB-PBI = 1-(4-vinylbenzyl)-(pyridin-2-yl)-1H-benzo[d]imidazole) with NVK (N-vinylcarbazole), the obtained Tb³⁺-containing metallopolymer poly(NVK-co-2) with an energy transfer from additional PVK to Tb³⁺ ion was used as the emitting material of polymer light-emitting diodes (PLEDs); it exhibited record renewed efficiencies (24.63 cd A^-1, 7.81%, and 6.45 lm W^-1) of Tb³⁺-based extreme colour-purity green-light PLEDs and superior performance of colour-stability and low efficiency roll-off to small-molecule Tb³⁺-complex's OLEDs.

Full text of DOI:10.1039/c7tc02092a

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Efficient and high colour-purity green-light polymer light-emitting
diodes (PLEDs) based on PVK-supported Tb3+-containing
metallopolymer
Received 00th January 20xx,
Accepted 00th January 20xx
Lin Liu,†a Mengyuan Pang,†a Hongting Chen,b Guorui Fu,a Baoning Li,a Xingqiang Lü*a and
Lei Wangb
DOI: 10.1039/x0xx00000x
www.rsc.org/
Based on the free-radical copolymerization of the green-light complex monomer [Tb(eb-PMP)3(4-VB-PBI)] (2; Heb-PMP =
4-(2-ethylbutanoyl)-1-phenyl-3-methyl-1H-pyrazol-5(4H)-one;
4-VB-PBI
=
1-(4-vinylbenzyl)-(pyridin-2-yl)-1H-
benzo[d]imidazole) with NVK (N-vinylcarbazole), the obtained Tb3+-containing metallopolymer Poly(NVK-co-2) with
additional PVK to Tb3+ ion energy transfer is used as the emitting material of polymer light-emitting diodes (PLEDs),
exhibiting record-renewed efficiencies (24.63 cd/A, 7.81% and 6.45 lm/W) of Tb3+-based extreme colour-purity green-light
PLEDs, and superior performance of colour-stability and low efficiency roll-off to small-molecule Tb3+-complex's OLEDs.
complexity from the required multilayer device structure and the
notorious efficiency roll-off from crystallization-undergoing and
thermal dissociation of small-molecule organo-Tb3+-complexes, the
1. Introduction
Due to large Stokes shift, µs- to ms-grade long lifetime and
characteristic narrow line-like emissions of Tb3+ ion,1 concerted
efforts have been devoted to the development of new kinds of
organo-Tb3+-based high colour-purity green-light materials with
potential applications in monochromatic2 or panchromatic3
electroluminescent (EL) devices, colour-tunable pigments4 and
fluoro-immunoassay.5 Especially in contrast to green-light organic
light-emitting diodes (OLEDs) from π-π*-transitioned organic
chromophores6 or MLCT-induced (metal-to-ligand charge-transfer)
Ir3+-complexes,7 organo-Tb3+-based phosphorescent OLEDs are
inevitable high-cost and the restriction of OLEDs' device size to
relatively small areas from the vacuum-deposition fabrication also
impede their popularity.
Conceivably, the circumvention of these problems, to some
extent, can rely on a solution-processing procedure through doping
organo-Tb3+-complex into polymeric matrix for cost-effective and
large-area polymer light-emitting diodes (PLEDs).14 However, this
strategy still suffers from
a phase-separation issue with the
discrepant compatibility of different materials, also endowing a
detrimental adventure of performance instability. Toward this end,
another promising approach to Tb3+-containing metallopolymer
should be much desirable, due to the advantages of colour-purity
green-light defined by Tb3+ ion and the polymer-assisted ideal
physical properties for PLEDs. More importantly, organo-Tb3+-
emitters can actually be covalently-grafted and molecularly
dispersed into the polymeric matrix in avoidance of emitters'
aggregation.15 Moreover, additional polymer to Tb3+ ion energy
transfer can be further activated in single-phase Tb3+-containing
metallopolymer. In this context, although there have few examples
of green-light PLED16 realized from the corresponding polyurea or
poly(norbornene)-supported Tb3+-containing metallopolymer, their
unsatisfactory efficiencies (0.07 a. u. of eternal quantum efficiency
at 0.5 mA/cm2 or 0.56 cd/A and 0.16 lm/W) are faced, probably
attributing to low photoluminescent efficiency of the optoelectron-
inert metallopolymers and the unbalanced carrier injection and
transport of their devices' design. Furthermore, compared with
worthy of
superiority to extreme colour-purity green-light achievable from
Tb3+-characteristic electronic transitions, besides possessing
a particular interest because of their prominent
a
theoretical 100% emission efficiency as phosphorescent Ir3+-
complexes while without the limitation of a maximum 25% to
fluorescent organic chromophores. As a matter of fact, since the
pioneer work8 of a green-light OLED consisting of the emitting Tb3+-
acetylacetonate-complex layer through vacuum deposition by Kido
et al, significant concerns have been focused on volatility-
dependent small-molecule organo-Tb3+-compounds, where the
maximum luminance of those Tb3+-based OLEDs are improved from
the unit8 (cd/m2), to the decade,9 to the hundred10 and to the
thousand place11 or above12, especially at a practical brightness of
100 cd/m2, the best current efficiency13 can be up to 30 cd/A at the
luminous efficiency of 20 lm/W. Nonetheless, besides the certain
much endeavour to colour-purity red-light PLEDs17 from Eu3+
-
grafted metallopolymers due to the scarce of saturated red-light in
nature, reports of Tb3+-metallopolymers' colour-purity green-light
PLEDs16 are very rare, and their electroluminescent performance
competitive to those8-13 of Tb3+-based OLEDs remains
a real
challenge. Herein, through the copolymerization of green-light-
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room temperature, each viscous mixture was diluted with absolute
toluene (10 mL) and precipitated with absolute n-hexane (100 mL)
emitting vinyl-modified complex monomer [Tb(eb-PMP)3(4-VB-PBI)]
(2) with NVK, new PVK-supported Tb3+-containing metallopolymers
with the deserved PVK to Tb3+ energy transfer are obtained, and
their colour-purity green-light PLEDs with significantly improved EL
performance are also expected.
2. Experimental section
Synthesis of the series of vinyl-containing complex monomers
[Ln(eb-PMP)3(4-VB-PBI)] (Ln = La, 1; Tb, 2 or Gd, 3)
To the stirred EtOH solution (15 mL) of the pyrazolonate ligand Heb-
PMP (0.6 mmol, 163 mg), an equimolar amount of solid NaOH (0.6
mmol, 24 mg) was added, and the resultant mixture was refluxed
for 3 h. Another aqueous solution (10 mL) of Ln(NO3)3∙6H2O (0.2
mmol, Ln = La, 87 mg; Ln = Tb, 91 mg or Ln = Gd, 91 mg) and solid 4-
VB-PBI (0.2 mmol, 62 mg) was added, and each of the resulting
solutions was refluxed under an N2 atmosphere for another 3 h,
respectively. After cooling to room temperature, each of the
obtained clear yellow solutions was filtered, and left to stand for
several days to give the pale yellow microcrystalline products of
complex monomers 1-3, respectively.
Scheme 1. Reaction scheme for the synthesis of the ligands Heb-PMP and 4-VB-
PBI, vinyl-modified complex monomers 1-3 and metallopolymers Poly(NVK-co-
[Ln(eb-PMP)3(4-VB-PBI)]).
for three times. The resulting solid products were collected by
filtration and dried at 45 °C under vacuum to constant weight,
respectively.
For [La(eb-PMP)3(4-VB-PBI)] (1): Yield: 192 mg (76%). Calc. for
C69H74N9O6La: C, 65.55; H, 5.90; N, 9.97%. Found: C, 65.34; H, 5.81;
N, 9.93%. FT-IR (KBr, cm-1): 2963 (w), 2923 (w), 2870 (w), 1635 (m),
1613 (vs), 1592 (s), 1534 (w), 1489 (s), 1437 (m), 1401 (w), 1370
(m), 1330 (w), 1257 (w), 1197 (w), 1131 (w), 1072 (m), 1030 (w),
988 (m), 907 (w), 827 (m), 784 (w), 756 (m), 693 (m), 653 (m), 620
(m), 572 (w), 516 (w), 436 (w). 1H NMR (400 MHz, DMSO-d6): δ
(ppm) 8.72 (d, 1H, -Py), 8.38 (d, 1H, -Py), 8.12 (b, 1H, -Py), 8.02 (b,
6H, -Ph), 7.77 (m, 1H, -Py), 7.60 (m, 1H, -Py), 7.53 (t, 1H, -Ph), 7.35
(d, 6H, -Ph), 7.29 (m, 3H, -Ph), 7.23 (m, 2H, -Ph), 7.11 (d, 2H, -Ph),
6.91 (s, 2H, -Ph), 6.65 (m, 1H, -CH=), 6.23 (d, 2H, -CH2), 5.77 (m, 1H,
=CH2), 5.20 (m, 1H, =CH2), 2.45 (b, 3H, -CH), 2.34 (b, 6H, -CH2), 2.18
(b, 6H, -CH2), 1.34 (s, 9H, -CH3), 1.10 (s, 18H, -CH3). ESI-MS (in
MeCN) m/z: 1264.35 (100%), [M-H]+.
For [Tb(eb-PMP)3(4-VB-PBI)] (2): Yield: 185 mg (72%). Calc. for
C69H74N9O6Tb: C, 64.53; H, 5.81; N, 9.82%. Found: C, 64.38; H, 5.73;
N, 9.69%. FT-IR (KBr, cm-1): 2961 (w), 2930 (w), 2872 (w), 1638 (m),
1615 (vs), 1593 (m), 1535 (w), 1496 (s), 1439 (m), 1400 (w), 1372
(m), 1331 (w), 1264 (w), 1202 (w), 1134 (w), 1073 (m), 1032 (w),
989 (m), 909 (w), 828 (m), 755 (m), 693 (w), 652 (m), 621 (m), 574
(w), 519 (w), 452 (w). ESI-MS (in MeCN) m/z: 1285.54 (100%), [M-
H]+.
For Poly(NVK-co-1) (100:1): Yield: 80%. FT-IR (KBr, cm-1): 3059
(w), 2928 (w), 1626 (w), 1595 (m), 1482 (m), 1451 (s), 1410 (w),
1331 (m), 1222 (m), 1159 (m), 1127 (w), 1076 (w), 1026 (w), 1000
(w), 925 (w), 869 (w), 836 (w), 780 (w), 745 (vs), 722 (s), 697 (w),
658 (m), 639 (m), 617 (w), 573 (w), 531 (w), 473 (w), 428 (w). 1H
NMR (400 MHz, DMSO-d6): δ (ppm) 8.58 (d, 1H, -Py), 8.22-3.95 (b,
640H + 26H), 3.95-2.15 (b, 80H + 20H), 2.15-0.50 (b, 160H +27 H).
For Poly(NVK-co-2) (50 : 1, 75:1, 100 : 1, 200 : 1 or 400 : 1):
Yield: 77% (50 : 1); 79% (75 : 1); 81% (100 : 1); 84% (200 : 1); 86%
(400 : 1). FT-IR (KBr, cm-1): 3055 (w), 2930 (w), 1623 (w), 1597 (m),
1482 (m), 1452 (s), 1410 (w), 1330 (m), 1222 (m), 1157 (m), 1124
(w), 1078 (w), 1027 (w), 1002 (w), 925 (w), 867 (w), 835 (w), 783
(w),745 (vs), 722 (s), 694 (w), 654 (m), 638 (m), 616 (w), 575 (w),
534 (w), 476 (w), 429 (w).
Structural design of PLEDs based on Poly(NVK-co-2) (50:1)
The two PLEDs were fabricated by spin-coating with the same
configuration of ITO/PEDOT:PSS (30 nm)/Poly(NVK-co-2) (40
nm)/BCP (10 nm or 20 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm)
while a thickness 10 or 20 nm of BCP (2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline) adopted, respectively. Among those
materials, ITO (indium tin oxide) is the coated glass substrate,
For [Gd(eb-PMP)3(4-VB-PBI)] (3): Yield: 192 mg (74 %). Calc. for
C69H74N9O6Gd: C, 64.61; H, 5.82; N, 9.83%. Found: C, 64.48; H, 5.91;
N, 9.79%. FT-IR (KBr, cm-1): 2922 (w), 2851 (w), 1642 (m), 1614 (vs),
1594 (m), 1536 (w), 1496 (s), 1437 (m), 1401 (w), 1371 (m), 1330
(w), 1261 (w), 1203 (w), 1153 (w), 1072 (m), 1034 (w), 988 (m), 909
(w), 827 (m), 755 (m), 693 (w), 650 (m), 621 (m), 578 (w), 524 (w),
449 (w). ESI-MS (in MeCN) m/z: 1283.26 (100%), [M-H]+.
PEDOT:PSS
ethylenedioxythiophene):poly(styrenesulfonate)) is used as the
hole-injecting material, TPBI (1,3,5-tris(2-N-
phenylbenzimidazolyl)benzene) is applied as the electronic-
transporting material, and BCP acts as the hole-blocking material.
The two devices' fabrication and testing in detailed are also
presented in ESI.
(poly(3,4-
Synthesis of metallopolymers Poly(NVK-co-[Ln(eb-PMP)3(4-VB-
PBI)]) (Ln = La, 1 or Tb, 2)
3. Results and discussion
A
mixture of NVK, and one of the vinyl-modified complex
monomers 1-2 at a stipulated feed molar ratio (50:1, 75:1, 100:1,
200:1 or 400:1) in the presence of AIBN initiator (1.5 mol% of NVK)
was dissolved in absolute toluene (15 mL), respectively. The
respective mixture was heated to 80 °C with continuous stirring for
48 h under a reduced N2 atmosphere. All the reaction mixtures
remained clear throughout the polymerization. After cooling to
Synthesis, characterization and photophysical property of complex
monomers [Ln(eb-PMP)3(4-VB-PBI) (Ln = La, 1; Tb, 2 or Gd, 3)
The pyrazolone ligand Heb-PMP was synthesized from the
nucleophilic replacement reaction18 of 1-phenyl-3-methyl-
pyrazolone-5 (PMP) and 2-ethylbutyryl chloride in a yield of 43%. As
to the vinyl-functionalized ancillary ligand 4-VB-PBI, it was
synthesized from the reaction19 of 1-(chloromethyl)4-vinylbenzene
2 | J. Name., 2012, 00, 1-3
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and 2-(pyridine-2-yl)-1H-benzo[d]imidazole in the presence of KOH.
As shown in Scheme 1, based on the self-assembly of the
deprotonated pyrazolonate ligand (eb-PMP)-, the ancillary ligand 4-
VB-PBI and Ln(NO3)3∙6H2O (Ln = La, Tb or Gd) in a molar ratio of
3:1:1, vinyl-containing complex monomers [Ln(eb-PMP)3(4-VB-PBI)
(Ln = La, 1; Ln = Tb, 2 or Ln = Gd, 3) were obtained and well-
characterized by EA, FT-IR, 1H NMR and ESI-MS, respectively.
Especially in the 1H NMR spectrum (Figure 1S) of anti-
ferromagnetic La3+-based complex monomer 1, besides the
combined (δ = 8.72-1.10 ppm) and stipulated proton molar ratio
(3:1) of (eb-PMP)- to 4-VB-PBI, the proton resonances (δ = 6.65,
Fig. 1. Emission and excitation spectra of complex monomers 2-3 in MeCN
5.77 and 5.20 ppm) of the functional vinyl group remain almost solution (2 × 10-5 M) at room temperature or 77 K.
identical to those (δ = 6.63, 5.73 and 5.19 ppm) of 4-VB-PBI despite
the coordination of La3+ ion. Moreover, the ESI-MS spectra of
complexes 1-3 in absolute MeCN display similar patterns, and
exhibit a strong mass peak at m/z 1264.35 (1), 1285.54 (2) or
1283.26 (3) assigned to the major species [M-H]+ of complex
monomers 1-3, respectively. These observations further indicate
that the respective discrete neutral tris-pyrazolonate-Ln-complex
unit of complexes 1-3 is stable in their respective dilute solution.
The photophysical properties of complex monomers 2-3 were
examined in dilute MeCN solution at room temperature or 77 K,
and summarized in Figures 1 and 2S. As shown in Figure 2S, the
similar ligands-centered absorption spectra (262-264 and 305-308
nm) of complex monomers 2-3 are observed, in which the lower
energy absorptions (305-308 nm) assigned to the ligands-based π-
π*-transition are distinctively blue-shifted upon the coordination of
the Ln3+ ions compared to that (378 nm) of the free ligand Heb-
PMP. For complex monomer 2, as shown in Figure 1, only Tb3+-
Fig. 2. Schematic energy level diagram and possible energy transfer process for
complex monomer 2 and Poly(NVK-co-2).
of complex monomer 2 is also validated from its Tb3+-based (7F5,
λem = 546 nm) long lifetime of 258 µs (Figure 3) and attractive
quantum yield (ΦLTb) of 34% in solution, which higher than those of
centered characteristic emissions (5D4
→
7FJ; J = 6, 5, 4, 3) are
tris-β
-diketonate-Tb complexes23 and among the top level of tris-
exhibited, endowing a bright colour-purity green-light with a CIE
(Commission International De L’Eclairage) chromatic coordinate x =
0.322 and y = 0.608. In contrast to the ligands-based fluorescence
(λem = 431 and τ = 1.64 ns) at room temperature, also shown in
Figure 1, complex monomer 3 exhibits the typical 0-0 transition
phosphorescence (λem = 424 and τ = 18.7 μs) at 77 K, from which the
triplet (3π–π*) energy level of 23585 cm-1 is obtained. With regard
to the singlet (1π–π*) energy level (28736 cm-1) estimated by the
lower wavelength of its UV-visible absorbance edge, the slightly
pyrazolonate-Tb complexes,24 should mainly be attributed to its
compatible triplet energy level. It is worth noting that the
involvement of one 4-VB-PBI ancillary ligand, not only effectively
prevents the further coordination of oscillator-vibrated solvates25
around Ln3+ ion within, but also renders each of the complex
monomers 1-2 active for the following copolymerization.
Synthesis, characterization and photophysical property of
metallopolymers Poly(NVK-co-2) with different feed molar ratios
In consideration of the excellent performance26 of semi-conducting
PVK as one of the popular polymer matrices with good mechanical
property and film-forming character, metallopolymers Poly(NVK-
co-2) with different feed molar ratios (50:1, 75:1, 100:1, 200:1 or
400:1) were obtained from the copolymerization (also in Scheme 1)
of NVK and the vinyl-containing complex monomer 2. As a matter of
fact, due to the formation of metallopolymer, not only does the PVK
matrix within act as a host to disperse the Tb3+-complex monomers
and effectively reduce the aggregation-induced self-quenching,15 it
also functions as an efficient fluorescence energy donor with a large
absorption cross-section to transfer energy to the Tb3+-complex-
acceptors.27 To elucidate their AIBN-initiated free-radical
copolymerization, the representative metallpolymer Poly(NVK-co-
1) (100:1) from the iso-structural and anti-ferromagnetic La3+-
basedcomplex monomer 1 was obtained for comparison. On one
hand, in the 1H NMR spectrum (also in Figure 1S) of Poly(NVK-co-1),
larger energy gap ΔE1 (5151 cm−1 1π–π*-3π–π*) than 5000 cm-1
,
endows the effective intersystem crossing process (Figure 2)
according to Reinhoudt’s empirical rule.20 Further checking the
energy level match between the ligand-based 3π–π* energy level
5
(23585 cm−1) and the first excited state (20545 cm-1 of D4) level of
Tb3+ ion, the energy gap (ΔE2 = 3040 nm) for complex monomer 2
falls well within the ideal 2500-4000 cm-1 range from Latva’s
empirical rule,21 confirming the suitability of the chromohpores for
sensitization of Tb3+-centered colour-purity green-light through
typical Dexter energy transfer process (also in Figure 2).22 Moreover,
the luminescent outstanding
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organic moieties of both PVK and the coordinated ligands. As to the
photoluminescence of Poly(NVK-co-2) (50:1, 75:1, 100:1, 200:1 or
400:1) in solid state, it (Figure 4) is strictly dependent on the
stipulated feed molar ratio. For Poly(NVK-co-2) (50:1), the multiple
peaks only related to Tb3+-centered 5D4
→
7FJ transitions give an
expected green-light (Point A with a CIE chromatic coordinate x =
0.335, y = 0.580). However, contributed to the PVK-matrixed
substrate with a larger refractive index than MeCN or PNBE16 and
the additional intra-chain efficient Förster energy transfer (also in
Figure 2) from PVK to Tb3+-complex-acceptors27 with significant
spectra overlap (Figure 6S) between the emission of PVK with the
absorption of complex monomer 2, the improved luminescent
property of Poly(NVK-co-2) (50:1) is verified from the distinctively
increased Tb3+-based (7F5, λem = 546 nm) lifetime (763 µs also
Fig. 3. Luminescent decay profiles for complex monomer 2 in solution and
Poly(NVK-co-2) (50:1, 75:1, 100:1, 200:1 or 400:1) in solid state with emission
monitored at 546 nm of Tb3+ ion.
shown in Figure 3) and the relatively larger quantum yield (ΦL
=
Tb
41%) than those (258 µs and 34%) of the corresponding complex
monomer 2 in solution, respectively. Interestingly, through grafting
of more PVK in Poly(NVK-co-2) (75:1, 100:1, 200:1 or 400:1), dual-
emitting behaviours associated with the PVK-characteristic broad
emission centered at 425 nm and the Tb3+-centered emissions are
critically exhibited, which is incomparable to the Tb3+-based colour-
purity green-light of Poly(NVK-co-2) (50:1) in solid state or complex
monomer 2 in solution. With the increase of the feed molar ratio
from 75:1 to 400:1, the PVK-based peak intensity monotonously
increases in an almost linear dependence, while the Tb3+-based
multiple emissions are identically consolidated. Therefore the
combination of retained blue-light from excess PVK with the green-
light of complex monomer 2 within is capable of colour-tuning from
green (Point B; 0.269, 0.387) to cyan (Points C-D; 0.216-0.237,
0.222-0.295) and to blue (Point E; 0.199, 0.211). Moreover, their
almost constant Tb3+-decay lifetimes (752-769 µs also in Figure 3) in
the whole grafting-content range (50:1, 75:1, 100:1, 200:1 or 400:1)
show no observed emitter's clustering,15 from which, the
incorporated Tb3+-chromophores are effectively blocked and
homogeneously dispersed into the PVK matrix.
Fig. 4. Emission and excitation spectra (Left) and corresponding CIE chromatic
coordinates (Right) of metallopolymers Poly(NVK-co-2) with different feed molar
ratios.
besides the combined and broadened proton resonances of the
polymerized complex monomer 1 and NVK, the absence of original
vinyl-based characteristic proton resonances suggests that the
vinyl-functionalized complex monomers are actually grafted into
the PVK matrix through covalently-bonded linkage.28 Moreover,
GPC results (Table 1S) show that almost similar Mn sizes (11168-
11279 g/mol) for Poly(NVK-co-1) and Poly(NVK-co-2) with the same
feed molar ratio of 100:1, and all the polydispersity indexes (PDI =
Mw/Mn) of Poly(NVK-co-2) with different feed molar ratios (50:1,
75:1, 100:1, 200:1 or 400:1) in the relatively narrow range of 1.17-
1.31 are actually resulted from their AIBN-initiated radical
copolymerization.29 On the other hand, their similar amorphous
PXRD patterns (Figure 3S) to that of PVK, also indicate the low-
concentration homogeneous distribution of the complex monomers
into PVK.30 Furthermore, TG analysis (Figure 4S) of the
representative Poly(NVK-co-2) (50:1) shows the distinctively
improved thermal stability (with a decomposition temperature
interval of 390−406 °C) than that (249−261 °C) of complex
monomer 2, and the desirable Tg (glass transformation
temperature) of 206 °C (also in Figure 4S) higher than those of PVK-
included organic polymers31 due to the restriction of the chain
mobility of PVK backbone, exhibiting a significant opportunity to
optoelectronic devices.
Device performance of colour-purity green-light PLEDs based on
Poly(NVK-co-2) (50:1)
To investigate the electroluminescent properties of the designed
metallopolymer Poly(NVK-co-2) (50:1), two single-layer spin-
coating PLEDs I-II with different thicknesses (10 and 20 nm) of BCP
were fabricated, respectively, and their EL performance is
summarized in Figure 5. According to the proposed energy level
diagram (Figure 5a) for the two PLEDs, the HOMO level (-5.1 eV) of
Poly(NVK-co-2) is well matched with that (-5.2 eV) of PEDOT:PSS,
and the LUMO level (-3.2 eV) of the hole-blocking BCP is between
those (-2.7 and -3.4 eV) of TPBI and PEDOT:PSS, and thus direct
exciton formation and following carrier trapping occur within the
As shown in Figure 5S, almost similar DR spectrum of the PVK-
supported metallopolymer from complex monomer 1 or 2 at the
same feed molar ratio of 100:1, indicates relatively broader
absorption bands than those of complex monomers 1-2 in solution,
where the absorptions at 264-266, 292-294 and 338-342 nm in the
UV-visible region should be assigned to electronic transitions from
4 | J. Name., 2012, 00, 1-3
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supported Tb3+-containing metallopolymers a new platform for
monochromatic green-light PLEDs, would be further improved
through the following device optimization.
4. Conclusions
In summary, using the PVK-supported Tb3+-containing
metallopolymer Poly(NVK-co-2) (50:1) with specific PVK to Tb3+ ion
energy transfer as the emitting material, the colour-purity green-
light PLEDs' EL efficiencies (24.63 cd/A, 7.81% and 6.45 lm/W)
renew the record of Tb3+-based PLEDs and bridge the gap to small-
molecule Tb3+-complex's OLEDs, which, together with the
advantageous performance of colour-stability and low efficiency
roll-off, indicate their potential application in future flat-display
area.
Fig. 5. PLEDs' structure with energy level diagram (a); EL Spectra from 10 nm (b)
or 20 nm (c) thickness of BCP; current density-voltage-luminance (d); EQE-current
density-current efficiency (e) and EQE-luminance-current efficiency (f).
Acknowledgements
This work is funded by the National Natural Science Foundation
(21373160, 91222201, 21173165), the Program for New Century
Excellent Talents in University from the Ministry of Education of
China (NCET-10-0936), the Doctoral Program (20116101110003) of
Higher Education, the Science, Technology and Innovation Project
(2012KTCQ01-37) of Shaanxi Province in P. R. of China
emitting Poly(NVK-co-2). As expected, the normalized EL spectra
(Figure 5b) of Device I with a BCP thickness of 10 nm show almost
voltage-independent (11-20 V) Tb3+-characteristic green-lights with
CIE chromatic coordinates of x = 0.311-0.383, y = 0.519-0.553. The
turn-on voltage (Figure 5d) of the highly monochromatic green-light
PLED I is 11 V at 1 cd/m2, and a maximum luminance of 81 cd/m2 is
achieved at 16 V with a current density of 1.22 mA/cm2. With an
increase of luminance or current density, as shown in Figures 5e
and 5f, the current efficiency, the eternal quantum yield (EQE) and
the luminous efficiency increase firstly and decrease insistently with
maximum values of 22.44 cd/A, 7.34% and 5.87 lm/W at 12 V,
respectively. Interestingly enough, upon a applied bias voltage up to
16 V, the corresponding efficiencies (6.61 cd/A and 1.30 lm/W) and
the EQE of 2.63% are retained.
Notes and references
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Table of content
For Tb3+-containing metallopolymer, colour-purity green-light PLEDs with record-renewed
efficiencies (24.63 cd/A, 7.81%, 6.45 lm/W) and low efficiency roll-off were realized.