The first solid-state LEC based on transition metal complexes was reported in 1996 [11], where a ruthenium poly-pyridyl complex was utilized as the emissive material.
Since then, many efforts have been made to improve performances of the LECs. In 1999, LECs based on low-molecular-weight ruthenium complexes were reported to have external quantum efficiency (EQE), which is defined as photons/electrons, of 1% [12]. In 2000, LECs using [Ru(bpy)3](ClO4)2(where bpy is 2,20-bipyridine) with the gallium–indium eutectic electrode showed an EQE up to 1.8% [13]. Later, [Ru(bpy)3](PF6)2blended with poly(methyl methacrylate) (PMMA) was reported to improve the film quality and increase the EQE to 3% [14]. In 2002, a single-crystal LEC was made by repeatedly filling nearly saturated solution of [Ru(bpy)3] (ClO4)2between two ITO (indium tin oxide) slides, followed by evaporating the solvent [15]. Such devices possessed low turn-on voltages and exhibited an EQE of 3.4%. Further improvement of performances was achieved by reducing self-quench-ing of excited states in [Ru(bpy)3]2+ with adding alkyl substituents on the bpy ligands [16], raising the EQE to 4.8% under the DC bias and to 5.5% under the pulsed driving. Yet a further improvement of the [Ru(bpy)3](ClO4)2 device was
achieved by forming a heterostructure device, thus moving the emission zone away from the electrode and giving an efficiency of 6.4% [17].
More efficient cationic iridium complexes have also been used in LECs. In 2004, LECs based on the yellow-emitting (560 nm) cationic iridium complex [Ir(ppy)2(dtb-bpy)]PF6, where ppy is phenylpyridine and dtb-bpy is 4,40 -di-tert-butyl-2,20-bipyridine, were reported, exhibiting efficiencies of 5% and 10 lm W1 [18, 19]. On the one hand, replacing the ppy ligands by F-mppy ones, where F-mppy is 2-(40-fluorophenyl)-5-methylpyridine, led to green emission (531 nm) and an EL efficiency of 1.8% [19,20]. On the other hand, employing the dF(CF3)ppy ligands, where dF(CF3)ppy is 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, increased the energy gap of the cationic iridium complexes and shifted the EL emission to blue-green (500 nm, with an EQE of 0.75%) [21]. Another series of cationic phenylpyrazole-based iridium complexes were also recently reported to give blue (492 nm), green (542 nm), and red (635 nm) emission [22]. Blue, green, and red devices made of these complexes on poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) coated ITO showed EQEs of 4.7%, 6.9%, and 7.4%, respectively.
Most LEC devices described above were made in a single-layered neat-film structure. In a neat film of an emissive material, interactions between closely packed molecules usually lead to quenching of excited states, detrimental to EQEs of devices. Thus, in addition to tuning of emission colors, the capability of the ligands to provide steric hindrance for suppressing self-quenching must also be carefully considered in designing ligands for emissive cationic metal complexes. In our previous studies of oligofluorenes, we had found that the introduction of aryl substitutions onto the tetrahedral C9 of oligofluorenes can provide effective hin-drance to suppress interchromophore packing and self-quenching yet without perturbing energy gaps of the molecules [23–27], making the photoluminescent quantum yields (PLQYs) of oligo(9,9-diarylfluorene)s in neat films rather close to those in dilute solutions. Thus, we introduced 4,5-diaza-9,90-spirobifluorene (SB) [28] as a steric and bulky auxiliary ligand of cationic iridium complexes, [Ir(ppy)2(SB)]PF6 (1) and [Ir(dFppy)2(SB)]PF6 (2) (Fig. 1), and investigate the influence of the SB ligand on reducing the self-quenching [29].
Spin-coated neat films of 1 and 2 exhibit PL spectra similar to those observed for their acetonitrile (MeCN) solutions (Fig.2). However, spin-coated neat films of 1 and 2 show higher PLQYs (0.316 for 1, 0.310 for 2) and longer excited-state lifetimes (0.60ms for 1, 0.59 ms for 2) than their MeCN solutions (Table1). Since MeCN is a strongly polar solvent and the emitters are ionic, the photophysical properties of complexes 1 and 2 perhaps are significantly perturbed by strong solute–solvent interaction, rendering the observed PLQYs and lifetimes of 1 and 2 in MeCN less intrinsic. This indeed can be verified by measuring photophysical properties of 1 and 2 in a less polar solvent (yet still with enough solubility for spectroscopic measurements), such as dichloromethane (DCM). In DCM (5 105M), both 1 and 2 exhibit longer excited-state lifetimes (0.79ms for 1, 0.42ms for 2) and higher PLQYs (0.467 for 1, 0.364 for 2) than in MeCN (Table1).
Thus, to better characterize the intrinsic photophysical properties of complexes 1 and 2 at the room temperature, complexes 1 and 2 were dispersed (with 1.5 mol%) in a large-gap thin-film host ofm-bis(N-carbazolyl)benzene (mCP), which is rather nonpolar and has a large triplet energy. The measured PLQYs and excited-state lifetimes of the dispersed mCP films are (0.667, 0.81ms) for 1 and (0.421, 0.74 ms) for 2 (Table1). Thus 1 and 2 dilutely dispersed in mCP exhibit higher PLQYs and longer excited-state lifetimes than those in neat films. Shorter lifetimes in neat films indicate that interaction between closely packed molecules provides additional deactivation pathways, shortening excited-state lifetimes, and lowering the PLQYs. However, PLQYs of complexes 1 and 2 in neat films still retain ~50%
and ~75% of those in mCP blend films. These are indeed rather high retaining percentages for PLQYs in neat films when compared with other phosphorescent molecules. For instance, [Ir(ppy)3] [1.5 mol% dispersed in 4,40-bis(carbazol-9-yl)
200 300 400 500 600 700 800 0
200 300 400 500 600 700 800 0
Fig. 2 Absorption and PL spectra in acetonitrile solutions and in neat films of (a) complex 1 and (b) complex 2. Also shown in (a) and (b) are EL spectra of complex 1 and complex 2, respectively.
Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim
Fig. 1 Molecular structures of complexes 1 and 2
biphenyl (CBP)] and bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium (III) [FIrpic] (1.4 mol% dispersed in mCP) films had been reported to have very high PLQYs of 97 2% and 99 1%, respectively [30], while PLQYs of [Ir(ppy)3)] and [FIrpic] neat films are only ~3% and ~15%, respectively. On the one hand, severe self-quenching in [Ir(ppy)3] and [FIrpic] implies that the ppy, dFppy, and picolinic acid ligands cannot provide enough hindrance against inter-molecular interactions in neat films. On the other hand, the high retaining percen-tages in neat-film PLQYs of complex 1 (with two ppy ligands and one SB ligand) and complex 2 (with two dFppy ligands and one SB ligand) compared with those in dispersed films indicate that SB ligands in these compounds provide effective steric hindrance and greatly reduce self-quenching. It is noted that self-quenching in neat films of [FIrpic] is not as severe as that in neat films of [Ir(ppy)3]. It is likely that the fluoro substituents on the ppy ligands somewhat hinder the intermolecular interac-tions [30]. A similar effect is also observed here for complex 2, which exhibits a higher PLQY retaining percentage (74%) than complex 1.
In the operation of LEC devices, when a constant bias voltage is applied, a delayed EL response that is associated with the time needed for counterions in the LECs to redistribute under a bias is typically observed. For the cases of neat films of complexes 1 and 2, the redistribution of the anions (PF6) leads to the formation of a region of Ir(IV)/Ir(III) complexes (p-type) near the anode and a region of Ir(III)/Ir Table 1 Summary of physical properties of complexes 1 and 2
Complex lmax, PL(nm)a PLQY, lifetime (ms)b Eox1=2(V)c Ered1=2(V)d DE1/2(V)e Solutionf Neat film Solution Film
1 605 593 0.226, 0.33f 0.316, 0.60g 1.35 1.33 2.64
0.467, 0.79h 0.667, 0.81i –, 4.31j 0.381, 0.72k –, 3.27l
2 535 535 0.278, 0.39f 0.310, 0.59g 1.70 1.26 2.92
0.364, 0.42h 0.421, 0.74i –, 4.49j 0.329, 0.55k –, 4.29l
aPL peak wavelength
bPhotoluminescence quantum yields and the excited-state lifetimes
cOxidation potential
dReduction potential
eThe electrochemical gapDE1/2is the difference betweenE1ox=2andE1red=2corrected by potentials of the ferrocenium/ferrocene redox couple
fMeasured in acetonitrile (5 105M) at room temperature
gNeat films
hMeasured in dichloromethane (5 105M) at room temperature
i1 and 2 were dispersed (1.5 mol%) in mCP films
jMeasured in acetonitrile (5 105M) at 77 K
kFilms with 0.75 mol [BMIM][PF6] per mole of 1 and 2
lMeasured in dichloromethane (5 105M) at 77 K
Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim
(II) complexes (n-type) near the cathode [31]. With the formation of p- and n-regions near the electrodes, carrier injection is enhanced, leading to a gradually increasing device current and emission intensity. Devices based on neat films of complexes 1 and 2 (with the structure of glass substrate/ITO/complex 1 or 2 (100 nm)/
Ag) exhibited very long response times. Very slow device response (e.g., tens of hours to reach the maximum brightness) had also been observed before for other LEC materials (e.g., [Ru(bpy)3](PF6)2 derivatives with esterified ligands) [12].
Plausibly, bulky side groups on molecules impede the migration of ions. Thus to accelerate formation of the p- and n-doped regions in the emissive layer, 0.75 mol ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] per mole of complex 1 or 2 was added to provide additional anions (PF6) [32].
It had been reported that in polymer LECs, incorporation of polar salts into conjugated polymer films might induce aggregates or phase separation due to discrepancy in polarity [33–35]. Thus, to study the effect of the [BMIM][PF6] addition on thin-film morphologies of complexes 1 and 2, atomic force microscopy (AFM) of thin films was performed. As shown in Fig.3, the AFM micrographs for films of complexes 1 and 2 with and without [BMIM][PF6] coated on ITO glass substrates show no significant differences and all give similar root-mean-square (RMS) roughness of ~1 nm. At this concentration of [BMIM][PF6] in complex 1 or 2 (0.75:1, molar ratio), no particular features of aggregation or phase separation were
a b
Fig. 3 3D AFM micrographs of (a) neat film of complex 1, (b) blend film of [BMIM][PF6] and complex 1 (0.75:1, molar ratio), (c) neat film of complex 2, and (d) blend film of [BMIM][PF6] and complex 2 (0.75:1, molar ratio). Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
observed, and uniform spin-coated thin films could be routinely obtained. Such characteristics may be associated with the ionic nature of complexes 1 and 2, which may make them more compatible with the added salts. Further, to examine the effects of the [BMIM][PF6] addition on photophysical properties of complexes 1 and 2, PLQYs and excited-state lifetimes of blend films were also measured and are shown in Table1. In general, films of both 1 and 2 containing [BMIM][PF6] (1:0.75, molar ratio) exhibit PLQYs and excited-state lifetimes comparable to those of neat films, indicating that the addition of [BMIM][PF6] does not induce particu-lar quenching of emission. Stable operation was also achieved in devices using such a formulation. In the following, device characteristics based on the structure of [glass substrate/ITO/1:[BMIM][PF6] or 2:[BMIM][PF6] (100 nm)/Ag] are dis-cussed and are summarized in Table2.
A distinct characteristic of LECs is that they can be operated under a bias voltage close toEg/e. As shown in Table1, the electrochemical gaps (DE1/2), which were derived from the difference betweenEox1=2andEred1=2corrected with potentials of the ferrocenium/ferrocene redox couple, for complexes 1 and 2 are 2.64 eV and 2.92 eV, respectively. The devices based on complexes 1 and 2 were thus first tested under the biases of 2.6 V and 2.9 V, respectively, although the energy gaps in films are usually smaller than those in solutions because of the environmental polarization. EL spectra of the devices based on complexes 1 and 2 (added with [BMIM][PF6] are shown in Figs.2aand2b, respectively, for comparison with their PL spectra. EL spectra are basically similar to PL spectra, indicating similar emission mechanisms. Commission internationale de l’Eclairage (CIE) coordinates for the EL spectra of complexes 1 and 2 are (0.51, 0.48) and (0.35, 0.57), respec-tively. Time-dependent brightnesses and current densities of the devices operated under bias conditions described above are shown in Figs.4aand4bfor complexes 1 and 2, respectively. Both devices exhibited similar electrical characteristics. On the one hand, the currents of both devices first increased with time after the bias was applied and then stayed at a constant level after 350–400 min. On the other hand,
Table 2 Summary of the LEC device characteristics based on complexes 1 and 2 Complex Bias (V) lmax, EL(nm)a tmax(min)b Lmaxcd m2 ext, max,p, max
(%, lm W1)d
Lifetime (h)e
1 2.6 580 170 330 6.2, 19.0 26
2.5 150 100 7.1, 22.6 54
2 2.9 535 85 145 6.6, 23.6 6.7
2.8 90 52 7.1, 26.2 12
aEL peak wavelength
bTime required to reach the maximal brightness
cMaximal brightness achieved at a constant bias voltage
dMaximal external quantum efficiency and maximal power efficiency achieved at a constant bias voltage
eThe time for the brightness of the device to decay from the maximum to half of the maximum under a constant bias voltage
Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim
the brightness first increased with the current and reached the maximum of 330 cd m2 for complex 1 at ~170 min and of 145 cd m2 for complex 2 at
~85 min. The brightness then decreased with time even though the device current stayed rather constant. The decrease in brightness was irreversible, that is, the maximum brightness obtained in the first measurement could not be fully recovered in the followed measurements even under the same driving conditions. It is ratio-nally associated with the degradation of the emissive material during the LEC operation, which was commonly seen in LEC devices [36].
Time-dependent EQEs and corresponding power efficiencies of the complex 1 device under the 2.6-V bias and the complex 2 device under the 2.9-V bias are shown in Figs.5aand5b, respectively. Both devices exhibited similar time evolu-tion in EQE. When a forward bias was just applied, the EQE was rather low due to unbalanced carrier injection. During the formation of the p- and n-type regions near electrodes, the balance of the carrier injection was improved and the EQE of the device thus increased rapidly. The peak EQE and the peak power efficiency are
0
Fig. 5 The time-dependent EQE and the corresponding power efficiency of the single-layered LEC device for (a) complex 1 driven at 2.6 or 2.5 V and (b) complex 2 driven at 2.9 or 2.8 V.
Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim
Current Density (mA/cm2) Current Density (mA/cm2)
0 100 200 300 400 500 600 0 0 100 200 300 400 500 600
Fig. 4 The time-dependent brightness and current density of the single-layered LEC device for (a) complex 1 driven at 2.6 or 2.5 V and (b) complex 2 driven at 2.9 or 2.8 V. Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(6.2%, 19.0 lm W1) for the complex 1 device under the 2.6-V bias and (6.6%, 23.6 lm W1) for the complex 2 device under the 2.9-V bias. One notices that the peak efficiencies occurred before the currents reached their final maximal values.
Such a phenomenon may be associated with two factors. First, although with the formation of the p- and n-type regions near electrodes, both contacts are becoming more ohmic and the carrier injection at both electrodes are becoming more bal-anced; however, the carrier recombination zone may consequently move toward one of the electrodes because of discrepancy in electron and hole mobilities. The recombination zone moving to the vicinity of an electrode may cause exciton quenching such that the EQE of the device would decrease with time while the current and the brightness are still increasing. Besides, degradation of the emissive material under a high field would also contribute to the decrease in the EQE when the recombination zone is still moving or when the recombination zone is fixed.
Both LEC devices driven under 0.1-V lower biases exhibit higher peak EQEs and lower degradation rates. As shown in Figs.5aand5b, the peak EQE and the peak power efficiency are (7.1%, 22.6 lm W1) for the complex 1 device under the 2.5-V bias and (7.1%, 26.2 lm W1) for the complex 2 device under the 2.8-V bias.
It is interesting to note that the peak EQEs of the single-layered devices (no PEDOT:PSS is used) based on complexes 1 and 2 approximately approach the upper limits that one would expect from the PLQYs of their neat films, when considering an optical out-coupling efficiency of ~20% from a typical layered light-emitting device structure. To our knowledge, these EQEs are among the highest values reported for orange-red (or yellow) and green solid-state LECs based on CTMCs. Such results indicate that CTMCs with superior steric hindrance are essential and useful for achieving highly efficient solid-state LECs.
Since these newly developed cationic complexes (1 and 2) are intrinsically efficient, to further reduce self-quenching and increase EL efficiency, one possible approach is to spatially disperse the emitting complex (guest) into a matrix complex (host), as previously reported for conventional OLEDs and solid-state LECs [37–39]. The mixed host–guest films (~100 nm) for PL studies were spin-coated onto quartz substrates using mixed solutions of various ratios. Since in LECs, a salt [BMIM][PF6] of 19 wt% (where BMIM is 1-butyl-3-methylimidazolium) was also added to provide additional mobile ions and to shorten the device response time [29,32], PL properties of the host–guest–salt three-component system were also characterized.
Figure6ashows the absorption spectrum of the guest and the PL spectra of the host–guest two-component systems having various guest concentrations. With the increase of the guest concentration, a gradual red shift from the host-like emission to the guest-like emission is observed. As shown in Fig.6b, the highest PLQY of
~37% (vs. ~31% of neat host and guest films) is obtained at the guest concentration of 25 wt%, at which the emission is nearly completely from the guest. Accompany-ing this enhanced PLQY is the longer excited-state lifetime (0.69ms) when com-pared with those of neat films (0.59ms for the host and 0.60 ms for the guest), indicating the effectiveness of the dispersion in suppressing quenching mechanisms of guest molecules. Less complete transfer is observed at lower guest concentrations,
0.00 0.04 0.08
a 0.12
0.0 0.2 0.4 0.6 0.8 1.0
Absorbance
Wavelength (nm) Guest
Absorption
Guest Concentration
0 wt.%
5 wt.%
15 wt.%
25 wt.%
50 wt.%
100 wt.%
PL Intensity (a. u.)
30 35 40 45 50
b
Photoluminescence Quantum Yield (%)
Guest Concentration (wt.%)
400 500 600 700 800
0 20 40 60 80 100
0 20 40 60 80 100
0.60 0.65 0.70 0.75 0.80 0.85
Excited-State Lifetime (μs) Without BMIM+(PF6–)
With BMIM+(PF6–) Without BMIM+(PF6–) With BMIM+(PF6–)
Guest Concentration (wt.%)
c
Fig. 6 (a) The absorption spectrum of the neat guest film and PL spectra of host–guest films with various guest concentrations (without [BMIM][PF6]. (b) Photoluminescence quantum yields and (c) excited-state lifetimes as a function of the guest concentration for host–guest films without and with [BMIM][PF6] (19 wt%). Reproduced with permission from [49]. Copyright 2006, American Institute of Physics
which may be associated with the less extensive overlap between the host emission and the weak guest absorption. A calculation of the F€orster radius for the host–guest energy transfer gives a small value of <16 A˚ [40] indicating inefficient energy transfer at lower guest concentrations. With the addition of [BMIM][PF6] (19 wt%), the trend in PL properties [spectra (not shown), PLQYs (Fig. 6b), and lifetimes (Fig.6c)] as a function of the guest concentration is similar to those of the two-component system. Yet, an even higher PLQY of ~50% (about 1.6 enhancement compared with PLQYs of neat host and guest films) and longer excited-state lifetime of ~0.82 ms are observed around the guest concentration of 25 wt%. It appears that [BMIM][PF6] not only provides additional mobile ions but is also effective in suppressing interchromophore quenching.
Figure7ashows the time-dependent brightness and current density under con-stant biases of 2.5–2.7 V for the LEC using the mixture giving the highest PLQY (i.e., with host, guest, and [BMIM][PF6] concentrations of 56 wt%, 25 wt%, and 19 wt%, respectively). After the bias was applied, on the one hand, the current first increased and then stayed rather constant. On the other hand, the brightness first increased with the current and reached the maxima of 10, 30, and 75 cd m2at<1 h under biases of 2.5 V, 2.6 V, and 2.7 V, respectively. The brightness then dropped with time with a rate significantly depending on the bias voltage (or current).
Corresponding time-dependent EQEs and power efficiencies of the same device are shown in Fig.7b. When a forward bias was just applied, the EQE was rather low due to poor carrier injection. During the formation of the p- and n-type regions near electrodes, the capability of carrier injection was improved and the EQE thus rose rapidly. The peak EQE, current, and peak power efficiencies at 2.5 V, 2.6 V, and 2.7 V are (10.4%, 29.3 cd A1, 36.8 lm W1), (9.9%, 27.9 cd A1, 33.7 lm W1), and (9.4%, 26.5 cd A1, 30.8 lm W1), respectively.
The maximum current density vs. voltage characteristics of LECs with various guest concentrations are shown in Fig. 8a. The bias voltage required for same
0
Fig. 7 (a) Brightness (solid symbols) and current density (open symbols) and (b) external quantum efficiency (solid symbols) and power efficiency (open symbols) as a function of time under a constant bias voltage of 2.5–2.7 V for the host–guest LEC with host, guest and [BMIM]
[PF6] concentrations of 56 wt%, 25 wt%, and 19 wt%, respectively. Reproduced with permission from [49]. Copyright 2006, American Institute of Physics
0.0 0.5 1.0 1.5 2.0 2.5
– 3.1 eV
Guest Host – 3.0 eV
– 6.0 eV – 5.7 eV
Guest (wt.%) 0 16 25 35 81
Maximum Current Density (mA/cm2)
Voltage (V)
0.0 0.2 0.4 0.6 0.8
1.0 Guest
Concentration 0 wt.%
7 wt.%
25 wt.%
81 wt.%
EL Intensity (a.u.)
Wavelength (nm)
2.5 2.6 2.7 2.8 2.9
400 500 600 700 800
0 20 40 60 80
7 8 9 10 11
16 20 24 28 32 36
Quantum Efficiency Peak External Quantum Efficiency (%)
Guest Concentration (wt.%) Power Efficiency
Peak Power Efficiency (lm/W)
a
b
c
Fig. 8 (a) Maximum current density vs. voltage characteristics for LECs with various guest concentrations. (b) EL spectra (at 2.8 V) for LECs with various guest concentrations. (c) Peak external quantum efficiencies and peak power efficiencies (at current densities<0.1 mA cm2) of host–guest LECs as a function of the guest concentration.Inset of (a): the energy level diagram of the host and guest molecules. Reproduced with permission from [49]. Copyright 2006, American Institute of Physics
current drops as the guest concentration is raised. This may be understood by the energy level diagram obtained from cyclic voltammetry [29] (inset of Fig.8a). Such an energy level alignment favors carrier injection/transport (at least for holes) through the smaller-gap guest and direct carrier recombination/exciton formation on the guest (rather than host–guest energy transfer) if the guest concentration is high enough. EL spectra of host–guest LECs with various guest concentrations (Fig. 8b) indeed support the mechanism of direct exciton formation on guest molecules, which would greatly reduce host emission due to incomplete energy transfer. Compared with PL spectra (e.g., Fig. 6), EL spectra are much less dependent on the guest concentration. Even with a guest concentration as low as 7 wt%, the EL spectrum is almost the same as that of the pure guest device [i.e., the LEC with 81 wt% of guest and 19 wt% of [BMIM][PF6], indicating predominant guest emission.
Figure8cshows peak EQEs and peak power efficiencies for host–guest LECs as
Figure8cshows peak EQEs and peak power efficiencies for host–guest LECs as