Cite this:
Phys. Chem. Chem. Phys
., 2012, 14, 1262–1269
Improving the balance of carrier mobilities of host–guest solid-state
light-emitting electrochemical cells
Chih-Teng Liao,
aHsiao-Fan Chen,
bHai-Ching Su*
aand Ken-Tsung Wong*
bReceived 21st October 2011, Accepted 15th November 2011 DOI: 10.1039/c1cp23336j
We report efficient host–guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the
host–guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host–guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host–guest LECs reaches 3.62% (7.36 lm W1), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (B0.2) and an optical out-coupling efficiency ofB20% and consequently indicates superior balance of carrier mobilities in such a host–guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host–guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.
Introduction
Solid-state light-emitting electrochemical cells (LECs) possess several advantages over conventional organic light emitting diodes (OLEDs). In LECs, electrochemically doped regions induced by spatially separated ions under a bias form ohmic contacts with electrodes, giving balanced carrier injection, low operating voltages, and consequently high power efficiencies.1,2 As such, LECs generally require only a single emissive layer, which can be easily processed from solutions and can con-veniently use air-stable electrodes, while OLEDs typically require more sophisticated multilayer structures and low-work-function cathodes.3,4Compared with conventional polymer LECs that are usually composed of an emissive conjugated polymer, a salt and an ion-conducting polymer,1,2LECs based on cationic transition metal complexes (CTMCs) show several further advantages and have attracted much attention in recent years.5–51In such devices,
no ion-conducting material is needed since these CTMCs are
intrinsically ionic. Furthermore, higher electroluminescent (EL) efficiencies are expected due to the phosphorescent nature of CTMCs.
In general, LECs are composed of neat films of emissive materials, which very often suffer self-quenching induced by interactions between closely packed molecules. Many efforts have been made to enhance device efficiencies of LECs based on CTMCs by reducing self-quenching of the emissive materials. Modifying the molecular structures such as adding bulky sub-stituents on ligands9,37,41or utilizing bulky auxiliary ligands25
has been shown to suppress interchromophore interaction to some degree, improving device efficiencies of LECs. However, self-quenching is still significant in neat films even composed of materials with bulky molecular structures and thus limits device efficiencies. To further reduce self-quenching and increase EL efficiency, one feasible approach is to spatially disperse an emitting guest into a transporting host matrix, as previously reported for conventional OLEDs4and solid-state LECs.14,26,30,36,41,45,52Among the reported host–guest LECs, CTMCs were the most commonly used host materials and high external quantum efficiency (EQE) up to 10.4% photon/ electron has been demonstrated in host–guest LECs based on CTMCs.26 To optimize device efficiencies, a sophisticated molecular design for CTMC-based host materials with balanced carrier mobilities is generally required to ensure the carrier aInstitute of Lighting and Energy Photonics, National Chiao Tung
University, Tainan 71150, Taiwan.
E-mail: [email protected]; Fax: +886-6-3032535; Tel: +886-6-3032121-57792
bDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan. E-mail: [email protected]; Fax: +886-2-33661667; Tel: +886-2-33661665
www.rsc.org/pccp
PAPER
recombination zone to situate at the center of the emissive layer and consequently to avoid exciton quenching near electrodes53
in single-layered LEC devices. Nevertheless, adjusting carrier transporting characteristics of CTMCs by modifying their ligands would simultaneously alter their energy gaps, influencing the effectiveness of energetic confinement for guest excitons when CTMCs are utilized as hosts. For instance, the neat-film photoluminescence (PL) maximum of the model compound [Ir(ppy)2(dtb-bpy)]+(PF6) (where ppy is 2-phenylpyridine and
dtb-bpy is 4,40-di-tert-butyl-2,20-dipyridyl) centers at 558 nm13
while the functionalized bipolar compound with dtb-bpy replaced by 4,5-diaza-20,70-bis(diphenylamino)-9,90-spirobifluorene, in
which the 4,5-diazafluorene and diphenylamino substituents act as electron and hole transporting moieties, respectively, exhibits significant bathochromic shift in neat-film PL (maximum at 638 nm).47Such reduced energy gap would impede function-alized bipolar CTMCs to be used as host materials for guest materials emitting in the visible region. Furthermore, utilizing expensive phosphorescent CTMCs containing rare metals as host materials, which require a vast amount of material usage to form the emissive layer, increases the fabrication costs of LECs.
Compared with CTMC-based hosts, relatively inexpensive fluorescent polymers54and small-molecule materials55are feasible
for independent tailoring of carrier transporting properties and energy gaps and thus would be more suitable for use as host materials in LECs. Host–guest phosphorescent LECs based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl] (BDOH-PF)54 as the host and a red-emitting iridium complex bis[2-(20
-benzo-thienyl)-pyridinato-N,C30]iridium(acetylacetonate) [Btp2Ir(acac)]
as the guest has been reported.52The host-only (BDOH-PF) LEC devices showed a high EQE up to 4%, which approaches the upper limit that one would expect from the photolumines-cence quantum yield (PLQY) of the neat host films (0.73) when considering spin statistics B25% for singlet excitons and an optical out-coupling efficiency ofB20% from a typical layered light-emitting device structure, suggesting balanced carrier mobilities in BDOH-PF neat films.54 However, the maximum power efficiency (1 lm W1) of the host–guest LECs based on BDOH-PF doped with [Btp2Ir(acac)]52 was much
lower than that obtained in [Btp2Ir(acac)] doped OLEDs
(4.6 lm W1),56in which multilayered structures were utilized to confine excitons in the emissive layer sandwiched between electron and hole transporting layers and thus to prevent exciton quenching near electrodes. These results reveal that when doped with guest, balance of carrier mobilities of host films would deteriorate due to carrier trapping induced by the offset in energy levels between the host and the guest and the carrier recombination zone would consequently move to the proximity of electrodes, leading to exciton quenching and thus reduced device efficiencies. Hence, to optimize the device efficiencies of the host–guest LECs, balancing carrier mobilities of the emissive layer to keep the carrier recombination zone away from electrodes would be a critical issue. In this work, we demonstrate improving the balance of carrier mobilities in host–guest LECs utilizing a cationic terfluorene derivative as the host and a red-emitting CTMC as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host–guest films and
thus improves balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting charac-teristics. PL measurements show efficient energy transfer in this host–guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host–guest LECs reaches 3.62% (7.36 lm W1), which approaches the upper limit that one would expect from the PLQY of the emissive layer (B0.2) and an optical out-coupling efficiency of B20% and consequently indicates superior balance of carrier mobilities in such a host–guest emissive layer. These results are among the highest reported for red-emitting LECs5–9,14,18,20,35–37,40and thus
confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host–guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.
Experimental
Materials
Molecular structures of the host and guest materials used in this study are shown in Fig. 1. All compounds were synthesized according to the procedures reported in the literature.30,55The
cationic terfluorene derivative (1) reported recently by Chen et al.to be used in saturated deep-blue-emitting LECs was used as the host.55 Compound 1 in dilute solutions possesses PL emission wavelengths in the deep-blue region with a high PLQY close to unity.55 The PLQY of 1 in the form of neat films
remained high (up to 0.76) despite the presence of intermolecular interactions.55More importantly, the addition of the ionic liquid BMIM+(PF6) (where BMIM is 1-butyl-3-methylimidazolium),
which provides additional mobile ions in the emissive layer to fasten the device response, in the neat film did not affect the emission properties of 1.55Thus, it would be suitable for use as
the host material in host–guest LECs. [Ir(ppy)2(biq)]+(PF6) (2)
(where biq is 2,20-biquinoline), which was used as the
red-emitting complex in white LECs reported by Su et al.,30 was utilized as the red-emitting guest. Complex 2 exhibits saturated red PL emission in both solutions and neat films and thus is suitable for use as the red-emitting guest in host–guest LECs. Photoluminescent characterization
Thin films for PL studies were spin-coated at 3000 rpm onto quartz substrates using mixed solutions (in acetonitrile) of
Fig. 1 Molecular structures of the host molecule, cationic terfluorene derivative (1) and the guest molecule, [Ir(ppy)2(biq)]+(PF6) (2).
various ratios. Since in LECs, an ionic liquid BMIM+(PF 6)
of 10 wt% was added to provide additional mobile ions and to shorten the device response time,16photophysical properties of the BMIM+(PF6) blended host–guest films were characterized.
The mass ratio of solute component [1 : 2: BMIM+(PF6)] in
acetonitrile solutions for spin coating of the host–guest films containing x wt% guest is (90 x) : x : 10. The thickness of each spin-coated film was ca. 200 nm, as measured using profilometry. The concentrations of all solutions for spin coat-ing are 80 mg mL1. UV-Vis absorption spectra were recorded using a Hitachi U2800A spectrophotometer. PL spectra were recorded using a Hitachi F9500 fluorescence spectrophoto-meter. PLQYs for thin-film samples were determined using a calibrated integrating sphere system (Hamamatsu C9920). LEC device fabrication and characterization
Indium tin oxide (ITO)-coated glass substrates were cleaned and treated with UV/ozone prior to use. A poly(3,4-ethylene-dioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS) layer was spin-coated at 4000 rpm onto the ITO substrate in air and baked at 150 1C for 30 min. The emissive layer (B200 nm, as measured by profilometry) was then spin-coated at 3000 rpm from the acetonitrile solutions under ambient conditions. To reduce the turn-on time of the LEC device, the ionic liquid [BMIM+(PF6)] (10 wt% for the host–guest devices and
20 wt% for the guest-only devices) was added to enhance the ionic conductivity of thin films.16The mass ratio of solute component and the concentrations of solutions for spin coat-ing of the emissive layers were the same as that used for spin coating of the host–guest films for PL studies described above. After spin coating, the thin films were then baked at 70 1C for 10 hours in a nitrogen glove box (oxygen and moisture levels below 1 ppm), followed by thermal evaporation of a 100 nm Al top contact in a vacuum chamber (B106 torr). The electrical and emission characteristics of LEC devices were measured using a source-measurement unit and a Si photo-diode calibrated with the Photo Research PR-650 spectro-radiometer. All device measurements were performed under a constant bias voltage (3.0, 3.2 and 3.4 V) in a nitrogen glove box. The EL spectra were taken with a calibrated CCD spectrograph.
Results and discussions
Photoluminescent studies of the host–guest system
PL spectra of the neat host and guest films are shown in Fig. 2. Neat films of the high-gap host (1) exhibit deep-blue fluorescent PL centered at 418 nm and yellow phosphorescent PL centered at 562 nm (the inset of Fig. 2, measured at 77 K), corresponding to a triplet energy of 2.21 eV. Neat films of the guest (2) show saturated red phosphorescent PL emission centered at 662 nm, which is equal to a triplet energy of 1.87 eV. Therefore, the triplet level of the host is higher than that of the guest and thus triplet–triplet back energy transfer from guest to host can be prevented, eliminating a potential pathway of energy loss in a host–guest system.57The absorption spectrum of the guest neat films is also shown in Fig. 2. Since the absorption spectrum of the guest and the PL spectrum of the host exhibit considerable
spectral overlap, efficient host–guest energy transfer is expected (calculated Fo¨rster radius ca. 3 nm for this host–guest system) and thus the guest emission could dominate PL at low guest concentrations, resulting in significantly reduced self-quenching of guest molecules.
Fig. 3 depicts the PL spectra of the emissive layers of the host–guest LECs, i.e. the host–guest films containing various guest concentrations and BMIM+(PF
6) (10 wt%). The ionic
liquid BMIM+(PF6) was added to provide additional mobile
ions and to shorten the device response time.16Addition of BMIM+(PF6) (10 wt%) has been reported not to affect
the PL emission properties of the neat host films.55With the increase of the guest concentration, the relative intensity of the guest emission with respect to the residual host emission is larger due to a higher host–guest energy transfer rate at a higher guest concentration. It is noted that the PL emissions of the guest at low concentrations (0.5–2.0 wt%, Fig. 3) center at ca. 610 nm and exhibit significant blue shift ca. 50 nm as compared to those of the neat guest films (Fig. 2), indicating reduced intermolecular interactions of dispersed guest mole-cules in a host matrix. PLQYs of the host–guest films contain-ing various guest concentrations and BMIM+(PF6) (10 wt%)
are shown in the inset of Fig. 3. PLQYs of the host–guest films decrease as the guest concentration increases from 0 to 2 wt% Fig. 2 Absorption spectrum of the neat guest films and PL spectra of the neat host and guest films. Inset: phosphorescence spectrum of the neat host films measured at 77 K.
Fig. 3 PL spectra of the host–guest films containing various guest concentrations and BMIM+(PF
6) (10 wt%). Inset: photoluminescence quantum yields vs. guest concentrations of the same films.
since the guest emission, which exhibits a lower PLQY than the host emission, dominates PL emission at relatively higher guest concentrations. At the guest concentration of 2 wt%, most PL emission (ca. 90%) of the host–guest films comes from the guest and the PLQY of the host–guest films is 0.2, which is compar-able with that of the guest in dilute (105M) dichloromethane solutions.30Thus, the PLQY of the guest molecules dispersed at
low concentrations in the host films would be estimated to be B0.2. Such PLQY is much higher as compared with that of the neat guest films (0.09)30and thus confirms significantly reduced self-quenching of guest molecules dispersed in the host matrix. EL characteristics of the host–guest LECs
To clarify the EL properties of the host–guest system, EL characteristics of host–guest LECs containing various guest concentrations were measured and are summarized in Table 1. The host–guest LECs have the structure of ITO/PEDOT : PSS (30 nm)/emissive layer (200 nm)/Al (100 nm), where the emissive layer contains [host (89.5 wt%), guest (0.5 wt%) and BMIM+(PF6) (10 wt%)] for Device I, [host (89.0 wt%),
guest (1.0 wt%) and BMIM+(PF6) (10 wt%)] for Device II,
[host (88.0 wt%), guest (2.0 wt%) and BMIM+(PF6) (10 wt%)]
for Device III and [host (0.0 wt%), guest (80.0 wt%) and BMIM+(PF
6) (20 wt%)] for Device IV. The ionic liquid
BMIM+(PF6) was added to provide additional mobile ions
and to shorten the device response time.16The EL spectra of the host–guest LECs with various guest concentrations under 3 V are shown in Fig. 4. The host–guest LECs with guest concentrations of 0.5–2.0 wt% exhibited similar saturated red EL spectra with a Commission Internationale de l’Eclairage (CIE)58 coordinate of (0.62, 0.37). The EL spectra resemble the guest emission in the PL spectra of the corresponding host–guest emissive layers (Fig. 3), indicating similar emission mechanisms. However, the residual host emissions appeared in the PL spectra of the host–guest films are absent in the EL spectra of the host–guest LECs. These results could be under-stood by energy level alignments of the host and guest molecules (estimated by cyclic voltammetry)30,55depicted in the inset of Fig. 4. For host–guest LECs, electrochemically doped regions of the emissive layer result in ohmic contact with metal electrodes and consequently facilitate carrier injection onto both
the host and the guest. Hence, both exciton formation on the host followed by host–guest energy transfer and direct exciton formation on the guest induced by charge trapping contribute to the guest emission. At lower biases, such energy level alignments favor electron injection and trapping on the smaller-gap guest, resulting in direct carrier recombination/exciton formation on the guest (rather than host–guest energy transfer). Thus, com-pared with the PL spectra (Fig. 3), the EL spectra (Fig. 4) are independent of the guest concentration and exhibit predominant guest emission even at a low guest concentration of 0.5 wt%.
The host–guest LECs with various guest concentrations exhibited similar time-dependent EL characteristics under constant-bias operation. Fig. 5(a) shows the time-dependent brightness and current density under constant biases of 3.0–3.4 V for Device I. After the bias was applied, 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 0.13, 0.57 and 2.77 cd m2 under biases of 3.0, 3.2 and 3.4 V, respectively. The brightness then dropped with time with a rate depending on the bias voltage (or current). Corresponding time-dependent EQEs and power efficiencies of the same device are shown in Fig. 5(b). 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 injec-tion was improved and the EQE thus rose rapidly. The peak EQE (peak power efficiency) at 3.0, 3.2 and 3.4 V are 3.62% (7.36 lm W1), 2.99% (3.33 lm W1) and 2.13% (2.81 lm W1), respectively. The drop of efficiency and brightness after reaching the peak value, as commonly seen in solid-state LECs5–51 may be associated with a few factors. Before the device current reaches a steady value, the carrier recombination zone may keep moving closer to one electrode due to discrepancy in electron and hole mobilities, which would induce exciton quenching. Further, the decrease in brightness and efficiency under a constant bias may be rationally attributed to the degradation of the emissive material during the LEC operation.10
For comparison, time-dependent brightness/current density and EQE/power efficiency of the guest-only LECs (Device IV) are shown in Fig. 6(a) and (b), respectively. The guest-only (Fig. 6(a)) and the host–guest LECs (Fig. 5(a)) exhibited
Table 1 Summary of the host–guest LEC device characteristics
Device (guest concentration) Bias/V tmaxa/min Lmaxb/cd m2 Zext,maxc(%) Zp,maxd/lm W1 Lifetimee/min
I (0.5 wt%) 3.0 V 97 0.13 3.62 7.36 527.f 3.2 V 33 0.57 2.99 3.33 248. 3.4 V 11 2.77 2.13 2.81 46 II (1.0 wt%) 3.0 V 132 0.04 2.37 3.25 539.f 3.2 V 78 0.59 1.74 2.15 245. 3.4 V 13 4.70 1.37 1.73 37. III (2.0 wt%) 3.0 V 131 0.02 1.01 1.18 617.f 3.2 V 114 0.38 0.98 1.14 399. 3.4 V 40 3.05 0.70 0.77 102. IV (80.0 wt%)g 2.2 V 600 1.09 0.04 0.05 —h 2.3 V 349 13.6 0.38 0.44 1221.f
aTime required to reach the maximal brightness.bMaximal brightness achieved at a constant bias voltage.cMaximal external quantum efficiency achieved at a constant bias voltage.dMaximal 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.fExtrapolated.gGuest-only device [mass ratio of host : guest : BMIM+(PF
6) = 0 : 80 : 20].hExtrapolation cannot be performed since brightness has not yet decreased after 10 h continuous operation.
similar characteristics in time-dependent brightness and current density. However, much larger current densities were measured in the guest-only devices even at significantly lower biases due to the lower electrochemical band gap of the guest complex 2. The peak EQE (peak power efficiency) of the guest-only devices at 2.3 V is 0.38% (0.44 lm W1), which is much lower than that of the host–guest LECs (Table 1). Further reducing the bias voltage (2.2 V) leads to an even lower EQE (0.04%). As revealed in previous studies,12 as bias voltage decreases, the
width of the intrinsic layer between the p- and n-type doped layers extends due to shrinking of the doped layers, resulting in reduced electric field in the recombination zone. Thus, reduced device efficiency under a lower bias may be attributed to deteriorated balance of carrier mobilities caused by field-dependent electron and hole mobilities.
Peak EQEs and peak power efficiencies (at current densities o0.003 mA cm2
) of the host–guest LECs with various guest concentrations are shown in Fig. 7. All LEC devices contain 10 wt% BMIM+(PF6) in the emissive layer. The peak device
efficiency first increases then decreases as the guest concen-tration increases from 0 to 90 wt%. The fluorescent host-only [mass ratio of host : guest : BMIM+(PF
6) = 90 : 0 : 10]
devices show an EQEB1%, which is much lower than that one would expect (3.8%) from the PLQY of the neat host films (0.76)55 when considering spin statistics B25% for singlet excitons and an optical out-coupling efficiency ofB20% from a layered structure. Since the electrochemically doped regions near electrodes of LECs ensure balanced carrier injection,2 such lowered device efficiency would be attributed to imperfect balance of carrier mobilities in the host films. As the carrier injection at both electrodes are becoming balanced, the carrier recombination zone may consequently locate near one of the electrodes due to discrepancy in electron and hole mobilities of the emissive layer. The recombination zone in the vicinity of an electrode may cause exciton quenching such that the EQE of the device would decrease. Terfluorene derivatives Fig. 5 (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 3.0–3.4 V for Device I.
Fig. 6 (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 2.2–2.3 V for Device IV.
Fig. 4 EL spectra (at 3.0 V) for the host–guest LECs with various guest concentrations and BMIM+(PF
6) (10 wt%). Inset: the energy level diagram of the host and guest molecules.
with alkyl substitutions on the tetrahedral C9 carbon have been reported to exhibit higher electron mobilities than hole mobilities.59 Furthermore, imidazole moieties, which are
tethered at the ends of the alkyl chains on 1, have been used in electron transporting materials for OLEDs.60 Therefore, electron preferred transporting characteristics of 1 would rationally be responsible for lowered device efficiencies of the host-only devices. The schematic diagram of the position of the exciton recombination zone for the host-only device is shown in Fig. 8(a). Since electron mobility is higher than hole mobility in the host, the exciton recombination zone would locate near the anode and thus exciton quenching occurs, deteriorating device efficiency. However, with a guest concen-tration of 0.5 wt%, the peak EQE (peak power efficiency) of the host–guest LECs reaches 3.62% (7.36 lm W1), which is among the highest reported for red-emitting LECs.5–9,14,18,20,35–37,40 It is noted that such an EQE approximately approaches the upper limit (B4%) that one would expect from the PLQY of the guest dispersed at low concentrations in host films (B0.2), when considering spin statistics B100% (both singlet and triplet excitons can be harvested for a CTMC) and an optical outcoupling efficiency of B20% from a typical layered light-emitting device structure. This result implies superior balance of carrier mobilities of the host–guest films with a guest concen-tration of 0.5 wt%. As shown in the inset of Fig. 4, balance of carrier mobilities would be significantly altered in the host–guest films due to the large energy offset (1.32 eV) in the LUMO levels between the host and the guest molecules. Electron trapping
induced by the host–guest energy offset in the LUMO levels would reduce the electron mobility while the hole mobility would remain relatively unchanged due to similar energies in the highest occupied molecular orbital (HOMO) levels of the host and the guest molecules. Since the host intrinsically possesses electron preferred transporting characteristics, balance of carrier mobili-ties would be improved in the host–guest devices doped with an electron-trapping guest at a proper concentration. As shown in Fig. 8(b), the exciton recombination zone of the host–guest LECs with a guest concentration of 0.5 wt% would be pushed toward the center of the emissive layer due to reduced electron mobility. Thus, exciton quenching would be effectively eliminated, leading to high device efficiency. However, over-doping of the electron-trapping guest would further impede electron transporting and consequently would result in higher hole mobility than electron mobility, deteriorating balance of carrier mobilities of the host–guest films as well. Thus, with a guest concentration higher than 0.5 wt%, the device efficiency of the host–guest devices decreases as the guest concentration further increases (Fig. 7). As shown in Fig. 8(c), the exciton recombination zone of the host–guest LECs with a guest concentration of 2.0 wt% would be pushed to the proximity of cathode due to reduced electron mobility. Exciton quenching occurs again and the device efficiency is consequently low.
On the other hand, the guest-only [mass ratio of host : guest : BMIM+(PF6) = 0 : 80 : 20] LECs under a low bias voltage
of 2.3 V exhibit a low EQE of 0.38%, which is approximately an order of magnitude lower than that achieved in the host– guest LECs doped with 0.5 wt% guest. However, the estimated upper limit of EQE of the guest-only LECs from the PLQY of the neat guest films (0.09)30when considering an optical out-coupling efficiencyB20% should reach ca. 1.8%. Thus, the low EQE obtained in the guest-only devices may be attributed to poor balance of carrier mobilities of the neat guest films. Except for a few reported LECs based on materials with balanced carrier mobilities,25,33,54 in which only PLQY of the emissive layer and optical outcoupling efficiency of layered device struc-ture limits the device efficiency, balance of carrier mobilities in single-layered LECs is a common bottleneck in optimizing device efficiencies. The research results of this work confirm that in addition to reducing self-quenching of guest molecules as revealed in previous reports,14,26,52 the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would also improve balance of carrier mobilities of the emissive layer and thus would be effective in optimizing device efficiencies of LECs. These device efficiencies obtained in host– guest LECs are comparable with those reported in polymer LEDs61–69and multi-layered OLEDs70–73doped with iridium(III)
emitters.
4.
Conclusions
In summary, we have demonstrated efficient host–guest solid-state LECs utilizing a cationic terfluorene derivative (1) as the host and a red-emitting CTMC [Ir(ppy)2(biq)]+(PF6) as the
guest (2). Carrier trapping induced by the offset in the LUMO levels between the host and the guest impedes electron transport in the host–guest films and thus improves balance of carrier mobilities of the host films intrinsically exhibiting electron Fig. 7 Peak external quantum efficiencies and peak power efficiencies
(at current densitieso0.003 mA cm2) of the host–guest LECs as a function of the guest concentration.
Fig. 8 Schematic diagrams of the position of the exciton recombina-tion zone for (a) host-only device, (b) Device I and (c) Device III. Electrochemically doped regions near electrodes are omitted for clarity.
preferred transporting characteristics. PL measurements show efficient host–guest energy transfer in this host–guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host–guest LECs reaches 3.62% (7.36 lm W1), which approaches the upper limit that one would expect from the PLQY of the emissive layer (B0.2) and an optical out-coupling efficiency of B20% and consequently indicates superior balance of carrier mobilities in such a host–guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host–guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.
Acknowledgements
The authors gratefully acknowledge the financial support from the National Science Council of Taiwan.
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![Fig. 1 Molecular structures of the host molecule, cationic terfluorene derivative (1) and the guest molecule, [Ir(ppy) 2 (biq)] + (PF 6 ) (2).](https://thumb-ap.123doks.com/thumbv2/9libinfo/7649412.139146/2.892.465.810.908.1054/molecular-structures-molecule-cationic-terfluorene-derivative-guest-molecule.webp)
