Efficient and color-stable solid-state white light-emitting
electrochemical cells employing red color conversion layers
Hung-Bao Wu
a, Hsiao-Fan Chen
b, Chih-Teng Liao
a, Hai-Ching Su
a,⇑, Ken-Tsung Wong
b,⇑ aInstitute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan
b
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e
i n f o
Article history:
Received 25 October 2011
Received in revised form 8 December 2011 Accepted 11 December 2011
Available online 3 January 2012 Keywords:
Light-emitting electrochemical cells Color conversion layer
White light
a b s t r a c t
We report efficient and color-stable white light-emitting electrochemical cells (LECs) by combining single-layered blue-emitting LECs with red-emitting color conversion layers (CCLs) on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive material results in efficient energy transfer and thus sufficient down-converted red emission at low doping concentrations of the red-emitting dye in the CCLs. Low doping concentration is beneficial in reducing self-quenching of the red-emitting dye, rendering efficient red emission. Elec-troluminescent (EL) measurements show that the peak external quantum efficiency and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm W1, respectively, which are among the highest reported for white LECs.
Further-more, these devices exhibit bias-insensitive white EL spectra, which are required for prac-tical applications, due to nondoped emissive layers. These results reveal that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
White organic light-emitting diodes (OLEDs) based on polymers and small-molecule materials have attracted in-tense attention due to their potential applications in flat-panel displays and solid-state lighting [1–5]. Recently, a power efficiency of white OLEDs higher than 120 lm W1
has been reported by Reineke et al. under laboratory conditions [6]. However, multi-layered structure and chemically p- and n-doped transporting layers for reducing operating voltages complicate fabrication processes. Com-pared with conventional white OLEDs, solid-state white light-emitting electrochemical cells (LECs) possess several
promising advantages. In solid-state LECs [7,8], electro-chemically doped regions, i.e. p-type doping near the an-ode and n-type doping near the cathan-ode, induced by spatially separated ions under a bias form Ohmic contacts with electrodes, giving balanced carrier injection, low operating voltages, and consequently high power efficien-cies[7,8]. Therefore, LECs generally require only a single emissive layer, which can be easily deposited by solution processes and can conveniently utilize air-stable elec-trodes, e.g., Au and Ag, while OLEDs typically require more sophisticated multilayer structures and low-work-function cathodes for balanced carrier injection[1–5].
Solid-state white LECs based on a phase-separated polyfluorene/poly(ethylene oxide) (PEO) mixture exhibit-ing an external quantum efficiency (EQE) of 2.4% pho-tons/electron were reported by Yang and Pei in 1997[9]. Recently, white LECs based on excimer emission of a fluo-rene-oxadiazole copolymer with a current efficiency of 0.15 cd/A have been reported by Sun et al.[10]. However, 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.12.011
⇑Corresponding authors. Tel.: +886 6 3032121x57792; fax: +886 6 3032535 (H.-C. Su), tel.: +886 2 33661665; fax: +886 2 33661667 (K.-T. Wong).
E-mail addresses: [email protected] (H.-C. Su), [email protected](K.-T. Wong).
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Organic Electronics
moderate efficiencies of polymer LECs showed that the fluorescent nature of conjugated polymers limits the even-tual electroluminescence (EL) efficiency due to the spin statistics. Compared with conventional polymer LECs that are usually composed of an emissive conjugated polymer, a salt and an ion-conducting polymer [7,8], LECs based on cationic transition metal complexes (CTMCs) show sev-eral further advantages and have attracted much attention in recent years[11–57]. In such devices, no ion-conducting material is needed since these CTMCs are intrinsically ionic. Furthermore, higher EL efficiencies are expected due to the phosphorescent nature of CTMCs. By employing a blue-emitting CTMC as the host and a red-emitting CTMC as the guest, solid-state white LECs with an EQE (power efficiency) of 4% (7.8 lm/W) were successfully demon-strated by Su et al. in 2008[36]. Recently, improving the EQE (power efficiency) of CTMC-based white LECs up to 5.6% (11.2 lm/W) has been achieved by He et al.[42,47]. However, low photoluminescence quantum yields (PLQYs) (<0.2 even in dilute solutions) of the red-emitting CTMCs used in previously reported host–guest white LECs
[36,42,47,55] preclude further enhancement in device
efficiencies. Furthermore, single-layered host–guest white LECs usually suffer bias-dependent color shift, which is undesired for practical applications [36,42,55]. This phenomenon results from excitons formed by direct carrier trapping on the guest under relatively lower biases due to significant energy offsets in the energy levels between the host and the guest. Direct exciton formation on the red-emitting guest owing to charge trapping domi-nates EL emission under lower biases while exciton formation on the blue-emitting host followed by host– guest energy transfer tends to be more significant under higher biases. As a result, the EL emission of the single-layered host–guest white LECs changes from red to white as the bias slightly increases (2.9–3.3 V) [36,55] and such color instability prohibits their usage in practical applications.
Color stability under varying bias conditions is also a critical issue in white OLEDs and one of the reported prac-tical ways to improve color stability is combination of blue-emitting devices with red color conversion layers (CCLs) on the reverse side of the glass substrates [58– 61]. White emission can be generated by the superposi-tion of nonabsorbed blue emission from the emissive layer and red emission, which is excited by the blue emis-sion, from the CCLs. This approach can be implemented by easy fabrication techniques and can provide better color stability since only one emitter is present in the emissive layer, eliminating carrier trapping effect induced by the low-gap red-emitting dyes in host–guest white LECs
[36,42]. Furthermore, highly efficient neutral red-emitting dye [4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetrame-thyljulolidyl-9-enyl)-4H-pyran, DCJTB] suitable for use in OLEDs exhibits significantly deteriorated PLQYs (<0.1) in ionic emissive materials used as emissive layers of LECs due to environmental polarity while they show high PLQYs (>0.5) in neutral polymer films [51]. Since the PLQYs of the reported red-emitting CTMCs used in white LECs[36,42,47,55]are rather low (<0.2), utilizing neutral polymer CCLs doped with a highly efficient neutral
red-emitting dye (DCJTB) would be beneficial in enhanc-ing device efficiencies of white LECs. To the best of our knowledge, no white LEC composed of a blue-emitting device combined with a down-conversion red CCL has been reported. In this work, we demonstrate efficient and color-stable white LECs by combining single-layered blue-emitting LECs with red-emitting CCLs on the inverse side of the glass substrate. Photoluminescent (PL) mea-surements reveal that good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive layer leads to efficient energy transfer and thus sufficient down-converted red emission at low doping concentra-tions of the red-emitting dye in the CCLs. It ensures re-duced self-quenching of dye molecules and consequently results in efficient red emission. EL measurements show that the peak EQE and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm W1, respectively, which are among the highest
reported for white LECs. Furthermore, these white LECs exhibit bias-insensitive white EL spectra, which are essen-tial for practical applications, due to nondoped emissive layers. These results confirm that single-layered blue-emitting LECs combined with red-blue-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices.
2. Experiment 2.1. Materials
Molecular structures of the blue-emitting CTMC (com-plex 1) used in the emissive layer of the LEC and the red-emitting dye DCJTB used in the CCL are shown in
Fig. 1(a). The blue-emitting complex 1 [Ir(dfppz)2
(dtb-bpy)]+(PF
6) (where dfppz is
1-(2,4-difluorophenyl)pyra-zole and dtb-bpy is [4,40-di(tert-butyl)-2,20-bipyridine])
re-ported previously by Tamayo et al. [24]was used as the Fig. 1. (a) Molecular structures of the blue-emitting CTMC (1) used in the emissive layer of the LEC and the red-emitting dye (DCJTB) used in the CCL. (b) Schematic structure of the LEC device combined with a CCL.
emissive material. Complex 1 was synthesized according to the procedures reported in the literature [24]. The red-emitting dye (DCJTB, LumTech) and poly(methyl methacrylate) (PMMA, weight-average molecular weight Mw= 350,000 g mol1, Alfa Aesar) were used as received.
2.2. Photoluminescent characterization
Thin films of the complex 1 for PL studies were spin-coated at 3000 rpm onto quartz substrates using acetonitrile solutions (150 mg/mL). Thin films of PMMA doped with DCJTB (0.3, 0.4 and 0.5 wt.%) were spin-coated at 1000 rpm onto quartz substrates using chlorobenzene solutions (180 mg/mL). The thicknesses of the spin-coated complex 1 and DCJTB-doped PMMA films were c.a. 400 nm and 6
l
m, respectively, as measured using profil-ometry. The absorption spectrum of DCJTB was measured using dichloromethane solutions (105M). UV–Visabsorp-tion spectra were recorded using a Hitachi U2800A spectro-photometer. PL spectra were recorded using a Hitachi F9500 fluorescence spectrophotometer. PLQYs for thin-film samples were determined using a calibrated integrating sphere system (Hamamatsu C9920).
2.3. CCL preparation, LEC device fabrication and characterization
The CCLs were spin-coated at 1000 rpm onto cover slips (1.8 cm 1.8 cm 0.15 mm) using chlorobenzene solu-tions (180 mg/mL) of PMMA doped with various concen-trations of DCJTB (0.3, 0.4 and 0.5 wt.%). The thickness of the CCL was c.a. 6
l
m as measured by profilometry. Indium tin oxide (ITO)-coated glass substrates were cleaned and treated with UV/ozone prior to use. A poly(3,4-ethylenedi-oxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer was spin-coated at 4000 rpm onto the ITO substrate in air and baked at 150 °C for 30 min. The emissive layer (400 nm, as measured by profilometry) was then spin-coated at 3000 rpm from the acetonitrile solutions of com-plex 1 under ambient conditions. To reduce the turn-on time of the LEC devices, the ionic liquid [BMIM+(PF6)]
(where BMIM is 1-butyl-3-methylimidazolium) of 20 wt.% was added to enhance the ionic conductivity of the emissive layers [22]. The concentration of the solutions used for spin coating of the emissive layers is 150 mg/ mL. After spin coating, the samples were then baked at 70 °C for 10 h in a nitrogen glove box (oxygen and moisture levels below 1 ppm), followed by thermal evaporation of a 100-nm Ag top contact in a vacuum chamber (106torr).
The schematic diagram of the LEC device combined with a CCL is shown inFig. 1(b). The LEC device was placed on the top of the cover slip containing a CCL during EL character-ization. No adhesive was used between the glass substrate and the cover slip. The electrical and emission characteris-tics of LEC devices were measured using a source-measurement unit and a Si photodiode calibrated with the Photo Research PR-650 spectroradiometer. All device measurements were performed under a constant bias voltage (3.1, 3.3, 3.5, 5, 7 and 9 V) in a nitrogen glove box. The EL spectra were taken with a calibrated CCD spectrograph.
3. Results and discussions 3.1. Photoluminescent studies
PL spectrum of the films of [Ir(dfppz)2(dtb-bpy)]+(PF6)
(complex 1) containing 20 wt.% BMIM+(PF
6), which has
the same components with those used in the emissive lay-ers of LECs is shown in Fig. 2. These films exhibit blue phosphorescent PL centered at 490 nm. Highly retained PLQY of complex 1 in neat films (0.75)[55]in comparison with that in dilute solutions (1.00) [55]reveals reduced self-quenching in neat films possibly resulting from the sterically bulky di-tert-butyl groups of the bipyridine li-gand[24], suggesting its suitability for use as the emissive material of neat-film blue-emitting LECs. To achieve effi-cient energy transfer from the emissive layer of LEC to the CCL layer, the red-emitting material doped in the CCL should exhibit absorption spectrum matched well with the emission spectrum of complex 1. As shown inFig. 2, good spectral overlap between the absorbance spectra of the red-emitting dye (DCJTB) dispersed in PMMA films (0.3, 0.4 and 0.5 wt.%) and the emission spectrum of the blue-emitting emissive layer (complex 1) would lead to efficient energy transfer and thus sufficient down-con-verted red emission at low doping concentrations of the red-emitting dye in the CCL layer. Low doping concentra-tions is beneficial in reducing self-quenching of dopants and consequently results in efficient red emission from the CCL layer. From the data of absorbance, which is de-fined as log10(I0/I) (where I0is the intensity of the light
be-fore it enters the CCL sample and I is the intensity of the light passed through the CCL sample), the percentages of absorption of the excitation source at 490 nm for CCLs with DCJTB at 0.3, 0.4 and 0.5 wt.% are estimated to be c.a. 50%, 62% and 89%, respectively. The PL spectra of the CCLs doped with various concentrations of DCJTB (0.3, 0.4 and 0.5 wt.%) under an excitation source at 490 nm are also de-picted inFig. 2. All PL spectra were collected under the same excitation intensity. The PL emissions of CCLs doped with 0.3–0.5 wt.% DCJTB center at c.a. 600 nm and exhibit slight bathochromic shift with increased DCJTB concentra-tions due to enhanced molecular polarization effect[62].
Fig. 2. Absorbance and PL spectra of DCJTB dispersed in PMMA films (0.3, 0.4 and 0.5 wt.%). PL spectrum of films of complex 1 containing 20 wt.% BMIM+
(PF
FromFig. 2, the relative values of integrated PL intensities of CCLs with DCJTB at 0.3, 0.4 and 0.5 wt.% are estimated to be c.a. 46, 64, and 89, respectively. These results reveal that PL intensities of CCLs with 0.3–0.5 wt.% DCJTB are in pro-portion to the amount of absorption of the excitation source at 490 nm, indicating similar PLQYs of PMMA films doped with DCJTB at 0.3–0.5 wt.%. White emission can be obtained by combining emissions of complex 1 and DCJTB in proper ratios. The PLQY of CCLs doped with 0.3–0.5 wt.% DCJTB were determined to be c.a. 0.6, which is much higher than the PLQYs of previously reported red-emitting CTMCs used in host–guest white LECs (<0.2)[36,42,47,55]. Thus, utilizing CCLs doped with DCJTB to generate red emission would be beneficial in further enhancing device efficien-cies of white LECs.
3.2. EL characteristics of the white LECs with CCLs
To clarify the EL properties of the white LECs with CCLs, EL characteristics were measured and are summarized in
Table 1. The LECs combined with CCLs have the structure
of CCL (6
l
m)/cover slip (0.15 mm)/glass (0.55 mm)/ITO (120 nm)/PEDOT:PSS (30 nm)/emissive layer [complex 1 (80 wt.%) and BMIM+(PF6) (20 wt.%)] (400 nm)/Ag
(100 nm), where the CCL/cover slip structure is absent for the reference Device I and the CCLs contain DCJTB of 0.3, 0.4 and 0.5 wt.% for Device II, Device III and Device IV, respectively. The ionic liquid BMIM+(PF
6) was added in
the emissive layer to provide additional mobile ions and to shorten the device response time[22]. The EL spectra of the white LECs with CCLs doped with various DCJTB con-centrations (0.3, 0.4 and 0.5 wt.%) under 3.3 V are shown in
Fig. 3. EL spectrum of the LECs based on complex 1 without CCLs under 3.3 V is also shown for comparison. For the EL emission coming from complex 1, EL spectra are basically similar to PL spectra, indicating similar emission mecha-nisms. When CCLs are incorporated with the LEC devices, some blue electrophosphorescence of complex 1 is
absorbed and then down-converted to red fluorescence of DCJTB, resulting in a mixture of white emission. As shown inFig. 3, the relative intensity of the red emission as com-pared to the blue emission, which was normalized for clar-ity, is enhanced as the doping concentration of DCJTB in the CCLs increases. It is noted that the red emission en-hances more significantly in EL spectra of LEC devices than in PL spectra of CCLs when the doping concentration of DCJTB increases. It is attributed that only excitation light at 490 nm accounts for PL emissions of CCLs while whole EL spectrum of complex 1 can be absorbed by DCJTB mol-ecules to generate red emission. Therefore, by considering the overlap integral of the absorbance spectra of CCLs (peak at c.a. 510 nm) and the PL spectrum of complex 1, which is similar to the EL spectrum of complex 1[57], rel-atively more significant enhancing in red emission at high-er DCJTB concentrations, e.g., at 0.5 wt.% (Fig. 3), is reasonable in EL spectra. Since absorption of DCJTB results in down-converted red emission, the color of white emission can be feasibly adjusted by tuning the DCJTB concentration in CCLs without altering balance of carrier
Table 1
Summary of the device characteristics of white LECs with red color conversion layers. Device (DCJTB concentration)a
Bias (V) tmax(min)b Lmax(cd m2)c gext, max(%)d gp, max(lm W1)e Lifetime (min)f CIE (x, y)g CRIg
I (without CCL)h 3.1 223 7.92 14.47 32.12 130 (0.195,0.362) – 3.3 104 23.42 14.35 29.92 46 3.5 61 38.33 13.28 26.19 27 II (0.3 wt.%) 3.1 146 4.63 8.54 23.11 113 (0.295,0.470) 50 3.3 71 11.90 8.50 21.60 47 3.5 51 19.79 7.22 17.23 27 III (0.4 wt.%) 3.1 129 2.87 5.93 15.34 111 (0.365,0.444) 66 3.3 103 9.45 5.49 13.35 47 3.5 66 16.21 5.53 12.67 23 IV (0.5 wt.%) 3.1 204 3.34 5.01 14.58 123 (0.435,0.493) 58 3.3 94 8.47 5.01 13.73 53 3.5 47 13.29 4.63 11.90 26 a
DCJTB concentrations of the color conversion layers.
b
Time required to reach the maximal brightness.
c
Maximal brightness achieved at a constant bias voltage.
d
Maximal external quantum efficiency achieved at a constant bias voltage.
e
Maximal power efficiency achieved at a constant bias voltage.
f
The time for the brightness of the device to decay from the maximum to half of the maximum under a constant bias voltage.
g
Evaluated from the EL spectra under 3.3 V.
h Devices without color conversion layers.
Fig. 3. EL spectra of the white LECs with CCLs doped with various DCJTB concentrations (0.3, 0.4 and 0.5 wt.%) under 3.3 V. EL spectrum of the LECs based on complex 1 without CCLs under 3.3 V is also shown for comparison. Blue emission peaks are normalized for clarity.
mobilities in the emissive layer, which may deteriorate de-vice efficiencies[55]. Independent color tuning of white EL emission and tailoring of balance of carrier mobilities in the emissive layer would facilitate optimizing device effi-ciencies of white LECs[55].
Another promising advantage of white LECs with CCLs is bias-insensitive EL spectra. As shown in Fig. 4, the EL spectra of Device III (DCJTB 0.4 wt.%) are approximately the same under biases of 3.1–9.0 V, which correspond to device current densities and maximum brightness of 0.04–16.17 mA/cm2 and 2.87–184.92 cd/m2. Commission
Internationale de l’Eclairage (CIE)[63]coordinates of these EL spectra are shown in the inset ofFig. 4. No significant color shift in EL spectra was observed for Device III under 3.1–9.0 V since the CIE coordinate migration (Dx, Dy) is less than (±0.009, ±0.005). Slight color shift may be attrib-uted to microcavity effect [64] of thickness variation in solution-processed emissive layer and CCL of each device. Similarly slight color shifts under the same range of bias voltages were also observed for Device II and Device IV. These results reveal good color stability of white LECs with CCLs. In contrast, typical host–guest white LECs usually ex-hibit significant color shift under similar bias voltages (or current densities). For instance, CIE coordinate migration up to (±0.124, ±0.029) has been reported for white LECs composed of a blue-emitting host CTMC doped with a red-emitting guest CTMC when bias voltage (current den-sity) varied from 2.9 to 3.3 V (0.01–0.12 mA/cm2) [55].
For host–guest LECs, electrochemically doped regions of the emissive layer result in ohmic contact with metal elec-trodes 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 di-rect exciton formation on the guest induced by charge trapping contribute to the guest emission. The later is more significant under relatively lower biases and thus varying bias condition leads to different fractions of guest emis-sion, which results in color shift in white EL spectrum. This is an inherent characteristic of host–guest white LECs and it is not easy to overcome this obstacle. Separating red-emitting materials from the active layer, e.g., utilizing CCLs, would be one of the promising techniques to achieve color-stable white LECs.
To examine the color stability of white LECs with CCLs under various bias voltages, time dependent EL spectra of Device III under 3.1, 3.3 and 3.5 V are shown inFig. 5(a– c), respectively. For each bias voltage, EL spectra were sam-pled when the brightness is increasing, approaching the maximum and decreasing finally. The CIE coordinate migrations of the EL spectra of Device III under 3.1, 3.3 and 3.5 V are within (±0.005, ±0.004), (±0.004, ±0.004) and (±0.003, ±0.008), respectively. Slight variation of color change in temporal EL spectra may result from extending of the doped regions and shrinking of the intrinsic layer with time under a bias [18], which perturbs the optical structure due to different refractive index of each layer
[64]. However, these color shifts are much smaller than those observed in host–guest white LECs under similar bias ranges [55]. These results confirm white LECs with CCLs
Fig. 4. EL spectra of Device III under various bias voltages. Inset: CIE coordinates of these EL spectra.
Fig. 5. Time dependent EL spectra of Device III under (a) 3.1 (b) 3.3 and (c) 3.5 V.
are capable of providing stable and bias insensitive white EL spectra.
The time-dependent brightness/current density and EQE under constant biases of 3.1–3.5 V for Device III are shown in Fig. 6(a) and (b), respectively. It is noted that unusual phenomena of a shoulder in brightness (e.g., at c.a. 30 min for 3.3 V) and a dip in device efficiency followed by a second rise (e.g., at c.a. 50 min for 3.3 V) were ob-served in LECs based on complex 1 with a relatively thicker emissive layer (400 nm). These phenomena were also found in LECs without CCLs (Device I). However, LECs based on complex 1 with relatively thinner emissive layer (200 nm) did not exhibit such phenomena[55,57]. Thus, this unusual behavior is related to the EL properties of a thicker emissive layer and has nothing to do with the PL properties of CCLs. We propose two schematic operation schemes shown inFig. 7to explain such unusual EL prop-erties. Since all LECs under 3.1, 3.3 and 3.5 V showed sim-ilar trends in EL properties (Fig. 6), LECs under 3.3 V are discussed for example. As shown inFig. 7(a), after the bias was applied, the brightness and device efficiency increased with time due to enhanced carrier injection induced by gradually formed p- and n-type doped layers near elec-trodes. Initially (scheme (1),Fig. 7(a)), the recombination zone would be relatively nearer the cathode since hole injection (injection barrier is 0.95 eV) is relatively easier than electron injection (injection barrier is 1.29 eV) (Fig. 7(b)). Due to a relatively smaller injection barrier, the required amount of accumulated anions near the anode for ohmic contact of holes would be less than the required amount of accumulated cations near the cathode for ohmic contact of electrons. Ohmic contact of hole would thus take place earlier than ohmic contact of electron. The injection efficiency of holes would consequently increase faster than
that of electrons at the initial stage of operation and the recombination zone would be gradually shifted toward the cathode with time (scheme (1), Fig. 7(b)). When the recombination zone is approaching the cathode, exciton quenching near electrode[65]occurs and the device effi-ciency degrades. As shown in Fig. 7(a), the increasing of brightness with time retarded at 30–50 min and simulta-neously the EQE significantly deteriorated. After 50 min, subsequent ohmic contact of electron injection is achieved and thus balanced carrier injection would push back the recombination zone toward the center of the emissive layer (scheme (2), Fig. 7(b)). As shown inFig. 7(a), after 50 min, the increasing of brightness recovered and the EQE increased again due to reduced exciton quenching near electrode[65]. However, the second peak of EQE is lower than the first one since degradation of complex 1 un-der electrical operation is continuous. The decrease in brightness and EQE after 100 min may also be rationally attributed to the degradation of the emissive material dur-ing the LEC operation[16]. In LECs based on complex 1 with a relatively thinner emissive layer (c.a. 200 nm)
[57], the turn-on time was much shorter under similar bias voltages due to much faster accumulation rates of mobile ions under higher electric fields. The difference in time re-quired for ohmic contact of hole and electron would be similar. Furthermore, recombination zone in a thinner emissive layer would be closer to electrodes and exciton quenching would be significant during the whole period of electrical operation. Therefore, LECs based on complex 1 with a relatively thinner emissive layer exhibited lower peak EQEs and did not show unusual electrical properties mentioned above[57].
Fig. 6. (a) Brightness (solid symbols) and current density (open symbols) and (b) external quantum efficiency as a function of time under a constant bias voltage of 3.1–3.5 V for Device III.
Fig. 7. (a) Brightness (solid symbols) and external quantum efficiency (open symbols) as a function of time under a constant bias voltage of 3.3 V for Device III. Two operation schemes are labeled: (1) t < 50 min and (2) t > 50 min. (b) Schematic energy levels of complex 1 and work functions of PEDOT:PSS and Ag. Moving of the recombination zone in scheme (1) and (2) are shown in the complex 1 layer.
Thicker emissive layer made by spin coating also leads to large variation in the time required to reach the maxi-mal brightness (tmax) between each individual device. As
shown inTable 1, tmax(shown as average and variation)
of LECs under 3.1, 3.3 and 3.5 V are 176 ± 27%, 87.5 ± 19% and 56.5 ± 18% min, respectively. Larger variation in turn-on time was found in lower bias voltages. Since the emis-sive layer was deposited by spin coating, spatial variation in thickness of thin film can not be easily avoided. When the applied bias voltage is lower and the film is thicker, the variation in film thickness would have significant effect on the speed of conductive doping processes due to a lower electric field. More time is needed to turn on thicker LECs since the accumulation rate of mobile ions is slower under a lower electric field. However, after the p- and n-type re-gions are well established, the EL properties are less dependent on device thickness. Thus maximum current density and consequent device lifetime are similar for LECs under the same bias voltage (Table 1).
Brightness and device efficiency of white LECs with CCLs under the same bias voltage decrease as the DCJTB concentration increases (Table 1). Since the refractive in-dex of PMMA is c.a. 1.50 at 490 nm, which is close to that of the ITO glass substrate and glass cover slips (both of them exhibit refractive indices of 1.52 at 490 nm), wave-guiding of blue light in glass substrate and glass cover slip due to total internal reflection at the interface between glass and PMMA is not significant (critical angle 81°). Thus, compared with blue-emitting LECs without CCLs, in which glass substrate is next to the air, more blue light can be coupled out of the glass substrate, resulting in exci-tation of DCJTB and red reemission. However, out-coupling efficiency of red light from the PMMA layer into the air in forward direction is only c.a. 23%, which is estimated by the formula 1/2n2 (where n = 1.49 for PMMA at 600 nm) [39]. More blue emission from complex 1 transferred to DCJTB leads to more red light trapped in the PMMA layer, which can only be out-coupled into the air from the device sides (lateral directions), consequently deteriorating vice efficiencies in the forward direction. The roll-off of de-vice efficiency as the DCJTB concentration increases is significant. Under 3.1 V, the peak EQE (power efficiency) is 8.54% (23.11 lm W1) when the DCJTB concentration is
0.3 wt.% while the peak EQE (power efficiency) reduces to 5.01% (14.58 lm W1) as the DCJTB concentration
in-creases to 0.5 wt.% (Table 1). However, the roll-off of brightness as the DCJTB concentration increases is rela-tively less significant. Under 3.1 V, the brightness is 4.63 cd m2 when the DCJTB concentration is 0.3 wt.%
while the brightness retains at 3.34 cd m2as the DCJTB
concentration increases to 0.5 wt.% (Table 1). When the DCJTB concentration increases, the number of total pho-tons from the LEC device in the forward direction reduces while the spectral overlap between the EL spectrum
(Fig. 3) and the luminosity function, which centers at
555 nm[66] increases, rendering a higher lumen or can-dela value. Thus, this effect retards the roll-off in bright-ness as compared to that in device efficiency. Since some energy loss occurs in CCLs, highly efficient white LECs with CCLs can only be achieved by utilizing highly efficient blue-emitting CTMCs as the emissive materials in LECs. To
enhance device efficiencies of white LECs with CCLs, highly efficient complex 1 was used as the emissive material of LECs and the thickness of the emissive layer was increased (c.a. 400 nm) to reduce exciton quenching in the recombi-nation zone near electrodes[65]. The blue-emitting LECs showed high EQEs up to 14.47% under 3.1 V (Table 1). Fur-thermore, highly efficient red-emitting DCJTB with a high PLQY up to 0.6 was used in the CCLs. Therefore, white LECs with CCLs (0.4 wt.% DCJTB) showing comparable blue and red EL emission (Fig. 3) still exhibited a high EQE (power efficiency) of 5.93% (15.34 lm W1), which is higher than
those of previously reported host–guest white LECs based on a blue-emitting host doped with a red-emitting guest
[36,42,47,55]. These results convince that combining
blue-emitting LECs with red-emitting CCLs would be a promising technique for providing efficient and color-sta-ble white EL emission in a simple device structure.
Although high device efficiencies can be obtained in white LECs with CCLs, their slow response and short lifetimes prohibit them from practical applications. These drawbacks are common for reported white LECs
[36,42,47,55]. Further studies for improving CTMC
mate-rials are still required to achieve faster response and bet-ter stability for practical applications. In this report, we have utilized the most efficient blue-emitting CTMC ever reported in white LECs [36,42,47,55] as the emissive material and thus high device efficiencies can be obtained. We tend to demonstrate a possible way to mitigate the problem of color instability, which is commonly observed in single-layered host–guest white
LECs [36,42,47,55]. Since the research of LEC is still in
the early stage and properties of materials used in LECs still have much room for improving, our report is not the total solution for white LECs used in lighting applica-tions. However, with rapid improvements of lifetimes of LECs in recent years[26,27,37,38,44,45,48,49], better LEC materials could be expected and thus white LECs with CCLs will be useful when more stable materials are used in the near future.
4. Conclusions
In summary, we have shown efficient and color-stable white LECs by combining single-layered blue-emitting LECs with red-emitting CCLs on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emis-sion spectrum of the blue-emitting emissive material re-sults in efficient energy transfer and thus sufficient down-converted red emission at low doping concentra-tions of the red-emitting dye in the CCLs. Self-quenching of dye molecules is reduced when the doping concentra-tions are low and the red emission is consequently effi-cient. EL measurements reveal that the peak EQE and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm W1, respectively, which
are among the highest reported for white LECs. Further-more, these white LECs exhibit bias-insensitive white EL spectra due to nondoped emissive layers. This device
characteristic is essential for practical applications. These results convince that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emit-ting devices.
Acknowledgement
The authors gratefully acknowledge the financial support from the National Science Council of Taiwan. References
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