Improving color saturation of blue light-emitting ...

Organic Electronics 51 (2017) 70e75

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Improving color saturation of blue light-emitting electrochemical cells by plasmonic filters Chien-Ming Fan Chiang a, 1, Bo-Ren Chang b, 1, Ya-Ju Lee c, Monima Sarma d, Zu-Po Yang a, *, Hai-Ching Su b, **, Hsyi-En Cheng e, Ken-Tsung Wong d a

Institute of Photonic System, National Chiao Tung University, Tainan, 71150, Taiwan Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan, 71150, Taiwan Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taipei, 11677, Taiwan d Department of Chemistry, National Taiwan University, Taipei, 10617, Taiwan e Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan, 71005, Taiwan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2017 Received in revised form 22 August 2017 Accepted 11 September 2017 Available online 12 September 2017

In consideration of the advantages of light-emitting electrochemical cells (LECs), it is desired to develop the saturated blue LECs for LEC display, which is hindered by the features of broad emission spectrum and emission peak not short enough. In this study, we demonstrated a novel method to improve blue saturation of the sky-blue LECs by engineering its emission spectrum through the plasmonic filters. These plasmonic filters composed of randomly distributed silver nanoparticles (Ag-NPs) can absorb the green and red emission tail of the sky-blue LECs due to localized surface plasmon resonance (LSPR). The LSPR wavelengths of Ag-NPs are tuned by manipulating the effective refractive index of materials around Ag-NPs through the accurate control of the TiO2 thickness using atomic layer deposition technique. By integrating with the plasmonic filters, the CIE1931 coordinate of the blue LECs can approach to (0.14, 0.22), which is comparable to or even better than the reported bluest values of blue LECs. Combination with the green and red LECs, the color gamut increases from 34% (without filters) to 54% of National Television System Committee (NTSC) color gamut, corresponding to 1.6 times enhancement. In addition, the blue LECs integrated with plasmonic filters still have better efficiency than those of the reported bluest LECs. © 2017 Elsevier B.V. All rights reserved.

Keywords: Light-emitting electrochemical cells Color saturation Nanoparticles

1. Introduction Solid-state light-emitting electrochemical cells (LECs) exhibit the characteristic of in-situ electrochemical doping under bias [1]. Therefore, p- and n-type doped layers form ohmic contacts with both electrodes to reduce the carrier injection barriers, rendering low operation voltage and good carrier balance. Simple single-layer structure of LECs is compatible with solution processes and facilitates large-area fabrication. Furthermore, applicability of employing inert metals as electrodes for LECs reduces the encapsulation cost because the carrier injection is relatively insensitive to the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-P. Yang), [email protected]. edu.tw (H.-C. Su). 1 Equal contribution. http://dx.doi.org/10.1016/j.orgel.2017.09.018 1566-1199/© 2017 Elsevier B.V. All rights reserved.

work functions of electrodes. Saturated blue emitting light sources are highly desired to realize full-color displays and solid-state lighting. Several works on blue polymer LECs have been reported [2e5]. However, green emission is often observed along with blue emission in these blue polymer-based LECs and thus color saturation was deteriorated. Recently, small-molecule materials have been employed in blue LECs [6e10]. Low device efficiency is the common disadvantage for polymer- and small-molecule-based blue LECs due to their fluorescent nature. Phosphorescent ionic transition metal complexes (iTMCs) are alternative candidates as the emissive materials of blue LECs [11e27]. Thanks to their phosphorescent nature, blue LECs based on iTMCs generally show better device efficiencies [11e21] and high external quantum efficiency (EQE) up to 14.5% has been achieved [17]. However, such blue LECs usually exhibit sky-blue electroluminescence (EL) with the emission peak at ca. 490 nm and an emission tail at longer wavelength, which is still not saturated blue enough. Further increasing the energy gap of iTMCs

C.-M. Fan Chiang et al. / Organic Electronics 51 (2017) 70e75

resulted in more saturated blue EL [22e27], but it destabilized the metal-to-ligand charge-transfer state such that the excited state became more ligand-centered, rendering lower emission efficiency [26]. To overcome the intrinsic difficulty, reducing the green part of EL spectrum from sky-blue LECs would be a possible approach to improve the color saturation of blue EL. Spectral tailoring via destructive interference from microcavity effect has been reported to suppress the green EL intensity [28,29]. However, temporal moving of the recombination zone in LECs [30e33] results in color migration [28,29]. To realize stable, efficient and saturated blue EL, we demonstrate a novel method that using plasmonic filters to tailor the EL spectrum of the highly efficient sky-blue LECs. By adjusting the effective refractive index of the surrounding medium near Ag nanoparticles (NPs), the resonant wavelength of localized surface plasmon resonance (LSPR) can be tuned to filter out the green part of EL emission. Therefore, the blue LECs integrated with the proposed plasmonic filters show EL with Commission Internationale de l’Eclairage (CIE) coordinate of (0.14, 0.22), which is comparable to or even bluer than those reported bluest values of blue LECs [22e27]. When combined with green and red LECs, the color gamut can be enhanced by 61% due to the improvement of blue saturation. These results confirm that efficient and saturated blue EL can be obtained without performing sophisticated molecular design. Plasmonic filtering offers a feasible way to obtain saturated blue EL from well-developed efficient sky-blue iTMCs and thus has potential for use in LEC displays.

2. Experimental section 2.1. Plasmonic filter fabrication and characterization The fabrication of Ag-NPs plasmonic filters include three phases, i.e. fabrication of randomly distributed Ag-NPs, thin TiO2 coating, and fabrication of anode contact. To fabricate the randomly distributed Ag-NPs, the thin sputtered Ag films on the glass substrates were subjected to rapid thermal annealing. Before sputtering, the glass substrates of dimension of 1.5 cm 1.5 cm were cleaned by solvent, hydrochloric acid, and DI water [34]. After cleaning, the glass substrates were immediately loaded into the sputter chamber and the chamber was pumped down to 5 106 torr. For the Ag film (10 nm) deposition, the deposition parameters of argon flow rate, chamber pressure, and RF power were set as 6 sccm, 5 106 torr, and 50 W, respectively. To obtain Ag-NPs, these deposited thin Ag films were annealed at 450 C for 1 min in nitrogen ambient. To tune the LSPR of Ag-NPs, various thicknesses of thin titanium oxide (TiO2) film were conformally coated on the fabricated Ag-NPs by a homemade atomic layer deposition (ALD) system. Titanium tetrachloride (TiCl4) and DI water (H2O) were used as the precursors and their flow rate were controlled by their temperatures, which were set as 31 C and 25 C, respectively. Argon (Ar) gas was used for purging gas. Each cycle includes 8 steps with equal duration of 1 s, i.e. TiCl4 injection, pumping down, Ar purging, pumping down, H2O injection, pumping down, Ar purging, and pumping down. The thicknesses of TiO2 films were controlled by the deposition cycles which are 38, 96, 192 and 288 cycles, corresponding to the thicknesses of 2, 5, 10 and 15 nm. All the TiO2 films were deposited at 200 C. Finally, a 30 nm-thick of ITO film was capped on these TiO2coated Ag-NPs to serve as the anode contact of LEC devices. For sputtering of ITO films, the argon flow rate and RF power was maintained at 10 sccm and 100 W. The sputtered ITO films were

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rapid thermal annealed at 450 C for 1 min in vacuum to increase the ITO conductivity. The fabricated Ag-NPs plasmonic filter were optically characterized by a commercial spectrometer (Shimadzu, UV-1800) and inspected by a scanning electron microscopy (SEM, JEOL, JSM-7600F). 2.2. LEC fabrication and characterization After the standard clean and UV/ozone treatment, the samples (plasmonic filters and ITO glasses) mentioned above were spin-coated with a 40 nm-thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron P VP Al 4083) layer at 4000 rpm and then were baked at 150 C for 30 min in ambient air. For blue LECs, the emissive layer was composed of complex 1 (Fig. 1), [Ir(dfppz)2(dtbbpy)]þ(PFe 6 ) [21] (where dfppz is 1-(2,4-difluorophenyl)pyrazole and dtb-bpy is 4,40 -di(tert-butyl)-2,20 -bipyridine) (80 wt%) and 1-butyl-3methylimidazolium hexafluorophosphate [BMIMþ(PFe 6 )] (20 wt%). The ionic liquid [BMIMþ(PF6)e] was added to enhance the ionic conductivity of thin films and hence reduced the turn-on time of the LEC devices. The mixture concentration of complexes 1 and [BMIMþ(PFe 6 )] in acetonitrile solution was 80 mg mL1. The emissive layer of green LECs contained complex 2 (Fig. 1), [Ir(dFppy)2(SB)]þ(PFe 6 ) [35] (where dFppy is 2-(2,4-difluorophenyl)pyridine and SB is 4,5-diaza-9,90 -spirobifluorene) (80 wt%) and [BMIMþ(PFe 6 )] (20 wt%). The mixture concentration of complexes 2 and [BMIMþ(PFe 6 )] in acetonitrile solution was 100 mg mL1. For red LECs, the emissive layer was based on complex 1 (75 wt%), complex 3, [Ir(ppy)2(biq)]þ(PFe 6 ) [36] (where ppy is 2-phenylpyridine and biq is 2,20 -biquinoline) (5 wt%) and [BMIMþ(PFe 6 )] (20 wt%). The mixture concentration of complexes 1,3 1 and [BMIMþ(PFe 6 )] in acetonitrile solution was 100 mg mL . All solutions were spun on top of PEDOT:DSS layer at 3000 rpm in ambient air. After spin coating of the emissive layer, the sample was then baked at 70 C for 10 h in a nitrogen glove box. Finally, a silver top contact was deposited by thermal evaporation in a vacuum chamber (~106 torr). For blue LECs integrated with single plasmonic filter (glass/Ag-NPs/ TiO2/ITO) and without plasmonic filter (i.e. 30 nm-thick ITO on glass), the blue LECs were directly fabricated on top the plasmonic filters and ITO glasses. For blue LECs integrated with 2 plasmonic filters, LECs were directly fabricated on one plasmonic filter and then were stacked on the other plasmonic filter. The electrical and emission properties of LEC devices were measured using a source-measurement unit and a Si photodiode calibrated with the Photo Research PR-650 spectroradiometer. All LECs were measured at a constant current density (0.4 mA cm2) in a nitrogen glove box. The EL spectra were taken with a calibrated CCD spectrograph. 3. Results and discussions Fig. 2(a) shows the top-view SEM image of glass/Ag-NPs/TiO2 (10 nm)/ITO (30 nm). The scale bar represents 100 nm. From the SEM image, the diameter of the Ag-NPs covered with TiO2 and ITO is ca. 140e150 nm, corresponding to the bare Ag-NPs with diameter of ca. 60e70 nm, which is consistent with our previous results [34]. The cross sectional SEM image of this plasmonic filter are shown in Fig. 2(b), which shows that the Ag-NPs are conformally coated by the deposited TiO2 and ITO. The deposited film of TiO2 and ITO with thickness of ca. 48 nm on the glass substrate are labeled, which is consistent with the expected thickness. From our previous results, these randomly distributed Ag-NPs capped with ITO, acting as a plasmonic filter, can produce white LEC by strongly absorbing the emitting green light from the non-doped sky-blue emissive material duo to its LSPR absorption. On the other hand, the previous results indicate that this ITO-capped Ag-NPs plasmonic filter is not

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F N

N

N

N

N

F F

Ir

PF6

Ir

N

N N

PF6

N

N

F

1

2 Fig. 1. Chemical structures of complexes 1 and 2.

suitable to improve the color saturation of this sky-blue LEC because this plasmonic filter does not filter out the red light emitting from the sky-blue emissive material. Therefore, red shifting the LSPR wavelength of Ag-NPs to absorb red light is one of routes to improve the blue saturation of this sky-blue LEC. Now we will show that the LSPR wavelength of Ag-NPs can be red shifted and finely tuned by accurately controlling the thickness

of TiO2 deposited on top of the Ag-NPs using ALD techniques. Fig. 3(a) shows the transmission spectra of bare glass/Ag-NPs, glass/ Ag-NPs/ITO (30 nm) and glass/Ag-NPs/TiO2 (2, 5, 10, and 15 nm)/ITO (30 nm). The transmission spectrum of glass/Ag-NPs/ITO (30 nm) is re-plotted from our previous result [34]. All samples show

Fig. 2. (a) Top-view and (b) cross sectional SEM images of glass/Ag-NPs/TiO2 (10 nm)/ ITO (30 nm). The scale bars represent 100 nm. The deposited film of TiO2 followed by ITO on glass are also labeled.

Fig. 3. (a) The transmission spectra of bare Ag-NPs, Ag-NPs/ITO (30 nm) and Ag-NPs/ TiO2 (2, 5, 10, and 15 nm)/ITO (30 nm). (b) LSPR wavelengths of Ag-NPs/TiO2/ITO as a function of TiO2 thickness.


Normalized EL intensity

1.0

w/o filter Ag NPs/TiO2 ( 2 nm) Ag NPs/TiO2 ( 10 nm) Ag NPs/TiO2 ( 2 nm) + Ag NPs/TiO2 (10 nm)

0.8 0.6 0.4

500

600

700

Wavelength (nm) Fig. 4. Normalized EL spectra of blue-green LEC without and with integration with TiO2-coated Ag-NPs plasmonic filters. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

NTSC w/o filter Ag NPs/TiO2 ( 2 nm) Ag NPs/TiO2 (10 nm) Ag NPs/TiO2 ( 2 nm) + Ag NPs/TiO2 (10 nm)

0.8 0.6 0.4 0.2 0.0 0.0

0.2 0.0 400

normalized EL spectra of the sky-blue LECs without and with various TiO2-coated Ag-NPs plasmonic filters, i.e. glass/Ag-NPs/TiO2 (2 nm)/ITO, glass/Ag-NPs/TiO2 (10 nm)/ITO and their combination. For the sky-blue LEC without Ag-NPs plasmonic filter, the EL spectrum shows a peak at ca. 490 nm and exhibits an emission tail extending over 600 nm. The full width at half maximum (FWHM) of the EL spectrum is 74 nm. This broad EL feature of the sky-blue LEC leads to poor performance on blue saturation. For the sky-blue LECs integrated with glass/Ag-NPs/TiO2 (2 nm)/ITO and glass/Ag-NPs/ TiO2 (10 nm)/ITO, not only the emission peak blue shifts to 479 nm but also the emission width become narrower (FWHM ¼ 63 and 67 nm, respectively), so that we can expect the better performance of blue saturation as the sky-blue LEC integrated with the either one plasmonic filters. The modification of the EL emission spectra is duo to the strong absorption of the green and red emission lights by AgNPs as the LSPR wavelength is red shifted by TiO2 coating. Since glass/Ag-NPs/TiO2 (10 nm)/ITO has longer LSPR wavelength (Fig. 3), it suppresses relatively stronger on red emission light but relatively weaker on green lights. For the sky-blue LEC integrated with the combination of these two plasmonic filters, the EL spectrum shows the shortest emission peak at 474 nm and the narrowest band width (FWHM ¼ 56 nm), meaning the best performance of blue saturation. To evaluate the improvement in blue saturation, the CIE1931 coordinates of EL from blue LECs without and with plasmonic filters are depicted in Fig. 5. Blue LECs without plasmonic filters exhibited sky-blue EL with a CIE1931 coordinate of (0.19, 0.36). When combined with the green (complex 2) and red [complex 3 (5 wt%): complex 1] LECs (EL spectra shown in Fig. 6), the LEC displays show only 34% National Television System Committee (NTSC) color gamut. With glass/Ag-NPs/TiO2 (2 nm) or glass/Ag-NPs/TiO2 (10 nm) plasmonic filters, green emission is reduced and thus blue saturation is improved, enhancing the percentage of NTSC color gamut to 45%. In comparison with the reported blue LECs with relatively higher EQE (ca. 3%) [24], these two blue LECs not only have

y

transmission dips at different wavelengths, corresponding to the LSPR of Ag-NPs under different environments. The bare Ag-NPs on glass substrate exhibit the shortest LSPR wavelength at ca. 452 nm. Then the LSPR wavelength red shifts to ca. 509 nm as Ag-NPs capped with 30 nm-thick ITO film. The LSPR wavelength of glass/Ag-NPs/ TiO2 (2 nm)/ITO is at ca. 535 nm, even longer than the one of glass/ Ag-NPs/ITO. The LSPR wavelength of glass/Ag-NPs/TiO2/ITO continuously red shifts as the thickness of TiO2 film increases up to 10 nm but eventually is nearly pinned at ca. 605 nm for thicker TiO2 film. Fig. 3(b) shows the LSPR wavelengths of glass/Ag-NPs/TiO2/ITO as a function of TiO2 thickness. The dashed lines are for the guide to the eyes. The LSPR wavelength almost increases linearly with the TiO2 thickness for TiO2 thickness less than 10 nm. According to the criteria of LSPR, namely εm(lR) ¼ -2ε [37e41], the LSPR wavelength of metallic nanoparticle is determined by its environment, where the εm, ε, and lR are the dielectric function of metal, (effective) dielectric constant of ambient material, and LSPR wavelength. More specifically, the environment of localized surface plasmon (LSP) sensing is limited by the penetration depth of its electric field. In general, this penetration depth is ca. 10 nm [42,43]. This qualitatively explains the LSPR wavelength pinning (ca. 605 nm) for the TiO2 film thicker than 10 nm. One the other hand, the LSP can sense all the materials within its penetration depth, i.e. glass, ITO, and TiO2 for our case. Hence, the effective dielectric constant (also effective refractive index) of Ag-NPs' environment is a combination of glass, ITO, and TiO2. According to the criteria of LSPR and simply using Drude model to describe the dielectric function of metal, we can pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi obtain lR ¼ lplasm 2ε þ 1 ¼ lplasm 2n2 þ 1, where lplasm is the plasma wavelength of metal and n is the (effective) index of refraction of the environment. Therefore, the LSPR wavelength red shifts as the effective index of refraction increases. From our previous result [44], the refractive index of TiO2 deposited by our ALD system at 200 C is ca. 2.3 for spectral range of 500e700 nm, which is larger than the refractive indices of glass (1.51) and ITO (1.9). Hence, the effective index of refraction increases as the thickness of TiO2 increases and lR red shifts consequently. These results demonstrate that the LSPR wavelength can be finely tuned by controlling the thickness of TiO2, which can be used to finely adjust the operation wavelength of plasmonic filter. To confirm that these TiO2-coated Ag-NPs plasmonic can improve the performance of blue saturation, the sky-blue LECs were directly fabricated on top of these plasmonic filters. Fig. 4 shows the

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[26] [24] [22] [27]

0.2

0.4

x

0.6

0.8

Fig. 5. CIE1931 coordinates of the EL from the blue LECs without filter and combined with Ag NPs/TiO2 (2 nm), Ag NPs/TiO2 (10 nm) and Ag NPs/TiO2 (2 nm) þ Ag NPs/TiO2 (10 nm) filters. CIE1931 coordinates of the EL from green and red LECs are shown for evaluating the color gamut. CIE1931 coordinates of the EL from reported blue LECs (Refs. [22,24,26,27]) are also labeled for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


Normalized EL intensity

74

1.0

Green LEC Red LEC

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) Fig. 6. EL spectra of the green and red LECs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

comparable EQE (Table 1) but also are bluer (Fig. 5). Incorporating both plasmonic filters further improves blue saturation and the CIE1931 coordinate approaches to (0.14, 0.22) and still maintains EQE >1% (Table 1), which is comparable to or even bluer than those reported bluest values of blue LECs [22,24,26,27] (Fig. 5). Consequently, 54% NTSC color gamut, which shows 1.6 times enhancement as compared to that of blue LECs without plasmonic filters, can be realized. These results confirm that plasmonic filters embedded in blue LECs effectively reduced green spectral intensity and thus improved blue EL saturation. To examine the device performance of blue LECs integrated with plasmonic filters, the EL characteristics of these devices under a constant current density of 0.4 mA cm2 were measured and are summarized in Table 1. As shown in Fig. 7a, the device voltage was initially high due to the poor carrier injection into the undoped emissive layer. When electrochemical doped layers were gradually formed, the device voltage reduced significantly under the measurement constrain of constant current density. Finally, the device voltage approached ca. 3.8 V after the p-i-n structure was well established. Time-dependent brightness for all devices is shown in Fig. 7(b). The brightness increased with the time and reached the peak value at 40e70 s. Since the current density was constant, the enhanced brightness was attributed to the improved balance in electron and hole currents. After reaching the peak value, the brightness decreased with the time gradually. Deteriorated

brightness was related to the exciton quenching near the extended doped layers [45,46] and/or to the degradation of the emissive material under electrical operation [47]. The brightness decreased when the plasmonic filters were used since some green EL was filtered out. Time-dependent EQE also showed similar temporal trend (Fig. 7(c)). Although blue color saturation was obtained at the expense of some brightness and efficiency, the proposed blue LECs integrated with plasmonic filters still possessed better blue saturation and higher device efficiency as compared to the reported blue LECs based on sophisticatedly designed iTMCs [22,24,26,27] (cf. Table 1 and Fig. 5). Hence, the blue LECs integrated with plasmonic filters offer a feasible and simple way to realize efficient and saturated blue EL by utilizing well the developed sky-blue iTMCs. Higher brightness from LECs can be obtained at the expense of device lifetime and efficiency. Such trade-off mainly results from poor stability of the ionic emissive materials used in LECs. Further improvements in LEC materials will be highly desired to meet practical requirements. This proposed technique would have great potential for use in LEC displays.

4. Conclusions In summary, we proposed and demonstrated a novel method to improve the blue saturation of the sky-blue LECs via plasmonic filters rather than performing sophisticated molecular design. These plasmonic filters composed of randomly distributed Ag-NPs have different LSPR wavelengths which can be finely controlled by tuning the effective refractive indices of the Ag-NPs’ surrounding materials. The effective refractive indices, so as LSPR wavelengths of Ag-NPs, are tuned by the accurately controlling the TiO2 thickness capped on top of Ag-NPs through a homemade ALD system. Hence, the LSPR wavelengths of Ag-NPs are red shifted as the TiO2 thickness increases so that can absorb the green and red emission tail of the blue-sky LECs. When combined with the green and red LECs, the color gamut is enhanced from 34% (without filter) to 45% of NTSC color gamut for the blue LECs integrated with one piece of plasmonic filter. Moreover, the blue LECs with one piece of plasmonic filter still have EQE around 2e3%. When integrated with two plasmonic filters, the CIE1931 coordinate of the blue LECs approaches to (0.14, 0.22), further improving the blue saturation, which is comparable to or even better than the reported bluest values of blue LECs. Hence, 54% of NTSC color gamut can be realized when combined with the green and red LECs. Thanks to the high efficiency of iTMCs, the blue LECs incorporated with two plasmonic filters still have EQE >1%. This proposed method would have potential using in the LEC display.

Table 1 Summary of EL characteristics of blue LECs integrated with various filters under a constant current density of 0.4 mA cm2. Filter

CIE (x, y)a

ELpeak (nm)b

FWHM (nm)c

tmax (sec)d

Lmax (cd m2)e

hext, max, hL, max, hp, max

t1/2 (sec)g

(5.0, (2.9, (2.5, (1.1,

280 260 290 270

(%, cd A1, lm W1)f w/o filter Ag NPs/TiO2 (2 nm) Ag NPs/TiO2 (10 nm) Ag NPs/TiO2 (2 nm)þAg NPs/TiO2 (10 nm) a b c d e f g

(0.19, (0.17, (0.15, (0.14,

0.36) 0.29) 0.29) 0.22)

485 479 479 474

74 63 67 56

Evaluated from the EL spectra. EL peak wavelength. Full width at half maximum of the EL spectrum. Time required to reach the maximal brightness. Maximal brightness. Maximal external quantum efficiency, current efficiency and power efficiency. The time for the brightness of the device to decay from the maximum to half of the maximum.

40 60 60 70

44.4 22.1 18.9 6.9

11.1, 7.3) 5.5, 3.8) 4.7, 3.2) 1.7, 1.2)


References

(a) 14 w/o filter Ag NPs/TiO2 ( 2 nm) Ag NPs/TiO2 ( 10 nm) Ag NPs/TiO2 ( 2 nm) + Ag NPs/TiO2 (10 nm)

12

Voltage (V)

10 8 6 4 2 0

0

2

4 6 Time (min)

8

10

0

2

4 6 Time (min)

8

10

0

2

4 6 Time (min)

8

10

(b) -2

Brightness (cd m )

50 40 30 20 10 0

(c) External Quantum Efficiency (%)

75

6 5 4 3 2 1 0

Fig. 7. Time-dependent (a) voltage, (b) brightness and (c) external quantum efficiency of the blue LECs without filter and combined with Ag NPs/TiO2 (2 nm), Ag NPs/TiO2 (10 nm) and Ag NPs/TiO2 (2 nm) þ Ag NPs/TiO2 (10 nm) filters under a constant current density of 0.4 mA cm2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Acknowledgements The authors would like to thanks the financial support from Ministry of Science and Technology (contract no. MOST 105-2221E-009-097-MY2 and MOST 105-2221-E-009-073).

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