Localized surface plasmons enhanced color conversion efficiency in organic light-emitting device with surface color conversion layer

Synthetic Metals 199 (2015) 69–73

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Localized surface plasmons enhanced color conversion efficiency in organic light-emitting device with surface color conversion layer Mei Tang a,b , Wenqing Zhu a,b, *, Liangliang Sun a,b , Jingting Yu b , Bingjie Qian b , Teng Xiao b a b

School of Material Science and Engineering, Shanghai University, Shanghai 200072, China Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 August 2014 Received in revised form 30 September 2014 Accepted 1 November 2014 Available online xxx

This paper demonstrates localized surface plasmons (LSPs) enhanced color conversion efficiency in organic light-emitting device (OLED) with surface color conversion layer (SCCL). Ag nanoparticles (AgNps) deposited by thermal evaporation between substrate and SCCL were used for localized surface plasmons. The high-density, strong photons-LSPs coupling achieved by AgNps leads to a photoluminescence (PL) enhancement in the SCCL. The electroluminescent (EL) measurements show that SCCL–OLED incorporated with AgNps has a 28% enhancement in color conversion efficiency compared with that of a conventional SCCL–OLED without AgNps which is attributed to strong electromagnetic field caused by LSP resonance of AgNps. Moreover, we verify that SCCL with LSPs of AgNps can effectively decrease the thickness of SCCL in the OLED. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Localized surface plasmons Ag nanopaticles Surface color conversion layer OLED

1. Introduction Organic light-emitting devices (OLEDs) have shown their excellent performance for a new illumination and flat-panel display technology [1], which has been attracting many researchers in recent years. To date, there are several main methods to fabricate OLEDs, including an emitting layer structure doped with different fluorescent materials [2–4], and a structure of multi-stacked emitting layers in which each layer emits a different color of light to generate incorporative light [5–7]. Without complicated fabricated process in the two methods above, another approach to fabricate OLED is to use a simple emitting layer of OLED in combination with a color conversion layer (CCL) on the outside of substrate [8,9], which was first demonstrated by Duggal et al. [10]. In general, the luminous mechanism of CCL–OLED is that the hole/electron carriers recombine in the emitting layer of OLED to produce blue electroluminescence (EL), and then part of blue EL is absorbed by color conversion layer and converted to red photoluminescence (PL), then unabsorbed blue EL from emitting layer and red PL from CCL are a mixture of white emission. Kumar et al. [11] have studied the white organic light-emitting diodes

* Corresponding author at: School of Material Science and Engineering, Shanghai University, Shanghai 200072, China. Tel.: +86 02156334387; fax: +86 02156334387. E-mail address: [email protected] (W. Zhu). http://dx.doi.org/10.1016/j.synthmet.2014.11.001 0379-6779/ ã 2014 Elsevier B.V. All rights reserved.

(WOLEDs) with color conversion layers whose thickness was about 4.5 mm. Yuan et al. [9] have reported that WOLED combined blue electro-phosphorescent device with red SCCL, and the thickness of SCCL was as thick as 10 mm. Low quantum efficiency of red-emitting color conversion materials used in previously reported semiconducting devices results in the increase of SCCL’s thickness. Moreover, low doping concentration of red-emitting material is beneficial in reducing self-quenching but consequently leads to thicker SCCL, which is undesired for practical application. Therefore, it is essential to study the enhancement of color conversion efficiency and the decrease of SCCL’s thickness. Localized surface plasmons (LSPs) are the collective oscillations of free electrons in metal nanoparticles interfaces, and the LSP resonance coupling between the electron oscillation and the light generates strong electromagnetic field close to the metal nanoparticles [12–14]. Recently, LSPR of metal nanoparticles has been widely used to enhance Raman scattering [15,16], optical absorption [17,18], and photoluminescence of fluorescent materials [19]. Especially, the coupling between metal nanoparticles and light-emitting device has been actively studied in semiconducting devices. Recently Kwon et al. [20] have reported enhancement of emitting intensity in GaN based blue LED with AgNps layer inserted between the n-GaN layer and the multiple quantum well layer. Yang et al. [21] have reported AgNps were embedded in the LiF layer on cathode structure (LiF/AgNps/LiF) to enhance plasmonic emission. The conventional mechanism of fluorescent

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intensity enhancement is metal enhanced fluorescence (MEF) by LSPR of metal nanoparticles, which has been studied both theoretically and experimentally [14,22–27]. To the best of our knowledge, there are many reports regarding the use of LSPs for enhancement of fluorescent intensity or efficiency in the emitting-layer of OLED [21,28–30], but few papers study LSPs of metal nanoparticles enhance color conversion efficiency of SCCL. In this paper, we fabricated a SCCL–OLED containing a AgNps layer inserted between substrate and SCCL, and chose the high efficient material (Aluminium tris(quinolin-8-olate) (Alq3) [31]) as a green-emitter layer in OLED, and the fluorescent dye Rhodamine B (RhB) doped into polymers polyvinyl alcohol (PVA) as a SCCL. SCCL–OLED with AgNps can effectively improve PL intensity in SCCL, enhance color conversion efficiency and decrease SCCL’s thickness. 2. Materials and methods AgNps were fabricated on the reverse side of indium tin oxide (ITO) coated substrate as following preparations. 16 nm Ag thin film (Sample 1) was fabricated by thermal evaporation at a base pressure of 5
104 Pa with a rate of 1 Å/s. Then the Ag thin film transformed into the AgNps (Sample 2) by thermal annealing in a hot air oven at 175  C under vacuum atmosphere for 20 min. The reason for the transformation was that large internal stress was produced inside the Ag thin film by thermal annealing, and diffused along the Ag atoms’ grain boundary [32]. The color of Ag thin film changed from blue to yellow after thermal annealing because large AgNps were generated. The size and surface characterization of AgNps were monitored directly by atomic force microscope (AFM) SPA400. The PVA doped with RhB composite thin film acts as a SCCL in the OLED. PVA (weight-average molecular weight Mw = 15000, % hydrolysis = 88, Aladdin), an environment-friendly reagent, is highly soluble in water, and its high transmittance makes it a good matrix of polymer film. So fluorescent dye RhB doped into PVA can effectively reduce self-quenching of RhB. Dye RhB and PVA were used as received without further purification. SCCL was prepared by the following procedure: PVA and deionized water were mixed with an initial weight ratio of 6% and well stirred to form the homogeneous aqueous solutions. PVA acts simultaneously as stabilizer for RhB molecules and matrix for homogeneous distribution. The PVA solution doped with RhB (concentration: 0.25
102 mol/L) was spin-coated at 4000 rpm for 60 s on the reverse side of ITO glass substrate. The samples were dried in the vacuum atmosphere for 30 min. Table 1 lists samples with various structures used in our experiments. The thickness of the spin-coated RhB-doped PVA film is about 320 nm, measured by using Alpha-Step 500. Fig. 1 shows a schematic of OLED thin-film structure with AgNps proposed in this study, and the AgNps layer was deposited on the reverse side of substrate. Organic layers were deposited on ITO glass substrates under high vacuum (5
104 Pa) with a

Fig. 1. Schematic structure of OLED combined with a SCCL coupling of AgNps.

deposition rate of 1 Å/s. The device structure was ITO (20 nm)/ MoO3 (5 nm)/NPB (40 nm)/Alq3 (60 nm)/LiF (0.6 nm)/Al (150 nm). Alq3 was used as the electron transporting layer (ETL) and green emitting layers (EML). N,N0 -bis-(naphthalen-1-yl)-N,N0 -bis(phenyl) benzidine (NPB) was used as the hole transporting layer (HTL). Lithium fluoride (LiF)/aluminium (Al) and ITO were used as cathode and anode, respectively. Table 2 lists kinds of OLEDs with/ without AgNps and with different SCCL’s thickness used in our experiments. The size of each pixel was 2mm
2 mm. UV–vis absorption spectra were recorded using a UV–vis spectrophotometer model No. U-3900H. Photoluminescence (PL) spectra were studied using a fluorescence spectrophotometer model No. FLSP920. The current density–luminance–voltage (J–L–V) characteristics, electroluminescence (EL) spectra, and CIE coordinates were measured and recorded by a testing setup comprising of a Keithley 2400 source-meter and a Minolta PR-6500 spectrometer. 3. Results and discussion Fig. 2 shows the two (a) and three-dimensional (b) AFM images of AgNps on the reverse side of substrates. The average diameter and height of relatively uniform AgNps seen from Fig. 2 are about 100  10 nm and 50  5 nm, respectively. The root mean square roughness (RMS) of Ag thin film before thermal annealing is 1.33 nm, while the RMS of AgNps is 10.58 nm after thermal annealing, which shows that surface structure of Ag film after annealing is coarser. Due to the collective oscillation of free electrons in metallic nanoparticles, the AgNps generate localized surface plasmons (LSPs) [18] which dramatically change the optical properties of adjacent fluorophores. The measurement of absorbance using a UV–vis spectrometer is the simplest method to confirm LSPR of AgNps. Fig. 3(a) shows the absorption spectra of 16 nm Ag thin film before (Sample 1) and after annealing (Sample 2) at 175  C for 20 min. It shows that LSPR absorption peak of AgNps is at about 444 nm, which was proved in other paper [33]. In contrast, the Sample 1 shows a flat absorption peak. It indicates that Ag thin film generates the AgNps by thermal annealing, which has been verified in the AFM images. The absorption spectra of SCCL (RhB doped into PVA) with and without AgNps are shown in Fig. 3(b). It shows that the LSPR absorption peak of AgNps in the structure substrate/AgNps/PVA

Table 1 Samples with various structures. Substrate

Ag thin film

AgNps

SCCL PVA

Sample Sample Sample Sample Sample Sample Sample

1 2 3 4 5 6 7

p p p p p p p

RhB

Table 2 Devices with different structures.

p p p p

p

p p p p p

p p p

Device Device Device Device Device Device

A B C D E F

Alq3–OLED p p p p p p

AgNps

SCCL (Xnm)

p

320 nm 320 nm 630 nm 930 nm 1050 nm 1500 nm

M. Tang et al. / Synthetic Metals 199 (2015) 69–73

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Fig. 2. AFM images of Ag thin film deposited on the ITO glass substrate after thermal annealing, (a) two-dimension image (b) three-dimensional image.

(a)

0.8

(b)

Sample 2 Sample 1

0.7

Sample 3 Sample 4 Sample 5 Sample 6 Calculation

0.8

Abs intensity

Abs intensity

0.6

1.0

0.5 0.4 0.3 0.2

0.6 0.4 0.2

0.1 350

400

450

500

550

600

650

0.0

700

400

wavelength (nm)

500

600

700

800

wavelength (nm)

Fig. 3. Absorption spectra of (a) Sample 1 and Sample 2 (b) four samples (Sample 3–Sample 6) of SCCL with/without AgNps and with/without RhB, and the green curve: sum by calculating. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(Sample 3) or structure substrate/AgNps/PVA-RhB (Sample 4) is at about 461 nm. Compared with Fig. 3(a), the LSPR absorption peak of AgNps in the SCCL generates red shift from 444 nm to 461 nm. This red shift may be caused by refractive index difference between PVA and AgNps which has been discussed by Xu et al. [34]. As shown in the Fig. 3(b), the absorption peak of RhB directly dissolved in the PVA (Sample 5) and Sample 4 are at about 556.5 nm, which is not change. Meanwhile, the absorption intensity of RhB in Sample 4 is obviously strengthened compared with Sample 5. We suppose the reason is that AgNps absorption spectrum will affect the RhB absorption intensity, so we calculate by adding the curve of Sample 3 and Sample 5 up together, and then minus curve of Sample 6 (only PVA), finally we get the curve (Calculation) in Fig. 3(b). Compared with absorption spectrum of

(b) 5

7x10

Sample 4 Sample 5 Sample 7

5

6x10

Abs&EL intensity(a.u.)

(a)

5

PL intensity

Sample 4, curve of Calculation is mainly consistent with Sample 4, so we consider that the RhB absorption intensity in the Sample 4 does not increase compared with Sample 5. The photoluminescence (PL) spectra of SCCL without/with Ag thin film or AgNps are shown in Fig. 4(a), and excited wavelength is localized at 528 nm, which matches with emitting wavelength from Alq3–OLED[31]. Those samples were pumped from the luminescent layer side and PL was collected from the same side. Sample 7 has a mildly stronger PL intensity than Sample 5 in Fig. 4(a). When the metal surface is flat, high momentum SPs generates at the interface between the metal layer and dielectric, and SPs are not effectively coupling to light because the SP is a nonpropagating evanescent wave [35]. However, the PL intensities of Sample 5 and Sample 4 are obviously shown in Fig. 4(a), and the

5x10

5

4x10

5

3x10

5

2x10

EL Alq3

1.0

Abs RhB

0.8 0.6 0.4 0.2 0.0

5

1x10

560

580

600 620 wavelength (nm)

640

660

400

500 600 wavelength(nm)

700

800

Fig. 4. (a) Emission spectra of Sample 4, Sample 5, Sample 7, excited wavelength is localized at 528 nm; (b) Abs spectrum of RhB dispersed in the PVA film and EL spectrum of Alq3.

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M. Tang et al. / Synthetic Metals 199 (2015) 69–73

(b) 10

1000

4

Device A Device B

1.0

Device A Device B

3

800

2

Luminance(cd/m )

EL intensity(a.u.)

10 0.8 0.6 0.4 0.2

2

600

1

400

0

200

10 10 10

0.0

10 300

400

500

600

700

800

wavelength(nm)

2

1.2

current density(mA/cm )

(a)

0

-1

4

5

6

7

8

9

10

11

12

13

voltage(V)

Fig. 5. (a) EL spectra normalized by the Alq3-peak intensity and (b) the current density and luminance with bias voltage (J–L–V curve) for both SCCL–OLED without (Device A) and with (Device B) AgNps.

PL intensity enhancement of Sample 4 can be attributed to the LSPR of AgNps. The reason for LSPR is that the roughness of metal layer causes the scattering of high momentum SPs to lose momentum and couples them to radiated light [35]. When light coupling with AgNps gives rise to LSPR, thus LSPR enhances the electromagnetic field around the RhB molecules, which increases radiative rate of RhB molecules so as to enhance PL intensity of SCCL. To achieve efficient energy from the emitting layer of OLED to the SCCL, absorption spectrum of red emission material doped into the SCCL should match well with the EL spectrum in the OLED. As shown in the Fig. 4(b), spectral overlap between the absorption spectrum of RhB dispersed in the PVA film and the EL spectrum of Alq3 as an emitting layer in the OLED would lead to efficient energy transfer. The OLED combined with surface CCL has the structure of SCCL (320 nm)/AgNps (16 nm)/glass/ITO/MoO3/NPB/Alq3/LiF/Al, where the AgNps layer is absent for the reference Device A. Considering identical absorption of Sample 5 and Sample 4 known from Fig. 3(b), the EL spectra normalized by the Alq3-peak intensity for both SCCL–OLED without (Device A) and with (Device B) at 200 mA/cm2 are shown in Fig. 5(a). The EL spectra of Device A and Device B have two peaks, the left peak generates from the emitting layer (Alq3), and the right peak generates from dye RhB in the SCCL. When SCCLs are incorporated with OLED, some green emission of Alq3 is absorbed and then converted to red fluorescence of RhB, resulting in a mixture of yellow emission. The red emission of SCCL with AgNps in OLED has been enhanced owing to LSPs of the AgNps. The relative intensity of the red emission as a reference to the green emission, which is normalized for clarity, is enhanced as the appearance of AgNps. The absorption intensity of RhB has been

demonstrated identical, so we calculate the enhancement of color conversion efficiency of the SCCL with AgNps by comparing the area of red emission spectra. Calculating from the Fig. 5, the enhancement of color conversion efficiency reaches about 28% using integral method. The CIE coordinates of Device A and Device B are (0.3915, 0.5139), (0.4451, 0.5079), respectively. Therefore, coupling of AgNps acts as an effective method in enhancing color conversion efficiency of SCCL and changing the CIE of SCCL–OLED. Fig. 5(b) shows the current density and luminance with bias voltage (J–L–V curve) of Device A and Device B. The J–V curves of Device A and Device B exhibit nearly similar current density– voltage characteristic, because AgNps are located on the outside of OLED, which can not affect the current density of internal OLED. Luminance–voltage (L–V) curves of OLED in the Fig. 5(b) shows that luminance of Device A and Device B are around 3880 cd/m2, 2700 cd/m2 at around 10.5 V and 200 mA/cm2. The main reason for lower luminance of SCCL–OLED with AgNps is that 16 nm AgNps can absorb the light from OLED. Further, in order to conform that SCCL with AgNps can effectively decrease SCCL’s thickness, we have fabricated different thickness of SCCL in OLED without AgNps. The thickness of SCCL (PVA–RhB) is controlled by rotate speed of spin-coated and the concentration of PVA dissolved in deionized water. It obviously shows that the thickness of SCCL decreases with the changing of PVA concentration from 12% to 6% and rotate speed of spin-coated from 1000 rpm to 4000 rpm. We fabricated green OLEDs with SCCL of different thickness (630 nm, 930 nm, 1050 nm, 1500 nm) which are named as Device C, Device D, Device E, Device F, and the EL spectra normalized by the Alq3-peak intensity at 100 mA/cm2 are shown in Fig. 6. The relative intensity of red emission as a reference

Fig. 6. (a) EL spectra normalized by the Alq3-peak intensity for different thickness of SCCL. Inset: EL spectra normalized for Device E (blue line) and Device B (red line). (b) A schematic diagram on the luminous mechanism of SCCL–OLED incorporated with AgNps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Tang et al. / Synthetic Metals 199 (2015) 69–73

to the green emission (I0 = Ired/Igreen) increases with the increase in SCCL’s thickness. From the inset and table of Fig. 6(a), we find that the normalized EL spectrum of Device E is mostly consistent with that of Device B, and the relative intensity of Device B (I0 = 1.098) and Device E (I0 = 1.075) is mostly identical. That is to say, the thickness of SCCL greatly decreases from 1050 nm (Device E) to 320 nm (Device B) coupling of AgNps. The reason for the difference in spectra localized at about 550 nm is that Device E needs to absorb more green-light to convert to red PL than Device B. Therefore, SCCL with the AgNps in the OLED acts as an effective way to the decrease of SCCL’s thickness. As for the luminous mechanism in this case (Fig. 6(b)), it should be that excitons generated in the Alq3 emitting layer are deactivated and thus radiate green fluorescence photons. Part of these photons directly radiate green light through the SCCL (Process I). Some green fluorescence photons excite RhB molecules in the SCCL and convert to red PL (Process II). The AFM images and absorption spectra of AgNps show that AgNps have the characteristics of LSPs. The fluorescent photons from EL in OLED coupling with LSPs of AgNps (lower right of Fig. 6(b)), which generate strong electromagnetic field near AgNps and then radiate more photos to excite RhB molecules, which causes that the extra RhB molecules are excited and radiative rate of RhB molecules in close proximity to AgNps is enhanced (Process III). Luminous mechanism includes Process I and II in SCCL–OLED without AgNps while that in SCCL–OLED coupling of AgNps contains Process I, II, III. Therefore, the process III contributes excited RhB to generate more red PL in SCCL–OLED with AgNps. In other words, to achieve the same yellow light, SCCL–OLED without AgNps needs more excited RhB molecules from thicker SCCL to generate enough red PL. 4. Conclusion In summary, we investigated the LSPs–enhanced color conversion efficiency of SCCL in OLED by depositing AgNps with thermal evaporation. Using AgNps layer inserted between substrate and SCCL, the PL intensity was increased significantly owing to photons-LSPs coupling. The normalized EL spectra show that LSPR of AgNps enhances the red emission and color conversion efficiency (28%) in SCCL, and effectively decreases SCCL’s thickness in the SCCL–OLED. Therefore, OLED combined with AgNps makes it promising for future research and practical application. In further work, we will study white OLED combine blue OLED with red SCCL coupling of AgNps to enhance color conversion efficiency and decrease the SCCL’s thickness. Acknowledgements This work was supported by the Key Innovation Project of Education Commission of Shanghai Municipality (12ZZ091, 12YZ021) and the project of Science and Technology Commission of Shanghai Municipality (10dz1140206). References [1] M.C. Gather, et al., White organic light-emitting diodes, Adv. Mater. 23 (2) (2011) 233–248. [2] Y.W. Ko, et al., Efficient white organic light emission by single emitting layer, Thin Solid Films 426 (1–2) (2003) 246–249.

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