Efficiency enhancement of organic light-emitting diodes (OLEDs) can be obtained by the combination of microcavity effect and Au nanoparticles based surface plasmons. Au nanoparticles are thermally deposited on distributed Bragg reflector (DBR)-coated glass substrate, leading to realization of microcavity effect and localized surface plasmon effect. Our results show the current efficiency of OLEDs with DBR/Au nanoparticles as anode is increased by 72% compared to that with ITO as anode.
© 2014 Optical Society of America
Organic light-emitting diodes (OLEDs) have made great progress since the first device was demonstrated , leading to successfully commercial applications. In particular, phosphorescent OLEDs with phosphorescent dyes-doped charge-transporting hosts as emissive layer have attracted intensive attention since they exhibit higher efficiency than fluorescent OLEDs [2–4]. Although the internal quantum efficiency (IQE) of phosphorescent OLEDs can be enable to approach 100%, the external quantum efficiency (EQE) is still limited significantly by light output coupling [3, 5], indicating that only a small fraction of light generated in the device can be extracted .
In conventional bottom-emitting OLEDs, due to the high refractive index (norganic = 1.8) of organic emissive layer compared with that of glass substrate (nglass = 1.5) , a large amount of luminescence is trapped in guided modes within the device . As a result, most of light generated is trapped inside the whole device. So far, there have been a few approaches to overcome this problem, such as surface roughening [8, 9], optical microcavity structure [10–12], and micro-structured scattering surfaces [13, 14].
It is well known that the introduction of dielectric distributed Bragg reflectors (DBRs) into OLEDs enhances the light-emitting properties via microcavity effect. The principal effect of microcavity on the spontaneous emission is to modify the spectrum and spatial distribution of the emission, i.e. only certain wavelength corresponds to allowed cavity modes in a given direction [15, 16]. Several types of microcavity OLEDs have been reported [12, 17]. Moreover, the luminescence intensity can also be enhanced by coupling of localized surface plasmons (SPs) in metallic nanostructures [18–23]. SPs excited on a rough metallic surface by the interaction between light and metal can significantly enhance light intensity. Transparent metal nanostructures have also been proposed as alternative electrodes in OLEDs due to their superior advantages over indium tin oxide (ITO) .
Although SPs and microcavity can remarkably enhance the efficiency of OLEDs, the fluorescence enhancement by the combination of SPs coupling from Au nanoparticles with microcavity structure has not been reported yet. In this work, we present enhanced light coupling out of bottom-emitting microcavity OLEDs by using semitransparent thin Au nanoparticle layers integrated with DBR as the anode.
Device fabrication: Control devices were fabricated on ITO (120 nm) coated glass substrates with a sheet resistance of 15 Ω sq−1. The substrates were cleaned in an ultrasonic bath with detergent, DI water, acetone, and isopropyl alcohol for 15 min, respectively. ITO substrates were treated by UV ozone for 15 min. Microcavity devices were fabricated on periodical SiO2/TiO2 multiple layers. SiO2/TiO2 layers were grown by sputtering. Vanadium oxide (V2O5) acts as hole injection layer , alpha-napthylphenylbiphenyl diamine (NPB) as hole transport layer, and Tris-8-hydroxyquinoline aluminum (Alq3) as electron transport layer and green light emission layer. All organic layers were thermally evaporated at a rate of 0.2-0.3 nm/s, LiF was at a rate of 0.05 nm/s, and Al electrode was evaporated at a rate of 2-3 nm/s in a high vacuum. The effective active area of all devices was 3.57 mm2.
Device characterization: Tapping-mode atomic force microscropy (AFM) was performed to measure the surface morphology. The current density-voltage (J-V) and luminance-voltage (L-V) characteristics were measured using a computer controlled sourcemeter (Keithley 2400) and photometer (IL1400A) with a calibrated silicon photodiode. The electroluminescence (EL) spectra were measured by Instaspec spectrometer integrated with CCD. The simulation was carried out by SimOLED. All measurements were carried out in ambient atmosphere at room temperature.
3. Results and discussion
Alq3 was used as the luminescent material, which has a rather broad free-space emission spectrum covering the range of 450–650 nm as shown in the inset of Fig. 1(b).
The design of DBR structure plays an important role in achieving good microcavity resonance effect. It is important that the stop-band of the DBR covers the intrinsic emission spectrum of organic materials, so that the resonant wavelength can be localized within the stop-band and the full width at half maximum (FWHM) of PL spectrum. Ideally, the stop-band center and the peak wavelength of emission spectrum overlap with each other and the resonant mode emerges, and consequently PL or EL intensity can be enhanced with possible single mode emission. Therefore, we designed a wide stop band with the lowest peak at 536 nm and 80% of reflectance. The transmission of the dielectric mirror is calculated by using transfer matrix method . These can be achieved by changing the thickness of each layer and periods of two materials forming DBR. Our DBR structure consists of four pairs of alternating quarter wavelength SiO2 and TiO2 layers . The calculated and measured transmission spectra of designed DBR are shown in Fig. 1(a). It can be seen that the stop-band centers of all optical cavity structures match well with the peak wavelength of Alq3 emission spectrum as shown in the inset of Fig. 1(b). The calculated and measured transmission spectra of DBR and DBR/ITO are shown in Fig. 1(a), exhibiting only a small variation in the intensity, indicating ITO has insignificant effect on the spectrum shape and resonance peak wavelength. Our calculated and measured results are in good agreement.
Figure 1(a) also shows the calculated and measured transmission spectra of Au nanoparticles layer and DBR/Au nanoparticles layer. Furthermore, the combination of semitransparent Au nanoparticles and DBR multilayer has similar central wavelength of the stop-band at 527 nm and transmittance of 18% to the central wavelength of stop-band at 538 nm and the transmittance of 19% of the DBR with ITO and without ITO. As a result, the experiment results of DBR/Au transmittance are in agreement with those of the simulation results. This should be attributed to higher transmission of 14 nm Au nanoparticle at 500 nm than that at 550 nm as shown in Fig. 1(a). However, the calculated and measured transmissions of thin Au films on glass and DBR are not completely consistent. It is obvious that the measured transmission is higher than the calculated one. Such certain of discrepancy might originate from the film quality on different substrates, leading to different refractive index. Au nanoparticles were grown by thermal deposition on glass substrate with/without DBR coated at a rate of ~0.1 nm s−1. A single layer of Au nanoparticles can remain the same or higher transparency of the substrate as reported .
Figure 2(a) and 2(b) show the AFM images of glass substrate and DBR substrate covered with Au nanoparticles layer, respectively. The density of the Au nanoparticles was found to be one particle per 100-200 nm2 . A smooth surface of the metal film has plasmon effect, however the enhancement effect is very weak. If the rough surface of metal film occurs, localized surface plasmon resonance effect will increase. In our case, the Au nanoparticles would be more likely to be formed on DBR substrate, partially due to the rougher surface of DBR film than that of the glass substrate. From AFM images, 14 nm Au nanoparticles covered DBR has a slightly larger root mean square (RMS) roughness (~2.68 nm) than 14 nm Au nanoparticles only on glass (~1.63 nm). The distribution of Au nanoparticles is uniform, and the gap between nanoparticles is rather small, hence it benefits to generating the SP resonance. In this case, the enhancement of the transmission can be attributed to the SPs caused by the Au nanoparticles. Figure 1(b) shows the absorption spectra of bare ITO and 14 nm Au nanoparticles on ITO. Obviously, the Au nanoparticles have a strong absorption from 450 nm to 650 nm due to SPs, which overlaps with the DBR resonance.
The device structures based on these substrates are shown in Fig. 3 and their corresponding configurations are as follows:
- A: DBR/Au (14 nm)/V2O5 (5 nm)/NPB (60 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (70 nm)
- B: DBR/ITO (120 nm)/V2O5 (5 nm)/NPB (60 nm)/Alq3 (50 nm)/LiF (1 nm)/ Al (70 nm)
- C: Au (14 nm)/V2O5 (5 nm)/NPB (60 nm)/Alq3 (50 nm)/LiF (1 nm)/ Al (70 nm)
- D: ITO (120 nm)/V2O5 (5 nm)/NPB (60 nm)/Alq3 (50 nm)/LiF (1 nm)/ Al (70 nm).
The current density-voltage (J-V) and luminance-voltage (L-V) characteristics of four OLEDs are shown in Fig. 4(a). Figure 4(b) compares the current efficiency-current density of all devices. The maximum current efficiency of device B (DBR/ITO: 4.0 cd/A) is 1.43 times as high as that of control device D (bare ITO: 2.8 cd/A). Similarly, that of device A is 1.46 times as high as that of control device D. The current efficiency (4.12 cd/A) of only semitransparent Au nanoparticles based device C is 1.46 times as high as that of control device D (2.8 cd/A) and also is slightly higher than that of device B as shown in Fig. 4(b). Although Au film as anode forms a rather weak optical microcavity effect, it has a great enhancement in the EL intensity. As a result, all improvement should be attributed to the microcavity effect through the redistribution of the photon density of state, the modification of spontaneous emissive characteristics in the emitter (i.e. the Purcell effect), and the resonance formation in the cavity. The resonant emission enhancement factor, Gcav, relative to free-space emission along the optical axis of a microcavity (in the forward direction) is defined as27] and Lu et al.  calculated a cavity-enhancement factor for microcavity OLEDs with two metal mirrors relative to a conventional OLED with ITO anode. Based on their calculations, device B has R1 = 0.9 (Al cathode), R2 = 0.80, and T2 = 0.16. While, device C (Au thin film) has R1 = 0.9 (Al cathode), R2 = 0.25, and T2 = 0.63. The radiative lifetime of Alq3 in the microcavity, τcav/τ = 0.9 for Alq3 . Finally, the calculated Ge is 1.36 for device B (DBR/ITO), consistent with our experimental efficiency enhancement factor of 1.40. Moreover, Lin et al calculated a cavity-enhancement factor for microcavity OLEDs with two metal mirrors relative to a conventional OLED with ITO anode . Based on their calculation, Au devices with R1 = 0.9 (Al cathode), R2 = 0.25 and T2 = 0.63 (experiment results) (Au anode) have an enhancement factor of 1.41. Our simulation result is slightly higher than that reported . It could be explained as: on one hand, this should be due to the higher transmission of our Au nanoparticles layer than that reported; on the other hand, it should be due to the SP metal nanoparticles excited efficiency enhancement. It can be observed that the peak absorption of Au nanoparticles (535 nm) and the peak fluorescence of Alq3 (525 nm) overlaps well, verifying the SP-enhanced effect, similar to the reported [19, 29, 30].
Moreover, we combine the Au nanoparticles based microcavity structure with the DBR mirror to further enhance the device efficiency. Therefore, the maximum current efficiency of 4.8 cd/A for device A (DBR/Au as anode) has been obtained, which is 1.7 times as high as that of control device D (ITO only: 2.8 cd/A).
The significant effect of SPs on the improvement of emission efficiency in our OLEDs can be attributed to the effective enhancement of OLED emissive sites, where a high density of excitons are localized within several nanometers away from the interface of hole/electron transport layers [12, 17], through coupling with localized SP in a single layer of Au nanoparticles.
The introduction of dielectric DBR reduces the spectral width of EL spectra significantly as shown in Fig. 5.The FWHM of green emissions in devices A and B at normal angle (FWHM: ~25 nm) is much narrower than those of noncavity devices (FWHM: ~104 nm). Obviously, the narrow emissive spectrum has a great advantage of improving the color purity for display. Device A has the maximum emission peak located at 522 nm (Fig. 5(a), at normal angle), which is with respect to the central wavelength of the stop-band at 527 nm.
In general, the cavity effect exhibits the angular dependence of the emission spectrum, which has also been observed in these two types of DBR microcavity devices. Figure 5(a) and 5(b) show the normalized EL spectra of microcavity devices A and B at various viewing angles, respectively. The peak emission shifts to shorter wavelength with the increase of the viewing angle, which can be explained by the change of the optical path inside the cavity with the increased angle . However, device A does not exhibit the change of the emission color with the viewing angle due to the compensation of blue-shifted color by the additional emission generated by resonant cavity mode at longer wavelength.
In conclusion, we have demonstrated significant efficiency enhancement and high color-purity OLEDs by combining DBR microcavity structure with SP-enhanced spontaneous emission. The current efficiency of DBR/Au anode based device increases by 70% due to strong microcavity effect in the whole device and also the coupling of SP effect by Au nanoparticles. Additionally, the emission spectrum has saturated color with better purity, which is attributed to efficient modification of the spontaneous emission through microcavity effect. Our work could be potential to further efficiency enhancement of OLEDs via light coupling by introducing various metal nanostructures and unique design of substrate structure.
This research work was financially supported by a grant from the Nanjing University of Telecommunication and Posts (NY212010), the National Natural Science Foundation of China (21161160442, 91233117, 51333007), and Natural Science Fund of in Jiangsu Province (BK2012834).
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