Surface plasmon-enhanced quantum dot lightemitting diodes by incorporating gold nanoparticles Jiangyong Pan,1 Jing Chen,1,4 Dewei Zhao,2 Qianqian Huang,1,3 Qasim Khan,1 Xiang Liu,1 Zhi Tao,1,3 Zichen Zhang,3,5 and Wei Lei1,6 1 School of Electronic Science and Engineering, Southeast University, Nanjing, 210096, China Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA 3 State Key Laboratory of Precision Measurement Technology and Instruments, Collaborative Innovation Center for Micro/Nano Fabrication, Device and System; Department of Precision Instrument, Tsinghua University, Beijing 100084, China 4 [email protected] 5 [email protected] 6 [email protected]
2
Abstract: Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED. ©2015 Optical Society of America OCIS codes: (230.5590) Quantum-well, -wire and -dot devices; (250.0250) Optoelectronics.
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1. Introduction In the past years, colloidal quantum dots (QDs) have received much attention due to their potential application as the new-generation light-emitting materials because of their superior size-dependent optoelectronic properties, such as narrow emission band width and tunable emission in the full visible spectral range, which offer significant advantages for QD based
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light-emitting devices (QLEDs) over liquid crystal displays or organic LEDs [1–3]. Since the first report on QLEDs in 1994, various approaches have been employed to improve device performance, such as application of new materials and employment of novel structures [4]. Despite great progress in the improvement of device performance, a few issues still exist such as lower electroluminescence (EL) efficiency compared with those of OLEDs due to the limitations of electrical properties and device structure. In order to further improve the luminance efficiency, more efforts have to be made to solve the issues of low quantum efficiency of QDs [5], unbalanced carrier injection [6], and poor outcoupling of efficiency due to the metal cathode [7]. Recently, the localized surface plasmon (SP) effect excited by metallic nanostructures becomes one of the most attractive approaches to boost the efficiency of OLEDs through the coupling between localized SP resonance and the excitons [8–11]. SPs are collective oscillations of free electrons at the interface between a metal and a dielectric material. The plasmonic coupling effect between excitons and SPs caused by the overlap between the local electromagnetic field of excitons in the emissive layer and SPs can lead to significant enhancement in radiative recombination rate through effective energy transfer in LEDs [12]. In an exciton–SP system, there are two competitive processes associated with the metal nanostructure; (i) radiative intensity enhancement due to the coupling of localized SPs with excitons and (ii) nonradiative losses due to metal nanostructure. On one hand, the efficiency of the coupling between localized SPs and excitons exponentially increases with decreasing distance between the metal nanostructure and the emissive layer [13]. On the other hand, nonradiative quenching of excitons at metal surface occurs when the distance between the metal surface and the emissive layer is very small [14]. Therefore, for practical application, an appropriate distance between the emissive material and metal surface should be optimized to obtain enhanced radiative intensity. Moreover, for the maximum coupling between exciton and SPs, the emission wavelength of excitons should be matched with the absorption wavelength of localized SPs, tunable by the size of nanostructures [15, 16]. Therefore, optimizing the size of nanostructures is an effective way to enhance the emission intensity . In this study, we demonstrate a SP-enhanced QLED consisting tunable sizes of Au nanoparticles (NPs). The size and distance between the Au NPs and the emissive layer have been optimized to reach the maximum EL enhancement. The remarkable enhancement in EL efficiency and stability can be achieved without negative effects on electrical properties due to the plasmonic near-field enhancement generated by the Au NPs. 2. Experimental section Yellow emitting ZnCdSeS QDs were synthesized according to a modified method reported previously. Here, 0.4 mmol of CdO, 4 mmol of zinc acetate, 4 mmol of oleic acid (OA), and 20 mL of 1-octadecne were mixed in a 100 mL round flask. The mixture was heated to 150 °C degassed under ~10 pa pressure for 30 min, filled with high-purity N2 flowing, and further heated to 300 °C to form a clear solution of Cd(OA)2 and Zn(OA)2. At this temperature, a stock solution containing 3 ml of trioctylphosphine, 0.4 mmol of Se, and 4 mmol of S was quickly injected into the reaction flask. After the injection, the reaction temperature was maintained for 10 min to promote the growth of QDs. The reaction was subsequently cooled down to room temperature to stop further growth. The QDs were washed with acetone three times, and finally dispersed in toluene with a concentration of 10 mg/ml. The ZnO NPs were synthesized by dropwise addition a solution of 0.5 M Tetramethylammonium Hydroxide (TMAH) in ethanol to 0.1 M zinc acetate in dimethyl sulphoxide (DMSO) and stirred for 1 hour in ambient condition. The prepared product was collected and then washed. The obtained transparent precipitate was dispersed in butanol with a concentration of 30 mg/ml. The Au NP layer was firstly deposited in a custom high-vacuum deposition chamber (background pressure, 6 × 10−4 torr) with a certain amount of Au (1mm Au silk of 0.5mm in diameter), then induced by the post-deposition annealing at 300°C for 1 hour to form Au NP layer.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A35
QLEDs were fabricated on ITO coated glass. The substrates were firstly cleaned with deionized water, acetone and iso-propanol, consecutively, for 15 min each, and then treated for 30 min with ozone generated by ultraviolet light. Poly (ethylenedioxythiophene): polystyrenesulphonate (PEDOT:PSS) solutions were filtered through a 0.22 mm filter and then spin-coated onto the ITO glass substrates for hole injection layer (HIL) at 5,000 r.p.m. for 30 s and baked at 150°C for 15 min under ambient conditions. Different sizes of Au NPs were formed by using different amount of Au on the PEDOT layer. The PEDOT: PSS-Au substrates were transferred into a nitrogen-filled glove box (O2 < 0.1 p.p.m., H2O < 0.1 p.p.m.) for spin-coating the sequential layers. The poly [(9,9-dioctylfluorenyl-2,7-diyl) -co(4,49- (N-(4-sec-butylphenyl)) diphenylamine)] (TFB) as hole transport layer (HTL) was spin-coated at 2000 r.p.m for 30 s, followed by baking at 150°C for 10 min. This was followed by spin-coating QDs (10 mg/ml in toluene) layer as emissive layer (EML) at 1100 r.p.m for 30 s followed by baking at 150°C. Next, the ZnO NPs (30 mg/ml in butanol) were spin coated as electron transport layer (ETL) at 4000 r.p.m for 30 s. Finally, the top Al cathode was deposited in a custom high-vacuum deposition chamber (background pressure, 6 × 10−4 torr) to form an active device area of 120 mm2. The control device without Au NPs was fabricated according to the method mentioned above for comparison. The shape and size information of the samples was analyzed using a scanning electron microscope (SEM). The current-voltage (I-V) characteristics were measured with a Keithley2400 source-meter unit. The absorption and photoluminescence spectra was measured using U-4100 UV-visible. The luminance of the devices was calibrated using a Minolta luminance meter (LS-100). 3. Results and discussion Figure 1(a) displays the schematic diagram of multilayer structure of ITO /PEDOT: PSS (40 nm) /Au NPs (single layer) /TFB (15 nm)/QD (20 nm)/ZnO NPs (30 nm)/Al. From scanning electron microscopic (SEM) images of Figs. 1(b) and 1(c) it can be seen that the relatively uniform surface morphologies of compactly packed ZnO NP and QD layers were fabricated by spin-coating. According to the schematic energy level diagram shown in Fig. 1(d), PEDOT:PSS was used as a hole injection layer and TFB as a hole transport layer to facilitate the hole injection and transport.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A36
Fig. 1. (a) Schematic illustration of all-solution-processed, multilayered QLED. Surface SEM images of uniformly, compactly packed (b) ETL of ZnO NPs and (c) EML of QDs.(d) Energy level diagram for the various layers.
It is noted that the TFB film used in the device not only acts as a hole transport layer but also as a dielectric spacer for adjusting the distance between the QDs and Au NPs to avoid the fluorescence quenching due to their extraordinarily high molar extinction coefficients and broad energy bandwidth of metals [17]. In addition, the ZnO nanoparticle layer not only provides efficient electron injection from the Al cathode into CdSe–ZnS QDs, but also facilitates to confine holes within the QD layer due to the valence band offset at the QD/ZnO nanoparticle interface, leading to improved charge recombination efficiency. Moreover, ZnO NPs was used as ETL due to its high electron mobility and insensitive to the air and moisture [18]. Figures 2(a)-2(c) show the SEM images of different sizes of Au NPs. It can be observed that the Au NPs are distributed uniformly on PEDOT:PSS film. The average size of the three kinds of Au NPs is 20 nm, 27 nm, and 77 nm respectively by statistical calculation. Figure 2(d) shows the PL spectra of QD film and the ultraviolet visible (UV-VIS) absorption spectra of Au NPs film. The maximum PL peak intensity of QD film is located at 567 nm with a narrow FWHM of 35 nm. The absorption peak undergoes a red shift from 529 to 604 nm with the increase of the particle size of Au NPs.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A37
Fig. 2. SEM images of (a) 20 nm Au NPs, (b) 27 nm Au NPs and (c) 77 nm Au NPs. (d) Normalized PL intensity of a QD film and absorption spectra of different size of Au NPs
For efficiently exciton-localized SP coupling, the SP absorption wavelength of the metal NPs should be matched with the PL emission wavelength of the QD film [19]. In this case, the 27 nm sized Au NPs is expected to lead to the best performance of QLED because the peak wavelength of its absorption matches well with the PL emission wavelength of the QD film and spectrum overlap is maximized between the absorption spectra of Au NPs and the PL emission spectra of the QD film among these three sizes of Au NPs. The large overlap of two spectra indicates the possible resonance between exticon generated in QDs and localized SPs excited by Au NPs, resulting in an effective energy transfer and therefore enhanced emission intensity [20]. Figure 3(a) depicts the PL intensity of QD film with and without Au NPs. It can be seen that the PL intensity of QD film with 27 nm Au NPs is much higher than those of the other QD films, achieving the enhancement ratio of 4.4 compared to the QD film without Au NPs.
Fig. 3. (a) PL intensity of QD films with and without Au NPs excited under 400 nm (The structure is ITO/PEDOT:PSS/Au NPs/TFB/QD), (b) Time-resolved fluorescence spectra of QD films with and without Au NPs at 10 K
The result indicates that the excitons of QDs are effectively excited at the frequency of surface plasmon resonance in Au NPs films. The coupling enhancement is more pronounced when the absorption peak of Au NPs is much closer to the PL emission peak of the QD film. However, the PL intensity of QD film with 77 nm Au NPs is decreased compared to that
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A38
without Au NPs. This may is ascribed to the occurrence of PL quenching dominant process when the coupling between exciton and SP is not strong enough due to the mismatch of SP absorption wavelength of the metal NPs with the PL emission wavelength of the QD film. In order to exclude the effect of slight difference of QD layer thickness among devices, the QD PL intensities of samples for different Au particle sizes were the average value over dozens of devices. In addition, as for controlling the thickness of QD layer, we use the same experiment parameter, such as the spin-coating speed, annealing temperature and time to guarantee the same thickness of the QD layer in all the devices. At the same time the layer thicknesses were monitored in situ using an oscillating quartz thickness monitor and the error bar of the layer thickness is ± 1 nm. Moreover, non-radiative energy transfer processes can be size-dependent, leading to the different enhancement of non-radiative rate, thereby changing the shape of the emssion spectral. In order to verify the origin of PL intensity enhancement, we performed the time-resolved PL (TR-PL) measurement of QD film with and without Au NPs shown as Fig. 3(b). It has been reported that the coupling process between SPs and excitons is much faster than spontaneous recombination of excitons, and the exciton lifetime in SP-enhanced devices should be reduced [21]. It is noticed that the decay time of QD without Au NPs is 15.50 ns, but it will decrease to 15.30, 9.54, and 12.20 ns after incorporation of 77, 27, and 20 nm Au NPs, respectively. The faster decay time for QLED with Au NPs is attributed to the coupling of exciton emission with localized SP in Au NPs. The fastest decay time of QD layer with 27 nm Au NPs indicates the strongest resonance coupling between QD excitons and localized SPs in the Au NPs in this case.When the energy of excitons in QD is closer to the electron vibrational energy of the localized SP in Au NPs, the energy of excitons can transfer to the localized SP and a new recombination path can be created [22]. A Purcell enhancement factor ( Fp ) can be calculated to describe the increase fold in spontaneous emission rate as formula (1): ∗ ∗ Fp = τ PL / τ PL = k PL / k PL ,
(1)
∗ PL
where k PL and k are the original and enhanced PL decay rate, respectively [23]. In this study, the value for Fp can be calculated as 1.62. As a result, internal quantum efficiency can be enhanced as the spontaneous recombination rate increases [24, 25]. Furthermore, the internal quantum efficiency (IQE) of the QD with and without excitonlocalized SP coupling can also be described approximately as the formula (2) and (3), respectively [20, 25]: IQEQD − Au = (krad + k sp ) / (krad + knon + ksp ),
(2)
IQEQD = krad / (krad + knon ),
(3)
Where krad and knon are the radiative and nonradiative recombination rates of QD, respectively, and ksp is the exciton-localized SP coupling rate. The exciton-localized SP coupling rate ksp is expected to be very fast compared to the radiative krad and nonradiative recombination rates knon [26]. It can be found that the value of IQEQD-Au is higher than that of IQEQD due to the existence of ksp by comparing the formula (2) and (3). Therefore, the spontaneous recombination rate can be increased by this new recombination path of the exciton-localized SP coupling, thereby significantly improving the IQE value after incorporation of Au NPs. In addition, as for the quantum yield typically falling down because of PL quenching, it is reported that the quenching process by the resonant energy transfer is a very short-range effect and can be reduced as the distance increases [27]. In order to solve the problem of PL quenching, we introduce the TFB film used in our device which not only acts as a hole
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A39
transport layer but also as a dielectric spacer for adjusting the distance between the QDs and Au NPs for avoiding the fluorescence quenching as discussed later. On the other hand, the large enhancement of PL intensity of QDs with Au NPs shown in the Fig. 3(a) indicates that many Au NPs are located at a distance between QD and Au NPs to realize the efficient exciton-SP resonance coupling which can override the quenching process. However, it is noted that the original quantum yield of QD was pretty low. Thus, it can be concluded that the PL quenching in our manuscript can be suppressed to a large extent and the PL intensity enhancement can be attributed to the improvement of internal quantum efficiency (IQE) in QDs due to an increase in the spontaneous emission rate by resonance coupling between the excitons in QDs and localized SP in the Au NPs In order to study the effect of the localized SP resonance on the EL enhancement of QLEDs, the current density–voltage (J–V), EL spectra and efficiency-current density characteristics are displayed in Fig. 4. In Fig. 4(a), it can be found that the presence of the Au NPs in device has slight effect on the charge transport since the current injection is identical for all devices. In other words, the performance enhancment of QLEDs with Au NPs incorporated does not result from the improved carrier injection efficiency due to the Au NPs. Figure 4(b) shows the EL spectra of the QLEDs with and without Au NPs at 5 V bias. The EL intensity of the QLED with Au NPs is remarkably enhanced compared with that of the QLED without Au NPs. In addition, the QLED with 27 nm Au NPs has the largest enhancement ratio of 1.7 in EL intensity, which is similar to the condition in the PL intensity of QD film. The significant enhancement of EL intensity without any peak shift can be attributed to the efficient resonance coupling between the excitons in the QDs and localized SPs in the Au NPs by a resonance energy matching between them as shown in Fig. 2(d). Figures 4(c) and 4(d) show the current efficiency-current density and power efficiency-current density, respectively. Obviously, the efficiency of QLED with 27 nm Au NPs increases significantly compared with the control device without Au NPs. The current efficiency and power efficiency are greatly enhanced to 4.56 cd/A and 2.98 lm/W with the enhancement of 2.0 and 2.2, respectively. The enhancement of efficiency can be attributed to the improvement of internal quantum efficiency in QDs due to the resonance coupling between the excitons in QDs and localized SP in the Au NPs [24]. However, it has been also reported that the use of arrays of metallic nanoantennas that sustain collective plasmonic resonances can shape the angular pattern of the emission, beaming most of the light into a very narrow angular range in a defined direction. It is also demonstrated the large impact that periodicity in the particle array exerts on the directional enhancement of the emission [28]. Therefore, it is a novel idea that modification of emission angular can be used to enhance the plasmonic LEDs. In our future work, some periodic Au NPs structure can be fabricated to verify the effect of surface plasmonic on the performance of device by modification of emission angular.
#248990 © 2016 OSA
Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A40
Fig. 4. (a) Current density–voltage (J–V), (b) EL spectra, (c) current efficiency-current density, and (d) power efficiency-current density of QLEDs without Au NPs and with different size of Au NPs.
However, the QLEDs with 77 and 20 nm Au NPs exhibit different trends. The efficiency of device with 20 nm Au NPs increases slightly compared with the control device because the absorption wavelength of Au NPs does not match well with the PL emission of the QD film and the coupling between the localized SP of Au NPs and exciton emission of QDs is not strong. Moreover, it is interesting that the efficiency of QLED with 77 nm Au NPs decreases compared with the control device. This may be ascribed to the PL quenching overriding the exciton-SP resonance coupling via nonradiative energy transfer from exciton emission of QDs to localized SP when the emissive materials and metal NPs are very close to each other. The performance of SP-enhanced device shows a strong dependence on the distance of Au NPs from the emissive layer [27, 29]. In this study, the distance between the emissive layer and the Au NPs is controlled by the thickness of TFB. Figure 5 shows the dependence of the
Fig. 5. Dependence of current efficiency and power efficiency on TFB layer thickness for the QLED with 27 nm Au NPs.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A41
current efficiency and power efficiency on the thickness of TFB layer for QLED with 27 nm Au NPs. The current efficiency increases from 1.05 to 4.56 cd/A and the corresponding power efficiency from 0.68 to 2.99 lm/W when the thickness of TFB layer is increased from 5 to 15 nm. However, a thicker TFB layer leads to a fast efficiency roll-off. This phenomenon can be explained as: although the coupling between the localized SP resonance of metal NPs and the exciton emission of QDs could leads to the EL enhancement, the penetration depth of the fringing field for localized SP resonance associated with metal NPs can be substantially tens of nm smaller than that for the SP resonance related to a planar metal surface [30]. This implies that metal NPs should be located in closer proximity to the active region in a device to obtain strong SP coupling. However, the emission quenching could be dominant at the metal surface by the energy transfer from exciton to localized SP when the emissive materials and metal NPs are too close as mentioned above. Therefore, it can be speculated that 15 nm TFB is an optimal distance between the Au NPs and the emissive layer to achieve the effective exciton-localized SP resonance coupling which can override the quenching loss, leading to the maximum enhancement of the device performance. In order to investigate the stability of SP enhanced QLED device, the luminance versus time for an unencapsulated QLED with and without Au NPs operated at a constant voltage of 5 V under ambient condition is shown in Fig. 6. Both devices have initially fast increase and then decrease in luminance, which is similar to the trend in the previous report [31].
Fig. 6. Luminance lifetime for the SP enhanced QLED with 27 nm Au NPs and the control device without Au NPs during operation in ambient condition at room temperature. Inset shows the electroluminescence picture of Au NPs based device under 2.5 V.
However, the maximum luminance of the device with Au NPs is much higher than that of the control device without Au NPs. Moreover, the luminance of the device with Au NPs degrades more slowly than that of the control device after reaching its maximum luminance. The enhancement ratio of lifetime (operating time corresponding to half of the initial luminance) for QLED device is calculated as 2.2. Therefore, the devices with Au NPs have higher performance and better stability due to the efficient resonance coupling between the excitons in the QDs and localized SPs in the Au NPs. 4. Conclusion
In summary, we demonstrated SP-enhanced QLEDs incorporating different size of Au NPs that is located away from the QD layer. The performance enhancement is ascribed to the increase of the spontaneous emission rate by strong localized SP resonance coupling of Au NPs with QDs, evidenced by the TR-PL results. The enhancement is maximized as the SP resonance wavelength for Au NPs matches exactly with the PL emission of the QD film and also the distance between them is optimal. The PL and EL intensity are enhanced by 4.4 and 1.7 fold respectively by incorporating Au NPs. Moreover, the enhancement ratio of 2.0 and
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A42
2.2 in the efficiency and stability of device respectively has been achieved, which is attributed to an improvement in the internal quantum efficiency of QD due to the coupling between the excitons in QD and the localized SP in Au NPs. These results indicate that noble metal nanostructures have great potential in the application of QLEDs. Acknowledgments
This work was supported partially by the National Key Basic Research Program 973(2013CB328804, 2013CB328803), the National High-Tech R&D Program 863 of China (2012AA03A302, 2013AA011004), National Natural Science Foundation Project (51120125001, 61271053, 61306140, 61405033, 91333118, 61372030, 61307077 and 51202028), Beijing Natural Science Foundation (4144076) and Natural Science Foundation Project of Jiangsu Province (BK20141390, BK20130629, and BK20130618).
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A43
2
Abstract: Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED. ©2015 Optical Society of America OCIS codes: (230.5590) Quantum-well, -wire and -dot devices; (250.0250) Optoelectronics.
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1. Introduction In the past years, colloidal quantum dots (QDs) have received much attention due to their potential application as the new-generation light-emitting materials because of their superior size-dependent optoelectronic properties, such as narrow emission band width and tunable emission in the full visible spectral range, which offer significant advantages for QD based
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light-emitting devices (QLEDs) over liquid crystal displays or organic LEDs [1–3]. Since the first report on QLEDs in 1994, various approaches have been employed to improve device performance, such as application of new materials and employment of novel structures [4]. Despite great progress in the improvement of device performance, a few issues still exist such as lower electroluminescence (EL) efficiency compared with those of OLEDs due to the limitations of electrical properties and device structure. In order to further improve the luminance efficiency, more efforts have to be made to solve the issues of low quantum efficiency of QDs [5], unbalanced carrier injection [6], and poor outcoupling of efficiency due to the metal cathode [7]. Recently, the localized surface plasmon (SP) effect excited by metallic nanostructures becomes one of the most attractive approaches to boost the efficiency of OLEDs through the coupling between localized SP resonance and the excitons [8–11]. SPs are collective oscillations of free electrons at the interface between a metal and a dielectric material. The plasmonic coupling effect between excitons and SPs caused by the overlap between the local electromagnetic field of excitons in the emissive layer and SPs can lead to significant enhancement in radiative recombination rate through effective energy transfer in LEDs [12]. In an exciton–SP system, there are two competitive processes associated with the metal nanostructure; (i) radiative intensity enhancement due to the coupling of localized SPs with excitons and (ii) nonradiative losses due to metal nanostructure. On one hand, the efficiency of the coupling between localized SPs and excitons exponentially increases with decreasing distance between the metal nanostructure and the emissive layer [13]. On the other hand, nonradiative quenching of excitons at metal surface occurs when the distance between the metal surface and the emissive layer is very small [14]. Therefore, for practical application, an appropriate distance between the emissive material and metal surface should be optimized to obtain enhanced radiative intensity. Moreover, for the maximum coupling between exciton and SPs, the emission wavelength of excitons should be matched with the absorption wavelength of localized SPs, tunable by the size of nanostructures [15, 16]. Therefore, optimizing the size of nanostructures is an effective way to enhance the emission intensity . In this study, we demonstrate a SP-enhanced QLED consisting tunable sizes of Au nanoparticles (NPs). The size and distance between the Au NPs and the emissive layer have been optimized to reach the maximum EL enhancement. The remarkable enhancement in EL efficiency and stability can be achieved without negative effects on electrical properties due to the plasmonic near-field enhancement generated by the Au NPs. 2. Experimental section Yellow emitting ZnCdSeS QDs were synthesized according to a modified method reported previously. Here, 0.4 mmol of CdO, 4 mmol of zinc acetate, 4 mmol of oleic acid (OA), and 20 mL of 1-octadecne were mixed in a 100 mL round flask. The mixture was heated to 150 °C degassed under ~10 pa pressure for 30 min, filled with high-purity N2 flowing, and further heated to 300 °C to form a clear solution of Cd(OA)2 and Zn(OA)2. At this temperature, a stock solution containing 3 ml of trioctylphosphine, 0.4 mmol of Se, and 4 mmol of S was quickly injected into the reaction flask. After the injection, the reaction temperature was maintained for 10 min to promote the growth of QDs. The reaction was subsequently cooled down to room temperature to stop further growth. The QDs were washed with acetone three times, and finally dispersed in toluene with a concentration of 10 mg/ml. The ZnO NPs were synthesized by dropwise addition a solution of 0.5 M Tetramethylammonium Hydroxide (TMAH) in ethanol to 0.1 M zinc acetate in dimethyl sulphoxide (DMSO) and stirred for 1 hour in ambient condition. The prepared product was collected and then washed. The obtained transparent precipitate was dispersed in butanol with a concentration of 30 mg/ml. The Au NP layer was firstly deposited in a custom high-vacuum deposition chamber (background pressure, 6 × 10−4 torr) with a certain amount of Au (1mm Au silk of 0.5mm in diameter), then induced by the post-deposition annealing at 300°C for 1 hour to form Au NP layer.
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QLEDs were fabricated on ITO coated glass. The substrates were firstly cleaned with deionized water, acetone and iso-propanol, consecutively, for 15 min each, and then treated for 30 min with ozone generated by ultraviolet light. Poly (ethylenedioxythiophene): polystyrenesulphonate (PEDOT:PSS) solutions were filtered through a 0.22 mm filter and then spin-coated onto the ITO glass substrates for hole injection layer (HIL) at 5,000 r.p.m. for 30 s and baked at 150°C for 15 min under ambient conditions. Different sizes of Au NPs were formed by using different amount of Au on the PEDOT layer. The PEDOT: PSS-Au substrates were transferred into a nitrogen-filled glove box (O2 < 0.1 p.p.m., H2O < 0.1 p.p.m.) for spin-coating the sequential layers. The poly [(9,9-dioctylfluorenyl-2,7-diyl) -co(4,49- (N-(4-sec-butylphenyl)) diphenylamine)] (TFB) as hole transport layer (HTL) was spin-coated at 2000 r.p.m for 30 s, followed by baking at 150°C for 10 min. This was followed by spin-coating QDs (10 mg/ml in toluene) layer as emissive layer (EML) at 1100 r.p.m for 30 s followed by baking at 150°C. Next, the ZnO NPs (30 mg/ml in butanol) were spin coated as electron transport layer (ETL) at 4000 r.p.m for 30 s. Finally, the top Al cathode was deposited in a custom high-vacuum deposition chamber (background pressure, 6 × 10−4 torr) to form an active device area of 120 mm2. The control device without Au NPs was fabricated according to the method mentioned above for comparison. The shape and size information of the samples was analyzed using a scanning electron microscope (SEM). The current-voltage (I-V) characteristics were measured with a Keithley2400 source-meter unit. The absorption and photoluminescence spectra was measured using U-4100 UV-visible. The luminance of the devices was calibrated using a Minolta luminance meter (LS-100). 3. Results and discussion Figure 1(a) displays the schematic diagram of multilayer structure of ITO /PEDOT: PSS (40 nm) /Au NPs (single layer) /TFB (15 nm)/QD (20 nm)/ZnO NPs (30 nm)/Al. From scanning electron microscopic (SEM) images of Figs. 1(b) and 1(c) it can be seen that the relatively uniform surface morphologies of compactly packed ZnO NP and QD layers were fabricated by spin-coating. According to the schematic energy level diagram shown in Fig. 1(d), PEDOT:PSS was used as a hole injection layer and TFB as a hole transport layer to facilitate the hole injection and transport.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A36
Fig. 1. (a) Schematic illustration of all-solution-processed, multilayered QLED. Surface SEM images of uniformly, compactly packed (b) ETL of ZnO NPs and (c) EML of QDs.(d) Energy level diagram for the various layers.
It is noted that the TFB film used in the device not only acts as a hole transport layer but also as a dielectric spacer for adjusting the distance between the QDs and Au NPs to avoid the fluorescence quenching due to their extraordinarily high molar extinction coefficients and broad energy bandwidth of metals [17]. In addition, the ZnO nanoparticle layer not only provides efficient electron injection from the Al cathode into CdSe–ZnS QDs, but also facilitates to confine holes within the QD layer due to the valence band offset at the QD/ZnO nanoparticle interface, leading to improved charge recombination efficiency. Moreover, ZnO NPs was used as ETL due to its high electron mobility and insensitive to the air and moisture [18]. Figures 2(a)-2(c) show the SEM images of different sizes of Au NPs. It can be observed that the Au NPs are distributed uniformly on PEDOT:PSS film. The average size of the three kinds of Au NPs is 20 nm, 27 nm, and 77 nm respectively by statistical calculation. Figure 2(d) shows the PL spectra of QD film and the ultraviolet visible (UV-VIS) absorption spectra of Au NPs film. The maximum PL peak intensity of QD film is located at 567 nm with a narrow FWHM of 35 nm. The absorption peak undergoes a red shift from 529 to 604 nm with the increase of the particle size of Au NPs.
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Fig. 2. SEM images of (a) 20 nm Au NPs, (b) 27 nm Au NPs and (c) 77 nm Au NPs. (d) Normalized PL intensity of a QD film and absorption spectra of different size of Au NPs
For efficiently exciton-localized SP coupling, the SP absorption wavelength of the metal NPs should be matched with the PL emission wavelength of the QD film [19]. In this case, the 27 nm sized Au NPs is expected to lead to the best performance of QLED because the peak wavelength of its absorption matches well with the PL emission wavelength of the QD film and spectrum overlap is maximized between the absorption spectra of Au NPs and the PL emission spectra of the QD film among these three sizes of Au NPs. The large overlap of two spectra indicates the possible resonance between exticon generated in QDs and localized SPs excited by Au NPs, resulting in an effective energy transfer and therefore enhanced emission intensity [20]. Figure 3(a) depicts the PL intensity of QD film with and without Au NPs. It can be seen that the PL intensity of QD film with 27 nm Au NPs is much higher than those of the other QD films, achieving the enhancement ratio of 4.4 compared to the QD film without Au NPs.
Fig. 3. (a) PL intensity of QD films with and without Au NPs excited under 400 nm (The structure is ITO/PEDOT:PSS/Au NPs/TFB/QD), (b) Time-resolved fluorescence spectra of QD films with and without Au NPs at 10 K
The result indicates that the excitons of QDs are effectively excited at the frequency of surface plasmon resonance in Au NPs films. The coupling enhancement is more pronounced when the absorption peak of Au NPs is much closer to the PL emission peak of the QD film. However, the PL intensity of QD film with 77 nm Au NPs is decreased compared to that
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without Au NPs. This may is ascribed to the occurrence of PL quenching dominant process when the coupling between exciton and SP is not strong enough due to the mismatch of SP absorption wavelength of the metal NPs with the PL emission wavelength of the QD film. In order to exclude the effect of slight difference of QD layer thickness among devices, the QD PL intensities of samples for different Au particle sizes were the average value over dozens of devices. In addition, as for controlling the thickness of QD layer, we use the same experiment parameter, such as the spin-coating speed, annealing temperature and time to guarantee the same thickness of the QD layer in all the devices. At the same time the layer thicknesses were monitored in situ using an oscillating quartz thickness monitor and the error bar of the layer thickness is ± 1 nm. Moreover, non-radiative energy transfer processes can be size-dependent, leading to the different enhancement of non-radiative rate, thereby changing the shape of the emssion spectral. In order to verify the origin of PL intensity enhancement, we performed the time-resolved PL (TR-PL) measurement of QD film with and without Au NPs shown as Fig. 3(b). It has been reported that the coupling process between SPs and excitons is much faster than spontaneous recombination of excitons, and the exciton lifetime in SP-enhanced devices should be reduced [21]. It is noticed that the decay time of QD without Au NPs is 15.50 ns, but it will decrease to 15.30, 9.54, and 12.20 ns after incorporation of 77, 27, and 20 nm Au NPs, respectively. The faster decay time for QLED with Au NPs is attributed to the coupling of exciton emission with localized SP in Au NPs. The fastest decay time of QD layer with 27 nm Au NPs indicates the strongest resonance coupling between QD excitons and localized SPs in the Au NPs in this case.When the energy of excitons in QD is closer to the electron vibrational energy of the localized SP in Au NPs, the energy of excitons can transfer to the localized SP and a new recombination path can be created [22]. A Purcell enhancement factor ( Fp ) can be calculated to describe the increase fold in spontaneous emission rate as formula (1): ∗ ∗ Fp = τ PL / τ PL = k PL / k PL ,
(1)
∗ PL
where k PL and k are the original and enhanced PL decay rate, respectively [23]. In this study, the value for Fp can be calculated as 1.62. As a result, internal quantum efficiency can be enhanced as the spontaneous recombination rate increases [24, 25]. Furthermore, the internal quantum efficiency (IQE) of the QD with and without excitonlocalized SP coupling can also be described approximately as the formula (2) and (3), respectively [20, 25]: IQEQD − Au = (krad + k sp ) / (krad + knon + ksp ),
(2)
IQEQD = krad / (krad + knon ),
(3)
Where krad and knon are the radiative and nonradiative recombination rates of QD, respectively, and ksp is the exciton-localized SP coupling rate. The exciton-localized SP coupling rate ksp is expected to be very fast compared to the radiative krad and nonradiative recombination rates knon [26]. It can be found that the value of IQEQD-Au is higher than that of IQEQD due to the existence of ksp by comparing the formula (2) and (3). Therefore, the spontaneous recombination rate can be increased by this new recombination path of the exciton-localized SP coupling, thereby significantly improving the IQE value after incorporation of Au NPs. In addition, as for the quantum yield typically falling down because of PL quenching, it is reported that the quenching process by the resonant energy transfer is a very short-range effect and can be reduced as the distance increases [27]. In order to solve the problem of PL quenching, we introduce the TFB film used in our device which not only acts as a hole
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A39
transport layer but also as a dielectric spacer for adjusting the distance between the QDs and Au NPs for avoiding the fluorescence quenching as discussed later. On the other hand, the large enhancement of PL intensity of QDs with Au NPs shown in the Fig. 3(a) indicates that many Au NPs are located at a distance between QD and Au NPs to realize the efficient exciton-SP resonance coupling which can override the quenching process. However, it is noted that the original quantum yield of QD was pretty low. Thus, it can be concluded that the PL quenching in our manuscript can be suppressed to a large extent and the PL intensity enhancement can be attributed to the improvement of internal quantum efficiency (IQE) in QDs due to an increase in the spontaneous emission rate by resonance coupling between the excitons in QDs and localized SP in the Au NPs In order to study the effect of the localized SP resonance on the EL enhancement of QLEDs, the current density–voltage (J–V), EL spectra and efficiency-current density characteristics are displayed in Fig. 4. In Fig. 4(a), it can be found that the presence of the Au NPs in device has slight effect on the charge transport since the current injection is identical for all devices. In other words, the performance enhancment of QLEDs with Au NPs incorporated does not result from the improved carrier injection efficiency due to the Au NPs. Figure 4(b) shows the EL spectra of the QLEDs with and without Au NPs at 5 V bias. The EL intensity of the QLED with Au NPs is remarkably enhanced compared with that of the QLED without Au NPs. In addition, the QLED with 27 nm Au NPs has the largest enhancement ratio of 1.7 in EL intensity, which is similar to the condition in the PL intensity of QD film. The significant enhancement of EL intensity without any peak shift can be attributed to the efficient resonance coupling between the excitons in the QDs and localized SPs in the Au NPs by a resonance energy matching between them as shown in Fig. 2(d). Figures 4(c) and 4(d) show the current efficiency-current density and power efficiency-current density, respectively. Obviously, the efficiency of QLED with 27 nm Au NPs increases significantly compared with the control device without Au NPs. The current efficiency and power efficiency are greatly enhanced to 4.56 cd/A and 2.98 lm/W with the enhancement of 2.0 and 2.2, respectively. The enhancement of efficiency can be attributed to the improvement of internal quantum efficiency in QDs due to the resonance coupling between the excitons in QDs and localized SP in the Au NPs [24]. However, it has been also reported that the use of arrays of metallic nanoantennas that sustain collective plasmonic resonances can shape the angular pattern of the emission, beaming most of the light into a very narrow angular range in a defined direction. It is also demonstrated the large impact that periodicity in the particle array exerts on the directional enhancement of the emission [28]. Therefore, it is a novel idea that modification of emission angular can be used to enhance the plasmonic LEDs. In our future work, some periodic Au NPs structure can be fabricated to verify the effect of surface plasmonic on the performance of device by modification of emission angular.
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Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A40
Fig. 4. (a) Current density–voltage (J–V), (b) EL spectra, (c) current efficiency-current density, and (d) power efficiency-current density of QLEDs without Au NPs and with different size of Au NPs.
However, the QLEDs with 77 and 20 nm Au NPs exhibit different trends. The efficiency of device with 20 nm Au NPs increases slightly compared with the control device because the absorption wavelength of Au NPs does not match well with the PL emission of the QD film and the coupling between the localized SP of Au NPs and exciton emission of QDs is not strong. Moreover, it is interesting that the efficiency of QLED with 77 nm Au NPs decreases compared with the control device. This may be ascribed to the PL quenching overriding the exciton-SP resonance coupling via nonradiative energy transfer from exciton emission of QDs to localized SP when the emissive materials and metal NPs are very close to each other. The performance of SP-enhanced device shows a strong dependence on the distance of Au NPs from the emissive layer [27, 29]. In this study, the distance between the emissive layer and the Au NPs is controlled by the thickness of TFB. Figure 5 shows the dependence of the
Fig. 5. Dependence of current efficiency and power efficiency on TFB layer thickness for the QLED with 27 nm Au NPs.
#248990 © 2016 OSA
Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A41
current efficiency and power efficiency on the thickness of TFB layer for QLED with 27 nm Au NPs. The current efficiency increases from 1.05 to 4.56 cd/A and the corresponding power efficiency from 0.68 to 2.99 lm/W when the thickness of TFB layer is increased from 5 to 15 nm. However, a thicker TFB layer leads to a fast efficiency roll-off. This phenomenon can be explained as: although the coupling between the localized SP resonance of metal NPs and the exciton emission of QDs could leads to the EL enhancement, the penetration depth of the fringing field for localized SP resonance associated with metal NPs can be substantially tens of nm smaller than that for the SP resonance related to a planar metal surface [30]. This implies that metal NPs should be located in closer proximity to the active region in a device to obtain strong SP coupling. However, the emission quenching could be dominant at the metal surface by the energy transfer from exciton to localized SP when the emissive materials and metal NPs are too close as mentioned above. Therefore, it can be speculated that 15 nm TFB is an optimal distance between the Au NPs and the emissive layer to achieve the effective exciton-localized SP resonance coupling which can override the quenching loss, leading to the maximum enhancement of the device performance. In order to investigate the stability of SP enhanced QLED device, the luminance versus time for an unencapsulated QLED with and without Au NPs operated at a constant voltage of 5 V under ambient condition is shown in Fig. 6. Both devices have initially fast increase and then decrease in luminance, which is similar to the trend in the previous report [31].
Fig. 6. Luminance lifetime for the SP enhanced QLED with 27 nm Au NPs and the control device without Au NPs during operation in ambient condition at room temperature. Inset shows the electroluminescence picture of Au NPs based device under 2.5 V.
However, the maximum luminance of the device with Au NPs is much higher than that of the control device without Au NPs. Moreover, the luminance of the device with Au NPs degrades more slowly than that of the control device after reaching its maximum luminance. The enhancement ratio of lifetime (operating time corresponding to half of the initial luminance) for QLED device is calculated as 2.2. Therefore, the devices with Au NPs have higher performance and better stability due to the efficient resonance coupling between the excitons in the QDs and localized SPs in the Au NPs. 4. Conclusion
In summary, we demonstrated SP-enhanced QLEDs incorporating different size of Au NPs that is located away from the QD layer. The performance enhancement is ascribed to the increase of the spontaneous emission rate by strong localized SP resonance coupling of Au NPs with QDs, evidenced by the TR-PL results. The enhancement is maximized as the SP resonance wavelength for Au NPs matches exactly with the PL emission of the QD film and also the distance between them is optimal. The PL and EL intensity are enhanced by 4.4 and 1.7 fold respectively by incorporating Au NPs. Moreover, the enhancement ratio of 2.0 and
#248990 © 2016 OSA
Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A42
2.2 in the efficiency and stability of device respectively has been achieved, which is attributed to an improvement in the internal quantum efficiency of QD due to the coupling between the excitons in QD and the localized SP in Au NPs. These results indicate that noble metal nanostructures have great potential in the application of QLEDs. Acknowledgments
This work was supported partially by the National Key Basic Research Program 973(2013CB328804, 2013CB328803), the National High-Tech R&D Program 863 of China (2012AA03A302, 2013AA011004), National Natural Science Foundation Project (51120125001, 61271053, 61306140, 61405033, 91333118, 61372030, 61307077 and 51202028), Beijing Natural Science Foundation (4144076) and Natural Science Foundation Project of Jiangsu Province (BK20141390, BK20130629, and BK20130618).
#248990 © 2016 OSA
Received 9 Sep 2015; revised 18 Nov 2015; accepted 18 Nov 2015; published 30 Nov 2015 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.000A33 | OPTICS EXPRESS A43
