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We report the investigation of plasmonic effect of array of aluminum nanoparticles (Al-NPs) on blue micro-OLED subject to exciplex emission. N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) andcarbazol derivative 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) have been used as the emitting layer (EML) and hole transport layer (HTL), respectively. For the reference µ-OLED without Al-NPs, we observed two emission peaks attributed to CBP emission and exciplex emission formed at the NPB/CBP (EML/HTL) interface. By the incorporation of the Al-NPs array, obtained by e-beam lithography technique on the ITO anode, the exciplex emission has been widely depressed. Moreover, thanks to localized surface plasmon resonance (LSPR), an enhancement of the CBP emission has been achieved indicating an efficient energy coupling between the LSPR of the Al-NPs and the CBP excitons. Thus, an enhancement of about 20% of the efficiency of the µ-OLED with Al-NPs in comparison to the reference device has been obtained.
© 2017 Optical Society of America
1. Introduction
The recent development of Organic Light-Emitting Diode (OLED) based lighting and display technology is actually driving more interest to efficient OLED heterostructures [1,2]. In this context, blue OLEDs are of a particular interest as they are needed in the development of full color display and white lighting. However, blue OLEDs have a very poor efficiency in comparison to green and red OLEDs [3]. In addition, the color of the emitted light is generally not stable due to the apparition of Green Emission Bands (GEB) allowing a broadened electroluminescence (EL) spectrum of the OLED [4]. These GEB are generally explained by the formation of the so-called exciplex [5, 6]. In fact, an exciplex is defined as an excited-state complex formed between two chemically dissimilar and neighboring molecules acting as the electron donor (excited state) and the electron acceptor (ground state), respectively. Its energy, determined by the difference between the LUMO (Lowest Unoccupied Molecular Orbital) of the donor and the HOMO (HeighstOccupeid Molecular Orbital) of the acceptor, is smaller than that of the exciton formed at each of the adjacent molecules. This allows a red shift and broadening of the OLED EL spectrum. The control of this effect can be of great interest for white lighting devices as it has been reported by several works [7–9]. However, the formation of the exciplex is very disturbing for applications that require high color purity like full color display. In this context, several works have been reported in order to eliminate the exciplex emission [10–12]. The main cause of the exciplex formation is the large difference between the frontier energy levels of the emitting layer (EML) acting as the donor and the hole transport layer (HTL) acting as the acceptor. This leads to the accumulation of charges on either side of the HTL/EML interface. The insertion of thin organic films with a high HOMO level between the donor and the acceptor is one of the most explored ways in order to decrease the charge accumulation and the probability of exciplex formation by favoring the hole injection inside the emitting layer [10–13]. However, this method allows the modification of the OLED heterostructure and requires a very high control of the band engineering of the organic materials in order to avoid a drop of the OLED efficiency [14]. Despite the progress achieved in this area, a pure efficient blue OLED is still needed.
Another exciting recent focus of organic photonics deals with the utilization of localized surface plasmon resonance (LSPR) ability to improve the OLED properties [15–20]. LSPR, generated by metallic nanoparticles (NPs), allows the modification of the photophysical properties of the narrowing activated molecules. It has been demonstrated that the exciton lifetime can be reduced when the LSPR wavelength matches the emission wavelength of the organic molecules. This leads to an enhancement of the OLED internal quantum efficiency. The LSPR wavelength can be controlled by varying the NPs parameters such as the shape, the size and the type of metal. The most reported results on the LSPR effect on the OLED properties have been focused on the study of green and red OLEDs [15–20]. The few existing reports on the investigation of the LSPR effect on blue OLEDs have been limited to the use of Random Metallic NPs (RMN) of silver and gold generally fabricated by thermal evaporation technique [21]. Thus, one cannot control the NPs parameters and the LSPR wavelength. Although, the study of Periodic Metallic NPs (PMN) in OLED is limited by the technological difficulties, they allow a precise control of the interaction between organic dye molecules and the metal. It must be noted that PMN structures can allow two different phenomena: LSPR and Surface Lattice Resonance (SLP) (or Plasmon Lattice Resonance (PLR)). The first one is connected to “small” periods of NPs arrays (in comparison to the resonance wavelength) and produces a strong electromagnetic field localized at the vicinity of the NP [22]. In the second case, the periodic nanoparticles are structured in such a way to get a hybrid surface plasmon and photonic modes. These SLR require greater periods similar to the resonance wavelength. They allow far-field photonic modes to interact with the plasmonic nanoparticles [23].
In this paper, we study the effect of Aluminum PMN structures on the properties of blue OLED subject to exciplex emission. As a first approach and in order to take advantage of only the LSPR effect of the NPs without considering the photonic modes of the PMN structure, we use arrays of Al-NPswith small periods. These NPs have been fabricated by electron beam lithography technique on an ITO coated glass substrate. As the NPs have been fabricated on micro-areas, we focused on the investigation of micro-OLED (µ-OLED) deposited on top of the Al-NPs and based on CBP (carbazol derivative 4,4′-bis(N-carbazolyl)-1,1′-biphenyl) as the emitting layer. In particular, the EL spectrum and the optical and electrical properties of the fabricated OLEDs have been studied and analyzed.
2. Numerical simulations
It is well known that the LSPR wavelength is determined by the NPs parameters like the shape, the size, the surrounding medium of the NPs and their chemical composition. Gold and silver are the most studied plasmonic materials in the visible spectrum. However, one has to note that, on the one hand, it is important to use large nanoparticles in order to maximize the ratio between scattering and absorption cross sections of metallic nanostructures. This is due to the fact that under quasi-static approximation, it can be obtained that the ratio between scattering and absorption of a spherical particle is proportional to a3, where a is the particle diameter [24]. On the other hand, increasing the size of the nanoparticle leads to a red shift of the LSPR wavelength. In addition to that, if we consider the influence of the refractive indices of the substrate and the organic material surrounding the metallic NPs, one can emphasizethat gold and silver are not the suitable material to obtain plasmonic resonances at short visible wavelengths [20]. Aluminum has been recently used as an alternative plasmonic material especially in the UV and blue regions where its plasmonic properties can be better than silver and gold [25, 26]. Therefore, Al has been selected to be used as the plasmonic material in this work.
In order to studythe LSPR effect of the Al-NPs and to optimize their parameters to match the LSPR with the emission wavelength of the blue OLED, we carried out numerical simulations based on the 3D FDTD (Finite-Difference Time-Domain) method using a home-made code [27]. The studied structure is shown in Fig. 1(a). The considered NPs are flattened elliptical nanocylinders labled nanorods in the rest of this manuscript. To take into account the experimental conditions, we have considered arrays of Al nanorods deposited on an ITO/Glass substrate and covered with an organic material. The refractive indices of the different materials are taken as 1.53, 2 and 1.7 for glass, ITO and the organic material, respectively. The dielectric function of Al are modeled using the Drude critical points model [27] by fitting the experimental data given by [28]. The structure is illuminated by a plane wave with a polarization along the long axis of the nanorods. We studied the effect of the aspect ratio (r = l/w) of the nanorods by varying the length of the nanorods (l) for a fixed height (h = 35 nm), width (w = 40 nm) and gap (g = 40 nm) between Al-NPs.
Fig. 1 (a) Schematic presentation of the simulated 3D structures, (b) Calculated extinction spectra of arrays of Al nanorods with different aspect ratios (r = l/w).
The calculated extinction spectra of the structures for different aspect ratios are shown in Fig. 1(b). One can see that the peak resonance is redshifted when the aspect ratio of the nanorods is increased. We can also see that, the extinction spectra of these NPs arrays have only one peak resonance corresponding to the LSPR mode generated by the nanorods. However, for the smallest aspect ratios, a secondary damped peak in the red part of the spectrum is observed. This damped peak is located very far from the working wavelength (the LSPR wavelength) and as we will see in the experimental section, it does not appear on the measured extinction spectrum. This peak might be due to a simulation artefact. Thus, we did not take it in consideration for our discussions.
Nevertheles, the photonic modes due to the periodic array do not appear, as the period (< 200 nm) is small in comparison to the wavelength. The nanorods with an aspect ratio of 2 have a peak resonance in the blue region (at about 460 nm) corresponding to the emission wavelength of blue OLEDs. The electromagnetic field distribution maps at the resonance wavelength of this NPs array in the xy and xz planes have been calculated andthey areshown in Figs. 2(a) and 2(b), respectively. As expected, the field is localized around the NPs confirming that the resonance is due to the surface plasmons generated by each NP. In addition, one can see that, due to the lighting rod effect, the field is maximum on the nanorods tips. It is worth noting that the map in the xy plane is carried out into the organic material at 10 nm away from the NPs. Despite that, we still have a strong electromagnetic field, which can be used to modify the optical properties of the organic molecules.
Fig. 2 Maps of the electromagnetic field distribution of the array of Al nanorods with an aspect ratio of 2 at the LSPR wavelength (460 nm) (a) in the XY plane (into the organic material 10 nm away from the NPs) and in (b) the XZ plane (median plane)
3. Experimental procedure
For the fabrication of our samples, 150 nm ITO coated glass substrate was used as a transparent anode. Thermal evaporation technique under a high vacuum was used to deposit the different organic layers consisting of: 4,4',4”,tris-(3-methylphenylphenylamino) triphenylamine (m-MTDATA) as a hole injection later (HIL) with a thickness of 25 nm, a 15 nm of N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as a hole transport layer (HTL), a carbazol derivative 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) with a thickness of 30 nm as the blue emitting layer (EML) with an EL spectrum centered at about 450 nm [29], a thin layer of Bathocupuroine (BCP) with a thickness of 10 nm as a hole blocking layer (HBL) and 40 nm of tris (8-hydroxyquinoline) aluminum (Alq3) as the electron transport layer (ETL). The organic layers were followed by 1 nm of Lithium Fluoride (LiF) in order to decrease the work function of the aluminum (Al) used as the OLED cathode (100 nm).
For the device including the Al-NPs, we have fabricated the NPs array on the ITO coated glass substrate and then we deposited on top of it the organic layers and the Al cathode as shown in Fig. 3(a). Based on the numerical simulations, the parameters of the nanorods array are fixed to 80 nm, 40 nm, 35 nm and 40 nm for the length, the width, the height and the interparticles gap, respectively, as shown by theSEM (Scanning Electron Microscope)image of Fig. 3(b). As the e-beam lithography technique is very time consuming, we concentrated on the fabrication and investigation of micro-OLEDs (100 x 200 µm2). In addition to that, µ-OLEDs can be used to obtain high current densities over 1 kA/cm2 [30], compared to OLED with standard dimensions; which might be of interest for organic laser diode quest.
Fig. 3 (a) Schematic structure of the fabricated µ-OLEDs, (b) SEM image of the fabricated Al nanorods (80 nm x 40 nm).
All the measurements were carried out at room temperature and under ambient conditions without any protective coatings. As the active area of the devices is very small, a homemade confocal microscope setup (see reference [20]) has been used to carry out the electrical-optical measurements. In fact, the light out-coupled from the µ-OLED is collected by usinga microscope objective (20x) and coupled to an optical fiber using a second objective of lower magnification (10x) and then is focused to a high speed avalanche detector to detect the optical power or to an optical spectrometer(OceanOptics HR 2000 + ) in order to get the OLED EL spectrum. The current-voltage-luminance curves were measured with Keithley 2635A Source Meter combined with a Power Meter Newport 2935C. The sample adjustment is done using a CCD camera.
4. Results and discussion
The obtained results are reported on Fig. 4. This latter shows the normalized EL spectra of µ-OLEDs with and without Al-NPs according to the area below the spectra in order to compare mutually the different EL peaks. We also display on the same figure the measured extinction spectrum of Al-NPs arrays recorded using a UV-Visible spectrometer.
Fig. 4 EL spectra of the µ-OLEDs with and without the Al NPs and the extinction spectrum of the Al NPs embedded into the OLED heterostructure.
Firstly, let us concentrate on the EL spectrum of the µ-OLED without Al-NPs. We obtained a broad spectrum covering the region from 400 nm to 600 nm with mainly two emission peaks centered at 442 nm and 490 nm. In fact, the second peak (490 nm) is large and ranges from 470 nm to 510 nm with a midlle of this interval situated at almost 490 nm. The first peak at 442 nm is attributed to the EL of CBP as reported by [29]. The second peak is the GEB generally appearing in the EL spectrum of blue OLEDs. In our case, this GEB can be attributed to the emission of exciplex formed at the interface between the CBP (EML) and the NPB (HTL) layers. The formation of exciplex needs that a sufficient number of charge carriers reaches the interface and accumulate at the interfacial region due to large differences between the frontiers energy levels of the two surrounding layers.
As shown on the energy level diagram presented in Fig. 5, we have a large energy barrier between the NPB and CBP molecules. This allows the accumulation of the negative charges on the CBP layer and the positive charges on the NPB layer at the NPB/CBP interface. The Coulomb interaction between these charges forms an excited state complex with emission energy lower than the exciton energy formed on the same molecule.
The exciplex emission energy can be directly related to the gap between the LUMO level of the donor (CBP) and the HOMO level of the acceptor (NPB). As reported by Peter et al. [31] the HOMO energy of molecular thin films can be determined with reasonable accuracy using ultraviolet photoelectron spectroscopy. The value of HOMO energy of NPB determined by this method is equal to 5.3 eV [31]. However, the LUMO level is more difficult to determine due the exciton bending energy [32]. Peter et al. [31] showed that the LUMO level of a variety of organic materials can be satisfactory determined by the electrochemical reduction potential in comparison to other methods. The measured LUMO level of CBP using this method was found equal to 2.75 eV. Thus, one can estimate the exciplex emission energy between the NPB and the CBP by:
This energy corresponds to an emission wavelength λexciplex (NPB/CBP) = 486 nm. This value is in a good agreement with the second peak emission (490 nm) obtained in the EL spectrum of the reference µ-OLED.
To further confirm that the emission of exciplex formed between CBP and NPB is at the origin of the red shifted peak obtained in the EL spectrum of the reference µ-OLED (Fig. 4), we have fabricated two OLED devices: in the first one, we used CBP as an emitting layer without introducing NPB and in the second device, CBP has been substituted by NPB in order to determine their EL spectra, separately. The detailed structures of these devices are as follows:
- Device (A): ITO/ m-MTDATA (40 nm)/ CBP (30 nm)/ BCP (10 nm)/ Alq3 (40 nm)/ LiF (1 nm)/ Al (100 nm)
- Device (B): ITO/ m-MTDATA (40 nm)/ NPB (30 nm)/ BCP (10 nm)/ Alq3 (40 nm)/ LiF (1 nm)/ Al (100 nm)
The measured EL spectra of the devices A and B are shown in Fig. 6. For comparison, we show also the exciplex emission spectrum separated from the global EL spectrum of the reference µ-OLED by subtracting the CBP EL. We can see that the EL spectra of both devices A and B are narrow with almost one peak emission centered at 442 nm corresponding to the EL of CBP and NPB.
Fig. 6 EL of an OLED based, in a first time, on CBP (no NPB) and, in a second time, on NPB (no CBP). The exciplex emission spectrum deduced from the EL of the reference µ-OLED is also plotted.
These results confirm that the peak emission observed in the EL spectrum of the studied µ-OLED is effectively due to the formation of exciplex between NPB and CBP molecules.
It is worth noting that the OLEDs fabricated by using NPB only or CBP only present a poor efficiency due to the charge imbalance into the EML caused by the difference between the holes and electrons mobilities and the high energy barriers at the different interfaces. The use of NPB as the HTL and CBP as the EML results in balanced charges leading to more efficient OLED in comparison to NPB and CBP only based-devices.
Let us now analyze the EL spectrum of the µ-OLED including the Al-NPs (red curve in Fig. 4). In this case, we obtained a narrower EL spectrum with mostly one emission peak centered at 442 nm, which corresponds to the CBP EL. These results indicate that the presence of the Al-NPs has two effects on the El spectrum of the blue µ-OLED: on one hand, the EL intensity of the µ-OLED including Al-NPs is enhanced in comparison to that of the reference device, and on the other hand, the exciplex emission peak observed in the EL spectrum of the reference µ-OLED is drastically reduced. As it can be seen from Fig. 4, the measured extinction spectrum of Al-NPs (blue curve) matches, very well, the EL spectrum of the µ-OLED with Al-NPs. It is well known that in the presence of plasmonic NPs at an optimal distance from an emitter, the lifetime of the excited state can be reduced by the Purcell effect [33] if the LSPR wavelength of the NP matches the emission wavelength of the emitter. In our case, the excitons emission wavelength of CBP is in an excellent agreement with the LSPR wavelength. In addition, if we take into account that during the evaporation of the organic materials, it is not deposited homogeneously on the nanorod surface i.e, when we deposit the two first layers (m-MTDATA and NPB) the center of the nanorods may be more covered then the tips. Consequently, the tips of the nanorods, where the field is maximum, are close enough to the emitting layer. Thus, the obtained CBP-EL enhancement can be attributed to the LSPR effect on the lifetime of the excitons generated into the CBP layer as it will be also discussed in the next section related to the I-V-L characteristics.
However, the presence of the metallic NPs at the vicinity of an emitter can also absorb the emitted light due to the quenching effect. The spectral overlap between the LSPR and the emission spectrum of the fluorophore plays a crucial role in light quenching process [34, 35]. This phenomenon known as nonradiative energy transfer to metallic NPs has been studied theoretically and experimentally in several works, such as that reported by Zhanget al. [36]. They studied the spectral and distance dependence of the fluorescence quenching of quantum dots emitting at different wavelengths by Au NPs with a LSP resonance centered at 534 nm. At a fixed separation distance between the NPs and the emitter, they showed that the quenching of fluorescence increases when moving away from on-resonance to off-resonance with respect to the LSPR peak. For a distance NP-emitter of 10 nm, the quenching is near to 0% when the emission wavelength matches the LSPR wavelength and increases to 50% when the emission wavelength is red-shifted by 25 nm with respect to the LSPR wavelength. This phenomenon may be at the origin of the suppression of the exciplex emission peak in the case of the µ-OLED with Al-NPs. As shown in Fig. 6, the exciplex spectrum is red-shifted by approximately 50 nm from the CBP EL (from the LSPR wavelength as well). This mismatch between the LSPR and the exciplex emission favors the nonradiative energy transfer allowing the quenching of practically the entire emitted light originated from the exciplex while the radiative coupling is the dominant effect in the case of excitons in the CBP.
In order to further investigate the effect of the Al-NPs on the optical and electrical properties of the µ-OLEDs, we measured the current density-voltage and luminance-voltage characteristics of the µ-OLED with and without Al-NPs. The results are shown in Fig. 7. It can be seen that the J-V characteristic (Fig. 7(a)) of the µ-OLED with Al-NPs is almost identical to the reference device. The variation of J-V curves of the two samples shows that the Al-NPs do not affect the charge injection and transport mechanism into the OLED heterostructure. In contrast to that, the luminance (Fig. 7(b)) of the µ-OLED with Al-NPs is higher than the reference device. Particularlly, for an applied voltage ranging from 14 V to 16 V, the luminance is almost two fold higherthan that of the OLED without Al-NPs. These behaviors of the J-V and L-V characteristics lead to an enhancement of the efficiency of the μ-OLED with Al-NPs by an average rate of 20% as shown in the inset of FEig. 7(b). These results emphasize that the presence of Al-NPs affect only the optical properties of the µ-OLED.This confirms that the obtained emission enhancement is a consequence of the coupling between electromagnetic field generated by the Al-NPs and the excitons of the CBP molecules which may reduce the excitons lifetime via the Purcell effect.
Fig. 7 (a) Current density and (b) luminance versus voltage of the µ-OLED with and without Al-NPs. Inset of (b) luminous efficiency versus current density for the µ-OLEDs with and without Al-NPs.
5. Conclusion
In conclusion, we have investigated the plasmonic effect of Al-NPs array on the properties of a blue µ-OLED subject to exciplex emission. The organic heterostructure studied was based on the utilization of NPB as the HTL and CBP as the EML that results in balanced charges leading to more efficient OLED in comparison to NPB or CBP only based-devices. The Al nanorods array has been fabricated by e-beam lithography technique on the ITO anode. The induced LSPR wavelength matches exactly the emission wavelength of CBP molecules. Two effects have been demonstrated: on one hand, an enhancement of the CBP excitonsemission thanks to the LSPR effect of the NPs, and on the other, the exciplex emission has been drastically reduced due to the presence of the Al-NPs. This is due to the quenching effect by the Al-NPs caused by the mismatch between the LSPR and exciplex emission wavelengths. In addition to that, we showed that the efficiency of the µ-OLED with Al-NPs was enhanced by about of 20% while keeping the electrical properties not affected. These results confirm that the obtained enhancement is due to the coupling between the LSPR and the excitons. It is suggested that the utilization of plasmonic nanostructures might decreases the lifetime of the excited state of emissive material.
Ultimately, the plasmonic NPs enhance the emission properties of blue OLEDs and suppress the unwanted exciplex emission to obtain a pure blue color, which can be of a great interest for many applications.
Acknowledgments
The Authors Knowledge Prof.AbderrahmaneBelkhir (Professor at the University of Tizi-Ouzou, Algeria) for his support in the numerical simulations performed by the FDTD method.
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