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Abstract
We report a highly efficient polariton organic light-emitting diode (POLED) based on an intracavity pumping architecture, where an absorbing J-aggregate dye film is used to generate polariton modes and a red fluorescent OLED is used for radiative pumping of emission from the lower polariton (LP) branch. To realize the device with large-area uniformity and adjustable coupling strength, we develop a spin-coating method to achieve high-quality J-aggregate thin films with controlled thickness and absorption. From systematic studies of the devices with different J-aggregate film thicknesses and OLED injection layers, we show that the J-aggregate film and the pump OLED play separate roles in determining the coupling strength and electroluminescence efficiency, and can be simultaneously optimized under a cavity design with a good LP-OLED emission overlap for effective radiative pumping. By increasing the absorption with thick J-aggregate film and improving the electron injection of pump OLED with Li2CO3 interlayer, we demonstrate the POLED with a large Rabi splitting energy of 192 meV and a maximum external quantum efficiency of 1.2%, a record efficiency of POLEDs reported so far. This POLED architecture can be generally applied for exploration of various organic materials to realize novel polariton devices and electrically pumped lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the past two decades, the study of strong light-matter interactions in organic microcavities has been a topic of great interest [1]. Extensive efforts have been made to understand polariton physics with organic materials and a number of fascinating phenomena have been demonstrated in the strong coupling regime, such as stimulated scattering [2], high-temperature polariton condensation and superfluid transition [3–5], and optically pumped polariton lasing with low threshold [6,7]. Realization of ultrastrong coupling in recent years has led to further discovery of nonclassical phenomena and novel applications [8–10]. Despite these remarkable advances, the research on electrically driven organic polariton devices, mainly the polariton OLEDs (POLEDs), is still very limited. The POLED was first demonstrated in 2005 [11], and is typically constructed with a p-i-n cavity architecture, where the strongly-coupled material also acts as the emitter [12–15]. Upon electrical excitation of the strongly-coupled emitter, incoherent exciton states will be generated first, and subsequently transfer into coherent polariton states via two possible pathways: phonon scattering by emitting vibrational quanta [16], and radiative decay by emitting photons [17]. Due to a slow exciton-phonon scattering rate in organic materials, radiative decay has been shown as a more effective pathway to populate the bottom of lower polariton (LP) branch and yield efficient polariton emission [18]. Therefore, the ideal strongly-coupled emitter used for a p-i-n cavity POLED requires not only the capabilities of high electroluminescence (EL) efficiency and high absorption, but also a moderate Stokes shift to favor radiative pumping of the LP branch by emissive excitons. Design of the film thickness also has a mixed effect on the coupling strength, cavity detuning, and eventual EL performance, although use of a thick film is benificial to reach the ultrastrong coupling regime. These stringent conditions in material selection and cavity design have brought challenges to the development of highly efficient POLEDs. Thus far, the highest external quantum efficiency (EQE) reported for the p-i-n cavity POLEDs is 0.2% based on coumarin molecules [15].
On the other hand, the intracavity pumping architecture with separate mediums for generation of strong coupling and radiative pumping has been proposed in the research of optically pumped organic polariton emission and lasing [19,20]. Introducing a second emitter in the strongly-coupled cavity enables a more relaxed material combination, and can resonantly populate the LP mode without the need for direct excitation of the strongly-coupled medium. This cavity architecture also allows spatial isolation of the strongly-coupled medium from the excited emitter, and therefore can bypass the losses such as exciton-exciton annihilation and collapse of strong coupling under high current excitation, which are inevitable in the p-i-n cavity POLEDs. With these great advantages, the intracavity pumping architecture could be a promising scheme for developing highly efficient POLEDs and electrically pumped organic lasers.
In this paper, we study the POLED based on the intracavity pumping architecture by employing a cyanine dye J-aggregate film as the strongly-coupled medium and a fluorescent OLED as the pumping emitter. Cyanine dye J-aggregate is an extensively used strongly-coupled material due to its strong, narrow-band absorption, but normally has a low EL efficiency [12,21]. This POLED architecture can isolate the J-aggregate film and the pump OLED, and make full use of their respective absorption and emission capabilities. Through a proper cavity design and device optimization, we shall demonstrate the POLED with a large coupling strength and a high EQE.
2. Cavity design and fabrication of POLED
Figure 1(a) illustrates the architecture of the studied POLED, mainly containing an absorbing J-aggregate thin film and a red fluorescent OLED in a λ-thick cavity. A 6.5 bilayer SiO2/TiO2 distributed Bragg reflector (DBR) deposited on B270 glass was used as the emission side mirror, and the thick Ag cathode of the OLED served as the top mirror. The J-aggregate thin film was fabricated based on an anionic cyanine dye molecule, DEDOC [5-chloro-2-(2-[(5-chloro-3-(3-sulfopropyl)-2(3H)-benzoxazolylidene)methyl]-1-butenyl)-3-(3-sulfopropyl)-benzoxazolium inner salt, sodium salt], with a narrow absorption J-band at the wavelength of 546 nm [22]. The red OLED incorporating DCJTB [4-(dicyanomethylene-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran] as the dopant exhibits the peak emission wavelength of 625 nm. All the spectra of DBR reflectance, DEDOC J-aggregate absorption, and OLED emission are shown in Fig. 1(b). The cavity design of POLED was aimed to have a good overlap between the low energy state of LP mode and the OLED emission for effective radiative pumping (λ∼625 nm). Moreover, in order to maximize the exciton-photon coupling while minimizing the absorption of the OLED anode, we positioned the J-aggregate film and the emissive layer of the OLED at the two antinodes of the cavity field and the OLED anode (semitransparent Au film) at the node. For this purpose, we incoporated two ∼λ/4 spacer layers (SiO2 and PMMA) below and above the J-aggregate film, and fixed the total thickness of the OLED to be ∼λ/2, in which the thicknesses of p-i-n layers were adjusted to meet the optical design while maintaining a good EL property. In this work, we controlled the coupling strength of POLED by varying the J-aggregate film thickness and adjusted the PMMA spacer thickness accordingly to fulfill a good LP-OLED emission overlap, without changing the OLED thickness. Here, the SiO2/TiO2 DBR and SiO2 spacer were continuously deposited by electron beam evaporation. The J-aggregate film with a controlled thickness was deposited by a spin-coated method as detailed later. The PMMA spacer was spin-coated on the J-aggregate film from toluene solution, and annealed at 80°C for 10 min in a nitrogen glove box. The substrate was then transferred to a high vacuum thermal evaporator to complete the red fluorescent OLED, which was configured as Au (20 nm)/MoO3 (10 nm)/NPB (40 nm)/TPB3:2 wt% DCJTB (40 nm)/Alq3 (40 nm)/LiF or Li2CO3 (1 nm)/Ag (150 nm). All the chemicals were used as purchased without further purification. MoO3 and LiF(/Li2CO3) were used as the hole and electron injection layers from Au anode and Ag cathode, respectively. NPB [N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine)] and Alq3 [Tris(8-hydroxyquinoline)aluminum] were used as the hole and electron transporting layers, respectively. TPB3 [1,3,5-tris(1-pyrenyl)benzene] was selected as the host material, which has been demonstrated to undergo efficient energy transfer to DCJTB and yield a high efficiency in a standard OLED with ITO anode [23]. The chemical structures and the measured optical constants of all the organic active layers were shown in Supplement 1, Figs. S1 and S2. The OLED with Au anode reveals a broad reflection dip near the wavelength of 700 nm (see Supplement 1, Fig. S3), which indicates the presence of a weak cavity effect and possibly results in a slightly red-shifted EL spectrum compared to the standard OLED with ITO anode (peak emission at 616 nm). Nevertheless, the EL spectrum of the OLED with Au anode does not have a strong angular dependence in our measurement range. Figure 1(c) shows the optimized cavity design of the POLED superimposed with a simulated electric field at the emission wavelength of 625 nm, calculated by finite-difference time-domain method. The active area of the devices is 3×3 mm2.
Fig. 1. (a) Schematic diagram of the cavity structure of POLED, consisting of a 6.5 bilayer SiO2/TiO2 distributed Bragg reflector (DBR) as the bottom mirror, an absorbing J-aggregate film sandwiched between SiO2 and PMMA spacer layers, and a red fluorescence OLED with 20 nm Au anode and 150 nm Ag cathode. (b) The spectra of DBR reflectance, DEDOC J-aggregate absorption, and OLED emission. The reflectance spectrum of DBR was measured at angle of incidence of 5°. The cavity was designed to yield LP mode emission at the peak emission wavelength (625 nm) of OLED. (c) The optimized cavity design of POLED and the electric field distribution along the cavity calculated by finite-difference time-domain method at the wavelength of 625 nm. The indicated position is relative to the glass surface.
As a key component in the POLED, the quality of J-aggregate film will have a strong impact on not only the coupling strength, but also the device uniformity and reproducibility, so a careful optimization is required. Layer-by-layer assembly is a well-known deposition method of nanoscale J-aggregate thin films [24], but the long-time sequential adsorption process may introduce environmental contaminants onto the substrate, forming large-scale defects and high roughness that can be detrimental to the device. To address this issue, we developed a simple, rapid, and well-controlled process of the nanoscale J-aggregate films based on spin-coating from DEDOC and Polyvinyl alcohol (PVA, Mw∼205 kD) blend aqueous solution. The DEDOC and PVA solutions were first prepared in alkaline water (pH=12), and then mixed at a fixed weight ratio of DEDOC:PVA be 3:2. Compared to the DEDOC film without PVA, we found that adding a small proportion of PVA in the DEDOC J-aggregate solution can significantly improve the large-area uniformity of J-aggregates dispersion in a rapid film formation, but does not much compromise the absorbance. As the optical image shown in Fig. 2(a), the DEDOC/PVA blend J-aggregate film generally exhibits a high uniformity over a large area. The AFM image in Fig. 2(b) also reveals a smooth surface of the blend J-aggregate film with a low RMS roughness (Rq) of 2.09 nm. Figure 2(c) shows the absorption spectra of the blend J-aggregate films extracted from the reflectance (R) and transmittance (T) spectra by 1-R-T. A sharp J-band absorption peaking at 546 nm indicates the formation of DEDOC J-aggregates in the spin-coated film [22]. Systematic increase of the DEDOC/PVA concentration from 1.5/1 mg/mL to 4.5/3 mg/mL could enhance the peak absorption from ∼20% to ∼40%. To evaluate the optical constants and thicknesses of the blend J-aggregate films with different concentrations, we used an iterative algorithm which combines the Kramers-Kronig (KK) transformation and transfer-matrix method to simulate the measured R and T spectra of various films, with the iteratively modified (n,k) values as inputs and the thickness as a fitting parameter [25]. The (n,k) spectra were obtained from the best fitting of the R and T spectra, as the results shown in Supplement 1, Figs. S2(a) and S4. The peak k value of 3.25 at λ=546 nm corresesponds to a peak absorption coefficient α=4πk/λ=7.48×105 cm−1, only marginally lower than the spin-coated film without PVA (α∼8.0×105 cm−1 in Ref. [21]). Moreover, the thickness of the blend J-aggregate film was extracted to be increased from 2 nm to 8.7 nm for the peak absorption enhanced from 20% to 40% with different concentrations, which is consistent with the AFM measurement [inset of Fig. 2(c)]. Based on these characterizations, we prepared the sufficiently thick J-aggregate films (6 and 8.7 nm) with a high absorption of 35–40% for the studied POLEDs.
Fig. 2. (a) Optical image and (b) AFM image of the spin-coated DEDOC/PVA blend J-aggregate film, showing a large-area uniformity and high surface smoothness. (c) Absorption spectra of the blend J-aggregate film with different DEDOC/PVA concentrations from 1.5/1 mg/mL to 4.5/3 mg/mL. The inset shows the peak absorption versus film thickness extracted from the AFM measurement and simulation of R and T spectra.
3. Spectral and optoelectronic characterizations of POLED
In this work, all the angle-resolved reflectivity and EL spectra of the POLEDs and reference devices were acquired by using a calibrated one-snap multi-angle spectroscopy imaged onto a 2D CMOS camera in ambient atmosphere [26]. This spectroscopy system enables a simultaneous measurement of relative EL intensity over a spectral range of 450–640 nm and an angular range of 0–60°. The optoelectronic characterization of the devices was performed by using an Agilent B1500A semiconductor parameter analyzer connected with a Si photodiode (S1227-1010BQ, Hamamatsu) in a nitrogen-filled glove box. The EQE was extracted from the measured photocurrent, average emission energy estimated from the angle-resolved EL spectra, and photodiode’s sensitivity at the average emission energy.
To quantify the coupling strength in the designed POLEDs, we first measured the angle-resolved reflectivity spectra of the devices containing J-aggregate films of different thicknesses (0, 6 nm, and 8.7 nm), as the results shown in Fig. 3. For the reference device without J-aggregate film, only one resonant dip is observed to blue-shift from 2.05 eV at angle of incidence θ=0° to 2.36 eV at θ=60°, which evidently corresponds to the uncoupled cavity mode (EC) following the angular dispersion given by
Fig. 3. (a)–(c) Reflectivity map as a function of angle and energy of the cavities containing (a) 0, (b) 6 nm, and (c) 8.7 nm J-aggregate film. The dispersion curves plotted in the images were extracted using a two-mode coupled oscillator model. (d)–(f) The corresponding angle-resolved reflectivity spectra of (a)–(c) between 0°−60° by a step of 5°. The variation of resonant dips with angle is visually guided by the dashed lines. The cavity without J-aggregate film exhibits only an uncoupled cavity mode, while the cavities with 6 nm and 8.7 nm J-aggregate film show upper and lower polariton branches, demonstrating the occurrence of strong exciton-photon coupling.
Figures 4(a)–4(f) show the angle-resolved EL spectra of the weakly-coupled device without J-aggregate film (denoted as WOLED) and two POLEDs with 6 nm and 8.7 nm J-aggregate film, where the dispersion curves were extracted from the reflectivity measurements. It can be observed that the emission intensity of the WOLED and POLEDs mainly comes from the low energy states of the EC and LP modes at low angles, suggesting an effective radiative pumping with the red OLED. However, there are some differences in the spectral characteristics of the POLED and WOLED. First, the linewidth of LP mode emission is slightly larger than that of EC mode emission, which may arise from an inhomogeneous broadening of the absorption band of J-aggregate film [Fig. 2(c)]. Increase of J-aggregate film thickness leads to a more broadening of absorption band, and hence a larger linewidth of LP mode emission. Second, the LP mode emission has a wider angular distribution compared to the EC mode emission. This result is not only due to a flatter angular dispersion of the LP mode, but also related to a better spectral overlap between the LP mode and the red OLED emission. As the spectra shown in Figs. 4(d)–4(f), EC(0°) of the WOLED is slightly higher than the peak emission energy of the red OLED, forcing the EC mode emission to fall rapidly in 20° along the high energy tail of the OLED emission. By contrast, incorporation of thickest J-aggregate film (8.7 nm) in the POLED would increase the cavity length such that ELP(0°) is lowered to nearly match the peak emission energy of the red OLED, resulting in a more effective LP mode emission over wider angles. A bright, uniform EL image of the POLED with 8.7 nm J-aggregate film can be seen in the inset of Fig. 4(f). Note that for all the devices, the blue-shifted reflectance of DBR mirror with angle causes leakage of the OLED emission from the DBR reflection band edge at high angles (>50°), which is estimated to account for about 20% to 30% of the total emission.
Fig. 4. (a)–(c) Normalized EL intensity as a function of angle and energy of (a) weakly-coupled device (WOLED) without J-aggregate film and POLEDs containing (b) 6 nm and (c) 8.7 nm J-aggregate film, also superimposed with the reflectivity dispersion curves. (d)–(f) The corresponding angle-resolved EL spectra of (a)-(c) at low angles. The zero-degree spectrum of the red OLED is also shown for comparison. The inset in (f) shows the EL image of POLED with 8.7 nm J-aggregate film. (g,h) Optoelectronic characteristics of POLED with 8.7 nm J-aggregate film and WOLED using LiF and Li2CO3 as the electron injection interlayer. The red OLED prepared on glass substrate is also shown for comparison. The inset shows the extracted EQE for various devices.
On the other hand, from the optoelectronic characterization we found that regardless of the coupling strength, the device performance of POLED resembles that of WOLED, and is closely correlated with the pump OLED. When LiF is used as the electron injection interlayer [Fig. 4(g)], the reference OLED (red OLED prepared on glass substrate) shows a relatively high turn-on voltage of >5 V, and the maximum EQE is only 1.6%. It can be noticed that the POLED with thickest J-aggregate film (8.7 nm) and WOLED have rather similar device characteristics, which basically follow the trend of the reference OLED but the EL intensities are 2–3 times lower. Owing to the poor performance of the pump OLED, the maximum EQE of the POLED and WOLED is limited to 0.6% and 0.5%, respectively. In general, the EQE of POLED may be several times or even an order of magnitude lower than that of the reference OLED, depending on the degree of spectral overlap between the LP dispersion and the uncoupled emission states. For example, the typical p-i-n cavity POLED with J-aggregates as the strongly-coupled emitter shows a 6–7 times decrease of EQE compared with the J-aggregates OLED, which is mainly due to a partial overlap of the LP mode and the J-aggregate emission at high angles, resulting in an inefficient radiative pumping [12]. For the POLEDs studied here, the low energy state of the LP mode closely matches the peak emission of the pump OLED, so the EQE is only reduced by 2–3 times compared with the OLED. A little higher EQE of the POLED than WOLED could also be attributed to a better LP-OLED emission overlap condition.
To further optimize the device performance without changing the cavity design, we replaced the 1 nm LiF interlayer with another lithium salt, Li2CO3, so the varied optical length can be negligible. Li2CO3 has been used as an n-dopant in the electron transporting layer of OLEDs and shown to lower the electron injection barrier and enhance the electron-transporting/hole-blocking properties [28,29]. Compared to those with LiF interlayer, all the devices with Li2CO3 interlayer systematically exhibit a much lower turn-on voltage, a higher EL intensity at a fixed current density, and an approximately twofold higher EQE [Fig. 4(h)]. This result clearly suggests that Li2CO3 facilitates electron injection from Ag cathode more effectively than LiF, leading to better charge carrier balance and higher recombination efficiency. In this case, the maximum EQE of the POLED can reach 1.2%, which is, to the best of our knowledge, the highest value reported so far for POLEDs. By fine-tuning other parameters such as transparency of Au anode and thickness of charge transporting layers, there is still room to improve the device performance further. Nevertheless, the present study demonstrates a remarkably increased EQE with a simple modification of injection interface, and highlights the importance of optimizing the pump OLED to enhance the overall EL performance of this cavity architecture.
It is worth mentioning that not limited to vacuum-sublimed small molecules, solution-processed materials can also be exploited as the pumping emitter in this cavity if insoluble spacers are incorporated. Therefore, a wide varieties of emissive materials with exceptionally high efficiencies can be further explored. Moreover, due to a weak correlation between coupling strength and EL performance of this POLED architecture, various parameters of the absorbing film and the pump OLED can be modified with high flexibility to maximize the coupling strength and EQE simultaneously.
4. Conclusion
We demonstrate a high performance POLED based on combination of an absorbing J-aggregate thin film and a red fluorescent OLED in a properly designed intracavity pumping architecture. We develop a facile spin-coating method to fabricate uniform J-aggregate thin films with controlled thickness and absorbance, which is suitable for large-area device applications. By systematically varying the J-aggregate film thicknesses and OLED injection layers under a good LP-OLED emission overlap condition, we clarify that the coupling strength is exclusively determined by the J-aggregate film, while the EL performance is primarily correlated with the property of pump OLED but little depends on the coupling strength. When using a thick J-aggregate film and an effective Li2CO3 electron injection layer, we achieve the optimal POLED with a Rabi splitting energy of 192 meV and a maximum EQE of 1.2%. Overall, the studied POLED architecture can be easily designed and fabricated, and can be applied for combinations of a wide range of absorbing dyes and OLEDs, which paves the way to develop POLEDs with higher coupling strength and EL efficiency, and even to realize electrically pumped organic polariton lasers.
Funding
Ministry of Science and Technology, Taiwan (108-2112-M-008-008-).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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