This study reports fabrication of white-emissive, tandem-type, hybrid organic/polymer light-emitting diodes (O/PLED). The tandem devices are made by stacking a blue-emissive OLED on a yellow-emissive phenyl-substituted poly(para-phenylene vinylene) copolymer-based PLED and applying an organic oxide/Al/molybdenum oxide (MoO3) complex structure as a connecting structure or charge-generation layer (CGL). The organic oxide/Al/MoO3 CGL functions as an effective junction interface for the transport and injection of opposite charge carriers through the stacked configuration. The electroluminescence (EL) spectra of the tandem-type devices can be tuned by varying the intensity of the emission in each emissive component to yield the visible-range spectra from 400 to 750 nm, with Commission Internationale de l’Eclairage chromaticity coordinates of (0.33, 0.33) and a high color rendering capacity as used for illumination. The EL spectra also exhibit good color stability under various bias conditions. The tandem-type device of emission with chromaticity coordinates, (0.30, 0.31), has maximum brightness and luminous efficiency over 25,000 cd/m2 and ~4.2 cd/A, respectively.
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White-emissive organic/polymer light-emitting diodes (O/PLEDs) have received considerable interest recently, because of their potential applications in back-lighting or as solid-state-lighting sources [1–3]. As a decent solid-state-lighting source, the electroluminescence (EL) of O/PLEDs should include broad emissions that cover the entire visible range of the spectrum with Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of close to (0.33, 0.33) . The EL spectra and the luminous efficiencies must also remain stable when the devices are operated in the high voltage and brightness regime (>1,000 cd/m2).
Several researchers have reported the generation of the white-emissive EL from O/PLEDs by the partial energy transfer processes, such as by using host materials that are doped with fluorescence/phosphorescence dyes [1–3,5–12]. or polymer/polymer blends [13–16]. Many groups have also demonstrated the mixing of the EL from the host molecules with the excimer/exciplex emissions to yield white-emissive output EL [17–22]. Other approaches are to synthesize or apply a single polymer layer, comprising the blue-, green- and red-emissive functional molecules to cover the whole visible spectrum [23–25]. Other researchers have presented another strategy based on stacking differently emissive components to fabricate white-emissive O/PLEDs, which has achieved promising results [1,26–31]. The luminous efficiencies of white-emissive OLEDs are substantially enhanced when devices (or emissive components) are serially stacked or linked by various connecting structure or charge-generation layer (CGL) to form sophisticated and tandem-type device configurations. The preparation of the proper connecting structure in the stacked devices is a major challenge to the effective transport of opposite charge carriers in the junction interface and to the overall performance.
Here we demonstrate the application of a poly(ethylene glycol) dimethyl ether (PEGDE)/Al/molybdenum oxide (MoO3) complex structure [28,32] as a CGL to stack serially one OLED and another PLED of complementary EL emissions to fabricate the white-emissive, tandem-type and hybrid devices. The application of the CGL layer in the tandem-type configuration facilitates the transport and injection of opposite charge carriers at the junction interface and the recombination in each emissive component. The output EL spectra of the tandem-type devices represent complementary colored emissions from O/PLEDs, which can be modulated by varying the charge-transport or –generation properties of the CGL. The device performances and the output EL spectra of the stacked O/PLEDs are verified to change with the thickness of the Al and hole-transport layers in the CGL and the OLED component, respectively. Our approach illustrates the fabrication of the PLED and OLED of different functionalities in a tandem configuration. The PLED of the single light-emissive layer prepared by the spin-coating process simplifies the deposition of multifunctional layers in OLED. The PLED presented in this study also has a wide-range EL emission, which is suitable for the fabrication as the decent white-lighting source. The tandem-type device with the optimized CGL structure yield an output EL spectrum with CIE chromaticity coordinates of (0.33, 0.33). The EL spectrum covers the visible range of the human’s eyes from 400 to 750 nm, corresponding to a high color rendering index (CRI) of ~90 for illumination . The EL spectra also exhibit good color stability in the high-current and high-brightness regime.
The white-emissive devices presented in this study are composed of a “high-yellow” phenyl-substituted poly(para-phenylene vinylene) (HY-PPV) copolymer-based PLED  as the bottom, yellow-emissive component, an PEGDE/Al/MoO3 structure as the CGL and an OLED as the top, blue-emissive part. Figure 1 schematically depicts the configuration of the tandem-type device. The bottom part of the stacked device comprises the pre-cleaned and -patterned indium-tin-oxide (ITO)/glass substrate as the anode, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS Bayer Corp. 4083) (400 Å) as the hole-transport layer and HY-PPV film (900 Å) as the light-emissive layer. Both the PEDOT:PSS and the HY-PPV layers are cast on the ITO/glass substrate by the typical spin-coating process. The CGL layer was prepared by thermally depositing a thin polymer layer (45 Å) of PEGDE (Aldrich, Mn ca. 2,000) onto the surface of the HY-PPV film inside a vacuum chamber (10−6 torr), following the deposition of an Al metal layer (30~90 Å) and an MoO3 layer (40 Å) on the PEGDE layer without breaking the vacuum. The PEGDE/Al complex structure, enabling the effective injection of electrons through the Al electrode at a low bias voltage, and suppressing the formation of metal-induced EL quenching sites in the HY-PPV layer, has been reported in our earlier works [Guo et al., Appl. Phys. Lett. 87(1), 013504 (2005), Appl. Phys. Lett. 88(11), 113501 (2006), and Adv. Funct. Mater. 18(19), 3036-3042 (2008)] [35–37] to be an alternative cathode in the fabrication of high-performance PLEDs. The PLED of HY-PPV as the active layer with a PEGDE(45 Å)/Al(800 Å) cathode was fabricated on the PEDOT:PSS/ITO/glass substrate to measure the individual EL spectrum of the yellow-emissive component. The top, blue-emissive OLED was made of N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4-4′-diamine (NPB) (200~500 Å) as the hole-transport layer, a host organic conjugated molecule (BH, as provided by Chi Mei Optoelectronics Corp., Taiwan) (350 Å) that is doped with a blue-emissive, organic fluorescent molecule (BD501, also provided by Chi Mei Optoelectronics Corp., Taiwan) in a ratio of approximately ~10:1 as the light-emissive layer, tris-(8-hydroxyquinoline) aluminum (Alq3) (250 Å) as the electron-transport layer, and lithium fluoride (LiF) (5 Å)/Al (800 Å) as the device cathode. These layers are thermally deposited on the CGL in series at high vacuum (10−6 torr). The OLED of the same configuration without the CGL was also fabricated on ITO/glass substrate to measure the EL spectrum of the blue-emissive component. All of these steps, except for the casting of the PEDOT:PSS layer, were implemented inside a nitrogen-filled glove box.
Although rubrene(buffer layer)/PEGDE/Al is a good candidate as the decent cathode structure for the fabrication of high-performance Alq3-based OLEDs, reported in our previous publication [Appl. Phys. Lett. 89(5), 053507 (2006)] , the LiF/Al cathode is applied for the top OLED component of tandem devices in this study to simplify the fabricating processes because it is still the most commonly used cathode for OLEDs. This will help readers clearly discuss the influence of the electrical and optical properties of CGL on the performance of tandem cells. Applying the rubrene(buffer layer)/PEGDE/Al as the cathode for the top component of tandem device is our future work and currently in progress. The active pixel area of the device was 0.04 cm2. The current-brightness-voltage (I-L-V) measurements were made using a Keithley 2400 source measuring unit and a Keithley 2000 digital multimeter, with a silicon photodiode, calibrated using a PR-650 luminosity meter (Photo Research, USA). The I-L-V curves were also obtained inside the glove box. The EL spectra, the CIE chromaticity coordinates and the CRI values of the devices were detected using a PR-650 luminosity meter.
3. Results and discussion
As presented in Fig. 1, the white-emissive EL from the tandem-type device is produced by mixing the two complementary colors (yellow and blue emissions) from the stacked PLED and OLED, respectively. Figure 2(a) shows the EL spectra of both the yellow-emissive HY-PPV-based PLED and the blue-emissive OLED, individually fabricated on ITO/glass substrates. The EL of the HY-PPV-based PLED has a broad emission maximized at ~550 nm with a full-width-half-maximum (FWHM) of approximately 90 nm, which covers the spectral range of 500~750 nm (green-yellow-red) and corresponds to a CIE coordinate of (0.46, 0.53). The EL of the single blue-emissive OLED has a peak at ~450 nm and a shoulder at ~480 nm. The CIE coordinates of the blue EL spectrum are (0.15, 0.14) and the FWHM is around ~60 nm. Figure 2(b) displays a CIE chromaticity diagram that marks the CIE coordinates of the EL emissions from the yellow- and blue-emissive devices. A straight line that connects these pairs of two CIE coordinates passes through the white-emissive zone near the CIE coordinates (0.33, 0.33) in the diagram, indicating that the stack of the yellow- and blue-emissive devices fabricated herein can generate white-emissive EL.
The bottom and top components are linked by a CGL to make a tandem-type structure. Since the opposite charge carriers can recombine in both diodes, the output EL spectrum represents the mixing of the emissions from the yellow-emissive bottom component and blue-emissive top part, and can be tuned by varying the composition of the CGL.
Figure 3 presents the EL spectra of the tandem-type, hybrid devices with the Al layers of different thicknesses in the CGL. Table 1 summarizes the corresponding device performance. In Fig. 3, the EL spectrum of the stacked device with the CGL of PEGDE(45 Å)/Al(30 Å)/MoO3(40 Å) is dominated by the blue-emissive OLED, and corresponds to CIE chromaticity coordinates of (0.26, 0.25). However, the EL, with CIE coordinates of (0.38, 0.40), is dominated by the yellow-emission from the bottom PLED when PEGDE(45 Å)/Al(90 Å)/MoO3(40 Å) was used as the CGL. Apparently, the relative intensities of the yellow- and blue-emissions from the bottom and top components, respectively, of the stacked O/PLED, vary with the thickness of the Al layer from 30 to 90Å. In Table 1, the maximum brightness and the luminous efficiency of the devices decline as the thickness of the Al layer in CGL structure increases from 30 to 90 Å. The light turn-on voltages also shift from 5.5 to 6.5 V. The bluish tandem-type device has the higher EL intensity, luminous efficiency, and the lower device turn-on voltage. It is found that the thickness of the Al layer determines the overall output performance of the device. Two possible mechanisms may be responsible for the variations in the output EL spectra. i) The thick Al layer (90Å) interferes with and reduces blue emission from the top OLED component of the stacked device, because of its low optical transmittance. ii) Our earlier studies had shown that the injection of electrons through the PEGDE/Al cathode into the HY-PPV film is associated with the formation of the poly(ethylene oxide)/Al complex at the polymer/metal junction. The Al layer should have sufficient coverage on the PEGDE film to form the poly(ethylene oxide)/Al complex layer in the CGL structure and thus to support the effective injection or generation of electrons to the bottom yellow-emissive PLED. [37, 39] Accordingly, the output EL spectra depend primarily on the tradeoff of the optical transmittance and electron-injection capability in CGL complex structure. In this study, the stacked device with PEGDE(45 Å)/Al(60 Å)/MoO3(40 Å) as the CGL produces moderate emissions from both its yellow- and blue-emissive components, yielding a decent white-emissive EL spectrum, which covers the entire visible region from 420 to 750 nm, as presented in Fig. 3. The CIE coordinates of (0.30, 0.31) are close to (0.33, 0.33) for perfect white emission. The CRI value measured by PR-650 is around ~88, which is also a good light source as for illumination.
Figure 4 plots the I-L-V curves of the tandem-type, hybrid device with PEGDE(45 Å)/Al(60 Å)/MoO3(40 Å) as the CGL and 350 Å of NBP as the hole-transport layer. The light turn-on voltage of the EL is about 5.8 V and the EL intensity exceeds 25,000 cd/m2, upon biasing at 15.5 V. Figure 5 plots the luminous efficiency versus the current density of the tandem device. The maximum luminous efficiency of the tandem-type device with PEGDE(45 Å)/Al(60 Å)/MoO3(40 Å) as the CGL is approximately 4.2 cd/A, and remains stable in the high-current density and high-brightness regime. The maximum luminous efficiencies for tandem-type devices of 30 and 90 Å of Al in the CGL is about 5.8 (bluish) and 1.7 cd/A (yellowish), respectively. The luminous efficiency for the device of 30 Å of Al in the CGL is higher, probably due to the better optical transmittance for the blue emission at the top component than that of device with 90 Å of Al in the CGL.
The tandem-type, hybrid devices with different configurations of the CGL are also fabricated for comparison. Figure 6 plots the EL spectra of the stacked device with PEGDE(45 Å)/Al(60 Å), Al(60 Å)/MoO3(40 Å) and PEGDE(45 Å)/MoO3(40 Å) as the CGL. Table 2 summarizes the corresponding device performance. The output EL spectra of stacked devices with Al(60 Å)/MoO3(40 Å) and PEGDE(45 Å)/MoO3(40 Å) as the CGL are dominated by the emission from the blue-emissive OLED component. The partial emissions from the yellow-emissive, bottom PLED are low, but sill can be observed at EL spectra as shown in Fig. 6. Based on our earlier studies , to take away either the PEGDE or the Al layer in the CGL structure ceases the formation of the poly(ethylene oxide)/Al complex interface and declines the effective injection of electrons through the connecting junction into HY-PPV film. Accordingly, the recombination of the opposite charge carriers at the yellow-emissive, bottom component is not comparable to that in the blue-emissive, top part, which results in the bluish output EL spectra of the stacked device. As presented in Table 2, the CIE chromaticity coordinates of devices with Al(60 Å)/MoO3(40 Å) and PEGDE(45 Å)/MoO3(40 Å) as the CGL are (0.21, 0.27) and (0.23, 0.27), respectively. The device with PEGDE(45 Å)/MoO3(40 Å) as the CGL has the higher luminous efficiency and brightness (6.5 cd/A, 11226.5 cd/m2) than those of device applying the Al(60 Å)/MoO3(40 Å) (0.9 cd/A, 5385.3 cd/m2) CGL. The decreased luminous efficiency and EL intensity probably are attributed to the lower optical transmittance in the CGL with 60 Å thickness of Al layer. On the other hand, the EL spectrum, CIE chromaticity coordinates (0.50, 0.49), for the stacked device with PEGDE(45 Å)/Al(60 Å) as the CGL is primarily dominated by the yellow emission from the bottom PLED component. The removal of 40 Å thickness of MoO3 layer in the CGL structure inhibits the generation or injection of holes into the blue-emissive, top OLED component. As shown in Fig. 6, almost no blue emission from the top OLED part is observed in the output EL spectrum of the stacked device with PEGDE(45 Å)/Al(60 Å) CGL. The consumption of the electrical power in the connecting junction results in the low luminous efficiency and output EL intensity, which are 0.6 cd/A and 907.6 cd/m2, respectively, as summarized in Table 2.
Figure 7 presents the EL spectra of the stacked devices of PEGDE(45 Å)/Al(60 Å)/MoO3(40 Å) as the CGL, but with varying thickness of the NPB hole-transport layer in the blue-emissive top component. Table 3 summarizes the device performance. In Fig. 7, the ratios of blue/yellow emissions in the output EL spectra of tandem-type devices increases with the thickness of the NPB hole-transport layer from 200 to 550 Å. The blue shift in the EL spectra probably follows from the fact; i) that the hole-transport capacity of the device with a thicker NPB layer is higher, as is therefore the effectiveness of the recombination of opposite charge carriers in the blue-emissive top component. ii) The change of the thickness of NPB layer alters the optical length of the light emissive components in the tandem configuration. This observation gives us a hint to fine tune the output EL spectra and optimize the device performance by manipulate the optical length of tandem devices in future work (in progress). In Table 3, the output EL spectrum, with CIE coordinates of (0.27, 0.29), of the stacked device with 550 Å of NPB as the hole-transport layer is bluish. An ideal white-emissive output EL spectrum, with CIE coordinates of (0.33, 0.33), is obtained from the stacked device with 200 Å of NPB. The maximal brightness and luminous efficiency are 9202.4 cd/m2 and 2.8 cd/A, respectively. Only slight variations (decrease) in the normalized intensity at the yellow-emissive region in the EL spectra are observed when the device is biased at a varying voltage from 10 to 14 V. However, all the CIE coordinates are very close to (0.33, 0.33), which are (0.33, 0.33), (0.32, 0.31), and (0.31, 0.31) for the stacked device biased at 10, 12, and 14 V, respectively. Figure 7 also shows the EL spectrum of a regular or commercialized, white-emissive inorganic LED lamp, with CIE coordinates of (0.28, 0.28), for comparison. Clearly, the EL spectrum of the tandem-type, hybrid device in this study covers the visible spectrum more effectively than the inorganic LED lamp, especially in the yellow-to-red region, corresponding to a superior CRI value as for illuminating applications. The CRI value measured by PR-650 for a tandem-type device of applying PEGDE(45 Å)/Al(60 Å)/MoO3(40 Å) as a CGL and 200 Å of NPB is ~90, which compares to only ~60 for the white-emissive inorganic LED lamp.
Figure 8 presents a photograph of the white-emissive device, with CIE coordinates (0.33, 0.33), which was fabricated herein. The stabilities and uniformities of the emissive color and EL intensity across the light-emissive region are excellent, as revealed by a microscopy and also measured by a PR-650 luminosity meter. We noted that the photograph, as shown in Fig. 8, is a decent white-emissive device. The yellow-emissive light as observed in the edge of the device is due the wave-guiding effect of the yellow emission from the bottom PLED device.
In conclusions, the fabrication of white-emissive, tandem-type and hybrid devices by the serial stacking of two light-emitting diodes with complementary emissions and a PEGDE/Al/MoO3 complex structure as the CGL is demonstrated. The performance of the device and the output EL spectra of the stacked O/PLEDs are modulated by changing the thicknesses of the Al layer and the hole-transport layer in the CGL and the OLED component, respectively. We understood the adjustment of the thickness in Al layer to reach the optimized optical performance as applied for the white-light illumination probably would sacrifice the current efficiency. The application of different CGL with or without thin Al and another yellow PLED with higher performance would be the strategies to enhance the efficiencies of our current tandem devices. Detailed studies of the CGL to optimize the white-emissive EL and performance of devices used for illumination are currently conducted.
The authors would like to thank the National Science Council (NSC) of Taiwan NSC96-2113-M-006-009-MY3, the Asian Office of Aerospace Research and Development (AOARD-09-4055) and NCKU Landmark project for financially supporting this research. Dr. Ruei-Tang Chen from Eternal Chemical Co., Ltd is highly appreciated for providing the HY-PPV polymer.
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