Reliable, all-phosphorescent stacked white organic light emitting

Nov 27, 2017 - High efficiency solid state lighting devices have the potential to significantly reduce lighting energy usage while also offering good ...
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Reliable, all-phosphorescent stacked white organic light emitting devices with a high color rendering index Caleb Coburn, Changyeong Jeong, and Stephen R. Forrest ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01213 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Reliable, all-phosphorescent stacked white organic light emitting devices with a high color rendering index Caleb Coburn,a Changyeong Jeong,b and Stephen Forresta,b,c,* a) b)

Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA

Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, USA

c)

Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA

* Corresponding author. Telephone: 1-734-647-1147; Fax: 1-734-763-0085; Email: [email protected]

Abstract High efficiency solid state lighting devices have the potential to significantly reduce lighting energy usage while also offering good color rendering and longer lifetimes than conventional lighting sources. While organic light emitting diodes are promising candidates for this application, their operational lifetime is limited by the blue phosphorescent chromophore. We demonstrate stacked white phosphorescent light emitting devices (SWOLEDs) with lifetimes (as determined from the time it takes to lose 30% of the initial luminance of 1000 cd/m2) of up to 80,000 hours. The correlated color temperature of the devices ranges between 2780-3300 K with color rendering index as high as 89. The three emitter (red, green, and blue) devices contain up to five stacked elements, and employ red emitting blocking layers, stable charge generation layers, graded doping, and hot excited state management to achieve long lifetime. The materials and layer structures used and design principles for SWOLEDs are discussed.



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Keywords: reliability, lifetime, stacked OLED, white OLED, exciton management TOC Graphic:

Organic light emitting devices (OLEDs) are attractive solid state lighting (SSL) sources due to their high power efficiency, broad and tunable spectra, Lambertian emission profiles, and potentially high reliability. Additionally, they offer unique aesthetics, flexible form factors, and compatibility with many substrates. While progress has been made in developing OLEDs for lighting applications,1-5 significant improvements in power efficiency and reliability remain possible.6 In this work, we focus on the reliability of all-phosphorescent, stacked white OLEDs (SWOLEDs)7-8. All-phosphorescent systems are attractive for their high internal quantum efficiencies (IQEs), which can approach 100%.9 However, blue phosphorescent OLED (PHOLED) reliability is typically poor compared to analogous red, yellow, or green PHOLEDs6, 10-11

that ultimately limits the useful lifetime of SWOLED lighting sources. We demonstrate reliable SWOLEDs incorporating a red-green element with red-emissive

blocking layers, as well as stable, low voltage charge generation layers (CGLs). Combining these with a blue PHOLED element employing strategies for extended lifetime, i.e. graded doping and hot excited state management11-12, we fabricated multi-element, stacked warm white SWOLEDs for SSL applications. A correlated color temperature (CCT) = 2780 and color rendering index (CRI) = 89 are obtained for a device comprising 4 red/green emitting sub-elements and a single



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blue PHOLED element. The maximum efficacy of the device is 50 lm/W with a T70 lifetime (defined as the time corresponding to 30% decrease in luminance from an initial value of L0 = 1000 cd/m2) of 80±20 khr, with minimal spectral shifts during aging. Results and Discussion The SWOLED structures comprising stacks of red, green, and blue elements are shown in Fig. 1. The devices are denoted as D3, D4, and D5, indicating the total number of stacked elements separated by low power and optical loss charge generation layers (CGLs). Device D5 consists of 48 individual layers, consisting of 4 mixed red and green emission layer (EML) based elements, and a single blue EML element. (See Methods for details of the structures.) The current-density voltage (J-V) characteristics of the devices are given in Fig. 2a. In the range of J = 1-50 mA/cm2, the voltage increase with number of stacks is 0.1-0.5 V less than the voltage of the red-green test device at the same current-density, indicating that the CGL voltage drop is less than that of the transport layers and contacts of the single EML test devices that lack a CGL (see Supporting Information: single element device and CGL performances). Emission between the wavelengths of 460 and 700 nm is shown in the spectra in Fig. 2a. Devices D4 and D5 exhibit balanced red, green, and blue emission. The peak near 520 nm in the D3 spectrum of blue-shifted by 10±1 nm compared to D4 and D5. There is also some position and width variation in the peak near 610 nm. The CCT, CRI, and 1931 CIE coordinates of the spectra of fresh devices, as well as after aging to T70 are summarized in Table 1. The external quantum efficiency (EQE) and luminance for D3 - D5 are shown vs. J in Fig. 2b. Here, dashed lines were obtained by outcoupling substrate modes using index matching fluid (IMF) between the device substrate and photodetector during the EQE measurement13 (see Methods), leading to an outcoupling improvement of 2.2±0.2 times over substrate emission



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without any outcoupling scheme. While simple outcoupling methods such as microlens arrays give smaller improvement factors (~1.5-1.8)14, similar improvements to that obtained with IMF are achievable with other, wavelength independent outcoupling schemes.1, 3, 13, 15-16 The EQE increases with the number of stacks, and assuming EQE = 10% for the blue element without outcoupling,10, 12

the average red-green element EQE in D5 is 16.4±0.1%, which is slightly lower than for the

single element red-green test structure (18.0±0.3%). Using index matching fluid (IMF) for substrate optical mode outcoupling, D5 reaches a maximum EQE = 171±1% (averaging 34.2% EQE per stacked element) and a luminance >200,000 cd/m2. The luminous power efficiency rolls off from LPE = 50±3 to 30±2 lm/W (24±2 lm/W to 13±1 lm/W without IMF) as the current-density increases from 0.1 to 10 mA/cm2, with an increasing number of stacks tending to slightly increase the power efficiency. Example spectra for D4, both fresh and aged to T70, are shown in Fig. 3a. The spectrum red shifts with aging due to the more rapid decrease in blue and green emission relative to red. The angular emission intensity is shown in Fig. 3a, inset. Devices D4 and D5 have nearly Lambertian emission profiles, while the stronger cavity effects of D3 result in a higher intensity at ~45o than expected for a Lambertian source. The devices exhibit color shifts with angle and brightness: as the current density increases from 1 to 10 mA/cm2, the color temperature increases by 338 ± 1 K. The spectral dependence on viewing angle is characterized by a standard deviation in color temperature of < 340 K from 0 to 60o. The color rendering fidelity provided by the SWOLEDs is demonstrated by the photograph in Fig. 3b. The luminaire comprises 36, 2 mm2 test coupons (12 each of D3-D5), each driven at 50,000-100,000 cd/m2. The luminance losses over time of devices D3-D5 at J = 5, 10, 20, 30, and 40 mA/cm2 are shown in Fig. 4a. Figure 4b gives the extrapolated lifetime to a brightness of 1,000 cd/m2 by fitting



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the relationship 𝑇70×𝐿'& = 𝑐𝑜𝑛𝑠𝑡 to the data, where n = 1.50-1.54, and L0 is the initial brightness.17-18 The extrapolated lifetime increases with outcoupling due to the smaller current required to achieve L0, as shown by the dotted line for D5. At L0=1,000 cd/m2, T70 = 26±7 khr, 50±15 khr, and 80±20 khr, for D3, D4, and D5, respectively. Error bars represent one standard deviation in the extrapolated lifetime determined from the least squares fit. At the increased luminance of L0 = 3,000 cd/m2, T70 = 5±1 khr, 10±2 khr, and 14±3 khr, for D3, D4, and D5. Voltage rise data are shown in Fig. 4c. The 2.5 V to 4.5 V increase in T70 for the devices represents 10-15% of the initial driving voltages. Constant current testing of a CGL-only device (see Methods) show that the voltage rise contribution from the CGL is 250 hr at 30 mA/cm2, which is longer than T70 of the device at the same drive current. This indicates that over 95% of the voltage rise is due to aging of the layers comprising the SWOLEDs other than due to the CGL (see Supporting Information, CGL performance). Device performance is summarized in Table 1. To achieve warm white emission, we calculated the CCT, CRI, and distance from the Planckian locus on the CIE color chart for linear combinations of the single emitter spectra of the B, G and R phosphors, iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f] phenanthridine] (Ir(dmp)3), iridium (III) tris[2-(5'-phenyl)phenylpyridine] (Ir(5’-Ph-ppy)3), and iridium (III) bis(2-phenyl quinolyl-N,C20) acetylacetonate (PQIr), respectively. We found that a warm white spectrum with CCT = 2750, CRI > 85, and within a seven-step McAdam’s ellipse19 of the Plankian locus is achieved for ratios of 4.2 red and 1.5 green photons emitted per blue photon. This spectrum can be achieved by stacking blue with red-green emission layers. Stacking lowers the required current density for a given luminance and thus increases the SWOLED lifetime and maximum brightness. Combining red and green emitters into a single element improves the spectral control compared to stacking separate red and green EMLs. Stacking also simplifies the



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design by allowing for separate optimization of each emitting element. Considering the lower IQE (45%) of the blue compared to the red-green element (>80%) and the expected outcoupling efficiencies of each based on optical modeling in Methods, we calculate that the desired spectrum requires three to four red-green EMLs per blue EML. The locations of the two elements nearest to the Al cathode are positioned to maximize outcoupling. The remaining elements were separated by CGLs to minimize voltage loss in the transport layers. We employ a bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (BAlq) HBL for the green element, while introducing a red phosphor doped region, leaving a thin BAlq spacer between the green EML and red doped region. This unusual combination serves as a red emitting blocking layer that takes advantage of the stability of BAlq18, 20-21 while reducing the loss of excitons transferred to its low triplet energy (ET)20. Green excitons that diffuse across the interface between the EML and HBL subsequently transfer to the PQIr where they recombine radiatively. Thus, nonradiative loss resulting from poor exciton confinement is prevented by recycling the green excitons to generate red photons. Similarly, by doping the red phosphor into a thin green EML adjacent to the hole transport layer (HTL), we achieve exciton confinement at the EML interface with 4,40-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPD). The layer thicknesses and doping concentrations of the spacer as well as the green and red emitting regions in the EML were optimized to give the desired red-to-green emission ratio and to minimize color shifts. Placing red emitting layers on both sides of the green EML reduces the color shift with current density: As the current density increases, the exciton recombination zone moves from the HBL to the HTL side of the EML,22 resulting in reduced emission from the PQIr doped at the HBL and increased emission from the PQIr-doped emitting layer adjacent to the HTL.



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The lifetime of the blue element is critical to prevent color shifts and a rapid decrease in luminance with aging. Hence, we use a graded dopant profile and hot excited state management to increase the blue element stability11-12. Dopant grading balances hole and electron transport in the EML, broadening the exciton recombination zone and reducing bimolecular annihilation rates that lead to molecular dissociation. In addition, the deep blue phosphor mer-tris-(N-phenyl, N-methylpyridoimidazol-2-yl)iridium (III) (mer-Ir(pmp)3) is used to improve the reliability of the blue element by reducing the probability that hot excited states degrade host or emitter molecules.11 Because the blue element only contributes 15% of the initial luminance, its degradation primarily impacts color rather than luminance intensity. Increasing the red-green element lifetime without also increasing that of the blue element would accelerate color shift with aging. The maximum allowable color shift depends on the application23, and can also limit the useful device lifetime. The small color shift of these devices with aging, shown in Fig. 3a and Table 1, is achieved by the balanced degradation rates of the emitting elements. The spectrum red-shifts slightly with time, due to the lower operational stability of the blue element: when D5 reaches T70 the blue emission is decreased by 50%. At the SWOLED T70, the blue cell loses about 5% more luminance without the hot excited state manager, and 40% more luminance without either grading or hot excited state manager. The CGL characteristics affect both lifetime and power efficiency of the SWOLEDs. We choose NPD/hexaazatriphenylene hexacarbonitrile (HATCN) as a stable charge generation interface,12 and Li-doped 2,7-bis(2,20-bipyridine-5-yl)triphenylene (BPyTP2) as the n-type electron transport layer (n-ETL)24 due to its high electron mobility and stability. The CGLs using BPyTP2 operate at low voltage ( 85 for both D4 and D5, with warm white CCTs. The power efficiency of is 30-50 lm/W, which may be improved by reducing operating voltage or better out coupling. Methods for decreasing operating voltage include higher mobility hosts/dopants with smaller energy gap differences, electron and hole conducting cohosts, and reducing the EML thickness. Also, separate red and green EMLs can improve outcoupling, however more reliable structures are required to realize these improvements without sacrificing lifetime. To our knowledge, device lifetime of T70 = 80±20 khr with L0 = 1000 cd/m2 is the longest for white PHOLEDs with reported materials and structures. Lifetimes of white PHOLEDs have been limited by rapid degradation of the blue EML, with T70 < 1 khr11. Blue fluorophores are therefore often used for their reliability in hybrid fluorescent/phosphorescent white OLEDs25. However, the longest fully reported hybrid fluorescent/phosphorescent device lifetime under the same test conditions is 31 khr26, which is less than half that of the PHOLEDs reported here. This indicates that blue PHOLED lifetime, when increased by excited state management, is sufficient



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for many lighting applications. Additionally, using CGLs to stack EMLs and red-emissive blocking layers are crucial to achieving long SWOLED lifetimes. Conclusion We have demonstrated all-phosphorescent warm white, stacked PHOLEDs with lifetime of T70 = 80,000 hr and high CRI of 89. This is achieved using a three phosphors in five stacked EMLs comprising a device with a total of 48 layers. The device features red emissive blocking layers in the red-green element, graded doping and hot excited state management in the blue element, and stable, low voltage CGLs. These devices demonstrate the potential of white PHOLEDs for solid state light sources. The design principles and strategies employed will undoubtedly lead to the further improvement of long lived SWOLEDs.

Acknowledgements This work was supported by the United States Department of Energy under award DE-EE0007077, Universal Display Corporation, and the Air Force Office of Scientific Research. The authors thank Dr. Julie Brown, Yue Qu and Tyler Fleetham for valuable discussions. Methods: Material Acronyms and Device Fabrication: All materials were purified via thermal gradient sublimation27 before device fabrication. Glass substrates with pre-patterned, 1 mm wide indium tin oxide (ITO) stripes were cleaned by sequential sonications in tergitol, deionized water, acetone, and isopropanol, followed by 20 min UV ozone exposure. Organic materials and metals were deposited at rates of 0.5-2 Å/s through shadow masks in a vacuum thermal evaporator with a base pressure of 10-7 Torr. Organic layers

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for D3, D4, and D5 were simultaneously deposited without breaking vacuum using in-situ substrate masking. Using a separate shadow mask, 1 mm wide stripes of 100 nm thick Al films were deposited perpendicular to the ITO stripes to form the cathode, resulting in 2 mm2 device area. Devices were encapsulated using a glass cover sealed to the substrate with a bead of UV cured epoxy around its periphery in a N2 environment with