Novel Emitting System Based on a Multifunctional Bipolar Phosphor: An Effective Approach for Highly Efficient Warm-White Light-Emitting Devices with High Color-Rendering Index at High Luminance

White organic light-emitting diodes (WOLEDs) are a very promising technology for next generation solid-state lighting. [1] High-quality illumination sources require WOLEDs with a high color rendering index (CRI) of >80. [1c] Although some two-color WOLEDs produced by an orange or yellow emitter complemented with a blue emitter, exhibit impressive electroluminescence (EL) efficiency, they have low CRI (≤70). [2] Therefore, three-color (red, green and blue) or more emitters is a prerequisite for high CRI. [3] Today, WOLEDs based on phosphorescent emitters with the CRI  80 [4] have achieved rather high external quantum efficiency (EQE) of  20% owing to near 100% internal quantum efficiency (IQE) of electrophosphorescence. However, they show only moderate peak power efficiencies (PEs) (40-60 lm W –1 ) and suffer from a pronounced efficiency

upon their transfer to the guest emitters. [4a] Thus, there is a need for conceptually new emitting systems, where the excited state energy will more efficiently transfer from the higher-energy (blue and/or green) to the lower-energy (yellow and/or red) emitters with little power losses due to a suitable E T (0.3 eV) between the respective host and guest.
Employing as few as possible components in the emitting system of WOLEDs is a means to reduce the energy losses through the simplified exciton-formation and energy-transfer processes. For achieving this, appropriate multi-functional emitter molecules are needed combined with a smart device design strategy.
A few efficient phosphorescent OLEDs (PhOLEDs) have been reported based on phosphorescent hosts. [7a,d] These no-fluorescent-host emitting systems are promising to simplify/optimize the electrophosphorescent process. [7] However, until now, integrating such an advanced doping model into the construction of the WOLEDs, has not yet been achieved.
In this work, three emitting complexes originating from our group: Bepp 2 ( max 450 nm), [8] FPPCA ( max 500 nm) and BZQPG ( max 605 nm), [9] which could provide essential colors of blue (B), green (G) and orange-red (OR), respectively, for white light, were well organized for realizing a simplified WOLED composed of two adjacent G-OR (FPPCA:BZQPG) and B-G (Bepp 2 :FPPCA) emitting layers (EMLs). In this strategy, phosphorescent (P) molecule FPPCA was distributed through both EMLs and showed an unprecedented multifunctional property by playing four key roles: (i) the charge-transporting host (ii) the green emitting host (iii) the sensitizer for the dopant P molecule BZQPG in the G-OR layer and (iv) the green dopant emitter in the B-G layer. This new method endowed the device with the advantage of a reduced number of constituent components and EMLs, which allows for a simplified fabrication processes and effectively reduces structural heterogeneity. Furthermore, careful manipulation for the well-matched FPPCA:BZQPG combination utilizes all the electrically generated excitons in the phosphor-phosphor type (PPT) G-OR layer, where the bipolar character of FPPCA results in a wide emission zone to enhance carrier and exciton utilization, thereby dominating the high electrophosphorescent efficiency. Meanwhile, the fluorescent (F) molecule Bepp 2 is used to generate blue singlet emission in the B-G layer and served as a host for sensitizing green emission, thereby achieving the broad white EL spectrum. An optimal management of B-G and G-OR layers aiming at balanced charge injection together with simultaneous efficient charge/exciton confinement, ensured this three-color device possesses stable and high EL performance as follows: very high forward-viewing EQE of 27.3% and PE of 74.5 lm W -1 at an illumination-relevant luminance of 1000 cd m -2 with a high CRI of 85 and desirable eye-friendly warm-white [5c,10] Commission Internationale de L'Eclairage (CIE x,y ) coordinates of (0.43, 0.46) were realized, which maintained the high levels of 26.3% and 56.5 lm W 1 at 5000 cd m 2 , 24.3% and 45.8 lm W 1 at the extremely high luminance of 10000 cd m 2 , with CRI in the range 86-87. To our knowledge, these values are competitive with, and even exceed, the best published data for a WOLED, [2,[4][5][6] simultaneously exhibiting a high efficiency and high CRI based on high luminance for the practical solid-state lighting.  Figure S1). Such high yields and short lifetimes are beneficial for high EL efficiency and reduced roll-off. [11] Moreover, the triplet energy alignment of 2.6, 2.4 and 2.1 eV for Bepp 2 , FPPCA and BZQPG respectively, [8,9] indicated that both triplet energy transfer processes are sufficient and losses are minimal due to their appropriate E T (0.2~0.3 eV). Time-of-flight (TOF) measurements revealed that FPPCA and BZQPG are bipolar molecules with comparable hole and electron mobility as high as 10 3 cm 2 V 1 s 1 , [9] while Bepp 2 is an electron-transporting molecule due to its high electron mobility of 1.3  10 3 cm 2 V 1 s 1 . The single-carrier devices based on active layers of FPPCA (x nm)/Bepp 2 (y nm) were fabricated, where ITO (indium tin oxide) and LiF/Al are the anode and the cathode, respectively; 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) and 4,4′-bis(N-(1naphthyl)-N-phenylamino)biphenyl (NPB) layers prevent injection of holes and electrons from the anode and cathode, respectively. [12] The current density (J) versus voltage (V) curves are shown in Figure 2a. For electron-only devices, the J-V characteristics are almost independent of the relative layer thicknesses, suggesting both molecules possess comparable electron-transporting properties. In contrast, in the hole-only devices the current decreases with increasing relative thickness of Bepp 2 at a constant driving voltage. This indicates that Bepp 2 has relatively weak hole transporting ability compared with FPPCA, which is in accord with the electron-transporting nature of Bepp 2 . [8] The recombination zone in the FPPCA-and Bepp 2 -based EMLs was further confirmed as follows. Four devices with a uniform configuration of ITO (100 ± 2 nm)/NPB (35 nm)/EML (30 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al were fabricated by introducing a thin layer (2 nm) of BZQPG (5 wt%) doped in FPPCA, and FPPCA (5 wt%) doped in Bepp 2 into different zones in the EML of the respective devices: namely devices F10 and F20 (F-series) based on FPPCA, and Be10 and Be20 (Be-series) based on Bepp 2 (Figure 2b). Here, NPB and TPBi are the hole-transporting layer (HTL) and the electron-transporting layer (ETL), respectively.
The EL spectra at a luminance of 1000 cd m -2 are shown in Figure 2c. In the F-series devices, the EL is almost independent of the different zone in the doping layer, where the relative intensities of the host FPPCA green emission and the dopant BZQPG orange-red emission are comparable. This demonstrates that both the holes and the electrons migrate freely and the excitons form and diffuse throughout the EMLs based on FPPCA host, rendering the FPPCA molecules bipolar. Be10 and Be20 showed different EL spectra: two emission peaks from the dopant FPPCA and the host Bepp 2 , respectively, are exhibited in Be10, whereas Be20 shows a slight green shoulder from the dopant, along with dominant blue emission from the host, indicating that the recombination region of electrons and holes in the Bepp 2 layer is close to the NPB/Bepp 2 interface and its optimized thickness is as narrow as ca. 10 nm.  (Figure S2). Here a novel phosphor-phosphor type (PPT) host-guest system contributed to G-OR EL, and an adjacent classical fluorophor-phosphor type (FPT) layer gave B-G EL, without any interlayer. Figure 3a shows the proposed energy diagram of Device 1.
Theoretically, both singlet and triplet excitons are created with a ratio of 1:3 in either of the EMLs. [13] In the FPPCA:BZQPG layer, all the generated excitons can produce either green emission through direct radiative decay on the host FPPCA molecule, or orange-red emission from the dopant BZQPG molecules through host-guest energy transfer. In another EML, the Bepp 2 molecules not only generate blue singlet emission, but also sensitize the emission of FPPCA in both EMLs, leading to the increase in total green emission intensity. and 3c show the important EL characteristics. The data in Table 1 show driving voltages of 2.4, 3.5, 4.8 and 6.7 V for luminances of 1000, 5000 and 10000 cd m 2 , respectively. Figure 3b shows warm-white EL due to the simultaneous fluorescent blue from  56.5 lm W 1 and 26.3%, respectively, with CRI of 86. Even at 10000 and 15000 cd m 2 , high PEs (45.8 and 38.5 lm W 1 ) and EQEs (24.3 and 22.1%) are maintained. Lighting sources are generally characterized by their total emitted light, which is a factor of 1.7-2.3 times more than the forward viewing efficiencies. [1a] Therefore, the present device should have total PE and EQE values approaching 100 lm W -1 and 50%, respectively, at 5000 cd m -2 , which are also the highest levels reported for WOLEDs with extra out-coupling and/or multi-layer tandem structures, [14] as well as comparable to the most efficient lighting technologies. [15] Table 1. Summary of EL performance for Devices 1, 2, 3 and 4.

Figures 3b
Device To elucidate the origin of the high performance of the Device 1, three reference devices  In the EL spectra of Devices 3 and 4, the emission peaks originating from both the host and dopant molecules are observed, due to suppression of energy transfer of singlet/triplet excitons at such low doping concentrations as 0.8 or 1.5 wt%. We attribute the increase of EQE for white Device 1 on shifting from low to high luminance to the following reasons: 1) the shorter emission wavelength of blue and/or green color results from the larger energy gaps compared to orange or red; therefore, increasing the driving voltage leads to more blue and/or green excitons, which in turn leads to the high EQE at high luminance.
2) The recombination zone consisting of double EMLs without an interlayer in Device 1, which could induce the undesirable charge accumulation as well as the subsequent triplet-polaron and/or polaronpolaron quenching, allows all the excitons to decay radiatively and is key to maximizing the overall quantum efficiency of the device.
3) The bipolar phosphor molecule FPPCA which is distributed through both EMLs has different roles, and is the essential feature for maximizing the overall quantum efficiency, especially retaining the high EQE at high luminance.
With increasing luminance as the driving voltage increased, the relative intensity of blue emission in Devices 2 and 3 also increased. This strongly supports the hypothesis for Device 1, namely that higher voltages favor the injection of holes into the B-G layer, which facilitates the creation of more blue excitons from Bepp 2 . For Device 4 increasing the voltage leads to gradually increased relative intensity of green emission indicating that the green excitons are not all captured by the BZQPG molecules and they decay radiatively. Such an EL process should exist in the G-OR layer in Device 1 with the same PPT system, where the enhancement of green emission is likely to cause the EL spectral variation with changing voltage.
The comparable high EL data of both Devices 1 and 4 (  transfer from FPPCA to BZQPG. [9] These factors combine to enable nearly all the electrically generated excitons to be employed for EL emission, leading to the high EL performance. On the other hand, the fluorophor-phosphor type (FPT) B-G layer in Device 1, based on a