Highly Efficient, Ultralow Turn-on Voltage Red and White Organic Light-Emitting Devices Based on Novel Exciplex Host

The exciplex forming co-host is one of the most promising candidates for developing high-performance organic light-emitting devices (OLEDs) that can implement an internal quantum efficiency of 100%. In this work, a novel exciplex co-host system by employing N-([1,1’-biphenyl]-2-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9’-spirobi[fluoren]-4-amine (FSF4A) and 2,4,6-tris[3-(diphenylphosphinyl) phenyl]-1,3,5-triazine (PO-T2T) is applied to design simplified red and white OLEDs with low turn-on voltage and high efficiency. The high performance red phosphorescent organic light-emitting diode (PhOLED) is achieved by employing exciplex co-host under a low guest doping level of 3%, showing the best performance with a maximum power efficiency of 38.5 lm W -1 , a maximum external quantum efficiency of 17.3%, and an ultralow turn-on voltage of 1.95 V, respectively. Based on the red device, the ultra-thin FIrPic layer is inserted to achieve high performance white OLED, exhibiting a low turn-on voltage of 2.2 V with a maximum power efficiency of 34.1 lm W -1 , and the Commission Internationale de’IEclairage (CIE) coordinate (0.33,0.33) at 1000 cd m -2 . These superior properties can be attributed to reduced barriers and the effective energy transfer by employing exciplex co-host. phenyl]-1,3,5-triazine (PO-T2T), the yellow exciplex emission can be observed in both photoluminescence (PL) and electroluminescence (EL). Compared with common mixed-host devices, the red PhOLED based on exciplex co-host demonstrates an ultralow turn-on voltage of 1.95 V and maximum power efficiency of 38.5 lm W -1 with external quantum efficiency of 17.3%. Moreover, its turn-on voltage is almost the same as the lowest turn-on voltage achieved by the red OLED based on exciplex co-host. 31 By further inserting a blue ultrathin layer, white OLED achieves ultralow turn-on voltage of 2.2 V and maximum power efficiency of 34.1 lm W -1 with external quantum efficiency of 12.4%, and the turn-on voltage is almost one of the lowest compared to previous references. 32, 33 Furthermore, the Commission (CIE) is (0.33,0.33) at 1000 cd m -2 , and CIE coordinates variation is only (0.026,0.003) over a large luminance range, which is better among the WOLEDs of exciplex co-hosts. It is found that the high performances of PhOLEDs are mainly attributed to balanced charge transport and proper energy transfer channels from exciplex co-host to dopant. This work may provide valuable clues for rational design of the exciplex system, as well as their application as co-host materials in PhOLEDs with high efficiency and ultralow turn-on voltage.


Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have attracted great attention due to their extensive application prospects in the field of solid-state lighting and flat-panel displays with the theoretical value of 100% exciton utilization by harvesting both singlet and triplet excitons for electroluminescence (EL). [1][2][3][4] In general, to improve device efficiency and stability, host-guest technology is a superior method for designing high performance phosphorescent devices. 5,6 The host materials usually need to have proper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to facilitate balanced charge injection and transportation capabilities, as well as high triplet energy levels (T1) to ensure effective energy transfer and confine excitons in the phosphorescent guest. [7][8][9][10][11] However, there are still huge challenges to achieve PhOLED with low driving voltage and low energy consumption. 12,13 In order to meet the needs of highquality red and white OLEDs, it is essential to design an advantageous emission layer structure, which can achieve high exciton utilization for both singlet and triplet, as well as effective charge injection and transportation at low voltage. 14 Recently, the co-hosts of exciplex formed by an acceptor and a donor, instead of traditional hole or electron type single hosts, has been regarded as a promising candidate for using as a host on phosphorescent OLEDs due to its good charge balance and exciton utilization. [15][16][17][18][19][20] Furthermore, it has been demonstrated that exciplex host with small singlet-triplet energy difference (ΔE st ) can notably reduce the turn-on voltage and improve the power efficiency (PE) and external quantum efficiency (EQE) of the PhOLEDs. [21][22][23] In general, exciplex systems are found to have thermally activated delayed fluorescence (TADF) effect through reverse intersystem crossing (RISC), triplet excitons can be upconverted to single excitons due to intrinsically smaller ΔE st values. 24,25 It is found that the exciplex co-host is more beneficial to facilitate effective Förster energy transfer, which can further improve device efficiency. 26,27 Many exciplex co-host systems have been reported to demonstrate high efficiency of red and white phosphorescent OLEDs with the development of exciplex co-host. Sheng et al. adopted exciplex cohost to achieve red phosphorescent OLEDs with maximum power efficiency of 35.3 lm W -1 and external quantum efficiency of 19.8%. 28 Xu et al. reported a red phosphorescent OLED through exciplex cohost system with maximum power efficiency of 31.8 lm W -1 and turnon voltage of 2.24 V. 29 Yao et al. realized a red phosphorescent device with a maximum power efficiency of 36.9 lm W -1 and external quantum efficiency of 15.5%. 30 However, despite the remarkable progress in the use exciplex co-hosts in OLEDs, it is still a great challenge to precisely predict the likelihood of electron hole pairs induce exciplex emission and achieve ultra-low turn-on voltage PhOLEDs.
In this work, a novel exciplex system is fabricated by employing new hole transport material of N-([1,1'-biphenyl]-2-yl)-N-(9,9dimethyl-9H-fluoren-2-yl)-9,9'-spirobi[fluoren]-4-amine (FSF4A) and electron transport material of 2,4,6-tris [3-(diphenylphosphinyl) phenyl]-1,3,5-triazine (PO-T2T), the yellow exciplex emission can be observed in both photoluminescence (PL) and electroluminescence (EL). Compared with common mixed-host devices, the red PhOLED based on exciplex co-host demonstrates an ultralow turn-on voltage of 1.95 V and maximum power efficiency of 38.5 lm W -1 with external quantum efficiency of 17.3%. Moreover, its turn-on voltage is almost the same as the lowest turn-on voltage achieved by the red OLED based on exciplex co-host. 31 By further inserting a blue ultrathin layer, white OLED achieves ultralow turn-on voltage of 2.2 V and maximum power efficiency of 34.1 lm W -1 with external quantum efficiency of 12.4%, and the turn-on voltage is almost one of the lowest compared to previous references. 32,33 Furthermore, the Commission Internationale de'IEclairage (CIE) coordinates is (0.33,0.33) at 1000 cd m -2 , and CIE coordinates variation is only (0.026,0.003) over a large luminance range, which is better among the WOLEDs of exciplex co-hosts. It is found that the high performances of PhOLEDs are mainly attributed to balanced charge transport and proper energy transfer channels from exciplex co-host to dopant. This work may provide valuable clues for rational design of the exciplex system, as well as their application as co-host materials in PhOLEDs with high efficiency and ultralow turn-on voltage.

Materials
FSF4A was purchased from Shenzhen PURI Materials Technologies Co., Ltd. Ir(MDQ) 2 acac and FIrPic were purchased from Luminescence Technology Corp. MoO 3 , TPBi, MCP, Liq, PO-T2T were purchased from Xi'an Polymer Light Technology Corp. For the basic parameters of FSF4A: S 1 is 3.0 eV, T 1 is 2.6 eV, HOMO is 5.3 eV, LUMO is 2.1 eV, and the absorption peak positions are 310 nm and 340 nm.

Device fabrication and characterization
The OLED were grown on pre-patterned ITO coated glass (20 Ω/square). Before depositing into the evaporation system, the ITO substrates were cleaned with acetone, ethyl alcohol, deionized water by ultrasonic cleaning machine for 20 min. All the devices were deposited sequentially under fine vacuum of 8×10-5 Pa. The organic transport materials were grown by the rate of 0.8-1.5 Å/s, while organic dopants Ir(MDQ) 2 acac and FIrPic were deposited at the rate of 0.01-0.1 Å/s, the FIrPic ultra-thin layer of 0.2 nm is deposited at a rate of 0.05 Å/s for 40 s, Liq and MoO 3 were deposited at the rate of 0.15-0.3 Å/s, Al was deposited by the rate of 3 Å/s. The photoluminescence (PL) spectra were acquired by an RF-5301PC fluorescence spectrophotometer. The transient PL decay curves were recorded by IHR320 spectrometer. The CIE coordinates, luminance and electroluminescent (EL) spectra were carries out by a PR655 spectra-scan photometer simultaneously. The CE, PE, and EQE were measured by a programmable Keithley 2400 source-meter and an absolute external quantum efficiency measurement system. All devices were characterized at room temperature without encapsulation. Recently, organic light-emitting diodes using exciplex as the host have been extensively researched compared with traditional host devices due to their excellent EL performance. Fig. 1 shows the energy level diagrams and chemical structures of the materials used in this work. From the orbital energy level diagram (Fig. 1a), the difference between LUMO of the donor of FSF4A and the acceptor of PO-T2T is 1.1 eV, which can effectively restrict the electrons from PO-T2T to FSF4A; meanwhile, their HOMO difference is 1.8 eV, which can also obviously block hole from FSF4A to PO-T2T. Therefore, we infer that such a large energy level difference may produce an exciplex system. 34 The photoluminescence (PL) spectra of FSF4A, PO-T2T (solution), TPBi, FSF4A:PO-T2T (molar ratio of 1:1) and FSF4A:TPBi (molar ratio of 1:1) are depicted in Fig. 2(a and b). As can been seen, the PL emission spectra (films measured at 300 K) of FSF4A, PO-T2T (solution) and FSF4A:PO-T2T mixed film (1:1, molar ratio) are completely different, and the PL emission peak position of FSF4A:PO-T2T film is 549 nm, which is obviously red-shifted relative to those of the pure FSF4A or PO-T2T (i.e., 407 nm for FSF4A and 419 nm for PO-T2T). The PL spectrum of FSF4A:PO-T2T mixed film also exhibits a full width at half maximum of 103 nm, and it is highly shifted to the long wavelength region due to its intermolecular charge transfer (CT) characteristics. 35 The exciplex photon energy of FSF4A:PO-T2T can be estimated to be 2.25 eV by the emission peak of mixed film. The value is quite close to the difference (2.1 eV) between the HOMO of FSF4A (donor) and the LUMO of PO-T2T (acceptor). The results indicate that an exciplex is formed between the FSF4A molecule and the PO-T2T molecule under photo excitation, and the FSF4A:PO-T2T mixed film generates a pure CT exciplex emission. The PL emission spectrum of peak position of FSF4A:TPBi film (1:1, molar ratio) at 409 nm and 385 nm is depicted in Fig. 2b, which is quite similar to those of pure FSF4A (407 nm) and TPBi (385 nm) films. Thus, we infer that the mixed film of FSF4A:TPBi cannot form exciplex emission. The formation process of the exciplex can be described as the following equation (1) 36

Please do not adjust margins
Please do not adjust margins  37 Meanwhile, the fitting formula for the tested data of the exciplex is as follows: (2) where A 1 and A 2 are the constants fitted according to the data of photoluminescence lifetime test, while τ 1 and τ 2 are the fitted prompt fluorescence and delayed fluorescence components respectively. The high RISC process from triplet to singlet is attributed to the small ΔE ST . The ΔE ST value can be obtained from the following formula: where R is the ideal gas constant, T is the thermodynamic temperature, K eq is the ratio of intersystem crossing process (k ISC ) and reverse intersystem crossing (k RISC ), k ISC and k RISC can be obtained by the following formulas 38 : where Φ PF and Φ DF are the photoluminescence quantum efficiency of PF and DF respectively, τ PF and τ DF can be revealed by fitting the decay curve in the time-resolved PL spectrum. We obtained k ISC of 2.57 × 10 7 S -1 and k RISC of 3.42 × 10 6 S -1 , and calculated that the singlet-triplet energy difference of the exciplex is 0.5 kcal/mol (0.022 eV). The photoluminescence quantum yield (η PL ) is 31% (5:5). Thus, the exciplex system can be realized by effectively converting the triplet CT into singlet CT through the reverse intersystem crossing (RISC). Fig. 2d shows the transient decay curve of mixed film FSF4A:TPBi, which has a significantly shorter exciton lifetime compared to FSF4A:PO-T2T. These results further support the formation of FSF4A:PO-T2T exciplex emission, while FSF4A:TPBi cannot form exciplex emission.   Table 1. The low turn-on voltage and high performance can be attributed to the "barrier-free" device structure, that is, holes and electrons can be injected into EML from FSF4A and PO-T2T without barrier, respectively. 39 Meanwhile, the device with the FSF4A:PO-T2T mixing ratio of 5:5 achieves the greatest luminance and efficiency, which is due to a more balanced carrier transport. Therefore, all the OLEDs based on FSF4A:PO-T2T emitting were developed with the optimized molar ratio of 5:5. Fig. 3d shows the electroluminescence spectra of different ratios of exciplex as emitting layer device. The emission peak position of the device with a mixing ratio of 5:5 is 550 nm at the voltage of 6 V, which is extremely consistent with the photoluminescence peak. The energy transfer characteristics of FSF4A:PO-T2T exciplex should be considered when it is used as the host: (1) Higher LUMO of FSF4A and deeper HOMO of PO-T2T limit the exciton recombination zone to avoid quenching during charge transport. (2) T1 level of FSF4A and PO-T2T is higher than the T1 level of the exciplex to effectively restrain the exciton energy transfer to the consisting donor or acceptor. (3) The exciplex need to have a higher T1 level than phosphorescent dopants to restrain energy transfer from the phosphors to the exciplex. 28,29 According to the PL spectrum (S1=2.258 eV) and ΔE ST (0.022 eV) of the exciplex, T1 can be estimated to be about 2.236 eV, which is obviously lower than FSF4A (2.6 eV) and PO-T2T (3.0 eV). 40 Therefore, the red dye Ir(MDQ) 2 acac (2.0 eV) is used as a dopant to design red PhOLEDs based on the characteristics of a novel exciplex co-host. 31 Table 2. Summary of EL performance of red OLEDs based on exciplex co-host   Fig. 4b, and the inset shows the emission peak at 616 nm with CIE coordinates of (0.61,0.37). It is encouraging that the turn-on voltage of A 1 -A 3 is extremely low as 1.95 V, which is even 0.05 V lower than the theoretical limit voltage corresponding to the emission photon energy of Ir(MDQ) 2 acac (2.0 eV). We believe that the extremely low turn-on voltage is due to the fact that the thermally activated carriers. 43 Such low turn-on voltage and high efficiency are a great improvement compared with previous reported red OLEDs based on the exciplex co-host (Table 2). Fig. 4c shows that device B 2 (with a doping concentration of 3 wt% Ir(MDQ) 2 acac) achieves a low turn-on voltage of 2.55 V and a maximum brightness of 24410 cd m -2 . The maximum CE, PE, EQE are 25.7 cd A -1 , 26.9 lm W -1 , 11.9%, which can be seen in Fig. 4d, and the inset displays the emission peak at 612 nm with CIE coordinates of (0.61,0.37). As can be seen, the device employing FSF4A : PO-T2T exciplex co-host exhibits superior electroluminescence performance compared to the device employing the FSF4A : TPBi common co-host. The EL performance of device A 2 and B 2 confirms the superiority of the exciplex as co-host in achieving ultralow turn-on voltage and high efficiency, which may be attributed to the carriers being more likely to cross the barrier and the excitons being effectively transferred to the dopant. It is also noticed that the J-V-L and CE-L-PE characteristics exhibited by device A and B with varied Ir(MDQ) 2 acac doping concentrations are quite different, as shown in Fig. 4. Detail characteristics for device A 1 -A 3 and B 1 -B 3 are summarized in Table 3.  It can be seen from the insets that the host emission peaks exist only at low doping concentration of 0.6% Ir(MDQ) 2 acac, which can be explained as incomplete energy transfer from host to the dopant. The EL efficiency declined as the doping concentration of Ir(MDQ) 2 acac increases to 8%, which is due to the strengthening of triplet-triplet annihilation, triplet-polaron quenching with a higher proportion of dye. Meanwhile, a slight red shift with increasing doping concentration can be observed in the insets of Fig. 4(b and d), which is due to reabsorption of the emitter emission. Based on the above results, it is confirmed that the FSF4A:PO-T2T exciplex co-host doped with 3% Ir(MDQ) 2 acac concentration achieves the optimal EL efficiency.

Von [V] a) Lmax[cd/m 2 ] b) CEmax [cd/A] c) PEmax [lm/W] d) EQEmax [%] e)
To Obviously we can notice that the charge carrier transmission of device tends to be more balanced as the doping concentration of Ir(MDQ) 2 acac is increased from 0% to 8%, it is due to Ir(MDQ) 2 acac has a strong hole trap effect and creats additional electron transport channel. Ir(MDQ) 2 acac has the highest occupied molecular orbital energy level of 5.1 eV, which is 0.2 eV difference from FSF4A (5.3 eV), indicating that Ir(MDQ) 2 acac acts as trapping sites for holes. On the contrary, the current density of electron-only devices increases with increasing the doping concentration of Ir(MDQ) 2 acac. The improvement can be attributed to the fact the Ir(MDQ) 2 acac molecules create an additional electron transport channel, which is beneficial for more balanced charge carriers in the devices. Overall, it is found that doping Ir(MDQ) 2 acac into exciplex co-host can simultaneously trap holes and facilitate electron transport. Both of these processes lead to a more balanced charge carriers, which causes a slight shift of the recombination zone to the center of the device. 44 Fig. 5 The current density-voltage characteristics of (a) 0% Ir(MDQ) 2 acac (b) 0.6% Ir(MDQ) 2 acac (c) 3% Ir(MDQ) 2 acac (d) 8% Ir(MDQ) 2 acac.
The energy transfer mechanism of red OLEDs based on the exciplex co-host is illustrated in Fig. 6. Excitons are produced by the direct recombination between holes on the donor (FSF4A) and electrons on the acceptor (PO-T2T), the exciplex excitons can also be divided into singlet excitons of 25% and triplet excitons of 75%. For singlet excitons of exciplex, one is converted to triplet excitons by intersystem crossing, and the other is to transfer energy to the dopant by Förster energy transfer (FRET); for triplet excitons of exciplex, part of which can diffuse into the dopant through Dexter energy transfer (DET), but the triplet energy is lost as the diffusion length increases. 45 Meanwhile, another triplet could upconvert into the singlet by the reverse intersystem crossing due to the small ΔE ST of exciplex. This process can effectively reduce energy loss due to improving FRET energy transfer and suppressing DET energy transfer. Therefore, we can achieve efficient and stable exciplex co-host PhOLEDs. Fig. 6 Operational mechanism of the exciplex type host.   Fig. 7a. The x is 1, 2, 3 and 4, corresponding to devices W 1 , W 2 , W 3 and W 4 , respectively. As can be seen from Fig.  7b, all WOLEDs realize an ultra-low turn-on voltage of 2.20 V due to the barrier-free charge transfer. The current density of the four devices decreases as the thickness of the spacer layer increases, which can be explained by the following formula (J-V curves) 52 : (8) device operating temperature). Meanwhile, the EL efficiency of the four devices has a significant improvement as the thickness of the spacer layer increases. As depicted in Fig. 7c, the maximum CE of 32.6 cd A -1 and PE of 34.1 lm W -1 , corresponding to an external quantum efficiency of 12.4%, are achieved in the W 4 . Such a low turn-on voltage and high efficiency when the standard CIE coordinate of (0.33,0.33) is achieved at a luminance of 1000 cd m -2 are a great enhancement compared to previous reported of WOLEDs (Table 4). The low CE roll-off is clearly observed in devices W 1 -W 4 , for device W 4 , the current efficiency drops to 30.3 cd A -1 at 1000 cd m -2 , corresponding to the roll-off of 7.1%. The low efficiency roll-off could be attributed to the extended exciton recombination zone due to balanced carrier transport. Meanwhile, according to reported reference, there is an interface exciplex between the interlayer MCP and the electron transport layer PO-T2T, and the emission peak position of exciplex is about 452 nm (2.74 eV). 53 The FIrPic emission peak position is about 476nm (2.6 eV). Therefore, we believe that there is energy transfer from interface exciplex to phosphorescent dyes, which can improve the efficiency of the devices. Detail characteristics of the W 1 -W 4 are summarized in Table 5. As shown in Fig. 7d, device W 1 shows an intense red emission peak and the quite weak blue emission, which is the result of energy transfer from the blue emission layer to the red emission layer caused by the thinner spacer layer. The blue emission gradually increases as the thickness of the spacer MCP increases. Device W 4 achieves a WOLED with CIE coordinates of (0.33,0.33) at 1000 cd m -2 , corresponding to the CRI of 52 and CCT of 5439.  a) The CE roll-off at 1000 cd m -2 ; b) The CIE coordinates at 1000 cd m -2 ; c) The CIE coordinates variation from 4 to 7 V. Fig. 8 exhibits the normalized EL spectra of devices W 1 -W 4 at different luminance. The device W 1 shows a rather weak blue emission, which can be attributed to the strong energy transfer due to the thinner spacer layer. Obviously, for white devices W 2 , W 3 , and W 4 , the relatively stable spectra are displayed in a large luminance range with a slight CIE coordinates shift. The CIE coordinates shift of device W 2 -W 4 is from (0.488,0.366), (0.414,0.353), (0.352,0.343) at 4V to (0.462,0.362), (0.386,0.350), (0.326,0.340) at 7V, revealing CIE variation is only (0.026,0.004), (0.028, 0.003), and (0.026, 0.003), respectively. The only slight colour shift in the EL spectra may be attributed to the bipolarity of the exciplex co-host, which reduces the recombination zone movement. Meanwhile, as the thickness of the spacer layer increases, the colour rending index of the devices W 2 -W 4 sequentially decreases, which is due to the red emission peak gradually decreases.

Conclusions
In summary, we have successfully achieved ultralow turn-on voltage, high-performance simplified red and white phosphorescent OLEDs based on a novel exciplex co-host. The energy transfer from the exciplex co-host to its constituents is completely suppressed due to the high E T of both FSF4A and PO-T2T, while from the host to the dopant is improved through long-range Förster energy transfer. The red device implements an ultra-low turn-on voltage of 1.95 V and the maximum EQE of 17.3% under a low doping level of 3 wt%, which is due to barrier-free charge transfer and effective energy transfer. Meanwhile, white OLEDs with a low turn-on voltage of 2.2 V is realized based on doping red dye and inserting ultra-thin blue layer, the optimized device shows relatively stable spectra and low efficiency roll-off. These superior performances can be attributed to the balanced charge transfer of exciplex co-host and the effective energy transfer from exciplex co-host to dopant. Such results indicate a promising method for designing a simplified highperformance OLEDs.