Efficient donor–acceptor host materials for green organic light-emitting devices: non-doped blue-emissive materials with dual charge transport properties

Comparative optical, electroluminescence and theoretical studies were performed for (E)-4′-(1-(4-(2-(1-(4-morpholinophenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1′-biphenyl]-4-amine (SMPI-TPA) and (E)-4-(4-(2-(4-(2-(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)phenyl)morpholine (SMPI-Cz). These compounds show excellent thermal properties, dual charge transport properties and form thin films under thermal evaporation. Blue OLEDs (CIE: 0.16, 0.08) based on SMPI-TPA show efficient device performance (ηex 6.1%; ηc 5.3 cd A−1; ηp 5.2 lm W−1) at low turn-on voltages. Both SMPI-TPA and SMPI-Cz were utilised as hosts for green OLEDs. The devices with SMPI-Cz (30 nm):5 wt% Ir(ppy)3 exhibit maximum luminance of 20 725 cd m−2, and ηc and ηp values of 61.4 cd A−1 and 63.8 lm W−1, respectively. In comparison, devices with SMPI-TPA (30 nm):5 wt% Ir(ppy)3 exhibit high ηc and ηp values of 65.2 cd A−1 and 67.1 lm W−1, respectively. Maximum ηex values of 19.6% and 23.4% were obtained from SMPI-TPA:Ir(ppy)3 and SMPI-Cz:Ir(ppy)3, respectively. These device performances indicate that the phenanthroimidazole unit is a tunable building unit for efficient carrier injection and it may also be employed as a host for green OLEDs.


Introduction
Efficient green or red OLEDs with pure color CIE coordinates have been reported [1][2][3][4] and blue emitters with less power consumption in organic optoelectronics have also been broadly studied. 5, 6 However, there is need for long-lifetime blue emitters with pure colour CIE coordinates due to wide band gaps (E g ), which require limited p-conjugation length. 7-9 Simultaneous carrier injection into a blue-emissive layer becomes very difficult due to its wide E g , resulting in a decrease in device efficiency. 10,11 Therefore, for OLED applications, highly efficient and low-cost blue OLEDs are of urgent demand. The external quantum efficiency (h ex ) can be deduced from the following equation: where h IQE is the internal quantum efficiency, h out ($1/2n 2 ) is the light outcoupling efficiency (n ¼ 1.5, h out $ 20%), h rec is the electronhole recombination efficiency (100%), h PL is the photoluminescence efficiency and h S is the utilization efficiency. The two key parameters h PL and h S required for high h ex can be tuned by altering the emitter molecular design. 12 A blue-emissive material with high triplet energy (E T ) may be employed as a host for green OLEDs. [13][14][15][16] However, the high triplet energy enables green emitters to harvest the triplet energy of a blue emitter; also, efficient non-doped blue emitters are not suitable as hosts for phosphorescent OLEDs due to their low E T as well as poor carrier transport properties. 17 Moreover, efficient hosts for green emitter exhibit low efficiency when they are used as emissive materials in blue OLEDs. 18-20 Hence, efforts are still required to achieve efficient OLEDs based on blue emissive materials.
Therefore, to develop dual-functional emissive materials, i.e., emitters for blue OLEDs and hosts for green OLEDs, synthesis of molecules with donor (D)/acceptor (A) (electron/ hole transport moieties) conguration has gained interest. To achieve deep blue emission, the donor-acceptor (D-A) molecule should have relatively weak charge transfer properties since a strong D-A geometry can induce red-shied emission. In addition, the singlet-triplet splitting should be small to ensure that the triplet-excited state energy is high enough to excite the green phosphorescent dopant.

Materials and measurements
The structure of emissive materials was conrmed by 1 H and 13 C NMR spectroscopies and mass spectrometry, recorded using a Bruker spectrometer (400 MHz) and Agilent (LCMS VL SD), respectively. Redox potentials were measured using a potentiostat CHI 630A electrochemical analyzer. The Lambda 35 PerkinElmer instrument spectrophotometer with integrated sphere (RSA-PE-20) was used to measure absorbance in both solution and lm states. Emissive properties (PL) were analyzed via PerkinElmer LS55 uorescence spectrometer measurements. Thermal characteristics such as decomposition (T d ) and glass transition (T g ) temperatures were analyzed using a Perki-nElmer thermal analysis system and NETZSCH-DSC-204, respectively, with heating rate of 10 C min À1 under N 2 atmosphere. Lifetime measurements of SMPI-TPA and SMPI-Cz were recorded using a time-correlated single-photon counting spectrometer (TCSPC: Horiba Fluorocube-01-NL lifetime system). The F (PL quantum yield) was measured in dichloromethane using 0.5 M H 2 SO 4 :quinine (0.54) as reference.

Theoretical calculations
Using the Gaussian 09 soware package, 24 the electron density on frontier molecular orbitals of SMPI-TPA and SMPI-Cz was identied.

Results and discussion
As depicted in Scheme S1, † the blue emitters used as hosts for green OLEDs, namely, (E)- The formed SMPI-TPA and SMPI-Cz were analysed by various spectroscopic methods, from which their molecular structures were conrmed.

HOMO-LUMO
The HOMO of both SMPI-TPA and SMPI-Cz is localized on the fragment attached to phenanthrimidazole nitrogen, while the LUMO is distributed on the phenanthrimidazole with TPA (SMPI-TPA) and Cz (SMPI-Cz) fragments (Fig. 2). The HOMO and LUMO of SMPI-TPA and SMPI-Cz display adequate separation in electron density features, which enhances the holeand electron-transport functions and also reduces the singlettriplet splitting. 22 Moreover, SMPI-TPA and SMPI-Cz exhibit redox waves, supporting their carrier transport abilities. The HOMO energies of À5.41 eV (SMPI-TPA) and À5.39 eV (SMPI-Cz) are determined from their respective oxidation onset potentials of 0.61 V (SMPI-TPA) and 0.59 V (SMPI-Cz) [E HOMO ¼ À(E ox + 4.8 eV)]. 23 The LUMO energies of À2.15 eV (SMPI-TPA) and À2.17 eV (SMPI-Cz) are calculated using the equation E LUMO ¼ E HOMO À 1239/l onset . The charge-transporting properties of SMPI-TPA and SMPI-Cz were investigated by fabricating single-carrier devices. The hole-only device has the conguration of ITO/NPB (50 nm)/SMPI-TPA or SMPI-Cz (30 nm)/NPB (50 nm)/Al (200 nm) and the electron-only device has the conguration of ITO/TPBI (20 nm) SMPI-TPA or SMPI-Cz (30 nm)/ TPBI(20 nm)/LiF (1 nm)/Al (200 nm). NPB on the cathode side of the hole-only device and TPBI (1,3,5-tri(phenyl-2benzimidazolyl)benzene) on the anode side of the electrononly device were used to block electron injection from Al and hole injection from ITO, respectively. Fig. 3 shows that both devices can signicantly conduct current, indicating that SMPI-TPA and SMPI-Cz are capable of transporting both holes and electrons and they exhibit a bipolar transporting nature. This is benecial for balancing the holes and electrons in the emitting layer based on SMPI-TPA and SMPI-Cz.

Photophysical properties
Electronic spectral studies of SMPI-TPA and SMPI-Cz were carried out in dichloromethane (Fig. 3). The results indicated that l abs at around 248 nm originated from the aryl group attached to the nitrogen of phenanthrimidazole plane, while l abs at around 355 nm was attributed to p / p* electronic transition of the styryl phenanthrimidazole ring. The origin of absorption of SMPI-TPA and SMPI-Cz was studied by computational methods. The outcomes of computational and experimental methods are similar [HOMO-LUMO+1 transition; Table  S1 Fig. 4 and Table 1 that the asfabricated novel SMPI-TPA-and SMPI-Cz-based blue OLEDs exhibit maximum brightness at low voltage. The blue EL and PL spectra of SMPI-TPA and SMPI-Cz in the solid state are similar (Fig. 3), and the hole injection barrier between SMPI-TPA and HTL (hole transport layer) is very small. Thus, effective electronhole radiative recombination occurs in SMPI-TPA and SMPI-Cz layers. The small injection barrier of 0.28 (SMPI-TPA) and 0.32 eV (SMPI-Cz) for charge carriers accounts for the observed low turn-on voltages. The SMPI-TPA lm exhibits smooth surface (roughness of 0.24 nm) and remains unchanged even aer annealing (100 C: 10 h) (Fig. 4: inset). The value of h ex is calculated using the formula h ex ¼ h out Â h rc Â h g Â F PL , 23 where h out is the light out-coupling efficiency (20%), h rc is the product of the charge recombination efficiency (100%), h g is the efficiency of radiative exciton formation (25%) and F PL is the photoluminescence quantum yield of emitters SMPI-TPA (0.56) and SMPI-Cz (0.71). Thus, the calculated h ex values of SMPI-TPAand SMPI-Cz-based devices are 2.8 and 3.6%, respectively. However, the obtained external quantum efficiencies (h ex ) of SMPI-TPA-and SMPI-Cz-based devices are 6.1 and 5.9%, respectively, and the current efficiencies (h c ) of SMPI-TPA-and SMPI-Cz-based devices are 5.3 and 5.1 cd A À1 , respectively. The harvested h ex exceeds the theoretical limit since delayed uorescence is not expected and the obtained high h ex is not in accordance with TADF. [25][26][27][28][29][30] Time-resolved lifetime studies revealed monoexponential decay for SMPI-TPA and SMPI-Cz  layer (HTL) to be deposited in proximity to the SMPI-TPA and SMPI-Cz emissive layers. Due to the relatively small energy gap of NPB (E g ¼ À3.00 eV) in comparison to the energy gaps of the emissive layers SMPI-TPA (E g ¼ À3.26 eV) and SMPI-Cz (E g ¼ À3.22 eV), the excitons generated in the emissive layer are more likely to leak into HTL, leading to loss of excitons. However, herein, we used TCTA with E g of À3.40 eV as a buffer layer to conne the excitons within the emissive layer, resulting in higher efficiency. 33 Further, the EL spectra of the SMPI-TPA-and SMPI-Cz-based devices are identical to the corresponding PL emission of the thin lm, implying balanced carrier transport and efficient connement of excitons 33 (Fig. 3). In the present study, some factors account for the efficiency of the devices: (i) bipolar carrier transporting properties of SMPI-TPA and SMPI-Cz, which contribute to better balance of carrier transport and wider distribution of recombination region within the emission layer (Fig. 3) and (ii) suitable HOMO and LUMO energies of SMPI-TPA and SMPI-Cz (Fig. 5); the hole injection barrier at TCTA : EML interface is 0.42 eV (SMPI-TPA) : 0.44 eV (SMPI-Cz) and electron injection barrier at TPBI : EML interface is 0.55 eV (SMPI-TPA) : 0.53 eV (SMPI-Cz). This reveals that there is only a small barrier for carrier injection, leading to high exciton formation even under high current density, resulting in higher device performances. 32,33 The PL spectra of SMPI-TPA or SMPI-Cz were recorded in THF/water mixture with different water fractions (f w ) to understand whether these materials show aggregation-induced emission. When a small amount of water (f w ¼ 10-90 vol%) was added to the THF solution of both SMPI-TPA and SMPI-Cz, the PL intensity remained unchanged, which shows that both SMPI-TPA and SMPI-Cz materials are AIEinactive (Fig. S1 †).
The small singlet-triplet splitting and good carrier transport properties allow both SMPI-TPA and SMPI-Cz to be used as hosts to fabricate red, green and yellow phosphorescenceemitting layers for PhOLEDs. The calculated PLQY of Ir(ppy) 3 -doped SMPI-TPA and Ir(ppy) 3 -doped SMPI-Cz was 0.61 and 0.79, respectively. In addition to high h ex and h c , SMPI-TPA-and SMPI-Cz-based devices show high h p (power efficiency) of 5.2 and 4.6 lm W À1 , respectively. The blue emitters exhibit CIE coordinates of (0.16, 0.08 -SMPI-TPA) and (0.15, 0.07 -SMPI-Cz). The device based on SMPI-TPA shows maximum luminance of 12 680 cd m À2 , and maximum current and power efficiencies of 5.3 cd A À1 and 5.2 lm W À1 , respectively, at 3.1 V. These SMPI-TPA and SMPI- Cz compounds were also utilised as hosts for green dopants in the fabricated devices (Fig. 4) with a conguration of ITO/ NPB (50 nm)/SMPI-TPA (30 nm):5 wt% Ir(PPy) 3 or (III) or SMPI-Cz (30 nm):5 wt% Ir(PPy) 3 (IV)/BCP (2,9-dimethyl-4,7diphenyl-1,10-phenanthroline) (15 nm)/Alq 3 (50 nm)/LiF (1 nm)/Al (100 nm). The energy level diagram of the asfabricated device and molecular structures of materials used in the devices are shown in Fig. 5. Devices with SMPI-Cz (30 nm):5 wt% Ir(ppy) 3 exhibit maximum luminance of 20 725 cd m À2 , and h c and h p of 61.4 cd A À1 and 63.8 lm W À1 , respectively, whereas SMPI-TPA:Ir(ppy) 3 -based devices exhibit high h c and h p values of 65.2 cd A À1 and 67.1 lm W À1 , respectively. The maximum h ex values of 19.6% and 23.4% were exhibited by SMPI-TPA:Ir(ppy) 3 and SMPI-Cz:Ir(ppy) 3 , respectively. The lower efficiency from SMPI-TPA:Ir(ppy) 3based devices could be attributed to the lower triplet energy (E T ¼ 2.32 eV) of SMPI-TPA, which may cause back energy transfer from the guest triplet states to the host, resulting in loss of efficiency. The device performances reveal that SMPI-TPA and SMPI-Cz are universal hosts for green emitters. The EL spectra of devices III and IV were consistent with their corresponding PL spectra (Fig. 3), and no new peaks were observed under different operation voltages (2.8-10 V: Fig. S1 †), implying that no excitons were wasted for host emission and effective exothermic energy transfer occurs from the host to the dopant in the emissive layer, which results in higher efficiency. 34,35 In the present study, some factors account for the higher efficiency of the devices: (i) high triplet energies of 2.32 eV (SMPI-TPA) and 2.40 eV (SMPI-Cz) efficiently suppress the energy return from the dopant to the host, resulting in higher efficiency and (ii) good thermal stability restrains the strong bimolecular interaction of the phosphorescent emitters to reduce the triplet-triplet annihilation. 34 These results indicate that the introduction of a bipolar molecule is a practical strategy for achieving highly efficient OLEDs both as a blue emitter and as a host for green emission.

Conclusion
In this study, we have reported the newly fabricated efficient deep blue-emissive materials with balanced transport and high thermal properties. Non-doped blue emitters with SMPI-TPA and SMPI-Cz exhibit higher electroluminescent efficiencies. Devices with SMPI-TPA show maximum luminance of 12 680 cd m À2 and maximum current (h c ) and power (h p ) efficiencies of 5.3 cd A À1 and 5.2 lm W À1 , respectively, at 3.1 V. The device with conguration of SMPI-Cz (30 nm):5 wt% Ir(ppy) 3 exhibits maximum luminance of 20 725 cd m À2 , whereas the h c and h p exhibited by the SMPI-TPA:Ir(ppy) 3 device are 65.2 cd A À1 and 67.1 lm W À1 , respectively. The maximum h ex of 19.6% and 23.4% were exhibited by SMPI-TPA:Ir(ppy) 3 -and SMPI-Cz:Ir(ppy) 3 -based devices, respectively. The lower efficiency from SMPI-TPA:Ir(ppy) 3 -based devices could be attributed to the lower triplet energy (E T ¼ 2.32 eV) of SMPI-TPA, which may cause back energy transfer from the guest triplet states to the host, resulting in loss of efficiency.
The device performance indicates that introduction of a bipolar molecule is a practical strategy for achieving efficient OLEDs both as a blue emitter and as a host for green emission.

Conflicts of interest
There are no conicts to declare.