Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Open AccessCCS ChemistryCOMMUNICATIONS2 Sep 2022Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes Xu-Lin Chen, Xiao-Dong Tao, Ya-Shu Wang, Zhuangzhuang Wei, Lingyi Meng, Dong-Hai Zhang, Fu-Lin Lin and Can-Zhong Lu Xu-Lin Chen State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 Fujian Science (b) frontier orbital distributions (left: DMAC-TPPPF6; right: DMAC-TPPPF6); (c) crystal structure (DMAC-TPPPF6). Download figure Download PowerPoint Results and Discussion The electronic properties of these compounds were investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the M06-2X/6-311G(d,p) level. As shown in Figure 1b, each compound shows spatially separated frontier molecular orbitals, with the HOMO predominantly distributed over the acridinyl unit and the LUMO mainly located on the remainder of the molecule. The TD-DFT calculations predict that the S1 and T1 states are characterized by HOMO–LUMO ICT transitions Supporting Information Figure S1, with S1–T1 energy difference (ΔEST) of 0.07 eV for DMAC-TPPPF6 and 0.01 eV for 2DMAC-TPPPF6, respectively. Owing to the symmetrical molecular structure, the excited states of 2DMAC-TPPPF6 appear in pairs degenerately (S1/S2, T1/T2, T3/T4, etc.). The calculated energy levels of excited states and spin–orbit coupling (SOC) constants are summarized in Supporting Information Table S1. For 2DMAC-TPPPF6, the total SOC between S1/S2 and T1/T2 states which have dominant CT exciton characters was calculated to be 0.145 cm−1, while the calculated SOC between S1/S2 and higher-lying triplet states, namely T3/T4 and T5/T6, reach up to 3.807 and 4.021 cm−1, respectively. The same situation exists in the case of DMAC-TPPPF6 with SOCS1–T1, SOCS1–T2, and SOCS1–T3 of 0.270, 1.946, and 1.171 cm−1, respectively. The first-order mixing coefficient between singlet and triplet states is proportional to their spin–orbit interaction and inversely proportional to the energy gap between them.2 Considering the large energy gaps (over 0.5 eV) between the higher-lying triplet states and S1 states, we speculate that the small ΔEST rather than the higher-lying triplet states play a key role in spin-flip conversion between the S1 and T1 states. These tetraarylphosphonium salts were synthesized in high yields via nickel-catalyzed coupling reactions52,54 between triarylphosphines and aryl bromides and the subsequent anion-exchange reactions (see Supporting Information for details). These compounds are quite air stable in the solid state as well as in solution. The powders survive in ambient conditions without decomposition over at least several months. As depicted in Supporting Information Figure S2, DMAC-TPPPF6 and 2DMAC-TPPPF6 show excellent thermal stability with high decomposition temperatures (Td, 5% weight loss) of 399 and 394 °C, respectively. Atomic-force microscopy (AFM) images ( Supporting Information Figure S3) reveal that the spin-coated doped films of these materials (30 wt % in PYD2, the same as the OLED-emitting layers) show fairly smooth surfaces with small root-mean-square surface roughness (Rq) values of 0.246 and 0.256 nm for DMAC-TPPPF6 and 2DMAC-TPPPF6, respectively. The excellent thermal stability and high-quality film morphologies support EL device fabrication via solution processes. The crystal structure of DMAC-TPPPF6 (Figure 1c) reveals that the phosphorus atom (sp3-hybridization) adopts tetrahedral geometry with tetrahedral angles of approximately 109°. The cationic moiety exhibits nearly perpendicular D–A linkage with a dihedral angle of 81.4°, which can result in spatially well-separated frontier molecular orbitals and small ΔEST. Moreover, there exist significant intramolecular (Figure 1c) and intermolecular interactions ( Supporting Information Figure S4) in the lattice, which are expected not only to suppress nonradiative deactivation by rigidifying molecular conformation but also to facilitate the formation of high-quality thin films.55,56 The photophysical properties of these emitters were investigated in dichloromethane and 30 wt %-doped polymethyl methacrylate (PMMA) films. Both compounds exhibit similar absorption profiles, composed of two types of absorption bands (Figure 2a). The intense absorptions below 370 nm are assigned to the π–π* transition originating from the donor moieties while the much weaker absorption bands between 370 and 460 nm are attributed to the ICT transitions from the DMAC donor(s) to the tetraphenylphosphonium acceptor. From the onset of absorption spectra ( Supporting Information Figure S5), optical bandgaps (Eg) were calculated to be 2.76 and 2.74 eV for DMAC-TPPPF6 and 2DMAC-TPPPF6, respectively. The increased number of donors results in slightly red-shifted absorption bands. These compounds exhibit strong yellow emission (λmax = 563 and 567 nm, respectively) in degassed dichloromethane at room temperature. The broad and structureless PL spectra as well as their solvent-polarity-dependent behaviors ( Supporting Information Figure S14 and Table S4) confirm CT characteristics of the emissive states. The transient PL decay curves of the investigated emitters in dichloromethane before and after Ar bubbling were compared to verify the involvement of triplet states in the light-emitting process (Figure 2b). A conspicuous delayed decay with significantly increased intensity was observed after 15 min of Ar bubbling to remove dissolving oxygen which can quench the triplet excited states of emitters. This behavior clearly confirms the contribution of triplet states to the fluorescence processes. Remarkably, the delayed decay components of DMAC-TPPPF6 and 2DMAC-TPPPF6 in degassed dichloromethane were fitted with ultrashort single-exponential lifetimes of 600 and 549 ns ( Supporting Information Figures S9 and S10), respectively. Figure 2 | (a) Absorption and PL spectra measured in dichloromethane (c = 2 × 10−5 M) at room temperature; (b) transient PL decay curves in dichloromethane (c = 2 × 10−5 M) before/after Ar bubbling for 15 min at room temperature; (c) transient PL decay curves of 2DMAC-TPPPF6 in 30 wt %-doped PMMA film at different temperatures; (d) time-resolved PL spectra in the 30 wt %-doped PMMA films at 77 K. Fluo.: fluorescence; Phos.: phosphorescence. The PL measurements were excited at 335 nm. Download figure Download PowerPoint The 30 wt %-doped PMMA films of DMAC-TPPPF6 and 2DMAC-TPPPF6 display bluish-green PL with emission maxima of 511 and 514 nm ( Supporting Information Figure S11 and Table 1) and PLQYs of 0.75 and 0.91, respectively. Compared with those recorded in dilute solution, for example, in dichloromethane, ethanol, and acetonitrile ( Supporting Information Figure S14 and Table S4), the PL spectra in 30 wt %-doped PMMA films significantly blue-shift. This behavior probably originates from two contributions. First, the solvation effect on the CT-excited states is considerably weakened in the doped nonpolar polymer films. Second, in the more rigid environment of the polymer films, intramolecular rotations and excited-state distortions are effectively restricted, thereby decreasing the vertical transition energies from the emissive ICT states (S1) to ground state (S0), resulting in significantly blue-shifted PL spectrum maxima. As shown in Figure 2c and Supporting Information Figure S12, temperature dependence of transient PL decay reveals that the intensity ratio of delayed fluorescence to prompt fluorescence increased when temperature was increased from 77 to 300 K. At ambient temperature (300 K), the observed emission originates from the S1 state, which is significantly populated via thermally-activated upconversion from the energetically lower-lying T1 state, demonstrating the TADF process. From onsets of the time-resolved PL spectra (fluorescence and phosphorescence spectra) taken at 77 K (Figure 2d and Supporting Information Figure S6), S1 and T1 energies and ΔEST are estimated to be 2.88, 2.82, and 0.06 eV for DMAC-TPPPF6 and 2.86, 2.81, and 0.05 eV for 2DMAC-TPPPF6, respectively. Such small ΔEST values would facilitate rapid upconversion of excitons from T1 to S1 through RISC process.57 At 300 K, the prompt fluorescence lifetimes (τPF) and delayed fluorescence lifetimes (τDF) were measured to be 16.6 ns and 1.67 μs for DMAC-TPPPF6, and 18.2 ns and 1.43 μs for 2DMAC-TPPPF6, respectively (see Supporting Information for detailed decay-curve fitting). The emission lifetimes are as short as those of representative phosphorescence Ir(III) complexes.58,59 Based on the PLQY and lifetime data, rate constants of the key photophysical processes were estimated using a previously reported method derived by Tsuchiya et al.60 (see Supporting Information for details). The radiative decay rate constants ( k r S ) from S1 to S0 are nearly the same and exceed 1.7 × 107 s−1 for both compounds, which are much higher than corresponding nonradiative rate constants ( k nr S ) (5.80 × 106 and 1.69 × 106 s−1 for DMAC-TPPPF6 and 2DMAC-TPPPF6, respectively). The ISC process of each compound (kISC = 2.75 × 107 s−1 for DMAC-TPPPF6 and kISC = 3.51 × 107 s−1 for 2DMAC-TPPPF6) is much faster than its competitive processes, namely the radiative and nonradiative transition of the S1 state, implying that the initially generated singlet excitons in these compounds are significantly transformed to triplet excitons. Notably, the rate constants of RISC (kRISC) of DMAC-TPPPF6 and 2DMAC-TPPPF6 reach up to 1.55 × 106 and 2.05 × 106 s−1, respectively. Fast radiative transition together with fast RISC of these emitters result in efficient utilization of excitons and short exciton lifetimes.61 Table 1 | Photophysical Data of the Invesigated Compounds in 30 wt %-Doped PMMA Films at 300 K Compound λPLa (nm) ΦPLb ΦPF/ΦDFc τPF/τDFd (ns/μs) ES1/ET1/ΔESTe (eV) k r S / k nr S f (106 s−1) kISC/kRISCg (106 s−1) DMAC-TPPPF6 511 0.75 0.29/0.46 16.6/1.67 2.88/2.82/0.06 17.5/5.80 27.5/1.55 2DMAC-TPPPF6 514 0.91 0.31/0.60 18.2/1.43 2.86/2.81/0.05 17.0/1.69 35.1/2.05 aThe wavelength at PL maximum (excited at 335 nm). bOverall PLQY. cΦPF and ΦDF are the quantum yields of prompt fluorescence and delayed fluorescence, respectively. dτPF and τDF are the lifetimes of prompt fluorescence and delayed fluorescence, respectively. eEnergy levels of S1 and T1 state were estimated from the onsets of time-resolved PL spectra at 77 K ( Supporting Information Figure S6). f k r S and k nr S represent the radiative and nonradiative rate constants of S1 states, respectively. gkISC and kRISC refer to the rate constants of intersystem crossing (ISC) and reverse ISC, respectively. The film thickness of samples was 100 nm. To evaluate EL properties of these emitters, partially solution-processed OLEDs were fabricated with a device structure of indium tin oxide (ITO)│poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (30 nm)│poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl) diphenylamine) (TFB) (10 nm)│PYD2: emitter (7:3, 40 nm)/m4PO (8 nm)│TPBi (45 nm)│LiF (1 nm)│Al (100 nm) (Figure 3a), where PEDOT:PSS, TFB, m4PO,62 TPBi, and LiF, act as the hole-injection, hole-transporting, hole-blocking, electron-transporting, and electron-injection layers, respectively ( Supporting Information Figure S16). PYD263 was selected as a host material due to its high triplet energy (T1 = 2.93 eV), which is conducive to confining triplet excitons of the investigated greenish-blue emitters (T1 = 2.81–2.82 eV), and due to its proper HOMO and LUMO energies (EHOMO/ELUMO = −5.7 eV/−2.3 eV) which fit the corresponding values of the emitters and adjacent layers for effective carrier transporting. The host–guest ratio was optimized ( Supporting Information Table S7 and Figures S17–S18). The results show that 30 wt % doped devices achieved the best device performance with efficient host to guest energy transfer. The HOMO and LUMO levels of DMAC-TPPPF6 and 2DMAC-TPPPF6 were estimated from the oxidation potentials and optical bandgaps ( Supporting Information Figures S15 and Tables S6). EL performances are shown in Figure 3b–d and Table 2. Figure 3 | (a) Energy-level diagram of the OLEDs; (b) current density–voltage–luminance characteristics; (c) EL spectra at various voltages; (d) EQE, current efficiency (CE) and power efficiency (PE) versus luminance characteristics. Download figure Download PowerPoint Table 2 | Summary of Device Performances Device (30 wt %-Doped) λELa (nm) Vonb (V) Lmaxc (cd/m2) EQEd (%) CEe (cd/A) PEf (lm/W) CIE1931g (x, y) DMAC-TPPPF6 508 5.0 9496 15.3/13.6/9.9 42.7/37.9/27.6 19.2/10.6/6.0 (0.25, 0.47) 2DMAC-TPPPF6 512 4.7 14532 18.3/17.0/13.2 53.4/49.5/38.5 26.0/15.2/9.3 (0.27, 0.49) aThe wavelength at EL maximum (recorded at 12 V). bTurn-on voltage at 1 cd/m2. cMaximum luminance. dEQE maximum value, value at 1000 cd/m2 and value at 5000 cd/m2. eCE maximum value, value at 1000 cd/m2, and value at 5000 cd/m2. fPE maximum value, value at 1000 cd/m2, and value at 5000 cd/m2. gCIE coordinates measured at 12 V. The 30 wt %-doped OLEDs employing DMAC-TPPPF6 and 2DMAC-TPPPF6 turned on at approximately 4.7 and 5.0 V, and exhibited bluish-green EL with emission maxima of 508 nm Commission Internatinale de L'Eclairage (CIE) = 0.25, 0.47 and 512 nm (CIE = 0.27, 0.49), respectively. It is particularly worth mentioning that these devices exhibited perfect emission-color stability over a wide range of operating voltages (Figure 3c). The DMAC-TPPPF6- and 2DMAC-TPPPF6-based devices showed maximum EQEs of 15.3% and 18.3% and peak luminances of 9496 and 14532 cd/m2, respectively. The DMAC-TPPPF6-based device exhibits efficiency roll-offs of 11.1% (EQE = 13.6%) and 35.3% (EQE = 9.9%) at luminances of 1000 and 5000 cd/m2, respectively. Notably, the 2DMAC-TPPPF6-based device reached the maximum EQE at a luminance of 117 cd/m2 and showed low efficiency roll-offs of 7.1% (EQE = 17.0%) and 27.9% (EQE = 13.2%) at practical high luminances of 1000 and 5000 cd/m2, respectively. These device efficiencies and efficiency roll-offs represent the best device performance of ionic-TADF-emitter-based OLEDs hitherto and are comparable with those state-of-the-art partially solution-processed OLEDs based on neutral TADF emitters ( Supporting Information Table S8). The small efficiency roll-offs obtained at high luminances can mainly be attributed to the short-lived TADF emitters, which can alleviate the exciton annihilation in the emitting layers. Conclusion Tetraphenylphosphonium cation has been used as the electron acceptor to construct highly efficient ionic TADF emitters with D–A+ and D–A+–D architectures. These ionic TADF emitters, namely DMAC-TPPPF6 and 2DMAC-TPPPF6, show high PLQYs of 0.75 and 0.91 and short decay fluorescence lifetimes of 1.67 and 1.43 μs in doped films, respectively. High EL performance has been achieved in partially solution-processed OLEDs. 2DMAC-TPPPF6-based device realized EQEmax of 18.3% and peak luminance of 14532 cd/m2. More importantly, the EQE values remain high with only tiny efficiency roll-off even at practical high luminances. Our results suggest that cationic acceptors are a promising choice for the design of high-performance TADF materials and that this opens an for ionic TADF materials. Supporting Information Supporting Information is and detailed and photophysical properties, of rate (see the Data at device fabrication and device performance and of is of to Information This was as a result of a from the Key Research of the Chinese Academy of the Science of the Science of Fujian the Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of and the Innovation of Xiamen and and the Research of Xiamen of a State and for Organic 2. 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