Accelerating reverse intersystem crossing (RISC) without sacrificing fast radiative decay would be effective in suppressing efficiency roll-off (eRO) in thermally activated delayed fluorescence (TADF)‐based organic light‐emitting diodes. We here report a TADF emitter, CC‐TXO‐I, combining a bicarbazole donor (CC) and a sulfur‐containing acceptor (TXO). The CC is used to accelerate radiative decay via moderate donor‐acceptor torsion angle, and the TXO is expected to provide fast RISC by the heavy atom effect of sulfur. We successfully realized very large rate constants for RISC (kRISC) of ~ 107 s− 1, and eROs were improved by increasing both kRISC and rate constants of radiative decays.
Organic light-emitting diodes (OLEDs) have several attractive characteristics such as high luminous efficiency, low power consumption, high color contrast, light weight, flexibility, and eco‐friendliness.1–5) To further improve the efficiency of noble‐metal‐free OLEDs, thermally activated delayed fluorescence (TADF) emitters have been actively investigated as new‐generation luminescent materials.6–8) Conventional fluorescent emitters can convert only singlet excitons into light, resulting in low internal quantum efficiency (IQE) of 25% at the maximum owing to spin statics.9) In contrast, TADF emitters can achieve IQE of 100% because they can convert not only singlet excitons but also triplet excitons into light via a reverse intersystem crossing (RISC) process from a triplet state (Tn, typically T1) to an excited singlet state (Sm, typically S1) (n and m are natural numbers with ≥ 1).6) Actually, some TADF emitters showed an IQE of ≈ 100%.10–12)
Nevertheless, TADF-based OLEDs have a serious problem of efficiency roll‐off (eRO) in the high luminance region.9,13) This is because triplet excitons generated on TADF emitters have relatively long lifetimes, which allow the quench processes such as triplet‐triplet annihilation (TTA) and singlet‐triplet annihilation (STA).14,15) Therefore, shortening an exciton lifetime can be an effective approach to reduce an exciton density in the emitting layer and then suppress eRO. A short exciton lifetime can be realized by fast RISC and fast radiative decay processes. When the transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) mainly contributes to the electronic configuration of S1, a large spatial overlap between the HOMO and LUMO leads to the large rate constant of radiative decay (kr).16,17) Besides, according to Fermi’s golden rule, a rate constant of RISC (kRISC) can be expressed as18)
$${k}_{RISC}\propto {\left|⟨S|{\widehat{H}}_{SOC}|T⟩\right|}^{2}exp\left(\frac{-{\varDelta E}_{ST}}{{k}_{B}T}\right)$$
1
where \(\left|⟨S|{\widehat{H}}_{SOC}|T⟩\right|\), ΔEST, kB, and T are the spin-orbit coupling matrix element values (SOCMEVs), the energy gap between S1 and T1, the Boltzmann constant, and temperature, respectively. This equation indicates that large SOCMEVs and small ΔEST are effective for fast RISC. Small ΔEST can be realized by a small spatial HOMO‐LUMO overlap and it is challenging to reconcile the large kRISC and large kr from the perspective of a HOMO‐LUMO overlap.19)
In this study, we designed a TADF emitter by combining a bicarbazoledonor and sulfur-containing acceptor, aiming to achieve an emitter with both large kRISC and large kr simultaneously. Since the heavy atom effect is expected to increase SOCMEVs, TADF emitters have been designed containing non‐metal heavy atoms, such as sulfur20–23), selenium24–26), and halogens27–29). Among them, thioxanthone (TXO)‐based emitters are especially noteworthy in terms of fast RISC.21,22) For instance, MCz‐TXO, reported by us,22) exhibited a very large kRISC of ~ 2×108 s− 1. MCz‐TXT, reported by Yasuda et al.21), similarly showed the large kRISC of 1.1×108 s− 1. Therefore, TXO was selected as the acceptor in this work to realize large kRISC.
Additionally, a moderate torsion angle between donor and acceptor is preferable to enlarge a HOMO-LUMO overlap and therefore kr.30) In one example, CCX‐I (Fig. S2), which was reported by us,31) containing 9H‐3,9'‐bicarbazole (CC) as a donor was determined to have a moderate torsion angle of 50° by the quantum chemical calculations because the carbazole unit contained in CC shows a relatively small steric hindrance. As a result, CCX‐I was calculated to have a sizable oscillator strength between S1 and ground state (S0), and experimentally exhibited the very large kr of 8.7×107 s− 1. On the basis of this report, we chose the CC as a donor.
On the basis of the above fact, a donor-acceptor type TADF emitter, 3‐(9H‐[3,9'‐bicarbazol]‐9‐yl)‐9H‐thioxanthen‐9‐one (CC‐TXO‐I) [Fig. 1(a)], was designed. Quantum chemical calculations based on density functional theory (DFT) and time‐dependent DFT with Tamm‐Dancoff approximation were performed using the Amsterdam Density Functional program package (ADF2019.302).32) With triple zeta basis set and PBE0 functional, the geometry optimization of the ground state, excitation energies, as well as the highest occupied natural transition orbital (HONTO) and lowest unoccupied natural transition orbital (LUNTO) of S1 and Tn (where n = 1, 2), together with SOCMEVs between S1 and Tn were calculated. The calculation results are shown in Fig. 1 and Table I. Table I also contains the results for CCX‐I, which has a similar chemical structure to CC‐TXO‐I but the acceptor contains oxygen, not sulfur. The S1 energy of CC‐TXO‐I was calculated to be 3.07 eV, which is similar to that of the blue emitter, CCX‐I (3.10 eV), predicting CC‐TXO‐I showed blue emission. As shown in Fig. 1(a), the torsion angle between CC and TXO is 60°. The HOMO and LUMO distributions are mostly spatially separated, but slightly overlapped as shown in Fig. 1(b). Besides, CC‐TXO‐I was calculated to possess relatively small ΔEST of 0.17 eV and the large oscillator strength of 0.1003, indicating that its torsion angle is suitable for the simultaneous realization of a large kRISC and large kr. Moreover, the oscillator strength of CC‐TXO‐I is close to that of CCX‐I (0.1050), evidencing that CC‐TXO‐I may show the large kr as we designed (kr of CCX‐I is 8.7×107 s− 1 in the doped film). Figure 1(d) shows natural transition orbital (NTO) results for CC‐TXO‐I. The S1 was attributed to a charger transfer (CT) characteristic, while T1 and T2 were assigned to hybridized local and charge transfer (HLCT) characteristics. In addition to this, large orbital coefficients were observed over the sulfur atom in HONTO of T1 and T2. As a result, notable SOCMEVs of 1.69, 2.01 cm− 1 were calculated for T1 → S1 and T2 → S1 transition, respectively (Table I). Compared to SOCMEVs of CCX‐I, the enhanced SOC of CC‐TXO‐I can be attributed to the heavy atom effect resulting from sulfur. A similar effect has been found in previous studies.24,25) From these calculations, CC‐TXO‐I are expected to exhibit blue emission with large kRISC and kr.
Prior to photophysical characterizations, CC-TXO‐I was synthesized and train sublimated (see detail in Supplementary Information). Figure 2 shows the fluorescence, phosphorescence, and UV‐vis absorption spectra for 1×10− 4 M CC‐TXO‐I in an oxygen‐free toluene. In the UV‐vis absorption spectrum, absorptions of CC and TXO are observed in the region of 300–350 nm33) and 350–380 nm34), respectively. Also, the absorption in the region around 400 nm could be assigned to intramolecular CT absorption. In addition, the HOMO‐LUMO gap of CC‐TXO‐I was determined to be 3.09 eV from the onset of the absorption spectra (the calculated value is 3.07 eV), resulting in blue emission with the emission peak maximum wavelength (λMAX) of 454 nm and CIE (x, y) = (0.17, 0.18) in fluorescence spectra. From the onset of both fluorescence and phosphorescence spectra, the experimental ΔEST was determined to be 0.19 eV, which agreed well with calculated ΔEST of 0.17 eV.
We prepared several CC-TXO‐I doped films with different host materials and conducted photophysical measurements. As shown in Table SI, we found that 2,8‐bis(diphenylphosphineoxide)dibenzofuran (PPF) was an appropriate host for CC‐TXO‐I in terms of the photoluminescence (PL) quantum yield (ΦPL). Table SII summarizes the doping concentration dependence of λMAX and ΦPLs for vacuum‐processed CC‐TXO‐I:PPF doped films. 6, 15, and 20 vol% CC‐TXO‐I:PPF exhibited similar ΦPL of 84, 83, and 84%, respectively, while the PL spectra were red‐shifted with increasing the concentration, probably due to the polarity of CC‐TXO‐I itself. Fig. S5(a) exhibits the transient PL decays of CC‐TXO‐I:PPF doped films at each doping concentration. All three concentrations of CC‐TXO‐I showed clear delayed emission of microsecond order, indicating TADF characteristics. The rate constants of CC‐TXO‐I:PPF doped film were determined with the equation reported in Ref. 34 and summarized in Table II. Owing to the heavy atom effect of sulfur in TXO, CC‐TXO‐I exhibited fast RISC with the kRISC of ~ 107 s− 1 as we designed, which is two orders of magnitude larger than that of the CCX‐I doped film (1.1×105 s− 1). In contrast, CC‐TXO‐I doped films showed moderate kr of ~ 106 s− 1. In comparison with the kr of the CCX‐I doped film at the same doping concentration (8.7×107 s− 1), that of CC‐TXO‐I was not large as expected from the calculation results, although the relatively high ΦPL of 84%. Additionally, we performed temperature‐dependent transient PL decay measurement for 6 vol% CC‐TXO‐I:PPF doped film. As shown in Fig. S6, the delayed component was increased with temperature increasing. This result confirmed that CC‐TXO‐I possessed TADF property.
Finally, we fabricated OLEDs using CC-TXO‐I‐doped films as emitting layers by vacuum deposition. The device structure is as follows: ITO (50 nm)/4,4ʹ‐cyclohexylidenebis[N,N‐bis(4‐methylphenyl)benzenamine] (TAPC) (60 nm)/1,3‐bis(9,9‐dimethylacridin‐10(9H)‐yl)benzene (mAP) (10 nm)/X vol% CC‐TXO‐I (X = 6, 15, 21) : PPF(40 nm)/PPF (10 nm)/1,3‐bis[3,5‐di(pyridin‐3‐yl)phenyl]benzene (BmPyPhB)35) (35 nm)/lithium quinolin‐8‐olate (Liq) (1 nm)/Al (80 nm) as shown in Fig. 3(a). TAPC and BmPyPhB were used as a hole transport layer and an electron transport layer, respectively. mAP and PPF layers were inserted to block electrons and holes, respectively. Figure 3(b), 3(c), and 3(d) show external quantum efficiency (EQE)‐luminance characteristics, electroluminescence (EL) spectra, and current density‐voltage‐luminance characteristics, respectively. The EL properties were summarized in Table III. CC‐TXO‐I‐based OLEDs showed blue to greenish blue emission with maximum EQE (EQEmaxs) of 19.0, 17.7, and 17.2% for 6, 15, and 21 vol%, respectively. Among them, 21 vol% device showed the smallest eRO; EQEs at 100 and 1000 cd m− 2 (EQE100 and EQE1000, respectively) were 15.3% and 13.1%; this means the EQE100 and EQE1000 dropped by 11.0% and 23.8% relative to its EQEmax, respectively. Although CC‐TXO‐I showed the very large kRISC of ~ 107 s− 1, eRO was still observed. The reason can be attributed to the relatively large delayed lifetime (\({\tau }_{d}\)) in the CC‐TXO‐I doped films, inducing TTA and STA in the high luminance region. In fact, the larger the \({\tau }_{d}\), the greater eRO (see Table II and Fig. 3; 6 vol% device exhibited the largest eRO). The moderate kr (~ 106 s− 1) and two orders of magnitude larger kISC (~ 109 s− 1) than kRISC are considered to be bottlenecks in achieving eRO‐free OLEDs.
In conclusion, we have developed a blue TADF emitter, CC-TXO‐I, by combining CC as a donor and TXO as an acceptor. TXO was selected to enhance RISC due to the heavy atom effect of sulfur and CC was selected to achieve a moderate HOMO‐LUMO overlap. The quantum chemical calculations indicated that CC‐TXO‐I had large SOCMEVs of 1.69 cm− 1 between S1 and T1, and 2.01 cm− 1 between S1 and T2. The doped films using CC‐TXO‐I as a dopant showed the TADF properties of the kRISC of ~ 107 s− 1, kr of ~ 106 s− 1, and the ΦPL of 84%. The EQEmax of 19.0%, and the maximum luminance of 17,200 cd m− 2 were obtained for CC‐TXO‐I‐based OLEDs at doping concentrations of 6 and 21 vol%, respectively. Both kRISC and kr increased with the doping concentration, resulting in improved eRO. This study suggests that accelerated RISC and radiative decay effectively realize OLEDs with reduced eROs.