Peng
Zuo
ab,
Yang-Kun
Qu
ab,
Qi
Zheng
ab,
Liang-Sheng
Liao
*abc and
Zuo-Quan
Jiang
*ab
aInstitute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, 215123, Jiangsu, P. R. China. E-mail: zqjiang@suda.edu.cn
bJiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, Jiangsu, P. R. China. E-mail: lsliao@suda.edu.cn
cMacao Institute of Materials Science and Engineering, Macau University of Science and Technology, Taipa 999078, Macau SAR, China
First published on 15th February 2023
Sensitized organic light-emitting diodes (OLEDs) have received extensive attention in recent years owing to their exciton utilization improvement. Along with the advances in fluorescent, phosphorescent, and thermally activated delayed fluorescent (TADF) OLEDs, a series of highly efficient sensitizers have been designed or selected to match them. Sensitized OLEDs realize dual improvements in efficiency and lifetime with the combination of high exciton utilization sensitizers and high-performance emitters. In this review, we systematically summarize the mechanisms of sensitized OLEDs, including phosphorescent, TADF, and exciplex sensitizers. On the basis of this classification, we further introduce the functions and selection criteria of sensitizers for phosphorescent, TADF, and exciplex-sensitized OLEDs. Finally, we hope that such a review will help researchers to design highly efficient and long lifetime sensitized OLEDs.
Besides light-emitting materials engineering, introducing high exciton utilization materials such as phosphorescent, TADF, and exciplex materials as sensitizers into conventional emitters is an additional method to harvest triplet excitons (Fig. 1). In the sensitized OLEDs, the sensitizers do not emit light but transfer energy to the emitters for radiative decay. The initial application of sensitizers in OLEDs can be traced back to 2000.13 Forrest et al. overcame the theoretical limitations of FOLEDs by doping both phosphorescent and fluorescent materials into the host. After the breakthrough concept of TADF reported by Adachi et al. in 2012, TADF materials have also been exploited as suitable sensitizers. In 2014, Adachi et al. proposed a double doping strategy using a wide-energy-gap host, a TADF-assistant dopant, and a fluorescence emitting dopant and observed hyperfluorescence (HF).14 Almost simultaneously, Duan et al. designed materials with small ΔEST as sensitizing hosts for conventional fluorescent dopants, and reported the TADF-sensitized fluorescence (TSF) mechanism.15 In the following year, the first OLED employing a conventional fluorescent dopant sensitized with a TADF exciplex host was reported by Lee et al.16 The sensitized fluorescent OLEDs doped with phosphorescent sensitizers, TADF assisted dopants/sensitizing hosts, and exciplex hosts broke the theoretical efficiency limits. In addition to being used as sensitizers for fluorescent materials, these high exciton utilization materials can also be used as sensitizers for phosphorescent materials. In 2013, Duan et al. first reported on using the TADF material as the host for phosphorescent materials, and fabricated TADF-sensitized phosphorescent (TSP) OLEDs.17 The TSP OLEDs can simultaneously realize high efficiency, low roll-off, and long lifetimes. Quite recently, the MR-TADF materials, with a high RISC process, high PLQY, and low ΔEST values, have also been considered ideal sensitizers for sensitized devices.18 TADF-sensitized MR-TADF OLEDs achieve higher efficiency, longer lifetimes, and higher color purity than conventional TADF OLEDs.
Herein, this review describes the mechanisms of phosphorescent, TADF, and exciplex sensitized OLEDs and provides details of the different functions of sensitizers in the corresponding OLEDs, including fluorescent, phosphorescent, and MR-TADF OLEDs. This review also discusses the prospects of the sensitized OLEDs.
Apart from the energy transfer, the formation of excitons on the emitter is also enabled by directing electron–hole recombination, which can be avoided by regulating the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) energy levels or by minimizing the doping concentration of the emitter.21–23 In this review, it is assumed that excitons are formed in the host and the sensitizer.
In the sensitized system, following the injection of electrons and holes from the cathode and anode, respectively, the excitons are classified into singlet and triplet excitons that are formed at a ratio of 25:
75. The initial S1 and T1 excitons of the host are transferred to phosphorescent sensitizers via the FET and DET processes, respectively. The S1 excitons will transfer to T1via an intersystem crossing (ISC) process in phosphorescent sensitizers. The T1 excitons in sensitizers are transferred to the S1 of the emitter by the FET process. The most critical step in the cascade energy transfer process is the transfer of energy from triplet excitons of sensitizers to the singlet excitons of the emitter via a FET process rather than a DET process. However, the FET rate (kFET) between the triplet and the singlet is very slow relative to the transfer between the singlet and the singlet because of the relatively low radiative rate (kr) in the phosphorescent sensitizer. As a result, owing to the high ISC rate (kISC), plenty of excitons will accumulate in the T1 of the phosphorescent material. Fortunately, the long decay time for the excited phosphorescent triplet state and the PLQY can compensate for the slow kFET.24,25
In a TADF-sensitized system, after excitons formed on the host and transferred to the sensitizer, a part of the singlet excitons can be converted to triplets on the sensitizer by a fast ISC process, followed by the triplet excitons reverting to the S1 by a relatively slow RISC process. Subsequently, the emitter can harvest the singlet excitons on the TADF sensitizer via the FET process. The energy transfer and the radiation decay process of excitons in the fluorescent and TADF emitters are similar to those of the phosphorescence-sensitized system. However, the DET process between the sensitizer and the phosphorescent emitter dominates in the sensitized OLEDs thanks to the nature of phosphorescence emission from the radiative decay of triplet excitons. The RISC process is the key to the energy transfer process in the TADF-sensitized systems. The kRISC of TADF materials is usually in the range of 105–106 s−1, which is slower than the kISC (107–1011 s−1). Therefore, even though the kFET (107–1011 s−1) of the systems with a TADF sensitizer is faster than that of the phosphorescence-sensitized systems, triplet excitons cannot be utilized in time on account of the slow RISC process.25
Exciplex acts as a sensitizer by a mechanism similar to the TADF sensitizer. The T1 exciton can be transferred to S1 through the RISC process in the exciplex sensitizer. The singlet excitons can be harvested through the FET of singlets between the exciplex and the emitter. Both exciplexes and TADF sensitizers focus on fast up-conversion processes. However, the up-conversion efficiency of exciplexes is lower than that of TADF materials. The exciplex can also be the co-host of the sensitized systems, whether TADF-sensitized or phosphorescence-sensitized. The exciplex:sensitizer:emitter system has an additional RISC process compared to the host:sensitizer:emitter system, facilitating the FET while suppressing the DET and avoiding charge recombination on the emitter.
In conclusion, some commonalities in the mechanisms of the sensitized OLEDs with different sensitizers can be found as follows: (i) the absorption spectrum of the emitter should have a suitable spectral overlap with the PL spectrum of the sensitizer to ensure the efficiency of the FET, (ii) the HOMO/LUMO levels between the sensitizer and the emitter should be matched or the doping concentration of the emitter should be reduced to avoid direct exciton formation by charge trapping on the emitter, and (iii) good thermal and morphological stability is necessary to extend the device lifetime. Equally important, the difference between the sensitizer and the emitter leads to a different characterization of the energy transfer process: (i) the DET process has different magnitudes of influence on the efficiency of the sensitized OLEDs due to the consideration of triplet excitons, (ii) the exciton transfer paths are different in different sensitized systems, and (iii) the rate-determining steps in the energy transfer process are different. In a phosphorescence-sensitized system, the slow kFET limits the utilization of triplet excitons. In a TADF/exciplex-sensitized device, the kRISC impedes high-efficiency energy transfer.
Host | Phosphorescent sensitizer | Fluorescent emitter | EQEmax/1000a (%) | PEmax/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PE max/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd −2. | ||||||
mCBP: TSPO1 | (dfpysipy)2Ir(mpic) | TBPe | 15.3/5.6 | 15.7/2.4 | 18.0/7.7 | 26 |
28CzDBF | Ir(cb)3 | TBPDP | 11.6/— | —/— | —/— | 22 |
TCTA: B4PyMPM | Ir(ppy)2tmd | DCJTB | 23.7/— | —/— | —/— | 28 |
TCTA: B4PYMPM | Ir(ppy)3 | TBRb | 25.0/24.8 | 100.4/82.2 | —/— | 29 |
TCTA: B4PYMPM | Ir(ppy)2tmd | TBRb | 26.1/26.0 | 114.3/91.9 | —/— | |
DIC-TRZ | Ir(mppy)3 | C545T | 25.1/24.4 | 95.0/— | 84.8/82.6 | 30 |
PCTF | Ir(ppy)2acac | TBRb | 24.2/— | 50.2/— | —/— | 31 |
In blue phosphorescence-sensitized fluorescent (PSF) OLEDs, the efficient phosphorescent sensitizer should emit light in the absorption spectrum of the emitter to ensure sufficient energy transfer. However, the design difficulties of deep-blue phosphorescent materials limit the development of blue PSF OLEDs. Besides, high exciton energy typically results in device instability. Several recent studies have addressed these obstacles. Kim et al. reported a highly efficient blue OLED constructed with a phosphorescent sensitizer and a conventional fluorescent emitter (TBPe).28 Even though the wavelength difference between the phosphorescence and fluorescence emissions was only 10 nm, the fluorescent dopant still predominated the emission. The EQE of the sensitized device was 15.3%, which is 2.2 times that of the non-sensitized device (7%).
Also, Robert Lovrincic et al. cleverly proposed a dual-emitting system based on the unicolored phosphorescence-sensitized fluorescence (UPSF) to achieve stable blue OLEDs.24,29 The sensitizer in the UPSF system has a similar emission spectrum to that of the emitter. The sky-blue emission color of the sensitizer was maintained by modulating the emission of the sensitizer and the emitter in the UPSF OLEDs. The EQE of the device at 1000 cd m−2 was 11.6%. In addition, to better comprehend the potential of the UPSF system, a multiscale model of the UPSF OLED was developed to demonstrate that neither the PLQY nor the radiation decay time can be improved by adding more acceptors. Further examination of the ideal UPSF system revealed that the suppression of Dexter transfer could result in high efficiency and a long lifetime. These studies demonstrate that when phosphorescent materials are used as sensitizers for blue fluorescent materials, it is possible to match the emission of the phosphorescent material with that of the fluorescent material and to adjust the FET and DET processes to design a suitable sensitized device without having to design a UV-emitting phosphorescent sensitizer, which significantly simplifies material design.
Increasing the additional exciton transfer pathway, also known as the multiple-sensitized process, is one method for further efficiency enhancement. Using a host with TADF properties in conjunction with phosphorescent materials to co-sensitize the dopant increases the additional transfer pathway of excitons. The maximum efficiency of PSF OLEDs with different emission wavelengths has been attained through a multiple-sensitized process.
In the red PSF OLEDs, Kim et al. reported a FOLED using an iridium (Ir) complex as a sensitizer in an exciplex co-host system.30 In contrast to conventional FOLEDs based on an exciplex co-host, when the Ir complex was co-doped, not only the triplet excitons can be converted into singlet excitons, but they can also be transferred to a triplet phosphorescent sensitizer. As a result, the singlet portion of the excitons in the fluorescent dopant increases. The FET process and sensitizer radiation decay rates were estimated and discovered that more than 80% of excitons were transferred to DCJDCB. The EQE of the device increased from 10.6% to 23.7%. This method had also been utilized in the development of yellow FOLEDs.31 Due to the high PLQY, the EQE of devices employing Ir(ppy)3 and Ir(ppy)2tmd as sensitizers reached 26.1% and 25%, respectively. The EQE of these works was the highest for the yellow and red PSF OLEDs.
Using DIC-TRZ as the TADF host, Ir(mppy)3 as the phosphorescent sensitizer, and C545T as the emitter, Wang et al. manufactured efficient green PSF OLEDs in 2021.32 The results demonstrated that the sensitized system, in conjunction with the TADF host and the phosphorescent sensitizer, was conducive to triplet excitons, thereby increasing the efficiency. Moreover, the increased HOMO energy inhibited the direct carrier capture of the dopant. The optimal C545T-based device achieved an EQE of 25.1%.
Recently, Duan et al. adopted this ternary system and dubbed this concept phosphor-assisted TADF-sensitized fluorescence.33 Unlike the widespread research into TADF materials in pursuit of a high RISC process, this work suggested that the ISC process also played a significant role in the energy transfer process, competing with the FET and influencing exciton utilization. Due to the unidirectional sensitization from phosphorescence (Ir(ppy)2acac) to fluorescence (TBRb) and the host of anti-aggregation-caused quenching characteristics (PCTF), the photoluminescence decay curve of TPSF had a nanosecond lifetime and no noticeable long delay tail. The maximum EQE of 24.2% was achieved for PSF OLEDs based on the TBRb emitter. Unbelievably, the device also exhibited a very low-efficiency roll-off, with an L90 (the luminance at which EQE falls to 90% of its maximum value) exceeding 190500 cd m−2.
Host | Phosphorescent sensitizer | Fluorescent emitter | EQE max/1000a (%) | PE max/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PE max/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd m−2. | ||||||
mCBP | Ir(cb)3 | t-DABNA | 24.8/18.4 | 20.2/8.5 | 22.6/16.7 | 36 |
DPEPO | m-tz1 | t-DABNA | 16.7/— | 14.8/— | 20.0/— | 37 |
DPEPO | m-tz2 | t-DABNA | 19.7/— | 17.5/— | 20.0/— | |
mCBP | f-tpb1 | t-DABNA | 29.6/— | 28.7/— | 19.6/— | 38 |
SiCzCz: SiTrzCz2 | PtON-TBBI | t-DABNA | 23.7/— | —/— | 26.2/— | 39 |
SiCzCz: SiTrzCz2 | PtON-TBBI | TBE01 | 25.4/— | —/— | 27.1/— | |
SiCzCz: SiTrzCz2 | PtON-TBBI | TBE02 | 25.8/— | —/— | 27.0/— | |
mCBP: SiCz2Trz | CN-Ir | ν-DABNA | 27.3/23.3 | —/— | 27.7/— | 40 |
oCBP: mCBP-4CN | PtON7-dtb | ν-DABNA | 32.2/25.4 | —/— | 32.0/25.1 | 41 |
In 2019, Lee et al. first synthesized t-DABNA with tert-butyl groups around the resonance core structures of B and N to avoid strong intermolecular interactions.35 The deep-blue TADF device emitted at 464 nm with a full width at half maximum (FWHM) of 27 nm achieved a maximum EQE of 22.8%. Due to the long-delayed fluorescence lifetime, the t-DABNA device suffered a significant efficiency roll-off. The group thus constructed deep-blue phosphorescence-sensitized MR-TADF OLEDs.36 Several blue phosphorescent materials were evaluated as potential sensitizers. Ir(cb)3 was the most favorable since its PL spectrum overlaps mostly with t-DABNA's absorption. The corresponding sensitized device achieved an EQE of 24.8% with the CIE coordinates (0.131, 0.107). The LT50 (time to 50% of the initial luminance) of the device was 293 h at 200 cd m−2, 17 times greater than the device without the sensitizer.
To further increase the EQE, Chi et al. designed a series of Ir compounds to sensitize t-DABNA in 2021.37,38 They proposed a series of homogeneous Ir(III) complexes with three 2-phenyl-1,2,4-triazole-3-ylidene cyclometalates. The deep-blue-emitting Ir(III) complexes (m-tz1 and m-tz2) were suitable candidates to sensitize the emitter as their emission peaks were located at 400 and 420 nm, respectively. Sensitized devices based on t-DABNA exhibited high EQE values of 16.7% and 19.7%, respectively, which were higher than devices without the sensitizer (14.8% for DPEPO: t-DABNA). Nonetheless, the EQE was not prominent in the sensitized device employing t-DABNA as the TADF terminal emitter due to the poor emission efficiency of the sensitizer and the unsuitable device structure. By increasing the π-conjugation of carbinol chelates, the group has recently synthesized multiple similar molecules (m-tpb1, m-tpb2, f-tpb1, and f-tpb2). These molecules exhibited superior photophysical properties compared to m-tz1 and m-tz2. The device employing f-tpb1 as a sensitizer achieved an exceptional EQE of 29.6%, the most remarkable performance of t-DABNA-based sensitized devices in recent years. It is a pity that neither of these articles mentions the relevant device's lifetime. A comparison of the two sensitized devices shows that a large PLQY value is a critical element in improving efficiency.
The sensitized t-DABNA devices achieved improvement in the EQE, but they still had a lifetime shortcoming. Chu et al. designed two new MR-TADF materials, TBE01, and TBE02, by adding a carbazole group to the periphery of t-DABNA.39 The materials exhibited a high molar extinction coefficient, a fast RISC rate, and weak hole-trapping characteristics. PtON-TBBI exhibiting suitable photocharacteristics and excellent stability was used as a phosphorescent sensitizer. SiCzCz:SiTrzCz2 showing a low ΔEST and a short exciton lifetime was selected as the host. In addition, the high molar extinction coefficient of MR-TADF also contributes to the FET process. The corresponding devices exhibited high EQEs of 25.4% and 25.8%, respectively. More importantly, the device's lifetime was improved by 6.6 times compared to the previously reported device. The top-emitting device shows state-of-the-art device performances with a lifetime of 681 h at 1260 cd m−2, a CCE of 441 cd A−1 CIEy−1and a CIEy value of 0.058.
v-DABNA was a molecule synthesized by Hatakeyama et al. in 2019 based on the DABNA molecule.11 The electroluminescence spectra of v-DABNA devices demonstrated deep blue emission with a peak maximum of 469 nm, an FWHM of 18 nm, and an EQE as high as 34.4%. Due to the small FWHM, the color coordinate of the device was (0.12, 0.11). However, the devices also experienced a severe efficiency roll-off. By doping the sensitizer, the sensitized v-DABAN devices also achieved high efficiency, narrow-band emission, and low roll-off. In addition, the EQE and lifetime of the sensitized v-DABAN devices were more remarkable than those of the t-DABNA-based devices.
In 2021, Lee et al. created energy exchange phosphorescent and TADF (EPHTADF) devices.40 The key to the EPHTADF mechanism is the simultaneous forward and backward energy transfer between the phosphorescence sensitizer and the TADF emitter. The extra pathways to the singlet of the TADF are created by giving the phosphorescent sensitizer the same triplet energy as the emitter's singlet energy. The process reduces the triplet exciton lifetime in the emitter. CN–Ir, showing a short phosphorescence lifetime, high EQE, and device stability, was selected as a sensitizer for v-DABNA. The EQE of the EPHTADF device was 27.3%. The LT50 of the EPHTADF-OLED was 121 h (1000 cd m−2), over five times longer than the device without a sensitizer. To further improve the purity and efficiency, a top-emitting device with a maximum CE value of 37.0 cd A−1 was developed. The LT50 of the top-emitting device was increased to 493 h at 1000 cd m−2, which can be converted to 31130 h at 100 cd m−2.
In addition to Ir complex sensitizers, Pt complexes have also shown superior performance as sensitizers. The PtON7-dtb molecules were used as sensitizers for v-DABNA by Hyeonho Choi et al.41 The OLED device achieved a maximum EQE as high as 32.2%, which was due to the high PLQY and outcoupling efficiency. This device also exhibited the LT50 of 156.3 h at a luminance of 1000 cd m−2. The group had also prepared a top emitting device. Compared to the bottom-emitting device, the top-emitting device exhibited a narrower FWHM, and the lifetime was increased to 253.8 h. The lifetimes of the devices are one of the highest values reported for the blue-sensitized OLEDs.
As described above, the introduction of phosphorescent sensitizers effectively improves the efficiency of fluorescent and MR-TADF OLEDs, while extending the lifetime of MR-TADF OLEDs. Phosphorescent sensitizers with high PLQY values are key for improving the EQE of sensitized OLEDs. Additionally, the large spectral overlap between the phosphorescent sensitizer and emitter can effectively facilitate FET energy transfer and thus increase EQE. However, the lack of efficient deep-blue phosphorescent materials has hindered the development of sensitized devices.
In the TSF OLEDs, the fast RISC process from the triplet to the singlet is crucial for efficiently utilizing triplet excitons for TADF sensitizers. Cui et al. designed and synthesized a TADF material, 5Cz-TRZ, consisting of multiple carbazole donors and one triazine acceptor.42 Due to the strong spin–orbit coupling between the local-excitation-dominated T1 state and the charge transfer-dominated S1 state and a dense manifold of triplet states resulting from the formation of charge-resonance-type hybrid triplet states, 5Cz-TRZ showed an extremely fast kRISC of 1.5 × 107 s−1. The EQE of TSF OLEDs employing this material to sensitize TBPe, TMDQA, TBRb, and DCJTB reached 24%, 22.3%, 24.9%, and 23.5%, respectively. The slightly lower EQE compared to the 5Cz-TRZ-only device (29.6%) was attributed to the shallower HOMO level of its fluorescent material compared to that of 5Cz-TRZ. In the blue TSF OLEDs, DMAC-DPS, a blue TADF material, was used as a sensitizer.43 The vertical alignment of the donor and the acceptor effectively suppressed the molecular packing and triplet exciton annihilation, even in the case of high doping. The device with DMAc-DPS as a sensitizer and TBPe as an emitter achieves a high EQE of 18.5%. CzAcSF, which replaces one acridine unit of DMAC-DPS with a weak electron-donating carbazole, was also used as a sensitizer.44 Benefiting from the PLQY and high doping ratio (CzAcSF:
TBPe =50
:
0.1), the TSF device based on TBPe as the emitter achieved a high EQE of 18.1%. Interestingly, the device used a single host system. The TADF-sensitized fluorescent device performances are shown in Table 3. The chemical structures of sensitizers and emitters are shown in Fig. 6.
Host | TADF sensitizer | Fluorescent emitter | EQEmax/1000a (%) | PE max/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PE max/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd m−2. | ||||||
DPEPO | ACRSA | TBPe | 13.4/8.7 | 18.0/7.0 | 27.0/18.0 | 14 |
mCP | ACRXTN | TTPA | 15.8/11.7 | 47.0/30.0 | 45.0/38.0 | |
mCBP | PXZ-TRX | TBRb | 18/17.2 | 58.0/33.0 | 60.0/56.0 | |
CBP | tri-PXZ-TRZ | DBP | 17.5/10.9 | 28.0/10.0 | 25.0/20.0 | |
— | DIC-TRZ | DDAF | 11.7/5.5 | 44.1/14.3 | 36.4/17.1 | 15 |
— | PIC-TRZ | DDAF | 4.5/3.9 | 12.3/7.5 | 12.7/11.2 | |
mCBP | 5Cz-TRZ | TBPe | 24.0/— | —/— | —/— | 42 |
mCBP | 5Cz-TRZ | TMDQA | 22.3/— | —/— | —/— | |
mCBP | 5Cz-TRZ | TBRb | 24.9/— | —/— | —/— | |
mCBP | 5Cz-TRZ | DCJTB | 23.5/— | —/— | —/— | |
DPEPO | CzAcSF | TBPe | 18.1/— | 31.2/— | 29.8/— | 44 |
DPEPO | DMAC-DPS | TBPe | 18.8/13.5 | 34.4/12.4 | —/— | 45 |
m32aICT | DACT-II | PhtBuPAD | 23.2/— | 76.9/— | —/— | 46 |
ACRXTO | DACT-II | TBRb | 19.6/14.2 | 79.0/28.7 | —/— | 47 |
8-MeACRXTO | DACT-II | TBRb | 25.5/18.6 | 109.0/44.4 | —/— | |
1-MeACRXTO | DACT-II | TBRb | 19.5/16.9 | 75.2/36.0 | —/— | |
dMeACRXTO | DACT-II | TBRb | 32.3/23.5 | 118.2/49.0 | —/— | |
32alCTRZ | DACz-TAZTRZ | TBRb | 23.7/23.2 | 82.1/58.1 | —/— | 48 |
B3PYMPM | TPA-DCPP | BPPC | 5.4/— | —/— | —/— | 49 |
CBP | TPAB-BF2 | BPPC-Ph | 3.8/— | —/— | —/— | 50 |
mCBP | TPA-PZTCN | BBT-TPA | 1.1/— | —/— | —/— | 51 |
SFBCz: SFTRZ | 5TCzBN | TBRb | 21.7/21.4 | 78.0/69.6 | —/— | 52 |
In addition to the efficient TADF sensitizer, energy transfer between the sensitizer and the emitter is also crucial in the sensitized system. Lee et al. constructed the multilayer luminescent layer structure by spatially separating the TADF sensitizer DMAC-DPS from the TBPe fluorescent molecule.45 The device with alternating DPEPO:DMAC-DPS and DPEPO:TBPe layers realized a maximum EQE of 18.8%, which was attributed to the suppression of the DET process by the separation of DMAC-DPS and TBPe fluorescent molecules.
A green TADF material with a high PLQY and a small ΔEST of 25.9 meV, DACT-II, was also selected for the sensitizer of the PhtBuPAD fluorescent molecules in the green TSF OLED.46 Four types of TSF OLEDs were designed based on these molecules by Duan et al. by considering the host type (TADF or non-TADF) and the carrier recombination position (on the host or the TADF sensitizer). The excitons on the host reduced the T1 exciton density on the TADF sensitizer while increasing the S1 exciton ratio and improving exciton utilization compared to recombination on the TADF sensitizer. When the TADF materials are used as the host, the multiple sensitized processes greatly enhance the FET rate while suppressing the DET rate. The results indicated that the EQE of the device based on the TADF host and the carrier form on the host reached a maximum value of 23.2%, with a power efficiency (PE) value of 76.9 lm W−1.
DACT-II was also used as a sensitizer for the yellow emitter (TBRb).47 You et al. designed and synthesized TADF molecules using a methyl-shielding strategy, 8-MeACRXTO, 1-MeACRXTO, and d-MeACRXTO, in which the xanthone (XTO) backbone was used as the electron acceptor and 9,9-dimethyl-9,10-dihydroacridine (ACR) as the donor. The device based on MeACRXTO as the host, and DACT-II as the sensitizer, achieved a record-high EQE of 32.3%, the highest reported for the TSF-OLEDs.
In the yellow TSF OLED, a new triazolotriazine (TAZTRZ) acceptor was designed to combine with the 3,6-(diphenylamine)carbazole (DACz) donor to form the TADF material.48 TAZTRZ has a strong electron-withdrawing ability, leading to a well-separated distribution of FMOs, thus reducing ΔEST and accelerating the RISC process. Based on DACz-TAZTRZ, a turn-on voltage of 2.1 V, a high EQE of 23.7%, and a PE of 82.1 lm W−1 are achieved. The device has one of the most advanced device performances currently available for the yellow FOLEDs.
Sensitizers are also deployed in near-infrared (NIR) OLEDs to improve the device's efficiency. In 2019, Cohen et al. prepared highly efficient NIR OLEDs using TPA-DCPP as a sensitizer.49 BPPC was selected as an emitter owing to easily tuned emission and high PLQY values. The sensitized NIR OLED achieved a maximum EQE of 5.4%, with an electroluminescence maximum at 790 nm. Subsequently, the group used an efficient TADF boron difluoride curcuminoid derivative (BF2) as a strong electron acceptor to attain deep-NIR wavelengths.50 To further red-shift the NIR wavelengths, a TADF sensitized system was employed. The sensitized NIR OLED emitting at 790 nm showed an EQE of up to 3.8%, with a FWHM < 40 nm. In addition, transport measurements demonstrated that holes are formed on the host and then transferred to the sensitizer to form electrons with electrons.
The same year, Adachi et al. also reported a donor–acceptor type sensitizer, TPA-PZTCN, for the deep-NIR-fluorophore.51 TPA-PZTCN had a high kRISC benefiting from a small ΔEST and the energetically closed T2 state. BBT-TPA, with relatively high PLQYs but no TADF characteristics, was used as the emitter. The films doped with TPA-PZTCN and BBT-TPA had higher PLQY values than that of BBT-TPA. The sensitized OLED achieved a maximum EQE of 1.1% with a peak wavelength of 901 nm.
Exciplexes as the host of TSF OLEDs also help achieving a highly efficient and long lifetime. In 2020, Duan et al. utilized the bulky spirofluorene unit as a π-spacer to expand the distance between the electron donor and the electron acceptor subunits of π-donor and π-acceptor molecules to construct the sky-blue exciplex (SFBCz:FTRZ) with high T1 energies and fast RISC rates.52 5TCzBN with high stability was selected as the sensitizer, and the surrounding tert-butyl units can eliminate the DET process. The white devices based on exciplex:5TCzBN:TBRb as the light-emitting layer recorded a high EQE of 21.7% and a record-high maximum PE of 78 lm W−1. The device exhibits excellent stability (a T80 > 8200 h at 1000 cd m−2).
Host | TADF sensitizer | Phosphorescent emitter | EQEmax/1000a (%) | PEmax/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PEmax/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd m−2. | ||||||
— | PIC–TRZ | Ir(ppy)3 | —/— | 60.0/— | 68.0/58.0 | 17 |
— | POBICT | PO-01 | 22.0/21.7 | 52.6/49.2 | —/— | 53 |
— | BICT | PO-01 | 16.8/16.6 | 38.4/34.1 | —/— | |
— | PBICT | PO-01 | 24.5/24.2 | 64.5/57.3 | —/— | |
— | BBICT | PO-01 | 13.7/13.6 | 19.7/19.0 | —/— | |
— | PBICT | Ir(ppy)3 | 23.9/— | —/— | —/— | 54 |
— | OSTFPCN | Ir(MDQ)2(acac) | 31.2/— | 63.6/— | —/— | 55 |
— | OSTFPB | Ir(MDQ)2(acac) | 29.1/— | 60.5/— | —/— | |
— | Trz–PhCz | Ir(ppy)2(acac) | 24.3/22.3 | 111.8/— | 90.0/— | 56 |
— | Trz–PhCz | PO-01 | 27.3/25.9 | 98.9/— | 80.0/— | |
— | Trz–PhCz | Ir(dmppm)2(acac) | 31.4/29.1 | 75.5/— | 60.0/— | |
— | Trz–PhCz | Ir-F-DHBA | 30.4/25.9 | 59.5/— | 50.0/— | |
— | DMAC-DPS | R-D2 | 22.2/9.1 | —/— | —/— | 57 |
— | 4CzDMAC-DPS | R-D2 | 14.6/10.1 | —/— | —/— | |
CBP: B4PyMPM | 4CzIPN | PtOEP | 21.5/— | —/— | —/— | 58 |
To address the problems of traditional PhOLEDs, Duan et al. proposed the TSP mechanism by combining TADF hosts with phosphorescent emitters. PIC-TRZ that exhibited a small ΔEST of 0.11 eV was selected as the sensitizer and Ir(ppy)3 as the phosphorescent emitter.17 The maximum CE of the TSP OLED was 68 cd A−1, and the roll-off was significantly suppressed (the CE of 62 cd A−1 at 10000 cd m−2) due to its more balanced bipolar characteristics. Subsequently, they designed and synthesized four TADF materials based on PIC-TRZ (POBICT, BICT, PBICT, and BICT).53 The ΔEST of these TADF materials decreases sequentially. PO-01 was selected as the phosphorescent emitter. Among the devices using these materials as the host, the orange TSP OLED based on PBICT as the host exhibited a maximum EQE of 24.5%, a maximum PE of 64.5%, a low drive voltage of 2.75 V, and an extremely small efficiency roll-off (the EQE of 23.8% at 10
000 cd m−2). Although BBICT has the smallest ΔEST, the carrier trapping is reduced due to the lower LUMO level. The device lifetimes of 955 and 1910 h were achieved in PBICT and BBICT-based devices, respectively. In these studies, it was shown that fine-tuning the ΔEST of the TADF sensitizer helps reduce the voltage and improve the device performance when the energy levels of the devices are matched.
Another function of the TADF sensitizer is to reduce the cost, which can be attributed to the fact that using hosts with TADF promotes energy transfer, resulting in a lower concentration of the dopant required for the system. Duan et al. chose PBICT as the host for green phosphor, Ir(ppy)3.54 The maximum EQE of the PBICT-based device is 23.9% at a low dopant concentration of 3 wt%. In contrast, for devices with CBP as the host, a similar EQE value is achieved with the dopant concentration in the range of 5–8 wt%. In addition, the device achieves a longer lifetime at low doping concentrations. Liao et al. showed the excellent device performance at much lower concentrations.55 TADF-type hosts (OSTFPCN and OSTFPB) were synthesized based on oxygenated spiro-triphenylamino-fluorenyl-triphenylamine (OSTF) and cyano- and bis-(2,4,6-trimethylphenyl) borane, which has a small ΔEST, balanced carrier transporting ability and effective ET ability. The device with Ir(MDQ)2(acac) as the emitter achieved 63.6 lm W−1 of PE and 31.2% of EQE at a low phosphorescent doping concentration (0.5 wt%). The underlying cause of the optimum performance of OLEDs with low phosphorescent concentrations is that the up-conversion process of the TADF material allows the devices to be expected to complete the energy transfer at lower dopant concentrations.
Fan et al. designed and synthesized a TADF material, namely Trz-PhCz, with a small ΔEST by combining the donor (carbazole) and acceptor (triazine) and used it as the sensitizer of the blue, green, yellow, orange, and red phosphorescent emitter based on FIrpic, Ir(ppy)2(acac), PO-01, Ir(dmppm)2(acac), and Ir-F-DHBA.56 Trz-PhCz exhibits a short triplet lifetime. The orange device based on Ir(dmppm)2(acac) as the emitter achieves a record-high EQE of 31.4%, while the yellow and red devices also achieved high EQEs of 27.3% and 30.4%. In particular, the green device with a maximum PE of 111.8 lm W−1 and a maximum EQE of 24.3% was developed. Notably, the device also shows an extremely low-efficiency roll-off. At a brightness of 10000 cd m−2, the EQE is maintained at 20.6%, with an EQE drop of merely 15.2%, which is attributed to the small dipole moment and a short triplet lifetime. The WOLED devices were prepared based on Trz-PhCz:FIrpic:PO-01 as the light-emitting layer, with a maximum EQE of 26.5% and a maximum PE of 102.5 lm W−1.
In TADF-sensitized phosphorescent systems, the short-range DET process between the TADF sensitizer and the emitter plays an important role. Yang et al. used DMAC-DPS and a similar molecule, 4CzDMAC-DPS, as the TADF sensitizer, where 4CzDMAC-DPS was surrounded by carbazole substituents to regulate the intermolecular distance.57 The iridium dendrimer R-D2 with the core tris(2-thienyl-4-phenyl) iridium [Ir (Th-PQ)3] was selected as the emitter. A higher energy transfer efficiency from DMAC-DPS to R-D2 was found by estimating the ratio of the PL intensity from the emission peak. The red device based on DMAC-DPS as the TADF sensitizer achieved a maximum EQE of 22.2%, which is higher than that of the 4CzDMAC-DPS-based device (EQE = 15.6%). This EQE is one of the highest values for the solution-processed red PhOLEDs. Due to the larger distance and the faster RISC rate, the 4CzDMAC-DPS-based device exhibits a rather low-efficiency roll-off of 9.0% at 1000 cd m−2. As a result, the TADF sensitizer with the additional hindrance units has a lower efficiency roll-off in the TSP OLED, while TADF hosts without any peripherals facilitate the DET and FET processes, thus achieving high efficiency.
The accumulation of large numbers of excitons in the triplet state of phosphorescent materials is one of the factors leading to the energy loss of excitons and device performance. Liao et al. used exciplex and TADF materials as the host and sensitizer, respectively, to effectively reduce the concentration of the triplet exciton on the emitter through multiple RISC processes.58 Transient PL spectroscopy found that the film of the CBP:B4PyMPM:4CzIPN:PtOEP had a short phosphorescence decay lifetime. Devices based on these materials achieved a maximum EQE of 22%.
Host | TADF sensitizes | MR-TADF emitter | EQEmax/1000a (%) | PE max/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PE max/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd m−2. d The top-emitting devices. e The two-unit-stacked tandem devices. | ||||||
DPEPO | DMAC-DPS | t-DABNA | 31.4/19.8 | 33.6/10.9 | 32.6/20.9 | 35 |
DPEPO | DMAC-DPS | DABNA-1 | 23.5/15.6 | —/— | —/— | |
mCPCz | p4TCzPhBN | t-DABNA | 32.5/23.2 | —/— | —/— | 59 |
oCBP: CNmCBPCN | PPCzTrz | ν-DABNA | 33.0/25.2 | —/— | 39.7/28.9 | 60 |
oCBP: CNmCBPCN | PCzTrz | ν-DABNAd | 33.5/23.8 | —/— | 35.5/23.4 | |
oCBP: CNmCBPCN | PPCzTrz | ν-DABNA | 29.3/21.4 | —/— | 32.8/23.6 | |
oCBP: CNmCBPCN | PCzTrz | ν-DABNAd | 34.4/21.9 | —/— | 38.9/24.4 | |
mCBP | HDT-1 | ν-DABNA | 27.0/20.0 | 41.0/16.0 | 39.0/31.0 | 62 |
mCBP | HDT-1 | ν-DABNAe | 41.0/32.0 | 23.0/10.0 | 72.0/59.0 | |
mCBP | 4PhCz2BN | ν-DABNA | 22.4/17.8 | 25.0/9.4 | 25.4/19.1 | 63 |
mCPBC | 23PCX | ν-DABNA | 25.1/23.4 | 40.4/29.9 | —/— | 64 |
mCPBC | 33PCX | ν-DABNA | 20.6/20.2 | 24.1/21.7 | —/— | |
DBFPO | DBA–BFICz | ν-DABNA | 38.8/29.1 | —/— | 30/22.5 | 65 |
DBFPO | DBA–BTICz | ν-DABNA | 37.3/27.5 | —/— | 28.3/21.4 | |
DBFPO | mMDBA-DI | t-Bu-ν-DABNA | 39.1/34.3 | —/— | 30.3/26.6 | 66 |
DBFPO | pMDBA-DI | t-Bu-ν-DABNA | 37.9/34.8 | —/— | 36.2/33.0 | |
PPF | dCz-Xo-TRZ | ν-DABNA | 33.5/27.3 | 35.8/— | 37.6/— | 71 |
PPF | mCz-Xo-TRZ | ν-DABNA | 24.5/19.2 | 24.1/— | 27.5/— | |
mCPBP | 5TCzBN | 2F-BN | 22.0/15.0 | 69.8/38.1 | —/— | 79 |
mCPBP | 5TCzBN | 3F-BN | 22.7/21.1 | 72.3/45.9 | —/— | |
mCPBP | 5TCzBN | 4F-BN | 20.9/16.4 | 51.3/29.4 | —/— | |
mCPBP | CTPCF3 | 2F-BN | 33.1/27.0 | 76.1/43.9 | —/— | 73 |
mCPBP | CNCTPCF3 | 3F-BN | 25.6/14.2 | 62.4/22.7 | —/— | |
mCPBP | TCTPCF3 | 4F-BN | 23.2/18.8 | 46.4/29.8 | —/— | |
mCPBC | 1CTF | AZA-BN | 27.6/20.1 | —/— | —/— | 74 |
mCPBC | 2CTF | AZA-BN | 30.3/22.8 | —/— | —/— | |
mCPBC | 3CTF | AZA-BN | 31.6/24.5 | —/— | —/— | |
mCBP | HDT-1 | CzBNCz | 21.9/19.8 | 47.8/— | 42.6/— | 75 |
mCBP | HDT-1 | CzBN | 20.6/17 | 40.1/— | 38.3/— | |
mCBP | HDT-1 | DABNA-2 | 24.1/18.4 | 45.4/— | 46.3/— | |
mCBP | HDT-1 | CzBN | 22.0/17.1 | 38.2/— | 48.7/— | 76 |
mCBP | HDT-1 | CzBNPyr | 19.4/15.1 | 37.1/— | 41.5/— | |
DMIC-TRZ | BNSeSe | BN3 | 40.5/32.4 | 205.8/197.7 | 164.5/162.4 | 18 |
In the TADF-sensitized MR-TADF devices with the t-DABNA emitter, Lee et al. introduced DMAC-DPS as a sensitizer.35 The EQE of sensitized devices is optimized from 25.1% to 31.4% compared to that of non-sensitized devices. Such a significant improvement in EQE is attributed to the increase in PLQY from DPEPO:t-DABNA to DPEPO:DMAC-DPS:t-DABNA films and close to 100% triplet-to-singlet exciton conversion efficiency.
Although the EQE of DMAC-DPS-sensitized t-DABNA devices was significantly improved, the poor stability of the DMAC-DPS material resulted in a device lifetime LT50 of only ∼35 h at 100 cd m−2. Duan et al. developed the advanced molecular motif based on a sterically crowded structure with multiple donors and acceptor-type carbazole–biphenyl carbonitrile derivative p4TCzPhBN.59 The p4TCzPhBN molecules that have unique molecular motifs with linearly localized donors and acceptors exhibited the most efficient TADF characteristics with a high RISC rate of 2.36 × 106 s−1. The sensitized device with t-DABNA as the emitter had the maximum EQE of 32.5%. The T80 of 60 h was obtained at 1000 cd m−2, which was six times higher than the device without the sensitizer. At an initial luminance of 100 cd m−2, the extrapolated blue OLED lifetime exceeded 3000 h. The long lifetime mainly benefits from the fast RISC rate of the sensitizer, which promotes the up-conversion of triplet excitons to single excitons and increases the exciton utilization.
TADF materials have also shown excellent electroluminescence performance as sensitizers for v-DABNA molecules, especially in the lifetime and EQE. In 2021, Lee et al. constructed the triplet exciton-distributed TADF (TED-TADF) device with a low-triplet energy host.60,61 The triplet energy of the host is slightly lower than that of the sensitizer and higher than that of the emitter. Compared with the high triplet energy of the host, the triplet excitons in the host with the low triplet energy can be recycled by the TADF sensitizer, thus suppressing the annihilation of triplet excitons while ensuring the stability of the material. Two deep-blue-emitting TADF materials, PPCzTrZ and PCzTrZ, were synthesized and selected as sensitizers. When employing these materials to sensitize the v-DABNA emitter, the corresponding device realized maximum EQEs of 33 ± 0.3% and 33.5 ± 0.1%, and at 1000 cd m−2 of 25.2 ± 0.8% and 23.8 ± 0.2%, respectively. The high EQE of the TED-TADF device can be explained by the high PLQY of the PPCzTrz:ν-DABNA-doped film. The device lifetimes (LT50) were 151 ± 3 h and 112 ± 3 h (at an initial luminance of 1000 cd m−2). To further improve the efficiency, a top-emitting device was developed, and the EQEs of the PPCzTrz and PCzTrz devices reached 43.8 ± 1.4% and 52.9 ± 1.5% with a CIEy value of 0.09 because of the reflecting microcavity effect, achieving the CCE values of 364 ± 10 cd A−1 and 432 ± 11 cd A−1, respectively. The results represent one of the highest EQE and the longest device lifetimes for deep-blue OLEDs with y color coordinates below 0.10.
Hatakeyama and Adachi et al. also reported efficient and stable deep-blue OLEDs, in which the electrically generated energy of the sky-blue TADF sensitizer called the hetero-donor-type (HDT) TADF is exothermically transferred to the excited state of the deep-blue emitter.61,62 The hetero-donor-type TADF molecules named TPh2Cz2DPhCzBN (HDT-1) with a sky-blue emission peak, fast kRISC, high PLQY, and short delayed lifetime were synthesized to sensitize v-DABNA. The sensitizer can be red-shifted relative to the emitter as the charge-transfer excited state of the TADF sensitizer results in a broad distribution of energy states and a small Stokes shift of v-DABNA. The device based on this material has obtained a maximum EQE of 27%, a remaining high EQE of 20% even at 1000 cd m−2, and an LT95 of over 10 h. The two-unit stacked tandem OLED was implemented to improve the device performance further. The device exhibits an outstanding EQE of 41% with an improved LT95 of 18 h at 1000 cd m−2.
The HDT-1 sensitized v-BABNA OLED achieved an extended lifetime compared to non-sensitized devices, but HDT-1 also exhibited incomplete energy transfer to v-DABNA. Recently, Adachi et al. developed multiple donor and acceptor blue TADF materials to sensitize v-DABNA.63 The four donors and two acceptors (4PhCz2BN) exhibited a fast RISC process (kRISC > 106 s−1) resulting from the dense alignments of 1CT, 3CT, and 3LE. The material also exhibited kr > 107 s−1. The sensitized devices had a maximum EQE of 22.4% with CIE coordinates (0.13, 0.15).
The stability of the molecular structure also exerts a positive influence on the extended lifetime of the device. A novel TADF structure, D-void-A, was designed by Duan et al.64 The D and A groups are connected through a vacant position in the π-principal chain of A (no HOMO-assigned carbon atom in the A group), which can inhibit the binding of the HOMO between D and A, while the deeper HOMO energy level of the acceptor becomes the energy level of the molecule, resulting in a larger blue emission energy gap. phCz is connected to the C1, C2, and C3 positions of the Xo group, where the C3 position has a deeper HOMO energy level. Using these molecules as a sensitizer for v-DABNA, the maximum EQE of the devices based on 23PCX and 33PCX were 25.1% and 20.6%, respectively. The device with 33PCX as a sensitizer exhibited worse performance mainly due to the larger ΔEST. It is worth mentioning that LT95 of ≈13 and ≈15 h were recorded for devices with 33PCX and 23PCX, which can be compared with the OLED lifetime of Adachi and Lee et al. This work provides that the highly efficient and stable blue TADF emitters should take TADF molecules with good stability into consideration.
The EQE of TADF-sensitized v-DABNA OLEDs needs to be further enhanced to compete with v-DABNA-based OLEDs. The recent work reported by Kwon et al. gave an idea of how to appropriately design TADF sensitizers to achieve a high EQE.65,66 In 2021, the group designed and synthesized two deep-blue TADF materials, DBA-BFIC and DBA-BTICz. The DBA moiety is one of the ideal acceptors for blue molecules, while the BFICz and BTICz donors were modified with the heteroatoms of oxygen and sulfur on the rigid donor to reduce the donor strength, increasing the spectral overlap area between the TADF material and the v-DABNA emitter. The corresponding deep-blue devices exhibited the maximum EQEs of 38% and 37.3%, respectively. Another TADF-sensitized device similar to this work exhibited a higher performance. The new TADF materials were synthesized, namely, pMDBA-DI and mMDBA-DI, with methyl substituted at the heteroatom's para and meta positions to reduce the acceptor strength. To effectively inhibit the DET process, tert-butyl groups were introduced on the v-DABNA to reduce aggregation quenching and enhance the PLQY. Utilizing the mMDBA-DI sensitized t-Bu-v-DABNA OLED results in a record-high EQE of 39.1%. Furthermore, the pMDBA-DI sensitized device with the DCzPy host exhibited a device lifetime of 440 h (LT50) at an initial luminance of 1000 cd m−2. Such high efficiency of the two reported works is mainly attributed to the high horizontal orientation factor and the PLQY of the TADF sensitizer, the low doping concentration, and the sufficient energy transfer between the sensitizer and the emitter, as well as the additional triplet excitons obtaining from the DET process and charge trapping on the emitter.
The through space charge transfer (TSCT) TADF materials also exhibit interesting properties in terms of sensitizers due to their extremely small ΔEST and dipole moments, but the kr and kRISC of the TSCT-TADF materials cannot be balanced.67–70 The group constructed a novel TSCT sensitizer material using the Xo group, mCz-Xo-TRZ, and dCz-Xo-TRZ.71 Benefiting from the short D–A distance, the device has a radiative decay rate close to 107 s−1 and achieves a high PLQY. The dCz-Xo-TRZ-based sensitized device has a maximum EQE of 34.7% when using v-DABNA as the emitter. In addition, the device exhibits a small efficiency roll-off (27.6% at 10000 cd m−2), and the fast FET rate promotes exciton transfer between the sensitizer and the emitter, alleviating the efficiency roll-off of the device.
In the case of the green color, Duan et al. synthesized the well-known green-emitting MR-TADF core (BCz-BN) by replacing the diphenylamine donor in the DABNA core with a carbazole.72 Based on the BCz-BN core, a range of green MR-TADF materials are designed and the corresponding sensitizers are selected or engineered to match them. In 2019, they introduced electron-absorbing groups at the para-position of BCz-BN to design MR-TADF materials 2F-BN, 3F-BN, and 4F-BN. The OLED utilizing 5TCzBN as the sensitizer and 3F-BN as the emitter showed a maximum EQE of 22.7%, a maximum PE of 72.3 lm W−1 and CIE coordinates of (0.20,0.58). And the lifetime LT90 based on 2F-BN reached 45.76 h, which is one of the most stable green emitters available.
In 2020, Duan et al. made the combination of TSCT and TBCT by designing donor–acceptor structures CTPCF3, CNCTPCF3, and TCTPCF3 with the neighbour arrangement.73 CTPCF3, with the highest kRISC, achieves the shortest lifetime of the delayed part and the relatively small molecular dipole moment. The highest EQE of 33.1% was achieved for the device with CTPCF3 selected as the sensitizer while again employing 2F-BN as the emitter, which far exceeds the rest of the work. This high efficiency can be attributed to the high PLQY resulting from the fast FET process. It is noteworthy that the TADF sensitizer uses the same emission spectrum as 2F-BN, which is not pursuing a large overlap with the absorption spectrum of the MR-TADF emitter. In addition, the group built on this works to design a TADF molecule containing ortho-linked multiple donor–acceptor motifs.74 Owing to the maximum oscillator strength value of 3CTF and its dense excited state manifold near S1 and T1 states, 3CTF achieves both high kr and kRISC. Unfortunately, the device efficiency is unsatisfactory owing to the low external coupling factor. They have hence applied it as a sensitizer for AZA-BN, reasonably avoiding that disadvantage. The corresponding green-sensitized OLED achieves a high EQE of 31.6% thanks to its high RISC rate. This work has demonstrated that the ideal sensitizer should have an efficient RISC rate and a high PLQY to ensure sufficient exciton harvesting.
The energy level matching between the sensitizer and the emitter contributes significantly to the device's lifetime. Adachi et al. achieved a low-efficiency roll-off and a long lifetime by modulating the HOMO energy level between the sensitizer and the emitter.75 The BN compounds with carbazole substitution (CzBN and CzBNCz) with deeper HOMO energy levels were synthesized, and the DABNA-2 material was chosen for comparison. Sensitized devices were produced employing TPh2Cz2DPhCzBN as the sensitizer, where the LT90 of CzBN and CzBNCz as the emitters, was 39.3 and 28.8 h at 1000 cd m−2, respectively, approximately twice those of DABNA-2 based devices. The analysis of the HOMO energy levels reveals that the energy levels of 5.56 and 5.71 eV for CzBN and CzBNCz, respectively, are closer to the sensitizer than that of 5.38 eV for DABNA-2. Meanwhile, the transient electroluminescence spectra confirm that the smaller HOMO energy level difference reduces the hole trapping in the luminescent layer. The reduced HOMO energy level differential also results in a decrease in the driving voltage and efficiency roll-off. Recently, Adachi et al. had also designed stable sensitized devices employing HDT-1 as the sensitizer.76 They designed CzBNPPr based on the CzBN molecule and the pyrene unit. CzBNPPr does not possess TADF activity owing to the introduction of the pyrene unit. Sensitized OLEDs with HDT-1 as the sensitizer showed excellent device lifetime (LT95 = 40 h) because of the low-T1 emitter.
In addition, to be an efficient emitter, the devices using MR-TADF materials as the sensitizer also show outstanding performance. Very recently, Yang et al. synthesized BNSeSe by embedding the non-metallic heavy atom Se atom into the multiple resonance skeleton.18 The heavy atom effect significantly increased the coupling between singlet and triplet orbits, achieving a high RISC rate and a high PLQY. Due to the short delay fluorescence lifetime, high PLQY, and excellent electroluminescence performance, BNSeSe was conceived and selected as the sensitizer of the yellow-emitting MR-TADF transmitter BN3. The EQE of the corresponding device exceeded 40%, the maximum PE exceeded 200 lm W−1, and the maximum brightness was close to 200000 cd m−2. This work was the first to design high-efficiency, low roll-off, high-purity OLEDs employing MR-TADF as the sensitizer.
TADF-sensitized OLEDs are more conducive to achieve the high EQE device than phosphorescence-sensitized OLEDs. Apart from the high PLQY value, the rapid RISC process is also key to improving EQE. The fast FET process also promotes exciton utilization and thus efficiency. In addition, TADF sensitizers are more favourable than phosphorescent sensitizers for the application of highly efficient deep-blue sensitized OLEDs.
Exciplex host | Exciplex sensitizer | Emitter | EQEmax/1000a (%) | PEmax/1000b (lm W−1) | CEmax/1000c (Cd A−1) | Ref. |
---|---|---|---|---|---|---|
a EQEmax/1000 – the maximum EQE and EQEs at 1000 cd m−2. b PE max/1000 – the maximum power efficiency and PEs at 1000 cd m−2. c CEmax/1000 – the maximum current efficiency and CEs at 1000 cd m−2. | ||||||
— | TAPC:DPTPCz | C545T | 14.5/— | 46.1/— | 44.0/— | 16 |
— | CDBP:PO-T2T | PO-T2T:DABNA-1 | 17.5/15 | —/— | 60.9/51.1 | 77 |
— | TPA-3:3CzFDPhTz | 3CzFDPhTz:PO-T2T | 18.8/8.9 | 43.0/16.3 | 63.0/29.6 | 78 |
— | TPA-3:9CzFDPhTz | 9CzFDPhTz:PO-T2T | —/— | 61.4/24.7 | 78.2/32.7 | |
CDBP:PO-T2T | PO-T2T:DABNA-1 | TBRb | 18.7/18.0 | 46.7/36 | 64.8/62.5 | 79 |
— | Tris-PCz:3Cz-TRZ | ν-DABNA | 19.0/18.0 | —/— | —/— | 80 |
In 2015, Lee et al. prepared the first OLED using an exciplex host to sensitize the fluorescent dopant.16 DPTPCz and TAPC were mixed 1:
1 (by weight) to form the exciplex host. C545T was selected as the fluorescent dopant. The TAPC:DPTPCz exciplex had a small ΔEST, a high PLQY, and an efficient RISC. The absorption spectrum of C545T overlaps well with the PL spectra of TAPC:DPTPCz. Unfortunately, the energy transfer between the excipient and the fluorescent dopant was insufficient, as the weak DPTPCz emission in the mixed films was observed. The OLEDs based on the exciplex sensitizer showed a maximum EQE of 14.5%, a maximum CE of 44 cd A−1, and a maximum PE of 46.1 lm W−1, which harvested nearly 100% of triplets.
The exciplex as a sensitizer can also increase the EQE of the exciplex emitter. In 2019, Lee et al. constructed a ternary exciplex system using the p-type donor, a carbazole-derived 4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl (CDBP) and the n-type acceptor, 2,4,6-tris[3-(diphenylphosphinyl) phenyl]-1,3,5-triazine (PO-T2T), to form a high energy exciplex.77 The n-type acceptor was mixed with the donor–acceptor TADF emitter DABNA-1 to generate a low-energy exciplex. Molecular dynamics simulations demonstrated the good dispersibility of the PO-T2T:DABNA-1 exciplex. The emission peak of the CDBP:PO-T2T film was 480 nm, which overlapped with the absorption peak of the PO-T2T:DABNA-1 exciplex (between 380 and 480 nm). The OLED based on a ternary exciplex film exhibited a high EQE of 17.8%. Similarly, Zhang et al. designed a donor that facilitates solution processing, 4-(9-(perfluoropyridin-4-yl)-9H-fluoren-9-yl)-N,N-diphenylamine, TPA-3, which forms green exciplexes with the acceptor PO-T2T.78 Two donors, 3CzFDPhTz (3-(9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-fluoren-9-yl)-9-phenyl-9H-carbazole) and 9PhFDPhTz (9-(4-(9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-fluoren-9-yl) phenyl)-9H-carbazole), with PO-T2T, are forming the sky blue exciplex. By analyzing the transient photoluminescence decay curve, the ternary exciplex has a faster RISC rate than the binary exciplex, while the additional RISC process makes more use of the triplet exciton. The solution-processed exciplex OLEDs based on the interfacial exciplex provided maximum EQEs of 18.8% and 24.0%, respectively.
In addition to the single-host exciplex sensitizing system, a high EQE in the fluorescent OLED was achieved using two exciplexes as the host and the sensitizer of the fluorescent emitter. Lee et al. developed the cascade singlet harvesting (CSH) OLEDs using the POT2T:CDBP high energy exciplex as the host, the POT2T:DABNA-1 low energy exciplex as the sensitizer, and TBRb as the emitter.79 Through analysis of singlet and triplet energy transfer between the exciton and TBRb, triplet exciton quenching by Dexter energy transfer, and molecular dynamics simulations indicated that the CSH mechanism suppressed the DET process. The maximum EQE of the device was 18.7% for DABNA-1 and TBRb at 10 and 0.5 wt%. Replacing DABNA-1 with t-DABNA, the maximum EQE of the TBRb-doped POT2T:
CDBP
:
t-DABNA (45
:
45
:
10) exciplex device was further increased to 19.9% in comparison with 10.4% of the common exciplex based fluorescent OLEDs.
Despite the improved EQE achieved by the exciplex-based system, the operational device stability of exciplex-based OLEDs is still far from practical applications. Adachi et al. have enhanced the device lifetime by rationally selecting TADF-type electron acceptors.80 When the TADF material is used as an acceptor for the exciplex, the recycling of triplet excitons between the TADF-type electron acceptor and the exciplex is carried out through the RISC process, thus reducing the density of triplet excitons. 9,9′,9′′-Triphenyl-9H,9′H,9′′H-3,3′:6′,3′′-tercarbazole (Tris-PCz) with a strong electron-donating group of tricarbazole had been selected as a donor molecule. The conventional acceptors, 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) and 2-(9,9′-spirobi [fluoren]-3-yl)-4,6-diphenyl-1,3,5-triazine (SF3-TRZ), bipolar-type acceptor, 9-[4-(4,6-diphenyl-1,3,5-triazin-2yl)phenyl]-9H-carbazole (Cz-TRZ), and TADF-type acceptors, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BCz-TRZ) and 3Cz-TRZ were compared with each other. It was found that the devices with the addition of the electron-donating group and those with TADF-type acceptors had significantly longer lifetimes. Compared to conventional and bipolar acceptors, TADF-type acceptors have a lower cumulative triplet density during OLED operation, resulting in a more stable device over the long term. The ν-DABNA doped OLED achieved an EQE of 19%. The blue OLED maintains stability after incorporating ν-DABNA (LT50 over 300 h at higher luminance). Regrettably, the energy transfer between the exciplex and v-DABNA is not sufficient, which reflects the potential of this system to achieve highly stable exciplex-sensitized devices.
In terms of sensitizers for sensitized devices, phosphorescence-sensitized OLEDs exhibit a lower EQE value than TADF-sensitized devices owing to the lack of the corresponding high-performance sensitizers. In the case of emitters for sensitized devices, the EQE value of the sensitized fluorescent OLEDs is generally lower than that of the phosphorescent, TADF, or exciplex OLEDs, because of the larger energy loss, such as the incomplete FET process, the DET process, and direct charge recombination on the emitter. In addition, sensitized fluorescent OLEDs with blue emission still require further research owing to the large Stokes shift of the fluorescent material and the poor performance of the blue phosphorescent, TADF, and exciplex sensitizers. MR-TADF materials with small Stokes shifts and triplet exciton availability are better suited than fluorescent materials as emitters for blue-sensitized devices. The maximum EQE values for sensitized MR-TADF OLEDs have surpassed 40% to date. However, the lifetime of the sensitized MR-TADF OLEDs still needs a further extension for commercial applications compared to those of the fluorescent and PhOLEDs. The device lifetime is closely related to the lifetime and density of the triplet exciton, with singlet–triplet annihilation, triplet–triplet annihilation, and triplet–polariton annihilation being the main processes which reduce the device lifetime. In addition, the stability of the materials, both sensitizers and emitters, is also necessary. The HOMO and LUMO energy level matching of the device also affects the lifetime to some extent.
The EQE and lifetime of exciplex-sensitized devices are less satisfactory than those of phosphorescent and TADF-sensitized devices. However, multiple sensitizing processes using exciplexes or TADF materials as the host in combination with TADF and phosphorescent sensitizers exhibit significant device performance and lifetime due to the full harnessing of triplet exciton and increased exciton transfer channels.
In the pursuit of high-performance sensitized OLEDs, we should take into account the sensitizer and energy transfer efficiency. An ideal sensitizer should have a high PLQY, short excited state lifetime, and low molecular dipole moment. A high RISC process is highly desired by TADF or exciplex sensitizers to suppress exciton annihilation. Next, efficient energy transfer efficiency should occur between the sensitizer and the emitter. A large degree of overlap between the sensitizer's emission spectrum and the emitter's absorption spectrum is necessary. Notably, this has not meant that a small degree of overlap would not allow for efficient energy transfer, especially for MR-TADFs with small Stokes shifts. The DET process is generally a pathway for exciton loss, which can be avoided by designing molecules with large steric groups to modulate the spatial distance between the sensitizers and the emitters. Moreover, the residual emission of the sensitizer is attributed to incomplete energy transfer. We can avoid the effect of residual emission by increasing the spectral overlap between the sensitizer and the emitter or by choosing an emitter with a small Stokes shift, which effectively increases the energy transfer efficiency.
Despite the great achievements in sensitized devices, efficient sensitized OLEDs still present challenging problems. In the future, the design strategy of efficient phosphorescent, TADF, and exciplex sensitizers should be paid more attention. In addition, energy transfer needs to be further investigated in detail to obtain high-performance sensitized OLEDs. In summary, we believe that sensitized OLEDs can achieve significant breakthroughs in efficiency and lifetime to enable commercial utilization.
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