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Highly Efficient Deep Blue Fluorescent Organic Light-emitting Diodes Boosted by Thermally Activated Delayed Fluorescence Sensitization Dae Hyun Ahn, Jae Ho Jeong, Jie Song, Ju Young Lee, and Jang Hyuk Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19030 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

Highly Efficient Deep Blue Fluorescent Organic Light-emitting Diodes Boosted by Thermally Activated Delayed Fluorescence Sensitization Dae Hyun Ahn,† Jae Ho Jeong,‡ Jie Song,‡ Ju Young Lee,*† and Jang Hyuk Kwon*† †

Department of Information Display, Kyung Hee University, Hoegi-dong, Dongdaemun-ku, Seoul, Republic of Korea. ‡

Material Science Co., ltd. (Ace Techno 10-cha, Gasan-dong) 805, 196, Gasandigital 1-ro, Geumcheon-gu, Seoul, Republic of Korea.

KEYWORDS : TADF OLED, assistant host, TADF sensitizer, deep blue, fluorescence

ABSTRACT

Highly efficient deep blue fluorescent material and various thermally activated delayed fluorescent (TADF) blue sensitization materials were synthesized for fluorescent deep blue organic light-emitting diodes (OLEDs). These materials were designed and selected by considering efficient energy transfer conditions (i.e. spectral overlap and quantum efficiency) between sensitizer and acceptor. Energy transfer process from TADF host sensitizers to deep blue fluorescent emitter have been investigated by measuring energy transfer rate. Measured

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energy transfer rate was to be 1.24ⅹ1010 s-1(mol/dm3)-1 for prompt decay of fluorescence and 2.61ⅹ108 s-1(mol/dm3)-1 for delayed fluorescence, which demonstrated efficient energy transfer. Indeed, highly efficient deep blue fluorescent OLEDs boosted by TADF host sensitization process were successfully fabricated. The maximum external quantum efficiency was 19.0% with color coordinates of (0.14, 0.15) and 15.5% with color coordinates of (0.15, 0.11) in the different host system, respectively. Efficiency roll-off characteristic and device operating lifetime were also improved by this efficient sensitization process.

1. INTRODUCTION Nowadays curved organic light-emitting diode (OLED) displays are already commercialized for mobile and large size TV application. Transparent, rollable, and foldable displays are in the developing stage based on OLED technologies.1-3 They are expected to be commercially available within a couple of years. Hence the importance of highly efficient OLEDs is increasing for these future displays. Until now phosphorescent red and green OLEDs have been already applied to mass production to achieve low power consumption because of its theoretical 100% of internal quantum efficiency (IQE). However, phosphorescent blue materials have not shown much progress due to several issues such as short operational lifetime and poor color purity. Therefore, blue OLEDs are still using fluorescent emitters for display and lighting applications due to their much better operational stability and deep blue color in spite of their low IQE of 25%. However, the use of fluorescent emitter for blue color results in increase of power consumption significantly because of low efficiency. Therefore, highly efficient and deep blue OLED materials are strongly demanded.4-6 For the last several years, many researchers have reported highly efficient phosphorescence blue OLEDs.7-9 The reported most blue


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phosphorescent OLEDs had sky blue colors but external quantum efficiency (EQE) values were very high as 27.2%~34.1% range. Although there have been a few reports about deep blue phosphorescent material with nearly National Television Systems Committee (NTSC) blue color coordinates, the reported EQEs do not exceed over 10% range.10 To solve current phosphorescent material issues, thermally activated delayed fluorescence (TADF) materials have been investigated in many research groups. Among many TADF materials, 9,9′-(5-(4,6-diphenyl1,3,5-triazin-2-yl)-1,3-phenylene) bis(9Hcarbazole) (DCzTrz) showed 17.8% of maximum EQE with color coordinate of (0.15, 0.16), 2,7-bis(9,9-dimethylacridin-10(9H)-yl)-9,9-dimethyl-9Hthioxanthene-10,10-dioxide (DMTDAc) showed 19.8% of maximum EQE with color coordinate of (0.15, 0.13), and 10,10'-(sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) showed 19.5% of maximum EQE with color coordinate of (0.16, 0.20).11-13 Moreover, highly efficient blue TADF material that indicates EQE of 37% also achieved for sky blue device with color coordinate of (0.18, 0.43).14 Very recently, a deep blue TADF material with the highest efficiency was reported by Adachi.15 It had 19.2% of maximum EQE with color coordinate of (0.15, 0.10).15 These results show that blue TADF materials could be more efficient than phosphorescent materials at the similar color. However, unlike phosphorescence materials, they show severe roll-off characteristic and poor device stability due to long exciton lifetime and easy material degradation. Moreover, their spectrum is too wide due to the intramolecular charge transfer (ICT) characteristics of TADF materials. To overcome these problems, TADF host sensitization (THS) systems with normal fluorescent emitter have been studied. In this system all electrically generated singlet and triplet exciton energies of TADF host can be transferred to fluorescent emitter as shown in Figure 1. Therefore, theoretically 100% of IQE is achievable. In addition, reduced stress to TADF materials by the


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fast energy transfer can improve roll-off characteristic and operational stability. Such THS system in the OLED devices initially have been studied by several groups.16-18 In green and red OLEDs, nearly 18.0% EQE demonstrated. In case of blue, 13.4% of EQE was reported by using 10-phenyl-10H,10’H-spiro[acridine-9,9’-anthracen]-10’-one (ACRSA) and 15.4% of EQE was also achieved by using 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-phenyl)-9,9-dimethyl-9,10dihydroacridine (CzAcSF).16-17 However, both devices used representative sky blue fluorescence material, 2,5,8,11-tetra-tert-butylperylene (TBPe) as an emitter, and reported color coordinate was (0.17, 0.30) and (0.15, 0.23), respectively. Herein, we report the first example of a deep blue THS system consisting of a new fluorescent emitter and TADF host sensitizer materials. We also report our investigation on energy transfer mechanism of this THS system.

2. RESULT AND DISCUSSION 2.1. Required host and dopant physical properties 2.1.1. Consideration items for effective energy transfer. In THS system, fluorescent emitter should be light emitting centers by accepting the energy from TADF host. Therefore, effective Förster energy transfer is important rather than Dexter energy transfer to prevent triplet excitons quenching by triplet state of fluorescent emitter. To achieve such condition for deep blue color, the fluorescent blue emitter should have emission peak at below 460 nm with narrow spectrum, and small Stokes shift of under 30 nm for effective energy transfer from TADF host. In such case, 400~450 nm absorption peak with small Stokes shift derived from fluorescent emitter gives an advantage for large spectral overlap between the donor emission spectrum and the acceptor absorption spectrum. In addition, PLQY term of TADF host can also affect energy transfer as expressed by following equation.19


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=

λ / = !

Where R0 is the distance that energy transfer efficiency is 50%, NA is Avogadro’s number, nD is the refractive index, κp2 is the dipole orientation factor, Qd is the fluorescence quantum yield of the donor in the absence of the acceptor, J(λ) is the spectral overlap, and kF is the energy transfer rate. Here most of the terms are constant and generally κp2 term does not have large difference. Accordingly, Qd and J(λ) are the main effective factors for energy transfer rate. In conclusion, PLQY of TADF host should be high to achieve effective energy transfer with large spectral overlapping between TADF host and fluorescent emitter. 2.1.2. Requirements of fluorescence material. Among the anthracene, pyrene and perylene core materials widely used in blue fluorescence, pyrene has appropriate deep blue emission with small Stokes shift of 26 nm.20 However, since a normal pyrene has emission peak of 375 nm, it is necessary to attach other moieties to red-shift to around 450 nm. Unfortunately, already reported pyrene-based deep blue dopants exhibited large Stokes shift, due to the large geometrical change between ground state and excited state.21-22 Therefore, by using density functional theory (DFT) calculation with B3LYP in Dmol3, we designed new blue fluorescent material, N,N'-bisdibenzofuran-4-yl-N,N'-bis-(2,5-dimethyl-phenyl)-pyrene-1,6-diamine

(BPPyA),

which

is

expected to have a small Stokes shift due to the small geometric change as shown in Figure 2a. The torsion angles of pyrene core and two moieties, phenyl group and dibenzofuran, are 82.1° and 115.2° in the ground state, respectively, and 77.2° and 120.1° in the singlet excited state. The difference in torsion angles between two states is only about 5.0°. Additionally, due to the introduction of rigid moieties, PLQY is also expected to be high. The synthesis of BPPyA is


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shown in supplementary information. Indeed, BPPyA showed deep blue emission peak at 458 nm with small Stokes shift of 28 nm as shown in Figure 2b and high PLQY of 98.5%. Molecular dipole moment is also an important characteristic to be considered. Generally, the TADF materials are highly polar, due to the strong ICT characteristic, which can influence the fluorescent emitter. Even in the same material, emission peak can be red shifted with increasing the polarity of host. This phenomenon can be confirmed by the solvatochromic effect.23 The solvatochromic effect was measured by increasing the polarity of solvent, from toluene to methylene chloride (MC). Toluene was used as the lowest polarity solvent, because the emission spectrum in toluene is generally similar to non-polar solid film. Likewise, considering of spectral change of BPPyA in polar TADF host environmental condition, the spectral change of BPPyA in the polar MC solvent was investigated. Prior to measuring this effect, dipole moment of the blue dopant was calculated. Generally, the materials exhibiting high polarity have more than 4 dipole moment value. However, BPPyA indicated very small dipole moment of 0.2 D, due to the reduced donor property of aryl amine moiety. As shown in Figure 2a dibenzofuran exists almost vertically to pyrene plane, which reduce the donor property. Indeed, BPPyA showed very small red shift of 3 nm from toluene to MC as shown in Figure 2c. Therefore, BPPyA was expected to maintain deep blue color even though it is doped into TADF materials. 2.1.3. Requirement of TADF materials. As mentioned above, high PLQY of TADF host and large spectral overlap between the fluorescent emitter, BPPyA, and TADF host is essential for highly efficient energy transfer. Therefore, the TADF materials indicating deep blue emission peak below 450 nm and high PLQY over 80% are required. Hence, we have synthesized new TADF

material

9,9-dimethyl-2,7-di(10H-spiro[acridine-9,9'-fluoren]-10-yl)-9H-thioxanthene

10,10-dioxide (SPAC-DMT). In addition, we also have synthesized well-known TADF materials


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DMAC-DPS and 2,7-bis(9,9-dimethylacridin-10(9H)-yl)-9,9-dimethyl-9H-thioxanthene-10,10dioxide (DMAC-DMT) for good sensitizer materials. Such materials with sulfone acceptor are already known as highly efficient deep blue TADF blue dopant. The PL emission peak of DMAC-DPS and DMAC-DMT were 455 and 447 nm, and the PLQY were 80% and 90.1%, respectively. Therefore, they were expected to be a good THS material. In addition, we newly synthesized SPAC-DMT with spiro-acridine donor for comparison with DMAC-DPS and DMAC-DMT. Spiro-acridine donor moiety could reduce conjugation length than acridine, which results in deeper blue color. It has also been reported that replacing the acridine moiety with a bulky and rigid spiro-acridine can enhance PLQY.14 Indeed, SPAC-DMT indicated the deepest emission peak at 440 nm among three materials, and showed the largest J(λ) value as shown in Figure 3a. However, unlike our expectation, SPAC-DMT had low PLQY of 38% which is significantly lower than DMAC-DPS and DMAC-DMT. Unlike previously reported material,24 our spiro-acridine TADF emitter, SPAC-DMT, results in poor TADF characteristics. In SPACDMT, an acceptor was changed from triazine to sulfone. Sulfone group has relatively weaker acceptor characteristic than triazine. Therefore, relatively weak donor of spiro-acridine seems to be insufficient to generate proper ICT characteristic as the TADF material. Furthermore, it had low T1 value of 2.80 eV as shown in Figure S1. Here △EST of DMAC-DPS and DMAC-DMT were measured as 0.09 eV and 0.01 eV, respectively, whereas SPAC-DMT indicated a large value of 0.25 eV. Due to these results, long delayed exciton lifetime of 5.19 µs was measured in SPAC-DMT, and the delayed component was relatively smaller than others as shown in Figure 3b. The thermal stability of electroluminescent materials is important for the morphological stability and device performances. In this regard, thermal properties of newly synthesized


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materials were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The decomposition temperatures (Td) of emitters measured by TGA at 5% weight loss were observed at 441.0 °C (SPAC-DMT) and 389.3 °C (BPPyA). The glass transition temperatures (Tg) of about 222.4 °C and 175.8 °C were indicated by DSC scan for SPAC-DMT and BPPyA, respectively as shown in Figure S2 and S3. The detailed properties are summarized in Table 1. 2.1.4. Requirement of emissive layer. Since TADF materials generally have strong polarity due to donor and acceptor structure, significant decrease of PLQY by the stabilization of singlet charge-transfer state and self-quenching seriously occurs in the film self-aggregation state. To develop a good deep blue THS system, it is necessary to avoid the efficiency decreasing process by increasing of vibronic decay and inefficient energy transfer by the self-quenching. To overcome these issues, the THS emissive layer (EML) system composed of three materials such as wide bandgap high triplet energy host, TADF host, and fluorescent emitter would be ideal. In such THS system the TADF material can be dispersed by wide bandgap host, which can prevent both of self-aggregation and self-quenching issues. Hence, fluorescent emitter can have almost 100% IQE without efficiency loss and original deep blue fluorescence characteristics with narrow emission can be kept.

2.2. Device performance 2.2.1. Fluorescent device performance. In order to see performances of the new fluorescent material BPPyA, fluorescent device (Device I) was firstly fabricated. Device I was fabricated with the configuration of ITO / 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN, 7 nm) / 1,1-bis[(di-4-tolylamino) phenyl] cyclo-hexane (TAPC, 50 nm) / 2-methyl-9,10-


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bis(naphthalen-2-yl) anthracene (MADN): 5% BPPyA (20 nm) / 2,2',2''-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi, 20 nm) / LiF (1.5 nm) / Al (100 nm). HATCN was served as a hole-injection material (HIL); TAPC was utilized as a hole-transport layer (HTL); MADN acted as host; TPBi was used as electron-transporting layer (ETL). The HTL and ETL were controlled under the thickness condition where the charge balance was well balanced and the optical condition is also considered to be deep blue color. In spite of the fluorescent device, it showed very high maximum EQE of 7.7% with deep blue color coordinates of (0.14, 0.10). Such high efficiency could be ascribed to triplet-triplet annihilation (TTA) and/or the horizontal molecular orientation influence in addition with high PLQY of BPPyA.25-26 2.2.2. TADF device performance. Highly efficient TADF devices are needed to consider very good energy transfer from TADF to fluorescent dopant. The device efficiency of various TADF device (Device II) with SPAC-DMT, DMAC-DPS and DMAC-DMT TADF emitters were examined. Device II was constructed in the following order of ITO / HATCN (7 nm) / TAPC (50 nm) / 3,5-di(9H-carbazol-9-yl)-N,N-diphenylaniline (DCDPA, 10 nm) / host: 40% TADF (25 nm) / diphenyl-4-triphenylsilylphenyl-phosphine oxide (TSPO1, 5 nm) / TPBi (15 nm) / LiF(1.5 nm) / Al (100 nm). DCDPA and TSPO1 were used as triplet exciton blocking layer (EBL); bis[2(diphenylphosphino) phenyl]ether oxide (DPEPO) and 2,8-bis (diphenylphosphine oxide) dibenzofuran (DBFPO) acted as host. To prevent triplet exciton quenching, high triplet energy materials, DCDPA (T1: 2.9 eV) and TSPO1 (T1: 3.36 eV), were used for EBLs and DPEPO (T1: 3.3 eV) and DBFPO (T1: 3.2 eV), were used for hosts. In results, fabricated devices with these emitters, DMAC-DPS and DMAC-DMT, exhibited high efficiency as known in previous study. Various doping concentration was examined and the best performance was achieved at 40% condition as shown in Figure S4. The maximum EQE of


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DMAC-DPS and DMAC-DMT devices are 18.5% and 20.1%, respectively, and the color coordinates are (0.16, 0.21) and (0.15, 0.16), respectively, as shown in Figure S5 and S6. Reported maximum EQE of these devices were 19.5% and 19.8%, respectively, and the color coordinates were slightly red shifted for DMAC-DMT. Due to the different host systems, the optical conditions have changed and the color was slightly red shifted. The DMAC-DMT exhibited somewhat higher performances in this work compared with reported results. The maximum EQE and efficiency roll-off characteristics were considerably improved. This may be due to high purity (99.9%) of DMAC-DMT of our synthesized material. However, since there was no problem in energy transfer comparison experiment, it was examined based on this reference. Here we can expect higher performance in THS device by using DMAC-DMT, due to higher EQE and deeper blue color. Meanwhile, SPAC-DMT device shows the deepest blue emission of (0.15, 0.13) as shown in PL spectrum. However, due to low PLQY and large △EST of SPAC-DMT, it indicated low maximum EQE of 9.0%. In addition, severe roll-off characteristic was observed, which was derived from the long delayed exciton lifetime as shown in Figure S7. Due to the poor device performance of the Device II, the characteristics in the THS device are expected to be low. 2.2.3. THS device performance. After the evaluation of the fluorescent device and TADF devices, THS devices were finally fabricated. THS devices (Device III) were fabricated by additionally doping with 0.7% of the fluorescent emitter in EML. Here triplet up-conversion efficiency is important to have an IQE of 100% in the THS device. Therefore, highly efficient and deep blue color TADF material that can effectively transfer energy to emitter was expected to show good THS device performance. The energy diagram of device and structure of materials are shown in Figure 4a and 4b, and current density-voltage-luminance characteristic is shown in


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Figure S8. The maximum EQE of SPAC-DMT device was 7.0%, which is similar to that of no sensitizer fluorescent device. Such low EQE was originated by poor energy transfer efficiency of SPAC-DMT. The SPAC-DMT has a large △EST value and low PLQY. Hence, triplet excitons formed in SPAC-DMT could not go rapidly to singlet state. In this case, the energy transfer to fluorescent emitter prefers via Dexter energy transfer process rather than Forster energy transfer. On the other hand, the maximum EQE of DMAC-DMT and DMAC-DPS THS devices were 15.5% and 11.8%, respectively. The efficiencies were decreased compared to Device II, and the loss was lower in the DMAC-DMT device. It can be seen that DMAC-DMT has higher PLQY and deeper blue color, so that energy transfer efficiency was better, which results in low energy loss. Measured EL spectra of THS and fluorescent devices were different even though we have improved color coordinate in the THS device as shown in Figure 4d. All THS devices have broader spectra than that of the fluorescent device. In general, the reason for the broadening of spectrum in blue devices are either change in the optical conditions due to different exciton recombination zones or influenced by high host polarity. However, our blue dopant is hardly affected by polarity. In our fabricated devices, the materials used for EML and HBL in the THS and fluorescence devices are different and the HTL thickness is also different. To investigate optical effect, various thickness of HTL and ETL were applied in fluorescent device. Modified device was fabricated with the configuration of ITO / HATCN (7 nm) / TAPC (40 nm) / MADN: 5% BPPyA (20 nm) / TPBi (20 nm) / LiF (1.5 nm) / Al (100 nm), and ITO / HATCN (7 nm) / TAPC (50 nm) / MADN: 5% BPPyA (20 nm) / TPBi (30 nm) / LiF (1.5 nm) / Al (100 nm), respectively. As shown in Figure S9, a blue shifted emission peak with shoulder variation near about 500 nm was observed with variation of HTL and ETL thickness. Our measured EL spectrum of THS device was similar to that of HTL thickness decreased fluorescent device.


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These result indicates that our measured spectrum of THS device is mainly originated from the blue dopant not from the sensitizer and emission zone variation give us these some spectral difference between THS and fluorescent devices. However, in the case of DMAC-DPS, due to the low spectral overlap, it was expected that fluorescence emission and DMAC-DPS host emission would be mixed. On the other hand, selecting wide bandgap host was also the issue for optimizing THS device. TADF materials can be dispersed to reduce self-quenching and vibronic decay by the singlet state stabilization, which results in high efficiency. In addition, energy transfer efficiency can be enhanced by reducing self-quenching. To confirm these effect, DBFPO host was replaced by DPEPO host.27 This host also has high triplet energy of 3.2 eV and more deep HOMO and LUMO level of 6.5 eV and 2.5 eV, respectively. Firstly, delayed exciton decay lifetime was measured in 40% doping film. In DBFPO host, slightly faster decay than DPEPO host was observed as shown in Figure S10. The decay lifetime was 2.49 µs, which was faster than that of DPEPO, 3.88 µs. Therefore, fast reverse intersystem crossing (RISC) can occur, and it should also improve energy transfer efficiency. Indeed, in DBFPO host device, maximum EQE was increased to 22.5% for Device II with almost same J-V characteristic, and Device III also showed an increased EQE of 19.0% as shown in Figure 4c. In comparison to Device II, the efficiency loss in Device III occurred due to introduction of fluorescent emitter. The DBFPO host showed a 15.5% efficiency loss and the DPEPO host had a 25.9% efficiency loss. Therefore, it was demonstrated that energy transfer occurred more effectively in DBFPO host device. However, DBFPO host devices showed red shifted spectrum than DPEPO host devices. DBFPO host is known as having higher electron mobility than DPEPO host. Therefore, the exciton recombination zone was shifted to the HTL side, thereby changing the optical condition


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and inducing the red shift phenomenon. Accordingly, we achieved almost same spectrum in similar optical condition by reducing ETL thickness as shown in Figure S11. This means that DBFPO host can improve energy transfer efficiency in THS device with maintaining deep blue color. The detailed device performances are summarized in Table 2. 2.2.4. Device operating stability. In all THS devices, roll-off characteristic was also improved. It was derived from decreased delayed exciton lifetime as shown in Figure 5a and Table 3. This is due to the rapid energy transfer from TADF to fluorescence in THS devices. From these results, it was expected that device stability would be improved in THS devices. To demonstrate this, the operating lifetimes of the TADF and THS devices were compared. Highly efficient devices containing DBFPO host and DMAC-DMT were used for comparison. Both of the TADF and THS devices started at the same initial luminance of 400 cd/m2. However, due to the deeper blue color of THS device, 1.5 times more current required to achieve the same luminance of 400 cd/m2. As a result, the operating lifetime of the TADF device was measured to be 0.7 hours, and that of the THS device was 2.8 hours as shown in Figure S12. Due to the unstable phosphine oxide series host and EBL and the sulfone series TADF materials, their lifetime was very short. Under these conditions, even though the 1.5 times higher current was applied, the lifetime of the THS device was 4 times longer than TADF device. Conventional TADF devices were suffered from singlet-triplet annihilation and triplet-triplet annihilation, but THS device can reduce these annihilations by rapid Förster energy transfer to fluorescent emitter.28 Therefore, enhanced lifetime could be achieved by using THS system.

2.3. Investigation of energy transfer mechanism


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2.3.1. Measurement of energy transfer mechanism. To confirm that the energy transfer occurs effectively in the THS device, various deposited films were fabricated and the change of exciton lifetime with the change of fluorescent material doping concentration were measured. The film conditions were DBFPO: 40% DMAC-DMT: X% BPPyA, and DBFPO: 1% BPPyA film was also fabricated to use for a reference. The PL spectrum was changed by adding just low concentration (0.4%) of fluorescent emitter, but spectra were same regardless of fluorescent emitter concentration as shown in Figure S13a. In addition, as shown in Figure S13b, the intensity of the spectrum was changed with increasing the fluorescent emitter doping concentration. Especially, it shows a much larger intensity in the thin film doped with TADF sensitizer at the same 1.0% condition compared to the film without TADF sensitizer. This demonstrates that energy transfer from TADF sensitizer to fluorescent emitter occurs well. In addition, the prompt and delayed exciton lifetime were also measured as shown in Figure 5a and 5b. Fluorescent lifetime of BPPyA was observed in the film without TADF sensitizer as 2.5 ns. In the film with TADF sensitizer, prompt and delayed exciton lifetimes were investigated with doping of BPPyA fluorescent emitter. Measured prompt and delayed lifetimes of DMAC-DMT with 0.4% BPPyA emitter were 12.0 ns and 1.09 µs, respectively. These values decreased to 7.2 ns and 0.41 µs at 1% doping condition, respectively, as shown in Figure 5a and Table 3. These decreased prompt and delayed exciton lifetimes in TADF sensitizer indicate that energy transfer rate from DMAC-DMT to BPPyA is very fast with Forster energy transfer. The energy transfer rate was calculated by applying the Stern-Volmer equation using the previously measured data.29 Generally, it is known that when the energy transfer rate is near 108 s-1 for singlet exciton and 106 s-1 for triplet exciton, energy transfer occurs well.30-31 Energy transfer rate was calculated by using the plot of the change in exciton lifetime ratio with doping concentration as shown in


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Figure 5c, and divided into prompt and delayed portion. Here molecular weight of DBFPO, DMAC-DMT and BPPyA are different, so concentration was calculated by converting the molar concentration ((mol/dm3)-1). As shown in Table 3, kET was calculated to 1.24ⅹ1010 s-1(mol/dm3)1

for prompt portion and 2.61ⅹ108 s-1(mol/dm3)-1 for delayed portion. Here by removing the

molar concentration term, the prompt portion is converted to 6.8ⅹ107 s-1 and the delayed portion to 1.4ⅹ106 s-1. These values indicate that energy transfer occurs effectively. However, since it can be inaccurate to confirm only by this factor, we have also compared it with the RISC rate constant. Assuming that effective energy transfer occurs in the THS device, the triplet excitons will firstly be upconverted to a singlet state and then very fast energy transfer to the fluorescence emitter will occur. Therefore, kET would be slightly slower or similar with reverse intersystem crossing rate (kRISC). We calculated the kRISC by using reported method and the result was 1.8ⅹ 106 s-1.32 Indeed, the kET value was calculated to be slightly lower than kRISC value. Therefore, it was demonstrated that very effective energy transfer occurred in our THS devices.

3. CONCLUSION In summary, we studied the THS device with focusing on Förster energy transfer mechanism. We established some criteria of selecting materials based on this mechanism, and designed the new deep blue THS device. Fluorescent blue emitter should have emission peak at below 460 nm with narrow spectrum, and small Stokes shift under 30 nm is required for effective energy accepting from host. Likewise, TADF materials indicating deep blue emission peak below 450 nm and high PLQY over 80% is required for large spectral overlap with fluorescent blue emitter. Under these conditions, the new deep blue fluorescent emitter, BPPyA, and various TADF material were synthesized and highly efficient deep blue fluorescent THS devices were


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successfully fabricated. Optimized device showed high EQE of 19.0% with color coordinates of (0.14, 0.15). We also investigated the energy transfer mechanism by calculating and analyzing the energy transfer rate in the THS devices. The energy transfer rates were 6.8ⅹ107 s-1 and 1.4 ⅹ106 s-1 for the prompt portion and the delayed portion, respectively. These values indicate that energy transfer occurs effectively. Due to fast energy transfer in THS device, improved roll-off characteristic and device operating lifetime were also observed. These results will be very useful to develop the highly efficient and stable deep blue OLEDs in the future.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, synthetic procedures, device fabrication and characterization, and the NMR and HRMS data

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Jang Hyuk Kwon) * E-mail: [email protected] (Ju Young Lee) ORCID Jang Hyuk Kwon: 0000-0002-1743-1486


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ACKNOWLEDGMENT This work was supported by Grant No. NRF-2016R1A6A3A11930666 and the Human Resources Development Program (Grant No. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government, Ministry of Trade, Industry and Energy. And this work was also supported by the Industrial Strategic Technology Development Program of MKE/KEIT (10048317).

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Table 1. Photo-physical and thermal data of used materials.

Td

Tg

(°C)

(°C)

2.85 (0.09)

-

3.08

3.01 (0.01)

-

6.03

2.98

2.80 (0.25)

5.90

3.20

-

Material

λmaxa) (nm)

Bandgap (eV)

HOMO (eV)

LUMO (eV)

T1 (∆EST)

DMAC-DPS

455

2.94

5.94

3.00

DMAC-DMT

447

3.02

6.10

SPAC-DMT

440

3.05

BPPyA

458

2.70

a)

ΦPL

τpb) (ns)

τdb) (µs)

-

0.80b)

22.5

4.39

-

0.90b)

24.5

2.49

441.0 222.4

0.38b)

31.5

5.19

389.3 175.8

0.985a)

-

-

Toluene solution (10-5 M), b)DBFPO: 40% doping film


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Table 2. Device performance of Device I, II and III.

Maximum efficiency

Device (EML structure)

I

Performance at 500 cd/m2

CEa)

EQEb)

CEa)

EQEb)

Color

(cd/A)

(%)

(cd/A)

(%)

coordinate

MADN: BDc)

7.1

7.7

6.8

7.3

(0.14, 0.10)

DPEPO: DMAC-DPS

25.5

18.5

18.9

13.7

(0.16, 0.21)

DPEPO: DMAC-DMT

23.1

20.1

20.4

17.9

(0.15, 0.16)

DBFPO: DMAC-DMT

27.3

22.5

24.2

20.8

(0.15, 0.18)

DBFPO: SPAC-DMT

11.4

9.0

1.6

1.3

(0.15, 0.13)

DPEPO: DMAC-DPS: BD

14.5

13.0

11.2

10.1

(0.15, 0.16)

DPEPO: DMAC-DMT: BD

12.9

15.5

11.4

13.7

(0.15, 0.11)

DBFPO: DMAC-DMT: BD

18.3

19.0

17.2

17.5

(0.14, 0.15)

DBFPO: SPAC-DMT: BD

5.8

7.0

2.6

3.0

(0.14, 0.11)

II

III

a)

CE : current efficiency, b)EQE : external quantum efficiency, c)BD : blue fluorescent dopant; BPPyA


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Table 3. Energy transfer rate measurement results from DMAC-DMT to BPPyA with doping concentration variation.

Molar Doping

Prompt Delayed

τ0/τ concentration

concentration

(ns)

(µs)

3 -1

τ0/τ

Prompt kET

Delayed kET

(prompt) (delayed) (s-1(mol/dm3)-1) (s-1(mol/dm3)-1)

((mol/dm ) ) 0%

24.5

2.49

-

-

-

0.4%

12

1.09

3.12x10-3

2.04

2.28

0.7%

9.3

0.62

5.46x10-3

2.63

4.02

1.0%

7.2

0.41

7.80x10-3

3.40

6.07

1.24ⅹ1010

2.61ⅹ108


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Figure 1. Electroluminescence emission mechanism in THS

device. FRET means Förster resonance energy transfer.


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Figure 2. (a) Molecular structure of BPPyA (black line), and calculated structure in ground state (blue line) and excited state (red line) by DFT simulation with B3LYP/631G* (b) UV-vis absorption spectrum and PL emission spectrum of BPPyA. (c) Solvatochromatic effect of BPPyA.


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Figure 3. (a) UV-Vis absorption spectrum of BPPyA and PL spectra of various TADF materials. It exhibits the largest spectral overlap in the SPAC-DMT. (b) Delayed exciton lifetime of three TADF materials in doping film.


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Figure 4. (a) The THS device structure and the energy diagram. The numbers in parentheses

indicate triplet energy. (b) The molecule structure of some materials including new ones. (c) Device performance of THS devices (Device III). Black line indicates the TADF Device II of DMAC-DMT used as a comparison. External quantum efficiency verses current density characteristic; (d) Normalized electroluminescence (EL) spectra;


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Figure 5. (a) Delayed exciton lifetime of DMAC-DMT film (DBFPO: 40% DMAC-DMT: X% BPPyA). (b) Exciton lifetime of BPPyA film (DBFPO: 1% BPPyA). (c) Stern-Volmer Plot with increasing BPPyA doping concentration.


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