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Stable Thermally Activated Delayed Fluorescence‐Sensitized Red Fluorescent Devices through Physical Suppression of Dexter Energy Transfer

Stable Thermally Activated Delayed Fluorescence‐Sensitized Red Fluorescent Devices through... IntroductionIn recent days, organic light‐emitting diodes (OLEDs) are evolving as promising optoelectronic devices owing to their high color purity, light weight, flexibility, wide viewing angle, and low cost. As of now, phosphorescent materials are used commercially in the OLED industries; however, they need highly cost noble metals.[1,2] Later on, the thermally activated delayed fluorescence (TADF) emitters were developed based on pure organic cores without needing any heavy metals. These TADF emitters can achieve even 100% internal quantum efficiency (IQE) through efficient reverse intersystem crossing (RISC).[3–6] However, the conventional donor–acceptor (D–A)‐based TADF materials tend to show intramolecular charge transfer (ICT), which results in full‐width half maximum (FWHM) of >50 nm of the emission spectra. Such broad FWHM leads to low color purity and fails to meet the Commission internationale de l'éclairage (CIE) coordinate criteria corresponding to BT.2020 (Rec.2020) for ultrahigh‐definition displays. Moreover, TADF materials have long‐lived excitons, which lead to annihilation processes such as triplet–triplet annihilation (TTA) and triplet‐polaron annihilation (TPA). Such processes lead to a reduction in the efficiency and operational lifetime of the devices.[7–15]In order to overcome the issues mentioned above, Adachi et al. developed the TADF‐sensitized fluorescent (TSF) system consisting of the TADF‐sensitizing host and fluorescence dopant (FD) as a final emitter.[16–18] In TSF OLEDs, TADF material upconverts all the dark triplet excitons into singlet excitons, which are transferred to the singlet state of the final emitter through the Förster resonance energy transfer (FRET) channel. Since the FRET rate is higher than the relaxation rate of the excited state of the TADF sensitizer, the triplet excitons up conversion happen more rapidly. Hence, TSF devices could efficiently transfer more singlet excitons to the final emitter, by which the TTA process is minimized and also improves the device stability.[19–21] The efficient FRET process can be achieved by the larger overlap between the emission spectra of the TADF sensitizer and the absorption spectrum of the final emitter. Parallelly, an unwanted energy loss from T1 state of the TADF sensitizer to T1 state of the final emitter occurs through a short‐range electron exchange mechanism called Dexter energy transfer (DET). DET is an electron exchange process involving p‐orbital overlap. It occurs well when the intermolecular distance is small as triplet exciton transition occurs mainly between the sensitizer and final emitter, which may accelerate the final emitter's material degradation, impacting the device's operational lifetime.[22,23] Hence, to achieve a highly efficient and stable TSF‐OLED device with a longer lifetime, DET should be mostly minimized while FRET should be enhanced simultaneously.On account of this issue, some approaches have been developed for reducing energy loss through the DET process, whereas the FRET process is unchanged. One approach is increasing intramolecular distance by inserting chemically inert‐bulky groups into the fluorescent/TADF emitter.[24–29] For example, Kwon et al. reported a pure red TSF‐OLED using the tert‐butyl‐substituted fluorescence emitter (4tBuMB) along with 4CzIPN and 4CzTPN TADF sensitizers. Such bulky group substitution increases the intramolecular distance and large spectral overlap with the sensitizers, which leads to a high FRET rate and reduced ISC/RISC cycles. As a result, the 4CzTPN‐based OLED device has shown better efficiency and lifetime (LT90) of 954 h at 3000 cd m‐2.[30] However, the DET process is not minimized well due to the low RISC rate and long‐delayed fluorescence lifetime of 4CzTPN. To overcome this issue, Lee et al. recently reported a TADF molecule (12BTCzTPN) by modifying the 4CzTPN with a sulfur atom in two of the donors. The heavy atom effect of sulfur increases the RISC rate about 4.5 times that of 4CzTPN and suppresses the DET process. Which results in improved device efficiency and lifetime by 1.4‐folds.[31] Apart from these chemical approaches, the improvement in the device architectures was also studied to alleviate the DET process. Liu et al. reported red TSF‐OLEDs with an assistant emission layer (AEL) showing a long operational device lifetime (LT95) of 900 h were achieved with corresponding CIE coordinates of (0.66, 0.33).[32] However, there is still a need for more improvements in highly efficient, stable red devices with long operational lifetimes for real‐time applications.In this work, we present a detailed mechanistic study of a TSF device with modified device architecture by inserting an additional layer called the ‘DET suppress layer (DSL)’ next to the final dopant without changing the emissive layer (EML) total thickness. The DSL consists of a host and a TADF sensitizer with a total thickness of 5 and 7 nm. Insertion of such DSL physically increases the distance between the excitons from the host‐TADF system to the final emitter. This increased exciton passage distance fairly suppresses the short‐range DET process while maintaining the FRET process. Since the DET is a distance‐sensitive process, the thickness of DSL was restricted to 5 and 7 nm that are not too thin to fabricate. Additionally, by taking into account excitons distribution, the position of the DSL was adjusted between the TSF‐emission layer (EML) and hole‐blocking layer (HBL) rather than between the electron‐blocking layer (EBL) and TSF‐EML. As a result, the optimized DSL‐TSF devices have shown comparatively higher efficiency, 17%, and a longer operational lifetime of over 370 h at an initial luminescence of 5000 cd m‐2 than the normal TSF devices.Results and DiscussionThe TSF devices were constructed utilizing a fluorescent emitter 4tBuMB as the final dopant, which has a LUMO of –3.8 eV.[30] To suppress electron trapping and for enhanced charge transfer properties, a TADF molecule (12BTCzTPN) with a very deep LUMO of –4.0 eV was used.[31] First, the TADF devices were fabricated using optimized doping concentrations of 10% and 15 % of 12BTCzTPN. The device structure is ITO (50 nm)/HATCN (7 nm)/PCBBiF (70 nm)/PCzAC (10 nm)/DIC‐TRZ: x% 12BTCzTPN (30 nm)/DDBFT (5 nm)/BPPB: Liq (60 nm)/LiF (1.5 nm)/Al (100 nm). The device structure with corresponding energy levels is depicted in Figure 1a, where indium‐tin‐oxide (ITO) and aluminum (Al) were used as anode and cathode, respectively. Dipyrazino[2,3‐f:2′,3′‐h]quinoxaline‐2,3,6,7,10,11‐hexacarbonitrile (HATCN), N‐(1,1′‐biphenyl‐4‐yl)‐N‐[4‐(9‐phenyl‐9H‐carbazol‐3‐yl)phenyl]‐9,9‐dimethyl‐9H‐fluoren‐2‐amine (PCBBiF), and 9,10‐dihydro‐9,9‐dimethyl‐10‐ (9‐phenyl‐9H‐carbazol‐3‐yl)‐acridine (PCzAC) were utilized as a hole‐ injection layer, transporting layer, and electron‐blocking layer, respectively. To prevent the carrier trap, 11‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐12‐phenyl‐11,12‐dihydroindolo[2,3‐a]carbazole (DIC‐TRZ) was used as the host material that has wide energy bandgap and bipolar characteristics.[33] 2,4‐Bis(dibenzo[b,d]furan‐2‐yl)‐6‐phenyl‐1,3,5‐triazine (DDBFT), 50% 8‐hydroxyquinolinolato‐lithium (Liq)‐doped 1,3‐bis(9‐phenyl‐1,10‐phenanthrolin‐2‐yl)benzene (BPPB) and lithium fluoride (LiF) were applied for a hole‐blocking and electron‐transporting layer, respectively.[34] The molecular structure of materials used in each layer is shown in Figure S1 (Supporting Information).1Figurea) The device structure of TADF and TSF devices, EL properties of 10%/15% TADF device, and TSF device with 10% TADF‐doped, b) current density–voltage–luminance (J–V–L) plot, c) EL spectrum of each device, d) EQE‐luminance plot, and e) LT95 devices operational lifetime at an initial luminance of 5000 cd m‐2.The EL characteristics of TADF and TSF devices are shown in Figure 1, and the data are summarized in Table S1 (Supporting Information). In contrast, from Figure 1b, both TADF devices with 10% and 15% of 12BTCzTPN showed relatively low turn‐on voltages of 2.7 V, indicating the low‐energy barriers in all layers of the device. As observed in Figure 1c, the EL spectra of 10% and 15% doped devices were at 571 and 574 nm with FWHM of 94 and 95 nm, respectively. An increase in the concentration resulted in little red‐shifted emission. These two TADF devices have shown maximum external quantum efficiency (EQEmax) of 8.3% and 6.9%, along with the operational lifetime of 206 and 187 h at an initial luminescence of 5000 cd m‐2 (Figure 1d,e). However, among the two devices, the 10% doped TADF device shows better efficiency. In general, the higher concentration of TADF sensitizer causes more aggregation quenching effect, resulting in poor efficiency.[35] Accordingly, the TSF device was constructed using 10% of 12BTCzTPN and 0.7% of 4tBuMB and achieved a high EQE of 16.3% at an EL peak of 618 nm with the FWHM of 44 nm. Which showed a slightly shorter lifetime (LT95) of 182 h at an initial luminescence of 5000 cd m‐2 compared with those TADF devices (Figure 1c–e).As aforementioned, we fabricated a TSF device by inserting an additional DSL consisting of 12BTCzTPN and DIC‐TRZ. In order to attain better results, the position of DSL is important. It was fixed after analyzing the exciton distribution and the carrier recombination zone by evaluating a hole‐only device (HOD) and electron‐only device (EOD) of 12BTCzTPN. The results are shown in Figure 2. It is visible from Figure 2a that the J–V curve of the HOD is lower than that of the EOD, which indicates that the exciton distribution and recombination zone were mostly on the HBL side. Furthermore, sensing layer experiments were carried out to confirm the exciton distribution.[36,37] A red fluorescent material, 5,10,15,20‐tetraphenylbisbenz[5,6]indeno[1,2,3‐cd:1′,2′,3′‐lm]perylene (DBP) (the structure in Figure 2c) was employed as the sensing layer at three positions in EML such as EBL side, middle, and HBL side with the thickness of 0.6 nm as shown in Figure 2b.[38,39] The device was fabricated with the same total thickness as the TSF device and the device structure is: ITO (50 nm)/HATCN (HIL, 7 nm)/PCBBiF (HTL, 70 nm)/PCzAC (EBL, 10 nm)/DIC‐TRZ: 10% 12BTCzTPN/DIC‐TRZ: 10% 12BTCzTPN (EML, x nm)/DBP (sensing layer, 0.6 nm)/DIC‐TRZ: 10% 12BTCzTPN (EML, 29.4‐x nm)/DDBFT (HBL, 5 nm)/BPPB: Liq (ETL, 60 nm)/LiF (EIL, 1.5 nm)/Al (100 nm). From Figure 2c, it is understood that the EL spectra were almost similar when the sensing layer (DBP) was moved from the EBL side to the middle side. A significant shift in EL spectra was observed when the sensing layer (DBP) was near HBL that further confirms that the exciton distribution was mainly on the HBL side. Based on this finding, we projected that the distribution of excitons in the TSF‐DSL device would be on the DSL side as shown in Figure 2d.2Figurea) J–V property of HOD and EOD of 12BTCzTPN, b) The sensing layer position in the EML, c) EL spectrum of the device with sensing layer at different positions and molecular structure of DBP, and d) Predicted exciton distribution in the TSF‐DSL device.The charge distribution results implemented the insertion of the additional layer on the HBL side fairly affect the device efficiency, and lifetime and also alters the emission spectra. Better outcomes would be obtained by inserting the DSL on the EBL side. TSF devices with DSL insertion on both the HBL and the EBL were fabricated under optimized device parameters and evaluated for comparable results. Initially, the DSL consisting of DIC‐TRZ: 10% 12BTCzTPN with the thickness of 5 and 7 nm was inserted in between TSF‐EML and HBL. To maintain the total EML thickness at 30 nm, the TSF‐EML was fabricated with thicknesses of 25 and 23 nm along with 5 and 7 nm of DSL, respectively. The structure and the mechanism of the DSL‐TSF‐EML‐based device have shown in Figure 3a,b. The evaluated results of these devices with 5 and 7 nm of DSL thickness have shown low turn‐on voltage of 2.7 V and EQEmax of 16.0% and 12.8% (Figure 3c,d). The two devices have almost similar EL peaks at 617 and 616 nm with FWHM of 48 and 54 nm, respectively. However, as shown in Figure 3e, the insertion of DSL on the HBL side results in poor color purity with intensified EL shoulder peak of 560–570 nm compared with the fabricated TSF. It was noticed that the shoulder peak is much improved in the device with the increased thickness of DSL. The respective operational lifetimes of devices are (LT95) 213 and 255 h at initial luminescence of 5000 cd m‐2 (Figure 3f). However, efficiency roll‐off and device lifetime improved by 3.42 and 1.40 times compared with the TSF device, respectively. The summary of EL performances of TSF‐DSL‐HBL devices is tabulated in Table S2 (Supporting Information).3FigureTSF‐DSL‐HBL device: a) structure, b) mechanism, c) current density–voltage–luminance (J–V–L) plot, d) EQE‐luminance plot, e) EL spectrum of each device, f) device operational lifetimes (LT95) at an initial luminance of 5000 cd m‐2.Subsequently, we fabricated and evaluated the device by repositioning the DSL between EBL and TSF‐EML to minimize the emission of TADF sensitizer. Figure 4a shows the detailed device structure and the related mechanism was depicted in Figure 4b. All the devices have low turn‐on voltage as shown in Figure 4c, which implies the low charge barriers in the device layers. The EBL‐DSL‐TSF‐based devices with DSL thicknesses of 5 and 7 nm have achieved almost similar EQEmax of 16.7% and 17.0%, respectively (Figure 4d). As observed from Figure 4e, EL peak is not much affected compared with TSF and is found at 617 and 618 nm with FWHM of 45 and 45 nm, respectively. Moreover, the DSL with 7 nm thickness has shown a noticeably longer operational lifetime (LT95) of 370 h than the 5 nm DSL‐device with a lifetime of 280 h at an initial luminance of 5000 cd m‐2 (Figure 4f). The most noteworthy lifetime improvement of the device was found to be 1.45 (7 nm DSL) times higher compared with the DSL on HBL side devices and 2.03 times higher than the TSF device. The EL characteristics are tabulated in Table 1.4FigureEL properties of EBL‐DSL‐TSF devices: a) Current density–voltage–luminance (J–V–L) plot, b) EQE‐luminance plot, c) EL spectrum of each device, d) Device operational lifetimes (LT95) at an initial luminance of 5000 cd m‐2.1TableSummary of performances of EBL‐DSL‐TSF devicesDSLEMLVon/Vd [@3000 cd m‐2]EQEmax /EQE [@3000 cd m‐2]EL peak [nm]FWHM [nm]CIE coordinatesLT95 [@5000 cd m‐2] hTADFThickness [nm]HF (TADF: FD)Thickness‐‐10%: 0.7%30 nm2.6/6.4 V16.3/12.3%61844(0.64, 0.36)18210%510%: 0.7%25 nm2.6/6.4 V16.7/12.8%61745(0.64, 0.36)28010%710%: 0.7%23 nm2.6/6.4 V17.0/14.3%61845(0.63, 0.37)370Furthermore, we calculated the FRET radius (R0) of 12BTCzTPN using equation (1). R0 signifies the distance at which the energy transition efficiency between the two molecules is 50%.[40]1R06=9000Φpκ2128π5n4JF\[\begin{array}{*{20}{c}}{R_0^6 = \frac{{9000{\Phi _{\rm{p}}}{\kappa ^2}}}{{128{\pi ^5}{n^4}}}{J_{\rm{F}}}}\end{array}\]where κ2 is the orientation factor, Φ is the quantum yield of the fluorescence, NA is the Avogadro number, n is the refractive index, and JF is the overlap integral between donor emission and acceptor absorption. The calculated R0 of 12BTCzTPN is 4.33 nm, which is lesser than the thickness of DSL (7 nm). Such a long distance between the two molecules than R0 results in less FRET process. Hence, thicker DSL can also emit light with improved device lifetime and efficiency roll‐off characteristics.We have investigated the energy transfer processes relative to the excitons generated in TADF‐sensitizer 12BTCzTPN as a donor and to the FD 4tBuMB as an acceptor in both TSF and DSL‐TSF devices using time‐resolved photoluminescence (TRPL) and PLQY measurements.[41] For the accurate kFRET and kDET evaluation, TSF thin film and DSL‐TSF thin film were fabricated with the same doping concentration of both TADF sensitizer and FD. The total thickness was maintained at 30 nm, like the EML thickness of the device (Figure 5a). Where the chosen DSL thickness is 7 nm. The transient PL decay curves of prompt and delayed fluorescence of both TSF and DSL‐TSF are shown in Figure 5b.5Figurea) The organization of TSF thin film, DSL film, and b) TRPL results of manufactured thin film.Both the TSF and DSL‐TSF films displayed clear exponential decay curves exhibiting the nanoscale of prompt and the microscale of delayed fluorescence, as shown in Figure 5b. The energy‐transfer rate constants kISC, kRISC, kr,, and knr of the TADF sensitizer are calculated using the equations S1–S6 (Supporting Information), and the corresponding results are tabulated in Table 2. Among these two films, the DSL‐TSF film shows an improved delayed lifetime showing a more population of triplet excitons in TADF. Such an increase in triplet states indicates that the DSL insertion improves the delayed components in the TADF by suppressing triplet exciton transfer to the final emitter. The exciton dynamics in the DSL‐TSF device compared with the normal TSF device shows increased ISC and RISC rates driven by the triplet population in the TADF. This generates more singlet excitons and enhances the FRET process to the final dopant. The improved radiative decay rate (kr) of the TADF confirms the rapid transfer of the singlet excitons (Table 2). Such enhanced efficiency by more generated singlet excitons is attributed to the improvement of device lifetime in the DSL‐TSF. Generally, the TADF triplet excitons lifetime is in microseconds and milliseconds in the final emitter. Such long‐lived triplet excitons generated in the final emitter increase the probability of material degradation through TTA and TPA processes. But, the TADF sensitizer possesses a 103‐fold lowered triplet exciton lifetime than the final emitter, and the probability of the exciton annihilation processes is reduced, enhancing device operational stability. Additionally, the TADF triplet excitons are rapidly transferred to the final emitter by faster energy transfer, which reduces the duration of triplet excitons in the TADF sensitizer. Hence, we believe that the energy stored in the TADF triplets will not cause a serious impact on the device's lifetime. Such enhanced charge dynamic properties reduce the electrically induced device degradation,[19] resulting in reduced efficiency roll‐off properties and an improved operational lifetime of 370 h (Table 1).2TableEnergy transfer rate constants, kFRET, and kDET values of TSF thin film and DSL‐TSF thin filmτp [ns]τd [µs]kr [x 107 s‐1]knr [x 104 s‐1]kISC [x 107 s‐1]kRISC [x 106 s‐1]kFRET [x 108 s‐1]kDET [x 105 s‐1]TSF film6.50.204.578.289.291.241.1413.6DSL‐TSF film6.40.246.195.9310.981.281.165.2Additionally, the kFRET and kDET were calculated using the following equations (2) and (3)[42–44] and tabulated in Table 2.2kFRET=kP−kr, S−kISC\[\begin{array}{*{20}{c}}{{k_{{\rm{FRET}}}} = {k_{\rm{P}}} - {k_{{\rm{r,}}\;{\rm{S}}}} - {k_{{\rm{ISC}}}}}\end{array}\]3kDET=kPkD−kRISC(kr, s+kFRET)kr,S+kISC+kFRET−knr, T\[\begin{array}{*{20}{c}}{{k_{{\rm{DET}}}} = \frac{{{k_{\rm{P}}}{k_{\rm{D}}} - {k_{{\rm{RISC}}}}\left( {{k_{{\rm{r,}}\;{\rm{s}}}} + {k_{{\rm{FRET}}}}} \right)}}{{{k_{{\mathop{\rm r}\nolimits} ,S}} + {k_{{\rm{ISC}}}} + {k_{{\rm{FRET}}}}}} - {k_{{\rm{nr,}}\;{\rm{T}}}}}\end{array}\]where kP and kD are prompt and delayed fluorescence rates, respectively. kr, S and knr, T express the radiative rate constant of the singlet and the nonradiative rate constant of the triplet, respectively. kISC and kRISC are intersystem crossing‐ and reverse intersystem crossing rate constant, respectively. From Table 2, the DSL thin film showed slightly improved kFRET values of 1.16 × 108 s‐1 and lowered kDET of 5.2 × 105 s‐1 compared with TSF thin film. As per our ideology, DSL insertion could suppress DET with a maintained FRET process. A remarkable DET suppression of 2.62 times that of TSF thin film is observed. Such a low DET minimizes the triplet exciton transfer and notably reduces the long‐lived triplet excitons in the final emitter and prohibits the destructive TTA and TPA processes. Hence, the final emitter degradation has significantly decreased, improving device stability (Table 1). This also induces better exciton generation that is transferred to the final emitter, resulting in improved device efficiency of 17%.On the other hand, we have also evaluated the singlet and triplet excitons decay rate for the comparison of the exciton distribution in the TADF sensitizer by using the following equations (4) and (5).[45,46]4dNS(t)dt=−(kFRET+kr+knr,s+kISC)NS(t)+kRISCT(t)\[\begin{array}{*{20}{c}}{\frac{{{\rm{d}}{{\rm{N}}_{\rm{S}}}\left( {\rm{t}} \right)}}{{{\rm{dt}}}} = - \left( {{k_{{\rm{FRET}}}} + {k_{\rm{r}}} + {k_{{\rm{nr,s}}}} + {k_{{\rm{ISC}}}}} \right){N_{\rm{S}}}\left( {\rm{t}} \right) + {k_{{\rm{RISC}}}}T\left( {\rm{t}} \right)}\end{array}\]5dNT(t)dt=−(kDET+knr,T+kRISC)NT(t)+kISCNS(t)\[\begin{array}{*{20}{c}}{\frac{{{\rm{d}}{{\rm{N}}_{\rm{T}}}\left( {\rm{t}} \right)}}{{{\rm{dt}}}} = - \left( {{k_{{\rm{DET}}}} + {k_{{\rm{nr,T}}}} + {k_{{\rm{RISC}}}}} \right){N_{\rm{T}}}\left( {\rm{t}} \right) + {k_{{\rm{ISC}}}}{N_{\rm{S}}}\left( {\rm{t}} \right)}\end{array}\]where NS and NT are singlet and triplet exciton densities, respectively and knr, S is the nonradiative decay rate constant of singlet excitons. The calculated values were plotted and shown in Figure 6a,b. As shown in Figure 6a, the singlet density of DSL‐TSF and TSF devices did not significantly vary, which confirms the almost same FRET process that occurred in both devices. As a result, there was little variation in the singlet exciton density with respect to time. As can be seen from Figure 6b, a long‐lived triplet exciton was detected in the DSL‐TSF device when compared with the decay curves of the triplet excitons of both devices. It occurs because the DET process transfers fewer triplet excitons to the final emitter. The observation strengthens our findings that inserting a DSL enhances the device's lifetime by lowering the DET process and the efficiency roll‐off characteristics (Figure 6c).6FigureThe decay rate of: a) singlet‐, b) triplet exciton density depending on time, c) EQE‐current density property of SDL (7 nm)/TSF‐EML device and TSF device.ConclusionIn summary, we have reported a clear mechanistic study of a TSF device that can remarkably suppress DET while maintaining the FRET process. This was achieved through the insertion of a “DSL” consisting of a host and a TADF sensitizer next to EML. The position of the DSL was optimized TSF‐EML and EBL using exciton distribution evaluation. Such DSL insertion resulted in the noticeable suppression of DET by 2.62‐fold compared with a standard TSF device with enhanced EQEmax of 17.0%. The DSL‐TSF showed a pure red emission at 618 nm with an FWHM of 45 nm, and the corresponding CIE coordinates are (0.63, 0.37). 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Shikita, T. Yasuda, Chem. Mater. 2017, 29, 7014. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Stable Thermally Activated Delayed Fluorescence‐Sensitized Red Fluorescent Devices through Physical Suppression of Dexter Energy Transfer

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© 2023 Wiley‐VCH GmbH
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2196-7350
DOI
10.1002/admi.202300147
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Abstract

IntroductionIn recent days, organic light‐emitting diodes (OLEDs) are evolving as promising optoelectronic devices owing to their high color purity, light weight, flexibility, wide viewing angle, and low cost. As of now, phosphorescent materials are used commercially in the OLED industries; however, they need highly cost noble metals.[1,2] Later on, the thermally activated delayed fluorescence (TADF) emitters were developed based on pure organic cores without needing any heavy metals. These TADF emitters can achieve even 100% internal quantum efficiency (IQE) through efficient reverse intersystem crossing (RISC).[3–6] However, the conventional donor–acceptor (D–A)‐based TADF materials tend to show intramolecular charge transfer (ICT), which results in full‐width half maximum (FWHM) of >50 nm of the emission spectra. Such broad FWHM leads to low color purity and fails to meet the Commission internationale de l'éclairage (CIE) coordinate criteria corresponding to BT.2020 (Rec.2020) for ultrahigh‐definition displays. Moreover, TADF materials have long‐lived excitons, which lead to annihilation processes such as triplet–triplet annihilation (TTA) and triplet‐polaron annihilation (TPA). Such processes lead to a reduction in the efficiency and operational lifetime of the devices.[7–15]In order to overcome the issues mentioned above, Adachi et al. developed the TADF‐sensitized fluorescent (TSF) system consisting of the TADF‐sensitizing host and fluorescence dopant (FD) as a final emitter.[16–18] In TSF OLEDs, TADF material upconverts all the dark triplet excitons into singlet excitons, which are transferred to the singlet state of the final emitter through the Förster resonance energy transfer (FRET) channel. Since the FRET rate is higher than the relaxation rate of the excited state of the TADF sensitizer, the triplet excitons up conversion happen more rapidly. Hence, TSF devices could efficiently transfer more singlet excitons to the final emitter, by which the TTA process is minimized and also improves the device stability.[19–21] The efficient FRET process can be achieved by the larger overlap between the emission spectra of the TADF sensitizer and the absorption spectrum of the final emitter. Parallelly, an unwanted energy loss from T1 state of the TADF sensitizer to T1 state of the final emitter occurs through a short‐range electron exchange mechanism called Dexter energy transfer (DET). DET is an electron exchange process involving p‐orbital overlap. It occurs well when the intermolecular distance is small as triplet exciton transition occurs mainly between the sensitizer and final emitter, which may accelerate the final emitter's material degradation, impacting the device's operational lifetime.[22,23] Hence, to achieve a highly efficient and stable TSF‐OLED device with a longer lifetime, DET should be mostly minimized while FRET should be enhanced simultaneously.On account of this issue, some approaches have been developed for reducing energy loss through the DET process, whereas the FRET process is unchanged. One approach is increasing intramolecular distance by inserting chemically inert‐bulky groups into the fluorescent/TADF emitter.[24–29] For example, Kwon et al. reported a pure red TSF‐OLED using the tert‐butyl‐substituted fluorescence emitter (4tBuMB) along with 4CzIPN and 4CzTPN TADF sensitizers. Such bulky group substitution increases the intramolecular distance and large spectral overlap with the sensitizers, which leads to a high FRET rate and reduced ISC/RISC cycles. As a result, the 4CzTPN‐based OLED device has shown better efficiency and lifetime (LT90) of 954 h at 3000 cd m‐2.[30] However, the DET process is not minimized well due to the low RISC rate and long‐delayed fluorescence lifetime of 4CzTPN. To overcome this issue, Lee et al. recently reported a TADF molecule (12BTCzTPN) by modifying the 4CzTPN with a sulfur atom in two of the donors. The heavy atom effect of sulfur increases the RISC rate about 4.5 times that of 4CzTPN and suppresses the DET process. Which results in improved device efficiency and lifetime by 1.4‐folds.[31] Apart from these chemical approaches, the improvement in the device architectures was also studied to alleviate the DET process. Liu et al. reported red TSF‐OLEDs with an assistant emission layer (AEL) showing a long operational device lifetime (LT95) of 900 h were achieved with corresponding CIE coordinates of (0.66, 0.33).[32] However, there is still a need for more improvements in highly efficient, stable red devices with long operational lifetimes for real‐time applications.In this work, we present a detailed mechanistic study of a TSF device with modified device architecture by inserting an additional layer called the ‘DET suppress layer (DSL)’ next to the final dopant without changing the emissive layer (EML) total thickness. The DSL consists of a host and a TADF sensitizer with a total thickness of 5 and 7 nm. Insertion of such DSL physically increases the distance between the excitons from the host‐TADF system to the final emitter. This increased exciton passage distance fairly suppresses the short‐range DET process while maintaining the FRET process. Since the DET is a distance‐sensitive process, the thickness of DSL was restricted to 5 and 7 nm that are not too thin to fabricate. Additionally, by taking into account excitons distribution, the position of the DSL was adjusted between the TSF‐emission layer (EML) and hole‐blocking layer (HBL) rather than between the electron‐blocking layer (EBL) and TSF‐EML. As a result, the optimized DSL‐TSF devices have shown comparatively higher efficiency, 17%, and a longer operational lifetime of over 370 h at an initial luminescence of 5000 cd m‐2 than the normal TSF devices.Results and DiscussionThe TSF devices were constructed utilizing a fluorescent emitter 4tBuMB as the final dopant, which has a LUMO of –3.8 eV.[30] To suppress electron trapping and for enhanced charge transfer properties, a TADF molecule (12BTCzTPN) with a very deep LUMO of –4.0 eV was used.[31] First, the TADF devices were fabricated using optimized doping concentrations of 10% and 15 % of 12BTCzTPN. The device structure is ITO (50 nm)/HATCN (7 nm)/PCBBiF (70 nm)/PCzAC (10 nm)/DIC‐TRZ: x% 12BTCzTPN (30 nm)/DDBFT (5 nm)/BPPB: Liq (60 nm)/LiF (1.5 nm)/Al (100 nm). The device structure with corresponding energy levels is depicted in Figure 1a, where indium‐tin‐oxide (ITO) and aluminum (Al) were used as anode and cathode, respectively. Dipyrazino[2,3‐f:2′,3′‐h]quinoxaline‐2,3,6,7,10,11‐hexacarbonitrile (HATCN), N‐(1,1′‐biphenyl‐4‐yl)‐N‐[4‐(9‐phenyl‐9H‐carbazol‐3‐yl)phenyl]‐9,9‐dimethyl‐9H‐fluoren‐2‐amine (PCBBiF), and 9,10‐dihydro‐9,9‐dimethyl‐10‐ (9‐phenyl‐9H‐carbazol‐3‐yl)‐acridine (PCzAC) were utilized as a hole‐ injection layer, transporting layer, and electron‐blocking layer, respectively. To prevent the carrier trap, 11‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐12‐phenyl‐11,12‐dihydroindolo[2,3‐a]carbazole (DIC‐TRZ) was used as the host material that has wide energy bandgap and bipolar characteristics.[33] 2,4‐Bis(dibenzo[b,d]furan‐2‐yl)‐6‐phenyl‐1,3,5‐triazine (DDBFT), 50% 8‐hydroxyquinolinolato‐lithium (Liq)‐doped 1,3‐bis(9‐phenyl‐1,10‐phenanthrolin‐2‐yl)benzene (BPPB) and lithium fluoride (LiF) were applied for a hole‐blocking and electron‐transporting layer, respectively.[34] The molecular structure of materials used in each layer is shown in Figure S1 (Supporting Information).1Figurea) The device structure of TADF and TSF devices, EL properties of 10%/15% TADF device, and TSF device with 10% TADF‐doped, b) current density–voltage–luminance (J–V–L) plot, c) EL spectrum of each device, d) EQE‐luminance plot, and e) LT95 devices operational lifetime at an initial luminance of 5000 cd m‐2.The EL characteristics of TADF and TSF devices are shown in Figure 1, and the data are summarized in Table S1 (Supporting Information). In contrast, from Figure 1b, both TADF devices with 10% and 15% of 12BTCzTPN showed relatively low turn‐on voltages of 2.7 V, indicating the low‐energy barriers in all layers of the device. As observed in Figure 1c, the EL spectra of 10% and 15% doped devices were at 571 and 574 nm with FWHM of 94 and 95 nm, respectively. An increase in the concentration resulted in little red‐shifted emission. These two TADF devices have shown maximum external quantum efficiency (EQEmax) of 8.3% and 6.9%, along with the operational lifetime of 206 and 187 h at an initial luminescence of 5000 cd m‐2 (Figure 1d,e). However, among the two devices, the 10% doped TADF device shows better efficiency. In general, the higher concentration of TADF sensitizer causes more aggregation quenching effect, resulting in poor efficiency.[35] Accordingly, the TSF device was constructed using 10% of 12BTCzTPN and 0.7% of 4tBuMB and achieved a high EQE of 16.3% at an EL peak of 618 nm with the FWHM of 44 nm. Which showed a slightly shorter lifetime (LT95) of 182 h at an initial luminescence of 5000 cd m‐2 compared with those TADF devices (Figure 1c–e).As aforementioned, we fabricated a TSF device by inserting an additional DSL consisting of 12BTCzTPN and DIC‐TRZ. In order to attain better results, the position of DSL is important. It was fixed after analyzing the exciton distribution and the carrier recombination zone by evaluating a hole‐only device (HOD) and electron‐only device (EOD) of 12BTCzTPN. The results are shown in Figure 2. It is visible from Figure 2a that the J–V curve of the HOD is lower than that of the EOD, which indicates that the exciton distribution and recombination zone were mostly on the HBL side. Furthermore, sensing layer experiments were carried out to confirm the exciton distribution.[36,37] A red fluorescent material, 5,10,15,20‐tetraphenylbisbenz[5,6]indeno[1,2,3‐cd:1′,2′,3′‐lm]perylene (DBP) (the structure in Figure 2c) was employed as the sensing layer at three positions in EML such as EBL side, middle, and HBL side with the thickness of 0.6 nm as shown in Figure 2b.[38,39] The device was fabricated with the same total thickness as the TSF device and the device structure is: ITO (50 nm)/HATCN (HIL, 7 nm)/PCBBiF (HTL, 70 nm)/PCzAC (EBL, 10 nm)/DIC‐TRZ: 10% 12BTCzTPN/DIC‐TRZ: 10% 12BTCzTPN (EML, x nm)/DBP (sensing layer, 0.6 nm)/DIC‐TRZ: 10% 12BTCzTPN (EML, 29.4‐x nm)/DDBFT (HBL, 5 nm)/BPPB: Liq (ETL, 60 nm)/LiF (EIL, 1.5 nm)/Al (100 nm). From Figure 2c, it is understood that the EL spectra were almost similar when the sensing layer (DBP) was moved from the EBL side to the middle side. A significant shift in EL spectra was observed when the sensing layer (DBP) was near HBL that further confirms that the exciton distribution was mainly on the HBL side. Based on this finding, we projected that the distribution of excitons in the TSF‐DSL device would be on the DSL side as shown in Figure 2d.2Figurea) J–V property of HOD and EOD of 12BTCzTPN, b) The sensing layer position in the EML, c) EL spectrum of the device with sensing layer at different positions and molecular structure of DBP, and d) Predicted exciton distribution in the TSF‐DSL device.The charge distribution results implemented the insertion of the additional layer on the HBL side fairly affect the device efficiency, and lifetime and also alters the emission spectra. Better outcomes would be obtained by inserting the DSL on the EBL side. TSF devices with DSL insertion on both the HBL and the EBL were fabricated under optimized device parameters and evaluated for comparable results. Initially, the DSL consisting of DIC‐TRZ: 10% 12BTCzTPN with the thickness of 5 and 7 nm was inserted in between TSF‐EML and HBL. To maintain the total EML thickness at 30 nm, the TSF‐EML was fabricated with thicknesses of 25 and 23 nm along with 5 and 7 nm of DSL, respectively. The structure and the mechanism of the DSL‐TSF‐EML‐based device have shown in Figure 3a,b. The evaluated results of these devices with 5 and 7 nm of DSL thickness have shown low turn‐on voltage of 2.7 V and EQEmax of 16.0% and 12.8% (Figure 3c,d). The two devices have almost similar EL peaks at 617 and 616 nm with FWHM of 48 and 54 nm, respectively. However, as shown in Figure 3e, the insertion of DSL on the HBL side results in poor color purity with intensified EL shoulder peak of 560–570 nm compared with the fabricated TSF. It was noticed that the shoulder peak is much improved in the device with the increased thickness of DSL. The respective operational lifetimes of devices are (LT95) 213 and 255 h at initial luminescence of 5000 cd m‐2 (Figure 3f). However, efficiency roll‐off and device lifetime improved by 3.42 and 1.40 times compared with the TSF device, respectively. The summary of EL performances of TSF‐DSL‐HBL devices is tabulated in Table S2 (Supporting Information).3FigureTSF‐DSL‐HBL device: a) structure, b) mechanism, c) current density–voltage–luminance (J–V–L) plot, d) EQE‐luminance plot, e) EL spectrum of each device, f) device operational lifetimes (LT95) at an initial luminance of 5000 cd m‐2.Subsequently, we fabricated and evaluated the device by repositioning the DSL between EBL and TSF‐EML to minimize the emission of TADF sensitizer. Figure 4a shows the detailed device structure and the related mechanism was depicted in Figure 4b. All the devices have low turn‐on voltage as shown in Figure 4c, which implies the low charge barriers in the device layers. The EBL‐DSL‐TSF‐based devices with DSL thicknesses of 5 and 7 nm have achieved almost similar EQEmax of 16.7% and 17.0%, respectively (Figure 4d). As observed from Figure 4e, EL peak is not much affected compared with TSF and is found at 617 and 618 nm with FWHM of 45 and 45 nm, respectively. Moreover, the DSL with 7 nm thickness has shown a noticeably longer operational lifetime (LT95) of 370 h than the 5 nm DSL‐device with a lifetime of 280 h at an initial luminance of 5000 cd m‐2 (Figure 4f). The most noteworthy lifetime improvement of the device was found to be 1.45 (7 nm DSL) times higher compared with the DSL on HBL side devices and 2.03 times higher than the TSF device. The EL characteristics are tabulated in Table 1.4FigureEL properties of EBL‐DSL‐TSF devices: a) Current density–voltage–luminance (J–V–L) plot, b) EQE‐luminance plot, c) EL spectrum of each device, d) Device operational lifetimes (LT95) at an initial luminance of 5000 cd m‐2.1TableSummary of performances of EBL‐DSL‐TSF devicesDSLEMLVon/Vd [@3000 cd m‐2]EQEmax /EQE [@3000 cd m‐2]EL peak [nm]FWHM [nm]CIE coordinatesLT95 [@5000 cd m‐2] hTADFThickness [nm]HF (TADF: FD)Thickness‐‐10%: 0.7%30 nm2.6/6.4 V16.3/12.3%61844(0.64, 0.36)18210%510%: 0.7%25 nm2.6/6.4 V16.7/12.8%61745(0.64, 0.36)28010%710%: 0.7%23 nm2.6/6.4 V17.0/14.3%61845(0.63, 0.37)370Furthermore, we calculated the FRET radius (R0) of 12BTCzTPN using equation (1). R0 signifies the distance at which the energy transition efficiency between the two molecules is 50%.[40]1R06=9000Φpκ2128π5n4JF\[\begin{array}{*{20}{c}}{R_0^6 = \frac{{9000{\Phi _{\rm{p}}}{\kappa ^2}}}{{128{\pi ^5}{n^4}}}{J_{\rm{F}}}}\end{array}\]where κ2 is the orientation factor, Φ is the quantum yield of the fluorescence, NA is the Avogadro number, n is the refractive index, and JF is the overlap integral between donor emission and acceptor absorption. The calculated R0 of 12BTCzTPN is 4.33 nm, which is lesser than the thickness of DSL (7 nm). Such a long distance between the two molecules than R0 results in less FRET process. Hence, thicker DSL can also emit light with improved device lifetime and efficiency roll‐off characteristics.We have investigated the energy transfer processes relative to the excitons generated in TADF‐sensitizer 12BTCzTPN as a donor and to the FD 4tBuMB as an acceptor in both TSF and DSL‐TSF devices using time‐resolved photoluminescence (TRPL) and PLQY measurements.[41] For the accurate kFRET and kDET evaluation, TSF thin film and DSL‐TSF thin film were fabricated with the same doping concentration of both TADF sensitizer and FD. The total thickness was maintained at 30 nm, like the EML thickness of the device (Figure 5a). Where the chosen DSL thickness is 7 nm. The transient PL decay curves of prompt and delayed fluorescence of both TSF and DSL‐TSF are shown in Figure 5b.5Figurea) The organization of TSF thin film, DSL film, and b) TRPL results of manufactured thin film.Both the TSF and DSL‐TSF films displayed clear exponential decay curves exhibiting the nanoscale of prompt and the microscale of delayed fluorescence, as shown in Figure 5b. The energy‐transfer rate constants kISC, kRISC, kr,, and knr of the TADF sensitizer are calculated using the equations S1–S6 (Supporting Information), and the corresponding results are tabulated in Table 2. Among these two films, the DSL‐TSF film shows an improved delayed lifetime showing a more population of triplet excitons in TADF. Such an increase in triplet states indicates that the DSL insertion improves the delayed components in the TADF by suppressing triplet exciton transfer to the final emitter. The exciton dynamics in the DSL‐TSF device compared with the normal TSF device shows increased ISC and RISC rates driven by the triplet population in the TADF. This generates more singlet excitons and enhances the FRET process to the final dopant. The improved radiative decay rate (kr) of the TADF confirms the rapid transfer of the singlet excitons (Table 2). Such enhanced efficiency by more generated singlet excitons is attributed to the improvement of device lifetime in the DSL‐TSF. Generally, the TADF triplet excitons lifetime is in microseconds and milliseconds in the final emitter. Such long‐lived triplet excitons generated in the final emitter increase the probability of material degradation through TTA and TPA processes. But, the TADF sensitizer possesses a 103‐fold lowered triplet exciton lifetime than the final emitter, and the probability of the exciton annihilation processes is reduced, enhancing device operational stability. Additionally, the TADF triplet excitons are rapidly transferred to the final emitter by faster energy transfer, which reduces the duration of triplet excitons in the TADF sensitizer. Hence, we believe that the energy stored in the TADF triplets will not cause a serious impact on the device's lifetime. Such enhanced charge dynamic properties reduce the electrically induced device degradation,[19] resulting in reduced efficiency roll‐off properties and an improved operational lifetime of 370 h (Table 1).2TableEnergy transfer rate constants, kFRET, and kDET values of TSF thin film and DSL‐TSF thin filmτp [ns]τd [µs]kr [x 107 s‐1]knr [x 104 s‐1]kISC [x 107 s‐1]kRISC [x 106 s‐1]kFRET [x 108 s‐1]kDET [x 105 s‐1]TSF film6.50.204.578.289.291.241.1413.6DSL‐TSF film6.40.246.195.9310.981.281.165.2Additionally, the kFRET and kDET were calculated using the following equations (2) and (3)[42–44] and tabulated in Table 2.2kFRET=kP−kr, S−kISC\[\begin{array}{*{20}{c}}{{k_{{\rm{FRET}}}} = {k_{\rm{P}}} - {k_{{\rm{r,}}\;{\rm{S}}}} - {k_{{\rm{ISC}}}}}\end{array}\]3kDET=kPkD−kRISC(kr, s+kFRET)kr,S+kISC+kFRET−knr, T\[\begin{array}{*{20}{c}}{{k_{{\rm{DET}}}} = \frac{{{k_{\rm{P}}}{k_{\rm{D}}} - {k_{{\rm{RISC}}}}\left( {{k_{{\rm{r,}}\;{\rm{s}}}} + {k_{{\rm{FRET}}}}} \right)}}{{{k_{{\mathop{\rm r}\nolimits} ,S}} + {k_{{\rm{ISC}}}} + {k_{{\rm{FRET}}}}}} - {k_{{\rm{nr,}}\;{\rm{T}}}}}\end{array}\]where kP and kD are prompt and delayed fluorescence rates, respectively. kr, S and knr, T express the radiative rate constant of the singlet and the nonradiative rate constant of the triplet, respectively. kISC and kRISC are intersystem crossing‐ and reverse intersystem crossing rate constant, respectively. From Table 2, the DSL thin film showed slightly improved kFRET values of 1.16 × 108 s‐1 and lowered kDET of 5.2 × 105 s‐1 compared with TSF thin film. As per our ideology, DSL insertion could suppress DET with a maintained FRET process. A remarkable DET suppression of 2.62 times that of TSF thin film is observed. Such a low DET minimizes the triplet exciton transfer and notably reduces the long‐lived triplet excitons in the final emitter and prohibits the destructive TTA and TPA processes. Hence, the final emitter degradation has significantly decreased, improving device stability (Table 1). This also induces better exciton generation that is transferred to the final emitter, resulting in improved device efficiency of 17%.On the other hand, we have also evaluated the singlet and triplet excitons decay rate for the comparison of the exciton distribution in the TADF sensitizer by using the following equations (4) and (5).[45,46]4dNS(t)dt=−(kFRET+kr+knr,s+kISC)NS(t)+kRISCT(t)\[\begin{array}{*{20}{c}}{\frac{{{\rm{d}}{{\rm{N}}_{\rm{S}}}\left( {\rm{t}} \right)}}{{{\rm{dt}}}} = - \left( {{k_{{\rm{FRET}}}} + {k_{\rm{r}}} + {k_{{\rm{nr,s}}}} + {k_{{\rm{ISC}}}}} \right){N_{\rm{S}}}\left( {\rm{t}} \right) + {k_{{\rm{RISC}}}}T\left( {\rm{t}} \right)}\end{array}\]5dNT(t)dt=−(kDET+knr,T+kRISC)NT(t)+kISCNS(t)\[\begin{array}{*{20}{c}}{\frac{{{\rm{d}}{{\rm{N}}_{\rm{T}}}\left( {\rm{t}} \right)}}{{{\rm{dt}}}} = - \left( {{k_{{\rm{DET}}}} + {k_{{\rm{nr,T}}}} + {k_{{\rm{RISC}}}}} \right){N_{\rm{T}}}\left( {\rm{t}} \right) + {k_{{\rm{ISC}}}}{N_{\rm{S}}}\left( {\rm{t}} \right)}\end{array}\]where NS and NT are singlet and triplet exciton densities, respectively and knr, S is the nonradiative decay rate constant of singlet excitons. The calculated values were plotted and shown in Figure 6a,b. As shown in Figure 6a, the singlet density of DSL‐TSF and TSF devices did not significantly vary, which confirms the almost same FRET process that occurred in both devices. As a result, there was little variation in the singlet exciton density with respect to time. As can be seen from Figure 6b, a long‐lived triplet exciton was detected in the DSL‐TSF device when compared with the decay curves of the triplet excitons of both devices. It occurs because the DET process transfers fewer triplet excitons to the final emitter. The observation strengthens our findings that inserting a DSL enhances the device's lifetime by lowering the DET process and the efficiency roll‐off characteristics (Figure 6c).6FigureThe decay rate of: a) singlet‐, b) triplet exciton density depending on time, c) EQE‐current density property of SDL (7 nm)/TSF‐EML device and TSF device.ConclusionIn summary, we have reported a clear mechanistic study of a TSF device that can remarkably suppress DET while maintaining the FRET process. This was achieved through the insertion of a “DSL” consisting of a host and a TADF sensitizer next to EML. The position of the DSL was optimized TSF‐EML and EBL using exciton distribution evaluation. Such DSL insertion resulted in the noticeable suppression of DET by 2.62‐fold compared with a standard TSF device with enhanced EQEmax of 17.0%. The DSL‐TSF showed a pure red emission at 618 nm with an FWHM of 45 nm, and the corresponding CIE coordinates are (0.63, 0.37). 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Journal

Advanced Materials Interfaces – Wiley

Published: May 1, 2023

Keywords: DET‐suppressing layer; dexter energy transfer; red OLED; TADF sensitizer; TSF

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