Controlling the exciton lifetime of blue thermally activated delayed fluorescence emitters using a heteroatom-containing pyridoindole donor moiety

Abstract

In this paper, we report a promising molecular design concept to attain short exciton lifetime, appropriate triplet energy and high photoluminescence quantum yield (PLQY) in blue thermally activated delayed fluorescence (TADF) emitters for high quantum efficiency and better efficiency roll-off characteristics in organic light emitting diodes (OLEDs). Herein, new TADF molecules with nitrogen at α- and δ-positions of the carboline donor moiety were synthesized and their effects on the photo-physical and electroluminescence properties were theoretically and experimentally investigated. The TADF molecule with a δ-carboline donor moiety was much favorable for short exciton lifetime, high delayed PLQY, good blue color, and small singlet and triplet splitting compared to the generally used carbazole. A maximum external quantum efficiency (EQE) of 22.5%, reduced efficiency roll-off and good color purity with Commission Internationale de l'Eclairage (CIE) 1931 color coordinates of (0.19, 0.34) were exhibited by the blue OLED using the TADF emitter, 4,5-bis(5H-pyrido[3,2-b]indol-5-yl)phthalonitrile (δ-2CbPN). Moreover, the effectiveness of the δ-carboline donor moiety was verified in another deep blue TADF emitter, 5,5′-(2-(9H-carbazol-9-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-pyrido[3,2-b]indole) (CzDCbTrz). A maximum EQE of 23.4%, deep blue color, and reduced efficiency roll-off were attained in the CzDCbTrz based OLED.

---

Conceptual insights

In recent years, several blue thermally activated delayed fluorescence (TADF) emitters have been investigated for improving the efficiencies of organic light emitting diodes (OLEDs). However, most of the previously reported TADF emitters demonstrate strong efficiency roll-off characteristics because of their long delayed fluorescence exciton lifetime. Indeed, there is no systematic evaluation and analysis available for controlling the exciton lifetime with high photoluminescence quantum yields (PLQYs) in TADF emitters. Herein, we present a promising molecular design concept for reducing the exciton lifetime without deteriorating the PLQYs of TADF emitters. The newly designed blue TADF emitters with pyridoindole containing a δ-nitrogen heteroatom as a donor moiety displayed appropriate triplet energy, improved reverse intersystem crossing due to small singlet–triplet energy splitting, and short delayed fluorescence exciton lifetime with high PLQYs. The OLEDs with new emitters revealed very high external quantum efficiencies of 22.5% (sky blue) and 23.4% (deep blue) with significantly reduced efficiency roll-off by reducing the delayed fluorescence exciton lifetime. The molecular design concept proposed here is helpful for the development of future high performance blue TADF emitters.

Organic light emitting diodes (OLEDs) with high quantum efficiency and desired electrical characteristics have been broadly required in the present flat-panel displays and general lighting. For many years, phosphorescent materials containing heavy noble-metals like iridium(III) and platinum(II) have shown high performances in the OLEDs.^1,2 Normally, OLEDs based on phosphorescent emitters can show 100% internal quantum efficiency by utilizing 25% of singlet (S₁) and 75% of triplet (T₁) excitons via strong spin–orbit coupling by heavy metal atoms as compared to those of fluorescence counterpart (25% of the singlet excitons).^3–5 However, blue phosphorescent emitter has not shown much progress because of several issues like short operational lifetime, poor color purity etc.⁶ Moreover, the high cost and rarity of the heavy noble-metal elements impedes the use of these phosphorescent materials in display as well as lighting applications. To address this issue, in recent times, heavy noble-metal free thermally activated delayed fluorescence (TADF) strategy have been explored as an alternative to phosphorescence. TADF emitters can show efficiencies comparable to those of phosphorescent OLEDs by harnessing both S₁ and T₁ excitons via enhanced reverse intersystem crossing (RISC) due to thermally accessible S₁ and T₁ energy splitting (ΔE_ST).^7–9 However, most TADF based OLEDs suffers from high efficiency roll-off at high luminescence because of the relatively long delayed fluorescence exciton lifetime of the emitters.^7,8,10,11

Several studies have been reported on enhancing the quantum efficiency of blue OLEDs by incorporating a properly designed efficient TADF emitter.^{8–10,12–14} However, the concept of reducing the exciton lifetime of TADF molecules for low efficiency roll-off characteristics has been poorly studied. Indeed, the long exciton lifetime directly affects the operational stability and efficiency roll-off characteristics of OLEDs by increasing the self-quenching at high current density.^15–17 Therefore, development of a new TADF emitter with a short exciton lifetime and a high photoluminescence quantum yield (PLQY) to realize extremely high external quantum efficiency (EQE) in TADF OLEDs is essentially required. Recently, heteroatom engineering was performed in carbazole based host materials to alter their photo-physical properties especially to improve the T₁ energy and the band-gap energy (E_g) for high performance phosphorescent blue OLEDs.^18,19 These materials (normally called as carboline) can have the heteroatom at a diverse location (α, β, γ, δ) of pyridoindole moiety with different physical properties. The β-carboline has a high T₁ energy and a low E_g compared to α-carboline. The α-carboline functions well in terms of good quantum efficiency in OLEDs due to its efficient electron accepting properties. On the other hand, γ-carboline shows a high E_g and T₁ but worst device performance due to its weak electron accepting properties. Here, we report a promising molecular design principle to obtain short exciton lifetime, appropriate T₁ energy and high PLQY in blue TADF emitters for high quantum efficiency and better efficiency roll-off characteristics in OLEDs.

Normally, TADF emitters with a carbazole donor moiety have a strong electron donor ability and thereby strong intramolecular CT characteristics. Hence, higher stabilization of the excited S₁ state with a long exciton lifetime can be observed in TADF emitters. In order to develop an efficient TADF emitter with a short exciton lifetime without reducing the PLQY, an appropriate material design concept to obtain proper intramolecular CT characteristics is necessary. The above mentioned crucial TADF properties can be properly controlled by incorporating a donor unit with a relatively weak electron donor ability. Herein, to design high performance blue TADF emitters with short exciton lifetime, the carboline moiety was investigated as a donor unit instead of carbazole. As a model compound, a carbazole combined phthalonitrile emitter, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), was used in TADF-OLEDs.¹¹ The incorporation of a carboline moiety with a relatively weak electron donor ability in the emitter could shorten the exciton lifetime of TADF materials because of weak intramolecular CT characteristics and the heteroatom effect.²⁰ Undeniably, the heteroatom in the aromatic compound can increase the molecular relaxation, which results in an increase in the non-radiative process with the reduction of the PLQY. However, interestingly the heteroatom in the carboline moiety can simultaneously increase the E_g and T₁ energies by ∼0.2 eV and ∼0.3 eV, respectively, compared to the carbazole moiety as previously reported.¹⁸ The influence of both S₁ and T₁ energies upon introduction of a carboline donor unit causes a deep blue color and a decrease in the ΔE_ST for efficient TADF materials. Such good chemical properties of the carboline moiety seem to be necessary for TADF performances due to the decreased ΔE_ST and properly controlled CT characteristics. On the other hand, the highly reduced donor ability may result in non-sufficient CT characteristics, and thereby poor TADF performances. In conclusion, incorporation of the carbazole donor moiety has both positive and negative effects on TADF performances. It could increase the E_g, T₁, reverse intersystem crossing (RISC), non-radiative process, and delayed PLQY, while other parameters like ΔE_ST, exciton lifetime, and prompt PLQY would be decreased. As shown in Fig. 1, an improvement in ΔE_ST and RISC is needed for the development of good TADF materials. To validate our assumption, the photo-physical properties as well as OLED performances of new TADF emitters were systematically investigated.

Fig. 1 Schematic representation of the pyridoindole effect in TADF emitters.

Two efficient blue TADF emitters in accordance with the molecular design approach stated above were synthesized using a phthalonitrile acceptor and α- and δ-carboline donors by a reported coupling reaction (Scheme S1, ESI†).⁷ The newly synthesized TADF emitters comprise two types of carboline derivatives, where the nitrogen heteroatom is at the α- or δ-position. A previous study on the heteroatom position in phosphorescent host molecules for donor and acceptor properties was reported.¹⁸ Normally, the electron donating ability of the carbolines (α-, β-, γ-, and δ-carbolines) can be determined from their basicity. Previously, Smirnova et al. reported the basicity of different carbolines based on their pK_a values, which denotes the deprotonation of the indole NH group.²¹ They found that δ- and α-carbolines are strong bases due to their high pK_a values compared to β- and γ-carboline moieties (pK_a: δ- > α- > β- > and γ-carbolines). Here, we have selected δ-, α-carbolines because of their relatively high electron donor properties among other carbolines. Although δ-carboline seems to be relatively better electron donor, both δ- and α-carboline donor based TADF emitters were synthesized for comparison and their photo-physical and electroluminescence (EL) properties were explored.

In order to understand the photo-physical properties like the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) distribution, the excited energy levels (S₁ and T₁) as well as the electronic characteristics of TADF emitters, 4,5-bis(9H-pyrido[2,3-b]indol-9-yl)phthalonitrile (α-2CbPN) and 4,5-bis(5H-pyrido[3,2-b]indol-5-yl)phthalonitrile (δ-2CbPN), molecular simulations were performed using density functional theory (DFT/B3LYP) and time dependent density functional theory (TD-DFT/GGA). As shown in Fig. 2, the HOMO and LUMO of δ-2CbPN and α-2CbPN are isolated from each other with partial overlap. However, α-2CbPN shows a relatively worse separation due to the weak donor ability of the α-carboline moiety. Although in previous studies, the carboline moiety was used as the acceptor, it can also be employed as a donor like the well-known carbazole. We anticipated that α- and δ-carbolines could function as good donors in combination with the strong electron acceptor moiety like phthalonitrile. Herein, the HOMO and LUMO of both TADF emitters are mainly localized on the electron donating moiety (carboline) and the electron accepting moiety (phthalonitrile), respectively. Such well separated HOMO and LUMO of α- and δ-carboline derivatives are supposed to have appropriate CT characteristics, which can be efficient TADF emitters with small ΔE_ST. The theoretically calculated T₁ energy (and E_g) of α-2CbPN and δ-2CbPN was found to be 2.81 eV (3.05 eV) and 2.77 eV (2.88 eV), which was higher than that of the carbazole based emitter 2.66 eV (2.82 eV). These values are in agreement with the reported carboline trends.^18,19

Fig. 2 Chemical structures and distribution of the HOMO and LUMO of α-2CbPN, and δ-2CbPN modified from 2CzPN.

To further understand the effect of heteroatom position in the carboline donor moiety of the TADF emitter, molecular relaxations of S₁ excited state geometry were evaluated using TD-DFT/GGA. In the case of α-2CbPN, a high molecular relaxation phenomenon was observed due to a large bond length change between carboline and phenyl group. The bond length changes in α-2CbPN, δ-2CbPN and 2CzPN between the ground and excited states were 0.048, 0.041 and 0.043 Å, respectively (Table S1, ESI†). A small molecular relaxation was obtained in δ-2CbPN compared to carbazole and α-2CbPN. Hence, the smaller molecular relaxation in δ-2CbPN is expected to decrease the non-radiative process. The changes in the dihedral angles of α-, δ-carboline and 2CzPN between the ground and excited states showed a similar tendency. The changes in the angles between phenyl and carboline moieties for α- and δ-carboline derivatives were 30.2° and 29.9°, respectively, while for 2CzPN it was 32.1° (Table S1, ESI†).

The UV-Vis absorption and photoluminescence (PL) spectra of both new TADF emitters in 10⁻⁵ M toluene and 6 wt% co-deposited 1,3-bis(9-carbazolyl)benzene (mCP) films were investigated at room temperature. Herein, the mCP was used as the host because of its wide E_g and high T₁.²² The relevant values of maximum absorption and PL peaks are presented in Table S2 (ESI†). Both α-2CbPN and δ-2CbPN showed absorption peaks at around ∼338 nm and ∼365 nm. The strong absorption peaks at 338–365 nm are ascribed to the π–π* transition from carboline to phthalonitrile. The relatively short absorption peak of α-2CbPN is attributed to its large E_g energy, and this is well matched with the simulation results. The maximum PL peak of α-2CbPN in 10⁻⁵ M toluene solution was observed at 417 nm, which shifted to the blue region by 36 nm as compared to δ-2CbPN (453 nm) (Fig. S18, ESI†). The PL spectra of the emitters in different solvents are shown in Fig. S11 (ESI†). The PL spectra demonstrated a strong solvatochromic effect.²³ The relative comparison of solvent polarity dependence on the PL characteristics of α-2CbPN and δ-2CbPN was investigated. As expected, δ-2CbPN was found to have stronger CT characteristics than α-2CbPN. By considering a previously reported method,⁹ the S₁ and T₁ energy values of TADF emitters were determined from the onset energy of the fluorescence (300 K) and phosphorescence (77 K) spectra of the 6 wt% co-deposited mCP film (Fig. S12, ESI†). The ΔE_ST values of α-2CbPN and δ-2CbPN were calculated from the energy gap between the S₁ and T₁. The measured ΔE_ST of α-2CbPN and δ-2CbPN was 0.26 and 0.13 eV, respectively. The ΔE_ST of δ-2CbPN was the smallest among the three TADF emitters and hence it was expected to show good TADF performance. The T₁ (S₁) energies of α-2CbPN and δ-2CbPN were evaluated to be 2.90 eV (3.18 eV) and 2.83 eV (2.96 eV), respectively. These T₁ energy values were higher than that of the 2CzPN emitter (2.59 eV), which has a similar molecular structure. The measured T₁ energy of both new TADF emitters was higher compared to that of the carbazole based 2CzPN due to the high T₁ energy of carboline. However, the T₁ energy of α-2CbPN was found to be slightly higher than that of δ-2CbPN and the ΔE_ST value of α-2CbPN was not improved because of the relatively poor donor ability.

The electrochemical properties of both α-2CbPN and δ-2CbPN were investigated by cyclic voltammogram (CV) measurements and their CV plots are shown in Fig. S13 (ESI†) and the comparative values are listed in Table 1. The HOMO levels of α-2CbPN and δ-2CbPN were obtained from the onset point of oxidation potential maxima and the assumed energy level of ferrocene/ferrocenium (Fc/Fc⁺) (i.e., 5.13 eV).²⁴ Both TADF emitters displayed HOMO levels of −6.21 eV (α-2CbPN) and −6.18 eV (δ-2CbPN), respectively. The LUMO energy level was calculated from the HOMO level and the E_g energy. The E_g was obtained from the onset edge of the UV-Vis absorption spectrum. The E_g values of α-2CbPN and δ-2CbPN were found to be 3.13 eV and 2.95 eV. These E_g values were slightly higher than that of the carbazole based emitter (2CzPN: −2.83 eV). The LUMO levels of α-2CbPN and δ-2CbPN were found to be −3.08 and −3.23 eV, respectively.

Table 1 Photo-physical properties and rate constants of TADF emitters

Emitters	HOMO^a (eV)	LUMO^b (eV)	T1^c (eV)	ΔEST^d (eV)	Φ PL (%)	Φ F (%)	Φ TADF (%)	τ p (ns)	τ d (μs)	k p (10^7 s^−1)	k d (10^4 s^−1)	k ISC (10^6 s^−1)	k ^Snr ^f (10^6 s^−1)
a Measured by the onset of the oxidation point. b Calculated from the HOMO − Eg. c Calculated from the S1 − ΔEST. d Determined from fluorescent λPL – phosphorescent λPL. e PLQYs and exciton lifetimes of 2CzPN, α-2CbPN, and δ-2CbPN were measured in mCP: 20% dopant thin film. PLQYs and exciton lifetimes of TCzTrz and CzDCbTrz were measured in DPEPO: 6% dopant thin film. f The rate constants of TADF emitters were calculated using the equations provided in the ESI.
2CzPN	−6.13	−3.30	2.59	0.21	89	51	38	25	270	4.0	0.4	15.2	4.4
α-2CbPN	−6.21	−3.08	2.90	0.28	37	30	7	86	57	1.2	1.7	0.8	7.3
δ-2CbPN	−6.18	−3.23	2.83	0.13	93	48	45	70	180	1.4	0.6	6.4	1.0
TCzTrz	−5.69	−2.87	2.80	0.14	82	68	14	13.3	9.5	7.5	10.5	10.5	13.5
CzDCbTrz	−5.75	−2.85	2.83	0.12	85	68	17	6.0	7.2	16.7	13.9	28.3	25.0

---

In order to confirm the delayed fluorescence behaviour of carboline based TADF emitters, transient and time-resolved PL measurements were carried out at room temperature. As shown in Fig. 3, transient PL measurements at 300 K clearly showed prompt decay components of 2CzPN, α-2CbPN and δ-2CbPN with lifetimes of 25 ns, 86 ns and 70 ns, respectively, and delayed decay components of 2CzPN and δ-2CbPN with lifetimes of 270 μs and 180 μs, respectively. The delayed decay component of α-2CbPN was very weak (57 μs) due to its large ΔE_ST. The lifetime of the delayed exciton decay of δ-2CbPN was much shorter than that of 2CzPN because of its enhanced RISC efficiency and smaller molecular relaxation. The improved RISC is directly related to the small ΔE_ST of δ-2CbPN, which decreases with increasing T₁ energy. The PLQYs of prompt (Φ_F) and delayed (Φ_TADF) components for new emitters were evaluated using a previously reported method.¹¹ The Φ_F PLQYs of 2CzPN and δ-2CbPN were derived to be 51% and 48%, whereas the Φ_TADF PLQY of δ-2CbPN (45%) was significantly higher with a short delayed exciton lifetime (180 μs) than that of 2CzPN (38%) because of its relatively improved CT characteristics and an increased RISC rate. In contrast, α-2CbPN showed a drastic drop in Φ_F (30%) and Φ_TADF (7%) PLQYs compared to the studied emitters due to its high ΔE_ST and decreased RISC efficiency, whereas it also showed a very short delayed exciton lifetime (57 μs). Indeed, the total PLQYs of 2CzPN, α-2CbPN and δ-2CbPN were measured to be 89%, 37%, and 93%, respectively. The PLQYs and exciton lifetimes of TADF emitters are listed in Table 1.

Fig. 3 Time-resolved PL decay of 2CzPN, α-2CbPN, and δ-2CbPN (20 wt% doped mCP thin film) at 300 K.

In order to understand the behaviour of the relatively short delayed exciton lifetime as well as the high PLQY in δ-2CbPN, rate constants like the ISC and RISC rate constant (K_ISC and K_RISC), the non-radiative decay rate constant of the singlet excited state (k^S_nr), and rate constants of the prompt and delayed components (k_p and k_d) were calculated from the PL efficiencies and decay times via previously reported equations (eqn (1)–(6), ESI†).^11,22,25 In the case of δ-2CbPN, a low k^S_nr (1.00 × 10⁶ s⁻¹) and a small ΔE_ST with a very high Φ_TADF results in a relatively high PLQY. Basically, the Φ_TADF value directly depends on the k_RISC from T₁ to S₁ and k^S_nr. Herein, a high Φ_TADF value was obtained for δ-2CbPN because of the high k_RISC and low k^S_nr. Likewise, herein the small ΔE_ST induced a high RISC efficiency of around 87% in δ-2CbPN, which is higher than that of 2CzPN (78%) with a similar molecular structure. The rate constants derived using previously reported equations are summarized in Table 1. On the other hand, the low PLQY in α-2CbPN is mostly due to the very high k^S_nr (7.33 × 10⁶ s⁻¹) and large ΔE_ST with a low Φ_TADF. These results prove that δ-2CbPN is an efficient emitter in terms of appropriate TADF properties.

In particular, although both new emitters exhibited higher T₁ energy and wide E_g compared to 2CzPN, only δ-2CbPN was feasible to attain short exciton lifetime with small ΔE_ST, high PLQY and high RISC. By considering the impressive photo-physical properties of δ-2CbPN, the same heteroatom concept was applied to the efficient blue TADF emitter, 9,9′,9′′-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) (TCzTrz) for high performance deep blue OLEDs.²⁶ The newly synthesized δ-carboline based emitter, 5,5′-(2-(9H-carbazol-9-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-pyrido[3,2-b]indole) (CzDCbTrz; Fig. S10, ESI†), exhibited excellent photo-physical properties in terms of PLQY (85%), ΔE_ST (0.12 eV) and short exciton lifetime (7.2 μs). In contrast, TCzTrz displayed a lower PLQY (82%) and a larger ΔE_ST (0.14 eV), as well as a longer exciton lifetime (9.5 μs). Furthermore, CzDCbTrz also displayed a wider E_g of 2.9 eV (Fig. S12(e) and S13(b), ESI†) and a higher T₁ of 2.83 eV than TCzTrz (E_g: 2.82 eV and T₁: 2.80 eV). The photo-physical properties of the deep blue emitters are shown in Fig. S11–S14 (ESI†). These excellent properties of CzDCbTrz are attributed to the good TADF properties of the δ-carboline moiety. As expected, CzDCbTrz was found to have a similar tendency in the photo-physical properties as studied in δ-2CbPN (Tables S1 and S2, ESI†).

To investigate the EL performances of carboline based TADF emitters, the blue OLEDs were fabricated. Three sky blue emitters (α-2CbPN, δ-2CbPN, and 2CzPN) (20 wt%) and two deep blue emitters (CzDCbTrz and TCzTrz) were doped in the mCP host and a high T₁ DBFPO (2,8-bis(diphenylphosphineoxide)-dibenzofuran)²⁷ host, respectively. The energy level diagram of the fabricated devices and the chemical structures of the studied organic materials are shown in Fig. S15 (ESI†). A maximum EQE of 22.5% was obtained in the δ-2CbPN based blue TADF–OLED (Fig. 4). This quantum efficiency is significantly higher than that of the 2CzPN device (19.2%). Such a high EQE in δ-2CbPN is attributed to the high PLQY and the low non-radiative process (k_nr). As expected, the efficiency roll-off phenomenon of the δ-2CbPN device was reduced by 83.1% at 3000 cd m⁻² relative to the 2CzPN device (92.7% at 3000 cd m⁻²) because of its short exciton lifetime. A slightly broad EL spectrum was obtained for δ-2CbPN over the 2CzPN based OLED as shown in the inset of Fig. 4. The EL peak and CIE color coordinates of the δ-2CbPN device were found to be 486 nm and (0.19, 0.34), respectively, and this EL peak was blue shifted by 5 nm compared to 2CzPN (491 nm). In contrast, the α-2CbPN device demonstrated a low quantum efficiency of 4.2% due to its low PLQY and poor TADF performances and the EL spectrum was blue shifted with a maximum EL peak at 473 nm. The current density versus voltage versus luminescence (J–V–L) characteristics and the relative parameters of the sky blue devices are displayed in Fig. S16 and Table S3 (ESI†). In addition, the device containing a deep-blue CzDCbTrz emitter (with a doping concentration of 40 wt%) displayed a significant improvement in the maximum EQE from 19.8% (TCzTrz, 40 wt% doping concentration) to 22.0% and a small blue shift in the EL peak (476 to 471 nm) with enhancement in color purity, shifted from (0.17, 0.29) to (0.16, 0.23). It is interesting to note that OLEDs with 20 wt% doping concentration of CzDCbTrz demonstrated a significantly high maximum EQE (23.4%) with color coordinates of (0.16, 0.19). The device performances with different doping concentrations of the emitters are summarized in Table S4 (ESI†). The efficiency roll-off of the deep-blue CzDCbTrz device was also reduced as compared to its counterpart TCzTrz based device (Fig. 4(b) and Fig. S17, ESI†). This low efficiency roll-off characteristic was ascribed to the short exciton lifetime of CzDCbTrz.

Fig. 4 External quantum efficiency (EQE) versus luminance characteristics of (a) 2CzPN, α-2CbPN, and δ-2CbPN OLEDs, (b) TCzTrz and CzDCbTrz OLEDs. Inset: normalized electroluminescence spectra at 1000 cd m⁻².

Conclusions

In conclusion, sky blue and deep blue TADF emitters were designed and synthesized by using carboline heteroatom techniques to attain high efficiency and good efficiency roll-off characteristics by reducing the exciton lifetime. The sky blue OLED containing the δ-2CbPN TADF emitter demonstrated a maximum EQE of 22.5% with improved CIE color coordinates (0.19, 0.34). Additionally, the effectiveness of this technique was also observed in the deep blue TADF emitter (CzDCbTrz), which was very efficient in terms of TADF characteristics and device performances with a maximum EQE of 23.4%. Therefore, we anticipate that the δ-carboline donor moiety will be useful to achieve high quantum efficiency and better roll-off characteristics in future TADF emitters.

Acknowledgements

This work was supported by the Human Resources Development program (no. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy. And this work was also supported by the Industrial Strategic Technology Development Program of MKE/KEIT (10048317).

Notes and references

1. D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 1999, 74, 442.
2. Y. Kawamura, K. Goushi, J. Brooks, J. J. Brown, H. Sasabe and C. Adachi, Appl. Phys. Lett., 2005, 86, 071104.
3. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys., 2001, 90, 5048.
4. M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403, 750.
5. A. Kohler, J. S. Wilson and R. H. Friend, Adv. Mater., 2002, 14, 701.
6. J. S. Lee, H. F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E. Thompson and S. R. Forrest, Nat. Mater., 2016, 15, 92.
7. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234.
8. Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka and C. Adachi, Nat. Photonics, 2014, 8, 326.
9. S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura, N. Nakamura, M. Yasumatsu, H. Nakanotani, Q. Zhang, K. Shizu, H. Miyazaki and C. Adachi, Nat. Mater., 2015, 14, 330.
10. Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and C. Adachi, J. Am. Chem. Soc., 2012, 134, 14706.
11. K. Masui, H. Nakanotani and C. Adachi, Org. Electron., 2013, 14, 2721.
12. R. Komatsu, H. Sasabe, Y. Seino, K. Nakao and J. Kido, J. Mater. Chem. C, 2016, 4, 2274.
13. J. W. Sun, J. Y. K. Baek, H. Kim, C. K. Moon, J. H. Lee, S. K. Kwon, Y. H. Kim and J. J. Kim, Chem. Mater., 2015, 27, 6675.
14. T. A. Lin, T. Chatterjee, W. L. Tsai, W. K. Lee, M. J. Wu, M. Jiao, K. C. Pan, C. L. Yi, C. L. Chung, K. T. Wong and C. C. Wu, Adv. Mater., 2016, 28, 6976.
15. T. Furukawa, H. Nakanotani, M. Inoue and C. Adachi, Sci. Rep., 2015, 5, 8429.
16. S. Wang, Y. Zhang, W. Chen, J. Wei, Y. Liu and Y. Wang, Chem. Commun., 2015, 51, 11972.
17. M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 10967.
18. Y. Im and J. Y. Lee, Chem. Commun., 2013, 49, 5948.
19. C. S. Oh, C. W. Lee and J. Y. Lee, Chem. Commun., 2013, 49, 3875.
20. M. Montaliti, A. Credi, L. Prodi and M. T. Gandolfi, Handbook of Photochemistry, Taylor & Francis, 2006, 3rd edn, p. 123.
21. O. B. Smirnova, T. V. Golovko and V. G. Granik, Pharm. Chem. J., 2011, 45, 389.
22. J. S. Kang, T. R. Hong, H. J. Kim, Y. H. Son, R. Lampande, B. Y. Kang, C. Lee, J. K. Bin, B. S. Lee, J. H. Yang, J. W. Kim, S. Park, M. J. Cho, J. H. Kwon and D. H. Choi, J. Mater. Chem. C, 2016, 4, 4512.
23. R. Ishimatsu, S. Matsunami, K. Shizu, C. Adachi, K. Nakano and T. Imato, J. Phys. Chem. A, 2013, 117, 5607.
24. G. Gritzner and J. Kuta, Pure Appl. Chem., 1984, 56, 461.
25. K. C. Pan, S. W. Li, Y. Y. Ho, Y. J. Shiu, W. L. Tsai, M. Jiao, W. K. Lee, C. C. Wu, C. L. Chung, T. Chatterjee, Y. S. Li, K. T. Wong, H. C. Hu, C. C. Chen and M. T. Lee, Adv. Funct. Mater., 2016, 26, 7560–7571.
26. D. R. Lee, M. Kim, S. K. Jeon, S. H. Hwang, C. W. Lee and J. Y. Lee, Adv. Mater., 2015, 27, 5861.
27. P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak and P. E. Burrows, Org. Lett., 2006, 8, 4211.

---

Footnote

† Electronic supplementary information (ESI) available: Material synthesis, NMR, MASS, computational and experimental data, photo-physical properties, device fabrication and performances. See DOI: 10.1039/c6mh00579a

---