Quinolinoacridine as High Efficiency Building Unit in Single‐Layer Phosphorescent Organic Light‐Emitting Diodes

The performances of simplified single‐layer phosphorescent organic light‐emitting diodes (SL‐PhOLEDs) have significantly increased and they now appear to be a promising alternative to multi‐layer PhOLEDs. The blue and white emissions, far more challenging than all the other colours, are still particularly desired. Herein, a high efficiency host material for blue emitting SL‐PhOLED using the blue emitter FIr6 is reported, which is particularly interesting as it displays an emission at shorter wavelengths than the well‐known FIrpic emitter, almost exclusively reported in the SL‐PhOLEDs literature. The host material investigated herein is constructed on the electron‐rich quinolinoacridine and displays when incorporated in FIr6‐based SL‐PhOLEDs, an external quantum efficiency (EQE)⟩10% and a low Von of 3.1 V. This is the first work passing an EQE of 10% with FIr6 as an emitter. This host also reaches a very high EQE of 19% when used with the green emitter Ir(ppy)2acac, this performance being among the highest recorded for green SL‐PhOLEDs. Finally, as white SL‐PhOLEDs involve blue emitting SL‐PhOLEDs, this host is also used with a combination of blue and yellow emitters. An extremely high EQE of 24% is reached with CIE coordinates of (0.40;0.48). These findings show the real potential of the quinolinoacridine fragment to reach high performance multi‐colour SL‐PhOLEDs.


Introduction
][8][9] This is particularly challenging for the new generations of simplified Single-Layer Phosphorescent OLEDs (SL-PhOLEDs), which have made great progress in the recent years. [10]These simplified devices do not possess any functional layers, which usually insure in a "classical" OLED (with a multi-layer stack) the injection, the transport and the recombination of the charges within the Emissive layer (EML).Consequently, all these abilities should be gathered in the EML itself (organic host material and organometallic phosphorescent emitter).The difficulty to efficiently host blue phosphorescent emitters is notably due to their high E T (above 2.6 eV) and their large HOMO/LUMO gap (above 3 eV).The most popular blue phosphor used in the field of PhOLEDs is the bis [2-(4,6-difluorophenyl)pyridinato-C, 2 N](picolinato)iridium(III) commonly abbreviated FIrpic. [11]However, FIrpic displays an E T of 2.67 eV (in 2-MeTHF at room temperature [12] ) and a broad emission shifting it into the sky blue or greenish-blue region of the chromatic diagram ( max = 465 and 497 nm in 2-MeTHF, CIE coordinates: 0.15; 0.37). [12]Its HOMO/LUMO energy levels are −5.55/−2.52eV (obtained from electrochemical studies in CH 2 Cl 2 [12] ), giving a gap of 3.03 eV.In addition to its greenish-blue emission, the main drawback of this phosphor is its instability when incorporated in an OLED.[15] Bis(2,4-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate iridium(III), FIr6, [16] displays a higher E T (2.72 eV measured in 2-MeTHF at room temperature [12] ) and bluer CIE coordinates (0.15;0.30) [12] than FIrpic.It also possesses an extended HOMO (−5.66 eV)/LUMO (−2.32 eV) gap (3.34 vs. 3.03 eV for FIrpic).Compared to FIrpic, its phosphorescence is therefore shifted to lower wavelengths ( max = 456 and 486 nm in 2-MeTHF [12] ).However, FIr6 has been barely studied to date, with only one example in SL-PhOLED. [12]Several examples nevertheless exist in multi-layer devices. [17,18]The difficulty to host such a type of blue phosphor is surely at the origin of its absence.21][22][23] In this SL-PhOLED technology, the molecular design of the material hosting the phosphor in the EML has been the driving force of the field as it has a strong impact on the device performance. [10]In this work, we report an efficient host material for FIr6-based SL-PhOLEDs, namely spiroquinolinoacridine-2-bis(diphenylphosphine oxide)-fluorene (SQA-2-FPOPh 2 ) constructed on a barely studied electron-rich 5,5-dimethyl-5,9dihydroquinolino[3,2,1-d]acridine (QA) fragment. [24,25]Thanks to the association with the well-known electron-deficient 2-bis(diphenylphosphineoxide)-fluorene, the potential of this molecular fragment in blue emitting SL-PhOLEDs is evidenced.Used as host, we report for the first time in FIr6-based SL-PhOLED, an External Quantum Efficiency (EQE) above 10% and a low threshold voltage (V on ) of 3.1 V. Thanks to a comparative study with structurally related hosts (one is the most efficient host reported to date for green SL-PhOLED, namely SQPTZ-2-FPOPh 2 [26] ), we show the influence of the photophysical (radiative deactivation), morphological (AFM) and charge transport (charge carrier mobilities) properties on the performances.In green emitting SL-PhOLEDs, SQA-2-FPOPh 2 also appears to be highly efficient with an EQE above 19%, in the same range than the best reported to date. [26]Finally, as the development of white SL-PhOLEDs involves blue emitting SL-PhOLEDs, this host was also used in a SL-PhOLED using a blue and a yellow emitter.An extremely high EQE of 24% was reached with CIE coordinates of (0.40;0.48).These performances show the potential of quinolinoacridine fragment to reach high performance single-layer devices with various phosphorescent emitters.
The determination of HOMO and LUMO energy levels has first been performed by cyclic voltammetry (CV) in CH 2 Cl 2 for oxidation and in DMF for reduction; potentials are given versus a saturated calomel electrode (SCE), Figure 1.Interestingly, SQA-2-FPOPh 2 displays first reversible oxidation and reduction waves, highlighting the stability of both radical cation and radical anion at the voltammetry time scale (Figure 1, top).The onset potentials were measured at 0.91 and −2.16 V respectively.The corresponding HOMO, imposed by the electron rich fragment, and LUMO, imposed by the electron-poor fragment, were then evaluated at −5.31 and −2.24 eV respectively.Molecular modelling shows that the HOMO and LUMO are almost exclusively spread out on the electron-rich and electron-poor units respectively, the spiro carbon insuring an efficient -conjugation breaking between the two fragments.This is one of the key characteristics of the Donorspiro-Acceptor molecular design and an important feature to keep a high triplet state energy (E T ), see below.
Compared to its quinolinophenothiazine analogue SQPTZ-2-FPOPh 2 , one can note that the HOMO energy level of SQA-2-FPOPh 2 is decreased (−5.20 vs. −5.31eV) due to the different electron donating effects of the bridge (CMe 2 vs. S).The inverse trend is observed when compared to indoloacridine SIA-2-FPOPh 2 , with a significant increase of the HOMO energy level (−5.53 vs. −5.31eV).This highlights the different electron rich behaviour of the QA, IA and QPTZ fragments.Regarding the LUMO energy levels, the values obtained are deep, −2.24/−2.26eV, and very similar as all centred on the same fragment, namely 2-bis(diphenylphosphineoxide)fluorene.
The difference in terms of HOMO energy levels induces different electrochemical gaps (ΔE El ): 3.07 eV for SQA-2-FPOPh 2 , 2.89 eV for SQPTZ-2-FPOPh 2 and 3.27 eV for SIA-2-FPOPh 2 .As the HOMO and LUMO are spatially separated thanks to the spiro carbon, this allows to gather in a single material i) HOMO/LUMO energy levels of the constituting building blocks, ii) a short -conjugation pathway and iii) a high E T (see below).
The UV-vis absorption and emission spectra of SQA-2-FPOPh 2 (black line), SIA-2-FPOPh 2 (red line), and SQPTZ-2-FPOPh 2 (blue line) were recorded in cyclohexane at room temperature (Figure 2).The optical properties are summarized in Table 1.The three molecules show absorption up to 365 nm, which can be attributed to −* transitions.The TD-DFT calculations (Figure 3) indicate that the HOMO-LUMO transition is almost forbidden (with oscillator strengths of 0.001-0.002for SQPTZ-2-FPOPh 2 and SQA-2-FPOPh 2 respectively).This is due to the spiro carbon, which allows for good spatial separation between the HOMO localized on the donor (QA or QPTZ) and the LUMO localized on the accepting fluorene core.In the case of SIA-2-FPOPh 2 , a small band is present at 355 nm corresponding to the HOMO-LUMO transition, which has a higher oscillator strength (f = 0.009).
Additionally, The TD-DFT calculations show that the bands seen at 318 nm in the experiments are the consequence of the HOMO-1→LUMO transition between orbitals that are both localized on the fluorene part.This transition is detected at 303-307 nm for all three compounds and shows good agreement between the theoretical and experimental results.The bands at lower energy in the case of SIA-2-FPOPh 2 are due to two transitions: HOMO→L+1 (localized on the indoloacridine part) and HOMO→L+2 (displaying a charge transfer character).
The E T of the three compounds measured at 77 K in 2Me-THF (Figure 2, bottom left) is almost identical and very high: 2.81-2.82eV.Indeed, the T 1 state is fully governed by the same fragment 2-fluorene(diphenylphosphineoxide) fragment.This is confirmed by the triplet spin density distribution (SDD) obtained by TD-DFT (b3lyp/6-311+g(d,p), Figure 2, bottom right), which shows for the three materials a triplet state density located on this fragment with no contribution of the donor unit.These values are very close to that of fluorene (2.93 eV), [36] with a decrease of ≈0.1 eV, which can be imputed to the presence of one phosphine oxide at C2 and to the interaction with the donor unit via spiro-conjugation.This feature has been previously observed with structurally related spiro compounds. [37]A very long lifetime of the emission at 77 K is measured for the three compounds, 4.3 s for SQA-2-FPOPh 2 , 4.0 s for SQPTZ-2-FPOPh 2 and 3.2 s for SIA-2-FPOPh 2 in accordance with a phosphorescent emission.One can note that, at 77 K, the fluorescence contribution of the three compounds is drastically different.Indeed, in the case of SIA-2-FPOPh 2 , the fluorescence contribution is far more intense than the phosphorescence one whereas for both SQA-2-FPOPh 2 and SQPTZ-2-FPOPh 2 , the fluorescence contribution is extremely weak.This can be correlated to the difference observed in terms of quantum yield.The fluorescence quantum yields of both SQA-2-FPOPh 2 and SQPTZ-2-FPOPh 2 are measured to be below 0.01 in solution in cyclohexane.This is expected in view of the very low oscillator strength arising from the spatial separation of the donor and the acceptor fragments, which promotes a vanishingly small transition dipole moment between the S 1 and S 0 states (see below).SIA-2-FPOPh 2 displays a different behaviour with a quantum yield slightly increased to ≈0.29, due to the higher oscillator strength of the lowest singlet excited state (f = 0.009 vs. f = 0.002 and f = 0.001, respectively).Finally, a host material for SL-PhOLED should present an excellent thermal stability for vacuum deposition and device operating.Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) provide very high decomposition (T d = 408 and 407 °C) and glassy transition (T g = 149 and 128 °C) temperatures for SQA-2-FPOPh 2 and SIA-2-FPOPh 2 respectively (Figures S1 and S2, Supporting Information).These values are in the same range than those of SQPTZ-2-FPOPh 2 [26]   and show that the QA unit is interesting for SL-PhOLED applications.
SQA-2-FPOPh 2 was finally incorporated as host in SL-PhOLEDs.The general SL-PhOLED architecture is ITO/PEDOT:PSS (40 nm)/EML: host + 18 wt.%FIr6 (100 nm)/LiF (1.2 nm)/Al (100 nm) with ITO/PEDOT:PSS as the anode and LiF/Al as the cathode.In order to obtain the highest performance, FIr6 doping rate has been optimized between 10 wt.% and 30 wt.%.Extracted characteristics (luminance, current efficiency, power efficiency, EQE, and threshold voltage V ON ) were compared to determine the optimal doping rate to be 18 wt.%(see Supporting Information).In these conditions, SQA-2-FPOPh 2 based SL-PhOLED exhibits a maximal external quantum efficiency (EQE max ) of 10.2 % at 0.01 mA cm −2 , a maximal luminance (L max ) of 7234 cd m −2 at 80 mA cm −2 and a low V ON of 3.1 V, Figure 5.Note that there is no contribution of the host in the electroluminescent (EL) spectrum, the devices exclusively exhibiting the blue emission of FIr6 (Figure 5, bottom right) with very similar CIE coordinates (see the emission spectrum of FIr6 thin-film dispersed in SQA-2-FPOPh 2 in Figure S27, Supporting Information).
For comparison purpose, SQPTZ-2-FPOPh 2 and SIA-2-FPOPh 2 have been incorporated in the same device architecture with the same FIr6 doping rate to show the influence of the three electron-rich units.The performances appear to be very different for both molecules with moderate EQE max , measured below 6%.It was particularly intriguing for SQPTZ-2-FPOPh 2 , which is the most efficient material reported to date in a green SL-PhOLED. [26]o interpret the different performances, photophysics and morphology of the EMLs were investigated as well as charge transport of host materials.The EMLs were exactly those used in the above-mentioned device.To get additional insights on excitons transfers and photophysical mechanisms, steady state and time resolved spectroscopy experiments of the three EML using either, SQA-2-FPOPh 2 , SQPTZ-2-FPOPh 2 or SIA-2-FPOPh 2 doped with 18% of FIr6 were performed.First of all, the three EMLs present photoluminescence spectra matching with their corresponding electroluminescence spectra (Figure S27, Supporting Information) showing the efficiency of the exciton's transfers.The lifetime decays of the different EMLs have been recorded at 1.0, 1.3, and 2.1 μs for SQPTZ-2-FPOPh 2 , SIA-2-FPOPh 2 and SQA-2-FPOPh 2 respectively (see Figure S28, Supporting Information).Decreasing the triplet-triplet annihilation is an important feature to lead to a high device performance, [38][39][40][41][42][43] and it has been previously observed that the shortest lifetime in a series can lead to the highest performance. [26,38]Herein, the lifetime of the EML does not follow the PhOLED performance and does not seem to be the major parameter.In this field of simplified SL-PhOLEDs, the ability of the host material to carry holes and electrons as balanced as possible is an important property to optimise the formation of excitons preferably within the centre of the EML. [10]This is due to the absence, in such a device, of functional organic layers that usually insure the injection, the transport and the blocking of the charges in order to maximize the exciton formation.Incorporation of the host materials in hole-only and electron-only space charge limited current (SCLC) diodes to extract charge carriers mobilities have been performed (see Supporting Information for device architectures, fabrication processes and electrical characteristics).The hole (μ h ) and electron (μ e ) mobilities of SQA-2-FPOPh 2 have been estimated at 7.25 × 10 −5 and 1.35 × 10 −7 cm 2 V −1 s −1 respectively.SIA-2-FPOPh 2 / SQPTZ-2-FPOPh 2 display μ h of 1.45 × 10 −2 / 5.10 × 10 −8 and μ e of 8.53 × 10 −8 / 1.10 × 10 −4 cm 2 V −1 s −1 .First, it is important to note that, despite very similar molecular structures, the mobilities are very different for both hole and electron ranging from ≈10 −2 to 10 −8 cm 2 V −1 s -1 for holes and from 10 −4 to 10 −8 cm 2 V −1 s −1 for electrons with ratios μ h /μ e of 537, 169, 988 and 0.0005 respectively.Thus, the highest hole mobility in the series, obtained for SIA-2-FPOPh 2 , is particularly high (1.45 × 10 −2 cm 2 V −1 s −1 ) for a spiro compound but leads to the lowest SL-PhOLEDs performance.The same remark can be done for the electron mobility of SQPTZ-2-FPOPh 2. However, in SL-PhOLEDs, as there are no other functional layers to insure the transport, the charges balance is more important than the intrinsic values to determine the OLED performance.In the case of SQA-2-FPOPh 2 and SIA-2-FPOPh 2 , holes are transported ≈500 and 100 000 times quicker than electrons while in the case of SQPTZ-2-FPOPh 2 electrons are ≈2000 times quicker than holes, reflecting well the different performances obtained in SL-PhOLEDs.The more balance the charge flow, the higher the SL-PhOLED performance.This shows the key role played by the charge transport in the efficiency of a SL-PhOLED.
Finally, as the morphology of the active layer is also known to have a great impact on the performance of an organic electronic device, [44] the roughness of the different EMLs were investigated by AFM studies, Figure 6.In order to well mimic the different samples, the exact architecture of the SL-PhOLEDs, i.e., ITO/PEDOT:PSS (40 nm)/EML (100 nm), was used.
The film surface of SQA-2-FPOPh 2 based-EML presents a very low root mean surface roughness (R q ) of 1.12 nm, whereas SQPTZ-2-FPOPh 2 and SIA-2-FPOPh 2 have a roughness of 3.18 and 2.67 nm respectively.As thin film organization has a major impact on recombination between hole and electron due to the trapping of carriers induced by structural defects, SQA-2-FPOPh 2 seems to promote the best structural organization.Thus, both charge transport and AFM studies correlate the highest performance measured for SQA-2-FPOPh 2 versus to the two other analogues.
The performance of SQA-2-FPOPh 2 -based devices was compared to benchmark devices fabricated with commonly used commercially available hosts for blue PhOLEDs, namely 1,3-bis(N-carbazolyl)benzene (mCP, HOMO/LUMO: −5.64/ −2.19 eV, E T = 3.05 eV) and 3,3′-bis(N-carbazolyl)−1,1′-biphenyl (mCBP, HOMO/LUMO −6.0/−2.447] The SL-PhOLED architecture of these benchmark devices is identical to that shown above.The origin of the different performance can then only be attributed to the host.For both compounds, the SL-PhOLEDs performance appear to be extremely low with EQE max of 1.4 (mCP) and 1.1% (mCBP) at 11 and 30 mA cm −2 respectively.V ON are very high, 6.9 and 7.4 V, and L max very low, 702 cd m −2 (at 60 mA cm −2 ) and 572 cd m −2 (at 80 mA cm −2 ) for mCP and mCBP respectively (see Figure S26 and Table S12, Supporting Information).As these materials are mainly "hole transporting", the origin of these low performance can only be ascribed to a poor charge balance within the EML, confirming how this parameter is important for high efficiency SL-PhOLEDs.The high performances obtained with SQA-2-FPOPh 2 show the efficiency of the Donor-spiro-Acceptor design to fabricate high performance SL-PhOLEDs.
As the efficiency of SQA-2-FPOPh 2 as host material for SL-PhOLED was high, it was finally interesting to test it in the challenging white-emitting SL-PhOLED.Indeed, the development of white SL-PhOLEDs, almost absent from literature, [49] involve blue emitting SL-PhOLEDs, Therefore, SQA-2-FPOPh 2 has been used as host in a device using two EML: a blue (FIrpic) and a yellow (PO-01),    performances get close to those of recently reported ML-PhOLEDs. [50]

Conclusion
In summary, we propose herein a new host material, namely SQA-2-FPOPh 2 for very high-performance SL-PhOLEDs.This molecule is constructed on the promising QA electron rich fragment.SQA-2-FPOPh 2 gathers an adequate combination of photophysical, electronic, thermal, and morphological properties for high performance multi-coulour SL-PhOLEDs.The blue emitter used is FIr6, which has been very rarely used in the field of SL-PhOLEDs.For the first time, an EQE above 10% at 0.01 mA cm −2 is recorded (CE = 20.5 cd A −1 , PE = 18.9 lm W −1 , V on = 3.1 V and L max = 7234 cd m −2 at 80 mA cm −2 , CIE coordinates: 0.17;0.35at 10 mA cm −2 ).In green emitting SL-PhOLEDs, SQA-2-FPOPh 2 displays a very high EQE, above 19% at 0.01 mA cm −2 , in the same range than the most efficient host (SQPTZ-2-FPOPh 2 ) reported to date for green SL-PhOLED.At 10 mA cm −2 , SQA-2-FPOPh 2 even displays higher performance than its QPTZ analogue.
A comparison with structurally related host materials (SQPTZ-2-FPOPh 2 and SIA-2-FPOPh 2 ) shed light on the different parameters (photophysical, charge transport and morphological) implied in the performance obtained.The balance of the charge transport appears as a key property to reach high performance in particular in the SL technology.
The performances of SQA-2-FPOPh 2 are also significantly higher than those of well-known commercially available host ma-trices, mCBP and mCP.Comparison with a ML-PhOLED was also performed and revealed a similar efficiency clearly showing the great potential of such a design to simplify the PhOLED technology.Finally, this host was also used in a SL-PhOLED built on the superposition of a blue and a yellow EML.A very high EQE of 24.4% was reached with CIE coordinates of (0.40;0.48).This work shows the potential and the versatility of quinolinoacridine fragment to reach high performance multi-colour single-layer devices with different phosphorescent emitters.As blue emission and simplified PhOLEDs are both important for the future of this technology (and notably for simplified white SL-PhOLEDs), designing highly efficient organic semi-conductors and rationalizing their performance are important steps.We keep working in this direction.

Figure 7 .
Figure 7. FIrPic/PO-01-SQA-2-FPOPh 2 based device performances.A) Current density and luminance as a function of voltage bias; B) power and current efficiencies as function of current density; C) Roll-off efficiency: EQE as function of luminance; D) Normalized electroluminescent spectra at 10 mA cm −2 .

Table 1 .
Selected electronic and physical data of SQA