Green Phosphorescent Organic Light-emitting Diodes Research Articles

Novel indolocarbazole-based bipolar host materials for fabricating green phosphorescent organic light-emitting diodes with high efficiency and low roll-off

Herein we report two novel host materials, viz., 2-(dibenzo[b,d]furan-2-yl)indolo[3,2,1-jk]carbazole (ICz-DBF) and 3-(indolo[3,2,1-jk]carbazol-2-yl)furo[2,3-b:5,4-b']dipyridine (ICz-PFP), for fabricating highly stable and efficient organic light-emitting diodes (OLEDs). Green phosphorescent OLEDs (PhOLEDs) based on ICz-DBF and ICz-PFP exhibited less efficiency roll-off than those based on 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host, and especially, the ICz-DBF-based device achieved a high external quantum efficiency (EQE) of 20.5%. Bipolar hosts composed of indolocarbazole (ICz) exhibit strong molecular rigidity, high triplet energy, and fast electron transport properties; therefore, they act as acceptors, but dibenzofuran (DBF) and dipyridofuran (PFP) substitutions endow them with controlled electron-donating characteristics. The bipolar properties of the new host materials were demonstrated by single-carrier device analysis, confirming the effectiveness of our research strategy for developing hosts for high-quality OLEDs.

Novel bipolar hosts for highly efficient green phosphorescent organic light-emitting diodes with slow efficiency roll-off

Organic light-emitting diodes (OLED) based on phosphorescent emitter promise 100 % internal quantum efficiency (IQE) theoretically, showing extensive applications in various fields. The guest phosphors are usually doped into appropriate host matrices to minimize unnecessary excitons quenching. Herein, three novel bipolar host materials (H1–H3) were developed. Pyridine or pyrimidine was selected as the electron-transporting n-type unit and indolocarbazole as hole-transporting p-type unit. Dibenzofuran was grafted onto the central pyridine or pyrimidine to enhance the electron-transporting ability based on its electron-deficient feature. Furthermore, dibenzofuran was also useful to enhance the thermal and amorphous stabilities of the generated host materials. These host materials are characterized by high glass transition temperatures over 150 °C and suitable energy levels. Thanks to strengthened microcavity effect, the top-emitting green phosphorescent OLEDs containing these hosts in emitting layer realized quite narrow spectra with full width at half maxima (FWHM) of ≤27 nm. Low turn on voltage of 2.01 V, high external quantum efficiency (EQE) up to 41.1 % and pretty stable emission color with ΔCIEy≤0.002 were achieved for these green devices. Moreover, these devices are characterized by extraordinary efficiency stability. A high EQE of 34.9 % was still remained at extremely high brightness of 10000 cd/m2, corresponding to an efficiency roll-off of less than 15 %. These host materials may find practical applications in highly efficient and stable OLED displays.

Highly efficient phosphorescent organic light-emitting diodes with low turn-on voltages using N-phenylcarbazole/pyrimidine-based bipolar host materials

Herein, three novel bipolar host materials with D-π-A structure, termed 2CzPm, 3CzPm, and 9CzPm, were successfully synthesized by incorporating a pyrimidine moiety as an accepter and N-phenylcarbazole group as a donor through different connection sites. The three hosts exhibited good thermal stability with a glass transition temperature (Tg) of 104, 103, and 102 °C, respectively. They also possessed high enough triplet energy level (ET = 2.58–2.75 eV) to be efficient hosts for red and green phosphorescent organic light-emitting diodes (PhOLEDs) (ETdopants <2.5 eV). Moreover, considerable hole and electron current density in the conventional voltage range were observed, suggesting their good bipolar transport properties. When utilized in green and red PhOLEDs, the devices hosted by 9CzPm exhibited the best electroluminescence (EL) performance with low turn-on voltages (Von) of 2.6 and 2.5 V, and maximum external quantum efficiency (EQEmax) of 21.79% and 25.03%, respectively, indicating that pyrimidine moiety is a promising building block for bipolar host materials.

Sterically Hindered Tetradentate [Pt(O^N^C^N)] Emitters with Radiative Decay Rates up to 5.3 × 105 s-1 for Phosphorescent Organic Light-Emitting Diodes with LT95 Lifetime over 9200 h at 1000cd m-2.

Described here are sterically hindered tetradentate [Pt(O^N^C^N)] emitters (Pt-1, Pt-2, and Pt-3) developed for stable and high-performance green phosphorescent organic light-emitting diodes (OLEDs). These Pt(II) emitters exhibit strong saturated green phosphorescence (λmax = 517-531nm) in toluene and mCP thin films with emission quantum yields as high as 0.97, radiative rate constants (kr) as high as 4.4-5.3 × 105 s-1 and reduced excimer emission, and with a preferential horizontally oriented transition dipole ratio of up to 84%. Theoreticalcalculations show that p-(hetero)arene substituents at the periphery of the ligand scaffolds in Pt-1, Pt-2, and Pt-3 can i) enhance the spin-orbit coupling (SOC) between the lower singlet excited states and the T1 state, and S0→Sn (n = 1 or 2) transition dipole moment, and ii) introducing additional SOC activity and the bright 1ILCT[π(carbazole)→π*(N^C^N)] excited state (Pt-2 and Pt-3), which are the main contributors to the increased kr values. Utilizing these tetradentate Pt(II) emitters, green phosphorescent OLEDs are fabricated with narrow-band electroluminescence (FWHM down to 36nm), high external quantum efficiency, current efficiency up to 27.6% and 98.7cd A-1, and an unprecedented device lifetime (LT95) of up to 9270 h at 1000cd m-2 under laboratory conditions.

Indolo[3,2,1-jk]carbazole-derived planar electron transport materials realizing high efficiency in green phosphorescent organic light-emitting diodes

Indolo[3,2,1-jk]carbazole-derived planar electron transport materials realizing high efficiency in green phosphorescent organic light-emitting diodes

Highly Efficient Phosphorescent Organic Light-Emitting Diodes Using Benzo[4',5']imidazo[2',1':2,3]imidazo[4,5,1-jk]carbazole as a Rigid and Effective Electron Acceptor in Bipolar Host.

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4',5']imidazo[2',1':2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

Effects of the change of isomers on room-temperature phosphorescence, thermally activated delayed fluorescence, and long persistent luminescence of organic hole-transporting materials with the selective potential for the application in electronic devices and optical sensors of oxygen

Competition of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) was investigated on the example of four newly synthesised organic isomeric compounds containing two electron-donating triphenylamine moieties and single electron-accepting dibenzothiophene-2-yl(phenyl)methanone unit by numerous theoretical and experimental approaches. The different emission origin (RTP or TADF) of the isomeric compounds was caused by the different energy gaps between the lowest singlet and triplet states locally exited and charge transfer in nature. Very different external quantum efficiencies ranging from 2.8 to 13.9% were observed for green phosphorescent organic light-emitting diodes fabricated using the isomeric compounds as the hosts. Such device efficiency differences were related to the different hole mobility values. The hole mobilities of ca. 1 × 10−3 cm2V−1s−1 were observed at an electric field of 6.4 × 105 V/cm for the derivatives having triphenylamine moieties at C-2, C-4, and at C-3, C-5 positions of the central benzene ring. Considerably lower hole mobilities of ca. 1 × 10−5 cm2 V−1 s−1 were recorded at the same electric field for the compounds with triphenylamine groups at C-2, C-3, and at C-2, C-6 positions. Depending to the great extent on RTP and TADF processes, different efficiencies of long persistent luminescence (LPL) of the exciplex-forming solid-state mixtures of the isomers and bis[2-(diphenylphosphine)phenyl] ether oxide (DPEPO) were detected. The compound with triphenylamine groups at C-3, and C-5 positions as RTP emitter and its molecular mixture with DPEPO as LPL emitter the active layers of optical sensors of oxygen were prepared. They showed the Stern–Volmer constant of 4.55 × 10−4 ppm in the range of oxygen concentrations up to 10000 ppm.

A symmetrical carbazole/benzonitrile-based high triplet energy bipolar host designed by a para-linkage strategy for efficient blue and green phosphorescent organic light-emitting diodes

Based on a para-linkage concept, a symmetrical carbazole-benzonitrile derivative of 4-(9H-carbazol-9-yl)-3,5-diphenyl-2,6-difluoronitrile (CzPhFBN) was designed and prepared as a host for phosphorescent organic light-emitting diodes (PhOLEDs). In the CzPhFBN molecular, a cyano (CN) unit and a carbazole (Cz) unit were attached to the central phenyl ring at the para positions. Simultaneously, the Cz group was surrounded by two phenyl units to increase structure distortion for high triplet energy and CN unit was surrounded by two F units to improve electron transporting behavior. The photophysical properties and carrier transport behavior of CzPhFBN were investigated. The results demonstrated that CzPhFBN offered a high triplet energy of exceeding 3.0 eV and bipolar carrier transporting property. By incorporating CzPhFBN as the host, the blue FIrpic- and green Ir(ppy)3-based PhOLEDs with a common device configuration delivered the maximum efficiencies of 11.1%/19.7 cd A−1/12.1 lm W−1 and/13.1%/43.6 cd A−1/28.6 lm W−1, respectively. These results indicate that this study provides a potential design strategy of high triplet energy bipolar host materials with a simple and elegant structure.

Highly Efficient and Stable Green Phosphorescent Light‐Emitting Diodes Based on Solution‐Processable Ir(III) Complexes with Electron‐Transporting Ancillary Ligands

AbstractTwo green heteroleptic Ir(III) complexes are reported, namely, bis[5‐(2‐ethylhexyl)‐8‐trifluoromethyl‐5H‐benzo[c][1,5]‐naphthyridin‐6‐one](2‐(5‐phenyl‐1,3,4‐oxadiazol‐2‐yl)phenolate)iridium (III) (Ir(CF3BNO)2‐pop) and bis[5‐(2‐ethylhexyl)‐5H‐benzo[c][1,5]naphthyridin‐6‐one](2‐(5‐phenyl‐1,3,4‐oxadiazol‐2‐yl)phenolate)iridium (III) (Ir(BNO)2‐pop) for solution‐processed phosphorescent organic light‐emitting diodes (PHOLEDs). Both the Ir(III) complexes exhibit high external quantum efficiency (EQE) and intense green emission upon using poly(3,4‐ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) as a hole injection layer (HIL); Ir(CF3BNO)2‐pop based PHOLEDs perform better with an EQE of 23% and a current efficiency of 82 cd A−1 than Ir(BNO)2‐pop based PHOLEDs. In addition, to improve device operational lifetime, a crosslinkable HIL material, 9,9'‐(cyclohexane‐1,1‐diylbis(4,1‐phenylene))bis(N‐phenyl‐N‐(4‐(((4‐vinylbenzyl)oxy)methyl)phenyl)‐9H‐carbazol‐3‐amine) (CPCzDPAX‐1) is designed and synthesized. Device operational lifetimes of Ir(CF3BNO)2‐pop and Ir(BNO)2‐pop PHOLEDs incorporating the CPCzDPAX‐1 HIL show 14‐fold and 6‐fold improvement, compared to the corresponding devices incorporating the PEDOT:PSS HIL. To the best of the authors’ knowledge, this is the best device lifetime performance by using the combination of new Ir(III) complexes and HIL materials for solution‐processed green PHOLEDs.

Tetrapyridine/triphenyltriazine-conjugated electron transporters for low-power-consumption, high-stability phosphorescent OLEDs

A series of tetrapyridine/triazine-conjugated electron transporters, TnPyTRZ, was developed. T3PyTRZ realized a turn-on voltage of 2.18 V, power efficiency of over 115 lm W−1, and lifetime of 30 000 h at 1000 cd m−2 in green phosphorescent OLED.

Robust Spirobifluorene Core Based Hole Transporters with High Mobility for Long-Life Green Phosphorescent Organic Light-Emitting Devices.

Using a tailored high triplet energy hole transport layer (HTL) is a suitable way to improve the efficiency and extend the lifetime of organic light-emitting devices (OLEDs), which can use all molecular excitons of singlets and triplets. In this study, dibenzofuran (DBF)-end-capped and spirobifluorene (SBF) core-based HTLs referred as TDBFSBF1 and TDBFSBF2 were effectively developed. TDBFSBF1 exhibited a high glass transition temperature of 178 °C and triplet energy of 2.5 eV. Moreover, a high external quantum efficiency of 22.0 %, long operational lifetime at 50 % of the initial luminance of 89,000 h, and low driving voltage at 1000 cd m-2 of 2.95 V were achieved in green phosphorescent OLEDs using TDBFSBF1. Further, a high-hole mobility μh value of 1.9×10-3  cm2  V-1  s-1 was recorded in TDBFSBF2. A multiscale simulation successfully reproduced the experimental μh values and indicated that the reorganization energy was the primary factor in determining the mobility differences among these SBF core based HTLs.

Acridone-Based Host Materials for Green Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs with Low-Efficiency Roll-Offs.

By linking the carbazole unit to the nitrogen atom of acridone through phenyl or pyridyl, two compounds, named 10-(4-(9H-carbazol-9-yl)phenyl)acridin-9(10H)-one (AC-Ph-Cz) and 10-(5-(9H-carbazol-9-yl)pyridin-2-yl)acridin-9(10H)-one (AC-Py-Cz) were designed and synthesized. These two materials, characterized with highly twisted and rigid structure, good thermal stability, and balanced carrier-transporting properties, were employed as host materials for green phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes (OLEDs). The carbazole group, despite its small contribution to the highest occupied molecular orbitals (HOMOs) of these two materials, plays an essential role as an intramolecular host in energy delivering and improving the hole transporting ability of these two hosts. The incorporation of the electron-deficient pyridyl group as a linking group slightly improves the electron transporting capability of AC-Py-Cz. The green phosphorescent OLED (PhOLED) based on AC-Py-Cz exhibited excellent device performance with a turn-on voltage of 2.5 V, a maximum power efficiency and an external quantum efficiency (ηext ) of 89.8 lm W-1 and 25.2 %, respectively, benefitting from the better charge-balancing ability of AC-Py-Cz host due to the presence of the pyridyl bridge. More importantly, all the devices based on these two hosts showed low efficiency roll-off at high brightness due to the suppressed non-radiative transition in the emitting layer. In particular, the AC-Py-Cz-hosted green PhOLED exhibited an efficiency roll-off of 1.6 % from the maximum next at a high brightness of 1000 cd m-2 and a roll-off of 15.9 % at an extremely high brightness of 10000 cd m-2 . This study manifests that acridone-based host materials have great potential in fabricating OLEDs with low efficiency roll-off.

Carbazole and triazine-based bipolar hosts with extremely low efficiency roll-offs for green PhOLEDs

In this paper, two new bipolar host materials (CZ-p-TRZ and CZ-m-TRZ) which consist of 9-phenylcarbazole as the donor and 2,4,6-triphenyl-1,3,5-triazine as the acceptor were synthesized and characterized. Two compounds exhibit excellent thermal stability and appropriate triplet energy levels (ET). Based on the above advantages, Ir(ppy)3-based green phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated with CZ-p-TRZ and CZ-m-TRZ as host. Both PhOLEDs show good performance with maximum external quantum efficiencies (EQEmax) of 16.83 and 19.40%, respectively. Especially, two devices also exhibit extremely small efficiency roll-off, and at a practical luminance of 1000 cd/m2, only 0.1% drop in efficiency was observed for two devices, which is among the best results ever reported as green bipolar hosts.

Carbazole-benzonitrile based organic semiconductors: Synthesis, characterization and electroluminescent property

In order to explore the relationship between molecular structure and the potential of being host materials, two carbazole-benzonitrile based molecules (DCzBNPh-1 and DCzBNPh-2) with possible high triplet energy and small energy gap (△ E ST ) between lowest singlet state (S 1 ) and lowest triplet state (T 1 ) were synthesized, forming the core moiety of terphenylene. Their initial difference is the positions of cyano (CN) groups. Studies on single crystal analysis, mechano-responsiveness, density functional theory (DFT) calculations, cyclic voltammetry (CV) measurements, thermal tests and solution photophysical properties verify their qualification of being host materials and clarify the CN units related difference. Then DCzBNPh-1 and DCzBNPh-2 were applied as host materials, matching tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) as the dopant to construct phosphorescent organic light-emitting diodes (PhOLEDs) with the same optimal structure. Despite their unbalanced carrier transporting character, for DCzBNPh-2 based device, the maximum emission brightness reaches 61,760 cd m −2 (weaker than DCzBNPh-1), while the maximum external quantum efficiency (EQE max ) of 22% and the turn-on voltage of 2.5 V surpass those of DCzBNPh-1. The satisfying result manifests DCzBNPh-2 should be the best host of this series. • Two host materials with high triplet energy has been designed and synthesized. • Terphenylene-cored CzBN isomers fill in the vacancy of multicore based CzBN derivatives. • Green PhOLEDs were prepared with the maximum external quanmtum efficiency about 22% and low turn-on voltage of 2.5 V.

Electroplex hosts for highly efficient phosphorescent organic light-emitting diodes with extremely small efficiency roll-offs

• Four bipolar molecules were designed and synthesized for accessing exciplex and electroplex host systems. • Highly efficient green PhOLED with EQE max of 29.9% was achieved. • Extremely small efficiency roll-offs (1.3% at 1,000 cd m −2 ; 5.4% at 5,000 cd m −2 ) were obtained. Host materials are of great significance to achieve high-performance host–guest organic light-emitting diodes (OLEDs) through restraining the exciton annihilation in the emissive layer. Herein, four bipolar molecules bearing benzothienocarbazole ( BTCz )/benzothienocarboline ( BTCb ) and benzimidazole ( Bi ) moieties were designed and synthesized. Using them as hosts, green phosphorescent OLEDs (PhOLEDs) with maximum current efficiency (CE max ) of 90.8 cd A −1 , power efficiency (PE max ) of 76.5 lm W −1 , and external quantum efficiency (EQE max ) of 27.9% were obtained. More importantly, it was found that these four molecules could be used as electron-donating materials to interact with electron-accepting triazine derivatives to form electroplexes. Employing these electroplexes as hosts, highly efficient green PhOLEDs with CE max of 104.4 cd A −1 , PE max of 85.1 lm W −1 , EQE max of 29.9%, and extremely small efficiency roll-offs (EQE = 29.5%, 1.3% roll-off at 1,000 cd m −2 ; EQE = 28.3%, 5.4% roll-off at 5,000 cd m −2 ) were achieved. The excellent device performances fully exhibit the promising potential of electroplexes as hosts for high-performing OLEDs.

Bicarbazole-cyanopyridine based bipolar host materials for green and blue phosphorescent OLEDs: influence of the linking style between P- and N-type units

Bicarbazole and cyanopyridine were used to construct bipolar host materials. Ortho-linkage generated more balanced charge transportations and sufficiently high triplet energy (3.05 eV), and finally led to high EQEs of 15.9% and 22.6% in sky-blue and green phosphorescent OLEDs.

Spirobifluorene modified electron transport materials for high efficiency in phosphorescent organic light-emitting diodes

Spirobifluorene modified materials developed as electron transport layers lowered device driving voltage, enhanced quantum efficiency, and more than doubled the lifetime of green phosphorescent organic light-emitting diodes.

Constructing all-solution-processed green phosphorescent organic light-emitting diodes based on zinc sulfide QDs as an electron injection layer

We have demonstrated an all-solution-processed multi-layer green phosphorescent organic light-emitting diodes(OLEDs) using zinc sulfide quantum dots (ZnS QDs) based electron injection layer (EIL) in the conventional device structure. The ZnS QDs EIL with proper work function offers efficient electron injection and retains the solvent resistance to the adjacent electron transportation layer. By inserting of a poly-ethyleneimine(PEI) interlayer between the emitting layer and electron transportation layer, solvent can be prevented from permeating through the emissive layers beneath and no additional injection barrier is formed. Thereby, the all-solution-processed OLED with a ZnS QDs EIL exhibits a highest current efficiency of 12.5 cd A−1 based on the PVK: Ir(ppy)3 emitter, achieving a 27.6% improvement in comparison to the reference without a ZnS QDs EIL. Our results provide a facile approach for the development of all-solution-processed OLEDs.

Novel benzonitrile- and benzo[d]imidazole-based bipolar hosts for green PhOLEDs with a low turn-on voltage

In this work, three bipolar host materials combined electron-rich carbazole moiety and electron-deficient benzonitrile and benzo[d]imidazole units were firstly designed and synthesized, namely 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-4-carbonitrile (p-PNBMCz), 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-3-carbonitrle (m-PNBMCz) and 3''-(9H-carbazol-9-yl)-5'-(1-phenyl-1H-benzo[d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-2-carbonitrle (o-PNBMCz), respectively. The glass transition temperatures (Tg) of all the materials are above 117 °C, demonstrating a good morphological stability, and the thermal decomposition temperatures (Td) are over 436 °C, reflecting remarkable thermal properties. Besides, their HOMO separated fully with LUMO, which is conducive to both hole and electron transporting. They also possess a triplet energy level (ET) of about 2.7 eV, which qualifies them to be hosts for green and red phosphorescent emitters. Green phosphorescent organic light-emitting diodes (PhOLEDs) hosted by these three materials were manufactured and they displayed good performance with very low turn-on voltages (Von). Remarkably, the device hosted by p-PNBMCz (G1) exhibits maximal external quantum efficiencies (EQE) of 17.1% and maximal power efficiency (ηp,max) of 69.2 lm/W with a minor efficiency roll-off, the ηext still remains to be 15.9% with only a 7% roll-off percentage as the luminance reaches 1000 cd/m2.

High-performance narrow spectrum green phosphorescent top-emitting organic light-emitting devices with external quantum efficiency up to 38%

In this work, we have experimentally demonstrated the efficacy of micro-cavity effect in realizing high-performance top-emitting organic light-emitting diodes (TEOLEDs). By optimizing the thickness of top Yb/Ag electrode and cavity length, highly efficient green TEOLED with external quantum efficiency as high as 38% was achieved. A strong dependence of electroluminescent (EL) performances and spectrum on cavity length was observed, and there was also a significant angle dependence of EL spectrum. Ultimately, ultra-high current efficiency up to 161.17 cd A−1 (3.2 V) was obtained by the device with emission peak at 552 nm, which is 35 nm longer than the intrinsic emission peak (517 nm) of utilized green emitter. Interestingly, this device displayed narrow emission with full-width at half-maximum of less than 20 nm, which was obtained by increasing the Ag layer thickness.