Bipolar Host Research Articles

Enhancing Stability in All-Vacuum-Evaporated Hybrid Perovskite Solar Cells via a Bipolar Host as a Hole-Transporting Layer.

Growing an ultrathin hybrid organic-inorganic perovskite film while maintaining high efficiency and addressing photostability challenges for commercial devices remains a significant hurdle. In this study, we explore the incorporation of organometallic copper phthalocyanine (CuPc) and MS-OC (a previously published spiro-based interfacial material for perovskite solar cells (PSCs), featuring an ortho-oriented carbazole donor) as an addition to the hole-transporting layer (HTL) in all-vacuum-deposited Cs0.06FA0.94Pb(I0.68Br0.32)3 PSCs. By innovatively introducing a 3 nm-thin MS-OC layer at the CuPc-perovskite interface, we achieve a deeper understanding of the crystallographic dynamics of perovskites, resulting in a uniform and pinhole-free film. We demonstrate that PSCs utilizing the CuPc HTL with an MS-OC interfacial layer in a p-i-n architecture achieve a power conversion efficiency (PCE) of up to 14.42%. Remarkably, the CuPc/MS-OC-based device exhibits outstanding long-term photostability, maintaining its initial PCE over 400 h (T100 = 400 h) under continuous sunlight illumination. By configuring the device architecture as ITO/MoO3/CuPc/MS-OC/perovskite/C60/BCP/Ag, we find that the evaporated MS-OC thin films effectively reduce nonradiative losses, passivate the perovskite, and enhance device performance. Our findings indicate that the polarity of the underlying surface significantly influences perovskite nucleation, underscoring the potential to improve photostability by controlling interfacial imperfections.

New Bipolar Host Materials Based on Indolocarbazole for Red Phosphorescent OLEDs.

We designed and synthesized new indolocarbazole-triazine derivatives, 9-di-tert-butyl-5,7-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5,7-dihydroindolo[2,3-b]carbazole (2TRZ-P-ICz) and 3,9-di-tert-butyl-5,7-bis(5'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1':3',1″-terphenyl]-2'-yl)-5,7-dihydroindolo[2,3-b]carbazole (2TRZ-TP-ICz), as new bipolar host materials for red phosphorescent OLEDs. In the film state, 2TRZ-P-ICz and 2TRZ-TP-ICz exhibited photoluminescence maxima at 480 nm and 488 nm, respectively. The dipole moment characteristics of the new compounds under various solvent conditions were investigated using the Lippert-Mataga equation. The results showed that the dipole moment of 2TRZ-P-ICz is 26.9D, while that of 2TRZ-TP-ICz is 21.3D. The delayed fluorescence lifetimes were 0.188 μs for 2TRZ-P-ICz and 2.080 μs for 2TRZ-TP-ICz, with 2TRZ-TP-ICz showing TADF characteristics. Additionally, 2TRZ-TP-ICz was found to have a ΔEST of less than 0.2 eV. The triplet energy levels of the newly synthesized bipolar host materials were found to be 2.72 and 2.75 eV, confirming their suitability for use in red phosphorescent OLEDs. To investigate the carrier mobility of the synthesized materials, hole-only devices and electron-only devices were fabricated and tested. The hole mobility value at 1V was found to be 3.43 × 10-3 cm2/Vs for 2TRZ-P-ICz and 2.16 × 10-3 cm2/Vs for 2TRZ-TP-ICz. For electron mobility at 1V, 2TRZ-P-ICz showed a value of 4.41 × 10-9 cm2/Vs, while 2TRZ-TP-ICz exhibited a value of 9.13 × 10-9 cm2/Vs. As a result, when the new material was used as a host in red phosphorescent OLEDs, 2TRZ-TP-ICz achieved a current efficiency of 9.92 cd/A, an external quantum efficiency of 13.7%, CIE coordinates of (0.679, 0.319), and an electroluminescence maximum wavelength of 626 nm.

White Organic Light-Emitting Diodes with External Quantum Efficiency of 20.77% at 10000cdm-2 and Ultra-High Color Stability by Controlling Exciton Distribution.

Although brightness and efficiency have been continually improved, the inability to achieve superior efficiency, color stability, and low-efficiency roll-off simultaneously in white organic light-emitting diodes (OLEDs) remains a knotty problem restricting the commercial application. In this paper, emission balance for two different horizontal orientation emitting molecules is maintained by using hole transport materials and bipolar host materials to control carriers' recombination and exciton diffusion. Impressively, the obtained devices exhibit extremely stable white emission with small chromaticity coordinates variation of (0.0023, 0.0078) over a wide brightness range from 1000 to 50000cdm-2. Meanwhile, the optimal white OLED realizes the power efficiency, current efficiency, and external quantum efficiency up to 70.68lmW-1, 85.53cdA-1, and 24.33%, respectively at the practical brightness of 1000cdm-2. Owing to reduced heterogeneous interfaces and broadening recombination region, this device exhibits a high EQE over 20% under high luminance of 10000cdm-2, demonstrating slight efficiency roll-off. The operating mechanism of the device is analyzed by versatile experimental and theoretical evidences, which concludes precise manipulation of charges and excitons is the key points to achieve these excellent performances. This work provides an effective strategy for the design of high-performance white OLEDs.

Novel bipolar host materials based on benzimidazole-hybridized phenanthridine for efficient red PhOLEDs

In this work, two innovative bipolar host materials, TPA-BIP and Cz-BIP, were designed and synthesized based on benzimidazole-hybridized phenanthridine (BIP) moiety as the electron transport unit. Compared to the widely used electronic transport unit benzimidazole, the BIP unit possesses a large planar rigid structure that can effectively enhance the thermal stability and the electron transfer capability of materials. Both materials exhibit good thermal stability respectively with Tg of 113 °C and 141 °C, proper HOMO/LUMO energy levels, and balance carrier transport capabilities. The red phosphorescent organic light-emitting diode prepared based on both materials achieve relatively high luminescence efficiency and relatively low efficiency roll-off. In particular, the EQEmax of the R1 device using TPA-BIP as the host material reaches 21.5 %, which is more superior than that of the conventional host material CBP (EQEmax of 9.84 %). Even at 1000 cd/m2, the device still achieves an EQE of 21.3 % with virtually no decrease. In conclusion, the results demonstrate the broad development potential of these BIP-based host materials for red PhOLEDs applications.

Two novel bipolar materials based on 9,9-dimethylfluorene bridges for non-doped blue and red phosphorescent OLED applications

The present study delineates the design and synthesis of two bipolar host materials, namely 4-(7-(9-([1,1′-biphenyl]-4-yl)-9H-carbazol-3-yl)-9,9-dimethyl-9H-fluoren-2-yl)benzonitrile (o-CzDFBN) and 4-(7-(9-([1,1′-biphenyl]-3-yl)-9H-carbazol-3-yl)-9,9-dimethyl-9H-fluoren-2-yl)benzonitrile (p-CzDFBN). By incorporating 9,9-dimethylfluorene as a π conjugated bridge, the fluorescence quantum yield, carrier mobility and balance can be improved. Both host materials have excellent thermal stability, with thermal decomposition temperature (Td) of exceeding 390 °C and glass transition temperature (Tg) of exceeding 125 °C. Electroluminescent devices using p-CzDFBN as the host material show better performance than those based on o-CzDFBN. The maximum external quantum efficiencyof red PhOLED reaches 27.17%. The non-doped blue OLED achieves a EQEmax of 6.38%, a maximum luminance (Lmax) of 20150 cd/m2, a maximum current efficiency of 4.81 cd/A, and a maximum power efficiency of 5.40 lm/W. This research contributes to the development of new materials for OLEDs, potentially leading to more efficient, stable, and high-performing OLEDs.

P‐206: Key factor of Device Lifetime in Exciplex Host Based Blue Phosphorescent Organic Light Emitting Diodes

In this study, we investigated the key factor of device lifetime in blue phosphorescent organic light emitting diodes (PhOLEDs) by managing hosts and dopants in the emitting layer. We manufactured four phosphorescent devices using the exciplex type mixed host and single bipolar host doped with either Pt type and Ir type phosphors. The mixed host was better than single host in the Pt emitter doped device, while it was similar to the single host in the Ir emitter doped device. Investigation of the light‐emission mechanism of the PhOLEDs revealed that the mixed host is essential in the device showing strong hole trapping behavior. The strong p‐type host in the mixed host relieves the hole trapping by the Pt phosphor, resulting in less polaron formation and polaron accumulation. As a result, the mixed host reduces triplet‐polaron annihilation and extends the device lifetime of the Pt emitter doped PhOLEDs.

Stable pure-green organic light-emitting diodes toward Rec.2020 standard

Manipulating dynamic behaviours of charge carriers and excitons in organic light-emitting diodes (OLEDs) is essential to simultaneously achieve high colour purity and superior operational lifetime. In this work, a comprehensive transient electroluminescence investigation reveals that incorporating a thermally activated delayed fluorescence assistant molecule with a deep lowest unoccupied molecular orbital into a bipolar host matrix effectively traps the injected electrons. Meanwhile, the behaviours of hole injection and transport are still dominantly governed by host molecules. Thus, the recombination zone notably shifts toward the interface between the emissive layer (EML) and the electron-transporting layer (ETL). To mitigate the interfacial carrier accumulation and exciton quenching, this bipolar host matrix could serve as a non-barrier functional spacer between EML/ETL, enabling the distribution of recombination zone away from this interface. Consequently, the optimized OLED exhibits a low driving voltage, promising device stability (95% of the initial luminance of 1000 cd m−2, LT95 > 430 h), and a high Commission Internationale de L’Éclairage y coordinate of 0.69. This indicates that managing the excitons through rational energy level alignment holds the potential for simultaneously satisfying Rec.2020 standard and achieving commercial-level stability.

Acceptor-σ-donor-σ-acceptor host material for red phosphorescent and thermally activated delayed fluorescent OLEDs

A novel bipolar host molecule di-spiro [fluorene-9,5′-quinolino [3,2,1-de] acridine-9′,9″-fluorene]-2,2″,7,7″-tetracarbonitrile (QACN) featuring an Acceptor-σ-Donor-σ-Acceptor structure was developed. Systematic investigations into its photophysical, electrochemical, and thermal properties unveiled exceptional thermal stability, a three-dimensional spatial configuration, and bipolar carrier transport capability. Employing QACN as the host material in red phosphorescent and thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) yielded a maximum external quantum efficiency of 25.3 % and 16.5 %, respectively. Compared with the commercial material TPBi (EQE = 12.0 %, λEL = 668 nm) and mCP (EQE = 14.5 %, λEL = 652 nm), the TADF OLEDs based on QACN achieved a higher EQE and a large red-shift emission (EQE = 16.5 %, λEL = 694 nm). These findings underscore the potential of QACN as an effective host material for red phosphorescent and TADF emitters and provide a new auxiliary finesse for realizing deep red and near-infrared (NIR) OLEDs.

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.

Derivatives of carbazole and quinazoline as host materials for red phosphorescent OLEDs showing high efficiency and brightness

Host materials in the emitting layer play an important role in phosphorescent organic light-emitting diodes (PhOLEDs). Two red-phosphorescent bipolar host materials, 9-phenyl-9'-(4-phenylquinazolin-2-yl)-9H,9′H-3,3′-bicarbazole (BQzB2Cz) and 10-(9-phenyl-9H-carbazol-3-yl)-7-(4-phenylquinazolin-2-yl)-7H-benzo[c]carbazole (BQzB2CzB), composing of carbazole moieties of 9-phenyl-9H,9′H-3,3′-bicarbazole and 10-(9-phenyl-9H-carbazol-3-yl)-7H-benzo[c]carbazole as hole-transporting groups and quinazoline moieties as electron-transporting groups, have been synthesized and characterized. The materials exhibit good thermal stability and suitable triplet-state energy levels, and the red PhOLED prepared from BQzB2Cz and BQzB2CzB showed good performance. The OLED prepared with 3 wt% guest material doped BQzB2CzB showed the optimized performance, with the turn-on voltage of 2.8 V, the maximum brightness of 114000 cd/m2, the maximum current efficiency of 21.60 cd/A, the maximum power efficiency of 24.24 lm/W, and the maximum external quantum efficiency (EQEmax) of 25.11 % (at 132 cd/m2). In addition, an EQEmax of 18.91 % was still achieved at 10,000 cd/m2. The lifetime and efficiency of OLED based on BQzB2CzB is improved compared to those of the OLED based on BQzB2Cz. The results indicate that the strategy of designing bipolar host materials with carbazole moieties as hole-transporting groups and quinazolines as electron-transporting units is a practical approach to realizing high-efficiency and high-brightness OLEDs.

Efficiency Roll-off Suppression in Solution-Processed Organic Light-Emitting Diodes through a Bipolar Host Design

Efficiency Roll-off Suppression in Solution-Processed Organic Light-Emitting Diodes through a Bipolar Host Design

Rational Design of Bipolar Host and Thermally Activated Delayed Fluorescence Materials in Donor-Acceptor Molecular Architecture: A Theoretical Study.

Donor-acceptor (D-A) molecules have drawn massive attention recently in the design of high-performance materials, but the underlying reasons for the magic abilities of D-A architecture in building very different organic semiconductors are still unclear. Here, based on a series of experimentally bipolar host and thermally activated delayed fluorescence (TADF) molecules with the same donor but different acceptor units, it was found that TADF emitters have more effective charge transfer between donor and acceptor units than bipolar host molecules. More efficient conjugation effects between the donor and acceptor units of host materials were identified from the lower dihedral angles of the D-A structure, smaller and even negative charge transfer amount, shorter charge-transfer length, and larger hole-electron overlap extent. These findings with in-depth insights into different interaction models of donor and acceptor units shed important light on the molecular design of TADF emitters and bipolar materials in a D-A architecture.

Highly efficient top-emitting green phosphorescent OLEDs with a narrow band and slow efficiency roll-off for high-definition displays

New bipolar hosts are used to fabricate green-phosphorescent OLEDs, realizing a narrow full width at half maximum of 28 nm, high efficiencies of 183.1 cd A−1 and 247.3 lm W−1, and slow efficiency roll-off with 132.2 cd A−1 at 66 000 cd m−2.

A chiral bipolar host for efficient solution-processed circularly polarized OLEDs via a chirality energy transfer process.

Chiral bipolar hosts of (R/S)-L-2mCPCN are synthesized, which show high singlet/triplet energy levels and clear circularly polarized luminescence. Employing racemic phosphorescent and TADF materials as emitting guest molecules, solution-processable CP-OLEDs based on such chiral hosts are obtained with an EQEmax of 10.7% and |gEL| values of 5.0 × 10-3.

Efficient and low roll-off deep-blue organic light-emitting diodes with anthracene-based compounds as hosts

We synthesize four anthracene-based bipolar host materials. The external quantum efficiency and the max luminance of the deep blue OLEDs reaches 8.10% and 7973 cd m−2, and the CIE is (0.1298, 0.1712) with a low efficiency roll-off of only 2.9%.

Highly efficient and stable pure-red phosphorescent organic light-emitting diodes based on heptacyclic bipolar hosts featuring an armor-like structure

The armor-like heptacyclic framework enhances the rigidity and stability of host materials named Hept-TRZ. The luminescent devices based on Hept-TRZ as the red host achieve high efficiency and long lifetime of PhOLEDs.

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 red and green phosphorescent OLEDs based on benzonitrile and carbazole as bipolar host materials

In this work, two novel bipolar host materials 3''-(9H-carbazol-9-yl)-5'-(2,6-diphenylpyridin-4-yl)-[1,1':3′,1″-terphenyl]-4-carbonitrile (p-PyCNCz) and 3''-(9H-carbazol-9-yl)-5'-(2,6-diphenylpyridin-4-yl)-[1,1':3′,1″-terphenyl]-3-carbonitrile (m-PyCNCz) were designed and synthesized by incorporating diphenylpyridine, benzonitrile and phenylcarbazole. Both materials exhibit excellent thermal stability with their glass transition temperatures (Tg) of exceeding 120 °C and thermal decomposition temperatures (Td) of reaching approximately 490 °C. The p-PyCNCz and m-PyCNCz not only possess a high ET of 2.75 eV and 2.84 eV, but their HOMO/LUMO are completely separated. Moreover, red and green phosphorescent OLEDs (PhOLEDs) hosted by the two materials were prepared. The m-PyCNCz exhibits better device performance, with the Ƞc,max, Ƞp,max and EQEmax of achieving 88.32 cd/A, 98.19 lm/W and 24.3 % for the green PhOLED, and 22.19 cd/A, 24.38 lm/W and 25.3 % for the red PhOLED, respectively.

Bipolar Deep-Blue Host for High-Performance Multicolor OLEDs by Peripheral Auxiliary Group Strategy

Bipolar Deep-Blue Host for High-Performance Multicolor OLEDs by Peripheral Auxiliary Group Strategy

Highly reduced efficiency roll-off with 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-containing unit on bipolar host for improved OLEDs device lifespan

The electrical, thermal, and optical properties of the host material used in the light-emitting layer of a thermally activated delayed fluorescence organic light-emitting diode (TADF-OLED) directly affect its efficiency, lifetime, and color. In this study, a new host material was designed, synthesized, and then applied to a green TADF-OLED device to evaluate its performance as a host. The host material was designed in a bipolar configuration, ensuring the balanced introduction and transport of electrons and holes. This balance was achieved by incorporating a carbazole unit to facilitate hole injection and transport. Moreover, a 5,9-dioxa-13b-boranaphtho [3,2,1-de]anthracene (DOBNA) unit containing a boron atom was introduced for electron injection and transport. These units were linked to ortho terphenyl spacers. The DOBNA unit improved the electron transport properties, the thermal stability of molecule, and anionic bond dissociation energy. Consequently, the device evaluated using the DOBNA-introduced material as a host for 4CzIPN, a TADF dopant, enhanced the performance by ∼ four times from LT80@1000 nit to 15.1 h compared to the unipolar host material comprising carbazole and ortho-terphenyl.