Bicarbazole-Benzophenone Based Twisted Donor-Acceptor Derivatives as Potential Blue TADF Emitters for OLEDs.

Over the past few decades, organic light-emitting diodes (OLEDs) find applications in smartphones, televisions, and the automotive sector. However, this technology is still not perfect, and its application for lighting purposes has been slow. For further development of the OLEDs, we designed twisted donor-acceptor-type electroactive bipolar derivatives using benzophenone and bicarbazole as building blocks. Derivatives were synthesized through the reaction of 4-fluorobenzophenone with various mono-alkylated 3,3'-bicarbazoles. We have provided a comprehensive structural characterization of these compounds. The new materials are amorphous and exhibit suitable glass transition temperatures ranging from 57 to 102 °C. They also demonstrate high thermal stability, with decomposition temperatures reaching 400 °C. The developed compounds exhibit elevated photoluminescence quantum yields (PLQY) of up to 75.5% and favourable HOMO-LUMO levels, along with suitable triplet-singlet state energy values. Due to their good solubility and suitable film-forming properties, all the compounds were evaluated as blue TADF emitters dispersed in commercial 4,4'-bis(N-carbazolyl)-1,10-biphenyl (CBP) host material and used for the formation of emissive layer of organic light-emitting diodes (OLEDs) in concentration-dependent experiments. Out of these experiments, the OLED with 15 wt% of the emitting derivative 4-(9'-{2-ethylhexyl}-[3,3']-bicarbazol-9-yl)benzophenone exhibited superior performance. It attained a maximum brightness of 3581 cd/m2, a current efficacy of 5.7 cd/A, a power efficacy of 4.1 lm/W, and an external quantum efficacy of 2.7%.

A Review of Benzophenone-Based Derivatives for Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) have garnered considerable attention in academic and industrial circles due to their potential applications in flat-panel displays and solid-state lighting technologies, leveraging the advantages offered by organic electroactive derivatives over their inorganic counterparts. The thin and flexible design of OLEDs enables the development of innovative lighting solutions, facilitating the creation of customizable and contoured lighting panels. Among the diverse electroactive components employed in the molecular design of OLED materials, the benzophenone core has attracted much attention as a fragment for the synthesis of organic semiconductors. On the other hand, benzophenone also functions as a classical phosphor with high intersystem crossing efficiency. This characteristic makes it a compelling candidate for effective reverse intersystem crossing, with potential in leading to the development of thermally activated delayed fluorescent (TADF) emitters. These emitting materials witnessed a pronounced interest in recent years due to their incorporation in metal-free electroactive frameworks and the capability to convert triplet excitons into emissive singlet excitons through reverse intersystem crossing (RISC), consequently achieving exceptionally high external quantum efficiencies (EQEs). This review article comprehensively overviews the synthetic pathways, thermal characteristics, electrochemical behaviour, and photophysical properties of derivatives based on benzophenone. Furthermore, we explore their applications in OLED devices, both as host materials and emitters, shedding light on the promising opportunities that benzophenone-based compounds present in advancing OLED technology.

Bicarbazole-Benzophenone-Based Twisted Donor-Acceptor-Donor Derivatives as Blue Emitters for Highly Efficient Fluorescent Organic Light-Emitting Diodes.

This paper delves into the development of a group of twisted donor-acceptor-donor (D-A-D) derivatives incorporating bicarbazole as electron donor and benzophenone as electron acceptor for potential use as blue emitters in OLEDs. The derivatives were synthesized in a reaction of 4,4'-difluorobenzophenone with various 9-alkyl-9'H-3,3'-bicarbazoles. The materials, namely, DB14, DB23, and DB29, were designed with different alkyl side chains to enhance their solubility and film-forming properties of layers formed using the spin-coating from solution method. The new materials demonstrate high thermal stabilities with decomposition temperatures >383 °C, glass transition temperatures in the range of 95-145 °C, high blue photoluminescence quantum yields (>52%), and short decay times, which range in nanoseconds. Due to their characteristics, the derivatives were used as blue emitters in OLED devices. Some of the OLEDs incorporating the DB23 emitter demonstrated a high external quantum efficiency (EQEmax) of 5.3%, which is very similar to the theoretical limit of the first-generation devices.

Ferrocene Derivatives for Improving the Efficiency and Stability of MA-Free Perovskite Solar Cells from the Perspective of Inhibiting Ion Migration and Releasing Film Stress.

Further improvement of the performance and stability of inverted perovskite solar cells (PSCs) is necessary for commercialization. Here, ferrocene derivative dibenzoylferrocene (DBzFe) is used as an additive to enhance the performance and stability of MA- and Br- free PSCs. The results show that the introduction of DBzFe not only passivates the defects in the film but also inhibits the ion migration in the film. The final device achieves a power conversion efficiency (PCE) of 23.53%, which is one of the highest efficiencies currently based on self-assembled monolayers (SAMs). Moreover, it maintains more than 96.4% of the original efficiency when running continuously for 400 h at the maximum power point.

Status and Challenges of Blue OLEDs: A Review.

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

Naphtalimide-Based Bipolar Derivatives Enabling High-Efficiency OLEDs.

Organic light-emitting diodes (OLEDs) have revolutionized the world of technology, making significant contributions to enhancing our everyday lives. With their exceptional display and lighting capabilities, OLEDs have become indispensable in various industries such as smartphones, tablets, televisions, and automotives. They have emerged as a dominant technology, inspiring continuous advancements, and improvements. Taking inspiration from the remarkable advancements in OLED advancements, we have successfully developed naphtalimide-based compounds, namely RB-08, RB-09, RB-10, and RB-11. These compounds exhibit desirable characteristics such as a wide bandgap, high decomposition temperatures (306-366 °C), and very high glass transition temperatures (133-179 °C). Leveraging these exceptional properties, we have harnessed these compounds as green emitters in the aforementioned devices. Among the various fabricated OLEDs, the one incorporating the RB-11 emitter has exhibited superior performance. This specific configuration achieved maximum power efficacy of 7.7 lm/W, current efficacy of 7.9 cd/A, and external quantum efficiency of 3.3%. These results highlight the outstanding capabilities of our synthesized emitter and its potential for further advancements in the field.

Efficiency Enhancement of Wide Bandgap Lead Perovskite Solar Cells with PTAA Surface-Passivated with Monomolecular Layer from the Viewpoint of PTAA Band Bending.

This report is on the efficiency enhancement of wide bandgap lead halide perovskite solar cells (WBG Pb-PVK PSCs) consisting of FA0.8Cs0.2PbI1.8Br1.2 as the light-harvesting layer. WGB Pb-PVK PSCs have attracted attention as the top layer of all perovskite-tandem solar cells. Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), a conductive polymer, is always used as the hole transporting layer (HTL) for Pb-PVK PSCs. Nevertheless, the hydrophobic surface of the PTAA sometimes destroys the growth of the FA0.8Cs0.2PbI1.8Br1.2 film. On the other hand, the Fermi level of PTAA is not well matched with that of perovskite film. Thus, the PCE of the WBG Pb-based PSCs with PTAA as the HTL was not very high. In this report, the efficiency of the FA0.8Cs0.2PbI1.8Br1.2 is improved by passivating the surface of the PTAA with a monomolecular layer, where the surface becomes hydrophilic, and the band bending of the PTAA layer is improved to cause swift hole collection. Finally, WBG Pb-PVK PSCs (1.77 eV) with 16.52% efficiency are reported.

Effect of substituent structure in fluorene based compounds: Experimental and theoretical study.

Fluorene-based low molar weight derivatives were synthesized in Suzuki reactions by using key starting materials 9-benzylidene-2,7-dibromofluorene or 3-(2,7-dibromofluoren-9-ylmethylen)-9-ethylcarbazole and various aryl boronic acids. Photophysical properties of the compounds were investigated in different solutions as well as in solid state. The thermal investigations showed that the obtained compounds are highly thermally stable with temperatures of 5% mass loss (T5%) in the range of 311-432 °C. Some of the compounds also exhibited very high glass transition temperatures exceeding 125 °C. The presented molecules were electrochemically active and showed the energy band gap below 2.97 eV. The investigations were supported by DFT calculations and the photovoltaic ability of the presented compounds was tested in the organic-inorganic solar cells.

Bifunctional Bicarbazole-Benzophenone-Based Twisted Donor-Acceptor-Donor Derivatives for Deep-Blue and Green OLEDs.

Organic light-emitting diodes (OLEDs) have played a vital role in showing tremendous technological advancements for a better lifestyle, due to their display and lighting technologies in smartphones, tablets, television, and automotive industries. Undoubtedly, OLED is a mainstream technology and, inspired by its advancements, we have designed and synthesized the bicarbazole-benzophenone-based twisted donor-acceptor-donor (D-A-D) derivatives, namely DB13, DB24, DB34, and DB43, as bi-functional materials. These materials possess high decomposition temperatures (>360 °C) and glass transition temperatures (~125 °C), a high photoluminescence quantum yield (>60%), wide bandgap (>3.2 eV), and short decay time. Owing to their properties, the materials were utilized as blue emitters as well as host materials for deep-blue and green OLEDs, respectively. In terms of the blue OLEDs, the emitter DB13-based device outperformed others by showing a maximum EQE of 4.0%, which is close to the theoretical limit of fluorescent materials for a deep-blue emission (CIEy = 0.09). The same material also displayed a maximum power efficacy of 45 lm/W as a host material doped with a phosphorescent emitter Ir(ppy)3. Furthermore, the materials were also utilized as hosts with a TADF green emitter (4CzIPN) and the device based on DB34 displayed a maximum EQE of 11%, which may be attributed to the high quantum yield (69%) of the host DB34. Therefore, the bi-functional materials that are easily synthesized, economical, and possess excellent characteristics are expected to be useful in various cost-effective and high-performance OLED applications, especially in displays.

Di(arylcarbazole) Substituted Oxetanes as Efficient Hole Transporting Materials with High Thermal and Morphological Stability for OLEDs.

A group of di(arylcarbazole)-substituted oxetanes has been prepared in Suzuki reactions by using the key starting material 3,3-di[3-iodocarbazol-9-yl]methyloxetane and various boronic acids (fluorophenylboronic acid, phenylboronic acid or naphthalene-1-boronic acid). Full characterization of their structure has been presented. The low molar mass compounds represent materials having high thermal stability with 5% mass loss thermal degradation temperatures in the range of 371-391 °C. Glass transition temperatures of the materials are also very high and range from 107 °C to 142 °C, which is a big advantage for formation of stable amorphous layers for optoelectronic devices, i.e., organic light emitting diodes. Hole transporting properties of the prepared materials were confirmed in formed organic light emitting diodes with tris(quinolin-8-olato)aluminium (Alq3) as a green emitter, which also served as an electron transporting layer. In the device's materials, 3,3-di[3-phenylcarbazol-9-yl]methyloxetane (5) and 3,3-di[3-(1-naphthyl)carbazol-9-yl]methyloxetane (6) demonstrated superior hole transporting properties than that of material 3,3-di[3-(4-flourophenyl)carbazol-9-yl]methyloxetane (4) based device. When material 5 was used in the device structure, the OLED demonstrated rather low turn-on voltage of 3.7 V, luminous efficiency of 4.2 cd/A, power efficiency of 2.6 lm/W and maximal brightness exceeding 11670 cd/m2. HTL of 6 based device also showed exclusive OLED characteristics. The device was characterized by turn-on voltage of 3.4 V, maximum brightness of 13193 cd/m2, luminous efficiency of 3.8 cd/A and power efficiency of 2.6 lm/W. An additional hole injecting-transporting layer (HI-TL) of PEDOT considerably improved functions of the device with HTL of compound 4. The modified OLED with a layer of the derivative 4 demonstrated exclusive characteristics with turn-on voltage of 3.9 V, high luminous efficiency of 4.7 cd/A, power efficiency of 2.6 lm/W and maximal brightness exceeding 21,000 cd/m2. These observations confirmed that the prepared materials have a big potential in the field of optoelectronics.

Highly Efficient Candlelight Organic Light-Emitting Diode with a Very Low Color Temperature.

Low color temperature candlelight organic light-emitting diodes (LEDs) are human and environmentally friendly because of the absence of blue emission that might suppress at night the secretion of melatonin and damage retina upon long exposure. Herein, we demonstrated a lighting device incorporating a phenoxazine-based host material, 3,3-bis(phenoxazin-10-ylmethyl)oxetane (BPMO), with the use of orange-red and yellow phosphorescent dyes to mimic candlelight. The resultant BPMO-based simple structured candlelight organic LED device permitted a maximum exposure limit of 57,700 s, much longer than did a candle (2750 s) or an incandescent bulb (1100 s) at 100 lx. The resulting device showed a color temperature of 1690 K, which is significantly much lower than that of oil lamps (1800 K), candles (1900 K), or incandescent bulbs (2500 K). The device showed a melatonin suppression sensitivity of 1.33%, upon exposure for 1.5 h at night, which is 66% and 88% less than the candle and incandescent bulb, respectively. Its maximum power efficacy is 23.1 lm/W, current efficacy 22.4 cd/A, and external quantum efficiency 10.2%, all much higher than the CBP-based devices. These results encourage a scalable synthesis of novel host materials to design and manufacture high-efficiency candlelight organic LEDs.

2,7(3,6)-Diaryl(arylamino)-substituted Carbazoles as Components of OLEDs: A Review of the Last Decade.

Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.

Synthesis and Thermal, Photophysical, Electrochemical Properties of 3,3-di[3-Arylcarbazol-9-ylmethyl]oxetane Derivatives.

Novel oxetane-functionalized derivatives were synthesized to find the impact of carbazole substituents, such as 1-naphtyl, 9-ethylcarbazole and 4-(diphenylamino)phenyl, on their thermal, photophysical and electrochemical properties. Additionally, to obtain the optimized ground-state geometry and distribution of the frontier molecular orbital energy levels, density functional theory (DFT) calculations were used. Thermal investigations showed that the obtained compounds are highly thermally stable up to 360 °C, as molecular glasses with glass transition temperatures in the range of 142-165 °C. UV-Vis and photoluminescence studies were performed in solvents of differing in polarity, in the solid state as a thin film on glass substrate, and in powders, and were supported by DFT calculations. They emitted radiation both in solution and in film with photoluminescence quantum yield from 4% to 87%. Cyclic voltammetry measurements revealed that the materials undergo an oxidation process. Next, the synthesized molecules were tested as hole transporting materials (HTM) in perovskite solar cells with the structure FTO/b-TiO2/m-TiO2/perovskite/HTM/Au, and photovoltaic parameters were compared with the reference device without the oxetane derivatives.

Pyridinyl-Carbazole Fragments Containing Host Materials for Efficient Green and Blue Phosphorescent OLEDs.

Pyridinyl-carbazole fragments containing low molar mass compounds as host derivatives H1 and H2 were synthesized, investigated, and used for the preparation of electro-phosphorescent organic light-emitting devices (PhOLEDs). The materials demonstrated high stability against thermal decomposition with the decomposition temperatures of 361-386 °C and were suitable for the preparation of thin amorphous and homogeneous layers with very high values of glass transition temperatures of 127-139 °C. It was determined that triplet energy values of the derivatives are, correspondingly, 2.82 eV for the derivative H1 and 2.81 eV for the host H2. The new derivatives were tested as hosts of emitting layers in blue, as well as in green phosphorescent OLEDs. The blue device with 15 wt.% of the iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) emitter doping ratio in host material H2 exhibited the best overall characteristics with a power efficiency of 24.9 lm/W, a current efficiency of 23.9 cd/A, and high value of 10.3% of external quantum efficiency at 100 cd/m2. The most efficient green PhOLED with 10 wt% of Ir(ppy)3 {tris(2-phenylpyridine)iridium(III)} in the H2 host showed a power efficiency of 34.1 lm/W, current efficiency of 33.9 cd/A, and a high value of 9.4% for external quantum efficiency at a high brightness of 1000 cd/m2, which is required for lighting applications. These characteristics were obtained in non-optimized PhOLEDs under an ordinary laboratory atmosphere and could be improved in the optimization process. The results demonstrate that some of the new host materials are very promising components for the development of efficient phosphorescent devices.

New Electroactive Polymers with Electronically Isolated 4,7-Diarylfluorene Chromophores as Positive Charge Transporting Layer Materials for OLEDs.

A group of polyethers containing electroactive pendent 4,7-diarylfluorene chromophores have been prepared by the multi-step synthetic route. Full characterization of their structures has been presented. The polymeric materials represent derivatives of high thermal stability with initial thermal degradation temperatures in a range of 392-397 °C. Glass transition temperatures of the amorphous polymers range from 28 °C to 63 °C and depend on structures of the 4,7-diarylfluorene chromophores. Electron photoemission spectra of thin layers of the electroactive derivatives showed ionization potentials in the range of 5.8-6.0 eV. Hole injecting/transporting properties of the prepared polymeric materials were confirmed during formation of organic light-emitting diodes with tris(quinolin-8-olato)aluminium (Alq3) as a green emitter, which also serves as an electron transporting layer. The device using hole-transporting polymer with electronically isolated 2,7-di(4-biphenyl)fluorene chromophores demonstrated the best overall performance with low turn on voltage of 3 V, high current efficiency exceeding 1.7 cd/A, and with maximum brightness over 200 cd/m2. The organic light-emitting diode (OLED) characteristics were measured in non-optimized test devices. The efficiencies could be further improved by an optimization of device structure, formation conditions, and encapsulation of the devices.

High light-quality OLEDs with a wet-processed single emissive layer.

High light-quality and low color temperature are crucial to justify a comfortable healthy illumination. Wet-process enables electronic devices cost-effective fabrication feasibility. We present herein low color temperature, blue-emission hazards free organic light emitting diodes (OLEDs) with very-high light-quality indices, that with a single emissive layer spin-coated with multiple blackbody-radiation complementary dyes, namely deep-red, yellow, green and sky-blue. Specifically, an OLED with a 1,854 K color temperature showed a color rendering index (CRI) of 90 and a spectrum resemblance index (SRI) of 88, whose melatonin suppression sensitivity is only 3% relative to a reference blue light of 480 nm. Its maximum retina permissible exposure limit is 3,454 seconds at 100 lx, 11, 10 and 6 times longer and safer than the counterparts of compact fluorescent lamp (5,920 K), light emitting diode (5,500 K) and OLED (5,000 K). By incorporating a co-host, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), the resulting OLED showed a current efficiency of 24.9 cd/A and an external quantum efficiency of 24.5% at 100 cd/m2. It exhibited ultra-high light quality with a CRI of 93 and an SRI of 92. These prove blue-hazard free, high quality and healthy OLED to be fabrication feasible via the easy-to-apply wet-processed single emissive layer with multiple emitters.

Incorporating a hole-transport material into the emissive layer of solid-state light-emitting electrochemical cells to improve device performance.

Solid-state light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) have several advantages such as high efficiency, low operation voltage and simple device structure. To improve the device efficiency of iTMC-based LECs for practical applications, improving the carrier balance to achieve a centered recombination zone would be an important issue. In this work, incorporating a hole-transport material (HTM) into the emissive layer of iTMC-based LECs is shown to improve device performance. When mixed with an HTM (12%), the LECs based on a Ru complex exhibit 1.9× and 1.5× enhancement in peak light output and peak external quantum efficiency (EQE) as compared to neat-film devices. Furthermore, over 2× enhancement in stabilized EQE can be achieved in LECs mixed with an HTM. It is attributed to that a more centered recombination zone in LECs mixed with an HTM is beneficial in reducing exciton quenching in the recombination zone approaching extended doped layers. Estimating the temporal evolution of the recombination zone in the LECs mixed with an HTM by employing the microcavity effect is demonstrated to confirm the physical origin for improved device performance. These results reveal that incorporating of an HTM in the emissive layer of LECs based on an iTMC is a feasible way to improve carrier balance and thus enhance light output and device efficiency.

Chemoselective assembly and immunological evaluation of multiepitopic glycoconjugates bearing clustered Tn antigen as synthetic anticancer vaccines.

In this paper we investigated the use of regioselectively addressable functionalized templates (RAFTs) as new scaffolds for the design of anticancer vaccine candidates. We report the synthesis of well-defined multiepitopic RAFT scaffolds and their immunological evaluation. These conjugates exhibit clustered Tn analogue as tumor-associated carbohydrate antigen (TACA, B-cell epitope) and the CD4+ helper T-cell peptide from the type 1 poliovirus. The saccharidic and peptidic epitopes were both synthesized separately and combined regioselectively to the RAFT core using a sequential oxime bond formation strategy. B- and T-antigenicity and immunogenicity of the vaccine candidates were investigated in vitro and in vivo. These studies clearly demonstrate that the saccharidic part of the conjugates is recognized by Tn-specific monoclonal antibodies. Moreover, the antibodies elicited by immunization of mice with our vaccine candidates recognize the native form of Tn epitope expressed on human tumor cells. Together with oxime ligation technique, these results suggest that the RAFT scaffold provides a promising and suitable tool for engineering potent synthetic anticancer vaccine.