A Method for Reducing the Singlet–Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency

We have successfully synthesized a series of blue thermally activated delayed fluorescence emitters, BPy-<i>p</i>C, BPy-<i>p</i>TC, BPy-<i>p</i>2C, and BPy-<i>p</i>3C, bearing a 4-benzoylpyridine core as the electron-accepting unit and carbazolyl, <i>tert</i>-butylcarbazolyl, dicarbazolyl, and tercarbazolyl groups as the electron-donating units, respectively. The density functional theory calculation shows that all of the compounds have their lowest unoccupied molecular orbitals on the benzoylpyridine moiety. However, the highest occupied molecular orbital (HOMO) of BPy-<i>p</i>3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-<i>p</i>C and BPy-<i>p</i>TC are mainly on the carbazolyl and extended to the phenyl ring. As a result, Δ<i>E</i>_ST is reduced from 0.29 eV for BPy-<i>p</i>C to 0.05 eV for BPy-<i>p</i>3C, and the organic light-emitting diodes using these materials as dopants emit blue light and their maximum external quantum efficiencies (EQEs) increase from 4.2% to 23.9% for BPy-<i>p</i>C and BPy-<i>p</i>3C, respectively. The EQE of the BPy-<i>p</i>3C-based device increases 2 times more than that of the BPy-<i>p</i>TC-based device without a significant change in the color coordinates

Vacuum-Deposited Organometallic Halide Perovskite Light-Emitting Devices

In this work, a sequential vacuum deposition process of bright, highly crystalline, and smooth methylammonium lead bromide and phenethylammonium lead bromide perovskite thin films are investigated and the first vacuum-deposited organometallic halide perovskite light-emitting devices (PeLEDs) are demonstrated. Exceptionally low refractive indices and extinction coefficients in the emission wavelength range are obtained for these films, which contributed to a high light out-coupling efficiency of the PeLEDs. By utilizing these perovskite thin films as emission layers, the vacuum-deposited PeLEDs exhibit a very narrow saturated green electroluminescence at 531 nm, with a spectral full width at half-maximum bandwidth of 18.6 nm, a promising brightness of up to 6200 cd/m², a current efficiency of 1.3 cd/A, and an external quantum efficiency of 0.36%

High-Speed Visible Light Communication Using Phenothiazine/Dimesitylborane Derivatives as Color Conversion Materials in Semipolar Micro-LED-Based White-Light Systems

Visible light communication (VLC) has emerged as a cutting-edge high-speed communication technology, poised to meet the surging capacity demands of 6G networks. Micro-light-emitting diodes (μLEDs) are considered as the light sources for achieving high-speed VLC, distinguished by their remarkable modulation bandwidths. However, achieving broadband white light emission hinges on the utilization of color-conversion materials with wide emission spectra. The transmission speed of the white-light system is inherently constrained by the characteristics of these color-conversion materials. In this work, we demonstrate CC-MP7 and CC-MP8, two derivatives of phenothiazine/dimesitylborane, as color conversion materials in a semipolar (20–21) micro-LED-based white-light system for high-speed VLC. The color conversion layers possess wide emission spectra, enabling them to achieve excellent color rendering performance when combined with blue micro-LEDs. CC-MP7 and CC-MP8 demonstrate rapid photoluminescence decay characteristics, thereby enhancing the modulation bandwidth of the color-conversion layer in the white-light system. The resulting bandwidths achieved by CC-MP7 and CC-MP8 are 210 and 240 MHz, respectively, which represents an approximately 45-fold increase compared to ordinary phosphors. By combining semipolar (20–21) micro-LEDs with CC-MP7 and CC-MP8, the resulting white-light systems exhibit correlated color temperatures of 6860 and 7500 K, CIE coordinates of (0.3009, 0.3577) and (0.2958, 0.3129), and color-rendering indexes of 80 and 85, respectively. Furthermore, both systems offer high bandwidths of 1063 and 1084 MHz with the data rates of 1.72 Gbps and 1.74 Gbps using non-return-to-zero on–off keying (NRZ-OOK) format, respectively, indicating the significant potential of CC-MP7 and CC-MP8 for practical applications in VLC

Bifacial Perovskite Solar Cells Featuring Semitransparent Electrodes

Inorganic–organic hybrid perovskite solar cells (PSCs) are promising devices for providing future clean energy because of their low cost, ease of fabrication, and high efficiencies, similar to those of silicon solar cells. These materials have been investigated for their potential use in bifacial PSCs, which can absorb light from both sides of the electrodes. Here, we fabricated bifacial PSCs featuring transparent BCP/Ag/MoO₃ rear electrodes, which we formed through low-temperature processing using thermal evaporation methods. We employed a comprehensive optical distribution program to calculate the distributions of the optical field intensities with constant thicknesses of the absorbing layer in the top electrode configuration. The best PSC having a transparent BCP/Ag/MoO₃ electrode achieved PCEs of 13.49% and 9.61% when illuminated from the sides of the indium tin oxide and BCP/Ag/MoO₃ electrodes, respectively. We observed significant power enhancement when operating this PSC using mirror reflectors and bifacial light illumination from both sides of the electrodes

Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics

Two D-π–A-A molecules (<b>MIDTP</b> and <b>TIDTP</b>) composed of an electron-rich ditolylamino group (D) and an electron-deficient 5-dicyanovinylenylpyrimidine (A-A) fragment bridged together with indeno­[1,2-<i>b</i>]­thiophene (IDT) were synthesized. These molecules provide an opportunity to examine in-depth the impact of side-chain variations (methyl vs <i>p</i>-tolyl) on the crystallization behaviors, solid-state morphology, physical properties, and optoelectronic characteristics relevant for practical applications. X-ray analyses on single-crystal structures indicate that methyl-substituted <b>MIDTP</b> forms “coplanar antiparallel dimers” via C–H···S interactions and organizes into an ordered slip-staircase arrays. In contrast, <i>p</i>-tolyl-bearing <b>TIDTP</b> shows “cofacial centrosymmetric dimers” via π–π interactions and packs into a less-ordered layered structures. The X-ray diffraction analyses upon thermal treatment are consistent with a superior crystallinity of <b>MIDTP</b>, as compared to that of <b>TIDTP</b>. This difference indicates a greater propensity to organization by introduction of the smaller methyl group versus the bulkier <i>p</i>-tolyl group. The increased propensity for order by <b>MIDTP</b> facilitates the crystallization of <b>MIDTP</b> in both solution-processed and vacuum-deposited thin films. <b>MIDTP</b> forms solution-processed single-crystal arrays that deliver OFET hole mobility of 6.56 × 10^–4 cm² V^–1 s^–1, whereas <b>TIDTP</b> only forms amorhpous films that gave lower hole mobility of 1.34 × 10^–5 cm² V^–1 s^–1. <b>MIDTP</b> and <b>TIDTP</b> were utilized to serve as donors together with C₇₀ as acceptor in the fabrication of small-molecule organic solar cells (SMOSCs) with planar heterojunction (PHJ) or planar-mixed heterojunction (PMHJ) device architectures. OPV devices based on higher crystalline <b>MIDTP</b> delivered power conversion efficiencies (PCEs) of 2.5% and 4.3% for PHJ and PMHJ device, respectively, which are higher than those of <b>TIDTP</b>-based cells. The improved PCEs of <b>MIDTP</b>-based devices are attributed to better hole-transport character

Top Illuminated Hysteresis-Free Perovskite Solar Cells Incorporating Microcavity Structures on Metal Electrodes: A Combined Experimental and Theoretical Approach

Further technological development of perovskite solar cells (PSCs) will require improvements in power conversion efficiency and stability, while maintaining low material costs and simple fabrication. In this Research Article, we describe top-illuminated ITO-free, stable PSCs featuring microcavity structures, wherein metal layers on both sides on the active layers exerted light interference effects in the active layer, potentially increasing the light path length inside the active layer. The optical constants (refractive index and extinction coefficient) of each layer in the PSC devices were measured, while the optical field intensity distribution was simulated using the transfer matrix method. The photocurrent densities of perovskite layers of various thicknesses were also simulated; these results mimic our experimental values exceptionally well. To modify the cavity electrode surface, we deposited a few nanometers of ultrathin MoO₃ (2, 4, and 6 nm) in between the Ag and poly­(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) layers provide hydrophobicity to the Ag surface and elevate the work function of Ag to match that of the hole transport layer. We achieved a power conversion efficiency (PCE) of 13.54% without hysteresis in the device containing a 4 nm-thick layer of MoO₃. In addition, we fabricated these devices on various cavity electrodes (Al, Ag, Au, Cu); those prepared using Cu and Au anodes displayed improved device stability of up to 72 days. Furthermore, we prepared flexible PSCs having a PCE of 12.81% after incorporating the microcavity structures onto poly­(ethylene terephthalate) as the substrate. These flexible solar cells displayed excellent stability against bending deformation, maintaining greater than 94% stability after 1000 bending cycles and greater than 85% after 2500 bending cycles performed with a bending radius of 5 mm

Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Three new tailor-made molecules (<b>DPDCTB</b>, <b>DPDCPB</b>, and <b>DTDCPB</b>) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor–acceptor–acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of <b>DTDCPB</b> and the previously reported <b>DTDCTB</b> were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure–property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing <b>DTDCPB</b> as the donor and C₇₀ as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (<i>V</i>_oc) of 0.93 ± 0.02 V, a short-circuit current density (<i>J</i>_sc) of 13.48 ± 0.27 mA/cm², and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm²) AM 1.5G simulated solar illumination

Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Three new tailor-made molecules (<b>DPDCTB</b>, <b>DPDCPB</b>, and <b>DTDCPB</b>) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor–acceptor–acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of <b>DTDCPB</b> and the previously reported <b>DTDCTB</b> were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure–property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing <b>DTDCPB</b> as the donor and C₇₀ as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (<i>V</i>_oc) of 0.93 ± 0.02 V, a short-circuit current density (<i>J</i>_sc) of 13.48 ± 0.27 mA/cm², and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm²) AM 1.5G simulated solar illumination

A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(<i>t</i>-butyl)­carbazolyl groups, respectively, at the <i>meta</i> and <i>ortho</i> carbons of the benzoyl ring, were synthesized. These molecules show very small Δ<i>E</i>_ST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the <i>ortho</i> donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small Δ<i>E</i>_ST with high photoluminescence quantum yield

A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(<i>t</i>-butyl)­carbazolyl groups, respectively, at the <i>meta</i> and <i>ortho</i> carbons of the benzoyl ring, were synthesized. These molecules show very small Δ<i>E</i>_ST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the <i>ortho</i> donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small Δ<i>E</i>_ST with high photoluminescence quantum yield