Xinyi Cai

Fluorescent Organic Planar pn Heterojunction Light‐Emitting Diodes with Simplified Structure, Extremely Low Driving Voltage, and High Efficiency

Fluorescent organic light-emitting diodes capable of radiative utilization of both singlet and triplet excitons are achieved via a simple double-layer planar pn hetero-junction configuration without a conventional emission layer, leading to high external quantum efficiency above 10% and extremely low driving voltages close to the theoretical minima.

High-Efficiency WOLEDs with High Color-Rendering Index based on a Chromaticity-Adjustable Yellow Thermally Activated Delayed Fluorescence Emitter.

A chromaticity-adjustable yellow thermally activated delayed fluorescence (TADF) material, PXZDSO2 as a triplet harvester provides a rational device concept, giving two-color and three-color pure organic white organic light-emitting diodes (WOLEDs) with unprecedented color-rendering index of 95 and external quantum efficiency of 19.2%.

Star-shaped isoindigo-based small molecules as potential non-fullerene acceptors in bulk heterojunction solar cells

Two novel, star-shaped isoindigo(II)-based small molecules with different cores of triphenylamine and benzene, namely P1 and P2, respectively, were designed and synthesized as non-fullerene acceptor materials in organic solar cells (OSCs). The impacts of the different cores combined with the II terminal groups on the optical absorption, electrochemical properties, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 400–650 nm and relatively high LUMO energy levels of 3.73 and 3.79 eV for P1 and P2, respectively. The power conversion efficiency (PCE) of the OSCs based on P2 as the acceptor material and P3HT as the donor material (1 : 1, w/w) is 0.19%. In contrast, a PCE of 0.81% was achieved for the device based on P1 as the acceptor and P3HT as the donor (1 : 1, w/w).

Deep blue fluorophores incorporating sulfone-locked triphenylamine: the key for highly efficient fluorescence–phosphorescence hybrid white OLEDs with simplified structure

Two novel bipolar isomeric blue fluorophores, PPI-TPA-SO2-1 and PPI-TPA-SO2-2, consisting of electron-withdrawing phenanthro[9,10-d]imidazole and sulfone-locked electron-donating triphenylamine, were designed and synthesized. The sulfone lock induces a more twisted molecular conformation, and thus a higher triplet energy level and better triplet exciton confining ability compared with the analogue TPA-PPI without the sulfone lock. In addition, the introduced sulfone lock also offers the developed materials improved electron affinities and an electron dominant transporting ability. They were utilized as the blue emitter and the host for a yellow phosphorescent emitter to fabricate fluorescence–phosphorescence (F–P) hybrid white organic light-emitting diodes (WOLEDs) in a single-emissive-layer architecture, giving forward-viewing maximum current efficiencies of 44.2 and 47.6 cd A−1, power efficiencies of 49.5 and 53.4 lm W−1, and external quantum efficiencies of 14.4% and 15.6%, respectively, which are much higher than those of the devices based on TPA-PPI (29.5 cd A−1, 33.1 lm W−1, and 9.6%) due to their superior singlet and triplet exciton separation and utilization ability over TPA-PPI. These efficiencies are also the highest values ever reported for the F–P hybrid WOLEDs in a similar architecture, and their power efficiencies are even comparable with most reported highly efficient all phosphorescent WOLEDs without using any out-coupling technology.

9,9-Diphenyl-thioxanthene derivatives as host materials for highly efficient blue phosphorescent organic light-emitting diodes

A series of 9,9-diphenyl-9H-thioxanthene derivatives with different valence states of sulfur atoms are reported as host materials in blue phosphorescent organic light-emitting diodes. Their photophysical, electrochemical and thermal properties, as well as device performance were thoroughly investigated to study their structure–property relationships, including the different carbazolyl linkage positions and valence states of sulfur atoms. Extremely low turn-on voltages of around 2.6 V for blue electrophosphorescence, which are already corresponding to the value of the emitted photon energy (hv)/electron charge (e), were achieved by utilizing the developed materials as hosts of the blue phosphor dopant iridium(III)bis(4,6-(difluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic). Notably, a maximal power efficiency of 69.7 lm W−1 and an external quantum efficiency of 29.0% were achieved for an optimal device based on m-DCz-S consisting of the bivalent sulfur atom and meta-combined carbazolyl.

Pyrene terminal functionalized perylene diimide as non-fullerene acceptors for bulk heterojunction solar cells

Two perylene diimide (PDI) based small molecules with different terminal groups of pyrene and tert-butyl pyrene, namely P1 and P2, respectively, were designed and synthesized as the acceptor materials in organic solar cells (OSCs). The impacts of the different terminal groups combined with the PDI core on the optical absorption and fluorescence, electrochemical properties, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 400–650 nm and a relatively high LUMO energy level of 3.77 eV. Power conversion efficiency (PCE) of the OSCs based on P2 as the acceptor material and PTB7 as the donor material (1:1, w/w) is 0.41%. In contrast, a PCE of 1.35% was achieved for the device based on P1 as the acceptor and PTB7 as the donor (1:1, w/w).

Structure-simplified and highly efficient deep blue organic light-emitting diodes with reduced efficiency roll-off at extremely high luminance

Based on a series of new fluorescent emitters, deep blue non-doped multilayer OLEDs with EQEs exceeding 5.10% and single layer devices excluding any charge carrier transporting materials with an EQE of 4.22% were obtained at an extremely high luminance of 10 000 cd m-2.

Synthesis and photovoltaic properties of A–D–A type non-fullerene acceptors containing isoindigo terminal units

Four solution-processable acceptor–donor–acceptor (A–D–A) structured organic molecules with isoindigo as terminal acceptor units and different aromatic rigid planar cores such as indacenodithiophene (IDT), dithienosilole (DTS), anthracene, and pyrene as donor units, namely P1, P2, P3, and P4, respectively, were designed and synthesized as the acceptor materials in organic solar cells (OSCs). The four compounds possess a broad absorption covering the wavelength range of 400–700 nm and a rather high-lying LUMO energy level, which is beneficial for achieving a high open circuit voltage (Voc). P1 and P2 with IDT and DTS central cores demonstrate stronger absorbance than the compounds P3 and P4 with anthracene and pyrene cores. A power conversion efficiency (PCE) of 1.39% was achieved for the OSC based on P1 as the acceptor material and P3HT as the donor material (1 : 1, w/w), which is the highest value for polymer bulk-heterojunction OSCs based on isoindigo small molecules as non-fullerene acceptors. The current results have demonstrated that isoindigo could be a useful building block for the synthesis of promising acceptor materials for OSCs.

Highly efficient thermally activated delayed fluorescence materials with reduced efficiency roll-off and low on-set voltages

Two carbazoles or diphenylamines were combined with carbazole to obtain two branch-shaped donor units and then connected to a benzophenone unit, acquiring two donor–acceptor (D–A) type thermally activated delayed fluorescence (TADF) materials named CCDC and CCDD. CCDC and CCDD demonstrate opposing phenomena in a THF/H2O mixed solvent despite their structural similarity. The photoluminescence (PL) of CCDC drops drastically as the water proportion goes up, while the PL of CCDD increases with an increasing water ratio. The lower PL emission of CCDD in good solvents may be attributed to the vibration and rotation of the phenyl rings in the diphenylamine donors, and the PL emission increases when the intramolecular movement is restricted. However, PL quantum yield (PLQY) tests and device performances indicate that both materials experience aggregation-caused quenching (ACQ) to some extent in a solid state film. This is because when intramolecular vibration and rotation is restrained in the solid state, intermolecular stacking still hampers the fluorescence emission of CCDD. Also, the materials perform well in doped devices. A maximal external quantum efficiency (EQE) of 15.9% is achieved in blue-emitting devices with CCDC as the emitter. Green-emitting devices where CCDD is utilized as the emitter perform well regarding their overall properties. Not only do their maximal EQEs exceed 22%, but they also achieve on-set voltages as low as 2.6 V with a 30% doping concentration. Moreover, the efficiency roll-off of the CCDD-based devices is well controlled. An EQE of nearly 20% is maintained at a luminance of 1000 cd m−2.

An ideal universal host for highly efficient full-color, white phosphorescent and TADF OLEDs with a simple and unified structure

The actualization of highly efficient full-color and white organic light-emitting diodes (OLEDs) in a simple and unified device architecture using host materials that are suitable for red, green, and blue dopants has been quite a challenge. Herein, a series of thioxanthene and 9-phenylcarbazole hybrids were designed and synthesized, among which 9,9′-((9H-thioxanthene-9,9-diyl)bis(3,1-phenylene))bis(9H-carbazole) (m-DCz-S) exhibited unique universal host characteristics. Full-color OLEDs were fabricated with phosphorescent and thermally activated delayed fluorescent (TADF) emitters, i.e. FIr6, Ir(ppy)3, PO-01, 2CzPN, ACRDSO2, and PyCN-ACR; among these, the FIr6-based blue device showed the best performance reported to date. In addition, the phosphorescent white OLED with complementary emitters, FIr6: PO-01, has high efficiencies of 61.4 cd A−1, 62.9 lm W−1, and 25.0% and extremely good color stability with a color variation less than (0.0007, 0.0007) in the whole range of luminance. Further improved efficiencies of 74.3 cd A−1, 82.7 lm W−1, and 26.4% have been achieved in the case of the phosphorescent white OLED based on FIrpic: PO-01, which is the most efficient white OLED with this simplified structure reported to date. It is worth noting that all the devices have been fabricated with the same extremely simple architecture by solely changing the emitters doped into the host; thus, the unique universal host material m-DCz-S is advantageous for future low-cost and highly efficient OLED application.