Dongcheng Chen

Nitrogen heterocycle-containing materials for highly efficient phosphorescent OLEDs with low operating voltage

Since their discovery, phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for more than ten years. To improve the electricity-to-light conversion efficiency of PHOLEDs, it is of great importance to improve their external quantum efficiency and to decrease their operating voltage. The improvement of functional materials such as host materials and carrier transport materials, especially electron transport materials, plays the most important role in reaching these targets. In this feature article, we provide a brief review of the recent developments in nitrogen heterocycle-containing functional materials for PHOLEDs. Special attention is paid to the issue of how to utilise the molecular design strategy in achieving the development of high-performance host and electron transport materials and of how to use these nitrogen heterocycle-containing materials to create PHOLEDs with efficiencies and operating voltages that reach the theoretical limits.

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%.

Modulation of Exciton Generation in Organic Active Planar pn Heterojunction: Toward Low Driving Voltage and High-Efficiency OLEDs Employing Conventional and Thermally Activated Delayed Fluorescent Emitters.

Organic light-emitting diodes (OLEDs) combining low driving voltage and high efficiency are designed by employing conventional and thermally activated delayed fluorescence emitters through modulation of excitons generated at the planar p-n heterojunction region. To date, this approach enables the highest power efficiency for yellow-green emitting fluorescent OLEDs with a simplified structure.

High-Performance Color-Tunable Perovskite Light Emitting Devices through Structural Modulation from Bulk to Layered Film

Adding 2-phenoxyethylamine (POEA) into a CH3 NH3 PbBr3 precursor solution can modulate the organic-inorganic hybrid perovskite structure from bulk to layered, with a photoluminescence and electroluminescence shift from green to blue. Meanwhile, POEA can passivate the CH3 NH3 PbBr3 surface and help to obtain a pure CH3 NH3 PbBr3 phase, leading to an improvement of the external quantum efficiency to nearly 3% in CH3 NH3 PbBr3 LED.

Three pyrido[2,3,4,5-lmn]phenanthridine derivatives and their large band gap copolymers for organic solar cells

Three novel pyrido[2,3,4,5-lmn]phenanthridine derivatives of 4,9-dialkylpyrido[2,3,4,5-lmn]phenanthridine-5,10-dione, 10-alkoxy-4-alkylpyrido[2,3,4,5-lmn]phenanthridin-5-one, and 5,10-dialkoxypyrido[2,3,4,5-lmn]phenanthridine were synthesized as building blocks of copolymers Pa, Pb and Pc, giving relatively large band gaps of 2.13, 2.19 and 2.21 eV, respectively. Bulk-heterojunction (BHJ) solar cells were fabricated in a typical device configuration by utilizing the resulting polymers as the donor of the active layer. A power conversion efficiency (PCE) of 3.47% was achieved from the Pa-based device without any additives or thermal annealing. A further improved PCE of 4.54% was achieved in inverted device architecture, and it is hitherto the highest efficiency of BHJ polymer solar cells using a donor polymer with a band gap above 2.1 eV.

Pyridinium salt-based molecules as cathode interlayers for enhanced performance in polymer solar cells

A series of water/alcohol-soluble small molecules based on electron-deficient pyridinium salts namely BTPS, BnPS and F8PS were successfully synthesized. Their photophysical and electrochemical properties were thoroughly studied. Due to its good film-forming ability, F8PS was employed as a cathode interlayer in an organic photovoltaic cell. Simultaneous enhancements in open-circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) were achieved, and the power conversion efficiency (PCE) was increased from 4.32% to 6.56% compared to the device based on the bare Al cathode. Voc was significantly improved from 0.76 V to 0.94 V, and it is one of the best results reported in literature to date for polymer solar cells (PSCs) based on the active layer of poly [N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′- di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methylester (PC71BM). The greatly increased Voc may be due to the interface dipoles generated by F8PS. It was also demonstrated that post treatment of the active layer with ethanol gave an improvement of the overall device efficiency from the initial 4.32% to 5.55%, compared to the device with the bare Al cathode. Therefore, the improvement in performance after pyridinium salt deposition may be due to a combination of the effects of ethanol treatment and the presence of the thin pyridinium salt layer. The good water/alcohol solubility, ideal HOMO/LUMO energy levels and the excellent electron transfer/collection ability of the hydrophilic pyridinium salt derivatives makes them a promising family of electron transport materials for highly efficient PSCs.

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.

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.