S-CQDs were chosen as a light emitting layer by virtue of their structure characteristics, optical properties and film-forming ability to obtain CQD-LEDs.

coupling effect induced by heavy-metal atoms. [5,6] For example, noble metal-based phosphors including iridium(III) complexes, [7][8][9][10][11][12][13][14][15][16] platinum(II) complexes, [17][18][19][20] and gold(III) complexes [21,22] have been widely used in phosphorescent OLEDs (PhOLEDs). However, these noble metals suffer from the low abundance and high cost. Hence, the relatively abundant, lowcost, and low-toxic phosphorescent metal complex have been drawing great interests for PhOLEDs.Recent explorations of phosphorescent manganese(II) complexes appear to be a new and attractive alternative toward highly efficient PhOLEDs. The manganese(II) complexes display strong photoluminescence in solid state originating from the metal-centered d-d ( 4 T 1 (G) → 6 A 1 ) radiative transition. [23][24][25] The well-known green light-emitting manganese(II) complexes are ionic compounds consisting of organic cations and inorganic tetrahalogenomanganate(II) anions. [26,27] Attributed to their excellent solid-state photophysical properties, this kind of organic-inorganic hybrid complexes have exhibited promising optoelectronic applications. For example, Chen and co-workers have realized the solution-processed PhOLEDs based on the ionic tetrabromide manganese(II) complex ((Ph 4 P) 2 (MnBr 4 )) as an emitting dopant, the external quantum efficiency (EQE) of this device can reach 9.6% for the doped OLEDs. [28] However, the ionic manganese(II) complexes often suffer from low stability and can be easily hydrolyzed Phosphorescent transition-metal complexes have played the vital role in the rapid development of organic light-emitting diodes (OLEDs) as the most promising candidates for next-generation flat-panel display and solid-state lighting techniques. In this work, novel and low-cost phosphorescent neutral tetrahedral manganese(II) complexes (DBFDPO-MnX 2 , X = Br, or Cl) based on dibenzofuran-based phosphine oxide derivative as ligand are designed and synthesized. The manganese(II) complexes exhibit intense green phosphorescence with high photoluminescence quantum yields (PLQYs) of as high as 81.4% (DBFDPO-MnBr 2 ). Using complex DBFDPO-MnBr 2 as dopant, a green OLED with current efficiency (CE max ) of 35.47 cd A −1 , power efficiency (PE max ) of 34.35 lm W −1 , and external quantum efficiency (EQE max ) of 10.49% is fabricated. Interestingly, red exciplex emission is also observed in electroluminescence, arising from the interaction between the host materials (bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp 2 ) or 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi)) and the dopant (DBFDPO-MnBr 2 ).The exciplex-based red OLED in this study exhibits the maximum CE and PE reaching 18.64 cd A −1 and 17.92 lm W −1 , respectively, which are among the up-to-date highest values for exciplex-based red OLEDs. Beneficial from the exciplex, it has the great potential to broaden the electroluminescent spectra with manganese(II) complex. Phosphorescent OLEDsThe ORCID identification number(s) for the author(s) of this article can be fou...

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In this work, we report the effort to develop high-efficiency inverted polymer solar cells (PSCs) by applying a solution-processable bilayer ZnO/carbon quantum dots (C-QDs) electron extraction layer (EEL). It is shown that the use of the bilayer EEL helps to suppress the exciton quenching by passivating the ZnO surface defects in the EEL, leading to an enhanced exciton dissociation, reduced charge recombination and more efficient charge extraction probability, and thereby achieving high power conversion efficiency (PCE). The inverted PSCs, based on the blend of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} and [6,6]-phenyl C71-butyric acid methyl ester, possess a significant improvement in PCE of ∼9.64%, which is >27% higher than that of a control cell (∼7.59%). The use of a bilayer ZnO/C-QD EEL offers a promising approach for attaining high-efficiency inverted PSCs.

Organic electroluminescence is considered as the most competitive alternative for the future solid-state displays and lighting techniques owing to many advantages such as selfluminescence, high efficiency, high contrast, high color rendering index, ultra-thin thickness, transparency, flat and flexibility, etc. The development of high-performance organic electroluminescence has become the continuing focus of research. In this personal account, a brief overview of representative achievements in our study on the design of highly efficient novel organic light-emitting materials (including fluorescent materials, phosphorescent iridium(III) complexes and conjugated polymers bearing phosphorescent iridium(III) complex) and highperformance device structures together with working principles are given. At last, we will give some perspectives on this fascinating field, and also try to provide some potential directions of research on the basis of the current stage of organic electroluminescence. Wiley Online Library 1537 Scheme 5. The chemical structures of cationic iridium(III) complexes based on various N N ligands (18-23).Figure 7. UV-vis absorption (a) and PL (b) of 18-23 in CH 2 Cl 2 . [30] Figure 35. (a) The device structure and proposed working mechanism for WOLED with sandwiched blue EML; (b) The normalized EL spectra of WOLED at the voltage of 5-7 V, and corresponding luminance, CIE, CCT, and CRI are also given. [65a]

First, oleophilic carbon dots (CDs) with a fluorescence quantum yield (QY) of 41% were synthesized by a one-pot microwave-assisted carbonization method. Then, CD-based electroluminescent light emitting diodes (CD-LEDs) were prepared. The impact of CD aggregation on the brightness of CD-LEDs was studied. The results show that, to some extent, with the decrease of the aggregation of the CD film, the luminescence quenching of the CD-LEDs gradually decreased, and the luminance of the CD-LEDs gradually increased. Hence, in order to improve the dispersion of CDs and reduce the aggregation of CDs and the luminescence quenching of the devices, host-guest doping was adopted to effectively improve the brightness of CD-LEDs. In this work, the yellow emission of the doped devices is mainly derived from the direct carrier trapping on CDs. Moreover, white and yellow CD-LEDs were obtained from the same oleophilic CDs by tuning the structure of the devices. The white CD-LEDs exhibit a high color rendering index (CRI) of 83 with a luminance of 455.2 cd m-2. The yellow CD-LEDs show the maximum brightness of 339.5 cd m-2 and excellent color stability. The results show that the luminescence quenching of CD-LEDs was resisted and the brightness of CD-LEDs was improved by using host-guest doping.

green, yellow, orange, and red hydrophilic CDs were synthesized by Yuan et al., and multicolor CD-LEDs were fabricated by using these CDs separately as emitting layer, with the maximum luminance of 136, 93, 60, 65, 12 cd m −2 , respectively. [11] However, the research of CD-LEDs is still at the initial stage, and the brightness of monochromatic CD-LEDs is still far from reaching commercialization level. Therefore, it is a great challenge for applying CDs to full-color displays.Currently, CDs in solid state always suffer from obvious fluorescence quenching. [14,15] Consequently, the prepared CD-LEDs exhibit a low luminance owing to fluorescence quenching. [16] In 2017, Yuan et al. obtained white CD-LED by doping green hydrophilic CDs in poly(N-vinylcarbazole) (PVK) as the emitting layer. [11] The high brightness of 2050 cd m −2 indicates that the method of hostguest doping can significantly improve luminance of the device. However, because of an incomplete energy transfer from PVK to CDs, the electroluminescent (EL) spectra for the fabricated CD-LEDs always contain the strong blue emission from PVK, which is probably related to the limited doping concentration of hydrophilic CDs. PVK is only soluble in chlorobenzene or chloroform, which makes it difficult to tune hydrophilic CDs' concentration during a doping process, because the precipitation from the mixed solutions at a high doping concentration of hydrophilic CDs could significantly reduce the performance of doped devices. So, oleophylic CDs are needed to obtain high brightness and pure emission of doped CD-LEDs.In present study, oleophylic CDs with QY of 41% were obtained using anhydrous citric acid as a carbon precursor and hexadecylamine as a passivation agent by one-step microwave carbonization method. Then, the strategy of host-guest doping was adopted to fabricate blue CD-LEDs by using the oleophylic CDs (guest) doped in PVK (host) as the emitting layer, because the oleophylic CDs and PVK could be well cosoluble in chlorobenzene at any doping concentration. Thus, the fabricated CD-LED realizes pure blue emission that dominantly originates from the emission of CDs by the host-guest energy transfer mechanism. Consequently, the CD-LED achieves the ultrahigh brightness of 569.8 cd m −2 , with the Internationale de L'Eclairage (CIE) coordinates of (0.22, 0.27), which is the highest value for monochromic CD-LEDs ever reported. Our result shows that oleophylic CDs are conducive to fabricating CD-LEDs with both high brightness and pure emission. Ultrahigh brightness blue carbon dot-based light-emitting diodes (CD-LEDs)are fabricated by a host-guest doping method with poly(N-vinylcarbazole) (PVK) as the host to realize the high brightness and pure emission of CD-LEDs. Oleophylic CDs are synthesized as the dopant to obtain a wide range of doping concentration of CDs because oleophylic CDs and PVK can be well cosoluble in chlorobenzene at any doping concentration. Therefore, the CD-LEDs emitting pure blue light that dominantly originates from CDs' emission are real...

Two highly efficient red neutral iridium(III) complexes, Ir1 and Ir2, were rationally designed and synthesized by selecting two pyridylimidazole derivatives as the ancillary ligands. Both Ir1 and Ir2 show nearly the same photoluminescence emission with the maximum peak at 595 nm (shoulder band at about 638 nm) and achieve high solution quantum yields of up to 0.47 for Ir1 and 0.57 for Ir2. Employing Ir1 and Ir2 as emitters, the fabricated red organic light-emitting diodes (OLEDs) show outstanding performance with the maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 20.98%, 33.04 cd/A, and 33.08 lm/W for the Ir1-based device and 22.15%, 36.89 cd/A, and 35.85 lm/W for the Ir2-based device, respectively. Furthermore, using Ir2 as red emitter, a trichromatic hybrid white OLED, showing good warm white emission with low correlated color temperature of <2200 K under the voltage of 4-6 V, was fabricated successfully. The white device also realizes excellent device efficiencies with the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. Such high electroluminescence performance for red and white OLEDs indicates that Ir1 and Ir2 as efficient red phosphors have great potential for future OLED displays and lightings applications.