Zhengfeng Chang

Series of Multifluorine Substituted Oligomers for Organic Solar Cells with Efficiency over 9% and Fill Factor of 0.77 by Combination Thermal and Solvent Vapor Annealing

We report the synthesis of a family of multifluorine substituted oligomers and the corresponding polymer that have the same backbones but different conjugation lengths and amounts of fluorine atoms on the backbone. The physical properties and photovoltaic performances of these materials were systematically investigated using optical absorption, charge mobility, atomic force microscopy, transmission electron microscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering methods, and photovoltaic devices. The power conversion efficiencies (PCEs) based on oligomers were much higher than that in the polymer. Moreover, the devices based on BIT6F and BIT10F, which have an axisymmetric electron-deficient difluorobenzothiadiazole as the central unit, gave slightly higher PCEs than those with centrosymmetric electron-rich indacenodithiophene (IDT) as the central unit (BIT4F or BIT8F). Using proper solvent vapor annealing (SVA), particularly using thermal annealing (TA) followed by SVA, the device performance could be significantly improved. Notably, the best PCE of 9.1% with a very high FF of 0.76 was achieved using the medium-sized oligomer BIT6F with the optimized film morphology. This efficiency is the highest value reported for organic solar cells from small-molecules without rhodanine terminal group. More excitingly, devices from the shortest oligomer BIT4F showed an impressively high FF of 0.77 (the highest FF value reported for solution-processed small-molecule organic solar cells). These results indicate that photovoltaic performances of oligomers can be modulated through successive change in chain-length and fluorine atoms, alternating spatial symmetric core, and combined post-treatments.

Using tetraphenylethene and carbazole to create efficient luminophores with aggregation-induced emission, high thermal stability, and good hole-transporting property

Tetraphenylethene (TPE) is an archetypal luminogen that exhibits a phenomenon of aggregation-induced emission (AIE), while carbazole is a conventional chromophore which shows the opposite effect of aggregation-caused quenching (ACQ) of light emission in the condensed phase. Melding the two units at the molecular level generates a group of new luminescent materials that suffer no ACQ effect but depict high solid-state fluorescence quantum yields up to unity, demonstrative of the uniqueness of the approach to solve the ACQ problem of traditional luminophores. All the TPE–carbazole adducts are thermally and morphologically stable, showing high glass-transition temperatures (up to 179 °C) and thermal-degradation temperatures (up to 554 °C). Multilayer electroluminescence devices with configurations of ITO/NPB/emitter/TPBi/Alq3/LiF/Al are constructed, which exhibit sky blue light in high luminance (up to 13 650 cd m−2) and high current and external quantum efficiencies (up to 3.8 cd A−1, and 1.8%, respectively). The devices of the luminogens fabricated in the absence of NPB or hole-transporting layer show even higher efficiencies up to 6.3 cd A−1 and 2.3%, thanks to the good hole-transporting property of the carbazole unit.

A tetraphenylethene-based red luminophor for an efficient non-doped electroluminescence device and cellular imaging

An efficient red luminophor (TTPEBTTD) consisting of a 4,7-di(thiophen-2-yl)benzo-2,1,3-thiadiazole core and tetraphenylethene peripheries is developed. The non-doped electroluminescence device based on TTPEBTTD radiates red light with high efficiency up to 3.7%. The nanoparticles of TTPEBTTD are promising fluorescent visualizers for cellular imaging with low cytotoxicity.

Aggregation‐Induced Emission and Efficient Solid‐State Fluorescence from Tetraphenylethene‐Based N,C‐Chelate Four‐Coordinate Organoborons

In it together: Thermally stable N,C-chelate four-coordinate organoborons were attained by grafting intramolecular B⋅ ⋅ ⋅N coordination into tetraphenylethene-pyridine and -quinoline adducts. They exhibit aggregation-induced emission characteristics (see figure), and high fluorescence quantum yields approaching unity in solid films.

Hexaphenylbenzene‐Based, π‐Conjugated Snowflake‐Shaped Luminophores: Tunable Aggregation‐Induced Emission Effect and Piezofluorochromism

In this work, two rigid, multiple tetraphenylethene (TPE)-substituted, π-conjugated, snowflake-shaped luminophores BT and BPT were facilely synthesized by using a 6-fold Suzuki coupling reaction. These molecules are constructed based on the nonplanar structure of propeller-shaped hexaphenylbenzene (HPB) or benzene as core groups and TPE as end groups. As a result, they reserve the intrinsic aggregation-induced emission (AIE) property of the TPE moiety. Meanwhile, both fluorescence quantum yield and piezochromic behavior in the solid state can be tuned or switched by inserting the phenyl bridges through changing the twisting conformation. The more extended structure BPT showed a much stronger AIE effect and higher ΦF,f in the solid state in comparison with that of BT. Furthermore, an excellent optical waveguide application of these molecules was achieved. However, the revisable piezofluorochromic behavior has only appeared when BT was ground using a pestle and treated with solvent.

A Fully Substituted 3‐Silolene Functions as Promising Building Block for Hyperbranched Poly(Silylenevinylene)

A 3-silolene derivative, 2,2,5,5-tetrakis(dimethylsilyl)-1,1-dimethyl-3,4-diphenyl-3-silolene (TDMSHS), is first synthesized and characterized by X-ray diffraction crystallography and spectroscopic methods. Hydrosilylation polymerization of TDMSHS with 1,1-dimethyl-2,5-bis(4-ethynylphenyl)-3,4-diphenylsilole in the presence of Karstedts catalyst generates a stereoregular silole-containing hyperbranched poly(silylenevinylene) (hb-SPSV) with a high molecular weight (M(w) = 146,000, M(w)/M(n) = 1.5) in high yield (≈95%). hb-SPSV exhibits excellent thermal stability and strong fluorescence, and the emission of its aggregates in aqueous mixture can be quenched efficiently by picric acid with large quenching constants K(SV) up to 414400 M(-1).

Toward high performance indacenodithiophene-based small-molecule organic solar cells: investigation of the effect of fused aromatic bridges on the device performance

Here we report the synthesis of a pair of D1–A-bridge–D2-bridge–A–D1 type small molecules BIT4FDT and BIT4FTT which have different π-conjugated bridges between indacenodithiophene (IDT) as the electron-donating core and the electron-deficient difluorobenzothiadiazole unit and investigated the effects of the π-conjugated bridges on their photovoltaic properties. We found that the molecule BIT4FTT, containing thieno[3,2-b]thiophene which has two fused thiophene rings as the π-conjugated bridges, exhibits different photophysical properties, HOMO/LUMO energy levels, charge carrier mobilities and morphologies of blend films, and photovoltaic properties compared with the analogous system BIT4FDT which has 2,2′-bithiophene rings as the conjugated bridges. Moreover, the devices based on the two molecules after CH2Cl2 solvent annealing exhibited superior device performance to those not subjected to CH2Cl2 solvent annealing. The PCE of BHJ-OSC devices based on BIT4FTT and PC71BM increased from 5.85% to 7.57% (Jsc = 11.33 mA cm−2, Voc = 0.89 V, and FF = 0.75) after exposure to CH2Cl2 vapor due to the obvious increase of both Jsc and FF. Interestingly, the devices based on BIT4FDT and PC71BM showed a weaker response to solvent vapor annealing and much lower PCEs in comparison with those based on BIT4FTT. The results indicate that highly efficient small-molecule solar cells can be achieved using fused aromatic bridges and a suitable solvent vapor annealing process.

Efficient electroluminescence from excimers of 1,3,6,8-tetrakis(3,5-dimethylphenyl)pyrene.

Excimers are generally considered as detrimental to OLEDs. For pyrene-based chromophores, however, this is not always true. In this contribution, two new methylated tetraphenylpyrenes, 1,3,6,8-tetra-o-tolylpyrene (TTPy) and 1,3,6,8-tetrakis(3,5-dimethylphenyl)pyrene (TDMPPy), were synthesized through Suzuki coupling reactions. TDMPPy absorbs and emits light at longer wavelengths than TTPy due to its more planar conformation and thus better conjugation. TDMPPy is prone to excimer formation, thus leading to a strong bathochromic shift (84 nm) in the photoluminescence spectrum of its film. TDMPPy exhibits efficient electroluminescence originating from pyrene excimers, affording a maximum luminance of 26,670 cd m(-2) and a current efficiency as high as 10.8 cd A(-1) in a non-doped OLED (ITO/PEDOT:PSS (50 nm)/NPB (30 nm)/TDMPPy (30 nm)/TPBI (40 nm)/Ca:Ag).

Constructing small molecular AIE luminophores through a 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene core and peripheral triphenylamine with applications in piezofluorochromism, optical waveguides, and explosive detection

In this work, we employ the Corey–Fuchs reaction followed by coupling reactions to develop a series of π-conjugated aggregation-induced emission (AIE) small-molecule luminophores (DT2A, DT3A and DT4A) through a 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT) core with different amounts and different strengths of TPA peripheral moieties. Interestingly, these molecules give obviously higher solid fluorescent quantum efficiency and AIE phenomena. In particular, the thin film of DT3A exhibited the highest fluorescent quantum efficiency of ca. 25% and the DT2A showed the highest αAIE of 78. Moreover, the excellent optical waveguide applications of DT2A and DT4A were achieved because of their excellent self-assembly properties. Meanwhile, piezofluorochromic behavior with a large red shift of 35 nm only appeared when DT4A was ground using a pestle, because DT4A inserted more TPA and changed the twisting conformation. The piezofluorochromic behavior can be recovered to its original color by fuming with solvent. Finally, DT2A and DT4A were utilized as fluorescent probes to detect nitroaromatic/nitroaliphatic compounds and showed evident fluorescence quenching. These results indicate a huge potential to develop bright AIEgens based on DPDT core units and also provide insights into understanding how piezofluorochromism, optical waveguides and explosive detection properties are influenced by alternating the spatial symmetry of AIE materials with different numbers of TPA terminal groups.

Rational design of diketopyrrolopyrrole-based oligomers with deep HOMO level and tunable liquid crystal behavior by modulating the sequence and strength of the donor moiety

A family of narrow-band gap π-conjugated oligomers and isomers based on diketopyrrolopyrrole and difluorobenzothiadiazole coupled with an oligothiophene or thiazole ring have been successfully synthesized. They exhibited intensive absorption bands (300–900 nm) and deep-lying HOMO energy levels (−5.41 to −5.60 eV) due to donor–acceptor interactions and multiple fluorine substituents. The sequence and intensity of the electron donor moiety play an important role in determining bulk molecular properties, such as the photophysical properties, the HOMO/LUMO energy levels and mesomorphic properties. Compared with the isomer DTFB2T, D2TFBT with two n-hexyl-substituted thiophenes as the terminal groups exhibited good liquid crystal behavior with Smectic phase when cooled from 180 °C, and it generated a large area of the liquid-crystalline phase at about 100 °C. However, we did not observe such behaviors in DTFB2T due to the different sequence of the moiety.