Yuguang Ma

Low Band‐Gap Conjugated Polymers with Strong Interchain Aggregation and Very High Hole Mobility Towards Highly Efficient Thick‐Film Polymer Solar Cells

Absorption spectra of polymer FBT-Th4 (1,4) (M n = 46.4 Kg/mol, E g = 1.62 eV, and HOMO = -5.36 eV) indicate strong interchain aggregation ability. High hole mobilities up to 1.92 cm(2) (V s)(-1) are demonstrated in OFETs fabricated under mild conditions. Inverted solar cells with active layer thicknesses ranging from 100 to 440 nm display PCEs exceeding 6.5%, with the highest efficiency of 7.64% achieved with a 230 nm thick active layer.

11% Efficient Ternary Organic Solar Cells with High Composition Tolerance via Integrated Near-IR Sensitization and Interface Engineering.

Highly efficient electron extraction is achieved by using a photoconductive cathode interlayer in inverted ternary organic solar cells (OSCs) where a near-IR absorbing porphyrin molecule is used as the sensitizer. The OSCs show improved device performance when the ratio of the two donors varies in a large region and a maximum power conversion efficiency up to 11.03% is demonstrated.

Multicolored-Fluorescence Switching of ICT-Type Organic Solids with Clear Color Difference: Mechanically Controlled Excited State

A donor-acceptor-type fluorophore containing a twisted diphenylacrylonitrile and triphenylamine has been developed by using the Suzuki reaction. The system indicates typical intramolecular charge-transfer properties. Upon mechanical grinding or hydrostatic pressure, the fluorophore reveals a multicolored fluorescence switching. Interestingly, a fluorescence color transition from green to red was clearly observed, and the change of photoluminescent (PL) wavelength gets close to 111 nm. The mechanisms of high-contrast mechanochromic behavior are fully investigated by techniques including powder XRD, PL lifetime, high-pressure PL lifetime, and Raman spectra analysis. The tremendous PL wavelength shift is attributed to gradual transition of excited states from the local excited state to the charge-transfer state.

Heating and mechanical force-induced luminescence on–off switching of arylamine derivatives with highly distorted structures

A triphenylamine-based organic luminophor (TPA-CO) with a highly distorted structure has been designed and effortlessly obtained by an Ullmann reaction. The luminophor exhibits a stimuli-induced emission enhancement effect and intramolecular charge transfer properties. The fluorescence efficiency of its crystals is dramatically increased from 0.4% to 12.3% upon grinding. The emission enhancement is also realized by a heating process. The “bright” state can recover its original state and turn “dark”. The luminescence “on–off” behaviour is repeatedly transformed by a grinding–vapour process or by a heating process. The XRD patterns of the “bright” and “dark” states show that the change of emission intensity is related to the reversible transition between the crystalline state and the metastable amorphous state. At the molecular level, the emission enhancement upon external stimuli may be attributed to conformational planarization and weak intermolecular interactions.

Porous Organic Polymer Films with Tunable Work Functions and Selective Hole and Electron Flows for Energy Conversions

Organic optoelectronics are promising technologies for energy conversion. However, the electrode interlayer, a key material between active layers and conducting electrodes that controls the transport of charge carriers in and out of devices, is still a chemical challenge. Herein, we report a class of porous organic polymers with tunable work function as hole- and electron-selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work-function-selective electron flow; while upon ionic ligation and electro-oxidation, the network significantly increases the work function and turns into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy-converting solar cells and light-emitting diodes.

Progress in small-molecule luminescent materials for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have been extensively studied since the first efficient device based on small molecular luminescent materials was reported by Tang. Organic electroluminescent material, one of the centerpieces of OLEDs, has been the focus of studies by many material scientists. To obtain high luminosity and to keep material costs low, a few remarkable design concepts have been developed. Aggregation-induced emission (AIE) materials were invented to overcome the common fluorescence-quenching problem, and cross-dipole stacking of fluorescent molecules was shown to be an effective method to get high solid-state luminescence. To exceed the limit of internal quantum efficiency of conventional fluorescent materials, phosphorescent materials were successfully applied in highly efficient electroluminescent devices. Most recently, delayed fluorescent materials via reverse-intersystem crossing (RISC) from triplet to singlet and the “hot exciton” materials based on hybridized local and charge-transfer (HLCT) states were developed to be a new generation of low-cost luminescent materials as efficient as phosphorescent materials. In terms of the device-fabrication process, solution-processible small molecular luminescent materials possess the advantages of high purity (vs. polymers) and low procession cost (vs. vacuum deposition), which are garnering them increasing attention. Herein, we review the progress of the development of small-molecule luminescent materials with different design concepts and features, and also briefly examine future development tendencies of luminescent materials.

Aqueous Solution Processed Photoconductive Cathode Interlayer for High Performance Polymer Solar Cells with Thick Interlayer and Thick Active Layer

An aqueous-solution-processed photoconductive cathode interlayer is developed, in which the photoinduced charge transfer brings multiple advantages such as increased conductivity and electron mobility, as well as reduced work function. Average power conversion efficiency over 10% is achieved even when the thickness of the cathode interlayer and active layer is up to 100 and 300 nm, respectively.

Polymorphic crystals and their luminescence switching of triphenylacrylonitrile derivatives upon solvent vapour, mechanical, and thermal stimuli

For piezo-, vapo-, and thermochromic materials, it remains a challenge to figure out the underlying reason for fluorescence color changes upon external stimulation and determine why only some fluorophores reveal emission switching. A novel triphenylacrylonitrile derivative (TPAN-MeO) with remarkably twisted conformations has been carefully prepared via the Suzuki coupling reaction. The fluorescence of TPAN-MeO in the aggregate state depends on the polymorphic forms: three crystalline forms BCrys, SCrys and YCrys exhibit bright blue, sky-blue and yellow emission, respectively; meanwhile the amorphous powders are also strongly fluorescent with green emission. The crystals BCrys and SCrys exhibit mechano- and piezochromism in that grinding and high pressure could alter the emission colour, respectively. In addition, the amorphous film exhibits vapo- and thermochromic behaviour in that organic vapour and heating could change the green colour into sky-blue. Interestingly, the solvent vapour and heating stimuli can trigger a crystal-to-crystal transformation between SCrys form and YCrys form.

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.

Fluorescence mutation and structural evolution of a π-conjugated molecular crystal during phase transition

Two thermodynamically stable crystalline phases (B- and G-phases) were found for a twistable π-conjugated molecule, CN-DSB, condensed from its solution. We investigated the structural evolution at the molecular and supramolecular levels as the crystalline phase transforms from the B-phase to G-phase under varied temperature or pressure. The intermolecular interactions were undermined before phase transition as the B-phase crystal was stimulated with an external energy. Heating the B-phase crystal up to 175 °C or applying stress up to its critical pressure (0.75 GPa) initially resulted in mixture phases or disordered state. At this stage, the molecules slightly adjust from a twisting configuration to a planar configuration, corresponding to the gradual red shift of the fluorescence spectra. Above the phase transition point, the initial intermolecular interaction of the B-phase is broken down, and the CN-DSB molecules re-assemble to the new phase—a new thermodynamic equilibrium state—corresponding to the sudden change of the emission color. Furthermore, the property of thermal-induced phase transition can be used to fabricate patterns on the CN-DSB crystal surface, and a uniform raster has been prepared by femtosecond laser direct writing (FsLDW) on the B-phase. The investigations provide new insight and understanding for the crystal phase transition and may contribute to process innovation in optical devices.