Xiaoxian Song

Terminal π–π stacking determines three-dimensional molecular packing and isotropic charge transport in an A–π–A electron acceptor for non-fullerene organic solar cells

In recent years, great progress has been achieved in the field of non-fullerene organic solar cells. In particular, the power conversion efficiencies for the photovoltaic devices based on A–π–A fused-ring electron acceptors, e.g. ITIC, can catch up with or even surpass the fullerene-based ones. However, the detailed molecular packing structures and charge transport properties of these acceptors are rarely studied and still unclear, which has become the major obstacle for rational molecular design to further improve the photovoltaic performance. Here, we have unravelled the intermolecular arrangements in the ITIC film via atomistic molecular dynamics simulations. The simulated results point to that three-dimensional molecular packing is formed in the ITIC film through local intermolecular π–π stacking between the terminal acceptor units. In sharp contrast, the ITIC crystal grown by the slow solvent vapor diffusion approach exhibits a one-dimensional edge-to-face stacking structure. Consequently, excellent isotropic electron mobilities along three dimensions are found for the film and unprecedentedly, the overall mobility is even higher than that of the crystal. Our work suggests that judicious modulation of the terminal acceptor unit to increase local intermolecular π–π interaction would be an effective way to improve the electron mobilities and photovoltaic performance of the A–π–A electron acceptors.

Efficient deep-blue OLEDs based on phenanthro[9,10-d]imidazole-containing emitters with AIE and bipolar transporting properties

Highly efficient deep-blue emitters, especially those matching the NTSC (National Television Standards Committee) blue standard CIE (Commission Internationale de L’Eclairage) coordinate of (0.14, 0.08), are crucial for full-colour displays and white lighting. However, deep-blue emitters with excellent electroluminescence performance are rare. According to the strategy of developing highly luminescent solids by constructing aggregation-induced emission (AIE) systems, two AIE-type blue emitters, mTPE-PPI and mTPE-DPI, have been synthesized by integrating a triphenylethylene (TPE) group with one or two phenanthro[9,10-d]imidazole (PI) groups. The adoption of the PI unit with a large optical bandgap, the meta-linkage of the PI and the TPE units on a benzene ring, and the twisted molecular conformations of present AIE systems endow these compounds with bright blue emission in the solid state. In addition, these compounds are capable of transporting both electrons and holes and possess appropriate HOMO and LUMO levels for carrier injections. These features enable them to serve as excellent blue-emitting materials in OLEDs. A non-doped OLED based on mTPE-PPI displayed deep-blue light with the CIE coordinate of (0.15, 0.09) and a maximum external quantum efficiency (EQEmax) of 2.30%. An analogous device with mTPE-DPI as the emitting layer emitted blue light (CIE: 0.15, 0.14) with a high EQEmax of 3.69% and low efficiency roll-off. Further improvement of device performance has been achieved by the doping technique. The doped device of mTPE-DPI was able to realize deep-blue emission (CIE: 0.15, 0.10) matching well the blue standard, meanwhile reaching a EQEmax of 5.52% that represents the highest value reported for deep-blue OLEDs based on AIE compounds.

Two-Dimensional Organic Single Crystals with Scale Regulated, Phase-Switchable, Polymorphism-Dependent, and Amplified Spontaneous Emission Properties

The successful preparation of two-dimensional (2D) single crystals can promote the development of organic optoelectronic devices with excellent performance. A Schiff base compound salicylidene(4-dimethylamino)aniline with aggregation induced emission (AIE) property was employed as the building block to fabricate 2D thin single crystal plates with scales from around 50 μm to 1.5 cm. Yellow and red emissive polymorphs were concomitantly obtained during crystallization. The single-crystal-to-single-crystal (SC-to-SC) transformation from yellow polymorph to red one was demonstrated. Furthermore, both polymorphs exhibited amplified spontaneous emission (ASE) properties. Interestingly, the red polymorph displayed size-dependent ASE characteristics. The larger red polymorph showed near-infrared ASE with maximum at 706 nm, whereas the smaller one presented red ASE with maximum at 610 nm. These results suggest that the different scale single crystalline thin films with perfect optoelectronic properties may be fabricated by using the organic molecules with 2D assembly feature.

Non-doped luminescent material based organic light-emitting devices displaying high brightness under very low driving voltage

Two phenanthro[9,10-d]imidazole (PI) derivatives, 9-N,N-diphenyl-amino-10-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)anthracene (DPAA-PPI) and 9-N,N-dipheny-amino-10-(4-(1-(4-tert-butylphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)anthracene (tBuDPAA-PPI), have been designed and synthesized. The two compounds exhibited high photoluminescence quantum yields (at around 0.80) in the solid state and bipolar carrier transport properties. The non-doped OLEDs using DPAA-PPI and tBuDPAA-PPI as emitting layers showed the maximum power efficiencies (PEs) of 13.9 lm W−1 and 11.4 lm W−1, corresponding to the maximum external quantum efficiencies (EQEs) of 5.2% and 4.1%, respectively. The DPAA-PPI-based OLED exhibited an extremely low turn-on voltage of 2.4 V and its luminescence reached 100, 1000, 10000 and over 100000 cd m−2 under driving voltages of 2.8, 3.6, 5.1 and 7.5 V, respectively.

Large π-Conjugated Quinacridone Derivatives: Syntheses, Characterizations, Emission, and Charge Transport Properties

Two 11-ring-fused quinacridone derivatives, TTQA and DCNTTQA, have been synthesized by ferric chloride mediated cyclization and Knoevenagel reaction. Replacement of the carbonyl groups (in TTQA) with dicyanoethylene groups (in DCNTTQA) not only red-shifted the emission to the near-infrared region but also led to a nonplanar skeleton that significantly improved the solubility of DCNTTQA. Moreover, dicyanoethylene groups rendered DCNTTQA low-lying HOMO and LUMO levels. DCNTTQA-based solution-processed field-effect transistors showed a hole mobility up to 0.217 cm(2) V(-1) s(-1).

Single-Molecule-based White-Light Emissive Organic Solids with Molecular-Packing-Dependent Thermally Activated Delayed Fluorescence

White-light-emitting single molecules have attracted broad attention because of their great potential for use in flat-panel displays and future light sources. We report a unique molecule of 3-(diphenylamino)-9H-xanthen-9-one (3-DPH-XO), which was found to exhibit bright white-light emission in the solid state caused by the spontaneous formation of a mixture with different polymorphs. Single-crystal analyses demonstrate that noncovalent interactions (such as π···π stacking, hydrogen bonding, and C-H···π interactions) induce different stacking arrangements (polymorphs A, B, and C) with different photophysical properties in a molecular solid. In addition, crystals B and C with the acceptor···acceptor stacking feature show the thermally activated delayed fluorescence (TADF) characteristics, indicating that appropriate noncovalent interactions could enhance the reverse intersystem crossing process and consequently lead to delayed fluorescence. This discovery provides an effective strategy for the design of new white-light-emitting single molecules as well as TADF materials.

Geometric Shape Regulation and Noncovalent Synthesis of One-Dimensional Organic Luminescent Nano-/Micro-Materials

Noncovalent synthesis of one-dimensional (1D) organic nano-/micro-materials with controllable geometric shapes or morphologies and special luminescent and electronic properties is one of the greatest challenges in modern chemistry and material science. Control of noncovalent interactions is fundamental for realizing desired 1D structures and crucial for understanding the functions of these interactions. Here, a series of thiophene-fused phenazines composed of a halogen-substituted π-conjugated plate and a pair of flexible side chains is presented, which displays halogen-dependent 1D self-assemblies. Luminescent 1D twisted wires, straight rods, and zigzag wires, respectively, can be generated in sequence when the halogen atoms are varied from the lightest F to the heaviest I. It was demonstrated that halogen-dependent anisotropic noncovalent interactions and mirror-symmetrical crystallization dominated the 1D-assembly behaviors of this class of molecules. The methodology developed in this study provides a potential strategy for constructing 1D organic materials with unique optoelectronic functions.

Controllable morphology and self-assembly of one-dimensional luminescent crystals based on alkyl-fluoro-substituted dithienophenazines

A class of dithienophenazine derivatives, 9,10-difluoro-2,5-dialkyldithieno[3,2-a:2′,3′-c]phenazine (F-n, n = 4, 5, 6, 7 and 8), modified with various lengths of linear alkyl chains were synthesized and used as building blocks to assemble luminescent one-dimensional (1D) nano/microcrystals. It was demonstrated that the side-chain length can dramatically influence the self-assembled morphologies of the 1D nano/micromaterials. The self-assembly behaviors of F-n have been studied based on the methods of crystallization in the solution phase and solvent evaporation on a substrate. For F-4, F-6 and F-7 molecules, well-defined 1D twisted microstructures (including ribbons, fibers and bundles) were obtained. F-5 molecules can self-assemble into nearly criss-cross (orthogonal) network patterns interconnected by 1D wires. F-8 molecules with longer alkyl chains predominantly formed flattened wires. Single crystal X-ray structure studies of F-n (n = 4, 5 and 8) demonstrated that alkyl chains with different lengths result in distinctly different intermolecular interactions and molecular packing modes, which provides a reasonable explanation for the alkyl chain length-dependent assembly morphologies and emission of F-n-based nano/micromaterials. It has been demonstrated that anisotropic noncovalent interactions and mirror-symmetrical crystallization dominated the 1D assembly behaviors of this class of molecules.