Yuewei Zhang

Structurally simple non-doped sky-blue OLEDs with high luminance and efficiencies at low driving voltages

Blue organic light-emitting diodes (OLEDs) are crucial for flat-panel displays and solid-state lighting. Regarding the practical application of OLEDs, high luminance and efficiencies at a low driving voltage, as well as a simple device structure, are highly desirable. However, blue OLEDs fulfilling these requirements are rare. Herein, two new sky-blue emitters, TPA-An-PPI and PCz-An-PPI, with the integration of wide-bandgap phenanthro[9,10-d]imidazole, anthracene and aromatic amine groups have been synthesized. They possess good thermal stability, bright emission in the solid state, as well as the bipolar carrier-transporting ability. Structurally simple non-doped sky-blue OLEDs with TPA-An-PPI as the emitting layer showed high performance, with the maximum luminance (Lmax) of 51 390 cd m−2, the maximum power efficiency (PEmax) of 10.42 lm W−1, and the maximum external quantum efficiency (EQEmax) of 4.61%. More importantly, at a practical high luminance of 1000 cd m−2, the PE and EQE retained high values of 8.48 lm W−1 and 4.60%, respectively, and the corresponding driving voltage was as low as 3.6 V. The performance of the non-doped device based on PCz-An-PPI was also high, although being lower than that of TPA-An-PPI.

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.

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.

An approach to high open-circuit voltage polymer solar cells via alcohol/water-soluble cathode interlayers based on anthrathiadiazole derivatives

Two small molecular cathode interlayer (CIL) materials with pyridinium ion or quaternary ammonium ion terminated 1,2,3-trihexyloxybenzene as pendant polar groups and anthrathiadiazole-4,11-dione (ATD) as a conjugated backbone, namely PBATD and TBATD, were synthesized for PCDTBT:PC71BM (PCDTBT:poly[poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]]; PC71BM:[6,6]-phenyl C71-butyric acid methyl ester) based polymer solar cells (PSCs). A dramatic improvement in the device performance was observed when the CILs were inserted between the active layer and the Al electrode. Especially, the device modified by quaternary ammonium ion terminated TBATD exhibits a power conversion efficiency (PCE) of 7.26%, which was 1.4 times that of the device with bare Al as the cathode, due to the simultaneously enhanced open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). Importantly, the Voc (0.93 V) achieved for the TBATD modified device was among the top values that used the same kind of active layer in conventional single-junction PSCs so far. It demonstrated that the CILs presented in this study may be a promising candidate for applications in high performance PSCs.

Constructing efficient organic photovoltaic devices with a spirobifluorene based water/alcohol-soluble cathode interlayer

A water/alcohol soluble small molecular cathode interlayer (CIL) with the quaternary ammonium ions terminated by trihydroxybenzene as the pendant polar functionalities and a commercially available spirobifluorene (SBF) as a 3D-structural conjugated backbone, namely, TBSBF, was synthesized and applied in conventional single-junction polymer solar cells (PSCs). TBSBF was found to effectively enhance the performance of the PSCs by simultaneously increasing the open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF). Combined measurements of the current density–voltage characteristics, ultraviolet photoemission spectroscopy (UPS) and the electron mobility demonstrated that TBSBF could effectively increase the built-in potential and electronic mobility, leading to higher performance of the devices. In addition, the atomic force microscopy (AFM) measurements showed that TBSBF could self-assemble into a leopard print-like pattern on the hydrophobic active layer surface. These results not only indicate TBSBF is a promising CIL material for the future solution-processed PSCs, but also provide a new way to design efficient CILs with good film-formation properties.

Efficient polymer solar cells based on a cathode interlayer of dicyanomethylenated indacenodithiophene derivative with large π-conjugation and electron-deficient properties

Two indacenodithiophene (IDT) based water/alcohol-soluble small molecular cathode interlayers (CILs), namely TBIDTD and TBIDTCN, were designed, synthesized and applied in typical PTB7:PC71BM-based conventional structural single-junction polymer solar cells (PSCs). This is the first report where the IDT core was used as a large π-conjugated backbone for CILs. These two molecules adopt the same polar end-groups. However, compared to TBIDTD, TBIDTCN exhibits a lower LUMO (lowest unoccupied molecular orbital) level due to the introduction of dicyanomethylene groups, which is beneficial for electron transport and extraction at the interface. Systematic optimizations were carried out to confirm the interfacial modification ability of the CILs. The results indicate that both CILs can significantly improve the performance of PSCs, and the TBIDTCN-modified devices presented better performance because of the further improved electron mobility and further reduced Rs (series resistance). A PCE (power conversion efficiency) of 9.19% was achieved by the TBIDTCN-modified devices, which was nearly 1.66 times that of the bare cathode device, due to the simultaneously enhanced Voc (circuit voltage), Jsc (short-circuit current) and FF (fill factor). Our work demonstrates that the IDT-based CILs are effective in improving the performance of PSCs and presents an attractive design strategy for CILs with large π-conjugation and electron-deficient properties.