Influence of Molecular Symmetry and Terminal Substituents on the Morphology and OFET Characteristics of S,N-Heteropentacenes.

Many heteroacenes have been extensively studied to improve device performances; however, the morphological effects stemmed from the chemical modification on a multiscale remain less explored. In this research, five axisymmetric S,N-heteropentacenes (DTPT, DTPT-Ph, DTPT-CN, DTPT-PYCN, and DTPT-BTCN) are studied to reveal the influences of molecular symmetry and end-capping substituents on the structure-property relationship, the thermal stability, crystallization behavior, film morphology, and OFET performance. Phase behavior was probed by differential scanning calorimetry (DSC), while the quality of the crystal array and structural details was investigated by optical microscopy (OM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). The analytic results reveal that (1) the parent axisymmetric S,N-heteropentacene, DTPT, is hard to crystallize, which hinders the preparation of high-quality crystal arrays for the OFET application. (2) The incorporation of π-conjugated electron-withdrawing (π-EW) endcaps that provide extended conjugation length and enhanced molecular polarity is required to form oriented crystal arrays to deliver reasonable OFET characteristics. (3) The π-EW endcaps with conformational freedom, such as -BTCN, due to the asymmetric feature of benzothiadiazole (BT), can hinder bulk phase crystallization and cause conformational disorder in the crystal array. Hence, the tradeoff of introducing the end-substituents to reinforce the poor crystalline nature of S,N-heteroacenes should be carefully considered.

Influences of Structural Modification of S, N-Hexacenes on the Morphology and OFET Characteristics.

Although chemical modifications on conjugated molecules are widely applied for the purpose of improving processability and device performances, the effect of the modification was far less investigated. Here, five S, N-hexacenes are studied to reveal the influences of (1) the lateral alkyl chain, (2) the terminal group (thiophene vs benzene), and (3) the end-capping phenyl group of the hexacenes on the morphology and organic field-effect transistor (OFET) performances. Crystal arrays of the hexacenes were prepared via polydimethylsiloxane (PDMS)-assisted crystallization (PAC) prior to morphological and OFET characterizations. The lattice structures and crystal quality of the hexacenes were evaluated by microscopy and diffraction techniques including single-crystal diffractometer, electron diffraction, and grazing incidence wide-angle X-ray scattering. The systematic analyses led to the following conclusions: (1) the bulkier alkyl side chain assists to form more densely packed crystals with less structural defects; (2) the terminal thiophene rings bring about higher-lying EHOMO, more ordered phase, and crystal orientation, whereas the terminal benzene rings deteriorate the structural order of the active layer and result in the liquid crystal phase; and (3) the phenyl end caps ameliorate the morphological order, intermolecular overlapping, thermal stability and elevate EHOMO. Thus, EH-DTPTt-Ph delivers the highest µh, contributing to high-lying EHOMO, well-oriented crystal array with a longer correlation length, and suitable lattice orientation. This systematic research provides the aspects about the effects of the functionalized S, N-hexacenes on the morphology and OFET characteristics, which is anticipated to be useful for the molecular design of heteroacenes.

S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field Effect Transistors and Organic Solar Cells: Critical Impact of Aromatic Subunits in the Ladder π-System.

Three novel donor-acceptor alternating polymers containing ladder-type pentacyclic heteroacenes (PBo, PBi, and PT) are synthesized, characterized, and further applied to organic field effect transistors (OFETs) and polymer solar cells. Significant aspects of quinoidal characters, electrochemical properties, optical absorption, frontier orbitals, backbone coplanarity, molecular orientation, charge carrier mobilities, morphology discrepancies, and the corresponding device performances are notably different with various heteroarenes. PT exhibits a stronger quinoidal mesomeric structure, linear and coplanar conformation, smooth surface morphology, and better bimodal crystalline structures, which is beneficial to extend the π-conjugation and promotes charge transport via 3-D transport pathways and in consequence improves overall device performances. Organic photovoltaics based on the PT polymer achieve a power conversion efficiency of 6.04% along with a high short-circuit current density (JSC) of 14.68 mA cm-2, and a high hole mobility of 0.1 cm2 V-1 s-1 is fulfilled in an OFET, which is superior to those of its counterparts, PBi and PBo.

Regioisomeric Effects of Donor-Acceptor-Acceptor' Small-Molecule Donors on the Open Circuit Voltage of Organic Photovoltaics.

Donor-acceptor-acceptor' small-molecule donors are synthesized to investigate regioisomeric effects on organic photovoltaic device performance. Cross-conjugation in 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[3,2-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile leads to an increased open-circuit voltage compared with its isomer 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[2,3-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile. A correlation is then established between molecular conjugation length and orbital energies, and hence open-circuit voltage.

Sky-Blue Organic Light Emitting Diode with 37% External Quantum Efficiency Using Thermally Activated Delayed Fluorescence from Spiroacridine-Triazine Hybrid.

Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

Easy Access to NO2 -Containing Donor-Acceptor-Acceptor Electron Donors for High Efficiency Small-Molecule Organic Solar Cells.

Two donor-acceptor-acceptor (D-A-A)-type molecules incorporating nitrobenzoxadiazole (NBO) as the A-A block and ditolylamine as the D block bridged through a phenylene (PNBO) and a thiophene (TNBO) spacer were synthesized in a one-step coupling reaction. Their electronic, photophysical, and thermal properties; crystallographic analysis; and theoretical calculations were studied to establish a clear structure-property relationship. The results indicate that the quinoidal character of the thiophene bridge strongly governs the structural features and crystal packings (herringbone vs. brickwork) and thus the physical properties of the compounds. PNBO and TNBO were utilized as electron donors combined with C70 as the electron acceptor in the active layer of vacuum-processed bulk heterojunction small-molecule organic solar cells (SMOSCs). The power conversion efficiency of both PNBO- and TNBO-based OSCs exceeded 5 %. The ease of accessibility of PNBO and TNBO demonstrates the potential for simple and economical synthesis of electron donors in vacuum-processed SMOSCs.