No abstract

We demonstrate a new method to reverse the polarity and charge transport behavior of naphthalenediimide (NDI)-based copolymers by inserting a vinylene linker between the donor and acceptor units. The vinylene linkers minimize the intrinsic steric congestion between the NDI and thiophene moieties to prompt backbone planarity. The polymers with vinylene linkers exhibit electron n-channel transport characteristics under vacuum, similar to the benchmark polymer, P(NDI2OD-T2). To our surprise, when the polymers are measured in air, the dominant carrier type switches from n-to p-type and yield hole mobilities up to 0.45 cm 2 V −1 s −1 with hole to electron mobility ratio of three (μ h /μ e , ∼3), which indicates that the hole density in the active layer can be significantly increased by exposure to air. This increase is consistent with the intrinsic more delocalized nature of the highest occupied molecular orbital of the charged vinylene polymer, as estimated by density functional theory (DFT) calculations, which facilitates hole transport within the polymer chains. This is the first demonstration of an efficient NDI-based hole semiconducting polymer, which will enable new developments in all-polymer solar cells, complementary circuits, and dopable polymers for use in thermoelectrics.

Ten methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized and studied using experimental and theoretical methods to examine how iodine substitution and loading influence the excited-state dynamics of the chromophores.

We report the synthesis and solution characterization of poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblock copolymers with high lysine weight fractions (>75 wt%). In contrast to PK diblock copolymers in this composition range, KPK triblock copolymers exhibit morphology transitions as a function of pH. Using a combination of light-scattering and microscopy techniques, we demonstrate spherical micelle-vesicle and spherical micelle-disk micelle transitions for different K fractions. We interpret these morphology changes in terms of the energy penalty associated with folding the core P block to form a spherical micelle in relation to the interfacial curvature associated with different charged states of the K block.

Control over the out-of-plane molecular orientation of solution-processed organic semiconductors is a long-standing challenge in the organic electronics community. Here, a generalizable strategy using nanoconfinement to direct the nucleation of small-molecule organic semiconductors during solution-phase deposition is presented. Using a facile dip-coating process, triisopropylsilylethynylderivatized acene molecules were deposited onto nanoporous anodized aluminum oxide (AAO) scaffolds with average pore diameters ranging from 60 to 200 nm. Preferentially oriented nuclei were found to form within the cylindrical AAO nanopores such that the fast growth direction (i.e., the π-stack direction) aligned with the long axes of the pores. Crystal growth then propagated above the scaffold, resulting in the formation of vertical crystal arrays with the high surface energy π-planes exposed at the crystal tips. The diameters and heights of these crystals were tunable over ranges of 100−600 nm and 0.8−6.7 μm, respectively, by varying the dip-coating speed and scaffold pore diameters. Photoluminescence (PL) experiments further revealed an 8-fold enhancement of the PL signal from vertical crystal arrays compared to horizontal crystals deposited on flat SiO 2 substrates due to waveguiding along the crystal length. Critically, this strategy is compatible with continuous deposition techniques that will enable the high-throughput, large-area manufacturing of flexible and inexpensive optoelectronic devices.

The crystallization of a series of triisopropylsilylethynyl (TIPS)-derivatized acene-based organic semiconductors drop cast from solution onto substrates was investigated as a function of the size of their conjugated cores. When drop cast onto a substrate, the molecules in TIPSpentacene crystals adopt a "horizontal" orientation, with the long axis of the pentacene core parallel to the substrate surface. For crystals comprising molecules with dibenzopyrene, anthanthrene, and pyranthrene cores, two-dimensional X-ray diffraction patterns revealed the existence of a second population of crystals adopting a "vertical" molecular orientation with the long axis of the acene core perpendicular to the substrate surface. The ratio of the population of TIPS-pyranthrene crystals with molecules adopting horizontal versus vertical orientations was controlled by varying the surface energy of the underlying substrate. These crystals displayed orientationdependent linear birefringence and linear dichroism, as observed by differential polarizing optical microscopy. Conductive atomic force microscopy (C-AFM) revealed a 42-fold improvement in out-of-plane hole mobility through crystals adopting the vertical molecular orientation compared to those adopting the horizontal molecular orientation.

An all-acceptor napthalenediimide-bithiazole-based copolymer, P(NDI2OD-BiTz), was synthesized and characterized for application in thin-film transistors. Density functional theory calculations point to an optimal perpendicular dihedral angle of 90°between acceptor units along isolated polymer chains; yet optimized transistors yield electron mobility of 0.11 cm 2 /(V s) with the use of a zwitterionic naphthalene diimide interlayer. Grazing incidence X-ray diffraction measurements of annealed films reveal that P(NDI2OD-BiTz) adopts a highly ordered edge-on orientation, exactly opposite to similar bithiophene analogs. This report highlights an NDI and thiazole allacceptor polymer and demonstrates high electron mobility despite its nonplanar backbone conformation.

Polycyclic aromatic hydrocarbons (PAHs) have been widely explored as molecular semiconductors in organic electronic devices such as field-effect transistors or solar cells. However, their tendency to undergo photooxidation is a primary limitation to their practical applications. Bistetracene derivatives have recently been demonstrated to possess much larger photooxidation stability than the widely investigated pentacene and rubrene, while maintaining high charge-carrier mobilities. Here, using several levels of density functional theory, we identify the origin of the increased stability of bistetracene with respect to molecular oxygen by systematically investigating the [4 + 2] cycloaddition (Diels–Alder) photooxidation reaction mechanism. Importantly, our computational results indicate that endoperoxide formation in bis­(2-(trimethylsilyl)­ethynyl) bistetracene (BT) occurs not on the ring with least aromaticity, but rather on the ring with smallest distortion energy. This feature was subsequently confirmed by experimental NMR analyses. The oxidation activation barriers of bistetracene, pentacene, and rubrene are found to be 17.7, 13.6, and 14.4 kcal/mol, respectively, in agreement with the observed order of stability of these molecules with respect to oxidation reactions in solution. In the cases of BT and pentacene, the rates of electron transfer to create charged species (PAH+ and O2 –) are at least two orders of magnitude lower than that of the charge recombination process (back to PAH and O2); for rubrene, both of these processes are calculated to be of the same order of magnitude, in agreement with experimental electron paramagnetic resonance spectroscopy observations.