A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.
A series of novel heterocycle-based dyes with donor-pi-bridge-acceptor-pi-bridge-donor (D-pi-A-pi-D) structural motif, where benzothiazole serves as an electron-withdrawing core, have been designed and synthesized via palladium-catalyzed Sonogashira and Suzuki-type cross-coupling reactions. All the target chromophores show strong one-photon and two-photon excited emission. The maximum two-photon absorption (TPA) cross sections delta(TPA) of the prepared derivatives bearing diphenylamino functionalities occur at wavelengths ranging from 760 to 800 nm and are as large as approximately 900-1100 GM. One- and two-photon absorption characteristics of the title dyes have also been investigated by using density functional theory (DFT) and the structure-property relationships are discussed. The TPA cross sections calculated by means of quadratic response time-dependent DFT using the Coulomb-attenuated CAM-B3LYP functional support the experimentally observed trends within the series, as well as higher delta(TPA) values of the title compounds compared to those of analogous fluorene or carbazole-derived dyes. In contrast, the traditional B3LYP functional was not successful in predicting the observed trend of TPA cross sections for systems with different central cores. In general, structural modification of the pi-bridge composition by replacement of ethynylene (alkyne) with E-ethenylene (alkene) linkages and/or replacement of dialkylamino electron-donating edge substituents by diarylamino ones results in an increase of delta(TPA) values. The combination of large TPA cross sections and high emission quantum yields makes the title benzothiazole-based dyes attractive for applications involving two-photon excited fluorescence (TPEF).
The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high‐efficiency solar cells. Here a strategy to passivate surface trap states of TiO2 films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al2O3) or zirconia (ZrO2) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO2 are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open‐circuit voltage and the short‐circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO2 surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO2 electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO2/ALD metal oxide/organic interface.
Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution‐processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol‐gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol‐based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short‐circuit current, open‐circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT‐PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s‐HxMoO2.75 as the hole extraction layer.
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