以昇陳

Selective increase in the Seebeck coefficient and improving the power factor by a factor of ≈5 in an n‐doped donor–acceptor copolymer are realized by the use of amphipathic side chains. This strategy can properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. Therefore, an excellent power factor of 18 µW m^–1 K^–2 is achieved.

Abstract

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n‐type donor–acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host–dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host–dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 µW m^–1 K^–2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n‐type polymeric thermoelectrics.

Nature Chemistry, Published online: 07 December 2020; doi:10.1038/s41557-020-00607-9

Electronic–vibrational interplay can enable electron and energy transfer processes to be regulated. Now, coherence spectroscopy has been used to disentangle two vibrational pathways that control an electron transfer reaction. It has been shown that a fast, effectively ballistic, electron transfer along one vibrational path acts like a pulse to generate a coherent wavepacket along another vibrational pathway.

Nature Chemistry, Published online: 07 December 2020; doi:10.1038/s41557-020-00593-y

The role of the biexcitonic triplet-pair state 1(TT) during triplet–triplet annihilation events in singlet-fission materials has been the subject of recent debate. Now, emissive 1(TT) states have been shown to be direct products of triplet–triplet annihilation in both endothermic and exothermic singlet-fission materials.

A series of PM6 polymers with different weight‐average molecular weights and polydispersity index are synthesized, and the effects of PM6 polymerization degree on the efficiency and degradation behaviors of the Y6‐based photovoltaic system are systematically studied.

Abstract

The degree of polymerization can cause significant changes in the blend microstructure and physical mechanism of the active layer of non‐fullerene polymer solar cells, resulting in a huge difference in device performance. However, the diversity of stability issues, including photobleaching stability, storage stability, photostability, thermal stability, and mechanical stability, and more, poses a challenge for the degree of polymerization to comprehensively address the trade‐off between device efficiency and stability and reasonably evaluate the application potential of polymer materials. Herein, a series of PM6 polymers with different weight‐average molecular weights (M _w) and polydispersity index (PDI) are synthesized. The effects of the degree of PM6 polymerization on the efficiency and degradation behaviors of the photovoltaic systems based on Y6 as acceptor are investigated systematically. The findings regarding stability issues, together with the trade‐offs in the efficiency‐stability gap, formulate a complete guideline for the material design and performance evaluation in a way that relies much less on trial‐and‐error efforts.

A palette of highly fluorogenic Huaxi‐Fluor probes are turned on by a tetrazine bioorthogonal reaction. The emission wavelengths are fine‐tuned from the far‐red to near‐infrared. These highly stable and biocompatible probes are suitable for visualizing organelles in live cells without a washing step and for imaging of tumors in live small animals to depths of 500 μm by two‐photon excitation.

Abstract

Highly fluorogenic tetrazine bioorthogonal probes emitting at near‐infrared wavelengths are in strong demand for biomedical imaging applications. Herein, we have developed a strategy for forming a palette of novel Huaxi‐Fluor probes in situ, whose fluorescence increases hundreds of times upon forming the bioorthogonal reaction product, pyridazine. The resulting probes show large Stokes shifts and high quantum yields. Manipulating the conjugate length and pull–push strength in the fluorophore skeleton allows the emission wavelength to be fine‐tuned from 556 to 728 nm. The highly photo‐stable and biocompatible probes are suitable for visualizing organelles in live cells without a washing step and for imaging of tumors in live small animals to depths of 500 μm by two‐photon excitation.

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Xunchang Wang, Jianxiao Wang, Jianhua Han, Da Huang, Pengchao Wang, Lixue Zhou, Chunming Yang, Xichang Bao, Renqiang Yang

The present work deconvolutes the electronic processes in organic solar cells under short‐circuit conditions by combining readily available experimental methods (current‐voltage characteristics, external quantum efficiency) with optical simulations. The proposed method allows the quantification of geminate recombination, to determine the mobility‐lifetime product, and to quantify extraction. The applicability of this new approach is demonstrated in three different organic photovoltaic systems.

Abstract

The short‐circuit current (J _sc) of organic solar cells is defined by the interplay of exciton photogeneration in the active layer, geminate and non‐geminate recombination losses and free charge carrier extraction. The method proposed in this work allows the quantification of geminate recombination and the determination of the mobility‐lifetime product (µτ) as a single integrated parameter for charge transport and non‐geminate recombination. Furthermore, the extraction efficiency is quantified based on the obtained µτ product. Only readily available experimental methods (current‐voltage characteristics, external quantum efficiency measurements) are employed, which are coupled with an optical transfer matrix method simulation. The required optical properties of common organic photovoltaic (OPV) materials are provided in this work. The new approach is applied to three OPV systems in inverted or conventional device structures, and the results are juxtaposed against the µτ values obtained by an independent method based on the voltage–capacitance spectroscopy technique. Furthermore, it is demonstrated that the new method can accurately predict the optimal active layer thickness.

A novel thermally activated delayed fluorescence (TADF) material that exhibits reversible crystal‐to‐crystal phase transition under multiple external stimuli and mechanochromic luminescence with a high contrast of 130 nm, is described. Highly efficient deep‐red TADF organic light‐emitting diode with a maximum external quantum efficiency up to 18.3% at a peak wavelength of 676 nm is achieved.

Abstract

Merging thermally activated delayed fluorescence (TADF) and mechanochromic luminescence (MCL) into one single molecule is a promising strategy for developing multifunctional organic materials. Herein, a unique multifunctional molecule TPA‐DQP, comprising a large π‐conjugated diquinoxalino[2,3‐a:2′,3′‐c]phenazine (DQP) as the acceptor and triphenylamine (TPA) as the donor, is designed and synthesized. TPA‐DQP possesses polymorphism, efficient TADF emission as well as MCL property with high‐contrast in emission colors from 576 to 706 nm. Reversible crystal‐to‐crystal phase transitions in response to external stimuli such as vapor fuming and heating are realized on the basis of the two polymorphs of TPA‐DQP. The distinct crystal‐to‐crystal phase transition is attributed ultimately to the change of packing arrangements and intermolecular interactions of the two polymorphs under stimuli. Furthermore, TPA‐DQP‐based organic light emitting diode (OLED) device achieves external quantum efficiency as high as 18.3% at 676 nm, which represents the best performance for deep‐red OLEDs based on MCL‐active TADF emitters. This study reports a novel MCL‐active TADF material that exhibits crystal‐to‐crystal phase transition and brings insight into the underlying relationship between molecular packing modes and the photoluminescent behavior.

A state‐of‐the‐art n‐type organic semiconductor, PhC₂−BQQDI, which can be used to form single‐crystalline thin films by printing, is identified as a band‐transport material by means of variable‐temperature gated Hall effect measurements. The printed PhC₂−BQQDI single crystal is also used to demonstrate a high‐frequency transistor with a capability of 4.3 MHz under ambient atmosphere.

Abstract

Organic semiconductors (OSCs) have attracted growing attention for optoelectronic applications such as field‐effect transistors (FETs), and coherent (or band‐like) carrier transport properties in OSC single crystals (SCs) have been of interest as they can lead to high carrier mobilities. Recently, such p‐type OSC SCs compatible with a printing technology have been used to achieve high‐speed FETs; therefore, developments of n‐type counterparts may be promising for realizing high‐speed complementary organic circuits. Herein, coherent electron transport properties in a printed SC of a state‐of‐the‐art, air‐stable n‐type OSC, PhC₂−BQQDI, by means of variable‐temperature gated Hall effect measurements and X‐ray single‐crystal diffraction analyses in conjunction with band structure calculations, are reported. Furthermore, the SC FET is tested for high‐speed operations, which obtains a cutoff frequency of 4.3 MHz at an operation voltage of 20 V in air. Thus, PhC₂−BQQDI is shown as a new candidate for practical applications of SC‐based, organic complementary devices.