Numerical study on high-frequency effect of rail corrugation on subway-induced environmental vibrations.

Rail corrugation is a common phenomenon in railway engineering, but its high-frequency effects on environmental vibrations are neglected in most previous research. Therefore, a hybrid numerical method was proposed in this paper to analyze subway-induced ground vibrations, especially in the high-frequency range caused by rail corrugation. The analysis model composed of a three-dimensional (3D) load generation subsystem and a two-dimensional (2D) wave propagation subsystem was established based on the vehicle-track coupling method and finite element method, and validated by the measured data. Then the high-frequency effects under different tunnel depths and rail fasteners were further studied. The results show that high-frequency vibrations propagate radially from the tunnel wall to the surrounding soil and transmit to the ground by the dominant path under different tunnel depths. The increase of tunnel depths could result in more serious high-frequency effects in the vibration amplification region. When the depth changes from 17 to 29 m, the 250 Hz ground vibration at around 30 m away from the track increases by 5.6 dB. Besides, it was found that in the commonly used range, the reduction of fastener stiffness can effectively eliminate high-frequency ground vibrations, while there is a significant nonlinear relationship between fastener damping and high-frequency vibration. The findings of this paper could provide references for parameter design in subway construction and rail corrugation remediation, and help create better living environments.

High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions.

Nonfullerene acceptors (MQ3, MQ5, MQ6) are synthesized using asymmetric and symmetric ladder-type heteroheptacene cores with selenophene heterocycles. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end-capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O⋅⋅⋅Se compared to that of O⋅⋅⋅S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport, leading to a higher power conversion efficiency (PCE) of 15.64 % compared to MQ3, which shows a PCE of 13.51 %. Based on the asymmetric heteroheptacene core, MQ6 shows an improved carrier transport induced by the reduced π-π stacking distance, related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. MQ6 exhibits a PCE of 16.39 % with a VOC of 0.88 V, a FF of 75.66 %, and a JSC of 24.62 mA cm-2 .

Ladder-Type Heteroheptacenes with Different Heterocycles for Nonfullerene Acceptors.

The design, synthesis, and characterization of two novel nonfullerene acceptors (M8 and M34) based on ladder-type heteroheptacenes with different heterocycles are reported. Replacing the furan heterocycles with the thiophene heterocycles in the heteroheptacene backbone leads to a hypsochromically shifted absorption band and greatly improved carrier transport for the resulting nonfullerene acceptor (M34) although the π-π-stacking distances are barely affected. Bulk-heterojunction polymer solar cells fabricated from M34 and a wide band gap polymer (PM6) as the donor showed a best power conversion efficiency (PCE) of 15.24 % with an open circuit voltage (VOC ) of 0.91 V, much higher than a PCE of 4.21 % and a VOC of 0.83 V for the counterparts based on M8:PM6. These results highlight the importance of key atoms in the construction of high-performance nonfullerene acceptors.

Control over π-π stacking of heteroheptacene-based nonfullerene acceptors for 16% efficiency polymer solar cells.

Nonfullerene acceptors are being investigated for use in polymer solar cells (PSCs), with their advantages of extending the absorption range, reducing the energy loss and therefore enhancing the power conversion efficiency (PCE). However, to further boost the PCE, mobilities of these nonfullerene acceptors should be improved. For nonfullerene acceptors, the π-π stacking distance between cofacially stacked molecules significantly affects their mobility. Here, we demonstrate a strategy to increase the mobility of heteroheptacene-based nonfullerene acceptors by reducing their π-π stacking distances via control over the bulkiness of lateral side chains. Incorporation of 2-butyloctyl substituents into the nonfullerene acceptor (M36) leads to an increased mobility with a reduced π-π stacking distance of 3.45 Å. Consequently, M36 affords an enhanced PCE of 16%, which is the highest among all acceptor-donor-acceptor-type nonfullerene acceptors to date. This strategy of control over the bulkiness of side chains on nonfullerene acceptors should aid the development of more efficient PSCs.