Yanqin Li

D–π–A–π–D type benzothiadiazole–triphenylamine based small molecules containing cyano on the π-bridge for solution-processed organic solar cells with high open-circuit voltage

Two novel D-π-A-π-D structured small molecules composed of benzothiadiazole and triphenylamine were designed and synthesized. with cyano on the π-bridge exhibited a deep HOMO energy level, resulting in an impressive V(OC) of up to 1.04 V with a PCE of 3.85%, while non-cyano substituted yielded a V(OC) of 0.94 V and a PCE of 1.99%.

The synthesis and photovoltaic properties of A–D–A-type small molecules containing diketopyrrolopyrrole terminal units

A series of novel A–D–A structured small molecule photovoltaic (PV) materials [CZ(TDPP)2, DPA(TDPP)2, PTZ(TDPP)2 and FL(TDPP)2] with diketopyrrolopyrrole (DPP) as an electron-withdrawing group were synthesized and characterized. These small molecular donors exhibit excellent solubility in common organic solvents. The density functional theory (DFT) calculations demonstrated the intramolecular charge transfer (ICT) behavior of the synthesized PV materials and an efficient charge separation was observed by a fluorescence quenching experiment. In addition, their photophysical and electrochemical properties show that they harvest sunlight over the entire visible spectrum range and keep appropriate energy levels to satisfy the requirement of solution-processable OSCs. Therefore, we explored the PV properties of the synthesized donors by fabricating BHJ solar cells with a typical structure of ITO/PEDOT:PSS/Donors:PC61BM/LiF/Al. Among them, CZ(TDPP)2 revealed a promising performance in PV devices with a power conversion efficiency (PCE) of 1.50%, along with an open-circuit voltage (VOC) of 0.66 V, a short-circuit current density (JSC) of 4.12 mA cm−2, and a fill factor (FF) of 0.44, under an illumination of AM 1.5G (80 mW cm−2).

D–π–A–π–D-type low band gap diketopyrrolopyrrole based small molecules containing an ethynyl-linkage: synthesis and photovoltaic properties

Two new D–π–A–π–D-type diketopyrrolopyrrole (DPP) based organic small molecules (OSMs), M3 and M4, were synthesized successfully, consisting of ethynyl as π-linkage, and containing alkylated carbazole and fluorene as terminal electron-donating groups, respectively. To investigate the triple-bond effect on the optical electronic properties, non-ethynyl analogues M1 and M2 with a single-bond-linkage were designed and synthesized as well. The relationship between molecular structure and property was thoroughly investigated by experimental and theoretical studies. In contrast, the ethynyl-linkage structural design could not only lower the highest occupied molecular orbital (HOMO) levels, but also delicately balance the relationship between the deep-lying HOMO and narrow band gap, thus improving the photovoltaic (PV) performance. As a result, compounds M3 and M4 exhibited relatively deep-lying HOMO levels relative to M1 and M2, resulting in the corresponding PV devices with an increased open-circuit voltage (VOC) of 0.84 V and 0.98 V, with power conversion efficiency (PCE) of 1.99% and 3.10%, respectively. Whereas M1 and M2 based devices showed a VOC of 0.46 V and 0.89 V, and a PCE of 1.48% and 2.23%, respectively. The best PV performance of M4 was primarily attributed to the deep-lying HOMO level and reasonably high hole mobility caused by the structural design with an ethynyl-linkage and a fluorene end-capping group. We found that the systematic investigation of the triple bond effect on novel OSMs could be critical for a deep understanding of the relationship between molecular structure and property. This work provided an important guide for the rational design of novel PV materials.

Linkage effects of linear D–π–A–π–D type diketopyrrolopyrrole-triphenylamine based solution-processable organic small molecule photovoltaic materials

Four novel D–π–A–π–D-type small molecules (SMs) were synthesized and their π-linkage effects were investigated. Among them, SM2 with vinylene as the π-linkage exhibited better molecular coplanarity, reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene and acrylonitrile as π-linkages exhibited relatively higher open-circuit voltages (VOC) of 0.93 V and 0.90 V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for developing new materials in solution-processed small molecule solar cells.

PdCl2 immobilized on metal–organic framework CuBTC with the aid of ionic liquids: enhanced catalytic performance in selective oxidation of cyclohexene

Metal–organic framework CuBTC was employed as a support to immobilize PdCl2 with the aid of ionic liquids (ILs). The as-synthesized PdCl2-ILs/CuBTC catalyst was studied in selective oxidation of cyclohexene with molecular oxygen as an oxidant and TBHP as an initiator. It is found that the allylic oxidation and radical oxidation processes are main reaction pathways. More importantly, the enhancement of catalytic activity in the oxidation of cyclohexene is observed over the PdCl2-IL/CuBTC catalyst due to Pd–Cu cooperative catalysis. Furthermore, ionic liquids could have a very favourable role in the stabilization of Pd(II) species and improvement of catalyst reusability.

Efficient small molecule photovoltaic donor based on 2,3-diphenyl-substituted quinoxaline core for solution-processed organic solar cells

A novel D–π–A–π–D-type organic small molecule (OSM) named (TPACN)2Qx was designed and synthesized for solution-processible organic solar cells (OSCs), which contained 2,3-diphenyl-substituted quinoxaline (Qx) as the central acceptor (A) unit, triphenylamine (TPA) as terminal electron-donor (D) moieties, and acrylonitrile as the π-linkage segment. With an effective electron-withdrawing property and relatively stable quinoid geometry of Qx, (TPACN)2Qx exhibits a narrow band-gap of 1.88 eV and strong and broad absorption at 300–700 nm. As a consequence, an excellent power conversion efficiency (PCE) of 6.25% was achieved based on the (TPACN)2Qx:PC61BM blend using a simple spin-coating process in a solution, which is the highest efficiency achieved to date for Qx-core based solution-processed OSM photovoltaic (PV) devices. The impressive result demonstrates that OSMs employing quinoxaline derivative as an electrophilic unit can compete with their polymer counterparts.

Nonlinear Vibration and Bifurcation Analysis for a Rotor with Seal and Bearing Excitations

This paper presents the nonlinear coupling vibration and bifurcation of a rotor system excited by both seal and journal bearing excitations. The rotor system is modeled as a Jeffcott rotor. The nonlinear transverse vibration of the Jeffcott rotor with labyrinth seal and two supporting journal bearings are investigated. The seal force with multiple parameters is presented by the Muszynska’s model [1, 2]. The excitation of unsteady oil-film forces is used based on the model proposed by Zhang [3]. In light of numerical computation, the Runge-Kutta method is used to determine the vibration responses at the disc center and the two bearings. When the rotation speed and the seal pressure change, bifurcation diagrams are shown, which prove the complexity of the rotor motions. There is an example with two different seal pressure pointing out the multiple periodic bifurcations. The complex bifurcation behavior explains inherent interactions between forces of oil-film and seal, which indicates much more intricate rotor dynamics than the one with either of the excitations alone.