Synthesis and Photovoltaic Effect of Electron-Withdrawing Units for Low Band Gap Conjugated Polymers Bearing Bi(thienylenevinylene) Side Chains

Boosting the Power Factor of Benzodithiophene Based Donor-Acceptor Copolymers/SWCNTs Composites through Doping

In this study, a benzodithiophene (BDT)-based donor (D)–acceptor (A) polymer containing carbazole segment in the side-chain was designed and synthesized and the thermoelectric composites with 50 wt % of single walled carbon nanotubes (SWCNTs) were prepared via ultrasonication method. Strong interfacial interactions existed in both of the composites before and after immersing into the 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) solution as confirmed by UV-Vis-NIR, Raman, XRD and SEM characterizations. After doping the composites by F₄TCNQ, the electrical conductivity of the composites increased from 120.32 S cm⁻¹ to 1044.92 S cm⁻¹ in the room temperature. With increasing the temperature, the electrical conductivities and Seebeck coefficients of the undoped composites both decreased significantly for the composites; the power factor at 475 K was only 6.8 μW m⁻¹ K⁻², which was about nine times smaller than the power factor at room temperature (55.9 μW m⁻¹ K⁻²). In the case of doped composites, although the electrical conductivity was deceased from 1044.9 S cm⁻¹ to 504.17 S cm⁻¹, the Seebeck coefficient increased from 23.76 μV K⁻¹ to 35.69 μW m⁻¹ K⁻², therefore, the power factors of the doped composites were almost no change with heating the composite films.

Substantial Improvement of the Dielectric Strength of Cellulose-Liquid Composites: Effects of Traps at the Nanoscale Interface

The dielectric strength of cellulose-liquid composites is always about several times higher than that of the cellulose paper and insulating liquids. However, this experimental phenomenon has not yet been demonstrated theoretically. Herein, the spectra characterization, molecular simulation, and wave function analysis method provide a new insight that the role of nanoscale interfacial adsorption of cellulose-liquid is exclusive for composites affecting the charge separation and producing the deep-level traps to seriously hinder electromigration under an electric field, which is responsible for the difference in dielectric strength. Meanwhile, the π conjugation and σ-π hyperconjugation effects enhance the electrical stability of aromatic hydrocarbon insulating liquids. In conclusion, interfacial trap theory can be used to explain the correlation of dielectric strength between cellulose-liquid composites and cellulose paper or dielectric liquids. It can be expected that materials with high dielectric strength can be manufactured according to the fundamental study of interfacial trap theory.

Fluorination Effect for Highly Conjugated Alternating Copolymers Involving Thienylenevinylene-Thiophene-Flanked Benzodithiophene and Benzothiadiazole Subunits in Photovoltaic Application

Two two-dimensional (2D) donor-acceptor (D-A) type conjugated polymers (CPs), namely, PBDT-TVT-BT and PBDT-TVT-FBT, in which two ((E)-(4,5-didecylthien-2-yl)vinyl)- 5-thien-2-yl (TVT) side chains were introduced into 4,8-position of benzo[1,2-b:4,5-b']dithiophene (BDT) to synthesize the highly conjugated electron-donating building block BDT-TVT, and benzothiadiazole (BT) and/or 5,6-difluoro-BT as electron-accepting unit, were designed to systematically ascertain the impact of fluorination on thermal stability, optoelectronic property, and photovoltaic performance. Both resultant copolymers exhibited the lower bandgap (1.60 ~ 1.69 eV) and deeper highest occupied molecular orbital energy level (EHOMO, -5.17 ~ -5.37 eV). It was found that the narrowed absorption, deepened EHOMO and weakened aggregation in solid film but had insignificant influence on thermal stability after fluorination in PBDT-TVT-FBT. Accordingly, a PBDT-TVT-FBT-based device yielded 16% increased power conversion efficiency (PCE) from 4.50% to 5.22%, benefited from synergistically elevated VOC, JSC, and FF, which was mainly originated from deepened EHOMO, increased μh, μe, and more balanced μh/μe ratio, higher exciton dissociation probability and improved microstructural morphology of the photoactive layer as a result of incorporating fluorine into the polymer backbone.