No abstract

Two triimide‐functionalized n‐type acceptor polymers are designed and synthesized, which show narrower bandgap, lower‐lying frontier molecular orbital energy levels, and improved film morphology than the diimide‐functionalized analogue polymers. When blended with a p‐type donor polymer semiconductor PTB7‐Th, an outstanding power conversion efficiency of 8.98% with a remarkable open‐circuit voltage of 1.03 V is attained. This efficiency is among the highest values in all‐polymer solar cells (all‐PSCs) reported till today, surpassing that (6.85%) of the diimide‐functionalized analogue polymers by a big margin and even higher than that (8.69%) of the fullerene‐based solar cells. The results demonstrate that the triimide‐functionalized f‐BTI3 is an excellent building block for developing n‐type polymer semiconductors, and the polymer f‐BTI3‐T is among the best‐performing n‐type polymers for applications in all‐PSCs. The structure–property correlations of these imide‐functionalized polymer semiconductors offer important guides for developing high‐performance n‐type polymer semiconductors.

Two new bithiophene imide (BTI)‐based n‐type polymers were synthesized. f‐BTI2‐FT based on a fused BTI dimer showed a smaller band gap, a lower LUMO, and higher crystallinity than s‐BTI2‐FT containing a BTI dimer connected through a single bond. s‐BTI2‐FT exhibited a remarkable electron mobility of 0.82 cm2 V−1 s−1, and f‐BTI2‐FT showed a further improved mobility of 1.13 cm2 V−1 s−1 in transistors. When blended with the polymer donor PTB7‐Th, f‐BTI2‐FT‐based all‐polymer solar cells (all‐PSCs) attained a PCE of 6.85 %, the highest value for an all‐PSC not based on naphthalene (or perylene) diimide polymer acceptors. However, s‐BTI2‐FT all‐PSCs showed nearly no photovoltaic effect. The results demonstrate that f‐BTI2‐FT is one of most promising n‐type polymers and that ring fusion offers an effective approach for designing polymers with improved electrical properties.

Spin coating is a simple process for rapidly depositing thin, solid polymeric films onto relatively flat substrates. Evaporation occurs during spinning of the relatively volatile species in any solution. The curing behavior of spin-coated polymeric film is influenced by the evaporation of any reactive component. An investigation was carried out on a silicon substrate to study the effects of spin coating on the curing behavior of the epoxy adhesive. The degree of curing for both spin and without spin-coated epoxy adhesive was measured by Fourier-transform infrared spectroscopy (FTIR). A slower curing reaction rate was observed for the spin-coated epoxy adhesive. The composition gradient established by solvent evaporation during spinning is responsible for the slower curing reaction rate of the spin-coated epoxy adhesive. From this study, it is proposed to use solvents that are less volatile and allow a greater part of the thinning behavior to occur without significant changes in the fluid properties during the spinning process.

Spin coating is a common method for depositing very thin polymeric film across a planar surface in a short period of time. Thinning occurs due to the combined effects of centrifugal spin-off and evaporation. The evaporation of any reactive component during spinning plays an important role on the stability of spin-coated polymeric film. An investigation was carried out to study the effects of spinning on the thermal and chemical stability of the epoxy adhesive. The thermal stability of both spin-coated and without spin-coated epoxy adhesive was measured by thermogravimetric analysis (TGA) at heating rate of 10 °C/min in an inert environment. A lower thermal stability was observed for the spin-coated epoxy adhesive. At the center of the substrate it is more stable than the other locations. Thermal stability greatly deviates at the border side of the spin-coated substrate. Higher chemical stability was also observed at the center than the other locations of the spin-coated layer when immersed in the metal (nickel) etchant chemical solution. The lower thermal and chemical stability is mainly due to changes in the material properties during the spinning process. From this study it is proposed to use the reactive components that are less volatile, having higher intermolecular forces, and allow a greater part of the thinning behavior to occur without significant changes in the fluid properties during the spinning process. Lower spin speed also suggested to reduce the mechanical degradation of the polymeric adhesive for the fabrication of a polymer optical waveguide.