5,6-Difluorobenzothiazole-Based Conjugated Polymers with Large Band Gaps and Deep Highest Occupied Molecular Orbital Levels

A 5,6-difluorobenzothiazole-based dibromo monomer was successfully synthesized, from which new fluorinated conjugated polymers PF-ffBTz and PFN-ffBTz were prepared via copolymerizations with two fluorene-based diboronic ester monomers. Twisted fluorene-ffBTz backbones enable PF-ffBTz and PFN-ffBTz with large band gaps up to 3.10 eV and deep-lying highest occupied molecular orbital levels down to −6.2 eV. The chemical structures of PF-ffBTz and PFN-ffBTz impart some new functionalities of fluorinated conjugated polymers. PF-ffBTz can show deep blue electroluminescent emission, with high external quantum efficiency of 3.71%. PFN-ffBTz, with amino-functionalized side chains on the fluorene unit, can serve as an efficient cathode interlayer in inverted polymer solar cells (PSCs), showing better photovoltaic performances if compared with a ZnO interlayer. In addition, it is found that using an optical filter to cut off the short wavelength section (≤380 nm) of incident light can significantly elevate photostability of PSCs under continuous illumination

Fused Perylene Diimide-Based Polymeric Acceptors for Efficient All-Polymer Solar Cells

Two polymeric electron acceptors (PFPDI-2T and PFPDI-2FT) based on the fused perylene diimide (PDI) and bithiophene or difluoro­bithiophene units were synthesized via the Stille polymerization. Both polymers exhibit the strong absorption between 350 and 650 nm, which have the good absorption compensation with the low band gap conjugated polymer in polymer solar cells (PSCs). PFPDI-2T and PFPDI-2FT have the LUMO energy levels of around −4.12 to −4.15 eV, which are comparable with other PDI-based polymers and fullerene derivatives. All-polymer solar cells (all-PSCs) based on PFPDI-2T or PFPDI-2FT as the polymeric electron acceptor were fabricated with PTB7-Th as the polymeric electron donor. Power conversion efficiency of as high as 6.39% based on PFPDI-2T/PTB7-Th was achieved under the standard illumination of simulated sunlight (AM 1.5, 100 mW cm^–2), which is significant higher than that of the all-PSC based on the nonfused PDI counterpart. The results demonstrate that the direct fusion of PDI unit is an effective design strategy to enhance the photovoltaic performances of all-PSCs

Fine-Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene Organic Solar Cells Based on Perylene Diimides

The geometries of acceptors based on perylene diimides (PDIs) are important for improving the phase separation and charge transport in organic solar cells. To fine-tune the geometry, biphenyl, spiro-bifluorene, and benzene were used as the core moiety to construct quasi-three-dimensional nonfullerene acceptors based on PDI building blocks. The molecular geometries, energy levels, optical properties, photovoltaic properties, and exciton kinetics were systematically studied. The structure–performance relationship was discussed as well. Owing to the finest phase separation, the highest charge mobility and smallest nongeminate recombination, the power conversion efficiency of nonfullerene solar cells using PDI derivatives with biphenyl core (BP-PDI₄) as acceptor reached 7.3% when high-performance wide band gap donor material poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene))-<i>alt</i>-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-<i>c</i>:4′,5′-<i>c</i>′]­dithiophene-4,8-dione))] was blended