Two‐dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar‐to‐hydrogen energy conversion owing to their pre‐designable structures and tailor‐made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py‐HTP‐BT‐COF) via chlorination (Py‐ClTP‐BT‐COF) and fluorination (Py‐FTP‐BT‐COF) can lead to dramatically enhanced photocatalytic H2 evolution rates (HER=177.50 μmol h−1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py‐ClTP‐BT‐COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar‐to‐chemical energy conversion.

Herein, trifluoromethylation has proven to be an effective strategy for ultra-narrow band-gap NFAs. A PCE of 15.59% is achieved from BTIC-CF 3 -g-based devices, which is the highest value in reported ultra-narrow band-gap acceptors. A ternary device with 16.50% efficiency is also obtained, resulting from its red-shifted absorption. Meanwhile, the single-crystal structure of BTIC-CF 3 -g has been successfully presented, which gives a deep understanding of the solid-state molecular packings in these highly efficient acceptors.

Halogenation is an effective way to tune the energy levels of organic semiconducting materials. To date, fluorination of organic semiconducting materials to fabricate polymer solar cells (PSCs) has been used far more than chlorination; however, fluorine exchange reactions suffer from low yields and the resulting fluorinated polymer always comes with a higher price, which will greatly hinder their commercial applications. Herein, we designed and synthesized a series of chlorinated donor–acceptor (D-A) type polymers, in which benzo­[1,2-b:4,5-b]­dithiophene and chlorinated benzothiadiazole units are connected by thiophene π-bridges with an asymmetric alkyl chain. These chlorinated polymers showed deep highest occupied molecular orbital (HOMO) energy levels, which promoted the efficiency of their corresponding PSCs by increasing the device open circuit voltage. The asymmetric alkyl chain on the thiophene moieties gave the final polymer sufficient solubility for solution processing and strong π–π stacking in films allowed for high mobility. Although the introduction of a large Cl atom increased the torsion angle of the polymer backbone, the chlorinated polymers maintained a favorable backbone orientation in blend films for efficient PSC application. These factors contributed to respectable device performances from thick-film devices, which showed PCEs as high as 9.11% for a 250-nm-thick active layer. These results demonstrate that chlorination is a promising method to fine-tune the energy levels of conjugated polymers, and chlorinated benzothiadiazole may be a versatile building block in materials for efficient solar energy conversion.

A chlorinated acceptor with 2-butyloctyl side chains and a 3D interpenetrated structure in the single crystal shows excellent PCE up to 16.43%.