Precisely Controlling Polymer Acceptors with Weak Intramolecular Charge Transfer Effect and Superior Coplanarity for Efficient Indoor All‐Polymer Solar Cells with over 27% Efficiency

Indoor photovoltaics (IPVs) are garnering increasing attention from both the academic and industrial communities due to the pressing demand of the ecosystem of Internet‐of‐Things. All‐polymer solar cells (all‐PSCs), emerging as a sub‐type of organic photovoltaics, with the merits of great film‐forming properties, remarkable morphological and light stability, hold great promise to simultaneously achieve high efficiency and long‐term operation in IPV's application. However, the dearth of polymer acceptors with medium‐bandgap has impeded the rapid development of indoor all‐PSCs. Herein, a highly efficient medium‐bandgap polymer acceptor (PYFO‐V) is reported through the synergistic effects of side chain engineering and linkage modulation and applied for indoor all‐PSCs operation. As a result, the PM6:PYFO‐V‐based indoor all‐PSC yields the highest efficiency of 27.1% under LED light condition, marking the highest value for reported binary indoor all‐PSCs to date. More importantly, the blade‐coated devices using non‐halogenated solvent (o‐xylene) maintain an efficiency of over 23%, demonstrating the potential for industry‐scale fabrication. This work not only highlights the importance of fine‐tuning intramolecular charge transfer effect and intrachain coplanarity in developing high‐performance medium‐bandgap polymer acceptors but also provides a highly efficient strategy for indoor all‐PSC application. Herein, a successful design strategy of medium‐bandgap polymer acceptor, PYFO‐V, is presented for indoor all‐polymer solar cell operation. By the synergy of alkoxy chains and vinylene spacers onto Y‐series polymer acceptors, PYFO‐V exhibits blue‐shifted absorption and upshifted energy levels. The binary devices realize an impressive efficiency of 27.1% under indoor illumination with great photostability, beneficial for the Internet‐of‐Things application.