Complementary Absorption Pseudo‐Ternary Blend Containing a Y‐Series Block Copolymer Acceptor for Indoor and Outdoor All‐Polymer Photovoltaics

All‐polymer photovoltaics (all‐PPVs) that operate under both indoor and outdoor lighting conditions require active layers with appropriately adjusted optical‐absorption ranges. However, the optical absorption of a conventional donor–acceptor binary blend is restricted to the combined absorption bands of its components. Herein, a new conjugated block copolymer (CBC) acceptor, b ‐PYT, is designed by integrating polymer acceptor blocks of wide and narrow bandgaps in a single structure. Such combination results in the wide absorption range (550–850 nm) of b ‐PYT that matches the emission of both artificial and solar light. The b ‐PYT CBC acceptor is more crystalline than the corresponding random terpolymer, r ‐PYT, owing to improved interactions between its macromolecular acceptor units. Despite exhibiting slightly inferior outdoor performance compared to that of devices using the homopolymer BTTP‐T, the PM6: b ‐PYT‐based devices deliver superior power conversion efficiency (PCE) under indoor light‐emitting diode (LED) light owing to better matched absorption and emission spectra of b ‐PYT and a cold white LED, respectively. Additionally, it is worth highlighting that PM6: b ‐PYT‐based all‐PPVs can maintain approximately 87% of the initial PCE even after 600 min of thermal aging at 150 °C, which demonstrates the superior thermal stability compared with those of all‐PPVs that use traditional binary active layers.