Developing Y‐Branched Polymer Acceptor with 3D Architecture to Reconcile Between Crystallinity and Miscibility Yielding >15% Efficient All‐Polymer Solar Cells

In all‐polymer solar cells (all‐PSCs), there remains such a dilemma that obtains good miscibility and crystallinity simultaneously. Herein a new family of Y‐shape polymer acceptor, namely PYTT is developed, which is copolymerized from Y6 and benzotrithiophene units in three‐way directions. Benefiting from its high‐density end‐chains and extended π‐conjugation thanks to highly‐branched 3D architecture, PYTT displays better organic solubility despite much higher molecular weights, larger crystallinity, and tighter π‐stacking than the linear counterpart—PYT comprising Y6 and thiophene moieties, while showing identical optical absorption yet threefold higher photoluminescence intensity. In PYTT blend film with PM6 polymer donor, the interpenetrating nano‐fibrillar structures are formed with well‐intermixed polymeric domain sizes close to the exciton diffusion length, which is greatly conducive to exciton dissociation and charge transport in device. Consequently, PYTT‐based all‐PSCs exhibit all increased photovoltaic parameters, yielding a decent power conversion efficiency of 15.60%, which is ≈20% enhancement over PYT‐based device, along with low nonradiative loss of 0.221 meV. Y‐branched polymer acceptor—PYTT featuring 3D highly branched structure displays good organic solubility yet strong crystallinity. When paired with PM6 donor, the reduced domain size is close to the exciton diffusion length in blend film. Thus, PYTT‐based all‐polymer solar cells exhibit an impressive power conversion efficiency of 15.60% and its high tolerability on the active layer thickness along with good photostability.