Modulating Coplanarity of Polymer Acceptor Facilitates Hierarchical All‐Polymer Heterojunction for Efficient and Stable Semitransparent Solar Cells

All‐polymer‐based semitransparent organic solar cells (ST‐OSCs) are better‐suited candidates than small‐molecule analogues for building‐integrated photovoltaics due to their superior operational stability. Their deployment is however hindered by low light utilization efficiency (LUE), mainly ascribed to intimating morphological control within bulk‐heterojunction architectures. To mitigate these constraints, here we synergize the polymer donor‐layer thinning strategy with a novel star‐shaped polymer acceptor in a quasi‐planar heterojunction (QPHJ) structure. Two such acceptors, PYBSe‐B and PYBSe‐L, are designed and synthesized with twisted π‐backbones based on a sterically hindered BTSe core flanked with tailorable side‐chains to regulate molecular planarity and self‐aggregation behavior. By inclusion into the bottom layer of PM6 donor in QPHJs, the relatively planar PYBSe‐L is aggregated between the PM6 domains whereas the amorphous PYBSe‐B is well‐mixed. PYBSe‐L is found to expand interdomain spacing and promote infiltration of the upper PYIT acceptor, strengthening optical transmittance and exciton dissociation. The champion devices achieved an efficiency of 11.16% and a record LUE of 4.80% for all‐polymer ST‐OSCs. Remarkably, the rigid hierarchical polymer–infiltration network formed by the QPHJ structure greatly reinforced storage stability of the unencapsulated device with an extrapolated T 80 lifetime (degradation to 80% of initial LUE values) of over 1310 h under ambient conditions.