Polymer self-assembly offers an important route to construct well-defined nanostructures. However, it remains challenging to assemble polymers into vertically oriented nanostructures. Here, we use a seed-induced confinement self-assembly strategy to construct vertically aligned semiconducting nanobrushes from polyfluorene-based polymers on conductive substrates. Mechanism studies elucidate that the immobilized seeds on the substrate initiate the vertical growth of nanobrushes, and supercritical drying as well as the rigid charged coronas collectively contribute to retaining the vertical architecture. This process enables nanobrushes with ∼40× higher charge mobilities than their bulk film counterparts. Thus, inverted organic solar cells using the nanobrushes as the electron transporting layer (ETL) exhibit a record power conversion efficiency of 18.51% as a result of increased ETL texturing and the ETL-active layer interface favoring electron extraction. Moreover, our approach also enables the uniform growth of nanobrushes on a nanoporous photoanode (bismuth vanadate) for photoelectrochemical water splitting, improving catalyst distribution and electron transfer. Our work presents a feasible approach to fabricating challenging vertical polymer nanostructures, thereby unlocking the tremendous potential of conjugated polymers in optoelectronic applications.