Phase‐Separation‐Driven Nanostructure Control for Highly Efficient Room‐Temperature Spin Transport in Molecular Spintronic Devices

Molecular semiconductors (MSCs) intrinsically exhibit weak spin–orbit coupling and suppressed spin relaxation, making them promising candidates for efficient room‐temperature (RT) spin transport and semiconductor‐based electronic device functionalities. However, the RT magnetoresistance, corresponding to the spin transport efficiency ( η s ) of MSC‐based devices has long been limited to ≈5%, highlighting the impact of extrinsic, morphology‐induced spin relaxation in molecular thin films. Here, we present a device‐compatible solid‐state nanostructure engineering strategy based on phase separation in blended MSC/insulating‐polymer systems to construct nanostructured transport channels and suppress extrinsic spin relaxation at RT. By systematically tuning the polymer blending ratio and molecular weight, spin relaxation is strongly suppressed while charge transport is concurrently regulated. At an optimized polymer content of 15 wt%, η s increases threefold to 12.26%, corresponding to a more than 16‐fold reduction in spin relaxation. Further increasing the polymer molecular weight enhances phase separation, yielding more well‐defined nanostructured MSC channels with simultaneously suppressed spin relaxation and improved carrier diffusion, ultimately achieving a record‐high RT η s of 18.64%, together with excellent environmental and operational stability. These results establish phase‐separation‐enabled nanostructure engineering as a robust and general strategy for realizing highly efficient RT spin transport in molecular spintronic devices.