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A double‐fibril network of the photoactive layer morphology is recognized as an ideal structure facilitating exciton diffusion and charge carrier transport for high‐performance organic solar cells (OSCs). However, in the layer‐by‐layer processed OSCs (LbL‐OSCs), polymer donors and small molecule acceptors (SMAs) are separately deposited, and it is challenging to realize a fibril network of pure SMAs with the absence of tight interchain entanglement as polymers. In this work, crystalline small molecule donors (SMDs), named TDZ‐3TR and SeDZ‐3TR, were designed and introduced into the L8‐BO acceptor solution, forcing the phase separation and molecular fibrilization. SeDZ‐3TR showed higher crystallinity and lower miscibility with L8‐BO acceptor than TDZ‐3TR, enabling more driving force to favor the phase separation and better molecular fibrilization of L8‐BO. On the other hand, two donor polymers of PM6 and D18 with different fibril widths and lengths were put together to optimize the fibril network of the donor layer. The simultaneously optimization of the acceptor and donor layers resulted in a more ideal double‐fibril network of the photoactive layer and an impressive power conversion efficiency (PCE) of 19.38 % in LbL‐OSCs. [ABSTRACT FROM AUTHOR]