Organic semiconductors are generally featured with low structure order in solid-state films, which leads to low charge-transport mobility and strong charge recombination in their photovoltaic devices. In this work, a "polycrystal-induced aggregation" strategy orders the polymer donor (PM6) and non-fullerene acceptor (L8-BO) molecules during solution casting with the assistance of PM6 polycrystals that are incubated through a vapor diffusion method, toward improved solar cell efficiency with either thin or thick photoactive layers. These PM6 polycrystals are redissolved in chloroform to prepare PM6 pre-aggregates (PM6-PA), and further incorporated into the conventional PM6:L8-BO blend solutions, which is found to prolong the molecular organization process and enhance the aggregation of both the PM6 and the L8-BO components. As the results, with the assistance of 10% PM6-PA, PM6:L8-BO solar cell devices obtain power conversion efficiencies (PCEs) from 18.0% and 16.2% to 19.3% and 17.2% with a 100 nm-thick and 300 nm-thick photoactive layer, respectively.