 Highlights a solid additive assisted layer by layer, SAALBL, processing was developed to facilitate the inter diffusion between polymer donor and acceptor and optimize the morphology of quasi planar heterojunction for high performance organic solar cells, OSCs. The pre-phase separation between fatty acid and polymer donor, easily tuned via controlling the cohesive energy, is critical to form the desired vertical phase separation morphology. The SAALBL is generally applicable to various OSC systems and enables the record efficiency of 19.02% among the binary OSCs. This article was authored by Guan Yu Ding, Tian Yi Chen, Meng Ting Wang and others.