The vertical composition distribution and crystallinity of photoactive layers are considered to have critical roles in photovoltaic performance. In this concise contribution, the layer-by-layer (LBL) solution process is used to fabricate efficient polymer solar cells. The results show that the vertical composition distribution can be finely regulated via employing solvent additive 1,8-diiodooctane (DIO). The favorable vertical component distribution in tandem with improved crystallinity induced by DIO contributes to the efficient exciton dissociation, charge transportation and extraction, and limited charge recombination loss. Therefore, the optimized LBL devices yield an efficiency of 16.5%, which is higher than that of the control bulk heterojunction solar cells with an efficiency of 15.8%. Importantly, the ternary solar cells based on PM6/ Y6:PC 71 BM LBL active layers demonstrate a promising efficiency of >17%, which is the record efficiency for LBL solar devices reported to date. These findings make clear that the solvent additive-assisted LBL solution process has broader implications for the further optimization of solar cells.

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