The power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film‐depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi‐empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6‐based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film‐depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well‐mixed ternary BHJ. Consequently, under this design guideline, the optimized film‐depth‐dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.

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Conjugated polymers are of interest for next‐generation electronics due to their improved flexibility, low cost, and solution processability. However, a conjugated polymer is not a chemically “pure” material because the polymer chains are polydispersed in terms of molecular weights and/or chemical stereoregularities, leading to dispersions of both localized order and electronic properties. Taking advantage of these properties, a one‐step self‐refinement method is proposed to induce film‐depth‐resolved composition and aggregation gradient of conjugated‐polymer@insulator‐matrix for high‐performance organic field‐effect transistors and nonvolatile memories. During solvent evaporation, the higher ordered sub‐components of conjugated polymers are substantially enriched at the top surface of the blend film to form a morphologically continuous sublayer serving as high‐quality transport pathways to warrant high‐performance transistors. The less ordered sub‐components are distributed within insulator‐matrix in the bottom part of the film without connection with each other, which guarantees a reliable composite electret to help store sufficient immobilized charges to tune the operation of the device. Moreover, these conjugated‐polymer@insulator‐matrix films with insulator content as high as 95% show excellent optical transparency (>90%) and environmental stability (for months), which demonstrates great potential for use in transparent electronics.