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Layer‐by‐layer (LbL) coating is becoming a widely used method to fabricate nonfullerene active layer films for organic solar cells. However, the vertical compositional distribution of the LbL‐coated active layer, particularly at the buried bottom surface, is not clear yet. In common sense, it is believed that the LbL‐coating yields a donor–mixture–acceptor (D–m–A) vertical distribution in the active layer, i.e., a thin polymer donor layer at the bottom surface, a thin acceptor layer at the top surface and a donor‐acceptor mixture in the middle. In this study, it is shown that the LbL active layer vertically is an entire donor:acceptor mixture. A pure layer of polymer donor doesn't exist at the bottom surface. The LbL active layer delivers high performance in both conventional and inverted device structures. A thin polymer layer with different thicknesses (2, 6, 12 nm) is inserted at the bottom surface to study their effects on the device performance. Those inserted layers substantially deteriorate the device's performance. Furthermore, the assumption is further confirmed by X‐ray photoelectron spectroscopy measurement on the exposed “originally buried” surface. This study sheds light on understanding the vertical compositional distribution of active layer via layer‐by‐layer solution processing.

Poly(3,4-ethylenedioxythiophere):poly(styrenesulfonate) (PEDOT:PSS) is a common solution-processed electrode for printable organic solar cells. However, the water-dispersed PEDOT:PSS displays de-wetting issues when deposited on active layer, which limits its application on printable...