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Abstract 2D‐on‐3D (2D/3D) perovskite heterostructures present a promising strategy to realize efficient and stable photovoltaics. However, their applicability in inverted solar cells is limited due to the quantum confinement of the 2D‐layer and solvent incompatibilities that disrupt the underlying 3D layer, hampering electron transport at the 2D/3D interface. Herein, solvent‐dependent formation dynamics and structural evolution of 2D/3D heterostructures are investigated via in situ X‐ray scattering. It is revealed that solvent interaction with the 3D surface determines the formation sequence and spatial distribution of quasi‐2D phases withn= 2–4. Isopropanol (IPA) reconstructs the perovskite into a PbI₂‐rich surface, forming a strata with smallernfirst, followed by a thinner substratum of largern. In contrast, 2,2,2‐Trifluoroethanol (TFE) preserves the 3D surface, promoting the formation of uniformly distributed largerndomains first, and smallernlast. Leveraging these insights, Dion–Jacobson perovskites are used with superior charge transport properties and structural robustness to fabricate 2D/3D heterostructures dominated byn≥ 3 and engineer a favorable energy landscape for electron tunneling. Inverted solar cells based on 3‐Aminomethylpyridine and TFE achieve a champion efficiency of 23.60%, withV_ocand FF of 1.19 V and 84.5%, respectively, and superior stabilities witht₉₄of 960 h under thermal stress.