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N,N-Dimethylformamide (DMF) has been shown to be an efficient precursor solvent for the one-step deposition of perovskite thin films in photovoltaic applications. Here, the specific advantage DMF introduces during the perovskite crystallization process is elucidated through comparison with dimethylacetamide (DMAc), one of its homologues. The unique presence of a DMF-induced intermediate phase was verified for the first time and its positive functions to inhibit uncontrolled perovskite precipitation and facilitate homogeneous nucleation were demonstrated. When combined with a double blocking layer structure to prevent shunting, our planar heterojunction (PHJ) perovskite solar cells achieved a high power conversion efficiency of up to 13.8%. Our results uncover the origin of the widespread adoption of DMF in perovskite thin film deposition, and represent a helpful step towards judicious perovskite morphological control. [ABSTRACT FROM AUTHOR]