Abstract Tin dioxide (SnO 2 ) electron transport layer (ETL) has become the most commonly used ETL in perovskite solar cells due to its excellent photoelectronic properties. Chemical bath deposition (CBD) is a common method for preparing SnO 2 ETL. However, the main challenge for this method is the poor reproducibility. Clarifying the key mechanisms that affect the growth of SnO 2 during the CBD process is crucial for building high‐quality and highly reproducible SnO 2 ETL. In this work, it is identified that the intrinsic limitations in SnO 2 ETL performance and reproducibility stem from the competitive coexistence of homogeneous and heterogeneous nucleation pathways. To address this challenge, a pre‐nucleation strategy to enable single heterogeneous nucleation‐driven SnO 2 deposition is proposed. This is achieved by employing a pre‐nucleated precursor solution that has already undergone homogeneous nucleation for the deposition of SnO 2 ETL. The resultant ETL exhibits ultrathin conformality, high surface coverage, uniform thickness, and reduced defect‐state density. This strategy not only enhances device performance but also significantly improves process reproducibility. Consequently, the hole transport layer‐free carbon‐based perovskite solar cells achieve a record efficiency of 21%. Furthermore, the device demonstrates remarkable operational stability, retaining 98% of the initial efficiency after 1600 h of continuous maximum power point operation.
Lin et al. (2025) studied this question.