An effective electron transport layer (ETL) plays a pivotal role in suppressing nonradiative recombination at the interface as well as promoting perovskite crystallization to facilitate electron extraction in perovskite solar cells (PSCs). Herein, a functional polymer, poly(amidoamine) (PM) dendrimer, is introduced to rationalize the morphology and electrical performance of SnO2 nanocrystals to construct an SnO2 charge transfer layer. PM offers an even SnO2 colloidal dispersion with a particle-size distribution of ∼10 nm, which prevents the agglomeration of nanocrystals significantly. The polymer-complexed SnO2 provides a uniform and dense ETL film without island-like agglomeration, yielding a large conductive layer superior to that of the control. Equally important, the wettability-improved SnO2 ETL with PM modification produces a high-quality perovskite film with larger grain size, resulting in a power conversion efficiency (PCE) of the champion n-i-p PSC of up to 22.93% with negligible hysteresis. Noticeably, the device based on SnO2-PM maintained 71% of its initial PCE (only 49% for the control device) after storing in the ambient environment for 45 days (relative humidity of 30%-80%) without packaging.