Quasi-two-dimensional (2D) perovskites are highly promising light-harvesting materials for commercialization of perovskite solar cells (PSCs) owing to the excellent materials stability. However, the coexistence of multiple n-value species in 2D perovskites often causes increased complexities in crystallization that can negatively affect the eventual photovoltaic performance. Herein, we present a binary solution based strategy via introducing nontoxic and widely accessible CH₃COOH (HAc) as a co-solvent for preparing high-quality 2D perovskite films. Based on a 2D perovskite model system, (AA)₂MA₄Pb₅I₁₆ (n = 5), we show that the prenucleation and grain growth kinetics are appreciably modified with HAc, which benefits from the strong electron-donating ability of HAc with the key component of PbI₂, leading to formation of favorable cluster aggregates and resultant modulation of crystal growth. With the HAc-based method, the devices yield a boosted photovoltaic efficiency of 18.55% with an impressive photovoltage of 1.26 V. The champion cells exhibit a supreme thermal stability, showing <3% efficiency degradation under continuous thermal aging for 800 h.