Perovskite photovoltaics have attracted remarkable attention recently due to their exceptional power conversion efficiencies (PCE). The quality of perovskite thin film is an important key for high-performance solar cells. For perovskite absorbers derived from solution-based deposition, thermal annealing is typically required to remove solvents and to achieve high crystallinity of the films. However, the annealing process can reduce device fabrication yield and the energy input increases the device pay-back time. Additionally, the thermal annealing may also hinder application of perovskite technology in tandem photovoltaics and flexible optoelectronics. Therefore, developing room-temperature method to deposit high-quality perovskite films is necessary. Here, we report an additive-based process to obtain high-quality methylammonium lead iodide films with micron-sized grains (>2 mm) and microsecond carrier lifetimes (τ1= 931.94 ± 89.43 ns; τ2 = 320.41 ± 43.69 ns) at room temperature. Solar cells employing such films demonstrate 18.22% PCE with significantly improved current-voltage hysteresis and stability without encapsulation. Moreover, we find that the grain size in perovskite film from solution process strongly depends on the precursor aggregate size in the film-deposition solution and tuning the aggregate properties enables enlarging grains to the micron scale. These results offer a new pathway for more versatile, cost-effective perovskite processing.