Cs2AgBiBr6, has recently gained wide attention as a possible alternative to lead-halide perovskites, considering the nontoxicity and improved stability. However, this double perovskite suffers from defects, especially deep electron traps, severely hampering the photovoltaic performance. This work reports a simple method to control the double perovskite crystallization by adding volatile salts into the precursor solution. X-ray diffraction patterns reveal that the organic cation with suitable radius (such as methylammonium, MA+) is introduced into the perovskite lattice, forming an organic/inorganic mixed double perovskite intermediate phase. The organic salt is thereafter fully evaporated during high temperature annealing, and the all-inorganic double perovskite is obtained with dense surface and less pin-holes. From optical and electrical characterization, it is concluded that the Cs2AgBiBr6 film exhibits high quality, with higher light absorptance and emission. Reduced trap density and longer carrier lifetime are also observed. The improved Cs2AgBiBr6 film is beneficial for efficient carrier collection with suppressed defect-assisted recombination. With this strategy, a power conversion efficiency (PCE) of 2.53% is achieved for the champion Cs2AgBiBr6-based solar cell device, which is significantly higher compared to the control device with 1.43% PCE. This work is therefore helpful for further improvement of inorganic lead-free perovskite materials for optoelectronic applications.