This work studied the influence of the methylammonium iodide/lead iodine precursor ratio on the perovskite film morphology and device performance. Using a non-stoichiometric precursor solution was demonstrated to be critical to form stoichiometric perovskite films. The compositions of the spun perovskite films were very sensitive to the surface of substrates, and can be very different from that in precursor solutions. Remarkably, we found that the unique double fullerene layers adopted could dramatically reduce dark current leakage by forming a Schottky junction with the anode, and effectively passivate traps in perovskite to increase the efficiency by boosting the fill factor to above 80% for perovskite solar cells.