We present a simple method for improving the thermal stability of organometal halide perovskite crystals that also improves the power conversion efficiencies of the associated perovskite solar cells. We demonstrate that the thermal degradation of perovskite crystals occurs predominantly at their grain boundaries due to the migration of components. To improve the thermal stability of the perovskite crystal, phenyl-C-butyric acid methyl ester (PCBM) is added to the perovskite precursor. It is observed that the decay of the power conversion efficiency of the solar cells slows and that the generation of the decomposed product PbI2 decreases as the amount of PCBM added increases. Moreover, by varying the grain size of perovskite crystals through the use of hot-casting method, we reveal that grain boundary significantly influences on their thermal stability. The improved thermal stability of perovskite crystals upon the addition of PCBM is attributed to the formation of PCBM-nX (n=1, 2, 3, ..., X= Cl, I), which chemically passivates perovskite grain boundaries and prevents halogens at the grain boundaries from exiting their crystal lattice. This study offers a simple method for improving thermal stability of perovskites without performance losses and opens up the possibility of the use of various molecular additives to achieve highly stable perovskite solar cells.