APL Materials, Volume 7, Issue 3, March 2019. In the inverted structure perovskite solar cells, a buffer layer is generally used at the interface between the n-type semiconductor layer and the metal electrode, but its design guidelines have not yet been established. Here, a series of inverted perovskite solar cells have been fabricated with the controlled thickness of bathocuproine (BCP) buffer layers deposited by thermal evaporation and validated the BCP buffer layer evaluation tool. The ideal factor was calculated from the gradient in the plot of Voc against the log of Jsc, and the effect of the BCP buffer layer on charge recombination was verified. Since the ideal factor greatly decreased from 5 to 1.4 by introducing the BCP buffer layer, it was confirmed that the interface between the n-type semiconductor layer and the metal electrode gradually changed from a Schottky barrier diode to an ohmic contact. On the other hand, it was found that an excessive BCP film thickness causes the series resistance to increase and induced recombination. Finally, as a result of optimizing the perovskite layer and the BCP buffer layer, respectively, the performance exceeding 17% was obtained. This study provides insight into the improvements in the conversion efficiency of perovskite solar cells by optimizing the thickness of the buffer layer using the ideal factor.