Perovskite solar cells (PSCs) have recently drawn much attention owing to their remarkable increase in power conversion efficiency (PCE) from less than 4％to over 15％in the past five years.1-3 Recently,a theoretical calculation by Yan et al.demonstrates that the maximum PCE of the perovskite solar cells can approach 26％,4 which indicates PSCs are economically viable with very fascinating prospect for their commercialization.