A phase-pure mixed-organic-cation perovskite absorber (MOC-PA) was fabricated by a one-step spin coating process with a toluene washing treatment, and the structural, optical and excitonic properties of the resultant MOC-PAs were then characterized by scanning electron microscopy, X-ray diffractometry, absorbance spectrometer, and time-resolved photoluminescence. The averaged power conversion efficiency (PCE) of inverted phase-pure MOC-PA based photovoltaics could be improved from 12.21% to 13.28% by tailing the surface roughness of the poly(3,4-ethylenedioxythiophene):poly(styrenesulfo nate) (PEDOT:PSS) thin film from 1.43 nm to 1.84 nm. The improved PCE is mainly due to the increased short-circuit current density (J(SC)) which is attributed to the improvement of the exciton dissociation, as confirmed by photoluminescence experiments. In addition, the experimental results indicate that the PCE can be further improved by increasing the crystallinity of MOC-PAs. (C) 2016 Elsevier Ltd. All rights reserved.