Amorphous molybdenum oxide (MoOx) as a hole collection layer (HCL) in polymer solar cells (PSCs) was prepared by annealing of a peroxo precursor. The power conversion efficiency of PSCs based on a blend of poly(3-hexylthiophene) and [6,6]-phenyl C-61 butyric acid methyl ester was improved to 3.19% by using an MoOx layer prepared by annealing at 200 degrees C in N-2 as an alternative for poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). X-ray photoelectron spectra of MoOx layers showed that the peroxide precursor was converted into MoOx by annealing at 100-150 degrees C and that annealing at a high temperature in N-2 under unaerated conditions promotes the reduction of Mo6+ species into Mo5+ species. In contrast, ultraviolet photoelectron spectra of an MoOx layer showed that the electric structure of MoOx depends on not the amount of Mo5+ species but the presence or absence of Mo5+ species. These results indicated that an appropriate amount of Mo5+ species in the MoOx layer is important for an efficient MoOx HCL A stability test revealed the PSCs with MoOx layers had relatively high device stability in air compared to that of PSCs with PEDOT:PSS. (C) 2015 Elsevier B.V. All rights reserved.