In recent years, organic solar cells have become the most potential technologies of the photovoltaic devices. In selection of the hole transport layer (HTL) in the inverted structure of organic solar cells, the use of molybdenum oxide (MoO3) has attracted more attention. However, the deposition of thermal evaporation in HTL fabrication requires highly vacuumed process, so it reduces the advantages in organic solar cells of large-area manufacture and low cost. Therefore, the solution process in MoO3 has been investigated. In this work, we dissolved MoO3 in isopropyl alcohol (IPA), spin-coated it on active layer Poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)thieno[3,4-b]thiophene))-2,6-diyl]): [6,6]-phenyl-C71-butyric acid methyl ester acid methyl ester (PBDTTT-C:PC71BM) as the anode modification layer in the inverted structure, and then modified further by air plasma before Ag(silver) electrodes were deposited. As a result, controlling the conditions of treating time, pressure and power of plasma, the efficiency of device with MoO3 solution process is improved from 2.50% to 4.65% under the plasma conditions 1.5Torr/18.0 W/1 min. (C) 2013 Elsevier B.V. All rights reserved.