A solution-processed neutral hole transport layer is developed by in situ formation of MoO3 in aqueous PEDOT:PSS dispersion (MoO3-PEDOT:PSS). This MoO3-PEDOT:PSS composite film takes advantage of both the highly conductive PEDOT:PSS and the ambient conditions stability of MoO3; consequently it possesses a smooth surface and considerably reduced hygroscopicity. The resulting bulk heterojunction polymer solar cells (BHJ PSC) based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1):[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) blends using MoO3-PEDOT:PSS composite film as hole transport layer (HTL) show considerable improvement in power conversion efficiency (PCE), from 5.5% to 6.4%, compared with the reference pristine PEDOT:PSS-based device. More importantly, the device with MoO3-PEDOT:PSS HTL shows considerably improved stability, with the PCE remaining at 80% of its original value when stored in ambient air in the dark for 10 days. In comparison, the reference solar cell with PEDOT:PSS layer shows complete failure within 10 days. This MoO3-PEDOT:PSS implies the potential for low-cost roll-to-roll fabrication of high-efficiency polymer solar cells with long-term stability at ambient conditions.