In poly(3-hexylthiophene):methanofullerene bulk-heterojunction polymer photovoltaic (PV) cells, we introduced nanoscale interfacial layers between the PV layer and the cathode. The nanoscale interfacial layers were made of ultrathin poly(oxyethylene tridecyl ether) surfactant and low-work-function alloy-metal of Al:Li layers. It was found that the nanoscale interfacial layers increase the photovoltaic performance: increasing short-circuit current density with fill factor and improving device stability. For PV cells with the nanoscale interfacial layers, an increase in power conversion efficiency of 4.18 +/- 0.24% was achieved, compared to that of the control devices (3.89 +/- 0.08%). (C) 2010 Elsevier B.V. All rights reserved.