We demonstrate the fabrication of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) nanogratings by a dehydration-assisted nanoimprint lithographic technique. Dehydration of PEDOT:PSS increases its cohesion to protect the nanostructures formed by nanoimprinting during demolding, resulting in the formation of high quality nanogratings of 60 nm in height, 70 nm in width and 70 nm in spacing (aspect ratio of 0.86). PEDOT:PSS nanogratings are used as hole transport and an electron blocking layer in blended poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) organic photovoltaic devices (OPV), showing enhancement of photocurrent and power efficiency in comparison to OPV devices with non-patterned PEDOT:PSS films.