We report the study of a process which enhances the power conversion efficiency (PCE) of solar cells employing poly(3-hexythiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). In this process, the spin-coated solution of the active material was maintained in the liquid state for a prolonged duration. It was observed that through this process, the PCE of the device was enhanced by 31% for the case of a fast-grown film. It also provided a further 19% enhancement on top of the enhancement obtained by the familiar solvent annealing process. We found that this process depends critically on the presence of a poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer. It is hypothesized that the action of this enhancement process involves the interfacial interactions between P3HT and PEDOT polymers.