Polymer solar cell (PSC) with a structure of ITO/PEDOT:PSS/P3HT:PCBM/Bphen/Ag was fabricated using spray coating method, and the influence of in situ annealing treatment on the PSC performance was studied. Comparing the performance of in situ annealed PSC with that of conventional post-annealed cell, the results showed that the in situ annealing treatment at low temperature not only corresponding to 12% enhancement of fill factor, but also resulting in 13% increase of open circuit voltage. The interpenetrating networks of P3HT:PCBM film were characterized by X-ray diffraction, and surface morphologies have been studied by atomic force microscope. The performance enhancement of PSC was attributed to the improved hole mobility of P3HT:PCBM blend and the increased interaction of active layer. (C) 2013 Elsevier B.V. All rights reserved.