The self-organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene)(RR-P3HT):[6, 6]-phenyl C61-butyric acid methyl ester is studied systematically as a function of the room temperature (RT) (varied from 300 K to 290 K). Optimal self-organized structures within the RR-P3HT:PCBM films are achieved by varying spin speed and time as well as the temperature at which the spin casting process occurs. These blend films are characterized by UV-vis absorption spectroscopy, atomic force microscopy, and X-ray diffraction measurements. The optimum device efficiency can be achieved in naturally dried devices when spin coating within the temperature range 292-294 K. Both the power conversion efficiency (PCE) and fill factor (FF) of the optimum devices show a plateau region, with PCEs exceed 4% and FFs close to 0.70. For RT < 290 K, the corresponding devices show a wide distribution of performance parameters for the unhomogeneous active layer. While for RT > 296 K, the short current density, FF and PCE of the corresponding devices are gradually decreased, suggesting that there is a major change in the ordered structure of the polymer. Based on the results, it is demonstrated that high performance device can be achieved just by natural drying the active layer at RT condition in air condition without further thermal treatments. (C) 2014 Elsevier Ltd. All rights reserved.