The thermal stability of organic photovoltaics (OPVs) is greatly enhanced by using an interdiffused polymer/fullerene bilayer (ID-BL) as a photoactive layer. The solutions of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and phenyl-C71-butyric-acid-methyl ester (PCBM) are sequentially deposited to form the ID-BL. For comparison, a single photoactive layer consisting of mixed domains of PCDTBT and PCBM (MX-SL) is prepared by depositing blended solution of PCDTBT and PCBM, which is the most widely used method for fabricating OPVs. After applying thermal stress at 80 degrees C for 10 days, the OPV utilizing the ID-BL photoactive layer maintained 97.2% of its initial efficiency, whereas the efficiency of the OPV utilizing the MX-SL photoactive layer decreased to 37.5% of its initial efficiency. The glazing angle X-ray diffraction (GIXRD) and Flory-Huggins interaction parameter analysis reveal that the ordered domain size of PCDTBT in ID-BL is greater than that of the MX-SL and the ordered domain is maintained after the thermal stress test. These findings imply that mixing of the PCDTBT and PCBM domains does not occur due to the enhanced ordering of PCDTBT in ID-BL during the thermal stress test. Meanwhile, the domain size of PCDTBT in MX-SL is reduced due to further mixing of the PCDTBT and PCBM domains during the thermal stress test, which deteriorates the optimized bulk heterojunction (BHJ) morphology. These results show that efficient and thermally stable OPVs can be realized by utilizing the ID-BL photoactive layer prepared by the sequential solution deposition. (C) 2016 Elsevier Ltd. All rights reserved.