In the pursuit of developing highly efficient polymer solar cells, it is indispensable to experimentally determine the molecular electronic and geometrical structures of distributed donor/acceptor bulk heterojunctions for understanding the processes inside the cell. In this article, substrate effect on interface energetics and film morphology of the poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C70 butyric acid methyl ester (PCDTBT:PC70BM) blends with various blending ratios on various conductive substrates is clarified based on the characterization of photoelectron spectroscopy and atomic force microscope, where the PCDTBT:PC70BM blend film serves as an important model system due to efficient charge generation and transport with low recombination. The energetics of the PCDTBT:PC70BM blend film is demonstrated to be highly dependent on the substrate work function, showing the transition from vacuum level alignment to Fermi level pinning with the variation of PC70BM ratio in the blend film. The resulting morphology is in good agreement with the observed formation of a PCDTBT-rich layer at the top of the PCDTBT:PC70BM blend film irrespective of the variation of the PC70BM blending ratio and annealing temperature. The results show the possibility of tuning the interfacial electronic structures by utilizing the substrate effects and potential applications on performance enhancement in polymer solar cells.