Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

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. The substrate effect on interface energetics and morphologies of the distributed polymer bulk heterojunction on various conductive substrates is investigated, revealing the dependence of energy level alignment of the polymer blend on the substrate work function. A polymer donor-rich layer at the top of polymer bulk heterojunction is formed due to the annealing-induced phase separation.