Molecular-Orientation-Dependent Interfacial Charge Transfer in Phthalocyanine/MoS2 Mixed-Dimensional Heterojunctions

Mixed-dimensional heterojunctions (MDHJs) combine the characteristics of component materials such as the discrete orbital energies of zero-dimensional (0D) molecules and the extended band structure of two-dimensional (2D) semiconductors. Here, time-resolved spectroscopy reveals sub-picosecond photoinduced hole-transfer and sub-320 fs photoinduced electron-transfer processes at the interfaces of type-II copper and free-base phthalocyanine/monolayer MoS2 MDHJs. In CuPc/MoS2 heterojunctions, charge separation lasts as long as 70 ns, which is a factor of 17 longer than that in H2Pc/MoS2 heterojunctions and a factor of 40 longer than that in previously reported transition-metal dichalcogenide-based heterojunctions. Preservation of the charge-separated state is attributed to the face-on orientation of CuPc on the MoS2 surface, which templates stacking of CuPc molecules and facilitates hole migration away from the interface, whereas H2Pc molecules adopt a mixed edge-on and face-on orientation. This work highlights the role of molecular structure in determining the interfacial geometry and, ultimately, charge-transfer dynamics in 0D/2D heterojunctions.