Theoretical study on photoinduced triplet electron transfer at the interface of Pd-octaethylporphyrin and tungsten disulfide

Heterostructures of organic semiconductors and transition metal dichalcogenides (TMDs) are viable candidates for superior optoelectronic devices. Photoinduced interfacial charge transfer is crucial for the performance efficiency of such devices, yet the underlying mechanism, especially the roles of optically dark triplets and spatially separated charge transfer states, is poorly understood. In the present work, we obtain the structures of distinct excited states and investigate how they are involved in the charge transfer process at the Pd-octaethylporphyrin (PdOEP) and WS2 interface in terms of their energies and couplings. The results show that electron transfer from the triplet PdOEP formed via intersystem crossing prevails over direct electron transfer from the singlet (two orders of magnitude faster). Further analysis reveals that the relatively higher rate of triplet electron transfer compared to singlet electron transfer is mainly attributed to a smaller reorganization energy, which is dominated by the out-of-plane vibrations of the organic component. The work emphasizes the important roles of the optically dark triplets in the electron transfer of the PdOEP@WS2 heterostructure, and provides valuable theoretical insights for further improving the optoelectronic performance of TMD-based devices.