Bimodal interfacial charge transfer in quantum dot heterostructures revealed by donor-/acceptor-specific broadband transient absorption spectroscopy

Quantum dot sensitized metal-oxide heterostructures offer an attractive platform for harvesting carriers from solar energy. Interfacial charge transfer processes lie at the heart of solar energy conversion using heterostructured nanomaterials, yet they are poorly understood and difficult to study. Here we employ broadband optical transient absorption spectroscopy, spanning a wavelength range that covers both quantum-dot donor and metal-oxide acceptor band gaps, enabling a selective view of charge transfer across the interface of CdSe-sensitized ZnO nanorods. This allows us to reveal a bimodal charge injection mechanism: a fast (<1 ps) indirect charge injection pathway populates an optically dark intermediate state, and a direct charge injection pathway on the tens of ps time scale without an observable intermediate state. The bimodal charge injection kinetics are attributed to heterogeneity of ZnO acceptor states. Our results deepen the understanding of the role of donor and acceptor states in heterostructured nanomaterials and pave the way for the rational design and control of charge transfer across complex interfaces and their application in optoelectronic devices.