Hybrid nanostructures attract considerable attention as candidates for practical photovoltaic (PV) devices. A promising approach is offered by non-contact energy transfer-based hybrid nanostructures combining strongly absorbing components, such as inorganic nanocrystal quantum dots (NQDs), with high-mobility semiconductor (SC) layers. In such hybrid systems, the excitonic energy is transferred via non-radiative (NRET) and radiative (RET) energy transfers across the interface with the subsequent separation and transport of charge carriers entirely within the SC-based component.

We have prepared hybrid structures consisting of several monolayers (ML) of the colloidal CdSe/ZnS NQDs attached to oxide-free Si surfaces. The bottom NQD ML is directly attached to the Si surface via self-assembled layer of amine-modified carboxy-alkyl chain linkers. Using hydrosilylation, H-terminated Si surfaces are fully functionalized through Si–C bonding (no interface oxide), providing functional headgroups for NQD’s attachment. The consequent, size-gradient NQD MLs are grafted via covalent linking by ethylene-diamine molecules. This approach results in the creation of tightly controlled, multilayer NQD structures with well determined distances to the Si surface, ideally suited for energy transfer studies.

We have demonstrated that efficient NRET and RET take place from NQD layers into Si substrates of various geometries. NRET, which is characterized by donor/acceptor dipole-dipole interaction, is shown to be an efficient mechanism for energy coupling for proximal NQDs (E_NRET~65%) in the visible portion¹, while radiative coupling into waveguiding modes of Si substrates² plays the major role in the near-infrared.³ Overall transfer efficiency of E_total~90% for a single NQD ML on Si has been obtained. However, to harvest the majority of sunlight, multilayer NQD films of several hundred nm thickness need to be created. We further utilize ET concepts to create graded multilayer NQD films with directed energy flow towards Si substrate via the cascaded interlayer NRET. We demonstrated that for bi-layer and tri-layer NQD structures, more than 80% of excitons originating in the outermost NQD ML are eventually transferred into Si substrate.⁴ These results are confirmed by theoretical calculations taking into account various rates of NRET between NQD MLs and direct RET into Si substrate and are consistent with the expectation of the defect-free layer grafting. Our spectroscopy studies are further confirmed by the measurements of ET-induced photocurrents in Si substrates. Such data further support the development of the efficient, thin film, ET-based hybrid structures for photovoltaic applications.