Effi cient Directed Energy Transfer through Size-Gradient Nanocrystal Layers into Silicon Substrates

wileyonlinelibrary.com placed on the materials involved, it becomes important to understand if those demands could be alleviated by employing different operational principles. Such an opportunity may arise in energy transfer (ET) based hybrid nanostructures [ 2,3 ] that seek a clear separation of the functionalities of the different materials components: one component of the hybrid structure is chosen for its strong light-matter interaction while the other for its high chargecarrier mobilities. The strong near-fi eld electromagnetic interaction is responsible for inter-conversion of neutral excitations, excitons and electron-hole pairs, between the components. In the photovoltaic (PV) mode of operation, solar light is harvested in the highly absorbing component followed by exciton diffusion and ET across the interface with the subsequent separation and transport of charge carriers entirely within the high-mobility semiconductor component. This separation of functionalities is conceptually reminiscent of photosynthesis, [ 4 ] where solar energy is absorbed in light-harvesting antennae and then relayed to reaction centers that enable charge separation. Hybrid architectures operating on ET principles should thus be contrasted with conventional [ 1 ] charge transfer based PV nanostructures. Many charge transfer based structures, for example, rely on exciton fi ssion at the interface resulting in charge carriers on both sides and therefore place high demands on carrier mobilities in both components as well as on the microscopic quality of the interface. We have recently shown [ 5,6 ] that a combination of colloidal nanocrystal quantum dots (NQDs) with ultrathin crystalline Si layers could be an attractive practical realization of ET-based hybrids for thin-fi lm solar cells that would take advantage of the benefi cial properties of both components. On the one hand, Si layers in such devices are effectively sensitized via ET from NQDs. Hence, the issue of weak solar light absorption in Si is no longer a defi ning factor in the overall design of the solar cell and the thickness of the crystalline Si layer can be substantially reduced to fractions of a micron in our experiments. [ 5 ] On the other hand, NQDs, well-known as good light absorbers and emitters, are no longer required to exhibit good charge carrier transport in their assemblies. We have experimentally demonstrated very effi cient (close to 90%) ET coupling between individual proximal NQDs and Si substrates over a wide spectral Effi cient Directed Energy Transfer through Size-Gradient Nanocrystal Layers into Silicon Substrates