This review summarizes progress made and challenges encountered in the development of photovoltaic structures based on energy transfer sensitization of semiconductor substrates emphasizing silicon (planar and nanostructured) based devices. Nanocrystal quantum dots are starting to be used as efficient light absorbers with subsequent transfer of their excitonic energy into Si substrates where charge separation and collection take place within carefully designed electrical structures. Moreover, Si nanostructuring (e.g., NW and NPs geometries) leads to substantially enhanced light absorption and provides a scaffold for enhanced placement of NQD absorbers. An important enabler of this technology is the wet chemical passivation of Si surfaces and their subsequent functionalization with SAM linkers via hydrosilylation that are used to reliably graft NQDs while maintaining an excellent interface quality (i.e., low density of defect states). Another challenge is to deposit NQD multilayers via grafting, layer-by-layer, or self-assembly techniques that are dense enough to allow for complete light absorption and high excitonic quantum yields. Energy transfer through both nonradiative and radiative ET processes is now recognized as an efficient mechanism to couple excitonic energy into Si as well as to funnel excitations between NQD layers. The demonstration of these mechanisms and the controlled modification of silicon surfaces highlights the potential of hybrid PV structures with separated functionalities, that may supplant more conventional solar cell geometries.

Despite these advantages, the field is still in its infancy. The development of practically relevant ET-based hybrid devices requires resolving a number of materials issues common across many research disciplines. It is important that any optically dense NQD solids developed can harvest most of the incident solar light while maintaining high luminescence quantum yields. Further progress may be achieved with novel types of nanocrystals that feature larger absorption cross sections that extend into the near-infrared spectral region, innovative that combine nanocrystals with organic absorbers, as well as, structures that exploit plasmonic enhancement and light trapping. Concurrently, progress in the passivation of Si NW and NP substrates will help to further reduce surface recombination velocities, while modifications to grafting and assembly strategies will improve NQDs placement in nanostructured environments. Radial junction Si NWs, in which the minority carrier diffusion lengths are comparable with the wire radius, could be advantageous for efficient dissociation and extraction of ET-induced charge carriers.

A number of concentrated efforts will have to be made before practical ET-based hybrid PV structures can be realized. But their distinct organization and architecture with separated material component functionalities are interesting to explore and hold potential for use in the creation of flexible, ultrathin crystalline silicon solar cells, providing an alternative to traditional photovoltaics.