Energy transfer from colloidal nanocrystals into silicon substrates: Toward its application in solar cells

The integration of organic and inorganic materials at the nanoscale offers the possibility of developing new photonic devices that could potentially combine the advantages of both classes of materials. Such optoelectronic structures could work both in photovoltaic as well as in light emitting modes depending on the direction of non-radiative exciton energy transfer (NRET). Physical principles of the hybrid systems were studied systematically in the structures consisting of a monolayer of the colloidal nanocrystal quantum dots (NQDs) grafted on hydrogenated Si surfaces via carboxydecyl linkers. Such an approach allowed us to passivate Si surfaces to suppress non-radiative surface state defects ( Ns < 1011 cm 2) and provide controllable spacer lengths between NQDs and Si. We performed measurements of ET via time-resolved and steady-state photoluminescence (PL) as a function of spacer thickness, emission wavelength of the NQDs and the thickness of the Si substrate. The experimental results are in a good agreement with the classical macroscopic electrodynamics picture of an oscillating electric dipole in the media characterized by their frequency-dependent complex dielectric function. Comparative analyses reveal separate contributions to ET coming from NRET and radiative energy waveguide coupling (RET) channels: while in the visible and at shorter separation (few nm) NRET is the dominant mechanism, RET starts to take over at near-infrared wavelengths and at larger separation distances (effective up to several tens of nm). Overall ET efficiency is estimated at the level of ~90% over broad spectrum range. We apply the ET mechanisms to the study and design of multilayer NQD hybrid structures assembled on the 3D nanopillar substrates in order to effectively utilize sunlight's spectrum.