Increased photo-stability of quantum dots in segregated bilayer films

We report a comparative study of photo-stability of CdSe/ZnS quantum dots (QDs) in a variety of thin film samples. These include electrophoretically deposited single and differently sized segregated bilayer films and self-assembled mixed-sized films. Our studies follow static and dynamic QD photoluminescence over prolonged periods of photo-excitation and find that compared to both single-sized and mixed-sized films, the segregated bilayer samples exhibit highest photo-stability. These films show a QD emission quench rate of ∼2.5 times slower than the others and have almost negligible spectral shifts (<2 nm). Time-resolved measurements indicate very short inter-layer energy transfer (ET) time for the acceptor QDs in the bilayer, coupled with low ET efficiency for the donor dots. Further analysis reveals a complex interplay of intra- and inter-ensemble ET, with ET rates that have disparate spectral dependence between the mixed and bilayer films, and we conclude that this leads to the enhanced photo stability in the latter. Our findings provide a vital clue to the optimal design of QD based energy-harvesting structures.