The performance of chemically synthesized lead sulfide ͑PbS͒ quantum dots ͑QDs͒ in planar, nontracking luminescent solar concentrators ͑LSCs͒ is evaluated using spectroscopic and photovoltaic techniques. Spatially resolved measurements are used to investigate and analyze the role of reduced self-absorption on the LSC efficiency. From comparative measurements of samples with Rhodamine B and CdSe/ZnS QDs it is established that PbS LSCs generate nearly twice the photocurrent in silicon cells than the other materials, achieving an integrated optical efficiency of 12.6%. This is attributed primarily to the broadband absorption of PbS which allows optimum harvesting of the solar spectrum.

We investigate the performance of cylindrical luminescent solar concentrators (CLSCs) with near-infrared lead sulfide quantum dots (QDs) in the active region. We fabricate solid and hollow cylinders from a composite of QDs in polymethylmethacrylate, prepared by radical polymerization, and characterize sample homogeneity and optical properties using spectroscopic techniques. We additionally measure photo-stability and photocurrent outputs under both laboratory and external ambient conditions. The experimental results are in good agreement with theoretical calculations which demonstrate that the hollow CLSCs have higher absorption of incident radiation and lower self-absorption compared to solid cylindrical and planar geometries with similar geometric factors, resulting in a higher optical efficiency.

We study photo-induced static and dynamic spectral changes in self-assembled CdSe=ZnS quantum dot (QD) thin films with varying QD concentrations under ambient conditions. Using spatially resolved scanning photoluminescence microscopy in conjunction with spectrally resolved time-correlated photon counting, we measure the variations in spectral intensity, emission wavelength, and recombination lifetimes as functions of photo-exposure time. We find that at low concentrations photo-darkening and photo-oxidation rates slow down with increasing QD density, but in the high concentration limit these rates are strongly enhanced. Our measurements lead us to conclude that the interplay of photo-induced surface trap discharging with preferential photo-oxidation of smaller QDs is further modulated by resonant energy transfer driven by strong inter-dot interactions in highly concentrated samples. Our results would imply that the efficiency and longevity of semiconducting nanoparticle based opto-electronic devices will be limited by the concentration of the active material V C 2011 American Institute of Physics.

We investigate the modulation of recombination lifetimes of CdSe/ZnS quantum dots (QDs) dispersed in a cholesteric liquid crystal (CLC) photonic cavity. Using ultrafast spectroscopic techniques we focus on the time-resolved emission from QD ensembles in CLC matrices with either planar or homeotropic alignment. In the case of planar alignment and a well-defined spectral stopband (reflection band) we observe the emergence of a second, faster decay time of less than 2 ns. This short recombination pathway is observed only in samples where the QD emission spectrum partially overlaps the CLC stop-band by 50% or more. Samples prepared with homeotropic alignment do not have a stop-band and, consequently, do not lead to spectral or dynamical modulation of the QD emission. Our observations indicate that coupling between the excitonic and the photonic cavity modes results in an enhancement and modulation of spontaneous emission in the liquid crystal medium.

We study photo-induced spectral changes in films containing two sizes of chemically synthesized CdSe/ZnS quantum dots (QDs) using static and time-resolved spectroscopies. As the concentration of the smaller (donor) QDs is varied over two orders of magnitude relative to the larger (acceptor) dots, we find that with decreasing proportion of donors, the photo-oxidation rate increases in acceptors but slows down in donors. We conclude that these differences originate from the variations in the amount of inter-dot energy transfer from donors to acceptors, and this tunability can be used to enhance the shelf-life of QD based opto-electronic and photovoltaic devices. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4721808] FIG. 2. (a) Emission intensity and (b) emission peak (k PEAK ) as functions of photo-excitation time for donors and acceptors in the 10:1 sample. The comparative curves for films with only donors (long dashes) and acceptors (short dashes) are also shown. Equivalent results for (c) emission intensity and (d) k PEAK for the 1:1 sample and the 0.1:1 sample.