Noble metal nanoparticles are strong absorbers of incident radiation in a tailorable fashion. Several solutionprocessed methods of fabricating metallic nanostructures exist. For large area solar cells, it is essential that the developed ink be cost-effective and reasonably overlap with the solar spectrum. In this work, we report the development of light-mediated synthetic method under various light sources, involving the evolution of spherical silver nanoparticles into large anisotropic silver nanoparticles of variable sizes. Reduction of silver nitrate in an aqueous sodium citrate solution results in spherical Ag nanoparticles that turned the stock solution yellow. The solution was aged under various light sources. Broadband (400-1400 nm) absorption was observed for the resulting ink. This synthesis method is thus potentially suitable for an industrial scale process for inks with a tailorable match with the solar spectrum (AM1.5G).
Selection of the correct crystalline phase in the NaYF4:Er nanomaterial system is expected to increase the upconversion photoluminescence efficiency for potential solar cell applications in the near IR region. Further, several recent reports involve the use of the cubic phase for biomedical in situ pressure sensing applications. Thus, it is vital to understand the precise annealing conditions necessary for the rational design of the nanomaterial species. We report the initial studies on phase purity (cubic and hexagonal phase) of NaYF4:Yb (18%), Er (2%) using thermal decomposition at 320°C. The as-synthesized (spherical and cubic) agglomerated nanoparticles were estimated to have a mean size of 200 nm from scanning electron microscopy (SEM), and present as aggregated particles in high-resolution transmission electron microscopy data (HRTEM). Powder X-ray diffraction (PXRD) measurements were carried out to infer the relative abundance of the two phases as a function of air annealing at different temperatures. Contrary to previously reported partial studies, we find that the initial mixed phase of 50:50 composition remains resistant to any change until 450°C, at which point the content of the hexagonal phase starts declining, resulting in a nearly pure cubic phase at 550°C. Thus, it is found that hexagonal phase does not dominate the product at any reasonably low processing temperature. Photoluminescence (PL) measurements on the unannealed material at 785 nm result in localized broadband emission in green (centered at 540 nm) and red (centered at 660 nm). This work establishes optimal annealing conditions for this important photonic nanomaterial for potential biomedical applications.
The realization of stable, high efficiency blue organic light emitting devices (OLEDs) remains
challenging. The efficiency of blue fluorescent OLEDs is fundamentally limited, and blue
phosphors are typically less stable. We discuss potential solutions to these problems. We show
that device engineering can be used to optimize the color of a relatively stable and efficient bluegreen
phosphor. In addition, we demonstrate the ability to manipulate the fraction of excitons
which form as singlets in fluorescent materials by inserting a mixing layer that affects only the
precursors to excitons.
Organic photovoltaics (PV) are constrained by a tradeoff between exciton diffusion and optical absorption. The short
exciton diffusion length within organic semiconductors demands the use of extremely absorptive materials.
Unfortunately, the excitonic character of most organic materials yields highly structured absorption spectra, with regions
of strong and weak absorption. Here, we describe a device architecture that decouples light absorption and exciton
diffusion in organic PV through the addition of a light absorbing 'antenna' layer external to the conventional charge
generating layers. Radiation absorbed by the antenna is transferred into the thin charge generating layers via surface
plasmon polaritons (SPP) in an interfacial thin silver contact and radiation into waveguide modes. SPPs are a
particularly effective energy transfer mechanism as they propagate in the plane of the PV rather than parallel to the
incident radiation, thereby providing a more efficient means of pumping thin charge generating structures. We exploit
efficient SPP-mediated energy transfer by attaching a resonant cavity antenna to a conventional small-molecular weight
organic PV. We find that the resonant cavity antenna boosts the performance of a phthalocyanine-based PV in the
absorption gap between the phthalocyanine Q and Soret bands. Off resonance the antenna serves as a mirror, but near the
resonant wavelength, the antenna absorption is significantly enhanced, and energy is fed back into the PV cell via SPP-mediated
energy transfer. Thus, the resonant antenna may be employed to supplement the performance of the PV cell at
resonance, with no degradation off-resonance.