KEYWORDS: Visible radiation, Ultraviolet radiation, Solar energy, Solar cells, Solar radiation, Near infrared, Nanoparticles, Nanocomposites, Ytterbium, Erbium
We report on organic-inorganic nanocomposites with combined properties of quantum cutting and down shifting of solar UV spectrum. They cut a single high energy UV photon of solar radiation in two near-IR (NIR) low energy photons and use the energy of the UV photon to produce low energy visible photons. The applications include solar spectrum down convertors and luminescent solar concentrators (LSCs). The nanocomposites were made of organic hosts filled the nanoparticles of Lanthanide doped fluoride phosphor NaYF4:Yb3+, Er3+. As an organic host for the nanoparticles in an LSC simulator 1-Propanol was used. The particles of the synthesized phosphor powder were ball-milled to an average size of~ 200-nm. The LSC filled with the phosphor nanocolloid produced 130% more electric power than in case of pure 1-Propanol. This effect was not observed when the LSC was illuminated with an incandescent light bulb with no UV component in its radiation spectrum. The LSC performance was thus due to down shifting and quantum cutting of the UV spectral component of solar radiation by the phosphor nanoparticles into visible and NIR radiation matching the spectral response of the silicon PV solar cells. The results are of interest for green solar power.
Sunlight spectrum down-converting films absorb UV/blue sunlight and convert it in near infrared (NIR) radiation that generates electricity in conventional photovoltaic (PV) cells more efficiently. In this paper we report on down-convertors in the form of polymer nanocomposite films impregnated with nanoparticles of lanthanide phosphor NaYF4:Yb3+, Tm3+ . This compound was synthesized using the wet method and ball-milled in nano-powder. The phosphor produced relatively intense NIR radiation if the region between 960 and 1100 nm with a quantum efficiency of ~5%. It was chemically stable and could be transferred in the polymer film using the open-air concurrent multi-beam multi-target pulsed laser deposition (CMBMT-PLD). Two beams from a Q-switched Nd:YAG laser at a wavelength of 1064 nm were used to ablate two targets. One PLD target was made of compressed nano-powder of the phosphor. The second target was made of polymer poly (methyl methacrylate) known as PMMA. The phosphor preserved its down-converting properties during the deposition and mixing with the polymer. The deposited PMMA+NaYF4:Yb3+, Tm3+ nanocomposite films responded with uniform, mostly blue upconversion radiation to the testing illumination with a NIR laser diode at 980 nm. This proved that the phosphor nanoparticles were evenly distributed in the polymer matrix and preserved the optical properties of the PLD target. Post-deposition heating was shown to significantly improve uniformity of the nanocomposite films.
There is a great interest in photonic substances with permittivity approaching zero, which are called the epsilon-nearzero (ENZ) materials. They have a potential for multiple applications in telecom industry. The newest ENZ materials based on transparent conductive oxides (TCOs) and transparent conductive nitrides (TCNs) still have limited spectral bands of the ENZ effect. We show with simulations based on the Effective Medium Theory that the limitation can be defeated by using nanocomposite films made of several TCOs/TCNs with the ENZ effect observed in different regions of optical spectrum that stand apart from each other. We proposed to make such composites with the concurrent multibeam multi-target pulsed laser deposition (CMBMT-PLD). The composite films of aluminum and gallium doped zinc oxide (AZO-GZO) at different proportions were made by concurrent PLD of AZO and GZO targets with two 532-nm laser beams from a frequency doubled Q-switched Nd:YAG laser in a 10-5-Torr vacuum. The deposition time varied from 10 to 50 min. The high-resolution scanning electron microscopy revealed that the films deposited on glass substrates were composed of nano-grains of the constituents with a size in the range 10-300 nm. Energy dispersive X-ray spectroscopy showed the presence of all the major constituents in the films. Optical absorption and reflection spectroscopy of the films in the visible and near-infrared regions demonstrated that they had a minimum of reflectance corresponding to the ENZ effect in a broad band (~ 200 nm) around 1200 nm in the agreement with theoretical predictions.
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