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Energy harvesting devices that are effective and efficient are key to this era. One particular direction is through use of luminescent solar concentrators. This research has focused on the use of direct-write (aerosol jet) printing to produce the packaging of luminescent solar concentrators
(LSC) and security printed films using upconversion .
The ink contains upconversion nanoparticles dispersed in a PMMA film. The particles absorb two lower energy photons and emit a higher energy photon in the visible region that can be transformed to a current by photovoltaic material. In this case, the lanthanide doped nanoparticles absorb 980 nm near-infrared wavelength and emit a green illumination at 530 nm. Using metal-enhancedfluorescence principles, a noble-metal layer can enhance and control the luminescence for higher efficiency.
Using direct-write printing, these films can be deposited homogenously over larger areas to produce LSCs as well as deposited over stencils to produce thin polymer security films on document style substrates.
The ink solutions were composed of toluene and methyl benzoate in a 9:1 mixture. This solution was found to control evaporation rate and reduce the “coffee ring” effect of the polymer to produce homogenous film.
Silver nanoparticles were capped with decanoic acid and are 4 - 7 nm in diameter. Lanthanide doped nanoparticles are capped in oleic acid and have a hexagonal shape (60 nm thickness – 120 nm diameter).
Current research has focused on determining specific amounts of material for optimal printing and illumination.
Polymer beads [poly(methyl methacrylate)] and silver nanoparticles were each added at 1 wt%, while upconverting particles (NaYF4:3%Er,17%Yb) are added at
2 wt%.
Printing was performed with Sono-Tek direct write machine which allows for large scale material deposition. As material reaches the ultrasonic nozzle orifice, a controllable flat jet air deflector deposits the material in fan shaped spray pattern. For security film applications the material is deposited over a stainless steel stencil with desired features. Solutions have been printed on bonded paper, Kapton, and glass slides at room temperature.
Slides were fractured and observed using a scanning electron microscope (SEM).
Upconversion was observed through he use of Nd:YAG infrared diode laser at 980 nm wavelength using 1-5 W through fiber coupling.
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Below in Figure 1 is a schematic of a theoretical LSC.
Preliminary work focused on the deposition process of the metal-enhancing surface and upconversion layer.
Solar Radiation
Upconversion
Particles in
Polymer
Metalenhancing
Surface
Photovoltaic
Material
Figure 1
Silver nanoparticles were printed in a PMMA polymer matrix and produced a film with different characteristics depending on the angle of incidence. Films can have transparent or “mirror –like” reflective surfaces, Figure 2.
(a) (b)
Figure 2
Figure 2: These images show the same slide at various incidents of light (a) Transparency of 3 layer film on glass slide. (b) Mirror-like surface can be seen.
SEM imaging revealed that during evaporation the decanoic-acid capped precipitated away from the polymer.
This effect resulted in an agglomerated sheets of silver on top each layer deposited, giving a “frosting” effect, Figure
3.
Silver Layers
PMMA
Glass
Figure 3
Figure 3: SEM image of a fractured glass slide with a 3 layer deposition to show layering of silver. The brighter regions that resemble frosting layers are the agglomerated silver nanoparticles
(4-7 nm) while the darker region areas are the PMMA matrices.
Printing upconverters in a PMMA matrix revealed even dispersion throughout polymer film, leaving no “frosting” effect as seen with silver. At 1 wt % of upconverting particles, the films are relatively transparent and show uniform upconversion as seen in Figure 4.
Figure 4
Figure 4: Photographs of upconverters printed in PMMA film onto glass slides. The upconverted light is refracted to edge of glass.
Power density of laser around 5 W/cm 2 , spot is 1 cm diameter.
Ink printed with both silver and upconverting particles gave little to no “frosting” effect. The oleic acid capping agent induced dispersion of the silver particles, Figure 5. With particles too close, the upconversion becomes quenched and reabsorbed.
Figure 5
Printed lanthanide doped nanoparticles in PMMA can be used to fabricate security printed films that are activated by
Near-IR diode lasers. Films are deposited over laser cut stainless steel stencil at 1 to 2 layers averaging 0.6 to1.2
µm in thickness. Upconverting nanoparticles were added at 2 wt%. Photographs of upconversion light of various features from stencil printing are shown below in Figure 6.
(a)
(c)
(b)
Figure 6:(a) and (b) represent the ability print letters using stencil style deposition of ink. (c) represents the ability to print various features at larger scales.
All samples were printed to high bond paper substrates. Note:
Scale bar printed using Optomec-
M 3 D aerosol jet printing.
Developed packaging parameters for printing metalenhancingsurfaces using PMMA. “Coffee ring” effect, build up of polymer at film edge, was negated by solvent selection. Based on decanoic capping agent on silver nanoparticles, the nanoparticle dissolution favors the solvent over the matrix forming the “frosting” effect.
Controlling amount of polymer can determine thickness for desired film.
With both silver and upconverting nanoparticles evenly dispersed throughout the polymer, the silver begins to quench the upconversion. The distance between both nanoparticles will need to be controlled to prevent quenching.
Films and patterned features printed with the upconverting particles emit a relatively strong green illumination when under 980 nm diode laser. Both films and features were successfully printed to glass, Kapton, and high bond paper substrates. Upconversion is best observed for security applications with opaque substrates due to refraction.
 Developing lanthanide doped nanoparticles that can absorb over a wide range of IR wavelengths
 Printing particles in separate layers as to control distance between silver an upconverting nanoparticles
 Alter chemistry of lanthanide doped nanoparticles to illuminate other specific colors of upconversion. Using stencil based security films, specific features with various colors would be illuminated under a single wavelength diode laser.
 Develop printing parameters to “frosting” effect of silver nanoparticles as to produce capacitors based on altering conductive and dielectric material composition.
 Test printing of Poly(lauryl methacrylate) for security printed films for its optimal dissolution of lanthanide doped nanoparticles in its matrices.
This material is based upon work supported by the
National Science Foundation/EPSCoR Grant No. 0903804 and by the State of South Dakota.
The authors would also like to thank Dr. Edward Duke, Mr.
Krishnamraju Ankireddy, Mr. James Randle, and Mr.
Jeevan Meruga.