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Micro/Nanostructured Composite TiO2 Optical Coatings for Light Manipulation Functions in LEDs and Solar Cells
KTH, School of Information and Communication Technology (ICT).
2017 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Optical coatings are interesting for light manipulation functions to meet the demand for high energy efficient devices in the field of optoelectronics. For example, light emitting devices show limited light extraction efficiency predominantly due to internal surface reflections. Surface microstructuring has been reported as an efficient way to enhance the light extraction efficiency of GaN-based LEDs. Direct wet etching of GaN, to form parabolic cone-shaped random structures, could offer ~60-200% more light extraction. Two-dimensional GaN photonic crystal patterns, fabricated by laser holography method, could provide ~200% enhancement of light extraction. Nanoimprinting ZnO polymer resin on GaN LEDs could offer ~20% enhancement of light extraction power. For solar cells, the use of high refractive index materials (for example Si) results in high surface reflections (about 30-35% in the solar spectral region) and thus lower light absorption in the cell. By appropriate surface micro/nanopatterning, surface reflection can be reduced significantly by light manipulation (anti-reflection, guiding and trapping).

In this thesis work, a simple and cost-effective table-top soft printing method has been developed and used for the fabrication of micro/nanostructured optical coatings created from assembled (composite) TiO2 nanoparticles (size <100 nm). Additionally, a smart force infilling technique was explored for fabricating the optical coatings. Bulk TiO2 has a refractive index of ~2.8 for rutile and ~2.6 for anatase TiO2, making it interesting for optical coatings on, e.g., GaN surfaces. An estimate for the refractive index for the porous composite rutile/anatase TiO2 was determined using reflectivity measurements by analyzing the Fabry Perot oscillations for different layer thicknesses. The estimated refractive index was

~2.1-2.3 in the NIR-visible wavelength region. A polydimethylsiloxane (PDMS) mold was created using pre-patterned micro-size parabolic cones and nano-size disk/pillar array structures. The fabricated PDMS master molds were used to directly print well-defined and large surface area micro/nanostructure arrays from colloidal TiO2 nanoparticles. This printing or the colloidal TiO2 nanoparticles molding prodedure was performed by using a simple weight suspension setup in a vacuum desiccator, where in the PDMS mold was attached at the bottom of the weight.

The applied pressure for the soft print approach was controlled by the size of the PDMS mold, related to the weight/area relation. Pressures of ~40-160 g/cm2 were investigated resulting in an optimal determined pressure of ~80 g/cm2, for which a weight of 80 g was used, and the PDMS mold sizes varied between 1.4x1.4 cm2 to 0.7x0.7 cm2. Furthermore, the optimal TiO2 (rutile/anatase) colloidal particle solution concentration was investigated, varying it between 5-35 wt%. From these experiments, an optimal concentration of ~35 wt% was determined. By this method, TiO2 buffer layers of the order of

~100 nm to ~2.5 μm thick (depending on structure size and substrate type) could be provided below the

patterns. The determined optimal parameters for the pressure and concentration were used to fabricate micro/nanostructured composite TiO2 optical coatings. Furthermore, coatings on different substrate surfaces were investigated for the following substrate materials: Si, glass, GaAs and GaN-based LEDs. During fabrication the samples were placed in a vacuum desiccator and the pressure was applied for ~30

s while kept in the desiccator, followed by taking out the sample (maintaining the pressure) and placing it on a heater set at 100 °C and kept for ~15 min, then removing the weight and finally the peeling-off the PDMS mold. The created parabolic microcone array coatings were used for light extraction enhancement purposes on GaN-based LEDs. The obtained nanosized disk/dome array coatings were investigated for anti-reflection (or light manipulation functions) purposes in the solar cell context.

Electromagnetic modeling and simulations were performed using structures similar to those fabricated, using Lumerical’s finite-difference time-domain (FDTD) software. The fabricated structures were optically characterized by spectrally-resolved total transmittance/reflectance measurements with regard to absorption (on glass) and surface reflection properties on different substrates. The simulated and the measured data show similar results. For the optical coatings on the LEDs, a semi-automatic probe system was used to determine the LED’s light extraction enhancement before and after embossing the microstructured composite TiO2 optical coating.

The parabolic microcone array structures (square array period of ~3.1 μm, height of 1.7 μm, base

diameter of 2.7 μm and apex angle of 80°; with a buffer layer of ~100-200 nm) on GaN-based LEDs resulted in a light extraction enhancement (LEE) of up to 210%, which is amongst the highest LEE reported in literature. The ordered TiO2 nanodisk (hexagonal array period of ~610 nm, height of ~240 nm and top-bottom diameter of ~220-310 nm; with a buffer layer of ~100 nm to ~2.5 μm) and nanodome (hexagonal array period of ~510 nm, height of ~260 nm and top-bottom diameter of ~200-300 nm; with a buffer layer of ~100 nm to ~2.5 μm) array structures showed the lowest surface reflection of ~5-15% for the wavelength range of 400-850 nm embossed on a c-Si surface. The printed nanodisk/dome structures on GaAs showed even a surface reflection as low a ~5-10% in the visible-NIR wavelength range. For glass, as expected, the surface reflection increased due to the higher refractive index of the composite TiO2 compared to glass. The bandgap of the composite TiO2 was determined to be ~3.35 eV using the transmittance/reflectance data on glass.

The cheap, simple and straightforward soft print method developed in this thesis work is shown to be a promising approach for creating a large area and well-defined micro/nanostructured optical coatings consisting of composite nanoparticles. Since different pattern geometries and nanoparticle materials can in principle be used, such imprinted optical coatings are very interesting for a wide number of applications in optoelectronics and nanophotonics.

Place, publisher, year, edition, pages
2017. , p. 69
Series
TRITA-ICT-EX ; 2017:143
Keyword [en]
Titanium dioxide (TiO2), anti-reflection, light extraction enhancement, soft imprinting, capillary force self-assembly, Gallium Nitride (GaN) thin film LED
National Category
Nano Technology
Identifiers
URN: urn:nbn:se:kth:diva-229452OAI: oai:DiVA.org:kth-229452DiVA, id: diva2:1213103
Subject / course
Engineering Physics
Educational program
Master of Science - Engineeering Physics
Examiners
Available from: 2018-06-04 Created: 2018-06-04 Last updated: 2018-06-04Bibliographically approved

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