Fabrication Of Light-scattering Porous Structures By Using Microcellular Foaming And Application On Quantum Dot Light-emitting Diodes

As a new nanomaterial,quantum dot（QD）has lots of advantages,including narrow emission linewidth,broad absorption spectra,high photoluminescence quantum yield（PLQY）and solution-processed characteristic and so on.With the expected replacement of conventional rare-earth-doped inorganic phosphors,QD is becoming the next-generation color conversion material in light emitting diodes（LEDs）and has shown promising application potential in display,lighting and visible light communication areas.On one hand,due to the weak scattering ability of sub-10 nm-size QD and the limited thickness of QD layer,the excitation light cannot maintain a long optical path in the QD layer,inducing the weak absorption by QDs.On the other hand,QD emission is isotropic and most of QD emission is coupled into waveguide mode and cannot be extracted into the forward direction,which limits the light extraction efficiency of QD layer.When QDs are applied on the LEDs as a color conversion material,the spectral and layer structure design of multiple-type QDs needs careful consideration.Additionally,the limited light extraction is also an issue for QD-LEDs.To solve such issues,microcellular foaming is proposed as a key methodology to optimize the performance of QD-LEDs from QD layer to device structure in this thesis.Note that this idea is inspired by the porous scale structure of Cyphochilus spp.The detailed research contents are summarized into several aspects:（1）Fabrication of porous structures by using microcellular foaming based on supercritical CO₂ and performance analysisPropose microcellular foaming method based on supercritical CO₂ to fabricate porous structures.Adopt related mathematical models to describe the bubble nucleation and growth behaviors during the foaming process.Build up a complete foaming apparatus and comprehensively study the influence of foaming pressure and temperature on the formed porous structures and its internal mechanism.With the optimized foaming parameters,fabricate micro-porous and nano-porous structures by utilizing polymethyl methacrylate（PMMA）with different molecular weight.Through morphological and spectroscopic characterizations,specify the relationship between the porous structure and the scattering properties.Investigate the influence of micro-porous and nano-porous structures on the PL performance of the hybrid QD-PMMA films with different pristine layer thickness and QD concentration.By characterizing the PL intensity,absorptance,and PLQY values of hybrid samples with spectroscopic measurement method,reveal the mechanism of porous-structure-enhanced PL intensity.（2）Fabrication of porous structures by using phase separation method and performance analysisPropose phase separation method based on acetone-water-PMMA three-phase system and secondary encapsulation method assisted by open porous network.Investigate the tuning of the molecular weight of PMMA by using sonication and study the influence of PMMA with different molecular weight on the foamed porous structures.Through morphological and spectroscopic characterizations,investigate the optical parameters variation of the porous PMMA films before and after encapsulation.Utilize cryo-milling to grind phase-separated PMMA films into micro-sized particles and investigate the influence of different particle size and nano-porous structure on the scattering performance of scattering films.By leveraging optical diffusion theory,calculate the optical mean free path values of porous films and films doped with milled particles.Finally,investigate the influence of encapsulated porous films on the PL characteristics of QD films and unveil the internal functional mechanism of porous-structure-enhanced PL intensity.（3）Light-scattering mechanism analysis of porous structuresAdopt Mie scattering theory to analyze the scattering properties of single spherical bubble to unveil the fundamental origin of scattering.By changing the polarization mode,light wavelength and pore diameter,investigate the variation of scattering characteristics and reveal the internal functional mechanism of optical and dimensional parameters.Leverage the particle cloud model to analyze the internal relationship between the scattering coefficients and pore size,pore density as well as pore filling ratio.Reveal the mechanism which explains the difference of scattering coefficients stemming from different size parameters under the same pore filling ratio.Set up a ray tracing model for porous structures.By comparing the difference of simulation and experimental results,analyze the reason of internal difference and put forward a corresponding revision method to achieve an accurate simulation of the light scattering behavior of porous structures and meanwhile,reveal the internal mechanism of porous-structure-enhanced PL intensity by introducing a fluorescence model into the ray tracing model.（4）Application of porous structures on the QD-LEDsFirst rationally design the spectral and phosphor layer structures for QD-LED devices to meet with the demands of display and lighting.Study the influence of full width at half maximum（FWHM）and peak wavelength of QD spectra on the color gamut and luminous efficiency of display panels.Obtain the optimized FWHM and peak wavelength values.In terms of lighting,investigate the influence of the FWHM of red and green spectra on the luminous efficacy of radiation（LER）of LED devices.Study the maximum LER of QD-LED devices with multiple QDs under constraints of correlated color temperature and color rendering index and so on.Regarding the QD-LED devices with several phosphor coatings,design horizontally-layered structures between different phosphors.Investigate the influence of different layered structures on the luminous efficiency,layer temperature and color performance of LED devices and analyze the internal difference and functional mechanism of different phosphor structures.To overcome the limited light-extraction efficiency issue of QD-LED device,propose nano-porous films made by supercritical-CO₂-enabled microcellular foaming as diffusing reflective layers to extract the total internal reflected rays and enhance the device efficiency.Through the optimization of the thickness of porous films,a tradeoff is achieved between the thickness and the diffusing reflectance.Study the influence of porous film with the optimized thickness on the optical and chromatic performance of QD-LED devices.Unveil the light extraction scheme of porous films.