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Luminescent Solar ContractorA value addition towards light harvesting technologiesPresented by:

Fahad MateenGraduate ScholarAdvance Electronic and Information Materials Labortary,Department of Chemical and Biochemical Engineering, Dongguk University, South Korea.

Session OverviewIntroduction to Luminescent solar contractorsNeed for Luminescent solar contractorOptimum design strategieskey toward successful light harvestingPrevious contributionsProposed objectives of our Project

Introduction to Luminescent solar contractors

A device used to absorb the light and isotropically emit it at a higher wavelength. The remitted light moves in a guided mode through the substrate and finally collected by a PV cell mounted at the edge of the LSC. Total internal reflection occurs when light impinges the LSC/air boundary with angles larger than the critical angle.n is the refractive index of the LSC layer and the fraction of photoluminescentlight which will be totally internally reflected is given by:

For n = 1.5, which is a common refractive index value for glasses, FTIR will be around 75%, which means that a quarter of the luminescence intensity is lost.

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Need for Luminescent solar contractorReduce the cost of solar cells by provide cheap solution solar energy conversionLSC can reduce the size of the PV cells more than 90% Reduction in overall weight of sysstem, so photovoltaics can be integrated into urban environments, such as windows and walls, at low cost.Sunlight can better penetrate the surface of the LSC waveguide from all angles, making them more appropriate for collecting non-direct sunlight.Both direct and diffuse light is collectedHeat is well dissipated into the large area of the collector plate

LSC designThe efficiency of an LSC is dependent on the light trapping capability of the waveguide, the optical properties of the fluorescent material

Waveguide:Should have high transparency throughout the visible spectrum, nearly perfect transparency at wavelengths within the emission spectrum of the fluorescent materialAn index of refraction greater than or equal to 1.5Good photo-stability and durability to achieve a life-span longer than 10 years, and low costFluorescent material :Absorption of all wavelengths < 950 nm with high absorption coefficients and an emission peak 1000 nm.Minimum reabsorption losses due to overlap of absorption and emission spectra.Near-unity fluorescence quantum yield (FQY).Long-term outdoor stability (more than ten years).

Thefluorescence quantum yield is the ratio of photons absorbed to photons emitted through fluorescence

Types of Photo luminescent material :

Organic Dyes:Mostly used ;Rhodamines, coumarins, and DCM, due to their near-unity fluorescence quantum yields and low cost.Advantageous compared to rare-earth metals and QDs due to their extremely high fluorescence quantum yield, low cost and availability. Disadvantageous in general due to their relatively narrow absorption spectra, relatively broad emission spectra, and absorption/emission spectrum overlap.

Rare earth metals: A class of inorganic fluorescent material, usually comprised of Neodymium (Nd), Europium, and/or Ytterbium (Yb). Surprisingly having non-overlapping absorption and emission spectraExcellent photostabilityThe FQY of RE materials vary greatly depending on host materials and concentration but values >90% have been reported in glass substrates.Disadvantages include that RE materials exhibit extremely low absorption coefficients. Thus, high concentrations are required to effectively harvest a significant fraction of sunlightExhibit numerous narrow absorption bands resulting in limited utilization of the solar spectrum

Quantum Dot Nanocrystals:Size of a QD dictates the degree of confinement; decreasing the size of a quantum dot increases the energy-band spacing and band-gap accordingly. Thus, the absorption and emission spectra of QDs can be adjusted by changing their size. PbS and lead selenide (PbSe) QDs have broad absorption spectra with high absorption coefficientsPrimary drawback is Absorption and emission spectra of QDs tends to overlap significantly and also less-than-unity fluorescence quantum yields

Future improvements in QD fabrication techniques could potentially reduce the reabsorption losses in QD samples by producing samples with smaller size distributions.

Performance evaluations:Total internal reflection is the basic principle behind wave guiding. Luminescent quantum efficiency:The ratio between the number of emitted photons per absorbed photon.organic dyes = 1, Rare-earth > 0.9, quantum dots < 0.8Stokes efficiency: The ratio between the energy of the emitted photon and the excitation photonEnergy difference concerned with this shift is lost to lattice vibrationsTrapping efficiency: The trapping efficiencyy of the light trapped in the collector given by:

n = refractive index of the light emitting medium.Optical Efficiency:the fraction incident photons collected by the concentrator

Previous contributions:Major contributions were done in three main areas.Development of Photo luminescent materials Development of various configuration and designs of LSC either by simulation or experimentsMaterials for back side coating in order to avoid emission losses from backside

Other than these major contributions research was also done for :fabrication techniques,measurement techniques matrix materials

Development of various configuration and designs of LSC:

Matrix choice:

PMMAsoft polydimethylsiloxane (PDMS) waveguidePolycarbonatesPMMA/SiO2 hybridsAcrylic glass

Proposed objectives of our Project:

Measure of spectrum obtained from illumination of the blank PMMA waveguide and from the dye-filled PMMA waveguide, PL, PMMA, SiO2, LC-PL-PMMA, LC-PL-PMMA, SiO2 and all samples with photo initiator. Effect of different concentrations of photo luminescent material on light collection efficiency.Calculated percentage of photons emitted outside the wave guiding of all samples as a function of the incidence angle (with respect to the waveguide normal) of the illumination source.Edge emission spectra of waveguides Total efficiency of LSC by using various bottom background e.g. Reflective mirrors, Al sheet, Black surface, white scattering surface (efficiency is more for scattering)Degradation kinetics using LC-PL system (no kinetics study was done for such system) Effect of photo stability of LSC by introducing LC with silica

FT-IR, SEM, TEM characterization of LC-PL PMMA/SiO2 nanohybrid coatingsDSC measurements revealed the increase of Tg values by increasing nano silica concentration promoting long-term thermal stability of nano hybrid film. Use of plasmons as a scattering medium. Previously it is reported that LC alignment retard the absorption of perpendicular falling light leading to less absorption events. So external diffuser is required. My suggestion is to use plasmons as a scattering particles for light, in this way use of external diffuser can be avoided. Effects of fabrication and curing method on stability of samples (Particularly LC-PL PMMA)FTIR, SEM, TEM analysis of films before and after checking photo stability specially in case of SiO2Addition of Au, Ag nano particles in film and determining the effect of their shapes on the stability of film. Introducing the sandwich design structure and test its validity

The idea is based on the fact that the scattering light from absorbing metal nanoparticles (plasmons) can interact with a dye molecules and increase its radiative transition probabilities, thus increase the collection efficiency of luminescent plate

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