Utilizing Quantum Dots to Enhance Solar Spectrum Conversion Efficiencies for Photovoltaics

Abstract

Silicon-based photovoltaics typically convert less than 30% of the solar spectrum into usable electric power. This study explores the utilization of CdSe based quantum dots as spectral converters that absorb the under utilized UV portion of the solar spectrum and fluoresce at wavelengths near the band-gap of silicon-based solar cells. A flexible 1 mm thick thin-film structure that contains an array of microfluidic channels is designed and fabricated in polydimethylsiloxane (PDMS) using soft-lithographic techniques. The channels are approximately 85 microns wide by 37 microns tall and are filled with a solution containing the quantum dots. The thin-film structure can easily be attached to the surface of a single-junction solar cell. As a result, solar energy striking the coated solar cell with wavelengths less than 450 nm, which would normally experience low conversion efficiency, are absorbed by the quantum dots which fluoresce at 620nm. The high energy photons are converted to photons near the band-gap which increase the overall conversion efficiency of the solar cell. The quantum dots employed in this study are fabricated with a CdSe core (5.2 nm) and a ZnS outer shell and they exhibit a 25 nm hydrodynamic diameter. The UV-VIS spectral transmission properties of PDMS, along with its refractive index, are determined in order to characterize the spectral conversion efficiency of the thin-film structure. A model is developed to predict the optimum path length and concentration of quantum dots required to improve the power output of an amorphous silicon solar cell by 10%. PHOTOVOLTAIC TECHNOLOGYPhotovoltaic solar cells are commonly employed to harness the enormous energy (600-800 watts/m 2 ) that strikes the earth each day from the sun. The first silicon solar cell was developed at Bell Labs in 1954 and there has been a steady increase in the utilization of this sustainable energy source over the past 50 years [1]. Today, only 0.03% of the world's total power requirements are provided by the conversion of solar energy into electrical power [2]. One of the most cost effective implementations involves the utilization of crystalline silicon with conversion efficiencies typically around 15% yielding power production rates of around $0.25/kWh, which is still high compared to $0.05/kWh for coal or gas fired power plants [2]. Over 90% of the photovoltaic power produced today is from single crystal silicon based panels; however, first generation silicon based solar cells do not utilize all of the radiant energy that is provided by the sun. Specifically, wavelengths below 450 nanometers are not converted effectively into usable electrical power and as much as 39% of the sun's energy is under utilized [3]. This research focuses on developing a method for improving solar cell efficiency by 10% through the process of spectral conversion, which would thereby make it competitive with thirdgeneration photovoltaics based on multi-junction thin films. The spectral conversion technique examined in this study employs an aque...

SYNTHESIS AND CHARACTERIZATION OF CdSe-ZnS CORE-SHELL QUANTUM DOTS FOR INCREASED QUANTUM YIELD

“…This study found that higher loading of quantum dots in the medium would be necessary to efficiently convert light. 16 The other primary objective for quantum dots at Cal Poly is in bioimaging. As described earlier, quantum dots can be used as fluorescent tags for imaging cells and other biological media.…”

Transparent glass-ceramics functionalized with EuSiO3constrained BaF2:Eu2+nanocrystals: theoretical design and experimental fulfillment towards an efficient spectral converter

“…1 During the past two decades, various materials have been developed as solar cells' spectral converters through the so-called upconversion, downshifting, and down-conversion photoluminescence (PL) processes. [2][3][4][5][6][7][8] Up-conversion materials mounted as rear panels 9 can convert several NIR photons into one photon at shorter wavelengths, but they require laser pumping with quantum yields (QYs) as low as o1%. In contrast, down-shifting or down-conversion materials 10 mounted as front panels can convert one ultraviolet (UV) into one or more photons at longer wavelengths with QYs as high as close to 100% or even beyond 100%.…”

“…Spectral conversion materials are generally constructed with active centers ( e.g. , quantum dots, 8 sensitized dyes, 19 and rare earth (RE) ions 2,3 ) and hosts ( e.g. , glasses/glass-ceramics, 20 gels, 6 and epoxy resins 21 ).…”

The Design and Fabrication of a Microfluidic Reactor for Synthesis of Cadmium Selenide Quantum Dots Using Silicon and Glass Substrates

“…A layer of quantum dots applied to a solar panel can convert otherwise unused UV light from the sun into visible light that can make electricity [33]. Quantum dots made of lead selenide have been shown experimentally to produce as many as seven excitons from one high energy photon of sunlight (7.8 times the band gap energy) [34].…”