Luminescent down-shifting is an optical approach to increase photovoltaic device efficiency and it consists of luminescent species such as quantum dots, organic dies and rare-earth complexes doped in a transport polymer sheet and deposited on top of photovoltaic cells.
Spraying of luminescent species on top of photovoltaic cells either by spray coating or incorporation into a multifunctional coating system based on photo-curable fluoropolymer have shown significant improvement in power conversion efficiency of uncoated dye sensitized solar cells devices thereby improving cell stability and prevention of photochemical and physical degradation.
In a recent article by Ahmed et al. (2016) and published in Solar Energy, investigations were made on plasmonic luminescent down-shifting (pLDS) layers applied to silicon cells (c-Si) and dye sensitized solar cells (DSSC) solar cells.
Quantum dots exhibits broad absorption spectra, high absorption coefficients and emission wavelength which can be tuned according to their size as a result of quantum confinement having advantages over organic dies due to their higher brightness and stability. However, the luminescent down-shifting suffer from self-absorption; a case where downshifted photons are reabsorbed by quantum dots or dye within the downshifting layer. Optical properties of luminescent species according to research were shown to exhibit dramatic emission enhancement in presence of metal nanoparticles on quantum dots ion dye emitters.
For the experiments, core-shell type Cadmium Selenide/Zinc Sulfide (CdSe/ZnS) quantum dots were used as fluorescent material which has a quantum yield of 0.7±0.07 measured in solution. Silver nanoparticles were used in plasmonic luminescent down-shifting composite layer and preparation of fluorescent species with silver nanoparticles composite layers followed.
The performance of c-Si and DSSC solar cells encapsulated with quantum dots luminescent down-shifting layer and plasmonic-quantum dots luminescent down-shifting composite layer was compared.
Absorption and emission measurement of quantum dots with/without silver nanoparticles revealed significant increase in emission for the plasmonic luminescent down-shifting layer when compared to layer with no silver nanoparticles. This result shows enhancement attributed to silver nanoparticles exhibiting strong scattering of incident light which greatly enhances local electric fields at surface plasmon resonance frequency.
Current-voltage curves for c-Si and DSSC solar cells showed electrical characterization increase of 1.92% in current density due to presence of quantum dots when compared to bare c-Si cells. Enhancement of 7.84% was calculated for plasmonic-quantum dots luminescent down shifting composite layer. There was also 5.81% increase in current density for plasmonic-quantum dots luminescent down shifting composite layer when compared with quantum dots luminescent down shifting layer.
Electrical characterization of DSSC solar cells showed a decrease of 8.03% in current density of quantum dots luminescent down shifting when compared to bare DSSC solar cells while enhancement of 3.31% was calculated for plasmonic-quantum dots luminescent down-shifting composite layer. There was also 11.29% increase in current density for plasmonic-quantum dots luminescent down shifting composite layer when compared with quantum dots luminescent down shifting layer.
External quantum efficiency of bare c-Si solar cells was poor reaching only 11% at wavelength below 400nm. Improvement of external quantum efficiency at the same wavelength for quantum dots luminescent down-shifting layer and plasmonic-quantum dots luminescent down-shifting layer were 23% and 52%, respectively. The high improvement in plasmonic-quantum dots luminescent down-shifting layer can be attributed to presence of silver nanoparticles in its composite layer.
The external quantum efficiency of DSSC solar cells had overall decrease between 300-800 nm for quantum dots luminescent downshifting layer. However, plasmonic-quantum dots composite layer show 3.03% increase when compared to DSSC bare solar cell and 11.71% when compared to quantum dots luminescent down-shifting device. Significant increase was calculated between 300 and 500nm where current density Jsc reached 21.64% and 5.16% for plasmonic-quantum dots composite layers compared to bare DSSC solar cell and quantum dots luminescent down-shifting device, respectively.
CdSe/ZnS quantum dots investigations in Ahmed et al. (2016) studies has the ability to absorb light below 465nm and emits at 500nm flows shifting the optical wavelength of the cell from poor optical response (short wavelengths) to external quantum efficiency (at longer wavelengths).
The research team for the first time demonstrated plasmonic luminescent down-shifting current density Jsc reaching up to 22% increase in region of 300-500nm for c-Si and DSSC solar cells.
Ahmed H1, Doran J2, McCormack S1. Increased Short-Circuit Current Density and External Quantum Efficiency of Silicon and Dye-Sensitized Solar Cells through Plasmonic Luminescent Down-Shifting Layers. Solar Energy, Volume 126, 2016, Pages 146–155.Show Affiliations
- School of Engineering, Trinity College Dublin, Dublin, Ireland
- Dublin Energy Lab, Dublin Institute of Technology, Dublin, Ireland
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