Luminescent Down-shifting Research Articles

Realizing luminescent downshifting in ZnO thin films by Ce doping with enhancement of photocatalytic activity

ZnO thin films doped with Ce at different concentration were deposited on glass substrates by spray pyrolysis technique. XRD analysis revealed the phase purity and polycrystalline nature of the films with hexagonal wurtzite geometry and the composition analysis confirmed the incorporation of Ce in the ZnO lattice in the case of doped films. Crystalline quality and optical transmittance diminished while electrical conductivity enhanced with Ce doping. Ce doping resulted in a red-shift of optical energy gap due to the downshift of the conduction band minimum after merging with Ce related impurity bands formed below the conduction band in the forbidden gap. In the room temperature photoluminescence spectra, UV emission intensity of the doped films decreased while the intensity of the visible emission band increased drastically implying the degradation in crystallinity as well as the incorporation of defect levels capable of luminescence downshifting. Ce doping showed improvement in photocatalytic efficiency by effectively trapping the free carriers and then transferring for dye degradation. Thus Ce doped ZnO thin films are capable of acting as luminescent downshifters as well as efficient photocatalysts.

Improving photovoltaic performance of silicon solar cells using a combination of plasmonic and luminescent downshifting effects

This paper reports on efforts to improve the photovoltaic performance of crystalline silicon solar cells by combining the plasmonic scattering of silver nanoparticles (Ag NPs) with the luminescent downshifting (LDS) effects of Eu-doped phosphors. The surface morphology was examined using a scanning electron microscope in conjunction with ImageJ software. Raman scattering and absorbance measurements were used to examine the surface plasmon resonance of Ag NPs of various dimensions in various dielectric environments. The fluorescence emission of the Eu-doped phosphors was characterized via photoluminescence measurements at room temperature. We examined the combination of plasmonic and LDS effects by measuring the optical reflectance and external quantum efficiency. Improvements in the photovoltaic performance of the solar cells were determined by photovoltaic current density-voltage under AM 1.5G illumination. A combination of plasmonic and LDS effects led to an impressive 26.17% improvement in efficiency, whereas plasmonic effects resulted in a 22.63% improvement compared to the cell with a SiO2 ARC of 17.33%.

Integrating Down-Shifting and Down-Conversion into Metal–Organic Frameworks to Enhance the Spectral Conversion for Solar Cells

Luminescent spectral conversion, including down-conversion (DC) and down-shifting (DS) is a potential route to overcome the spectral mismatch between solar photons and the semiconductor energy gap. Here, we introduce metal organic frameworks (MOFs) that integrate DC and DS processes into one system. Three different MOFs (Gd(BTC)(1), Gd(BPT)(2), and Gd(OBT)(3)) were synthesized doped with lanthanide ions couples. The DS–DC synergistic fluorescent processes were observed in all the MOFs. The overall absorption and near-infrared (NIR) emission intensity are dramatically enhanced (by factors of ∼4 800 and 550) due to the DS-sensitized effect for DC. As for DS process, the DC-bridge effect is quantified for the first time by comparing the NIR quantum yield (QYNIR). Benefiting from the bridge effect, QYNIR (5.30%) of Dy0.01Yb0.10-3 is increased around 3 times greater than the system without DC process, and it is close to the highest value reported for the C–H bond containing Yb3+ emitter. Our study demonstrated...

Photovoltaic performance of textured silicon solar cells with MAPbBr3 perovskite nanophosphors to induce luminescent down-shifting

This study employed a two-step multi-cycle spin-coating method for the application of MAPbBr3 perovskite nanophosphors on textured silicon solar cells with the aim of enhancing photovoltaic performance through luminescent down-shifting (LDS). The surface morphology and dimensions of the MAPbBr3 perovskite nanophosphors were examined using scanning electron microscopy in conjunction with ImageJ software. The LDS effects of the nanophosphors were revealed by measuring photo-luminance, optical reflectance, and external quantum efficiency. The photovoltaic performance of cells with and without MAPbBr3 perovskite nanophosphors was evaluated according to photovoltaic current density-voltage (J–V) under AM 1.5 G solar illumination. Compared to uncoated cells, two-layer and one-layer coatings of MAPbBr3 perovskite nanophosphors were shown to enhance conversion efficiency by 4.56% and 3.38%, respectively.

Enhancing Photovoltaic Performance Using Broadband Luminescent Down-Shifting by Combining Multiple Species of Eu-Doped Silicate Phosphors.

This paper demonstrates the application of a broadband luminescent downshifting (LDS) layer with multiple species of europium (Eu)-doped silicate phosphors using spin-on film technique to enhance the photovoltaic efficiency of crystalline silicon solar cells. The surface morphology of the deposited layer was examined using a scanning electron microscope (SEM). The chemical composition of the Eu-doped silicate phosphors was analyzed using energy-dispersive X-ray spectroscopy (EDS). The fluorescence emission of the Eu-doped silicate phosphors was characterized using photoluminescence (PL) measurements at room temperature. We also compared the optical reflectance and external quantum efficiency (EQE) response of cells with combinations of various Eu-doped phosphors species. The cell coated with two species of Eu-doped phosphors achieved a conversion efficiency enhancement (∆η) of 19.39%, far exceeding the ∆η = 15.08% of the cell with one species of Eu-doped phosphors and the ∆η = 8.51% of the reference cell with the same silicate layer without Eu-doped phosphors.

Efficient Cu+ to Ho3+ Energy Transfer in Highly CuO/SnO Co-doped Phosphate Glass

Glasses activated with Cu+ ions and rare-earths are attractive materials for luminescent down-shifting (LDS) in solar cells. This calls for investigating on fundamental interactions between dopants aiming to understand the factors underpinning resulting optical properties. Accordingly, this Communication reports for the first time on the determination of Cu+ → Ho3+ energy transfer efficiencies in melt-quenched phosphate glass containing holmium(III) together with a high concentration of Cu+ ions obtained through CuO/SnO co-doping. Material characterization was carried out by UV/Vis absorption and photoluminescence spectroscopy including a Cu+ emission decay assessment. The data revealed suppressed Cu+ visible emission and reduced lifetimes in the presence of Ho3+ ions consistent with an efficient Cu+ → Ho3+ energy transfer, most significant for Cu+ emission in resonance with Ho3+ absorption. The findings point to Cu+ ions being efficient energy donors to Ho3+ which may have implications for the development of glasses for LDS and laser materials as well.

AC/DC electrical conduction and dielectric properties of PMMA/PVAc/C60 down-shifting nanocomposite films

Polymer nanocomposite films were prepared by doping fullerene C60 in polymer blend composed of polymethacrylate/polyvinyl acetate blends (PMMA/PVAc) using solution cast technique. The films were characterized by differential scanning calorimeter (DSC), Transmission electron microscope (TEM), DC/AC electrical conductivity and dielectric measurements in the frequency range (100 Hz- 1 MHz). The glass transition temperature, Tg, was increased by increasing the concentration of fullerene C60; this property reflects the increase of thermal stability by increasing the nanofiller content. The DC and AC electrical conductivities were enhanced by increasing C60 concentration due to the electron hopping or tunneling between filled and empty localized states above Tg. The relaxation time was determined from the αβ -relaxations and found to be attenuated by increasing the temperature as a typical behavior of amorphous polymers. The calculated values of thermodynamic parameters revealed the increase of molecular stability by increasing the doping concentration; this feature supports the application of PMMA/PVAc/C60 nanocomposite films in a wide scale of solar energy conversion applications such as luminescent down-shifting (LDS) coatings for photovoltaic cells.

Plasmon-Assisted Efficiency Enhancement of Eu3+-Doped Tellurite Glass-Covered Solar Cells

Rare-earth-doped tellurite glass containing metallic nanoparticles can be exploited to manage the solar spectrum in order to increase solar cell efficiency. It is therefore possible to modify the incident solar spectrum profile to the spectrum that optimizes the solar cell recombination process by covering the solar cell with plasmonic luminescent downshifting layers. With this approach, the losses due to thermalization are minimized and the efficiency is increased. Due to the down-conversion process that couples the plasmon resonance of the metallic nanoparticles and the rare-earth electronic energy levels, it is possible to convert photons from the ultraviolet region to the visible and near-band-gap region of the semiconductor. It is demonstrated here that plasmon-assisted efficiency enhancements of 14.0% and 34.5% can be obtained for commercial Si and GaP solar cells, respectively, covered with Eu3+-doped TeO2-ZnO glass containing silver nanoparticles.

Key parameters of efficient phosphor-filled luminescent down-shifting layers for photovoltaics

Spectral management is one of the promising ways to increase the efficiency of modern photovoltaic devices. We study the performance of phosphor-filled luminescent down-shifting (LDS) layers. We focus on four powder phosphors with refractive indices in the range of 1.66–1.84 and similar particle size distributions. Using experimental characterization as well as 3D optical simulations, we identify key parameters of the phosphor particles and LDS layers that primarily affect the optical transmittance, absorptance, and photoluminescence quantum yield of the layers. We investigate the influence of the medium located beneath the LDS layer and reveal a strong increase in the performance when the layer is applied directly onto the solar cell. Finally, the optimal combination of the particle, binder and layer parameters that render the highest performance of the LDS layers are also indicated and discussed.

Ultrawide Spectral Response of CIGS Solar Cells Integrated with Luminescent Down-Shifting Quantum Dots.

Conventional Cu(In1-x,Gax)Se2 (CIGS) solar cells exhibit poor spectral response due to parasitic light absorption in the window and buffer layers at the short wavelength range between 300 and 520 nm. In this study, the CdSe/CdZnS core/shell quantum dots (QDs) acting as a luminescent down-shifting (LDS) layer were inserted between the MgF2 antireflection coating and the window layer of the CIGS solar cell to improve light harvesting in the short wavelength range. The LDS layer absorbs photons in the short wavelength range and re-emits photons in the 609 nm range, which are transmitted through the window and buffer layer and absorbed in the CIGS layer. The average external quantum efficiency in the parasitic light absorption region (300-520 nm) was enhanced by 51%. The resulting short circuit current density of 34.04 mA/cm2 and power conversion efficiency of 14.29% of the CIGS solar cell with the CdSe/CdZnS QDs were improved by 4.35 and 3.85%, respectively, compared with those of the conventional solar cells without QDs.

Large‐area flexible, transparent, and highly luminescent films containing lanthanide (III) complex‐doped ionic liquids for efficiency enhancement of silicon‐based heterojunction solar cell

ABSTRACTIonic liquid (IL)‐based lanthanide luminescent materials with luminescent downshifting properties are beneficial for enhancing the photovoltaic energy conversion efficiency (ηpv) of silicon‐based solar cells. Herein, novel lanthanide (III) complex‐doped ILs have been successfully prepared by simply dissolving the complex Ln(pybox)3 (Ln = Eu or Tb, pybox = 2,6‐bis[(4R)‐4‐phenyl‐2‐oxazolinyl]pyridine) into a bidentate organophosphine functionalized IL (1,3‐bis‐[3‐(diphenylphosphinyl)propyl]imidazolebis (trifluoromethylsulfonyl)imide), showing improved luminescence efficiency attributed to the coordination of ILs with Ln3+ ions and thus occurrence of the energy transfer from ILs to Ln3+ ions and very high absolute quantum efficiency (up to 97.22%). Large area (17 × 17 cm2) flexible, transparent, and luminescent poly(methyl methacrylate) thin films were prepared through a simple drop‐casting method and were applied as luminescent downshifting coatings to the silicon‐based heterojunction solar cells (active area: 235 cm2). In the best case using the Tb3+‐containing film, the ηpv of the solar cell increases from 16.676 to 16.774%.

Giant Enhancement of Luminescence Down-Shifting by a Doubly Resonant Rare-Earth-Doped Photonic Metastructure

Phosphors based on rare earth ions are used to convert photons to useful wavelengths for a wide range of applications, but the improvement of their emission efficiency remains a key challenge in thin films. In this paper, we demonstrate theoretically and experimentally that a photonic crystal (PC) structure can efficiently enhance the luminescence properties of a down-shifting layer (DSL). Our new metastructure combines a DSL (Y2O3:Eu3+) and a two-dimensional (2D) planar PC (SiNx), the latter being specifically engineered to control simultaneously the absorption in the DSL and the emission into free space. A global emission enhancement of 77 times is obtained due to PC Bloch mode resonances. Additionally, this metastructure exhibits high transmission above 450 nm and favors the emission of converted light at specific angles, with an enhancement factor of up to 690. We explain how the slow and fast Bloch modes of the PC structure induce these giant absorption and extraction enhancements of light.

Application of luminescence downshifting materials for enhanced stability of CH3NH3PbI3(1-x)Cl3x perovskite photovoltaic devices

The application of luminescent down shifting (LDS) layers as alternative UV filters for CH3NH3PbI3(1-x)Cl3x perovskite solar cell (PSC) devices is reported. A combination of photo-absorption measurements and of device decay measurements during light soaking are used to verify the stability. The application of a UV filter or LDS layer was able to significantly retard photo-induced degradation with ∼18% drop in device power conversion efficiency (PCE) observed over 30 h for non-encapsulated devices, which is compared to ∼97% for an un-filtered device, also without encapsulation. Whilst the PCE of the PSC device decreases with the application of the LDS layer, the drop is not as significant as when a commercial UV filter is used. Considering that UV filters will be essential for the commercialization of PSCs, the work provides evidence that the LDS layer can act as an alternative UV filter in PSCs and can limit the drop in PCE that can be expected from the inclusion of a UV filter, thus providing an added benefit over commercial UV filters.

Layered Tb‐Doped Yttrium Carboxyphosphonate Nanocrystals as Efficient Filler for PEDOT:PSS Composite Films

AbstractA layered yttrium phosphonate based on N‐(phosphonomethyl)glycine (H3L) and of formula Y(PO3CH2NHCH2COOH)(HPO3CH2NHCH2COOH) was synthesized in mild hydrothermal conditions. Its crystal structure was first determined by single‐crystal X‐ray diffraction. Then, the synthetic parameters were changed in order to reduce the particle size of the compound. The use of microwave (MW) irradiation during the synthesis allowed a) to obtain the compound in form of nanoparticles (NP) in the 50–300 nm range dispersed in water, b) to strongly reduce the reaction time by a factor 150, and c) to successfully introduce into the structure small amounts of a luminescent ion like terbium(III) as an isomorphic substituent of yttrium(III). The obtained NP were used to prepare composite materials by dispersing them in poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The resulting optical, photophysical, morphological and electrical properties of modified and pristine PEDOT:PSS films were investigated and compared, in order to test the potential use of Y(H2L)(HL) NP as luminescent downshifting materials in optoelectronic devices.

Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices.

Single-junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. Spectral converters can be used to convert solar photons into energies that are more effectively captured by the photovoltaic device through a photoluminescence process. Here, recent advances in the fields of luminescent solar concentration, luminescent downshifting, and upconversion are discussed. The focus is specifically on the role that materials science has to play in overcoming barriers in the optical performance in all spectral converters and on their successful integration with both established (e.g., c-Si, GaAs) and emerging (perovskite, organic, dye-sensitized) cell types. Current challenges and emerging research directions, which need to be addressed for the development of next-generation luminescent solar devices, are also discussed.

Procedure to decouple reflectance and down-shifting effects in luminescent down-shifting enhanced photovoltaics.

The down-shifting (DS) process is a purely optical approach used to improve the short-wavelength response of a solar cell by shifting high-energy photons to the visible range, which can be more efficiently absorbed by the solar cell. In addition to the DS effect, coupling a DS layer to the top surface of a solar cell results in a change in surface reflectance. The two effects are intermixed and therefore, usually reported as a single effect. Here we propose a procedure to decouple the two effects. Analytical equations are derived to decouple the two effects, that consider the experimentally measured quantum efficiency of the solar cell with and without the DS layer, in addition to transfer matrix simulations of the parasitic absorption in the device structure. In this work, an overall degradation of 0.46 mA/cm2 is observed when adding a DS layer composed of silicon nanocrystals embedded in a quartz matrix to a silicon solar cell of 11% baseline efficiency. To fully understand the contribution from each effect, the surface reflectance and DS effects are decoupled and quantified using the described procedure. We observe an enhancement of 0.27 mA/cm2 in short-circuit current density due to the DS effect, while the surface reflectance effect leads to a degradation of 0.73 mA/cm2 in short-circuit current density.

Photovoltaic Performance Characterization of Textured Silicon Solar Cells Using Luminescent Down-Shifting Eu-Doped Phosphor Particles of Various Dimensions.

This paper reports on efforts to enhance the photovoltaic performance of textured silicon solar cells through the application of a layer of Eu-doped silicate phosphor with particles of various dimensions using the spin-on film technique. We examined the surface profile and dimensions of the Eu-doped phosphors in the silicate layer using optical microscopy with J-image software. Optical reflectance, photoluminescence, and external quantum efficiency were used to characterize the luminescent downshifting (LDS) and light scattering of the Eu-doped silicate phosphor layer. Current density-voltage curves under AM 1.5G simulation were used to confirm the contribution of LDS and light scattering produced by phosphor particles of various dimensions. Experiment results reveal that smaller phosphor particles have a more pronounced effect on LDS and a slight shading of incident light. The application of small Eu-doped phosphor particles increased the conversion efficiency by 9.2% (from 12.56% to 13.86%), far exceeding the 5.6% improvement (from 12.54% to 13.32%) achieved by applying a 250 nm layer of SiO2 and the 4.5% improvement (from 12.37% to 12.98%) observed in cells with large Eu-doped phosphor particles.

Plasmonic luminescent down shifting layers for the enhancement of CdTe mini-modules performance

In this investigation, plasmonic coupling between luminescent species (lumogen yellow, and silver nanoparticles (Ag NPs) has been exploited to produce significant enhancement in absorption and fluorescence emission of lumogen yellow dye luminescent downshifting (LDS) layers. The optimum dye concentration in a poly(methyl,methacrylate) polymer LDS layer of ∼100μm thickness was established. Subsequently, plasmonic coupling with Ag NPs was introduced and optimum concentration has been determined for the composite plasmonic Luminescent Down-Shifting (pLDS) layers. The optimum plasmonic coupling has been observed to produce fluorescence emission enhancement of up to 81% for the dye-pLDS layers. The pLDS layers were deposited on top of CdTe mini-modules to investigate the action of plasmonic interaction in LDS layers. It has been demonstrated that the addition of pLDS layers containing lumogen yellow dye increases the short circuit current density (Jsc) of CdTe devices between 300 and 500nm. An increase of ∼100% has been achieved. External quantum efficiency (EQE) measurements of the pLDS layers have also shown significant improvement below 500nm reaching 25–40%, while increased efficiency was confirmed with current–voltage (J–V) measurements.

Development of multidye UV filters for OPVs using luminescent materials

Luminescence down-shifting (LDS) is used in several photovoltaic technologies aiming to improve the photon conversion efficiency (PCE) of the devices through the increase of the light harvesting in the regions of the electromagnetic spectrum where the EQE of the solar cells is poor. The aim of this work was to produce films of mixtures (blends) of two luminescent materials, dispersed in a poly-methyl methacrylate (PMMA) matrix, hoping to improve their properties both as LDS layer and as UV filter when applied on the clear, external surface of P3HT:PC61BM photovoltaic devices. The best results led to an increment of 7.4% in the PCE of the devices, and a six fold enhancement in their half-life (T50%). This study indicates that multidye LDS layers with optimized optical properties can lead to an effective improvement in the performance and operational stability of OPVs.

Luminescent Downshifting by Photo‐Induced Sol‐Gel Hybrid Coatings: Accessing Multifunctionality on Flexible Organic Photovoltaics via Ambient Temperature Material Processing

A novel high‐durability multifunctional organic–inorganic hybrid coating material is presented in this work as luminescent down‐shifting (LDS) host matrix system for flexible organic photovoltaic (OPV) devices. Such new LDS coating is obtained by incorporating a convenient fluorescent organic dye in an appropriately functionalized fluoropolymeric resin that can be readily crosslinked by means of a dual‐cure mechanism with a single‐step ambient‐temperature photo‐induced sol–gel process. Due to its peculiar characteristics, the newly proposed system may be readily implemented in heat‐sensitive flexible devices. By carefully tuning the amount of organic fluorophore in the hybrid coating material, a maximum increase in power conversion efficiency exceeding 4% is achieved on devices incorporating the new LDS layer with respect to control systems. This represents the highest efficiency enhancement reported to date on flexible OPVs by means of a polymer‐based LDS layer. In addition, long‐term accelerated weathering tests (>550 h) highlight the excellent stability of LDS‐coated OPV devices, which can retain 80% of their initial performance, as opposed to the dramatic efficiency decay experienced by control uncoated devices. The approach presented here opens the way to the straightforward incorporation of versatile multifunctional light‐managing layers on flexible OPV systems for improved device efficiency and lifetime.