Spectral Converters Research Articles

A quantum dot-silica composite as an efficient spectral converter in a luminescent down-shifting layer of organic photovoltaic devices

The power conversion efficiency of organic photovoltaic (OPV) devices with a luminescent down-shifting layer was enhanced by 8.9% compared to pristine OPV devices.

Synthesis and Photoluminescence Properties of Double Perovskite Phosphor Ba6Y2W3O18: Mg2+, Mn4+ for Plant Cultivation

A novel far-red emission phosphor Ba6Y2W3O18: Mg2+, Mn4+ is firstly synthesized through high temperature solid reaction and its luminescence properties are investigated. The results show that Mn4+ doped Ba6Y2W3O18 phosphors have intense absorption in both ultraviolet and blue region, and can exhibit efficient far-red emission centered at 692 nm, which has large overlapping with the characteristic absorption bands of photosensitive pigments PFR. The optimum Mn4+ doping content, internal quantum efficiency, the content and temperature dependent luminescence behavior are discussed. The effects of “Mn-Mn” bond and crystal field strength on the luminescent property of Mn4+ are discussed in detail. Moreover, it is impressive that the co-doping of Mg2+ can significantly improve the emission intensity of Mn4+ and the related mechanisms are studied systematically. Finally, a red light emitting diode is fabricated based on Ba6Y2W3O18: Mn4+, Mg2+ phosphor and a 365 nm light emitting diode chip. The results indicate Ba6Y2W3O18: Mn4+, Mg2+ phosphor can be used as a potential spectral converter for plant growing.

Efficient Visible-to-NIR Spectral Conversion for Polycrystalline Si Solar Cells and Revisiting the Energy Transfer Mechanism from Ce3+ to Yb3+ in Lu3Al5O12 Host.

The so-called Shockley-Queisser converting efficiency limit of Si solar cells is believed to be surpassed by using the spectral converter. However, searching for efficient spectral converting materials is still a challenging task. In this paper, efficient visible-to-NIR spectral conversion for polycrystalline Si solar cells has been demonstrated in Ce3+ and Yb3+ codoped Lu3Al5O12. Moreover, the underlying energy transfermechanism from Ce3+ to Yb3+ is systematically re-investigated by the detailed excitation and emission spectra as well as fluorescent decay curves, and our results demonstrate that fast metal-to-metal charge transfer from Ce3+ to nearby Yb3+ is the dominant energy transfermechanism. Finally, we provide new evidence that Ce4+-Yb2+ charge-transfer state is responsible for the relatively low quantum efficiency of NIR emission in Ce3+ and Yb3+ codoped system.

Dual-mode downconversion luminescence with broad near-ultraviolet and blue light excitation in Tm3+/Yb3+ codoped oxy-fluoride glasses for c-Si solar cells

In this work, we demonstrated an efficient dual-mode solar spectral converter Tm3+/Yb3+ co-doped oxy-fluoride glasses synthesized using high temperature melt-quenching method. The transmittance, photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra and decay dynamics have been systematically investigated. The experimental results revealed that two distinct solar spectral converting processes, quantum cutting (QC) and down-shifting (DS) were achieved through the sensitization of Tm3+ excited multiplets and a broad band attributing to Yb3+-O2− charge transfer state (CTS), respectively. The prepared materials could convert the UV (250–370 nm) and blue (450–490 nm) photons into broad NIR emission (900–1100 nm), which coincides well with the most efficient spectral response of Si-based solar cells. The resultant Tm3+/Yb3+ co-doped oxy-fluoride glasses with dual-mode solar spectral modification indicate its potential application in improving the energy conversion efficiency of Si-based solar cells.

Study of the converter based on photonic crystals filters and quantum dots for solar blind ultraviolet imaging system

Miniaturization and high accuracy are the future directions of the solar-blind ultraviolet imaging system. Based on the principle of quantum dot photoluminescence and one-dimensional photonic crystal, we design an ultraviolet–visible spectral converter for an ultraviolet imaging system. In addition to the advantages of small size, high precision, and low-voltage power requirement, our design avoids the use of image intensifiers and light-cone couples. We propose three stacks of photonic crystals structures, which provide band-pass in the solar-blind range and a wide stopband in the visible range. This structure has a local effect on the excitation light of the quantum dots and can effectively enhance the brightness. The spectral converter utilizes the light-to-light conversion, which breaks away from the photoelectric conversion mode of the past and puts forward ideas for the future development.

NIR emitting Bi2MoO6:Nd3+/Yb3+ phosphor as a spectral converter for solar cells

Bi2MoO6 had been studied as a photocatalyst, ferroelectric, anti-bacterial, microwave dielectric, etc. No significant studies on the luminescence in Bi2MoO6 had been previously reported. In this work, NIR emission of Nd3+ activator sensitized by Bi2MoO6 host is reported. The excitation is in form of a broad band covering entire nUV-blue region of the spectrum. NIR emission around 1 µm and the broad band excitation is highly suitable for several applications like modification of solar spectrum for solar photovoltaics. Apart from Nd3+ emission, NIR emission around 979 nm resulting from Nd3+→ Yb3+ energy transfer is also reported. Mechanisms of host → Nd3+ and Nd3+→ Yb3+ energy transfer are suggested.

Photo‐Thermo‐Refractive Glasses Doped with Silver Molecular Clusters as Luminescence Downshifting Material for Photovoltaic Applications

AbstractSilver nanostructures, including molecular clusters and plasmonic nanoparticles, attract much attention due to their unique optical properties and potential applications in various fields. Glasses doped with silver molecular clusters are known to possess bright luminescence under UV excitation, which opens up prospects of using the glasses as solar spectral converters to overcome poor spectral response for most of photovoltaic cells in the short‐wavelength region. In this study, silver molecular clusters are formed in photo‐thermo‐refractive glass matrix both by UV irradiation and by Na+↔Ag+ion exchange and optically characterized. Photo‐thermo‐refractive glasses doped with silver molecular clusters, independently on silver introduction method, are shown to demonstrate broad absorption in the UV, intense white emission under excitation at 360 nm, and high photoluminescence quantum yield that meet requirements for luminescence downshifting materials. Despite this, only photo‐thermo‐refractive glass with silver molecular clusters uniformly distributed in the glass volume enhances the output power of a silicon photovoltaic cell, thus showing the potential for future application of photo‐thermo‐refractive glasses in photovoltaics, particularly in case of further experiments.

Ultrathin mono-resonant nano photovoltaic device for broadband solar conversion.

Nano-resonators can be used in photovoltaics to drastically improve the ability of the device to absorb light and generate photo-carriers, therefore enabling a reduction of the absorber volume. Conventionally, the harvest of the spectrally broad solar spectrum is achieved via the tedious engineering of multiple optical resonances. In this paper, we propose a breakthrough approach, which consists in reducing the solar spectral range with a spectral conversion layer to match only one resonance that can then be easily designed. We use a Maxwell solver and a ray-tracing code to optimize the nano-resonator and its spectral converter. We show that 66.2% optical efficiency can be theoretically achieved in less than 40 nm mean thick absorber while leading to device design enabling collection of photo-generated carriers.

Sol-gel preparation and near-infrared emission properties of Yb3+ sensitized by Mn4+ in double-perovskite La2ZnTiO6

Yb3+/Mn4+ co-doped La2ZnTiO6 (brief as LZT:Yb3+/Mn4+) phosphors with xYb3+ (x = 0–0.12) and yMn4+ (y = 0–0.01) doping concentrations have been synthesized via a sol-gel method. X-ray diffraction (XRD) and photoluminescence spectra were employed to characterize the properties of final products. The LZT:Yb3+/Mn4+ material can efficiently convert the short-wavelength sunlight in wide spectral ranges (250–600 nm) into near-infrared emission around 990 nm which matches the higher photosensitivity region of Si-based solar cells. A dipole-dipole interaction plays an important role for the energy transfer process from Mn4+ to Yb3+ ions, which has been verified by Dexter's theory. Under the 390 nm excitation, the energy transfer efficiency is 38.6% in the La1·9ZnTi0·998O6:0.1Yb3+/0.002Mn4+ co-doped sample. Due to the effective absorption of Mn4+ in the visible region and charge transfer transitions (O2−→Yb3+ and O2−→Mn4+) in the UV region for LZT and the efficient energy transfer to Yb3+, this material can be developed as spectral convertors to improve silicon solar cell photovoltaic conversion efficiency.

Enhancement of dye sensitized solar cell efficiency through introducing concurrent upconversion/downconversion core/shell nanoparticles as spectral converters

Extending the spectral absorption of dye-sensitized solar cells from the visible into near-infrared and ultra-violet range enables the minimization of non-absorption loss of solar photons. Here, we report a viable strategy to implement simultaneously near-infrared upconversion and ultra-violet downconversion for dye-sensitized solar cells through constructing a type of upconversion-core/downconversion-shell-structured nanoparticles. For the first time, NaYF4:20%Yb,2%Er@NaYF4:7%Eu core/shell nanoparticles are applied to TiO2 photoanode for fabricating near-infrared/ultra-violet-enabled dye-sensitized solar cell devices. The incorporation of designed nanoparticles into TiO2 photoanode of dye-sensitized solar cells achieves high efficiency of 7.664% under one sun illumination, increasing the power conversion efficiency by about 13.95%. We confirm that the enhancement of overall efficiency includes 4.82% upconversion contribution, 7.58% downconversion function and 1.55% scattering effect. Our strategy opens the path for further broadening the solar spectral use to improve the performance of photovoltaic devices.

Tailoring UV-blue sensitization effect in enhancing near infrared emission in X, Yb3+: CaGa2O4 (X = 0, Eu3+, Bi3+, Cr3+) phosphor for solar energy conversion

This work presents a strategy to modify the UV–vis part of solar spectrum into near infrared (NIR) region- an operational region for silicon solar cells. CaGa2O4, a wide band gap material, has been doped with Yb3+ ion and different sensitizers (Bi3+, Eu3+ and Cr3+) in order to probe the NIR emitting properties through downconversion (DC) process. Of these, Cr3+ ion serves the purpose of maximum conversion efficiency in a best way owing to its wide excitation spectrum spanning over the entire UV-blue region. The mechanisms involved in obtained results are supported by the decay curve measurement and energy transfer studies. Further, germainum ion is also doped to get long afterglow Yb3+ ion emission. This doping develops deep-traps that can store excitation energy and transfer it cooperatively via DC to Yb3+ for a longer time. Thus, the material is a promising spectral converter for c-Si solar cells and other photochemical processes.

NIR emitting K2SrCl4:Eu2+, Nd3+ phosphor as a spectral converter for CIGS solar cell

Intense near-infrared emitting phosphor K2SrCl4:Eu2+,Nd3+ with various concentrations of Nd3+ were synthesized. These are characterized with X-ray diffraction, reflectance, photoluminescence emission and photoluminescence excitation spectroscopy, PL lifetime measurements. The emission can be excited by a broad band in near ultra violet region as a consequence of Eu2+→Nd3+ energy transfer. The efficiency of Eu2+→Nd3+ energy transfer is as high as 95%. Fluorescence decay curves for Eu2+ doped samples are almost exponential and described by τ = 500 ns. Eu2+ lifetimes are shortened after Nd3+ doping. Near infrared Emission intensity is limited by Nd3+→Nd3+ energy transfer and the consequent concentration quenching. Nd3+ emission matches well with the spectral response of CIGS and CIS solar cells. Absorption of near ultra violet radiations followed by conversion to near infrared indicates the potential application in solar photovoltaics.

Investigation of Upconversion, downshifting and quantum –cutting behavior of Eu3+, Yb3+, Bi3+ co-doped LaNbO4 phosphor as a spectral conversion material

This work presents the spectral conversion characteristics [upconversion (UC), downshifting (DS) and quantum–cutting (QC) optical processes] of Eu3+, Yb3+ and Bi3+ co-doped LaNbO4 (LBO) phosphor samples synthesized by solid state reaction technique. The crystal structure and the pure phase formation have been confirmed by x-ray diffraction (XRD) measurements. The surface morphology and particle size are studied by scanning electron microscopy (SEM). The rarely observed intense red UC emission from Eu3+ ion has been successfully obtained in Eu3+/Yb3+ co-doped LaNbO4 phosphor (on excitation with 980 nm) by optimizing the concentrations of Eu3+ and Yb3+ ions. The downshifting (DS) behavior has been studied by photoluminescence (PL) measurements on excitation with 265 nm wavelength from a Xe lamp source. A broad blue emission in the region 300–550 nm with its maximum ∼415 nm due to charge transfer band (CTB) of the host and large number of sharp peaks due to f-f transitions of Eu3+ ion have been observed. The energy transfer has been observed from (NbO4)3− to Eu3+ ion and the fluorescence emission has been optimized by varying the concentration of Eu3+ ion. An intense red emission has also been observed corresponding to 5D0 → 7F2 transition of Eu3+ ion at 611 nm in LBO: 0.09Eu3+ phosphor on excitation with 394 nm. The luminescence properties of Eu3+ ion are enhanced further through the sensitization effect of Bi3+ ion. The near infra-red (NIR) quantum cutting (QC) behavior due to Yb3+ ion has been monitored on excitation with 265 as well as 394 nm. The NIR QC is observed due to 2F5/2 → 2F7/2 transition of Yb3+ ion via co-operative energy transfer (CET) process from (NbO4)3− as well as Eu3+ ions to Yb3+ ion. This multimodal behavior (UC, DS and QC) makes this a promising phosphor material for multi-purpose spectral converter.

Potential energy yield increase of a solar spectra down-converter equipped photovoltaic device in real operational conditions

In this work, we evaluate the impact of solar spectrum down-converter (DC) on an energy yield of solar cells working in real world meteorological conditions. For this purpose, a simple model of PV device with a down-converting layer is used, with inputs (spectra, irradiance, temperature) taken from the public NREL database for their outdoor test facility. The model assumes ideal external quantum efficiency (EQE). The analysis showed that in these conditions the energy yield increase from the use of DC system is 23.64% which is bigger than improvement due to DC calculated for the standard test conditions (20.1%). This effect is more profound in summer (24.8% for June-August) than winter months (22.1% for December-February). The average photon energy (APE) turned out to reasonably well reflect the overall impact of the DC layer on the energy yields with R2 = 0.96. The primary conclusion is that the impact of spectral converters on PV device performance should be analyzed not only in the standard test conditions as in the real operating conditions the result of spectra converting layer may turn out substantially better, lowering the bar for commercialization and broad application of such layers.

Impact of firing temperature on multi-wavelength selective Stokes and anti-Stokes luminescent behavior by Gd2O2S:Er,Yb phosphor and its application in solar energy harvesting

Erbium (Er3+)–ytterbium (Yb3+) doped gadolinium oxysulphide (Gd2O2S) phosphor has been developed via a facile method of solid-state flux fusion, and offers two-fold spectrum modification with highly intense Stokes and anti-Stokes shift. The effect of the firing cycle on the photoluminescent response and morphology of Gd2O2S:Er,Yb is scrutinized, wherein the firing temperature was varied (1000 °C–1250 °C), keeping firing time and all other parameters constant. Interestingly, the nanostructures fired below 1150 °C showed nanorods of diameter ~200 nm and length ~1–2 µm, whereas firing at 1150 °C and above rendered nanospheres with small diameter, ~350 nm. Highly bright upconversion (UC) emission was achieved even under an extremely low excitation power density of 800 µW cm−2 from a 980 nm laser, and was comfortably visible to the naked eye. The incident power dependent studies disclosed increase in UC-emission intensity with increasing excitation power and a quasi-linear dependence on excitation power density. Intense characteristic UC-emission of Er3+ excited states at 525 nm, 556 nm and 668 nm were observed, and the green emission band was found to be dominant over the red band in intensity. Concurrently, downconversion (DC) emission at 556 nm and 669 nm was also exhibited under ultraviolet excitation (285 nm and 380 nm), with the red band being more powerful than the green, unlike UC-emission. Firing temperature dependent studies divulged the dependence of luminescence intensity on the firing cycle of the luminophore and formation of the respective luminescent phase. The UC-emission intensity was found to be maximum for samples fired at 1150 °C, whereas samples fired at 1000 °C showed the highest DC-emission intensity. The excitation and emission profile of single Gd2O2S:Er,Yb phosphor lying in the desired spectral region and as a dual spectral converter marks its possible application for enhanced harvesting of sunlight.

Preparation and characterization of Y2O3:Bi3+,Yb3+ nanosheets with wavelength conversion from near-ultraviolet to near-infrared

Y2O3:Bi3+,Yb3+ nanophosphors, which exhibit near-infrared (NIR) emission under near-ultraviolet (near-UV) excitation, are promising candidates for spectral converters in crystalline silicon solar devices. Herein, Y2O3:Bi3+,Yb3+ nanosheets were prepared by calcination of hydrothermally-synthesized precursors. The effects of the calcination temperature on their properties were investigated. The nanosheets calcined at ≤ 800 °C had sheet-like, square morphologies with average lateral sizes of 210–240 nm, which were smaller than precursor nanosheet with 257 nm attributed to shrinkage through the calcination. The thickness of the nanosheet before and after calcination at 800 °C was ~20 nm. Moreover, the nanosheet had a single-crystal nature. However, at calcination temperatures ≥ 900 °C, the nanosheets lost their morphologies, and transformed into irregular particles. The Y2O3:Bi3+,Yb3+ nanosheets showed visible emission from Bi3+ and NIR emission from Yb3+ under near-UV excitation. The PL intensity and PL decay time of the NIR emission monotonically increased with increasing calcination temperature. Both increases can be explained by the following effects: (i) removal of H2O molecules adsorbed on the nanosheet surface, (ii) improvement of crystallinity and decrease of specific surface area, (iii) increase in probability of energy transfer from Bi3+ to Yb3+, and (iv) oxidation of reduced bismuth during calcination. Furthermore, the photostability of the nanosheets under near-UV excitation was improved by increasing calcination temperature.

Lanthanide complexes embedded in silicone resin as a spectral converter for solar cells

The performance of solar cell devices is dependent on various factors, and the spectral mismatch limits the upper limit of performance. Using a spectral converter to manipulate solar radiation is one way to improve solar cell efficiency. We present herein a spectral converter to move the short-wavelength (< 400 nm) range of solar radiation to the longer-wavelength range. The spectral converter comprises fluorescent lanthanide complexes uniformly embedded in silicone resin. A successful spectral converter showed transmittance of over 85%, and when applied in a Si solar cell, its relative efficiency was increased up to 4%.

Flame synthesized Y2O3:Tb3+–Yb3+ phosphors as spectral convertors for solar cells

Near-infrared (NIR) quantum cutting phosphors serve as a potential material for fabricating photovoltaic spectral convertors. In many cases, quantum cutting phosphors are obtained via a wet chemical method coupled with a post-annealing treatment—a very costly process. In this report, we used continuous flame spray pyrolysis (FSP) for fabricating Y2O3:Tb3+–Yb3+ quantum-cutting phosphors without any post-treatment. Based on characterizations by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, we found that as-synthesized Y2O3:Tb3+–Yb3+ phosphors exhibit hollow and shell-like micro-structures composed of highly crystalline and pure cubic-phase nanoparticles ( 500 nm), making them suitable as spectral photovoltaic convertors.

Conversion of broadband UV-visible to near infrared emission by LaMgAl11O19: Cr3+, Yb3+ phosphors

The Cr3+, Yb3+ co-doped LaMgAl11O19 spectral conversion materials have been synthesized by solid-state reaction method. The phosphor can absorb photons in UV–vis spectral region and then give intense near-infrared emission around 980 nm originated from Yb3+: 2F5/2 → 2F7/2 transition. Phonon-assisted energy transfers from Cr3+ ions and Cr3+-Cr3+ ion pairs to Yb3+ ions are both witnessed in LaMgAl11O19 host. The doping concentration and temperature dependent luminescence properties have been investigated. It is believed that Cr3+, Yb3+ co-doped LaMgAl11O19 phosphor might potentially act as spectral convertor for silicon-based solar cells to enhance the photovoltaic conversion efficiency.

Far-red-emitting double-perovskite CaLaMgSbO6:Mn4+ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs.

A series of Mn4+-activated CaLaMgSbO6 far-red-emitting phosphors were synthesized by a solid-state reaction route and the microstructure and optical characterizations were investigated in detail. Upon excitation at 370 and 469 nm, the samples showed intense far-red emission at about 708 nm originating from the 2Eg → 4A2g transition and the optimal Mn4+ concentration was 0.7 mol%. The as-prepared phosphors also exhibited excellent internal quantum efficiency (88%) and high thermal stability. The emission intensity at room temperature dropped to 54% when the temperature rose to 423 K and the activation energy was 0.34 eV. The outstanding optical properties and the fact that the emission band of the obtained phosphors had a broad overlap with the absorption band of phytochrome PFR demonstrated that the CaLaMgSbO6:Mn4+ phosphors may be promising potential spectral converters for applying to indoor plant cultivation light-emitting diodes.