Luminescent Down-shifting Research Articles

Tuning solar absorption spectra via carbon quantum dots/VAE composite layer and efficiency enhancement for crystalline Si solar module

AbstractIn this work, fluorescent carbon quantum dots (CQDs) composite transparent film with blue emission was prepared via hydrothermal fabrication, mixing with vinyl acetate‐ethylene (VAE) emulsion, coated and cross‐linked on the glass in sequence. This CQD/VAE film has good sticking ability and compatibility of sealing ethylene‐vinyl acetate (EVA) material and works well as luminescent downshifting (LDS) materials. Based on these features, we did a set of optical characteristics analysis for CQD/VAE film and inserted a CQD/VAE film between glass and EVA encapsulation in a mini metal wrap through (MWT) solar module. This LDS could convert UV part of solar spectra, which are normally blocked by EVA, into the blue range and improve the light absorption by solar cell. Enhancement in the performance of solar modules was proved, especially the improvement of current of short circuit (Isc), external quantum efficiency (EQE) of the UV wavelength and solar module efficiency. This material provides the chance for prompt application, since it simply solves the dispersion problem of LDS material inside encapsulation and keeps the transparency of sealing polymer materials.

Orange-red fluorescent polymer nanocomposite films with large stokes shift: An opto-electronic exercise

Herein, we report the successful fabrication of orange-red fluorescent polymer nanocomposite (PNC) hybrids, that envisage efficient photon management and appreciable UVA-protection, besides high design flexibilities and ease of self-cleaning. The visibly transparent PNC thick films were developed via aqueous solution casting of -OH backboned poly (vinyl alcohol) (PVA) with nanostructured sodium zincate (Na2ZnO2). The as developed PNC films offer appreciable photonic down-conversion of high energy UVA-radiations to relatively lower energy Orange-red light via fluorescent emission. The optical band gap studies reveal a direct band gap relationship with valence band maxima and conduction band minima occurring at same wave vectors. While, static contact angle measurements support nanofiller induced wettability transitions (hydrophilic to near hydrophilic). The fluorescence spectral (excitation and emission) studies of PVA/Na2ZnO2 NC films validate a promisingly large Stokes shift (~245 nm), which visualises a fair separation between the photonic absorption and emission bands, that may further aid an exponential minimization of optical losses due to re-absorption. The thermogravimetric studies support their excellent thermal stabilities. The relatively large Stokes shift, appreciable thermal stability and excellent UV harvesting ability of PVA/Na2ZnO2NC films construct them as competent luminescent down-shifting (LDS) materials for possible solar cell applications.

Efficient light harvesting in hybrid quantum dot-interdigitated back contact solar cells via resonant energy transfer and luminescent downshifting.

In this paper, we propose a hybrid quantum dot (QD)/solar cell configuration to improve performance of interdigitated back contact (IBC) silicon solar cells, resulting in 39.5% relative boost in the short-circuit current (JSC) through efficient utilisation of resonant energy transfer (RET) and luminescent downshifting (LDS). A uniform layer of CdSe1-xSx/ZnS quantum dots is deposited onto the AlOx surface passivation layer of the IBC solar cell. QD hybridization is found to cause a broadband improvement in the solar cell external quantum efficiency. Enhancement over the QD absorption wavelength range is shown to result from LDS. This is confirmed by significant boosts in the solar cell internal quantum efficiency (IQE) due to the presence of QDs. Enhancement over the red and near-infrared spectral range is shown to result from the anti-reflection properties of the QD layer coating. A study on the effect of QD layer thickness on solar cell performance was performed and an optimised QD layer thickness was determined. Time-resolved photoluminescence (TRPL) spectroscopy was used to investigate the photoluminescence dynamics of the QD layer as a function of AlOx spacer layer thickness. RET can be evoked between the QD and Si layers for very thin AlOx spacer layers, with RET efficiencies of up to 15%. In the conventional LDS architecture, down-converters are deposited on the surface of an optimised anti-reflection layer, providing relatively narrowband enhancement, whereas the QDs in our hybrid architecture provide optical enhancement over the broadband wavelength range, by simultaneously utilising LDS, RET-mediated carrier injection, and antireflection effects, resulting in up to 40% improvement in the power conversion efficiency (PCE). Low-cost synthesis of QDs and simple device integration provide a cost-effective solution for boosting solar cell performance.

Lanthanide-based downshifting layers tested in a solar car race

The mismatch between the AM1.5G spectrum and the photovoltaic (PV) cells absorption is one of the most limiting factors for PV performance. To overcome this constraint through the enhancement of solar energy harvesting, luminescent downshifting (LDS) layers are very promising to shape the incident sunlight and, thus, we report here the use of Tb3+- and Eu3+-doped organic-inorganic hybrid materials as LDS layers on Si PV cells. Electrical measurements on the PV cell, done before and after the deposition of the LDS layers, confirm the positive effect of the coatings on the cell’s performance in the UV spectral region. The maximum delivered power and the maximum absolute external quantum efficiency increased 14% and 27%, respectively. Moreover, a solar powered car race was organized in which the small vehicle containing the coated PV cells presented a relative increase of 9% in the velocity, when compared to that with the uncoated one.

Design and Color Flexibility for Inkjet-Printed Perovskite Photovoltaics

In this work we report on the coloring of perovskite solar cells (PSC) by combining the ease of freedom in design of the solar cell’s shape with the bright color of luminescent down-shifting (LDS) layers. Both the perovskite solar cell and the LDS layers are fabricated with digital inkjet-printing processes, such that the perceived color of the devices can be tuned independently from the shape of the device. The results demonstrate that a strong color perception of the PSCs with the use of luminescent materials of various colors can be achieved at a relatively small (∼17%) reduction of power conversion efficiency.

Colloidal N-Doped Graphene Quantum Dots with Tailored Luminescent Downshifting and Detection of UVA Radiation with Enhanced Responsivity.

Luminescent downshifting (LDS) materials are in great demand for their applications in light conversion devices. In this work, by an ingenious chemical approach of in situ doping, N-doped graphene quantum dots (N-GQDs) have been synthesized with tailored green photoluminescence (PL) under ultraviolet (UV) light excitation. The incorporation of N atoms in the form of pyridinic and graphitic C–N bonding into the sp2-hybridized graphitic framework of N-GQDs has led to tailored LDS via PL emissions. The LDS property of synthesized N-GQDs has been advantageously utilized to demonstrate enhanced responsivity (R) of a low-cost commercially available photoconductive cell (PC) for detection of UVA radiation through an indigenous technique. The linear optical responses of samples are optimized by varying the concentration and the dispersing medium. Also the N-GQDs are shown to be photostable in poly(vinyl alcohol) (PVA) hydrogel. A 60% enhancement in photocurrent of the PC-based photodetector under UV radiation has been obtained here by using N-GQDs/PVA as LDS material. Thus, detection of UVA radiation with a high specific detectivity (D*) of 9 × 1013 Jones and responsivity (R) of 3 A W–1 has been demonstrated, which might open the opportunity of using this material in future energy conversion devices.

Enhanced performance of a solar cell based on a layer-by-layer self-assembled luminescence down-shifting layer of core-shell quantum dots**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2017PF011), the National Natural Science Foundation of China (Grant No. E020701), and the Doctoral Scientific Research Foundation of Binzhou University, China (Grant No. 2014Y10).

In this paper, core–shell quantum dots (QDs) with two polar surface functional groups (ZnSe/ZnS–COOH QDs and ZnSe/ZnS–NH2 QDs) are synthesized in an aqueous phase. Photoluminescence (PL) and absorption spectra clearly indicate luminescence down-shifting (LDS) properties. On the basis of QDs, surface functional group multilayer LDS films (M-LDSs) are fabricated through an electrostatic layer-by-layer (LBL) self-assembly method. The PL intensity increases linearly with the number of bilayers, showing a regular and uniform film growth. When the M-LDS is placed on the surface of a Si-based solar cell as an optical conversion layer for the first time, the external quantum efficiency (EQE) and short-circuit current density (Jsc) notably increases for the LDS process. The EQE response improves in a wavelength region extending from the UV region to the blue region, and its maximum increase reaches more than 15% between 350 nm and 460 nm.

Fullerene C60 doped polymeric nanocomposite coatings: moving solar spectra from ultraviolet to the deep red

One of the main techniques in the state of the art of photovoltaic (PV) cells is their spectral loss towards the ultraviolet solar photons and the lack of absorption of infrared photons by PV cells. In this study, polymer nanocomposite luminescent down-shifting (LDS) films were prepared by doping fullerene C60 in polymethyl methacrylate (PMMA), polyvinyl acetate (PVAc) and PMMA/PVAc blend. The homogeneity and weathering stability of the films were characterized using AFM, FT-IR, UV–Vis absorption and fluorescence measurements. The influence of such coatings on the power conversion efficiency (PCE) of commercial PV cells was studied and showed the maximum percentage increase of the order of 15.72% and 19.23% for c-Si and GaAs PV cells. This result evinces that these LDS coatings represent a potential way to improve the efficiency of commercial PV cells.

The Analysis of Dual-Junction Tandem Solar Cells Enhanced by Surface Dispensed Quantum Dots

The luminescent down-shifting (LDS) effect of a colloidal quantum dot layer on a dual junction solar cell is explored and analyzed. A 5.47% enhancement of power conversion efficiency is recorded when green quantum dots are dispersed on device surface. The surface reflectance spectra before and after the quantum dot dispenses are necessary to decouple the different mechanisms between antireflective and LDS effects. A further deduction of formulation on the dual-junction device with antireflection and LDS effect is presented and as high as 44.6% of short-circuit current increase in the green quantum dot case can be attributed to the LDS effect according to our calculation.

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.

Improving spectral modification for applications in solar cells: A review

Abstract The spectral mismatch between solar cells and incident radiation is a fundamental factor limiting their efficiencies. There exist materials and luminescent processes which can modify the incident sunlight’s properties to better suit the cell’s optimal absorption regions. This makes for an interesting area of research and promising technique for enhancing the efficiency of solar cells which is important for environmental reasons. It is intended for this review to provide the reader with historical and up-to-date developments of the application of spectral modification to solar cells and contribute to growing its impact on real-world PV devices. We concisely outline the underlying principles of three spectral modification processes: upconversion (UC), downconversion (DC) and luminescent downshifting (LDS). For each section we present up to date experimental results for applications to a range of solar PV technologies and discuss their drawbacks. With particular focus on UC, we then review how nanostructures or integrated optics might overcome these problems. Finally, we discuss practical challenges associated with advancing this approach for commercialisation and opportunities spectral modification presents; namely where future research should focus and via a cost analysis with a simple formula that can be used to determine financial viability for the deployment of this technology.

Tuning the optical properties of luminescent down shifting layers based on organic dyes to increase the efficiency and lifetime of P3HT: PCBM photovoltaic devices

Mixtures of luminescence downshifting (LDS) materials has been used to increase the efficiency of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) bulk heterojunction solar cell. This layers convert more energetic photons to lower energies that are better matched with wavelength peak of the external quantum efficiency (EQE) of a P3HT:PC61BM solar cell. Experimental studies were used to optimise the optical properties of LDS layers including the maximum of absorbance and the photoluminescence quantum yield (PLQY). To provide the significant improvements, combinations of LDS mixtures were prepared to provide the greatest absorption and PLQY. The approach is shown to simultaneously improve the photocurrent and increase the lifetime of the device by absorbing UV light. By optimising the optical properties of the LDS mixture, a relative increase of about 20% in the photocurrent density produced by the P3HT:PCBM cell could be achieved, which to our knowledge is one of the most significant reported for OPVs.

Spectral Dependence of Degradation under Ultraviolet Light in Perovskite Solar Cells

Perovskite solar cells (PSCs) demonstrate excellent power conversion efficiencies (PCEs) but face severe stability challenges. One key degradation mechanism is exposure to ultraviolet (UV) light. However, the impact of different UV bands is not yet well established. Here, we systematically study the stability of PSCs on the basis of a methylammonium lead iodide (CH3NH3PbI3) absorber exposed to (i) 310-317 (UV-B range) and (ii) 360-380 nm (UV-A range), under accelerated conditions. We demonstrate that the investigated UV-B band is detrimental to the stability of PSCs, resulting in PCE degradation by more than 50% after an exposure period >1700 sun-hours. This finding is valid for architectures with a range of electron transport layers, including SnO2, compact-TiO2, electron-beam TiO2, and nanoparticle-TiO2. We also show that photodegradation is apparent for high, as well as for low illumination intensities of UV-B light, but not for illumination with UV-A wavelengths. Finally, we show that degradation of PSCs is preventable at the cost of a small fraction of photocurrent by using UV-filtering or luminescent downshifting layers.

Perovskite Quantum Dots Embedded Composite Films Enhancing UV Response of Silicon Photodetectors for Broadband and Solar‐Blind Light Detection

AbstractIn this work, the integration of in situ fabricated perovskite quantum dots embedded composite films (PQDCFs) as downshifting materials is first reported for enhancing the ultraviolet (UV) response of silicon (Si) photodetectors toward broadband and solar‐blind light detection. External quantum efficiency measurements show that the UV response of PQDCF coated Si photodiodes greatly improves from near 0% to at most of 50.6% ± 0.5% @ 290 nm. As compared to the calculated maximum value of 87%, the light coupling efficiency of the integrated device is determined to be 80%@395 nm, suggesting an efficient downshifting process. Furthermore, PQDCF is also successfully adapted for electron multiplying charge coupled device (EMCCD) based image sensor. The PQDCF coated EMCCD shows linear response with high‐resolution imaging under illumination at 360, 620, and 960 nm, implying the ability of broadband light detection in the UV, visible (VIS), and near infrared (NIR) region. Furthermore, a solar‐blind UV detection is demonstrated by integrating a solar‐blind UV filter with PQDCF coated EMCCD. In all, the use of PQDCF as luminescent downshifting materials provides an effective and low‐cost way to improve the UV response of Si photodetectors.

Photovoltaic Performance Enhancement of Silicon Solar Cells Based on Combined Ratios of Three Species of Europium-Doped Phosphors.

This paper presents a scheme for the enhancement of silicon solar cells in terms of luminescent emission band and photovoltaic performance. The proposed devices are coated with an luminescent down-shifting (LDS) layer comprising three species of europium (Eu)-doped phosphors mixed within a silicate film (SiO2) using a spin-on film deposition. The three species of phosphor were mixed at ratios of 0.5:1:1.5, 1:1:1, or 1.5:1:0.5 in weight percentage (wt %). The total quantity of Eu-doped phosphors in the silicate solution was fixed at 3 wt %. The emission wavelengths of the Eu-doped phosphors were as follows: 518 nm (specie-A), 551 nm (specie-B), and 609 nm (specie-C). We examined the extended luminescent emission bands via photoluminescence measurements at room temperature. Closely matching the luminescent emission band to the high responsivity band of the silicon semiconductor resulted in good photovoltaic performance. Impressive improvements in efficiency were observed in all three samples: 0.5:1:1.5 (20.43%), 1:1:1 (19.67%), 1.5:1:0.5 (16.81%), compared to the control with a layer of pure SiO2 (13.80%).

Luminescent down shifting CdTe colloidal quantum dots for enhancing polycrystalline silicon solar cells

In this paper, CdTe quantum dots (QDs) with emission bands at 762 nm and 812 nm were investigated for their use in luminescent down shifting (LDS) layers. Spectral location of emission bands and quantum yield of luminescence of the QDs in solution proposed that they were suitable nanoparticles for inclusion in LDS layers. QDs were dispersed in poly (methylmethacrylate) (PMMA) polymer matrix and thin films were fabricated approximately of 70 microns. Significant solar cell efficiency enhancements (approximately 5%–10%) under normal (AM1.5, 25 °C) condition were observed after QDs doped PMMA coatings. The experimental results showed increments in the Voc from 478 to 541 mV, Isc from 35 to 42 mA, Jsc from 8.96 to 10.51 mA/sm2, FF from 69.1 to 77.3% and efficiency from 13.12 to 19.48%. PMMA thin film doped with CdTe QDs was spin-coated on the front surface of Si solar cell for enhancing the solar cell efficiency via photoluminescence (PL) conversion and antireflection effects.

Improving Efficiency and Light Stability of Perovskite Solar Cells by Incorporating YVO4:Eu3+, Bi3+ Nanophosphor into the Mesoporous TiO2 Layer

Even though a high power conversion efficiency (PCE) of 22.1% is achieved in the perovskite solar cells (PSCs), the ultraviolet (UV) light stability in PSCs is still a challenge that needs to be overcome before their actual application in an outdoor environment. In this work, we incorporated the YVO4:Eu3+, Bi3+ luminescent down-shifting (LDS) material into the mesoporous TiO2. The YVO4:Eu3+, Bi3+ would convert the high UV photons into the low energy visible photons (red light) and improve the utilization of UV light in PSCs, which results in PCE enhancement and UV light stability improvement in PSCs. By controlling the YVO4:Eu3+, Bi3+ concentrations, an optimized PCE of 17.9% is obtained, which shows an improvement of 9.8% compared with the bare device. More importantly, the device based on the m-TiO2/YVO4:Eu3+, Bi3+ layer has distinguished stability under UV light irradiation, which could maintain 70% initial efficiency after 60 h of UV irradiation.

Efficiency enhancement of single-junction GaAs solar cells coated with europium-doped silicate-phosphor luminescent-down-shifting layer

In this study, we investigated the electrical and optical performance of single-junction GaAs solar cells coated with an antireflective layer of indium tin oxide (ITO) via thermal sputtering deposition followed by a layer of SiO2 doped with 3 wt% europium-doped (Eu-doped) silicate phosphors via spin-on film technique. The chemical composition of the Eu-doped silicate phosphors was analyzed using energy-dispersive X-ray spectroscopy and the luminescent downshifting (LDS) characteristics were examined in terms of photoluminescence, optical reflectance, and external quantum efficiency (EQE) response. Reverse saturation-current and ideality factor were used to evaluate the passivation performance of ITO films thermal sputtered on GaAs solar cells. The antireflective performance of the ITO film and the LDS effects of the Eu-doped silicate phosphor coatings were respectively evaluated in terms of optical reflectance and EQE response. The enhancement of photovoltaic performance due to LDS effects was confirmed by photovoltaic current density–voltage characteristics of cells under one-sun air mass 1.5G solar simulations. The efficiency enhancement of the cell with only an ITO/SiO2 antireflective layer was 18.39%, whereas the cells coated a SiO2 layer that included various species of Eu-doped phosphors (species-A, species-B, or species-C) on ITO achieved efficiency enhancements of 19.83%, 20.29%, and 21.07%, respectively.

Praseodymium – A Competent Dopant for Luminescent Downshifting and Photocatalysis in ZnO Thin Films

Abstract Highly transparent and conducting Zinc oxide (ZnO) thin films doped with Praseodymium (Pr) were deposited on glass substrates by using the spray pyrolysis method. The X-ray diffraction (XRD) analysis revealed the polycrystallinity of the deposited films with a hexagonal wurtzite structure, whereas the energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the incorporation of Pr in the films. The optical energy gap decreased by Pr doping due to the merging of the conduction band with the impurity bands formed within the forbidden gap. The room temperature photoluminescence spectra of the Pr-doped film showed enhancement of visible emission, suggesting efficient luminescent downshifting. The photocatalytic activity of the Pr-doped films is higher than that of undoped films due to the effective suppression of the rapid recombination of the photo-generated electron-hole pairs. The impurity levels formed within the forbidden gap act as efficient luminescent centers and electron traps, which lead to luminescent downshifting and enhanced photocatalytic activity.

ZnSe/ZnS core-shell quantum dots incorporated with Ag nanoparticles as luminescent down-shifting layers to enhance the efficiency of Si solar cells

Luminescent down-shifting (LDS) is a spectral conversion method for improving the short-wavelength response of Si-based solar cell by transforming shorter wavelength (200 nm-400 nm) into longer wavelength (>400 nm). In this paper, LDS efficiency was enhanced through incorporating Ag nanoparticles into ZnSe/ZnS quantum dots (QDs), as demonstrated by photoluminescence (PL) spectra and finite-difference time-domain (FDTD) simulation results. The mixed Ag/ZnSe/ZnS QDs were spin-coated on the surface of manufactured Si-solar cell so as to efficiently utilize LDS properties. The external quantum efficiency (EQE) and short-circuit current density (Jsc) of Ag/ZnSe/ZnS QDs–Si hybrid solar cell were respectively increased by 30% and 7.5% compared to those of ZnSe/ZnS–Si hybrid solar cell. These improvements were mainly ascribed to localized surface plasmonic (LSP) enhancement effect of Ag nanoparticles. Such small but distinct improvements are expected to provide the theoretical basis and technical support for the development of photovoltaic devices in the future.