Luminescent Down-shifting Layer Research Articles

Synthesis and photophysical properties of tetraphenylethylene derivatives as luminescent downshifting materials for organic photovoltaic applications

Luminescent Down-Shifting (LDS) is an optical approach applied in several photovoltaic (PV) technologies in which high energy solar radiation is converted to a wavelength region where the response of the photovoltaic devices is better. The use of LDS layers on organic photovoltaics (OPV) could serve two purposes: to prevent cell degradation by filtering the incident ultraviolet (UV) radiation and to improve the spectral response of PV cells at short-wavelength. This work reports, the design and synthesis of a series of tetraphenylethylene (TPE) derivatives based on TPE-A or A-π-TPE-π-A molecular structure featuring various electron-acceptor (A) groups. The photophysical properties of the new LDS compounds were systematically studied in 1,4-dioxane solution and film (Zeonex) by UV–visible absorption and fluorescence spectroscopy, and electrochemical properties studied by cyclic voltammetry. Thermal stability of the new LDS compounds was evaluated by Thermogravimetric analysis (TGA). Theoretical computational studies provided evidence of existence of intramolecular charge-transfer (ICT) between frontier orbitals of donor and acceptor moieties. The good photophysical and thermal properties of the synthesized TPE derivatives, associated with high molar absorption coefficients in UV spectrum and emission maximum in the range of 476–531 nm, make them promising candidates for LDS layers in OPV application.

Application of heterostructured CdS/ZnS quantum dots as luminescence down-shifting layer in P3HT:PCBM solar cells

Organic photovoltaic devices (OPVs) have a shorter working lifespan and lower power conversion efficiency if compared to traditional silicon-based solar cells. Intending to increase the lifespan and improve the performance of OPVs, in this work we have applied a layer of Cadmium Sulfide (CdS) Quantum Dots (QDs) encapsulated in Zinc Sulfide (CdS/ZnS) to act as a luminescence down-shifting (LDS) layer in poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric-acid-methyl ester (PCBM) organic solar cell. The QDs were synthesized by the hot injection route and deposited by casting on a device with ITO/PEDOT:PSS/P3HT:PCBM/Al structure. OPVs with and without the LDS layer of QDs were tested over time of exposure to the illumination of an AM1.5 solar simulator. The results showed that in the cells with the QDs layer, there was an increase in Open Circuit Voltage (Voc), Short Circuit Current Density (Jsc), Fill Factor (FF), and Power Conversion Efficiency (η), after 70 min of exposure, reaching a peak of these parameter values around 120 min of light exposure, while in the cells without the LDS layer, these parameters have decreased over the entire exposure time. This behavior was explained taking into account the improvement of the photoluminescence (PL) yield by the QDs film and the illumination process.

Improving Ultraviolet Responses in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells Using Quantum Dot-Based Luminescent Down-Shifting Layer.

Quantum dot (QD)-based luminescent down-shifting (LDS) layers were deposited on Cu2ZnSn(S,Se)4 (CZTSSe) solar cells via the drop-casting method. The LDS layers can easily widen the narrow absorption wavelength regions of single-junction solar cells and enable improvement of the short-circuit current. The optical properties of LDS layers deposited on glass and containing different QD contents were analyzed based on their transmittance, reflectance, and absorbance. The absorber films to be used in the CZTSSe solar cells were determined by X-ray diffraction measurements and Raman spectroscopy to determine their crystal structures and secondary phases, respectively. The completed CZTSSe solar cells with LDS layers showed increased ultraviolet responses of up to 25% because of wavelength conversion by the QDs. In addition, the impact of the capping layer, which was formed to protect the QDs from oxygen and moisture, on the solar cell performance was analyzed. Thus, a maximal conversion efficiency of 7.3% was achieved with the 1.0 mL QD condition; furthermore, to the best of our knowledge, this is the first time that LDS layers have been experimentally demonstrated for CZTSSe solar cells.

Ultraviolet-Filtering Luminescent Transparent Coatings for High-Performance PTB7-Th:ITIC–Based Organic Solar Cells

Photovoltaic (PV) devices based on organic heterojunctions have recently achieved remarkable power conversion efficiency (PCE) values. However, photodegradation is often a cause of dramatic drops in device performance. The use of ultraviolet (UV)-absorbing luminescent downshifting (LDS) layers can be a mitigation strategy to simultaneously filter UV radiation reaching the device and reemit it with lower energy in the visible spectral range, matching the maximum spectral response of the PV cells and thus enabling the increase of the photocurrent generated by the cell. In this work, we report the use of a Eu3+-doped siliceous-based organic–inorganic hybrid as a coating on organic solar cells based on the PTB7-Th:ITIC bulk heterojunction with the purpose of increasing their performance. We found that the applied coatings yield a PCE enhancement of ∼22% (from 3.1 to 3.8%) in solar cells with spin-coated layers, compared with the bare solar cells, which is among the highest performance enhancements induced by plastic luminescent coatings.

Microwave-assisted synthesis of multifunctional fluorescent carbon quantum dots from A4/B2 polyamidation monomer sets

Carbon quantum dots (CQDs) are becoming a desirable alternative to metal-based QDs due to their high biocompatibility, low toxicity, ease of preparation, and unique photophysical properties. In the present study, a simple microwave-assisted synthesis using a domestic oven has been adopted to produce multifunctional CQDs from the ethlyenediaminetetraacetic acid (EDTA) and 4, 7, 10-trioxa-1, 13-tridecanediamine (TTDDA) monomers. The molecular surface of the prepared CQDs is composed of hyperbranched polyamides obtained from the polymerization reaction of both EDTA and TTDDA. The controlled functionalities and chemical structures of the molecular surface of CQDs are attained by varying the initial feed ratio of EDTA and TTDDA from 1:1 to 1:2, 1:3, 1:4, 1:5, and 3:1. The T-richCD3 (EDTA: TTDDA = 1:4) with highest mass yield of 25 wt% exhibited five-folds higher photoluminescence quantum yield (PLQY) (53.3%) compared to E-richCD1 (EDTA: TTDDA = 3:1). All synthesized CQDs revealed good sensitivity and selectivity to a Cu2+ ion that is prone to cause different neurodegenerative diseases and obstruct many biological activities, while the sensitivity increased with the decrease in EDTA to TTDDA ratio. The zebrafish model has been used to demonstrate the toxicity, adverse effects, transport, and biocompatibility of CQDs. At higher concentrations (1.0 mg/mL), the T-richCDs exhibited superior cell viability than the E-richCDs for Madin-Darby Canine Kidney (MDCK) and human breast cancer MDA-MB-231 (MDAMB) cells. Because of the excellent down conversion fluorescence ability, T-richCD3 were employed as luminescent down-shifting (LDS) layer in CuInSe2 solar cell to enhance its performance.

Improvement of the current generation in silicon solar cells by utilizing graphene quantum dot as spectral converter

This paper deals with a luminescent layer applied on a commercial Silicon (Si) solar cell by spin coating of an aqueous solution of graphene quantum dots (GQDs), synthesized by the hydrothermal method at 200 °C for 24 h. The GQDs layer, acted as a luminescent down-shifting (LDS) layer, causes a relative enhancement in the short circuit current density (JSC) of mc-Si and c-Si solar cells up to 5.14% and 6.82%, respectively. Remarkable relative enhancements in JSC under UV illumination showed that the LDS layer increases the JSC by downshifting effect; Diffuse reflectance spectroscopy (DRS) of the solar cells with and without the LDS layers showed that the GQD filled LDS layers do not have a considerable effect on the light reflection from the Si solar cell to affect the JSC.

Colorful opaque photovoltaic modules with down-converting InP/ZnSexS1-x quantum dot layers

Abstract The luminescent down-shifting (LDS) layer, which transforms incoming high energy solar photons to visible ones, promises augmentation of both photoelectric performance and aesthetic appeals of photovoltaic (PV) modules. For efficient, colored PVs with LDS layer, luminophores with high photoluminescence quantum yield (PL QY), small overlap between absorption and emission spectra, and proven photostability are prerequisites. Here, we demonstrate colorful, opaque PV modules with LDS layers of minimized photon sacrifice enabled by structurally-engineered, eco-friendly InP/ZnSexS1-x quantum dots (QDs). Specifically, composition-controlled, thick shells allow enhanced absorption in the UV region and improved PL QY of QDs. Additionally, the ligand-engineering guarantees the stability of LDS layers after the damp heat test. Benefited from QD-LDS layers, commercially available CIGS and c-Si PV modules are awarded with 40% EQE enhancement in the ultraviolet region and wide-ranging color tunability over the entire visible region by QD-LDS layers. Hence, this integrated approach for desirable luminophores will contribute to the realization of highly-efficient, aesthetically-appealing opaque PV modules.

Quantum yield enhancement of Mn-doped CsPbCl3 perovskite nanocrystals as luminescent down-shifting layer for CIGS solar cells

The emission and absorption properties of Mn doped CsPbCl3 NC make it ideal candidates as luminescent down shifting layer material for solar cells, However, effective Mn doping and further quantum yield improvement remains challenging. We present our modified hot injection route to obtain Mn doped CsPbCl3 nanocrystals with different morphologies and enhanced quantum yield. The samples presented were synthesized with precursor MnCl2 and PbCl2 with the ratio of 1:1, 1:2, 1:2.5 and 1:3. For samples with lower Mn doping level, the as prepared samples are uniform nanocubes with average size of 11 nm. When MnCl2 amount is increased beyond 2.5, large piece of nanosheets were found by TEM and AFM with ultrathin thickness and strong quantum confinement effect. CsPbCl3 nanocrystals with optimal Mn doping level show enhanced quantum yield of 69%. Luminescent down shifting layers prepared from Mn doped CsPbCl3 nanocrystals successfully increased short-circuit current (6.67%) and PCE (1.67%) of Cu(In,Ga)Se2 solar cell.

The Viability of Organic Dyes in Luminescent Down-Shifting Layers for the Enhancement of Si Solar Cell Efficiency

The main energy losses in solar cells are related to spectral losses where high energy photons are not used efficiently, and energy is lost via thermalization which reduces the solar cell’s overall efficiency. A way to tackle this is to introduce a luminescent down-shifting layer (LDS) to convert these high energy photons into a lower energy bracket helping the solar cell to absorb them and thus generating a greater power output. In this paper, lumogen dye Violet 570 has been used as LDS coated films of 10μm and 60μm placed on top of Si solar cells. The dye was incorporated into polymer films of Polyvinyl Butyral (PVB) and Polymethyl Methacrylate (PMMA) after which they were tested for their absorption, transmission and emission properties. Once optimised layers had been determined, they were deposited directly onto silicon solar cells and the external quantum efficiency (EQE) of the Si solar cells were measured with and without the LDS layers. The resulting graphs have shown an increase of up to 2.9% in the overall EQE efficiency after the lumogen films had been applied.

Eu(III) complex-polymer composite luminescence down-shifting layers for reducing the blue-losses in thin film solar cells

In this work a luminescence down-shifting (LDS) layer, which can reduce significantly the blue-loss due to absorption in the n-type layer of popular thin-film solar cells such as CIGS, CdTe and CZTS is demonstrated. The potential benefits of the developed LDS layer is demonstrated in a prototype CdTe/CdS solar cell. The blue response of the devices coated with Eu(TTA)3Phen-based LDS layer exhibit an improvement by a factor of two to three times, reaching almost 6% relative enhancement in the short-circuit photocurrent density, compared with the reference device without LDS. The spectral characteristics of the Eu-complex were studied by dissolving in different organic solvents and polymer matrices. The microstructure of the polymer matrix plays a key role on the size, shape and spatial distribution of Eu-complex microparticles, which enables a control of photoluminescence properties at optimal conditions of agglomeration, isotropic luminescence and reduced quenching. The results indicate that the solvents affect differently the spectral characteristics of the emission spectra of the Eu-complex, which show a hypsochromic band shift as the solvent polarity increases. The analysis of the relative intensity ratio (R) of 5D0 → 7F2 to 5D0 → 7F1 transition indicated the deterioration of local symmetry around europium ions with the reduction of solvent polarity. These results provide a means to improve the emission efficiency of fluorophores embedded in porous matrices and also bring new insights into LDS materials for photovoltaic devices. The developed LDS layer will have applications in devices which use a high band gap heterojunction partner.

Solar spectral conversion based on plastic films of lanthanide-doped ionosilicas for photovoltaics: Down-shifting layers and luminescent solar concentrators

The mismatch between the photovoltaic (PV) cells absorption and the solar irradiance on earth is one of the major limitations towards more efficient PV energy conversion. This aspect was addressed by down-shifting the solar irradiance on Earth through luminescent down-shifting layers based on lanthanide-doped surface-functionalized ionosilicas (ISs) embedded in poly(methyl methacrylate) (PMMA) coated on the surface of commercial Si-based PV cells. The IS-PMMA hybrid materials exhibit efficient solar radiation harvesting (spectral overlap of ∼9.5 × 1019 photons/(s·m2)) and conversion (quantum yield ∼52%). The direct solar radiation and the down-shifted radiation are partially guided and lost through total internal reflection to the layer edges being unavailable for PV conversion of the coated PV cell. By tuning the down-shifting layer thickness, it also acts as luminescent solar concentrator enabling the collection of the guided radiation by flexible PV cells applied on the borders of the down-shifting layer leading to an enhancement of the PV energy conversion from ∼5% (in the case of the single-use of the luminescent down-shifting layer) to ∼13% comparing with the bare PV cell. The overall electrical output of the device resulted in an absolute external quantum efficiency increase of ∼32% for the optimized Eu3+-based films in the UV spectral region (compared with the bare PV device, which is among the best values reported so far).

Luminescent down-shifting CsPbBr3 perovskite nanocrystals for flexible Cu(In,Ga)Se2 solar cells

To overcome the parasitic absorption of ultraviolet (UV) light in the transparent conductive oxide (TCO) layer of flexible Cu(In,Ga)Se2 (CIGS) thin film solar cells, a CsPbBr3 perovskite nanocrystal based luminescent down-shifting (LDS) layer was integrated on CIGS solar cells fabricated on a stainless steel foil. The CsPbBr3 perovskite nanocrystal absorbs solar irradiation at wavelengths shorter than 520 nm and emits photons at a wavelength of 532 nm. These down-shifted photons pass the TCO layer without parasitic absorption and are absorbed in the CIGS absorber layer where they generate photocurrent. By minimizing the parasitic absorption in the TCO layer, the external quantum efficiency (EQE) of the CIGS solar cell with the CsPbBr3 perovskite nanocrystal layer is highly improved in the UV wavelength range between 300 and 390 nm. Additionally, in the wavelength range between 500 and 1100 nm, the EQE is improved since the surface reflectance of the CIGS device with the CsPbBr3 perovskite LDS layer was reduced. This is because the CsPbBr3 perovskite nanocrystal layer, which has an effective refractive index of 1.82 at a wavelength of 800 nm, reduces the large refractive index mismatch between air (nair = 1.00) and the TCO layer (nZnO = 1.96 at a wavelength of 800 nm). Both the short circuit current density and power conversion efficiency of the flexible CIGS solar cell integrated with the CsPbBr3 perovskite are improved by 4.5% compared with the conventional CIGS solar cell without the CsPbBr3 perovskite LDS layer.

A large-area luminescent downshifting layer containing an Eu3+ complex for crystalline silicon solar cells.

The spectral mismatch between the distribution of sunlight (AM1.5G) and crystalline silicon (c-Si) solar cells is one of the most important limiting factors of the conversion efficiency of photovoltaic (PV) devices. As an effective solution, the use of the luminescence down-shifting (LDS) technique is an important way to improve the short-wavelength response of a solar cell by shifting high-energy photons to the visible range. Herein, a large-area (17 × 17 cm2) luminescent thin film consisting of a ternary europium (Eu3+) complex and polyvinyl alcohol (PVA) was successfully constructed through a solution casting method and further developed as an effective LDS layer to improve the photoelectric conversion efficiency of c-Si solar cells with a large active area (235 cm2). The self-standing LDS layer is flexible, transparent and easily attachable to the surface of the solar cell module. Compared with the uncoated c-Si solar cell, the one coated with the LDS layer displayed an enhancement of ∼15% in external quantum efficiency (EQE) due to the high luminescence quantum yield of the Eu3+ complex doped inside the layer. These results demonstrate that use of a large area luminescent film embedding an Eu3+ complex is a versatile and effective strategy to improve the conversion efficiency of large size PV devices, giving rise to its great potential application as an LDS material.

Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells

Despite the remarkable photovoltaic characteristics and printability of perovskite solar cells, their intrinsic instability has been the most serious drawback toward future commercialization. In this work, we have investigated the stability of perovskite films in terms of morphology, electronic properties and chemical compositions. Specifically, the chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, superior durability and attractive PCE of 21.5%. Femto-second transient absorption spectra and GIWAXS measurements provide deep insight into the reduced carrier recombination and indicate the improved crystallinity of the modified perovskite films. Furthermore, an efficient hole-transporting material, PDPP4T, without using any doping process is applied to achieve PSCs with enhanced open-circuit voltage and better repeatability. As a consequence, the modified PSCs could maintain 82% of their initial efficiency after 5000 h of storage in ambient conditions and 90% of their initial efficiency after 1000 h of light soaking process. An excellent water resistance up to 100 h of the PSCs is also obtained by encapsulation technology. Besides, after coating Ce3+-CsPbI3 nanocrystals as luminescent down-shifting layers on the front side of the PSCs, the PCE of the device was further improved to 22.16%.

Enhance short-wavelength response of CIGS solar cell by CdSe quantum disks as luminescent down-shifting material

As the luminescent down-shifting (LDS) material, Cadmium selenide (CdSe) quantum disks (QDs) have been synthesized for improving the short-wavelength response of Cu(In, Ga)Se2 (CIGS) solar cells. The peak of the photoluminescence spectra of the as-prepared QDs is coinciding with the peak of the external quantum efficiency (EQE) of CIGS solar cell. The absorption and photoluminescence spectra of the as-prepared CdSe QDs show that the photons of wavelength below 460 nm, which could not be converted into electricity by CIGS solar cell, would be absorbed by the QDs and generating longer wavelength photons that could contribute to charge carrier generation by the cell. The QDs were embedded into the polymethylmethacrylate (PMMA) forming the colloidal fluorescent mixture. Subsequently, the mixture was applied to the surface of the solar cell as the LDS layer. It is found that the EQE of CIGS devices increases up to about 25% in the range of 300–460 nm due to LDS. The placement of the LDS layer upon the CIGS solar cell improved the electrical power output of the bare cell to 114%.

Optimizing the optical properties of a dye-based luminescent down-shifter to improve the performance of organic photovoltaic devices

Through luminescence downshifting (LDS) effect, high energy solar radiation is converted to a wavelength region where the response of the photovoltaic devices is better. Additionally, LDS layers deposited on organic materials-based devices could provide a protective feature for the atmospheric agents, like ultraviolet photons, oxygen and moisture, that cause material degradation. This work reports an optimization procedure for the optical properties of a LDS layer composed by the organic dye commercially called Kremer Blue dispersed on poly-methyl methacrylate (PMMA) inert matrix. With optimized values for photoluminescence quantum yield (PLQY) and absorbance parameters, this LDS layer became an efficient, low cost UV filter for solar cells based on poly (3-hexylthiophene):[6,6]‐phenyl C61‐butyric acid methyl ester (P3HT:PC61BM) blend, at the same time that provoked a significant increasing (around 6%) in the device's photocurrent production. Such optimization procedure can be applied to other materials that could be candidates as luminescence down-shifters for the various photovoltaic technologies currently available.

Enhanced spectral response of CIGS solar cells with anti-reflective subwavelength structures and quantum dots

A cover glass integrated with subwavelength structures (SWSs) and quantum dots (QDs) was realized to enhance the spectral response of Cu(In1-xGax)Se2 (CIGS) thin film solar cells. The nanoscale SWSs was produced on the front and backside of cover glass using the self-masked etching method. The fabricated SWSs-integrated glass (SIG) exhibited superior anti-reflective properties over a broad wavelength range and a broad range of incident angle. The average transmittance of the single and double-sided SIG was increased from 92.4% to 95.2% and 97.8% in the wavelength range between 300 and 1200 nm, as compared to that of a flat surface glass. QDs were formed between the fabricated SIG and ZnO layer to act as a luminescent down-shifting (LDS) layer. A large amount of light can pass through the SIG, enhancing the efficiency of the LDS by improving light absorption and lowering reflection. After successful integration of the SIG with QDs (QD/SIG) on top of the CIGS devices, photovoltaic performance was significantly improved. The power conversion efficiency of the CIGS devices integrated with QD/double-sided SIG was improved by 7.41%, compared with that of a reference device (without QD/cover glass). The efficiency improvement is attributed to the enhanced solar energy harvesting by the efficient LDS and the minimized surface reflection loss.

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.

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.

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.