Upconversion Materials Research Articles

Upconversion Properties and Temperature-Sensing Behaviors of Alkaline-Earth-Metal Scandate Nanocrystals Doped with Er3+/Yb3+ Ions in the Presence of Alkali Ions (Li+, Na+, and K+).

Temperature-sensing media based on the fluorescence intensity ratio (FIR) of upconversion materials that suffer from low sensitivity owing to the small energy gap still have a need for new compounds with strong upconversion luminescence (UCL). In this work, a series of MSc2O4:Er3+/Yb3+ (M = Mg, Ca, Sr, and Ba) nanocrystals were prepared by a hydrothermal method using NaOH alkaline solution. The structure, morphology, and UCL characteristics of materials were investigated, and the red UCL of the CaSc2O4:Er3+/Yb3+ sample was dramatically enhanced by a factor of ∼12, ∼23, and ∼2000 compared with SrSc2O4, MgSc2O4, and BaSc2O4 samples, respectively. By adjusting alkali ions (Li+, Na+, K+), the UCL intensities of CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ samples were further improved, especially in the presence of Li+ ions. Excellent temperature-sensing behaviors are realized for CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ samples in the presence of Li+ ions, in which the maximum absolute sensitivity SA values are about 0.0041 and 0.0036 K-1 at 600 K and the corresponding relative sensitivity SR values are expressed as 1197/T2 and 1129/T2 (the current optimal SR = 1289/T2), respectively. The intense UCL and excellent SA and SR values indicate that CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ materials are promising candidates for application in high-temperature sensors working under 980 nm excitation.

Study of synthesis temperature effect on β-NaGdF4: Yb3+, Er3+ upconversion luminescence efficiency and decay time using maximum entropy method

Upconversion materials have several advantages for many applications due to their great potential in converting infrared light to visible. For practical use, it is necessary to achieve high intensity of UC luminescence, so the studies of the optimal synthesis parameters for upconversion nanoparticles are still going on. In the present work, we analyzed the synthesis temperature effect on the efficiency and luminescence decay of β-NaGd0.78Yb0.20Er0.02F4 (15–25 nm) upconversion nanoparticles with hexagonal crystal structure synthesized by anhydrous solvothermal technique. The synthesis temperature was varied in the 290 °C–320 °C range. The synthesis temperature was shown to have a significant influence on the upconversion luminescence efficiency and decay time. The coherent scattering domain linearly depended on the synthesis temperature and was in the range 13.1–22.3 nm, while the efficiency of the upconversion luminescence increases exponentially from 0.02 to 0.10% under 1 W cm−2 excitation. For a fundamental analysis of the reasons for the upconversion luminescence intensity dependence on the synthesis temperature, it was proposed to use the maximum entropy method for luminescence decay kinetics processing. This method does not require a preliminary setting of the number of exponents and, due to this, makes it possible to estimate additional components in the luminescence decay kinetics, which are attributed to different populations of rare-earth ions in different conditions. Two components in the green luminescence and one component in the red luminescence decay kinetics were revealed for nanoparticles prepared at 290 °C–300 °C. An intense short and a weak long component in green luminescence decay kinetics could be associated with two different populations of ions in the surface quenching layer and the crystal core volume. With an increase in the synthesis temperature, the second component disappears, and the decay time increases due to an increase in the number of ions in the crystal core volume and a more uniform distribution of dopants.

Influence on the Apparent Luminescent Lifetime of Rare-Earth Upconversion Nanoparticles by Quenching the Sensitizer's Excited State for Hypochlorous Acid Detection and Bioimaging.

Lanthanide-ion-doped upconversion materials have been widely used in biological detection, bioimaging, displays, and anticounterfeiting due to their abilities of real-time readings, high spatial resolution, and deep tissue penetration. The typically long fluorescence lifetimes of rare-earth nanoparticles, in the microsecond to millisecond range, make them useful in interference-free lifetime detection imaging. Most detection systems are accompanied by fluorescence resonance energy transfer (FRET), in which the lifetime of the luminescence center can be used as a signal to reveal the degree of FRET. Due to the complex energy level structure and complex energy transfer processes, the apparent lifetimes of upconversion nanoparticles (UCNPs) do not simply equal the decay time of the corresponding energy level, inducing an insignificant lifetime change in the upconversion detection system. In this study, the relationship between the apparent luminescence lifetime of upconversion and the decay rate of each energy level was studied by numerical simulations. It was proved that the apparent lifetime of the emission at 540 nm was mainly affected by the decay rate of Yb3+. We then constructed a nanocomposite with Rh1000 fluorophores loaded onto the surface of UCNPs to quench the sensitizer Yb3+. We found that the lifetime of the emission at 540 nm from Er3+ was affected to a large extent by the number of attached Rh1000 molecules, proving the greater influence on the apparent luminescent lifetime of Er3+ at 540 nm caused by quenching the Yb3+ excited state. The qualitative detection of hypochlorous acid (HClO) in vivo was also achieved using the luminescent lifetime as the signal.

Strong interface contact between NaYF4:Yb,Er and CdS promoting photocatalytic hydrogen evolution of NaYF4:Yb,Er/CdS composites

• NaYF 4 :Yb,Er/CdS composite was prepared by growing CdS nanoparticles on the NaYF 4 :Yb,Er nanorods by ion adsorption/precipitation process. • Annealing induced CdS phase transformation and strong interface contact between CdS and NaYF 4 :Yb,Er greatly enhance photocatalytic hydrogen of the composite. To acquire efficient photocatalysts, it is necessary to make effective use of visible light/Near Infrared (NIR) light, which takes up a large percentage of sunlight. Integrating upconversion materials with visible light active photocatalysts has attracted much attention in this regard. The interface contact between upconversion material and photocatalyst has potential influence on the properties and thus the performance of the system. In this work, NaYF 4 :Yb,Er/CdS composites of the upconversion material NaYF 4 :Yb,Er nanorods and CdS nanoparticles were synthesized by ion adsorption/precipitation process and were then annealed in an argon atmosphere at different temperatures to modulate the microstructures. The annealing process endows the crystal transformation of cubic CdS with low crystallinity to hexagonal CdS with high crystallinity and, importantly, good interface contact between NaYF 4 :Yb,Er and CdS. Consequently, the hydrogen evolution activity greatly increases from 171 to 2539 μmol h −1 g −1 under the light irradiation of λ > 400 nm, and from 0 to 19 μmol h −1 g −1 under the light irradiation of λ > 600 nm. This work might provide a useful reference for the rational design of promising photocatalyst involving upconversion materials.

Biocompatible Yb3+/Er3+ Co-activated La2(WO4)3 Upconversion Nanophosphors for Optical Thermometry, Biofluorescent, and Anticancer Agents.

Non-cytotoxic upconversion nanocrystals are preferred candidates because they offer exceptional advantages for numerous applications, ranging from optical thermometry to bioimaging/biomedical applications. In this report, we demonstrate the luminescence characteristics and practical utility of a multifunctional upconversion nanophosphor based on Yb3+/Er3+:La2(WO4)3 (LWO) flakes. Strong upconversion green emission was observed from 6-mol % Er3+-doped LWO nanophosphor flakes excited by a 980 nm laser. We further enhanced the upconversion emission considerably by co-doping LWO nanophosphors with Yb3+/Er3+ to exploit energy migration from Yb3+ to Er3+ ions. The exceptional improvement in upconversion green and near-infrared emission was achieved by Yb3+ ion co-doping up to 6 mol %; beyond 6 mol %, emission intensities remarkably dropped due to concentration quenching. Photometric parameters were evaluated with and without Yb3+ ion-doped LWO nanophosphors, which exhibited a high green color purity of 95.6%, to elucidate their energy transfer mechanism. In addition, temperature-dependent upconversion emission trends were evaluated by analyzing the fluorescence intensity ratio, exhibiting higher temperature sensitivity than that previously reported. This suggests the applicability of our proposed nanophosphors to optical thermometry. As for bioimaging applications, the non-cytotoxicity of the optimized nanophosphor was confirmed based on distinct fluorescence images of a normal fibroblast cell line (L929). Furthermore, we demonstrated the strong cytotoxicity of nanophosphors against human colon cancer (HCT-116) cells. Based on the results, non-cytotoxic Yb3+(6 mol %)/Er3+ (6 mol %):LWO upconversion nanophosphor flakes are expected to be exceptional candidates owing to their extensive suitability to the fields of upconversion lasers, optical thermometry, and biomedical and anticancer applications. The results indicate the potential of upconversion materials in the effective execution of multiple strategic applications.

Recent progress of upconversion nanoparticles in the treatment and detection of various diseases

Rare earth-doped upconversion nanoparticles (UCNPs) have the advantages of a narrow emission peak, large anti-Stokes displacement, negligible autofluorescence and photobleaching, which make them popular in biomedical fields. With the development of biological technology and nanotechnology, great progress has been achieved in application of rare earth-doped UCNPsfor disease diagnosis and treatment. To date, based on the light conversion characteristics of upconverted rare earth nanocrystals, their medical applications are mainly focused on tumor detection and treatment. In view of the above situation, this paper focuses on reviewing UCNPs applications for the treatment and detection of various diseases, including atherosclerosis, inflammation, bacterial infections, viruses, and tumors. Finally, challenges and opportunities of UCNPs applicaiton in disease treatment and detection are discussed. Overall, our goal is to provide readers with a fresh perspective on developments in the treatment and detection of various diseases with rare earth-doped upconversion materials.

Tuning the Photoluminescence Properties of β‐NaYF4:Yb,Er by Bi3+ Doping Strategy

AbstractThe low luminous efficiency of upconversion materials limits their practical applications in various fields. The present study focuses on the efficient upconversion enhancement of β‐NaYF4:Yb,Er induced by Bi3+ doping. The XRD and SEM results show that introducing Bi3+ into the matrix lattice changes the microstructure and particle size of the crystal. The photoluminescence spectra show that at 6% Bi3+ doping, the red and green emission intensities are enhanced by 12 and 8 times, respectively. The reason could be the lattice distortion caused by Bi3+ doping, breaking the partial forbidden transition of rare‐earth ions. The fluorescence lifetime measurement reveals that the Bi3+ doping changes the environment around erbium ions, affecting the energy level lifetime of erbium ions, explaining the luminescence dynamics law. The present study provides a facile route to achieve nano/microstructures with intense upconversion luminescence with potential applications in optoelectronic devices.

Functional Materials with Wide‐Spectral‐Responsive Photocatalytic Activity and Real‐Time Temperature Feedback: The Electrospun Fibers Embedded with NaGdF4‐Tm‐Yb@TiO2 Nanocrystals

AbstractIn this work, combining the advantages of upconversion materials, nanofibers, and TiO2 semiconductors, the composite fibers embedded with NaGdF4: Tm3+, Yb3+@TiO2 nanocrystals (NGF‐TY@TiO2 NCs) are prepared as wide‐spectral (UV‐NIR) response photocatalysts and temperature sensing materials. Based on the FIR technique, both the thermally (3F2,3 and 3H4) and nonthermally (1G4 and 3F2,3) coupled levels of Tm3+ are confirmed to be applicable as temperature probes for real‐time monitoring and feedback of the working temperature of the catalytic degradation process, and the maximum relative sensitivity values of SR1 and SR2 are as high as 2.760%K−1 and 3.019%K−1 at 304 K, respectively. Under 977 nm laser excitation, the energy migration from NGF‐T0.4Y6 to TiO2 is demonstrated to a fluorescence resonance energy transfer process. The degradation ratio of rhodamine B (RhB) is 19.3% in the presence of NGF‐T0.4Y6@TiO2/PAN fibers and the degradation constant K reaches 0.00117 min−1 after 3 h irradiating from Xe lamp with a UV cutoff filter. Accordingly, this study suggests a new perspective to utilize the NIR energy of sunlight for photocatalytic application based on TiO2, and the addition of dual temperature feedback provides a reliable reference for the development of safer and more efficient catalytic materials.

NIR-to-NIR and NIR-to-Vis up-conversion of SrF2:Ho3+ nanoparticles under 1156 nm excitation

Recently, the up-converting (UC) materials, containing lanthanide ions (Ln3+)have attracted considerable attention because of the multitude of their potential applications. The most frequently investigated are UC systems based on the absorption of near-infrared (NIR) radiation by Yb3+ ions at around 975–980 nm and emission of co-dopants, usually Ho3+, Er3+ or Tm3+ ions. UC can be observed also upon excitation with irradiation with a wavelength different than around 980 nm. The most often studied systems capable of UC without the use of Yb3+ ion are those based on the properties of Er3+ ions, which show luminescence resulting from the excitation at 808 or 1532 nm. However, also other Ln3+ ions are worth attention. Herein, we focus on the investigation of the UC phenomenon in the materials doped with Ho3+ ions, which reveal unique optical properties upon the NIR irradiation. The SrF2 NPs doped with Ho3+ ions in concentrations from 4.9% to 22.5%, were synthesized by using the hydrothermal method. The structural and optical characteristics of the obtained SrF2:Ho3+ NPs are presented. The prepared samples had crystalline structure, were built of NPs of round shapes and their sizes ranged from 16.4 to 82.3 nm. The NPs formed stable colloids in water. Under 1156 nm excitation, SrF2:Ho3+ NPs showed intense UC emission, wherein the brightest luminescence was recorded for the SrF2:10.0%Ho3+ compound. The analysis of the measured lifetime profiles and dependencies of the integral luminescence intensities on the laser energy allowed proposing the mechanism, responsible for the observed UC emission. It is worth mentioning that the described SrF2:Ho3+ samples are one of the first materials for which the UC luminescence induced by 1156 nm excitation was obtained.

Improved Response of Upconversion Luminescence Color to Pump Power through the Coupling of Er3+ and Tm3+

Upconversion materials activated with Er3+ have been demonstrated to be potential candidates for multi-color displays, dynamic fluorescence anti-counterfeiting, and so forth. However, due to the similar upconversion pathways for green and red emissions, it remains a challenge to realize highly sensitive multi-color upconversion luminescence by adjusting the laser power. Here, based on Er2Mo4O15 without concentration quenching of luminescence, we demonstrated that the incorporation of heterogeneous Tm3+ impurities could significantly improve the response of luminescence color to laser power. Through solving a set of rate equations, it was clarified that the power-dependent depletion of the 4I11/2 level was responsible for the modulation of luminescence color. Also, it was proved that the enhancement of luminescence was in relation to the blocked energy migration and enhanced radiative transition probability. Our work provides an effective approach to improve the power response of luminescence colors based on the photon upconversion of Er3+.

Design Guidelines for Rigid Epoxy Resins with High Photon Upconversion Efficiency: Critical Role of Emitter Concentration.

For the practical application of triplet-triplet annihilation-based photon upconversion (TTA-UC), the development of rigid, transparent, air-stable, and moldable materials with a high TTA-UC efficiency remains a challenging issue. In addition to the noncovalent introduction of ionic liquid emitters into the epoxy network, we covalently introduce emitters with polymerization sites to increase the emitter concentration to 35.6 wt %. A TTA-UC quantum yield ΦUC of 5.7% (theoretical maximum: 50%) or a TTA-UC efficiency ηUC of 11.4% (theoretical maximum: 100%) is achieved, which is the highest value ever achieved for a rigid polymer material. More importantly, the high emitter concentration speeds up the triplet diffusion and suppresses the back energy transfer from the emitter to sensitizer so that the sensitized emitter triplet can be effectively utilized for TTA. The generality of our finding is also confirmed for epoxy resins of similar emitter unit concentrations without the ionic liquid. This work provides important design guidelines for achieving highly efficient TTA-UC in rigid solid materials, which has been very difficult to achieve in the past. Furthermore, the solid-state TTA-UC exhibits high air stability, reflecting the high oxygen barrier performance of epoxy resins. The high moldability of epoxy resins allows the construction of upconversion materials with complex geometries at nano- to macroscopic scales.

Direct Electron Transfer from Upconversion Graphene Quantum Dots to TiO2 Enabling Infrared Light-Driven Overall Water Splitting.

Utilization of infrared light in photocatalytic water splitting is highly important yet challenging given its large proportion in sunlight. Although upconversion material may photogenerate electrons with sufficient energy, the electron transfer between upconversion material and semiconductor is inefficient limiting overall photocatalytic performance. In this work, a TiO2/graphene quantum dot (GQD) hybrid system has been designed with intimate interface, which enables highly efficient transfer of photogenerated electrons from GQDs to TiO2. The designed hybrid material with high photogenerated electron density displays photocatalytic activity under infrared light (20 mW cm−2) for overall water splitting (H2: 60.4 μmol gcat.−1 h−1 and O2: 30.0 μmol gcat.−1 h−1). With infrared light well harnessed, the system offers a solar-to-hydrogen (STH) efficiency of 0.80% in full solar spectrum. This work provides new insight into harnessing charge transfer between upconversion materials and semiconductor photocatalysts and opens a new avenue for designing photocatalysts toward working under infrared light.

Suppressing singlet-triplet annihilation processes to achieve highly efficient deep-blue AIE-based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for the fabrication of high efficiency organic light-emitting diodes (OLEDs) by harnessing "hot excitons" from the high-lying triplet exciton states (Tn, n ≥ 2) and high photoluminescence (PL) quantum efficiency in solid films. However, the electroluminescence (EL) efficiency of most AIE-based OLEDs does not meet our expectation due to some unrevealed exciton loss processes. Herein, we further enhance the efficiency of blue AIE-based OLEDs, and find experimentally and theoretically that the serious exciton loss is caused by the quenching of radiative singlet excitons and long-lived triplet excitons [singlet-triplet annihilation (STA)]. In order to suppress the STA process, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) with triplet-triplet annihilation up-conversion was doped in two AIE emitters to reduce the triplet excitons on the lowest triplet excited state (T1) of AIE molecules. It can be seen that the external quantum efficiency (EQE) of the resulting blue OLEDs was enhanced to 11.8% with CIE coordinates of (0.15, 0.07) and a negligible efficiency roll-off, realizing the efficiency breakthrough of deep-blue AIE-based OLEDs. This work establishes a physical insight in revealing the exciton loss processes and the fabrication of high-performance AIE-based OLEDs.

High-level information encryption based on optical nanomaterials with multi-mode luminescence and dual-mode reading

High-level information encryption based on a visible up-conversion and invisible persistent luminescence material.

Infrared-Visible Wavelength Conversion by Upconversion Phosphors and Materials Development

Infrared-Visible Wavelength Conversion by Upconversion Phosphors and Materials Development

Expanding the toolbox of photon upconversion for emerging frontier applications.

Photon upconversion in lanthanide-based materials has recently shown compelling advantages in a wide range of fields due to their exceptional anti-Stokes luminescence performances and physicochemical properties. In particular, the latest breakthroughs in the optical manipulation of photon upconversion, such as the precise tuning of switchable emission profiles and lifetimes, open up new opportunities for diverse frontier applications from biological imaging to therapy, nanophotonics and three-dimensional displays. A summary and discussion on the recent progress can provide new insights into the fundamental understanding of luminescence mechanisms and also help to inspire new upconversion concepts and promote their frontier applications. Herein, we present a review on the state-of-the-art progress of lanthanide-based upconversion materials, focusing on the newly emerging approaches to the smart control of upconversion in aspects of light intensity, colors, and lifetimes, as well as new concepts. The emerging scientific and technological discoveries based on the well-designed upconversion materials are highlighted and discussed, along with the challenges and future perspectives. This review will contribute to the understanding of the fundamental research of photon upconversion and further promote the development of new classes of efficient upconversion materials towards diversities of frontier applications in the future.

Unravelling the benefits of transition-metal-co-doping in lanthanide upconversion nanoparticles.

Recent advances reveal that upconversion (UC) luminescent materials are highly important not just from a scientific, but also from a future application standpoint. Although significant progress has been made in this field in recent decades, still their versatile applications are hindered by the low upconversion luminescence intensity and low tuneability, which has hampered further implementation in real life applications. We find it highly beneficial to compile a summary of recent relevant literature and propose ways to enhance upconversion efficiency in lanthanide nanomaterials. One very promising way to tackle this problem is through implementing transition metal ion co-dopants into the materials, which is the focus of this tutorial review. In this review, the recent studies related to the tailored design of UC materials with transition metal ion co-dopants have been summarized, and the desirable functionality of transition metal ions in the host matrixes has been discussed. Apart from improving the upconversion efficiency, the implementation of transition metal-co-dopants into lanthanide upconversion materials has recently sparked interest in applications such as in vivo imaging, drug delivery or nanothermometers.

Determination of radiative and multiphonon non-radiative relaxation rates of upconversion materials.

The radiative and multiphonon non-radiative relaxation rates of lanthanide ions are intrinsic parameters to characterize the optical properties, which are the basic data for the theoretical model and numerical simulation of lanthanide upconversion systems. However, there are complex energy transfer processes, such as energy migration, energy transfer upconversion, and cross-relaxation in the lanthanide-doped upconversion materials, so it is difficult to accurately measure the intrinsic radiative and multiphonon relaxation rates. Therefore, a method to determine the relaxation rates of multi-level upconversion systems is proposed based on multi-wavelength excitation and level-by-level parameter calculations in this paper. For a dilute doped multi-level luminescence system excited at low powers, a model based on the measurements of steady-state emission spectra and luminescence decay curves is established through the macroscopic rate equations at multi-wavelength excitation, which can be used for the level-by-level calculation of the multi-level radiative and multiphonon relaxation rates. With the dilute doped β-NaYF4:Er3+ six-level luminescence system as an example, the measurement method and the model are introduced in detail. Under the experimental conditions of neglecting the energy transfer effect between ions, the materials are excited by five lasers with central wavelengths of 1523 nm, 980 nm, 808 nm, 660 nm, and 520 nm to form five subsystems. The steady-state emission spectra and luminescence decay curves of the luminescence system excited by each wavelength were recorded. The intrinsic relaxation rates including 11 radiative relaxation rates and 4 multiphonon relaxation rates in the β-NaYF4:Er3+ six-level system were determined based on the established model and method, which experimentally verified the applicability of the method proposed in this paper. This work will provide basic data for the analysis and regulation of the luminescence properties of lanthanide upconversion systems.

Tunable Anti‐Stokes‐Shift Behaviors Based on Intramolecular Charge Transfer Characteristics of Diarylethene Derivatives

AbstractUnderstanding the rules for designing anti‐Stokes emission induced by hot‐band absorption is crucial for developing novel materials for optical devices and biology. However, there are very few published reports on hot‐band‐absorption‐based single‐photon upconversion (HBA‐UC) materials, mainly because the rules for designing these molecules are poorly understood. Herein, HBA‐UC is investigated in a series of diarylethene derivatives (DAEs), namely benzothiophene S, S‐dioxide derivatives, which possess photochromic and photofluorochromic properties. All these DAEs show photoswitchable Stokes emission and HBA‐UC emission. Regulating the substituent groups enables flexibly adjustment of the HBA‐UC properties. Density functional theory (DFT) calculations reveal that the intramolecular charge transfer (ICT) between benzothiophene 1,1‐dioxide core and substituent groups is responsible for the emission properties of HBA‐UC. Furthermore, it is confirmed that the strong intensity of the ICT in DAEs can improve the scale of the anti‐Stokes shift. These findings provide a new direction for exploiting ICT feature in the design of HBA‐UC materials.

Improved luminescence performance of Yb3+-Er3+-Zn2+: Y2O3 phosphor and its application to solar cells

Upconversion materials (UC) can convert low-energy photons into visible light and, therefore, can be incorporated in solar cells to increase the absorption of visible light. This study synthesized UC nanophosphors Yb3+, Er3+: Y2O3 and Yb3+, Er3+, Zn2+: Y2O3 by a simple co-precipitation method for application in dye-sensitized solar cells (DSSCs). The impact of the enhancement in the concentration of Zn2+ on the photoluminescence (PL) and color point of the synthesized nanophosphors was also investigated. The synthesized nanophosphors emitted intense red and weaker green emissions upon excitation at 980 nm. The incorporation of Zn2+ to the Yb3+, Er3+: Y2O3 nanophosphors leads to color tunability in the red and yellow regions. Furthermore, the synthesized nanophosphors were incorporated into the DSSC photoanode to form a TiO2-UC-based DSSC for converting near-infrared (NIR) into visible light. We observed that the TiO2-UC-based DSSC showed an enhancement ratio in current density and power conversion efficiency of 17.4% and 16.6%, respectively, compared to the bare TiO2-based DSSC. These results reveal that UC-based Yb3+, Er3+, Zn2+: Y2O3 nanophosphors are useful in improving the efficiency of DSSCs and in color tunability applications.