Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF:Yb,Er phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.
a green-selective (G) OPD layer on top of blue (B) and red (R) color filters. Removing the G filter in the horizontally integrated R/G/B color filters resulted in a 1.5-fold increase in image resolution. [4] Siegmund et al. achieved narrowband near-infrared photodetectors using small-molecule p-type ZnPc and n-type C60 by tuning the resonant cavity thickness of the photoconductive layer. [5] Yoon et al. reported blue-selective OPDs using a novel polymer donor and PC 60 BM that were fabricated via a simple bulk heterojunction solution process. [6] These successful reports on the development of wavelength-selective OPDs were exclusively achieved by using a photoconductive layer composed of all small molecules, polymer donor/small molecule acceptors or single polymer; [7] polymer/polymer blends have been rarely studied for wavelength-selective OPDs. All-polymer photodetecting layers have the unique advantages of strong absorption coefficients, easy color tunability, solution processability, and especially strong mechanical properties owing to their strongly entangled nanomorphology. [8] However, their relatively disordered backbone structures compared to small molecules broaden their absorption spectra, and thus the blending of polymer donor and polymer acceptor is regarded to be a challenging issue for the realization of narrowwavelength OPDs. In this study, we suggest a new strategy for the development of p-and n-type polymers in order to achieve G-selective all-polymer OPDs while obtaining a low dark current density (J D). Both the polymer donor and acceptor were designed to have a similar G absorption wavelength. Importantly and in addition, we controlled the molar absorption coefficients of both polymers to achieve the desired wavelength selectivity in the OPDs. The absorbance of the polymer donor was maximized by increasing backbone planarity and degree of polymerization, whereas the absorbance of the polymer acceptor was minimized by breaking the π-conjugation via the inclusion of insulating alkyl chains within the conjugated polymers. The significant difference in molar absorption coefficients resulted in G selectivity in the p-n junction photoconductive layer, while the introduced alkyl chains in the main chains effectively suppressed the leakage current in OPDs. The low J D and G selectivity are highly advantageous to detect weak G light signals. Currently available wavelength-selective p-n junction organic photodetectors (OPDs) nearly exclusively use small molecules. In this study, green (G) selective all-polymer p-n junction OPDs are developed by engineering the π-conjugation networks and insulating properties of p-and n-type polymers. Enhanced intermolecular ordering of p-n junction blend films compared to pristine polymer films results in superior hole/electron mobilities and low bimolecular recombination in the devices. Notably, similar G absorption ranges and the huge difference in their absorption coefficients between p-and n-type polymers make excellent G selectivity in the p-n junction OPDs. Thus,...
A novel conjugated polymer, poly(4‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b']dithiophen‐2‐yl)‐2,6‐dioctylbenzo[1,2‐d:4,5‐d']bis(oxazole)) (PBB), is synthesized for use in ultrafast‐response organic photodetectors (OPDs). The PBB synthesis is considered facile because a monomer could be isolated without column chromatography purification. The excellent solubility of PBB allows easy dissolution in both halogenated and nonhalogenated solvents, facilitating the fabrication of p‐n heterojunction OPDs based‐on PBB and 6,6‐phenyl‐C71‐butyric acid methyl ester (PC71BM) using either chloroform (CF) or o‐xylene (Xy). Notably, Xy‐processed OPDs show superior performance with a responsivity (R) of 0.385 A W−1 and specific detectivity (D*) of 1.33 × 1013 Jones at –1 V, whereas CF‐based devices showed an R of 0.349 A W−1 and D* of 5.86 × 1012 Jones. More importantly, Xy‐processed OPDs exhibited an ultrafast signal response time of 3 µs, which is much faster than that of CF‐processed OPDs (42 µs). These superior static and dynamic properties of PBB:PC71BM devices are achieved by the inverted hierarchical arrangement of PBB and PC71BM via a biased orientation of PC71BM toward the ZnO layer as well as a homogenous nanomorphology. This ideal morphology is spontaneously induced due to the difference in interfacial energy between PBB and PC71BM toward the ZnO surface and a well‐mixed PBB:PC71BM blend in Xy solvent.
Lanthanide phosphors have attracted great interest because of their highly efficient photonic upconversion processes. The upconversion spectra and time-resolved relaxation are influenced not only by the chemical composition of phosphors but also by the excitation characteristics of incident radiation. Despite numerous reports that upconversion spectra strongly depend on the experimental conditions, little research has been reported on the relationship between the temporal profile of excitation and the resulting luminescence spectra. In this study, we introduced time-and frequency-domain pulsed modulation spectroscopy to investigate statistical energy accumulation and relaxation in a NaYF 4 :Yb 3+ ,Er 3+ upconversion process. The modulation of a laser pulse width and interval showed that complete depopulation of hot phosphors required more than 5 ms. Furthermore, we demonstrated that the intermediate state ( 4 I 13/2 ) is populated in hot phosphors and affects upconversion luminescence.
The photonic upconversion in rare earth atoms is widely used to convert "invisible" near infrared photons to "visible" photons with continuous wave light. By using a patterned substrate, upconversion become a route for creating new information-incorporating security codes. The amount of information in the cipher increases in proportion to the number of emission colors as well as the pattern structure. Subsequently, changing the chemical composition of upconversion phosphors on 2 D substrates is required to manufacture information-rich upconversion cryptography. In this study, we exploited temperature-controlled thermal reaction on upconversion films deposited on a quartz substrate to prepare security information codes. Multiple color emission was generated from upconversion films as the result of inserting high-frequency molecular oscillators into the film structures. Fourier-transform infrared (FTIR) and time-resolved study corroborated the mechanism of spectral variation of upconversion films.
Since the fat content of pork is a deciding factor in meat quality grading, the use of a noninvasive subcutaneous probe for real-time in situ monitoring of the fat components is of importance to vendors and other interested parties.
Spectral upconversion systems placed underneath solar cells have the considerable potential for enhancement of the photovoltaic performance as they allow additional absorption for the solar photons with energy below the bandgap of active materials. However, their application to a type of ultrathin solar cells for achieving the meaningful benefit of the efficiency improvement is challenging, because a pre‐existing rear‐side reflector that substantially increases the photon absorption needs to be eliminated for photonic interaction between photovoltaic active layer and upconversion medium, and hence a level of cell efficiency becomes limited. Herein a facile strategy is presented that can circumvent the issue of performance deterioration arising from the expelled reflector for integrating plasmonically enhanced upconversion systems with ultrathin nonfullerene‐based polymer solar cells. By employing a wavelength‐selectively reflective rear electrode of metal/dielectric multilayer that enables the photon penetration only at excitation and emission wavelengths of the upconversion process, the effect of photocurrent improvement with uncompromising efficiency levels can be expected from the plasmonic upconversion backplane comprising NaYF4:Yb3+,Er3+ core‐shell nanoparticles and metallic nanostructure. Systematic studies of optical process and resulting device performance in both experiments and numerical modeling provide the optimal design scheme for high‐performance polymer solar cells assisted with upconversion systems.
It has been reported that the scattering cross-sections of resonance Raman spectra strongly depend on the resonance between the laser's excitation energy and the electronic absorption band of pigments in...
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