Gencai Pan

Doping Lanthanide into Perovskite Nanocrystals: Highly Improved and Expanded Optical Properties

Cesium lead halide (CsPbX3) perovskite nanocrystals (NCs) have demonstrated extremely excellent optical properties and great application potentials in various optoelectronic devices. However, because of the anion exchange, it is difficult to achieve white-light and multicolor emission for practical applications. Herein, we present the successful doping of various lanthanide ions (Ce3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+, and Yb3+) into the lattices of CsPbCl3 perovskite NCs through a modified hot-injection method. For the lanthanide ions doped perovskite NCs, high photoluminescence quantum yield (QY) and stable and widely tunable multicolor emissions spanning from visible to near-infrared (NIR) regions are successfully obtained. This work indicates that the doped perovskite NCs will inherit most of the unique optical properties of lanthanide ions and deliver them to the perovskite NC host, thus endowing the family of perovskite materials with excellent optical, electric, or magnetic properties.

Cerium and Ytterbium Codoped Halide Perovskite Quantum Dots: A Novel and Efficient Downconverter for Improving the Performance of Silicon Solar Cells

Quantum cutting can realize the emission of multiple near-infrared photons for each ultraviolet/visible photon absorbed, and has potential to significantly improve the photoelectric conversion efficiency (PCE) of solar cells. However, due to the lack of an ideal downconversion material, it has merely served as a principle in the laboratory until now. Here, the fabrication of a novel type of quantum cutting material, CsPbCl1.5 Br1.5 :Yb3+ , Ce3+ nanocrystals is presented. Benefiting from the larger absorption cross-section, weaker electron-phonon coupling, and higher inner luminescent quantum yield (146%), the doped perovskite nanocrystals are successfully explored as a downconverter of commercial silicon solar cells (SSCs). Noticeably, the PCE of the SSCs is improved from 18.1% to 21.5%, with a relative enhancement of 18.8%. This work exhibits a cheap, convenient, and effective way to enhance the PCE of SSCs, which may be commercially popularized in the future.

Fabrication of Au-Ag nanocage@NaYF4@NaYF4:Yb,Er core-shell hybrid and its tunable upconversion enhancement

Localized electric filed enhancement by surface plasmon resonance (SPR) of noble metal nanoparticles is an effective method to amplify the upconversion luminescence (UCL) strength of upconversion nanoparticles (UCNPs), whereas the highly effective UCL enhancement of UCNPs in colloids has not been realized until now. Here, we designed and fabricated the colloidal Au-Ag nanocage@NaYF4@NaYF4:Yb,Er core-shell hybrid with different intermediate thickness (NaYF4) and tunable SPR peaks from visible wavelength region to NIR region. After the optimization of the intermediate spacer thickness (~7.5 nm) of NaYF4 NPs and the SPR peak (~950 nm) of noble metal nanoparticles, an optimum enhancement as high as ~25 folds was obtained. Systematic investigation indicates that UCL enhancement mainly originates from the influence of the intermediate spacer and the coupling of Au-Ag nanocages with the excitation electromagnetic field of the UCNPs. Our findings may provide a new thinking on designing highly effective metal@UCNPs core-shell hybrid in colloids.

Carbon dots with efficient solid-state photoluminescence towards white light-emitting diodes

Carbon dots (CDs) exhibit excellent ultraviolet (UV) absorption and tunable photoluminescence over the full visible light range, which endows CDs with huge potential to be designed as efficient full-color emitting phosphors for UV to white light conversion. However, the low quantum yield (QY) for white light emission and solid-state quenching dramatically limit their optoelectronic applications. We proposed an effective strategy for modulating the emitting states of colloidal CDs by introducing hexadecyltrimethyl ammonium bromide. Consequently, white light emission with tunable correlated color temperature from 8121 K to 3623 K was realized. Furthermore, we dispersed CDs in a PVP matrix for solid-state films, where the solid-state quenching was effectively avoided. A white light-emitting QY of 38.7% was thus achieved through the inhibition of non-radiative electron–hole recombination as well as the cooperation between the intrinsic state of the carbogenic cores and the surface-related state of the organic ligands. The white light emitting QY is much higher than that of other reported CDs (ca. 15% in the soluble state and not reported in the solid-state) and is comparable to that of the nanophosphors with the highest UV pumped single-component white light emissions reported in the literature.

Semiconductor plasmon-sensitized broadband upconversion and its enhancement effect on the power conversion efficiency of perovskite solar cells

Photon upconversion (UC) is an attractive strategy to substantially enhance the power conversion efficiency (PCE) of solar cells via upconverting unavailable near-infrared sunlight to available visible light. However, to date, it is almost infeasible to achieve effective PCE improvement of solar cells with the assistance of UC materials, limited by their poor UC efficiency and extremely weak and narrowband near-infrared absorption. Here, we demonstrate the efficient photon energy UC in semiconductor plasmon mCu2−xS@SiO2@Er2O3 (mCSE) nanocomposites, where the broadband semiconductor plasmon (800–1600 nm) of mCu2−xS serves as an antenna to sensitize UC of Er2O3 nanoparticles. The overall upconversion luminescence (UCL) of the composites was dramatically enhanced by a factor of ∼1000, with a maximal inner quantum efficiency of 14.3%. The excitation range was expanded, ranging from 800 to 1600 nm. As a proof-of-concept, the highly efficient mCSE nanocomposites were utilized to improve the PCE of perovskite solar cells (PSCs). The expansion of the near-infrared response (800–1000 nm) and considerable improvement of the PCE were obtained, with an optimum PCE of 17.8%. The mCSE composites in PSCs enhanced the photocurrent via electron transfer from oxygen defects to the conduction band of TiO2 under irradiation of one sunlight. Under irradiation of 15 suns, the electron transfer and reabsorption of UCL both contributed to the enhancement of PCE. Our work can provide an insightful thought on boosting UC efficiency as well as broadening the PCE of PSCs.

Size-dependent downconversion near-infrared emission of NaYF4:Yb3+,Er3+ nanoparticles

Near-infrared-downconversion-near-infrared (NIR-DC-NIR) bioimaging based on lanthanide doped upconversion nanoparticles (UCNPs) has received much attention due to its deeper penetration, and higher contrast imaging and signal-to-noise ratio in biological tissues. The size of UCNPs determines the mechanism and rate of cell uptake of the nanoparticles and their ability to permeate through biological tissues. Herein, we experimentally and theoretically demonstrate downconversion-near-infrared (DC-NIR) emission behavior in different sized UCNPs ranging from 5–150 nm. Interestingly, 15–40 nm UCNPs have more effective DC-NIR emissions than 150 nm UCNPs and an extremely high excitation threshold, which is entirely different from the size-dependent upconversion-visible (UC-VIS) emissions usually observed in UCNPs. We also observed that the intensity ratio of the DC-NIR emission to the UC-VIS emission decreases with the increase of the particle size and the excitation power, attributed to the more efficient upconversion (UC) process. Finally, we further confirmed that the competition process between the UC population and non-radiative relaxation to the DC-NIR level plays a key role in size-independent DC-NIR emissions. Our discovery would provide guidance for optimizing and designing NIR-DC-NIR NPs for bioimaging applications.

Carbon dot/polyvinylpyrrolidone hybrid nanofibers with efficient solid-state photoluminescence constructed using an electrospinning technique

Carbon dots (CDs) are the promising candidates for application in optoelectronic and biological areas due to their excellent photostability, unique photoluminescence, good biocompatibility, low toxicity and chemical inertness. However, the self-quenching of photoluminescence as they are dried into the solid state dramatically limits their further application. Therefore, realizing efficient photoluminescence and large-scale production of CDs in the solid state is an urgent challenge. Herein, solid-state hybrid nanofibers based on CDs and polyvinylpyrrolidone (PVP) are constructed through an electrospinning process. The resulting solid-state hybrid PVP/CD nanofibers present much enhanced photoluminescence performance compared to the corresponding pristine colloidal CDs due to the decrease in non-radiative recombination of electron-holes. Owing to the suppressed self-quenching of CDs, the photoluminescence quantum yield is considerably improved from 42.9% of pristine CDs to 83.5% of nanofibers under the excitation wavelength of 360 nm. This has great application potential in optical or optoelectronic devices.

Modulation of the photoluminescence in carbon dots through surface modification: from mechanism to white light-emitting diodes

Carbon dots (CDs) have emerged as a new type of fluorescent material because of their unique optical advantages, such as high photoluminescence quantum yields (QYs), excellent photo-stability, excitation-dependent emissions, and low toxicity. However, the photoluminescence mechanism for CDs remains unclear, which limits their further practical application. Here, CDs were synthesized via a solvothermal route from citric acid and urea. Through the oxidation and reduction treatment of pristine CDs, the origin of the photoluminescence and the involved mechanism were revealed. We found that the blue/green/red emissions originated from three diverse emitting states, i.e. the intrinsic state, and C=O- and C=N-related surface states, respectively. Based on the as-prepared CDs, a pH sensor depending on the radiometric luminescence detection was developed. Furthermore, we constructed CD/PVP (PVP, polyvinylpyrrolidone) composite films, which exhibited white light emission with photoluminescence QYs of 15.3%. The white light emission with different correlated color temperatures (CCTs), from 4807 K to 3319 K, was obtained by simply changing the amount of PVP solution. Benefiting from the white light-emitting solid-state films, single-component white light-emitting diodes were fabricated with an average color rendering index value (Ra) of 80.0, luminous efficiency of 10.2 lm W-1, and good working stability, thus indicating a promising potential for practical lighting applications.

Luminescence carbon dot-based nanofibers for a water-insoluble drug release system and their monitoring of drug release

Drug release systems with fluorescence detection have emerged as a potential application for the biological area of diagnosis and therapy. Carbon dots (CDs) are a promising fluorescence probe for application in a drug release system due to their excellent biocompatibility, low toxicity, chemical inertness, and non-blinking fluorescence emission. Herein, we developed a composite nanocarrier based on fluorescent CDs and polyvinylpyrrolidone (PVP) through an electrospinning technology. The as-prepared PVP/CD-ketoprofen (PVP/CD-KET) composite nanofiber presents bright blue-light fluorescence with a photoluminescence quantum yield (QY) of 65.7% and was utilized for loading a water-insoluble drug and moreover for monitoring the drug release process. In vitro tests indicate that the photoluminescence emission intensity of the CDs and the cumulative amount of KET released from the PVP/CD-KET composite nanofiber gradually increase with the release time. Furthermore, the emission intensity of the CDs as a function of the cumulative released amount of KET can be summarized by a power function. The correlation between the emission intensity of CDs and drug release amount can be potentially used to monitor the drug release process.

Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals with Bright White Light Emission towards UV-WLED

White light-emitting diodes (WLEDs) based on all-inorganic perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have attracted extensive interests. However, the native ion exchange among halides makes them extremely difficult to realize the white emission. Herein, we demonstrate a novel strategy to obtain WLED phosphors based on the codoping of different metal ion pairs, such as Ce3+/Mn2+, Ce3+/Eu3+, Ce3+/Sm3+, Bi3+/Eu3+, and Bi3+/Sm3+ into stable CsPbCl3 and CsPbCl xBr3- x NCs. Notably, by the typical anion exchange reaction, the highly efficient white emission of Ce3+/Mn2+-codoped all-inorganic CsPbCl1.8Br1.2 perovskite NCs was achieved, with an optimal photoluminescence quantum yield of 75%, which is much higher than the present record of 49% for single perovskite phosphors. Moreover, the WLED with a luminous efficiency of 51 lm/W based on the 365 nm ultraviolet chip and CsPbCl1.8Br1.2:Ce3+/Mn2+ nanophosphor was achieved. This work represents a novel device for perovskite-based phosphor-converted WLEDs.