D. C. Yu

Spectral conversion for solar cell efficiency enhancement using YVO4:Bi3+,Ln3+ (Ln = Dy, Er, Ho, Eu, Sm, and Yb) phosphors

Bi3+–Ln3+ (Ln = Dy, Er, Ho, Eu, and Sm) co-doped YVO4 phosphors are proposed as UV-absorbing luminescent converter candidate to enhance the power conversion efficiency and photochemical stability of dye-sensitized solar cells (DSSCs). The phosphors can efficiently convert UV photons in a broad range from 250 to 400 nm into visible emissions, which can be absorbed by DSSCs. Efficient broadband down-conversion UV light into near-infrared emission around 1000 nm was achieved in the YVO4:Bi3+,Yb3+ phosphors. The energy transfer from V5+–Bi3+ charge-transfer state to Yb3+ was shown to be a cooperative down-conversion type by the luminescence spectra, energy transfer efficiency, and luminescence decay curves. The YVO4:Bi3+,Yb3+ phosphors are promising for boosting the efficiency of crystalline silicon solar cells by down-converting the UV part of the solar spectrum to near-infrared photons with a twofold increase in the photon number. This research may open up promising new perspectives for designing novel lumi...

Enhanced near-infrared quantum cutting in GdBO3:Tb3+,Yb3+ phosphors by Ce3+ codoping

A near-infrared quantum cutting phosphor GdBO3:Tb3+,Yb3+ shows poor optical absorption in the ultraviolet-visible excitation spectral range and weak near-infrared luminescence in the 900–1100 nm range, which strongly limit its practical application in silicon-based solar cells, due to the inefficient excitation of the Tb3+ ions for their intra-4f forbidden transitions. Herein, Ce3+ ion has been codoped as a sensitizer in GdBO3:Tb3+,Yb3+, thus enlarging the absorption linewidth and increasing the absorption cross-sections in the ultraviolet-visible region. The presence of Ce3+ results in an enhancement in Yb3+ near-infrared emission intensity by a factor of ∼30, which makes these phosphors more attractive as luminescent downconversion materials for enhanced silicon-based solar cell performance. The relevant luminescence and energy transfer mechanisms involved have also been investigated. The present results demonstrate that the 4f−5d luminescence of Ce3+ may be used to sensitize the Tb3+–Yb3+ downconversio...

Room-temperature upconverted white light from GdMgB5O10 : Yb3+, Mn2+

Room-temperature upconverted white light from GdMgB5O10:Yb3+, Mn2+ has been demonstrated upon 976 nm diode laser excitation, which is low for rare-earth (RE) ion and transition metal (TM) ion codoping systems. The white light emission is composed of two broad bands peaked at 490 and 620 nm, originating from the upconverted emissions of Yb3+–Yb3+ and Yb3+–Mn2+ dimer ions, respectively. The cooperative luminescence and cooperative sensitization mechanisms are excluded and exchange interaction models are proposed for these two-photon processes, based on the measured luminescence spectra and crystal structure. The large deviations from two for the plot slopes of the upconversion (UC) intensities as a function of pump power are quantitatively interpreted by a large UC rate for both cases and an additional depletion pathway of relaxation from the upper excited state |2F7/2,4T1> to the intermediate state |2F5/2,6A1> for the Yb3+–Mn2+ dimer according to the exchange interaction model. The color of the UC luminescence can be tuned by varying the content of Yb3+ and Mn2+, indicating that GdMgB5O10:Yb3+, Mn2+ would be a potential candidate for lighting and displays.

Three-photon near-infrared quantum splitting in β-NaYF4:Ho3+

We report on three-step sequential three-photon near-infrared (NIR) quantum splitting in β-NaYF4:Ho3+, where one absorbed ultraviolet photon is split into three NIR photons with wavelengths 850, 1015, and 1180 nm. The underlying mechanism is analyzed by static and dynamic photoemission and excitation spectroscopy. An internal quantum yield of 124% is estimated on the basis experimental data and theoretical considerations. Further development of an efficient triply splitting NIR phosphor might open up an approach in achieving efficient photonic devices, which enables more photons emitted than absorbed in the excitation process.

Structural effects on Stokes and anti-Stokes luminescence of double-perovskite (Ba,Sr)2CaMoO6: Yb3+,Eu3+

This paper deals with the investigation of structural effects on Stokes and anti-Stokes luminescence properties of double-perovskite (Ba,Sr)2CaMoO6: Eu3+,Yb3+. It is found that the tilting of the (Ca/Mo)O6 octahedra framework favors energy transfer from the MoO6 group to Eu3+/Yb3+ at the B site for Stokes luminescence processes, in which the structural connectivity angle ∠Mo–O–Eu/Yb is slightly smaller than 180°. While Eu3+/Yb3+ at the A site with a structural connectivity angle ∠Mo–O–Eu/Yb of around 90° least benefits the energy transfer. The linear connectivity of Yb–O–Mo–O–Eu with or without tilting of the (Ca/Mo)O6 framework for Eu3+ and Yb3+ at the B site strongly restrains the anti-Stokes luminescence. Whereas, it shows normal anti-Stokes luminescence for Eu3+ or Yb3+ at the A site and Yb3+ or Eu3+ at the B site with ∠Yb–O–Eu of around 90°. This research may open up promising new perspectives for designing novel luminescent materials with high efficiency.

Sequential three-step three-photon near-infrared quantum splitting in β-NaYF4:Tm3+

We report on sequential three-step three-photon near-infrared (NIR) quantum splitting in Tm3+-doped β-NaYF4, where an incident blue photon around 470 nm is split into three NIR photons (1165, 1466, and 1800 nm). The underlying mechanism is analyzed by means of static and dynamic photoemission spectroscopy. Here, an experimental total quantum yield of ∼32% is obtained. When quenching due to residual hydroxyl groups and other defect species can be overcome, numerical analyses indicate a theoretical maximum quantum yield of 158%, suggesting application in efficient spectral converters.

Anomalous tunable visible to near infrared emission in the Mn2+-doped spinel MgGa2O4 and room-temperature upconversion in the Mn2+ and Yb3+-codoped spinel

In contrast to common visible emission, an anomalous near-infrared (NIR) emission band at 790 nm has been demonstrated in the spinel structure MgGa2O4:Mn2+ with heavy Mn2+ doping. Tunable single-band visible to visible-NIR and single-band NIR emission are easily realised upon tuning the doping concentration of Mn2+. Careful investigation of the crystal structure, fluorescence lifetime and excitation & emission spectra indicates that the NIR emission might be ascribed to the 6A1(6S)4T1(4G) → 6A1(6S)6A1(6S) transitions of Mn2+–Mn2+ dimers. When some Yb3+ ions are codoped into the spinel MgGa2O4:Mn2+, room temperature visible upconversion (UC) emission is realised upon excitation with a 976 nm laser diode. The temperature-dependent UC emission properties as well as the related UC mechanism have been investigated. Understanding the nature of the Mn2+ Stokes and UC emissions is the key to developing advanced photonic devices with improved properties and manufacturability for engineering applications.

Three-photon near-infrared quantum cutting in Tm3+-doped transparent oxyfluoride glass ceramics

Efficient three-step sequential three-photon near-infrared (NIR) quantum cutting in Tm3+-doped transparent oxyfluoride glass ceramics has been demonstrated, where an absorbed blue photon could be cut into three NIR photons at 1190, 1460, and 1800 nm with quantum yield greater than unity. On the basis of static and dynamic photoemission, monitored excitation, and time-resolved fluorescence spectra, we investigate in detail the underlying optoelectronic mechanism. Further development of an efficient triply-cutting material might open up a path towards ultra-efficient photonic devices, which enables more photons emitted than absorbed in the excitation process.

Ultrabroadband sensitization of near infrared emission through energy transfer from Pb to Yb ions in LiYbMo2O8:Pb

Ultrabroadband sensitization of near infrared emission through energy transfer from Pb to Yb ions in LiYbMo2O8:Pb has been achieved upon ultraviolet (UV)-blue light excitation varying from 300 to 450 nm. The excitation band at 300–370 nm can be assigned to the charge transfer state of MoO42− groups, while the band at 370–450 nm could be mainly ascribed to charge transfer from Pb2+ to Yb3+ with a possible additional partial absorption contribution of Pb3+. Application of these phosphors in silicon-based solar cells might enhance its UV-blue response via spectrum modification.

Sequential three-photon near-infrared quantum cutting in transparent fluorogermanate glass-ceramics containing LaF 3 :Tm 3+ nanocrystals

Transparent fluorogermanate glass-ceramics singly doped with 0.25 mol.%Tm3+ have been prepared through melt quenching and sequential thermal treatment. The structure and composition of the glass-ceramics have been characterized by means of X-ray diffraction, Raman spectroscopy and transmission electron microscopy. Efficient three-step sequential three-photon near-infrared (NIR) quantum cutting has been demonstrated, where an absorbed blue photon at 468 nm could be cut into three NIR photons at 1190, 1462 and 1800 nm, respectively. The underlying mechanism has been analyzed in terms of the static and dynamic spectra measurements. Based on the experimental data and theoretical consideration, an internal quantum yield has been estimated to be about 160%. Further development of such a triply-cutting material might explore a way to design high efficient photonic devices, which harvest more photons emitted than absorbed in the excitation process.