Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Synthesis, Luminescent Properties, and Biomedical Applications Shili Gai,†,‡ Chunxia Li,† Piaoping Yang,*,‡ and Jun Lin*,† †State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, P. R. China

Recent Progress in Rare Earth Micro/Nanocrystals: Soft Chemical Synthesis, Luminescent Properties, and Biomedical Applications

From its initial use to describe hydrolysis and condensation processes, the term ‘sol–gel’ is now used for a diverse range of chemistries. In fact, it is perhaps better defined more broadly as covering the synthesis of solid materials such as metal oxides from solution-state precursors. These can include metal alkoxides that crosslink to form metal–oxane gels, but also metal ion–chelate complexes or organic polymer gels containing metal species. What is important across all of these examples is how the choice of precursor can have a significant impact on the structure and composition of the solid product. In this review, we will attempt to classify different types of sol–gel precursor and how these can influence a sol–gel process, from self-assembly and ordering in the initial solution, to phase separation during the gelation process and finally to crystallographic transformations at high temperature.

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The evolution of 'sol-gel' chemistry as a technique for materials synthesis

White light was generated from a single silica thin film made with Yb0.75La0.2Eu0.05F3, La0.45Yb0.5Er0.05F3, and La0.75Yb0.2Tm0.05F3 nanoparticles by exciting with a single source near-infrared light (980 nm CW diode laser). Eu3+ and Tm3+ ions are responsible for red and blue emission, respectively. Er3+ ion is responsible for green as well as red emission. The Commission Internationale de l'Eclairage (CIE) coordinates of the resulting light were easily adjusted by controlling the concentration of Ln3+ (Eu3+, Er3+, Tm3+) ions in the nanoparticles as well as the concentration of Ln3+-doped nanoparticles in the sol−gel thin layer.

Bright white light through up-conversion of a single NIR source from sol-gel-derived thin film made with Ln3+-doped LaF3 nanoparticles.

Lanthanide upconversion nanoparticles and applications in bioassays and bioimaging: A review

Rare earth ions (Tm3+, Er3+, and Yb3+)-doped cubic Gd2O3 nanocrystals were prepared by a simple sol−gel method. Raman and FT-IR spectra were measured to evaluate the vibrational feature of the samples. Under 980 nm laser excitation, blue (488 nm), green (564 nm), and red (661 nm) upconversion has been recorded in Gd2O3:Tm+Yb and Gd2O3:Er (Gd2O3:Er+Yb), respectively. A great enhancement of red emission and diminishment of green emission of Er3+ in Gd2O3:Er+Yb have been observed. Laser power and doping concentration dependence of the upconverted emissions were studied to understand the upconversion mechanisms. Excited absorption and energy-transfer processes are discussed as the possible mechanisms for the visible emissions.

Ning Dong

Papers

Visible Upconversion in Rare Earth Ion-Doped Gd2O3 Nanocrystals

Effect of heat-treatment temperature on the luminescent properties of Lu2O3:Eu film prepared by Pechini sol–gel method

Green and red upconversion luminescence in Er3+-doped and Er3+/Yb3+-codoped SrTiO3 ultrafine powders

Preparation and characterization of optical spectroscopy of Lu2O3:Eu nanocrystals

Spectroscopic studies and crystal field calculation for Nd3+ in single crystal K2YF5