Synthesis And Application Of The Graphene/Upconversion Nanomaterials

The lanthanide-doped NaYF4 upconversion nanoparticles (UCNPs) possess the ability that can emit high energy photon after sequentially absorb two or more low energy photons. This excellent property makes the UCNPs an ideal candidate to improve the light harvest of the absorber for solar cell, and lots of advancement has been achieved. For example, the problem that the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the absorber in the near infrared region is ameliorated. However, the poor conductivity seriously restricts the application of UCNPs, and the performance of solar cell is not so wonderful. SO it is urgent to improve the conductivity of UCNPs so as to put forward the application of them in solar cell to anew stage.Graphene is a good candidate to hydrid with upconversion nanoparticles due to its high charge carrier mobilities and conductivity, and efficient energy transfer between them. Here we propose the graphene-UCNPs composites should enhance the efficiency of solar cell by making better use of NIR light and at the same time overcome the low emission efficiency and electrical conductivity (high resistance) of UCNPs.Graphene oxide (GO)-NaYF4:Yb3+/Er3+ composites are in situ synthesized by hydrothermal method. One composite contains large micro-disk UC particles (marked as GO-LUC) and the other containes small UCNPs (marked as GO-SUC), GO-MUC is prepared by physical mix SUC and GO. Field emission scanning electron microscopy (FE-SEM) images confirm the formation of GO-NaYF4:Yb3+/Er3+ composites and sufficient binding between them. Importantly, efficient energy transfer from NaYF4:Yb3+/Er3+ to graphene is proved by the emission spectra of the composites, which will facilitate the utilizing of NIR light. Furthermore, electrochemical impedance spectroscopy (EIS) proves that the in situ combination of graphene with UCNPs indeed improved the charge transfer ability of UCNPs.Thanks to the fully energy transfer from UCNPs to GO and low resistance of the composite, the Jsc and η of the DSSCs containing RGO-SUC are 10% higher than that of the device without the composite and the device containing physically mixed GO and UCNPs. This highlights the importance of the good binding of UCNPs with GO. It is worth noting that although by in situ binding, the use of RGO-LUC containing upconversion microdisk about 500 nm in DSSCs barely improves the performance due to the low upconverted ability and unsatisfactory energy transfer to GO. Our results demonstrate that utilizing the in situ prepared GO-UCNPs composites with appropriate particle size and good upconverted ability in DSSC can result in an enhancement of Jsc and η by completely transferring the upconverted NIR light energy in UCNPs to GO and then to TiO2, and at the same time avoiding the poor electrical conductivity induced by UCNPs. The composites present here could be well suited for other solar cells, including QDs solar cell and the most recently developed organic-inorganic lead halide perovskites solar cell to further improve their performance.