Effectively Utilizing NIR Light Using Direct Electron Injection from Up-Conversion Nanoparticles to the TiO2 Photoanode in Dye-Sensitized Solar Cells

TiO 2 /NaYF 4 :Yb 3+ ,Er 3+ nano-heterostructures are prepared in situ on the TiO 2 photoanode of dye-sensitized solar cells (DSCs). Transmission electron microscopy (TEM) and high-resolution (HR)-TEM confirm the formation of TiO 2 /NaYF 4 :Yb 3+ ,Er 3+ nano-heterostructures. The up-converted fluorescence spectrum of the photoanode containing the nano-heterostructure confirms electron injection from NaYF 4 :Yb 3+ ,Er 3+ to the condution band (CB) of TiO 2 . When using a photoanode containing the nano-heterostructure in a DSC, the overall efficiency ( η ) of the device is 17% higher than that of a device without the up-conversion nanoparticles (UCNPs) and 13% higher than that of a device containing mixed TiO 2 and UCNPs. Nano-heterostructures of TiO 2 /NaYF 4 :Yb 3+ ,Tm 3+ and TiO 2 /NaYF 4 :Yb 3+ ,Ho 3+ can also be prepared in situ on TiO 2 photoanodes. The overall efficiency of the device containing TiO 2 /NaYF 4 :Yb 3+ ,Ho 3+ nano-heterostructures is 15% higher than the control device without UCNPs. When nano-heterostructures of TiO 2 /NaYF 4 :Yb 3+ ,Tm 3+ are used, the open-circuit voltage ( V oc ) and the short-circuit current density ( J sc ) are all slightly decreased. The effect of the different UCNPs results from the different energy levels of Er 3+ , Tm 3+ , and Ho 3+ . These results demonstrate that utilizing the UCNPs with the apporpriate energy levels can lead to effective electron injection from the UCNPs to the CB of TiO 2 , effectively improving the photocurrent and overall efficiency of DSCs while using NIR light.