β-NaYF4:Yb3+, Tm3+@TiO2 core-shell nanoparticles incorporated into the mesoporous layer for high efficiency perovskite solar cells

β-NaYF4:Yb3+, Tm3+ @ TiO2 core-shell upconversion nanoparticles (UCNPs) were synthesized and incorporated into the TiO2 mesoporous layer of perovskite solar cells (PSCs). Changes of the upconversion spectra of the UCNPs suggest the existence of the electron injection from NaYF4:Yb3+, Tm3+ to the conduction band (CB) of TiO2 through the NaYF4:Yb3+, Tm3+/TiO2 nano-heterojunctions. The core-shell UCNPs that incorporated into the TiO2 mesoporous layer not only act as the upconversion centers to use the NIR light but also serve as the light scatter centers to enhance the light harvesting. When using mesoporous layer containing the core-shell UCNPs in PSCs, the efficiency of the best devices is 16.27%, which is about 16.38% higher than that of the devices without core-shell UCNPs (13.98%). The significant enhancement in the performance of the PSCs is attributed to light scattering, NIR light upconversion and electron injection from the UCNPs to the CB of TiO2.