Upconversion properties of Er3+-Yb3+:NaYF4 phosphors with a wide range of Yb3+ concentration

► Er3+-Yb3+:NaYF4 with a wide range of Yb3+ codoping concentration were synthesized. ► Green and red UC emissions were strongly dependent on the Yb codoping concentration. ► Intensity ratio of green emissions was related to Yb3+ concentration and temperature. ► Upconversion properties of Er3+-Yb3+:NaYF4 were explained by steady-state equations.