Structural and luminescence properties of NaYF4:Yb3+/Er3+ and NaGdF4:Yb3+/Nd3+ nanocrystals for possible application in photovoltaic solar cells

Abstract

NaYF4:Yb3+/Er3+ and NaGdF4:Yb3+/Nd3+ nanocrystals were prepared using a solution combustion (SC) method and their properties were evaluated for potential use as photon harvesters in photovoltaic solar cells (PSCs). The dopant concentrations were varied (10–30 mol% Yb3+, 1–3 mol% Er3+/Nd3+), and the samples went through an annealing process at 600 and 700 °C. The effect of doping and annealing on their structure, morphology, photon absorbance, photoluminescence (PL), and thermoluminescence (TL) was investigated. The α-cubic and β-hexagonal phases of the nanocrystals was confirmed by X-ray diffractometer (XRD). Changing the dopant concentration and annealing temperature, respectively caused a distinct change in morphology (cubic to hexagonal and rod-like). It was observed that photon absorption occurred in the range from visible to near infrared (NIR), and bandgap values were greatly reduced. PL demonstrated upconversion (UC) and effective transfer of energy from Yb3+ to Er3+ ions, and also from Yb3+ to Nd3+ ions. The UC luminescence was boosted by annealing the nanocrystals. The thermogravimetric analysis (TGA) of the nanocrystals shed light on their stable temperatures, while the defects within the materials were investigated using TL. The kinetics involved in the electron trapping centres were found to be that of first order, and the depth of the electron trapping centres was determined using the initial rise (IR) method.