Citric acid-assisted growth of lanthanide ions co-doped one-dimensional upconversion microcrystals and their photovoltaic applications

We report for the first time the preparation of Yb³⁺-Er ³⁺ co-doped β-phase sodium gadolinium fluoride (NaGdF ₄:Yb/Er) microcrystals using citric acid (CA) as a shape modifier and the application of NaGdF₄:Yb/Er to amorphous silicon solar cells. The proposed hydrothermal method was facile and eco-friendly. It was found that CA played a critical role in the shape evolution of the final products. The related synthesis mechanisms are discussed. Under 980 nm laser excitation, the optimized NaGdF₄:Yb/Er crystals exhibited strong upconversion (UC) emissions at visible wavelengths. Amorphous silicon thin film solar cells were designed and fabricated for investigating different UC configurations. In combination with a metallic back reflector, the cell with the front side UC layer achieved a six-fold improvement of the photocurrent while the cell with the rear side UC layer exhibited a ten-fold enhancement of the photocurrent, under 980 nm light, compared with the cell without upconverter.