Competing effects of sensitization and energy dissipation by Ni2+ incorporation in La(Ga0.5Sc0.5)O-3:Er,Ni,Nb upconverters

We investigated three pathways of energy transfer between Ni2+ sensitizers and Er3+ emitters in broadband-sensitive La(Ga0.5Sc0.5)O-3: Er,Ni,Nb upconverters emitting at 0.98 mu m for crystalline silicon solar cells. The results were interpreted based on simplified models of the emission mechanisms. A high efficiency close to unity was successfully achieved for the Ni2+-> Er3+ (I-4(13/ 2)) energy transfer, meaning efficient sensitization. Energy dissipation caused by the Er3+ (I-4(13/2))-> Ni2+ back transfer was minor. However, detrimental impact of another back transfer from the initial state of the upconversion emission Er3+ (I-4(11/2)) competed against improvements in the absorbance and energy transfer upconversion with increasing Ni2+ and Er3+ concentrations, which limited the upconversion efficiency. Precise tuning of the Ni2+ absorption band to suppress the second back transfer can improve the upconversion performance.