Upconversion mechanisms in rare-earth doped glasses to improve the efficiency of silicon solar cells

The electronic energy transfer properties between Ho3+ and Yb3+ ions have been studied in a fluoroindate glass for solar cell applications. The Ho3+ ions absorb infrared radiation at around 1150 nm, below the energy gap of Si solar cells. Energy transfer between Ho3+ and Yb3+ ions produces an upconversion emission in the visible and in the near infrared spectral range just above the Si bandgap. When these glasses are placed at the rear of a bifacial Si solar cell, the upconverted radiation can be absorbed by Si and generate electron–hole pairs that contribute to enhance the cell efficiency. An estimation of the expected increase in photo-current has been calculated when the upconverter material is used in a solar concentrator. Besides, they can be used alone or together with other Er3+ doped phosphors for the same purpose. The Ho3+–Yb3+ upconversion emission characteristics have been investigated as a function of the doping ion concentrations. Excitation, pump power dependency and dynamic experiments have been performed to determine the electronic energy transfer mechanism that is responsible of the upconversion. A rate equation analysis shows a reasonable agreement between the model and the experimental data.