3.4. Near infrared upconversion
Fig. 7 shows the NIR UC emission spectra for the 4I9/2→4I15/2 of Er3+-doped glasses for various concentrations under 1523 nm excitation. The characteristic emission bands of Er3+ ions centered at around 975 nm correspond to the 4I11/2→4I15/2 transitions. It is interesting to note that the UC emission increases enormously with increase in Er3+ ion concentration (0.01–3.0 mol%) and no luminescence quenching (to be expected for higher concentration) was noticed in the investigated range of concentrations. The FWHM increases with increase in concentration and is represented in Table 6. These highly Er3+-doped glasses could be useful for c-Si solar cell applications as they emit intense emission at 1.0 mm region. Moreover, they present a high refractive index that diminishes reflection losses when integrated with a silicon cell, and they show a broad absorption band at 4I13/2, which is crucial for UC photovoltaic’s (PV) [45].