Geometrical concentration for enhanced up-conversion: A review of recent results in energy and biomedical applications

Up-conversion is one of the promising approaches for energy and biomedical applications. To enable significant power conversion efficiencies, high irradiance is required. In solar energy applications, this irradiance translates to solar concentrations higher than one sun, unable to be obtained without geometrical or local-field concentration. For biomedical applications, enhanced sensitivity and imaging resolution are required, accompanied by low sample volumes. In this review, the performance of approaches with geometrical concentration on up-conversion processes is reviewed based on recently reported results. Compound parabolic concentrators in c-Si and β-NaYF4: 25%Er3+ is to-date the most promising approach, with a device external quantum efficiency of 1.8% at incident irradiance of 240 W/m2.