Upconverted fluorescence in Er-doped ZBLAN glasses for high efficiency solar cells

Transparent erbium-doped fluorozirconate (FZ) glasses are attractive systems for upconversion-based solar cells. Upconverted fluorescence intensity vs. excitation power dependence was investigated for a series of erbium-doped FZ glasses. It was found that the ratio of the 2-photon upconverted emission in the near infrared at 980 nm to the 3-photon upconverted emissions in the visible at 530, 550, and 660 nm decreases with increasing excitation power. The integrated upconverted fluorescence intensity per excitation power shows "saturation" upon increasing the excitation power, while the point of saturation shifts to lower excitation power with increasing erbium doping level. To demonstrate the potential of these upconverters for photovoltaic applications, the external quantum efficiency (EQE) of a commercial monocrystalline silicon solar cell with an Er-doped FZ glass on top of it was measured. For an excitation power of 1 mW at a wavelength of 1540 nm an EQE of 1.6% was found for a 9.1 mol% Er-doped FZ glass. The samples investigated were not optically coupled to the solar cell and no optical coating was applied to the glass surface.