Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap infrared photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er³⁺-doped Gd₂O₂S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics experiments. Excitation into the maximum of the ⁴I_15/2 → ⁴I_13/2 Er³⁺ absorption band around 1500 nm induces very efficient UC emission from different Er³⁺ excited states with energies above the silicon bandgap, in particular, the emission originating from the ⁴I_11/2 state around 1000 nm. Concentration dependent studies reveal that the highest UC quantum yield is realized for a 10% Er³⁺-doping concentration. The UC luminescence is compared to the well-known Er³⁺-doped β-NaYF₄ UC material for which the highest UC quantum yield has been reported for 25% Er³⁺. The UC internal quantum yields were measured in this work for Gd₂O₂S: 10%Er³⁺ and β-NaYF₄: 25%Er³⁺ to be 12 ± 1% and 8.9 ± 0.7%, respectively, under monochromatic excitation around 1500 nm at a power of 700 W/m². The UC quantum yield reported here for Gd₂O₂S: 10%Er³⁺ is the highest value achieved so far under monochromatic excitation into the ⁴I_13/2 Er³⁺ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding ⁴I_11/2 UC emission is energy transfer UC.