Many aspects of solar energy and policies to tackle the energy transition have been neglected. Even though the earth is plenty of sun energy, our planet is not plenty of resources to transform that energy into electricity. This is a case between many others where an strongly optimistic bias is shadowing the white elephant in the room.

Upconversion of low‐energy photons into high‐energy photons increases the efficiency of photovoltaic devices by converting photons with energies below the absorption threshold of the solar cell into photons that can be utilized. In this review, an overview is provided of quantitative studies of the upconversion quantum yield of upconverter materials, and of the achieved efficiency enhancements in upconverting solar cell devices. Different materials and devices are compared based on well‐defined figures‐of‐merit and the challenges to their accurate measurement are discussed. Internal upconversion quantum yields above 13% have been reported both for Er3+‐based materials as well as for organic upconverters, using irradiance values below 0.4 W cm−2. On the upconverting solar cell device level, relative enhancements of the solar cells' short‐circuit currents by up to 0.55% have been achieved. These values document progress by orders of magnitude achieved in the last years. However, they also show that the field of upconversion needs further development to become a relevant technology option in photovoltaics. Different options regarding how upconversion performance can be increased further in the future are outlined.

Improving costs and scale reflect looming opportunities

Upconversion (UC) of subband-gap photons is a promising possibility to enhance solar cell efficiency by making also the subband-gap photons useful. For this application, we investigate the material system of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+), which shows efficient UC suitable for silicon solar cells. We determine the optical UC efficiency by calibrated photoluminescence measurements. Because these data are free from any influence of losses associated with the application of the upconverter to the solar cell, the obtained values constitute the upper limit that can be achieved with an optimized device. Subsequently, we compare the results of the optical measurements with the results obtained by using solar cells as detectors on which the upconverter material is applied. We find an optical UC quantum efficiency of 5.1% at a monochromatic irradiance of 1880 W m(-2) (0.27 cm(2) W-1) at 1523 nm. The device of silicon solar cell and applied upconverter showed an external quantum efficiency of 0.34% at an irradiance of 1090 W m(-2) (0.03 cm(2) W-1) at 1522 nm. The differences are explained by the optical losses occurring in the upconverter solar cell device, which are dominated by the transmission of the solar cell and the incomplete absorption of the upconverting layer, and the nonlinear behavior of the upconverter

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 4I15/2 → 4I13/2 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 4I11/2 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 4I13/2 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 4I11/2 UC emission is energy transfer UC.