The efficiency of single-junction photovoltaic (PV) devices is fundamentally limited by spectral mismatch between the incident solar radiation and the semiconductor bandgap. In silicon photovoltaics, sub-bandgap near-infrared photons are transmitted without generating electron–hole pairs, while excess photon energy above the bandgap is lost through thermalisation. Spectral conversion offers a route to mitigate these losses by modifying the incident photon distribution before it reaches the photovoltaic absorber. Among the available approaches, up-conversion is particularly attractive because it can combine two or more low-energy photons into a higher-energy photon capable of being absorbed by the semiconductor. This article reviews the scientific basis for Yb³⁺/Er³⁺-doped borate glasses as up-conversion materials for photovoltaic applications. The roles of Yb³⁺ as a sensitiser and Er³⁺ as an activator, the relevant energy-transfer pathways, host-glass characteristics, concentration effects, and the limitations imposed by phonon-assisted non-radiative relaxation are discussed. Previous studies demonstrate characteristic green and red emissions from Yb³⁺/Er³⁺ systems under near-infrared excitation, while borate glasses offer advantages in transparency, rare-earth solubility, chemical durability, cost, and scalable fabrication. However, the literature indicates a need for systematic optimisation under PV-relevant, low-irradiance conditions. The article therefore identifies optimal dopant concentration, energy-transfer efficiency, and spectral overlap with silicon response, low-intensity up-conversion performance, and optical stability as important directions for future experimental investigation.