Important for increasing the efficiency of solar cells, multifunctional near- stoichiometric LiNbO3 doped with Er3+ ions (Er:NSLN) could be considered as a luminescent layer to improve the solar spectrum response. The upconversion emission spectra show that Er:NSLN crystal converts the near infrared wavelength light which cannot be absorbed by the solar cells into the visible upconversion luminescence. The increased green and red upconversion emissions are observed in Er:NSLN crystal, and both green and red emitting states are populated by the three-photon process at the low pump power. As for the high pump power, the one-photon process is attributed to the “saturation” of the upconversion process. The Judd–Ofelt intensity parameters of Er:NSLN are calculated to be Ω2 = 9.17 × 10−20 cm2, Ω4 = 2.47 × 10−20 cm2 and Ω6 = 1.00 × 10−20 cm2. Based on McCumber theory, the enhanced emission and absorption cross-sections of the green and red emissions indicate that Er:NSLN used as the upconversion luminescent layer could emit high efficiency of visible luminescence re-absorbed by the solar cells.