Upconversion nanoparticles (UCNPs) hold promise as near-infrared light converters to enhance the efficiency of solar cells. However, the prevalent use of UCNPs in solar cells is restricted by their poor electrical conductivity and low emission efficiency. Here reduced graphene oxide (rGO)-NaYF4:Yb(3+)/Er(3+) composites are proposed to achieve good electrical conductivity due to the high charge carrier mobility of rGO. Composites of rGO and UCNPs combined by a chemical bond are in situ synthesized by the hydrothermal method, followed by a reduction process. The contact of UCNPs with rGO is proved by SEM, and the binding between the rGO-UCNP composites is confirmed by Fourier transform infrared spectroscopy. The composites are doped into the photoanode of a solar cell. As anticipated, electrochemical impedance spectroscopy confirms the good electrical conductivity of the in situ synthesized rGO-UCNPs. Furthermore, the use of rGO-UCNPs in solar cells enables an enhancement in short-circuit current density and overall efficiency by about 10%. These findings reveal that the combination of UCNPs with rGO opens up new opportunities of extending the use of UCNPs in the area of solar energy harvesting.