Herein, we describe an efficient way to engineer the electronic energy defects and achieve the defect passivation of codoping of two rare-earth (RE) ions, namely, neodymium (Nd3+) and erbium (Er3+), into the TiO2 lattice. It is shown that the PEC performance is significantly improved using the RE-coated TiO2 photoanode. X-Ray diffraction (XRD), transmission electron microscopy (TEM), and Raman study have confirmed the doping concentration's upper limit without modifying the single-phase structure of the TiO2 lattice. The Brunauer-Emmett-Teller (BET) study established the optimal codoping composition of “Er0.003Nd0.001Ti0.996O2” with enriched surface properties. Photoluminescence (PL) investigations indicated defect passivation, which enabled a favourable energy landscape via intermediate trap-sites, improving charge transport characteristics. The photoelectrochemical (PEC) device with the optimized codoped TiO2 photoanode has a higher increased photocurrent density (Jph) of 16 μA cm−2 in comparison to the device with pristine TiO2 (Jph: 7 μA cm−2). As a proof of concept, a simple sensitization of these engineered TiO2 photoanodes with the cadmium sulfide (CdS) quantum dots resulted in a Jph of 2.8 mA cm−2, and with N719 ruthenium dye, a Jph of 0.8 mA cm−2 was obtained, indicating the improved performance of sensitized codoped TiO2 photoanodes for PEC devices. As a proof of concept, N719 dye-based dye-sensitized solar cells with doped TiO2 photoanodes demonstrated an enhanced power conversion efficiency. © 2022 The Royal Society of Chemistry.