In this paper, we synthesized manganese-substituted cerium oxide (CeMnO2) electron transport layer (ETL) nanomaterial via the co-precipitation method using various Mn compositions. These variations resulted in significant changes in the physical, optical, electronic, and dielectric properties of the CeMnO2 nanoparticles. The hexagonal structure of the CeMnO2 nanoparticles was validated using X-ray diffraction (XRD). Their morphology and surface characteristics were analyzed through scanning electron microscopy (SEM). Ultraviolet–visible (UV–Vis) spectrophotometry was utilized to explore their optoelectronic properties, revealing an increase in transmittance from 75 % to 90 %, along with corresponding decreases in reflectance and absorbance. Moreover, the energy bandgap (Eg) increased from 3.68 to 3.86 eV, while the Urbach energy (EU) rose from 0.18 to 0.6 eV with varying Mn compositions. The increase in defect quantity is associated with the reduction in both DC and AC conductivity observed with higher Mn compositions. Using a Hall effect instrument, we measured the sheet resistance, Hall mobility (ranging from 35.20 to 62.32 cm2 V−1 s−1), and carrier concentration (from 8.71 × 1018 to 1.22 × 1020 cm−3) of the CeMnO2 nanomaterials. The second stage focused on developing a high-efficiency (CeMnO2/MAFASnBrI3/BaSi2) solar cell based on experimental data from the CeMnO2 ETL and BaSi2 HTL materials. After multiple optimizations of the proposed solar cells, we achieved a 27.73 % efficiency.