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Co/B co-doped ZnO (Zn 0.93-x Co x B 0.07 O, x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) nanoparticles were synthesized by the hydrothermal method to investigate the effect of cobalt and boron on the structural, microstructural, and optoelectronic properties of ZnO nanoparticles. The X-ray diffraction method was used for the structural analysis and single phases were found for all Co/B co-doped ZnO nanoparticles. Scanning Electron Microscope (SEM) technique was used to determine the surface morphology, particle size, and the shapes of the nanoparticles. The elemental compositions of the nanoparticles were obtained by electron dispersive spectroscopy (EDS). Hexagonal Wurtzite structure was proved by c/a ratios of the ZnCoBO nanoparticles. The Fourier transform infrared (FTIR) studies were performed and explained. The energy band gaps of the samples were calculated and the effects of dopant elements on optical properties were discussed. The maximum band gap occurred for Zn 0.93 B 0.07 O with a band gap energy of E g = 3.26 eV. The refractive index was calculated using the energy band gap with five different models. The grain sizes and the band gap energies fluctuated as the doping ratio increased. The results showed that the refractive index strongly depends on the Co concentration nonlinearly. It was found that doping cobalt increased the Urbach energy value of B-doped ZnO nanoparticles. The increase in E u indicates that the structural disorder and the number of defects in the Zn 0.93-x Co x B 0.07 O structures increased with increasing concentration of Co in the Zn 0.93-x Co x B 0.07 O structures. The highest value of Urbach energy was approximately found in the range of 1259 and 1469 meV for 3% Co. Moreover, for 3% Co the concentration-dependent microstrain (Ɛ) values also reached the maximum, dislocation density δ had also the maximum value. [ABSTRACT FROM AUTHOR]