Zinc oxide nanoparticles (ZnO NPs) were synthesized using a hydrothermal route. The prepared ZnO NPs were characterized by x-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), UV–vis spectroscopy, and photoluminescence (PL) spectroscopy. The XRD patterns confirmed the standard hexagonal wurtzite structure of ZnO NPs, and the calculated value of the average particle size was 23.34 nm. HR-TEM micrographs of ZnO NPs showed semispherical particle morphologies and their sizes lie between 10 and 40 nm. The estimated average size distribution of ZnO NPs was 21.35±6.01 nm. UV–vis spectrum of ZnO NPs revealed the highest absorption band at 360.5 nm, and the Eg was 3.70±0.01 eV. The PL spectrum emission was deconvoluted by eight peaks into two regions [near-ultraviolet (NUV) and visible that caused from the defects]. Two groups of dye-sensitized solar cells (DSSCs) thin film devices based on ZnO NPs were sensitized in different concentration solutions of 0.1, 0.32, and 0.5 mM of eosin B (EB) and eosin Y (EY) dyes. The sensitized DSSCs device with 0.32-mM dye of EY displayed higher efficiency and its performance parameters are much better among all other fabricated DSSCs devices. The short current density (Jsc) increased from 1.59 to 4.97 mA/cm2 and the Voc enhanced from 0.36 to 0.46 V. The conversion efficiency from light to electricity showed a significant improvement from 0.29% to 0.94%. The transient open circuit photovoltage decay (TOCPVD) was measured to estimate the apparent electron lifetime or response time (τn) or the electron recombination rate (krec), using the double exponential function for first time to fit the experiment data of TOCPVD. The results revealed that the EY dye can be used as an efficient and an inexpensive dye for DSSCs.