One-Diode Model Equivalent Circuit Analysis for ZnO Nanorod-Based Dye-Sensitized Solar Cells: Effects of Annealing and Active Area

Electrical characteristics of 1-D zinc oxide (ZnO) nanorod-based dye-sensitized solar cells (DSSCs) were experimentally measured and followed by theoretical analysis using simple one-diode model. Defect sites (mostly oxygen vacancies) in ZnO are typically responsible for lower DSSC performance, which are removed by annealing the ZnO nanorods at high temperatures up to 450 °C. The DSSC performances with respect to the different annealing temperatures (250 °C, 350 °C, and 450°C) were determined by measuring their I-V characteristics at 1-sun irradiation (AM 1.5G). The variations in series and shunt resistances of DSSC were estimated by fitting the experimental I-V characteristics with the ideal I-V curve obtained from the one-diode equivalent model of the DSSC. By increasing annealing temperature, reduction in the series resistance R_s of the DSSCs with a subsequent increase in the shunt resistance R_sh was obtained. Annealing temperature of 350 °C was found to be optimum at which maximum DSSC performances with 1-cm² cell active area showing minimum R_s (0.02 kΩ) with high R_sh (1.08 kΩ) values were observed. Reduction in the active area of the DSSCs from 1 to 0.25 cm² and further to 0.1 cm² demonstrated improved device performance with ~56% and ~24% enhancement in the fill factor and open-circuit voltage V_oc, respectively, due to the reduced sheet resistance and lower recombination rate resulting low series resistance and high shunt resistance, respectively. At the optimum annealing temperature, maximum DSSC efficiency of 4.60% was obtained for the 0.1-cm² cell active area.