An organo-sulfur compound, 5-methylthio-1,3,4-thiadiazole-2-thiol and its oxidized dimer 5-methylthio-1,3,4-thiadiazolium disulfide dication are used as a novel and universal redox couple in both dye-sensitized and quantum-dot sensitized solar cells, in conjunction with a conducting poly(3,4-ethylenedioxythiophene) counter electrode. The 5-methylthio-1,3,4-thiadiazole-2-thiol/5-methylthio-1,3,4-thiadiazolium disulfide dication redox couple is low-cost and easily processable, and exhibits nonabsorption of visible light, showing improved redox behavior at an electrochemically deposited poly(3,4-ethylenedioxythiophene) counter electrode. Its redox potential is ca. 60 mV negatively and ca. 170 mV (vs. normal hydrogen electrode) positively positioned than those of iodide/tri-iodide and polysulfide electrolytes, respectively. The maximum power conversion efficiency of (3.55 and 1.20) % is obtained in dye and quantum-dot sensitized solar cells, respectively, sensitized by 5-[[4-[4-(2, 2-Diphenylethenyl) phenyl]-1,2,3,3a,4,8b hexahydrocyclopent [b] indol-7-yl] methylene] 2 (3 octyl-4-oxo-2-thioxo-5-thiazolid dye and CdS-quantum dot, respectively, together with the optimized poly(3,4ethylenedioxythiophene) counter electrode. These results clearly outperform the performance of identical dye and quantum-dot sensitized solar cells with Pt and Cu2S-counter electrodes. Thus, this new redox couple and poly(3,4-ethylenedioxythiophene) counter electrode pair is expected to offer a promising and universal alternative to the conventional iodide/tri-iodide and polysulfide electrolytes for sensitization-based solar cells. (C) 2018 Elsevier Ltd. All rights reserved.