Influence of the Donor Size in D−π–A Organic Dyes for Dye-Sensitized Solar Cells

Abstract

We report two new molecularly engineered push–pull dyes, i.e., <b>YA421</b> and <b>YA422</b>, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported <b>IQ4</b> dye. Benefitting from increased steric hindrance with the introduction of bis­(2,4-dihexyloxy)­benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO₂ surface is reduced, especially in redox electrolyte employing Co­(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of <b>IQ4</b>, <b>YA421</b>, and <b>YA422</b> devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, <b>YA422</b> devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor−π–acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of <b>YA422</b> is slower than that of the other two dyes, the slower back electron transfer of <b>YA422</b> contributes to the higher device performance