Comparative study on the interactions of sulfide and iodine mediators with a dye in p-type dye-sensitized solar cells

Abstract The intermolecular interactions of a typical triphenylamine-based push-pull dye (P1) with three sulfide species T 3 − , T 2 , and T 2 − and with the iodine species I 3 − , I 2 , and I 2 − were investigated by means of density functional theory. The reduced form of P1 interacts with both T 3 − (I 3 − ) and T 2 − (I 2 − ) through reductive cleavage of the S S (I I) bond, whereas T 2 (I 2 ) interactions involving single-electron transfer occur without S S (I I) bond breakage. Therefore, regeneration with a thiolate–disulfide redox couple (as well as with iodide–triiodide) proceeds by three different mechanisms: by interaction with T 3 − (I 3 − ) via the transient intermediate complex T 2 − ⋯P1⋯ T − (I 2 − ⋯P1⋯I − ) to give T 2 − (I 2 − ) and T − (I − ); by interaction with T 2 (I 2 ) via P1⋯ T 2 − (P1⋯I 2 − ) to give T 2 − (I 2 − ); or by interaction with T 2 − (I 2 − ) via T − ⋯P1⋯ T − (I − ⋯P1⋯I − ) to give two T − (I − ) moieties. The P1– T 2 − complexes have far fewer configurations, and the T 2 − species are much farther away from the semiconductor surface than are the I 2 − species in P1–I 2 − . This suggests that recombination of injected holes with electron donors in the thiolate–disulfide electrolyte is suppressed relative to that in iodide–triiodide, leading to a greater than expected increase in the open-circuit photovoltage for a p-type dye-sensitized solar cell.