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

摘要

The intermolecular interactions of a typical triphenylamine-based push-pull dye (P1) with three sulfide species T-3(-), T-2, and T-2(-)center dot and with the iodine species I-3(-), I-2, and I-2(-)center dot were investigated by means of density functional theory. The reduced form of P1 interacts with both T-3(-) (I-3(-)) and T-2(-) center dot (I-2(-)center dot) 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(-)center dot center dot center dot P1 center dot center dot center dot T- (I-2(-)center dot center dot center dot P1 center dot center dot center dot I-) to give T-2(-) center dot (I-2(-) center dot) and T- (I-); by interaction with T-2 (I-2 via PI center dot center dot center dot T-2(-) (P1 center dot center dot center dot I-2(-)) to give T-2(-) center dot (I-2(-)center dot); or by interaction with T-2(-)center dot (I-2(-)center dot) via T-center dot center dot center dot P1 center dot center dot center dot T- (I-center dot center dot center dot P1 center dot center dot center dot I-) to give two T- (I-) moieties. The P1-T2(-)center dot complexes have far fewer configurations, and the T-2(-) center dot species are much farther away from the semiconductor surface than are the I-2(-) center dot species in P1-I-2(-) center dot. 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.