[Show abstract][Hide abstract] ABSTRACT: Electron diffusion coefficient, lifetime, and density in the TiO(2) electrode of dye-sensitized TiO(2) solar cells (DSCs) employing I(-)/I(3)(-) redox couples were measured with eight different metal-free organic dyes and three Ru complex dyes. At matched electron density, all DSCs using organic dyes (ODSCs) showed shorter electron lifetime with comparable or larger diffusion coefficients in comparison to the DSCs using the Ru dyes (RuDSC). The shorter lifetime was attributed partially to the slower dye cation reduction rate of the organic dyes by I(-), faster electron diffusion coefficient in the TiO(2), and mostly higher I(3)(-) concentration in the vicinity of the TiO(2) surface. Whereas a slight shift of the conduction band edge potential (E(cb)) of the TiO(2) was seen with a few organic dyes, no correlation was found with the dipole moment of the adsorbed dyes. This implies that the adsorbed dyes interact with cations in the electrolyte, so the direction of the dipole is altered or simply screened. The increase of [I(3)(-)] in the vicinity of the TiO(2) surface was interpreted with partial charge distribution of the dyes. Under one-sun conditions, less electron density due to shorter electron lifetime was found to be the main reason for the lower values of V(oc) for all ODSCs in comparison to that of RuDSCs. Among the organic dyes, having larger molecular size and alkyl chains showed longer electron lifetime, and thus higher V(oc). Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.
No preview · Article · Jan 2009 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: We fabricated dye-adsorbed NiO solar cells (pDSCs) with six different metal-free organic dyes having various ground-state oxidation potentials. Under monochromatic light irradiation, all dyes showed cathodic current at wavelengths where the dyes absorb, suggesting hole injection occurred from the adsorbed dyes to the valence band of NiO. Absorbed photon-to-current conversion efficiency (APCE) tends to increase with the increase of energy difference (ΔE) between the valence band edge of NiO and the ground state of the dyes. The maximum APCE of 30% was obtained with 0.6 eV of the ΔE. The apparent hole diffusion coefficient in the NiO electrode was nearly independent from light intensity, and the values were estimated to be 4 × 10−8 cm2/s. On the other hand, hole lifetime depends on light intensity, ranging from 3 × 10−2 to 1 × 100 s. Investigation of the anchoring site of the dyes and the results of molecular orbital calculations suggested that electron injection from the dye to the valence band of NiO, occurring just after the hole injection, is the major factor of the relatively low efficiency even with the case of large ΔE.
No preview · Article · Sep 2008 · The Journal of Physical Chemistry C