Complete Photodynamics of the Efficient YD2-o-C8-Based Solar Cell

Abstract

The interplay between the different channels involved in the dye-sensitized solar cells (DSSCs) photodynamic cycle (photon absorption, dye aggregation and emission quenching, e– injection, dye regeneration, e––dye recombination) is the determining factor for their efficiency and performance. Here, we report on studies of the photodynamics of complete solar cells sensitized with 5,15-bis(2,6-dioctoxyphenyl)-10-(bis(4-hexylphenyl)amino-20-4-carboxyphenyl ethynyl)porphyrinato]zinc(II) (YD2-o-C8), adsorbed on TiO2 and Al2O3 nanoparticles in the presence of [Co(bpy)3]3+/2+ electrolyte, with and without a disaggregating agent (chenodeoxycholic acid, CDCA). We used steady-state and femtosecond to millisecond time-resolved techniques to interrogate the related dynamics. The steady-state UV–vis results show a change in the YD2-o-C8 molecules distribution and orientation as well as a decrease in the ligand to metal coordination between the porphyrin molecules due to the effect of the [Co(bpy)3]3+/2+ redox couple. The picosecond to nanosecond emission results for cells using Al2O3 electrode (as a reference) indicate the increase in emission lifetime (from 260 to 460 ps) upon addition of CDCA. This decreases the population of the dye aggregates, thus reducing the efficiency of the nonradiative processes. For YD2-o-C8/Al2O3 cell, we observed vibrational cooling in the 3.5–5 ps range and vibrational energy transfer (27–30 ps) to the alumina NPs film. However, for YD2-o-C8/TiO2 these processes occur in ∼1 and 6 ps, respectively, due to a difference in the dye/electrode interactions. For the DSSC using TiO2 nanoparticles, electron injection from YD2-o-C8 to titania occurs in ∼100 fs, while from the relaxed vibrational state it is much slower (6 ps). The back electron transfer from the TiO2 trap states to the porphyrin radical cation happens in ∼9 ps. Electron injection efficiencies for the full cell with and without CDCA are different (85–89%), reflecting the effect of the aggregates on the cell performance. In addition, the back electron transfer from the titania conduction band to the dye radical cation is slower in the presence of CDCA (878 and 1577 μs at 550 and 1400 nm, respectively) than without coadsorbent (143 and 252 μs at 550 and 1400 nm, respectively). Thus, the quantum yield of dye regeneration amounts to ∼93% and ∼99% in the absence and presence of the coadsorbent, respectively. These results indicate the role of the YD2-o-C8 aggregation on TiO2 electrode in the DSSCs performance. The findings relevant to the photocycle of porphyrins-based solar cells can help in better design and optimization of such devices.