Long-Wavelength Sensitization of TiO₂ by Ruthenium Diimine Compounds with Low-Lying π* Orbitals

Abstract

The role of low-lying π* orbitals in dye-sensitized solar cells based on mesoporous thin films of anatase TiO(2) nanocrystallites remains unknown. Herein we report three ruthenium compounds, cis-Ru(dcbq)(2)(NCS)(2), cis-Ru(dcbq)(bpy)(NCS)(2), and cis-Ru(dcb)(bq)(NCS)(2), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO(2)H)(2)-2,2'-bipyridine, bq is 2,2'-biquinoline, and dcbq is 4,4'-(CO(2)H)(2)-2,2'-biquinoline, that were synthesized, characterized, and contrasted with the well-known N3 compound (i.e., cis-Ru(dcb)(2)(NCS)(2)) in dye-sensitized solar cells. These compounds maintain the same cis-Ru(NCS)(2) core with a systematic variation in the energy of the π* orbitals of the diimine ligand: bpy > dcb > bq > dcbq. The lowered π* orbitals resulted in enhanced red absorption relative to N3. With HCl pretreated TiO(2) in regenerative solar cells, sensitization from 400 to 900 nm was realized with cis-Ru(dcb)(bq)(NCS)(2) and global power conversion efficiencies as high as 6.5% were achieved under 1 sun of AM 1.5 irradiation. The energy conversion efficiency was found to be acutely sensitive to the presence of p-tert-butylpyridine (TBP) in a 0.5 M LiI/0.05 M I(2) acetonitrile electrolyte. Nanosecond transient absorption studies revealed that the addition of TBP decreased the excited-state injection yield for the compounds with biquinoline ligands. Spectro-electrochemical studies showed that the HCl pretreatment lowered the effective density of TiO(2) acceptor states and confirmed that the presence of TBP raised them toward the vacuum level. There was no spectroscopic data to support the hypothesis that the π* levels of the diimine ligand mediate back-electron transfer to the oxidized dye or the redox mediator was found.

Kinetic pathway for interfacial electron transfer from a semiconductor to a molecule

“…Kinetic-data fitting was performed in Origin 9 using the Levenberg-Marquardt iteration method for least-squares error minimization, and spectral modelling was performed using a code written in Mathematica 10. Excited-state injection yields for all the compounds were determined by comparative actinometry using cis-Ru(dcb) 2 (NCS) 2 as the reference with an injection yield equal to one (Supplementary Table 1 47 ).…”

Enhancing the photoresponse by CdSe‐Dye‐TiO2‐based multijunction systems for efficient dye‐sensitized solar cells: A theoretical outlook

“…The semiconductor quantum dots (QDs) show strong absorption in the visible region and have hence grabbed the attention toward the development of better photovoltaic devices . The OD sensitizers are of immense importance owing to the tunable bandgap in the visible region, a large intrinsic dipole moment, a high extinction coefficient, and relatively simple and cost‐effective production . The widely used QD sensitizers include the metal chalcogenides, such as CdS, CdSe, CdTe, PbS, PbSe, and Sb 2 S 3 …”

Design, fabrication and modification of metal oxide semiconductor for improving conversion efficiency of excitonic solar cells

“…DSC is a category of photovoltaic device based on a photo-electrochemical system in which a porous MOS film with dye molecules adsorbed on the surface serving as the working electrode for light harvesting and the generation of photoexcited electrons [24]. Many efforts have been focused on improving light absorption of sensitizers [25][26][27][28]. In 2014, a recorded PCE of 13% of DSC has been obtained by using a molecularly engineered porphyrin dye coded SM315 and the cobalt (II/III) redox electrolyte [29].…”