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Adjacent- versus remote-site electron injection in TiO2 surfaces modified with binuclear ruthenium complexes
Bobak Gholamkhass1, Kazuhide Koike, Nobuaki Negishi
1Photoenergy Application Group, Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba West, Ibaraki 305-8569, Japan.
Inorganic Chemistry
|April 29, 2003
Summary
Ruthenium complexes on TiO2 films show efficient energy transfer for solar cells. The best performance came from a heterotriad assembly with fast electron injection and controlled orientation for reduced recombination.
Area of Science:
- Materials Science
- Photochemistry
- Nanotechnology
Background:
- Ruthenium complexes are crucial sensitizers in dye-sensitized solar cells (DSSCs).
- Understanding the photophysical properties and surface interactions of these complexes is key to improving DSSC efficiency.
- Heterobinuclear ruthenium complexes offer tunable electronic properties for enhanced light harvesting and charge transfer.
Purpose of the Study:
- To synthesize and characterize novel homo- and heterobinuclear ruthenium complexes for TiO2-based solar cells.
- To investigate the photophysical behavior and electron injection dynamics of these complexes adsorbed on nanocrystalline TiO2.
- To correlate molecular structure and surface orientation with photovoltaic performance.
Main Methods:
- Synthesis of ruthenium homo- and heterobinuclear complexes with varying ligands (dcb, bpy) and bridging units (tpphz, bfimbz).
- Adsorption of complexes onto nanocrystalline TiO2 films on ITO glass.
- Characterization using nanosecond time-resolved transient absorption and emission spectroscopy.
- Photocurrent measurements and photovoltaic performance evaluation.
Main Results:
- Intercomponent energy transfer was observed in solution, from Ru --> bpy to Ru --> dcb excited states.
- Adsorption on TiO2 occurred via dcb ligands, with perpendicular or parallel orientations.
- Fast, unresolved electron injection was observed for bfimbz-bridged complexes due to direct metal-to-dcb promotion.
- Slower injection rates (5.5-9.5 x 10^7 s^-1) were found for tpphz-bridged complexes, with excited state localization on the bridging ligand.
- The heterotriad assembly [(bpy)(2)Ru(bfimbz)Ru(bpy)(dcb(2-))](2+)/TiO2 exhibited the best photovoltaic performance.
Conclusions:
- The choice of bridging ligand (BL) significantly influences electron injection kinetics and photovoltaic performance.
- Fast electron injection is achieved when the excited state promotes electron transfer directly to a dcb ligand attached to TiO2.
- Optimized molecular orientation, facilitated by rigid and linear heterotriad structures, can reduce charge recombination and enhance device efficiency.