The N3/TiO₂ interfacial structure is dependent on the pH conditions during sensitization

Area of Science:

• Materials Science
• Electrochemistry
• Surface Chemistry

Background:

• The performance of dye-sensitized solar cells (DSSCs) is critically dependent on the electronic structure at the N3 dye/TiO2 semiconductor interface.
• Understanding dye orientation on the semiconductor surface is crucial for optimizing DSSC efficiency.

Purpose of the Study:

• To investigate how the pH of the N3 sensitizing solution influences the protonation states of the N3 dye.
• To determine the binding geometry of N3 dye on a TiO2 substrate under varying pH conditions.
• To elucidate the relationship between dye orientation and interfacial electronic structure.

Main Methods:

• Utilized heterodyne detected vibrational sum frequency generation spectroscopy, a surface-specific technique.
• Analyzed N3 dye adsorption on TiO2 under different pH conditions of the sensitizing solution.
• Correlated dye protonation states with anchoring group behavior.

Main Results:

• Observed significant reorientation of the N3 dye on the TiO2 surface at pH ≤2.0.
• Identified that at low pH, N3-dye carboxylate anchoring groups are less involved in adsorption to the TiO2 substrate.
• Demonstrated a change in interfacial electronic structure resulting from altered molecular geometry.

Conclusions:

• Dye protonation state, controlled by solution pH, dictates N3 dye anchoring and orientation on TiO2.
• Dye reorientation at low pH alters interfacial electronic structure, potentially impeding electron transfer in DSSCs.
• Optimizing sensitizing solution pH is essential for controlling dye adsorption and enhancing DSSC performance.