(311d) Interfacial Electron Transfer in Dye Sensitized Solar Cells Measured by Time Resolved Terahertz Spectroscopy

Electron injection times from the ruthenium organometallic dyes N3, "red dye," and N749, "black dye," vary from time scales of hundreds of femtoseconds to hundreds of picoseconds depending on the combination of dye, semiconductor, and excitation wavelength. Injection dynamics and quantum efficiency are determined by the electronic coupling between available states in the semiconductor conduction band and the dye's excited states. We find that injection into TiO₂ is generally ultrafast, approaching the ~500 fs instrument response time. Injection dynamics into TiO₂ are independent of dye with 400 nm excitation. However, with 800 nm excitation, injection from black dye takes ~10's of ps while injection from red dye remains ultrafast. Injection into ZnO is much slower than into TiO₂, taking ~100 ps. The dynamics are independent of dye and excitation wavelength. The exception that is that is essentially no injection from red dye into ZnO using 800 nm excitation since there is insufficient photon energy to excite dye states that are above the ZnO conduction band. Literature suggests that faster injection into TiO₂ than ZnO is expected because electrons are injected into a high density of TiO₂ 3d orbitals while injection into ZnO is into a lower density of s and p orbitals. All of the measured time scales are much faster than the 59 nanosecond relaxation time of the excited dye molecule in solution and should result in efficient interfacial electron transfer in DSSCs.

We have also investigated injection into TiO₂/ZnO, ZnO/TiO₂, and ZnO/Al₂O₂ core/shell nanoparticles. The use of a shell layer with higher conduction band potential than the core can reduce recombination in DSSCs by confining electrons to the nanoparticle core. However, the presence of the shell can change the injection dynamics as well. Shell layers were grown on nanoparticle films in solution by successive ionic layer adsorption and reaction (SILAR). Increasingly thick TiO₂ shells on ZnO nanoparticles show the development of an ultrafast injection component in addition to the slower injection found in ZnO. However, the injected electron density decreases compared to uncoated ZnO, indicating that electrons may become trapped at the interface or remain in TiO₂instead of proceeding to the ZnO core. TiO₂/ZnO and ZnO/Al₂O₂ core/shell nanoparticles show decreased electron intensity without any significant change in dynamics. The lower electron density in coated nanoparticles indicates that electrons can easily become trapped at the core-shell interface. Studies of the microstructure of these interfaces are required for further interpretation of the data.