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43036-AEF
Investigating Dynamics of Dye Sensitized Solar Cells Using Time Resolved Terahertz Spectroscopy

Jason B. Baxter, Drexel University

During the second year of my PRF AEF fellowship, I have studied the time-dependent photoconductivity of ZnO wafers and interfacial electron transfer (IET) using time-resolved terahertz spectroscopy (TRTS). Recently, TRTS has emerged as an immensely useful method for understanding phenomena on sub-picosecond time scales using far-infrared radiation (0.2-2 THz, or 6-66 cm^-1). TRTS is ideal for probing photoexcited electrons because scattering rates are typically in the 1-10 THz regime. Using TRTS, ZnO or adsorbed dye molecules are pumped with near-UV or visible light with pulse durations under 100 fs, and the complex permittivity is obtained.

We measured permittivity of a ZnO wafer as a function of temperature from 10 K to 130 K. At temperatures below 40 K, there is essentially zero THz absorbance and the refractive index is 2.9 over the range 0.2 – 2 THz. However, as temperature increases there is an increase in absorbance and a minimum develops in the refractive index curve that signifies the presence of free electrons. These electrons were trapped at low temperature but at high temperature have gained sufficient energy to escape these traps. At 80 K, we measure an electron density of 7 x 10¹⁵ cm^-3 with mobility of 1400 cm²/V-s. This mobility is similar to what is found by Hall measurements at this temperature. Both these native conductivity measurements and photoconductivity measurements are fit well by the Drude conductivity model. We also found that the decay in photoconductivity is strongly temperature dependent, with half times of 150 ps below 40 K but only 20 ps at 80 K. The conductivity of ZnO is a critical parameter because of the use of ZnO in a variety of optoelectronic materials and as a transparent conducting oxide layer in solar cells.

We have also investigated IET from dye molecules adsorbed onto semiconductor nanoparticles into the bulk semiconductor. This work examined two different types of sensitizers, ruthenium bipyridal dyes commonly used in DSSCs and oxomanganese sensitizers for use in photocatalytic systems. In the ruthenium work, we have found that injection occurs on sub-picosecond timescales for dyed TiO₂ but on hundred-picosecond timescales for dyed ZnO. We measured injection from N3 and “black dye” into TiO2 and ZnO with 400 and 800 nm photoexcitation. Injection into ZnO occurs on 100 ps timescales for all combinations of dye and photoexcitation wavelength. However, injection into TiO₂ occurs on sub-ps timescales with 400 nm excitation, but on 10 ps timescales for 800 nm excitation. We believe that the differences in injection kinetics arise from the overlap of the density of states of the semiconductor with excited states of the dye. We have also investigated the effect on IET of adding a thin shell of another semiconductor to the nanoparticles, including ZnO-TiO₂, TiO₂-ZnO, and ZnO-Al₂O₃ core-shell systems. The concentration of mobile injected electrons decreases with the addition of a shell layer in each case, likely because defects formed at the interface trap electrons so that they are not visible to TRTS. The dynamics of injection remain dominated by the core material, although for shell thickness on the order of 10 nm, the dynamics begin to shift to a mixture of core and shell properties. The dynamics of the IET process are an important factor in DSSC performance.

I have also contributed to a collaborative project on photocatalytic water splitting for hydrogen production. My role has been to study IET from adsorbed oxomanganese-based compounds into TiO₂ nanoparticles. IET is the first step in a process to increase the oxidation state of the catalyst to the point where it has sufficient oxidizing power to split water. The reduction reaction at the counterelectrode can then produce hydrogen. IET from the oxomanganese compound through a terpy-catechol linker into the semiconductor occur on sub-picosecond timescales. IET must be fast to prevent re-reduction of the photocatalyst.

I finished my post-doctoral fellowship on June 30, and I am now an assistant professor in the Department of Chemical and Biological Engineering at Drexel University. During my fellowship, I have gained great experience with a new and important spectroscopy technique, TRTS. I plan to acquire an ultrafast laser system to do TRTS in my lab at Drexel. TRTS is an excellent complement to the materials and solar cell characterization skills that I developed as a Ph.D. student. The insight that I will gain into the mechanisms underlying DSSC processes using TRTS will be of great interest, and not accessible by other experimental means. I presented my work on ZnO photoconductivity and on photocatalytic hydrogen production at the American Institute of Chemical Engineers (AICHE) Annual Meeting in November 2006. I will present my work on IET for DSSCs at the 2007 Annual Meeting. So far I have published two papers on my post-doctoral work, with three more in preparation.

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