Ultrafast Terahertz Conductivity Dynamics in Mesoporous TiO₂: Influence of Dye Sensitization and Surface Treatment in Solid-State Dye-Sensitized Solar Cells

Abstract

We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO 2 films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm 2 /(V s) and diffusion length of ∼20 nm for electron motion through the TiO 2 matrix. The photoconductivity dynamics in TiO 2 films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70-200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl 4 on early-time charge dynamics. In the solar cells, surface treatment of the mesoporous TiO 2 with TiCl 4 is critical to enable efficient operation. Here, we find that neither early-time charge mobility nor charge injection rate or decay times are significantly affected by the treatment, which suggests that it may, instead, have an impact on phenomena occurring on longer time scales.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…It is clear that, given our current state of understanding of THz spectroscopy to these types of systems, future applications of time-resolved terahertz spectroscopy can focus on systems of increasing complexity, such as those encountered in photovoltaic devices where different semiconductor phases are intermixed on nanometer lengths scales (Esenturk et al, 2009). Such complex systems may include, for instance, semiconductor nanostructures (quantum dots or nanorods) embedded in an organic polymer semiconductor phase and quantum dot or dye-sensitized transition metal oxide systems (Tiwana et al, 2009;Nemec, Rochford et al, 2010;Pijpers et al, 2010Pijpers et al, , 2011. The ability to monitor exciton and carrier dynamics on ultrafast time scales, across a wide frequency range, without the necessity to apply contacts to the material makes time-resolved terahertz spectroscopy a rather unique tool for studying carrier dynamics, for instance, allowing us to monitor the conversion of excitons into free charge carriers and vice versa.…”

The origin of an efficiency improving “light soaking” effect in SnO2 based solid-state dye-sensitized solar cells

“…The excited sample was then probed after time s with picosecond duration terahertz (THz) pulses generated using optical rectification by a ZnTe crystal with the Spitfire 800 nm pulses. 42 Part of the THz pulses interacts with and is absorbed by any mobile charge carriers present in the photoexcited sample. The change in the amplitude and phase of the THz transmitted signal is measured using a double lock-in technique and gives a direct measure of the sample photoconductivity.…”

Improved conductivity in dye-sensitised solar cells through block-copolymer confined TiO2crystallisation

“…While the electron mobility in crystalline anatase TiO 2 is usually on the order of µ ∼ 10 cm 2 /Vs, this value decreases by five to six orders of magnitude in nanoparticle films. 7,8 The low electron mobility in nanoparticle TiO 2 films appears to limit charge collection and thereby power conversion efficiency in solid-state DSCs. 9 Furthermore, the lack of control over the pore size distribution may hinder pore infiltration by the viscous materials required to assemble this promising type of DSCs.…”