Professor Gregory Hartland
(Ed.)
An improved optical design for nanosecond diffuse reflectance (DR)
spectroscopy is presented. The in-situ analysis of the electron back-reaction and dye
regeneration processes in efficient opaque dye-sensitized solar cell devices (DSCs) was
scrutinized for the first time using nanosecond DR spectroscopy. The efficient DSC device is
based on an opaque TiO2 double-layer film comprising 400 nm light-scattering particles and 20
nm optically transparent particles. Transmission-based laser techniques are not suitable for
studying these or other devices by using the opaque morphologies of TiO2 films. However,
time-resolved DR flash photolysis enables the exploration of photophysical processes in a broad
variety of opaque or highly light-absorbing and light-scattering materials. We experimentally
verified the three important components of DR-based spectroscopy: optical configuration,
sample condition, and theoretical quantitative optical models. The large optical angle for
diffusive light enables efficient light collection and measurement at a relatively low power. We
tested the steady-state and time-resolved concentration dependence of the Kubelka−Munk
theory for the quantitative analysis of time-resolved results and observed that the dynamics of electron back-reactions are strongly affected by the morphological parameters of the TiO2 films. With a lifetime of 50 μs, the kinetics of electron back-recombination in the device’s photoanode, which is manufactured with 400 nm TiO2 particles and 20 nm TiO2 particles, are 2 orders of magnitude faster than what has been reported to date for 20 nm particles (1 ms). In contrast to electron back-recombination, the dye regeneration process is not influenced by the TiO2 film morphology.
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