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Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy.

Ghadiri E, Zakeeruddin SM, Hagfeldt A, Grätzel M, Moser JE - Sci Rep (2016)

Bottom Line: This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds.The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply.The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

View Article: PubMed Central - PubMed

Affiliation: Photochemical Dynamics Group , Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

ABSTRACT
Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

No MeSH data available.


Related in: MedlinePlus

Transient absorptance on complete device.Normalized transient absorptance deduced from femtosecond diffuse reflectance measurements on a DSC composed of Z907 dye-sensitized TiO2 double layer. (a) Probed at 840 nm monitoring the kinetics of oxidized dye molecules in the presence of MPN solvent (red markers) and Iodide based electrolyte Z946 (black markers); the solid lines correspond the fit to the result by a two exponential function. The inset shows the short delay time scan of the observed kinetics at 840 nm in the presence of MPN solvent and electrolyte. (b) The observed kinetics in the vicinity of electrolyte measured at 1200 nm monitoring the kinetics of photo-injected electrons in TiO2. The solid lines correspond the fit to the result by exponential function. The time constant of exponential fit to the measurements at 840 nm is τ1 = 9.22 ps and for trace measured at 1200 nm is τ1 = 8.9 ps. The pump wavelength is 530 nm and pulse intensity is 200 nJ.
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f3: Transient absorptance on complete device.Normalized transient absorptance deduced from femtosecond diffuse reflectance measurements on a DSC composed of Z907 dye-sensitized TiO2 double layer. (a) Probed at 840 nm monitoring the kinetics of oxidized dye molecules in the presence of MPN solvent (red markers) and Iodide based electrolyte Z946 (black markers); the solid lines correspond the fit to the result by a two exponential function. The inset shows the short delay time scan of the observed kinetics at 840 nm in the presence of MPN solvent and electrolyte. (b) The observed kinetics in the vicinity of electrolyte measured at 1200 nm monitoring the kinetics of photo-injected electrons in TiO2. The solid lines correspond the fit to the result by exponential function. The time constant of exponential fit to the measurements at 840 nm is τ1 = 9.22 ps and for trace measured at 1200 nm is τ1 = 8.9 ps. The pump wavelength is 530 nm and pulse intensity is 200 nJ.

Mentions: The excitation intensity dependence of the observed kinetics is also presented in Supplementary information, Figure S4. All traces measured at low excitation intensities can be fitted by a single exponential function. This early decay kinetics is again observed when the evolution of oxidized dye molecules is monitored in the visible wavelength region at 670 nm (Supplementary Figure S5). Moreover, the same depleting kinetics can be observed (Fig. 3b) when photo-injected electrons are monitored at 1200 nm in the presence of the electrolyte. Therefore, we assign the early decay kinetics observed for the complete opaque DSC in the presence of MPN solvent, to the early back recombination of photo-injected electrons with oxidized dye molecules.


Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy.

Ghadiri E, Zakeeruddin SM, Hagfeldt A, Grätzel M, Moser JE - Sci Rep (2016)

Transient absorptance on complete device.Normalized transient absorptance deduced from femtosecond diffuse reflectance measurements on a DSC composed of Z907 dye-sensitized TiO2 double layer. (a) Probed at 840 nm monitoring the kinetics of oxidized dye molecules in the presence of MPN solvent (red markers) and Iodide based electrolyte Z946 (black markers); the solid lines correspond the fit to the result by a two exponential function. The inset shows the short delay time scan of the observed kinetics at 840 nm in the presence of MPN solvent and electrolyte. (b) The observed kinetics in the vicinity of electrolyte measured at 1200 nm monitoring the kinetics of photo-injected electrons in TiO2. The solid lines correspond the fit to the result by exponential function. The time constant of exponential fit to the measurements at 840 nm is τ1 = 9.22 ps and for trace measured at 1200 nm is τ1 = 8.9 ps. The pump wavelength is 530 nm and pulse intensity is 200 nJ.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837338&req=5

f3: Transient absorptance on complete device.Normalized transient absorptance deduced from femtosecond diffuse reflectance measurements on a DSC composed of Z907 dye-sensitized TiO2 double layer. (a) Probed at 840 nm monitoring the kinetics of oxidized dye molecules in the presence of MPN solvent (red markers) and Iodide based electrolyte Z946 (black markers); the solid lines correspond the fit to the result by a two exponential function. The inset shows the short delay time scan of the observed kinetics at 840 nm in the presence of MPN solvent and electrolyte. (b) The observed kinetics in the vicinity of electrolyte measured at 1200 nm monitoring the kinetics of photo-injected electrons in TiO2. The solid lines correspond the fit to the result by exponential function. The time constant of exponential fit to the measurements at 840 nm is τ1 = 9.22 ps and for trace measured at 1200 nm is τ1 = 8.9 ps. The pump wavelength is 530 nm and pulse intensity is 200 nJ.
Mentions: The excitation intensity dependence of the observed kinetics is also presented in Supplementary information, Figure S4. All traces measured at low excitation intensities can be fitted by a single exponential function. This early decay kinetics is again observed when the evolution of oxidized dye molecules is monitored in the visible wavelength region at 670 nm (Supplementary Figure S5). Moreover, the same depleting kinetics can be observed (Fig. 3b) when photo-injected electrons are monitored at 1200 nm in the presence of the electrolyte. Therefore, we assign the early decay kinetics observed for the complete opaque DSC in the presence of MPN solvent, to the early back recombination of photo-injected electrons with oxidized dye molecules.

Bottom Line: This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds.The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply.The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

View Article: PubMed Central - PubMed

Affiliation: Photochemical Dynamics Group , Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

ABSTRACT
Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

No MeSH data available.


Related in: MedlinePlus