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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 different nanostructured opaque TiO2 films based devices.Transient absorptance change deduced from transient diffuse reflectance measurements on DSCs based on different TiO2 opaque films. λex = 530 nm and λobs = 840 nm. (a) Standard DSC based on TiO2 double layer dipped in MPN solvent (red) and iodide-based redox electrolyte (black). 30% of the signal decays in 20 ps in the vicinity of electrolyte. (b) DSC based on anodized TiO2 nanotubes on Ti foil dipped in MPN solvent (red) and iodide-based redox electrolyte (green). 16% of the signal decays in 20 ps after excitation in the vicinity of the electrolyte. (c) DSC based on TiO2 fibers in MPN solvent (blue) and cobalt-based redox electrolyte (pink). In the presence of cobalt electrolyte, 20% of signal decays in 200 ps after excitation. (d) DSC based on TiO2 fibers dipped in redox-inactive ionic liquid, EMITFSI (black), 28% of the signal decays in 20 ps after excitation. The measurements are performed at the same excitation intensities.
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f4: Transient absorptance on different nanostructured opaque TiO2 films based devices.Transient absorptance change deduced from transient diffuse reflectance measurements on DSCs based on different TiO2 opaque films. λex = 530 nm and λobs = 840 nm. (a) Standard DSC based on TiO2 double layer dipped in MPN solvent (red) and iodide-based redox electrolyte (black). 30% of the signal decays in 20 ps in the vicinity of electrolyte. (b) DSC based on anodized TiO2 nanotubes on Ti foil dipped in MPN solvent (red) and iodide-based redox electrolyte (green). 16% of the signal decays in 20 ps after excitation in the vicinity of the electrolyte. (c) DSC based on TiO2 fibers in MPN solvent (blue) and cobalt-based redox electrolyte (pink). In the presence of cobalt electrolyte, 20% of signal decays in 200 ps after excitation. (d) DSC based on TiO2 fibers dipped in redox-inactive ionic liquid, EMITFSI (black), 28% of the signal decays in 20 ps after excitation. The measurements are performed at the same excitation intensities.

Mentions: The kinetics of back recombination is even more strongly accelerated in the presence of the electrolyte. Traces in Fig. 3a compare the measurements in the presence (black markers) and absence of redox electrolyte (red markers) on double layer TiO2 film. In the vicinity of the redox electrolyte, the formation of the signal is again ultrafast, and the sub-200 fs ultrafast component is still present. As it can be observed, 26% of the signal of the oxidized dye molecule decays with a time constant of τ1 = 9 ps. The same fast decay kinetics are also present in the samples dipped in the redox-inactive ionic liquid, 3-methyl-1-ethylimidazolium bis (trifluoromethane) sulfonimide (EMITFSI), (see Fig. 4d). Therefore, the observed kinetics cannot be assigned to processes like reductive quenching of the excited-state of the dye molecules by redox electrolyte. The accelerated charge recombination in the presence of both redox active and redox inactive electrolyte in some picoseconds after excitation is rationalized by electric fields induced by the charges in the electrolyte and charge screening effects. The local electric field induced by ions present in the redox electrolyte or ionic liquid at the surface is accelerating the charge recombination between electron-hole geminate pair after initial charge separation.


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 different nanostructured opaque TiO2 films based devices.Transient absorptance change deduced from transient diffuse reflectance measurements on DSCs based on different TiO2 opaque films. λex = 530 nm and λobs = 840 nm. (a) Standard DSC based on TiO2 double layer dipped in MPN solvent (red) and iodide-based redox electrolyte (black). 30% of the signal decays in 20 ps in the vicinity of electrolyte. (b) DSC based on anodized TiO2 nanotubes on Ti foil dipped in MPN solvent (red) and iodide-based redox electrolyte (green). 16% of the signal decays in 20 ps after excitation in the vicinity of the electrolyte. (c) DSC based on TiO2 fibers in MPN solvent (blue) and cobalt-based redox electrolyte (pink). In the presence of cobalt electrolyte, 20% of signal decays in 200 ps after excitation. (d) DSC based on TiO2 fibers dipped in redox-inactive ionic liquid, EMITFSI (black), 28% of the signal decays in 20 ps after excitation. The measurements are performed at the same excitation intensities.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC4837338&req=5

f4: Transient absorptance on different nanostructured opaque TiO2 films based devices.Transient absorptance change deduced from transient diffuse reflectance measurements on DSCs based on different TiO2 opaque films. λex = 530 nm and λobs = 840 nm. (a) Standard DSC based on TiO2 double layer dipped in MPN solvent (red) and iodide-based redox electrolyte (black). 30% of the signal decays in 20 ps in the vicinity of electrolyte. (b) DSC based on anodized TiO2 nanotubes on Ti foil dipped in MPN solvent (red) and iodide-based redox electrolyte (green). 16% of the signal decays in 20 ps after excitation in the vicinity of the electrolyte. (c) DSC based on TiO2 fibers in MPN solvent (blue) and cobalt-based redox electrolyte (pink). In the presence of cobalt electrolyte, 20% of signal decays in 200 ps after excitation. (d) DSC based on TiO2 fibers dipped in redox-inactive ionic liquid, EMITFSI (black), 28% of the signal decays in 20 ps after excitation. The measurements are performed at the same excitation intensities.
Mentions: The kinetics of back recombination is even more strongly accelerated in the presence of the electrolyte. Traces in Fig. 3a compare the measurements in the presence (black markers) and absence of redox electrolyte (red markers) on double layer TiO2 film. In the vicinity of the redox electrolyte, the formation of the signal is again ultrafast, and the sub-200 fs ultrafast component is still present. As it can be observed, 26% of the signal of the oxidized dye molecule decays with a time constant of τ1 = 9 ps. The same fast decay kinetics are also present in the samples dipped in the redox-inactive ionic liquid, 3-methyl-1-ethylimidazolium bis (trifluoromethane) sulfonimide (EMITFSI), (see Fig. 4d). Therefore, the observed kinetics cannot be assigned to processes like reductive quenching of the excited-state of the dye molecules by redox electrolyte. The accelerated charge recombination in the presence of both redox active and redox inactive electrolyte in some picoseconds after excitation is rationalized by electric fields induced by the charges in the electrolyte and charge screening effects. The local electric field induced by ions present in the redox electrolyte or ionic liquid at the surface is accelerating the charge recombination between electron-hole geminate pair after initial charge separation.

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