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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

Femtosecond diffuse reflectance measurements on DSC devices based on organic D- π-A dye, Y123.Measurements are in the presence of cobalt-based redox mediator, Z1148 electrolyte. Red and blue markers are the measurements on the double layer and scattering layer films respectively and trace with green markers correspond to the transparent layer. Pump wavelength is 600 nm, and the probe wavelength is 840 nm. (a) The kinetics is drawn until 25 ps after excitation. In all three TiO2 films, the amplitude of the signal decays to half of its value in short time scale after excitation. (b) The kinetics are drawn until 500 ps after excitation. Solid lines are the result of fitting two exponential functions to the data.
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f6: Femtosecond diffuse reflectance measurements on DSC devices based on organic D- π-A dye, Y123.Measurements are in the presence of cobalt-based redox mediator, Z1148 electrolyte. Red and blue markers are the measurements on the double layer and scattering layer films respectively and trace with green markers correspond to the transparent layer. Pump wavelength is 600 nm, and the probe wavelength is 840 nm. (a) The kinetics is drawn until 25 ps after excitation. In all three TiO2 films, the amplitude of the signal decays to half of its value in short time scale after excitation. (b) The kinetics are drawn until 500 ps after excitation. Solid lines are the result of fitting two exponential functions to the data.

Mentions: One of the most significant advances in design of light-harvesting materials is the so-called donor-conjugated linker-acceptor (D-π-A) organic dyes. In comparison with Ru-based dyes, organic dyes have higher molar extinction coefficient and can be readily designed for a desired absorption spectrum1353637. These molecular structures look attractive in terms of electron donor-acceptor interactions35. In order to understand the electron injection dynamics in these systems, femtosecond diffuse reflectance spectroscopy is applied to DSC devices containing Y123 and a cobalt complex based redox electrolyte. Three different morphologies of the photoanode are used, and the results are compared in Fig. 6. In these measurements, the pump beam wavelength is 600 nm to excite the dye, and the probe beam is 840 nm. Upon laser excitation, an ultrafast formation of the signal happens in 200 fs, followed by a fast decay of the signal to 50% of its amplitude in 2 ps. After two picoseconds, the signal reaches a plateau in all the 3 morphologies of the TiO2 layers. Unlike Ru-based dyes, no obvious difference is observed in the kinetics for measurements in the presence of MPN solvent and redox electrolyte and the medium has no influence on the observed kinetics (measurements in the presence of MPN solvent are provided in Supplementary information Figure S9). The slow growth component of the signal in hundred picoseconds time scale seen in the films made of scattering particles or the double layer film, is assigned to the electron injection from dye aggregates. This component is removed from the signal when the sample is immersed in acetonitrile solvent for several hours (Supplementary Figure S10). TiO2 films prepared with scattering particles might have enough space to accommodate aggregated dye molecules within the pores, which are loosely in contact with the TiO2 layer. This makes a larger distance for electron transfer between the dye and TiO2.


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)

Femtosecond diffuse reflectance measurements on DSC devices based on organic D- π-A dye, Y123.Measurements are in the presence of cobalt-based redox mediator, Z1148 electrolyte. Red and blue markers are the measurements on the double layer and scattering layer films respectively and trace with green markers correspond to the transparent layer. Pump wavelength is 600 nm, and the probe wavelength is 840 nm. (a) The kinetics is drawn until 25 ps after excitation. In all three TiO2 films, the amplitude of the signal decays to half of its value in short time scale after excitation. (b) The kinetics are drawn until 500 ps after excitation. Solid lines are the result of fitting two exponential functions to the data.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Femtosecond diffuse reflectance measurements on DSC devices based on organic D- π-A dye, Y123.Measurements are in the presence of cobalt-based redox mediator, Z1148 electrolyte. Red and blue markers are the measurements on the double layer and scattering layer films respectively and trace with green markers correspond to the transparent layer. Pump wavelength is 600 nm, and the probe wavelength is 840 nm. (a) The kinetics is drawn until 25 ps after excitation. In all three TiO2 films, the amplitude of the signal decays to half of its value in short time scale after excitation. (b) The kinetics are drawn until 500 ps after excitation. Solid lines are the result of fitting two exponential functions to the data.
Mentions: One of the most significant advances in design of light-harvesting materials is the so-called donor-conjugated linker-acceptor (D-π-A) organic dyes. In comparison with Ru-based dyes, organic dyes have higher molar extinction coefficient and can be readily designed for a desired absorption spectrum1353637. These molecular structures look attractive in terms of electron donor-acceptor interactions35. In order to understand the electron injection dynamics in these systems, femtosecond diffuse reflectance spectroscopy is applied to DSC devices containing Y123 and a cobalt complex based redox electrolyte. Three different morphologies of the photoanode are used, and the results are compared in Fig. 6. In these measurements, the pump beam wavelength is 600 nm to excite the dye, and the probe beam is 840 nm. Upon laser excitation, an ultrafast formation of the signal happens in 200 fs, followed by a fast decay of the signal to 50% of its amplitude in 2 ps. After two picoseconds, the signal reaches a plateau in all the 3 morphologies of the TiO2 layers. Unlike Ru-based dyes, no obvious difference is observed in the kinetics for measurements in the presence of MPN solvent and redox electrolyte and the medium has no influence on the observed kinetics (measurements in the presence of MPN solvent are provided in Supplementary information Figure S9). The slow growth component of the signal in hundred picoseconds time scale seen in the films made of scattering particles or the double layer film, is assigned to the electron injection from dye aggregates. This component is removed from the signal when the sample is immersed in acetonitrile solvent for several hours (Supplementary Figure S10). TiO2 films prepared with scattering particles might have enough space to accommodate aggregated dye molecules within the pores, which are loosely in contact with the TiO2 layer. This makes a larger distance for electron transfer between the dye and TiO2.

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