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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 diffuse reflectance of dye-sensitized TiO2 photoanodes.Samples are covered with MPN solvent. λex = 600 nm and λobs = 850 nm. (a) Evolution of Z907 oxidized dye molecule anchored on transparent film (blue markers), scattering film (red marker) and double layer film (green marker) recorded up to 50 ps after excitation. Lines are fitted to convoluted exponential function. (b) Kinetics of oxidized dye molecule anchored on scattering film (red) and complete photoanode (green) up to 2 ps after excitation. Fitting parameters for complete photoanode are A = 0.11, τ1 = 1.08 ps, σ = 0.158 ps and μ = −0.05 ps. Fitting parameters for scattering layer are A = 0.15, τ1 = 1.75 ps, σ = 0.109 ps and μ = −0.024 ps. (c) Kinetics of oxidized dye molecule in full photoanode until 500 ps after excitation. Fitting parameters are A = 0.1, τ1 = 1.18 ps, σ = 0.164 ps, μ = −0.05 ps, B = 0.9 and τ2 = 4 ns. A and B are the pre-exponential factors and τ is the time constant. σ and μ are the broadening and zero onset of Gaussian function fit to cross-correlation of the pump and probe.
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f2: Transient diffuse reflectance of dye-sensitized TiO2 photoanodes.Samples are covered with MPN solvent. λex = 600 nm and λobs = 850 nm. (a) Evolution of Z907 oxidized dye molecule anchored on transparent film (blue markers), scattering film (red marker) and double layer film (green marker) recorded up to 50 ps after excitation. Lines are fitted to convoluted exponential function. (b) Kinetics of oxidized dye molecule anchored on scattering film (red) and complete photoanode (green) up to 2 ps after excitation. Fitting parameters for complete photoanode are A = 0.11, τ1 = 1.08 ps, σ = 0.158 ps and μ = −0.05 ps. Fitting parameters for scattering layer are A = 0.15, τ1 = 1.75 ps, σ = 0.109 ps and μ = −0.024 ps. (c) Kinetics of oxidized dye molecule in full photoanode until 500 ps after excitation. Fitting parameters are A = 0.1, τ1 = 1.18 ps, σ = 0.164 ps, μ = −0.05 ps, B = 0.9 and τ2 = 4 ns. A and B are the pre-exponential factors and τ is the time constant. σ and μ are the broadening and zero onset of Gaussian function fit to cross-correlation of the pump and probe.

Mentions: Figure 2a,b compare the early and later time evolution of absorptance of oxidized dye molecules anchored on three different TiO2 films in the presence of MPN solvent. The samples are excited at 600 nm. The time delayed diffuse reflected probe beam is measured at 840 nm. Transient absorptance change is extracted from the measured transient diffuse reflectance change given by equation (2), depicted in the method section. In Fig. 2a,b, it is seen that in the presence of MPN solvent, the kinetics of electron injection in double layer film resembles that of transparent film made of small TiO2 particles. All samples have an instrument response-limited transient absorptance onset within 200 fs and a slow rise of the signal with a time constant of 1.1 ps.


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 diffuse reflectance of dye-sensitized TiO2 photoanodes.Samples are covered with MPN solvent. λex = 600 nm and λobs = 850 nm. (a) Evolution of Z907 oxidized dye molecule anchored on transparent film (blue markers), scattering film (red marker) and double layer film (green marker) recorded up to 50 ps after excitation. Lines are fitted to convoluted exponential function. (b) Kinetics of oxidized dye molecule anchored on scattering film (red) and complete photoanode (green) up to 2 ps after excitation. Fitting parameters for complete photoanode are A = 0.11, τ1 = 1.08 ps, σ = 0.158 ps and μ = −0.05 ps. Fitting parameters for scattering layer are A = 0.15, τ1 = 1.75 ps, σ = 0.109 ps and μ = −0.024 ps. (c) Kinetics of oxidized dye molecule in full photoanode until 500 ps after excitation. Fitting parameters are A = 0.1, τ1 = 1.18 ps, σ = 0.164 ps, μ = −0.05 ps, B = 0.9 and τ2 = 4 ns. A and B are the pre-exponential factors and τ is the time constant. σ and μ are the broadening and zero onset of Gaussian function fit to cross-correlation of the pump and probe.
© Copyright Policy - open-access
Related In: Results  -  Collection

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f2: Transient diffuse reflectance of dye-sensitized TiO2 photoanodes.Samples are covered with MPN solvent. λex = 600 nm and λobs = 850 nm. (a) Evolution of Z907 oxidized dye molecule anchored on transparent film (blue markers), scattering film (red marker) and double layer film (green marker) recorded up to 50 ps after excitation. Lines are fitted to convoluted exponential function. (b) Kinetics of oxidized dye molecule anchored on scattering film (red) and complete photoanode (green) up to 2 ps after excitation. Fitting parameters for complete photoanode are A = 0.11, τ1 = 1.08 ps, σ = 0.158 ps and μ = −0.05 ps. Fitting parameters for scattering layer are A = 0.15, τ1 = 1.75 ps, σ = 0.109 ps and μ = −0.024 ps. (c) Kinetics of oxidized dye molecule in full photoanode until 500 ps after excitation. Fitting parameters are A = 0.1, τ1 = 1.18 ps, σ = 0.164 ps, μ = −0.05 ps, B = 0.9 and τ2 = 4 ns. A and B are the pre-exponential factors and τ is the time constant. σ and μ are the broadening and zero onset of Gaussian function fit to cross-correlation of the pump and probe.
Mentions: Figure 2a,b compare the early and later time evolution of absorptance of oxidized dye molecules anchored on three different TiO2 films in the presence of MPN solvent. The samples are excited at 600 nm. The time delayed diffuse reflected probe beam is measured at 840 nm. Transient absorptance change is extracted from the measured transient diffuse reflectance change given by equation (2), depicted in the method section. In Fig. 2a,b, it is seen that in the presence of MPN solvent, the kinetics of electron injection in double layer film resembles that of transparent film made of small TiO2 particles. All samples have an instrument response-limited transient absorptance onset within 200 fs and a slow rise of the signal with a time constant of 1.1 ps.

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