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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

J-V analysis, pump-probe diffuse reflectance spectroscopy, and energy level diagram of the full device.(a) The J-V characteristics of an optimized standard DSC device based on Z907 dye and Z946 electrolyte measured in dark and under irradiance of AM 1.5G sunlight of 100 mW cm−2. (b) diffuse reflectance measurements on DSC under apply bias voltage. Signals are recorded at 840 nm reflecting the kinetic of oxidized dye molecules, Green circle: short circuit, Red circle: −500 mV, blue diamond: +500 mV, pink triangle: −320 mV, black triangle: −690 mV. (c) Energy level diagram of complete DSC. The energy level of quasi-Fermi level Efn at the bias voltage of 300 mV and 700 mV are depicted. Trap state density exponentially increases with increasing the energy level. The Efn level respect to Ef0 at 3 different biases is illustrated. Trap density at voltage difference of 500 meV, 600 meV and 700 meV is approximately respectively 1 × 1019, 2 × 1019 and 7 × 1019 cm−3. Arrows show the shift in the quasi-Fermi level position and the energy difference of quasi-Fermi level and dye LUMO level.
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f5: J-V analysis, pump-probe diffuse reflectance spectroscopy, and energy level diagram of the full device.(a) The J-V characteristics of an optimized standard DSC device based on Z907 dye and Z946 electrolyte measured in dark and under irradiance of AM 1.5G sunlight of 100 mW cm−2. (b) diffuse reflectance measurements on DSC under apply bias voltage. Signals are recorded at 840 nm reflecting the kinetic of oxidized dye molecules, Green circle: short circuit, Red circle: −500 mV, blue diamond: +500 mV, pink triangle: −320 mV, black triangle: −690 mV. (c) Energy level diagram of complete DSC. The energy level of quasi-Fermi level Efn at the bias voltage of 300 mV and 700 mV are depicted. Trap state density exponentially increases with increasing the energy level. The Efn level respect to Ef0 at 3 different biases is illustrated. Trap density at voltage difference of 500 meV, 600 meV and 700 meV is approximately respectively 1 × 1019, 2 × 1019 and 7 × 1019 cm−3. Arrows show the shift in the quasi-Fermi level position and the energy difference of quasi-Fermi level and dye LUMO level.

Mentions: We have combined the diffuse reflectance spectroscopy with potentiometric techniques to monitor the electron injection process in DSC standard device under working condition. Figure 5a shows the photovoltaic characteristics of the Z907 sensitizer and iodide based redox electrolyte device. The photovoltaic parameters of the device at full sunlight are; short circuit current density (Jsc) of 15.5 mA/cm2, open circuit photovoltage (Voc) of 698 mV, fill factor (FF) of 0.71 and power conversion efficiency (PCE) of 7.6%. Figure 5b presents the typical transient absorptance of the cell in short circuit condition and under different bias voltages of +500 mV, −500 mV (close to max power point) and −690 mV. Transient absorptance traces are not easily distinguishable. All traces in this figure can be fitted by exponential function with close fitting parameters; a component with a lifetime of 50–70 ps, and the flat behavior until hundred picoseconds, which is shown in inset. By raising the bias to −690 mV close to open circuit condition, the amplitude of the observed signal is almost half of the others while the kinetics remain similar. In other words, as the amplitude of the pump-probe signal is proportional to the number of oxidized dye molecules, with increasing the applied bias voltage the quantum yield of electron injection is reduced. The energy level diagram, which contains the energy level of the conduction band of TiO2 (CB), HOMO and LUMO level of Z907 dye and iodide-based redox electrolyte (Eredox) and trap state distribution are illustrated in Fig. 5c. Increasing the forward bias voltage would raise the quasi-Fermi level position by filling up the trap states to some extent in TiO2, which is highlighted in Fig. 5c. As it is observed by shifting the quasi-Fermi level toward the LUMO level of the dye, the energy difference gets noticeably smaller. In example the energy difference at the bias level of −600 mV is 75% of the energy difference at the bias level of −500 mV and further reduces to 50% at a higher bias level of −700 mV.


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)

J-V analysis, pump-probe diffuse reflectance spectroscopy, and energy level diagram of the full device.(a) The J-V characteristics of an optimized standard DSC device based on Z907 dye and Z946 electrolyte measured in dark and under irradiance of AM 1.5G sunlight of 100 mW cm−2. (b) diffuse reflectance measurements on DSC under apply bias voltage. Signals are recorded at 840 nm reflecting the kinetic of oxidized dye molecules, Green circle: short circuit, Red circle: −500 mV, blue diamond: +500 mV, pink triangle: −320 mV, black triangle: −690 mV. (c) Energy level diagram of complete DSC. The energy level of quasi-Fermi level Efn at the bias voltage of 300 mV and 700 mV are depicted. Trap state density exponentially increases with increasing the energy level. The Efn level respect to Ef0 at 3 different biases is illustrated. Trap density at voltage difference of 500 meV, 600 meV and 700 meV is approximately respectively 1 × 1019, 2 × 1019 and 7 × 1019 cm−3. Arrows show the shift in the quasi-Fermi level position and the energy difference of quasi-Fermi level and dye LUMO level.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4837338&req=5

f5: J-V analysis, pump-probe diffuse reflectance spectroscopy, and energy level diagram of the full device.(a) The J-V characteristics of an optimized standard DSC device based on Z907 dye and Z946 electrolyte measured in dark and under irradiance of AM 1.5G sunlight of 100 mW cm−2. (b) diffuse reflectance measurements on DSC under apply bias voltage. Signals are recorded at 840 nm reflecting the kinetic of oxidized dye molecules, Green circle: short circuit, Red circle: −500 mV, blue diamond: +500 mV, pink triangle: −320 mV, black triangle: −690 mV. (c) Energy level diagram of complete DSC. The energy level of quasi-Fermi level Efn at the bias voltage of 300 mV and 700 mV are depicted. Trap state density exponentially increases with increasing the energy level. The Efn level respect to Ef0 at 3 different biases is illustrated. Trap density at voltage difference of 500 meV, 600 meV and 700 meV is approximately respectively 1 × 1019, 2 × 1019 and 7 × 1019 cm−3. Arrows show the shift in the quasi-Fermi level position and the energy difference of quasi-Fermi level and dye LUMO level.
Mentions: We have combined the diffuse reflectance spectroscopy with potentiometric techniques to monitor the electron injection process in DSC standard device under working condition. Figure 5a shows the photovoltaic characteristics of the Z907 sensitizer and iodide based redox electrolyte device. The photovoltaic parameters of the device at full sunlight are; short circuit current density (Jsc) of 15.5 mA/cm2, open circuit photovoltage (Voc) of 698 mV, fill factor (FF) of 0.71 and power conversion efficiency (PCE) of 7.6%. Figure 5b presents the typical transient absorptance of the cell in short circuit condition and under different bias voltages of +500 mV, −500 mV (close to max power point) and −690 mV. Transient absorptance traces are not easily distinguishable. All traces in this figure can be fitted by exponential function with close fitting parameters; a component with a lifetime of 50–70 ps, and the flat behavior until hundred picoseconds, which is shown in inset. By raising the bias to −690 mV close to open circuit condition, the amplitude of the observed signal is almost half of the others while the kinetics remain similar. In other words, as the amplitude of the pump-probe signal is proportional to the number of oxidized dye molecules, with increasing the applied bias voltage the quantum yield of electron injection is reduced. The energy level diagram, which contains the energy level of the conduction band of TiO2 (CB), HOMO and LUMO level of Z907 dye and iodide-based redox electrolyte (Eredox) and trap state distribution are illustrated in Fig. 5c. Increasing the forward bias voltage would raise the quasi-Fermi level position by filling up the trap states to some extent in TiO2, which is highlighted in Fig. 5c. As it is observed by shifting the quasi-Fermi level toward the LUMO level of the dye, the energy difference gets noticeably smaller. In example the energy difference at the bias level of −600 mV is 75% of the energy difference at the bias level of −500 mV and further reduces to 50% at a higher bias level of −700 mV.

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