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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

Device architecture and optical properties of complete photoanode.(a) Schematic of the optimized highly efficient liquid DSC based on single layer and double layer TiO2 films. This represents examples of the devices that have been studied. (b) Steady-state optical parameters of a Z907 sensitized TiO2 double layer photoanode applied in highly efficient DSC devices. Total transmittance (Red), diffuse reflectance (blue), absorptance (green) and Kubelka- Munk function (dashed green) are depicted. Kubelka-Munk function and absorptance are defined according to equations (1) and (3) depicted in the method section. The total transmittance of the cell in the visible and infrared region is less than 15%. The Kubelka-Munk function spectrum follows the shape of the absorptance spectrum.
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f1: Device architecture and optical properties of complete photoanode.(a) Schematic of the optimized highly efficient liquid DSC based on single layer and double layer TiO2 films. This represents examples of the devices that have been studied. (b) Steady-state optical parameters of a Z907 sensitized TiO2 double layer photoanode applied in highly efficient DSC devices. Total transmittance (Red), diffuse reflectance (blue), absorptance (green) and Kubelka- Munk function (dashed green) are depicted. Kubelka-Munk function and absorptance are defined according to equations (1) and (3) depicted in the method section. The total transmittance of the cell in the visible and infrared region is less than 15%. The Kubelka-Munk function spectrum follows the shape of the absorptance spectrum.

Mentions: Figure 1a shows the schematics of the standard optimized high-performance liquid solar cell. In the conventional DSC scheme, the mesoporous layer is made of 20 nm-diameter interconnected TiO2 particles. Although this structure offers a large surface area for dye adsorption, Rayleigh scattering with this size of TiO2 particles is small, resulting in high transparency of the dye-sensitized film in a broad spectral region. A significant amount of light (70% in the near infrared region) is transmitted without interacting with dye molecules in the cell. The working electrode applied in highly efficient devices is based on a TiO2 double layer film18, sensitized with dye molecules on top of a TiCl4-treated conductive glass. The structure of these samples is shown in Fig. 1a. The first layer is a transparent mesoporous anatase TiO2 film, consisting of interconnected spherical nanoparticles (20 nm). Another layer made of 400 nm-diameter TiO2 particles is deposited on top of the transparent layer. Figure 1b shows the total transmittance, total reflectance and total absorptance of the Z907 dye-sensitized TiO2 double layer film based DSC photoanode. The 400 nm particles act as light scattering centers enhancing light absorption by increasing the light pathway within the film. Consequently, the total transmittance of the cell in the visible and near-infrared region is less than 15% as it can be seen in Fig. 1b. This suggests that the diffuse reflectance spectroscopy is the only versatile optical laser spectroscopy technique capable of studying such devices. The Kubelka-Munk function, F(R) spectra is derived from diffuse reflectance of the film according to equation (3), presented in the method section. The F(R) spectrum is compared with the absorptance spectrum of the opaque photoanode in Fig. 1b. As it is seen, the Kubelka-Munk spectrum follows the shape of the absorptance curve, and the similarity in both spectra is observed. The peak around 520 nm corresponds to the Z907 dye ground state absorption that serves as an absorbing medium. The shoulder at 380 nm corresponds to the absorption of TiO2 substrate that serves as the scattering media in Kubelka-Munk theory.


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)

Device architecture and optical properties of complete photoanode.(a) Schematic of the optimized highly efficient liquid DSC based on single layer and double layer TiO2 films. This represents examples of the devices that have been studied. (b) Steady-state optical parameters of a Z907 sensitized TiO2 double layer photoanode applied in highly efficient DSC devices. Total transmittance (Red), diffuse reflectance (blue), absorptance (green) and Kubelka- Munk function (dashed green) are depicted. Kubelka-Munk function and absorptance are defined according to equations (1) and (3) depicted in the method section. The total transmittance of the cell in the visible and infrared region is less than 15%. The Kubelka-Munk function spectrum follows the shape of the absorptance spectrum.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Device architecture and optical properties of complete photoanode.(a) Schematic of the optimized highly efficient liquid DSC based on single layer and double layer TiO2 films. This represents examples of the devices that have been studied. (b) Steady-state optical parameters of a Z907 sensitized TiO2 double layer photoanode applied in highly efficient DSC devices. Total transmittance (Red), diffuse reflectance (blue), absorptance (green) and Kubelka- Munk function (dashed green) are depicted. Kubelka-Munk function and absorptance are defined according to equations (1) and (3) depicted in the method section. The total transmittance of the cell in the visible and infrared region is less than 15%. The Kubelka-Munk function spectrum follows the shape of the absorptance spectrum.
Mentions: Figure 1a shows the schematics of the standard optimized high-performance liquid solar cell. In the conventional DSC scheme, the mesoporous layer is made of 20 nm-diameter interconnected TiO2 particles. Although this structure offers a large surface area for dye adsorption, Rayleigh scattering with this size of TiO2 particles is small, resulting in high transparency of the dye-sensitized film in a broad spectral region. A significant amount of light (70% in the near infrared region) is transmitted without interacting with dye molecules in the cell. The working electrode applied in highly efficient devices is based on a TiO2 double layer film18, sensitized with dye molecules on top of a TiCl4-treated conductive glass. The structure of these samples is shown in Fig. 1a. The first layer is a transparent mesoporous anatase TiO2 film, consisting of interconnected spherical nanoparticles (20 nm). Another layer made of 400 nm-diameter TiO2 particles is deposited on top of the transparent layer. Figure 1b shows the total transmittance, total reflectance and total absorptance of the Z907 dye-sensitized TiO2 double layer film based DSC photoanode. The 400 nm particles act as light scattering centers enhancing light absorption by increasing the light pathway within the film. Consequently, the total transmittance of the cell in the visible and near-infrared region is less than 15% as it can be seen in Fig. 1b. This suggests that the diffuse reflectance spectroscopy is the only versatile optical laser spectroscopy technique capable of studying such devices. The Kubelka-Munk function, F(R) spectra is derived from diffuse reflectance of the film according to equation (3), presented in the method section. The F(R) spectrum is compared with the absorptance spectrum of the opaque photoanode in Fig. 1b. As it is seen, the Kubelka-Munk spectrum follows the shape of the absorptance curve, and the similarity in both spectra is observed. The peak around 520 nm corresponds to the Z907 dye ground state absorption that serves as an absorbing medium. The shoulder at 380 nm corresponds to the absorption of TiO2 substrate that serves as the scattering media in Kubelka-Munk theory.

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