Correlations between Photovoltaic Characteristics, Adsorption Number, and Regeneration Kinetics in Dye-Sensitized Solar Cells Revealed by Scanning Photocurrent Microscopy

Abstract

Newly developed simultaneous scanning photocurrent and luminescence microscopy was applied to ruthenium-based dye-sensitized solar cells (DSCs) comprising a cover glass photoanode with a 100 nm thick TiO2 layer. Using this, we have investigated the lateral variations of several parameters of these DSCs under short-circuit conditions. Simultaneous measurement of photocurrent and luminescence images for the same area of the DSC demonstrated submicrometric lateral resolution of our photocurrent microscopy, which is approximately 10 times better than the resolution of photocurrent microscopy used in past studies. The photovoltaic parameters, such as short-circuit current density, open-circuit voltage, and charge-collection efficiency, were thus evaluated for local (or submicrometric) regions of the DSCs. Furthermore, the photocurrent saturation behavior of the DSCs was examined as a function of the excitation rate and analyzed on the basis of a three-state kinetic model. This protocol allowed for quantification of the dye-adsorption number and dye-regeneration rate constant for any local area of the DSCs. Consequently, the correlations between the dye adsorption number, photovoltaic parameters, and regeneration rate constant, which are difficult to address through examination of the entire cell, were revealed by the "zoom-in" approach utilizing this high-resolution photocurrent microscopy.

Quantification of Dyes Generating Photocurrent and/or Photoluminescence in Dye-Sensitized Solar Cells Using Laser Scanning Microscopy

“…However, such a thin film significantly shortens the electron diffusion length in TiO 2 , and reduces the probability of recombination of injected electrons with oxidized dye molecules and/or oxidized redox species. Consequently, the DSSCs prepared for LSM possess near 100% collection efficiency of injected electrons at the submicrometric laser illumination (i.e.,~10 −9 cm 2 ) [20]. To verify this for the present MK-2-DSSC, the corrected current (I dark ), which was obtained by subtracting the dark current from the light current, was plotted against the applied voltage, as shown in Figure 2b.…”

“…Figure 3a,b shows PC and PL images simultaneously recorded for the same area in a MK-2-DSSC under short-circuit condition. Owing to the very fast PC response (<1 ms) of the present cells under local (or submicrometric) laser illumination conditions, the PC image can be directly obtained without any corrections of the effect of PC response delay [20]. Several (unintentional) submicrometric structures are observed at the same positions in the PC and PL images.…”

“…Simultaneous PC and PL measurements were performed on a homemade LSM. A detailed description of the laser scanning method and optical setup of our microscope has been published elsewhere [20,24]. The excitation light was provided by a 478 nm pulsed diode laser (PIL048X, Advanced Laser Diode Systems, Berlin, Germany) operating at 50 kHz-40 MHz and a pulse width of approximately 80 ps full-width at half-maximum (FWHM).…”

“…As shown in Figure 1a, we fabricated a DSSC comprising a cover glass photoanode with a 100-nm thick TiO2 layer, which allows analyzing the DSSC devices using an oil-immersion objective lens with a high numerical aperture (N.A. > 1), as schematically depicted in Figure 1b [20]. This setup provides a spatial resolution of ca.…”

“…Using this setup, we evaluated photovoltaic parameters such as short-circuit current density (Jsc) and open-circuit voltage (Voc) for a local area of the DSSCs. Additionally, analysis of the PC saturation behavior based on a three-state kinetic model quantitatively provides the dye adsorption number (Nc) and dye regeneration rate constant (kreg) at a submicrometric area of the DSSCs [20]. In this contribution, our newly developed LSM for determining PC and PL is applied for DSSCs using a metal-free organic dye sensitizer, MK-2 ( Figure 1c), which is an alkyl-functionalized-carbazole dye that exhibits a photoelectric conversion efficiency of 6-8% [21][22][23].…”