Effects of Post-Assembly Molecular and Atomic Passivation of Sensitized Titania Surface: Dynamics of Electron Transfer Measured from Femtoseconds to Seconds

Abstract

The dynamics of electron transfer at the dye-titania and titania-electrolyte interfaces is investigated in two post-sensitization processes: (i) atomic layer deposition of blocking alumina coating and (ii) hierarchical molecular multicapping. To measure the electron transfer dynamics, time-resolved spectroscopic methods (femtosecond transient absorption on the time scale from femtoseconds to nanoseconds and electrochemical impedance spectroscopy on the time scale from milliseconds to seconds) are applied to the complete dye-sensitized solar cells with cobalt-based electrolyte and champion ADEKA-1 dye (with silyl-anchor unit) or its popular carboxyl-anchor analogue, MK-2 dye. Both molecular capping and alumina blocking layers slow down the electron injection process (the average rate constant decreases from 1.1 ps to 0.4 ps) and partial sub-nanosecond back electron transfer from titania to the dye (from ca. 10 ns to 5 ns). Very small alumina layers (of 0.1 nm thickness) have the highest impact on reducing the rate constants of these electron transfer processes, and for the thicknesses greater than 0.3 nm the rate constants hardly change. In contrast, the electron recombination between titania and electrolyte, occurring on the millisecond time scale, starts to be significantly suppressed for the blocking layers of 0.3 nm or more in thickness (up to ca. 20 times for 0.5 nm thickness with respect to that for untreated sample), improving open circuit voltage and fill factor of the cells. The amplitude of the relative photocurrent (short circuit current per number of absorbed photons) is found to depend almost exclusively on the ultrafast and fast processes taking place in the first nanoseconds after dye excitation. The positive impact of coadsorbents on the solar cells performance for both ADEKA-1 and MK-2 is also studied.

Complexity of Electron Injection Dynamics and Light Soaking Effects in Efficient Dyes for Modern DSSC

“…Our interpretation of the transient absorption measurements collected for Y123 sensitized solar cells is provided in the previous part of this paper. On the other hand, MK2 dye was extensively studied by our group and we based our interpretation on the previous reports [41][42][43][44][45]. Namely, we attributed the two faster (3-4 ps and 30-35 ps, black and red lines in Figure 4C,D) components to electron injection process.…”

“…Besides the Y123 based cells, we used well-described systems based on carbazole MK2 dye (Supplementary Scheme S1B), extensively studied by our group for the last few years [41][42][43][44][45]. Both dyes were studied in combination with [Co(bpy) 3 ](B(CN) 4 ) 2/3 (bpy = 2,2 -bipyridine) [9] and [Cu(tmby) 2 ](TFSI) 1/2 (tmby = 4,4 ,6,6 tetramethyl-2,2bipyridine, TFSI = bis(trifluoromethane)sulfonamide) [15], providing new insights into the dynamics of electron separation and recombination in Y123 and their impact on photocurrent generation.…”

Understanding the Interfaces between Triple-Cation Perovskite and Electron or Hole Transporting Material

“…Therefore, in our works we introduce the parameter called total APCE (APCE is absorbed photon to current efficiency). 28 It can be calculated based on the measured short-circuit current of the cell, J SC , and the number of absorbed photons, N ph (obtained from integration of stationary absorptance spectrum multiplied by the photon flux spectrum from AM1.5G data): where e is the elementary charge. In the present work we consider the correlation between total APCE and time constants or perovskite layer thickness.…”

Interfacial Modification of Photoanode|Electrolyte Interface Using Oleic Acid Enhancing the Efficiency of Dye-Sensitized Solar Cells

“…While increasing the bias potential, the TiO 2 becomes conductive and the electrons are allowed to diffuse through the TiO 2 film, and now the recombination takes place at the TiO 2 surface. 37,38 Figure 5B shows an increase in the charge recombination resistance at photoanode|electrolyte interface for the OA-modified device while forwarding the bias potential from 0.80 to 0.85 V, suggesting that the adsorbed OA molecules over the naked surface of TiO 2 passivate the exposed TiO 2 surface and prevent the electron transport to the electrolyte. In addition, the electron transport resistance is lower in the entire range of the bias potentials for the OA-modified DSSC (Figure 5C) and these results are in good agreement with the calculated R t values under illumination (see Table 2).…”