New molecular design of donor-π-acceptor dyes for dye-sensitized solar cells: control of molecular orientation and arrangement on TiO₂surface

Abstract

A series of benzofuro [2,3-c]oxazolo [4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton are synthesized and applied to dye-sensitized solar cells (DSSCs) as a new class of donor-p-acceptor (D-p-A) photosensitizers. In the dye OH1, a carboxyl group acts as not only the anchoring group for attachment on TiO 2 surface but also the electron acceptor. For OH2, OH4, and OH7, on the other hand, a carboxyl group is an anchoring group, but the electron acceptor is not a carboxyl group but a cyano group. The dye OH17 has two carboxyl groups, which are located at the same positions as those of OH1 and OH7. The absorption and fluorescence spectra and cyclic voltammograms of these fluorescent dyes resemble very well, showing a negligible influence of the position of carboxyl group on photophysical and electrochemical properties of these dyes. When these dyes are used in DSSCs, however, their photovoltaic performances differ considerably. In order to elucidate the difference in the DSSC performance among the five dyes, kinetics of the electron injection from the conduction band of TiO 2 to dye cation and to I 3 À ions in solution were studied by employing the transient absorption spectroscopy and the transient photovoltage technique, respectively. It is found that the charge recombination rate for OH2 is similar to that of OH1 and is much slower than those of OH4 and OH17, consistent with the high short-circuit photocurrent densities for OH1 and OH2. On the basis of the MO calculations (AM1 and INDO/S) and the charge recombination kinetics, the differences of the photovoltaic performances among the five dyes are discussed from the viewpoint of configuration of the dyes adsorbed on TiO 2 surface, and a new molecular design for D-p-A dye sensitizers based on a control of molecular orientation and arrangement of dye adsorbed on TiO 2 surface is proposed.

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…This can be attributed to (1) the degradation of catalytic activity after the introduction of functional groups, leading to changes in electron distribution and energetics in electrocatalysts,, , and (2) the vulnerability of ester linkages to hydrolytic degradation, resulting in detachment of electrocatalysts from photoelectrodes , . It is well‐known that the introduction of electron withdrawing and donating groups has a significant effect on their redox potentials and catalytic activity , , . To address the former issue, Wang et al suggested the utilization of anchoring groups with long alkyl chains , , .…”

Control of Molecular Arrangement and/or Orientation of D–π–A Fluorescent Dyes for Dye-sensitized Solar Cells

“…Thus, in order to provide a further confirmation for this view and to gain additional insights about the differences in the DSSC performance among the D³A fluorescent dyes, kinetics of the charge recombination between the injected electrons in the CB of TiO 2 and the dye cations [(5) in Figure 1] was studied by employing transient absorption spectroscopy. 65 The charge recombination half-time (t 50% ) increases in the order of OH17 (21¯s) < OH4 (195¯s) < OH2 (1.8 ms)¯OH1 (2.5 ms) < OH7 (9 ms) ( Figure 7a). It is found that the charge recombination rate for OH2 is similar to that of OH1 and is much slower than those of OH4 and OH17, consistent with the high J sc values for OH1 and OH2 (Table 1).…”

Transforming Benzophenoxazine Laser Dyes into Chromophores for Dye‐Sensitized Solar Cells: A Molecular Engineering Approach

“…However, it is worth noting that a Co (II/III) tris(bipyridyl) redox couple is gaining traction as an alternative, given that it has a higher redox potential ( E redox ≈ +0.535 V) compared to I − / I 3 − ( E redox ≈ +0.35 V) and it mitigates better against energy losses in the dye regeneration process. [ 1,3,4 ] As the most effi cient, and hence most commonly used dyes contain the expensive and relatively rare transition metal ruthenium, [ 5,6 ] metal-free organic dyes have become attractive candidates due to their low cost, high molar extinction coeffi cients, and inherent design fl exibility. [ 5,6 ] Most organic dyes are based on donor-(π-bridge)-acceptor ( D -π-A ) architectures, in which a π-conjugated system connects an electron-donating group ( D ) with an electron-accepting group ( A ).…”

“…[ 1,3,4 ] As the most effi cient, and hence most commonly used dyes contain the expensive and relatively rare transition metal ruthenium, [ 5,6 ] metal-free organic dyes have become attractive candidates due to their low cost, high molar extinction coeffi cients, and inherent design fl exibility. [ 5,6 ] Most organic dyes are based on donor-(π-bridge)-acceptor ( D -π-A ) architectures, in which a π-conjugated system connects an electron-donating group ( D ) with an electron-accepting group ( A ). Upon photoexcitation, the resulting "push-pull" effect is characterized by anisotropic intramolecular charge transfer (ICT) from the donor to the acceptor, which facilitates electron injection into the semiconductor.…”