Relating Electron Donor and Carboxylic Acid Anchoring Substitution Effects in Azo Dyes to Dye-Sensitized Solar Cell Performance

Abstract

The relationship between the molecular structures of a series of azo dyes and their operational performance when applied to dye-sensitized solar cells (DSSCs) is probed via experimental and computational analysis. Seven azo dyes, with three different donating groups (dimethylamino, diethylamino, and dipropylamino) and carboxylic acid anchoring positions (ortho-, meta-, and para-substituted phenyl rings) are studied. Single-crystal X-ray diffraction is employed in order to analyze the effects of conformation and quantify the contribution of quinoidal resonance forms to the intramolecular charge transfer (ICT), which controls their intrinsic photovoltaic potential from an electronic standpoint. Harmonic oscillator stabilization energy (HOSE) calculations indicate that the para- and ortho-azo dyes exhibit potential for DSSC application. However, from a geometrical standpoint, the crystal structure data, proton nuclear magnetic resonance spectroscopy (1H NMR), and density functional theory (DFT) all indicate that intramolecular hydrogen bonds form in ortho-dyes within both solid and solution states, impeding their intrinsic ICT-based photovoltaic potential, and offering insights into the photostability of azo dyes and the dye···TiO2 anchoring mechanism in DSSCs. Donor effects are manifested in the packing mode and molecular planarity revealed by X-ray crystallography and in the UV/vis absorption spectra. DFT and time-dependent density functional theory (TDDFT) were performed to understand the electronic and optical properties of these azo dyes; these calculations compare well with experimental findings. Operational tests of DSSCs, functionalized by these azo dyes, show that the carboxylic acid anchoring position plays a crucial role in DSSC performance, while donating groups offer a much less obvious effect on the overall DSSC device efficiency.

Incorporation of Kojic Acid-Azo Dyes on TiO2 Thin Films for Dye Sensitized Solar Cells Applications

“…Extracted natural dye, however, has low yields of extract and scarce resources [10]. Organic dye such as azobenzene has outstanding chromophores with strong absorption in visible region and intrinsic advantages of good photo and thermal stability [11,12]. Azobenzene is a reactive precursor for functional group conversion and attachment such as vinyl and alkoxy chain with enhanced optical properties [13,14].…”

Can nitro groups really anchor onto TiO2? Case study of dye-to-TiO2adsorption using azo dyes with NO2substituents

“…η = 0.29-0.32%. 24 These photovoltaic test results are corroborated by a visual inspection of the DSSC working electrodes involving 1-3 after dye sensitization (see Figure S3 of the Electronic Supplementary Information). This confirms that the -NO 2 anchoring group offers a viable binding site for the dye on the TiO 2 surface, as judged by the color difference before and after the sensitization process.…”

“…60 In each case, the main peak in 1-3 is red-shifted by ca. 50-100 nm compared to the analogous case where -COOH rather than -NO 2 is the substituent (464 nm) 24 . This is presumably because the nitro group has a better electron withdrawing ability than the -COOH group, as judged by the corresponding Hammett values.…”

“…This demonstrates the electron 'push-pull' effect, which is an indicator of efficient intramolecular charge-transfer. 24 A more extended set of molecular orbital distributions for each corresponding dye/TiO 2 nanocomposite is provided in Figure 3. This illustrates that these NO 2 -based azo dyes have similar molecular distributions of occupied orbitals with respect to their COOH-based azo dyes; 58 and yet, the associated LUMO distributions are highly disparate: they are distributed wholly within the organic dyes in 1-3, as opposed to residing inside the TiO 2 region of the nanocomposite, as per the case of the COOH-containing azo dyes.…”

Bis{μ-ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate-κO}bis[aqua(4-{[4-(ethoxycarbonyl)phenyl]diazenyl}phenolato-κO){ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate-κO}potassium]

“…Azo dyes are a well known family of organic dyes including the azo group (-N N-) bonded to aromatic ring systems (Zhang et al, 2013). There are several compounds reported containing even more than one azo chromophore per molecule, also linked to heterocyclic systems (Patni & Patni, 2016).…”