Role of energy level alignment in solar cells sensitized with a metal‐free organic dye: A combined experimental and theoretical approach

Abstract

We report results of combined experimental and theoretical studies of dye‐sensitized solar cells (DSSCs) using 5‐(4‐sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY‐10), as TiO2 sensitizer. We focus on a single dye but vary the solvent and the pH of the solution as well as the photoelectrode preparation conditions to determine the conditions for best photovoltaic conversion efficiency. We found experimentally that the efficiency, measured under standard air mass 1.5 global (AM 1.5G) conditions, was higher in solutions of ethanol than of water, but still small (up to 0.174%), although the fill factor (FF) was large (up to 0.73). Of the dyes in ethanol, MY‐10 in alkaline solution showed the best matching of the solar spectrum but displayed the lowest efficiency. Density functional theory (DFT) calculations provided the optimized geometry, electronic structure, and electronic spectrum of the dye in fully protonated as well as partially and totally deprotonated forms, in solution. The calculated optical spectra are consistent with the experimental data, with strong absorption in the visible range only for the alkaline dye solution. The low device efficiency is very likely related to the weak optical absorption in the visible range. The much higher photovoltaic conversion efficiency of the DSSCs fabricated using acid or roughly neutral pH solutions, corresponding to the protonated and partially deprotonated forms of MY‐10, respectively, is likely caused by the better alignment of the ground state of the dye with the redox level of the electrolyte. The decrease with pH of the dye solution of the short‐circuit current was linked to a weaker charge injection from the excited state of the dye to the conduction band of the oxide, which is correlated with the shifting of the excited state of the dye deeper into the CB edge of the semiconductor. The variation of the open‐circuit voltage with the pH of the solution was linked to the adjustment of the conduction band edge of TiO2, depending on the number of protons transferred from the dye to the oxide surface. Based on our results, we analyze the relative importance of the main criteria that should be met by a dye to be used in a DSSC.

A combined experimental and theoretical study of natural betalain pigments used in dye-sensitized solar cells

“…One possible explanation of this effect may be the hindering of the charge injection from the excited state of the dye to the conduction band of the oxide when the energy difference between the two is larger. The importance of the good matching between the electron donor (excited dye) and the acceptor (semiconductor conduction band) in order for injection to occur at a sufficiently rapid rate was previously mentioned by us [24,25,49] and other authors [46,47].…”

“…The values on the axes in Fig. 4 are relative to the vacuum (on the left axis) and to the normal hydrogen electrode (NHE, on the right axis) [24,25].…”

“…The search for new dyes is, however, time and resource consuming, making the theoretical computation a promising alternative. Although the theoretical investigations have not matched the extent of the experimental ones, the interest for dye modelling [5,24,25] and for more complex dye-substrate computational studies has been steadily increasing [5,26,27]. Achieving accurate results is challenging because of the size of the system, which in many cases consists of a relatively large number of atoms [28,29].…”

DFT study of binding and electron transfer from colorless aromatic pollutants to a TiO2 nanocluster: Application to photocatalytic degradation under visible light irradiation

“…The higher performance of the dyes with both carboxy and hydroxy anchoring groups [28] has led us to revisit earlier studies of a dye with three types of anchoring groups: –OH, –COOH and –SO 3 H [29–30]. We showed that although the salicylate does use both the carboxy and hydroxy substituent groups, the binding configuration is not bidentate chelate, as previously thought [14,18–20].…”

“…Similarly, the binding of SA was thought as bidentate chelate through the oxygen atoms of –OH and of –COOH [14,20]. In a previous paper [29] we showed that for Mordant Yellow-10 anchoring through the SA part can lead to three binding modes, depending on the degree of deprotonation, but none of them was bidentate chelate. Returning to the present study, for SA we performed geometry optimization calculations in three cases.…”