Interfacial charge separation and photovoltaic efficiency in Fe(ii)–carbene sensitized solar cells

Abstract

The first combined theoretical and photovoltaic characterization of both homoleptic and heteroleptic Fe(ii)-carbene sensitized photoanodes in working dye sensitized solar cells (DSSCs) has been performed. Three new heteroleptic Fe(ii)-NHC dye sensitizers have been synthesized, characterized and tested. Despite an improved interfacial charge separation in comparison to the homoleptic compounds, the heteroleptic complexes did not show boosted photovoltaic performances. The ab initio quantitative analysis of the interfacial electron and hole transfers and the measured photovoltaic data clearly evidenced fast recombination reactions for heteroleptics, even associated with un unfavorable directional electron flow, and hence slower injection rates, in the case of homoleptics. Notably, quantum mechanics calculations revealed that deprotonation of the not anchored carboxylic function in the homoleptic complex can effectively accelerate the electron injection rate and completely suppress the electron recombination to the oxidized dye. This result suggests that introduction of strong electron-donating substituents on the not-anchored carbene ligand in heteroleptic complexes, in such a way of mimicking the electronic effects of the carboxylate functionality, should yield markedly improved interfacial charge generation properties. The present results, providing for the first time a detailed understanding of the interfacial electron transfers and photovoltaic characterization in Fe(ii)-carbene sensitized solar cells, open the way to a rational molecular engineering of efficient iron-based dyes for photoelectrochemical applications.

NHC-Based Iron Sensitizers for DSSCs

“…Strikingly, the IPCE spectrum of C4 does not show any sensitization activity of the MLCT transitions of this complex (Figure 15a). To gain insight into the peculiar interfacial properties of the C2-and C4-sensitized TiO2 interfaces and hence provide a fundamental understanding of the charge generation and sensitization capabilities, we reported a quantitative analysis based on high-level DFT calculations and the modelling of the relevant interfacial electron-transfer processes [35]. To gain insight into the peculiar interfacial properties of the C2-and C4-sensitized TiO 2 interfaces and hence provide a fundamental understanding of the charge generation and sensitization capabilities, we reported a quantitative analysis based on high-level DFT calculations and the modelling of the relevant interfacial electron-transfer processes [35].…”

“…At a first stage, the photovoltaic properties for the homoleptic complexes C2 and C4 were investigated, since they both bear carboxylic functionalities acting as grafting groups for the TiO 2 substrate [30,35].…”

“…To investigate this, complex C5 [35] was prepared bearing the COOH group on only one ligand (see Scheme 3). In contrast to C1 and C2, absorption spectra of complex C5 exhibit one additional band in the visible region, that is, at 440 nm, that has no correspondence with those of the former (Figure 8a).…”

“…To investigate this, complex C5 [35] was prepared bearing the COOH group on only one ligand (see Scheme 3). .…”

First Principle Modelling of Materials and Processes in Dye-Sensitized Photoanodes for Solar Energy and Solar Fuels

“…Most of the published computational modeling works so far are, indeed, focused on the characterization of the water From a computational point of view, modern first principles computational methodologies, essentially relying on density functional theory (DFT) and its time dependent extension (TDDFT), are able to describe most of the target characteristics of the individual systems (dyes, catalysts, semiconductors, etc.) [16,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] and of their active interfaces [16,41,43,47,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79]. The information extracted from these calculations serve as the basis for the explicit simulation of the photo-induced electron transfer by means of quantum or non-adiabatic dynamics.…”