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

Abstract

The nitro group has recently been suggested as a new type of anchor for dye-sensitized solar cells (DSSCs) and has shown promising optoelectronic properties. Considering the excellent electron withdrawing ability of the nitro group and wider materials selection brought about by this substituent, it is helpful to evaluate the interfacial structures and photophysics of more organic dyes where NO2 poses as the dye-to-TiO2 anchor. A computational study on a family of azo dyes bearing a nitro group is presented, where the effect of certain side groups on their optical properties is examined. Both isolated dye molecules and dye/TiO2 nanocomposites are studied via density functional theory and time-dependent density functional theory, with complementary experimental UV/vis absorption spectroscopy and photovoltaic device testing. Results demonstrate that these nitro-containing dyes prefer a monodentate anchoring mode on a TiO2 cluster. These nitro dyes reveal weak, but non-negligible, adsorption onto TiO2; yet, very low photovoltaic performance once incorporated into a DSSC device. This poor delivery of nitro groups as DSSC anchors is ostensibly inconsistent with previous findings; but is rationalized via the "auxiliary anchor" concept.

Dye aggregation in dye-sensitized solar cells

“…67 Azo dyes such as para-methyl (25) and para-ethyl red ( 26) exhibit substantial H-aggregation on TiO2 substrates due to their azobenzene backbone, which is well-suited to engage in π•••π interactions. 48,[68][69][70][71] The phenothiazine-based dye 27 exhibits H-aggregation on TiO2 films in DSSCs, and the aggregation can be modulated by the presence of thiophene and ethylenedioxythiophene. 72 Apart from its influence on electron injection, dye aggregation can significantly reduce photodegradation by pollutants adsorbed onto TiO2; see for example the case of dyes 19-21 where their interaction with the photocatalyst is inhibited.…”

Successive Pd‐Catalyzed Decarboxylative Cross‐Couplings for the Modular Synthesis of Non‐Symmetric Di‐Aryl‐Substituted Thiophenes

“…groups have potential as anchors for pigment molecules within dye-sensitized solar cells. [78] All non-symmetric di-aryl-substituted thiophenes synthesized also exhibited fluorescence, and so the emission spectra for each was also measured. Most of the compounds exhibit similar fluorescence emissions around 470-495 nm.…”

Novel Azocoumarin Derivatives—Synthesis and Characterization

“…Much worse performance of BHJ has been reported, when azo dyes have been considered for a non-fullerene acceptor, with poly(3-octylthiophene) donor, where such devices have shown efficiency up to 0.65% [ 11 ]. Apart from this, compounds consisting of azo-moiety have been investigated as a sensitizer in dye-sensitized solar cells (DSSC) [ 12 , 13 ] or as a hole transporting material (HTM) in perovskite solar cells (PSC) [ 14 ].…”