Zinc phthalocyanines as light harvesters for SnO2-based solar cells: a case study

Toward the search for new photosensitizers for DSSCs: theoretical study of both substituted Zn(II) and Si(IV) phthalocyanines

We report a density functional theory (DFT) study performed for a set of 66 compounds based on zinc(II) and silicon(IV) phthalocyanines (Pcs) with potential applications in dye-sensitized solar cells (DSSCs). The effect of the metal center (Zn, Si), periplanar and axial substituents, and anchor groups like anhydrous, carboxyl, and catechol on the electronic, optical, photovoltaics, and adsorption properties is investigated. Using the TD-DFT methodology and M06 and CAM-B3LYP functionals, we calculated the absorption spectra on optimized structures and in the solution phase but not on structures relaxed in the solvent. We obtained a strong Q band and a weak Soret band in the UV-Vis region, which are attributed to the transitions of type π-π* as described by the Gouterman orbitals. Q bands calculated show absorption up to 667 nm for ZnPcs and up to 769 nm for SiPcs, suggesting an essential role of the metal atom. The systems have a bathochromic effect in the order of secondary amine > primary amine > hydroxyl > amide > ester. We also found that the anhydrous and carboxyl groups favor absorption at longer wavelengths than the catechol group. The ZnPc systems show a slightly larger electron injection ΔGinj (∼1.1 eV) than SiPcs (∼0.9 eV), with similar values for the three anchor groups. The interaction energies (Eint) between ZnPcs/SiPcs and TiO2 in molecular and periodic configuration and corrected by the counterpoise method indicate that SiPcs predict more negative values than ZnPcs. The anchor group effect is relevant; the carboxyl moiety leads to stronger interactions than the anhydrous moiety. The strategies used could help to identify new photosensitizers for DSSCs.

Self-Assembled Monolayers of Push-Pull Chromophores as Active Layers and Their Applications

In recent decades, considerable attention has been focused on the design and development of surfaces with defined or tunable properties for a wide range of applications and fields. To this end, self-assembled monolayers (SAMs) of organic compounds offer a unique and straightforward route of modifying and engineering the surface properties of any substrate. Thus, alkane-based self-assembled monolayers constitute one of the most extensively studied organic thin-film nanomaterials, which have found wide applications in antifouling surfaces, the control of wettability or cell adhesion, sensors, optical devices, corrosion protection, and organic electronics, among many other applications, some of which have led to their technological transfer to industry. Nevertheless, recently, aromatic-based SAMs have gained importance as functional components, particularly in molecular electronics, bioelectronics, sensors, etc., due to their intrinsic electrical conductivity and optical properties, opening up new perspectives in these fields. However, some key issues affecting device performance still need to be resolved to ensure their full use and access to novel functionalities such as memory, sensors, or active layers in optoelectronic devices. In this context, we will present herein recent advances in π-conjugated systems-based self-assembled monolayers (e.g., push-pull chromophores) as active layers and their applications.

Surface Morphology of Textured Transparent Conductive Oxide Thin Film Seen by Various Probes: Visible Light, X-rays, Electron Scattering and Contact Probe

Fluorine-doped tin oxide thin films (SnO₂:F) are widely used as transparent conductive oxide electrodes in thin-film solar cells because of their appropriate electrical and optical properties. The surface morphology of these films influences their optical properties and therefore plays an important role in the overall efficiencies of the solar cells in which they are implemented. At rough surfaces light is diffusely scattered, extending the optical path of light inside the active layer of the solar cell, which in term improves light absorption and solar cell conversion efficiency. In this work, we investigated the surface morphology of undoped and doped SnO₂ thin films and their influence on the optical properties of the films. We have compared and analysed the results obtained by several complementary methods for thin-film surface morphology investigation: atomic force microscopy (AFM), transmission electron microscopy (TEM), and grazing-incidence small-angle X-ray scattering (GISAXS). Based on the AFM and TEM results we propose a theoretical model that reproduces well the GISAXS scattering patterns.

Low-Symmetry Phthalocyanines Bearing Carboxy-Groups: Synthesis, Spectroscopic and Quantum-Chemical Characterization

The synthesis and characterization of A₃B-type phthalocyanines, ZnPc1–4, bearing bulky 2,6-diisopropylphenoxy-groups or chlorine atoms on isoindoline units “A” and either one or two carboxylic anchors on isoindoline unit “B” are reported. A comparison of molecular modelling with the conventional time dependent—density functional theory (TD-DFT) approach and its simplified sTD-DFT approximation provides further evidence that the latter method accurately reproduces the key trends in the spectral properties, providing colossal savings in computer time for quite large molecules. This demonstrates that it is a valuable tool for guiding the rational design of new phthalocyanines for practical applications.