Increased solar-driven chemical transformations through surface-induced benzoperylene aggregation in dye-sensitized photoanodes

The impact of benzo[ghi]perylenetriimide (BPTI) dye aggregation on the performance of photoelectrochemical devices was explored, through imide-substitution with either alkyl (BPTI-A, 2-ethylpropyl) or bulky aryl (BPTI-B, 2,6-diisopropylphenyl) moieties, to, respectively, enable or suppress aggregation. While both dyes demonstrated similar monomeric optoelectronic properties in solution, adsorption onto mesoporous SnO2 revealed different behavior, with BPTI-A forming aggregates via π-stacking and BPTI-B demonstrating reduced aggregation in the solid state. BPTI photoanodes were tested in dye-sensitized solar cells (DSSCs) before application to dye-sensitized photoelectrochemical cells (DSPECs) for Br2 production (a strong oxidant) coupled to H2 generation (a solar fuel). BPTI-A demonstrated a twofold higher dye loading of the SnO2 surface than BPTI-B, resulting in a fivefold enhancement to both photocurrent and Br2 production. The enhanced output of the photoelectrochemical systems (with respect to dye loading) was attributed to both J- and H- aggregation phenomena in BPTI-A photoanodes that lead to improved light harvesting. Our investigation provides a strategy to exploit self-assembly via aggregation to improve molecular light-harvesting and charge separation properties that can be directly applied to dye-sensitized photoelectrochemical devices.

Automated assessment of redox potentials for dyes in dye-sensitized photoelectrochemical cells

Sustainable solutions for hydrogen production, such as dye-sensitized photoelectrochemical cells (DS-PEC), rely on the fundamental properties of its components whose modularity allows for their separate investigation. In this work, we design and execute a high-throughput scheme to tune the ground state oxidation potential (GSOP) of perylene-type dyes by functionalizing them with different ligands. This allows us to identify promising candidates which can then be used to improve the cell's efficiency. First, we investigate the accuracy of different theoretical approaches by benchmarking them against experimentally determined GSOPs. We test different methods to calculate the vertical oxidation potential, including GW with different levels of self-consistency, Kohn-Sham (KS) orbital energies and total energy differences. We find that there is little difference in the performance of these methods. However, we show that it is crucial to take into account solvent effects as well as the structural relaxation of the dye after oxidation. Other thermodynamic contributions are negligible. Based on this benchmark, we decide on an optimal strategy, balancing computational cost and accuracy, to screen more than 1000 dyes and identify promising candidates which could be used to construct more robust DS-PECs.

Systematic Computational Design and Optimization of Light Absorbing Dyes

We present a workflow to aid the discovery of new dyes for the role of a photosensitive unit in the dye-sensitized photo-electrochemical cells (DS-PECs). New structures are generated in a fully automated way using the Compound Attachment Tool (CAT) introduced in this work. These structures are characterized with efficient approximate density functional theory (DFT) methods, and molecules with favorable optical properties are suggested for possible further use in DS-PECs. As around 2500 structures are generated in this work, and as we aim for still larger volumes of compounds to screen in subsequent applications, we have assessed the reliability of low-cost screening methods and show that simplified time-dependent density functional theory (sTDDFT) provides a satisfying accuracy/cost ratio. From the dyes considered, we propose a set that can be suitable for panchromatic sensitization of the photoelectrode in DS-PECs to further increase DS-PEC efficiency.

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoOx Catalyst

A planar organic thin film composed of a perylene diimide dye (N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide, PMPDI) with photoelectrochemically deposited cobalt oxide (CoO_x) catalyst was previously shown to photoelectrochemically oxidize water (DOI: 10.1021/am405598w). Herein, the same PMPDI dye is studied for the sensitization of different nanostructured metal oxide (nano-MO_x) films in a dye-sensitized photoelectrochemical cell architecture. Dye adsorption kinetics and saturation decreases in the order TiO₂ > SnO₂ ≫ WO₃. Despite highest initial dye loading on TiO₂ films, photocurrent with hydroquinone (H₂Q) sacrificial reductant in pH 7 aqueous solution is much higher on SnO₂ films, likely due to a higher driving force for charge injection into the more positive conduction band energy of SnO₂. Dyeing conditions and SnO₂ film thickness were subsequently optimized to achieve light-harvesting efficiency >99% at the λ_max of the dye, and absorbed photon-to-current efficiency of 13% with H₂Q, a 2-fold improvement over the previous thin-film architecture. A CoO_x water-oxidation catalyst was photoelectrochemically deposited, allowing for photoelectrochemical water oxidation with a faradaic efficiency of 31 ± 7%, thus demonstrating the second example of a water-oxidizing, dye-sensitized photoelectrolysis cell composed entirely of earth-abundant materials. However, deposition of CoO_x always results in lower photocurrent due to enhanced recombination between catalyst and photoinjected electrons in SnO₂, as confirmed by open-circuit photovoltage measurements. Possible future studies to enhance photoanode performance are discussed, including alternative catalyst deposition strategies or structural derivatization of the perylene dye.