Homoleptic and Heteroleptic Copper Complexes as Redox Couples in Dye‐Sensitized Solar Cells

Influence of Redox Couple on the Performance of ZnO Dye Solar Cells and Minimodules with Benzothiadiazole-Based Photosensitizers

ZnO-based dye-sensitized solar cells exhibit lower efficiencies than TiO₂-based systems despite advantageous charge transport dynamics and versatility in terms of synthesis methods, which can be primarily ascribed to compatibility issues of ZnO with the dyes and the redox couples originally optimized for TiO₂. We evaluate the performance of solar cells based on ZnO nanomaterial prepared by microwave-assisted solvothermal synthesis, using three fully organic benzothiadiazole-based dyes YKP-88, YKP-137, and MG-207, and alternative electrolyte solutions with the I^-/I₃ ^-, Co(bpy)₃ ^2+/3+, and Cu(dmp)₂ ^1+/2+ redox couples. The best cell performance is achieved for the dye-redox couple combination YKP-88 and Co(bpy)₃ ^2+/3+, reaching an average efficiency of 4.7% and 5.0% for the best cell, compared to 3.7% and 3.9% for the I^-/I₃ ^- couple with the same dye. Electrical impedance spectroscopy highlights the influence of dye and redox couple chemistry on the balance of recombination and regeneration kinetics. Combined with the effects of the interaction of the redox couple with the ZnO surface, these aspects are shown to determine the solar cell performance. Minimodules based on the best systems in both parallel and series configurations reach 1.5% efficiency for an area of 23.8 cm².

Quasi-Restricted Orbital Description of the Copper(I) Photoredox Catalytic Cycle

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis[2-(diphenylphosphino)phenyl]ether) are scrutinized and characterized using quasi-restricted orbitals (QRO), electron density differences and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet 3MLCT (d9L-1), Dox (d9L0) and Dred (d10L-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Amongst others, we reveal the molecular-scale delocalization of the ligand-centered radical in the a 3MLCT (d9L-1) and Dred (d9L-1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

Thermally activated delayed fluorescence in luminescent cationic copper(i) complexes

In this work, the photophysical characteristics of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ complexes were described. The concept of thermally activated delayed fluorescence (TADF) and its development throughout the years was also explained. The importance of ΔE_(S1–T1) and spin-orbital coupling (SOC) values on the TADF behavior of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ complexes is discussed. Examples of ΔE_(S1–T1) values reported in the literature were collected and some trends were proposed (e.g. the effect of the substituents at the 2,9 positions of the phenanthroline ligand). Besides, the techniques (or calculation methods) used for determining ΔE_(S1–T1) values were described. The effect of SOC in TADF was also discussed, and examples of the determination of SOC values by DFT and TD-DFT calculations are provided. The last chapter covers the applications of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ TADF complexes and the challenges that are still needed to be addressed to ensure the industrial applications of these compounds.

Bibliographic review of cationic Cu(i) complexes that undergo Thermally Activated Delayed Fluorescence (TADF). From the first findings found in the homoleptic and heteroleptic Cu(i)-TADF complexes to the use of this property in lighting devices.

Recent Advances on Copper Complexes as Visible Light Photoinitiators and (Photo) Redox Initiators of Polymerization

Metal complexes are used in numerous chemical and photochemical processes in organic chemistry. Metal complexes have not been excluded from the interest of polymerists to convert liquid resins into solid materials. If iridium complexes have demonstrated their remarkable photochemical reactivity in polymerization, their high costs and their attested toxicities have rapidly discarded these complexes for further developments. Conversely, copper complexes are a blooming field of research in (photo) polymerization due to their low cost, easy syntheses, long-living excited state lifetimes, and their remarkable chemical and photochemical stabilities. Copper complexes can also be synthesized in solution and by mechanochemistry, paving the way towards the synthesis of photoinitiators by Green synthetic approaches. In this review, an overview of the different copper complexes reported to date is presented. Copper complexes are versatile candidates for polymerization, as these complexes are now widely used not only in photopolymerization, but also in redox and photoassisted redox polymerization processes.