Dye-sensitized solar cells with carboxylate-derivatized {Cu^IL₂} complexes are surprisingly efficient and offer a long-term alternative approach to ruthenium-functionalized systems.

Since the discovery of Grätzel-type dye sensitized solar cells (DSCs) in the early 1990s, there has been an exponential growth in the number of publications dealing with their optimization and new design concepts. Conventional Grätzel DSCs use ruthenium(II) complexes as sensitizers, and the highest photon-to-electrical current conversion efficiency for a ruthenium dye is ≈12%. However, ruthenium is both rare and expensive, and replacement by cheaper and more sustainable metals is desirable. In this Tutorial Review, we describe strategies for assembling copper(I) complexes for use as dyes in DSCs, a research area that has been active since ≈2008. We demonstrate design principles for (i) ligands to anchor the complex to a semiconductor surface and promote electron transfer from dye to semiconductor, and (ii) ancillary ligands to tune the light absorption properties of the dye and facilitate electron transfer from electrolyte to dye in the DSC. We assess the progress made in terms of light-harvesting and overall photoconversion efficiencies of copper(I)-containing DSCs and highlight areas that remain ripe for development and improvement.

Four prototype heteroleptic copper(I) complexes [Cu(bpy)(pop)][PF₆] (bpy = 2,2′-bipyridine, pop = bis(2-(diphenylphosphino)phenyl)ether), [Cu(phen)(pop)][PF₆] (phen = 1,10-phenanthroline), [Cu(bpy)(pdpb)][PF₆] (pdpb = 1,2-bis(diphenylphosphino)benzene) and [Cu(phen)(pdpb)][PF₆] are presented. The synthesis, X-ray structures, solution and solid-state photophysical studies, and the performance in light-emitting electrochemical cells (LECs) of these complexes are described. Their photophysical properties are interpreted with the help of density functional theory (DFT) calculations. The photophysical studies in solution and in the solid-state indicate that these copper(I) complexes show good luminescent properties which allow them to be used as active materials in electroluminescent devices such as LECs. Additionally, these materials are very attractive since we can take advantage of their low-cost, due to the copper abundance, and their limited environmental damaging effects for producing cheap large-area panels based on the LEC technology for lighting applications. LEC devices were fabricated using the four prototype copper(I) complexes together with an ionic liquid (IL), 1-ethyl-3-methylimidazolium hexafluoridophosphate, at a molar ratio of 1 : 1. They yield devices that are comparable to those obtained for most LEC devices based on ruthenium(II) and iridium(III) complexes. Hence, this work shows that promising electroluminescent devices can be prepared using cheap and environmentally friendly copper(I) complexes.